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Evolution of Oxygen Vacancy Sites in Ceria-Based High-Entropy Oxides and Their Role in N2 Activation
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Energy, Environmental, and Catalysis Applications

Evolution of Oxygen Vacancy Sites in Ceria-Based High-Entropy Oxides and Their Role in N2 Activation
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  • Omer Elmutasim
    Omer Elmutasim
    Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
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  • Aseel G. Hussien
    Aseel G. Hussien
    Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
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  • Abhishek Sharan
    Abhishek Sharan
    Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    Physics Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
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  • Sara AlKhoori
    Sara AlKhoori
    Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
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  • Michalis A. Vasiliades
    Michalis A. Vasiliades
    Department of Chemistry, Heterogeneous Catalysis Laboratory, University of Cyprus, 1 University Avenue, University Campus, 2109 Nicosia, Cyprus
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    Orcidhttps://orcid.org/0000-0002-1568-1158
  • Inas Magdy Abdelrahman Taha
    Inas Magdy Abdelrahman Taha
    Physics Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
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  • Seokjin Kim
    Seokjin Kim
    Oxide & Organic Nanomaterials for Energy & Environment (ONE) Laboratory, Advanced Membranes & Porous Materials (AMPM) Center, and KAUST Catalysis Center (KCC), Physical Science & Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia
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    Orcidhttps://orcid.org/0000-0002-6052-547X
  • Messaoud Harfouche
    Messaoud Harfouche
    Synchrotron-Light for Experimental Science and Applications in the Middle East (SESAME), Allan 19252, Jordan
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  • Abdul-Hamid Emwas
    Abdul-Hamid Emwas
    Core Laboratories, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
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  • Dalaver H. Anjum
    Dalaver H. Anjum
    Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    Physics Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
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    Orcidhttps://orcid.org/0000-0003-2336-2859
  • Angelos M. Efstathiou
    Angelos M. Efstathiou
    Department of Chemistry, Heterogeneous Catalysis Laboratory, University of Cyprus, 1 University Avenue, University Campus, 2109 Nicosia, Cyprus
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    Orcidhttps://orcid.org/0000-0001-8393-8800
  • Cafer T. Yavuz
    Cafer T. Yavuz
    Oxide & Organic Nanomaterials for Energy & Environment (ONE) Laboratory, Advanced Membranes & Porous Materials (AMPM) Center, and KAUST Catalysis Center (KCC), Physical Science & Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia
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    Orcidhttps://orcid.org/0000-0003-0580-3331
  • Nirpendra Singh*
    Nirpendra Singh
    Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    Physics Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-8043-0403
  • Kyriaki Polychronopoulou*
    Kyriaki Polychronopoulou
    Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-0723-9941
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ACS Applied Materials & Interfaces

Cite this: ACS Appl. Mater. Interfaces 2024, 16, 18, 23038–23053
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https://doi.org/10.1021/acsami.3c16521
Published April 29, 2024

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Abstract

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In this work, a relatively new class of materials, rare earth (RE) based high entropy oxides (HEO) are discussed in terms of the evolution of the oxygen vacant sites (Ov) content in their structure as the composition changes from binary to HEO using both experimental and computational tools; the composition of HEO under focus is the CeLaPrSmGdO due to the importance of ceria-related (fluorite) materials to catalysis. To unveil key features of quinary HEO structure, ceria-based binary CePrO and CeLaO compositions as well as SiO2, the latter as representative nonreducible oxide, were used and compared as supports for Ru (6 wt % loading). The role of the Ov in the HEO is highlighted for the ammonia production with particular emphasis on the N2 dissociation step (N2(ads) → Nads) over a HEO; the latter step is considered the rate controlling one in the ammonia production. Density functional theory (DFT) calculations and 18O2 transient isotopic experiments were used to probe the energy of formation, the population, and the easiness of formation for the Ov at 650 and 800 °C, whereas Synchrotron EXAFS, Raman, EPR, and XPS probed the Ce–O chemical environment at different length scales. In particular, it was found that the particular HEO composition eases the Ov formation in bulk, in medium (Raman), and in short (localized) order (EPR); more Ov population was found on the surface of the HEO compared to the binary reference oxide (CePrO). Additionally, HEO gives rise to smaller and less sharp faceted Ru particles, yet in stronger interaction with the HEO support and abundance of Ru–O–Ce entities (Raman and XPS). Ammonia production reaction at 400 °C and in the 10–50 bar range was performed over Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO2 catalysts; the Ru/HEO had superior performance at 10 bar compared to the rest of catalysts. The best performing Ru/HEO catalyst was activated under different temperatures (650 vs 800 °C) so to adjust the Ov population with the lower temperature maintaining better performance for the catalyst. DFT calculations showed that the HEO active site for N adsorption involves the Ov site adjacent to the adsorption event.

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1. Introduction

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Ammonia is maybe the most important chemical for humankind. The widely used, industrial-scale process for ammonia production is still dependent on the energy-demanding Haber–Bosch (HB) process (temperatures in the 723–773 K and pressures in the 15–30 MPa); the latter is consuming high amounts of energy (nearly above 1% of the global energy) in annual basis. Therefore, the design of processes that are more environmentally friendly and less energy-hungry is of pivotal importance. (1) Shifting the energy demanding HB process to milder conditions would require a catalyst surface where N and H atoms are not bind strongly, while the activation barrier for the N2 to N dissociation is rather low; the latter is inversely dependent to the adsorbed atomic N stability (scaling relations). (2)
Fe and Ru are among the traditional catalysts for ammonia production with Fe being more acceptable for industrial use due to its lower costs. (3,4) The dissociative mechanism involves the adsorption of N2 followed by its dissociation into 2N (atomic nitrogen) before its reaction with H2 and the production of NH3(g). The energy-demanding step is that of N2 activation where the breaking of the N–N stable triple bond is involved; the latter is considered the rate-determining step on Ru and Fe widely used, catalysts. Therefore, a thorough understanding of the active sites and/or the catalyst design criteria involved in the N2 activation is crucial for optimizing the ammonia production rate.
Cerium dioxide (CeO2) is a widely used catalyst component for a span of applications from industrial and automobile exhausts (5) to promote the water gas shift reaction, (6) and hydrogen production reactions (7) as well as solid electrolyte in fuel cells, (8) due to its superior ability to recycle oxygen through redox reactions and high mobility of oxygen ion. The rapid formation and elimination of oxygen vacancy defects plays a crucial role in all these catalytic reactions. Ceria-based high-entropy oxides are expected to withhold great potential for catalytic reactions where the high oxygen mobility is a significant catalyst treatment, (9,10) likely due to the theoretically predicted easiness of formation of the oxygen vacancies. Other properties of HEO that make them attractive candidates for catalysis/electrocatalysis are their high ionic/mixed conductivity due to their high lattice strain (11) as well as the presence of multiple luminescent centers (origin of exceptional optical properties). In particular, the lattice distortion in the HEO contributes to properties such as good transparency, high refractive index, overall enhancing the imaging quality, and expanding viewing angle lens. Moreover, the stabilization of unconventional spin-electronic states can lead to noteworthy magnetic applications. It has been reported that the HEO with spinel structure responds quickly to small magnetic field changes. Additionally, the combination of morphological features with the tunability of the lattice distortion can give rise to distinct values of strength and elastic modulus (mechanical properties). Last but not least, the low thermal conductivity of the high entropy oxides is usually combined with fairly good electrical conductivity reaching the levels of steel and lead. (12−14) In a recent review article by our group, the design criteria of the HEO are explicitly presented and discussed toward tuning the features that dictate their catalysis chemistry. (10) As it has also been eloquently discussed in (15) the multielemental nature of the HEO gives space for tunability of the active sites and/or vacancies location, whereas understanding the defects formation plays a major role in unveiling their functional properties and driving new discoveries. From a thermodynamic perspective, the driving force for defect formation is the configurational entropy increase, without undermining the role of the constituent elements. (9)
In the present study, the evolution of the formation of oxygen vacancy (Ov) defects as a result of stepwise transition from pure ceria (single oxide), to binary, to ternary systems, and eventually to quinary rare earth high entropy oxide (HEO) systems is explored. The CeLaPrSmGdO composition was used as model one. Computational ab initio tools, as well as experimental techniques, such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and synchrotron X-ray absorption spectroscopy (XAS) along with detailed electron microscopy tools involving HRTEM, STEM-HAADF, RGB analysis, FFT analysis, and isotopic oxygen exchange transient experiments, are employed. Following the investigation of the Ov formation in the HEO composition, a demonstration of the role of Ov in the ammonia thermal production is provided over Ru based HEO supported catalyst with a critical comparison with Ru-based catalysts where the support is either binary reducible oxide (CePrO, CeLaO) or nonreducible one (SiO2). The impact of the support (HEO vs CePrO) on the growth of Ru particles shape and size is critically discussed whereas insights on the N2 dissociation over the Ru/HEO catalyst and the role of the Ov proximity to the N2 dissociation site (HEO active site) are provided.

2. Methods

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2.1. Computational Studies

The first-principles calculations are based on density functional theory as this is developed in the Vienna ab initio Simulation package (VASP), (16,17) using generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (18) as the exchange-correlation functional. Projector-augmented-wave potentials are used to account for the valence electrons and ionic core interactions. (19,20) 4f electrons of the rare-earth elements are considered core electrons for structural relaxations, while for computation of the total energy, 4f electrons are considered valence electrons. For computational efficacy, soft pseudopotentials of oxygen and nitrogen are used. The GGA+U method (21) is used to push the unoccupied 4f orbitals of the rare-earth elements away from the Fermi level, and the Hubbard U parameter is set to 5 eV for all the rare-earth elements. Spin-polarized calculations are performed for structural relaxations, while for computations of the total energy spin–orbit interaction is taken into account. A fluorite-type crystal structure (with space group 225) of pure CeO2 is used for computations. The structure of the ternary and quinary high-entropy oxide (HEO) is generated using special quasirandom structures (SQS) (22) based on a 144 and 180-atom supercell respectively on a parent CeO2 prototype structure, using alloy theoretic automated toolkit (ATAT) code. (23) SQS has been successful in describing the electronic and thermodynamic properties of various disordered systems. (24) Energy cutoff of 350 eV is used for plane wave basis set expansion and Γ-centered k-point mesh of 4 × 4 × 4, 3 × 5 × 2, 3 × 2 × 2, and 2 × 1 × 1 is used for pure CeO2, binary, ternary and quinary HEO, respectively. The oxygen vacancies on the (111) surface of these systems are considered, where a vacuum region of 20 Å is used in the supercell containing the surface to avoid spurious interactions with the periodic images. The formation energy (ΔEform) of oxygen vacancy is computed as follows:
ΔEf=Esp:Ovac+12EO2−Esp
(1)
where Esp:Ovac is the energy of the supercell with one oxygen vacancy, EO2 is the energy of one oxygen molecule, and Esp is the energy of the supercell without any oxygen vacancy.

2.1.1. Ru on HEO Calculations

The literature revealed that the ruthenium cluster, having a pyramid structure composed of 4 atoms, supported on ceria is an energetically stable structure. (25) Likewise, in this work, a similar Ru4 cluster was deposited atop the oxygen vacancy site on reduced quinary LaPrCeSmGdO10(111) surface (hereafter named the Ru4/HEO surface).

2.2. Experimental Studies

2.2.1. Catalysts Preparation

2.2.1.1. Metal Oxide Synthesis
In this study, the high entropy oxide (CeLaPrSmGdO) support and two reference (reducible) supports (i.e., CeLaO and CePrO) were synthesized via the coprecipitation method. (26) The total moles of the metal nitrate precursors were maintained at 0.023 mol. For the CeLaPrSmGdO (HEO) support, the metal nitrate precursors with equimolar composition were dissolved in 45 mL DI water and placed on an orbital shaker with RPM set to 340 at 65 °C for 15 h. Then, 12.5 mL of ammonium hydroxide was added to the solution while stirring for an extra 2 h. The mixture was dried overnight at 100 °C, then calcined at 900 °C at a rate of 5 °C/min. A similar procedure was followed for the synthesis of reference supports, CeLaO and CePrO, except that the molar ratio of Ce to the metal (i.e., La and Pr) was 4:1. The SiO2 support was provided by Zeofree (commercial) and it was used as received.
2.2.1.2. Ru-Based Catalyst Synthesis
Ruthenium nanoparticles were prepared by chemical reduction of RuCl3·xH2O using NaBH4 as a reducing agent and KOH as a stabilizer (in situ reduction). First, 50 mL of 2-propanol, 50 mL of DI water, and 360 mg of oxide support (HEO, CePrO, CeLaO, SiO2) were mixed together and sonicated for 20 min. Then, 0.047 g of RuCl3·xH2O was added to the mixture and sonicated for 5 more mins. The mixture was stirred on a high magnetic stirrer at 500 rpm for 10 min. Then, a mix of 0.1 M KOH and 0.1 M of NaBH4 (total 50 mL) was slowly added and stirred for an additional 1 h. The final mix was washed 3 times in DI water and then dried overnight at 90 °C. Therefore, the prepared Ru-based catalysts are the: Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO2 all with 6 wt % Ru loading.

2.2.2. Characterization of Catalysts

Most of the techniques used in this study are described in previous studies. (27,28) More details about Raman, XPS, EPR, and HRTEM are given in the SI.
2.2.2.1. Synchrotron EXAFS
The X-ray absorption fine structure (XAFS) allows to assess the oxidation state by taking into account the X-ray absorption near edge structure (XANES) segment of the spectra; additionally, it helps to unveil structural characteristics at different length scales- short and medium range- through utilizing the extend X-ray absorption fine structure (EXAFS) part of data; the latter were collected on the BM08-XAFS/XRF beamline (29) at Synchrotron-light for Experimental Science and Applications in the Middle East (SESAME), Jordan for all catalysts of this study following their reduction (10 vol % H2/Ar, 650 °C, 120 min). XAFS spectra were recorded, at room temperature, in step-by-step scanning mode at the Ce LIII-edge (5723 eV). Transmission and fluorescence modes of the signal were collected in XAFS in both ionization chambers and the energy-selective single-element silicon drift detector (SDD) from KETEK GmbH, Germany. For scanning the energy, at the XANES region, with a step size of 0.2 and 0.5 eV for the pre-edge and white line, respectively, a double crystal monochromator equipped with Si(111) crystal was used. Variable energy steps using a fixed k space wavenumber of 0.03 A–1 were utilized for recording the EXAFS region of the spectra. Demeter (30) and WinXAS (31) software packages were used for the collection and processing of the XAFS data. The reference spectra used for XANES are from the databases (32,33) for the CeO2, CePO4, and CeF4.
2.2.2.2. 16O/18O Surface Isotopic Exchange
The solid in powder form (50 mg, dp <106 μm) was introduced into a quartz CSTR microreactor, (34) where the absence of interparticle (external) and intraparticle (internal) concentration gradients was verified. (35) Both catalysts were pretreated as follows: (i) Exp 1: Calcination (5% O2/He) at 600 °C/1 h; (ii) Exp 2: Calcination (5% O2/He) at 600 °C for 1 h, Reduction (5% H2/He) at 600 °C/2 h; (iii) Exp 3: Calcination (5% O2/He) at 600 °C for 1 h, Reduction (5% H2/He) at 800 °C/2 h. Transient Isothermal Oxidation (TIO) was used to investigate the formation of oxygen vacancies as a function of the pretreatment procedure over the two solids. More specifically, following the catalyst pretreatment, the reactor was purged in He for 30 min at 600 °C, followed by a step-gas switch to 1% 16O2/1% Kr/Ar/He (FT = 100 N mL min–1). It should be mentioned that at the end of TIO, steady-state rates of oxygen transfer between gas-phase oxygen and lattice oxygen (OL)/oxygen vacancies (Ov) of the solid are established. TIO experiment was followed by a transient isothermal isotopic exchange with 18O2 experiment (18O2-TIIE) in order to probe the kinetics of 16O/18O surface isotopic exchange and bulk oxygen diffusion (oxygen mobility) under equilibrium or pseudo equilibrium conditions, (36) verified by the sum (1 mol %) of all three isotopologues oxygen gas compositions during the whole transient. It is worth mentioning that the shape and the position of the 16O18O species strongly depend on the bulk oxygen diffusion coefficient (Deff, cm2 s–1) of the solid, but also the amount of oxygen (N16O, mmol g–1) available to be exchanged. (37) Specifically, during 18O2-TIIE the following gas step switch was applied: 1% 16O2/1% Kr/Ar/He → 1% 18O2/Ar/He at 600 °C. During the 18O gas mixture, the transient responses of 16O2, 16O18O, 18O2, and Kr (m/z = 32, 34, 36, and 84, respectively) were continuously monitored via an online mass spectrometer (Balzers, Quadrupole 1–200 amu), and converted into concentration (mol %) by using certified gas mixtures (i.e., 2% 16O2/He, 5% 18O2/He, and the impurity concentration of 16O18O(g) present in the 5% 18O2/ He).
The N16O (mmol g–1) amount of 16O/18O exchanged (oxygen storage capacity, OSC) was estimated via the material balance depicted in eq 2:
N16O[mol g−1]=2yf16O2FTW∫0t(ZO162(t)−ZKr(t))dt+FTW∫0t(yO16O18(t)−yO16O18f[1−ZKr])dt
(2)
where Zi and yi are the dimensionless response and mole fraction of gaseous species i, respectively.
The dimensionless αg(18)(t) descriptor function illustrates both the surface 16O/18O exchange (initial period under 18O gas), which is crucial from an industrial point of view, and the oxygen diffusion in the bulk (prolonged time on 18O gas), as obtained via eq 3. It should be noted that the lower the value of αg(18)(t), the faster the oxygen diffusion in the bulk of the solid.
αg(18)(t)=yO16O18+2yO1822∑yOiOj
(3)

2.2.3. Catalytic Evaluation

Catalytic ammonia synthesis tests were conducted within an Avantium system (Flowrence XD, Netherlands) using a quartz tube reactor with an I.D. of 2 mm and an O.D. of 3 mm, equipped with a porosity 3 filter. Four reactors were simultaneously employed for screening purposes, with each reactor loaded with 25 mg of catalyst, which was diluted with 600 μL of silicon carbide (SiC) grit (Lot: 10226827, Alfa Aesar). One reactor was kept as a blank control, containing only SiC. Before the activity measurement, the catalysts were treated in an in situ reduction/activation process. This was achieved under a gas mixture consisting of hydrogen (99.999%) and nitrogen (99.999%) at a 3:1 ratio, with a total flow rate of 15 mL min–1 at 650 °C (ramping rate: 5 °C min–1) for 2 h. Catalytic performance assessments were conducted at 10 bar, 400 °C. For these experiments, precise gas flow rates of 11.25 for hydrogen and 3.75 mL min–1 for nitrogen (corresponding to a weight hourly space velocity of 36,000 mL gcat–1 h–1) were maintained using mass flow controllers. An online gas chromatogram (8890 GC system, Agilent) was employed to analyze the reaction products under isothermal conditions. Helium (99.999%) at a flow rate of 1.25 mL min–1 was used as an internal standard for quantitative analysis. All experiments were replicated three times, and the average values were utilized for generating graphical representations and data analysis. To eliminate the scenario of spurious NH3 presence (e.g., due to possible impurities in the lines), control experiments were performed following the procedure: the same experimental protocol (reduction at 75% H2 −25% N2 at 650 °C for 2 h) followed by the reaction conducted in the absence of N2 in the feed (control experiment), where gas flow of H2 15 mL/min, and He 1.25 mL/min, at 400 °C, and pressure of 10 bar was used.

3. Results and Discussion

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3.1. Computational Studies

3.1.1. Structural Stability

Ceria exists in fluorite cubic crystal structure with space group no. 225 as shown in Figure 1a. The formation energy of ceria is −4.04 eV/atom (shown in Table 1). To understand the formation of oxygen vacancy in high-entropy systems calculations for Ce-based binary systems, CeGdO4 (coded as CeGdO), CePrO4 (coded as CePrO), ternary systems, CeLaGdO6, CeLaPrO6, (coded as CeLaGdO and CeLaPrO) and quinary high-entropy oxide CeLaPrSmGdO10 (coded as CeLaPrSmGdO) were performed. The crystal structure of the binary system is derived from the ordered arrangement of rare earth atoms in the crystal structure of ceria. For ternary and quinary high entropy systems, crystal structure is derived by SQS (22) using the ATAT package. (23) The bulk crystal structures for pure CeO2, binary CeGdO, ternary CeLaGdO, and quinary CeLaPrSmGdO are shown in Figure 1. The formation energy for binary, ternary, and quinary bulk phases is mentioned in Table 1. It is to be noted that the formation energy is negative even for high-entropy systems, suggesting that although configurational entropy contributes to the stability of these systems, it is not the governing criterion for stability, as these materials are stable even at T = 0 K.
Table 1. Formation Energy in eV/fu for CeO2 and CeO2-Based High Entropy Systems
 ΔHf (eV/atom)
CeO2–4.04
binary (CeGdO)–4.83
binary (CePrO)–3.76
ternary (CeLaGdO)–4.26
ternary (CeLaPrO)–3.54
quinary (CeLaPrSmGdO)–4.13

Figure 1

Figure 1. Unit cells of (a) pure CeO2, (b) binary CeGdO, (c) ternary CeLaGdO, and (d) quinary CeLaPrSmGdO.

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3.1.2. Evolution of Oxygen Vacant Sites

To understand the evolution of oxygen vacancy formation in ceria-based high entropy systems, the oxygen vacancy formation energy (ΔEf) along (111) surface for each of the cases, pure CeO2, binary, ternary, and quinary systems was computed. As shown in Figure 2a the formation energy of oxygen vacancy defect using eq 1 in bulk, sublayer, and top-layer along (111) surface of various systems was computed. For pure CeO2 the ΔEf in bulk is 4.71 eV, on the top-layer is 2.93 eV and the sublayer is 3.21 eV, which is in agreement with previous studies. (38,39) Similarly, ΔEf is computed for different systems (shown in Figure 1) along (111) surface, values shown in Table 2 and graphically shown in Figure 2b.
Table 2. Formation Energy (ΔHf) in Top-Layer, Sub-Layer, and Bulk for Various Systems
  ΔEf (eV)
CeO2top-layer2.93
 sublayer3.21
 bulk4.71
binary (CeGdO)top-layer–0.28
 sublayer0.18
 bulk4.15
binary (CePrO)top-layer–0.24
 sublayer–0.13
 bulk2.17
ternary (CeLaGdO)top-layer–0.20
 sublayer–0.09
 bulk0.07
ternary (CeLaPrO)top-layer–0.62
 sublayer–0.50
 bulk0.29
quinarytop-layer–0.43
(CeLaPrSmGdO)sublayer0.01
 bulk0.21

Figure 2

Figure 2. (a) Unit cell of CeO2 along (110) surface with vacuum. Bulk region, sublayer, and top-layers are shown. (b) Evolution of oxygen vacancy formation energy in pure CeO2, binary systems (CeGdO, CePrO), ternary systems (CeLaGdO, CeLaPrO), and quinary (CeLaPrSmGdO) high entropy system.

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Several possible sites of oxygen vacancy in bulk and on (111) surface are considered and the one with the lowest energy is considered for further analysis. It is observed that as the number of heteroatoms increases, moving from pure ceria to mixed cation systems, ΔEf reduces and eventually reaches values below zero, suggesting that in high-entropy systems (HEO) oxygen vacancy (Ov) formation is energetically more favorable as compared to pure CeO2. This is due to the ease of transition of cation from +4 oxidation state to +3 oxidation state. Pr, Sm, and Gd exhibit various oxidation states, including +3 and +4. + 3 oxidation state of La, Pr, and Gd is more favorable as they exhibit fully filled [Xe], [Xe] 6s2, and [Xe] 4f7 electron configuration. Therefore, replacing Ce with these rare-earth elements tends to ease the formation of oxygen vacancy both in bulk and on the surface. It is also noticeable that ΔEf is highest in bulk, and gradually reduces in sublayer, and is lowest at the surface. This trend is observed in all the systems. This can be explained by the fact that the oxygen atoms on the (111) surface of these ceria-based systems contain dangling bonds, such that the surface is electron-poor. Removal of an oxygen atom on the surface provides extra charge from the cations which are near to the vacancy site, to the already electron-poor surface, thereby favoring the formation of oxygen vacancy.

3.2. Experimental Studies: Monitoring of the HEO vs Binary Oxides Differences

Further understanding of the fundamental differences of the HEO, compared to (1) simpler structures of ceria-based oxides (e.g., binary supports, such as CeLaO, CePrO) and (2) nonreducible ones (Silica) was herein attempted using a toolkit of analytical techniques. For this reason, the catalysts with Ru/CeLaO, Ru/CePrO, Ru/SiO2, and Ru/HEO compositions were investigated; more details are provided in what follows.

3.2.1. Overview of Structural/Textural and Morphological Features

Figure 3A presents the XRD patterns of the Ru-based catalysts on different supports of this study following their reduction at 650 °C for 2 h under an H2 atmosphere; the supports were CePrO, CeLaO, HEO, and silica. All the ceria-based supports present the characteristic reflections of the fluorite ceria lattice; namely 33.2°, 47.6°, 56.5°, 59.1°, 69.4°, 76.6°, and 79.2° are assigned to the diffractions of (111), (200), (220), (311), (222), (400), (311), and (420) crystallographic planes of CeO2. (40) The catalyst where silica was used as support presents a rather amorphous pattern, whereas the Ru/HEO catalyst presents no reflection due to a dopant relevant phase (e.g., La2O3, Sm2O3, Gd2O3, Pr2O3, or combination/solid solution of them). On the other hand, despite the relatively high Ru loading (6 wt %), there is no obvious reflection peak corresponding to Ru/RuO2 phases, demonstrating the high dispersion of Ru and/or RuO2 phases in all the supports herein used (regardless of their nature: reducible or nonreducible). N2 adsorption at 77 K (Figure S1) over all the ceria-based supported catalysts showed similar BET surface areas (Table S1) of the catalysts Ru/CeLaO (18 m2/g), Ru/CePrO (6 m2/g), and Ru/HEO (18 m2/g), whereas the pore volume was found to be in the 0.032–0.048 cm3/g range for the ceria-based catalysts and 0.936 cm3/g for the silica-based catalyst. The Ru/SiO2 catalyst had a surface area of 102 m2/g. All the catalysts presented similar morphological features (rather aggregates formation based on the SEM studies with the Ru/SiO2 catalyst presenting a bit more spongy morphology (Figure S2). The latter is in agreement with the high surface area of this catalyst. Elemental EDS mapping measurements, by SEM, were carried out to confirm the presence of elements, and the results are presented in Table S2. Based on the values reported, the EDX-derived values for the Ce/La and Ce/Pr ratios are close to the nominal one (4/1); the same applies to the Ce/La/Sm/Gd/Pr ratio which reflects the equimolar composition of the HEO (design criterion is fulfilled).

Figure 3

Figure 3. (A) XRD diffractograms; (B–C) EPR spectra collected at 100 K; (D) Raman spectra collected over the Ru/CePrO catalyst; (E) Raman spectra collected over the Ru/HEO catalyst; (F) H2-temperature-programmed desorption (H2-TPD) profiles obtained over all the Ru catalysts of the present study.

