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Recent Progress in Pd-Based Nanocatalysts for Selective Hydrogenation
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  • Xiaojing Zhao*
    Xiaojing Zhao
    College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China
    *Email: [email protected]
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  • Yandong Chang
    Yandong Chang
    College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China
    College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China
    More by Yandong Chang
  • Wen-Jie Chen
    Wen-Jie Chen
    College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China
    More by Wen-Jie Chen
  • Qingshi Wu
    Qingshi Wu
    College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China
    More by Qingshi Wu
  • Xiaoyang Pan
    Xiaoyang Pan
    College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China
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  • Kongfa Chen
    Kongfa Chen
    College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China
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    Orcidhttps://orcid.org/0000-0001-7809-675X
  • Bo Weng*
    Bo Weng
    cMACS, Department of Microbial and Molecular Systems, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-8337-219X
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ACS Omega

Cite this: ACS Omega 2022, 7, 1, 17–31
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https://doi.org/10.1021/acsomega.1c06244
Published December 20, 2021

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Abstract

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Selective hydrogenation plays an important role in the chemical industry and has a wide range of applications, including the production of fine chemicals and petrochemicals, pharmaceutical synthesis, healthcare product development, and the synthesis of agrochemicals. Pd-based catalysts have been widely applied for selective hydrogenation due to their unique electronic structure and ability to adsorb and activate hydrogen and unsaturated substrates. However, the exclusive and comprehensive summarization of the size, composition, and surface and interface effect of metal Pd on the performance for selective hydrogenation is still lacking. In this perspective, the research progress on selective hydrogenation using Pd-based catalysts is summarized. The strategies for improving the catalytic hydrogenation performance over Pd-based catalysts are investigated. Specifically, the effects of the size, composition, and surface and interfacial structure of Pd-based catalysts, which could influence the dissociation mode of hydrogen, the adsorption, and the reaction mode of the catalytic substrate, on the performance have been systemically reviewed. Then, the progress on Pd-based catalysts for selective hydrogenation of unsaturated alkynes, aldehydes, ketones, and nitroaromatic hydrocarbons is revealed based on the fundamental principles of selective hydrogenation. Finally, perspectives on the further development of strategies for chemical selective hydrogenation are provided. It is hoped that this perspective would provide an instructive guideline for constructing efficient heterogeneous Pd-based catalysts for various selective hydrogenation reactions.

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1. Introduction

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Catalysis in the 20th century was focused primarily on the activity and turnover rate to produce more molecules per unit time; however, due to increased processing costs and the negative ecological impacts of undesirable byproducts, the present focus and future direction of catalysis science pursue high selectivity in all catalyst-based chemical processes. (1) With the development of societies, the waste and depletion of natural resources have prompted the synthesis of many artificial chemicals to meet human needs. This situation requires improved catalytic selectivity and a decrease in the formation of byproducts in accordance with the principles of green chemistry. (2) Hydrogenation is the core of many industrial processes. (3) To address the low-carbon environmental protection, selective hydrogenation, which refers to a reaction in which only one target functional group is reduced while all other functional groups are still unsaturated in the substrate, is arguably the most important. (4) For instance, during fine chemical synthesis, various functional groups, such as −C≡C, −C═O, −NO2, −C≡N, −COOR, and −CONH2, can be selectively reduced by clean and cheap H2 to their corresponding alkenes, alcohols, and amines, as shown in Figure 1, which are key intermediates for fine chemicals, agrochemicals, polymers, and pharmaceuticals. (5) More specifically, the hydrogenation of acetylene and propyne in petroleum cracking gases and the conversion of alkynes to olefins can be achieved by selectively hydrogenating C≡C to C═C for producing high-purity ethylene and propylene. (6,7) Notably, the crucial point is to promote the hydrogenation reaction of C≡C into C═C while inhibiting the overhydrogenation to form C–C compounds. Heterogeneous catalysts using metal nanoparticles as the main active components are more favorable in the chemical industry due to their high activity, reusability, and stability. However, traditional heterogeneous catalysts often lead to overhydrogenation, resulting in low selectivity. Thus, improving the product selectivity remains a major challenge for the hydrogenation reactions.

Figure 1

Figure 1. Selective hydrogenation of unsaturated bonds by palladium-based catalysts.

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To achieve selective hydrogenation, a suitable catalyst must be chosen to obtain the target products. Pd-based catalysts have been widely used in metal-organic chemistry, and they present fantastic performance in a variety of organic chemistry reactions such as selective hydrogenation, oxidative dehydrogenation, coupling, and cycloaddition. (8−10) Due to their unique electronic structures and their ability to adsorb and activate hydrogen and unsaturated substrates, Pd-based catalysts are also widely used in selective hydrogenation. In the past decades, many efforts have been made to design highly selective and active Pd-based catalysts for selective hydrogenation. (11−17) For instance, Grabovskii et al. have described the recent advances in palladium catalysts deposited on various inorganic and organic for C═C bond hydrogenation. (18) Also, Sajiki and co-workers have also outlined the effect of support, such as chelate resin, ceramic, spherically shaped activated carbon, 3 Å molecular sieves, and boron nitride, on the selective hydrogenation performance over heterogeneous palladium catalysts. (19) Moreover, McCue et al. have introduced the selective hydrogenation of acetylene using Pd-containing catalysts. (20) These works strongly indicate that Pd-based catalysts play a crucial role in the field of selective hydrogenation reactions. However, the exclusive summarization of the size, composition, and surface and interface effect of metal Pd, which could significantly affect the performance for selective hydrogenation, is still lacking. Moreover, a comprehensive overview on the recent progress of different selective hydrogenation reactions over Pd-based catalysts is also unavailable.
In this context, we present such a timely perspective on recent advances in Pd-based catalysts for selective hydrogenation reactions. We first introduce the advantages of Pd-based catalysts for selective hydrogenation and the effects of the size, composition, and surface and interfacial structure on influencing their catalytic performance. Then, the progress on Pd-based catalysts for selective hydrogenation of unsaturated alkynes, aldehydes, ketones, and nitroaromatic hydrocarbons is revealed based on the fundamental principles of selective hydrogenation. Finally, on the basis of the current progress in this area, future perspectives in the aspects of synthesizing efficient Pd-based catalysts have been proposed. It is hoped that this perspective would provide an instructive guideline for constructing efficient heterogeneous Pd-based catalysts toward various selective hydrogenation reactions.

2. Advantages of Pd-Based Catalysts and the Factors Influencing Their Catalytic Performance

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2.1. Advantages of Pd-Based Catalysts

Commonly used selective hydrogenation catalysts are composed of noble metals such as Pd, Pt, Ru, and Rh and a base metal such as Fe or Ni. Although Fe-based catalysts are inexpensive, their activity is low, and the resulting iron mud is difficult to treat and easily causes pollution. Ni-based catalysts generally exhibit low selectivities with poor stability and require harsh reaction conditions. The precious metal palladium (Pd) is widely used in metal-organic chemistry due to its excellent catalytic performance. (21) This is mainly because of the unfilled d-electron orbital of Pd, which easily adsorbs reactants on its surface. Pd0 (zero-valent Pd) is a good nucleophile and could be easily oxidized to Pd1+ or Pd2+. Based on the nucleophilic addition of Pd0 to various organic groups, Pd0 has been widely studied for chemical synthesis. Pd1+ and Pd2+ are good electrophiles, are air-stable, and can be dissolved in a variety of common organic solvents. Pd2+ shows strong electrophilic properties toward electron-rich compounds such as alkenes and alkynes. Pd1+ and Pd2+ compounds can react with alkenes or alkynes to form unstable intermediates that are then attacked and released by nucleophilic reagents to carry out catalytic reactions. (22,23) These characteristics have resulted in the wide use of Pd in catalytic reactions.

2.2. Homolytic and Heterolytic Dissociation of H2

The activation manner and the types of H species (i.e., H+, H–) also profoundly influence the selectivity during hydrogenation. During this reaction, H2 dissociates on the surface of a catalyst and then interacts with the functional groups to be hydrogenated to obtain the target product. (24) H2 on Pd sites usually dissociates in two ways: homolysis and heterolysis (Figure 2). The partially occupied d-orbitals of Pd can accept the σ electrons of H2, and d-electrons can be donated to the σ* antibonding orbital of H2. Therefore, the H–H bond is weakened and cracked to form two hydrides via H2 dissociation. Generally, increasing the electron density of Pd sites promotes the homogeneous dissociation of H2. (25) Different to homolytic dissociation, the heterolysis of H2 does not involve charge transfer. H2 is cleaved to H+ and H– when it is activated on the surface of Pd and other coordination heteroatoms. Then, it combines with heteroatoms and Pd, respectively. In general, heterolysis more easily reduces polar groups, and H+ and H– can be directly transferred to substrates. Due to the noncompetitive adsorption of the substrate and H2, there is sufficient space for the activation of H2. Besides, H+/H– pairs tend to kinetically reduce polar groups, which greatly improves the chemical selectivity of hydrogenation. (26) For example, Zheng’s group found that the Pd/Cu2O catalyst showed excellent performance for semihydrogenation of terminal alkynes. This was mainly because the unique interface structure of Pd/Cu2O and Pd1+ promoted the heterolysis of H2 at Pd–O–Cu sites. This formed Pd–Hδ− and O–Hδ+, which promoted the hydrogenation of alkynes. Since the hydrogenation energy barrier of olefin intermediates is higher than its desorption energy barrier at the active site with isomeric hydrogen species, the formation of alkanes is inhibited; thus, olefin products can be obtained with high selectivity. (27)

Figure 2

Figure 2. Heterolysis of H2. (28) Reproduced with permission from ref (28). Copyright 2007 American Association for the Advancement of Science.

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2.3. Factors Affecting the Catalytic Performance of Pd-Based Catalysts

The selective hydrogenation performance of Pd-based catalysts is related to the size, composition, and surface–interface structure (Figure 3). First, when the particle size of Pd-based catalysts is decreased, their morphology and the spatial distribution of components change, resulting in significant differences in their activity and selectivity. Second, the composition of Pd-based catalysts significantly impacts their selective hydrogenation ability (for example, different Pd-based alloy catalysts like PdCu, PdAg, PdAu, PdZn, etc.). The arrangement of surface atoms and the coordination structure of Pd-based catalysts also influence the catalytic activity; thus, the addition of appropriate ligands, modifiers, and carriers plays an important role in improving the catalyst performance. Moreover, the different synthesis methods of Pd-based catalysts are also related to the selective hydrogenation performance of the catalysts; therefore, the main goal of selective hydrogenation research is to clarify the structure–activity relationship of Pd-based catalysts and prepare Pd catalysts with high activity, high selectivity, and high stability. (29−31)

Figure 3

Figure 3. (A) Size effect, (B) composition effect, and (C) surface and interface effect over Pd-based catalysts.

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2.3.1. Size Effects

Surface-coordinated unsaturated atoms often interact with other atoms. Decreasing the size of Pd-based catalysts introduces surface atomic coordination structures, increases the specific surface area, as well as introduces unsaturated coordination sites. (32) When the particle size decreases, the defects on the catalyst surface increase. Decreasing the particle size also increases the number of defects on the catalyst surface, and terrace easily forms, which forms catalytic active sites that are different from planar sites. (33) Abdollahi’s group calculated the activation energy of acetylene hydrogenation to ethylene on small Pd2 by density functional theory (DFT), which was 23.96 kJ/mol lower than that of Pd12 nanoclusters. The ethylene adsorption energy on Pd2 was lower than that on Pd12, so ethane was more difficult to synthesize using Pd2 compared with Pd12; therefore, decreasing the size of nanoparticles improved the hydrogenation selectivity. (34)
Selectivity depends on the adsorption strength and configuration of reactants or intermediates on a catalyst surface. The atom located at the edge, corner, and terrace of the nanoparticle shows different coordination environments and electronic structures. The absorption strength and configuration depend on the electronic and geometric structure of active sites. For example, depending on the assemblies of Pd atoms, ethylene has three adsorption modes: the ethylidyne mode on threefold Pd sites, the di-σ mode on bridged Pd dimers, and the π-bonded mode on isolated Pd atoms. The adsorption strength decreases in the order ethylidyne > di-σ > π-bonded. (35,36) When two or more functional groups that can be adsorbed on the catalyst simultaneously exist in the substrate, multiple absorption modes will also exist; thus, various byproducts are produced, leading to poor chemical selectivity. To achieve excellent chemical selectivity, the catalytic active sites should have uniform geometric and electronic structures so that the target functional groups can be adsorbed on the catalyst and avoid multiple absorption modes. If the size of nanoparticles is reduced to a minimum, i.e., atomic dispersion, then the metal atoms will directly interact with heteroatoms in the carrier. Single-atom catalysts have unique geometric and electronic structures that can isolate the adsorption sites and prevent multisite adsorption, which effectively controls the catalyst selectivity. (37−39) Single-atom catalysts also reduce the noble metal loading. Therefore, controlling the size of Pd-based catalysts and changing the geometric and electronic structures of catalysts help improve their catalytic activity and selectivity.

2.3.2. Composition Effects

The composition of a catalyst changes its absorption capacity for substrates and intermediates, which produces different catalytic activities and selectivities. The incorporation of other metals into Pd catalysts reduces costs and also changes the electronic structure of the surface atomic coordination structure due to changes in the chemical composition and microstructure. (40) The selective hydrogenation ability of a catalyst can be improved through synergistic and electronic effects. For gas-phase acetylene hydrogenation, Pd–Ag, Pd–Au, Pd–Bi, Pd–Cu, Pd–Ga, Pd–Sn, and Pd–Zn are often used in the industry. (41−44) For liquid-phase alkyne hydrogenation, Pd/CaCO3 (Lindlar catalyst) whose surface has been poisoned by Pb and quinoline is generally used. By doping with other metals, Pd sites separated by intermetallic compounds are formed, which changes the adsorption energy of the substrate and intermediate products and further improves the selectivity. For instance, the selectivity of PdCu/CNT nanoparticles synthesized by a one-pot method reported by the Godard group was >90% toward ethylene during acetylene hydrogenation. This was attributed to the electronic and geometric effects of Cu, which promoted the dissociation of H2 on Pd and resulted in the weak adsorption of ethylene on Pd. Ethylene displayed good selectivity even at a high acetylene conversion rate. This method has already been extended to the preparation of colloidal and carbon-supported single-metal Cu catalysts, whose catalytic performance is much higher than that of ordinary acetylene semihydrogenation catalysts. (45) Besides, constructing Pd intermetallic to make the continuous Pd ensembles separated or even totally isolated can largely enhance the hydrogenation selectivity. Single-atom alloy (SAA) catalysts are promising candidates for selective hydrogenation because the active metal atoms are exclusively surrounded by the second metal atoms at proper compositions and the interaction between each component leads to high thermal stability and structural integrity under reaction conditions. (37,46)

2.3.3. Surface and Interface Effects

The interface structure of a catalyst affects its selectivity. A fine surface interfacial structure includes the arrangement of surface atoms and the coordination structure of surface atoms. During the preparation of Pd-based catalysts, the introduction of suitable ligands produces catalysts with good morphology and appropriate size and the ligands adsorbed on the surface of the catalysts change the adsorption mode of the substrate and intermediate products, which significantly improves the catalytic selectivity. (47−49) Common organic ligands include surfactants and polymers, e.g., oil amines, oleic acid, trioctylphosphine, dodecyl mercaptan, cetyltrimethylammonium bromide, poly(vinylpyrrolidone) (PVP), poly(vinyl alcohol) (PVA), and polyamidoamine (Table 1).
Table 1. Partial Pd-Based Catalysts Modified by Ligands
ligandmetal precursorreducing agentsize (nm)ref
ethylene glycolPdCl2 2.4 ± 0.4 (50)
ethylene glycolNa2PdCl4 14.9 ± 2.3 (51)
trisodium citrate (Na3C6H5O7)Na2PdCl4alcohol15 (52)
diethylene glycolNa2PdCl4 19 (53)
oleylamine (OAM)Pd(acac)2borane tributylamine complex4.5 (54)
poly(N-vinyl-2-pyrrolidone) (PVP)Na2PdCl4acetaldehyde18 (55)
poly(vinyl alcohol) (PVA)Na2PdCl4·2H2O 3.1 ± 0.7 (56)
dendritic phosphine ligandPd(acac)2H23.2–5.0 (57)
n-didocosyl sulfidePd(CH3CN)2Cl2NaBH44–6 (58)
thiolPd(acac)2CO5 (59)
For example, poly(vinylpyrrolidone) (PVP) is a polymer end-capping ligand, in which an O atom is firmly bonded to the (100) surface of Pd. (60) During the growth of PVP-protected Pd nanoparticles, Pd atoms must first bond to the (111) surface and then the attached atoms migrate to the surface edge, extending to form nanocubes on the (100) surface. A capping agent acts as a suitable ligand to form a complex with the metal precursor. For example, during the synthesis of Pd nanocrystals in OAM, Pd(acac)2 first reacted with OAM to form an intermediate complex, Pd(ACAC)x(OAM)y. Then, formaldehyde was added, and the rate of catalysis decreased upon increasing the amount of OAM. (61) Although traditional studies suggest that the presence of ligands affects the catalytic activity, more recent studies have shown that appropriate ligand modification affects the spatial arrangement and coordination structure of the exposed atoms on the surface, which positively impacts the adsorption of substrates and intermediate products, thus improving the product selectivity.
On the one hand, the surface coordination bond structure is closely related to the types of coordination atoms and metals, and the arrangement structure between ligands is often affected by the peripheral noncoordination groups or the surface curvature of nanomaterials, thereby affecting the enrichment process and catalytic reaction kinetics of different reaction species on the metal surface. On the other hand, the coordination of organic molecules with surface metal atoms will also change the electronic structure of surface metal atoms and adjust the adsorption strength of different types of reaction substrates and intermediates on the metal surface. In addition, the coordination of organic ligands with metal atoms also segments the continuous arrangement of metal atoms into smaller active sites, thus affecting the adsorption form and reaction process of different reaction groups on the surface. More importantly, surface organic matter can also form an active interface with metal atoms, changing the catalytic reaction pathway. For example, Medlin’s group modified thiol molecules with different degrees of steric hindrance on the surface of Pd nanoparticles, which improved the selectivity of furan alcohols and methyl furans during furfural hydrogenation, as well as cinnamyl alcohol during cinnamaldehyde hydrogenation. (62) They believed that the self-assembled monolayers (SAMs) formed from thiols and metals through covalent binding of sulfur groups changed the adsorption mode of the substrate and intermediate products to provide selectivity.
Understanding the catalytic mechanism at the molecular level is the key to improving catalyst performance. So, the well-defined catalysts need to be prepared to regulate the active sites. As we know, clusters have accurate atomic structure and can be characterized by mass spectrometry, nuclear magnetic resonance (NMR), and other characterization methods. Using metal nanoclusters with clear compositions and structures as model catalysts helps to understand the structure–activity relationship behind the catalytic reaction of surface ligands. (63,64) Besides, we can first synthesize clean-surface nanoparticles with clear structures and then modify ligands on them to improve their selectivity.
Pd nanoparticles are often loaded on supports as catalysts. Common supports include acidic or alkaline oxides, natural minerals (TiO2, pumice, etc.), silica gel, and various types of carbon materials (activated carbon, graphene, silicon carbide), etc. The use of a carrier improves the dispersion of Pd-based catalysts, changes the electronic structure and spatial distribution of Pd, and forms strong metal–support interactions. (65) This affects the arrangement and coordination structure of the surface atoms of the catalyst, which permits the selective adsorption of functional groups of the substrate and changes the adsorption energy of the substrate and intermediate. (66−69) In recent years, a large number of studies have shown that the catalytic performance of supported metal catalysts is highly dependent on the support. Therefore, it is particularly important to fabricate model catalysts with well-defined and abundant metal–support interfacial sites for their involvement in hydrogenation. Several strategies have been developed to enhance the performance: (1) decorating catalytic metal particles with species having similar composition to the supports to create abundant metal–support interfaces, (2) designing reverse structure which reducible metal oxide supports metal catalysts adopted in metal catalysts so that the interfacial O2–/OH– species can involve hydrogenation, and (3) developing atomically dispersed metal catalysts (ADCs) for revealing the molecular functions of supports in hydrogenation reactions.
Moreover, when metal nanoparticles are confined to porous materials (such as metal-organic frameworks (MOFs), zeolites, etc.), the functional groups of the substrate can be selectively adsorbed at metal active centers by adjusting the pore size to selectivity obtain the target product. Gong et al. reported a Pd nanocluster catalyst confined by a periclase (sodalite (SOD)) zeolite, named Pd@SOD. In the reaction, H2 dissociated on Pd@SOD to form H species (H+, H–), which overflowed to the surface of SOD, and the ethylene selectivity reached 94.5%. Other studies have shown that the main reason for such high selectivity is that H2 can enter the pores, activate Pd, and prevent deep hydrogenation of ethylene. (21)

3. Selective Hydrogenation Using Pd-Based Catalysts

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In both bulk and fine chemical industries, selective hydrogenation plays an important role. As the most widely used hydrogenation catalysts, Pd-based catalysts require high activity and stability as well as high selectivity.

3.1. Application of Pd-Based Catalysts in Selective Hydrogenation of Alkynes

The selective hydrogenation of C≡C to C═C is an important industrial process for the production of fine chemical intermediates. (70) Industrial olefin feedstock often contains a small amount of unsaturated alkynes, and ethylene doping in the feed affects the synthesis and product quality; therefore, it is necessary to selectively hydrogenate acetylene mixed in the ethylene flow to ethylene and remove it to the ppm level to meet quality requirements. Among the various methods to eliminate alkynes, semihydrogenation of alkynes to alkenes has proven to be the most efficient one. A variety of catalysts have been explored for the selective hydrogenation of alkynes, among which Pd-based catalysts have attracted the most attention due to their high intrinsic activity. (71) The performance of some Pd-based catalysts in selective hydrogenation of alkynes is shown in Table 2.
Table 2. Activity and Selectivity of Selective Hydrogenation of Some Alkynes
substrateproductcatalystsel./conv. (%)ref
acetyleneethylenePd4S/CNF94/100 (72)
acetyleneethylenePd>91/99 (73)
acetyleneethyleneAgPd0.005/SiO292.6/93.6 (37)
acetyleneethylenePd/ND@C90/100 (74)
phenylacetylenestyrenePd–Au>80/100 (65)
phenylacetylenestyrenePd@Ag99/99 (75)
phenylacetylenestyrenePd-Cu2O98/99.2 (76)
2-butyne-1,4-diol(Z)-2-butene-1,4-diolPd/Boehmite>60/100 (77)
3-hexyn-1-ol3-hexen-1-olPd/TiO288/100 (78)
1-hexyne1-hexenePdAu85/100 (79)
butynebutenesPdS4/C98/100 (80)
propynepropenePd/Al2O397/97 (81)
diphenylacetylenestilbenePdNP95/99 (82)
diphenylacetylenestilbenePd + PEI@HSS94/100 (83)
diphenylacetylenestilbeneFFSienPd94/100 (31)
1-phenyl-1-propyne1-phenyl-1-propenePdS97/100 (59)
The C≡C bonds contain one σ bond and two π bonds. The electrons of the π bonds are mobile and tend to attack electrophilic reagents. When a π bond is broken, C is added to the active hydrogen species to form an olefin containing a σ bond and a π bond to complete the addition reaction; however, olefins can also add to saturated alkanes. Semihydrogenation of alkynes generally follows the Horiuti–Polanyi (H–P) mechanism in which H2 is first adsorbed and dissociated on the catalyst and then an alkyne is adsorbed and two hydrides are continuously added to the unsaturated bond. In the hydrogenation reaction following the H–P mechanism, the adsorption mode of the acetylene substrate on the catalyst surface plays a decisive role in product selectivity. For example, when controlling the hydrogenation of acetylene to ethylene, the desorption energy barrier of the π bond of ethylene is lower than that of hydrogenation, and ethylene is desorbed from the surface, which avoids excessive hydrogenation to ethane. Zhang’s research group prepared a PdZn/ZnO catalyst, which achieved >90% conversion and selectivity of ethylene. This was mainly attributed to the special active sites of Pd–Zn–Pd (Figure 4A). Initially, ethylene is adsorbed on isolated Pd sites by weak π bonds, which allows it to easily desorb from the PdZn surface, thus preventing its further hydrogenation to ethane. Second, single Pd sites on the Pd–Zn–Pd provide two adjacent isolated Pd sites for the σ absorption of acetylene, which activates C≡C through an ethylene-like intermediate to efficiently convert C≡C to ethylene. The Pd active sites are also favorable for H2 activation. Therefore, the PdZn catalyst showed superior activity and selectivity during acetylene hydrogenation due to its good thermodynamics and kinetics for the conversion of acetylene to ethylene. (84)

Figure 4

Figure 4. (A) PdZn intermetallic nanostructure with Pd–Zn–Pd ensembles are both highly active and selective for semihydrogenation of acetylene to ethylene; (B) high-resolution transmission electron microscopy (HRTEM) image of Pd/ZnO sample reduced at 400 °C; (C) conversion and selectivity with time on stream in absence of ethylene over PdZn; (84) (D) thiol treatment is demonstrated as a highly effective strategy for promoting the catalytic selectivity of Pd nanocatalysts in the hydrogenation of internal alkynes to alkenes; (E) energy barriers of transition states of PhC≡CCH3 hydrogenation on Pd4S@SPhF2, Pd3S@SPhF2, and Pd(111); (F) catalytic stability and selectivity of the Pd@SPhF2(1:1) obtained; (59) (G) a defective nanodiamond-graphene (ND@G) to prepare an atomically dispersed metal catalyst which is used for selective hydrogenation; (H) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Pd1/ND@G at high magnifications, (I) durability test on Pd1/ND@G at 180 °C for 30 h. (Reaction conditions: 1% C2H2, 10% H2, 20% C2H4 gas mix balanced with He; gas hourly space velocity (GHSV) = 60 000 h–1.). (74) (A–C) Reproduced with permission from ref (84). Copyright 2016 American Chemical Society. (D–F) Reproduced with permission from ref (59). Copyright 2018 Elsevier. (G–I) Reproduced with permission from ref (74). Copyright 2018 American Chemical Society.

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The alkynyl hydrogenation selectivity can be improved by doping other metals into Pd to form an alloy, which segments the surface Pd sites. In addition, the doping of main group nonmetals into the surface and lattice of Pd-based catalysts can also regulate the coordination and electronic structure of Pd sites on the surface. Besides, the steric hindrance induced by an ordered array of surface ligands can also manipulate the binding of hydrogenation intermediates to improve selectivity. For example, Zheng’s research group used thiol-treated ultrathin Pd nanosheets as a model catalyst and demonstrated the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward 1-phenyl-1-propene and no obvious decay in the activity and selectivity after 10 cycles (Figure 4B). It was found that a mercaptan dissociated on the surface of Pd nanosheets, the C–S bond was broken, and then S diffused into Pd nanosheets to form palladium sulfide. The unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to semihydrogenation. The electronic effect produced by PdSx and the steric hindrance produced by thiol caused the thiol-modified palladium nanosheets to reduce the reaction energy barrier of the first step in the hydrogenation of phenylpropyne and increase the reaction energy barrier of the third step of hydrogenation. This made the subsequent hydrogenation energy barrier of olefins much higher than that of alkyne hydrogenation and the olefin surface desorption energy barrier, which finally realized the selective hydrogenation of alkynes to olefins. More importantly, this thiol treatment strategy is applicable to commercial Pd/C catalysts for semihydrogenation of internal alkynes. (59)
It has been well documented that the adsorption and dissociation of H2 are closely related to the electronic structure and coordination environment of catalytic metal centers. (85) The catalytic metal center with a low valence state and an unsaturated coordination structure is conducive to the activation of H2, thus showing good hydrogenation activity. (86,87) The supported atomically dispersed catalysts where all active metal atoms are exposed have been fabricated for selective hydrogenation of alkynes. Professor Martin prepared an atomically dispersed metal catalyst Pd/nanodiamond-graphene (ND@G) by an impregnation–reduction method by controlling the pH. Acetylene could be reduced by a mixture of H2 and He in 30 min with 100% conversion and 90% ethylene selectivity. Compared with Pd foils, the X-ray absorption near-edge spectroscopy (XANES) peaks of Pd1/ND@G and Pdn/ND@G shifted to a higher valence state, which demonstrated the existence of slightly positively charged Pd species. The absorption peak of Pd1/ND@G was located between those of Pdn/ND@G and PdO, suggesting that the interactions of atomically dispersed Pd species and ND@G were stronger than those of Pd in Pdn/ND@G. Extended X-ray absorption fine structure (EXAFS) contained peaks of Pd–C or Pd–O at 1.5 Å, which provided key evidence for the coordination environments of Pd atoms anchored on ND@G. The Pd–Pd coordination signal at 2.4 Å in Pdn/ND@G confirmed the existence of Pd nanoclusters (Figure 4C). According to DFT calculations, acetylene gas is preferentially adsorbed on the Pd atoms of Pd1@Gr. The adsorption energy of acetylene on Pd atoms of Pd1@Gr (−0.61 eV) was weaker than that on the Pd(111) surface (−1.79 eV). Then, hydrogen underwent heterolytic dissociation, in which one atom combined with a C atom and the other combined with a Pd atom. The energetic barrier of further hydrogenation of the adsorbed C2H4 intermediate to ethane (1.17 eV) was much higher than the desorption energy of surface C2H4 atoms. The atomically dispersed Pd atoms blocked the formation of unselective subsurface hydrogen species and promoted the facile desorption of ethylene, which prevented its overhydrogenation to ethane. (74)
In short, the selective hydrogenation of alkynes is of great significance to both basic research and industrial applications. Based on the understanding of the adsorption behavior of olefins and alkynes onto catalyst surfaces, some strategies have been developed to improve the selectivity of catalysts. For example, other metal atoms can be incorporated into Pd, forming alloys or intermetallic compounds, to isolate Pd active sites and change the adsorption behavior of intermediate products, which effectively inhibits side reactions. Another efficient strategy involves modifying the Pd surface with thiols, amines, phosphines, or other ligands, which selectively poisons active sites and induces electronic and steric effects, thus changing the adsorption mode of substrates or intermediates. To maximize the efficiency for metal utilization, various monoatomic catalysts, including catalysts anchored on the supports or alloyed with the second metal, have been prepared, showing good activity that is superior to nanoparticle catalysts in the semihydrogenation reactions in gas or liquid phases. In addition, the structure–activity relationship of the catalyst can be clearly analyzed by spherical differential electron microscopy and EXAFS when the size of the catalyst is reduced to the single-atom level.

3.2. Selective Hydrogenation of Aldehydes and Ketones

The selective hydrogenation of unsaturated aldehydes/ketones (C═O and C═C coexisting in one molecule) usually requires a long reaction time and harsh conditions, (24) such as high temperatures and high pressures; therefore, new efficient systems are needed to realize the selective conversion of aldehydes and ketones. Because C═C hydrogenation is thermodynamically favored over C═O hydrogenation, more efforts were made to selective hydrogenation of C═O to unsaturated alcohols (UOLs). The rational design of heterogeneous catalysts is a crucial step toward high selectivity. Some strategies like reducing the size to the atomic level, changing the electronic properties, and forming a steric effect have been used to design a catalyst. Pd-based catalysts are widely studied for selective hydrogenation of aldehydes and ketones, and the performance of selected Pd catalysts is listed in Table 3.
Table 3. Activity and Selectivity of Selective Hydrogenation of Some Unsaturated Aldehydes and Ketones
substrateproductcatalystsel./conv. (%)ref
cinnamaldehyde3-phenyl-1-propanalNa2[Pd(HSS)]92.5/100 (88)
cinnamaldehydephenylpropionaldehydePdAu90/100 (89)
1-phenyl-1-propanone1-phenyl-1-propanolPd/TiO299.7/100 (90)
cinnamaldehydehydrocinnamaldehydePdZn70/90 (91)
cinnamaldehydehydrocinnamaldehydePd-NMC93/100 (92)
benzaldehydebenzyl alcoholPd/MIL-101(Fe)-NH277/100 (93)
5-hydroxymethylfurfural2,5-dimethylfuranPd–Co9S8/S-CNT83.7/96 (94)
furfuralfurfurylPd/Cu96.5/96.4 (95)
chalconedihydrochalconePd99/98 (96)
cinnamaldehydecinnamyl alcoholPd/Al2O390/100 (62)
When the size of Pd decreases, especially to the single-atom level, its catalytic performance changes, and the catalytic effect is improved after being dispersed on a support. In the target C═O group, the C-terminal is positively charged and the O-terminal is electron-rich, so the positive center in the catalyst can promote the polarization and activation of C═O bonds. (97) So, the objectives of catalyst design should be preferentially to adsorb C═O bonds and to prevent the C═C bonds from approaching the metal surface. A Pd1+NPs/TiO2 catalyst was synthesized by Li’s research group to explore the performance of ketone/aldehyde hydrogenation catalyzed by a synergic catalyst (Figure 5A). They found that the catalytic activity of Pd1+NPs/TiO2 was 3.2 times higher than that of the commercial Pd/C (5.2 wt %). Under 0.3 MPa H2 and 40 °C, Pd1+NPs/TiO2 showed excellent catalytic activity with 100% conversion of 4-methylacetophenone within 20 min and an alcohol selectivity up to 98%. EXAFS analysis showed that Pd1 and Pdn had a synergistic effect (Figure 5B). Compared with Pd1/TiO2 and PdNPs/TiO2 catalysts, Pd1+NPs/TiO2 catalysts synthesized by a simple large-scale spray pyrolysis method combined the advantages of Pd1 and PdNPs on mesoporous TiO2 supports. By exploiting the synergistic effect of Pd1 and PdNPs, some Pd1 sites contributed dispersion sites to activate C═O groups, while the PdNPs sites promoted the dissociation of H2 molecules to H atoms. According to DFT results, H2 was preferentially dissociated on metal PdNPs sites, while ketones and aldehydes were adsorbed at Pd1 sites. Pd1 dispersed around PdNPs helped to improve the hydrogenation activity and selectivity of acrolein. The synergistic effect of Pd1 and PdNPs on TiO2 improved the reaction activity. A large number of Pd1 sites activated C═O groups, and PdNPs sites promoted the dissociation of H2. (90)

Figure 5

Figure 5. (A) HAADF-STEM energy-dispersive spectrometry (EDS) mapping of Pd1+NPs/TiO2 and AC-HAADF-STEM image of Pd1+NPs/TiO2, (B) R-spaced Pd K-edge Fourier transform (FT)-EXAFS spectra of Pd1+NPs/TiO2 (green line) referred to bulk Pd foil (black line) and PdO (magenta line), (90) (C) cinnamaldehyde hydrogenation pathways, and (D) cinnamaldehyde hydrogenation performance profile using Pd–Au catalysts. (98) (A, B) Reproduced with permission from ref (90). Copyright 2020 Springer Nature. (C, D) Reproduced with permission from ref (98). Copyright 2006 Wiley-VCH.

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Reducing the size of a catalyst to the single-atom level changes the adsorption energy of the C═O bond and improves the catalyst selectivity. Changing the catalyst composition by constructing a bimetallic catalyst also changes the selectivity of the catalyst through geometric and electronic effects. The Au–Pd alloy catalyst synthesized by Pârvulescu’s research group improved the selectivity of cinnamaldehyde hydrogenation into cinnamic alcohol. (98) A series of bimetallic Au–Pd colloidal nanoparticles with different Au/Pd molar ratios were synthesized by a sol–gel method by adjusting the concentration of metal ions in an aqueous solution. On monometallic Pd- and Au-embedded colloids, 3-phenylpropanal and 3-phenyl-1-propanol were found to be the predominant reaction products. Alloying of the two nanometer-sized metals in colloids led to a very important enhancement of the selectivity, with cinnamyl alcohol being the major product (Figure 5C,D). Besides the influence of the Au–Pd ratio on the catalytic performance, other factors also affect the selectivity. For example, increasing the H2 pressure can improve the gas concentration in the reaction medium, thereby enhancing the addition of H2 to functional groups. The solvent also affects the catalyst. For instance, a polar solvent can promote the hydrogenation of carbonyl groups. (99) It is clear from these results that Au, although totally inactive when it is present alone, plays an important role when alloyed with Pd, increasing the catalytic activity and selectivity with respect to those of monometallic Pd.
In addition, modifying a catalyst by adding proper ligands can change the sites that adsorb functional groups during catalytic reactions, thus affecting the reaction pathway during selective hydrogenation. At present, researchers have used organic ligands such as mercaptans, (100) amines, (101) and organic phosphines (102) to modify the surface of a catalyst to introduce steric and electronic effects, which affect the adsorption mode of functional groups and change the catalyst selectivity. When thiols are exposed to the metal surface, they tend to occupy the terrace sites, forming densely stacked monolayers, thus covering continuous metal sites and leaving discontinuous corner/edge sites. Due to the decrease of the assemblies of accessible metal atoms, the adsorption mode of the reactants is limited, thus improving the selectivity. This is called the “active-site selection” strategy (Figure 6). (103) Medlin’s group reported a SAM of thiols with varying surface densities to tune the selectivity of supported Pd catalysts. As confirmed by vibrational spectroscopy, while 1-adamantanethiol (AT) prefers to coordinate to corner sites of Pd nanoparticles, octadecanethiol (C18) preferentially coordinates with terrace sites. In the hydrogenation of furfural in a fixed-bed reactor at 190 °C and 0.1 MPa H2, the selectivity to furfural alcohol was below 5% over unmodified and AT-modified Pd/Al2O3, while over octadecanethiol-modified Pd/Al2O3, the selectivity was increased to above 70% (Figure 6B).

Figure 6

Figure 6. Illustrations of approaches for tuning the selectivity of nanocatalysts in hydrogenation of α,β-unsaturated aldehydes using organic modifiers. (A) Proposed schematic depicting active-site selection, (B) product selectivity for furfural hydrogenation over uncoated and alkanethiolate SAM-coated Pd/Al2O3 catalysts, (C) thiol SAMs used to coat the Pt/Al2O3 surface, and (D) selectivity to cinnamyl alcohol for the hydrogenation of cinnamaldehyde over Pt/Al2O3 catalysts. (A, B) Reproduced with permission from ref (103). Copyright 2013 Springer Nature. (C, D) Reproduced with permission from ref (104). Copyright 2014 American Chemical Society.

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It has been shown previously that binding in a horizontal configuration favors C═C hydrogenation, while binding in a vertical orientation favors C═O hydrogenation. In cinnamaldehyde hydrogenation, the authors chose thiol molecules with a tail structure similar to that of the substrate. While linear alkyl ligands are not expected to interact preferentially with a particular region of the cinnamaldehyde reactant, phenylated ligands can interact with cinnamaldehyde’s phenyl group through aromatic π–π stacking. The cinnamic alcohol selectivity reached 90% when 3-phenylpropanethiol was used as the modifier during selective hydrogenation of cinnamaldehyde under conditions of 50 °C and 4 MPa H2. The cinnamyl alcohol selectivity was very poor when a long-chain alkyl mercaptan or short-chain 2-phenylethyl thiol and long-chain 4-phenylbutanthiol were used. The selectivity was mainly due to π–π stacking between cinnamyl aldehyde and 3-phenyl thiol. (104)
Selective hydrogenation of unsaturated aldehydes/ketones C═O to unsaturated alcohols is an important reaction in the pharmaceutical industry, but it is still challenging because C═C groups are more easily saturated. Among them, selective poisoning of a catalyst is used to modify thiols, amines, and other organic ligands on a catalyst’s surface to promote selective adsorption of C═O. Selective hydrogenation of C═O can be achieved by reducing the catalytic active components to the single-atom scale and inhibiting the adsorption of C═C. Similarly, the selectivity can also be increased by constructing bimetallic nanoparticles by exploiting electronic effects and by changing the substrate and intermediate adsorption energy. Besides, more investigations on the effect of the valence state of transition-metal cations and the solvent on the adsorption strength of the carbonyl group also need to be done.

