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Tuning the Size of TiO2-Supported Co Nanoparticle Fischer–Tropsch Catalysts Using Mn Additions
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Cite this: ACS Catal. 2024, 14, 14, 10648–10657
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https://doi.org/10.1021/acscatal.4c02721
Published June 30, 2024

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Abstract

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Modifying traditional Co/TiO2-based Fischer–Tropsch (FT) catalysts with Mn promoters induces a selectivity shift from long-chain paraffins toward commercially desirable alcohols and olefins. In this work, we use in situ gas cell scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS) elemental mapping, and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to demonstrate how the elemental dispersion and chemical structure of the as-calcined materials evolve during the H2 activation heat treatment required for industrial CoMn/TiO2 FT catalysts. We find that Mn additions reduce both the mean Co particle diameter and the size distribution but that the Mn remains dispersed on the support after the activation step. Density functional theory calculations show that the slower surface diffusion of Mn is likely due to the lower number of energetically accessible sites for the Mn on the titania support and that favorable Co–Mn interactions likely cause greater dispersion and slower sintering of Co in the Mn-promoted catalyst. These mechanistic insights into how the introduction of Mn tunes the Co nanoparticle size can be applied to inform the design of future-supported nanoparticle catalysts for FT and other heterogeneous catalytic processes.

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Introduction

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Fischer–Tropsch (FT) synthesis is a heterogeneous catalytic process that can convert syngas (CO/H2), typically derived from fossil fuels, into high-quality liquid hydrocarbon transport fuels. (1) However, the environmental impact (2) and energy security concerns (3) of using fossil fuels are pushing the need for alternative, renewable sources of energy to meet with ever-increasing global demand. (4) Recent technological developments have enabled the use of waste products, such as municipal solid waste, biomass, and natural gas, as an alternative feedstock, making FT synthesis a greener and more sustainable route to clean fuels and value-added chemicals. (5,6)
The traditional method for FT synthesis is to flow syngas over a metallic catalyst within a fixed-bed reactor. (7) Several transition metals are active catalysts for FT, but the desire for high activity, selectivity, and stability makes cobalt-based catalysts the practical choice to produce high-molecular-weight paraffin fuels. (8) Cobalt particle size is a key factor in FT performance, and diameters of 6 nm have been found to be optimal; (9) larger particles have reduced surface area that leads to reduced activity, while smaller particles have a larger number of edge sites that favor chain termination and the formation of methane. (9,10) In practice, it may be desirable to have a particle size below 6 nm in the starting activated catalyst if it provides longer onstream performance and if the selectivity for undesirable methane remains below an acceptable level, since particle size tends to increase during FT reaction conditions. (11)
Industrial FT processes rely on supported nanoparticle systems synthesized via wet impregnation, where the support is typically a metal oxide such as titania, silica, or alumina in the form of a nanoparticle powder. (12) The support provides the catalytic material with mechanical integrity for the extruded pellets when loaded within the reactor as well as encouraging good dispersion and hindering metal particle agglomeration. (13) The as-synthesized wet impregnated materials are then extruded into pellets, calcined at ∼300 °C in air for up to 12 h, and then subjected to a activation heat treatment inside the FT reactor (typically heating in hydrogen at 200–400 °C for 1–5 h) to produce the active metal phase of the catalyst. (14) Metal–support interactions can also play a potential role in FT performance, such as modifying the charge transfer to create an interfacial layer with increased activity, reported previously for CO oxidation (15) and CO2 hydrogenation; (16) or through the use of reducible oxide supports, which form suboxide species during hydrogen spillover from the metal nanoparticle and that can migrate onto the metal catalyst surface, (17) potentially enhancing the catalyst performance (18) or blocking active sites and inhibiting chemisorption of CO and H2. (19)
The addition of alloying promoters, such as Mn, (20−22) Re, (23−25) or Zr, (26−28) provides opportunities to tune the catalytic performance of the Co. For example, Re enhances the reducibility of cobalt, which produces energy savings by lowering the temperature required for catalyst activation (26,29) and decreases the FT reaction selectivity to undesirable methane. (30) The present study focuses on the use of Mn as a promoter, specifically industrial catalysts composed of wet impregnation-synthesized 10 wt % Co on TiO2 with and without 5 wt % Mn promoter. Previous investigations of 10 wt % Co on TiO2 have demonstrated that the addition of up to 10 wt % Mn as a promoter can modify the FT reaction selectivity toward long-chain oxygenates and liquid petroleum gas (LPG, C2–C4), with the addition of 5 wt % Mn identified as having unusually high selectivity toward desirable long-chain alcohols. (31,32) The reason for the high selectivity of 10 wt % Co and 5 wt % Mn on TiO2 compared to that of the unpromoted catalyst is still debated despite the industrial importance of these materials for the FT reaction. Previous extended X-ray adsorption fine structure (EXAFS) analysis of the Co and Mn coordination in the catalysts has suggested a mixed metal oxide spinel (CoxMn3–xO4) with a preference toward MnCo2O4 (i.e., MnO·Co2O3) for the 5% Mn catalyst. (32) The selectivity change on addition of Mn is thus suggested to result from the presence of MnO, which inhibits CO dissociation and instead favors CO insertion. (32) Nonetheless, synthesis of catalysts using physical mixtures of Co-only and Mn-only catalysts on TiO2 have not shown similar catalytic behavior to those impregnated with a mixed solution, indicating that close interaction of Co–Mn is key to the differences in FT functionality. (31) Similar EXAFS studies for γ-Al2O3-supported CoMn catalysts report that the presence of Mn leads to a stabilization effect of CO, C, H, O, and CHx adsorption relative to unpromoted Co, in turn decreasing the CO dissociation barrier and increasing the intrinsic activity. (33) However, Tucker et al. have used density functional theory (DFT) calculations to conclude that manganese promotion cannot be linked to enhanced CO adsorption or promotion of the C–O bond cleavage and that selectivity improvements toward liquid fuels are related to improved oxygen removal from the catalytically active surface. (22)
Both Co and Mn adatoms on anatase (101) have been studied previously with DFT. Yang and Zhou explored the binding sites of single Co adatoms on the surface as a part of the first-principles study with relevance to the hydrogen evolution reaction; (34) similarly, Kalantari, Tran, and Blaha computed the stable configurations of clusters of metals and their oxides, including up to five atoms of Co and Mn. (35) Previous computational studies have not, however, extensively considered coadsorption of Co and Mn adatoms as necessary to understand the current catalytic materials. Understanding the interplay of catalytic species is key to improving selectivity and long-term stability of the materials under FT conditions, but little is known about the nanoscale interactions of the active species in these materials. Spatially resolved analytical studies of how the material composition evolves during activation treatment have not been previously reported and could provide key new insights.
In situ transmission electron microscopy (TEM) is a powerful approach for imaging of supported nanoparticle catalysts, (36) overcoming the limitations of traditional vacuum TEM studies by using ultrathin, electron-transparent silicon nitride (SiNx) membranes to separate the reactive gas environment from the microscope vacuum and using a local heating element to allow temperature control. (37,38) Commercially available gas cell holders allow in situ TEM observations to be performed at pressures typically up to 1 bar and temperatures up to 1000 °C, which is highly desirable for studying the structure and composition of heterogeneous catalysts where the local environment and presence of surface adsorbates can strongly affect the surface chemistry and nanoparticle morphology. (39−41) In situ TEM is particularly valuable for reactive metal nanoparticle catalysts, like Co, where performing the activation of the catalyst inside the electron microscope prevents the exposure of the activated material to the atmosphere when transferring it to the transmission electron microscope. (42) However, in situ TEM struggles to measure the local oxidation state of the materials due to the combined effects of the reactive gas environment and the amorphous SiNx windows of commercial gas cells, producing significant electron scattering that degrades the quality of in situ electron energy loss spectroscopy (EELS) compared to ex situ studies. Complementary characterization using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) allows measurements of the changes in the cobalt oxidation state during reduction. Here, we apply a combination of in situ TEM and NAP-XPS to perform element-sensitive imaging and oxidation state analysis of wet impregnation-synthesized 10 wt % Co catalysts on titania with and without 5 wt % Mn (denoted Co-0Mn and Co-5Mn, respectively). We study these materials while heating in hydrogen across a range of elevated temperatures to reveal the evolution of the Co-rich nanoparticles during the catalyst activation step. DFT calculations are performed to understand the interactions of Co and Mn species with each other and with the titania support and thereby shed light on the mechanism for the observed decrease in nanoparticle size induced by Mn promotion.