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3.2.2. Probing the Oxygen Vacant Sites and the Metal–Support Interactions

Figures 3B,C and S3 present the EPR spectra of the Ru/CeLaO (reducible oxide, reference), Ru/HEO, and Ru/SiO2 (nonreducible, reference) obtained at 100 K and room temperature, respectively, following their reduction at 650 °C for 2h in H2 atmosphere. EPR is used in this study as a diagnostic tool for the Ce3+ species; given their paramagnetic nature, Ce3+ ions can be detectable using this technique. From the catalysis perspective, it is of high importance to be able to probe the presence of oxygen vacancies (Ov); the formation of the latter is associated with the Ce3+ presence. It is worthwhile to discuss in brief the mechanisms of Ov formation in these materials; according to the first mechanistic scenario, as the lattice oxygen leaves its original position, the two electrons in the vacant site localize on two Ce4+ ions and they reduce them forming Ce3+–VO–Ce3+ pairs. (41) Usually, conventional EPR at RT (298 K) cannot detect these species due to the strong spin–spin interaction. An alternative mechanism of oxygen vacancy (Ov) formation is when one electron, originating from lattice oxygen, localizes on a Ce4+ leading to the formation of Ce4+–VO–Ce3+ species; in this scenario, the other electron is trapped in an adjacent to the vacancy position, leading to the formation of F+ center; (41) the latter can be probed by RT (298 K) EPR. In the open literature, there is a lot of discussion on the origin of peak with gav ∼ 1.97 which is usually assigned to Ce3+. Based on the literature, the symmetry around the Ce3+ ions dictates the g parameter value. As Ce4+ to Ce3+ reduction takes place, it leads to the formation of oxygen vacancies accompanied by lattice distortion. In the case of CaF2 (isomorphic) for Ce3+ ions in a coordination environment of low symmetry, g ∼ 3.67 has been reported, whereas lines with g values of 2.4 for the trigonal site of Ce3+ ions have also been reported. (41) As can be seen in Figure 3C, EPR signals at different g-values confirm the various paramagnetic centers; of high significance is the signal at g ∼2.000 which is assigned to superoxide anions (•O2–) attached to Ce4+ ions; the intensity of this pulse is correlated with the concentration of oxygen vacant sites. (42) This signal is of high intensity in the case of the Ru/HEO catalyst.
Additional structural information was acquired utilizing Raman spectroscopy over the Ru-based catalysts of this study following their reduction at 650 °C for 2h in H2 atmosphere. In particular, Raman spectroscopic results (Figure 3D,E) obtained over the Ru/CePrO and Ru/HEO catalysts give insight into the effect of dopants on the multi-elemental HEO structure and defective sites formation as well as how the two different supports interact with the metal sites (Ru) through strong metal support interactions (SMSI). A comparative Raman plot is given in Figure S4 to assist with the peaks’ assignment. It is known that the Raman spectra for typical ceria-based materials have a dominant peak, the F2g peak, usually found at about 464 cm–1 and assigned to the symmetric stretch mode of the Ce–O8 crystal unit, characteristics of the fluorite lattice structure. (43) A close look at the Raman spectra of the catalysts (Figure S4) reveals significant changes in them indicating the remarkable impact of the dopants in the ceria microstructure owing to higher defectiveness and a rise in topographical/local disorder. Particularly, the F2g peak appears quite sharp in the case of Ru/CePrO catalyst and rather suppressed in the case of Ru/HEO catalyst (see Figures 3E and S4); this is associated with the high degree of oxygen sublattice distortion in the latter case (HEO) and the high sensitivity of Raman spectroscopy to capture the oxygen sublattice disturbances. The herein Raman spectra over the HEO agree quite well with the literature reported by Sarkar et al. (44) Apart from the suppressed intensity of the F2g peak, changes in the defect-induced bands were found in the 500 to 700 cm–1 range, which was being misinterpreted in the open literature as oxygen vacancies associated with the presence of reduced Ce3+ (45−47) species or to oxygen vacancies (Ov) involving movement of an oxygen atom into an octahedral interstitial position to obtain vacancy. (48,49) The characteristic features have rightly been considered intrinsic properties of a pure ceria structure; however, intensity and broadness of the defects band are traced to extrinsic defects, attributed to the addition of dopant (Gd, Sm, La, and Pr for HEO; Pr for CePrO). (50) The abundant defects as those identified in the Raman spectra are linked to the strong interaction between ceria and doped elements, thereby obtaining a microstructural change. The introduction of dopant into the ceria microstructure causes a steady increase in the amount of intrinsic and extrinsic defects. In a more quantitative manner, this is reflected in the value of the IOv/IF2g ratio, the latter being a descriptor of the Ov abundance. In particular, the IOv/IF2g ratio equals 0.68 and 3.2 in the cases of Ru/CePrO and Ru/HEO catalysts, respectively, demonstrating the abundance of Ov in the HEO lattice (bulk) and in the medium range that is probed by Raman scattering effects. (51) Additionally, the Raman band at 1100 cm–1 corresponding to the Ru–O–Ce bond environment demonstrates strong metal–support interactions (SMSI); this band is sharp (higher signal-to-noise ratio) in the case of quinary HEO-based catalyst compared to the binary CePrO-supported one. The presence of Ru–O–Ce species is a critical factor for the Ru dispersion, as this has been assessed through H2 chemisorption studies (Figure 3F), and catalytic activity as it will be discussed later. (52,53)

3.2.3. Surface Composition and Coordination Environment

To delve deeper into the chemical states of Ru species in the as-prepared catalysts, X-ray photoelectron spectroscopy (XPS) measurements were conducted (see Figure 4A). Of particular interest in the XPS studies was to shed light on the effectiveness of the in situ Ru reduction as well as on the Ru-support interaction while Ru growth and reduction simultaneously take place during the synthesis. The peaks observed at 461.9 and 484.1 eV are attributed to metallic Ru, while those at 463.7 and 485.9 eV are associated with RuO2, as reported by Wang et al. (54) A shift to lower binding energy in the Ru 3p core level spectra, particularly when compared to the Ru 3p of the reference Ru/SiO2 catalyst suggests changes in the electronic structure or chemical environment of Ru in the ceria-supported catalysts. The surface analysis reveals an increase in Ru/Ce ratio (Ru surface enrichment) as we transition from the binary system (i.e., Ru/CePrO or Ru/CeLaO) to the high-entropy system (Ru/HEO), as shown in Table 3. This observation suggests a different interaction among the three under study supports (CePrO, CeLaO, HEO) and the Ru species, promoting a high dispersion of Ru on the surface (in the case of HEO); these results are consistent with H2-TPD (chemisorption) results and the existing literature. (55) Possible parameters contributing to the support-metal interaction are surface termination (dangling bonds, functional groups) and surface energy (J/m2). The Ru 3p spectra of Ru/CePrO, Ru/CeLaO, and Ru/HEO catalysts were fitted by CasaXPS software using Lorentzian Asymmetric line shape with Voigt function, and results are shown in Figure S5. Indeed, Ru species exist as Ru0 and Ru4+ (in RuO2), with the latter species being more dominant on the surface of the Ru/CeLaO supported catalysts (Ru4+/Ru0=15.6), as can be seen in Table 3. The ratios of Ru4+/Ru0 in Ru/CePrO, Ru/CeLaO, and Ru/HEO were found to be 5.5, 15.6, and 6.2, respectively. The increased presence of Ru4+ species in the Ru/CeLaO system, as indicated by the higher Ru4+/Ru0 ratio, suggests that the specific combination of dopants in the CeLaO system has a pronounced effect on the reducibility of Ru due to the strong metal–support interactions (SMSI).
Table 3. XPS Data of the Prepared Catalysts
catalystO (at %)Ru (at %)Ce (at %)Ru/CeRu4+/Ru0a
Ru/SiO250.530.37   
Ru/CePrO13.0124.319.822.55.5
Ru/CeLaO22.707.977.781.015.6
Ru/HEO26.583.480.93.96.2
a

Values were estimated by XPS peak fitting using CasaXPS software.

Figure 4

Figure 4. (A) Ru 3p, (B) Ce 3d, and (C) O 1s XPS core-level spectra of the prepared catalysts.

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The Ce 3d spectra depicted in Figure 4B are intricate, featuring multiple peaks that are labeled based on previous XPS studies on cerium oxide. (55,56) The prominent peaks appearing at 916.2, 907, 900.3, 897.8, 888.7, and 882 eV are ascribed to Ce4+ species, whereas the two peaks observed at 902.2 and 884.6 eV are contributions of Ce3+ species. With respect to the binary systems, the Ru/HEO catalyst exhibits a shift of binding energy in higher values indicating a change in the Ce environment upon doping. Overall, a remarkable decrease in surface Ce species is observed (see Table 3), as well as intensity increase in Ce3+-related peaks can be visualized based on its spectrum in Figure 4B, likely attributed to the formation of extrinsic defects like oxygen vacancies due to doping with multiple elements.
In the O 1s spectra (Figures 4C and S5), the peak at approximately ∼528.8 eV is attributed to lattice oxygen, while the peaks around ∼530 eV, 531, and 533 eV are linked to oxygen vacancies, surface hydroxyl species, and carbonates, respectively. (57) A significant change in the O environment is noticed across the catalysts. Moving from binary systems (e.g., CePrO and CeLaO) to the HEO system reveals increased peak intensities. This indicates that varying the chemical compositions has an impact on the type and concentration of surface oxygen species due to the formation of distinct metal oxides or mixed-metal oxides. Ru/HEO presents the highest content of surface oxygen species, likely attributed to the optimal formation of Ru–O–Ce bonds. Deconvolution of the O 1s peak (Figure S6 and Table S3), although, revealed that for the Ru/CePrO, Ru/CeLaO, and Ru/HEO catalysts the lattice oxygen (OL) abundance on the surface is 20, 28, and 42%, respectively. This result leads to the conclusion that the Ru/HEO catalyst has the least Ov surface concentration. Caution should be exercised in the interpretation of this data due to the fact that these values are only a rough estimation and will probably not give the correct stoichiometry, which presumably is (Ce, X, Y, Z)O2 in the case of HEO. Furthermore, the higher abundance of Olattice (associated with lower abundance of Ov) in the case of HEO catalyst can be understood by taking into consideration the following: (1) the XPS studies were performed on the as-prepared catalyst; (2) the XPS experiment was of ex-situ nature; (3) the fact that Ov creation can happen under the vacuum-catalyst interaction (ultrahigh vacuum in the XPS experiment) and (4) XPS experiment captures only the surface, so Ov at the different locations (subsurface/bulk) escape the detection depth of XPS. Additionally, the dynamics of Ov is a parameter to not be ignored. As pointed out by Younis et al. (58) at high levels of doping (>20%) the vacancies become immobile due to their clustering and this can lead to deterioration of catalytic activity, as it will be discussed later.

3.2.4. Coordination Environment through Synchrotron EXAFS Studies

Long and medium-range features, as those can be derived using XRD (long-range) and Raman (medium), can be hardly used to describe structural differences for materials in the nm-range. Local distortion with no periodicity can dictate the local structure and the ultimate functionality of a material. (59) Synchrotron EXAFS was used to study the Ce–O correlations in a changing environment from the binary Ce–Pr oxide to the HEO. (60)
The linear combination fitting (Figure 5A,B) of the XANES data collected at the L3 edge of Ce reveals a mixture of Ce3+ and Ce4+ oxidation states. In particular, the dominating oxidation state in the Ru/CePrO catalyst is Ce3+ at an abundance of ∼96%; in the case of the Ru/HEO catalyst, the Ce3+/Ce4+ ratio is about 1.2. The above results are important, though should be interpreted given the short-range sensitivity of the technique (different length scale with the focus being primarily around the atoms of interest).

Figure 5

Figure 5. (A, B) Linear combination fitting of the XANES spectra collected at the LIII edge of Ce in the HEO sample (left) and CePrO sample (right) to determine all the oxidation states of Ce in the samples; (C) Fourier transforms with their respective real part of the k3 weighted EXAFS spectra collected at the Ce LIII edge in CLPSG-HEO and CePr samples.

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Analyzing a short EXAFS signal can give good information on the first coordination shell around Ce (nearest neighbor, NN). Derived structural parameters from the EXAFS fitting show a coordination number of about 7 oxygen atoms with an interatomic distance that is close to that of CeO2 with a slight contraction in CePr to 2.30 Å and even shorter in the case of the HEO sample. In particular, the Ce–O distance becomes smaller as we move from ceria (2.34 Å) to CePrO (2.30 Å) to HEO (2.27 Å) (Table 4). The short distance coincides also with a reduction of the coordination number from 8 oxygen atoms for CeO2 to an average number of 7.4 and 7.1 atoms in CePr and HEO samples, respectively. These results may suggest a defect in the local structure at short-medium (could be defects or distortion). It should be mentioned here that, it was not possible to collect long EXAFS data (signal) at the L3 edge of Ce (due to the presence of the L2 edge at around 400 eV), and thus the coordination number is largely affected by the noise level which is reflected in the higher standard deviation (2.4 for N and 0.036 for the R, Table 4). This can be considered as a limitation of the above data, making EXAFS results only complementary to the above XANES ones. Moreover, based on the above HEO seems to be evolving as a triclinic CeO2 which has a shorter Ce–O distance (2.27) than the CeO2 cubic (2.34).
Table 4. Structural Parameters Derived by EXAFS Fitting of the First Shell around the Ce Atom in CLPSG-HEO and CePr Samples
samplebondN (atom)R (Å)σ2 (Å2)ΔE(eV)
HEOCe–O7.1 ± 2.42.27 ± 0.0360.01460 ± 0.0043.99 ± 1.6
CePrOCe–O7.4 ± 2.02.30 ± 0.0100.01542 ± 0.0014.05 ± 0.5
CeO2 (cubic)Ce–O8a2.34248a  
a

Theoretical value.

3.2.5. Ru Particle Shape and Size on the Different Supports (Electron Microscopy and Chemisorption Studies)

In the present study, the Ru dispersion and particle size were evaluated using H2 chemisorption studies, where the H atoms dissociative adsorb onto the Ru metal with a H:Ru stoichiometry of 1:1. Additionally, HRTEM analysis was employed to verify the chemisorption studies. Figure 3F displays the H2-TPD profiles of the Ru-based catalysts of this study. According to the open literature, the peak at low temperature (75 °C, 135 °C) is assigned to the desorption of hydrogen from various active sites (metallic Ru of different sizes), whereas the peak with Tmax > 380 °C corresponds to the spillover hydrogen from the support. The corresponding Ru particle size was found to be in the 4–11 nm range demonstrating different extents of Ru dispersion as shown in Table S4. This Ru particle size variation, as probed using chemisorption, is expected to have a profound impact on the NH3 synthesis catalytic activity due to the well-known structural and size sensitivity of the reaction at hand and particularly of the N2 activation step. (61)
In a complementary fashion to the H2 chemisorption studies, HRTEM studies were performed and the results are presented in Figures 6, 7, 8, 9, and 10 along with their STEM-HAADF Red Green Blue (RGB) and Fast Fourier Transform (FFT) analysis which allows us to comment on the distribution of the elements in the oxide matrix and the exposed facets in the particles, respectively. In particular, in Figures 6 and 7, and Figures 8, 9, and 10 the HRTEM and STEM-HAADF RGB analysis of the Ru/CePrO and Ru/HEO catalysts, respectively, are presented, following their reduction at 650 °C for 2 h. Comparing the HRTEM images in Figure 6 with Figures 8 and 9 it can be concluded that Ru particles grow in a more faceted manner (sharp planes) over the CePrO compared to HEO support. In the Figure 6, lattice fringes with interplanar spacing of 0.196 nm, 0.227 nm can be assigned to the Ru(101) and Ru(100) planes, respectively.

Figure 6

Figure 6. (A) HRTEM images along with Fast Fourier Transform (FFT) pattern; (B, C) facets analysis of the Ru/CePrO catalyst.

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Figure 7

Figure 7. (A–E) STEM-HAADF RGB analysis and (F) selected area electron diffraction (SAED) over the Ru/CePrO catalyst.

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Figure 8

Figure 8. (A–C) HRTEM images of the Ru/HEO catalyst with emphasis on Ru particles at different areas (B) and (C).

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Figure 9

Figure 9. (A) HRTEM images of the Ru/HEO catalyst along with Fast Fourier Transform (FFT) pattern and (B–D) facets analysis.

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Figure 10

Figure 10. (A–H) STEM-HAADF RGB analysis and (I) selected area electron diffraction (SAED) over the Ru/HEO catalyst.

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Ammonia synthesis reaction and its rate-determining step, that of N2 activation, exhibits high sensitivity in particle shape and faceting. (62) The facets exposed in the cases of Ru/CePrO and Ru/HEO were confirmed through FFT analysis (Figures 7 and 9 and Tables S5, S6). Regarding the Ru particle size, histogram analysis (Figure S7), based on the HRTEM studies, also showed that Ru particle size is ∼6 and ∼12 nm in the cases of HEO and CePrO-supported catalysts, respectively. These values of particle size are quite in agreement with the H2 chemisorption studies above; some overestimation of dispersion can be observed in the chemisorption studies due to the well-known phenomenon of atomic hydrogen spillover in the ceria-based supports. In brief, it is reported by Rarog-Pilecka et al. (63) that the ammonia synthesis rate increased with the particle size increasing from 0.7 to 4 nm, whereas smaller sizes of Ru crystallites (smaller than 0.7 nm) can be totally inactive; this is linked to the lack of B5 sites. Kim et al. reported that double-stepped Ru(109) sites are more active than the stepped Ru(0001) surface. (64−70)

3.2.6. Catalytic Performance toward NH3 Production

All the Ru-based catalysts of this study were tested following their reduction at 650 °C for 2h for ammonia production at different reaction pressures (Figure 11A), whereas the best-performing one was evaluated following different activation conditions (Figure 11B). In particular, reaction pressures in the 10–50 bar were used; additionally, the catalysts were subjected to different activation temperatures so to induce different populations of Ov sites; namely, reduction at 650 °C (1) and reduction at 800 °C (2). The Ru/HEO catalyst following reduction at 650 °C (catalyst 1, Figure 11A) led to the production of 1236 μmols NH3/g-h at a pressure of 10 bar. Increasing the reaction pressure results in a drop in catalytic activity for the Ru/HEO catalyst, whereas the rest of the catalysts presented activity close to zero. Following its activation/reduction to 800 °C for 2 h and keeping the reaction pressure at 10 bar, the NH3 production drops to 954 μmols NH3/g-h (Figure 11B). It is expected that two major parameters are playing a key role in the measured catalytic activity, as presented in Figure 11 toward ammonia production; Ru particle size as this is controlled by the sintering extent (650 vs 800 °C) and active sites population as this dictated by the Ru particle shape; additionally, it is expected that oxygen vacant sites from the HEO are also a critical activity descriptor. It is well documented that N2 dissociation (bond energy 945 kJ/mol) is the rate-limiting step in the NH3 production. (68,61) Under reduction conditions it is expected that electron donation is enhanced from Ru to the π* antibonding orbital of N2 facilitating the N2 activation.

Figure 11

Figure 11. Catalytic production of NH3 (A) over Ru-based catalysts of this study in the 10–40 bar pressure range and 400 °C; (B) over the Ru/HEO catalyst at 10 bar, 400 °C following two different activation conditions (650 vs 800 °C).

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3.2.7. Intertwined Activity Descriptors

In particular, it has been demonstrated by Jacobsen et al. (61) that the Ru particle size in the 1.8–2.5 nm range contains the maximum number of B5 sites; the latter are the ones favoring the N2 dissociative adsorption. It is believed that the results of Figure 11 are the outcome of two competitive trends; the B5 sites drop in population due to Ru sintering and the Ov increase in population due to the reduction conditions (Ov-650 °C > Ov-800 °C). The trend observed in NH3 production is in agreement with the findings of the 18O2 transient isotopic isothermal experiments and the population of the surface Ov (discussed later). For study completeness, and to ensure that there is no NH3/N2 impurity that can affect the obtained measurements of our study (overestimate of ammonia production), the above experiments were also run under the same conditions in the absence of N2 in the feed; no NH3 production was found, thus confirming the lack of ambiguity in our measurements as per the open literature. (69,70)

3.2.8. Ru Metal Particles’ Role

In an effort to untangle the role of Ru particle size on the N2 dissociation, it is worth revisiting the recently published report by Yanliang Zhou, (71) where it is clearly stated that Ru size reduction, in the case of Ru/BaCeO3 catalysts, enhances the formation of Ce3+ species and oxygen vacancies (Ov) sites; the latter facilitates the donation of electrons to the Ru sites and thus enhances the N2 dissociation step. Additionally, the structure sensitivity of the ammonia synthesis reaction has been highlighted in the study of Peng et al. (72) at mild conditions; it is manifested by significant changes in catalyst activity at only small structural alterations. In particular, it was reported that the population of corner sites (low coordination sites) increases with dropping Ru particle size with a simultaneous drop of the terrace sites population; additionally, due to the geometric changes (shape and size), the Ru electronic structure is tailored with its size tuning. This causes a reduction to the catalyst work function, ϕcat, which subsequently facilitates the electron donation from the d-orbitals of Ru to the N2, enhancing N2 activation and N–H bond formation. Isotopic labeling experiments coupled with DRIFTS can be used to trace the size of the pool of the key N-containing intermediates and the mechanism followed (associative vs dissociative); this is important, particularly due to the fact that Ru size can be the crucial factor determining the mechanism too; namely, Ru-based catalysts with particle size >2 nm favor a dissociative pathway, (73) whereas associative pathway is followed in the case of almost atomically dispersed Ru catalysts. (74,75) Moreover, smaller Ru size (higher dispersion) can facilitate the H-spillover from Ru to support/interface and induce the H trapping in Ov-H entities, thus blocking the H-poisoning (parasitic) effect of Ru (76) which takes place under the reaction conditions. It has to be mentioned that the location (surface/subsurface) and dynamics (clustering) of the Ov in trapping H atoms in Ov-H entities are yet to be determined.

3.2.9. Oxygen Vacancies’ Role

It is noteworthy that Ov plays a crucial role and it is almost synonymous with the ceria-based materials’ functionality. However, caution should be exercised when a direct correlation of Ov presence and catalytic activity is attempted for reasons commented in what follows and related to Ov location and dynamics. Jennifer L. M. Rupp and her collaborators in an enlightening review (77) discuss the structural arrangements of the Ov in the ceria-doped materials (Ce3+ = 128 pm, Ce4+ = 111 pm). Authors point out that elements/dopants smaller than Gd (Gd3+ = 119 pm) favor the placement of the vacancy at the nearest neighbor (NN) site, whereas the elements that are larger than Sm (Sm3+ = 96 pm) repel the vacancy from this location. Themselves, Gd and Sm, exhibit neither attractive nor repulsive traits toward vacancies location in the NN site. An opposite trend has been found for the case of the next nearest neighbor (NNN) site, where locking the Ov at this site seems never to be preferred. Computational studies show that as the population of NN rare earth ions surrounding the vacancy increases, the defect association energy increases too. Therefore, the local configurations of dopants and oxygen vacancies in doped ceria are of particular importance, yet some details are still not completely understood. Particularly for the N2 reduction step, there are many studies trying to unresolve the Ov role in the reaction. In the work by Yanliang Zhou (71) authors investigated the effect of different contents of Ov (equivalent to different contents of Ce3+ entities) through Bader charge analysis. In the structural models (zero Ov, 1 Ov, and 2 Ov) adopted, the Ov content was 0, 3.3, and 6.7%, respectively, giving rise to 2.3–5% Ov content in the Ru/BaCeO2 catalyst. It was found that the charge donation from Ce3+ and Ov to the Ru increases as the Ov content increases. Oxygen vacancies dictate the charge accumulation to Ru particles, while they can be the drive for mechanism pathway change. (78) Though, another characteristic of Ov that needs to be discussed herein is their clustering at a certain level of doping. Younis et al. report the drop in the catalytic activity of Gd-doped ceria when the dopant level passes 15% where the Ov vacancies become immobile due to their clustering, stressing the importance of vacancies dynamics. (58)

3.3. Insights into the Oxygen Mobility and Vacancy Formation

To get a thorough understanding of the fine differences in terms of oxygen mobility and vacancy formation of the HEO vs CePrO binary oxide, a 18O2 transient isotopic isothermal exchange (TIIE) experiment was performed. The transient rates of 16O2 consumption (μmol g–1 s–1) obtained during TIO at 650 °C followed by catalyst pretreatment (Exp 1–3) over the HEO solid, are shown in Figure 12A. It can be clearly seen that the rate of oxygen consumed (and its maximum) is lower in the case where no reduction (Exp 1) was performed during the pretreatment of the solid, compared to the cases where H2 reduction was performed. It is evident that by applying H2 reduction after calcination, oxygen vacancies were formed, the amount of which depends on the temperature of reduction. At the initial stage (before maximum), Exp 2 and Exp 3 exhibit similar rates, indicating similar amounts of surface oxygen vacancies. Whereas, with time under 16O2/Kr/Ar/He gas stream, the amount of oxygen vacancies in the bulk is higher in the case where the catalyst reduced at 650 °C, instead of 800 °C. It should be noted that the higher the O2 consumption amount, the more oxygen vacancies exist. Figure 12B presents the total amount of O2 consumed (NV, μmol g–1) as a function of pretreatment conditions applied over the two solids. More precisely, independent of the catalyst pretreatment protocol followed, the HEO exhibits more oxygen vacant sites, compared to the CePrO, with an optimum condition being that of catalyst reduction with 5% H2/He at 650 °C (Exp 2).

Figure 12

Figure 12. (A) Transient rates of 16O2 (μmol g–1 s–1) consumed during transient isothermal oxidation (TIO) at 650 °C, following the different pretreatment conditions, over HEO solid; and (B) amounts of O2 consumed (μmol g–1) during TIO, over both solids.

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Figure 13A displays the transient concentration (mol %) response curves of Kr, 16O2, 16O18O, and 18O2 obtained during the 18O2-TIIE switch from 16O2/Kr/Ar/He to 18O2/Ar/He gas mixture at 650 °C, over HEO pretreated via Exp 1. After the switch to the 18O2/Ar/He, the 16O2(g) and 18O2(g) concentrations decrease and increase, respectively, while the formation of 16O18O(g) starts immediately and passes through a maximum at about 450 s, following by a decreasing tail controlled mostly from the bulk 16O diffusion toward the surface. The large difference between 16O2 decay and that of inert Kr, which lasts for ∼15 s, can be attributed to the surface 16O, 18O, and 16O18O exchange on the catalytic surface, considering that the amount of 16O exchanged reached the ML = 1 in the first 20 s of the transient. For t > 20 s, subsurface and bulk 16O diffusion and exchange toward 16O18O formation is dominant. The total amount of 16O18O exchanged (N16O), estimated via eq 2, was found ∼1.2 times lower in the case of HEO compared to CePrO, while increases by increasing the H2 reduction temperature (from 650 to 800 °C) during the pretreatment process (9.75, 10.21, and 11.99 mmol g–1 for HEO, respectively, compared to 11.92, 12.02, and 12.48 mmol g–1 for CePrO).

Figure 13

Figure 13. (A) Transient concentration (mol %) response curves of Kr, 16O2, 16O18O, and 18O2 gaseous species recorded during the 18O2-TIIE experiment at 650 °C, over the calcined HEO; (B) αg(18) descriptor as a function of time and pretreatment procedure, estimated during 18O2-TIIE over HEO; and (C) αg(18) descriptor obtained over the both reduced at 650 °C solids.

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The effect of H2 reduction temperature on the αg(18)(t) descriptor function, over the HEO is illustrated in Figure 13B. It can be seen that different shapes and positions are obtained in the whole transient by varying the catalyst pretreatment (similar results were obtained over CePrO, not shown). Getting more specific, at the initial period after the 18O2/Ar/He gas switch, where the 16O/18O exchange on the surface takes place, a steeper response is obvious in the absence of H2 pretreatment (Exp 1), indicating a higher Deff value, (37) opposed to the Exp 2 and 3, where H2 reduction was applied after catalyst calcination. The latter behavior could be partly attributed to catalyst sintering during reduction, limiting the surface 16O/18O exchange. However, for prolonged times under the 18O2/Ar/He gas mixture (t > 450 s), it is clear that when the catalyst is reduced at 650 °C, enhances the oxygen diffusion in the bulk, compared to Exp 3 and 1. Based on the above-offered discussion, and comparing the HEO and CePrO pretreated with 5% H2 at 800 °C (see Figure 13C), it is rather clear that the former catalyst promotes to a higher extent the surface 16O/18O exchange (for t < 450 s, steeper and lower αg(18)), opposed to the bulk oxygen diffusion (t > 450 s), which is higher for the latter Ce20Pr catalytic system.