3.3. Selective Hydrogenation of Nitroaromatic Compounds

Selective hydrogenation of nitroarenes is a simple, economic, and effective way to synthesize aromatic amines. (105) At present, high H2 pressures and high temperatures are often used in the industry, and the process is mainly realized by reducing agents or organic solvents that are environmentally unfriendly. Although Pt-based catalysts have a good effect on nitro hydrogenation, research into Pd-based catalysts is still challenging and has focused on the development of Pd-based catalysts with high catalytic activity, good sustainability, and environmental friendliness. When using Pd-based catalysts, it is necessary to ensure that other functional groups are not destroyed during −NO2 reduction. Taking nitrostyrene as an example, C═C is more easily reduced than −NO2. It has been pointed out that the adsorption of C═C is very sensitive to the catalyst structure, and −NO2 preferentially undergoes monodentate adsorption on the catalyst surface; therefore, researchers have studied the geometry and electronic structure of a catalyst to regulate the adsorption mode of C═C and −NO2 to preferentially adsorb nitro groups for selective hydrogenation. Table 4 shows the performance of some Pd-based catalysts for selective hydrogenation of nitro and nitrile groups.
Table 4. Activity and Selectivity of Selective Hydrogenation of Some Nitro and Nitrile Groups
substrateproductcatalystsel./conv. (%)ref
4-nitrophenol4-aminophenolPd/CeO299.9/100 (106)
nitrobenzeneazoxybenzenePd(acac)296.8/100 (107)
nitrobenzeneazoxybenzenePd87.6/100 (108)
2-nitroaniline2-phenylenediamineVPY-NVP-Pd98/100 (109)
nitrobenzenesanilineSiO2-BisILS[Cl]R-Pd100/100 (110)
nitrophenolaminophenolPd/GO99/100 (111)
2-methylnitrobenzene2-methylanilinePd100/100 (112)
4-chloronitrobenzene4-chloroaminobenzenePd/B-MCM-41100/100 (113)
1-chloro-4-nitrobenzene4-chloro-anilinePdFe99/100 (114)
nitrobenzeneaminobenzenePd/PBA98/100 (115)
nitroarenesazoxybenzenePd/SiO292/100 (116)
4-vinyl nitrobenzene4-vinylaminobenzenePd/CSs100/100 (117)
4-chloronitrobenzene4-chloroanilinePd-BNNS>99/100 (118)
mandelonitrilephenylethylaminePd/C87/100 (119)
benzonitrilebenzylaminePd/C90/99 (120)
benzonitrilebenzylaminePdNi94/100 (121)
4-nitrostyrene4-aminostyrenePdNPs/MAX93/100 (122)
Taking the hydrogenation of acetonitrile to ethyl amine as an example, the Pd(111) surface provides multiple adsorption sites. The acetonitrile reaction substrate and hydrogen are coadsorbed at the active sites of the catalyst. If the dissociated H is used exclusively for conversion to the intermediate of the desired amine product, the selectivity can be improved. The imine intermediate CH3CH═NH is crucial for the formation of secondary and tertiary amines. If there are few dissociated hydrogen atoms on the surface or if the active sites are difficult to access, these strongly bound intermediates may poison the catalyst’s surface, which reduces the selectivity. (123) Pd in the catalyst mainly determines the activity of the catalyst. When an alloy is formed by doping other metals in a catalyst, changes in its chemical composition and microstructure will change its catalytic performance. Single-atom Pd1 is prepared by atomic layer deposition (ALD) on the surface of SiO2 loaded with Ni nanoparticles. Quasi-atomically dispersed Pd was formed on the outer layer of Ni nanoparticles, and a core–shell quasi-PdNi single-atom structure was formed by adjusting the Pd coverage on NiNPs (Figure 7A). The yield of dibenzylamine increased sharply from 5 to about 97% under H2/0.6 MPa and 80 °C. Studies have shown that using Ni to isolate Pd destroys the strong metal selectivity during the hydrogenation of −C≡N and promotes the yield of secondary amines to >94% using a wide range of −C≡N groups (Figure 7B). More importantly, the obtained material also showed excellent recyclability and completely inhibited the formation of hydrolysate byproducts, demonstrating its potential in practical applications. (121)

Figure 7

Figure 7. (A) Schematic diagram for synthesis of the Pd–Ni/SiO2 bimetallic catalyst and (B) Pd/SiO2, Pt/SiO2, and PdNi/SiO2 catalytic performance diagram. (121) Reproduced with permission from ref (121). Copyright 2019 Springer Nature.

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The presence of a surface and an interface during a catalytic reaction directly affects a catalyst’s performance. The arrangement of surface atoms and coordination structures directly affects the adsorption mode of the substrate, thus affecting the activity and selectivity of a catalyst. Qu’s group synthesized a graphene oxide-supported Pd nanocatalyst (Pd/GO) by a one-step method. This Pd nanocatalyst, in which Pd nanoparticles were uniformly dispersed in GO, was cost-effective and environmentally friendly and efficiently reduced nitroaromatic compounds. Pd/GO nanocatalysts in aqueous solution converted nitroaromatic compounds into corresponding amino-aromatic compounds with up to 99% yield (Figure 8). (111) Researchers from the University of Drexel reported the preparation of Ti3SiC2, Ti3AlC2, and Ti2AlC at 1500 °C with trace Pd doping. Ti3SiC2 without Pd loading and Ti3SiC2 loaded with Pd by an impregnation method were mixed mechanically to achieve more selective reduction. After optimizing the amount of Pd, the results showed that when the Pd content was 130 ppm, the conversion of 4-nitroacetylene reached 100%, and the selectivity was 93%. The well-dispersed Pd on the support and its support composition, surface, and metal–support interactions all played important roles in the reaction. The high chemoselectivity of Pd/MAX was attributed to the synergistic effect between the Pd nanoparticle size and dispersion and the non-Ti-containing oxides formed on the MAX phase, which preferentially activated the nitro group. H2 activation was a key step in chemical selective hydrogenation, and the role of Pd in the catalytic reaction was mainly to cleave and activate H2 molecules. They speculated that due to its low content, Pd may exist as single atoms, which enabled it to serve as a hydride receptor to activate H2, which is very effective for selective hydrogenation. (122)

Figure 8

Figure 8. Pd loading supported on MAX phases for chemoselective hydrogenation. (122) Reproduced with permission from ref (122). Copyright 2020 American Chemical Society.

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In summary, for selective hydrogenation of nitro compounds, based on the current understanding of the reaction mechanism, the activity and selectivity of catalysts can be regulated from several aspects. First, since the hydrogenation of C═C is structurally sensitive and the hydrogenation of NO2 is not, the reaction of C═C can be inhibited by reducing the aggregation of metal atoms, such as by poisoning or modifying with organic ligands, forming alloys, or enhancing the selectivity of catalysts through strong metal–support interactions. Second, C═C is electron-rich, while −NO2 is electron-deficient; thus, we can construct nucleophilic sites on catalysts that strongly and selectively adsorb NO2 and weakly adsorb other groups by using oxygen-rich supports or by modifying the support surface with −NH2. Third, when the catalyst is decreased to the single-atom scale, hydrogen undergoes heterolytic cleavage, which allows it to more easily adsorb nitro groups to achieve selective hydrogenation.

4. Summary and Perspectives

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Selective hydrogenation is a very important hydrogenation reaction that is widely used to synthesize fine chemicals, drugs, healthcare products, and agricultural chemicals. Typical reactions include partial hydrogenation of alkynes to alkenes, hydrogenation of aldehydes and ketones to unsaturated alcohols, and hydrogenation of nitro groups to amino groups. The grim state of the environment and the need to achieve “peak carbon dioxide emission, carbon neutralization” as soon as possible put forward unprecedented challenges for catalytic materials. Catalysts must have high chemical reactivity and high stability to save energy and high selectivity to achieve atom economy and reduce pollution and byproducts generated to achieve green chemical processes.
The performance of catalytic hydrogenation mainly depends on the interaction and reaction between H2 and metal unsaturated substrates. As pointed out by the Sabatier principle, the high activity requires that the interaction between the substrate and the metal site is neither too strong nor too weak. (124) In addition, for substrates containing multiple unsaturated groups, the scaling relationship between the relevant adsorption groups on similar active metal sites often leads to inevitable overhydrogenation. (125) To prevent overhydrogenation, it is important to understand the dissociation mode of H2 on the catalyst surface and the adsorption mode and reaction mechanism of the substrate at active sites during hydrogenation. With the progress of various characterization techniques and the guidance of DFT simulations, the structure of catalysts has become clearer; therefore, the rational design of the electronic and coordination structure is essential to optimize their hydrogenation performance.
First, the use of atomically dispersed catalysts reduces metal loading, and the separation of active sites determines the substrate’s adsorption mode, which prevents the occurrence of side reactions and is closer to achieving ″green catalysis″. The coordination environments of those atomically dispersed metal species, including the central metal atom and its oxidation state, coordination number and geometry, play together to induce steric and electronic effects to determine the overall catalytic performance. Moreover, atomically dispersed catalysts are desirable because the active metal atoms are 100% exposed and accessible to reactants and thus greatly improve the atom utilization efficiency of noble metal catalysts.
Second, changing the composition of the catalyst, forming alloys or intermetallic compounds, creating electronic and geometric ensemble effects that control the binding energetics of reactants or hydrogenated intermediates on metal surfaces can also be used to improve selectivity. Among these, single-atom alloy (SAA) catalysts are an ideal system because the electronic structure of the isolated catalytic sites tailored by the host metal can also regulate the adsorption structure of reactants or intermediates, therefore impacting both the activity and selectivity during hydrogenation.
Third, organic ligands containing N, P, and S atoms were used to modify the catalyst surface to change the atomic arrangement and coordination of the active elements to produce electronic or build up a confinement space generating steric hindrance effects that affected the substrate adsorption and increased the catalyst’s selectivity. Besides, the supported metal catalysts constructing metal–support interfaces not only make the heterogeneous activation of H2 possible to enhance the hydrogenation of polar unsaturated bonds but also make it easy to generate unsaturated low-valence metal sites on the reducible supports. Finally, catalyst selectivity can also be improved by confining metal atoms to a porous structure, forming strong metal–support interactions, and adjusting the reaction parameters.
Although these strategies for chemical selective hydrogenation have made exciting and encouraging progress, there are still some challenges to be overcome in fundamental research and practical applications. (1) Successful preparation of well-defined catalysts and accurate characterization of its active sites: The key to preparing atomically dispersed catalysts is that the supports need to provide atoms to coordinate with the catalytic single atoms. So, different supports such as ionic supports, metallic supports, and MOFs should be chosen according to metal valence. (2) Achieving both high activity and selectivity: In most situations, high chemoselectivity is often accompanied by a certain loss of activity by means of surface modifiers or doping other elements or oxides. Therefore, to achieve high chemical selectivity without affecting the activity, a promising way to develop in this direction is to construct well-defined catalysts with an adjustable coordination environment. (3) Mechanism understanding at the molecular level: The key factors including metal size, support, and ligand modifiers determine the performance of heterogeneous metal nanocatalysts, which manipulate the activation path of H2 and the interaction between the activated hydrogen species and the substrate on the surface of catalysts. The atomic dispersion feature can be identified by aberration-corrected TEM-STEM, and the average coordination structure can be identified by EXAFS. Precise in situ local characterization techniques such as Mössbauer spectroscopy, temperature-programmed desorption-mass spectrometry (TPD-MS), and nuclear magnetic resonance (NMR) should be applied for chemical environments of active sites in different structures. Combining DFT calculations, constructing a structure model reflecting its active coordination structure, and providing experimental evidence of the intermediates or labeled products expected from the proposed mechanism can help to incisively understand the catalytic mechanism.
In conclusion, rational design of the electronic and coordination structure of Pd-based catalysts, understanding the structure–activity relationships of Pd-based catalysts, and expanding large-scale application prospects for industrial applications are efficient ways to achieve green industrial hydrogenation techniques.

Author Information

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  • Corresponding Authors
  • Authors
    • Yandong Chang - College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China;  College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China
    • Wen-Jie Chen - College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China
    • Qingshi Wu - College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China
    • Xiaoyang Pan - College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China
    • Kongfa Chen - College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China;  Orcidhttps://orcid.org/0000-0001-7809-675X
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the NSF of China (21802085, 21805164), the General Project of the NSF of Fujian Province (2019J01730, 2020J01776, 2019J01735), and the Innovation and Entrepreneurship Projects for High-Level Talents of Quanzhou (2017Z028).