Results

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Catalytic Testing

Catalytic testing was undertaken to investigate the conversion/product selectivity differences between the Co-0Mn and Co-5Mn catalysts (31,32) (see Sections S1.1 and S1.2 for further details of the catalyst synthesis and reactor testing methods, respectively). The benefit of the Mn promotor is to enhance the selectivity to high-value alcohols and other oxygen containing products compared to the unpromoted Co-0Mn (Figure 1b). Unfortunately, this is only achieved at the expense of increased selectivity for undesirable C2–C4 products and methane (C1), as well as a slight overall drop of preferred C5+ products relative to Co-0Mn (Figure 1a). The 10% methane selectivity is slightly improved compared to the 18% reported for CoMn/TiO2 catalysts by Morales et al., (43) likely due to the larger particle size in our materials. However, Morales et al. (43) found the catalyst’s methane selectivity was not influenced by the addition of Mn, which we attribute to their use of a consecutive loading method of Co and Mn solutions via incipient wetness impregnation, as opposed to the simultaneous loading method (via mixed solution) employed for our investigations. This emphasizes the importance of understanding the interaction mechanisms for Co and Mn in tuning the catalytic performance, which is the focus of this study. A 0Co-5Mn catalyst previously analyzed by temperature-programmed reduction (TPR), presented in Figure S2, was also tested during the same reactor conditions, which showed practically no CO conversion and hence no catalytic activity toward FT synthesis (Table S1).

Figure 1

Figure 1. Catalytic testing of the 10 wt % Co/TiO2 catalyst with and without 5 wt % Mn. (a) The addition of Mn induces a selectivity shift from long-chain (C5+) products to those with shorter chains (C2–C4) as well as a slight increase in methane production. (b) The yield in alcohol selectivity for the C5+ products are significantly enhanced with Mn addition.