3.3.1. Insights into the Adsorption of N2/N Species on Ru/HEO Catalysts

The understanding of the interaction between the Ru/HEO catalytic surface and Nx (x = 1 or 2) species is expected to avail valuable insights into ammonia synthesis reaction and the experimental results presented above (Figure 13). Therefore, the ab initio studies were performed to examine the catalytic activity of the Ru nanoparticles and HEO support in the adsorption of N2 molecules in the presence and absence of hydrogen. Considering the first step for ammonia synthesis is the activation of the N2 molecule, the adsorption of the N2 molecule on the reduced HEO surface (Ru4/HEO) is considered by placing the N2 molecule at different sites on the surface consisting of Ru4 cluster. The configuration with the minimum energy is considered for further analysis and is shown in Figure 14a. As shown in the figure the N2 molecule prefers to bind to the surface over the Ru4 cluster. This suggests that the presence of the Ru4 cluster facilitates the adsorption of N2 molecules on the HEO surface, thereby initiating the first step in ammonia formation. The adsorption energy of the N2 molecule is calculated to be −0.35 eV, with a Ru–N bond length of 2.02 Å. The N–N bond length decreased slightly to 1.17 eV compared to the N–N bond length in the gas phase (1.19 Å). For the next step in ammonia synthesis, the adsorption of N2 molecules in the presence of H2 is analyzed. The adsorption energy of N2 further reduces to −0.68 eV in the presence of H2 compared to −0.35 eV in the absence of H2. The adsorption configuration is shown in Figure 14b. In this step, the N≡N is broken into double bonds, with top N bonded to H with a bond length of 0.89 Å and bottom N bonded to the Ru4 cluster with a bond length of 2.02 Å.

Figure 14

Figure 14. Top and side views of N2 Adsorption configurations on Ru4/reduced HEO surface: (a) in absence of hydrogen and (b) in the presence of hydrogen. Gd is shown in purple, La in dark green, Pr in yellow, Sm in pink, Ce in light green, Ru in orange, N in white, H in blue, and O in red.

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It is expected that upon the N2 adsorption onto the Ru/HEO catalyst, N2 is being dissociated onto Ru sites followed by N≡N bond scission and N atom spillover onto the HEO support. Due to the above findings and including the N spillover effect the binding strength of atomic N on reduced quinary CeLaPrSmGdO (111) surface (i.e., HEO surface) is investigated. The adsorption of atomic N over a reduced HEO surface is studied by placing N over different sites over the reduced HEO surface. Our analysis shows that N over the oxygen-vacant site is most energetically favorable with an adsorption energy of −0.70 eV. The adsorption configuration is shown in Figure 15a. The adsorption of atomic N over the HEO surface is also studied in the presence of H2. The adsorption energy of N2 over the HEO surface in the presence of H2 reduced to −1.18 eV with the H atom bonded to the surface N atom with an N–H bond length of 0.89 Å.

Figure 15

Figure 15. Top and side views of N Adsorption configurations on reduced quinary CeLaPrSmGdO (111) surface: (a) in absence of hydrogen and (b) in the presence of hydrogen. Gd is shown in purple, La in dark green, Pr in yellow, Ce in light green, Sm in pink, H in blue, N in white, and O in red.

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4. Conclusions

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Briefly, the main conclusions that can be withdrawn from the study are 2-fold. First, in terms of the understanding of the structure of the HEO compared to reference binary oxides, CePrO and CeLaO, the following remarks can be pointed out based on the multirange analysis that was performed, XRD (long-range), Raman (medium range) and EXAFS (short range): (1) according to the XRD studies, the HEO reported herein seems to adopt a cubic lattice being isostructural with the CePrO and CeLaO (reference, reducible oxides); (2) EPR and Raman demonstrated that HEO structure bears higher population of oxygen vacancies (Ov) compared to the binary CePrO under operating conditions (reduction at 650 °C/2 h); though, the abundance of Ov varies depending on the length scale probed and the surface/subsurface. In the case of Ru-supported catalysts over the HEO and CePrO supports the quinary one induces strong metal–support interactions (SMSI) and those are manifested through the formation of Ru–O–Ce interfacial species to a greater extent than in the binary oxide (Raman), and the different Ru/Ce and Ru4+/Ru0 surface intrinsic ratios (XPS); (3) HRTEM studies revealed the effect of support (HEO vs CePrO) on the different mode of growth of Ru particles shape and size (geometrical factor) and demonstrated the role of HEO in getting smaller Ru particle sizes ∼6 nm which are less sharply faceted; (4) In order to induce different degree of Ov formation in the oxide lattice, different conditions of H2 reduction, after calcination, were applied, driving the formation of Ov, the amount of which depends on the temperature of reduction (600 vs 800 °C); (5) HEO exhibits more oxygen vacant sites, compared to the reference binary CePrO, yet reducible oxide, with an optimum condition being that of catalyst reduction with 5% H2/He at 650 °C due to the extensive sintering happening at 800 °C; (6) The total amount of 16O18O exchanged (N16O), was found ∼1.2 times lower in the case of HEO compared to CePrO, while increases by increasing the H2 reduction temperature (650 °C, and 800 °C) during the pretreatment process; (7) HEO promotes in a higher extend the surface 16O/18O exchange (for t < 450 s, steeper and lower αg(18)), opposed to the bulk oxygen diffusion (t > 450 s), which is higher for the CePrO catalytic system.
Second, these Ru-based catalysts were evaluated toward their activity for ammonia production; the latter being a probe reaction targeting to unveil important features of the above catalysts and guide potential catalysts’ design. In terms of the role of the HEO in a Ru/HEO catalyst for ammonia production the following remarks can be made: (1) In the HEO, the higher amount of oxygen vacancies formed at 650 °C, and higher extend of Deff, promotes the surface 16O/18O exchange and leads to surface Ov (potential active sites for N2 and H2 activation); (2) the adsorption of N atom (being spilled over from the Ru) is more favorable at the neighboring site to oxygen vacancy (location of oxygen vacancy) as compared to the one far from the Ov; (3) The intertwined connection of Ru particle size and population/local environment of surface Ov in the HEO facilitates the efficient N2 dissociation and thus leads to higher NH3 yield (μmol/g-h); (4) The different degree of Ru particles faceting is associated with the variation of B5 sites population and thus the different performance of the catalysts for the reaction at hand.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.3c16521.

  • Experimental protocol for catalysts characterization that was followed in Raman spectroscopy; X-ray photoelectron spectroscopy; high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR), and H2 chemisorption (H2-temperature-programmed desorption, TPD); textural analysis results as obtained after N2 adsorption at 77 K; SEM microphotographs obtained over the Ru catalysts of this study; EPR spectra obtained at room temperature (298 K) over 6Ru/SiO2, 6Ru/CePrO, and 6Ru/HEO catalysts; comparative raman spectra of Ru/CePrO and Ru/HEO catalysts; deconvolution of Ru 3p core-level spectra of Ru/CePrO and Ru/HEO; deconvolution of O 1s core-level spectra of Ru/CePrO, Ru/CeLaO and Ru/HEO catalysts; areas of the O 1s deconvoluted peaks; Ru dispersion and particle size as obtained from the H2 chemisorption studies; and FFT analysis of the Ru/HEO catalyst (PDF)

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Author Information

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  • Corresponding Authors
    • Nirpendra Singh - Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Physics Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Orcidhttps://orcid.org/0000-0001-8043-0403;  Email: [email protected]
    • Kyriaki Polychronopoulou - Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Orcidhttps://orcid.org/0000-0002-0723-9941;  Email: [email protected]
  • Authors
    • Omer Elmutasim - Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    • Aseel G. Hussien - Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    • Abhishek Sharan - Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Physics Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    • Sara AlKhoori - Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    • Michalis A. Vasiliades - Department of Chemistry, Heterogeneous Catalysis Laboratory, University of Cyprus, 1 University Avenue, University Campus, 2109 Nicosia, Cyprus;  Orcidhttps://orcid.org/0000-0002-1568-1158
    • Inas Magdy Abdelrahman Taha - Physics Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
    • Seokjin Kim - Oxide & Organic Nanomaterials for Energy & Environment (ONE) Laboratory, Advanced Membranes & Porous Materials (AMPM) Center, and KAUST Catalysis Center (KCC), Physical Science & Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia;  Orcidhttps://orcid.org/0000-0002-6052-547X
    • Messaoud Harfouche - Synchrotron-Light for Experimental Science and Applications in the Middle East (SESAME), Allan 19252, Jordan
    • Abdul-Hamid Emwas - Core Laboratories, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
    • Dalaver H. Anjum - Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Physics Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates;  Orcidhttps://orcid.org/0000-0003-2336-2859
    • Angelos M. Efstathiou - Department of Chemistry, Heterogeneous Catalysis Laboratory, University of Cyprus, 1 University Avenue, University Campus, 2109 Nicosia, Cyprus;  Orcidhttps://orcid.org/0000-0001-8393-8800
    • Cafer T. Yavuz - Oxide & Organic Nanomaterials for Energy & Environment (ONE) Laboratory, Advanced Membranes & Porous Materials (AMPM) Center, and KAUST Catalysis Center (KCC), Physical Science & Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia;  Orcidhttps://orcid.org/0000-0003-0580-3331
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors highly acknowledge the support of Khalifa University through the RC2-2018-024 grant as well as the support from Khalifa University’s high-performance computing and research computing facilities.