References

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    A review of recent trends and developments for the selective hydrogenation of multifunctional mols. from the point of view of fine chem. synthesis. In a first part, the design and prepn. of catalysts and ligands with interesting properties are summarized, particularly for the catalysis specialist. The following topics are described in some detail: how enantioselective homogeneous catalysts are designed and tested; new effective chiral monodentate phosphines; successful bidentate phosphine ligand families (with axially chiral biaryl- and ferrocenyl-based backbones, new phospholanes and with stereogenic phosphorus); novel bidentate ligand families with P-O and P-N bonds; and oxazoline-based ligands. A short overview on immobilized chiral complexes and of the toolbox of heterogeneous catalysis (bimetallic, colloidal and modified catalysts) concludes this chapter. In a second part, progress for selected catalytic transformations and generic selectivity problems is described, intended mainly for the org. chemist who has to solve specific synthetic problems. Emphasis is on the following topics: the enantioselective hydrogenation of olefins with various substitution patterns; the chemo- and enantioselective hydrogenation of ketones; the diastereo- and enantioselective hydrogenation of C:N functions; the stereoselective hydrogenation of arom. rings; chemoselectivity and hydroxylamine accumulation in the redn. of functionalized nitroarenes; chemoselectivity and new protecting groups for catalytic debenzylation; the mild hydrogenation of carboxylic acid derivs.; and the chemoselective hydrogenation of nitriles. In the last parts of the review, transfer hydrogenation and mechanistic issues are discussed.
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  5. 5
    Meemken, F.; Baiker, A. Recent Progress in Heterogeneous Asymmetric Hydrogenation of C═O and C═C Bonds on Supported Noble Metal Catalysts. Chem. Rev. 2017, 117, 11522– 11569,  DOI: 10.1021/acs.chemrev.7b00272
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    5
    Recent Progress in Heterogeneous Asymmetric Hydrogenation of C=O and C=C Bonds on Supported Noble Metal Catalysts
    Meemken, Fabian; Baiker, Alfons
    Chemical Reviews (Washington, DC, United States) (2017), 117 (17), 11522-11569CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. In this review, a systematic account of the research accomplished in the past decade on noble metal based heterogeneous asym. hydrogenation of prochiral C=O and C=C bonds, including all important facets of these catalytic systems is provided. The advances made are critically analyzed and future research challenges are identified.
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  6. 6
    Sankar, M.; Dimitratos, N.; Miedziak, P. J.; Wells, P. P.; Kiely, C. J.; Hutchings, G. J. Designing bimetallic catalysts for a green and sustainable future. Chem. Soc. Rev. 2012, 41, 8099– 8139,  DOI: 10.1039/c2cs35296f
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    Designing bimetallic catalysts for a green and sustainable future
    Sankar, Meenakshisundaram; Dimitratos, Nikolaos; Miedziak, Peter J.; Wells, Peter P.; Kiely, Christopher J.; Hutchings, Graham J.
    Chemical Society Reviews (2012), 41 (24), 8099-8139CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    This crit. review provides an overview of the recent developments in the prepn. and characterization of bimetallic nanoparticles. Initially the review follows a materials science perspective on prepg. bimetallic nanoparticles with designer morphologies, after which the emphasis shifts towards recent developments in using these bimetallic particles for catalyzing either oxidn. or redn. In the final part of this review an overview is presented of the utilization of bimetallic catalyst systems for the transformation of bio-renewable substrates and reactions related to the realization of a bio-refinery. Because of the sheer no. of examples of transformations in this area, a few key examples, namely selective oxidn., hydrogenation/hydrogenolysis and reforming of biomass derived mols., were chosen for this review. Reports of bimetallic catalysts being used for the aforementioned transformations are critically analyzed and the potential for exploiting such bimetallic catalysts were also highlighted. A specific objective of this review article is to motivate researchers to prep. some of the "designer" bimetallic catalysts with specific nanostructures, inspired from recent advances in the area of materials chem., and to utilize them for the transformation of biomass derived materials that are very complex and pose different challenges compared to those of simple org. mols. Supported bimetallic nanoparticles have an important role to play as catalysts in our quest for a more green and sustainable society.
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  7. 7
    Michaelides, I. N.; Dixon, D. J. Catalytic stereoselective semihydrogenation of alkynes to E-alkenes. Angew. Chem., Int. Ed. 2013, 52, 806– 808,  DOI: 10.1002/anie.201208120
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    Catalytic Stereoselective Semihydrogenation of Alkynes to E-Alkenes
    Michaelides, Iacovos N.; Dixon, Darren J.
    Angewandte Chemie, International Edition (2013), 52 (3), 806-808CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. The authors focus on the significant advancements made in the recent years toward the development of trans-selective, catalytic, functional group-tolerant semihydrogenation of alkynes with transition metal catalysts.
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  8. 8
    Seifert, W. K.; Condit, P. C. Selective Catalytic Hydrogenation of Nitroölefins. J. Org. Chem. 1963, 28, 265– 267,  DOI: 10.1021/jo01036a536
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    Selective catalytic hydrogenation of nitroolefins
    Seifert, Wolfgang K.; Condit, Paul C.
    Journal of Organic Chemistry (1963), 28 (), 265-7CODEN: JOCEAH; ISSN:0022-3263.
    cf. CA 58, 6709g. Hydrogenations of 1-nitrocyclooctene (I) and 1-nitro-1-octadecene (II) were carried out on a 2-50 millimole scale at 20°/10-60 lb./in.2 with 5% Pd-C, and the mole-% compns. of crude products were detd. by quant. infrared analysis in CCl4. I (35 millimoles) in 175 ml. MeOH contg. 35 millimoles dry HCl hydrogenated with 3.25 g. catalyst 16 min., 35 millimoles NaOAc added, the mixt. filtered, the catalyst washed with MeOH, the combined solns. concd. to 20 ml. in vacuo, dild. with 200 ml. H2O and extd. with Et2O and the product (96%) analyzed by infrared detn. showed the presence of 83% cyclooctanone oxime (III) and 17% cyclooctanone (IV). Equimolar amts. of IV, HONH2.HCl, and NaOAc.3H2O in H2O-MeOH kept 16 hrs. at 20° and the product isolated as above gave III, b0.08 63°, m. 41.7-2.7°, ε2.74μ 1.19 × 102 1. cm.-1 mole-1, ε2.74 μ/3.4 μ 0.52. In another run 4.05 g. product (contg. 70% III, 23% IV) treated at 50° in 40 ml. MeOH and 15 ml. H2O with 2.8 g. HONH2.HCl and 10.8 g. NaOAc.3H2O and the product dried 24 hrs. at 5 mm. yielded 91% material contg. 93% III and 0% IV. III was also prepd. by hydrogenating 5.5 wt.-% I in C5H5N with 1.3% Pd 16 hrs., the residue on filtration washed with Et2O, and the C5H5N removed by azeotropic distn. with C7H16. II (2 millimoles) in 10 ml. MeOH contg. 1 millimole dry HCl hydrogenated 20 min. with 183 mg. catalyst, the mixt. treated with 2 millimoles NaOAc.3H2O, and the product isolated (94%) and analyzed showed the presence of 73% stearaldoxime (V), recryst. from MeOH and C6H14 to give a pure sample, m. 88.0-9.8°, ε2.74 μ 1.30 × 102 l. cm.-1mole-1. If the CO absorption at 5.75 μ is assigned to stearaldehyde (VI), the amt. present was estd. at 13%. I (13.8 millimoles) in 100 ml. MeOH and 1 g. C5H5N was hydrogenated 10 min. with 0.94 g. catalyst and, after filtration, MeOH extn. of the catalyst, vacuum evapn. of solvents with added C7H16, the product (96%) analyzed for 83% nitrocyclooctane (VII) and 10% IV. Purification by preparative vapor phase chromatography (6 ft. column packed with 25% GE-30 silicone gum rubber on Chromosorb W, flow rate 200 ml. He/min. at 165° retention time 37 min.) gave pure VII, n20D 1.4819, ε6.45μ/ε3.40μ 3.29. II (2.26 millimoles) in 10 ml. MeOH and 0.17 g. C5H5N hydrogenated with 1.3% Pd 11 min. and the product (90%) isolated, chromatographed on silica gel, and eluted with 9:1 C6H14-C6H6 yielded 50% pure 1-nitrooctadecane, m. 39.5-41.0°, ε6.44μ/ε340 0.91. The origin of IV in he hydrogenation of I in MeOH contg. HCl was discussed. IV was not formed by hydrolysis of III by the H2O produced in the reaction. The further hydrogenation of III to the imine and subsequent hydrolysis was considered as a possible mechanism.
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  9. 9
    Chernichenko, K.; Madarász, Á.; Pápai, I.; Nieger, M.; Leskelä, M.; Repo, T. A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes. Nat. Chem. 2013, 5, 718– 723,  DOI: 10.1038/nchem.1693
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    9
    A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes
    Chernichenko, Konstantin; Madarasz, Adam; Papai, Imre; Nieger, Martin; Leskelae, Markku; Repo, Timo
    Nature Chemistry (2013), 5 (8), 718-723CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Frustrated Lewis pairs are compds. contg. both Lewis acidic and Lewis basic moieties, where the formation of an adduct is prevented by steric hindrance. They are therefore highly reactive, and are capable of heterolysis of mol. hydrogen, a property that led to their use in hydrogenation reactions of polarized multiple bonds. Here, the authors describe a general approach to the hydrogenation of alkynes to cis-alkenes under mild conditions using the unique ansa-aminohydroborane as a catalyst. The approach combines several reactions as the elementary steps of the catalytic cycle: hydroboration (substrate binding), heterolytic hydrogen splitting (typical frustrated-Lewis-pair reactivity) and facile intramol. protodeborylation (product release). The mechanism is verified by exptl. and computational studies.
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  10. 10
    Furukawa, S.; Komatsu, T. Selective Hydrogenation of Functionalized Alkynes to (E)-Alkenes, Using Ordered Alloys as Catalysts. ACS Catal. 2016, 6, 2121– 2125,  DOI: 10.1021/acscatal.5b02953
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    10
    Selective Hydrogenation of Functionalized Alkynes to (E)-Alkenes, Using Ordered Alloys as Catalysts
    Furukawa, Shinya; Komatsu, Takayuki
    ACS Catalysis (2016), 6 (3), 2121-2125CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Intermetallic Pd3Pb acts as a highly selective alkyne semihydrogenation catalyst that is greatly superior to the conventional Lindlar catalyst. D. functional theory (DFT) calcns. demonstrate an ideal adsorption property of Pd3Pb, where the surface holds alkynes while releasing alkenes. A tandem catalytic system that is comprised of Pd3Pb/SiO2 for alkyne semihydrogenation and RhSb/SiO2 for alkene isomerization allows one-pot (E)-alkene synthesis from a functionalized alkyne, which is the first success using heterogeneous catalysts. A variety of functionalized alkynes with aldehyde, ketone, carboxylic acid, and ester moieties are hydrogenated into the corresponding (E)-alkene in good to excellent yields under 1 atm H2 at room temp.
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  11. 11
    Chen, X.; Engle, K. M.; Wang, D.-H.; Yu, J.-Q. Palladium(II)-catalyzed C-H activation/C-C cross-coupling reactions: versatility and practicality. Angew. Chem., Int. Ed. 2009, 48, 5094– 5115,  DOI: 10.1002/anie.200806273
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    11
    Palladium(II)-Catalyzed C-H Activation/C-C Cross-Coupling Reactions: Versatility and Practicality
    Chen, Xiao; Engle, Keary M.; Wang, Dong-Hui; Yu, Jin-Quan
    Angewandte Chemie, International Edition (2009), 48 (28), 5094-5115CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Recent studies concerning the palladium(II)-catalyzed C-H activation/C-C cross-coupling reaction with organometallic reagents through a PdII/Pd0 catalytic cycle was reviewed. The versatility and practicality of this new mode of catalysis were also reviewed, including unaddressed questions and the potential for the development in the field.
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  12. 12
    Melo, I. E. M. d. S.; de Sousa, S. A. A.; Pereira, L. N. d. S.; Oliveira, J. M.; Castro, K. P. R.; Costa, J. C. S.; de Moura, E. M.; de Moura, C. V. R.; Garcia, M. A. S. Au–Pd Selectivity-switchable Alcohol-oxidation Catalyst: Controlling the Duality of the Mechanism using a Multivariate Approach. ChemCatChem 2019, 11, 3022– 3034,  DOI: 10.1002/cctc.201900512
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    Tessonnier, J.-P.; Pesant, L.; Ehret, G.; Ledoux, M. J.; Pham-Huu, C. Pd nanoparticles introduced inside multi-walled carbon nanotubes for selective hydrogenation of cinnamaldehyde into hydrocinnamaldehyde. Appl. Catal., A 2005, 288, 203– 210,  DOI: 10.1016/j.apcata.2005.04.034
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    13
    Pd nanoparticles introduced inside multi-walled carbon nanotubes for selective hydrogenation of cinnamaldehyde into hydrocinnamaldehyde
    Tessonnier, Jean-Philippe; Pesant, Laurie; Ehret, Gabrielle; Ledoux, Marc J.; Pham-Huu, Cuong
    Applied Catalysis, A: General (2005), 288 (1-2), 203-210CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    Palladium nanoparticles (4-6 nm) were deposited inside multi-walled carbon nanotubes (MWNTs) via impregnation of aq. Pd salt, PdNO3.6H2O. The low surface tension of the solvent allowed complete filling of the tube, leading, after thermal treatments, to formation of small and homogeneous Pd particles decorating the inner cavity of the support. The impregnation method was extremely efficient as no Pd particles were obsd. on the outer surface of the tubes. The catalyst was tested for the selective hydrogenation of cinnamaldehyde which contains both a C=C and a C=O bond. The nanotube based catalyst exhibited high catalytic activity an extremely high selectivity towards the C=C bond hydrogenation when compared to a com. catalyst supported on a high surface area activated carbon. A peculiar metal-support interaction and the absence of micropores and of oxygenated surface groups on the carbon nanotubes support are proposed to explain these results.
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    Schoenbaum, C. A.; Schwartz, D. K.; Medlin, J. W. Controlling the Surface Environment of Heterogeneous Catalysts Using Self-Assembled Monolayers. Acc. Chem. Res. 2014, 47, 1438– 1445,  DOI: 10.1021/ar500029y
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    Controlling the Surface Environment of Heterogeneous Catalysts Using Self-Assembled Monolayers
    Schoenbaum, Carolyn A.; Schwartz, Daniel K.; Medlin, J. Will
    Accounts of Chemical Research (2014), 47 (4), 1438-1445CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Modification approaches for tuning self-assembled monolayer structure to improve catalytic performance for hydrogenation reactions on palladium and platinum catalysts are summarized. Each approach serves to direct selectivity by tuning a particular aspect of the system including the availability of specific active sites (active-site selection), intermol. interactions between the reactants and modifiers (mol. recognition), and general steric or crowding effects. The tail moiety can be tuned to control the d. of SAM modifiers on the surface. IR spectra of adsorbed CO probe mols. reveal that increasing the d. of the thiols restricts the availability of contiguous active sites on catalyst terraces while maintaining accessibility to sites located at particle edges and steps. This technique was utilized to direct selectivity for the hydrogenation of furfural. Results obtained from SAM coatings with different surface densities indicated that, for this reaction, formation of the desirable products occurs primarily at particle edges and steps, whereas the undesired pathway occurs on particle terrace sites. As an alternative approach, the tail structure of the SAM precursor can be tuned to promote specific intermol. interactions between the modifier and reactant in order to position reactant mols. in a desired orientation. This technique was utilized for the hydrogenation of cinnamaldehyde, which contains an arom. Ph moiety. By using a phenyl-contg. SAM modifier with an appropriate tether length, > 90% selectivity toward reaction of the aldehyde group was achieved. In contrast, employing a modifier where the Ph moiety was closer to the catalyst surface biased selectivity toward the hydrogenation of the C=C bond due to reorienting the mol. to a more "lying down" conformation. In addn. to approaches that target specific interactions between the reactant and modified catalyst, we have demonstrated the use of SAMs to impose a steric or blocking effect, for example, during the hydrogenation of polyunsatd. fatty acids. The SAMs facilitated hydrogenation of polyunsatd. to monounsatd. fatty acids but inhibited further hydrogenation to the completely satd. species due to the sterically hindered, single "kink" shape of the monounsatd. product.
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  15. 15
    Vilé, G.; Almora-Barrios, N.; Mitchell, S.; Lopez, N.; Perez-Ramirez, J. From the Lindlar Catalyst to Supported Ligand-Modified Palladium Nanoparticles: Selectivity Patterns and Accessibility Constraints in the Continuous-Flow Three-Phase Hydrogenation of Acetylenic Compounds. Chem.─Eur. J. 2014, 20, 5926– 5937,  DOI: 10.1002/chem.201304795
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    15
    From the Lindlar Catalyst to Supported Ligand-Modified Palladium Nanoparticles: Selectivity Patterns and Accessibility Constraints in the Continuous-Flow Three-Phase Hydrogenation of Acetylenic Compounds
    Vile, Gianvito; Almora-Barrios, Neyvis; Mitchell, Sharon; Lopez, Nuria; Perez-Ramirez, Javier
    Chemistry - A European Journal (2014), 20 (20), 5926-5937CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Site modification and isolation through selective poisoning comprise an effective strategy to enhance the selectivity of palladium catalysts in the partial hydrogenation of triple bonds in acetylenic compds. The recent emergence of supported hybrid materials matching the stereo- and chemoselectivity of the classical Lindlar catalyst holds promise to revolutionize palladium-catalyzed hydrogenations, and will benefit from an in-depth understanding of these new materials. In this work, we compare the performance of bare, lead-poisoned, and ligand-modified palladium catalysts in the hydrogenation of diverse alkynes. Catalytic tests, conducted in a continuous-flow three-phase reactor, coupled with theor. calcns. and characterization methods, enable elucidation of the structural origins of the obsd. selectivity patterns. Distinctions in the catalytic performance are correlated with the relative accessibility of the active site to the org. substrate, and with the adsorption configuration and strength, depending on the ensemble size and surface potentials. This explains the role of the ligand in the colloidally prepd. catalysts in promoting superior performance in the hydrogenation of terminal and internal alkynes, and short-chain alkynols. In contrast, the greater accessibility of the active surface of the Pd-Pb alloy and the absence of polar groups are favorable in the conversion of alkynes contg. long aliph. chains and/or ketone groups. These findings provide detailed insights for the advanced design of supported nanostructured catalysts.
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  16. 16
    Niu, W.; Gao, Y.; Zhang, W.; Yan, N.; Lu, X. Pd-Pb Alloy Nanocrystals with Tailored Composition for Semihydrogenation: Taking Advantage of Catalyst Poisoning. Angew. Chem., Int. Ed. 2015, 54, 8271– 8274,  DOI: 10.1002/anie.201503148
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    16
    Pd-Pb Alloy Nanocrystals with Tailored Composition for Semihydrogenation: Taking Advantage of Catalyst Poisoning
    Niu, Wenxin; Gao, Yongjun; Zhang, Weiqing; Yan, Ning; Lu, Xianmao
    Angewandte Chemie, International Edition (2015), 54 (28), 8271-8274CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Metallic nanocrystals (NCs) with well-defined sizes and shapes represent a new family of model systems for establishing structure-function relationships in heterogeneous catalysis. Here in this study, we show that catalyst poisoning can be utilized as an efficient strategy for nanocrystals shape and compn. control, as well as a way to tune the catalytic activity of catalysts. Lead species, a well-known poison for noble-metal catalysts, was investigated in the growth of Pd NCs. We discovered that Pb atoms can be incorporated into the lattice of Pd NCs and form Pd-Pb alloy NCs with tunable compn. and crystal facets. As model catalysts, the alloy NCs with different compns. showed different selectivity in the semihydrogenation of phenylacetylene. Pd-Pb alloy NCs with better selectivity than that of the com. Lindlar catalyst were discovered. This study exemplified that the poisoning effect in catalysis can be explored as efficient shape-directing reagents in NC growth, and more importantly, as a strategy to tailor the performance of catalysts with high selectivity.
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  17. 17
    Liu, K.; Qin, R.; Zheng, N. Insights into the Interfacial Effects in Heterogeneous Metal Nanocatalysts toward Selective Hydrogenation. J. Am. Chem. Soc. 2021, 143, 4483– 4499,  DOI: 10.1021/jacs.0c13185
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    17
    Insights into the Interfacial Effects in Heterogeneous Metal Nanocatalysts toward Selective Hydrogenation
    Liu, Kunlong; Qin, Ruixuan; Zheng, Nanfeng
    Journal of the American Chemical Society (2021), 143 (12), 4483-4499CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A review. Heterogeneous metal catalysts are distinguished by their structure inhomogeneity and complexity. The chameleonic nature of heterogeneous metal catalysts have prevented us from deeply understanding their catalytic mechanisms at the mol. level and thus developing industrial catalysts with perfect catalytic selectivity toward desired products. This Perspective aims to summarize recent research advances in deciphering complicated interfacial effects in heterogeneous hydrogenation metal nanocatalysts toward the design of practical heterogeneous catalysts with clear catalytic mechanism and thus nearly perfect selectivity. The mol. insights on how the three key components (i.e., catalytic metal, support, and ligand modifier) of a heterogeneous metal nanocatalyst induce effective interfaces detg. the hydrogenation activity and selectivity are provided. The interfaces influence not only the H2 activation pathway but also the interaction of substrates to be hydrogenated with catalytic metal surface and thus the hydrogen transfer process. As for alloy nanocatalysts, together with the electronic and geometric ensemble effects, spillover hydrogenation occurring on catalytically "inert" metal by utilizing hydrogen atom spillover from active metal is highlighted. The metal-support interface effects are then discussed with emphasis on the mol. involvement of ligands located at the metal-support interface as well as cationic species from the support in hydrogenation. The mechanisms of how org. modifiers, with the ability to induce both 3D steric and electronic effects, on metal nanocatalysts manipulate the hydrogenation pathways are demonstrated. A brief summary is finally provided together with a perspective on the development of enzyme-like heterogeneous hydrogenation metal catalysts.
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    Grabovskii, S. A.; Akchurin, T. I.; Dokichev, V. A. Heterogeneous Palladium Catalysts in the Hydrogenation of the Carbon-carbon Double Bond. Curr. Org. Chem. 2021, 25, 315– 329,  DOI: 10.2174/1385272824999201202084812
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    18
    Heterogeneous Palladium Catalysts in the Hydrogenation of the Carbon-carbon Double Bond
    Grabovskii, Stanislav A.; Akchurin, Timur I.; Dokichev, Vladimir A.
    Current Organic Chemistry (2021), 25 (2), 315-329CODEN: CORCFE; ISSN:1385-2728. (Bentham Science Publishers Ltd.)
    A review. The results of studies over the past ten years in the field of C=C bond hydrogenation in the presence of palladium catalysts deposited on various inorg. and org. carriers such activated carbons, carbon nanotubes, alumina, zeolites, or composite materials based on Al2O3-SiO2, polystyrene, polypropyleneimine, polyamidoamine and hybrid inorg./ polymer-carriers, are presented. The selectivity and rates of the hydrogenation process are considered and some comparisons are made. Porous supports and contg. dendrimers generally retain palladium particles more effectively. Nanosized palladium stabilized by different dendrimers catalyzes the hydrogenation of C=C bonds in polyfunctional compds. chemoselectively without affecting functional groups, such as CHO, C=O, C(O)OR, CN, NO2, and halogens.
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    Monguchi, Y.; Ichikawa, T.; Sajiki, H. Recent Development of Palladium-Supported Catalysts for Chemoselective Hydrogenation. Chem. Pharm. Bull. 2017, 65, 2– 9,  DOI: 10.1248/cpb.c16-00153
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    Recent development of palladium-supported catalysts for chemoselective hydrogenation
    Monguchi, Yasunari; Ichikawa, Tomohiro; Sajiki, Hironao
    Chemical & Pharmaceutical Bulletin (2017), 65 (1), 2-9CODEN: CPBTAL; ISSN:0009-2363. (Pharmaceutical Society of Japan)
    A review. This paper describes practical and selective hydrogenation methodologies using heterogeneous palladium catalysts. Chemoselectivity develops dependent on the catalyst activity based on the characteristic of the supports, derived from structural components, functional groups, and/or morphologies. We esp. focus on our recent development of heterogeneous palladium catalysts supported on chelate resin, ceramic, and spherically shaped activated carbon. In addn., the application of flow technol. for chemoselective hydrogenation using the palladium catalysts immobilized on mol. sieves 3A and boron nitride is outlined.
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  20. 20
    McCue, A. J.; Anderson, J. A. Recent advances in selective acetylene hydrogenation using palladium containing catalysts. Front. Chem. Sci. Eng. 2015, 9, 142– 153,  DOI: 10.1007/s11705-015-1516-4
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    Recent advances in selective acetylene hydrogenation using palladium containing catalysts
    McCue, Alan J.; Anderson, James A.
    Frontiers of Chemical Science and Engineering (2015), 9 (2), 142-153CODEN: FCSEA3; ISSN:2095-0187. (Springer)
    Recent advances with Pd contg. catalysts for the selective hydrogenation of acetylene are described. The overview classifies enhancement of catalytic properties for monometallic and bimetallic Pd catalysts. Activity/selectivity of Pd catalysts can be modified by controlling particle shape/morphol. or immobilization on a support which interacts strongly with Pd particles. In both cases enhanced ethylene selectivity is generally assocd. with modifying ethylene adsorption strength and/or changes to hydride formation. Inorg. and org. selectivity modifiers (i.e., species adsorbed onto Pd particle surface) have also been shown to enhance ethylene selectivity. Inorg. modifiers such as TiO2 change Pd ensemble size and modify ethylene adsorption strength whereas org. modifiers such as diphenylsulfide are thought to create a surface template effect which favors acetylene adsorption with respect to ethylene. A no. of metals and synthetic approaches have been explored to prep. Pd bimetallic catalysts. Examples where enhanced selectivity is obsd. are generally assocd. with decreased Pd ensemble size and/or hindering of the ease with which an unselective hydride phase is formed for Pd. A final class of bimetallic catalysts are discussed where Pd is not thought to be the primary reaction site but merely acts as a site where hydrogen dissocn. and spillover occurs onto a second metal (Cu or Au) where the reaction takes place more selectively. [Figure not available: see fulltext.].
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    Wang, S.; Zhao, Z. J.; Chang, X.; Zhao, J.; Tian, H.; Yang, C.; Li, M.; Fu, Q.; Mu, R.; Gong, J. Activation and Spillover of Hydrogen on Sub-1 nm Palladium Nanoclusters Confined within Sodalite Zeolite for the Semi-Hydrogenation of Alkynes. Angew. Chem., Int. Ed. 2019, 58, 7668– 7672,  DOI: 10.1002/anie.201903827
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    Activation and Spillover of Hydrogen on Sub-1 nm Palladium Nanoclusters Confined within Sodalite Zeolite for the Semi-Hydrogenation of Alkynes
    Wang, Shuai; Zhao, Zhi-Jian; Chang, Xin; Zhao, Jiubing; Tian, Hao; Yang, Chengsheng; Li, Mingrun; Fu, Qiang; Mu, Rentao; Gong, Jinlong
    Angewandte Chemie, International Edition (2019), 58 (23), 7668-7672CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The search for efficient nontoxic catalysts able to perform industrial hydrogenations is a topic of interest, with relevance to many catalytic processes. Herein, we describe a mechanistic phenomenon for the activation and spillover of hydrogen for remarkable selectivity in the semi-hydrogenation of acetylene over sub-1 nm Pd nanoclusters confined within sodalite (SOD) zeolite (Pd@SOD). Specifically, hydrogen is dissocd. on the Pd nanoclusters to form hydrogen species (i.e., hydrogen atoms and hydroxyl groups) that spill over the SOD surfaces. The design and utilization of the small-pore zeolite SOD (six-membered rings with 0.28×0.28 nm channels) is crucial as it only allows H2 diffusion into the channels to reach the encapsulated Pd nanoclusters and thus avoids over-hydrogenation to form ethane. Pd@SOD exhibits an ethylene selectivity of over 94.5 %, while that of conventional Pd/SOD is approx. 21.5 %.
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    Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds. Chem. Rev. 1995, 95, 2457– 2483,  DOI: 10.1021/cr00039a007
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    Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds
    Miyaura, Norio; Suzuki, Akira
    Chemical Reviews (Washington, D. C.) (1995), 95 (7), 2457-83CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review with >250 refs. including title reactions and their mechanisms, prepn. of organoboron reagents, alkoxycarbonylation and dimerization.
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    Teschner, D.; Borsodi, J.; Wootsch, A.; Revay, Z.; Havecker, M.; Knop-Gericke, A.; Jackson, S. D.; Schlogl, R. The roles of subsurface carbon and hydrogen in palladium-catalyzed alkyne hydrogenation. Science 2008, 320, 86– 89,  DOI: 10.1126/science.1155200
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    The Roles of Subsurface Carbon and Hydrogen in Palladium-Catalyzed Alkyne Hydrogenation
    Teschner, Detre; Borsodi, Janos; Wootsch, Attila; Revay, Zsolt; Haevecker, Michael; Knop-Gericke, Axel; Jackson, S. David; Schloegl, Robert
    Science (Washington, DC, United States) (2008), 320 (5872), 86-89CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Alkynes can be selectively hydrogenated into alkenes on solid palladium catalysts. This process requires a strong modification of the near-surface region of palladium, in which carbon (from fragmented feed mols.) occupies interstitial lattice sites. In situ X-ray photoelectron spectroscopic measurements under reaction conditions indicated that much less carbon was dissolved in palladium during unselective, total hydrogenation. Addnl. studies of hydrogen content using in situ prompt gamma activation anal., which allowed us to follow the hydrogen content of palladium during catalysis, indicated that unselective hydrogenation proceeds on hydrogen-satd. β-hydride, whereas selective hydrogenation was only possible after decoupling bulk properties from the surface events. Thus, the population of subsurface sites of palladium, by either hydrogen or carbon, governs the hydrogenation events on the surface.
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    Zhang, L.; Zhou, M.; Wang, A.; Zhang, T. Selective Hydrogenation over Supported Metal Catalysts: From Nanoparticles to Single Atoms. Chem. Rev. 2020, 120, 683– 733,  DOI: 10.1021/acs.chemrev.9b00230
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    Selective hydrogenation over supported metal catalysts: from nanoparticles to single atoms
    Zhang, Leilei; Zhou, Maoxiang; Wang, Aiqin; Zhang, Tao
    Chemical Reviews (Washington, DC, United States) (2020), 120 (2), 683-733CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. In this review article, the approaches to tackle challenges in selective catalytic hydrogenation, including adsorption/grafting of N/S-contg. org. mols. on the metal surface, partial covering of active metal surface by metal oxides either via doping or through strong metal-support interaction, confinement of active metal nanoparticles in micro- or meso-pores of the supports, formation of bimetallic alloys or intermetallics or core@shell structures with a relatively inert metal (IB and IIB) or nonmetal element (B, C, S, etc.), and construction of single-atom catalysts on reducible oxides or inert metals were summarized. Both advantages and disadvantages of each approach toward the site isolation were discussed for three types of chemoselective hydrogenation reactions, including alkynes/dienes to monoenes, α, β-unsatd. aldehydes/ketones to the unsatd. alcs., and substituted nitroarenes to the corresponding anilines. The key factors affecting the catalytic activity/selectivity, in particular, the geometric and electronic structure of the active sites, were discussed with the aim to ext. fundamental principles for the development of efficient and selective catalysts in hydrogenation as well as other transformations.
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    Alshakova, I. D.; Gabidullin, B.; Nikonov, G. I. Ru-Catalyzed Transfer Hydrogenation of Nitriles, Aromatics, Olefins, Alkynes and Esters. ChemCatChem 2018, 10, 4860– 4869,  DOI: 10.1002/cctc.201801039
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    Jagtap, S. A.; Bhanage, B. M. Ligand Assisted Rhodium Catalyzed Selective Semi-hydrogenation of Alkynes Using Syngas and Molecular Hydrogen. ChemistrySelect 2018, 3, 713– 718,  DOI: 10.1002/slct.201702976
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    Ligand Assisted Rhodium Catalyzed Selective Semi-hydrogenation of Alkynes Using Syngas and Molecular Hydrogen
    Jagtap, Samadhan A.; Bhanage, Bhalchandra M.
    ChemistrySelect (2018), 3 (2), 713-718CODEN: CHEMUD; ISSN:2365-6549. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The rhodium catalyzed selective semi-hydrogenation of alkynes to (E) and (Z)-alkenes in the presence of various Buchwald phosphine ligands such as S-Phos, t-Bu XPhos, Ru-Phos, Johnphos and DavePhos was reported. Selective formation of E-alkenes in high activity with use of H2 gas (5 bar) was an alternative method for conventional Birch redn. High selectivity obtained towards Z-alkenes with use of syngas (10 bar) was an alternative method for Lindlar reaction. (E)-alkenes were obtained in high yield due to the steric crowding present in Johnphos ligand. The carbon monoxide present in syngas controlled over-hydrogenation of alkynes and (Z)-alkenes were obtained as major product due to cis addn. of mol. hydrogen to alkynes. The use of additives, base, co-catalyst and excess use of chems. was prevented with this protocol for semi-hydrogenation reaction. With this simple and effective catalyst system and mild reaction conditions, internal as well as terminal alkynes were smoothly converted to resp. alkenes in high yield (>90%).
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    Liu, K.; Qin, R.; Zhou, L.; Liu, P.; Zhang, Q.; Jing, W.; Ruan, P.; Gu, L.; Fu, G.; Zheng, N. Cu2O-Supported Atomically Dispersed Pd Catalysts for Semihydrogenation of Terminal Alkynes: Critical Role of Oxide Supports. CCS Chem. 2019, 1, 207– 214,  DOI: 10.31635/ccschem.019.20190008
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    Cu2O-supported atomically dispersed Pd catalysts for semihydrogenation of terminal alkynes: critical role of oxide supports
    Liu, Kunlong; Qin, Ruixuan; Zhou, Lingyun; Liu, Pengxin; Zhang, Qinghua; Jing, Wentong; Ruan, Pengpeng; Gu, Lin; Fu, Gang; Zheng, Nanfeng
    CCS Chemistry (2019), 1 (2), 207-214CODEN: CCCHB2 ISSN:. (Chinese Chemical Society)
    Atomically dispersed catalysts have demonstrated superior catalytic performance in many chem. transformations. However, limited success has been achieved in applying oxide-supported atomically dispersed catalysts to semihydrogenation of alkynes under mild conditions. By utilizing various metal oxides (e.g., Cu2O, Al2O3, ZnO, and TiO2) as support for atomically dispersed Pd catalysts, we demonstrate herein the crit. role of the oxidn. state and coordination environment of Pd centers in their catalytic performance, thus leading to the discovery of an "oxide-support effect" on atomically dispersed metal catalysts. Pd atomically dispersed on Cu2O exhibits far better catalytic activity in the hydrogenation of alkynes, with an extremely high selectivity toward alkenes, compared with catalysts on other oxides. Pd species galvanically displace surface Cu(I) sites on Cu2O to create two-coordinated Pd(I), which is a crit. step for the activation and heterolytic splitting of H2 into Pd-Hδ- and O-Hδ+ species for the selective hydrogenation of alkynes. Moreover, the adsorption of alkenes on H2-preadsorbed Pd(I) is relatively weak, preventing deeper hydrogenation and increased selectivity during semihydrogenation. We demonstrate that the local coordination environment of active metal centers plays a crucial role in detg. the catalytic performance of an oxide supported atomically dispersed catalyst.
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    Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Facile Splitting of Hydrogen and Ammonia by Nucleophilic Activation at a Single Carbon Center. Science 2007, 316, 439– 441,  DOI: 10.1126/science.1141474
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    Facile splitting of hydrogen and ammonia by nucleophilic activation at a single carbon center
    Frey, Guido D.; Lavallo, Vincent; Donnadieu, Bruno; Schoeller, Wolfgang W.; Bertrand, Guy
    Science (Washington, DC, United States) (2007), 316 (5823), 439-441CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    In possessing a lone pair of electrons and an accessible vacant orbital, singlet carbenes resemble transition metal centers and thus could potentially mimic their chem. behavior. Although singlet di(amino)carbenes are inert toward dihydrogen, it is shown that more nucleophilic and electrophilic (alkyl)(amino)carbenes can activate H2 under mild conditions, a reaction that has long been known for transition metals. However, in contrast to transition metals that act as electrophiles toward dihydrogen, these carbenes primarily behave as nucleophiles, creating a hydride-like hydrogen, which then attacks the pos. polarized carbon center. This nucleophilic behavior allows these carbenes to activate NH3 as well, a difficult task for transition metals because of the formation of Lewis acid-base adducts.
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    An, K.; Somorjai, G. A. Size and Shape Control of Metal Nanoparticles for Reaction Selectivity in Catalysis. ChemCatChem 2012, 4, 1512– 1524,  DOI: 10.1002/cctc.201200229
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    Size and Shape Control of Metal Nanoparticles for Reaction Selectivity in Catalysis
    An, Kwangjin; Somorjai, Gabor A.
    ChemCatChem (2012), 4 (10), 1512-1524CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review; a nanoparticle with well-defined surfaces, prepd. through colloidal chem., enables it to be studied as a model heterogeneous catalyst. The colloidal synthetic approach provides versatile tools to control the size and shape of nanoparticles. Traditional nucleation and growth mechanisms have been utilized to understand how nanoparticles can be uniformly synthesized and unprecedented shapes can be controlled. Now, the size of metal particles can be controlled to cluster regimes by using dendrimers. By using seeds and foreign atoms, specific synthetic environments such as seeded growth and crystal overgrowth can be induced to generate various shaped mono- or bi-metallic, core/shell, or branched nanostructures. For green chem., catalysis in 21st century is aiming for 100 % selectivity to produce only one desired product at high turnover rates. Recent studies on nanoparticle catalysts clearly demonstrate size and shape dependent selectivity in many catalytic reactions. By combining in situ surface characterization techniques, real-time monitoring of nanoparticles can be performed under reaction environments, thus identifying several mol. factors affecting catalytic activity and selectivity.
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    Roldan Cuenya, B.; Behafarid, F. Nanocatalysis: size- and shape-dependent chemisorption and catalytic reactivity. Surf. Sci. Rep. 2015, 70, 135– 187,  DOI: 10.1016/j.surfrep.2015.01.001
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    Nanocatalysis: size- and shape-dependent chemisorption and catalytic reactivity
    Roldan Cuenya, Beatriz; Behafarid, Farzad
    Surface Science Reports (2015), 70 (2), 135-187CODEN: SSREDI; ISSN:0167-5729. (Elsevier B.V.)
    A review. In recent years, the field of catalysis has experienced an astonishing transformation, driven in part by more demanding environmental stds. and crit. societal and industrial needs such as the search for alternative energy sources. Thanks to the advent of nanotechnol., major steps have been made towards the rational design of novel catalysts. Striking new catalytic properties, including greatly enhanced reactivities and selectivities, have been reported for nanoparticle (NP) catalysts as compared to their bulk counterparts. However, in order to harness the power of these nanocatalysts, a detailed understanding of the origin of their enhanced performance is needed. The present review focuses on the role of the NP size and shape on chemisorption and catalytic performance. Since homogeneity in NP size and shape is a prerequisite for the understanding of structure-reactivity correlations, we first review different synthesis methods that result in narrow NP size distributions and shape controlled NPs. Next, size-dependent phenomena which influence the chem. reactivity of NPs, including quantum size-effects and the presence of under-coordinated surface atoms are examd. The effect of the NP shape on catalytic performance is discussed and explained based on the existence of different at. structures on the NP surface with distinct chemisorption properties. The influence of addnl. factors, such as the oxidn. state of the NPs and NP-support interactions, is also considered in the frame of the size- and shape-dependency that these phenomena present. Ultimately, our review highlights the importance of achieving a systematic understanding of the factors that control the activity and selectivity of a catalyst in order to avoid trial and error methods in the rational design of the new generation of nanocatalysts with properties tunable at the at. level.
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    da Silva, F. P.; Fiorio, J. L.; Rossi, L. M. Tuning the Catalytic Activity and Selectivity of Pd Nanoparticles Using Ligand-Modified Supports and Surfaces. ACS Omega 2017, 2, 6014– 6022,  DOI: 10.1021/acsomega.7b00836
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    Tuning the Catalytic Activity and Selectivity of Pd Nanoparticles Using Ligand-Modified Supports and Surfaces
    da Silva, Fernanda Parra; Fiorio, Jhonatan Luiz; Rossi, Liane Marcia
    ACS Omega (2017), 2 (9), 6014-6022CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    The org. moiety plays an essential role in the design of homogeneous catalysts, where the ligands are used to tune the catalytic activity, selectivity, and stability of the transition metal centers. The impact of ligands on the catalytic performance of metal nanoparticle catalysts is still less understood. Here, we prepd. supported nanoparticle (NP) catalysts by the immobilization of preformed Pd NPs on the ligand-modified silica surfaces bearing amine, ethylenediamine, and diethylenetriamine groups. After excluding any size effect, we were able to study the influence of the ligands grafted on the support surface on the catalytic activity of the supported nanoparticles. Higher activity was obsd. for the Pd NPs supported on propylamine-functionalized support, whereas the presence of ethylenediamine and diethylenetriamine groups was detrimental to the activity. Upon the addn. of excess of these amine ligands as surface modifiers, the hydrogenation of alkene to alkane was fully suppressed and, therefore, we were able to tune Pd selectivity. The selective hydrogenation of alkynes into alkenes, although a considerable challenge on the traditional palladium catalysts, was achieved here for a range of alkynes by combining Pd NPs and amine ligands.
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    Zheng, N.; Zhang, T. Preface: single-atom catalysts as a new generation of heterogeneous catalysts. Natl. Sci. Rev. 2018, 5, 625,  DOI: 10.1093/nsr/nwy095
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    Li, Z.; Ji, S.; Liu, Y.; Cao, X.; Tian, S.; Chen, Y.; Niu, Z.; Li, Y. Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites. Chem. Rev. 2020, 120, 623– 682,  DOI: 10.1021/acs.chemrev.9b00311
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    Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites
    Li, Zhi; Ji, Shufang; Liu, Yiwei; Cao, Xing; Tian, Shubo; Chen, Yuanjun; Niu, Zhiqiang; Li, Yadong
    Chemical Reviews (Washington, DC, United States) (2020), 120 (2), 623-682CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single at. sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.
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    Abdollahi, T.; Farmanzadeh, D. Selective hydrogenation of acetylene in the presence of ethylene on palladium nanocluster surfaces: A DFT study. Appl. Surf. Sci. 2018, 433, 513– 529,  DOI: 10.1016/j.apsusc.2017.10.085
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    Selective hydrogenation of acetylene in the presence of ethylene on palladium nanocluster surfaces: A DFT study
    Abdollahi, Tahereh; Farmanzadeh, Davood
    Applied Surface Science (2018), 433 (), 513-529CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    In this work, by d. functional theory, the palladium nanoclusters were investigated in order to design new catalysts for the selective hydrogenation of acetylene present in olefin feeds. At first, the palladium nanoclusters were studied using PBE-G functional with DNP-ECP basis set. According to the performed calcns., among all the Pdn (n = 2-15) nanoclusters, two Pd12 and Pd2 nanoclusters can be used as catalysts in the reactions of hydrogenation of acetylene and ethylene. The adsorption energy of hydrogen on the Pd12 nanocluster is higher than that of acetylene and ethylene, and therefore, the Pd12 nanocluster is more appropriate for the hydrogenation of acetylene and ethylene. However, the calcd. activation energy barriers for the reactions of hydrogenation of acetylene and ethylene showed that the Pd2 nanocluster has more selectivity in comparison to the Pd12 nanocluster. According to our results, the activation energy of the hydrogenation of acetylene to vinyl on the Pd2 nanocluster is 23.96 kJ/mol lower than that on the Pd12 nanocluster. Also, the activation energy of the hydrogenation of ethylene to Et on the Pd2 nanocluster is higher than that on the Pd12 nanocluster Therefore, it seems that the Pd2 surface can be used as a catalyst for the selective hydrogenation of acetylene.
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    Li, M.; Shen, J. Microcalorimetric studies of O2 and C2H4 adsorption on Pd/SiO2 catalysts modi ed by Cu and Ag. Thermochim. Acta 2001, 379, 45– 50,  DOI: 10.1016/S0040-6031(01)00600-1
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    Microcalorimetric studies of O2 and C2H4 adsorption on Pd/SiO2 catalysts modified by Cu and Ag