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Catalyst Dispersion and Chemical State in the As-Calcined Materials

High-angle annular dark field (HAADF) scanning TEM (STEM) images of the Co-0Mn and Co-5Mn catalysts after calcination, but prior to activation, are shown in the left-hand column of Figure 2a,b, respectively. These images reveal roughly spherical nanoparticles with diameters of 10–30 nm, in agreement with the expected particle size for the P25 titania support. For common scattering angles, the intensity of the HAADF-STEM signal scales with the local density, thickness, and atomic number, Z1.7. (44) As the Co, Mn, and Ti oxides all have similar atomic numbers and density, but variable thickness, it is not possible to unambiguously distinguish the different oxide phases with only HAADF-STEM imaging. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping and EELS were therefore required to characterize the dispersion of Co and Mn on the titania support.

Figure 2

Figure 2. Ex situ structural and chemical characterization of the TiO2-supported Co/Mn catalysts after calcination (prior to activation). (a,b) HAADF-STEM imaging and STEM-EDS elemental mapping of the (a) Co-0Mn and (b) Co-5Mn catalysts. (c) Corresponding EDS spectra from the imaged regions in (a,b). (d,e) High-resolution HAADF-STEM imaging of the supported Co clusters in (d) Co-0Mn and (e) Co-5Mn. Fourier transforms of the images are shown as insets. Co-0Mn is indexed to the Co3O4 spinel viewed along [111], while Co-5Mn is indexed to CoO viewed along [110]. (f) XRD analysis highlighting the presence of Co3O4 in the Co-0Mn catalyst and the disappearance of any discernible cobalt oxide structure in the Co-5Mn catalyst. (g–i) Summed STEM-EELS edges from the (g) Co-0Mn and (h,i) Co-5Mn catalysts. (j,k) XPS spectra of the cobalt peaks from the (j) Co-0Mn and (k) Co-5Mn catalysts. The appropriate experimentally measured spectra taken from standard reference materials (Co3O4 spinel, CoO, metallic cobalt, Mn3O4 spinel, Mn2O3, MnO2, and MnO) are included in (g–k) for comparison.

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The STEM-EDS elemental mapping of the as-calcined catalysts demonstrates the dramatic increase in the uniformity of Co dispersion when Mn is added (Figure 2a,b, with further examples in Figure S3). For the Co-0Mn catalyst, the Co forms irregular-shaped porous structures 20–100 nm in diameter (Figure 2a), which are much larger than the individual titania particles; in contrast, in the Co-5Mn catalyst, the Co is more uniformly dispersed across the support surface (Figure 2b). Comparing the elemental maps for Co and Mn in the Co-5Mn catalyst shows colocation of both elements, with high-resolution maps confirming that this is maintained at the nanoscale (Figure S4), consistent with previous studies of similarly prepared mixed-solution wet-impregnated catalysts at a lower spatial resolution. (31,32) Figure 2c shows the EDS spectra for both catalyst samples, confirming the presence of Mn only in Co-5Mn. A catalyst with 0 wt % Co and 1 wt % Mn (0Co-1Mn) on TiO2 was also prepared and demonstrated similarly high Mn dispersion to that of the Co-5Mn catalyst, confirming that Mn is likely the underlying cause of the increased Co dispersion effect (Figure S5).
Atomic-resolution HAADF-STEM imaging of the Co-rich regions in Co-0Mn and Co-5Mn (Figure 2d,e) shows that these also have different crystal structures, with Co-0Mn matching a Co3O4 spinel structure, while the smaller Co-5Mn particle closely matches the CoO/MnO rock salt phase; however, the lattice spacing of CoO and MnO are too similar to allow the particle composition to be distinguished using only HAADF-STEM imaging (Figure S6a,b). X-ray diffraction (XRD) analysis of the Co-0Mn catalyst confirms the Co3O4 spinel structure (Figure 2f); in contrast, no cobalt oxide (Co3O4 or CoO), manganese oxide (MnO, MnO2, Mn2O3, or Mn3O4), or mixed Co–Mn spinel (CoMn2O4 or MnCo2O4) phases are observed for the Co-5Mn material. The absence of crystallinity observed by XRD is likely due to the very small size of the crystallites or absence of long-range ordering; the particle in Figure 2e is ∼3 nm in diameter, and in other areas, the particle size was smaller or poorly defined, existing in a semicrystalline or amorphous state (Figure S6c) supported by the lack of Co/Mn peaks and increased background of the XRD profiles before subtraction (Figure S6d). Atomic-resolution HAADF-STEM imaging of the Co3O4 spinel in Co-0Mn suggests that the Co-rich particles have a crystallite diameter of the order of 10 nm. This is consistent with previous work on similar 10 wt % Co catalysts on titania, which also saw a reduction in the Co-containing oxide particle size in the promoted catalyst and where Scherrer analysis indicated a decrease in the average Co3O4 crystallite size from ∼11 nm when no Mn is present to ∼2 nm when 5 wt % Mn is incorporated. (31)
To establish the oxidation state of Co and Mn in the catalysts, STEM-EELS was used to obtain the Co and Mn L2 and L3 core-loss edges from the as-calcined catalysts (at energy losses of ∼780 and ∼640 eV, respectively), where these are then compared to Co and Mn oxide/metallic references (Figure 2g–i). For the Co-0Mn catalyst, the Co is a good match to the Co3O4 reference (Figure 2g), whereas for the Co-5Mn catalyst, the Co edges are best matched to CoO (Figure 2h) and the Mn edge is a good fit to MnO (Figure 2i). The change in the oxidation state of Co, from a mixed +3/+2 state (Co3O4) in Co-0Mn to being primarily +2 (CoO) in Co-5Mn, is also demonstrated by XPS analysis of the catalysts (Figure 2j,k) highlighted by the shakeup satellite in Co-5Mn at ∼787 eV, characteristic of Co2+. (45) A full comparison of the catalyst Co and Mn STEM-EELS edges and Co XPS peaks to the full range of oxide/metallic reference materials can be found in Figure S7. Together, our HR-STEM, XRD, EELS, and XPS observations all suggest that, before activation, the Co and Mn species in Co-5Mn are dispersed across the surface of the titania support and are present as a disordered Co1–xMnxO-type solid solution with some ultrafine-scale rock salt crystallites. This differs from the mixed spinel structure previously suggested for similar materials as the presence of CoO has only previously been reported for higher Mn promoter loadings and in Co catalysts made via a different synthesis route. (43,46)