References

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    Entropic stabilization has evolved into a strategy to create new oxide materials and realize novel functional properties engineered through the alloy compn. Achieving an atomistic understanding of these properties to enable their design, however, has been challenging due to the local compositional and structural disorder that underlies their fundamental structure-property relationships. Here, we combine high-throughput atomistic calcns. and linear regression algorithms to investigate the role of local configurational and structural disorder on the thermodn. of vacancy formation in (MgCoNiCuZn)O-based entropy-stabilized oxides (ESOs) and their influence on the elec. properties. We find that the cation-vacancy formation energies decrease with increasing local tensile strain caused by the deviation of the bond lengths in ESOs from the equil. bond length in the binary oxides. The oxygen-vacancy formation strongly depends on structural distortions assocd. with the local configuration of chem. species. Vacancies in ESOs exhibit deep thermodn. transition levels that inhibit elec. conduction. By applying the charge-neutrality condition, we det. that the equil. concns. of both oxygen and cation vacancies increase with increasing Cu mole fraction. Our results demonstrate that tuning the local chem. and assocd. structural distortions by varying alloy compn. acts an engineering principle that enables controlled defect formation in multi-component alloys.
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    Ye, S.; Zhu, J.; Zhu, S.; Zhao, Y.; Li, M.; Huang, Z.; Wang, H.; He, J. Design strategies for perovskite-type high-entropy oxides with applications in optics. ACS Appl. Mater. Interfaces 2023, 15, 47475– 47486,  DOI: 10.1021/acsami.3c09447
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    Sarkar, A.; Kruk, R.; Hahn, H. Magnetic properties of high entropy oxides. Dalton Transactions 2021, 50, 1973– 1982,  DOI: 10.1039/D0DT04154H
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    Sarkar, A.; Wang, Q.; Schiele, A.; Chellali, M. R.; Bhattacharya, S. S.; Wang, D.; Brezesinski, T.; Hahn, H.; Velasco, L.; Breitung, B. High-entropy oxides: Fundamental aspects and electrochemical properties. Adv. Mater. 2019, 31, e1806236  DOI: 10.1002/adma.201806236
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    Brahlek, M.; Gazda, M.; Keppens, V.; Mazza, A. R.; McCormack, S. J.; Gryn, A. M.; Musico, B.; Page, K.; Rost, C. M.; Sinnott, S. B.; Toher, C.; Ward, T. Z.; Yamamoto, A. What is in a Name: Defining “High Entropy” Oxides. APL Mater. 2022, 10, 110902  DOI: 10.1063/5.0122727
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    What is in a name: Defining "high entropy" oxides
    Brahlek, Matthew; Gazda, Maria; Keppens, Veerle; Mazza, Alessandro R.; McCormack, Scott J.; Mielewczyk-Gryn, Aleksandra; Musico, Brianna; Page, Katharine; Rost, Christina M.; Sinnott, Susan B.; Toher, Cormac; Ward, Thomas Z.; Yamamoto, Ayako
    APL Materials (2022), 10 (11), 110902CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    A review. High entropy oxides are emerging as an exciting new avenue to design highly tailored functional behaviors that have no traditional counterparts. Study and application of these materials are bringing together scientists and engineers from physics, chem., and materials science. The diversity of each of these disciplines comes with perspectives and jargon that may be confusing to those outside of the individual fields, which can result in miscommunication of important aspects of research. In this Perspective, we provide examples of research and characterization taken from these different fields to provide a framework for classifying the differences between compositionally complex oxides, high entropy oxides, and entropy stabilized oxides, which is intended to bring a common language to this emerging area. We highlight the crit. importance of understanding a material's crystallinity, compn., and mixing length scales in detg. its true definition. (c) 2022 American Institute of Physics.
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    Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169– 11186,  DOI: 10.1103/PhysRevB.54.11169
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    Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
    Kresse, G.; Furthmueller, J.
    Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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    Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15– 50,  DOI: 10.1016/0927-0256(96)00008-0
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    Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
    Kresse, G.; Furthmuller, J.
    Computational Materials Science (1996), 6 (1), 15-50CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
    The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
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    Pedrew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868,  DOI: 10.1103/PhysRevLett.77.3865
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    Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953,  DOI: 10.1103/PhysRevB.50.17953
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    Projector augmented-wave method
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    Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
    There is no expanded citation for this reference.
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    Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. Phys. Rev. B 1999, 59, 1758,  DOI: 10.1103/PhysRevB.59.1758
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    From ultrasoft pseudopotentials to the projector augmented-wave method
    Kresse, G.; Joubert, D.
    Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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    Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-energy-loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 1505,  DOI: 10.1103/PhysRevB.57.1505
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    Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study
    Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P.
    Physical Review B: Condensed Matter and Materials Physics (1998), 57 (3), 1505-1509CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    By taking better account of electron correlations in the 3d shell of metal ions in Ni oxide it is possible to improve the description of both electron energy loss spectra and parameters characterizing the structural stability of the material compared with local spin d. functional theory.
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    Zunger, A.; Wei, S. H.; Ferreira, L. G.; Bernard, J. E. Special Quasi-random Structures. Phys. Rev. Lett. 1990, 65, 353
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    Special quasirandom structures
    Zunger, Alex; Wei, S. H.; Ferreira, L. G.; Bernard, James E.
    Physical Review Letters (1990), 65 (3), 353-6CODEN: PRLTAO; ISSN:0031-9007.
    Structural models used in calcns. of properties of substitutionally random A1-xBx alloys are usually constructed by randomly occupying each of the N sites of a periodic cell by A or B. It is possible to design "special quasirandom structures" (SQS's) that mimic for small N (even N = 8) the first few, phys. most relevant radial correlation functions of a perfectly random structure far better than the std. technique does. The usefulness is demonstrated of these SQS's by calcg. optical and thermodn. properties of a no. of semiconductor alloys in the local-d. formalism.
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    Walle, A. V.; Tiwary, P.; Jong, M. D.; Olmsted, D. L.; Asta, M.; Dick, A.; Shin, D.; Wang, Y.; Chen, L. Q.; Liu, Z. K. Efficient Stochastic Generation of Special Quasi-random Structures. Calphad 2013, 42, 13– 18,  DOI: 10.1016/j.calphad.2013.06.006
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    Ghosh, G.; Walle, A. V.; Asta, M. First-principles Calculations of the Structural and Thermodynamic Properties of bcc, fcc and hcp Solid Solutions in the Al–TM (TM = Ti, Zr and Hf) Systems: A Comparison of Cluster Expansion and Supercell Methods. Acta Mater. 2008, 56, 3202,  DOI: 10.1016/j.actamat.2008.03.006
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    First-principles calculations of the structural and thermodynamic properties of bcc., fcc. and hcp. solid solutions in the Al-TM (TM=Ti, Zr and Hf) systems: A comparison of cluster expansion and supercell methods
    Ghosh, G.; van de Walle, A.; Asta, M.
    Acta Materialia (2008), 56 (13), 3202-3221CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
    The thermodn. properties of bcc., fcc. and hcp. solid solns. in the Al-TM systems with TM = Ti, Zr or Hf are calcd. from first-principles by using cluster expansion, Monte-Carlo simulation and supercell methods. The 32-atom special quasi-random structure supercells are used to compute properties of alloys contg. 25, 50 and 75 at.% TM, and 64-atom supercells have been used to compute properties of the dil. alloys with one solute and 63 solvent atoms. In general, the energy of mixing calcd. by cluster expansion and dil. supercells are in agreement. In the concd. region, the mixing energy values calcd. by both methods also agree in many cases, but noteworthy discrepancies are found in some cases, which originate from inherent elastic and dynamic instabilities in the relevant parent lattices. The importance of short-range order on the calcd. values of mixing energy for hcp. Al-Ti alloys is demonstrated. Calcd. results are presented for the compn. dependence of the at. vols. in random solid solns. with bcc., fcc. and hcp. structures. The reported properties may be integrated within the CALPHAD formalism to develop reliable thermodn. databases to facilitate (1) calcns. of stable and metastable phase diagrams of binary and multicomponent systems (2) alloy design (3) processing of Al-TM-base alloys.
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    Qu, P. F.; Wang, G. C. Theoretical Insight into the Strong Size-dependence of Dry Reforming of Methane Over Ru/CeO2. J. CO2 Util. 2022, 65, 102221,  DOI: 10.1016/j.jcou.2022.102221
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    Theoretical insight into the strong size-dependence of dry reforming of methane over Ru/CeO2
    Qu, Peng-Fei; Wang, Gui-Chang
    Journal of CO2 Utilization (2022), 65 (), 102221CODEN: JCUOAJ; ISSN:2212-9839. (Elsevier Ltd.)
    Transitional metals catalyzed dry reforming of methane (DRM) usually shows strongly size-dependence behavior, but the at. level understanding is lacking. The DRM over Run/CeO2(111) (n = 1,2,4,7) were studied systematically by using d. functional theory calcn. combined with microkinetic modeling anal. in order to explore the nature behind the Ru size effect on the DRM catalytic activity. By d. functional theory calcn., it was found that the CH4 activation and CO2 activation processes in DRM reaction are size-dependence and the Ru coverage of optimal chem. performance is around 0.1-0.2 Monolayer (i.e., Ru2 and Ru4 models). The main reason is that Ru2 and Ru4 models could activate CO2 much more easily than other models either with lower Ru coverage like Ru1 or higher Ru coverage like Ru7 model. By using the energy decompn. scheme, it was shown that the Pauli repulsion between two dissocd. species and the absorption energy of CO* have crucial effects for the chem. performance of the CO2 activation step, and Ru2 and Ru4 models have weak Pauli repulsion and strong absorption ability of CO. The Ru2 and Ru4 models have optimal chem. performance than other models by DRM free energy profiles. By coke formation anal., the Ru2 model is difficult to form coke on surface owing to the similar ability for the carbon-carbon coupling step and carbon-oxygen step. Microkinetic model simulation shown that the reaction rates of DRM reaction are also size-dependence and the Ru coverage of highest rate is around 0.1-0.2 Monolayer. By those results, the Ru2 and Ru4 models has the optimal chem. performance for DRM reaction. The high catalytic activity of Ru2(Ru4) model can be attributed to the properties of balances the relationships between cleavage steps (including CH4 and CO2 cleavage) and oxidn. processes (mainly CH* oxidn. process). It was found that CO2 activation is the rate-controlling step of DRM in terms of the microkinetic modeling results, and the 2D cluster model (such as Ru1, Ru2) could form the interfacial oxygen vacancy more easily than 3D cluster model (such as Ru4, Ru7) by oxygen reverse spillover mechanism. Our results provide further mechanistic understanding for the behaviors of DRM reaction over various Ru coverage on CeO2(111) in order to design more efficient catalysts for DRM by adjusting the loading of transition metals.
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    Zafar, H.; Albedwawi, S.; Polychronopoulou, K.; Anjum, D. H. Characterization of Lanthanide High Entropy Oxides Using Transmission Electron Microscopy. Mater. Lett. 2022, 324, 132628  DOI: 10.1016/j.matlet.2022.132628
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    Hussien, A. G.; Damaskinos, C.; Dabbawala, A. A.; Anjum, D. H.; Vasiliades, M.; Khaleel, M.; Wehbe, N.; Efstathiou, A. M.; Polychronopoulou, K. Elucidating the Role of La3+/Sm3+ in the Carbon Paths of Dry Reforming of Methane Over Ni/Ce-La(Sm)-Cu-O Using Transient Kinetics and Isotopic Techniques. Appl. Catal., B 2022, 304, 121015  DOI: 10.1016/j.apcatb.2021.121015
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    Elucidating the role of La3+/Sm3+ in the carbon paths of dry reforming of methane over Ni/Ce-La(Sm)-Cu-O using transient kinetics and isotopic techniques
    Hussien, Aseel G. S.; Damaskinos, Constantinos M.; Dabbawala, Aasif; Anjun, Dalaver H.; Vasiliades, Michalis A.; Khaleel, Maryam T. A.; Wehbe, Nimer; Efstathiou, Angelos M.; Polychronopoulou, Kyriaki
    Applied Catalysis, B: Environmental (2022), 304 (), 121015CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    The different effects of the presence of La3+ and Sm3+ heteroatoms in the 5 wt% Ni/45Ce-45(Sm or La)-10Cu-O catalytic system on the carbon deposition and removal reaction paths in the dry reforming of methane (DRM) at 750 °C were investigated using transient kinetic and isotopic expts. The relative initial rates of carbon oxidn. by lattice oxygen of support and that by oxygen derived from CO2 dissocn. under DRM reaction conditions were quantified. Ni nanoparticles (23-nm) supported on La3+-doped ceria exhibited at least 3 times higher initial rates of carbon oxidn. to CO by lattice oxygen, and ∼ 13 times lower rates of carbon accumulation than Ni (18-nm) supported on Sm3+-doped ceria. The concn. and mobility of labile surface oxygen at the Ni-support interface region seems to correlate with carbon accumulation. Ni/Ce-La(or Sm)--10Cu-O formed NiCu alloy nanoparticles, partly responsible for lowering carbon deposition and increasing carbon oxidn. rates to CO.
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    Polychronopoulou, K.; Dabbawala, A. A.; Sajjad, M.; Singh, N.; Anjum, D. H.; Baker, M. A.; Charisiou, N.; Goula, M. Hydrogen Production via Steam Reforming of Glycerol Over Ce-La-Cu-O Ternary Oxide Catalyst: An Experimental and DFT Study. Appl. Surf. Sci. 2022, 586, 152798,  DOI: 10.1016/j.apsusc.2022.152798
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    Harfouche, M.; Abdellatief, M.; Momani, Y.; Abbadi, A.; Al Najdawi, M.; Al Zoubi, M.; Aljamal, B.; Matalgah, S.; Khan, L. U.; Lausi, A.; Paolucci, G. Emergence of the First XAFS/XRF Beamline in the Middle East: Providing Studies of Elements and Their Atomic/Electronic Structure in Pluridisciplinary Research Fields. J. Synchrotron Rad. 2022, 29, 1107– 1113,  DOI: 10.1107/S1600577522005215
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    Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Rad. 2005, 12, 537– 541,  DOI: 10.1107/S0909049505012719
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    ATHENA, ARTEMIS, HEPHAESTUS: data analysis for x-ray absorption spectroscopy using IFEFFIT
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    Journal of Synchrotron Radiation (2005), 12 (4), 537-541CODEN: JSYRES; ISSN:0909-0495. (Blackwell Publishing Ltd.)
    A software package for the anal. of x-ray absorption spectroscopy (XAS) data is presented. This package is based on the IFEFFIT library of numerical and XAS algorithms and is written in the Perl programming language using the Perl/Tk graphics toolkit. The programs described here are: (i) ATHENA, a program for XAS data processing, (ii) ARTEMIS, a program for EXAFS data anal. using theor. stds. from FEFF and (iii) HEPHAESTUS, a collection of beamline utilities based on tables of at. absorption data. These programs enable high-quality data anal. that is accessible to novices while still powerful enough to meet the demands of an expert practitioner. The programs run on all major computer platforms and are freely available under the terms of a free software license.
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    Ressler, T. WinXAS: A Program for X-ray Absorption Spectroscopy Data Analysis Under MS-Windows. J. Synchrotron Rad. 1998, 5, 118– 122,  DOI: 10.1107/S0909049597019298
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    Journal of Synchrotron Radiation (1998), 5 (2), 118-122CODEN: JSYRES; ISSN:0909-0495. (Munksgaard International Publishers Ltd.)
    WinXAS is a new x-ray absorption spectroscopy (XAS) data anal. program. It runs under the operating system MS-Windows 95/NT and offers several unique features. It has a user-friendly graphical environment and is capable of reading a variety of data formats. It contains a no. of useful numerical algorithms beyond those used in conventional XAS anal. and offers a simple interface to the ab-initio theor. code FEFF. The availability of fast macros in WinXAS makes it particularly useful for online data examn. at synchrotron radiation facilities during XAS expts. as well as for the anal. of multiple-scan data such as those from time-resolved expts.
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    Costa, C. N.; Christou, S. Y.; Georgiou, G.; Efstathiou, A. M. Mathematical Modeling of the Oxygen Storage Capacity Phenomenon Studied by CO Pulse Transient Experiments Over Pd/CeO2 Catalyst. J. Catal. 2003, 219, 259– 272,  DOI: 10.1016/S0021-9517(03)00151-9
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    Mathematical modeling of the oxygen storage capacity phenomenon studied by CO pulse transient experiments over Pd/CeO2 catalyst
    Costa, C. N.; Christou, S. Y.; Georgiou, G.; Efstathiou, A. M.
    Journal of Catalysis (2003), 219 (2), 259-272CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Science)
    A math. model has been developed for the first time to study the oxygen storage capacity (OSC) phenomenon by the CO pulse injection technique over a 1 wt% Pd/CeO2 model catalyst in the 500-700 °C range. A two-step reaction path that involves the reaction of gaseous CO with the oxygen species of PdO (pre-oxidized supported palladium particles in the 500-700 °C range) and of the back-spillover of the oxygen process from ceria to the oxygen vacant sites of surface PdO has been proven to better describe the outlet CO pulse transient response and the exptl. measured quantity of OSC (μatoms of O/g) obtained in a CSTR microreactor. With the proposed math. model, the transient rates of the CO oxidn. reaction and of the back-spillover of the oxygen process can be calcd. In the 500-700 °C range, the transient rate of CO oxidn. was always greater than that of the back-spillover of oxygen. The ratio, ρ, of the max. CO oxidn. rate to the max. back-spillover of the oxygen rate was found to decrease with increasing reaction temp. in the 500-700 °C range. In particular, at 500 and 700 °C the value of ρ was found to be 1.6 and 1.2, resp. The present math. model allows also the calcn. of the intrinsic rate const. k1 (s-1) of the Eley-Rideal step for the reaction of gaseous CO with surface oxygen species of PdO to form CO2. An activation energy of 9.2 kJ/mol was estd. for this reaction step. In addn., an apparent rate const. k2app (s-1) was estd. for the process of back-spillover of oxygen. The ratio of the two rate consts. (k1/k2app) was found to be greater than 100 in the 500-700 °C range. A Langmuir-Hinshelwood surface elementary reaction step of adsorbed CO with at. oxygen of PdO failed to describe the exptl. transient kinetics of CO oxidn. in the 500-700 °C range. The results of the present work provide the means for a better understanding of the effects of various additives and contaminants present in a three-way com. catalytic converter and other related model catalysts on their OSC kinetic behavior. In addn., intrinsic effects of a given regeneration method for a com. three-way catalyst on the OSC phenomenon could better be studied by making use of the results of the present math. model.
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    Perego, C.; Peratello, S. Experimental Methods in Catalytic Kinetics. Catal. Today 1999, 52, 133– 145,  DOI: 10.1016/S0920-5861(99)00071-1
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    Experimental methods in catalytic kinetics
    Perego, C.; Peratello, S.
    Catalysis Today (1999), 52 (2-3), 133-145CODEN: CATTEA; ISSN:0920-5861. (Elsevier Science B.V.)
    A review with 32 refs.; selecting an appropriate exptl. procedure and a suitable lab. reactor is crucial for evaluating the kinetics of a catalytic process. In this paper, practical guidelines are suggested for carrying out a kinetic study for a given reaction system. Comparison of various lab. reactors is presented and some of the pitfalls and limitations are discussed. The influence of transport phenomena due to the flow, the catalyst and the reactor geometry is analyzed and the criteria for their absence are given in a convenient form.
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    Vasiliades, M. A.; Harris, D.; Stephenson, H.; Boghosian, S.; Efstathiou, A. M. Novel Analysis of Transient Isothermal 18O Isotopic Exchange on Commercial CexZr1-xO2-Based OSC Materials. Top. Catal. 2019, 62, 219– 226,  DOI: 10.1007/s11244-018-1116-x
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    Sadovskaya, E. M.; Bobin, A. S.; Skazka, V. V. Isotopic Transient Analysis of Oxygen Exchange Over Oxides. Chem. Eng. J. 2018, 348, 1025– 1036,  DOI: 10.1016/j.cej.2018.05.027
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    Isotopic transient analysis of oxygen exchange over oxides
    Sadovskaya, E. M.; Bobin, A. S.; Skazka, V. V.
    Chemical Engineering Journal (Amsterdam, Netherlands) (2018), 348 (), 1025-1036CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    We investigated the model of oxygen isotopic exchange on solid oxides in flow reactor using numerical and anal. approaches. The study concerns the effect of the exchange mechanism (mono- or diat.) and of the rate of exchange as well as the oxygen diffusion rate in the bulk on the isotopic response after 16O/18O switch. Ratio of 16O18O mol. fraction to 18O at. fraction is shown to depend on the kinetics of gas-surface exchange, and does not depend on the oxygen diffusion in the oxide bulk. Algorithm of isotopic transient anal. without use of numerical methods was developed. Criterion to discriminate between the types of isotopic exchanges was proposed. Formulas for estn. of exchange rate and diffusion coeff. of oxygen in oxides were derived. The ests. of the surface exchange rate and of the bulk diffusion coeff. of the complex oxide La0.3Bi0.7MnO3 were calcd. applying proposed algorithm. The comparison with the results of numerical modeling verified the reliability and sufficiently high accuracy of obtained ests.
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    Skorodumova, N. V.; Simak, S. I.; Lundqvist, B. I.; Abrikosov, I. A.; Johansson, B. Quantum Origin of the Oxygen Storage Capability of Ceria. Phys. Rev. Lett. 2002, 89, 166601  DOI: 10.1103/PhysRevLett.89.166601
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    Quantum origin of the oxygen storage capability of ceria
    Skorodumova, N. V.; Simak, S. I.; Lundqvist, B. I.; Abrikosov, I. A.; Johansson, B.
    Physical Review Letters (2002), 89 (16), 166601/1-166601/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    The microscopic mechanism behind the extraordinary ability of ceria to store, release, and transport oxygen is explained on the basis of first-principles quantum mech. simulations. The oxygen-vacancy formation energy in ceria is calcd. for different local environments. The reversible CeO2-Ce2O3 redn. transition assocd. with oxygen-vacancy formation and migration is shown to be directly coupled with the quantum process of electron localization.
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    Hinuma, Y.; Toyao, T.; Kamachi, T.; Maeno, Z.; Takakusagi, S.; Furukawa, S.; Takigawa, I.; Shmizu, K. Density Functional Theory Calculations of Oxygen Vacancy Formation and Subsequent Molecular Adsorption on Oxide Surfaces. J. Phys. Chem. C 2018, 122, 29435– 29444,  DOI: 10.1021/acs.jpcc.8b11279
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    Density Functional Theory Calculations of Oxygen Vacancy Formation and Subsequent Molecular Adsorption on Oxide Surfaces
    Hinuma, Yoyo; Toyao, Takashi; Kamachi, Takashi; Maeno, Zen; Takakusagi, Satoru; Furukawa, Shinya; Takigawa, Ichigaku; Shimizu, Ken-ichi
    Journal of Physical Chemistry C (2018), 122 (51), 29435-29444CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The surface oxygen vacancy formation energy (EOvac) is an important parameter in detg. the catalytic activity of metal oxides. Estg. these energies can therefore lead to data-driven design of promising catalyst candidates. In the present study, we det. EOvac for various insulating and semiconducting oxides. Statistical investigations indicate that the band gap, bulk formation energy, and electron affinity are factors that strongly influence EOvac. Electrons enter defect states after O desorption, and these states can be in the valence band, mid-gap, or in the conduction band. Subsequent adsorption of O2, NO, CO, CO2, and H2 mols. on an O-deficient surface is also investigated. These mols. become preferentially adsorbed at the defect sites, and EOvac is identified as the dominant factor that dets. the adsorption mode as well as a descriptor that shows good correlation with the adsorption energy.
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  40. 40
    AlKhoori, A. A.; Polychronopoulou, K.; Belabbes, A.; Jaoude, M. A.; Vega, L. F.; Sebastian, V.; Hinder, S.; Baker, M. A.; Zedan, A. F. Cu, Sm co-doping effect on the CO oxidation activity of CeO2. A combined experimental and density functional study. Appl. Surf. Sci. 2020, 521, 146305,  DOI: 10.1016/j.apsusc.2020.146305
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    40
    Cu, Sm co-doping effect on the CO oxidation activity of CeO2. A combined experimental and density functional study
    AlKhoori, Ayesha A.; Polychronopoulou, Kyriaki; Belabbes, Abderrezak; Jaoude, Maguy Abi; Vega, Lourdes F.; Sebastian, Victor; Hinder, Steven; Baker, Mark A.; Zedan, Abdallah F.
    Applied Surface Science (2020), 521 (), 146305CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    The co-doping effect of a rare earth (RE) metal and a transition metal (TM) on ceria oxidn. catalysis through the evaluation of samarium-copper co-doped catalysts with Ce-Sm-xCu-O (x: 0-20 at.%, Ce/Sm = 1) nominal compns., is discussed. The CO oxidn. reaction was used as a prototype reaction due to its pivotal role in the fuel cell technol. Ce-Sm-20Cu-O catalyst presented a 64% increase in the CO oxidn. activity compared to that of pristine ceria. Diffraction and Raman studies proved that the Cu, Sm co-doping induces many defects related to the dopants (Sm, Cu) and the oxygen vacant sites, while the presence of hybrid CuO/Ce-Sm(Cu)-O fluorite/SmO8 (cubic metastable) phases is the most representative scenario of this oxide microstructure. A size polydispersity of CuO phases was achieved by introducing air cooling during the microwave heating. Cu, Sm atoms were uniformly doped in CeO2 structure according to the HAADF-STEM studies. These results are in agreement with EDS anal., where Cu, Sm and Ce are located in all the analyzed areas without any preferential distribution. The XPS studies demonstrated the co-presence of Cu2+/Cu1+ and Ce4+/Ce3+ redox couples in agreement with the Bader charge anal. from the ab initio calcns., the latter influencing greatly the oxidn. activity of the catalysts. D. functional theory (DFT) calcns. shed light on the oxide surface and the underlying mechanism governing the oxidn. catalysis taking place. In particular, Cu2+ and Sm3+ dopants were found to be located in the nearest neighbor (NN) sites of oxygen vacancies. Different oxygen vacancies configurations were studied (single vs. double, surface vs. subsurface), where the single vacancies are more stable on the surface, whereas the double vacancies configurations are more stable on the subsurface. Regarding the Ce3+ location, in the presence of single and double oxygen vacancy, the Ce3+ ions prefer to be located in the 1st NN/2nd NN and 2nd NN of the first Ce layer, relative to the oxygen vacancy, resp. The total D. of States (DOS) anal. of the co-doped systems revealed that the dopants induced new surface states inside the ceria band gap, which can accommodate the unpaired electrons of the vacant oxygen sites. These electronic modifications justify the much lower energy of oxygen vacancy formation (Evf) in both cases, the Sm-doped, and Cu, Sm -doped CeO2 (1 1 1) geometries. Specifically, the Evf lowering upon doping was found to be almost two times larger for the Cu adjacent oxygen vacancies (Cu2+-.box.) compared to the Sm ones (Sm3+-.box.), consistent with the CO adsorption trend as the Cu-Sm-CeO2 (1 1 1) system is energetically more favorable than the Sm-CeO2 (1 1 1) and pure CeO2 (1 1 1) surfaces.
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  41. 41
    Rakhmatullin, R.M.; Semashko, V.V.; Korableva, S.L.; Kiiamov, A.G.; Rodionov, A.A.; Tschaggelar, R.; van Bokhoven, J. A.; Paun, C. EPR study of ceria nanoparticles containing different concentration of Ce3+ ions. Mater. Chem. Phys. 2018, 219, 251– 257,  DOI: 10.1016/j.matchemphys.2018.08.028
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    EPR study of ceria nanoparticles containing different concentration of Ce3+ ions
    Rakhmatullin, R. M.; Semashko, V. V.; Korableva, S. L.; Kiiamov, A. G.; Rodionov, A. A.; Tschaggelar, R.; van Bokhoven, J. A.; Paun, C.
    Materials Chemistry and Physics (2018), 219 (), 251-257CODEN: MCHPDR; ISSN:0254-0584. (Elsevier B.V.)
    The ESR of Ce3+ ions in ceria is a rather controversial issue. In this report we analyze the EPR line with g ∼1.97 in the absorption spectra of ceria powders, which has been assigned in many studies to Ce3+ ions. Such an assignment is not in agreement with the exptl. data nor with the theory of spin-lattice relaxation of rare earth ions in solids. Our statement is supported by our exptl. results based on EPR spectroscopy of nanocryst. ceria, synthesized in various ways, as well as by magnetization measurements of as-prepd. and vacuum-annealed ceria nanoparticles. The possible model of the paramagnetic center, which is assigned to the EPR line with g ∼1.97 in ceria, is an electron trapped near the surface Ce3+/Ce4+ redox pair.
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    Alkhoori, A. A.; Elmutasim, O.; Dabbawala, A. A.; Vasiliades, M. A.; Petallidou, K. C.; Emwas, A.-H.; Anjum, D. H.; Singh, N.; Baker, M. A.; Charisiou, N. D.; Goula, M. A.; Efstathiou, A. M.; Polychronopoulou, K. Mechanistic Features of the CeO2-Modified Ni/Al2O3 Catalysts for the CO2Methanation Reaction: Experimental and Ab Initio Studies. ACS Applied Energy Materials 2023, 6 (16), 8550– 8571,  DOI: 10.1021/acsaem.3c01437
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    Agarwal, S.; Zhu, X.; Hensen, E. J.; Lefferts, L.; Mojet, B. L. Defect Chemistry of Ceria Nanorods. J. Phys. Chem. C 2014, 118, 4131– 4142,  DOI: 10.1021/jp409989y
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    Defect Chemistry of Ceria Nanorods
    Agarwal, S.; Zhu, X.; Hensen, E. J. M.; Lefferts, L.; Mojet, B. L.
    Journal of Physical Chemistry C (2014), 118 (8), 4131-4142CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Ceria nanorods were investigated using in situ Raman and FTIR spectroscopies for CO adsorption and subsequent reaction with water at 200 and 350 °C. The involvement of defects in ceria nanorods during CO adsorption and reaction with H2O is dependent on the temp. At 200 °C, most of the carbonate and formate species formed in CO do not involve the formation of defects, while at 350 °C all of the carbonates and formates formed can be correlated to the formation of defects (15% by formates and 85% by mono/bidentate carbonates). Finally, at 350 °C very stable polydentate carbonates are formed that do not induce defects and cannot be regenerated with water.
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    Sarkar, A.; Loho, C.; Velasco, L.; Thomas, T.; Bhattachary, S. S.; Hahn, H.; Djenadic, R. Multicomponent equiatomic rare earth oxides with a narrow band gap and associated Praseodymium Multivalency. Dalton Trans. 2017, 46, 12167– 12176,  DOI: 10.1039/C7DT02077E
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    Multicomponent equiatomic rare earth oxides with a narrow band gap and associated praseodymium multivalency
    Sarkar, Abhishek; Loho, Christoph; Velasco, Leonardo; Thomas, Tiju; Bhattacharya, Subramshu S.; Hahn, Horst; Djenadic, Ruzica
    Dalton Transactions (2017), 46 (36), 12167-12176CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    New multicomponent equiat. rare earth oxides (ME-REOs) contg. 3-7 rare earth elements (Ce, Gd, La, Nd, Pr, Sm and Y) in equiat. proportions are synthesized using nebulized spray pyrolysis. All the systems crystd. as a phase pure fluorite type (Fm‾3m) structure in spite of the high chem. complexity. A nominal increase in the lattice parameter compared to CeO2 is obsd. in all ME-REOs. XPS performed on the ME-REOs confirmed that all the constituent rare earth elements are present in the 3+ oxidn. state, except for Ce and Pr which are present in 4+ and in a mixed (3+/4+) oxidn. state, resp. The presence of Ce4+ contributes substantially to the obsd. stability of the single phase structure. These new oxide systems have narrow direct band gaps in the range of 1.95-2.14 eV and indirect band gaps in the range of 1.40-1.64 eV, enabling light absorption over the entire visible spectral range. Furthermore, the oxygen vacancy concn. rapidly increases and then sats. with the no. of rare earth elements that are incorporated into the ME-REOs. The lowering of the band gap is found to be closely related to the presence of multivalent Pr. Interestingly, the band gap values are relatively invariant with respect to the compn. or thermal treatments. Considering the high level of oxygen vacancies present and the obsd. low band gap values, these new material systems can be of importance where the presence of oxygen vacancies is essential or in applications where a narrow band gap is desirable.