    Li, Mingshi; Shen, Jianyi
    Thermochimica Acta (2001), 379 (1-2), 45-50CODEN: THACAS; ISSN:0040-6031. (Elsevier Science B.V.)
    The microcalorimetric adsorption of H2 and O2 has been employed to probe the dispersion of metals and surface compn. of the bimetallic Pd-Cu/SiO2 and Pd-Ag/SiO2 catalysts. In addn., the microcalorimetric adsorption of ethylene was carried out to study the effects of Cu and Ag on the adsorption of ethylene on reduced and oxidized Pd surfaces. It was found that the addn. of Cu or Ag dild. the surface Pd sites. The heats of O2 adsorption for Pd-Cu and Pd-Ag were approx. the avs. between those for Pd and the second metal Cu and Ag, resp., when the ratios of Cu/Pd and Ag/Pd are low (1:1). Moreover, the O/H ratio was found to be about 0.5 for the Pd-Cu/SiO2 (1:1) and Pd-Ag/SiO2 (1:1) samples, which was about the same for the Pd/SiO2 sample. These results suggest the enrichment of Pd on the surfaces in the bimetallic Pd-Cu/SiO2 (1:1) and Pd-Ag/SiO2 (1:1) samples. On the other hand, the initial heats of O2 adsorption for the Pd-Cu/SiO2 (1:4) and Pd-Ag/SiO2 (1:4) are approx. equal to those of the second metal Cu and Ag, resp., revealing the enrichment of the second metals in these samples. The addn. of Cu or Ag inhibited the formation of ethylidyne on the reduced Pd surface, and Ag was found to be more effective than Cu for this effect. The adsorption of ethylene on the oxidized Pd resulted in the serious oxidn. of ethylene. The addn. of Cu or Ag greatly reduced the oxidn. of ethylene on the oxidized Pd. In this case, Cu was more effective than Ag in reducing the activity of oxygen for the oxidn. of ethylene on the Pd surface.
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    Hamm, G.; Schmidt, T.; Breitbach, J.; Franke, D.; Becker, C.; Wandelt, K. The Adsorption of Ethene on Pd(111) and Ordered Sn/Pd(111) Surface Alloys. Surf. Sci. 2004, 562, 170– 182,  DOI: 10.1016/j.susc.2004.05.119
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    The adsorption of benzene on Pd(1 1 1) and ordered Sn/Pd(1 1 1) surface alloys
    Hamm, G.; Schmidt, T.; Breitbach, J.; Franke, D.; Becker, C.; Wandelt, K.
    Surface Science (2004), 562 (1-3), 170-182CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
    The adsorption of benzene on Pd(1 1 1) and ordered Sn/Pd(1 1 1) surface alloys was studied using UPS (UPS), high-resoln. electron energy loss spectroscopy (HREELS), temp. programmed desorption (TPD) and LEED. Two ordered surface alloys were prepd. by thermal treatment of vapor deposited Sn-films on Pd(1 1 1). Depending on the prepn. conditions, the surface exhibited a p(2×2) or a (√3×√3)R30° LEED pattern, corresponding to surface alloys of compn. Pd3Sn and Pd2Sn, resp. Benzene adsorbs molecularly on Pd(1 1 1) at temps. below 400 K and that the mol. is π-bonded to the surface with the ring plane parallel to the surface. At low initial coverage, heating leads to complete decompn. of the adsorbed benzene, while at higher coverage addnl. desorption of mol. benzene occurs in 2 desorption states at ∼400 and ∼500 K, resp. The evolution of the low temp. state is correlated with variations in the electronic states of the adsorbed benzene and attributed to a change in the adsorption configuration. For the first time a CO-free (√19×√19)R±23.4° superstructure was discovered on Pd(1 1 1) at benzene monolayer satn., allowing for a detn. of the adsorption sites. In comparison to the pure Pd(1 1 1) surface, alloying leads to successive weakening of the benzene-surface bond with increasing Sn-content in the topmost layer of the substrate. Decompn. is suppressed on both surface alloys. Finally there was no evidence for a tilted benzene configuration on either of the surfaces investigated.
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  37. 37
    Pei, G. X.; Liu, X. Y.; Wang, A.; Lee, A. F.; Isaacs, M. A.; Li, L.; Pan, X.; Yang, X.; Wang, X.; Tai, Z.; Wilson, K.; Zhang, T. Ag Alloyed Pd Single-Atom Catalysts for Efficient Selective Hydrogenation of Acetylene to Ethylene in Excess Ethylene. ACS Catal. 2015, 5, 3717– 3725,  DOI: 10.1021/acscatal.5b00700
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    37
    Ag Alloyed Pd Single-Atom Catalysts for Efficient Selective Hydrogenation of Acetylene to Ethylene in Excess Ethylene
    Pei, Guang Xian; Liu, Xiao Yan; Wang, Aiqin; Lee, Adam F.; Isaacs, Mark A.; Li, Lin; Pan, Xiaoli; Yang, Xiaofeng; Wang, Xiaodong; Tai, Zhijun; Wilson, Karen; Zhang, Tao
    ACS Catalysis (2015), 5 (6), 3717-3725CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Semihydrogenation of acetylene in an ethylene-rich stream is an industrially important process. Conventional supported monometallic Pd catalysts offer high acetylene conversion, but they suffer from very low selectivity to ethylene due to overhydrogenation and the formation of carbonaceous deposits. Herein, a series of Ag alloyed Pd single-atom catalysts, possessing only ppm levels of Pd, supported on silica gel were prepd. by a simple incipient wetness compregnation method and applied to the selective hydrogenation of acetylene in an ethylene-rich stream under conditions close to the front-end employed by industry. High acetylene conversion and simultaneous selectivity to ethylene was attained over a wide temp. window, surpassing an analogous Au alloyed Pd single-atom system we previously reported. Restructuring of AgPd nanoparticles and electron transfer from Ag to Pd were evidenced by in situ FTIR and in situ XPS as a function of increasing redn. temp. Microcalorimetry and XANES measurements support both geometric and electronic synergetic effects between the alloyed Pd and Ag. Kinetic studies provide valuable insight into the nature of the active sites within these AgPd/SiO2 catalysts, and hence, they provide evidence for the key factors underpinning the excellent performance of these bimetallic catalysts toward the selective hydrogenation of acetylene under ethylene-rich conditions while minimizing precious metal usage.
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  38. 38
    Vilé, G.; Albani, D.; Nachtegaal, M.; Chen, Z.; Dontsova, D.; Antonietti, M.; Lopez, N.; Perez-Ramirez, J. A stable single-site palladium catalyst for hydrogenations. Angew. Chem., Int. Ed. 2015, 54, 11265– 11269,  DOI: 10.1002/anie.201505073
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    A Stable Single-Site Palladium Catalyst for Hydrogenations
    Vile, Gianvito; Albani, Davide; Nachtegaal, Maarten; Chen, Zupeng; Dontsova, Dariya; Antonietti, Markus; Lopez, Nuria; Perez-Ramirez, Javier
    Angewandte Chemie, International Edition (2015), 54 (38), 11265-11269CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The prepn. and hydrogenation performance of a single-site palladium catalyst that was obtained by the anchoring of Pd atoms into the cavities of mesoporous polymeric graphitic carbon nitride is reported. The characterization of the material confirmed the at. dispersion of the palladium phase throughout the sample. The catalyst was applied for three-phase hydrogenations of alkynes and nitroarenes in a continuous-flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. D. functional theory calcns. provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites.
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  39. 39
    Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations. Science 2012, 335, 1209– 1212,  DOI: 10.1126/science.1215864
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    Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations
    Kyriakou, Georgios; Boucher, Matthew B.; Jewell, April D.; Lewis, Emily A.; Lawton, Timothy J.; Baber, Ashleigh E.; Tierney, Heather L.; Flytzani-Stephanopoulos, Maria; Sykes, E. Charles H.
    Science (Washington, DC, United States) (2012), 335 (6073), 1209-1212CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Facile dissocn. of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resoln. scanning tunneling microscopy, the authors show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissocn. at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.
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    Zhang, J.; Xu, W.; Xu, L.; Shao, Q.; Huang, X. Concavity Tuning of Intermetallic Pd–Pb Nanocubes for Selective Semihydrogenation Catalysis. Chem. Mater. 2018, 30, 6338– 6345,  DOI: 10.1021/acs.chemmater.8b02337
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    40
    Concavity Tuning of Intermetallic Pd-Pb Nanocubes for Selective Semihydrogenation Catalysis
    Zhang, Junbo; Xu, Weiwei; Xu, Lai; Shao, Qi; Huang, Xiaoqing
    Chemistry of Materials (2018), 30 (18), 6338-6345CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Although considerable studies on pursuing high-performance Pd-based catalysts for the semihydrogenation of alkynes have been carried out, the creation of catalyst with high activity, selectivity and stability simultaneously toward semihydrogenation reactions remains a significant challenge. Herein, for the first time a facile synthetic strategy is reported to realize the intermetallic Pd-Pb nanocubes with different concave degree by selectively utilizing small mols. These obtained Pd-Pb nanocrystals exhibit high activity in the semihydrogenation of alkynes, where their performances are highly shape- and compn.-dependent with Pd-Pb concave nanocubes showing the optimized alkene selectivity of 94.6% and activity of 179.2 h-1, much higher than those of 10% Pd/C. Detailed XPS results show that the higher ratio of metallic Pd results in the higher activity for semihydrogenation of phenylacetylene and the higher ratio of Pb2+ and Pb/Pd contribute to higher styrene selectivity. The d. functional theory calcns. further reveal that the favorable adsorption energy of phenylacetylene and desirable desorption energy of styrene on the Pd3Pb surface are crit. for the phenylacetylene semihydrogenation with excellent activity and high selectivity. Furthermore, the Pd-Pb concave nanocube can endure at least five cycles with very limited conversion and selectivity decays, representing an efficient Pd-based catalyst for selective hydrogenation and beyond.
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    Saranya, A.; Vivekanandan, G.; Thirunavukkarasu, K.; Krishnamurthy, K. R.; Viswanathan, B. Studies on palladium based bimetallic catalysts Pd-M/TiO2 (M = Cu, Ag & Au): I-Selective hydrogenation of 1-heptyne. Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem. 2019, 58, 271– 280
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    41
    Studies on palladium based bimetallic catalysts Pd-M/TiO2 (M = Cu, Ag & Au): i-selective hydrogenation of 1-heptyne
    Saranya, A.; Vivekanandan, G.; Thirunavukkarasu, K.; Krishnamurthy, K. R.; Viswanathan, B.
    Indian Journal of Chemistry, Section A: Inorganic, Bio-inorganic, Physical, Theoretical & Analytical Chemistry (2019), 58A (2), 271-280CODEN: ICACEC; ISSN:0376-4710. (National Institute of Science Communication and Information Resources)
    Two series of palladium based bi-metallic catalysts, Pd((1-x))Aux (x = 0.1, 0.12, 0.15 and 0.2 ) and PdmMn (M = Cu/Ag/Au; m, n = 0.9, 0.1) supported on TiO2-P-25, have been prepd. and characterized by X-ray diffraction (XRD), Diffuse Reflectance Spectroscopy (DRS), Transmission Electron Microscopy (TEM), X-ray Photo-electron Spectroscopy (XPS) and Temp. Programmed Redn. (TPR). DRS and XPS studies indicate formation of nano scale alloys involving redistribution of charges within the metals. Selective hydrogenation of 1-heptyne in liq. phase has been studied on these catalysts at atm. pressure and in the temp. range 293-313 K. In the Pd((1-x))Aux series, the catalyst compn. Pd0.9Au0.1 > displays max. activity, expressed as TOF. Activity pattern in Pd0.9Au0.1 series follows the trend, Pd-Au > Pd-Ag = Pd-Cu > Pd. Selectivity for heptene formation is maintained at > 95% on all catalysts up to 60 min reaction time. Interplay of ensemble as well as ligand effects, acting simultaneously, influences the adsorption and activation of 1-heptyne, leading to higher activity on Pd-Au bimetallic catalyst vis-a-vis other bimetallic and mono metallic catalysts.
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    Zhang, R. G.; Xue, M. F.; Wang, B. J.; Ling, L. X.; Fan, M. H. C2H2 Selective Hydrogenation over the M@Pd and M@Cu (M = Au, Ag, Cu, and Pd) Core-Shell Nanocluster Catalysts: The Effects of Composition and Nanocluster Size on Catalytic Activity and Selectivity. J. Phys. Chem. C 2019, 123, 16107– 16117,  DOI: 10.1021/acs.jpcc.9b01757
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    C2H2 Selective Hydrogenation over the M@Pd and M@Cu (M = Au, Ag, Cu, and Pd) Core-Shell Nanocluster Catalysts: The Effects of Composition and Nanocluster Size on Catalytic Activity and Selectivity
    Zhang, Riguang; Xue, Mifeng; Wang, Baojun; Ling, Lixia; Fan, Maohong
    Journal of Physical Chemistry C (2019), 123 (26), 16107-16117CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    To clarify the effects of the compn. and nanocluster size of the core-shell catalysts on C2H4 selectivity and activity in C2H2 selective hydrogenation, the kinetic mechanisms of C2H2 selective hydrogenation over different compns. of M@Pd (M = Au, Ag, and Cu) and M@Cu (M = Au, Ag, and Pd) nanoclusters with different sizes are studied using d. functional theory calcns. Probably the compn. and nanocluster size of the core-shell catalyst affect C2H4 selectivity and activity, and Cu as the core for M@Pd catalysts exhibits excellent C2H4 selectivity and activity than that of Au and Ag; also, M@Pd catalysts show better C2H4 selectivity and activity than M@Cu. Namely, the core-shell nanocluster catalyst with Cu as the core and Pd as the shell is beneficial to improve C2H4 selectivity and activity in C2H2 selective hydrogenation. However, C2H4 selectivity and activity increase over M@Pd catalysts with the increase in the nanocluster size, which means that it is necessary to have the catalyst with a larger cluster size in the prepn. of Cu@Pd core-shell catalysts. The electronic structure anal. revealed the microscopic reasons about the effects of core-shell catalyst compns. and nanocluster size on the catalytic performance of C2H2 selective hydrogenation. This study can provide theor. guidance for the design of core-shell nanocluster catalysts to improve C2H4 selectivity and activity in C2H2 selective hydrogenation by adjusting the compn. and nanocluster size in an efficient way.
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    Yang, J. Y.; Fan, Y. P.; Li, Z. L.; Peng, Z. K.; Yang, J. H.; Liu, B. Z.; Liu, Z. Y. Bimetallic Pd-M (M= Pt, Ni, Cu, Co) nanoparticles catalysts with strong electrostatic metal-support interaction for hydrogenation of toluene and benzene. Mol. Catal. 2020, 492, 110992,  DOI: 10.1016/j.mcat.2020.110992
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    Bimetallic Pd-M (M = Pt, Ni, Cu, Co) nanoparticles catalysts with strong electrostatic metal-support interaction for hydrogenation of toluene and benzene
    Yang, Jingyi; Fan, Yanping; Li, Zhong-Li; Peng, Zhikun; Yang, Jing-He; Liu, Baozhong; Liu, Zhongyi
    Molecular Catalysis (2020), 492 (), 110992CODEN: MCOADH ISSN:. (Elsevier B.V.)
    Herein, we reported an effective method to synthesis a series of supported bimetallic Pd-M/SiO2 (M = Pt, Ni, Cu, Co) catalysts with highly exposed and well-alloyed nanoparticles via simultaneous strong electrostatic adsorption (co-SEA), which controlled metal precursors strongly anchored onto the oppositely charged support (SiO2) by changing the pH relative to the surface point of zero charge (PZC). The toluene and benzene hydrogenation were used as probe reactions to evaluate the catalytic hydrogenation activity performance of the Pd-M/SiO2 catalysts. Compared to the catalysts synthesized by the common technique simultaneous dry impregnation (co-DI), the co-SEA Pd-Ni/SiO2 catalyst exhibited the highest turnover frequencies (TOFs) of 3.7 s-1 for benzene hydrogenation and 3.2 s-1 for toluene hydrogenation. This was attributed to the large amt. of electron-deficient Pdδ+ sites on the surface of SEA Pd-M/SiO2 that easily for electron-rich benzene and toluene to adsorb. Besides, the co-SEA catalysts produced well stability and reusability without significant decrease in the catalytic activity after six runs.
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    Cao, X. X.; Mirjalili, A.; Xie, W. T.; Jang, W. L. Selective hydrogenation of acetylene in ethylene over Cu-Pd catalysts. Abstr. Pap. Am. Chem. Soc. 2016, 251
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    Lomelí-Rosales, D. A.; Delgado, J. A.; Diaz de Los Bernardos, M.; Perez-Rodriguez, S.; Gual, A.; Claver, C.; Godard, C. A General One-Pot Methodology for the Preparation of Mono- and Bimetallic Nanoparticles Supported on Carbon Nanotubes: Application in the Semi-hydrogenation of Alkynes and Acetylene. Chem.─Eur. J. 2019, 25, 8321– 8331,  DOI: 10.1002/chem.201901041
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    A General One-Pot Methodology for the Preparation of Mono- and Bimetallic Nanoparticles Supported on Carbon Nanotubes: Application in the Semi-hydrogenation of Alkynes and Acetylene
    Lomeli-Rosales, Diego A.; Delgado, Jorge A.; Diaz de los Bernardos, Miriam; Perez-Rodriguez, Sara; Gual, Aitor; Claver, Carmen; Godard, Cyril
    Chemistry - A European Journal (2019), 25 (35), 8321-8331CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A facile and straightforward methodol. for the prepn. of monometallic (copper and palladium) and bimetallic nanocatalysts (NiCu and PdCu) stabilized by a N-heterocyclic carbene ligand is reported. Both colloidal and supported nanoparticles (NPs) on carbon nanotubes (CNTs) were prepd. in a one-pot synthesis with outstanding control on their size, morphol. and compn. These catalysts were evaluated in the selective hydrogenation of alkynes and alkynols. PdCu/CNTs revealed an efficient catalytic system providing high selectivity in the hydrogenation of terminal and internal alkynes. Moreover, this catalyst was tested in the semi-hydrogenation of acetylene in industrially relevant acetylene/ethylene-rich model gas feeds and showed excellent stability even after 40 h of reaction.
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    Pei, G. X.; Liu, X. Y.; Yang, X.; Zhang, L.; Wang, A.; Li, L.; Wang, H.; Wang, X.; Zhang, T. Performance of Cu-Alloyed Pd Single-Atom Catalyst for Semihydrogenation of Acetylene under Simulated Front-End Conditions. ACS Catal. 2017, 7, 1491– 1500,  DOI: 10.1021/acscatal.6b03293
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    46
    Performance of Cu-Alloyed Pd Single-Atom Catalyst for Semihydrogenation of Acetylene under Simulated Front-End Conditions
    Pei, Guang Xian; Liu, Xiao Yan; Yang, Xiaofeng; Zhang, Leilei; Wang, Aiqin; Li, Lin; Wang, Hua; Wang, Xiaodong; Zhang, Tao
    ACS Catalysis (2017), 7 (2), 1491-1500CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Selective hydrogenation of acetylene to ethylene is an industrially important reaction. Pd-based catalysts have been proved to be efficient for the acetylene conversion, while enhancing the selectivity to ethylene is challenging. Here, we chose Cu as the partner of Pd, fabricated an alloyed Pd single-atom catalyst (SAC), and investigated its catalytic performance for the selective hydrogenation of acetylene to ethylene under a simulated front-end hydrogenation process in industry: i.e., with a high concn. of hydrogen and ethylene. The Cu-alloyed Pd SAC showed ∼85% selectivity to ethylene and 100% acetylene elimination. In comparison with the Au- or Ag-alloyed Pd SAC, the Cu-alloyed analog exceeded both of them in conversion, while the selectivity rivaled that of the Ag-alloyed Pd SAC and surpassed that of the Au-alloyed Pd SAC. As Cu is a low-cost metal, Cu-alloyed Pd SAC would minimize the noble-metal usage and possess high utilization potential for industry. The Cu-alloyed Pd SAC was verified by EXAFS, with the Pd/Cu at. ratio lowered to 0.006, corresponding to the loading of Pd at 494 ppm. The microcalorimetric measurement results demonstrated that the adsorption of C2H4 over the Cu-alloyed Pd SAC was weaker than that over the catalyst with large Pd ensembles; thus, the selectivity to ethylene was greatly enhanced. At the same time, the adsorption of H2 was stronger than that over the corresponding monometallic Cu catalyst; thus, the activation of H2 was obviously promoted. On the basis of the above results, a possible reaction path over the Cu-alloyed Pd SAC was proposed. Furthermore, by systematic comparison of the IB-metal-alloyed Pd SACs, we found that the apparent activation energies of the IB-metal-alloyed Pd SACs were close to each other, indicating similar active sites and/or catalytic mechanisms over the three catalysts. The isolation of the Pd atoms by the IB metal distinctly contributed to both the conversion and the selectivity. Further DFT calcn. results suggested that electron transfer between the IB metal and Pd might be responsible for their different selectivities to ethylene.
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    Dong, Z.; Dong, C.; Liu, Y.; Le, X.; Jin, Z.; Ma, J. Hydrodechlorination and further hydrogenation of 4-chlorophenol to cyclohexanone in water over Pd nanoparticles modified N-doped mesoporous carbon microspheres. Chem. Eng. J. 2015, 270, 215– 222,  DOI: 10.1016/j.cej.2015.02.045
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    47
    Hydrodechlorination and further hydrogenation of 4-chlorophenol to cyclohexanone in water over Pd nanoparticles modified N-doped mesoporous carbon microspheres
    Dong, Zhengping; Dong, Chunxu; Liu, Yansheng; Le, Xuanduong; Jin, Zhicheng; Ma, Jiantai
    Chemical Engineering Journal (Amsterdam, Netherlands) (2015), 270 (), 215-222CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    The development of new catalysts with high catalytic activity for the hydrodechlorination (HDC) of chlorophenols (CPs) in the industrial polluted water has recently triggered various research interests. In this study, a novel N-doped mesoporous carbon microspheres (N-MCM) was synthesized through a nanocasting route using the low-cost and widely available dicyandiamide as precursor and MCM-48 nanospheres as the hard template. The synthesized N-MCM material exhibited a large surface area of 120.8 m2 g-1, pore vol. of 0.24 cm3 g-1, av. pore size of 9.9 nm and rich N content of 27.2 wt%. The characterization results of FT-IR, Raman spectrum and XPS illustrated that N-MCM had graphitic-like structures consisted of carbon nitride heterocycles, as well as amino groups. When N-MCM was used as catalyst support, Pd nanoparticles could be well dispersed in the mesopores and on the surface of the N-MCM. And the Pd/N-MCM nanocatalyst showed excellent catalytic activity for the eco-friendly HDC of CPs under green conditions in 1 h. While interestingly, when the reaction time was continued, the HDC product phenol can be further hydrogenated to a much more useful product (cyclohexanone) with selectivity above 85%.
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    Ding, S.; Zhang, C.; Liu, Y.; Jiang, H.; Chen, R. Selective hydrogenation of phenol to cyclohexanone in water over Pd@N-doped carbons derived from ZIF-67: Role of dicyandiamide. Appl. Surf. Sci. 2017, 425, 484– 491,  DOI: 10.1016/j.apsusc.2017.07.068
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    48
    Selective hydrogenation of phenol to cyclohexanone in water over Pd@N-doped carbons derived from ZIF-67: Role of dicyandiamide
    Ding, Shuaishuai; Zhang, Chunhua; Liu, Yefei; Jiang, Hong; Chen, Rizhi
    Applied Surface Science (2017), 425 (), 484-491CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Highly efficient Pd@CN catalysts for selective hydrogenation of phenol to cyclohexanone in water were successfully fabricated by loading Pd nanoparticles (NPs) in N-doped carbons (CN) derived from ZIF-67 with dicyandiamide (DICY) as the addnl. nitrogen source. For comparison, polyvinylpyrrolidone (PVP) was also used as the addnl. nitrogen source during the ZIF-67 synthesis. The results showed that the PVP and DICY had significantly different impacts on the microstructures of as-obtained CN materials and the catalytic performance of Pd@CN catalysts in the phenol hydrogenation. The addn. of DICY had the pos. promotion effect on the surface area of the obtained CN materials. Moreover, the introduction of DICY could increase the nitrogen content of CN and then prevent the re-oxidn. of Pd NPs during air contact, resulting in higher Pd0 ratio. In comparison with PVP, the DICY was more suitable as the addnl. nitrogen source for the formation of CN and Pd@CN (Pd@CND, Pd@CNP). The Pd@CND exhibited superior catalytic activity as compared to Pd@CNP (phenol conversion 96.9% vs. 67.4%). More importantly, the as-prepd. Pd@CND catalyst could be reused for four times without catalytic performance redn. The work would aid the development of Pd@CN catalysts with superior catalytic properties.
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    Zhang, C.; Zhang, J.; Shao, Y.; Jiang, H.; Chen, R.; Xing, W. Controllable Synthesis of 1D Pd@N-CNFs with High Catalytic Performance for Phenol Hydrogenation. Catal. Lett. 2021, 151, 1013,  DOI: 10.1007/s10562-020-03374-x
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    Controllable Synthesis of 1D Pd@N-CNFs with High Catalytic Performance for Phenol Hydrogenation
    Zhang, Chunhua; Zhang, Jiuxuan; Shao, Yanhua; Jiang, Hong; Chen, Rizhi; Xing, Weihong
    Catalysis Letters (2021), 151 (4), 1013-1024CODEN: CALEER; ISSN:1011-372X. (Springer)
    Achieving both high conversion and selectivity under mild conditions still remains a big challenge in the selective hydrogenation of phenol to cyclohexanone. Herein, one-dimensional (1D) N-doped carbon nanofibers (N-CNFs) were successfully fabricated by electrospinning with one-step carbonization, and used to load Pd nanoparticles for synthesizing Pd@N-CNFs catalysts. The dicyandiamide (DICY) and citric acid in the spinning soln. exhibited a significant synergistic effect in controlling the morphol. and surface property of N-CNFs and the corresponding catalytic activity of Pd@N-CNFs in the selective hydrogenation of phenol to cyclohexanone. The as-prepd. [email protected] catalyst possessed good fibrous morphol., larger surface area, and more amts. of surface N and OH group, and exhibited a phenol conversion of 99.7% with a cyclohexanone selectivity of 97.3% under mild reaction conditions. In addn., the catalytic activity of [email protected] increased by 2.75 times as compared to Pd@N-CNFs-0 and 1.22 times in comparison with Pd@N-CNFs-100. Furthermore, the 1D [email protected] was easy to be recovered from the reaction mixt., and showed good reusability. The proper molar ratio of dicyandiamide (DICY) and citric acid could significantly adjust the fibrous characteristic and surface properties of the Pd@CNFs catalyst, which contributed to the higher N and OH group contents, thereby improving the loading and distribution of Pd nanoparticles and the dispersibility of Pd@CNFs in water. These properties jointly led to the superior phenol hydrogenation efficiency of the Pd@CNFs catalyst.
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  50. 50
    Chen, L. J.; Wan, C. C.; Wang, Y. Y. Chemical preparation of Pd nanoparticles in room temperature ethylene glycol system and its application to electroless copper deposition. J. Colloid Interface Sci. 2006, 297, 143– 150,  DOI: 10.1016/j.jcis.2005.10.029
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    Chemical preparation of Pd nanoparticles in room temperature ethylene glycol system and its application to electroless copper deposition
    Chen, Li-Jung; Wan, Chi-Chao; Wang, Yung-Yun
    Journal of Colloid and Interface Science (2006), 297 (1), 143-150CODEN: JCISA5; ISSN:0021-9797. (Elsevier)
    Room-temp. synthesis of Pd nanoparticles protected by polyvinylpyrrolidone (PVP) was successfully achieved by merely adding NaOH acting as accelerator for the redn. of Pd(II) in ethylene glycol (EG) without any externally added reducing agent. The Pd particle sizes were controlled at 8.6-2.4 nm by changing the concn. of NaOH from 0 to 3.2 × 10 -1 M. The particle formation was monitored by UV-visible spectroscopy and the microstructure of Pd nanoparticles was analyzed by TEM and XRD. The product of adding NaOH in EG was characterized by FTIR and a -CHO group which possesses reductive ability was identified. The prepd. Pd nanoparticle could serve as an effective activator for electroless Cu deposition (ECD) on epoxy substrate, which is an essential process in the printed circuit board (PCB) industry. In contrast to existing com. activators, the new activator shows superior stability and excellent performance for ECD.
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    Wang, X.; Choi, S. I.; Roling, L. T.; Luo, M.; Ma, C.; Zhang, L.; Chi, M.; Liu, J.; Xie, Z.; Herron, J. A.; Mavrikakis, M.; Xia, Y. Palladium-platinum core-shell icosahedra with substantially enhanced activity and durability towards oxygen reduction. Nat. Commun. 2015, 6, 7594  DOI: 10.1038/ncomms8594
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    Palladium-platinum core-shell icosahedra with substantially enhanced activity and durability towards oxygen reduction
    Wang Xue; Zhang Lei; Choi Sang-Il; Luo Ming; Roling Luke T; Herron Jeffrey A; Mavrikakis Manos; Ma Cheng; Chi Miaofang; Liu Jingyue; Xie Zhaoxiong; Xia Younan
    Nature communications (2015), 6 (), 7594 ISSN:.
    Conformal deposition of platinum as ultrathin shells on facet-controlled palladium nanocrystals offers a great opportunity to enhance the catalytic performance while reducing its loading. Here we report such a system based on palladium icosahedra. Owing to lateral confinement imposed by twin boundaries and thus vertical relaxation only, the platinum overlayers evolve into a corrugated structure under compressive strain. For the core-shell nanocrystals with an average of 2.7 platinum overlayers, their specific and platinum mass activities towards oxygen reduction are enhanced by eight- and sevenfold, respectively, relative to a commercial catalyst. Density functional theory calculations indicate that the enhancement can be attributed to the weakened binding of hydroxyl to the compressed platinum surface supported on palladium. After 10,000 testing cycles, the mass activity of the core-shell nanocrystals is still four times higher than the commercial catalyst. These results demonstrate an effective approach to the development of electrocatalysts with greatly enhanced activity and durability.
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    Wang, X.; Vara, M.; Luo, M.; Huang, H.; Ruditskiy, A.; Park, J.; Bao, S.; Liu, J.; Howe, J.; Chi, M.; Xie, Z.; Xia, Y. Pd@Pt Core–Shell Concave Decahedra: A Class of Catalysts for the Oxygen Reduction Reaction with Enhanced Activity and Durability. J. Am. Chem. Soc. 2015, 137, 15036– 15042,  DOI: 10.1021/jacs.5b10059
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    52
    Pd@Pt Core-Shell Concave Decahedra: A Class of Catalysts for the Oxygen Reduction Reaction with Enhanced Activity and Durability
    Wang, Xue; Vara, Madeline; Luo, Ming; Huang, Hongwen; Ruditskiy, Aleksey; Park, Jinho; Bao, Shixiong; Liu, Jingyue; Howe, Jane; Chi, Miaofang; Xie, Zhaoxiong; Xia, Younan
    Journal of the American Chemical Society (2015), 137 (47), 15036-15042CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We report a facile synthesis of multiply twinned Pd@Pt core-shell concave decahedra by controlling the deposition of Pt on preformed Pd decahedral seeds. The Pt atoms are initially deposited on the vertices of a decahedral seed, followed by surface diffusion to other regions along the edges/ridges and then across the faces. Different from the coating of a Pd icosahedral seed, the Pt atoms prefer to stay at the vertices and edges/ridges of a decahedral seed even when the deposition is conducted at 200 °C, naturally generating a core-shell structure covered by concave facets. The nonuniformity in the Pt coating can be attributed to the presence of twin boundaries at the vertices, as well as the {100} facets and twin defects along the edges/ridges of a decahedron, effectively trapping the Pt adatoms at these high-energy sites. As compared to a com. Pt/C catalyst, the Pd@Pt concave decahedra show substantial enhancement in both catalytic activity and durability toward the oxygen redn. reaction (ORR). For the concave decahedra with 29.6% Pt by wt., their specific (1.66 mA/cm2Pt) and mass (1.60 A/mgPt) ORR activities are enhanced by 4.4 and 6.6 times relative to those of the Pt/C catalyst (0.36 mA/cm2Pt and 0.32 A/mgPt, resp.). After 10 000 cycles of accelerated durability test, the concave decahedra still exhibit a mass activity of 0.69 A/mgPt, more than twice that of the pristine Pt/C catalyst.
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    Wang, Z.; Zheng, S.; Wan, N.; Zhang, L.; Wang, Q.; He, N.; Huang, Y. Synthesis of a Au-on-Pd Heteronanostructure Stabilized by Citrate and its Catalytic Application. Part. Part. Syst. Charact. 2013, 30, 905– 910,  DOI: 10.1002/ppsc.201300152
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    Synthesis of a Au-on-Pd Heteronanostructure Stabilized by Citrate and its Catalytic Application
    Wang, Zhifei; Zheng, Shuang; Wan, Neng; Zhang, Liming; Wang, Qilong; He, Nongyue; Huang, Yaji
    Particle & Particle Systems Characterization (2013), 30 (10), 905-910CODEN: PPCHEZ; ISSN:1521-4117. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The prepn. of Au-on-Pd heteronanostructure (HNS) using citrate-stabilized polycryst. Pd nanoparticles (NPs) as the seeds is described. The resulting Au-on-Pd HNS is characterized and it is found that the formation of Au-on-Pd HNS depends greatly on a ratio between Pd seeds and AuCl4- ions added and the optimal molar ratio is 10:1. If fewer AuCl4- ions are added (Pd/Au ratio is 100:1), the growth of Au NPs only occurs on part of the Pd seeds' surface. The addn. of more AuCl4- ions (Pd/Au ratio is 5:1) hinders the growth of Au NPs on the Pd seeds' surface. To demonstrate the catalytic performance, the electrochem. oxidn. of ethanol and the redn. of p-nitrophenol by NaBH4 are chosen to examine the catalytic activity of Au-on-Pd HNS. Pd seeds, Au NPs, and poly(vinyl pyrrolidone) (PVP)-stabilized PdAu nanoalloy are used as the refs. for comparison. In the first reaction, the catalytic reactivity of Au-on-Pd HNS is better than that of corresponding pure Pd or Au NPs, while the opposite occurs for the latter reaction. The catalytic activity of Au-on-Pd HNS is much higher than that of PVP-stabilized PdAu nanoalloy.
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    Effenberger, F. B.; Sulca, M. A.; Machini, M. T.; Couto, R. A.; Kiyohara, P. K.; Machado, G.; Rossi, L. M. Copper nanoparticles synthesized by thermal decomposition in liquid phase: the influence of capping ligands on the synthesis and bactericidal activity. J. Nanopart. Res. 2014, 16, 2588  DOI: 10.1007/s11051-014-2588-7
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    Crespo-Quesada, M.; Yarulin, A.; Jin, M.; Xia, Y.; Kiwi-Minsker, L. Structure sensitivity of alkynol hydrogenation on shape- and size-controlled palladium nanocrystals: which sites are most active and selective?. J. Am. Chem. Soc. 2011, 133, 12787– 12794,  DOI: 10.1021/ja204557m
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    Structure Sensitivity of Alkynol Hydrogenation on Shape- and Size-Controlled Palladium Nanocrystals: Which Sites Are Most Active and Selective?
    Crespo-Quesada, Micaela; Yarulin, Artur; Jin, Mingshang; Xia, Younan; Kiwi-Minsker, Lioubov
    Journal of the American Chemical Society (2011), 133 (32), 12787-12794CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The activity and selectivity of structure-sensitive reactions are strongly correlated with the shape and size of the nanocrystals present in a catalyst. This correlation can be exploited for rational catalyst design, esp. if each type of surface atom displays a different behavior, to attain the highest activity and selectivity. In this work, uniform Pd nanocrystals with cubic (in two different sizes), octahedral, and cuboctahedral shapes were synthesized through a soln.-phase method with poly(vinyl pyrrolidone) (PVP) serving as a stabilizer and then tested in the hydrogenation of 2-methyl-3-butyn-2-ol (MBY). The obsd. activity and selectivity suggested that two types of active sites were involved in the catalysis - those on the planes and at edges - which differ in their coordination nos. Specifically, semihydrogenation of MBY to 2-methyl-3-buten-2-ol (MBE) occurred preferentially at the plane sites regardless of their crystallog. orientation, Pd(111) and/or Pd(100), whereas overhydrogenation occurred mainly at the edge sites. The exptl. data can be fitted with a kinetic modeling based on a two-site Langmuir-Hinshelwood mechanism. By considering surface statistics for nanocrystals with different shapes and sizes, the optimal catalyst in terms of productivity of the target product MBE was predicted to be cubes of roughly 3-5 nm in edge length. This study is an attempt to close the material and pressure gaps between model single-crystal surfaces tested under ultra-high-vacuum conditions and real catalytic systems, providing a powerful tool for rational catalyst design.
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    Chan-Thaw, C. E.; Villa, A.; Wang, D.; Dal Santo, V.; Biroli, A. O.; Veith, G. M.; Thomas, A.; Prati, L. PdHx Entrapped in a Covalent Triazine Framework Modulates Selectivity in Glycerol Oxidation. ChemCatChem 2015, 7, 2149– 2154,  DOI: 10.1002/cctc.201500055
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    PdHx Entrapped in a Covalent Triazine Framework Modulates Selectivity in Glycerol Oxidation
    Chan-Thaw, Carine E.; Villa, Alberto; Wang, Di; Dal Santo, Vladimiro; Orbelli Biroli, Alessio; Veith, Gabriel M.; Thomas, Arne; Prati, Laura
    ChemCatChem (2015), 7 (14), 2149-2154CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Pd nanoparticles within a nitrogen-contg. covalent triazine framework (CTF) material are investigated to understand if the highly tunable CTF chem. mediates the catalytic properties of the Pd nanoparticles. Surprisingly, our results demonstrate that the CTF stabilizes the formation of 2.6 nm PdHx particles within the pores. These confined PdHx particles are very active for the liq.-phase oxidn. of glycerol and promote C-C cleavage, probably connected with the enhanced in situ formation of H2O2. During recycling tests, the confined particles are transformed progressively to very stable Pd0 particles. This stability has been attributed mainly to a confinement effect as nanoparticles trapped outside the pores lose activity rapidly. These results indicate that there is a potential to tune CTF chem. to modify the chem. of the catalytic metals significantly.
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    Wu, L.; Li, Z.-W.; Zhang, F.; He, Y.-M.; Fan, Q.-H. Air-Stable and Highly Active Dendritic Phosphine Oxide- Stabilized Palladium Nanoparticles: Preparation, Characterization and Applications in the Carbon-Carbon Bond Formation and Hydrogenation Reactions. Adv. Synth. Catal. 2008, 350, 846– 862,  DOI: 10.1002/adsc.200700441
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    57
    Air-stable and highly active dendritic phosphine oxide-stabilized palladium nanoparticles: preparation, characterization and applications in the carbon-carbon bond formation and hydrogenation reactions
    Wu, Lei; Li, Zhi-Wei; Zhang, Feng; He, Yan-Mei; Fan, Qing-Hua
    Advanced Synthesis & Catalysis (2008), 350 (6), 846-862CODEN: ASCAF7; ISSN:1615-4150. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Dendrimer-stabilized palladium nanoparticles were formed in the redn. of palladium bis(acetylacetonate) [Pd(acac)2] in the presence of phosphine dendrimer ligands using hydrogen in THF. The resulting Pd nanoparticles were characterized by TEM, 31P NM,R and 31P MAS NMR. The results indicated that the dendritic phosphine ligands were oxidized to phosphine oxides. These dendrimer-stabilized Pd nanoparticles were demonstrated to be efficient catalysts for Suzuki and Stille coupling reactions and hydrogenations. The dendritic wedges served as a stabilizer for keeping the nanoparticles from aggregating, and as a vehicle for facilitating the sepn. and/or the recycling of the Pd catalyst. In the case of the Suzuki coupling reaction, these Pd nanoparticles exhibited high catalytic efficiency (TON up to 65,000) and air stability as compared with the commonly used homogeneous catalyst tetrakis(triphenylphosphine)palladium [Pd(PPh3)4]. In addn., the results obtained from the bulky dendritic substrate suggest that the Pd nanoparticles might act as reservoir of catalytically active species, and that the reaction is actually catalyzed by the sol. Pd(0) and/or Pd(II) species leached from the nanoparticle surface.
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    Kumar, S.; Rao, G. K.; Kumar, A.; Singh, M. P.; Saleem, F.; Singh, A. K. Efficient catalytic activation of Suzuki–Miyaura C–C coupling reactions with recyclable palladium nanoparticles tailored with sterically demanding di-n-alkyl sulfides. RSC Adv. 2015, 5, 20081– 20089,  DOI: 10.1039/C5RA00441A
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    Efficient catalytic activation of Suzuki-Miyaura C-C coupling reactions with recyclable palladium nanoparticles tailored with sterically demanding di-n-alkyl sulfides
    Kumar, Satyendra; Rao, G. K.; Kumar, Arun; Singh, Mahabir P.; Saleem, Fariha; Singh, Ajai K.
    RSC Advances (2015), 5 (26), 20081-20089CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The compd. n-bromodocosane reacts with Na2S, generated in situ by the redn. of elemental sulfur with NaBH4, to give n-didocosyl sulfide (L1), which acts as a protector for palladium nanoparticles that are prepd. using different palladium precursors in the presence of L1 (Pd : L1 ratio 1 : 2 and 4 : 1) and the nanoparticles have been characterized with powder X-ray diffraction. The size (nm) ranges for the majority of spherical nanoparticles are ~18-19, 4-5, 5-7, 4-6, 7-9 and 4-6 resp. The precursor of palladium affects the size, shape and dispersion of the nanoparticles. The nanoparticles show good catalytic activity for the Suzuki-Miyaura coupling of various aryl chlorides/bromides ArCl/ArBr [Ar = 4-O2NC6H4, 4-H(O)CC6H4, pyridin-2-yl, etc.] with phenylboronic acid at low catalyst loading (0.1-0.5 mol% of Pd). The distinct advantage of nanoparticles is that they can be sepd. and reused at least up to five times. The compd. didocosyl sulfide and its complex [Pd(L1)2Cl2] have also been synthesized by the reaction of Na2PdCl4 with didocosyl sulfide and the complex [Pd(L1)2Cl2], equiv. to 0.001 mol% Pd, is efficient for the Suzuki-Miyaura coupling of some aryl halides, as good conversion into coupled products has been obsd. Two phase tests, conducted for some nanoparticles suggest the contribution of both homogeneous and heterogeneous catalytic pathways in overall catalysis.
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    Zhao, X.; Zhou, L.; Zhang, W.; Hu, C.; Dai, L.; Ren, L.; Wu, B.; Fu, G.; Zheng, N. Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes. Chem 2018, 4, 1080– 1091,  DOI: 10.1016/j.chempr.2018.02.011
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    Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes
    Zhao, Xiaojing; Zhou, Lingyun; Zhang, Wuyong; Hu, Chengyi; Dai, Lei; Ren, Liting; Wu, Binghui; Fu, Gang; Zheng, Nanfeng
    Chem (2018), 4 (5), 1080-1091CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    Surface and interfacial engineering of heterogeneous metal catalysts is effective and crit. for optimizing selective hydrogenation for fine chems. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from com. Pd/C catalysts for semihydrogenation of internal alkynes.
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    Jin, M.; Zhang, H.; Xie, Z.; Xia, Y. Palladium nanocrystals enclosed by {100} and {111} facets in controlled proportions and their catalytic activities for formic acid oxidation. Energy Environ. Sci. 2012, 5, 6352– 6357,  DOI: 10.1039/C2EE02866B
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    60
    Palladium nanocrystals enclosed by {100} and {111} facets in controlled proportions and their catalytic activities for formic acid oxidation
    Jin, Mingshang; Zhang, Hui; Xie, Zhaoxiong; Xia, Younan
    Energy & Environmental Science (2012), 5 (4), 6352-6357CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A seed-mediated approach to polyhedral nanocrystals of Pd with controlled sizes, shapes, and different proportions of {100} to {111} facets on the surface is reported. The success of this synthesis relies on the use of Pd nanocubes with different sizes as the seeds and the use of formaldehyde as a relatively mild reducing agent. By controlling the ratio of Pd precursor to the seed, the authors obtained uniform polyhedrons such as truncated cubes, cuboctahedrons, truncated octahedrons, and octahedrons in a purity approaching 100%. The sizes of these polyhedrons were detd. by the edge length of the cubic seeds. Since these Pd polyhedrons were characterized by different proportions of {111} to {100} facets, they could serve as model catalysts to uncover the correlation between the surface structure and the catalytic performance for formic acid oxidn. Pd nanocubes exhibited the highest max. c.d. in the forward anodic scan, but the peak position was also located at a potential higher than those of the other polyhedrons. When both the c.d. and the operation potential are taken into consideration, Pd nanocubes with slight truncation at the corners become the best choice of catalyst for formic acid oxidn. The size of Pd polyhedrons had essentially no effect on the activity for formic acid oxidn.
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    Rossi, L. M.; Fiorio, J. L.; Garcia, M. A. S.; Ferraz, C. P. The role and fate of capping ligands in colloidally prepared metal nanoparticle catalysts. Dalton Trans. 2018, 47, 5889– 5915,  DOI: 10.1039/C7DT04728B
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    The role and fate of capping ligands in colloidally prepared metal nanoparticle catalysts
    Rossi, Liane M.; Fiorio, Jhonatan L.; Garcia, Marco A. S.; Ferraz, Camila P.
    Dalton Transactions (2018), 47 (17), 5889-5915CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    A review. Metal nanoparticles have received intense scientific attention in the field of catalysis. Precise engineering of nanomaterials' size, shape and surface compn., including adsorbed capping ligands, is of utmost importance to control activity and selectivity, and distinguish colloidally prepd. metal nanoparticle catalysts from traditional heterogeneous catalysts. The interface between the material and the reaction medium is where the key interactions occur; therefore, catalysis occurs under the influence of capping ligands. In this Perspective review, we focus on the choice of capping ligands (or stabilizing agents), and their role and fate in different steps from prepn. to catalysis. Evaluating the influence of the ligands on the catalytic response is not trivial, but the literature provides examples where the ligands adsorbed on the nanoparticle surface dramatically change the activity and selectivity for a particular reaction, while acting either as a dynamic shell or a passivation coating. Steric and electronic effects resulting from the presence of adsorbed ligands have been proposed to influence the catalytic properties. Attempts to remove the capping ligands are discussed, even though they are not always successful or even necessary. Finally, we outline our personal understanding and perspectives on the use of ligands or functionalized supports to tune the activity and selectivity of supported metal nanoparticles.
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    Kahsar, K. R.; Johnson, S.; Schwartz, D. K.; Medlin, J. W. Hydrogenation of Cinnamaldehyde over Pd/Al2O3 Catalysts Modified with Thiol Monolayers. Top. Catal. 2014, 57, 1505– 1511,  DOI: 10.1007/s11244-014-0325-1
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    62
    Hydrogenation of Cinnamaldehyde over Pd/Al2O3 Catalysts Modified with Thiol Monolayers
    Kahsar, Karl R.; Johnson, Stephanie; Schwartz, Daniel K.; Medlin, J. Will
    Topics in Catalysis (2014), 57 (17-20), 1505-1511CODEN: TOCAFI; ISSN:1022-5528. (Springer)