Evolution of the Catalyst Structure and Chemistry during In Situ Activation

We now seek to investigate the catalyst structure after the catalyst activation step, which is used to reduce the cobalt oxides to the metallic phase that is active for FT. HAADF-STEM and STEM-EDS/EELS investigations of the materials after an ex situ activation treatment (∼1 bar H2 at 300 °C for 12 h) revealed that Co-0Mn contains Co nanoparticles with a mean diameter of 17.3 nm (standard deviation, σ = 3.9 nm, and number of particles analyzed, n = 116, Figure S8a,c), consisting of metal cores and oxide shells (Figure S9c). The promoted Co-5Mn showed smaller particles, 11.4 nm in diameter (σ = 5.3 nm, n = 113, Figure S8b,d), with EELS providing a best fit to CoO and MnO (Figure S10). The high level of oxide is indicative of how readily Co can oxidize with minimal air exposure even though a vacuum transfer holder was employed to move the sample from the furnace to the transmission electron microscope (see Section S2.3 for STEM-EDS/EELS results and further details). Studying the nanoscale evolution of the catalyst structure and chemistry during activation therefore requires in situ techniques to be employed. In situ HAADF-STEM with EDS was performed using a Protochips Atmosphere gas cell holder, which has been designed to allow X-rays to reach the Titan scanning transmission electron microscope’s Super-X EDS detector geometry. (47) The activation treatment was performed under ∼1 bar of hydrogen pressure at 150, 250, and 350 °C, with a 1 °C/s temperature ramp and a dwell time of 30 min at each temperature step, where the 150 °C starting temperature was chosen to avoid contaminant deposition prevalent during room temperature EDS (Figure S11). When the Co (or Mn) oxides transform to the metallic state during reduction, the increase in the local density of the material is visible as an increase in the HAADF-STEM intensity on the oxide support. Nonetheless, it is not possible to distinguish which metal species the reduced material contains, so STEM-EDS or EELS analysis is required. Unlike previous ex situ TEM characterization, using in situ STEM-EDS elemental mapping allowed us to investigate the evolution of the nanoscale dispersion of all constituent elements during the activation step (Figure 3, with EDS spectra presented in Figure S12), which has rarely been achieved. Tang et al. have recently applied in situ STEM-EDS elemental mapping to study a mixed Co and Pt system; (48) however, this methodology has not previously been applied to CoMn/TiO2 catalysts, a system used industrially for FT synthesis.

Figure 3

Figure 3. In situ structural and chemical characterization of the TiO2-supported Co/Mn catalyst dispersion during activation. (a,b) HAADF-STEM imaging and STEM-EDS elemental mapping of the (a) Co-0Mn and (b) Co-5Mn catalysts. Temperatures are shown inset on the corresponding HAADF images. (c,d) NAP-XPS analysis of the in situ activation of (c) Co-0Mn and (d) Co-5Mn in hydrogen. The presence of 5 wt % Mn induces a transition from a three-stage reduction to a two-stage process, where the presence of Co3+ indicates a spinel structure, Co3O4, which transitions to Co2+, indicative of CoO rock salt, in the promoted (as-calcined) catalyst and lowers the Co reduction temperature during activation. The reference data for the Co standards are presented in Figures 2i,j and S7d,e. (e,f) Histograms illustrating the Co particle size distributions for both catalysts at (e) 250 and (f) 350 °C (see Section S1.5 and Figure S16 for details).