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    Acharya, S. A.; Gaikwad, V.; D’Souza, S. W.; Barman, S. R. Gd/Sm Dopant-modified Oxidation State and Defect Generation in Nano-Ceria. Solid State Ionics 2014, 260, 21– 29,  DOI: 10.1016/j.ssi.2014.03.008
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    Gd/Sm dopant-modified oxidation state and defect generation in nano-ceria
    Acharya, S. A.; Gaikwad, V. M.; D'Souza, S. W.; Barman, S. R.
    Solid State Ionics (2014), 260 (), 21-29CODEN: SSIOD3; ISSN:0167-2738. (Elsevier B.V.)
    Gd/Sm-modified oxidn. state and defect in nano-ceria are studied by X-ray photoelectron (XPS) and Raman spectroscopy technique to investigate their influence on oxy-ionic cond. of the system. Nanosize Sm and Gd doped ceria (SDC and GDC) are synthesized in different compns., Ce(1 - x) (Sm / Gd)xO3 - δ (x = 0.05 to 0.30) under identical conditions. XPS study of doped ceria system indicates enrichment of Ce3+ ions in GDC as compared to that in SDC. The quant. anal. of Raman spectra also predicted the same trend of oxidn. state of cerium. The ionic cond. measurement by electrochem. impedance spectroscopy is demonstrated to verify the results for all compns. of dopant (x = 0.05 - 0.30).
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    Filtschew, A.; Hofmann, K.; Hess, C. Ceria and Its Defect Structure: New Insights From a Combined Spectroscopic Approach. J. Phys. Chem. C 2016, 120, 6694– 6703,  DOI: 10.1021/acs.jpcc.6b00959
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    Ceria and Its Defect Structure: New Insights from a Combined Spectroscopic Approach
    Filtschew, Anastasia; Hofmann, Kathrin; Hess, Christian
    Journal of Physical Chemistry C (2016), 120 (12), 6694-6703CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Ceria is an interesting component for a variety of catalytic and fuel cell applications. In the study described here, ten different com. ceria samples as well as synthesized ceria samples were investigated in detail regarding their (defect) structure and characteristic properties using XRD, N2 adsorption-desorption, and optical spectroscopy (Raman, DRIFTS, UV-vis). The investigations revealed correlations of surface defect features (Raman, DRIFTS) as well as those of bulk defects (Raman, UV-vis). The Raman feature at around 250 cm-1 was demonstrated to be related to surface defects rather than a 2TA vibration as described in the literature. A correlation between UV-vis band gap values and the presence of Raman bulk defects was established based on the obsd. decrease of the band gap energy with increasing no. of defects. Detailed Raman anal. revealed that the frequently mentioned linear equation for the detn. of the crystal size from the half-width of the F2g Raman feature is erroneous, since the F2g half-width depends on ceria bulk defects. Apart from these universal observations, differences in the properties depending on synthesis conditions were obsd. In particular, it is shown that the type and quantity of ceria defects are influenced not only by crystal size but also by the prepn. method.
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    Andriopoulou, C.; Trimpalis, A.; Petallidou, K. C.; Sgoura, A.; Efstathiou, A. M.; Boghosian, S. Structural and Redox Properties of Ce1–xZrxO2−δ and Ce0. 8Zr0.15RE0.05O2−δ (RE: La, Nd, Pr, Y) Solids Studied by High Temperature in Situ Raman Spectroscopy. J. Phys. Chem. C 2017, 121, 7931– 7943,  DOI: 10.1021/acs.jpcc.7b00515
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    Lee, Y.; He, G.; Akey, A. J.; Si, R.; Stephanopoulos, M. F.; Herman, I. P. Raman Analysis of Mode Softening in Nanoparticle CeO2−δ and Au-CeO2−δ During CO Oxidation. J. Am. Chem. Soc. 2011, 133, 12952– 12955,  DOI: 10.1021/ja204479j
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    Raman Analysis of Mode Softening in Nanoparticle CeO2-δ and Au-CeO2-δ during CO Oxidation
    Lee, Youjin; He, Guanghui; Akey, Austin J.; Si, Rui; Flytzani-Stephanopoulos, Maria; Herman, Irving P.
    Journal of the American Chemical Society (2011), 133 (33), 12952-12955CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Oxygen vacancy levels are monitored during the oxidn. of CO by CeO2-δ nanorods and Au-CeO2-δ nanorods, nanocubes, and nanopolyhedra by using Raman scattering. The first-order CeO2 F2g peak near 460 cm-1 decreases when this reaction is fast (fast redn. and relatively slow reoxidn. of the surface), because of the lattice expansion that occurs when Ce3+ replaces Ce4+ during oxygen vacancy creation. This shift correlates with reactivity for CO oxidn. Increases in the oxygen deficit δ as large as ∼0.04 are measured relative to conditions when the ceria is not reduced.
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    Wu, Z.; Li, M.; Howe, J.; Meyer, H. M.; Overbury, S. Probing Defect Sites on CeO2 Nanocrystals with Well-defined Surface Planes by Raman Spectroscopy and O2 Adsorption. Langmuir 2010, 26, 16595– 16606,  DOI: 10.1021/la101723w
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    Probing Defect Sites on CeO2 Nanocrystals with Well-Defined Surface Planes by Raman Spectroscopy and O2 Adsorption
    Wu, Zili; Li, Meijun; Howe, Jane; Meyer, Harry M., III; Overbury, Steven H.
    Langmuir (2010), 26 (21), 16595-16606CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Defect sites play an essential role in ceria catalysis. Ceria nanocrystals with well-defined surface planes were synthesized and used for studying defect sites with both Raman spectroscopy and O2 adsorption. Ceria nanorods ({110} + {100}), nanocubes ({100}), and nano-octahedra ({111}) are employed to analyze the quantity and quality of defect sites on different ceria surfaces. On oxidized surfaces, nanorods have the most abundant intrinsic defect sites, followed by nanocubes and nano-octahedra. When reduced, the induced defect sites are more clustered on nanorods than on nanocubes, although similar amts. (based on surface area) of such defect sites are produced on the 2 surfaces. Very few defect sites can be generated on the nano-octahedra due to the least reducibility. These differences can be rationalized by the crystallog. surface terminations of the ceria nanocrystals. The different defect sites on these nanocrystals lead to the adsorption of different surface dioxygen species. Superoxide on 1-electron defect sites and peroxide on 2-electron defect sites with different clustering degree are identified on the ceria nanocrystals depending on their morphol. Also, the stability and reactivity of these oxygen species are also surface-dependent, which is of significance for ceria-catalyzed oxidn. reactions.
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    Sartoretti, E.; Novara, C.; Giorgis, F.; Piumetti, M.; Bensaid, S.; Russo, N.; Fino, D. In Situ Raman Analyses of the Soot Oxidation Reaction over Nanostructured Ceria-based Catalysts. Sci. Rep. 2019, 9, 3875,  DOI: 10.1038/s41598-019-39105-5
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    50
    In situ Raman analyses of the soot oxidation reaction over nanostructured ceria-based catalysts
    Sartoretti Enrico; Novara Chiara; Giorgis Fabrizio; Piumetti Marco; Bensaid Samir; Russo Nunzio; Fino Debora
    Scientific reports (2019), 9 (1), 3875 ISSN:.
    To reduce the emissions of internal combustion engines, ceria-based catalysts have been widely investigated as possible alternatives to the more expensive noble metals. In the present work, a set of four different ceria-based materials was prepared via hydrothermal synthesis, studying the effect of Cu and Mn as dopants both in binary and ternary oxides. In situ Raman analyses were carried out to monitor the behaviour of defect sites throughout thermal cycles and during the soot oxidation reaction. Despite ceria doped with 5% of Cu featured the highest specific surface area, reducibility and amount of intrinsic and extrinsic defects, a poor soot oxidation activity was observed through the standard activity tests. This result was confirmed by the calculation of soot conversion curves obtained through a newly proposed procedure, starting from the Raman spectra collected during the in situ tests. Moreover, Raman analyses highlighted that new defectiveness was produced on the Cu-doped catalyst at high temperature, especially after soot conversion, while a slight increase of the defect band and a total reversibility were observed in case of the ternary oxide and pure/Mn-doped ceria, respectively. The major increment was related to the extrinsic defects component; tests carried out in different atmospheres suggested the assignment of this feature to vacancy-free sites containing oxidized doping cations. Its increase at the end of the tests can be an evidence of peroxides and superoxides deactivation on catalysts presenting excessive oxygen vacancy concentrations. Instead, ceria doped with 5% of Mn exhibited the best soot oxidation activity, thanks to an intermediate density of oxygen vacancies and to its well-defined morphology.
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    Purans, J.; Menushenkov, A. P.; Besedin, S. P.; Ivanov, A. A.; Minkov, V. S.; Pudza, I.; Kuzmin, A.; Klementiev, K. V.; Pascarelli, S.; Mathon, O.; Rosa, A. D.; Irifune, T.; Eremets, M. I. Local electronic structure rearrangements and strong anharmonicity in YH3 under pressures up to 180 GPA. Nat. Commun. 2021, 12, 1765,  DOI: 10.1038/s41467-021-21991-x
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    Satsuma, A.; Yanagihara, M.; Ohyama, J.; Shimizua, K. Oxidation of CO over Ru/Ceria prepared by self-dispersion of Ru metal powder into nano-sized particle. Catal. Today 2013, 201, 62– 67,  DOI: 10.1016/j.cattod.2012.03.048
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    Oxidation of CO over Ru/Ceria prepared by self-dispersion of Ru metal powder into nano-sized particle
    Satsuma, Atsushi; Yanagihara, Masatoshi; Ohyama, Junya; Shimizu, Kenichi
    Catalysis Today (2013), 201 (), 62-67CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    The oxidn. of CO was performed using phys. mixt. of Ru metal powder and various metal oxide supports after calcination in air and redn. in H2. Among various supports (CeO2, ZrO2, MgO, Al2O3, TiO2, SnO2, and SiO2), only Ru + CeO2 showed very high catalytic activity for CO oxidn. with the light-off temp. below 100 °C, which was comparable to Ru/CeO2 prepd. by a conventional impregnation method. The optimum conditions for the prepn. were loading amt. of 2 wt%, calcination at 700 °C in air followed by redn. at 400 °C in 5%H2. It was revealed that the high catalytic activity for Ru + CeO2 is attributed to the high dispersion of Ru on CeO2. The original Ru metal powder having diam. of 36 nm was dispersed during the calcination leading into small Ru particles having diam. around 2 nm. Raman spectra indicated that the formation of RuOCe bond after the calcination is one of the key for the Ru dispersion. The self-dispersion of Ru was not achieved by the calcination in N2. It was proposed that the self-dispersion of Ru into nano-particles was caused by oxidn. of Ru metal into Ru oxide in air, then dispersion of nano-sized Ru oxide by forming RuOCe bond, and finally conversion of Ru oxide particles into nano-sized Ru metal particles in a flow of H2.
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    Loridant, S. Raman spectroscopy as a powerful tool to characterize ceria-based catalysts. Catal. Today 2021, 373, 98– 111,  DOI: 10.1016/j.cattod.2020.03.044
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    Raman spectroscopy as a powerful tool to characterize ceria-based catalysts
    Loridant, Stephane
    Catalysis Today (2021), 373 (), 98-111CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    A review. CeO2 is widely used and investigated as an oxide catalyst or support due to its unique redox property of oxygen storage and release. In this paper, the different opportunities offered by Raman spectroscopy for advanced characterization of ceria-based catalysts are reviewed: spectral modifications induced by nanocrystallinity, defects, doping and redn., identification of supported mol. species, isolated atoms and nanoclusters, characterization of surface modes, hydroxyl groups, reaction intermediates such as peroxo and superoxo species. Finally, in situ/operando studies for environmental catalysis are summarized illustrating Raman spectroscopy as a powerful tool to characterize ceria-based catalysts.
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    Wang, H.; Li, X.; Ruan, Q.; Tang, J. Ru and Ruoxdecorated carbon nitride for efficient ammonia photosynthesis. Nanoscale 2020, 12, 12329– 12335,  DOI: 10.1039/D0NR02527E
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    Gu, Y. 1,2-dichloroethane deep oxidation over bifunctional Ru/CExAly Catalysts. ACS Omega 2018, 3, 8460– 8470,  DOI: 10.1021/acsomega.8b00592
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    1,2-Dichloroethane Deep Oxidation over Bifunctional Ru/CexAly Catalysts
    Gu, Yufeng; Jiang, Xingxing; Sun, Wei; Bai, Shuxing; Dai, Qiguang; Wang, Xingyi
    ACS Omega (2018), 3 (8), 8460-8470CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    Ru/CexAly catalysts were synthesized with impregnation of RuCl3 aq. soln. on CexAly (Al2O3-CeO2) and used in 1,2-dichloroethane (1,2-DCE) oxidn. Characterization by X-ray diffraction, Raman, NH3-temp.-programmed desorption (TPD), CO2-TPD, XPS, and H2-temp.-programmed redn. indicates that CeO2 exists as a form of face-centered cubic fluorite structure, whereas the chem. states and the structure of Ru species are dependent on the Ce content. The reducibility and acidity of catalysts increase with Ce/Ce + Al ratio. However, the latter is promoted only in a Ce/Ce + Al range of 0-0.25 and then decreases quickly. Ru/CexAly catalysts have considerable activity for 1,2-DCE combustion. TOFRu of 1,2-DCE oxidn. increases with strong acid, which is ascribed to a synergy of reducibility and acidity. Ru greatly inhibits the chlorination through the decreases in both Cl deposition and CH2 = CHCl formation. High stability of Ru/Ce10Al90 maintains at 280 °C for at least 25 h with CO2 selectivity of 99% or higher. In situ Fourier transform IR spectroscopy indicates that 1,2-DCE dissocs. to form ClCH2-CH2O- species, which is an intermediate species for the prodn. of CH3CHO and CH2 = CHCl, the former responsible for deep oxidn.
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    Siakavelas, G. I. Highly selective and stable nickel catalysts supported on Ceria promoted with SM2O3, PR2O3 and mgo for the CO2Methanation reaction. Applied Catalysis B: Environmental 2021, 282, 119562,  DOI: 10.1016/j.apcatb.2020.119562
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    Polychronopoulou, K. Decoupling the chemical and mechanical strain effect on steering the CO2 activation over CeO2-based oxides: An experimental and DFT approach. ACS Appl. Mater. Interfaces 2022, 14, 33094– 33119,  DOI: 10.1021/acsami.2c05714
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    Decoupling the Chemical and Mechanical Strain Effect on Steering the CO2 Activation over CeO2-Based Oxides: An Experimental and DFT Approach
    Polychronopoulou, Kyriaki; AlKhoori, Sara; AlBedwawi, Shaima; Alareeqi, Seba; Hussien, Aseel G. S.; Vasiliades, Michalis A.; Efstathiou, Angelos M.; Petallidou, Klito C.; Singh, Nirpendra; Anjum, Dalaver H.; Vega, Lourdes F.; Baker, Mark A.
    ACS Applied Materials & Interfaces (2022), 14 (29), 33094-33119CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO2 is involved. Thus, it is important to understand how to tailor their CO2 adsorption behavior. In this work, steering the CO2 activation behavior of Ce-La-Cu-O ternary oxide surfaces through the combined effect of chem. and mech. strain was thoroughly examd. using both exptl. and ab initio modeling approaches. Doping with aliovalent metal cations (La3+ or La3+/Cu2+) and post-synthetic ball milling were considered as the origin of the chem. and mech. strain of CeO2, resp. Exptl., microwave-assisted reflux-prepd. Ce-La-Cu-O ternary oxides were imposed into mech. forces to tune the structure, redox ability, defects, and CO2 surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chem. strain (εC) and mech. strain (εM) on the modification of the Ce-La-Cu-O surface reactivity toward CO2 activation. During the ab initio calcns., the stability (energy of formation, EOvf) of different configurations of oxygen vacant sites (Ov) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO2-philicity of the surface was assessed at different levels of the imposed mech. strain. The EOvf values were found to decrease with increasing tensile strain. The Ce-La-Cu-O(111) surface exhibited the lowest EOvf values for the single subsurface sites, implying that Ov may occur spontaneously upon Cu addn. The mobility of the surface and bulk oxygen anions in the lattice contributing to the Ov population was measured using 16O/18O transient isothermal isotopic exchange expts.; the max. in the dynamic rate of 16O18O formation, Rmax(16O18O), was 13.1 and 8.5μmol g-1 s-1 for pristine (chem. strained) and dry ball-milled (chem. and mech. strained) oxides, resp. The CO2 activation pathway (redox vs. associative) was exptl. probed using in situ diffuse reflectance IR Fourier transform spectroscopy. It was demonstrated that the mech. strain increased up to 6 times the CO2 adsorption sites, though reducing their thermal stability. This result supports the mech. actuation of the "carbonate"-bound species; the latter was in agreement with the d. functional theory (DFT)-calcd. C-O bond lengths and O-C-O angles. Ab initio studies shed light on the CO2 adsorption energy (Eads), suggesting a covalent bonding which is enhanced in the presence of doping and under tensile strain. Bader charge anal. probed the adsorbate/surface charge distribution and illustrated that CO2 interacts with the dual sites (acidic and basic ones) on the surface, leading to the formation of bidentate carbonate species. D. of states (DOS) studies revealed a significant Eg drop in the presence of double Ov and compressive strain, a finding with design implications in covalent type of interactions. To bridge this study with industrially important catalytic applications, Ni-supported catalysts were prepd. using pristine and ball-milled oxides and evaluated for the dry reforming of methane reaction. Ball milling was found to induce modification of the metal-support interface and Ni catalyst reducibility, thus leading to an increase in the CH4 and CO2 conversions. This study opens new possibilities to manipulate the CO2 activation for a portfolio of heterogeneous reactions.
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    Younis, A.; Shirsath, S. E.; Shabbir, B.; Li, S. Controllable dynamics of oxygen vacancies through extrinsic doping for superior catalytic activities. Nanoscale 2018, 10, 18576– 18585,  DOI: 10.1039/C8NR03801E
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    Controllable dynamics of oxygen vacancies through extrinsic doping for superior catalytic activities
    Younis, Adnan; Shirsath, Sagar E.; Shabbir, Babar; Li, Sean
    Nanoscale (2018), 10 (39), 18576-18585CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Due to its strong redox ability, high stability, cost effectiveness and non-toxicity, cerium oxide (CeO2) has been extensively researched as an active photocatalyst material. The underlying photocatalytic reactions are mostly assocd. with the transportation of oxygen ions through vacancies, but the actual transport phenomenon had not been clearly understood. In this work, gadolinium (Gd) is sequentially doped into CeO2 to investigate how extrinsic doping can modulate oxygen vacancies in CeO2 and influence photocatalytic activities. From our investigations, it was found that the Gd doping may induce structural symmetry breaking leading to a pure CeO2 fluorite structure that transforms mobile oxygen vacancies into clustered or immobile vacancies. When the vacancies were set as "mobile" (for Gd doping levels ≤15 at%), max. photocatalytic activities were obtained. In contrast, suppressed photocatalytic efficiencies were noted for higher Gd doping levels (20 at% or more). The results reported in this research may provide an extra degree of freedom in the form of extrinsic doping to configure the oxygen vacancy defects and their mobility to achieve better catalytic efficiencies.
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    Holder, C. F.; Schaak, R. E. Tutorial on Powder X-ray Diffraction for Characterizing Nanoscale Materials. ACS Nano 2019, 13, 7359– 7365,  DOI: 10.1021/acsnano.9b05157
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    Tutorial on Powder X-ray Diffraction for Characterizing Nanoscale Materials
    Holder, Cameron F.; Schaak, Raymond E.
    ACS Nano (2019), 13 (7), 7359-7365CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A review. The authors provide the broad nanoscience and nanotechnol. communities with a brief tutorial on some of the key aspects of powder XRD data that are often encountered when analyzing samples of nanoscale materials, with an emphasis on inorg. nanoparticles of various sizes, shapes, and dimensionalities.
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    Schmitt, R.; Nenning, A.; Kraynis, O.; Korobko, R.; Frenkel, A. I.; Lubomirsky, I.; Haile, S. M.; Rupp, J. L. M. A Review of Defect Structure and Chemistry in Ceria and Its Solid Solutions. J. Chem. Soc. Rev. 2020, 49, 554– 592,  DOI: 10.1039/C9CS00588A
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    Jacobsen, J.; Dahl, S.; Hansen, P. L.; Tornqvist, E.; Jensen, L.; Topsoe, H.; Prip, D. V.; Moenshaug, P. B.; Chorkendorff, I. Structure Sensitivity of Supported Ruthenium Catalysts for Ammonia Synthesis. J. Mol. Catal. A: Chem. 2000, 163, 19– 26,  DOI: 10.1016/S1381-1169(00)00396-4
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    Structure sensitivity of supported ruthenium catalysts for ammonia synthesis
    Jacobsen, Claus J. H.; Dahl, Soren; Hansen, Poul L.; Tornqvist, Eric; Jensen, Lone; Topsoe, Henrik; Prip, Dorthe V.; Moenshaug, Pernille B.; Chorkendorff, Ib
    Journal of Molecular Catalysis A: Chemical (2000), 163 (1-2), 19-26CODEN: JMCCF2; ISSN:1381-1169. (Elsevier Science B.V.)
    The catalytic ammonia synthesis activities of four supported ruthenium catalysts are reported. It is seen that Ru/MgAl2O4 is more active than two similar Ru/C catalysts, which are significantly more active than Ru/Si3N4. The activity differences cannot be satisfactorily explained solely by the differences in dispersion. Recent results from single crystal studies and DFT calcns. have shown that ammonia synthesis over ruthenium catalysts is a very structure sensitive reaction, more so than on iron catalysts. It is suggested that special B5-type sites are primarily responsible for the catalytic activity of the present supported Ru catalysts. It is shown how the no. of such B5-type sites depends on the Ru crystal size for a given crystal morphol. We have found that the activity of the Ru/MgAl2O4 catalyst increases significantly during the initial part of a test run. This activity increase is paralleled by the disappearance of crystals smaller than ca. 1.0 nm due to sintering and a resulting formation of larger crystals. We conclude that there exists a lower limit to the desired crystal size of Ru in supported ammonia synthesis catalysts. This is in agreement with a low no. of B5-type sites expected for such crystal sizes. Furthermore, we suggest that the support plays a decisive role in controlling the morphol. of the Ru crystals and the resulting change in abundance of B5-type sites is the main cause for the significant activity variations obsd. for Ru catalysts with different supports. Finally, the support may also influence the electronic and catalytic properties of neighboring B5-type sites.
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    Lingling, L.; Ya-Fei, J.; Tianhua, Z.; Hongfang, C.; Yanliang, Z.; Bingyu, L.; Xingyi, L.; Ying, Z.; Lirong, Z.; Xiuyun, W.; Cong-Qiao, X.; Chak-tong, A.; Lilong, J.; Jun, L. Size sensitivity of supported Ru catalysts for ammonia synthesis: From nanoparticles to subnanometric clusters and atomic clusters. Chem 2022, 8, 749– 768,  DOI: 10.1016/j.chempr.2021.11.008
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    Raróg-Pilecka, W.; Miśkiewicz, E.; Szmigiel, D.; Kowalczyk, Z. Structure sensitivity of ammonia synthesis over promoted ruthenium catalysts supported on graphitised carbon. J. Catal. 2005, 231, 11– 19,  DOI: 10.1016/j.jcat.2004.12.005
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    Structure sensitivity of ammonia synthesis over promoted ruthenium catalysts supported on graphitised carbon
    Rarog-Pilecka, Wioletta; Miskiewicz, Elzbieta; Szmigiel, Dariusz; Kowalczyk, Zbigniew
    Journal of Catalysis (2005), 231 (1), 11-19CODEN: JCTLA5; ISSN:0021-9517. (Elsevier)
    A series of carbon-based ruthenium catalysts differing in Ru loading (1-32 wt%) was characterized (XRD, TEM, O2 and CO chemisorption) and, after promotion with Ba or Cs or both, was studied in NH3 synthesis. Partly graphitized carbon with a high surface area (S BET = 1310 m2/g) was used as a support for catalyst prepn. Ruthenium chloride and barium nitrate and/or cesium nitrate were precursors of the active phase and promoters, resp. The chemisorption expts. have shown that the mean size of ruthenium particles (d) increases monotonically with Ru loading, from about 1 nm for 1 wt% Ru to about 4 nm for 32 wt% Ru. The NH3 synthesis studies have revealed that the reaction rates (400 °C, 63 bar, 8.5% NH3 or 400 °C, 90 bar, 11.5% NH3), expressed in terms of TOF, increase vs. particle size, regardless of the promoter type. Extrapolation to lower sizes indicates that crystallites smaller than 0.7-0.8 nm might be totally inactive. The co-promoted catalysts (Ba + Cs) were found to be more active than the singly doped systems over the whole range of the particle diam. The trends in TOF vs. d have been attributed to the promoter/promoters location, on the faces of the Ru crystallites, or, alternatively, to changes in crystallite morphol.-larger particles (3-4 nm) may expose more B5 sites than the smaller ones (1-2 nm). The effect of co-promotion has been ascribed to different modes of the promoters' action: whereas the main role of Cs is to lower the barrier for N2 dissocn. (electronic effect), barium acts predominantly as a structural promoter-the reconstructed surfaces in Ba-Ru/C are more resistant to poisoning by hydrogen when operating, thus making more sites available for N2 adsorption.
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    Kim, Y. K.; Morgan, G. A.; Yates, J. T. Site-specific dissociation of N-2 on the stepped Ru(109). Surf. Sci. 2005, 14, 598
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    Lin, H.; Liu, J. X.; Fan, H.-J.; Li, W.-X. Crystallographic and morphological sensitivity of N2 activation over ruthenium. Chin. J. Chem. Phys. 2021, 34, 263– 272,  DOI: 10.1063/1674-0068/cjcp2009171
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    Crystallographic and morphological sensitivity of N2 activation over ruthenium
    Lin, Hao; Liu, Jin-xun; Fan, Hong-jun; Li, Wei-xue
    Chinese Journal of Chemical Physics (2021), 34 (3), 263-272CODEN: CJCPA6; ISSN:1674-0068. (American Institute of Physics)
    Ruthenium (Ru) serves as a promising catalyst for ammonia synthesis via the Haber-Bosch process, identification of the structure sensitivity to improve the activity of Ru is important but not fully explored yet. We present here d. functional theory calcns. combined with micro-kinetic simulations on nitrogen mol. activation, a crucial step in ammonia synthesis, over a variety of hcp. (hcp) and face-center cubic (fcc) Ru facets. Hcp {2130} facet exhibits the highest activity toward N2 dissocn. in hcp Ru, followed by the (0001) monat. step sites. The other hcp Ru facets have N2 dissocn. rates at least three orders lower. Fcc {211} facet shows the best performance for N2 activation in fcc Ru, followed by {311}, which indicates stepped surfaces make great contributions to the overall reactivity. Although hcp Ru {2130} facet and (0001) monat. step sites have lower or comparable activation barriers compared with fcc Ru {211} facet, fcc Ru is proposed to be more active than hcp Ru for N2 conversion due to the exposure of the more favorable active sites over step surfaces in fcc Ru. This work provides new insights into the crystal structure sensitivity of N2 activation for mechanistic understanding and rational design of ammonia synthesis over Ru catalysts. (c) 2021 American Institute of Physics.
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    Jiang, Rongzhong; Tran, Dat T.; Li, Jiangtian; Chu, D. Ru@RuO2 Core-Shell Nanorods: A Highly Active and Stable Bifunctional Catalyst for Oxygen Evolution and Hydrogen Evolution Reactions. Energy Environ. Mater. 2019, 2, 201– 208,  DOI: 10.1002/eem2.12031
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    Ru@RuO2 Core-Shell Nanorods: A Highly Active and Stable Bifunctional Catalyst for Oxygen Evolution and Hydrogen Evolution Reactions
    Jiang, Rongzhong; Tran, Dat T.; Li, Jiangtian; Chu, Deryn
    Energy & Environmental Materials (2019), 2 (3), 201-208CODEN: EEMNA3; ISSN:2575-0356. (John Wiley & Sons, Inc.)
    Ru@RuO2 core-shell nanorods were successfully synthesized by heat-treating Ru nanorods with air oxidn. through an accurate control of the temp. and time. The structure, compn., dimension, and adsorption property of the core-shell nanorods were well characterized with XRD and TEM. The catalytic activity and stability were electrochem. evaluated with a rotating disk electrode, a rotating ring-disk electrode, and chronopotentiometric methods. The Ru@RuO2 nanorods reveal excellent bifunctional catalytic activity and robust stability for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The overpotentials for OER and HER are 320 mV and 137 mV at the c.d. of 10 mA cm-2, resp. The catalytic activity of Ru@RuO2 nanorods for OER is 6.5 times higher than that of the state-of-the-art catalyst IrO2 according to the catalytic c.d. measured at 1.60 V (vs. RHE). The catalytic activity of Ru@RuO2 nanorods for HER is comparable to 40% Pt/C by comparing the catalytic current densities at -0.2 V.
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    Chandra, D.; Inoue, Y.; Sasase, M.; Kitano, M.; Bhaumik, A.; Kamata, K.; Hosono, H.; Hara, M. Chem. Sci. 2018, 9, 5949– 5956,  DOI: 10.1039/C8SC01197D
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    Stevens, C. A. C.; Lambie, S. G.; Ruffman, C.; Skulason, E.; Garden, A. L. Geometric and Electronic Effects Contributing to N2 Dissociation Barriers on a Range of Active Sites on Ru Nanoparticles. J. Phys. Chem. C 2019, 123, 30458– 30466,  DOI: 10.1021/acs.jpcc.9b09563
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    Tang, C.; Qiao, S.-Z. How to explore ambient electrocatalytic nitrogen reduction reliably and insightfully. Chem. Soc. Rev. 2019, 48, 3166– 3180,  DOI: 10.1039/C9CS00280D
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    How to explore ambient electrocatalytic nitrogen reduction reliably and insightfully
    Tang, Cheng; Qiao, Shi-Zhang
    Chemical Society Reviews (2019), 48 (12), 3166-3180CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. The efficient activation of dinitrogen for the prodn. of ammonia plays a crucial role in our modern society, considering the significant impact of ammonia on food, chems., and energy. As an attractive alternative to the century-old Haber-Bosch process which is responsible for 1-2% of global energy consumption, utilization of half the hydrogen produced globally, and ∼1% of global energy-related CO2 emissions, the ambient electrocatalytic nitrogen redn. reaction has attracted tremendous interest during the past few years. Some achievements have revealed the possibility of this process, but have also identified great challenges. The activity and selectivity of the nitrogen redn. reaction are fundamentally limited by competing hydrogen evolution and nitrogen scaling relations, while low prodn. rates and ubiquitous contaminants plague exptl. practices. Aiming toward higher accuracy and reproducibility of claimed results, and more meaningful, impactful, and insightful research, this tutorial review summarizes the present status and challenges in the study of ambient electrocatalytic nitrogen redn., followed by a thorough discussion of various exptl. parameters. We then recommend a series of protocols and best practices for expts., and also highlight some potential directions for future research in this exciting and important field.
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    Singh, S.; Mohammed, A. K.; AlHammadi, A. A.; Shetty, D.; Polychronopoulou, K. Hypes and hopes on the materials development strategies to produce ammonia at mild conditions. Int. J. Hydrogen Energy 2023, 48, 34700– 34739,  DOI: 10.1016/j.ijhydene.2023.05.206
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    Zhou, Y.; Wang, J.; Liang, L.; Sai, Q.; Ni, J.; Au, C. T.; Lin, X.; Wang, X.; Zheng, Y.; Zheng, L.; Jiang, L. Unraveling the size-dependent effect of RU-based catalysts on ammonia synthesis at mild conditions. J. Catal. 2021, 404, 501– 511,  DOI: 10.1016/j.jcat.2021.10.024
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    Unraveling the size-dependent effect of Ru-based catalysts on Ammonia synthesis at mild conditions
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    Identifying optimal Ru size in NH3 synthesis can improve reaction activity and maximize the utilization of Ru to reduce catalyst cost. However, previous researches are focused on large Ru particle size (≥2 nm) while that below 2 nm in NH3 synthesis is unclear. Here we synthesized a series of Rux/BaCeO3 with different Ru sizes (x = 1.1-3.0 nm) through size-controlled Ru colloid. With the decrease of Ru size, NH3 synthesis rate over Ru1.1/BaCeO3 increases to 19.4 mmol g-1cath-1 at 400°C and 1 MPa, which is 5.7 times that of Ru3.0/BaCeO3 and superior to most of Ru-based catalysts previously reported. It reveals that the redn. of Ru size enhances the generation of Ce3+ and oxygen vacancies in BaCeO3, which can donate electron to Ru centers and promote N2 dissocn. Moreover, the small Ru size enhances hydrogen spillover from Ru to BaCeO3 to alleviate hydrogen poisoning, resulting in efficient NH3 synthesis.
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    The electrochem. redn. of N to NH3 on Au-based catalysts showed a reasonably high Coulombic efficiency. The pathway of this promising reaction, however, is not clear partially due to the lack of information on reaction intermediates. Herein, surface-enhanced IR absorption spectroscopy (SEIRAS) was employed to study the reaction mechanisms of N redn. on an Au thin film for the 1st time. During the N redn., the N2Hy species was detected with bands at 1453 (H-N-H bending), 1298 (-NH2 wagging), and 1109 cm-1 (N-N stretching) at potentials <0 V against reversible H electrode. The N redn. reaction on Au surfaces follows an associative mechanism, and the N≡N bond in N2 tends to break simultaneously with the H addn. By comparison, no absorption band assocd. with N was obsd. on Pt surfaces under the same reaction condition. This result is consistent with the low efficiency of N redn. on Pt due to the much faster kinetics of H evolution reaction.
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  • Abstract