    Modification of supported Pt catalysts with thiols has recently been shown to improve the hydrogenation selectivity of α,β-unsatd. aldehydes to unsatd. alcs. Here, we apply a variety of org. thiol coatings to Pd/Al2O3 catalysts that typically have a much lower intrinsic selectivity for desired product formation. Thiol monolayers were found to increase hydrogenation selectivity to cinnamyl alc.; however, unlike with Pt catalysts, the increase was independent of the identity of the org. tail.
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    Wang, Y.; Wan, X. K.; Ren, L. T.; Su, H. F.; Li, G.; Malola, S.; Lin, S. C.; Tang, Z. C.; Hakkinen, H.; Teo, B. K.; Wang, Q. M.; Zheng, N. F. Atomically Precise Alkynyl-Protected Metal Nanoclusters as a Model Catalyst: Observation of Promoting Effect of Surface Ligands on Catalysis by Metal Nanoparticles. J. Am. Chem. Soc. 2016, 138, 3278– 3281,  DOI: 10.1021/jacs.5b12730
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    Atomically precise alkynyl-protected metal nanoclusters as a model catalyst: observation of promoting effect of surface ligands on catalysis by metal nanoparticles
    Wang, Yu; Wan, Xian-Kai; Ren, Liting; Su, Haifeng; Li, Gang; Malola, Sami; Lin, Shuichao; Tang, Zichao; Hakkinen, Hannu; Teo, Boon K.; Wang, Quan-Ming; Zheng, Nanfeng
    Journal of the American Chemical Society (2016), 138 (10), 3278-3281CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Gold-silver acetylide nanoclusters Au34Ag28(PhC≡C)34 were prepd. by reaction of ClAuSMe2 and AgOAc with phenylacetylene and by redn. of acetylides PhC≡CAuPPh3, PhC≡CAu and PhC≡CAg with NaBH4. Metal nanoclusters whose surface ligands are removable while keeping their metal framework structures intact are an ideal system for investigating the influence of surface ligands on catalysis of metal nanoparticles. We report in this work an intermetallic nanocluster contg. 62 metal atoms, Au34Ag28(PhC≡C)34, and its use as a model catalyst to explore the importance of surface ligands in promoting catalysis. As revealed by single-crystal diffraction, the 62 metal atoms in the cluster are arranged as a four-concentric-shell Ag@Au17@Ag27@Au17 structure. All phenylalkynyl (PA) ligands are linearly coordinated to the surface Au atoms with staple "PhC≡C-Au-C≡CPh" motif. Compared with reported thiolated metal nanoclusters, the surface PA ligands on Au34Ag28(PhC≡C)34 are readily removed at relatively low temps., while the metal core remains intact. The clusters before and after removal of surface ligands are used as catalysts for the hydrolytic oxidn. of organosilanes to silanols. It is, for the first time, demonstrated that the org.-capped metal nanoclusters work as active catalysts much better than those with surface ligands partially or completely removed.
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    Sun, C.; Mammen, N.; Kaappa, S.; Yuan, P.; Deng, G.; Zhao, C.; Yan, J.; Malola, S.; Honkala, K.; Häkkinen, H.; Teo, B. K.; Zheng, N. Atomically Precise, Thiolated Copper–Hydride Nanoclusters as Single-Site Hydrogenation Catalysts for Ketones in Mild Conditions. ACS Nano 2019, 13, 5975– 5986,  DOI: 10.1021/acsnano.9b02052
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    Atomically Precise, Thiolated Copper-Hydride Nanoclusters as Single-Site Hydrogenation Catalysts for Ketones in Mild Conditions
    Sun, Cunfa; Mammen, Nisha; Kaappa, Sami; Yuan, Peng; Deng, Guocheng; Zhao, Chaowei; Yan, Juanzhu; Malola, Sami; Honkala, Karoliina; Hakkinen, Hannu; Teo, Boon K.; Zheng, Nanfeng
    ACS Nano (2019), 13 (5), 5975-5986CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Copper-hydrides are known catalysts for several technol. important reactions such as hydrogenation of CO, hydroamination of alkenes and alkynes, and chemoselective hydrogenation of unsatd. ketones to unsatd. alcs. Stabilizing copper-based particles by ligand chem. to nanometer scale is an appealing route to make active catalysts with optimized material economy; however, it was long believed that the ligand-metal interface, particularly if sulfur-contg. thiols were used as stabilizing agent, may poison the catalyst. The authors report here a discovery of an ambient-stable thiolate-protected copper-hydride nanocluster [Cu25H10(SPhCl2)18]3- that readily catalyzes hydrogenation of ketones to alcs. in mild conditions. A full exptl. and theor. characterization of its at. and electronic structure shows that the 10 hydrides are instrumental for the stability of the nanocluster and are in an active role being continuously consumed and replenished in the hydrogenation reaction. D. functional theory computations suggest, backed up by the exptl. evidence, that the hydrogenation takes place only around a single site of the 10 hydride locations, rendering the [Cu25H10(SPhCl2)18]3- one of the first nanocatalysts whose structure and catalytic functions were characterized fully to at. precision. Understanding of a working catalyst at the atomistic level helps to optimize its properties and provides fundamental insights into the controversial issue of how a stable, ligand-passivated, metal-contg. nanocluster can be at the same time an active catalyst.
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    Wang, S.; Xin, Z.; Huang, X.; Yu, W.; Niu, S.; Shao, L. Nanosized Pd-Au bimetallic phases on carbon nanotubes for selective phenylacetylene hydrogenation. Phys. Chem. Chem. Phys. 2017, 19, 6164– 6168,  DOI: 10.1039/C6CP08805H
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    Nanosized Pd-Au bimetallic phases on carbon nanotubes for selective phenylacetylene hydrogenation
    Wang, Shenghua; Xin, Zhiling; Huang, Xing; Yu, Weizhen; Niu, Shuo; Shao, Lidong
    Physical Chemistry Chemical Physics (2017), 19 (8), 6164-6168CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Palladium (Pd)-catalyzed selective hydrogenation of alkynes has been one of the most studied hydrogenation reactions in the last century. However, kinetic studies conducted to reveal the catalyst's active centers have been hindered because of dynamic surface changes on Pd during the reaction. In the present study, bimetallic Pd-Au nanoparticles supported on carbon nanotubes have been synthesized at room temp. as catalysts for selective hydrogenation of phenylacetylene, which show effectively enhanced selectivity compared to their monometallic counterparts. Structural and surface analyses of fresh and reacted catalysts reveal that selective hydrogenation of phenylacetylene is favored over nanosized Pd-Au bimetallic phases due to modifications in the Pd surface in terms of neighboring site isolation and electron d. redn.
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    Zhang, L.; Ding, Y.; Wu, K. H.; Niu, Y.; Luo, J.; Yang, X.; Zhang, B.; Su, D. Pd@C core-shell nanoparticles on carbon nanotubes as highly stable and selective catalysts for hydrogenation of acetylene to ethylene. Nanoscale 2017, 9, 14317– 14321,  DOI: 10.1039/C7NR04992G
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    Pd@C core-shell nanoparticles on carbon nanotubes as highly stable and selective catalysts for hydrogenation of acetylene to ethylene
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    Nanoscale (2017), 9 (38), 14317-14321CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Developing highly selective and stable catalysts for acetylene hydrogenation is an imperative task in the chem. industry. Herein, core-shell Pd@carbon nanoparticles supported on carbon nanotubes (Pd@C/CNTs) were synthesized. During the hydrogenation of acetylene, the selectivity of Pd@C/CNTs to ethylene was distinctly improved. Moreover, Pd@C/CNTs showed excellent stability during the hydrogenation reaction.
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    Zhou, S.; Shang, L.; Zhao, Y.; Shi, R.; Waterhouse, G. I. N.; Huang, Y. C.; Zheng, L.; Zhang, T. Pd Single-Atom Catalysts on Nitrogen-Doped Graphene for the Highly Selective Photothermal Hydrogenation of Acetylene to Ethylene. Adv. Mater. 2019, 31, 1900509,  DOI: 10.1002/adma.201900509
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    Khouya, A. A.; Ba, H.; Baaziz, W.; Nhut, J.-M.; Rossin, A.; Zafeiratos, S.; Ersen, O.; Giambastiani, G.; Ritleng, V.; Pham-Huu, C. Palladium Nanosheet-Carbon Black Powder Composites for Selective Hydrogenation of Alkynes to Alkenes. ACS Appl. Nano Mater. 2021, 4, 2265– 2277,  DOI: 10.1021/acsanm.1c00002
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    Palladium Nanosheet-Carbon Black Powder Composites for Selective Hydrogenation of Alkynes to Alkenes
    Khouya, Ahmed Ait; Ba, Housseinou; Baaziz, Walid; Nhut, Jean-Mario; Rossin, Andrea; Zafeiratos, Spyridon; Ersen, Ovidiu; Giambastiani, Giuliano; Ritleng, Vincent; Pham-Huu, Cuong
    ACS Applied Nano Materials (2021), 4 (2), 2265-2277CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    Ultrathin palladium nanosheets on carbon black grains have been straightforwardly prepd. under reductant-free conditions using ultrasonication. The as-prepd. composites (showing a quite unusual morphol. of Pd-deposits) have been scrutinized as catalysts in the selective alkyne-to-alkene hydrogenation under mild operational conditions. Their activity, selectivity, and robustness in the process are outstanding, opening new horizons for simpler synthetic procedures for this class of composites. Moreover, the tight anchorage of the large and thin metal sheets to the carbon grains of the support dramatically reduces the active phase surface mobility during catalysis. Besides preserving the catalyst performance, this peculiar palladium morphol. strongly limits the environmental risks stemming from metal nanoparticles leaching in heterogeneous catalysts.
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    Lee, H.; Nguyen-Huy, C.; Jeong Jang, E.; Lee, J.; Yang, E.; Lee, M. S.; Kwak, J. H.; An, K. Interfacial effect of Pd supported on mesoporous oxide for catalytic furfural hydrogenation. Catal. Today 2021, 365, 291– 300,  DOI: 10.1016/j.cattod.2020.02.035
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    Interfacial effect of Pd supported on mesoporous oxide for catalytic furfural hydrogenation
    Lee, Hojeong; Nguyen-Huy, Chinh; Jang, Eun Jeong; Lee, Jihyeon; Yang, Euiseob; Lee, Man Sig; Kwak, Ja Hun; An, Kwangjin
    Catalysis Today (2021), 365 (), 291-300CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    Highly dispersed Pd is loaded onto different types of mesoporous oxide supports to investigate the synergetic metal-support effect in catalytic furfural (FAL) hydrogenation. Ordered mesoporous Co3O4, MnO2, NiO, CeO2, and Fe2O3 are prepd. by the nanocasting and the supported Pd on mesoporous oxide catalysts are obtained by the chem. redn. method. It is revealed that mesoporous oxides play an important role on Pd dispersion as well as the redox behavior of Pd, which dets. the final FAL conversion. Among the catalysts used, Pd/Co3O4 shows the highest conversion in FAL hydrogenation and distinct product selectivity toward 2-methylfuran (MF). While FAL is converted via two distinct pathways to produce either furfuryl alc. (FA) via aldehyde hydrogenation or MF via hydrogenolysis, MF as a secondary product is derived from FA via the hydrogenolysis of C-O over the Pd/Co3O4 catalyst. It is revealed that FAL is hydrogenated to FA preferentially on the Pd surface; then the secondary hydrogenolysis to MF from FA is further promoted at the interface between Pd and Co3O4. We confirm that the reaction pathway over Pd/Co3O4 is totally different than other catalysts such as Pd/MnO2, which produces FA dominantly. The characteristics of the mesoporous oxides influence the Pd-oxide interfaces, which det. the activity and selectivity in FAL hydrogenation.
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    Crespo-Quesada, M.; Cardenas-Lizana, F.; Dessimoz, A.-L.; Kiwi-Minsker, L. Modern Trends in Catalyst and Process Design for Alkyne Hydrogenations. ACS Catal. 2012, 2, 1773– 1786,  DOI: 10.1021/cs300284r
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    Modern Trends in Catalyst and Process Design for Alkyne Hydrogenations
    Crespo-Quesada, Micaela; Cardenas-Lizana, Fernando; Dessimoz, Anne-Laure; Kiwi-Minsker, Lioubov
    ACS Catalysis (2012), 2 (8), 1773-1786CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A review. This review provides an overview of the recent achievements in catalytic process development for alkyne hydrogenations. It underlines the necessity of simultaneous optimization over different length scales from mol./nanoscale of active phase, up-to macro-scale of catalytic reactor design. One case study, the hydrogenation of 2-methyl-3-butyn-2-ol, is analyzed in detail to illustrate the practical application of this approach. Finally, it presents the personal view of the authors concerning the new trends and paths available in the field.
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    McCue, A. J.; Anderson, J. A. Recent advances in selective acetylene hydrogenation using palladium containing catalysts. Front. Chem. Sci. Eng. 2015, 9, 142– 153,  DOI: 10.1007/s11705-015-1516-4
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    Recent advances in selective acetylene hydrogenation using palladium containing catalysts
    McCue, Alan J.; Anderson, James A.
    Frontiers of Chemical Science and Engineering (2015), 9 (2), 142-153CODEN: FCSEA3; ISSN:2095-0187. (Springer)
    Recent advances with Pd contg. catalysts for the selective hydrogenation of acetylene are described. The overview classifies enhancement of catalytic properties for monometallic and bimetallic Pd catalysts. Activity/selectivity of Pd catalysts can be modified by controlling particle shape/morphol. or immobilization on a support which interacts strongly with Pd particles. In both cases enhanced ethylene selectivity is generally assocd. with modifying ethylene adsorption strength and/or changes to hydride formation. Inorg. and org. selectivity modifiers (i.e., species adsorbed onto Pd particle surface) have also been shown to enhance ethylene selectivity. Inorg. modifiers such as TiO2 change Pd ensemble size and modify ethylene adsorption strength whereas org. modifiers such as diphenylsulfide are thought to create a surface template effect which favors acetylene adsorption with respect to ethylene. A no. of metals and synthetic approaches have been explored to prep. Pd bimetallic catalysts. Examples where enhanced selectivity is obsd. are generally assocd. with decreased Pd ensemble size and/or hindering of the ease with which an unselective hydride phase is formed for Pd. A final class of bimetallic catalysts are discussed where Pd is not thought to be the primary reaction site but merely acts as a site where hydrogen dissocn. and spillover occurs onto a second metal (Cu or Au) where the reaction takes place more selectively. [Figure not available: see fulltext.].
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    McCue, A. J.; Guerrero-Ruiz, A.; Rodríguez-Ramos, I.; Anderson, J. A. Palladium sulphide – A highly selective catalyst for the gas phase hydrogenation of alkynes to alkenes. J. Catal. 2016, 340, 10– 16,  DOI: 10.1016/j.jcat.2016.05.002
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    Palladium sulphide - A highly selective catalyst for the gas phase hydrogenation of alkynes to alkenes
    McCue, Alan J.; Guerrero-Ruiz, Antonio; Rodriguez-Ramos, Inmaculada; Anderson, James A.
    Journal of Catalysis (2016), 340 (), 10-16CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    A particular palladium sulfide phase (Pd4S) supported on carbon nanofibers is shown to be one of the most selective alkyne hydrogenation catalysts reported to date. Propyne and ethyne (in the absence of the corresponding alkene) can be converted in the gas phase to the corresponding alkene with 96% and 83% selectivity at 100% alkyne conversion. A bulk phase PdS powder (pre-reduced at 523 K) also demonstrated excellent performance (79% ethene selectivity). Other bulk phase metal sulfides (Ni2S3 and CuS) were tested and while the nickel analog was found to be active/selective the performance was poorer than obsd. with either supported or unsupported Pd sulfide. Exceptional alkene selectivity extends to mixed alkyne/alkene feeds using the Pd4S/CNF catalyst - 86% and 95% alkene selectivity for C3 and C2 mixes, resp. This report opens up exciting possibilities for using metal sulfides as highly selective hydrogenation catalysts.
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    Wei, S.; Li, A.; Liu, J. C.; Li, Z.; Chen, W.; Gong, Y.; Zhang, Q.; Cheong, W. C.; Wang, Y.; Zheng, L.; Xiao, H.; Chen, C.; Wang, D.; Peng, Q.; Gu, L.; Han, X.; Li, J.; Li, Y. Direct observation of noble metal nanoparticles transforming to thermally stable single atoms. Nat. Nanotechnol. 2018, 13, 856– 861,  DOI: 10.1038/s41565-018-0197-9
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    Direct observation of noble metal nanoparticles transforming to thermally stable single atoms
    Wei, Shengjie; Li, Ang; Liu, Jin-Cheng; Li, Zhi; Chen, Wenxing; Gong, Yue; Zhang, Qinghua; Cheong, Weng-Chon; Wang, Yu; Zheng, Lirong; Xiao, Hai; Chen, Chen; Wang, Dingsheng; Peng, Qing; Gu, Lin; Han, Xiaodong; Li, Jun; Li, Yadong
    Nature Nanotechnology (2018), 13 (9), 856-861CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
    Single noble metal atoms and ultrafine metal clusters catalysts tend to sinter into aggregated particles at elevated temps., driven by the decrease of metal surface free energy. Herein, we report an unexpected phenomenon that noble metal nanoparticles (Pd, Pt, Au-NPs) can be transformed to thermally stable single atoms (Pd, Pt, Au-SAs) above 900 °C in an inert atm. The at. dispersion of metal single atoms was confirmed by aberration-cor. scanning transmission electron microscopy and X-ray absorption fine structures. The dynamic process was recorded by in situ environmental transmission electron microscopy, which showed competing sintering and atomization processes during NP-to-SA conversion. Further, d. functional theory calcns. revealed that high-temp. NP-to-SA conversion was driven by the formation of the more thermodynamically stable Pd-N4 structure when mobile Pd atoms were captured on the defects of nitrogen-doped carbon. The thermally stable single atoms (Pd-SAs) exhibited even better activity and selectivity than nanoparticles (Pd-NPs) for semi-hydrogenation of acetylene.
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    Huang, F.; Deng, Y.; Chen, Y.; Cai, X.; Peng, M.; Jia, Z.; Ren, P.; Xiao, D.; Wen, X.; Wang, N.; Liu, H.; Ma, D. Atomically Dispersed Pd on Nanodiamond/Graphene Hybrid for Selective Hydrogenation of Acetylene. J. Am. Chem. Soc. 2018, 140, 13142– 13146,  DOI: 10.1021/jacs.8b07476
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    Atomically Dispersed Pd on Nanodiamond/Graphene Hybrid for Selective Hydrogenation of Acetylene
    Huang, Fei; Deng, Yuchen; Chen, Yunlei; Cai, Xiangbin; Peng, Mi; Jia, Zhimin; Ren, Pengju; Xiao, Dequan; Wen, Xiaodong; Wang, Ning; Liu, Hongyang; Ma, Ding
    Journal of the American Chemical Society (2018), 140 (41), 13142-13146CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We reported here a strategy to use a defective nanodiamond-graphene (ND@G) to prep. an atomically dispersed metal catalyst, i.e., in the current case atomically dispersed palladium catalyst which is used for selective hydrogenation of acetylene in the presence of abundant ethylene. The catalyst exhibits remarkable performance for the selective conversion of acetylene to ethylene: high conversion (100%), ethylene selectivity (90%), and good stability. The unique structure of the catalyst (i.e., atomically dispersion of Pd atoms on graphene through Pd-C bond anchoring) blocks the formation of unselective subsurface hydrogen species and ensures the facile desorption of ethylene against the overhydrogenation to undesired ethane, which is the key for the outstanding selectivity of the catalyst.
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    Mitsudome, T.; Urayama, T.; Yamazaki, K.; Maehara, Y.; Yamasaki, J.; Gohara, K.; Maeno, Z.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Design of Core-Pd/Shell-Ag Nanocomposite Catalyst for Selective Semihydrogenation of Alkynes. ACS Catal. 2016, 6, 666– 670,  DOI: 10.1021/acscatal.5b02518
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    Design of Core-Pd/Shell-Ag Nanocomposite Catalyst for Selective Semihydrogenation of Alkynes
    Mitsudome, Takato; Urayama, Teppei; Yamazaki, Kenji; Maehara, Yosuke; Yamasaki, Jun; Gohara, Kazutoshi; Maeno, Zen; Mizugaki, Tomoo; Jitsukawa, Koichiro; Kaneda, Kiyotomi
    ACS Catalysis (2016), 6 (2), 666-670CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    We designed core-Pd/shell-Ag nanocomposite catalyst (Pd@Ag) for highly selective semihydrogenation of alkynes. The construction of the core-shell nanocomposite enables a significant improvement in the low activity of Ag NPs for the selective semihydrogenation of alkynes because hydrogen is supplied from the core-Pd NPs to the shell-Ag NPs in a synergistic manner. Simultaneously, coating the core-Pd NPs with shell-Ag NPs results in efficient suppression of overhydrogenation of alkenes by the Pd NPs. This complementary action of core-Pd and shell-Ag provides high chemoselectivity toward a wide range of alkenes with high Z-selectivity under mild reaction conditions (room temp. and 1 atm H2). Moreover, Pd@Ag can be easily sepd. from the reaction mixt. and is reusable without loss of catalytic activity or selectivity.
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    Yang, S.; Cao, C.; Peng, L.; Zhang, J.; Han, B.; Song, W. A Pd-Cu2O nanocomposite as an effective synergistic catalyst for selective semi-hydrogenation of the terminal alkynes only. Chem. Commun. 2016, 52, 3627– 3630,  DOI: 10.1039/C6CC00143B
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    A Pd-Cu2O nanocomposite as an effective synergistic catalyst for selective semi-hydrogenation of the terminal alkynes only
    Yang, Shuliang; Cao, Changyan; Peng, Li; Zhang, Jianling; Han, Buxing; Song, Weiguo
    Chemical Communications (Cambridge, United Kingdom) (2016), 52 (18), 3627-3630CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A palladium-Cu2O nanocomposite with Fe3O4 on multiwalled carbon nanotubes (MWCNT) was prepd. as a recyclable catalyst for the chemoselective hydrogenation of terminal alkynes in acetonitrile to terminal alkenes. Internal alkynes and nitrobenzene were not hydrogenated or reduced in the presence of terminal alkynes under hydrogenation conditions using the nanocomposite catalyst.
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    Wu, Z.; Calcio Gaudino, E.; Rotolo, L.; Medlock, J.; Bonrath, W.; Cravotto, G. Efficient partial hydrogenation of 2-butyne-1,4-diol and other alkynes under microwave irradiation. Chem. Eng. Process. 2016, 110, 220– 224,  DOI: 10.1016/j.cep.2016.10.016
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    Efficient partial hydrogenation of 2-butyne-1,4-diol and other alkynes under microwave irradiation
    Wu, Zhilin; Calcio Gaudino, Emanuela; Rotolo, Laura; Medlock, Jonathan; Bonrath, Werner; Cravotto, Giancarlo
    Chemical Engineering and Processing (2016), 110 (), 220-224CODEN: CENPEU; ISSN:0255-2701. (Elsevier B.V.)
    The microwave-assisted partial hydrogenation of 2-butyne-1,4-diol, diphenylacetylene and phenylacetylene with novel lead-free Pd/Boehmite catalysts were investigated. Obtaining high alkyne conversion and good selectivity of the product alkenes required optimization of several reaction parameters: substrate concn., amt. of catalyst, temp., hydrogen pressure, solvent and reaction scale. Dielec. heating strongly enhanced the conversion while maintaining good selectivity to alkenes (over 92%). Excellent 2-butyne-1,4-diol conversions, with 100% alkene selectivity, were achieved in water (20 mL) at 90° at high substrate concns. (20 wt%).
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    Sittikun, J.; Boonyongmaneerat, Y.; Weerachawanasak, P.; Praserthdam, P.; Panpranot, J. Pd/TiO2 catalysts prepared by electroless deposition with and without SnCl2 sensitization for the liquid-phase hydrogenation of 3-hexyn-1-ol. React. Kinet., Mech. Catal. 2014, 111, 123– 135,  DOI: 10.1007/s11144-013-0634-6
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    Pd/TiO2 catalysts prepared by electroless deposition with and without SnCl2 sensitization for the liquid-phase hydrogenation of 3-hexyn-1-ol
    Sittikun, Jarutphon; Boonyongmaneerat, Yuttanant; Weerachawanasak, Patcharaporn; Praserthdam, Piyasan; Panpranot, Joongjai
    Reaction Kinetics, Mechanisms and Catalysis (2014), 111 (1), 123-135CODEN: RKMCAJ; ISSN:1878-5190. (Springer)
    Palladium-titanium oxide catalysts were prepd. by an electroless deposition method with and without SnCl2 sensitization and the catalysts thus formed were employed in the liq.-phase selective hydrogenation of 3-hexyn-1-ol under mild reaction conditions. As revealed by inductively coupled plasma optical emission spectroscopy, transmission electron microscopy and CO pulse chemisorption, the use of a conventional SnCl2/PdCl2 soln. resulted in a larger amt. of Pd being deposited (0.98 wt%) on the TiO2 and higher Pd dispersion (6.6 × 1017 mols. CO/g Pd) than the SnCl2-free activation (0.87 wt%, 5.3 × 1017 mols. CO/g Pd). However, tin was found to remain on the catalyst in the form of Sn4+ and partially covered the Pd surface. Large Pd particles were formed after electroless plating of Pd when the TiO2 was activated by PdCl2 followed by a redn. using NaH2PO2 (SnCl2-free process). The selectivity towards cis-hexen-1-ol after complete conversion of 3-hexyn-1-ol was much improved using an electroless deposition catalyst (selectivity 60-87%) compared to catalysts prepd. by a conventional impregnation method (selectivity 4%). The results in this study show the advantages of electroless deposition technique as a simple method to synthesize highly selective catalysts for the prodn. of cis-alkene in an alkyne semi-hydrogenation. The target compd. thus formed was (3Z)-3-hexen-1-ol (green leaf alc., folic alc.). (3E)-3-Hexen-1-ol and 1-hexanol were formed as byproducts.
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    Liu, J.; Uhlman, M. B.; Montemore, M. M.; Trimpalis, A.; Giannakakis, G.; Shan, J.; Cao, S.; Hannagan, R. T.; Sykes, E. C. H.; Flytzani-Stephanopoulos, M. Integrated Catalysis-Surface Science-Theory Approach to Understand Selectivity in the Hydrogenation of 1-Hexyne to 1-Hexene on PdAu Single-Atom Alloy Catalysts. ACS Catal. 2019, 9, 8757– 8765,  DOI: 10.1021/acscatal.9b00491
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    Integrated Catalysis-Surface Science-Theory Approach to Understand Selectivity in the Hydrogenation of 1-Hexyne to 1-Hexene on PdAu Single-Atom Alloy Catalysts
    Liu, Jilei; Uhlman, Matthew B.; Montemore, Matthew M.; Trimpalis, Antonios; Giannakakis, Georgios; Shan, Junjun; Cao, Sufeng; Hannagan, Ryan T.; Sykes, E. Charles H.; Flytzani-Stephanopoulos, Maria
    ACS Catalysis (2019), 9 (9), 8757-8765CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The selective hydrogenation of alkynes to alkenes is an important industrial process. However, achieving high selectivity and reducing the usage of precious platinum group metals are still challenging for the conventional hydrogenation catalysts. With atomically dispersed active metal atoms on the surface of a host metal, single-atom alloys (SAAs) have shown excellent hydrogenation selectivity and activity, but their hydrogenation mechanism is not fully understood. This work reports on the selective hydrogenation of 1-hexyne to 1-hexene on PdAu SAA catalysts. Au is a highly selective hydrogenation catalyst, but it is not active at low temps. Through measurements of reaction kinetics and in operando spectroscopy studies, we follow the much more facile activation of PdAu SAA catalysts and demonstrate the different hydrogenation chem. of single Pd atoms and Pd nanoparticles (NPs). We further investigate the role of Pd atoms and the mechanism behind the improved hydrogenation selectivity through surface science and d. functional theory. These studies indicate that the difference in reactivity stems from the relative energy barrier heights for over-hydrogenating the terminal C atom. The complementary catalysis-surface science-theory investigation described here is a powerful and general approach for understanding and controlling NP performance. The selective hydrogenation on PdAu SAAs is demonstrated and understood fundamentally, which serves as a guide for future designs of this type of catalyst.
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  80. 80
    Liu, Y.; Li, Y.; Anderson, J. A.; Feng, J.; Guerrero-Ruiz, A.; Rodríguez-Ramos, I.; McCue, A. J.; Li, D. Comparison of Pd and Pd4S based catalysts for partial hydrogenation of external and internal butynes. J. Catal. 2020, 383, 51– 59,  DOI: 10.1016/j.jcat.2020.01.010
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    80
    Comparison of Pd and Pd4S based catalysts for partial hydrogenation of external and internal butynes
    Liu, Yanan; Li, Yinwen; Anderson, James A.; Feng, Junting; Guerrero-Ruiz, Antonio; Rodriguez-Ramos, Inmaculada; McCue, Alan J.; Li, Dianqing
    Journal of Catalysis (2020), 383 (), 51-59CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    The partial hydrogenation of but-1-yne and but-2-yne was studied with a view to probing the difference between external and internal alkynes. Catalysts with Pd and Pd4S active phases were prepd. on a carbon nanofiber support. Over the simple Pd catalyst over-hydrogenation was common which restricted alkene selectivity greatly - 25-35% depending on temp. In contrast, the Pd4S active phase offered exceptional alkene selectivity (max. of 92-93% alkene selectivity for both the external and internal alkyne). DFT calcns. were subsequently used to rationalise this difference in product selectivity - sulfur appears to change the geometry of the active site in Pd4S and create a surface which favors alkene desorption relative to over-hydrogenation. This work further emphases the potential of palladium sulfide phases as an alternative to purely metallic palladium catalysts for partial alkyne hydrogenation.
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  81. 81
    Friedrich, M. F.; Lucas, M.; Claus, P. Selective hydrogenation of propyne on a solid Pd/Al2O3 catalyst modified with ionic liquid layer (SCILL). Catal. Commun. 2017, 88, 73– 76,  DOI: 10.1016/j.catcom.2016.09.036
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    81
    Selective hydrogenation of propyne on a solid Pd/Al2O3 catalyst modified with ionic liquid layer (SCILL)
    Friedrich, Michael F.; Lucas, Martin; Claus, Peter
    Catalysis Communications (2017), 88 (), 73-76CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
    The successful application of solid catalyst with ionic liq. layer (SCILL) concept in the selective hydrogenation of propyne is presented in this work. A Pd-based SCILL catalyst was prepd. by spray impregnation of com. available Pd/Al2O3 egg-shell catalyst and characterized by nitrogen sorption. Catalytic testing showed that even small amts. of ionic liq. were sufficient to suppress the consecutive reaction of propane formation, while propene selectivity was increased up to 97%. C6 oligomer formation, which is known to induce deactivation of the catalyst, could be prevented by excess hydrogen in the feed.
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  82. 82
    Lu, Y.; Feng, X.; Takale, B. S.; Yamamoto, Y.; Zhang, W.; Bao, M. Highly Selective Semihydrogenation of Alkynes to Alkenes by Using an Unsupported Nanoporous Palladium Catalyst: No Leaching of Palladium into the Reaction Mixture. ACS Catal. 2017, 7, 8296– 8303,  DOI: 10.1021/acscatal.7b02915
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    82
    Highly Selective Semihydrogenation of Alkynes to Alkenes by Using an Unsupported Nanoporous Palladium Catalyst: No Leaching of Palladium into the Reaction Mixture
    Lu, Ye; Feng, Xiujuan; Takale, Balaram S.; Yamamoto, Yoshinori; Zhang, Wei; Bao, Ming
    ACS Catalysis (2017), 7 (12), 8296-8303CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    We report the highly chemoselective and stereoselective semihydrogenation of alkynes to Z-internal and terminal alkenes by using unsupported nanoporous palladium (PdNPore) as a heterogeneous catalyst under mild reaction conditions (room temp. and 1 atm of H2). The semihydrogenation of various terminal/internal and arom./aliph. alkynes afforded the corresponding alkenes in good chem. yields with high selectivities. PdNPore further showed high chemoselectivity toward terminal alkynes in the presence of internal alkynes, which has not yet been achieved using supported palladium nanoparticle catalysts. H-H heterolysis of H2 on the surface of PdNPore was strongly suggested by deuterium labeling expts. No Pd leached from PdNPore during the reaction, and the catalyst was easily recovered and reused without a loss of activity.
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  83. 83
    Kuwahara, Y.; Kango, H.; Yamashita, H. Pd Nanoparticles and Aminopolymers Confined in Hollow Silica Spheres as Efficient and Reusable Heterogeneous Catalysts for Semihydrogenation of Alkynes. ACS Catal. 2019, 9, 1993– 2006,  DOI: 10.1021/acscatal.8b04653
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    83
    Pd Nanoparticles and Aminopolymers Confined in Hollow Silica Spheres as Efficient and Reusable Heterogeneous Catalysts for Semihydrogenation of Alkynes
    Kuwahara, Yasutaka; Kango, Hiroto; Yamashita, Hiromi
    ACS Catalysis (2019), 9 (3), 1993-2006CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A yolk-shell nanostructured composite composed of Pd nanoparticles (NPs) and aminopolymers, poly(ethylenimine) (PEI), confined in hollow silica spheres which act as an efficient and stable heterogeneous catalyst for semihydrogenation of alkynes is reported herein. The yolk-shell nanostructured Pd-PEI-silica composite catalysts (Pd+PEI@HSS), consisting of Pd NPs core ca. 5-9 nm in diam. and a porous silica shell ca. 30-50 nm in shell thickness, are fabricated by a facile one-pot method using linear- or branched-type PEI (Mw = 1,800-2,500) as an org. template. On the basis of comprehensive structural analyses by FE-SEM, TEM, N2 physisorption, IR, TG, and Pd K-edge XAFS, we show that metal Pd NPs and PEI mols. are encapsulated in the hollow silica sphere with a size of ca. 100-160 nm. The Pd+PEI@HSS composite shows an activity at near room temp. in the liq.-phase hydrogenation of diphenylacetylene to selectively produce cis-stilbene with 95% yield, which outperforms those of the previously reported Pd/PEI and Lindlar catalysts. Interestingly, the catalyst encapsulating linear-type PEI provides a markedly high alkene selectivity in the semihydrogenation of phenylacetylene to produce styrene owing to the strong poisoning effect of linear PEI, which is clearly revealed by an isotope study using H2/D2/acetylene (or ethylene) gases. The catalyst synthesized with optimum silica shell thickness can be easily recovered and recycled without any loss of palladium species and PEI and retaining high activities and selectivities over multiple cycles owing to the ability of the protective effect of silica shell, rendering this material an efficient and stable catalyst for semihydrogenation of alkynes.
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  84. 84
    Zhou, H.; Yang, X.; Li, L.; Liu, X.; Huang, Y.; Pan, X.; Wang, A.; Li, J.; Zhang, T. PdZn Intermetallic Nanostructure with Pd–Zn–Pd Ensembles for Highly Active and Chemoselective Semi-Hydrogenation of Acetylene. ACS Catal. 2016, 6, 1054– 1061,  DOI: 10.1021/acscatal.5b01933
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    84
    PdZn Intermetallic Nanostructure with Pd-Zn-Pd Ensembles for Highly Active and Chemoselective Semi-Hydrogenation of Acetylene
    Zhou, Huiran; Yang, Xiaofeng; Li, Lin; Liu, Xiaoyan; Huang, Yanqiang; Pan, Xiaoli; Wang, Aiqin; Li, Jun; Zhang, Tao
    ACS Catalysis (2016), 6 (2), 1054-1061CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Intermetallic alloying of one active metal to another inert metal provides not only the improved dispersion of active centers but also a unique and homogeneous ensemble of active sites, thus offering new opportunities in a variety of reactions. Herein, we report that PdZn intermetallic nanostructure with Pd-Zn-Pd ensembles are both highly active and selective for the semihydrogenation of acetylene to ethylene, which is usually inaccessible due to the sequential hydrogenation to ethane. Microcalorimetric measurements and d. functional theory calcns. demonstrate that the appropriate spatial arrangement of Pd sites in the Pd-Zn-Pd ensembles of the PdZn alloy leads to the moderate σ-bonding mode for acetylene with two neighboring Pd sites while the weak π-bonding pattern of ethylene adsorption on the single Pd site, which facilitates the chemisorption toward acetylene and promotes the desorption of ethylene from the catalyst surface. As a result, it leads to the kinetic favor of the selective conversion of acetylene to ethylene.
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    Liu, P.; Zheng, N. Coordination chemistry of atomically dispersed catalysts. Natl. Sci. Rev. 2018, 5, 636– 638,  DOI: 10.1093/nsr/nwy051
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    85
    Coordination chemistry of atomically dispersed catalysts
    Liu, Pengxin; Zheng, Nanfeng
    National Science Review (2018), 5 (5), 636-638CODEN: NSRACI; ISSN:2053-714X. (Oxford University Press)
    There is no expanded citation for this reference.
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  86. 86
    Rossell, M. D.; Caparrós, F. J.; Angurell, I.; Muller, G.; Llorca, J.; Seco, M.; Rossell, O. Magnetite-supported palladium single-atoms do not catalyse the hydrogenation of alkenes but small clusters do. Catal. Sci. Technol. 2016, 6, 4081– 4085,  DOI: 10.1039/C6CY00596A
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    86
    Magnetite-supported palladium single-atoms do not catalyse the hydrogenation of alkenes but small clusters do
    Rossell, Marta D.; Caparros, Francisco J.; Angurell, Inmaculada; Muller, Guillermo; Llorca, Jordi; Seco, Miquel; Rossell, Oriol
    Catalysis Science & Technology (2016), 6 (12), 4081-4085CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    The activity of supported noble metal catalysts strongly depends on the particle size. The ultimate small-size limit is the single-atom catalyst (SAC), which maximizes the catalytic efficiency in the majority of the examples. Here, we investigate the catalytic behavior of Pd SACs supported on magnetite nanoparticles and we unambiguously demonstrate that Pd SACs are absolutely inactive in the hydrogenation of various alkene substrates. Instead, Pd clusters of low atomicity exhibit outstanding catalytic performance.
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  87. 87
    Vorobyeva, E.; Chen, Z.; Mitchell, S.; Leary, R. K.; Midgley, P.; Thomas, J. M.; Hauert, R.; Fako, E.; López, N.; Pérez-Ramírez, J. Tailoring the framework composition of carbon nitride to improve the catalytic efficiency of the stabilised palladium atoms. J. Mater. Chem. A 2017, 5, 16393– 16403,  DOI: 10.1039/C7TA04607C
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    87
    Tailoring the framework composition of carbon nitride to improve the catalytic efficiency of the stabilised palladium atoms
    Vorobyeva, E.; Chen, Z.; Mitchell, S.; Leary, R. K.; Midgley, P.; Thomas, J. M.; Hauert, R.; Fako, E.; Lopez, N.; Perez-Ramirez, J.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (31), 16393-16403CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Graphitic carbon nitride (g-C3N4) exhibits unique properties for the prepn. of single-atom heterogeneous catalysts (SAHCs) due to the presence of sixfold nitrogen-based coordination sites in the lattice. Despite the potential to profoundly affect the metal stabilization and resulting catalytic properties, no work has previously investigated the effect of modifying the carrier compn. Here, we study the impact of doping carbon in g-C3N4 on the interaction with palladium. This is achieved by introducing carbon-rich heterocycles (barbituric acid or 2,4,6-triaminopyrimidine) during the synthesis of bulk and mesoporous g-C3N4. Palladium is subsequently introduced via microwave-irradn.-assisted deposition, which emerges as a highly effective route for the dispersion of single atoms. Detailed characterization confirms the controlled variation of the C/N ratio of the lattice and reveals the complex interplay with the crystal size, surface area, amt. of defects, basic properties and thermal stability of the carrier. Atomic dispersions of palladium with similar surface densities could be obtained on both the stoichiometric and carbon-doped carriers in mesoporous form, but appreciable differences are obsd. in the ratio of Pd2+/Pd4+. The latter, which provides a measure of the degree of electron transfer from the metal to the carrier, is found to correlate with the activity in the continuous flow semi-hydrogenation of 2-methyl-3-butyn-2-ol. D. functional theory calcns. support the decreased adsorption energy of palladium upon doping with carbon and reveal the potentially significant impact of oxygen-contg. defects. The findings demonstrate the importance of understanding the metal-carrier interaction to optimize the catalytic efficiency of SAHCs.
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  88. 88
    Gombos, R.; Nagyházi, B.; Joó, F. Hydrogenation of α,β-unsaturated aldehydes in aqueous media with a water-soluble Pd(II)-sulfosalan complex catalyst. React. Kinet., Mech. Catal. 2019, 126, 439– 451,  DOI: 10.1007/s11144-018-1488-8
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    88
    Hydrogenation of α,β-unsaturated aldehydes in aqueous media with a water-soluble Pd(II)-sulfosalan complex catalyst
    Gombos, Reka; Nagyhazi, Brigitta; Joo, Ferenc
    Reaction Kinetics, Mechanisms and Catalysis (2019), 126 (1), 439-451CODEN: RKMCAJ; ISSN:1878-5190. (Springer)
    A water-sol. Pd(II)-salan complex Na2[Pd(HSS)] (HSS = sulfonated tetrahydrosalen or sulfosalan) was examd. as a hydrolytically stable catalyst for hydrogenation of various aldehydes. Na2[Pd(HSS)] was found to be a highly selective catalyst towards hydrogenation of C=C over C=O bonds in cinnamaldehyde and crotonaldehyde (used as representative α,β-unsatd. aldehydes). Kinetic measurements revealed an important role of protonation/deprotonation of one of the phenolate oxygens of the N2O2 coordination framework in the reaction mechanism. Na2[Pd(HSS)] was also found an efficient catalyst for transfer hydrogenation of aldehydes from isopropanol in the presence of various bases. The results show, for the first time, the usefulness of easily accessible, hydrolytically stable Pd(II)-salan type catalysts in aq. catalytic organometallic hydrogenations.
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  89. 89
    Zhang, Y.; Yang, X.; Zhou, Y.; Li, G.; Li, Z.; Liu, C.; Bao, M.; Shen, W. Selective hydrogenation of the C═C bond in α,β-unsaturated aldehydes and ketones over ultra-small Pd–Au clusters. Nanoscale 2016, 8, 18626– 18629,  DOI: 10.1039/C6NR07013B
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    89
    Selective hydrogenation of the C=C bond in α,β-unsaturated aldehydes and ketones over ultra-small Pd-Au clusters
    Zhang, Yifei; Yang, Xiujuan; Zhou, Yan; Li, Gao; Li, Zhimin; Liu, Chao; Bao, Ming; Shen, Wenjie
    Nanoscale (2016), 8 (44), 18626-18629CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Pd-Au clusters of 1.8 nm preferentially catalyzed the hydrogenation of the C=C bond in α,β-unsatd. aldehydes and ketones with a selectivity of ~ 99%, primarily because of the unique electronic properties and isolated Pd ensembles on the ultra-small alloyed particles.
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  90. 90
    Kuai, L.; Chen, Z.; Liu, S.; Kan, E.; Yu, N.; Ren, Y.; Fang, C.; Li, X.; Li, Y.; Geng, B. Titania supported synergistic palladium single atoms and nanoparticles for room temperature ketone and aldehydes hydrogenation. Nat. Commun. 2020, 11, 48  DOI: 10.1038/s41467-019-13941-5
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    90
    Titania supported synergistic palladium single atoms and nanoparticles for room temperature ketone and aldehydes hydrogenation
    Kuai, Long; Chen, Zheng; Liu, Shoujie; Kan, Erjie; Yu, Nan; Ren, Yiming; Fang, Caihong; Li, Xingyang; Li, Yadong; Geng, Baoyou
    Nature Communications (2020), 11 (1), 48pp.CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    A synergistic function of single-atom palladium (Pd1) and nanoparticles (PdNPs) on TiO2 for highly efficient ketone/aldehydes hydrogenation to alcs. at room temp. Compared to simple but inferior Pd1/TiO2 and PdNPs/TiO2 catalysts, more than twice activity enhancement was achieved with the Pd1+NPs/TiO2 catalyst that integrated both Pd1 and Pd NPs on mesoporous TiO2 supported, obtained by a simple but large-scaled spray pyrolysis route. The synergistic function of Pd1 and PdNPs was assigned so that the partial Pd1 dispersion contributed enough sites for the activation of C=O group while PdNPs site boosts the dissocn. of H2 mols. to H atoms. This work not only contributed a superior catalyst for ketone/aldehydes hydrogenation, but also deepens the knowledge on their hydrogenation mechanism and guides people to engineer the catalytic behaviors as needed.
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  91. 91
    Fujita, S.-i.; Mitani, H.; Zhang, C.; Li, K.; Zhao, F.; Arai, M. Pd and PdZn supported on ZnO as catalysts for the hydrogenation of cinnamaldehyde to hydrocinnamyl alcohol. Mol. Catal. 2017, 442, 12– 19,  DOI: 10.1016/j.mcat.2017.08.018
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    91
    Pd and PdZn supported on ZnO as catalysts for the hydrogenation of cinnamaldehyde to hydrocinnamyl alcohol
    Fujita, Shin-ichiro; Mitani, Haruka; Zhang, Chao; Li, Kai; Zhao, Fengyu; Arai, Masahiko
    Molecular Catalysis (2017), 442 (), 12-19CODEN: MCOADH ISSN:. (Elsevier B.V.)
    Liq. phase selective hydrogenation of cinnamaldehyde (CAL) was investigated over ZnO-supported Pd and PdZn catalysts different in the Pd loading. The former monometallic catalyst was less selective to the formation of cinnamyl alc. (COL) irresp. of the Pd loading (5 and 30 wt.-% Pd). When the Pd loading was small (5 wt.-%), PdZn catalyst (PdZn-5) indicated similar catalytic actions. However, PdZn catalyst contg. Pd in a larger content of 30 wt.-% (PdZn-30) showed different results: the COL selectivity was about 20% at low conversion but it increased with CAL conversion, reaching to >50% at a conversion of 60%. The COL selectivity was likely to change depending on the concn. of a product of hydrocinnamaldehyde (HCAL). The coadsorption of HCAL should control the orientation of CAL mols. adsorbed on the PdZn-30 catalyst. This may assist the adsorption of CAL via its aldehyde group on the surface of catalyst, resulting in an increase in the COL selectivity. Unique catalysis of PdZn-30 may result from structural features of the surface of its large PdZn particles, which are different from those of PdZn-5 having smaller PdZn particles.
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  92. 92
    Nagpure, A. S.; Gurrala, L.; Gogoi, P.; Chilukuri, S. V. Hydrogenation of cinnamaldehyde to hydrocinnamaldehyde over Pd nanoparticles deposited on nitrogen-doped mesoporous carbon. RSC Adv. 2016, 6, 44333– 44340,  DOI: 10.1039/C6RA04154J
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    92
    Hydrogenation of cinnamaldehyde to hydrocinnamaldehyde over Pd nanoparticles deposited on nitrogen-doped mesoporous carbon
    Nagpure, Atul S.; Gurrala, Lakshmiprasad; Gogoi, Pranjal; Chilukuri, Satyanarayana V.
    RSC Advances (2016), 6 (50), 44333-44340CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Palladium nanoparticles deposited on nitrogen-doped mesoporous carbon (NMC) were synthesized by simple ultrasonic-assisted method. This novel Pd-NMC catalyst was highly active and selective for the hydrogenation of cinnamaldehyde (CA) to hydrocinnamaldehyde (HCA) at room temp. (30 °C) under low H2 pressure. The nitrogen-free mesoporous carbon (MC) and activated carbon (AC) were also employed as the support for Pd in the liq.-phase hydrogenation of CA. The incorporation of nitrogen into carbon matrix remarkably enhanced the catalytic activity and C=C bond hydrogenation selectivity (HCA selectivity of 93% with 100% CA conversion for Pd-NMC) in CA hydrogenation compared to the catalysts with no nitrogen (HCA selectivity of 66 and 47% for Pd-MC and Pd-AC, resp.). Moreover, Pd-NMC catalyst demonstrated an excellent recyclability without any loss in activity and HCA selectivity when it was reused for six times. The superior catalytic performance of Pd-NMC catalyst in CA hydrogenation is attributed to the small size of Pd nanoparticles due to presence of high nitrogen content (11.6 wt%) and mesoporous nature of NMC support.