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STEM-EDS images of the Co-0Mn catalyst heated at 150 °C (Figure 3a) show a reduction of volume and porosity for the Co material compared to the room temperature morphology, although the Co-rich regions still have similar dimensions to those of the unreduced material (20–100 nm in diameter; for a comparison of ex situ and in situ imaging at room temperature, see Figures 2a and S13a, respectively). The sample was then heated to higher temperatures (250 °C and then 350 °C), causing the Co-rich regions to start sintering, forming distinct nanoparticles. To confirm that sintering was an effect of the activation treatment and not from the increased electron beam dose used during EDS, the regions in Figure 3 were imaged both before and after EDS acquisition at each temperature step (Figure S14). Particle size distributions were measured from the HAADF-STEM images using the Co EDS maps as a reference (maps are presented in Figures 3a and S15a, with a representative range of size analysis presented in Figure 3e,f and a complete range in Figure S16). The results reveal mean diameters of 11.3 nm at 250 °C and 10.9 nm at 350 °C, with standard deviations in the size distributions of 6.3 and 9.3 nm, respectively. Comparison of the same region at 250 and 350 °C shows several examples where individual particles have combined to a single particle, demonstrating that particle migration and coalescence occur during activation. The mean particle size does not increase despite the particle coalescence we observe. This is because new particles continue to form during activation, generating a bimodal particle size distribution; the peak particle size (mode) has decreased from 6.5 nm at 250 °C to 3.5 nm at 350 °C, but a second peak has also appeared at 20.5 nm after 350 °C activation. The number of particles (n) in the field of view has also decreased from 58 at 250 °C to 44 at 350 °C.
STEM-EDS images of the Co-5Mn catalyst activated at 150 °C in hydrogen show some clustering of the Co and Mn, although defined nanoparticles are not visible and both elements remain widely dispersed on the TiO2 (Figure 3b). After heating to 250 °C, distinct particles become visible in the STEM-EDS map for the cobalt distribution, and these become slightly larger and more clearly defined after further heating to 350 °C. Size analysis of the Co EDS maps reveals the mean diameter of these Co-rich particles to be 3.4 nm at 250 °C and 3.9 nm at 350 °C, with a standard deviation of 2.1 nm at both temperatures. More particles are visible for a region with the same field of view in Co-5Mn compared to that in Co-0Mn (for Co-5Mn, n = 69 at 250 °C and n = 92 at 350 °C), but care needs to be taken when comparing different samples as Co-0Mn is highly heterogeneous at the 100 nm length scale (Figure S3a).
Interestingly, comparison of the STEM-EDS mapping images for Mn and Co reveals a correlation between the two distributions at 150 °C, in agreement with our ex situ STEM-EDS measurements of the as-calcined sample. At 250 °C, there are two particles visible in the Mn map, both of which are close to particles that are visible in the Co elemental map. However, at 350 °C, the Mn is more uniformly dispersed than at 250 °C, and the Co and Mn distributions are no longer colocated but remain in close contact due to the high dispersion of Mn (in agreement with our STEM-EDS measurements after ex situ activation, as shown in Figure S8b). Together, these measurements show that the addition of 5 wt % Mn causes a significant decrease in the Co particle size and a narrowing of the particle size distribution in Co-5Mn compared to that in the unpromoted Co-0Mn catalyst. There is no evidence of alloying of Co and Mn in the particles formed at 350 °C. Previous studies have shown that Co particle growth can occur in supported catalysts through both particle migration and coalescence (11,49) and via Ostwald ripening, (50,51) and it seems likely that both also occur here in the Co-0Mn catalyst. Interestingly, no particle migration was observed in the Co-5Mn catalyst, although this cannot be ruled out to occur at higher temperatures.
In situ analysis of the local oxidation state of the Co and Mn in the catalyst particles during activation was not possible with STEM-EELS due to the poor signal-to-noise ratio in the summed spectra, which prevented the energy loss edge structure being distinguished for highly dispersed materials, such as Co in the Co-5Mn catalyst (Figure S17). Instead, NAP-XPS was used to measure the surface oxidation state of the catalysts as a function of temperature in hydrogen (Figure 3c,d). The NAP-XPS data demonstrate that the Co-0Mn material undergoes a transition from Co3+ to Co2+ between 242 and 303 °C and then from Co2+ to Co0 metal between 303 and 372 °C. In contrast, the Co-5Mn sample undergoes a single reduction from Co2+ to Co0 metal between 333 and 363 °C. The result is consistent with Co in the as-calcined Co-5Mn sample being present as CoO rock salt whereas the as-calcined Co-0Mn material is Co3O4. The result also demonstrates that at 250 °C, the Co-rich nanoparticles in the STEM-EDS, as shown in Figure 3a,b, are partially reduced for both Co-0Mn and Co-5Mn, whereas they are expected to be fully reduced at 350 °C. In situ XANES/EXAFS have also been used to confirm the predominantly metallic nature of Co in both catalysts postreduction in H2 at 300 °C for 8 h (Figure S18). Quantitative estimation of the oxidation state using linear combination fitting reveals that the contents of metallic cobalt in Co-0Mn and Co-5Mn were 98.3 and 96.8%, respectively (Table S2), suggesting that the presence of Mn acts to slightly inhibit the amount of Co that can be reduced. In the EXAFS, the Mn promotion also leads to a slight attenuation in the most intense peak at ∼2.2 Å, associated with scattering from the Co–Co bond, which can be explained by the smaller size of the Co particles (for further details, see Section S2.5).