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    Figure 1

    Figure 1. Unit cells of (a) pure CeO2, (b) binary CeGdO, (c) ternary CeLaGdO, and (d) quinary CeLaPrSmGdO.

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    Figure 2

    Figure 2. (a) Unit cell of CeO2 along (110) surface with vacuum. Bulk region, sublayer, and top-layers are shown. (b) Evolution of oxygen vacancy formation energy in pure CeO2, binary systems (CeGdO, CePrO), ternary systems (CeLaGdO, CeLaPrO), and quinary (CeLaPrSmGdO) high entropy system.

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    Figure 3

    Figure 3. (A) XRD diffractograms; (B–C) EPR spectra collected at 100 K; (D) Raman spectra collected over the Ru/CePrO catalyst; (E) Raman spectra collected over the Ru/HEO catalyst; (F) H2-temperature-programmed desorption (H2-TPD) profiles obtained over all the Ru catalysts of the present study.

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    Figure 4

    Figure 4. (A) Ru 3p, (B) Ce 3d, and (C) O 1s XPS core-level spectra of the prepared catalysts.

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    Figure 5

    Figure 5. (A, B) Linear combination fitting of the XANES spectra collected at the LIII edge of Ce in the HEO sample (left) and CePrO sample (right) to determine all the oxidation states of Ce in the samples; (C) Fourier transforms with their respective real part of the k3 weighted EXAFS spectra collected at the Ce LIII edge in CLPSG-HEO and CePr samples.

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    Figure 6

    Figure 6. (A) HRTEM images along with Fast Fourier Transform (FFT) pattern; (B, C) facets analysis of the Ru/CePrO catalyst.

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    Figure 7

    Figure 7. (A–E) STEM-HAADF RGB analysis and (F) selected area electron diffraction (SAED) over the Ru/CePrO catalyst.

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    Figure 8

    Figure 8. (A–C) HRTEM images of the Ru/HEO catalyst with emphasis on Ru particles at different areas (B) and (C).

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    Figure 9

    Figure 9. (A) HRTEM images of the Ru/HEO catalyst along with Fast Fourier Transform (FFT) pattern and (B–D) facets analysis.

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    Figure 10

    Figure 10. (A–H) STEM-HAADF RGB analysis and (I) selected area electron diffraction (SAED) over the Ru/HEO catalyst.

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    Figure 11

    Figure 11. Catalytic production of NH3 (A) over Ru-based catalysts of this study in the 10–40 bar pressure range and 400 °C; (B) over the Ru/HEO catalyst at 10 bar, 400 °C following two different activation conditions (650 vs 800 °C).

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    Figure 12

    Figure 12. (A) Transient rates of 16O2 (μmol g–1 s–1) consumed during transient isothermal oxidation (TIO) at 650 °C, following the different pretreatment conditions, over HEO solid; and (B) amounts of O2 consumed (μmol g–1) during TIO, over both solids.

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    Figure 13

    Figure 13. (A) Transient concentration (mol %) response curves of Kr, 16O2, 16O18O, and 18O2 gaseous species recorded during the 18O2-TIIE experiment at 650 °C, over the calcined HEO; (B) αg(18) descriptor as a function of time and pretreatment procedure, estimated during 18O2-TIIE over HEO; and (C) αg(18) descriptor obtained over the both reduced at 650 °C solids.

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    Figure 14

    Figure 14. Top and side views of N2 Adsorption configurations on Ru4/reduced HEO surface: (a) in absence of hydrogen and (b) in the presence of hydrogen. Gd is shown in purple, La in dark green, Pr in yellow, Sm in pink, Ce in light green, Ru in orange, N in white, H in blue, and O in red.

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    Figure 15

    Figure 15. Top and side views of N Adsorption configurations on reduced quinary CeLaPrSmGdO (111) surface: (a) in absence of hydrogen and (b) in the presence of hydrogen. Gd is shown in purple, La in dark green, Pr in yellow, Ce in light green, Sm in pink, H in blue, N in white, and O in red.