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  93. 93
    Dong, S.; Liu, Z.; Liu, R.; Chen, L.; Chen, J.; Xu, Y. Visible-Light-Induced Catalytic Transfer Hydrogenation of Aromatic Aldehydes by Palladium Immobilized on Amine-Functionalized Iron-Based Metal–Organic Frameworks. ACS Appl. Nano. Mater. 2018, 1, 4247– 4257,  DOI: 10.1021/acsanm.8b01039
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    93
    Visible-Light-Induced Catalytic Transfer Hydrogenation of Aromatic Aldehydes by Palladium Immobilized on Amine-Functionalized Iron-Based Metal-Organic Frameworks
    Dong, Shenghong; Liu, Zhen; Liu, Ruihan; Chen, Limin; Chen, Jinzhu; Xu, Yisheng
    ACS Applied Nano Materials (2018), 1 (8), 4247-4257CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    Visible-light-induced selective transfer hydrogenation of arom. aldehyde to the corresponding alc. was achieved by using Pd nanocatalyst supported on amine-functionalized iron-based metal-org. frameworks [Pd/MIL-101(Fe)-NH2] with triethylamine (TEA) as an electron donor and HCOOH as a proton source. The Pd/MIL-101(Fe)-NH2, obtained by an in situ photodeposition method, showed homogeneously and highly dispersed Pd nanoparticles (NPs) with a uniform size throughout the MIL-101(Fe)-NH2 support due to an effective stabilization role of amine groups on the backbone linkage of MIL-101(Fe)-NH2. The resulting Pd/MIL-101(Fe)-NH2 exhibited excellent catalytic performance toward a visible-light-induced transfer hydrogenation of benzaldehyde by producing a benzyl alc. yield of 77% with a full benzaldehyde conversion in the presence of TEA-HCOOH. In addn. to benzaldehyde, biomass-based renewable platform mols. such as furfural and 5-hydroxymethylfurfural (HMF) were successively converted into the corresponding alcs. with the yields of 29% for furfuryl alc. and 27% for 2,5-dihydroxymethylfuran (DHMF), resp., which are the highest yields reported so far by visible-light-induced transfer hydrogenation method. Our exptl. investigation reveals that a preliminary photoirradn. promotes in situ photodeposition of Pd salt [MIL-101(Fe)-NH3]+·1/2[PdCl4]2- to form Pd catalyst Pd/MIL-101(Fe)-NH2 in the presence of TEA-HCOOH, and a further photoirradn. successively triggers Pd/MIL-101(Fe)-NH2-promoted transfer hydrogenation of aldehyde, again, with the help of TEA-HCOOH. Our theor. research based on d. functional theory (DFT) further confirms a dual function of amine group in the Pd/MIL-101(Fe)-NH2 for Pd NPs stabilization as well as for enhancement of the electron d. of the Pd center upon light adsorption. The photocatalytic system of Pd nanocatalyst and TEA-HCOOH thus demonstrates an environmentally friendly and efficient strategy for aldehyde hydrogenation by using renewable solar energy as a driving force.
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  94. 94
    Liao, W.; Zhu, Z.; Chen, N.; Su, T.; Deng, C.; Zhao, Y.; Ren, W.; Lü, H. Highly active bifunctional Pd-Co9S8/S-CNT catalysts for selective hydrogenolysis of 5-hydroxymethylfurfural to 2,5-dimethylfuran. Mol. Catal. 2020, 482, 110756  DOI: 10.1016/j.mcat.2019.110756
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    94
    Highly active bifunctional Pd-Co9S8/S-CNT catalysts for selective hydrogenolysis of 5-hydroxymethylfurfural to 2,5-dimethylfuran
    Liao, Weiping; Zhu, Zhiguo; Chen, Naimeng; Su, Ting; Deng, Changliang; Zhao, Yuchao; Ren, Wanzhong; Lu, Hongying
    Molecular Catalysis (2020), 482 (), 110756CODEN: MCOADH ISSN:. (Elsevier B.V.)
    A series of Pd-Co bimetallic catalysts were smoothly synthesized using sulfur-modified carbon nanotubes (S-CNT) as support by impregnation method. Those catalysts were characterized by XRD, XPS, TEM, SEM, H2-TPR, TGA, and nitrogen adsorption-desorption. Selective catalytic hydrogenolysis of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF) was realized over these synthesized sulfur-modified Pd-Co bimetallic supported on carbon nanotubes (PdCox/S-CNT) under mild conditions. Among them, PdCo8/S-CNT catalyst possessed extremely active and selective for the prodn. of DMF, giving 96.0% conversion of HMF with 83.7% selectivity to DMF at the temp. of 120°C with 0.3 MPa H2 for 13 h. It is found that the formed Co9S8 species are related to the high catalytic activity in PdCo8/S-CNT. Control expts. for evaluating the roles of palladium and Co9S8 revealed that Pd catalyzed the hydrogenation of the aldehyde group in HMF, whereas Co9S8 was beneficial for hydrogenolysis of the hydroxy group in HMF. The remarkable hydrogenolysis/hydrogenation performance of the bifunctional Pd-Co9S8/S-CNT was due to support-enhanced adsorption effect and the effective synergy between highly dispersed metallic Pd and the Co9S8 nanoparticles.
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  95. 95
    Deng, Q.; Wen, X.; Zhang, P. Pd/Cu-MOF as a highly efficient catalyst for synthesis of cyclopentanone compounds from biomass-derived furanic aldehydes. Catal. Commun. 2019, 126, 5– 9,  DOI: 10.1016/j.catcom.2019.04.008
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    95
    Pd/Cu-MOF as a highly efficient catalyst for synthesis of cyclopentanone compounds from biomass-derived furanic aldehydes
    Deng, Qiang; Wen, Xiaohan; Zhang, Ping
    Catalysis Communications (2019), 126 (), 5-9CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
    Herein, a series of Pd nanoparticles supported on Cu-MOFs (Cu3(BTC)2, FeCu-DMC) with pure Lewis acidity are synthesized for hydrogenative ring-rearrangement reaction of furanic aldehydes (furfural, 5-hydroxymethyl furfural) to cyclopentanone compds. (cyclopentanone, 3-hydroxymethyl cyclopentanone). For both furfural and 5-hydroxymethyl furfural, the hydrogenation rate catalyzed by the Pd/Cu-BTC is six times faster than Pd/FeCu-DMC, due to the higher dispersion of Pd nanoparticles. Meanwhile, Pd/Cu-BTC with strong acidity for the selectivity of cyclopentanone compds. is more than 90%, while as for Pd/FeCu-DMC, furanic alcs. (furfuryl alc., 2,5-bis(hydroxymethyl)furan) are only actually be obtained because weak acidity cannot incur subsequent hydrolysis reaction.
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  96. 96
    Chen, P.; Li, W.; Wang, Y. Atmospheric hydrogenation of α, β-unsaturated ketones catalyzed by highly efficient and recyclable Pd nanocatalyst. Catal. Commun. 2019, 125, 10– 14,  DOI: 10.1016/j.catcom.2019.03.008
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    96
    Atmospheric hydrogenation of α,β-unsaturated ketones catalyzed by highly efficient and recyclable Pd nanocatalyst
    Chen, Pu; Li, Wenjiang; Wang, Yanhua
    Catalysis Communications (2019), 125 (), 10-14CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
    A thermoregulated phase-transfer Pd nanocatalyst was explored firstly and shown to be highly efficient and recyclable in the atm. hydrogenation of α,β-unsatd. ketones. Under optimized reaction conditions, the conversion of chalcone and the selectivity of dihydrochalcone were 99% and 98%, resp. The catalyst can be easily sepd. from the product and used directly for four times without evident loss in activity and selectivity. The turnover frequency (TOF) for the atm. hydrogenation of chalcone was 870 h-1, which to the best of knowledge was the highest value ever reported among transition metal nanocatalysts.
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  97. 97
    Han, J.; Kim, Y.-H.; Jang, H.-S.; Hwang, S.-Y.; Jegal, J.; Kim, J. W.; Lee, Y.-S. Heterogeneous zirconia-supported ruthenium catalyst for highly selective hydrogenation of 5-hydroxymethyl-2-furaldehyde to 2,5-bis(hydroxymethyl)furans in various n-alcohol solvents. RSC Adv. 2016, 6, 93394– 93397,  DOI: 10.1039/C6RA18016G
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    97
    Heterogeneous zirconia-supported ruthenium catalyst for highly selective hydrogenation of 5-hydroxymethyl-2-furaldehyde to 2,5-bis(hydroxymethyl)furans in various n-alcohol solvents
    Han, Jusung; Kim, Yo-Han; Jang, Hyung-Seok; Hwang, Sung-Yeon; Jegal, Jonggeon; Kim, Jung Won; Lee, Yoon-Sik
    RSC Advances (2016), 6 (96), 93394-93397CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    5-Hydroxymethyl-2-furaldehyde (HMF) was hydrogenated to 2,5-bis(hydroxymethyl)furan (BHMF) (>99% yield) in various n-alc. solvents using a Ru(OH)x/ZrO2 catalyst. The TON and TOF for the hydrogenation were calcd. to be 912 and 304 h-1, resp.
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  98. 98
    Pârvulescu, V. I.; Pârvulescu, V.; Endruschat, U.; Filoti, G.; Wagner, F. E.; Kübel, C.; Richards, R. Characterization and Catalytic-Hydrogenation Behavior of SiO2-Embedded Nanoscopic Pd, Au, and Pd–Au Alloy Colloids. Chem.─Eur. J. 2006, 12, 2343– 2357,  DOI: 10.1002/chem.200500971
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    98
    Characterization and catalytic-hydrogenation behavior of SiO2-embedded nanoscopic Pd, Au, and Pd-Au alloy colloids
    Parvulescu, Vasile I.; Parvulescu, Viorica; Endruschat, Uwe; Filoti, George; Wagner, Friedrich E.; Kubel, Christian; Richards, Ryan
    Chemistry - A European Journal (2006), 12 (8), 2343-2357CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Nanoparticle Colloids embedded in a silica sol-gel matrix were prepd. by using Pd-48% Au alloy colloids, and Pd and Au colloids stabilized with tetraalkylammonium bromide following a modified sol-gel procedure with THF as the solvent. Tetraethoxysilicate (TEOS) was used as the precursor for the silica support. The molar compn. of the sol was TEOS/THF/H2O/HCl = 1:3.5:4:0.05 for the Pd-Au alloy and TEOS/THF/H2O/HCl = 1:4.5:4:0.02 for Pd and Au monometallic systems. After refluxing, the colloid was added as a 4.5 wt.% soln. in THF for Pd-Au, 10.2 wt.% soln. in THF for Pd and 8.4 wt.% soln. in THF for Au at room temp. The gelation was carried out with vigorous stirring for 4 days under an Ar atm. Following these procedures, Pd-Au/SiO2 catalysts with 0.6 and 1 wt.% metal, and monometallic Pd-SiO2 and Au-SiO2 catalysts with 1 wt.% metal were prepd. These were further treated by (1) simple drying (2) calcining in air at 723 K (3) reducing in hydrogen at 723 K (4) extg. the surfactant by using an ethanol-heptane azeotropic mixt. The catalysts were characterized by nitrogen adsorption-desorption isotherms at 77 K, H2 chemisorption measurements, solid-state 1H, 13C, 29Si-CP/MAS-NMR spectroscopy, powder x-ray diffraction, SAXS, XPS, TEM and 197Au Mossbauer spectroscopy. The phys. characterization by a combination of these techniques has shown that the size and microstructural characteristics of the Pd-Au colloid precursor are preserved when embedded in an SiO2 matrix. Catalytic tests were carried out in selective hydrogenation of 3-hexyn-1-ol, cinnamaldehyde, and styrene. These data showed evidence that alloying Pd with Au in in the colloids leads to enhanced activity and most importantly to improved selectivity. Also, the combination of the two metals resulted in catalysts that were stable against poisoning, as was evidenced for the hydrogenation of styrene in the presence of thiophene.
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  99. 99
    Cattaneo, S.; Freakley, S. J.; Morgan, D. J.; Sankar, M.; Dimitratos, N.; Hutchings, G. J. Cinnamaldehyde hydrogenation using Au–Pd catalysts prepared by sol immobilisation. Catal. Sci. Technol. 2018, 8, 1677– 1685,  DOI: 10.1039/C7CY02556D
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    99
    Cinnamaldehyde hydrogenation using Au-Pd catalysts prepared by sol immobilisation
    Cattaneo, Stefano; Freakley, Simon J.; Morgan, David J.; Sankar, Meenakshisundaram; Dimitratos, Nikolaos; Hutchings, Graham J.
    Catalysis Science & Technology (2018), 8 (6), 1677-1685CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    We report the catalytic performance of Au-Pd nanoparticles prepd. via a sol immobilization technique for the catalytic hydrogenation of cinnamaldehyde under mild reaction conditions. We synthesized a series of bimetallic Au-Pd colloidal supported nanoparticles with different Au : Pd molar ratios and optimized the exptl. parameters to achieve the best catalyst performance. The optimum catalytic activity for the hydrogenation of cinnamaldehyde was obsd. for Au50Pd50/TiO2 (with a Au : Pd molar ratio of 1 : 1), while the monometallic Pd/TiO2 was the most selective towards hydrocinnamaldehyde. The catalysts have been structurally characterized and FTIR anal. showed that the presence of adsorbed carbonyl surface species in used catalyst materials is coupled with Pd leaching, which is the main reason for catalyst deactivation. The effect of calcination on the most active Au-Pd/TiO2 was studied in the range 110-400 °C and a direct correlation between the rise in calcination temp. and catalyst stability and selectivity was obsd. These results emphasize the importance of tuning the Au-Pd molar ratio and understanding the metal-support interaction of catalysts synthesized for hydrogenation reactions, such as cinnamaldehyde hydrogenation.
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  100. 100
    Schoenbaum, C. A.; Schwartz, D. K.; Medlin, J. W. Controlling the surface environment of heterogeneous catalysts using self-assembled monolayers. Acc. Chem. Res. 2014, 47, 1438– 1445,  DOI: 10.1021/ar500029y
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    100
    Controlling the Surface Environment of Heterogeneous Catalysts Using Self-Assembled Monolayers
    Schoenbaum, Carolyn A.; Schwartz, Daniel K.; Medlin, J. Will
    Accounts of Chemical Research (2014), 47 (4), 1438-1445CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Modification approaches for tuning self-assembled monolayer structure to improve catalytic performance for hydrogenation reactions on palladium and platinum catalysts are summarized. Each approach serves to direct selectivity by tuning a particular aspect of the system including the availability of specific active sites (active-site selection), intermol. interactions between the reactants and modifiers (mol. recognition), and general steric or crowding effects. The tail moiety can be tuned to control the d. of SAM modifiers on the surface. IR spectra of adsorbed CO probe mols. reveal that increasing the d. of the thiols restricts the availability of contiguous active sites on catalyst terraces while maintaining accessibility to sites located at particle edges and steps. This technique was utilized to direct selectivity for the hydrogenation of furfural. Results obtained from SAM coatings with different surface densities indicated that, for this reaction, formation of the desirable products occurs primarily at particle edges and steps, whereas the undesired pathway occurs on particle terrace sites. As an alternative approach, the tail structure of the SAM precursor can be tuned to promote specific intermol. interactions between the modifier and reactant in order to position reactant mols. in a desired orientation. This technique was utilized for the hydrogenation of cinnamaldehyde, which contains an arom. Ph moiety. By using a phenyl-contg. SAM modifier with an appropriate tether length, > 90% selectivity toward reaction of the aldehyde group was achieved. In contrast, employing a modifier where the Ph moiety was closer to the catalyst surface biased selectivity toward the hydrogenation of the C=C bond due to reorienting the mol. to a more "lying down" conformation. In addn. to approaches that target specific interactions between the reactant and modified catalyst, we have demonstrated the use of SAMs to impose a steric or blocking effect, for example, during the hydrogenation of polyunsatd. fatty acids. The SAMs facilitated hydrogenation of polyunsatd. to monounsatd. fatty acids but inhibited further hydrogenation to the completely satd. species due to the sterically hindered, single "kink" shape of the monounsatd. product.
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  101. 101
    Wu, B.; Huang, H.; Yang, J.; Zheng, N.; Fu, G. Selective Hydrogenation of α,β-Unsaturated Aldehydes Catalyzed by Amine-Capped Platinum-Cobalt Nanocrystals. Angew. Chem., Int. Ed. 2012, 51, 3440– 3443,  DOI: 10.1002/anie.201108593
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    Selective Hydrogenation of α,β-Unsaturated Aldehydes Catalyzed by Amine-Capped Platinum-Cobalt Nanocrystals
    Wu, Binghui; Huang, Huaqi; Yang, Jing; Zheng, Nanfeng; Fu, Gang
    Angewandte Chemie, International Edition (2012), 51 (14), 3440-3443, S3440/1-S3440/15CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Selective hydrogenation of α,β-unsatd. aldehydes catalyzed by amine-capped platinum-cobalt nanocrystals is discussed.
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  102. 102
    Guo, M.; Li, H.; Ren, Y.; Ren, X.; Yang, Q.; Li, C. Improving Catalytic Hydrogenation Performance of Pd Nanoparticles by Electronic Modulation Using Phosphine Ligands. ACS Catal. 2018, 8, 6476– 6485,  DOI: 10.1021/acscatal.8b00872
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    102
    Improving Catalytic Hydrogenation Performance of Pd Nanoparticles by Electronic Modulation Using Phosphine Ligands
    Guo, Miao; Li, He; Ren, Yiqi; Ren, Xiaomin; Yang, Qihua; Li, Can
    ACS Catalysis (2018), 8 (7), 6476-6485CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Tuning the activity and selectivity of metal nanoparticles (NPs) is a long-term pursuit in the field of catalysis. Herein, we report successfully improving both the activity and chemoselectivity of Pd NPs (1.1 nm) with triphenylphosphine (PPh3) cross-linked in the nanopore of FDU-12. The electron-donating effect of PPh3 increases the surface electronic d. of Pd NPs and weakens the Pd-H bond, as evidenced by the results of XPS, in situ FT-IR adsorption of CO, and H2-D2 exchange reactions. Consequently, Pd NPs modified with PPh3 obtain >99% selectivity to 1-phenylethanol in acetophenone hydrogenation and 94% selectivity to styrene in phenylacetylene hydrogenation. Furthermore, the activity of Pd NPs is enhanced and suppressed by PPh3, resp., in the hydrogenation of electrophilic nitro compds. and nucleophilic carbonyl substrates. Our primary results shed some light on judiciously choosing org. ligands for modifying the catalytic performance of metal NPs toward specific chem. transformations.
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  103. 103
    Pang, S. H.; Schoenbaum, C. A.; Schwartz, D. K.; Medlin, J. W. Directing reaction pathways by catalyst active-site selection using self-assembled monolayers. Nat. Commun. 2013, 4, 2448  DOI: 10.1038/ncomms3448
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    Directing reaction pathways by catalyst active-site selection using self-assembled monolayers
    Pang Simon H; Schoenbaum Carolyn A; Schwartz Daniel K; Medlin J Will
    Nature communications (2013), 4 (), 2448 ISSN:.
    One key route for controlling reaction selectivity in heterogeneous catalysis is to prepare catalysts that exhibit only specific types of sites required for desired product formation. Here we show that alkanethiolate self-assembled monolayers with varying surface densities can be used to tune selectivity to desired hydrogenation and hydrodeoxygenation products during the reaction of furfural on supported palladium catalysts. Vibrational spectroscopic studies demonstrate that the selectivity improvement is achieved by controlling the availability of specific sites for the hydrogenation of furfural on supported palladium catalysts through the selection of an appropriate alkanethiolate. Increasing self-assembled monolayer density by controlling the steric bulk of the organic tail ligand restricts adsorption on terrace sites and dramatically increases selectivity to desired products furfuryl alcohol and methylfuran. This technique of active-site selection simultaneously serves both to enhance selectivity and provide insight into the reaction mechanism.
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    Kahsar, K. R.; Schwartz, D. K.; Medlin, J. W. Control of Metal Catalyst Selectivity through Specific Noncovalent Molecular Interactions. J. Am. Chem. Soc. 2014, 136, 520– 526,  DOI: 10.1021/ja411973p
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    Control of Metal Catalyst Selectivity through Specific Noncovalent Molecular Interactions
    Kahsar, Karl R.; Schwartz, Daniel K.; Medlin, J. Will
    Journal of the American Chemical Society (2014), 136 (1), 520-526CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The specificity of chem. reactions conducted over solid catalysts can potentially be improved by utilizing noncovalent interactions to direct reactant binding geometry. Here we apply thiolate self-assembled monolayers (SAMs) with an appropriate structure to Pt/Al2O3 catalysts to selectively orient the reactant mol. cinnamaldehyde in a configuration assocd. with hydrogenation to the desired product cinnamyl alc. While nonspecific effects on the surface active site were shown to generally enhance selectivity, specific arom. stacking interactions between the Ph ring of cinnamaldehyde and phenylated SAMs allowed tuning of reaction selectivity without compromising the rate of desired product formation. IR spectroscopy showed that increased selectivity was a result of favorable orientation of the reactant on the catalyst surface. In contrast, hydrogenation of an unsatd. aldehyde without a Ph ring showed a nontunable improvement in selectivity, indicating that thiol SAMs can improve reaction selectivity through a combination of nonspecific surface effects and ligand-specific near-surface effects.
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    Alexander, A.-M.; Hargreaves, J. S. J. Alternative catalytic materials: carbides, nitrides, phosphides and amorphous boron alloys. Chem. Soc. Rev. 2010, 39, 4388– 4401,  DOI: 10.1039/b916787k
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    Alternative catalytic materials: carbides, nitrides, phosphides and amorphous boron alloys
    Alexander, Anne-Marie; Hargreaves, Justin S. J.
    Chemical Society Reviews (2010), 39 (11), 4388-4401CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review; catalysts generated by the addn. of carbon, nitrogen or phosphorus to transition metals have interesting properties and potential applications. The addn. of carbon, nitrogen or phosphorus can lead to substantial modification of the catalytic efficacy of the parent metal and some carbides and nitrides are claimed to be comparable to noble metals in their behavior. Amorphous boron transition metal alloys are also a class of interesting catalyst, although their structures and phase compn. are more difficult to define. In this crit. review, the prepn. of these catalysts is described and brief details of their application given. To date, attention has largely centered upon the application of these materials as alternatives for existing catalysts. However, novel approaches towards their utilization can be envisaged. For example, the extent to which it is possible to utilize the "activated" carbon and nitrogen species within the host lattices of carbides and nitrides, resp., as a reactant remains largely unexplored.
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  106. 106
    Zhang, S.; Chang, C. R.; Huang, Z. Q.; Li, J.; Wu, Z.; Ma, Y.; Zhang, Z.; Wang, Y.; Qu, Y. High Catalytic Activity and Chemoselectivity of Sub-nanometric Pd Clusters on Porous Nanorods of CeO2 for Hydrogenation of Nitroarenes. J. Am. Chem. Soc. 2016, 138, 2629– 2637,  DOI: 10.1021/jacs.5b11413
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    106
    High Catalytic Activity and Chemoselectivity of Sub-nanometric Pd Clusters on Porous Nanorods of CeO2 for Hydrogenation of Nitroarenes
    Zhang, Sai; Chang, Chun-Ran; Huang, Zheng-Qing; Li, Jing; Wu, Zhemin; Ma, Yuanyuan; Zhang, Zhiyun; Wang, Yong; Qu, Yongquan
    Journal of the American Chemical Society (2016), 138 (8), 2629-2637CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Sub-nanometric Pd clusters on porous nanorods of CeO2 (PN-CeO2) with a high Pd dispersion of 73.6% exhibit the highest catalytic activity and best chemoselectivity for hydrogenation of nitroarenes to date. For hydrogenation of 4-nitrophenol, the catalysts yield a TOF of ∼44059 h-1 and a chemoselectivity to 4-aminophenol of >99.9%. The superior catalytic performance can be attributed to a cooperative effect between the highly dispersed sub-nanometric Pd clusters for hydrogen activation and unique surface sites of PN-CeO2 with a high concn. of oxygen vacancy for an energetically and geometrically preferential adsorption of nitroarenes via nitro group. The high concn. of surface defects of PN-CeO2 and large Pd dispersion contribute to the enhanced catalytic activity for the hydrogenation reactions. The high chemoselectivity is mainly governed by the high Pd dispersion on the support. The catalysts also deliver high catalytic activity and selectivity for nitroaroms. with various reducible substituents into the corresponding aminoarenes.
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    Wang, J.; Hu, L.; Cao, X.; Lu, J.; Li, X.; Gu, H. Catalysis by Pd nanoclusters generated in situ of high-efficiency synthesis of aromatic azo compounds from nitroaromatics under H2 atmosphere. RSC Adv. 2013, 3, 4899,  DOI: 10.1039/c3ra23004j
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    107
    Catalysis by Pd nanoclusters generated in situ of high-efficiency synthesis of aromatic azo compounds from nitroaromatics under H2 atmosphere
    Wang, Jiaqing; Hu, Lei; Cao, Xueqin; Lu, Jianmei; Li, Xinming; Gu, Hongwei
    RSC Advances (2013), 3 (15), 4899-4902CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    A facile and efficient catalytic system based on Pd nanoclusters generated in situ from Pd(acac)2 was developed for the synthesis of various arom. azo compds. by hydrogenation of the corresponding nitroaroms., using H2 as the sole reductant, under mild reaction conditions.
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    Hu, L.; Cao, X.; Shi, L.; Qi, F.; Guo, Z.; Lu, J.; Gu, H. A Highly Active Nano-Palladium Catalyst for the Preparation of Aromatic Azos under Mild Conditions. Org. Lett. 2011, 13, 5640– 5643,  DOI: 10.1021/ol202362f
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    A Highly Active Nano-Palladium Catalyst for the Preparation of Aromatic Azos under Mild Conditions
    Hu, Lei; Cao, Xue-Qing; Shi, Lin-Yan; Qi, Fen-Qiang; Guo, Zhi-Qiang; Lu, Jian-Mei; Gu, Hong-Wei
    Organic Letters (2011), 13 (20), 5640-5643CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
    A worm-like Pd nanocatalyst has been prepd. and used in the prepn. of azo compds. from nitroaroms. under mild reaction conditions. This highly dispersible nano-Pd catalyst shows high activity toward the synthesis of both sym. arom. azo compds. and a range of asym. arom. azo compds.
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    Xu, S.; Xi, X.; Shi, J.; Cao, S. A homogeneous catalyst made of poly 4-vinylpyridine-co-N-vinylpyrrolidone -Pd 0 complex for hydrogenation of aromatic nitro compounds. J. Mol. Catal. A: Chem. 2000, 160, 287– 292,  DOI: 10.1016/S1381-1169(00)00263-6
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    109
    A homogeneous catalyst made of poly(4-vinylpyridine-co-N-vinylpyrrolidone)-Pd(0) complex for hydrogenation of aromatic nitro compounds
    Xu, S.; Xi, X.; Shi, J.; Cao, S.
    Journal of Molecular Catalysis A: Chemical (2000), 160 (2), 287-292CODEN: JMCCF2; ISSN:1381-1169. (Elsevier Science B.V.)
    Poly(4-vinylpyridine-co-N-vinylpyrrolidone)(VPy-co-NVP) and its palladium complex (VPy-NVP-Pd) were prepd. The palladium complex was used as catalyst for the hydrogenation of some nitro aroms. The molar content of VPy units in VPy-co-NVP was detd. as 31.25% by 1H NMR. VPy-NVP-Pd can be easily resolved in ethanol forming a homogeneous catalytic hydrogenation system together with substrates. The optimum catalytic activity for hydrogenation of nitrobenzene appeared when VPy/Pd molar ratio was 2. The catalytic behavior of the catalyst was found to be greatly affected by the type and concn. of added alkalies. The highest hydrogenation rate for nitrobenzene was found in a 0.1 mol/l ethanol soln. of potassium hydroxide. The catalytic stability was examd. by using nitrobenzene and 4-nitroanisole as substrates.
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    Li, J.; Shi, X.-Y.; Bi, Y.-Y.; Wei, J.-F.; Chen, Z.-G. Pd Nanoparticles in Ionic Liquid Brush: A Highly Active and Reusable Heterogeneous Catalytic Assembly for Solvent-Free or On-Water Hydrogenation of Nitroarene under Mild Conditions. ACS Catal. 2011, 1, 657– 664,  DOI: 10.1021/cs200105u
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    110
    Pd Nanoparticles in Ionic Liquid Brush: A Highly Active and Reusable Heterogeneous Catalytic Assembly for Solvent-Free or On-Water Hydrogenation of Nitroarene under Mild Conditions
    Li, Jing; Shi, Xian-Ying; Bi, Yuan-Yuan; Wei, Jun-Fa; Chen, Zhan-Guo
    ACS Catalysis (2011), 1 (6), 657-664CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A heterogeneous catalytic assembly of Pd nanoparticles-in-SiO2 supported ionic liq. brushes has been developed. The catalytic assembly showed uniform Pd particles size distribution. The catalyst allowed the hydrogenation of nitrobenzenes to arylamines to proceed under solvent-free or in neat water, room temp., and normal pressure, giving nearly 100% yield and selectivity. A clean and energy-efficient process can be thus created over the robust catalysts. The process employing such a catalytic assembly overcomes simultaneously the disadvantages of the requirement for org. solvents in the liq. hydrogenation process and for high temp. in the vapor hydrogenation process.
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    Zhang, K.; Hong, K.; Suh, J. M.; Lee, T. H.; Kwon, O.; Shokouhimehr, M.; Jang, H. W. Facile synthesis of monodispersed Pd nanocatalysts decorated on graphene oxide for reduction of nitroaromatics in aqueous solution. Res. Chem. Intermed. 2019, 45, 599– 611,  DOI: 10.1007/s11164-018-3621-8
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    Research on Chemical Intermediates (2019), 45 (2), 599-611CODEN: RCINEE; ISSN:0922-6168. (Springer)
    We synthesized the reproducible heterogeneous catalyst of graphene oxide (GO)-supported palladium nanoparticles (NPs) via a simple and green process. The structure, morphol. and physicochem. properties of the synthesized heterogeneous catalyst were characterized by the latest techniques such as high-resoln. transmission electron microscopy (TEM), scanning TEM, energy-dispersive x-ray spectroscopy, x-ray diffraction anal., and XPS. The GO-supported Pd NPs (Pd/GO nanocatalyst) exhibited excellent catalytic activity for the redn. of nitroaroms. to aminoaroms. in aq. sodium borohydride. The nitroaroms. were converted to corresponding aminoaroms. with high yields (up to 99%) using Pd/GO nanocatalyst in aq. soln. The hybrid heterogeneous catalyst showed 83% of conversion after six cycles in the redn. of nitrobenzene to aminobenzene. These features ensured the high catalytic activity of the introduced graphene oxide supported Pd nanocatalysts. Graphical abstr.: [Figure not available: see fulltext.].
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    Tetrahedron Letters (2015), 56 (25), 3913-3915CODEN: TELEAY; ISSN:0040-4039. (Elsevier Ltd.)
    Palladium nanoparticles stabilized by alkyne derivs. catalyzed the hydrogenation of nitroarenes to the aryl amines in homogeneous phase. The reaction of nitrobenzene proceeded smoothly with a substrate-to-palladium molar ratio (S/Pd) of 51,000 under 8 atm of H2. The reaction under 1 atm of H2 with an S/Pd of 1030 was completed in 4 h. A series of substituted nitroarenes, including 4-acetyl- and 4-formylnitrobenzenes, were converted to the aryl amines with high chemoselectivity.
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    Green Chemistry (2012), 14 (12), 3415-3422CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
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    Gabriel, C. M.; Parmentier, M.; Riegert, C.; Lanz, M.; Handa, S.; Lipshutz, B. H.; Gallou, F. Sustainable and Scalable Fe/ppm Pd Nanoparticle Nitro Group Reductions in Water at Room Temperature. Org. Process Res. Dev. 2017, 21, 247– 252,  DOI: 10.1021/acs.oprd.6b00410
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    Sustainable and Scalable Fe/ppm Pd Nanoparticle Nitro Group Reductions in Water at Room Temperature
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    Organic Process Research & Development (2017), 21 (2), 247-252CODEN: OPRDFK; ISSN:1083-6160. (American Chemical Society)
    An operationally simple and general process for the safe and selective redn. of nitro groups utilizing ppm Pd supported on Fe nanomaterials in aq. soln. of designer surfactant TPGS-750-M has been developed and successfully carried out at a 100 mmol scale. Preferred use of KBH4 as the hydride source, at ambient temp. and pressure, lends this process suitable for a std. reaction vessel alleviating the need for specialized hydrogenation equipment. Calorimetry data parallel those expected for a classical nitro group redn. when measuring the heat of reaction (-896 to -850 kJ/mol).
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    Zhang, K.; Cha, J. H.; Jeon, S. Y.; Kirlikovali, K. O.; Ostadhassan, M.; Rasouli, V.; Farha, O. K.; Jang, H. W.; Varma, R. S.; Shokouhimehr, M. Pd modified Prussian blue frameworks: Multiple electron transfer pathways for improving catalytic activity toward hydrogenation of nitroaromatics. Mol. Catal. 2020, 492, 110967,  DOI: 10.1016/j.mcat.2020.110967
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    Molecular Catalysis (2020), 492 (), 110967CODEN: MCOADH ISSN:. (Elsevier B.V.)
    Prussian blue analogs (PBAs) exhibit potential as low-cost and eco-friendly nanocatalysts that can be fabricated with ease. However, the PBA framework structure suffers from poor electronic cond., which limits the catalytic efficiency for this class of materials. Noble metals represent an alternative class of materials that display inherent catalytic activity but suffer from aggregation, ultimately reducing the amt. of accessible catalytic sites. Herein, we demonstrate a combinatory approach that circumvents the known disadvantages with these classes of catalytic materials in which PBA-supported nanocatalysts were synthesized. These composite materials exhibit excellent catalytic activity for the redn. of nitroaroms. to aminoaroms. while displaying long-term cycling stability, which is attributed to the availability of multiple electron transfer pathways. Overall, this work opens the study on the assembly of PBA-supported heterogeneous nanocatalysts and potentially paves the way toward future applications.
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    Catalysis Letters (2020), 150 (8), 2309-2321CODEN: CALEER; ISSN:1011-372X. (Springer)
    In this study, we synthesized a robust and sustainable Pd/SiO2 nanospheres catalyst. Further, its catalytic activity was demonstrated for the direct reductive coupling of nitroarenes under mild conditions. While the reaction with Pd nanoparticles on other supporting materials such as modifed carbon materials and TiO2, under similar conditions, resulted formation of amines exclusively. Therefore, it was confirmed that the SiO2 was found to be the best supporting material towards the selective reductive coupling of nitroarenes. Also, the catalyst could be recycled up to five cycles with a marginal loss of product yield (<2% yield).
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    In this paper, monodisperse Pd nanocrystals were immobilized on previously reported carbon nanospheres via in situ adsorption and redn. In this protocol, no excess reductant and capping reagents were necessary, which made the surface of the as-prepd. nanocatalysts very clean. Using sodium tetrachloropalladate(II) as the metal precursor yielded palladium nanocrystals with a size around 5 nm regardless of the metal loading, while the use of palladium chloride resulted in a size increase to 18.1 nm. Moreover, the additives used during prepn. have been proven to be of great importance in controlling the av. particle size. It was suggested that the pattern of the adsorbed palladium ions or the surface environment of support was greatly influenced. Strong adsorption of the palladium ions on the carbon spheres led to a decrease in size. The nanocrystals exhibited excellent catalytic activity for transfer hydrogenation under ambient conditions. The conversion was 83.1% to 100% for several nitroaroms. with moderate to excellent selectivity. More importantly, these nanocatalysts are promising for renewable catalysis owing to their sustainable support, green catalyst fabrication and ease of handling.
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    ACS Applied Materials & Interfaces (2016), 8 (15), 9881-9888CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Single- or few-layered h-BN nanosheets (BNNSs) are analogous to graphene and possess unique properties. However, their technol. applications were severely hindered by the low prodn. efficiency of BNNSs. We reported here a study in which BNNSs were efficiently produced by exfoliating bulk h-BN powder in thionyl chloride without using any dispersion agents. The BNNSs yield was as high as 20%, and it could be doubled through the second round of exfoliation of the h-BN ppt. Microscopic results revealed that the BNNSs generally consisted of 3-20 layers. Pd nanoparticles were successfully immobilized and uniformly distributed on BNNS surfaces through the deposition-pptn. method. The resultant Pd-BNNS catalyst exhibited high catalytic activity and recyclability for the hydrogenation of nitro aroms., demonstrating that BNNSs served as a promising platform to fabricate heterogeneous catalysts.
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    McAllister, M. I.; Boulho, C.; Brennan, C.; Parker, S. F.; Lennon, D. Toward Sustained Product Formation in the Liquid-Phase Hydrogenation of Mandelonitrile over a Pd/C Catalyst. Org. Process Res. Dev. 2020, 24, 1112– 1123,  DOI: 10.1021/acs.oprd.0c00111
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    Toward Sustained Product Formation in the Liquid-Phase Hydrogenation of Mandelonitrile over a Pd/C Catalyst
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    Organic Process Research & Development (2020), 24 (6), 1112-1123CODEN: OPRDFK; ISSN:1083-6160. (American Chemical Society)
    The liq. phase hydrogenation of the arom. cyanohydrin mandelonitrile (C6H5CH(OH)CH2CN) over a carbon supported Pd catalyst to produce the primary amine phenethylamine (C6H5CH2CH2NH2) is investigated with respect to the transition from operation in single batch mode to repeat batch mode. While a single batch reaction returns a complete mass balance, product anal. alongside mass balance measurements for a 6 addn. repeat batch procedure shows an attenuation in the rate of product formation and an incomplete mass balance from the 4th addn. onwards. This scenario potentially hinders possible com. operation of the phenethylamine synthesis process, so it is investigated further. With ref. to a previously reported reaction scheme, the prospects of sustained catalytic performance are examd. in terms of acid concn., stirrer agitation rate, catalyst mass, and hydrogen availability. Gas-liq. mass transfer coeff. measurements indicate efficient gas-liq. transfer kinetics within the exptl. constraints of the Henry's law limitation for hydrogen soly. in the process solvent (methanol). Deviations from optimized product selectivity are attributed to mass transport constraints, specifically the transition of H2(solv) 2H(ads), which is ultimately restrained by the availability of H2(solv). Finally, in an attempt to better understand the deactivation pathways, inelastic neutron scattering measurements on a comparable industrial grade catalyst, operated in an analogous reaction in fed-batch mode, indicate the presence of an oligomeric overlayer post-reaction. This overlayer is thought to be formed via oligomerization of hydroxyimine or imine species via specific pathways that are identified within a postulated global reaction scheme.
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    Applied Catalysis, A: General (2005), 296 (2), 209-215CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    A method for selective liq.-phase heterogeneous catalytic hydrogenation of nitriles to primary amines was developed. Benzonitrile (BN) was hydrogenated to benzylamine (BA) under mild reaction conditions (30°, 6 bar), over Pd/C catalysts, in a mixt. of two immiscible solvents (e.g., water/dichloromethane) and in the presence of sodium dihydrogen phosphate (NaH2PO4). Complete conversion, high selectivity (95% to BA) and isolated yield (85-90%) could be achieved by using this process. Pure product (>99% BA-content) was prepd. without applying any special purifn. procedures.
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    Nature communications (2019), 10 (1), 4998 ISSN:.
    Hydrogenation of nitriles represents as an atom-economic route to synthesize amines, crucial building blocks in fine chemicals. However, high redox potentials of nitriles render this approach to produce a mixture of amines, imines and low-value hydrogenolysis byproducts in general. Here we show that quasi atomic-dispersion of Pd within the outermost layer of Ni nanoparticles to form a Pd1Ni single-atom surface alloy structure maximizes the Pd utilization and breaks the strong metal-selectivity relations in benzonitrile hydrogenation, by prompting the yield of dibenzylamine drastically from ∼5 to 97% under mild conditions (80 °C; 0.6 MPa), and boosting an activity to about eight and four times higher than Pd and Pt standard catalysts, respectively. More importantly, the undesired carcinogenic toluene by-product is completely prohibited, rendering its practical applications, especially in pharmaceutical industry. Such strategy can be extended to a broad scope of nitriles with high yields of secondary amines under mild conditions.
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    ACS Catalysis (2020), 10 (10), 5899-5908CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Pd is one of the most efficient metals for the hydrogenation of org. compds. However, when mols., such as nitroaroms., with several reducible functionalities, are hydrogenated, Pd, like any other very active metal, such as Ni or Pt, often behaves unselectively. One strategy to render Pd more selective is to choose the proper support. Herein, MAX phase powders of Ti3SiC2, Ti2AlC, or Ti3AlC2 can chemoselectively hydrogenate 4-nitrostyrene to 4-aminostyrene, with 100% selectivity, at ∼3-4% conversion. To boost the latter, the authors loaded Ti3SiC2 with 0.0005% Pd and increased the conversion to 100% while maintaining the 4-AS selectivity at >90%. By optimizing the Pd loading, the authors were also able to increase the turnover frequency 100-fold relative to previous literature results. The identification of this highly efficient and chemoselective system has broad implications for the design of cost-effective, earth-abundant, nontoxic, metal catalysts, with ultralow noble metal loadings.
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  5. Miao Guo, Huicong Dai, Qihua Yang. Promotion Effect of Pd in the Ru/C-Catalyzed Hydrogenation of Benzofurans. ACS Catalysis 2024, 14 (4) , 2719-2729. https://doi.org/10.1021/acscatal.3c05429
  6. Hao Ge, Yasutaka Kuwahara, Masahiro Okada, Hiromi Yamashita. Mild Hydrodeoxygenation of Aromatic Ketones by Pd/HxWO3–y with Plasmonic Features Assisted by Visible-NIR Light Irradiation. ACS Sustainable Chemistry & Engineering 2024, 12 (6) , 2162-2171. https://doi.org/10.1021/acssuschemeng.3c04978
  7. Rajat Ghalta, Arzoo Chauhan, Rajendra Srivastava. Selective Hydrogenation of Cinnamaldehyde, Furanic, and Aromatic Aldehydes over a Z-Scheme Heterojunction Photocatalyst Constituted of Pd NPs Supported on g-C3N4/ZnO Nanocomposite. ACS Applied Nano Materials 2024, 7 (1) , 1462-1476. https://doi.org/10.1021/acsanm.3c05787
  8. Jordi Ballesteros-Soberanas, Antonio Leyva-Pérez. Electron-Poor Phosphines Enable the Selective Semihydrogenation Reaction of Alkynes with Pd on Carbon Catalysts. The Journal of Physical Chemistry Letters 2023, 14 (4) , 965-970. https://doi.org/10.1021/acs.jpclett.2c03428
  9. Shuai Chen, Tianxing Yang, Hao Lu, Yanan Liu, Yufei He, Qiang Li, Junxian Gao, Junting Feng, Hong Yan, Jeffrey T. Miller, Dianqing Li. Increased Hydrogenation Rates in Pd/La-Al2O3 Catalysts by Hydrogen Transfer O(-La) Sites Adjacent to Pd Nanoparticles. ACS Catalysis 2022, 12 (24) , 15696-15706. https://doi.org/10.1021/acscatal.2c04920
  10. Wen-Zhong Jiao, Peng Yin, Lei Tong, Shi-Long Xu, Chang-Song Ma, Lu-Jie Zuo, Ao Wang, Hai-Wei Liang. Pentacoordinate Al3+ Sites Anchoring Synthesis of Palladium Intermetallic Catalysts on Al2O3 Supports. Inorganic Chemistry 2022, 61 (18) , 6706-6710. https://doi.org/10.1021/acs.inorgchem.2c00796
  11. Ourmaz Dehghani, Akbar Bolhasani, Shahin Hosseini, Ali Darvishi. Process intensification of selective acetylene hydrogenation reactor by bed configuration change: A case study of an ethylene plant. Case Studies in Chemical and Environmental Engineering 2023, 7 , 100321. https://doi.org/10.1016/j.cscee.2023.100321
  12. Yue Zhang, Chunyu Zhang, Guozhu Liu, Li Wang, Zhiyong Pan. Ce-doped SBA-15 supported Pd catalyst for efficient hydrogenation of 2-ethyl-anthraquinone. Applied Surface Science 2023, 616 , 156515. https://doi.org/10.1016/j.apsusc.2023.156515
  13. Xinxiang Cao, Ben W.-L. Jang, Jiaxue Hu, Lei Wang, Siqi Zhang. Synthetic Strategies of Supported Pd-Based Bimetallic Catalysts for Selective Semi-Hydrogenation of Acetylene: A Review and Perspectives. Molecules 2023, 28 (6) , 2572. https://doi.org/10.3390/molecules28062572
  14. Akkenapally Swamy, Kempanna S. Kanakikodi, Vasudeva Rao Bakuru, Bhavana B. Kulkarni, Sanjeev P. Maradur, Suresh Babu Kalidindi. Continuous Flow Liquid‐Phase Semihydrogenation of Phenylacetylene over Pd Nanoparticles Supported on UiO‐66(Hf) Metal‐Organic Framework. ChemistrySelect 2023, 8 (5) https://doi.org/10.1002/slct.202203926
  15. Menglu Wang, Lingling Liang, Xia Liu, Qi Sun, Mingrui Guo, Shuxing Bai, Yong Xu. Selective semi-hydrogenation of alkynes on palladium-selenium nanocrystals. Journal of Catalysis 2023, 418 , 247-255. https://doi.org/10.1016/j.jcat.2023.01.024
  16. Minglin Chen, Jinfang Kou, Haowen Ma, Yongsheng Xiang, Ping Ma, Limin Sun, Xuecheng Zhan, Junyi Zhang, Huan Zhang, Fushan Wang, Zhengping Dong. Acceleration of the semi-hydrogenation of alkynes over an N-doped porous carbon sphere-confined ultrafine PdCu bimetallic nanoparticle catalyst. Physical Chemistry Chemical Physics 2023, 25 (5) , 4201-4210. https://doi.org/10.1039/D2CP04845K
  17. A. V. Rassolov, G. O. Bragina, G. N. Baeva, I. S. Mashkovsky, N. S. Smirnova, E. Yu. Gerasimov, A. V. Bukhtiyarov, Ya. V. Zubavichus, A. Yu. Stakheev. Highly Active Bimetallic Single-Atom Alloy PdAg Catalysts on Cerium-Containing Supports in the Hydrogenation of Alkynes to Alkenes. Kinetics and Catalysis 2022, 63 (6) , 756-764. https://doi.org/10.1134/S0023158422060118
  18. Azina Rahmani, Taylor M. Currie, Lorianne R. Shultz, Jacob T. Bryant, Melanie J. Beazley, Fernando J. Uribe-Romo, Laurene Tetard, Nicholas G. Rudawski, Shaohua Xie, Fudong Liu, Ting-Hsuan Wang, Tiow-Gan Ong, Lei Zhai, Titel Jurca. Robust palladium catalysts on nickel foam for highly efficient hydrogenations. Catalysis Science & Technology 2022, 12 (23) , 6992-6997. https://doi.org/10.1039/D2CY01082H
  19. Krishna Manna, Basuvaraj Suresh Kumar, Tanmoy Maity, Srinivasan Natarajan. C−C Coupling of Aryl Chlorides and Reduction of Nitroarenes to Amines Employing Recyclable Heterogeneous Green Catalysts. ChemNanoMat 2022, 8 (6) https://doi.org/10.1002/cnma.202200081
  20. Linda Z. Nikoshvili, Alexander Y. Popov, Alexey V. Bykov, Alexander I. Sidorov, Lioubov Kiwi-Minsker. Hybrid Pd-Nanoparticles within Polymeric Network in Selective Hydrogenation of Alkynols: Influence of Support Porosity. Molecules 2022, 27 (12) , 3842. https://doi.org/10.3390/molecules27123842
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  • Abstract