Theoretical Modeling of Co/Mn Interactions on the TiO2 Surface

To understand the two key observations of the Mn additions in (i) improving the distribution of Co in the calcined material and (ii) significantly decreasing the size of the Co-rich nanoparticles after reduction, DFT calculations were performed to determine the nature of the Co and Mn atomic interactions on the TiO2 support. Calculations considered metal atom coordination on the TiO2 anatase (101) surface, chosen due to the prevalence of the anatase TiO2 phase in our material and the high stability of this surface facet. (52,53) An extensive range of adsorption complexes were considered for the Co and Mn species, with specific favorability observed toward the adsorption positions at various hollow and bridge sites between surface oxygen atoms.
The single-atom adsorption complexes for Co and Mn that result from our comprehensive configuration survey are shown in Figure S19 along with relative energies; atomic charges and preferred spin configurations are shown in Table S3. For both Co and Mn, positive atomic charge is observed, which indicates the favorability of oxidation states above zero. The most stable adatom configurations for both Co and Mn are with four nearest oxygen atoms, at distances of 1.87–4.12 Å for Mn and 1.81–3.06 Å for Co. For the Co adatom calculations, the five most stable adsorption positions have an energy range of 0.83 eV, with the two lowest energy Co adsorption complexes being notably close in geometry and energy, yet clearly distinguishable by oxygen bond lengths and distortion of the anatase lattice (Figure S19a,b); in contrast, for the Mn adsorption complexes, the four most stable structures have a large energy range over 1.80 eV, with the difference between the two most stable adsorption sites being 1.25 eV (Figure S19f,g). The much greater range of energies for Mn adatoms is indicative that they are anchored more strongly to the anatase (101) surface than Co and less likely to sinter at elevated temperatures. This is in excellent agreement with our STEM-EDS observations, where Mn was found to persist as a dispersed layer covering the titania support even after the Co has coalesced into clearly segregated nanoparticles. The most stable configuration of the Co and Mn adatoms have been previously identified as the hollow site between four oxygens, (34,35) in agreement with our results in Figure S19a–c for Co and Figure S19f for Mn.
Diatomic adsorption complexes were further considered to understand the interactions between Co and Mn on the TiO2 anatase surface. The diatomic adsorption configurations considered favorability on available oxygen species at the TiO2 surface, guided by our observations for the monatomic adsorption (see Section S1.9 experimental method for details). Here, the chemical formula of the dimeric clusters is given as CoxMn2–x, where x ranges from 0 to 2, and the relative energies of the configurations were considered by calculating the mixing energy, Emix, which is defined as
Emix=Etot−(ECox2+EMn2−x2)
where Etot is the total energy of the diatomic configuration, and ECo and EMn correspond to the lowest energy configurations of the pure Co and Mn diatomic clusters, respectively. Figure 4a plots this mixing energy as a convex hull, with mixed heteronuclear diatomic clusters demonstrating energetic preference over homonuclear diatomic clusters. The most stable pairwise atomic configurations (with the lowest potential energy) are shown schematically in Figure 4b–d and demonstrate that Co–Co and Co–Mn interactions are readily formed for these species, while Mn–Mn interactions are unfavorable.

Figure 4

Figure 4. Most stable configurations for diatomic Co and Mn clusters. (a) Plot of the mixing energy, Emix against diatomic composition for CoxMn2–x clusters. Emix of the most stable mixed CoMn cluster is −0.14 eV, i.e., more stable than the homonuclear diatomic clusters. The most stable configuration is indicated by the red dots, while other configurations are shown in blue. (b–d) Illustrations demonstrating the most stable configuration for (b) Mn2, (c) CoMn, and (d) Co2 on the anatase (101) TiO2 surface. Colors for Co, Mn, Ti, and O in the schematics are yellow, purple, blue, and red, respectively.

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Discussion

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The DFT calculations provide important insights to explain the experimental changes in the catalyst morphology that we observe on addition of Mn to the Co-TiO2 catalysts. In situ and ex situ observations of the as-calcined catalysts reveal a dramatic increase in the dispersion of Co on the titania support in the presence of Mn and showed close mixing of Mn and Co to form CoO (and MnO) on the titania surface. Our DFT calculations show fewer stable lattice configurations for Mn than for Co, with large energy differences between metastable configurations, which suggests that Mn will be strongly bonded to the titania surface and cannot readily diffuse. As the most stable configuration for the CoMn clusters is found to be heteronuclear dimers, distributed Mn on the titania surface may act as anchors for Co species rather than favoring the nucleation of small homogeneous Mn clusters. These favored interactions are likely to lead to the experimentally observed improved Co dispersion on the TiO2 in the presence of Mn for the as-calcined material.
During the H2 activation treatment, STEM-EDS elemental mapping showed that Co sintered to form the active phase of cobalt metal nanoparticles, with the differences in the morphologies of the as-calcined material leading to a reduction in diameter from ∼11 to ∼4 nm through the addition of Mn, while the Mn remained dispersed over the support. Our DFT calculations also provide insights into the slowed coarsening of Co; the Co adatoms have several energetically accessible sites, which is likely to favor their diffusion across the surface to form Co-rich nanoparticles observed by STEM-EDS. Co–Co dimers are more stable than isolated Co adatoms on the titania, which indicates the favorability of nucleation for Co clusters. In contrast, Mn adatoms are constrained on the surface, with the next energy minimum 1.25 eV above the ground state, making surface migration of the Mn less likely than for Co; the Mn–Mn dimers are also unfavorable compared to the separated single Mn adatoms, which inhibits their agglomeration to form clusters and larger nanoparticles. These results may explain the persistent dispersion of Mn observed experimentally after the activation treatment.
The Co–Mn heterodimer pairs are found to be the most energetically preferable compared to any isolated adatom or homodimer species, which leads to a conclusion that the dispersed Mn adatoms on the TiO2 support act as surface anchoring points. During activation, these Mn adatoms provide multiple nucleation sites from which Co clusters can form, which together with the improved dispersion leads to the experimental observation of the active Co being present as smaller, more disperse nanoparticles. The Co particles are also more resistant to sintering in the Mn-promoted Co/TiO2 system as the Mn provides anchoring points on the titania, which slows surface diffusion of the Co and inhibits Ostwald ripening. Our STEM-EDS observations show that Mn remains distributed across the support after catalyst activation, so this effect will also likely continue to slow the sintering degradation of the catalyst during FT operation, helping to provide these industrial catalysts with a long lifetime on stream.