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      Petallidou, Klito C.; Polychronopoulou, Kyriaki; Boghosian, Soghomon; Garcia-Rodriguez, Sergio; Efstathiou, Angelos M.
      Journal of Physical Chemistry C (2013), 117 (48), 25467-25477CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Pt nanoparticles (1.2-2.0 nm size) supported on Ce1-xTixO2-δ (x = 0, 0.2, 0.5, 0.8, and 1.0) carriers synthesized by the citrate sol-gel method were tested toward the water-gas shift (WGS) reaction in the 200-350° range. A deep insight into the effect of two structural parameters, the chem. compn. of support (Ce/Ti atom ratio), and the Pt particle size on the catalytic performance of Pt-loaded catalysts was realized after employing in situ X-ray diffraction (XRD), high-resoln. transmission electron microscopy (HR-TEM) and HAADF/STEM, SEM, in situ Raman and diffuse reflectance IR Fourier transform (DRIFT) spectroscopies under different gas atmospheres, H2 temp.-programmed redn. (H2-TPR), and temp.-programmed desorption (NH3-TPD and CO2-TPD) techniques. The 0.5 wt. % Pt/Ce0.8Ti0.2O2-δ solid (dPt = 1.7 nm) was found to be by far the best catalyst among all the other solids investigated. In particular, at 250° the CO conversion over Pt/Ce0.8Ti0.2O2-δ was increased by a factor of 2.5 and 1.9 compared to Pt/TiO2 and Pt/CeO2, resp. The catalytic superiority of the Pt/Ce0.8Ti0.2O2-δ solid is the result of the support's (i) robust morphol. preserved during the WGS reaction, (ii) moderate acidity and basicity, and (iii) better reducibility at lower temps. and the significant redn. of "coking" on the Pt surface and of carbonate accumulation on the Ce0.8Ti0.2O2-δ support. Several of these properties largely affected the reactivity of sites (k, s-1) at the Pt-support interface. In particular, the specific WGS reaction rate at 200° expressed per length of the Pt-support interface (μmol CO cm-1 s-1) was found to be 2.2 and 4.6 times larger on Pt supported on Ce0.8Ti0.2O2-δ (Ti4+-doped CeO2) compared to TiO2 and CeO2 alone, resp.
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    7. 7
      Dou, B.; Zhao, L.; Zhang, H.; Wu, K.; Zhang, H. Renewable hydrogen production from chemical looping steam reforming of biodiesel byproduct glycerol by mesoporous oxygen carriers. Chemical Engineering Journal 2021, 416, 127612,  DOI: 10.1016/j.cej.2020.127612
      There is no corresponding record for this reference.
    8. 8
      Kim, Y. M.; Lohsoontorn, P. K.; Bae, J. Effect of Unsintered Gadolinium-doped Ceria Buffer Layer on Performance of Metal-supported Solid Oxide Fuel Cells Using Unsintered Barium Strontium Cobalt Ferrite Cathode. J. Power Sources 2010, 195, 6420– 6427,  DOI: 10.1016/j.jpowsour.2010.03.095
      There is no corresponding record for this reference.
    9. 9
      Sarkar, A.; Velasco, L.; Wang, D.; Wang, Q.; Talasila, G.; Biasi, L. D.; Kubel, C.; Brezesinski, T.; Bhattacharya, S. S.; Hahn, H.; Breitung, B. High Entropy Oxides for Reversible Energy Storage. Nat. Commun. 2018, 9, 3400,  DOI: 10.1038/s41467-018-05774-5
      9
      High entropy oxides for reversible energy storage
      Sarkar Abhishek; Velasco Leonardo; Wang Di; Wang Qingsong; Talasila Gopichand; de Biasi Lea; Kubel Christian; Brezesinski Torsten; Hahn Horst; Breitung Ben; Wang Di; Kubel Christian; Breitung Ben; Kubel Christian; Hahn Horst; Bhattacharya Subramshu S
      Nature communications (2018), 9 (1), 3400 ISSN:.
      In recent years, the concept of entropy stabilization of crystal structures in oxide systems has led to an increased research activity in the field of "high entropy oxides". These compounds comprise the incorporation of multiple metal cations into single-phase crystal structures and interactions among the various metal cations leading to interesting novel and unexpected properties. Here, we report on the reversible lithium storage properties of the high entropy oxides, the underlying mechanisms governing these properties, and the influence of entropy stabilization on the electrochemical behavior. It is found that the stabilization effect of entropy brings significant benefits for the storage capacity retention of high entropy oxides and greatly improves the cycling stability. Additionally, it is observed that the electrochemical behavior of the high entropy oxides depends on each of the metal cations present, thus providing the opportunity to tailor the electrochemical properties by simply changing the elemental composition.
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    10. 10
      Albedwawi, S. H.; Aljaberi, A.; Haidemenopoulos, G. N.; Polychronopoulou, K. High Entropy Oxides-Exploring a Paradigm of Promising Catalysts: A Review. Materials & Design 2021, 202, 109534,  DOI: 10.1016/j.matdes.2021.109534
      10
      High entropy oxides-exploring a paradigm of promising catalysts: A review
      Albedwawi, Shaima H.; AlJaberi, Asala; Haidemenopoulos, Gregory N.; Polychronopoulou, Kyriaki
      Materials & Design (2021), 202 (), 109534CODEN: MADSD2; ISSN:0264-1275. (Elsevier Ltd.)
      A review. This review presents the structural elements and discusses the great potential of high entropy oxides (HEO) as promising catalysts. A crit. comparison is provided with the medium and low entropy metal oxides in terms of the important functionality of HEO to exhibit higher oxygen mobility and withhold high population of oxygen vacancies as well as leading to high metal dispersion. This review study critically compares the performance of the thermal, electro- and photo- high entropy oxides catalysts with the conventional metal oxides and demonstrates their superiority over them while discussing the governing characteristics of HEO. The HEO complex structure is highlighted using ab initio calcns. on understanding and tuning their electronic/structural properties. Catalysts' design criteria and direction in the studies of the HEO as catalysts for energy and sustainability are proposed.
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    11. 11
      Chae, S.; Williams, L.; Lee, J.; Heron, J. T.; Kioupakis, E. Effects of Local Compositional and Structural Disorder on Vacancy Formation in Entropy-stabilized Oxides from First-principles. npj Comput. Mater. 2022, 8, 95,  DOI: 10.1038/s41524-022-00780-0
      11
      Effects of local compositional and structural disorder on vacancy formation in entropy-stabilized oxides from first-principles
      Chae, Sieun; Williams, Logan; Lee, Jihang; Heron, John T.; Kioupakis, Emmanouil
      npj Computational Materials (2022), 8 (1), 95CODEN: NCMPCS; ISSN:2057-3960. (Nature Portfolio)
      Entropic stabilization has evolved into a strategy to create new oxide materials and realize novel functional properties engineered through the alloy compn. Achieving an atomistic understanding of these properties to enable their design, however, has been challenging due to the local compositional and structural disorder that underlies their fundamental structure-property relationships. Here, we combine high-throughput atomistic calcns. and linear regression algorithms to investigate the role of local configurational and structural disorder on the thermodn. of vacancy formation in (MgCoNiCuZn)O-based entropy-stabilized oxides (ESOs) and their influence on the elec. properties. We find that the cation-vacancy formation energies decrease with increasing local tensile strain caused by the deviation of the bond lengths in ESOs from the equil. bond length in the binary oxides. The oxygen-vacancy formation strongly depends on structural distortions assocd. with the local configuration of chem. species. Vacancies in ESOs exhibit deep thermodn. transition levels that inhibit elec. conduction. By applying the charge-neutrality condition, we det. that the equil. concns. of both oxygen and cation vacancies increase with increasing Cu mole fraction. Our results demonstrate that tuning the local chem. and assocd. structural distortions by varying alloy compn. acts an engineering principle that enables controlled defect formation in multi-component alloys.
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    12. 12
      Ye, S.; Zhu, J.; Zhu, S.; Zhao, Y.; Li, M.; Huang, Z.; Wang, H.; He, J. Design strategies for perovskite-type high-entropy oxides with applications in optics. ACS Appl. Mater. Interfaces 2023, 15, 47475– 47486,  DOI: 10.1021/acsami.3c09447
      There is no corresponding record for this reference.
    13. 13
      Sarkar, A.; Kruk, R.; Hahn, H. Magnetic properties of high entropy oxides. Dalton Transactions 2021, 50, 1973– 1982,  DOI: 10.1039/D0DT04154H
      There is no corresponding record for this reference.
    14. 14
      Sarkar, A.; Wang, Q.; Schiele, A.; Chellali, M. R.; Bhattacharya, S. S.; Wang, D.; Brezesinski, T.; Hahn, H.; Velasco, L.; Breitung, B. High-entropy oxides: Fundamental aspects and electrochemical properties. Adv. Mater. 2019, 31, e1806236  DOI: 10.1002/adma.201806236
      There is no corresponding record for this reference.
    15. 15
      Brahlek, M.; Gazda, M.; Keppens, V.; Mazza, A. R.; McCormack, S. J.; Gryn, A. M.; Musico, B.; Page, K.; Rost, C. M.; Sinnott, S. B.; Toher, C.; Ward, T. Z.; Yamamoto, A. What is in a Name: Defining “High Entropy” Oxides. APL Mater. 2022, 10, 110902  DOI: 10.1063/5.0122727
      15
      What is in a name: Defining "high entropy" oxides
      Brahlek, Matthew; Gazda, Maria; Keppens, Veerle; Mazza, Alessandro R.; McCormack, Scott J.; Mielewczyk-Gryn, Aleksandra; Musico, Brianna; Page, Katharine; Rost, Christina M.; Sinnott, Susan B.; Toher, Cormac; Ward, Thomas Z.; Yamamoto, Ayako
      APL Materials (2022), 10 (11), 110902CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      A review. High entropy oxides are emerging as an exciting new avenue to design highly tailored functional behaviors that have no traditional counterparts. Study and application of these materials are bringing together scientists and engineers from physics, chem., and materials science. The diversity of each of these disciplines comes with perspectives and jargon that may be confusing to those outside of the individual fields, which can result in miscommunication of important aspects of research. In this Perspective, we provide examples of research and characterization taken from these different fields to provide a framework for classifying the differences between compositionally complex oxides, high entropy oxides, and entropy stabilized oxides, which is intended to bring a common language to this emerging area. We highlight the crit. importance of understanding a material's crystallinity, compn., and mixing length scales in detg. its true definition. (c) 2022 American Institute of Physics.
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    16. 16
      Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169– 11186,  DOI: 10.1103/PhysRevB.54.11169
      16
      Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
      Kresse, G.; Furthmueller, J.
      Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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    17. 17
      Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15– 50,  DOI: 10.1016/0927-0256(96)00008-0
      17
      Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
      Kresse, G.; Furthmuller, J.
      Computational Materials Science (1996), 6 (1), 15-50CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
      The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
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    18. 18
      Pedrew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868,  DOI: 10.1103/PhysRevLett.77.3865
      There is no corresponding record for this reference.
    19. 19
      Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953,  DOI: 10.1103/PhysRevB.50.17953
      19
      Projector augmented-wave method
      Blochl
      Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
      There is no expanded citation for this reference.
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    20. 20
      Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. Phys. Rev. B 1999, 59, 1758,  DOI: 10.1103/PhysRevB.59.1758
      20
      From ultrasoft pseudopotentials to the projector augmented-wave method
      Kresse, G.; Joubert, D.
      Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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    21. 21
      Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-energy-loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 1505,  DOI: 10.1103/PhysRevB.57.1505
      21
      Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study
      Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P.
      Physical Review B: Condensed Matter and Materials Physics (1998), 57 (3), 1505-1509CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      By taking better account of electron correlations in the 3d shell of metal ions in Ni oxide it is possible to improve the description of both electron energy loss spectra and parameters characterizing the structural stability of the material compared with local spin d. functional theory.
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    22. 22
      Zunger, A.; Wei, S. H.; Ferreira, L. G.; Bernard, J. E. Special Quasi-random Structures. Phys. Rev. Lett. 1990, 65, 353
      22
      Special quasirandom structures
      Zunger, Alex; Wei, S. H.; Ferreira, L. G.; Bernard, James E.
      Physical Review Letters (1990), 65 (3), 353-6CODEN: PRLTAO; ISSN:0031-9007.
      Structural models used in calcns. of properties of substitutionally random A1-xBx alloys are usually constructed by randomly occupying each of the N sites of a periodic cell by A or B. It is possible to design "special quasirandom structures" (SQS's) that mimic for small N (even N = 8) the first few, phys. most relevant radial correlation functions of a perfectly random structure far better than the std. technique does. The usefulness is demonstrated of these SQS's by calcg. optical and thermodn. properties of a no. of semiconductor alloys in the local-d. formalism.
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    23. 23
      Walle, A. V.; Tiwary, P.; Jong, M. D.; Olmsted, D. L.; Asta, M.; Dick, A.; Shin, D.; Wang, Y.; Chen, L. Q.; Liu, Z. K. Efficient Stochastic Generation of Special Quasi-random Structures. Calphad 2013, 42, 13– 18,  DOI: 10.1016/j.calphad.2013.06.006
      There is no corresponding record for this reference.
    24. 24
      Ghosh, G.; Walle, A. V.; Asta, M. First-principles Calculations of the Structural and Thermodynamic Properties of bcc, fcc and hcp Solid Solutions in the Al–TM (TM = Ti, Zr and Hf) Systems: A Comparison of Cluster Expansion and Supercell Methods. Acta Mater. 2008, 56, 3202,  DOI: 10.1016/j.actamat.2008.03.006
      24
      First-principles calculations of the structural and thermodynamic properties of bcc., fcc. and hcp. solid solutions in the Al-TM (TM=Ti, Zr and Hf) systems: A comparison of cluster expansion and supercell methods
      Ghosh, G.; van de Walle, A.; Asta, M.
      Acta Materialia (2008), 56 (13), 3202-3221CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
      The thermodn. properties of bcc., fcc. and hcp. solid solns. in the Al-TM systems with TM = Ti, Zr or Hf are calcd. from first-principles by using cluster expansion, Monte-Carlo simulation and supercell methods. The 32-atom special quasi-random structure supercells are used to compute properties of alloys contg. 25, 50 and 75 at.% TM, and 64-atom supercells have been used to compute properties of the dil. alloys with one solute and 63 solvent atoms. In general, the energy of mixing calcd. by cluster expansion and dil. supercells are in agreement. In the concd. region, the mixing energy values calcd. by both methods also agree in many cases, but noteworthy discrepancies are found in some cases, which originate from inherent elastic and dynamic instabilities in the relevant parent lattices. The importance of short-range order on the calcd. values of mixing energy for hcp. Al-Ti alloys is demonstrated. Calcd. results are presented for the compn. dependence of the at. vols. in random solid solns. with bcc., fcc. and hcp. structures. The reported properties may be integrated within the CALPHAD formalism to develop reliable thermodn. databases to facilitate (1) calcns. of stable and metastable phase diagrams of binary and multicomponent systems (2) alloy design (3) processing of Al-TM-base alloys.
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    25. 25
      Qu, P. F.; Wang, G. C. Theoretical Insight into the Strong Size-dependence of Dry Reforming of Methane Over Ru/CeO2. J. CO2 Util. 2022, 65, 102221,  DOI: 10.1016/j.jcou.2022.102221
      25
      Theoretical insight into the strong size-dependence of dry reforming of methane over Ru/CeO2
      Qu, Peng-Fei; Wang, Gui-Chang
      Journal of CO2 Utilization (2022), 65 (), 102221CODEN: JCUOAJ; ISSN:2212-9839. (Elsevier Ltd.)
      Transitional metals catalyzed dry reforming of methane (DRM) usually shows strongly size-dependence behavior, but the at. level understanding is lacking. The DRM over Run/CeO2(111) (n = 1,2,4,7) were studied systematically by using d. functional theory calcn. combined with microkinetic modeling anal. in order to explore the nature behind the Ru size effect on the DRM catalytic activity. By d. functional theory calcn., it was found that the CH4 activation and CO2 activation processes in DRM reaction are size-dependence and the Ru coverage of optimal chem. performance is around 0.1-0.2 Monolayer (i.e., Ru2 and Ru4 models). The main reason is that Ru2 and Ru4 models could activate CO2 much more easily than other models either with lower Ru coverage like Ru1 or higher Ru coverage like Ru7 model. By using the energy decompn. scheme, it was shown that the Pauli repulsion between two dissocd. species and the absorption energy of CO* have crucial effects for the chem. performance of the CO2 activation step, and Ru2 and Ru4 models have weak Pauli repulsion and strong absorption ability of CO. The Ru2 and Ru4 models have optimal chem. performance than other models by DRM free energy profiles. By coke formation anal., the Ru2 model is difficult to form coke on surface owing to the similar ability for the carbon-carbon coupling step and carbon-oxygen step. Microkinetic model simulation shown that the reaction rates of DRM reaction are also size-dependence and the Ru coverage of highest rate is around 0.1-0.2 Monolayer. By those results, the Ru2 and Ru4 models has the optimal chem. performance for DRM reaction. The high catalytic activity of Ru2(Ru4) model can be attributed to the properties of balances the relationships between cleavage steps (including CH4 and CO2 cleavage) and oxidn. processes (mainly CH* oxidn. process). It was found that CO2 activation is the rate-controlling step of DRM in terms of the microkinetic modeling results, and the 2D cluster model (such as Ru1, Ru2) could form the interfacial oxygen vacancy more easily than 3D cluster model (such as Ru4, Ru7) by oxygen reverse spillover mechanism. Our results provide further mechanistic understanding for the behaviors of DRM reaction over various Ru coverage on CeO2(111) in order to design more efficient catalysts for DRM by adjusting the loading of transition metals.
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    26. 26
      Zafar, H.; Albedwawi, S.; Polychronopoulou, K.; Anjum, D. H. Characterization of Lanthanide High Entropy Oxides Using Transmission Electron Microscopy. Mater. Lett. 2022, 324, 132628  DOI: 10.1016/j.matlet.2022.132628
      There is no corresponding record for this reference.
    27. 27
      Hussien, A. G.; Damaskinos, C.; Dabbawala, A. A.; Anjum, D. H.; Vasiliades, M.; Khaleel, M.; Wehbe, N.; Efstathiou, A. M.; Polychronopoulou, K. Elucidating the Role of La3+/Sm3+ in the Carbon Paths of Dry Reforming of Methane Over Ni/Ce-La(Sm)-Cu-O Using Transient Kinetics and Isotopic Techniques. Appl. Catal., B 2022, 304, 121015  DOI: 10.1016/j.apcatb.2021.121015
      27
      Elucidating the role of La3+/Sm3+ in the carbon paths of dry reforming of methane over Ni/Ce-La(Sm)-Cu-O using transient kinetics and isotopic techniques
      Hussien, Aseel G. S.; Damaskinos, Constantinos M.; Dabbawala, Aasif; Anjun, Dalaver H.; Vasiliades, Michalis A.; Khaleel, Maryam T. A.; Wehbe, Nimer; Efstathiou, Angelos M.; Polychronopoulou, Kyriaki
      Applied Catalysis, B: Environmental (2022), 304 (), 121015CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      The different effects of the presence of La3+ and Sm3+ heteroatoms in the 5 wt% Ni/45Ce-45(Sm or La)-10Cu-O catalytic system on the carbon deposition and removal reaction paths in the dry reforming of methane (DRM) at 750 °C were investigated using transient kinetic and isotopic expts. The relative initial rates of carbon oxidn. by lattice oxygen of support and that by oxygen derived from CO2 dissocn. under DRM reaction conditions were quantified. Ni nanoparticles (23-nm) supported on La3+-doped ceria exhibited at least 3 times higher initial rates of carbon oxidn. to CO by lattice oxygen, and ∼ 13 times lower rates of carbon accumulation than Ni (18-nm) supported on Sm3+-doped ceria. The concn. and mobility of labile surface oxygen at the Ni-support interface region seems to correlate with carbon accumulation. Ni/Ce-La(or Sm)--10Cu-O formed NiCu alloy nanoparticles, partly responsible for lowering carbon deposition and increasing carbon oxidn. rates to CO.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXivVagurfO&md5=d127ab544fc23876e521e85c087d90ef
    28. 28
      Polychronopoulou, K.; Dabbawala, A. A.; Sajjad, M.; Singh, N.; Anjum, D. H.; Baker, M. A.; Charisiou, N.; Goula, M. Hydrogen Production via Steam Reforming of Glycerol Over Ce-La-Cu-O Ternary Oxide Catalyst: An Experimental and DFT Study. Appl. Surf. Sci. 2022, 586, 152798,  DOI: 10.1016/j.apsusc.2022.152798
      There is no corresponding record for this reference.
    29. 29
      Harfouche, M.; Abdellatief, M.; Momani, Y.; Abbadi, A.; Al Najdawi, M.; Al Zoubi, M.; Aljamal, B.; Matalgah, S.; Khan, L. U.; Lausi, A.; Paolucci, G. Emergence of the First XAFS/XRF Beamline in the Middle East: Providing Studies of Elements and Their Atomic/Electronic Structure in Pluridisciplinary Research Fields. J. Synchrotron Rad. 2022, 29, 1107– 1113,  DOI: 10.1107/S1600577522005215
      There is no corresponding record for this reference.
    30. 30
      Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Rad. 2005, 12, 537– 541,  DOI: 10.1107/S0909049505012719
      30
      ATHENA, ARTEMIS, HEPHAESTUS: data analysis for x-ray absorption spectroscopy using IFEFFIT
      Ravel, B.; Newville, M.
      Journal of Synchrotron Radiation (2005), 12 (4), 537-541CODEN: JSYRES; ISSN:0909-0495. (Blackwell Publishing Ltd.)
      A software package for the anal. of x-ray absorption spectroscopy (XAS) data is presented. This package is based on the IFEFFIT library of numerical and XAS algorithms and is written in the Perl programming language using the Perl/Tk graphics toolkit. The programs described here are: (i) ATHENA, a program for XAS data processing, (ii) ARTEMIS, a program for EXAFS data anal. using theor. stds. from FEFF and (iii) HEPHAESTUS, a collection of beamline utilities based on tables of at. absorption data. These programs enable high-quality data anal. that is accessible to novices while still powerful enough to meet the demands of an expert practitioner. The programs run on all major computer platforms and are freely available under the terms of a free software license.
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    31. 31
      Ressler, T. WinXAS: A Program for X-ray Absorption Spectroscopy Data Analysis Under MS-Windows. J. Synchrotron Rad. 1998, 5, 118– 122,  DOI: 10.1107/S0909049597019298
      31
      WinXAS: a program for x-ray absorption spectroscopy data analysis under MS-Windows
      Ressler, T.
      Journal of Synchrotron Radiation (1998), 5 (2), 118-122CODEN: JSYRES; ISSN:0909-0495. (Munksgaard International Publishers Ltd.)
      WinXAS is a new x-ray absorption spectroscopy (XAS) data anal. program. It runs under the operating system MS-Windows 95/NT and offers several unique features. It has a user-friendly graphical environment and is capable of reading a variety of data formats. It contains a no. of useful numerical algorithms beyond those used in conventional XAS anal. and offers a simple interface to the ab-initio theor. code FEFF. The availability of fast macros in WinXAS makes it particularly useful for online data examn. at synchrotron radiation facilities during XAS expts. as well as for the anal. of multiple-scan data such as those from time-resolved expts.
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    32. 32
      IXAS: X-ray Absorption Data Library Online, https://xaslib.xrayabsorption.org/elem/ (accessed January 2023).
      There is no corresponding record for this reference.
    33. 33
      F. W. Lytle database Online, http://ixs.iit.edu/database/data/Farrel_Lytle_data/RAW/periodic-table.html (accessed January 2023).
      There is no corresponding record for this reference.
    34. 34
      Costa, C. N.; Christou, S. Y.; Georgiou, G.; Efstathiou, A. M. Mathematical Modeling of the Oxygen Storage Capacity Phenomenon Studied by CO Pulse Transient Experiments Over Pd/CeO2 Catalyst. J. Catal. 2003, 219, 259– 272,  DOI: 10.1016/S0021-9517(03)00151-9
      34
      Mathematical modeling of the oxygen storage capacity phenomenon studied by CO pulse transient experiments over Pd/CeO2 catalyst
      Costa, C. N.; Christou, S. Y.; Georgiou, G.; Efstathiou, A. M.
      Journal of Catalysis (2003), 219 (2), 259-272CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Science)
      A math. model has been developed for the first time to study the oxygen storage capacity (OSC) phenomenon by the CO pulse injection technique over a 1 wt% Pd/CeO2 model catalyst in the 500-700 °C range. A two-step reaction path that involves the reaction of gaseous CO with the oxygen species of PdO (pre-oxidized supported palladium particles in the 500-700 °C range) and of the back-spillover of the oxygen process from ceria to the oxygen vacant sites of surface PdO has been proven to better describe the outlet CO pulse transient response and the exptl. measured quantity of OSC (μatoms of O/g) obtained in a CSTR microreactor. With the proposed math. model, the transient rates of the CO oxidn. reaction and of the back-spillover of the oxygen process can be calcd. In the 500-700 °C range, the transient rate of CO oxidn. was always greater than that of the back-spillover of oxygen. The ratio, ρ, of the max. CO oxidn. rate to the max. back-spillover of the oxygen rate was found to decrease with increasing reaction temp. in the 500-700 °C range. In particular, at 500 and 700 °C the value of ρ was found to be 1.6 and 1.2, resp. The present math. model allows also the calcn. of the intrinsic rate const. k1 (s-1) of the Eley-Rideal step for the reaction of gaseous CO with surface oxygen species of PdO to form CO2. An activation energy of 9.2 kJ/mol was estd. for this reaction step. In addn., an apparent rate const. k2app (s-1) was estd. for the process of back-spillover of oxygen. The ratio of the two rate consts. (k1/k2app) was found to be greater than 100 in the 500-700 °C range. A Langmuir-Hinshelwood surface elementary reaction step of adsorbed CO with at. oxygen of PdO failed to describe the exptl. transient kinetics of CO oxidn. in the 500-700 °C range. The results of the present work provide the means for a better understanding of the effects of various additives and contaminants present in a three-way com. catalytic converter and other related model catalysts on their OSC kinetic behavior. In addn., intrinsic effects of a given regeneration method for a com. three-way catalyst on the OSC phenomenon could better be studied by making use of the results of the present math. model.
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    35. 35
      Perego, C.; Peratello, S. Experimental Methods in Catalytic Kinetics. Catal. Today 1999, 52, 133– 145,  DOI: 10.1016/S0920-5861(99)00071-1
      35
      Experimental methods in catalytic kinetics
      Perego, C.; Peratello, S.
      Catalysis Today (1999), 52 (2-3), 133-145CODEN: CATTEA; ISSN:0920-5861. (Elsevier Science B.V.)
      A review with 32 refs.; selecting an appropriate exptl. procedure and a suitable lab. reactor is crucial for evaluating the kinetics of a catalytic process. In this paper, practical guidelines are suggested for carrying out a kinetic study for a given reaction system. Comparison of various lab. reactors is presented and some of the pitfalls and limitations are discussed. The influence of transport phenomena due to the flow, the catalyst and the reactor geometry is analyzed and the criteria for their absence are given in a convenient form.
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    36. 36
      Vasiliades, M. A.; Harris, D.; Stephenson, H.; Boghosian, S.; Efstathiou, A. M. Novel Analysis of Transient Isothermal 18O Isotopic Exchange on Commercial CexZr1-xO2-Based OSC Materials. Top. Catal. 2019, 62, 219– 226,  DOI: 10.1007/s11244-018-1116-x
      There is no corresponding record for this reference.
    37. 37
      Sadovskaya, E. M.; Bobin, A. S.; Skazka, V. V. Isotopic Transient Analysis of Oxygen Exchange Over Oxides. Chem. Eng. J. 2018, 348, 1025– 1036,  DOI: 10.1016/j.cej.2018.05.027
      37
      Isotopic transient analysis of oxygen exchange over oxides
      Sadovskaya, E. M.; Bobin, A. S.; Skazka, V. V.
      Chemical Engineering Journal (Amsterdam, Netherlands) (2018), 348 (), 1025-1036CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      We investigated the model of oxygen isotopic exchange on solid oxides in flow reactor using numerical and anal. approaches. The study concerns the effect of the exchange mechanism (mono- or diat.) and of the rate of exchange as well as the oxygen diffusion rate in the bulk on the isotopic response after 16O/18O switch. Ratio of 16O18O mol. fraction to 18O at. fraction is shown to depend on the kinetics of gas-surface exchange, and does not depend on the oxygen diffusion in the oxide bulk. Algorithm of isotopic transient anal. without use of numerical methods was developed. Criterion to discriminate between the types of isotopic exchanges was proposed. Formulas for estn. of exchange rate and diffusion coeff. of oxygen in oxides were derived. The ests. of the surface exchange rate and of the bulk diffusion coeff. of the complex oxide La0.3Bi0.7MnO3 were calcd. applying proposed algorithm. The comparison with the results of numerical modeling verified the reliability and sufficiently high accuracy of obtained ests.
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    38. 38
      Skorodumova, N. V.; Simak, S. I.; Lundqvist, B. I.; Abrikosov, I. A.; Johansson, B. Quantum Origin of the Oxygen Storage Capability of Ceria. Phys. Rev. Lett. 2002, 89, 166601  DOI: 10.1103/PhysRevLett.89.166601
      38
      Quantum origin of the oxygen storage capability of ceria
      Skorodumova, N. V.; Simak, S. I.; Lundqvist, B. I.; Abrikosov, I. A.; Johansson, B.
      Physical Review Letters (2002), 89 (16), 166601/1-166601/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      The microscopic mechanism behind the extraordinary ability of ceria to store, release, and transport oxygen is explained on the basis of first-principles quantum mech. simulations. The oxygen-vacancy formation energy in ceria is calcd. for different local environments. The reversible CeO2-Ce2O3 redn. transition assocd. with oxygen-vacancy formation and migration is shown to be directly coupled with the quantum process of electron localization.
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    39. 39
      Hinuma, Y.; Toyao, T.; Kamachi, T.; Maeno, Z.; Takakusagi, S.; Furukawa, S.; Takigawa, I.; Shmizu, K. Density Functional Theory Calculations of Oxygen Vacancy Formation and Subsequent Molecular Adsorption on Oxide Surfaces. J. Phys. Chem. C 2018, 122, 29435– 29444,  DOI: 10.1021/acs.jpcc.8b11279
      39
      Density Functional Theory Calculations of Oxygen Vacancy Formation and Subsequent Molecular Adsorption on Oxide Surfaces
      Hinuma, Yoyo; Toyao, Takashi; Kamachi, Takashi; Maeno, Zen; Takakusagi, Satoru; Furukawa, Shinya; Takigawa, Ichigaku; Shimizu, Ken-ichi
      Journal of Physical Chemistry C (2018), 122 (51), 29435-29444CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The surface oxygen vacancy formation energy (EOvac) is an important parameter in detg. the catalytic activity of metal oxides. Estg. these energies can therefore lead to data-driven design of promising catalyst candidates. In the present study, we det. EOvac for various insulating and semiconducting oxides. Statistical investigations indicate that the band gap, bulk formation energy, and electron affinity are factors that strongly influence EOvac. Electrons enter defect states after O desorption, and these states can be in the valence band, mid-gap, or in the conduction band. Subsequent adsorption of O2, NO, CO, CO2, and H2 mols. on an O-deficient surface is also investigated. These mols. become preferentially adsorbed at the defect sites, and EOvac is identified as the dominant factor that dets. the adsorption mode as well as a descriptor that shows good correlation with the adsorption energy.
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    40. 40
      AlKhoori, A. A.; Polychronopoulou, K.; Belabbes, A.; Jaoude, M. A.; Vega, L. F.; Sebastian, V.; Hinder, S.; Baker, M. A.; Zedan, A. F. Cu, Sm co-doping effect on the CO oxidation activity of CeO2. A combined experimental and density functional study. Appl. Surf. Sci. 2020, 521, 146305,  DOI: 10.1016/j.apsusc.2020.146305
      40
      Cu, Sm co-doping effect on the CO oxidation activity of CeO2. A combined experimental and density functional study
      AlKhoori, Ayesha A.; Polychronopoulou, Kyriaki; Belabbes, Abderrezak; Jaoude, Maguy Abi; Vega, Lourdes F.; Sebastian, Victor; Hinder, Steven; Baker, Mark A.; Zedan, Abdallah F.
      Applied Surface Science (2020), 521 (), 146305CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      The co-doping effect of a rare earth (RE) metal and a transition metal (TM) on ceria oxidn. catalysis through the evaluation of samarium-copper co-doped catalysts with Ce-Sm-xCu-O (x: 0-20 at.%, Ce/Sm = 1) nominal compns., is discussed. The CO oxidn. reaction was used as a prototype reaction due to its pivotal role in the fuel cell technol. Ce-Sm-20Cu-O catalyst presented a 64% increase in the CO oxidn. activity compared to that of pristine ceria. Diffraction and Raman studies proved that the Cu, Sm co-doping induces many defects related to the dopants (Sm, Cu) and the oxygen vacant sites, while the presence of hybrid CuO/Ce-Sm(Cu)-O fluorite/SmO8 (cubic metastable) phases is the most representative scenario of this oxide microstructure. A size polydispersity of CuO phases was achieved by introducing air cooling during the microwave heating. Cu, Sm atoms were uniformly doped in CeO2 structure according to the HAADF-STEM studies. These results are in agreement with EDS anal., where Cu, Sm and Ce are located in all the analyzed areas without any preferential distribution. The XPS studies demonstrated the co-presence of Cu2+/Cu1+ and Ce4+/Ce3+ redox couples in agreement with the Bader charge anal. from the ab initio calcns., the latter influencing greatly the oxidn. activity of the catalysts. D. functional theory (DFT) calcns. shed light on the oxide surface and the underlying mechanism governing the oxidn. catalysis taking place. In particular, Cu2+ and Sm3+ dopants were found to be located in the nearest neighbor (NN) sites of oxygen vacancies. Different oxygen vacancies configurations were studied (single vs. double, surface vs. subsurface), where the single vacancies are more stable on the surface, whereas the double vacancies configurations are more stable on the subsurface. Regarding the Ce3+ location, in the presence of single and double oxygen vacancy, the Ce3+ ions prefer to be located in the 1st NN/2nd NN and 2nd NN of the first Ce layer, relative to the oxygen vacancy, resp. The total D. of States (DOS) anal. of the co-doped systems revealed that the dopants induced new surface states inside the ceria band gap, which can accommodate the unpaired electrons of the vacant oxygen sites. These electronic modifications justify the much lower energy of oxygen vacancy formation (Evf) in both cases, the Sm-doped, and Cu, Sm -doped CeO2 (1 1 1) geometries. Specifically, the Evf lowering upon doping was found to be almost two times larger for the Cu adjacent oxygen vacancies (Cu2+-.box.) compared to the Sm ones (Sm3+-.box.), consistent with the CO adsorption trend as the Cu-Sm-CeO2 (1 1 1) system is energetically more favorable than the Sm-CeO2 (1 1 1) and pure CeO2 (1 1 1) surfaces.
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    41. 41
      Rakhmatullin, R.M.; Semashko, V.V.; Korableva, S.L.; Kiiamov, A.G.; Rodionov, A.A.; Tschaggelar, R.; van Bokhoven, J. A.; Paun, C. EPR study of ceria nanoparticles containing different concentration of Ce3+ ions. Mater. Chem. Phys. 2018, 219, 251– 257,  DOI: 10.1016/j.matchemphys.2018.08.028
      41
      EPR study of ceria nanoparticles containing different concentration of Ce3+ ions
      Rakhmatullin, R. M.; Semashko, V. V.; Korableva, S. L.; Kiiamov, A. G.; Rodionov, A. A.; Tschaggelar, R.; van Bokhoven, J. A.; Paun, C.
      Materials Chemistry and Physics (2018), 219 (), 251-257CODEN: MCHPDR; ISSN:0254-0584. (Elsevier B.V.)
      The ESR of Ce3+ ions in ceria is a rather controversial issue. In this report we analyze the EPR line with g ∼1.97 in the absorption spectra of ceria powders, which has been assigned in many studies to Ce3+ ions. Such an assignment is not in agreement with the exptl. data nor with the theory of spin-lattice relaxation of rare earth ions in solids. Our statement is supported by our exptl. results based on EPR spectroscopy of nanocryst. ceria, synthesized in various ways, as well as by magnetization measurements of as-prepd. and vacuum-annealed ceria nanoparticles. The possible model of the paramagnetic center, which is assigned to the EPR line with g ∼1.97 in ceria, is an electron trapped near the surface Ce3+/Ce4+ redox pair.
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    42. 42
      Alkhoori, A. A.; Elmutasim, O.; Dabbawala, A. A.; Vasiliades, M. A.; Petallidou, K. C.; Emwas, A.-H.; Anjum, D. H.; Singh, N.; Baker, M. A.; Charisiou, N. D.; Goula, M. A.; Efstathiou, A. M.; Polychronopoulou, K. Mechanistic Features of the CeO2-Modified Ni/Al2O3 Catalysts for the CO2Methanation Reaction: Experimental and Ab Initio Studies. ACS Applied Energy Materials 2023, 6 (16), 8550– 8571,  DOI: 10.1021/acsaem.3c01437
      There is no corresponding record for this reference.
    43. 43
      Agarwal, S.; Zhu, X.; Hensen, E. J.; Lefferts, L.; Mojet, B. L. Defect Chemistry of Ceria Nanorods. J. Phys. Chem. C 2014, 118, 4131– 4142,  DOI: 10.1021/jp409989y
      43
      Defect Chemistry of Ceria Nanorods
      Agarwal, S.; Zhu, X.; Hensen, E. J. M.; Lefferts, L.; Mojet, B. L.
      Journal of Physical Chemistry C (2014), 118 (8), 4131-4142CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Ceria nanorods were investigated using in situ Raman and FTIR spectroscopies for CO adsorption and subsequent reaction with water at 200 and 350 °C. The involvement of defects in ceria nanorods during CO adsorption and reaction with H2O is dependent on the temp. At 200 °C, most of the carbonate and formate species formed in CO do not involve the formation of defects, while at 350 °C all of the carbonates and formates formed can be correlated to the formation of defects (15% by formates and 85% by mono/bidentate carbonates). Finally, at 350 °C very stable polydentate carbonates are formed that do not induce defects and cannot be regenerated with water.
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    44. 44
      Sarkar, A.; Loho, C.; Velasco, L.; Thomas, T.; Bhattachary, S. S.; Hahn, H.; Djenadic, R. Multicomponent equiatomic rare earth oxides with a narrow band gap and associated Praseodymium Multivalency. Dalton Trans. 2017, 46, 12167– 12176,  DOI: 10.1039/C7DT02077E
      44
      Multicomponent equiatomic rare earth oxides with a narrow band gap and associated praseodymium multivalency
      Sarkar, Abhishek; Loho, Christoph; Velasco, Leonardo; Thomas, Tiju; Bhattacharya, Subramshu S.; Hahn, Horst; Djenadic, Ruzica
      Dalton Transactions (2017), 46 (36), 12167-12176CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      New multicomponent equiat. rare earth oxides (ME-REOs) contg. 3-7 rare earth elements (Ce, Gd, La, Nd, Pr, Sm and Y) in equiat. proportions are synthesized using nebulized spray pyrolysis. All the systems crystd. as a phase pure fluorite type (Fm‾3m) structure in spite of the high chem. complexity. A nominal increase in the lattice parameter compared to CeO2 is obsd. in all ME-REOs. XPS performed on the ME-REOs confirmed that all the constituent rare earth elements are present in the 3+ oxidn. state, except for Ce and Pr which are present in 4+ and in a mixed (3+/4+) oxidn. state, resp. The presence of Ce4+ contributes substantially to the obsd. stability of the single phase structure. These new oxide systems have narrow direct band gaps in the range of 1.95-2.14 eV and indirect band gaps in the range of 1.40-1.64 eV, enabling light absorption over the entire visible spectral range. Furthermore, the oxygen vacancy concn. rapidly increases and then sats. with the no. of rare earth elements that are incorporated into the ME-REOs. The lowering of the band gap is found to be closely related to the presence of multivalent Pr. Interestingly, the band gap values are relatively invariant with respect to the compn. or thermal treatments. Considering the high level of oxygen vacancies present and the obsd. low band gap values, these new material systems can be of importance where the presence of oxygen vacancies is essential or in applications where a narrow band gap is desirable.
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    45. 45
      Acharya, S. A.; Gaikwad, V.; D’Souza, S. W.; Barman, S. R. Gd/Sm Dopant-modified Oxidation State and Defect Generation in Nano-Ceria. Solid State Ionics 2014, 260, 21– 29,  DOI: 10.1016/j.ssi.2014.03.008
      45
      Gd/Sm dopant-modified oxidation state and defect generation in nano-ceria
      Acharya, S. A.; Gaikwad, V. M.; D'Souza, S. W.; Barman, S. R.
      Solid State Ionics (2014), 260 (), 21-29CODEN: SSIOD3; ISSN:0167-2738. (Elsevier B.V.)
      Gd/Sm-modified oxidn. state and defect in nano-ceria are studied by X-ray photoelectron (XPS) and Raman spectroscopy technique to investigate their influence on oxy-ionic cond. of the system. Nanosize Sm and Gd doped ceria (SDC and GDC) are synthesized in different compns., Ce(1 - x) (Sm / Gd)xO3 - δ (x = 0.05 to 0.30) under identical conditions. XPS study of doped ceria system indicates enrichment of Ce3+ ions in GDC as compared to that in SDC. The quant. anal. of Raman spectra also predicted the same trend of oxidn. state of cerium. The ionic cond. measurement by electrochem. impedance spectroscopy is demonstrated to verify the results for all compns. of dopant (x = 0.05 - 0.30).
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    46. 46
      Filtschew, A.; Hofmann, K.; Hess, C. Ceria and Its Defect Structure: New Insights From a Combined Spectroscopic Approach. J. Phys. Chem. C 2016, 120, 6694– 6703,  DOI: 10.1021/acs.jpcc.6b00959
      46
      Ceria and Its Defect Structure: New Insights from a Combined Spectroscopic Approach
      Filtschew, Anastasia; Hofmann, Kathrin; Hess, Christian