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    Figure 1

    Figure 1. Selective hydrogenation of unsaturated bonds by palladium-based catalysts.

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    Figure 2

    Figure 2. Heterolysis of H2. (28) Reproduced with permission from ref (28). Copyright 2007 American Association for the Advancement of Science.

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    Figure 3

    Figure 3. (A) Size effect, (B) composition effect, and (C) surface and interface effect over Pd-based catalysts.

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    Figure 4

    Figure 4. (A) PdZn intermetallic nanostructure with Pd–Zn–Pd ensembles are both highly active and selective for semihydrogenation of acetylene to ethylene; (B) high-resolution transmission electron microscopy (HRTEM) image of Pd/ZnO sample reduced at 400 °C; (C) conversion and selectivity with time on stream in absence of ethylene over PdZn; (84) (D) thiol treatment is demonstrated as a highly effective strategy for promoting the catalytic selectivity of Pd nanocatalysts in the hydrogenation of internal alkynes to alkenes; (E) energy barriers of transition states of PhC≡CCH3 hydrogenation on Pd4S@SPhF2, Pd3S@SPhF2, and Pd(111); (F) catalytic stability and selectivity of the Pd@SPhF2(1:1) obtained; (59) (G) a defective nanodiamond-graphene (ND@G) to prepare an atomically dispersed metal catalyst which is used for selective hydrogenation; (H) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Pd1/ND@G at high magnifications, (I) durability test on Pd1/ND@G at 180 °C for 30 h. (Reaction conditions: 1% C2H2, 10% H2, 20% C2H4 gas mix balanced with He; gas hourly space velocity (GHSV) = 60 000 h–1.). (74) (A–C) Reproduced with permission from ref (84). Copyright 2016 American Chemical Society. (D–F) Reproduced with permission from ref (59). Copyright 2018 Elsevier. (G–I) Reproduced with permission from ref (74). Copyright 2018 American Chemical Society.

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    Figure 5

    Figure 5. (A) HAADF-STEM energy-dispersive spectrometry (EDS) mapping of Pd1+NPs/TiO2 and AC-HAADF-STEM image of Pd1+NPs/TiO2, (B) R-spaced Pd K-edge Fourier transform (FT)-EXAFS spectra of Pd1+NPs/TiO2 (green line) referred to bulk Pd foil (black line) and PdO (magenta line), (90) (C) cinnamaldehyde hydrogenation pathways, and (D) cinnamaldehyde hydrogenation performance profile using Pd–Au catalysts. (98) (A, B) Reproduced with permission from ref (90). Copyright 2020 Springer Nature. (C, D) Reproduced with permission from ref (98). Copyright 2006 Wiley-VCH.

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    Figure 6

    Figure 6. Illustrations of approaches for tuning the selectivity of nanocatalysts in hydrogenation of α,β-unsaturated aldehydes using organic modifiers. (A) Proposed schematic depicting active-site selection, (B) product selectivity for furfural hydrogenation over uncoated and alkanethiolate SAM-coated Pd/Al2O3 catalysts, (C) thiol SAMs used to coat the Pt/Al2O3 surface, and (D) selectivity to cinnamyl alcohol for the hydrogenation of cinnamaldehyde over Pt/Al2O3 catalysts. (A, B) Reproduced with permission from ref (103). Copyright 2013 Springer Nature. (C, D) Reproduced with permission from ref (104). Copyright 2014 American Chemical Society.

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    Figure 7

    Figure 7. (A) Schematic diagram for synthesis of the Pd–Ni/SiO2 bimetallic catalyst and (B) Pd/SiO2, Pt/SiO2, and PdNi/SiO2 catalytic performance diagram. (121) Reproduced with permission from ref (121). Copyright 2019 Springer Nature.

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    Figure 8

    Figure 8. Pd loading supported on MAX phases for chemoselective hydrogenation. (122) Reproduced with permission from ref (122). Copyright 2020 American Chemical Society.