Conclusions

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In this work, we provide new insights to understand the effects that 5 wt % Mn addition has on the structure and chemistry of an industrial Co/TiO2 FT catalyst. The size of metal nanoparticle catalysts is known to be essential for optimizing the activity and selectivity of catalysts for FT as well as for other reactions in heterogeneous catalysis. Our complementary in situ analysis and DFT calculations illustrate how interactions at the atomic scale can improve the dispersion of wet impregnated elements on a substrate, reduce the particle size after activation, and inhibit the sintering of nanoparticle catalysts during operation. It is feasible that our observations of the Mn acting as the anchor site to disperse and inhibit the diffusion of the active catalyst element on the support could be expanded to provide small, sintering-resistant nanoparticles for other industrially relevant catalyst systems.

Experimental Methods

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Catalysts were prepared from mixed solutions of cobalt nitrate hexahydrate and manganese acetate tetrahydrate precursors dissolved in water at 40 °C. The solutions were incipient wetness coimpregnated onto a supporting P25 titania powder to yield 10 wt % Co + (n) wt % Mn/TiO2 catalysts, where n = 0 and 5. The resulting mixtures were extruded into pellets and calcined in air with a stepped heating profile (5 h at 60 °C, 5 h at 120 °C and 2 h at 300 °C). STEM with EDS and EELS was performed using a probe aberration-corrected FEI Titan G2 80–200 ChemiSTEM operated at 200 kV. In situ STEM analysis was performed using a Protochips Atmosphere gas cell system with H2 at pressures up to 1 bar and temperatures of 150–350 °C. Full details of STEM experimental measurements, DFT, TPR, XPS, and complementary X-ray absorption spectroscopy (XAS) methods are available in Section S1.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.4c02721.

  • Experimental methods (catalyst synthesis/testing, TPR, STEM-EDS/EELS, XRD, XPS, XAS, and DFT) and additional results (STEM-EDS/EELS, XAS, and DFT) (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • Matthew Lindley - Department of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.;  Orcidhttps://orcid.org/0000-0002-9116-3862
    • Pavel Stishenko - Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, U.K.
    • James W. M. Crawley - Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, U.K.
    • Fred Tinkamanyire - Department of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
    • Matthew Smith - Department of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
    • James Paterson - bp, Applied Sciences, Innovation & Engineering, Saltend, Hull HU12 8DS, U.K.;  Orcidhttps://orcid.org/0000-0003-1016-5776
    • Mark Peacock - bp, Applied Sciences, Innovation & Engineering, Saltend, Hull HU12 8DS, U.K.
    • Zhuoran Xu - bp, Applied Sciences, Innovation & Engineering, Chicago, Illinois 60606, United States
    • Christopher Hardacre - Department of Chemical Engineering and Analytical Science, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.;  Orcidhttps://orcid.org/0000-0001-7256-6765
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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All authors acknowledge funding from BP through the BP-International Centre for Advanced Materials (ICAM53 project). S.J.H. thanks the Engineering and Physical Sciences Research Council (EPSRC) for funding under grants EP/M010619/1 and EP/P009050/1 and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant ERC-2016-STG-EvoluTEM-715502). TEM and NAP-XPS access was supported by the Henry Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P025021/1, and EP/P025498/1. P.S. and A.J.L. acknowledge funding by the UKRI Future Leaders Fellowship program (MR/T018372/1). The authors acknowledge computational resources and support from the Supercomputing Wales project, which is part-funded by the European Regional Development Fund (ERDF) via the Welsh Government; and the UK National Supercomputing Services ARCHER and ARCHER2, accessed via membership of the Materials Chemistry Consortium, which is funded by Engineering and Physical Sciences Research Council (EP/L000202/1, EP/R029431/1, and EP/T022213/1). Thanks are due to the Argonne Advanced Photon Source and beamline 10-BM for their support in collecting the XAS data.