      Journal of Physical Chemistry C (2016), 120 (12), 6694-6703CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Ceria is an interesting component for a variety of catalytic and fuel cell applications. In the study described here, ten different com. ceria samples as well as synthesized ceria samples were investigated in detail regarding their (defect) structure and characteristic properties using XRD, N2 adsorption-desorption, and optical spectroscopy (Raman, DRIFTS, UV-vis). The investigations revealed correlations of surface defect features (Raman, DRIFTS) as well as those of bulk defects (Raman, UV-vis). The Raman feature at around 250 cm-1 was demonstrated to be related to surface defects rather than a 2TA vibration as described in the literature. A correlation between UV-vis band gap values and the presence of Raman bulk defects was established based on the obsd. decrease of the band gap energy with increasing no. of defects. Detailed Raman anal. revealed that the frequently mentioned linear equation for the detn. of the crystal size from the half-width of the F2g Raman feature is erroneous, since the F2g half-width depends on ceria bulk defects. Apart from these universal observations, differences in the properties depending on synthesis conditions were obsd. In particular, it is shown that the type and quantity of ceria defects are influenced not only by crystal size but also by the prepn. method.
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    47. 47
      Andriopoulou, C.; Trimpalis, A.; Petallidou, K. C.; Sgoura, A.; Efstathiou, A. M.; Boghosian, S. Structural and Redox Properties of Ce1–xZrxO2−δ and Ce0. 8Zr0.15RE0.05O2−δ (RE: La, Nd, Pr, Y) Solids Studied by High Temperature in Situ Raman Spectroscopy. J. Phys. Chem. C 2017, 121, 7931– 7943,  DOI: 10.1021/acs.jpcc.7b00515
      There is no corresponding record for this reference.
    48. 48
      Lee, Y.; He, G.; Akey, A. J.; Si, R.; Stephanopoulos, M. F.; Herman, I. P. Raman Analysis of Mode Softening in Nanoparticle CeO2−δ and Au-CeO2−δ During CO Oxidation. J. Am. Chem. Soc. 2011, 133, 12952– 12955,  DOI: 10.1021/ja204479j
      48
      Raman Analysis of Mode Softening in Nanoparticle CeO2-δ and Au-CeO2-δ during CO Oxidation
      Lee, Youjin; He, Guanghui; Akey, Austin J.; Si, Rui; Flytzani-Stephanopoulos, Maria; Herman, Irving P.
      Journal of the American Chemical Society (2011), 133 (33), 12952-12955CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Oxygen vacancy levels are monitored during the oxidn. of CO by CeO2-δ nanorods and Au-CeO2-δ nanorods, nanocubes, and nanopolyhedra by using Raman scattering. The first-order CeO2 F2g peak near 460 cm-1 decreases when this reaction is fast (fast redn. and relatively slow reoxidn. of the surface), because of the lattice expansion that occurs when Ce3+ replaces Ce4+ during oxygen vacancy creation. This shift correlates with reactivity for CO oxidn. Increases in the oxygen deficit δ as large as ∼0.04 are measured relative to conditions when the ceria is not reduced.
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    49. 49
      Wu, Z.; Li, M.; Howe, J.; Meyer, H. M.; Overbury, S. Probing Defect Sites on CeO2 Nanocrystals with Well-defined Surface Planes by Raman Spectroscopy and O2 Adsorption. Langmuir 2010, 26, 16595– 16606,  DOI: 10.1021/la101723w
      49
      Probing Defect Sites on CeO2 Nanocrystals with Well-Defined Surface Planes by Raman Spectroscopy and O2 Adsorption
      Wu, Zili; Li, Meijun; Howe, Jane; Meyer, Harry M., III; Overbury, Steven H.
      Langmuir (2010), 26 (21), 16595-16606CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Defect sites play an essential role in ceria catalysis. Ceria nanocrystals with well-defined surface planes were synthesized and used for studying defect sites with both Raman spectroscopy and O2 adsorption. Ceria nanorods ({110} + {100}), nanocubes ({100}), and nano-octahedra ({111}) are employed to analyze the quantity and quality of defect sites on different ceria surfaces. On oxidized surfaces, nanorods have the most abundant intrinsic defect sites, followed by nanocubes and nano-octahedra. When reduced, the induced defect sites are more clustered on nanorods than on nanocubes, although similar amts. (based on surface area) of such defect sites are produced on the 2 surfaces. Very few defect sites can be generated on the nano-octahedra due to the least reducibility. These differences can be rationalized by the crystallog. surface terminations of the ceria nanocrystals. The different defect sites on these nanocrystals lead to the adsorption of different surface dioxygen species. Superoxide on 1-electron defect sites and peroxide on 2-electron defect sites with different clustering degree are identified on the ceria nanocrystals depending on their morphol. Also, the stability and reactivity of these oxygen species are also surface-dependent, which is of significance for ceria-catalyzed oxidn. reactions.
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    50. 50
      Sartoretti, E.; Novara, C.; Giorgis, F.; Piumetti, M.; Bensaid, S.; Russo, N.; Fino, D. In Situ Raman Analyses of the Soot Oxidation Reaction over Nanostructured Ceria-based Catalysts. Sci. Rep. 2019, 9, 3875,  DOI: 10.1038/s41598-019-39105-5
      50
      In situ Raman analyses of the soot oxidation reaction over nanostructured ceria-based catalysts
      Sartoretti Enrico; Novara Chiara; Giorgis Fabrizio; Piumetti Marco; Bensaid Samir; Russo Nunzio; Fino Debora
      Scientific reports (2019), 9 (1), 3875 ISSN:.
      To reduce the emissions of internal combustion engines, ceria-based catalysts have been widely investigated as possible alternatives to the more expensive noble metals. In the present work, a set of four different ceria-based materials was prepared via hydrothermal synthesis, studying the effect of Cu and Mn as dopants both in binary and ternary oxides. In situ Raman analyses were carried out to monitor the behaviour of defect sites throughout thermal cycles and during the soot oxidation reaction. Despite ceria doped with 5% of Cu featured the highest specific surface area, reducibility and amount of intrinsic and extrinsic defects, a poor soot oxidation activity was observed through the standard activity tests. This result was confirmed by the calculation of soot conversion curves obtained through a newly proposed procedure, starting from the Raman spectra collected during the in situ tests. Moreover, Raman analyses highlighted that new defectiveness was produced on the Cu-doped catalyst at high temperature, especially after soot conversion, while a slight increase of the defect band and a total reversibility were observed in case of the ternary oxide and pure/Mn-doped ceria, respectively. The major increment was related to the extrinsic defects component; tests carried out in different atmospheres suggested the assignment of this feature to vacancy-free sites containing oxidized doping cations. Its increase at the end of the tests can be an evidence of peroxides and superoxides deactivation on catalysts presenting excessive oxygen vacancy concentrations. Instead, ceria doped with 5% of Mn exhibited the best soot oxidation activity, thanks to an intermediate density of oxygen vacancies and to its well-defined morphology.
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    51. 51
      Purans, J.; Menushenkov, A. P.; Besedin, S. P.; Ivanov, A. A.; Minkov, V. S.; Pudza, I.; Kuzmin, A.; Klementiev, K. V.; Pascarelli, S.; Mathon, O.; Rosa, A. D.; Irifune, T.; Eremets, M. I. Local electronic structure rearrangements and strong anharmonicity in YH3 under pressures up to 180 GPA. Nat. Commun. 2021, 12, 1765,  DOI: 10.1038/s41467-021-21991-x
      There is no corresponding record for this reference.
    52. 52
      Satsuma, A.; Yanagihara, M.; Ohyama, J.; Shimizua, K. Oxidation of CO over Ru/Ceria prepared by self-dispersion of Ru metal powder into nano-sized particle. Catal. Today 2013, 201, 62– 67,  DOI: 10.1016/j.cattod.2012.03.048
      52
      Oxidation of CO over Ru/Ceria prepared by self-dispersion of Ru metal powder into nano-sized particle
      Satsuma, Atsushi; Yanagihara, Masatoshi; Ohyama, Junya; Shimizu, Kenichi
      Catalysis Today (2013), 201 (), 62-67CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      The oxidn. of CO was performed using phys. mixt. of Ru metal powder and various metal oxide supports after calcination in air and redn. in H2. Among various supports (CeO2, ZrO2, MgO, Al2O3, TiO2, SnO2, and SiO2), only Ru + CeO2 showed very high catalytic activity for CO oxidn. with the light-off temp. below 100 °C, which was comparable to Ru/CeO2 prepd. by a conventional impregnation method. The optimum conditions for the prepn. were loading amt. of 2 wt%, calcination at 700 °C in air followed by redn. at 400 °C in 5%H2. It was revealed that the high catalytic activity for Ru + CeO2 is attributed to the high dispersion of Ru on CeO2. The original Ru metal powder having diam. of 36 nm was dispersed during the calcination leading into small Ru particles having diam. around 2 nm. Raman spectra indicated that the formation of RuOCe bond after the calcination is one of the key for the Ru dispersion. The self-dispersion of Ru was not achieved by the calcination in N2. It was proposed that the self-dispersion of Ru into nano-particles was caused by oxidn. of Ru metal into Ru oxide in air, then dispersion of nano-sized Ru oxide by forming RuOCe bond, and finally conversion of Ru oxide particles into nano-sized Ru metal particles in a flow of H2.
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    53. 53
      Loridant, S. Raman spectroscopy as a powerful tool to characterize ceria-based catalysts. Catal. Today 2021, 373, 98– 111,  DOI: 10.1016/j.cattod.2020.03.044
      53
      Raman spectroscopy as a powerful tool to characterize ceria-based catalysts
      Loridant, Stephane
      Catalysis Today (2021), 373 (), 98-111CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      A review. CeO2 is widely used and investigated as an oxide catalyst or support due to its unique redox property of oxygen storage and release. In this paper, the different opportunities offered by Raman spectroscopy for advanced characterization of ceria-based catalysts are reviewed: spectral modifications induced by nanocrystallinity, defects, doping and redn., identification of supported mol. species, isolated atoms and nanoclusters, characterization of surface modes, hydroxyl groups, reaction intermediates such as peroxo and superoxo species. Finally, in situ/operando studies for environmental catalysis are summarized illustrating Raman spectroscopy as a powerful tool to characterize ceria-based catalysts.
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    54. 54
      Wang, H.; Li, X.; Ruan, Q.; Tang, J. Ru and Ruoxdecorated carbon nitride for efficient ammonia photosynthesis. Nanoscale 2020, 12, 12329– 12335,  DOI: 10.1039/D0NR02527E
      There is no corresponding record for this reference.
    55. 55
      Gu, Y. 1,2-dichloroethane deep oxidation over bifunctional Ru/CExAly Catalysts. ACS Omega 2018, 3, 8460– 8470,  DOI: 10.1021/acsomega.8b00592
      55
      1,2-Dichloroethane Deep Oxidation over Bifunctional Ru/CexAly Catalysts
      Gu, Yufeng; Jiang, Xingxing; Sun, Wei; Bai, Shuxing; Dai, Qiguang; Wang, Xingyi
      ACS Omega (2018), 3 (8), 8460-8470CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      Ru/CexAly catalysts were synthesized with impregnation of RuCl3 aq. soln. on CexAly (Al2O3-CeO2) and used in 1,2-dichloroethane (1,2-DCE) oxidn. Characterization by X-ray diffraction, Raman, NH3-temp.-programmed desorption (TPD), CO2-TPD, XPS, and H2-temp.-programmed redn. indicates that CeO2 exists as a form of face-centered cubic fluorite structure, whereas the chem. states and the structure of Ru species are dependent on the Ce content. The reducibility and acidity of catalysts increase with Ce/Ce + Al ratio. However, the latter is promoted only in a Ce/Ce + Al range of 0-0.25 and then decreases quickly. Ru/CexAly catalysts have considerable activity for 1,2-DCE combustion. TOFRu of 1,2-DCE oxidn. increases with strong acid, which is ascribed to a synergy of reducibility and acidity. Ru greatly inhibits the chlorination through the decreases in both Cl deposition and CH2 = CHCl formation. High stability of Ru/Ce10Al90 maintains at 280 °C for at least 25 h with CO2 selectivity of 99% or higher. In situ Fourier transform IR spectroscopy indicates that 1,2-DCE dissocs. to form ClCH2-CH2O- species, which is an intermediate species for the prodn. of CH3CHO and CH2 = CHCl, the former responsible for deep oxidn.
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    56. 56
      Siakavelas, G. I. Highly selective and stable nickel catalysts supported on Ceria promoted with SM2O3, PR2O3 and mgo for the CO2Methanation reaction. Applied Catalysis B: Environmental 2021, 282, 119562,  DOI: 10.1016/j.apcatb.2020.119562
      There is no corresponding record for this reference.
    57. 57
      Polychronopoulou, K. Decoupling the chemical and mechanical strain effect on steering the CO2 activation over CeO2-based oxides: An experimental and DFT approach. ACS Appl. Mater. Interfaces 2022, 14, 33094– 33119,  DOI: 10.1021/acsami.2c05714
      57
      Decoupling the Chemical and Mechanical Strain Effect on Steering the CO2 Activation over CeO2-Based Oxides: An Experimental and DFT Approach
      Polychronopoulou, Kyriaki; AlKhoori, Sara; AlBedwawi, Shaima; Alareeqi, Seba; Hussien, Aseel G. S.; Vasiliades, Michalis A.; Efstathiou, Angelos M.; Petallidou, Klito C.; Singh, Nirpendra; Anjum, Dalaver H.; Vega, Lourdes F.; Baker, Mark A.
      ACS Applied Materials & Interfaces (2022), 14 (29), 33094-33119CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO2 is involved. Thus, it is important to understand how to tailor their CO2 adsorption behavior. In this work, steering the CO2 activation behavior of Ce-La-Cu-O ternary oxide surfaces through the combined effect of chem. and mech. strain was thoroughly examd. using both exptl. and ab initio modeling approaches. Doping with aliovalent metal cations (La3+ or La3+/Cu2+) and post-synthetic ball milling were considered as the origin of the chem. and mech. strain of CeO2, resp. Exptl., microwave-assisted reflux-prepd. Ce-La-Cu-O ternary oxides were imposed into mech. forces to tune the structure, redox ability, defects, and CO2 surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chem. strain (εC) and mech. strain (εM) on the modification of the Ce-La-Cu-O surface reactivity toward CO2 activation. During the ab initio calcns., the stability (energy of formation, EOvf) of different configurations of oxygen vacant sites (Ov) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO2-philicity of the surface was assessed at different levels of the imposed mech. strain. The EOvf values were found to decrease with increasing tensile strain. The Ce-La-Cu-O(111) surface exhibited the lowest EOvf values for the single subsurface sites, implying that Ov may occur spontaneously upon Cu addn. The mobility of the surface and bulk oxygen anions in the lattice contributing to the Ov population was measured using 16O/18O transient isothermal isotopic exchange expts.; the max. in the dynamic rate of 16O18O formation, Rmax(16O18O), was 13.1 and 8.5μmol g-1 s-1 for pristine (chem. strained) and dry ball-milled (chem. and mech. strained) oxides, resp. The CO2 activation pathway (redox vs. associative) was exptl. probed using in situ diffuse reflectance IR Fourier transform spectroscopy. It was demonstrated that the mech. strain increased up to 6 times the CO2 adsorption sites, though reducing their thermal stability. This result supports the mech. actuation of the "carbonate"-bound species; the latter was in agreement with the d. functional theory (DFT)-calcd. C-O bond lengths and O-C-O angles. Ab initio studies shed light on the CO2 adsorption energy (Eads), suggesting a covalent bonding which is enhanced in the presence of doping and under tensile strain. Bader charge anal. probed the adsorbate/surface charge distribution and illustrated that CO2 interacts with the dual sites (acidic and basic ones) on the surface, leading to the formation of bidentate carbonate species. D. of states (DOS) studies revealed a significant Eg drop in the presence of double Ov and compressive strain, a finding with design implications in covalent type of interactions. To bridge this study with industrially important catalytic applications, Ni-supported catalysts were prepd. using pristine and ball-milled oxides and evaluated for the dry reforming of methane reaction. Ball milling was found to induce modification of the metal-support interface and Ni catalyst reducibility, thus leading to an increase in the CH4 and CO2 conversions. This study opens new possibilities to manipulate the CO2 activation for a portfolio of heterogeneous reactions.
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    58. 58
      Younis, A.; Shirsath, S. E.; Shabbir, B.; Li, S. Controllable dynamics of oxygen vacancies through extrinsic doping for superior catalytic activities. Nanoscale 2018, 10, 18576– 18585,  DOI: 10.1039/C8NR03801E
      58
      Controllable dynamics of oxygen vacancies through extrinsic doping for superior catalytic activities
      Younis, Adnan; Shirsath, Sagar E.; Shabbir, Babar; Li, Sean
      Nanoscale (2018), 10 (39), 18576-18585CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Due to its strong redox ability, high stability, cost effectiveness and non-toxicity, cerium oxide (CeO2) has been extensively researched as an active photocatalyst material. The underlying photocatalytic reactions are mostly assocd. with the transportation of oxygen ions through vacancies, but the actual transport phenomenon had not been clearly understood. In this work, gadolinium (Gd) is sequentially doped into CeO2 to investigate how extrinsic doping can modulate oxygen vacancies in CeO2 and influence photocatalytic activities. From our investigations, it was found that the Gd doping may induce structural symmetry breaking leading to a pure CeO2 fluorite structure that transforms mobile oxygen vacancies into clustered or immobile vacancies. When the vacancies were set as "mobile" (for Gd doping levels ≤15 at%), max. photocatalytic activities were obtained. In contrast, suppressed photocatalytic efficiencies were noted for higher Gd doping levels (20 at% or more). The results reported in this research may provide an extra degree of freedom in the form of extrinsic doping to configure the oxygen vacancy defects and their mobility to achieve better catalytic efficiencies.
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    59. 59
      Holder, C. F.; Schaak, R. E. Tutorial on Powder X-ray Diffraction for Characterizing Nanoscale Materials. ACS Nano 2019, 13, 7359– 7365,  DOI: 10.1021/acsnano.9b05157
      59
      Tutorial on Powder X-ray Diffraction for Characterizing Nanoscale Materials
      Holder, Cameron F.; Schaak, Raymond E.
      ACS Nano (2019), 13 (7), 7359-7365CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A review. The authors provide the broad nanoscience and nanotechnol. communities with a brief tutorial on some of the key aspects of powder XRD data that are often encountered when analyzing samples of nanoscale materials, with an emphasis on inorg. nanoparticles of various sizes, shapes, and dimensionalities.
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    60. 60
      Schmitt, R.; Nenning, A.; Kraynis, O.; Korobko, R.; Frenkel, A. I.; Lubomirsky, I.; Haile, S. M.; Rupp, J. L. M. A Review of Defect Structure and Chemistry in Ceria and Its Solid Solutions. J. Chem. Soc. Rev. 2020, 49, 554– 592,  DOI: 10.1039/C9CS00588A
      There is no corresponding record for this reference.
    61. 61
      Jacobsen, J.; Dahl, S.; Hansen, P. L.; Tornqvist, E.; Jensen, L.; Topsoe, H.; Prip, D. V.; Moenshaug, P. B.; Chorkendorff, I. Structure Sensitivity of Supported Ruthenium Catalysts for Ammonia Synthesis. J. Mol. Catal. A: Chem. 2000, 163, 19– 26,  DOI: 10.1016/S1381-1169(00)00396-4
      61
      Structure sensitivity of supported ruthenium catalysts for ammonia synthesis
      Jacobsen, Claus J. H.; Dahl, Soren; Hansen, Poul L.; Tornqvist, Eric; Jensen, Lone; Topsoe, Henrik; Prip, Dorthe V.; Moenshaug, Pernille B.; Chorkendorff, Ib
      Journal of Molecular Catalysis A: Chemical (2000), 163 (1-2), 19-26CODEN: JMCCF2; ISSN:1381-1169. (Elsevier Science B.V.)
      The catalytic ammonia synthesis activities of four supported ruthenium catalysts are reported. It is seen that Ru/MgAl2O4 is more active than two similar Ru/C catalysts, which are significantly more active than Ru/Si3N4. The activity differences cannot be satisfactorily explained solely by the differences in dispersion. Recent results from single crystal studies and DFT calcns. have shown that ammonia synthesis over ruthenium catalysts is a very structure sensitive reaction, more so than on iron catalysts. It is suggested that special B5-type sites are primarily responsible for the catalytic activity of the present supported Ru catalysts. It is shown how the no. of such B5-type sites depends on the Ru crystal size for a given crystal morphol. We have found that the activity of the Ru/MgAl2O4 catalyst increases significantly during the initial part of a test run. This activity increase is paralleled by the disappearance of crystals smaller than ca. 1.0 nm due to sintering and a resulting formation of larger crystals. We conclude that there exists a lower limit to the desired crystal size of Ru in supported ammonia synthesis catalysts. This is in agreement with a low no. of B5-type sites expected for such crystal sizes. Furthermore, we suggest that the support plays a decisive role in controlling the morphol. of the Ru crystals and the resulting change in abundance of B5-type sites is the main cause for the significant activity variations obsd. for Ru catalysts with different supports. Finally, the support may also influence the electronic and catalytic properties of neighboring B5-type sites.
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    62. 62
      Lingling, L.; Ya-Fei, J.; Tianhua, Z.; Hongfang, C.; Yanliang, Z.; Bingyu, L.; Xingyi, L.; Ying, Z.; Lirong, Z.; Xiuyun, W.; Cong-Qiao, X.; Chak-tong, A.; Lilong, J.; Jun, L. Size sensitivity of supported Ru catalysts for ammonia synthesis: From nanoparticles to subnanometric clusters and atomic clusters. Chem 2022, 8, 749– 768,  DOI: 10.1016/j.chempr.2021.11.008
      There is no corresponding record for this reference.
    63. 63
      Raróg-Pilecka, W.; Miśkiewicz, E.; Szmigiel, D.; Kowalczyk, Z. Structure sensitivity of ammonia synthesis over promoted ruthenium catalysts supported on graphitised carbon. J. Catal. 2005, 231, 11– 19,  DOI: 10.1016/j.jcat.2004.12.005
      63
      Structure sensitivity of ammonia synthesis over promoted ruthenium catalysts supported on graphitised carbon
      Rarog-Pilecka, Wioletta; Miskiewicz, Elzbieta; Szmigiel, Dariusz; Kowalczyk, Zbigniew
      Journal of Catalysis (2005), 231 (1), 11-19CODEN: JCTLA5; ISSN:0021-9517. (Elsevier)
      A series of carbon-based ruthenium catalysts differing in Ru loading (1-32 wt%) was characterized (XRD, TEM, O2 and CO chemisorption) and, after promotion with Ba or Cs or both, was studied in NH3 synthesis. Partly graphitized carbon with a high surface area (S BET = 1310 m2/g) was used as a support for catalyst prepn. Ruthenium chloride and barium nitrate and/or cesium nitrate were precursors of the active phase and promoters, resp. The chemisorption expts. have shown that the mean size of ruthenium particles (d) increases monotonically with Ru loading, from about 1 nm for 1 wt% Ru to about 4 nm for 32 wt% Ru. The NH3 synthesis studies have revealed that the reaction rates (400 °C, 63 bar, 8.5% NH3 or 400 °C, 90 bar, 11.5% NH3), expressed in terms of TOF, increase vs. particle size, regardless of the promoter type. Extrapolation to lower sizes indicates that crystallites smaller than 0.7-0.8 nm might be totally inactive. The co-promoted catalysts (Ba + Cs) were found to be more active than the singly doped systems over the whole range of the particle diam. The trends in TOF vs. d have been attributed to the promoter/promoters location, on the faces of the Ru crystallites, or, alternatively, to changes in crystallite morphol.-larger particles (3-4 nm) may expose more B5 sites than the smaller ones (1-2 nm). The effect of co-promotion has been ascribed to different modes of the promoters' action: whereas the main role of Cs is to lower the barrier for N2 dissocn. (electronic effect), barium acts predominantly as a structural promoter-the reconstructed surfaces in Ba-Ru/C are more resistant to poisoning by hydrogen when operating, thus making more sites available for N2 adsorption.
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    64. 64
      Kim, Y. K.; Morgan, G. A.; Yates, J. T. Site-specific dissociation of N-2 on the stepped Ru(109). Surf. Sci. 2005, 14, 598
      There is no corresponding record for this reference.
    65. 65
      Lin, H.; Liu, J. X.; Fan, H.-J.; Li, W.-X. Crystallographic and morphological sensitivity of N2 activation over ruthenium. Chin. J. Chem. Phys. 2021, 34, 263– 272,  DOI: 10.1063/1674-0068/cjcp2009171
      65
      Crystallographic and morphological sensitivity of N2 activation over ruthenium
      Lin, Hao; Liu, Jin-xun; Fan, Hong-jun; Li, Wei-xue
      Chinese Journal of Chemical Physics (2021), 34 (3), 263-272CODEN: CJCPA6; ISSN:1674-0068. (American Institute of Physics)
      Ruthenium (Ru) serves as a promising catalyst for ammonia synthesis via the Haber-Bosch process, identification of the structure sensitivity to improve the activity of Ru is important but not fully explored yet. We present here d. functional theory calcns. combined with micro-kinetic simulations on nitrogen mol. activation, a crucial step in ammonia synthesis, over a variety of hcp. (hcp) and face-center cubic (fcc) Ru facets. Hcp {2130} facet exhibits the highest activity toward N2 dissocn. in hcp Ru, followed by the (0001) monat. step sites. The other hcp Ru facets have N2 dissocn. rates at least three orders lower. Fcc {211} facet shows the best performance for N2 activation in fcc Ru, followed by {311}, which indicates stepped surfaces make great contributions to the overall reactivity. Although hcp Ru {2130} facet and (0001) monat. step sites have lower or comparable activation barriers compared with fcc Ru {211} facet, fcc Ru is proposed to be more active than hcp Ru for N2 conversion due to the exposure of the more favorable active sites over step surfaces in fcc Ru. This work provides new insights into the crystal structure sensitivity of N2 activation for mechanistic understanding and rational design of ammonia synthesis over Ru catalysts. (c) 2021 American Institute of Physics.
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    66. 66
      Jiang, Rongzhong; Tran, Dat T.; Li, Jiangtian; Chu, D. Ru@RuO2 Core-Shell Nanorods: A Highly Active and Stable Bifunctional Catalyst for Oxygen Evolution and Hydrogen Evolution Reactions. Energy Environ. Mater. 2019, 2, 201– 208,  DOI: 10.1002/eem2.12031
      66
      Ru@RuO2 Core-Shell Nanorods: A Highly Active and Stable Bifunctional Catalyst for Oxygen Evolution and Hydrogen Evolution Reactions
      Jiang, Rongzhong; Tran, Dat T.; Li, Jiangtian; Chu, Deryn
      Energy & Environmental Materials (2019), 2 (3), 201-208CODEN: EEMNA3; ISSN:2575-0356. (John Wiley & Sons, Inc.)
      Ru@RuO2 core-shell nanorods were successfully synthesized by heat-treating Ru nanorods with air oxidn. through an accurate control of the temp. and time. The structure, compn., dimension, and adsorption property of the core-shell nanorods were well characterized with XRD and TEM. The catalytic activity and stability were electrochem. evaluated with a rotating disk electrode, a rotating ring-disk electrode, and chronopotentiometric methods. The Ru@RuO2 nanorods reveal excellent bifunctional catalytic activity and robust stability for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The overpotentials for OER and HER are 320 mV and 137 mV at the c.d. of 10 mA cm-2, resp. The catalytic activity of Ru@RuO2 nanorods for OER is 6.5 times higher than that of the state-of-the-art catalyst IrO2 according to the catalytic c.d. measured at 1.60 V (vs. RHE). The catalytic activity of Ru@RuO2 nanorods for HER is comparable to 40% Pt/C by comparing the catalytic current densities at -0.2 V.
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    67. 67
      Chandra, D.; Inoue, Y.; Sasase, M.; Kitano, M.; Bhaumik, A.; Kamata, K.; Hosono, H.; Hara, M. Chem. Sci. 2018, 9, 5949– 5956,  DOI: 10.1039/C8SC01197D
      There is no corresponding record for this reference.
    68. 68
      Stevens, C. A. C.; Lambie, S. G.; Ruffman, C.; Skulason, E.; Garden, A. L. Geometric and Electronic Effects Contributing to N2 Dissociation Barriers on a Range of Active Sites on Ru Nanoparticles. J. Phys. Chem. C 2019, 123, 30458– 30466,  DOI: 10.1021/acs.jpcc.9b09563
      There is no corresponding record for this reference.
    69. 69
      Tang, C.; Qiao, S.-Z. How to explore ambient electrocatalytic nitrogen reduction reliably and insightfully. Chem. Soc. Rev. 2019, 48, 3166– 3180,  DOI: 10.1039/C9CS00280D
      69
      How to explore ambient electrocatalytic nitrogen reduction reliably and insightfully
      Tang, Cheng; Qiao, Shi-Zhang
      Chemical Society Reviews (2019), 48 (12), 3166-3180CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. The efficient activation of dinitrogen for the prodn. of ammonia plays a crucial role in our modern society, considering the significant impact of ammonia on food, chems., and energy. As an attractive alternative to the century-old Haber-Bosch process which is responsible for 1-2% of global energy consumption, utilization of half the hydrogen produced globally, and ∼1% of global energy-related CO2 emissions, the ambient electrocatalytic nitrogen redn. reaction has attracted tremendous interest during the past few years. Some achievements have revealed the possibility of this process, but have also identified great challenges. The activity and selectivity of the nitrogen redn. reaction are fundamentally limited by competing hydrogen evolution and nitrogen scaling relations, while low prodn. rates and ubiquitous contaminants plague exptl. practices. Aiming toward higher accuracy and reproducibility of claimed results, and more meaningful, impactful, and insightful research, this tutorial review summarizes the present status and challenges in the study of ambient electrocatalytic nitrogen redn., followed by a thorough discussion of various exptl. parameters. We then recommend a series of protocols and best practices for expts., and also highlight some potential directions for future research in this exciting and important field.
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    70. 70
      Singh, S.; Mohammed, A. K.; AlHammadi, A. A.; Shetty, D.; Polychronopoulou, K. Hypes and hopes on the materials development strategies to produce ammonia at mild conditions. Int. J. Hydrogen Energy 2023, 48, 34700– 34739,  DOI: 10.1016/j.ijhydene.2023.05.206
      There is no corresponding record for this reference.
    71. 71
      Zhou, Y.; Wang, J.; Liang, L.; Sai, Q.; Ni, J.; Au, C. T.; Lin, X.; Wang, X.; Zheng, Y.; Zheng, L.; Jiang, L. Unraveling the size-dependent effect of RU-based catalysts on ammonia synthesis at mild conditions. J. Catal. 2021, 404, 501– 511,  DOI: 10.1016/j.jcat.2021.10.024
      71
      Unraveling the size-dependent effect of Ru-based catalysts on Ammonia synthesis at mild conditions
      Zhou, Yanliang; Wang, Junjie; Liang, Lili; Sai, Qianjin; Ni, Jun; Au, Chak-tong; Lin, Xingyi; Wang, Xiuyun; Zheng, Ying; Zheng, Lirong; Jiang, Lilong
      Journal of Catalysis (2021), 404 (), 501-511CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Identifying optimal Ru size in NH3 synthesis can improve reaction activity and maximize the utilization of Ru to reduce catalyst cost. However, previous researches are focused on large Ru particle size (≥2 nm) while that below 2 nm in NH3 synthesis is unclear. Here we synthesized a series of Rux/BaCeO3 with different Ru sizes (x = 1.1-3.0 nm) through size-controlled Ru colloid. With the decrease of Ru size, NH3 synthesis rate over Ru1.1/BaCeO3 increases to 19.4 mmol g-1cath-1 at 400°C and 1 MPa, which is 5.7 times that of Ru3.0/BaCeO3 and superior to most of Ru-based catalysts previously reported. It reveals that the redn. of Ru size enhances the generation of Ce3+ and oxygen vacancies in BaCeO3, which can donate electron to Ru centers and promote N2 dissocn. Moreover, the small Ru size enhances hydrogen spillover from Ru to BaCeO3 to alleviate hydrogen poisoning, resulting in efficient NH3 synthesis.
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    72. 72
      Peng, X.; Chen, X.; Zhou, Y.; Sun, F.; Zhang, T.; Zheng, L.; Jiang, L.; Wang, X. Size-dependent activity of supported RU catalysts for ammonia synthesis at mild conditions. J. Catal. 2022, 408, 98– 108,  DOI: 10.1016/j.jcat.2022.02.024
      72
      Size-dependent activity of supported Ru catalysts for ammonia synthesis at mild conditions
      Peng, Xuanbei; Chen, Xiaochuan; Zhou, Yanliang; Sun, Fuxiang; Zhang, Tianhua; Zheng, Lirong; Jiang, Lilong; Wang, Xiuyun
      Journal of Catalysis (2022), 408 (), 98-108CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Ammonia (NH3) synthesis is a structure-sensitive reaction, where a minute variation of catalyst structure would lead to a dramatic change of activity. To date, the roles of Ru size on NH3 synthesis still remain elusive, and it remains a major challenge. Herein, a series of Ru catalysts with sizes ranging from 1.4 to 5.0 nm were prepd. by colloidal Ru deposition, which allows the investigation of size influence on NH3 synthesis. Through analyses of the geometric and electronic properties of variable-sized Ru particles, it shows that the redn. of Ru particle size could increase the proportion of corner sites while decrease that of terrace sites, which can decrease the work function and thus promote N2 for its activation. Isotopic investigations show that the decrease of dissociative contribution but the increase of associative contribution as Ru particle size was decreased from 5.0 to 1.4 nm. Meanwhile, the NH3 synthesis rate increases when the size of Ru catalysts decreases from 5.0 to 1.4 nm. Using a suite of elaborate characterizations, we have obsd. that 1.4 nm Ru nanoclusters (NCs) with ample corner sites are conducive to activate N2 via an associative route. As a result, the 1.4 nm Ru NCs catalyst shows the highest NH3 synthesis rate (up to 17.13 mmolNH3 g-1cath-1) at 400°C and 1 MPa.
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    73. 73
      Hinrichsen, O.; Rosowski, F.; Hornung, A.; Muhler, M.; Ertl, G. The kinetics of ammonia synthesis over RU-based catalysts. J. Catal. 1997, 165, 33– 44,  DOI: 10.1006/jcat.1997.1447
      There is no corresponding record for this reference.
    74. 74
      Qiu, J.-Z.; Hu, J.; Lan, J.; Wang, L.-F.; Fu, G.; Xiao, R.; Ge, B.; Jiang, J. Pure siliceous zeolite-supported RU single-atom active sites for ammonia synthesis. Chem. Mater. 2019, 31, 9413– 9421,  DOI: 10.1021/acs.chemmater.9b03099
      74
      Pure Siliceous Zeolite-Supported Ru Single-Atom Active Sites for Ammonia Synthesis
      Qiu, Jiang-Zhen; Hu, Jibo; Lan, Jinggang; Wang, Long-Fei; Fu, Guangying; Xiao, Rujian; Ge, Binghui; Jiang, Jiuxing
      Chemistry of Materials (2019), 31 (22), 9413-9421CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      As the most active new frontier, a single-atom catalyst (SAC) combining the merits of heterogeneous and homogeneous catalysts would have a significant effect on a 100-yr history of ammonia synthesis research. It is commonly accepted that B5 is the active site of Ru catalysts for ammonia synthesis. Here, the Ru single atoms catalyst has been demonstrated active and efficient for ammonia synthesis. To this end, an ideal model catalyst, pure siliceous zeolite-supported Ru SAC (Ru SAs/S-1), which shows surprising catalytic ammonia synthesis activity compared to that of a conventional Ru catalyst was designed. Both at. resoln. scanning transmission electron microscopy and X-ray absorption spectrometric anal. identify the single-Ru-atom nature of Ru SAs/S-1 before and after the reaction. Further DFT calcns. reveal that the reaction mechanism is different from traditional mechanisms. Therefore, beyond B5 sites, this paper provides an alternative SAC strategy to design high-performance Ru catalysts for ammonia synthesis.
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    75. 75
      Wang, X.; Li, L.; Fang, Z.; Zhang, Y.; Ni, J.; Lin, B.; Zheng, L.; Au, C. T.; Jiang, L. Atomically dispersed Ru catalyst for low-temperature nitrogen activation to ammonia via an associative mechanism. ACS Catal. 2020, 10, 9504– 9514,  DOI: 10.1021/acscatal.0c00549
      75
      Atomically Dispersed Ru Catalyst for Low-Temperature Nitrogen Activation to Ammonia via an Associative Mechanism
      Wang, Xiuyun; Li, Lingling; Fang, Zhongpu; Zhang, Yongfan; Ni, Jun; Lin, Bingyu; Zheng, Lirong; Au, Chak-tong; Jiang, Lilong
      ACS Catalysis (2020), 10 (16), 9504-9514CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The industrial synthesis of NH3 using Fe- or Ru-based catalysts usually requires harsh reaction conditions. It is desirable to develop catalysts that perform well at low temp. and pressure (250-400°C, < 2 MPa). The main challenge of low-temp. NH3 synthesis is the dissocn. of the extremely stable N≡N triple bond. Herein, we report the design of homogeneous single-atom Ru centers on an H-ZMS-5 (HZ) support with the Ru atoms individually anchored in the micropores of HZ, effectively boosting NH3 synthesis under mild conditions via an associative pathway. Synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) and in situ DRIFTS analyses show that =N- groups are the primary intermediates, and DFT calcns. further show that, unlike Ru nanoclusters, the cooperation of a single Ru atom and hydrogen species in HZ leads to N2 hydrogenation rather than direct N2 dissocn., and the indirect N-N bond dissocn. occurs much more easily via the formation of the NHNH3* intermediate; the energy barrier for breaking the N-N bond keeps falling from 2.90 eV for *N2 to 0.04 eV for *NHNH3, showing that N2 hydrogenation is an effective way for sharp weakening of N-N bonds. Moreover, the rate-detg. step is shifted from the dissocn. of the N≡N triple bond to the formation of *N2H2. As a consequence, the single-atom 0.2 wt % Ru/H-ZSM-5 catalyst shows the highest NH3 synthesis rate per g of Ru (1.26 molNH3 gRu-1 h-1 at 300°C and 1 MPa) among the Ru-based catalysts ever reported.
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    76. 76
      Yao, Y.; Zhu, S.; Wang, H.; Li, H.; Shao, M. A spectroscopic study on the nitrogen electrochemical reduction reaction on Gold and platinum surfaces. J. Am. Chem. Soc. 2018, 140, 1496– 1501,  DOI: 10.1021/jacs.7b12101
      76
      A Spectroscopic Study on the Nitrogen Electrochemical Reduction Reaction on Gold and Platinum Surfaces
      Yao, Yao; Zhu, Shangqian; Wang, Haijiang; Li, Hui; Shao, Minhua
      Journal of the American Chemical Society (2018), 140 (4), 1496-1501CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The electrochem. redn. of N to NH3 on Au-based catalysts showed a reasonably high Coulombic efficiency. The pathway of this promising reaction, however, is not clear partially due to the lack of information on reaction intermediates. Herein, surface-enhanced IR absorption spectroscopy (SEIRAS) was employed to study the reaction mechanisms of N redn. on an Au thin film for the 1st time. During the N redn., the N2Hy species was detected with bands at 1453 (H-N-H bending), 1298 (-NH2 wagging), and 1109 cm-1 (N-N stretching) at potentials <0 V against reversible H electrode. The N redn. reaction on Au surfaces follows an associative mechanism, and the N≡N bond in N2 tends to break simultaneously with the H addn. By comparison, no absorption band assocd. with N was obsd. on Pt surfaces under the same reaction condition. This result is consistent with the low efficiency of N redn. on Pt due to the much faster kinetics of H evolution reaction.
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    77. 77
      Schmitt, R.; Nenning, A.; Kraynis, O.; Korobko, R.; Frenkel, A. I.; Lubomirsky, I.; Haile, S. M.; Rupp, J. L. M. A review of defect structure and chemistry in Ceria and its solid solutions. Chem. Soc. Rev. 2020, 49, 554– 592,  DOI: 10.1039/C9CS00588A
      77
      A review of defect structure and chemistry in ceria and its solid solutions
      Schmitt, Rafael; Nenning, Andreas; Kraynis, Olga; Korobko, Roman; Frenkel, Anatoly I.; Lubomirsky, Igor; Haile, Sossina M.; Rupp, Jennifer L. M.
      Chemical Society Reviews (2020), 49 (2), 554-592CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Ceria and its solid solns. play a vital role in several industrial processes and devices. These include solar energy-to-fuel conversion, solid oxide fuel and electrolyzer cells, memristors, chem. looping combustion, automotive 3-way catalysts, catalytic surface coatings, supercapacitors and recently, electrostrictive devices. An attractive feature of ceria is the possibility of tuning defect-chem. to increase the effectiveness of the materials in application areas. Years of study have revealed many features of the long-range, macroscopic characteristics of ceria and its derivs. In this review we focus on an area of ceria defect chem. which has received comparatively little attention - defect-induced local distortions and short-range assocs. These features are non-periodic in nature and hence not readily detected by conventional X-ray powder diffraction. We compile the relevant literature data obtained by thermodn. anal., Raman spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. Each of these techniques provides insight into material behavior without reliance on long-range periodic symmetry. From thermodn. analyses, assocn. of defects is inferred. From XAFS, an element-specific probe, local structure around selected at. species is obtained, whereas from Raman spectroscopy, local symmetry breaking and vibrational changes in bonding patterns is detected. We note that, for undoped ceria and its solid solns., the relationship between short range order and cation-oxygen-vacancy coordination remains a subject of active debate. Beyond collating the sometimes contradictory data in the literature, we strengthen this review by reporting new spectroscopy results and anal. We contribute to this debate by introducing addnl. data and anal., with the expectation that increasing our fundamental understanding of this relationship will lead to an ability to predict and tailor the defect-chem. of ceria-based materials for practical applications.
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    78. 78
      Cheng, S.; Gao, Y.-J.; Yan, Y.-L.; Gao, X.; Zhang, S.-H.; Zhuang, G.-L.; Deng, S.-W.; Wei, Z.-Z.; Zhong, X.; Wang, J.-G. Oxygen vacancy enhancing mechanism of nitrogen reduction reaction property in Ru/TiO2. Journal of Energy Chemistry 2019, 39, 144– 151,  DOI: 10.1016/j.jechem.2019.01.020
      There is no corresponding record for this reference.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.3c16521.

    • Experimental protocol for catalysts characterization that was followed in Raman spectroscopy; X-ray photoelectron spectroscopy; high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR), and H2 chemisorption (H2-temperature-programmed desorption, TPD); textural analysis results as obtained after N2 adsorption at 77 K; SEM microphotographs obtained over the Ru catalysts of this study; EPR spectra obtained at room temperature (298 K) over 6Ru/SiO2, 6Ru/CePrO, and 6Ru/HEO catalysts; comparative raman spectra of Ru/CePrO and Ru/HEO catalysts; deconvolution of Ru 3p core-level spectra of Ru/CePrO and Ru/HEO; deconvolution of O 1s core-level spectra of Ru/CePrO, Ru/CeLaO and Ru/HEO catalysts; areas of the O 1s deconvoluted peaks; Ru dispersion and particle size as obtained from the H2 chemisorption studies; and FFT analysis of the Ru/HEO catalyst (PDF)


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