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      cf. CA 58, 6709g. Hydrogenations of 1-nitrocyclooctene (I) and 1-nitro-1-octadecene (II) were carried out on a 2-50 millimole scale at 20°/10-60 lb./in.2 with 5% Pd-C, and the mole-% compns. of crude products were detd. by quant. infrared analysis in CCl4. I (35 millimoles) in 175 ml. MeOH contg. 35 millimoles dry HCl hydrogenated with 3.25 g. catalyst 16 min., 35 millimoles NaOAc added, the mixt. filtered, the catalyst washed with MeOH, the combined solns. concd. to 20 ml. in vacuo, dild. with 200 ml. H2O and extd. with Et2O and the product (96%) analyzed by infrared detn. showed the presence of 83% cyclooctanone oxime (III) and 17% cyclooctanone (IV). Equimolar amts. of IV, HONH2.HCl, and NaOAc.3H2O in H2O-MeOH kept 16 hrs. at 20° and the product isolated as above gave III, b0.08 63°, m. 41.7-2.7°, ε2.74μ 1.19 × 102 1. cm.-1 mole-1, ε2.74 μ/3.4 μ 0.52. In another run 4.05 g. product (contg. 70% III, 23% IV) treated at 50° in 40 ml. MeOH and 15 ml. H2O with 2.8 g. HONH2.HCl and 10.8 g. NaOAc.3H2O and the product dried 24 hrs. at 5 mm. yielded 91% material contg. 93% III and 0% IV. III was also prepd. by hydrogenating 5.5 wt.-% I in C5H5N with 1.3% Pd 16 hrs., the residue on filtration washed with Et2O, and the C5H5N removed by azeotropic distn. with C7H16. II (2 millimoles) in 10 ml. MeOH contg. 1 millimole dry HCl hydrogenated 20 min. with 183 mg. catalyst, the mixt. treated with 2 millimoles NaOAc.3H2O, and the product isolated (94%) and analyzed showed the presence of 73% stearaldoxime (V), recryst. from MeOH and C6H14 to give a pure sample, m. 88.0-9.8°, ε2.74 μ 1.30 × 102 l. cm.-1mole-1. If the CO absorption at 5.75 μ is assigned to stearaldehyde (VI), the amt. present was estd. at 13%. I (13.8 millimoles) in 100 ml. MeOH and 1 g. C5H5N was hydrogenated 10 min. with 0.94 g. catalyst and, after filtration, MeOH extn. of the catalyst, vacuum evapn. of solvents with added C7H16, the product (96%) analyzed for 83% nitrocyclooctane (VII) and 10% IV. Purification by preparative vapor phase chromatography (6 ft. column packed with 25% GE-30 silicone gum rubber on Chromosorb W, flow rate 200 ml. He/min. at 165° retention time 37 min.) gave pure VII, n20D 1.4819, ε6.45μ/ε3.40μ 3.29. II (2.26 millimoles) in 10 ml. MeOH and 0.17 g. C5H5N hydrogenated with 1.3% Pd 11 min. and the product (90%) isolated, chromatographed on silica gel, and eluted with 9:1 C6H14-C6H6 yielded 50% pure 1-nitrooctadecane, m. 39.5-41.0°, ε6.44μ/ε340 0.91. The origin of IV in he hydrogenation of I in MeOH contg. HCl was discussed. IV was not formed by hydrolysis of III by the H2O produced in the reaction. The further hydrogenation of III to the imine and subsequent hydrolysis was considered as a possible mechanism.
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    9. 9
      Chernichenko, K.; Madarász, Á.; Pápai, I.; Nieger, M.; Leskelä, M.; Repo, T. A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes. Nat. Chem. 2013, 5, 718– 723,  DOI: 10.1038/nchem.1693
      9
      A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes
      Chernichenko, Konstantin; Madarasz, Adam; Papai, Imre; Nieger, Martin; Leskelae, Markku; Repo, Timo
      Nature Chemistry (2013), 5 (8), 718-723CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Frustrated Lewis pairs are compds. contg. both Lewis acidic and Lewis basic moieties, where the formation of an adduct is prevented by steric hindrance. They are therefore highly reactive, and are capable of heterolysis of mol. hydrogen, a property that led to their use in hydrogenation reactions of polarized multiple bonds. Here, the authors describe a general approach to the hydrogenation of alkynes to cis-alkenes under mild conditions using the unique ansa-aminohydroborane as a catalyst. The approach combines several reactions as the elementary steps of the catalytic cycle: hydroboration (substrate binding), heterolytic hydrogen splitting (typical frustrated-Lewis-pair reactivity) and facile intramol. protodeborylation (product release). The mechanism is verified by exptl. and computational studies.
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    10. 10
      Furukawa, S.; Komatsu, T. Selective Hydrogenation of Functionalized Alkynes to (E)-Alkenes, Using Ordered Alloys as Catalysts. ACS Catal. 2016, 6, 2121– 2125,  DOI: 10.1021/acscatal.5b02953
      10
      Selective Hydrogenation of Functionalized Alkynes to (E)-Alkenes, Using Ordered Alloys as Catalysts
      Furukawa, Shinya; Komatsu, Takayuki
      ACS Catalysis (2016), 6 (3), 2121-2125CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Intermetallic Pd3Pb acts as a highly selective alkyne semihydrogenation catalyst that is greatly superior to the conventional Lindlar catalyst. D. functional theory (DFT) calcns. demonstrate an ideal adsorption property of Pd3Pb, where the surface holds alkynes while releasing alkenes. A tandem catalytic system that is comprised of Pd3Pb/SiO2 for alkyne semihydrogenation and RhSb/SiO2 for alkene isomerization allows one-pot (E)-alkene synthesis from a functionalized alkyne, which is the first success using heterogeneous catalysts. A variety of functionalized alkynes with aldehyde, ketone, carboxylic acid, and ester moieties are hydrogenated into the corresponding (E)-alkene in good to excellent yields under 1 atm H2 at room temp.
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    11. 11
      Chen, X.; Engle, K. M.; Wang, D.-H.; Yu, J.-Q. Palladium(II)-catalyzed C-H activation/C-C cross-coupling reactions: versatility and practicality. Angew. Chem., Int. Ed. 2009, 48, 5094– 5115,  DOI: 10.1002/anie.200806273
      11
      Palladium(II)-Catalyzed C-H Activation/C-C Cross-Coupling Reactions: Versatility and Practicality
      Chen, Xiao; Engle, Keary M.; Wang, Dong-Hui; Yu, Jin-Quan
      Angewandte Chemie, International Edition (2009), 48 (28), 5094-5115CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Recent studies concerning the palladium(II)-catalyzed C-H activation/C-C cross-coupling reaction with organometallic reagents through a PdII/Pd0 catalytic cycle was reviewed. The versatility and practicality of this new mode of catalysis were also reviewed, including unaddressed questions and the potential for the development in the field.
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    12. 12
      Melo, I. E. M. d. S.; de Sousa, S. A. A.; Pereira, L. N. d. S.; Oliveira, J. M.; Castro, K. P. R.; Costa, J. C. S.; de Moura, E. M.; de Moura, C. V. R.; Garcia, M. A. S. Au–Pd Selectivity-switchable Alcohol-oxidation Catalyst: Controlling the Duality of the Mechanism using a Multivariate Approach. ChemCatChem 2019, 11, 3022– 3034,  DOI: 10.1002/cctc.201900512
      There is no corresponding record for this reference.
    13. 13
      Tessonnier, J.-P.; Pesant, L.; Ehret, G.; Ledoux, M. J.; Pham-Huu, C. Pd nanoparticles introduced inside multi-walled carbon nanotubes for selective hydrogenation of cinnamaldehyde into hydrocinnamaldehyde. Appl. Catal., A 2005, 288, 203– 210,  DOI: 10.1016/j.apcata.2005.04.034
      13
      Pd nanoparticles introduced inside multi-walled carbon nanotubes for selective hydrogenation of cinnamaldehyde into hydrocinnamaldehyde
      Tessonnier, Jean-Philippe; Pesant, Laurie; Ehret, Gabrielle; Ledoux, Marc J.; Pham-Huu, Cuong
      Applied Catalysis, A: General (2005), 288 (1-2), 203-210CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      Palladium nanoparticles (4-6 nm) were deposited inside multi-walled carbon nanotubes (MWNTs) via impregnation of aq. Pd salt, PdNO3.6H2O. The low surface tension of the solvent allowed complete filling of the tube, leading, after thermal treatments, to formation of small and homogeneous Pd particles decorating the inner cavity of the support. The impregnation method was extremely efficient as no Pd particles were obsd. on the outer surface of the tubes. The catalyst was tested for the selective hydrogenation of cinnamaldehyde which contains both a C=C and a C=O bond. The nanotube based catalyst exhibited high catalytic activity an extremely high selectivity towards the C=C bond hydrogenation when compared to a com. catalyst supported on a high surface area activated carbon. A peculiar metal-support interaction and the absence of micropores and of oxygenated surface groups on the carbon nanotubes support are proposed to explain these results.
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    14. 14
      Schoenbaum, C. A.; Schwartz, D. K.; Medlin, J. W. Controlling the Surface Environment of Heterogeneous Catalysts Using Self-Assembled Monolayers. Acc. Chem. Res. 2014, 47, 1438– 1445,  DOI: 10.1021/ar500029y
      14
      Controlling the Surface Environment of Heterogeneous Catalysts Using Self-Assembled Monolayers
      Schoenbaum, Carolyn A.; Schwartz, Daniel K.; Medlin, J. Will
      Accounts of Chemical Research (2014), 47 (4), 1438-1445CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Modification approaches for tuning self-assembled monolayer structure to improve catalytic performance for hydrogenation reactions on palladium and platinum catalysts are summarized. Each approach serves to direct selectivity by tuning a particular aspect of the system including the availability of specific active sites (active-site selection), intermol. interactions between the reactants and modifiers (mol. recognition), and general steric or crowding effects. The tail moiety can be tuned to control the d. of SAM modifiers on the surface. IR spectra of adsorbed CO probe mols. reveal that increasing the d. of the thiols restricts the availability of contiguous active sites on catalyst terraces while maintaining accessibility to sites located at particle edges and steps. This technique was utilized to direct selectivity for the hydrogenation of furfural. Results obtained from SAM coatings with different surface densities indicated that, for this reaction, formation of the desirable products occurs primarily at particle edges and steps, whereas the undesired pathway occurs on particle terrace sites. As an alternative approach, the tail structure of the SAM precursor can be tuned to promote specific intermol. interactions between the modifier and reactant in order to position reactant mols. in a desired orientation. This technique was utilized for the hydrogenation of cinnamaldehyde, which contains an arom. Ph moiety. By using a phenyl-contg. SAM modifier with an appropriate tether length, > 90% selectivity toward reaction of the aldehyde group was achieved. In contrast, employing a modifier where the Ph moiety was closer to the catalyst surface biased selectivity toward the hydrogenation of the C=C bond due to reorienting the mol. to a more "lying down" conformation. In addn. to approaches that target specific interactions between the reactant and modified catalyst, we have demonstrated the use of SAMs to impose a steric or blocking effect, for example, during the hydrogenation of polyunsatd. fatty acids. The SAMs facilitated hydrogenation of polyunsatd. to monounsatd. fatty acids but inhibited further hydrogenation to the completely satd. species due to the sterically hindered, single "kink" shape of the monounsatd. product.
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    15. 15
      Vilé, G.; Almora-Barrios, N.; Mitchell, S.; Lopez, N.; Perez-Ramirez, J. From the Lindlar Catalyst to Supported Ligand-Modified Palladium Nanoparticles: Selectivity Patterns and Accessibility Constraints in the Continuous-Flow Three-Phase Hydrogenation of Acetylenic Compounds. Chem.─Eur. J. 2014, 20, 5926– 5937,  DOI: 10.1002/chem.201304795
      15
      From the Lindlar Catalyst to Supported Ligand-Modified Palladium Nanoparticles: Selectivity Patterns and Accessibility Constraints in the Continuous-Flow Three-Phase Hydrogenation of Acetylenic Compounds
      Vile, Gianvito; Almora-Barrios, Neyvis; Mitchell, Sharon; Lopez, Nuria; Perez-Ramirez, Javier
      Chemistry - A European Journal (2014), 20 (20), 5926-5937CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Site modification and isolation through selective poisoning comprise an effective strategy to enhance the selectivity of palladium catalysts in the partial hydrogenation of triple bonds in acetylenic compds. The recent emergence of supported hybrid materials matching the stereo- and chemoselectivity of the classical Lindlar catalyst holds promise to revolutionize palladium-catalyzed hydrogenations, and will benefit from an in-depth understanding of these new materials. In this work, we compare the performance of bare, lead-poisoned, and ligand-modified palladium catalysts in the hydrogenation of diverse alkynes. Catalytic tests, conducted in a continuous-flow three-phase reactor, coupled with theor. calcns. and characterization methods, enable elucidation of the structural origins of the obsd. selectivity patterns. Distinctions in the catalytic performance are correlated with the relative accessibility of the active site to the org. substrate, and with the adsorption configuration and strength, depending on the ensemble size and surface potentials. This explains the role of the ligand in the colloidally prepd. catalysts in promoting superior performance in the hydrogenation of terminal and internal alkynes, and short-chain alkynols. In contrast, the greater accessibility of the active surface of the Pd-Pb alloy and the absence of polar groups are favorable in the conversion of alkynes contg. long aliph. chains and/or ketone groups. These findings provide detailed insights for the advanced design of supported nanostructured catalysts.
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    16. 16
      Niu, W.; Gao, Y.; Zhang, W.; Yan, N.; Lu, X. Pd-Pb Alloy Nanocrystals with Tailored Composition for Semihydrogenation: Taking Advantage of Catalyst Poisoning. Angew. Chem., Int. Ed. 2015, 54, 8271– 8274,  DOI: 10.1002/anie.201503148
      16
      Pd-Pb Alloy Nanocrystals with Tailored Composition for Semihydrogenation: Taking Advantage of Catalyst Poisoning
      Niu, Wenxin; Gao, Yongjun; Zhang, Weiqing; Yan, Ning; Lu, Xianmao
      Angewandte Chemie, International Edition (2015), 54 (28), 8271-8274CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Metallic nanocrystals (NCs) with well-defined sizes and shapes represent a new family of model systems for establishing structure-function relationships in heterogeneous catalysis. Here in this study, we show that catalyst poisoning can be utilized as an efficient strategy for nanocrystals shape and compn. control, as well as a way to tune the catalytic activity of catalysts. Lead species, a well-known poison for noble-metal catalysts, was investigated in the growth of Pd NCs. We discovered that Pb atoms can be incorporated into the lattice of Pd NCs and form Pd-Pb alloy NCs with tunable compn. and crystal facets. As model catalysts, the alloy NCs with different compns. showed different selectivity in the semihydrogenation of phenylacetylene. Pd-Pb alloy NCs with better selectivity than that of the com. Lindlar catalyst were discovered. This study exemplified that the poisoning effect in catalysis can be explored as efficient shape-directing reagents in NC growth, and more importantly, as a strategy to tailor the performance of catalysts with high selectivity.
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    17. 17
      Liu, K.; Qin, R.; Zheng, N. Insights into the Interfacial Effects in Heterogeneous Metal Nanocatalysts toward Selective Hydrogenation. J. Am. Chem. Soc. 2021, 143, 4483– 4499,  DOI: 10.1021/jacs.0c13185
      17
      Insights into the Interfacial Effects in Heterogeneous Metal Nanocatalysts toward Selective Hydrogenation
      Liu, Kunlong; Qin, Ruixuan; Zheng, Nanfeng
      Journal of the American Chemical Society (2021), 143 (12), 4483-4499CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A review. Heterogeneous metal catalysts are distinguished by their structure inhomogeneity and complexity. The chameleonic nature of heterogeneous metal catalysts have prevented us from deeply understanding their catalytic mechanisms at the mol. level and thus developing industrial catalysts with perfect catalytic selectivity toward desired products. This Perspective aims to summarize recent research advances in deciphering complicated interfacial effects in heterogeneous hydrogenation metal nanocatalysts toward the design of practical heterogeneous catalysts with clear catalytic mechanism and thus nearly perfect selectivity. The mol. insights on how the three key components (i.e., catalytic metal, support, and ligand modifier) of a heterogeneous metal nanocatalyst induce effective interfaces detg. the hydrogenation activity and selectivity are provided. The interfaces influence not only the H2 activation pathway but also the interaction of substrates to be hydrogenated with catalytic metal surface and thus the hydrogen transfer process. As for alloy nanocatalysts, together with the electronic and geometric ensemble effects, spillover hydrogenation occurring on catalytically "inert" metal by utilizing hydrogen atom spillover from active metal is highlighted. The metal-support interface effects are then discussed with emphasis on the mol. involvement of ligands located at the metal-support interface as well as cationic species from the support in hydrogenation. The mechanisms of how org. modifiers, with the ability to induce both 3D steric and electronic effects, on metal nanocatalysts manipulate the hydrogenation pathways are demonstrated. A brief summary is finally provided together with a perspective on the development of enzyme-like heterogeneous hydrogenation metal catalysts.
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    18. 18
      Grabovskii, S. A.; Akchurin, T. I.; Dokichev, V. A. Heterogeneous Palladium Catalysts in the Hydrogenation of the Carbon-carbon Double Bond. Curr. Org. Chem. 2021, 25, 315– 329,  DOI: 10.2174/1385272824999201202084812
      18
      Heterogeneous Palladium Catalysts in the Hydrogenation of the Carbon-carbon Double Bond
      Grabovskii, Stanislav A.; Akchurin, Timur I.; Dokichev, Vladimir A.
      Current Organic Chemistry (2021), 25 (2), 315-329CODEN: CORCFE; ISSN:1385-2728. (Bentham Science Publishers Ltd.)
      A review. The results of studies over the past ten years in the field of C=C bond hydrogenation in the presence of palladium catalysts deposited on various inorg. and org. carriers such activated carbons, carbon nanotubes, alumina, zeolites, or composite materials based on Al2O3-SiO2, polystyrene, polypropyleneimine, polyamidoamine and hybrid inorg./ polymer-carriers, are presented. The selectivity and rates of the hydrogenation process are considered and some comparisons are made. Porous supports and contg. dendrimers generally retain palladium particles more effectively. Nanosized palladium stabilized by different dendrimers catalyzes the hydrogenation of C=C bonds in polyfunctional compds. chemoselectively without affecting functional groups, such as CHO, C=O, C(O)OR, CN, NO2, and halogens.
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    19. 19
      Monguchi, Y.; Ichikawa, T.; Sajiki, H. Recent Development of Palladium-Supported Catalysts for Chemoselective Hydrogenation. Chem. Pharm. Bull. 2017, 65, 2– 9,  DOI: 10.1248/cpb.c16-00153
      19
      Recent development of palladium-supported catalysts for chemoselective hydrogenation
      Monguchi, Yasunari; Ichikawa, Tomohiro; Sajiki, Hironao
      Chemical & Pharmaceutical Bulletin (2017), 65 (1), 2-9CODEN: CPBTAL; ISSN:0009-2363. (Pharmaceutical Society of Japan)
      A review. This paper describes practical and selective hydrogenation methodologies using heterogeneous palladium catalysts. Chemoselectivity develops dependent on the catalyst activity based on the characteristic of the supports, derived from structural components, functional groups, and/or morphologies. We esp. focus on our recent development of heterogeneous palladium catalysts supported on chelate resin, ceramic, and spherically shaped activated carbon. In addn., the application of flow technol. for chemoselective hydrogenation using the palladium catalysts immobilized on mol. sieves 3A and boron nitride is outlined.
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    20. 20
      McCue, A. J.; Anderson, J. A. Recent advances in selective acetylene hydrogenation using palladium containing catalysts. Front. Chem. Sci. Eng. 2015, 9, 142– 153,  DOI: 10.1007/s11705-015-1516-4
      20
      Recent advances in selective acetylene hydrogenation using palladium containing catalysts
      McCue, Alan J.; Anderson, James A.
      Frontiers of Chemical Science and Engineering (2015), 9 (2), 142-153CODEN: FCSEA3; ISSN:2095-0187. (Springer)
      Recent advances with Pd contg. catalysts for the selective hydrogenation of acetylene are described. The overview classifies enhancement of catalytic properties for monometallic and bimetallic Pd catalysts. Activity/selectivity of Pd catalysts can be modified by controlling particle shape/morphol. or immobilization on a support which interacts strongly with Pd particles. In both cases enhanced ethylene selectivity is generally assocd. with modifying ethylene adsorption strength and/or changes to hydride formation. Inorg. and org. selectivity modifiers (i.e., species adsorbed onto Pd particle surface) have also been shown to enhance ethylene selectivity. Inorg. modifiers such as TiO2 change Pd ensemble size and modify ethylene adsorption strength whereas org. modifiers such as diphenylsulfide are thought to create a surface template effect which favors acetylene adsorption with respect to ethylene. A no. of metals and synthetic approaches have been explored to prep. Pd bimetallic catalysts. Examples where enhanced selectivity is obsd. are generally assocd. with decreased Pd ensemble size and/or hindering of the ease with which an unselective hydride phase is formed for Pd. A final class of bimetallic catalysts are discussed where Pd is not thought to be the primary reaction site but merely acts as a site where hydrogen dissocn. and spillover occurs onto a second metal (Cu or Au) where the reaction takes place more selectively. [Figure not available: see fulltext.].
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    21. 21
      Wang, S.; Zhao, Z. J.; Chang, X.; Zhao, J.; Tian, H.; Yang, C.; Li, M.; Fu, Q.; Mu, R.; Gong, J. Activation and Spillover of Hydrogen on Sub-1 nm Palladium Nanoclusters Confined within Sodalite Zeolite for the Semi-Hydrogenation of Alkynes. Angew. Chem., Int. Ed. 2019, 58, 7668– 7672,  DOI: 10.1002/anie.201903827
      21
      Activation and Spillover of Hydrogen on Sub-1 nm Palladium Nanoclusters Confined within Sodalite Zeolite for the Semi-Hydrogenation of Alkynes
      Wang, Shuai; Zhao, Zhi-Jian; Chang, Xin; Zhao, Jiubing; Tian, Hao; Yang, Chengsheng; Li, Mingrun; Fu, Qiang; Mu, Rentao; Gong, Jinlong
      Angewandte Chemie, International Edition (2019), 58 (23), 7668-7672CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The search for efficient nontoxic catalysts able to perform industrial hydrogenations is a topic of interest, with relevance to many catalytic processes. Herein, we describe a mechanistic phenomenon for the activation and spillover of hydrogen for remarkable selectivity in the semi-hydrogenation of acetylene over sub-1 nm Pd nanoclusters confined within sodalite (SOD) zeolite (Pd@SOD). Specifically, hydrogen is dissocd. on the Pd nanoclusters to form hydrogen species (i.e., hydrogen atoms and hydroxyl groups) that spill over the SOD surfaces. The design and utilization of the small-pore zeolite SOD (six-membered rings with 0.28×0.28 nm channels) is crucial as it only allows H2 diffusion into the channels to reach the encapsulated Pd nanoclusters and thus avoids over-hydrogenation to form ethane. Pd@SOD exhibits an ethylene selectivity of over 94.5 %, while that of conventional Pd/SOD is approx. 21.5 %.
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    22. 22
      Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds. Chem. Rev. 1995, 95, 2457– 2483,  DOI: 10.1021/cr00039a007
      22
      Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds
      Miyaura, Norio; Suzuki, Akira
      Chemical Reviews (Washington, D. C.) (1995), 95 (7), 2457-83CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review with >250 refs. including title reactions and their mechanisms, prepn. of organoboron reagents, alkoxycarbonylation and dimerization.
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    23. 23
      Teschner, D.; Borsodi, J.; Wootsch, A.; Revay, Z.; Havecker, M.; Knop-Gericke, A.; Jackson, S. D.; Schlogl, R. The roles of subsurface carbon and hydrogen in palladium-catalyzed alkyne hydrogenation. Science 2008, 320, 86– 89,  DOI: 10.1126/science.1155200
      23
      The Roles of Subsurface Carbon and Hydrogen in Palladium-Catalyzed Alkyne Hydrogenation
      Teschner, Detre; Borsodi, Janos; Wootsch, Attila; Revay, Zsolt; Haevecker, Michael; Knop-Gericke, Axel; Jackson, S. David; Schloegl, Robert
      Science (Washington, DC, United States) (2008), 320 (5872), 86-89CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Alkynes can be selectively hydrogenated into alkenes on solid palladium catalysts. This process requires a strong modification of the near-surface region of palladium, in which carbon (from fragmented feed mols.) occupies interstitial lattice sites. In situ X-ray photoelectron spectroscopic measurements under reaction conditions indicated that much less carbon was dissolved in palladium during unselective, total hydrogenation. Addnl. studies of hydrogen content using in situ prompt gamma activation anal., which allowed us to follow the hydrogen content of palladium during catalysis, indicated that unselective hydrogenation proceeds on hydrogen-satd. β-hydride, whereas selective hydrogenation was only possible after decoupling bulk properties from the surface events. Thus, the population of subsurface sites of palladium, by either hydrogen or carbon, governs the hydrogenation events on the surface.
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    24. 24
      Zhang, L.; Zhou, M.; Wang, A.; Zhang, T. Selective Hydrogenation over Supported Metal Catalysts: From Nanoparticles to Single Atoms. Chem. Rev. 2020, 120, 683– 733,  DOI: 10.1021/acs.chemrev.9b00230
      24
      Selective hydrogenation over supported metal catalysts: from nanoparticles to single atoms
      Zhang, Leilei; Zhou, Maoxiang; Wang, Aiqin; Zhang, Tao
      Chemical Reviews (Washington, DC, United States) (2020), 120 (2), 683-733CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. In this review article, the approaches to tackle challenges in selective catalytic hydrogenation, including adsorption/grafting of N/S-contg. org. mols. on the metal surface, partial covering of active metal surface by metal oxides either via doping or through strong metal-support interaction, confinement of active metal nanoparticles in micro- or meso-pores of the supports, formation of bimetallic alloys or intermetallics or core@shell structures with a relatively inert metal (IB and IIB) or nonmetal element (B, C, S, etc.), and construction of single-atom catalysts on reducible oxides or inert metals were summarized. Both advantages and disadvantages of each approach toward the site isolation were discussed for three types of chemoselective hydrogenation reactions, including alkynes/dienes to monoenes, α, β-unsatd. aldehydes/ketones to the unsatd. alcs., and substituted nitroarenes to the corresponding anilines. The key factors affecting the catalytic activity/selectivity, in particular, the geometric and electronic structure of the active sites, were discussed with the aim to ext. fundamental principles for the development of efficient and selective catalysts in hydrogenation as well as other transformations.
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    25. 25
      Alshakova, I. D.; Gabidullin, B.; Nikonov, G. I. Ru-Catalyzed Transfer Hydrogenation of Nitriles, Aromatics, Olefins, Alkynes and Esters. ChemCatChem 2018, 10, 4860– 4869,  DOI: 10.1002/cctc.201801039
      There is no corresponding record for this reference.
    26. 26
      Jagtap, S. A.; Bhanage, B. M. Ligand Assisted Rhodium Catalyzed Selective Semi-hydrogenation of Alkynes Using Syngas and Molecular Hydrogen. ChemistrySelect 2018, 3, 713– 718,  DOI: 10.1002/slct.201702976
      26
      Ligand Assisted Rhodium Catalyzed Selective Semi-hydrogenation of Alkynes Using Syngas and Molecular Hydrogen
      Jagtap, Samadhan A.; Bhanage, Bhalchandra M.
      ChemistrySelect (2018), 3 (2), 713-718CODEN: CHEMUD; ISSN:2365-6549. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The rhodium catalyzed selective semi-hydrogenation of alkynes to (E) and (Z)-alkenes in the presence of various Buchwald phosphine ligands such as S-Phos, t-Bu XPhos, Ru-Phos, Johnphos and DavePhos was reported. Selective formation of E-alkenes in high activity with use of H2 gas (5 bar) was an alternative method for conventional Birch redn. High selectivity obtained towards Z-alkenes with use of syngas (10 bar) was an alternative method for Lindlar reaction. (E)-alkenes were obtained in high yield due to the steric crowding present in Johnphos ligand. The carbon monoxide present in syngas controlled over-hydrogenation of alkynes and (Z)-alkenes were obtained as major product due to cis addn. of mol. hydrogen to alkynes. The use of additives, base, co-catalyst and excess use of chems. was prevented with this protocol for semi-hydrogenation reaction. With this simple and effective catalyst system and mild reaction conditions, internal as well as terminal alkynes were smoothly converted to resp. alkenes in high yield (>90%).
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXovFahug%253D%253D&md5=1146b3201d5d45aa8a0d9beec997816f
    27. 27
      Liu, K.; Qin, R.; Zhou, L.; Liu, P.; Zhang, Q.; Jing, W.; Ruan, P.; Gu, L.; Fu, G.; Zheng, N. Cu2O-Supported Atomically Dispersed Pd Catalysts for Semihydrogenation of Terminal Alkynes: Critical Role of Oxide Supports. CCS Chem. 2019, 1, 207– 214,  DOI: 10.31635/ccschem.019.20190008
      27
      Cu2O-supported atomically dispersed Pd catalysts for semihydrogenation of terminal alkynes: critical role of oxide supports
      Liu, Kunlong; Qin, Ruixuan; Zhou, Lingyun; Liu, Pengxin; Zhang, Qinghua; Jing, Wentong; Ruan, Pengpeng; Gu, Lin; Fu, Gang; Zheng, Nanfeng
      CCS Chemistry (2019), 1 (2), 207-214CODEN: CCCHB2 ISSN:. (Chinese Chemical Society)
      Atomically dispersed catalysts have demonstrated superior catalytic performance in many chem. transformations. However, limited success has been achieved in applying oxide-supported atomically dispersed catalysts to semihydrogenation of alkynes under mild conditions. By utilizing various metal oxides (e.g., Cu2O, Al2O3, ZnO, and TiO2) as support for atomically dispersed Pd catalysts, we demonstrate herein the crit. role of the oxidn. state and coordination environment of Pd centers in their catalytic performance, thus leading to the discovery of an "oxide-support effect" on atomically dispersed metal catalysts. Pd atomically dispersed on Cu2O exhibits far better catalytic activity in the hydrogenation of alkynes, with an extremely high selectivity toward alkenes, compared with catalysts on other oxides. Pd species galvanically displace surface Cu(I) sites on Cu2O to create two-coordinated Pd(I), which is a crit. step for the activation and heterolytic splitting of H2 into Pd-Hδ- and O-Hδ+ species for the selective hydrogenation of alkynes. Moreover, the adsorption of alkenes on H2-preadsorbed Pd(I) is relatively weak, preventing deeper hydrogenation and increased selectivity during semihydrogenation. We demonstrate that the local coordination environment of active metal centers plays a crucial role in detg. the catalytic performance of an oxide supported atomically dispersed catalyst.
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    28. 28
      Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Facile Splitting of Hydrogen and Ammonia by Nucleophilic Activation at a Single Carbon Center. Science 2007, 316, 439– 441,  DOI: 10.1126/science.1141474
      28
      Facile splitting of hydrogen and ammonia by nucleophilic activation at a single carbon center
      Frey, Guido D.; Lavallo, Vincent; Donnadieu, Bruno; Schoeller, Wolfgang W.; Bertrand, Guy
      Science (Washington, DC, United States) (2007), 316 (5823), 439-441CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      In possessing a lone pair of electrons and an accessible vacant orbital, singlet carbenes resemble transition metal centers and thus could potentially mimic their chem. behavior. Although singlet di(amino)carbenes are inert toward dihydrogen, it is shown that more nucleophilic and electrophilic (alkyl)(amino)carbenes can activate H2 under mild conditions, a reaction that has long been known for transition metals. However, in contrast to transition metals that act as electrophiles toward dihydrogen, these carbenes primarily behave as nucleophiles, creating a hydride-like hydrogen, which then attacks the pos. polarized carbon center. This nucleophilic behavior allows these carbenes to activate NH3 as well, a difficult task for transition metals because of the formation of Lewis acid-base adducts.
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    29. 29
      An, K.; Somorjai, G. A. Size and Shape Control of Metal Nanoparticles for Reaction Selectivity in Catalysis. ChemCatChem 2012, 4, 1512– 1524,  DOI: 10.1002/cctc.201200229
      29
      Size and Shape Control of Metal Nanoparticles for Reaction Selectivity in Catalysis
      An, Kwangjin; Somorjai, Gabor A.
      ChemCatChem (2012), 4 (10), 1512-1524CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review; a nanoparticle with well-defined surfaces, prepd. through colloidal chem., enables it to be studied as a model heterogeneous catalyst. The colloidal synthetic approach provides versatile tools to control the size and shape of nanoparticles. Traditional nucleation and growth mechanisms have been utilized to understand how nanoparticles can be uniformly synthesized and unprecedented shapes can be controlled. Now, the size of metal particles can be controlled to cluster regimes by using dendrimers. By using seeds and foreign atoms, specific synthetic environments such as seeded growth and crystal overgrowth can be induced to generate various shaped mono- or bi-metallic, core/shell, or branched nanostructures. For green chem., catalysis in 21st century is aiming for 100 % selectivity to produce only one desired product at high turnover rates. Recent studies on nanoparticle catalysts clearly demonstrate size and shape dependent selectivity in many catalytic reactions. By combining in situ surface characterization techniques, real-time monitoring of nanoparticles can be performed under reaction environments, thus identifying several mol. factors affecting catalytic activity and selectivity.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xht1Kitb7E&md5=632e1ae98580c41623984912e897f324
    30. 30
      Roldan Cuenya, B.; Behafarid, F. Nanocatalysis: size- and shape-dependent chemisorption and catalytic reactivity. Surf. Sci. Rep. 2015, 70, 135– 187,  DOI: 10.1016/j.surfrep.2015.01.001
      30
      Nanocatalysis: size- and shape-dependent chemisorption and catalytic reactivity
      Roldan Cuenya, Beatriz; Behafarid, Farzad
      Surface Science Reports (2015), 70 (2), 135-187CODEN: SSREDI; ISSN:0167-5729. (Elsevier B.V.)
      A review. In recent years, the field of catalysis has experienced an astonishing transformation, driven in part by more demanding environmental stds. and crit. societal and industrial needs such as the search for alternative energy sources. Thanks to the advent of nanotechnol., major steps have been made towards the rational design of novel catalysts. Striking new catalytic properties, including greatly enhanced reactivities and selectivities, have been reported for nanoparticle (NP) catalysts as compared to their bulk counterparts. However, in order to harness the power of these nanocatalysts, a detailed understanding of the origin of their enhanced performance is needed. The present review focuses on the role of the NP size and shape on chemisorption and catalytic performance. Since homogeneity in NP size and shape is a prerequisite for the understanding of structure-reactivity correlations, we first review different synthesis methods that result in narrow NP size distributions and shape controlled NPs. Next, size-dependent phenomena which influence the chem. reactivity of NPs, including quantum size-effects and the presence of under-coordinated surface atoms are examd. The effect of the NP shape on catalytic performance is discussed and explained based on the existence of different at. structures on the NP surface with distinct chemisorption properties. The influence of addnl. factors, such as the oxidn. state of the NPs and NP-support interactions, is also considered in the frame of the size- and shape-dependency that these phenomena present. Ultimately, our review highlights the importance of achieving a systematic understanding of the factors that control the activity and selectivity of a catalyst in order to avoid trial and error methods in the rational design of the new generation of nanocatalysts with properties tunable at the at. level.
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    31. 31
      da Silva, F. P.; Fiorio, J. L.; Rossi, L. M. Tuning the Catalytic Activity and Selectivity of Pd Nanoparticles Using Ligand-Modified Supports and Surfaces. ACS Omega 2017, 2, 6014– 6022,  DOI: 10.1021/acsomega.7b00836
      31
      Tuning the Catalytic Activity and Selectivity of Pd Nanoparticles Using Ligand-Modified Supports and Surfaces
      da Silva, Fernanda Parra; Fiorio, Jhonatan Luiz; Rossi, Liane Marcia
      ACS Omega (2017), 2 (9), 6014-6022CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      The org. moiety plays an essential role in the design of homogeneous catalysts, where the ligands are used to tune the catalytic activity, selectivity, and stability of the transition metal centers. The impact of ligands on the catalytic performance of metal nanoparticle catalysts is still less understood. Here, we prepd. supported nanoparticle (NP) catalysts by the immobilization of preformed Pd NPs on the ligand-modified silica surfaces bearing amine, ethylenediamine, and diethylenetriamine groups. After excluding any size effect, we were able to study the influence of the ligands grafted on the support surface on the catalytic activity of the supported nanoparticles. Higher activity was obsd. for the Pd NPs supported on propylamine-functionalized support, whereas the presence of ethylenediamine and diethylenetriamine groups was detrimental to the activity. Upon the addn. of excess of these amine ligands as surface modifiers, the hydrogenation of alkene to alkane was fully suppressed and, therefore, we were able to tune Pd selectivity. The selective hydrogenation of alkynes into alkenes, although a considerable challenge on the traditional palladium catalysts, was achieved here for a range of alkynes by combining Pd NPs and amine ligands.
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    32. 32
      Zheng, N.; Zhang, T. Preface: single-atom catalysts as a new generation of heterogeneous catalysts. Natl. Sci. Rev. 2018, 5, 625,  DOI: 10.1093/nsr/nwy095
      There is no corresponding record for this reference.
    33. 33
      Li, Z.; Ji, S.; Liu, Y.; Cao, X.; Tian, S.; Chen, Y.; Niu, Z.; Li, Y. Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites. Chem. Rev. 2020, 120, 623– 682,  DOI: 10.1021/acs.chemrev.9b00311
      33
      Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites
      Li, Zhi; Ji, Shufang; Liu, Yiwei; Cao, Xing; Tian, Shubo; Chen, Yuanjun; Niu, Zhiqiang; Li, Yadong
      Chemical Reviews (Washington, DC, United States) (2020), 120 (2), 623-682CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single at. sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.
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    34. 34
      Abdollahi, T.; Farmanzadeh, D. Selective hydrogenation of acetylene in the presence of ethylene on palladium nanocluster surfaces: A DFT study. Appl. Surf. Sci. 2018, 433, 513– 529,  DOI: 10.1016/j.apsusc.2017.10.085
      34
      Selective hydrogenation of acetylene in the presence of ethylene on palladium nanocluster surfaces: A DFT study
      Abdollahi, Tahereh; Farmanzadeh, Davood
      Applied Surface Science (2018), 433 (), 513-529CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      In this work, by d. functional theory, the palladium nanoclusters were investigated in order to design new catalysts for the selective hydrogenation of acetylene present in olefin feeds. At first, the palladium nanoclusters were studied using PBE-G functional with DNP-ECP basis set. According to the performed calcns., among all the Pdn (n = 2-15) nanoclusters, two Pd12 and Pd2 nanoclusters can be used as catalysts in the reactions of hydrogenation of acetylene and ethylene. The adsorption energy of hydrogen on the Pd12 nanocluster is higher than that of acetylene and ethylene, and therefore, the Pd12 nanocluster is more appropriate for the hydrogenation of acetylene and ethylene. However, the calcd. activation energy barriers for the reactions of hydrogenation of acetylene and ethylene showed that the Pd2 nanocluster has more selectivity in comparison to the Pd12 nanocluster. According to our results, the activation energy of the hydrogenation of acetylene to vinyl on the Pd2 nanocluster is 23.96 kJ/mol lower than that on the Pd12 nanocluster. Also, the activation energy of the hydrogenation of ethylene to Et on the Pd2 nanocluster is higher than that on the Pd12 nanocluster Therefore, it seems that the Pd2 surface can be used as a catalyst for the selective hydrogenation of acetylene.
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    35. 35
      Li, M.; Shen, J. Microcalorimetric studies of O2 and C2H4 adsorption on Pd/SiO2 catalysts modi ed by Cu and Ag. Thermochim. Acta 2001, 379, 45– 50,  DOI: 10.1016/S0040-6031(01)00600-1
      35
      Microcalorimetric studies of O2 and C2H4 adsorption on Pd/SiO2 catalysts modified by Cu and Ag
      Li, Mingshi; Shen, Jianyi
      Thermochimica Acta (2001), 379 (1-2), 45-50CODEN: THACAS; ISSN:0040-6031. (Elsevier Science B.V.)
      The microcalorimetric adsorption of H2 and O2 has been employed to probe the dispersion of metals and surface compn. of the bimetallic Pd-Cu/SiO2 and Pd-Ag/SiO2 catalysts. In addn., the microcalorimetric adsorption of ethylene was carried out to study the effects of Cu and Ag on the adsorption of ethylene on reduced and oxidized Pd surfaces. It was found that the addn. of Cu or Ag dild. the surface Pd sites. The heats of O2 adsorption for Pd-Cu and Pd-Ag were approx. the avs. between those for Pd and the second metal Cu and Ag, resp., when the ratios of Cu/Pd and Ag/Pd are low (1:1). Moreover, the O/H ratio was found to be about 0.5 for the Pd-Cu/SiO2 (1:1) and Pd-Ag/SiO2 (1:1) samples, which was about the same for the Pd/SiO2 sample. These results suggest the enrichment of Pd on the surfaces in the bimetallic Pd-Cu/SiO2 (1:1) and Pd-Ag/SiO2 (1:1) samples. On the other hand, the initial heats of O2 adsorption for the Pd-Cu/SiO2 (1:4) and Pd-Ag/SiO2 (1:4) are approx. equal to those of the second metal Cu and Ag, resp., revealing the enrichment of the second metals in these samples. The addn. of Cu or Ag inhibited the formation of ethylidyne on the reduced Pd surface, and Ag was found to be more effective than Cu for this effect. The adsorption of ethylene on the oxidized Pd resulted in the serious oxidn. of ethylene. The addn. of Cu or Ag greatly reduced the oxidn. of ethylene on the oxidized Pd. In this case, Cu was more effective than Ag in reducing the activity of oxygen for the oxidn. of ethylene on the Pd surface.
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    36. 36
      Hamm, G.; Schmidt, T.; Breitbach, J.; Franke, D.; Becker, C.; Wandelt, K. The Adsorption of Ethene on Pd(111) and Ordered Sn/Pd(111) Surface Alloys. Surf. Sci. 2004, 562, 170– 182,  DOI: 10.1016/j.susc.2004.05.119
      36
      The adsorption of benzene on Pd(1 1 1) and ordered Sn/Pd(1 1 1) surface alloys
      Hamm, G.; Schmidt, T.; Breitbach, J.; Franke, D.; Becker, C.; Wandelt, K.
      Surface Science (2004), 562 (1-3), 170-182CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
      The adsorption of benzene on Pd(1 1 1) and ordered Sn/Pd(1 1 1) surface alloys was studied using UPS (UPS), high-resoln. electron energy loss spectroscopy (HREELS), temp. programmed desorption (TPD) and LEED. Two ordered surface alloys were prepd. by thermal treatment of vapor deposited Sn-films on Pd(1 1 1). Depending on the prepn. conditions, the surface exhibited a p(2×2) or a (√3×√3)R30° LEED pattern, corresponding to surface alloys of compn. Pd3Sn and Pd2Sn, resp. Benzene adsorbs molecularly on Pd(1 1 1) at temps. below 400 K and that the mol. is π-bonded to the surface with the ring plane parallel to the surface. At low initial coverage, heating leads to complete decompn. of the adsorbed benzene, while at higher coverage addnl. desorption of mol. benzene occurs in 2 desorption states at ∼400 and ∼500 K, resp. The evolution of the low temp. state is correlated with variations in the electronic states of the adsorbed benzene and attributed to a change in the adsorption configuration. For the first time a CO-free (√19×√19)R±23.4° superstructure was discovered on Pd(1 1 1) at benzene monolayer satn., allowing for a detn. of the adsorption sites. In comparison to the pure Pd(1 1 1) surface, alloying leads to successive weakening of the benzene-surface bond with increasing Sn-content in the topmost layer of the substrate. Decompn. is suppressed on both surface alloys. Finally there was no evidence for a tilted benzene configuration on either of the surfaces investigated.
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    37. 37
      Pei, G. X.; Liu, X. Y.; Wang, A.; Lee, A. F.; Isaacs, M. A.; Li, L.; Pan, X.; Yang, X.; Wang, X.; Tai, Z.; Wilson, K.; Zhang, T. Ag Alloyed Pd Single-Atom Catalysts for Efficient Selective Hydrogenation of Acetylene to Ethylene in Excess Ethylene. ACS Catal. 2015, 5, 3717– 3725,  DOI: 10.1021/acscatal.5b00700
      37
      Ag Alloyed Pd Single-Atom Catalysts for Efficient Selective Hydrogenation of Acetylene to Ethylene in Excess Ethylene
      Pei, Guang Xian; Liu, Xiao Yan; Wang, Aiqin; Lee, Adam F.; Isaacs, Mark A.; Li, Lin; Pan, Xiaoli; Yang, Xiaofeng; Wang, Xiaodong; Tai, Zhijun; Wilson, Karen; Zhang, Tao
      ACS Catalysis (2015), 5 (6), 3717-3725CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Semihydrogenation of acetylene in an ethylene-rich stream is an industrially important process. Conventional supported monometallic Pd catalysts offer high acetylene conversion, but they suffer from very low selectivity to ethylene due to overhydrogenation and the formation of carbonaceous deposits. Herein, a series of Ag alloyed Pd single-atom catalysts, possessing only ppm levels of Pd, supported on silica gel were prepd. by a simple incipient wetness compregnation method and applied to the selective hydrogenation of acetylene in an ethylene-rich stream under conditions close to the front-end employed by industry. High acetylene conversion and simultaneous selectivity to ethylene was attained over a wide temp. window, surpassing an analogous Au alloyed Pd single-atom system we previously reported. Restructuring of AgPd nanoparticles and electron transfer from Ag to Pd were evidenced by in situ FTIR and in situ XPS as a function of increasing redn. temp. Microcalorimetry and XANES measurements support both geometric and electronic synergetic effects between the alloyed Pd and Ag. Kinetic studies provide valuable insight into the nature of the active sites within these AgPd/SiO2 catalysts, and hence, they provide evidence for the key factors underpinning the excellent performance of these bimetallic catalysts toward the selective hydrogenation of acetylene under ethylene-rich conditions while minimizing precious metal usage.
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    38. 38
      Vilé, G.; Albani, D.; Nachtegaal, M.; Chen, Z.; Dontsova, D.; Antonietti, M.; Lopez, N.; Perez-Ramirez, J. A stable single-site palladium catalyst for hydrogenations. Angew. Chem., Int. Ed. 2015, 54, 11265– 11269,  DOI: 10.1002/anie.201505073
      38
      A Stable Single-Site Palladium Catalyst for Hydrogenations
      Vile, Gianvito; Albani, Davide; Nachtegaal, Maarten; Chen, Zupeng; Dontsova, Dariya; Antonietti, Markus; Lopez, Nuria; Perez-Ramirez, Javier
      Angewandte Chemie, International Edition (2015), 54 (38), 11265-11269CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The prepn. and hydrogenation performance of a single-site palladium catalyst that was obtained by the anchoring of Pd atoms into the cavities of mesoporous polymeric graphitic carbon nitride is reported. The characterization of the material confirmed the at. dispersion of the palladium phase throughout the sample. The catalyst was applied for three-phase hydrogenations of alkynes and nitroarenes in a continuous-flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. D. functional theory calcns. provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites.
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    39. 39
      Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations. Science 2012, 335, 1209– 1212,  DOI: 10.1126/science.1215864
      39
      Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations
      Kyriakou, Georgios; Boucher, Matthew B.; Jewell, April D.; Lewis, Emily A.; Lawton, Timothy J.; Baber, Ashleigh E.; Tierney, Heather L.; Flytzani-Stephanopoulos, Maria; Sykes, E. Charles H.
      Science (Washington, DC, United States) (2012), 335 (6073), 1209-1212CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Facile dissocn. of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resoln. scanning tunneling microscopy, the authors show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissocn. at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.
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    40. 40
      Zhang, J.; Xu, W.; Xu, L.; Shao, Q.; Huang, X. Concavity Tuning of Intermetallic Pd–Pb Nanocubes for Selective Semihydrogenation Catalysis. Chem. Mater. 2018, 30, 6338– 6345,  DOI: 10.1021/acs.chemmater.8b02337
      40
      Concavity Tuning of Intermetallic Pd-Pb Nanocubes for Selective Semihydrogenation Catalysis
      Zhang, Junbo; Xu, Weiwei; Xu, Lai; Shao, Qi; Huang, Xiaoqing
      Chemistry of Materials (2018), 30 (18), 6338-6345CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Although considerable studies on pursuing high-performance Pd-based catalysts for the semihydrogenation of alkynes have been carried out, the creation of catalyst with high activity, selectivity and stability simultaneously toward semihydrogenation reactions remains a significant challenge. Herein, for the first time a facile synthetic strategy is reported to realize the intermetallic Pd-Pb nanocubes with different concave degree by selectively utilizing small mols. These obtained Pd-Pb nanocrystals exhibit high activity in the semihydrogenation of alkynes, where their performances are highly shape- and compn.-dependent with Pd-Pb concave nanocubes showing the optimized alkene selectivity of 94.6% and activity of 179.2 h-1, much higher than those of 10% Pd/C. Detailed XPS results show that the higher ratio of metallic Pd results in the higher activity for semihydrogenation of phenylacetylene and the higher ratio of Pb2+ and Pb/Pd contribute to higher styrene selectivity. The d. functional theory calcns. further reveal that the favorable adsorption energy of phenylacetylene and desirable desorption energy of styrene on the Pd3Pb surface are crit. for the phenylacetylene semihydrogenation with excellent activity and high selectivity. Furthermore, the Pd-Pb concave nanocube can endure at least five cycles with very limited conversion and selectivity decays, representing an efficient Pd-based catalyst for selective hydrogenation and beyond.
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    41. 41
      Saranya, A.; Vivekanandan, G.; Thirunavukkarasu, K.; Krishnamurthy, K. R.; Viswanathan, B. Studies on palladium based bimetallic catalysts Pd-M/TiO2 (M = Cu, Ag & Au): I-Selective hydrogenation of 1-heptyne. Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem. 2019, 58, 271– 280
      41
      Studies on palladium based bimetallic catalysts Pd-M/TiO2 (M = Cu, Ag & Au): i-selective hydrogenation of 1-heptyne
      Saranya, A.; Vivekanandan, G.; Thirunavukkarasu, K.; Krishnamurthy, K. R.; Viswanathan, B.
      Indian Journal of Chemistry, Section A: Inorganic, Bio-inorganic, Physical, Theoretical & Analytical Chemistry (2019), 58A (2), 271-280CODEN: ICACEC; ISSN:0376-4710. (National Institute of Science Communication and Information Resources)
      Two series of palladium based bi-metallic catalysts, Pd((1-x))Aux (x = 0.1, 0.12, 0.15 and 0.2 ) and PdmMn (M = Cu/Ag/Au; m, n = 0.9, 0.1) supported on TiO2-P-25, have been prepd. and characterized by X-ray diffraction (XRD), Diffuse Reflectance Spectroscopy (DRS), Transmission Electron Microscopy (TEM), X-ray Photo-electron Spectroscopy (XPS) and Temp. Programmed Redn. (TPR). DRS and XPS studies indicate formation of nano scale alloys involving redistribution of charges within the metals. Selective hydrogenation of 1-heptyne in liq. phase has been studied on these catalysts at atm. pressure and in the temp. range 293-313 K. In the Pd((1-x))Aux series, the catalyst compn. Pd0.9Au0.1 > displays max. activity, expressed as TOF. Activity pattern in Pd0.9Au0.1 series follows the trend, Pd-Au > Pd-Ag = Pd-Cu > Pd. Selectivity for heptene formation is maintained at > 95% on all catalysts up to 60 min reaction time. Interplay of ensemble as well as ligand effects, acting simultaneously, influences the adsorption and activation of 1-heptyne, leading to higher activity on Pd-Au bimetallic catalyst vis-a-vis other bimetallic and mono metallic catalysts.
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    42. 42
      Zhang, R. G.; Xue, M. F.; Wang, B. J.; Ling, L. X.; Fan, M. H. C2H2 Selective Hydrogenation over the M@Pd and M@Cu (M = Au, Ag, Cu, and Pd) Core-Shell Nanocluster Catalysts: The Effects of Composition and Nanocluster Size on Catalytic Activity and Selectivity. J. Phys. Chem. C 2019, 123, 16107– 16117,  DOI: 10.1021/acs.jpcc.9b01757
      42
      C2H2 Selective Hydrogenation over the M@Pd and M@Cu (M = Au, Ag, Cu, and Pd) Core-Shell Nanocluster Catalysts: The Effects of Composition and Nanocluster Size on Catalytic Activity and Selectivity
      Zhang, Riguang; Xue, Mifeng; Wang, Baojun; Ling, Lixia; Fan, Maohong
      Journal of Physical Chemistry C (2019), 123 (26), 16107-16117CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      To clarify the effects of the compn. and nanocluster size of the core-shell catalysts on C2H4 selectivity and activity in C2H2 selective hydrogenation, the kinetic mechanisms of C2H2 selective hydrogenation over different compns. of M@Pd (M = Au, Ag, and Cu) and M@Cu (M = Au, Ag, and Pd) nanoclusters with different sizes are studied using d. functional theory calcns. Probably the compn. and nanocluster size of the core-shell catalyst affect C2H4 selectivity and activity, and Cu as the core for M@Pd catalysts exhibits excellent C2H4 selectivity and activity than that of Au and Ag; also, M@Pd catalysts show better C2H4 selectivity and activity than M@Cu. Namely, the core-shell nanocluster catalyst with Cu as the core and Pd as the shell is beneficial to improve C2H4 selectivity and activity in C2H2 selective hydrogenation. However, C2H4 selectivity and activity increase over M@Pd catalysts with the increase in the nanocluster size, which means that it is necessary to have the catalyst with a larger cluster size in the prepn. of Cu@Pd core-shell catalysts. The electronic structure anal. revealed the microscopic reasons about the effects of core-shell catalyst compns. and nanocluster size on the catalytic performance of C2H2 selective hydrogenation. This study can provide theor. guidance for the design of core-shell nanocluster catalysts to improve C2H4 selectivity and activity in C2H2 selective hydrogenation by adjusting the compn. and nanocluster size in an efficient way.
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    43. 43
      Yang, J. Y.; Fan, Y. P.; Li, Z. L.; Peng, Z. K.; Yang, J. H.; Liu, B. Z.; Liu, Z. Y. Bimetallic Pd-M (M= Pt, Ni, Cu, Co) nanoparticles catalysts with strong electrostatic metal-support interaction for hydrogenation of toluene and benzene. Mol. Catal. 2020, 492, 110992,  DOI: 10.1016/j.mcat.2020.110992
      43
      Bimetallic Pd-M (M = Pt, Ni, Cu, Co) nanoparticles catalysts with strong electrostatic metal-support interaction for hydrogenation of toluene and benzene
      Yang, Jingyi; Fan, Yanping; Li, Zhong-Li; Peng, Zhikun; Yang, Jing-He; Liu, Baozhong; Liu, Zhongyi
      Molecular Catalysis (2020), 492 (), 110992CODEN: MCOADH ISSN:. (Elsevier B.V.)
      Herein, we reported an effective method to synthesis a series of supported bimetallic Pd-M/SiO2 (M = Pt, Ni, Cu, Co) catalysts with highly exposed and well-alloyed nanoparticles via simultaneous strong electrostatic adsorption (co-SEA), which controlled metal precursors strongly anchored onto the oppositely charged support (SiO2) by changing the pH relative to the surface point of zero charge (PZC). The toluene and benzene hydrogenation were used as probe reactions to evaluate the catalytic hydrogenation activity performance of the Pd-M/SiO2 catalysts. Compared to the catalysts synthesized by the common technique simultaneous dry impregnation (co-DI), the co-SEA Pd-Ni/SiO2 catalyst exhibited the highest turnover frequencies (TOFs) of 3.7 s-1 for benzene hydrogenation and 3.2 s-1 for toluene hydrogenation. This was attributed to the large amt. of electron-deficient Pdδ+ sites on the surface of SEA Pd-M/SiO2 that easily for electron-rich benzene and toluene to adsorb. Besides, the co-SEA catalysts produced well stability and reusability without significant decrease in the catalytic activity after six runs.
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    44. 44
      Cao, X. X.; Mirjalili, A.; Xie, W. T.; Jang, W. L. Selective hydrogenation of acetylene in ethylene over Cu-Pd catalysts. Abstr. Pap. Am. Chem. Soc. 2016, 251
      There is no corresponding record for this reference.
    45. 45
      Lomelí-Rosales, D. A.; Delgado, J. A.; Diaz de Los Bernardos, M.; Perez-Rodriguez, S.; Gual, A.; Claver, C.; Godard, C. A General One-Pot Methodology for the Preparation of Mono- and Bimetallic Nanoparticles Supported on Carbon Nanotubes: Application in the Semi-hydrogenation of Alkynes and Acetylene. Chem.─Eur. J. 2019, 25, 8321– 8331,  DOI: 10.1002/chem.201901041
      45
      A General One-Pot Methodology for the Preparation of Mono- and Bimetallic Nanoparticles Supported on Carbon Nanotubes: Application in the Semi-hydrogenation of Alkynes and Acetylene
      Lomeli-Rosales, Diego A.; Delgado, Jorge A.; Diaz de los Bernardos, Miriam; Perez-Rodriguez, Sara; Gual, Aitor; Claver, Carmen; Godard, Cyril
      Chemistry - A European Journal (2019), 25 (35), 8321-8331CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A facile and straightforward methodol. for the prepn. of monometallic (copper and palladium) and bimetallic nanocatalysts (NiCu and PdCu) stabilized by a N-heterocyclic carbene ligand is reported. Both colloidal and supported nanoparticles (NPs) on carbon nanotubes (CNTs) were prepd. in a one-pot synthesis with outstanding control on their size, morphol. and compn. These catalysts were evaluated in the selective hydrogenation of alkynes and alkynols. PdCu/CNTs revealed an efficient catalytic system providing high selectivity in the hydrogenation of terminal and internal alkynes. Moreover, this catalyst was tested in the semi-hydrogenation of acetylene in industrially relevant acetylene/ethylene-rich model gas feeds and showed excellent stability even after 40 h of reaction.
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    46. 46
      Pei, G. X.; Liu, X. Y.; Yang, X.; Zhang, L.; Wang, A.; Li, L.; Wang, H.; Wang, X.; Zhang, T. Performance of Cu-Alloyed Pd Single-Atom Catalyst for Semihydrogenation of Acetylene under Simulated Front-End Conditions. ACS Catal. 2017, 7, 1491– 1500,  DOI: 10.1021/acscatal.6b03293
      46
      Performance of Cu-Alloyed Pd Single-Atom Catalyst for Semihydrogenation of Acetylene under Simulated Front-End Conditions
      Pei, Guang Xian; Liu, Xiao Yan; Yang, Xiaofeng; Zhang, Leilei; Wang, Aiqin; Li, Lin; Wang, Hua; Wang, Xiaodong; Zhang, Tao
      ACS Catalysis (2017), 7 (2), 1491-1500CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Selective hydrogenation of acetylene to ethylene is an industrially important reaction. Pd-based catalysts have been proved to be efficient for the acetylene conversion, while enhancing the selectivity to ethylene is challenging. Here, we chose Cu as the partner of Pd, fabricated an alloyed Pd single-atom catalyst (SAC), and investigated its catalytic performance for the selective hydrogenation of acetylene to ethylene under a simulated front-end hydrogenation process in industry: i.e., with a high concn. of hydrogen and ethylene. The Cu-alloyed Pd SAC showed ∼85% selectivity to ethylene and 100% acetylene elimination. In comparison with the Au- or Ag-alloyed Pd SAC, the Cu-alloyed analog exceeded both of them in conversion, while the selectivity rivaled that of the Ag-alloyed Pd SAC and surpassed that of the Au-alloyed Pd SAC. As Cu is a low-cost metal, Cu-alloyed Pd SAC would minimize the noble-metal usage and possess high utilization potential for industry. The Cu-alloyed Pd SAC was verified by EXAFS, with the Pd/Cu at. ratio lowered to 0.006, corresponding to the loading of Pd at 494 ppm. The microcalorimetric measurement results demonstrated that the adsorption of C2H4 over the Cu-alloyed Pd SAC was weaker than that over the catalyst with large Pd ensembles; thus, the selectivity to ethylene was greatly enhanced. At the same time, the adsorption of H2 was stronger than that over the corresponding monometallic Cu catalyst; thus, the activation of H2 was obviously promoted. On the basis of the above results, a possible reaction path over the Cu-alloyed Pd SAC was proposed. Furthermore, by systematic comparison of the IB-metal-alloyed Pd SACs, we found that the apparent activation energies of the IB-metal-alloyed Pd SACs were close to each other, indicating similar active sites and/or catalytic mechanisms over the three catalysts. The isolation of the Pd atoms by the IB metal distinctly contributed to both the conversion and the selectivity. Further DFT calcn. results suggested that electron transfer between the IB metal and Pd might be responsible for their different selectivities to ethylene.
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    47. 47
      Dong, Z.; Dong, C.; Liu, Y.; Le, X.; Jin, Z.; Ma, J. Hydrodechlorination and further hydrogenation of 4-chlorophenol to cyclohexanone in water over Pd nanoparticles modified N-doped mesoporous carbon microspheres. Chem. Eng. J. 2015, 270, 215– 222,  DOI: 10.1016/j.cej.2015.02.045
      47
      Hydrodechlorination and further hydrogenation of 4-chlorophenol to cyclohexanone in water over Pd nanoparticles modified N-doped mesoporous carbon microspheres
      Dong, Zhengping; Dong, Chunxu; Liu, Yansheng; Le, Xuanduong; Jin, Zhicheng; Ma, Jiantai
      Chemical Engineering Journal (Amsterdam, Netherlands) (2015), 270 (), 215-222CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      The development of new catalysts with high catalytic activity for the hydrodechlorination (HDC) of chlorophenols (CPs) in the industrial polluted water has recently triggered various research interests. In this study, a novel N-doped mesoporous carbon microspheres (N-MCM) was synthesized through a nanocasting route using the low-cost and widely available dicyandiamide as precursor and MCM-48 nanospheres as the hard template. The synthesized N-MCM material exhibited a large surface area of 120.8 m2 g-1, pore vol. of 0.24 cm3 g-1, av. pore size of 9.9 nm and rich N content of 27.2 wt%. The characterization results of FT-IR, Raman spectrum and XPS illustrated that N-MCM had graphitic-like structures consisted of carbon nitride heterocycles, as well as amino groups. When N-MCM was used as catalyst support, Pd nanoparticles could be well dispersed in the mesopores and on the surface of the N-MCM. And the Pd/N-MCM nanocatalyst showed excellent catalytic activity for the eco-friendly HDC of CPs under green conditions in 1 h. While interestingly, when the reaction time was continued, the HDC product phenol can be further hydrogenated to a much more useful product (cyclohexanone) with selectivity above 85%.
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    48. 48
      Ding, S.; Zhang, C.; Liu, Y.; Jiang, H.; Chen, R. Selective hydrogenation of phenol to cyclohexanone in water over Pd@N-doped carbons derived from ZIF-67: Role of dicyandiamide. Appl. Surf. Sci. 2017, 425, 484– 491,  DOI: 10.1016/j.apsusc.2017.07.068
      48
      Selective hydrogenation of phenol to cyclohexanone in water over Pd@N-doped carbons derived from ZIF-67: Role of dicyandiamide
      Ding, Shuaishuai; Zhang, Chunhua; Liu, Yefei; Jiang, Hong; Chen, Rizhi
      Applied Surface Science (2017), 425 (), 484-491CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Highly efficient Pd@CN catalysts for selective hydrogenation of phenol to cyclohexanone in water were successfully fabricated by loading Pd nanoparticles (NPs) in N-doped carbons (CN) derived from ZIF-67 with dicyandiamide (DICY) as the addnl. nitrogen source. For comparison, polyvinylpyrrolidone (PVP) was also used as the addnl. nitrogen source during the ZIF-67 synthesis. The results showed that the PVP and DICY had significantly different impacts on the microstructures of as-obtained CN materials and the catalytic performance of Pd@CN catalysts in the phenol hydrogenation. The addn. of DICY had the pos. promotion effect on the surface area of the obtained CN materials. Moreover, the introduction of DICY could increase the nitrogen content of CN and then prevent the re-oxidn. of Pd NPs during air contact, resulting in higher Pd0 ratio. In comparison with PVP, the DICY was more suitable as the addnl. nitrogen source for the formation of CN and Pd@CN (Pd@CND, Pd@CNP). The Pd@CND exhibited superior catalytic activity as compared to Pd@CNP (phenol conversion 96.9% vs. 67.4%). More importantly, the as-prepd. Pd@CND catalyst could be reused for four times without catalytic performance redn. The work would aid the development of Pd@CN catalysts with superior catalytic properties.
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    49. 49
      Zhang, C.; Zhang, J.; Shao, Y.; Jiang, H.; Chen, R.; Xing, W. Controllable Synthesis of 1D Pd@N-CNFs with High Catalytic Performance for Phenol Hydrogenation. Catal. Lett. 2021, 151, 1013,  DOI: 10.1007/s10562-020-03374-x
      49
      Controllable Synthesis of 1D Pd@N-CNFs with High Catalytic Performance for Phenol Hydrogenation
      Zhang, Chunhua; Zhang, Jiuxuan; Shao, Yanhua; Jiang, Hong; Chen, Rizhi; Xing, Weihong
      Catalysis Letters (2021), 151 (4), 1013-1024CODEN: CALEER; ISSN:1011-372X. (Springer)
      Achieving both high conversion and selectivity under mild conditions still remains a big challenge in the selective hydrogenation of phenol to cyclohexanone. Herein, one-dimensional (1D) N-doped carbon nanofibers (N-CNFs) were successfully fabricated by electrospinning with one-step carbonization, and used to load Pd nanoparticles for synthesizing Pd@N-CNFs catalysts. The dicyandiamide (DICY) and citric acid in the spinning soln. exhibited a significant synergistic effect in controlling the morphol. and surface property of N-CNFs and the corresponding catalytic activity of Pd@N-CNFs in the selective hydrogenation of phenol to cyclohexanone. The as-prepd. [email protected] catalyst possessed good fibrous morphol., larger surface area, and more amts. of surface N and OH group, and exhibited a phenol conversion of 99.7% with a cyclohexanone selectivity of 97.3% under mild reaction conditions. In addn., the catalytic activity of [email protected] increased by 2.75 times as compared to Pd@N-CNFs-0 and 1.22 times in comparison with Pd@N-CNFs-100. Furthermore, the 1D [email protected] was easy to be recovered from the reaction mixt., and showed good reusability. The proper molar ratio of dicyandiamide (DICY) and citric acid could significantly adjust the fibrous characteristic and surface properties of the Pd@CNFs catalyst, which contributed to the higher N and OH group contents, thereby improving the loading and distribution of Pd nanoparticles and the dispersibility of Pd@CNFs in water. These properties jointly led to the superior phenol hydrogenation efficiency of the Pd@CNFs catalyst.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhslKjtLnN&md5=aed040d71b16a1c90de9128b018115f6
    50. 50
      Chen, L. J.; Wan, C. C.; Wang, Y. Y. Chemical preparation of Pd nanoparticles in room temperature ethylene glycol system and its application to electroless copper deposition. J. Colloid Interface Sci. 2006, 297, 143– 150,  DOI: 10.1016/j.jcis.2005.10.029
      50
      Chemical preparation of Pd nanoparticles in room temperature ethylene glycol system and its application to electroless copper deposition
      Chen, Li-Jung; Wan, Chi-Chao; Wang, Yung-Yun
      Journal of Colloid and Interface Science (2006), 297 (1), 143-150CODEN: JCISA5; ISSN:0021-9797. (Elsevier)
      Room-temp. synthesis of Pd nanoparticles protected by polyvinylpyrrolidone (PVP) was successfully achieved by merely adding NaOH acting as accelerator for the redn. of Pd(II) in ethylene glycol (EG) without any externally added reducing agent. The Pd particle sizes were controlled at 8.6-2.4 nm by changing the concn. of NaOH from 0 to 3.2 × 10 -1 M. The particle formation was monitored by UV-visible spectroscopy and the microstructure of Pd nanoparticles was analyzed by TEM and XRD. The product of adding NaOH in EG was characterized by FTIR and a -CHO group which possesses reductive ability was identified. The prepd. Pd nanoparticle could serve as an effective activator for electroless Cu deposition (ECD) on epoxy substrate, which is an essential process in the printed circuit board (PCB) industry. In contrast to existing com. activators, the new activator shows superior stability and excellent performance for ECD.
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    51. 51
      Wang, X.; Choi, S. I.; Roling, L. T.; Luo, M.; Ma, C.; Zhang, L.; Chi, M.; Liu, J.; Xie, Z.; Herron, J. A.; Mavrikakis, M.; Xia, Y. Palladium-platinum core-shell icosahedra with substantially enhanced activity and durability towards oxygen reduction. Nat. Commun. 2015, 6, 7594  DOI: 10.1038/ncomms8594
      51
      Palladium-platinum core-shell icosahedra with substantially enhanced activity and durability towards oxygen reduction
      Wang Xue; Zhang Lei; Choi Sang-Il; Luo Ming; Roling Luke T; Herron Jeffrey A; Mavrikakis Manos; Ma Cheng; Chi Miaofang; Liu Jingyue; Xie Zhaoxiong; Xia Younan
      Nature communications (2015), 6 (), 7594 ISSN:.
      Conformal deposition of platinum as ultrathin shells on facet-controlled palladium nanocrystals offers a great opportunity to enhance the catalytic performance while reducing its loading. Here we report such a system based on palladium icosahedra. Owing to lateral confinement imposed by twin boundaries and thus vertical relaxation only, the platinum overlayers evolve into a corrugated structure under compressive strain. For the core-shell nanocrystals with an average of 2.7 platinum overlayers, their specific and platinum mass activities towards oxygen reduction are enhanced by eight- and sevenfold, respectively, relative to a commercial catalyst. Density functional theory calculations indicate that the enhancement can be attributed to the weakened binding of hydroxyl to the compressed platinum surface supported on palladium. After 10,000 testing cycles, the mass activity of the core-shell nanocrystals is still four times higher than the commercial catalyst. These results demonstrate an effective approach to the development of electrocatalysts with greatly enhanced activity and durability.
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    52. 52
      Wang, X.; Vara, M.; Luo, M.; Huang, H.; Ruditskiy, A.; Park, J.; Bao, S.; Liu, J.; Howe, J.; Chi, M.; Xie, Z.; Xia, Y. Pd@Pt Core–Shell Concave Decahedra: A Class of Catalysts for the Oxygen Reduction Reaction with Enhanced Activity and Durability. J. Am. Chem. Soc. 2015, 137, 15036– 15042,  DOI: 10.1021/jacs.5b10059
      52
      Pd@Pt Core-Shell Concave Decahedra: A Class of Catalysts for the Oxygen Reduction Reaction with Enhanced Activity and Durability
      Wang, Xue; Vara, Madeline; Luo, Ming; Huang, Hongwen; Ruditskiy, Aleksey; Park, Jinho; Bao, Shixiong; Liu, Jingyue; Howe, Jane; Chi, Miaofang; Xie, Zhaoxiong; Xia, Younan
      Journal of the American Chemical Society (2015), 137 (47), 15036-15042CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We report a facile synthesis of multiply twinned Pd@Pt core-shell concave decahedra by controlling the deposition of Pt on preformed Pd decahedral seeds. The Pt atoms are initially deposited on the vertices of a decahedral seed, followed by surface diffusion to other regions along the edges/ridges and then across the faces. Different from the coating of a Pd icosahedral seed, the Pt atoms prefer to stay at the vertices and edges/ridges of a decahedral seed even when the deposition is conducted at 200 °C, naturally generating a core-shell structure covered by concave facets. The nonuniformity in the Pt coating can be attributed to the presence of twin boundaries at the vertices, as well as the {100} facets and twin defects along the edges/ridges of a decahedron, effectively trapping the Pt adatoms at these high-energy sites. As compared to a com. Pt/C catalyst, the Pd@Pt concave decahedra show substantial enhancement in both catalytic activity and durability toward the oxygen redn. reaction (ORR). For the concave decahedra with 29.6% Pt by wt., their specific (1.66 mA/cm2Pt) and mass (1.60 A/mgPt) ORR activities are enhanced by 4.4 and 6.6 times relative to those of the Pt/C catalyst (0.36 mA/cm2Pt and 0.32 A/mgPt, resp.). After 10 000 cycles of accelerated durability test, the concave decahedra still exhibit a mass activity of 0.69 A/mgPt, more than twice that of the pristine Pt/C catalyst.
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    53. 53
      Wang, Z.; Zheng, S.; Wan, N.; Zhang, L.; Wang, Q.; He, N.; Huang, Y. Synthesis of a Au-on-Pd Heteronanostructure Stabilized by Citrate and its Catalytic Application. Part. Part. Syst. Charact. 2013, 30, 905– 910,  DOI: 10.1002/ppsc.201300152
      53
      Synthesis of a Au-on-Pd Heteronanostructure Stabilized by Citrate and its Catalytic Application
      Wang, Zhifei; Zheng, Shuang; Wan, Neng; Zhang, Liming; Wang, Qilong; He, Nongyue; Huang, Yaji
      Particle & Particle Systems Characterization (2013), 30 (10), 905-910CODEN: PPCHEZ; ISSN:1521-4117. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The prepn. of Au-on-Pd heteronanostructure (HNS) using citrate-stabilized polycryst. Pd nanoparticles (NPs) as the seeds is described. The resulting Au-on-Pd HNS is characterized and it is found that the formation of Au-on-Pd HNS depends greatly on a ratio between Pd seeds and AuCl4- ions added and the optimal molar ratio is 10:1. If fewer AuCl4- ions are added (Pd/Au ratio is 100:1), the growth of Au NPs only occurs on part of the Pd seeds' surface. The addn. of more AuCl4- ions (Pd/Au ratio is 5:1) hinders the growth of Au NPs on the Pd seeds' surface. To demonstrate the catalytic performance, the electrochem. oxidn. of ethanol and the redn. of p-nitrophenol by NaBH4 are chosen to examine the catalytic activity of Au-on-Pd HNS. Pd seeds, Au NPs, and poly(vinyl pyrrolidone) (PVP)-stabilized PdAu nanoalloy are used as the refs. for comparison. In the first reaction, the catalytic reactivity of Au-on-Pd HNS is better than that of corresponding pure Pd or Au NPs, while the opposite occurs for the latter reaction. The catalytic activity of Au-on-Pd HNS is much higher than that of PVP-stabilized PdAu nanoalloy.
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    54. 54
      Effenberger, F. B.; Sulca, M. A.; Machini, M. T.; Couto, R. A.; Kiyohara, P. K.; Machado, G.; Rossi, L. M. Copper nanoparticles synthesized by thermal decomposition in liquid phase: the influence of capping ligands on the synthesis and bactericidal activity. J. Nanopart. Res. 2014, 16, 2588  DOI: 10.1007/s11051-014-2588-7
      There is no corresponding record for this reference.
    55. 55
      Crespo-Quesada, M.; Yarulin, A.; Jin, M.; Xia, Y.; Kiwi-Minsker, L. Structure sensitivity of alkynol hydrogenation on shape- and size-controlled palladium nanocrystals: which sites are most active and selective?. J. Am. Chem. Soc. 2011, 133, 12787– 12794,  DOI: 10.1021/ja204557m
      55
      Structure Sensitivity of Alkynol Hydrogenation on Shape- and Size-Controlled Palladium Nanocrystals: Which Sites Are Most Active and Selective?
      Crespo-Quesada, Micaela; Yarulin, Artur; Jin, Mingshang; Xia, Younan; Kiwi-Minsker, Lioubov
      Journal of the American Chemical Society (2011), 133 (32), 12787-12794CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The activity and selectivity of structure-sensitive reactions are strongly correlated with the shape and size of the nanocrystals present in a catalyst. This correlation can be exploited for rational catalyst design, esp. if each type of surface atom displays a different behavior, to attain the highest activity and selectivity. In this work, uniform Pd nanocrystals with cubic (in two different sizes), octahedral, and cuboctahedral shapes were synthesized through a soln.-phase method with poly(vinyl pyrrolidone) (PVP) serving as a stabilizer and then tested in the hydrogenation of 2-methyl-3-butyn-2-ol (MBY). The obsd. activity and selectivity suggested that two types of active sites were involved in the catalysis - those on the planes and at edges - which differ in their coordination nos. Specifically, semihydrogenation of MBY to 2-methyl-3-buten-2-ol (MBE) occurred preferentially at the plane sites regardless of their crystallog. orientation, Pd(111) and/or Pd(100), whereas overhydrogenation occurred mainly at the edge sites. The exptl. data can be fitted with a kinetic modeling based on a two-site Langmuir-Hinshelwood mechanism. By considering surface statistics for nanocrystals with different shapes and sizes, the optimal catalyst in terms of productivity of the target product MBE was predicted to be cubes of roughly 3-5 nm in edge length. This study is an attempt to close the material and pressure gaps between model single-crystal surfaces tested under ultra-high-vacuum conditions and real catalytic systems, providing a powerful tool for rational catalyst design.
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    56. 56
      Chan-Thaw, C. E.; Villa, A.; Wang, D.; Dal Santo, V.; Biroli, A. O.; Veith, G. M.; Thomas, A.; Prati, L. PdHx Entrapped in a Covalent Triazine Framework Modulates Selectivity in Glycerol Oxidation. ChemCatChem 2015, 7, 2149– 2154,  DOI: 10.1002/cctc.201500055
      56
      PdHx Entrapped in a Covalent Triazine Framework Modulates Selectivity in Glycerol Oxidation
      Chan-Thaw, Carine E.; Villa, Alberto; Wang, Di; Dal Santo, Vladimiro; Orbelli Biroli, Alessio; Veith, Gabriel M.; Thomas, Arne; Prati, Laura
      ChemCatChem (2015), 7 (14), 2149-2154CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Pd nanoparticles within a nitrogen-contg. covalent triazine framework (CTF) material are investigated to understand if the highly tunable CTF chem. mediates the catalytic properties of the Pd nanoparticles. Surprisingly, our results demonstrate that the CTF stabilizes the formation of 2.6 nm PdHx particles within the pores. These confined PdHx particles are very active for the liq.-phase oxidn. of glycerol and promote C-C cleavage, probably connected with the enhanced in situ formation of H2O2. During recycling tests, the confined particles are transformed progressively to very stable Pd0 particles. This stability has been attributed mainly to a confinement effect as nanoparticles trapped outside the pores lose activity rapidly. These results indicate that there is a potential to tune CTF chem. to modify the chem. of the catalytic metals significantly.
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    57. 57
      Wu, L.; Li, Z.-W.; Zhang, F.; He, Y.-M.; Fan, Q.-H. Air-Stable and Highly Active Dendritic Phosphine Oxide- Stabilized Palladium Nanoparticles: Preparation, Characterization and Applications in the Carbon-Carbon Bond Formation and Hydrogenation Reactions. Adv. Synth. Catal. 2008, 350, 846– 862,  DOI: 10.1002/adsc.200700441
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      Air-stable and highly active dendritic phosphine oxide-stabilized palladium nanoparticles: preparation, characterization and applications in the carbon-carbon bond formation and hydrogenation reactions
      Wu, Lei; Li, Zhi-Wei; Zhang, Feng; He, Yan-Mei; Fan, Qing-Hua
      Advanced Synthesis & Catalysis (2008), 350 (6), 846-862CODEN: ASCAF7; ISSN:1615-4150. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Dendrimer-stabilized palladium nanoparticles were formed in the redn. of palladium bis(acetylacetonate) [Pd(acac)2] in the presence of phosphine dendrimer ligands using hydrogen in THF. The resulting Pd nanoparticles were characterized by TEM, 31P NM,R and 31P MAS NMR. The results indicated that the dendritic phosphine ligands were oxidized to phosphine oxides. These dendrimer-stabilized Pd nanoparticles were demonstrated to be efficient catalysts for Suzuki and Stille coupling reactions and hydrogenations. The dendritic wedges served as a stabilizer for keeping the nanoparticles from aggregating, and as a vehicle for facilitating the sepn. and/or the recycling of the Pd catalyst. In the case of the Suzuki coupling reaction, these Pd nanoparticles exhibited high catalytic efficiency (TON up to 65,000) and air stability as compared with the commonly used homogeneous catalyst tetrakis(triphenylphosphine)palladium [Pd(PPh3)4]. In addn., the results obtained from the bulky dendritic substrate suggest that the Pd nanoparticles might act as reservoir of catalytically active species, and that the reaction is actually catalyzed by the sol. Pd(0) and/or Pd(II) species leached from the nanoparticle surface.
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    58. 58
      Kumar, S.; Rao, G. K.; Kumar, A.; Singh, M. P.; Saleem, F.; Singh, A. K. Efficient catalytic activation of Suzuki–Miyaura C–C coupling reactions with recyclable palladium nanoparticles tailored with sterically demanding di-n-alkyl sulfides. RSC Adv. 2015, 5, 20081– 20089,  DOI: 10.1039/C5RA00441A
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      Efficient catalytic activation of Suzuki-Miyaura C-C coupling reactions with recyclable palladium nanoparticles tailored with sterically demanding di-n-alkyl sulfides
      Kumar, Satyendra; Rao, G. K.; Kumar, Arun; Singh, Mahabir P.; Saleem, Fariha; Singh, Ajai K.
      RSC Advances (2015), 5 (26), 20081-20089CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The compd. n-bromodocosane reacts with Na2S, generated in situ by the redn. of elemental sulfur with NaBH4, to give n-didocosyl sulfide (L1), which acts as a protector for palladium nanoparticles that are prepd. using different palladium precursors in the presence of L1 (Pd : L1 ratio 1 : 2 and 4 : 1) and the nanoparticles have been characterized with powder X-ray diffraction. The size (nm) ranges for the majority of spherical nanoparticles are ~18-19, 4-5, 5-7, 4-6, 7-9 and 4-6 resp. The precursor of palladium affects the size, shape and dispersion of the nanoparticles. The nanoparticles show good catalytic activity for the Suzuki-Miyaura coupling of various aryl chlorides/bromides ArCl/ArBr [Ar = 4-O2NC6H4, 4-H(O)CC6H4, pyridin-2-yl, etc.] with phenylboronic acid at low catalyst loading (0.1-0.5 mol% of Pd). The distinct advantage of nanoparticles is that they can be sepd. and reused at least up to five times. The compd. didocosyl sulfide and its complex [Pd(L1)2Cl2] have also been synthesized by the reaction of Na2PdCl4 with didocosyl sulfide and the complex [Pd(L1)2Cl2], equiv. to 0.001 mol% Pd, is efficient for the Suzuki-Miyaura coupling of some aryl halides, as good conversion into coupled products has been obsd. Two phase tests, conducted for some nanoparticles suggest the contribution of both homogeneous and heterogeneous catalytic pathways in overall catalysis.
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    59. 59
      Zhao, X.; Zhou, L.; Zhang, W.; Hu, C.; Dai, L.; Ren, L.; Wu, B.; Fu, G.; Zheng, N. Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes. Chem 2018, 4, 1080– 1091,  DOI: 10.1016/j.chempr.2018.02.011
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      Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes
      Zhao, Xiaojing; Zhou, Lingyun; Zhang, Wuyong; Hu, Chengyi; Dai, Lei; Ren, Liting; Wu, Binghui; Fu, Gang; Zheng, Nanfeng
      Chem (2018), 4 (5), 1080-1091CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      Surface and interfacial engineering of heterogeneous metal catalysts is effective and crit. for optimizing selective hydrogenation for fine chems. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from com. Pd/C catalysts for semihydrogenation of internal alkynes.
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    60. 60
      Jin, M.