References

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    Journal of Catalysis (2007), 246 (1), 91-99CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Ltd.)
    The adsorption properties of manganese-promoted Co/TiO2 Fischer-Tropsch (FT) catalysts were investigated by diffuse reflectance IR spectroscopy (DRIFTS) using CO and H2 as probe mols. Manganese was found to be closely assocd. to the FT active Co0 sites at the surface of the catalysts. With increased MnO loading, CO preferentially bound linearly to surface metal sites. Manganese also decreased the extent of Co-TiO2 interactions, increasing the Co0 dispersion, resulting in higher H2 chemisorption uptake. Furthermore, with increasing MnO loading, FT catalytic tests at 1 bar and 220° revealed an increase in C5+ selectivity and olefinic products. These findings suggest that MnO species induce both structural and electronic promotion effects, resulting in higher metal dispersions and lower hydrogenation activity of the catalyst, ultimately enhancing the overall FT catalytic performance. The findings also suggest that MnO catalyzes the water-gas shift reaction, thereby changing the syngas feed compn. and affecting overall catalyst performance.
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    Thiessen, J.; Rose, A.; Meyer, J.; Jess, A.; Curulla-Ferré, D. Effects of manganese and reduction promoters on carbon nanotube supported cobalt catalysts in Fischer–Tropsch synthesis. Microporous Mesoporous Mater. 2012, 164, 199– 206,  DOI: 10.1016/j.micromeso.2012.05.013
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    Effects of manganese and reduction promoters on carbon nanotube supported cobalt catalysts in Fischer-Tropsch synthesis
    Thiessen, J.; Rose, A.; Meyer, J.; Jess, A.; Curulla-Ferre, D.
    Microporous and Mesoporous Materials (2012), 164 (), 199-206CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier Inc.)
    Carbon nanotube (CNT) supported Co catalysts were promoted with manganese and noble metals (Pt, Ru) and tested in Fischer-Tropsch synthesis at temps. between 200 and 250° and pressures of 1 and 30 bar. A significant decrease in methane selectivity and with that an increase in C5+ selectivity was found due to promotion with manganese. In addn., the activity (syngas conversion) slightly increases and the olefin selectivity drastically increases. The promotion with noble metals (Pt, Ru) resulted in a higher degree of catalyst redn., yet otherwise only had small effects. The double promotion of Co with Mn and Pt or Ru lead to a higher activity and activation energy in the case of Pt, and in both cases to a lower olefin selectivity compared to the catalyst promoted with manganese only. For a Mn promoted and an unpromoted Co/CNT catalyst, an increase of total pressure from 1 to 30 bar leads to a lower methane selectivity and higher chain growth probability. The rate of syngas consumption was similar at 30 bar compared to 1 bar.
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    Tucker, C. L.; Ragoo, Y.; Mathe, S.; Macheli, L.; Bordoloi, A.; Rocha, T. C. R.; Govender, S.; Kooyman, P. J.; Van Steen, E. Manganese promotion of a cobalt Fischer–Tropsch catalyst to improve operation at high conversion. J. Catal. 2022, 411, 97– 108,  DOI: 10.1016/j.jcat.2022.05.006
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    Vada, S.; Hoff, A.; Ådnanes, E.; Schanke, D.; Holmen, A. Fischer–Tropsch synthesis on supported cobalt catalysts promoted by platinum and rhenium. Top. Catal. 1995, 2, 155– 162,  DOI: 10.1007/BF01491963
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    Fischer-Tropsch synthesis on supported cobalt catalysts promoted by platinum and rhenium
    Vada, S.; Hoff, A.; Aadnanes, E.; Schanke, D.; Holmen, A.
    Topics in Catalysis (1995), 2 (1-4, Fischer-Tropsch and Methanol Synthesis), 155-62CODEN: TOCAFI; ISSN:1022-5528. (Baltzer)
    An investigation of the CO hydrogenation on Pt- or Re-promoted 8.7 wt.% Co/Al2O3 (1.0 wt.% Pt or 1.0 wt.% Re) has been carried out at two different conditions: 473 K, 5 bar, H2/CO = 2 and 493 K, 1 bar, H2/CO = 7.3. The addn. of Pt or Re significantly increased the CO hydrogenation rate (based on wt. of Co), but the selectivity was not changed. The results show that the obsd. increases in the reaction rates are caused by increased reducibility and increased no. of surface exposed Co-atoms. Steady-state isotopic transient kinetic anal. (SSITKA) with carbon tracing was used to decouple the effects of the concn. of active surface intermediates and the av. site reactivity of intermediates during steady-state CO hydrogenation. The SSITKA results show that the concn. of active surface intermediates leading to CH4 increased as a result of the addn. of a noble metal promoter. However, the av. site activity was not significantly affected upon Re or Pt addn.
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    Bertole, C. J.; Mims, C. A.; Kiss, G. Support and rhenium effects on the intrinsic site activity and methane selectivity of cobalt Fischer–Tropsch catalysts. J. Catal. 2004, 221, 191– 203,  DOI: 10.1016/j.jcat.2003.08.006
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    Ma, W.; Jacobs, G.; Keogh, R. A.; Bukur, D. B.; Davis, B. H. Fischer–Tropsch synthesis: Effect of Pd, Pt, Re, and Ru noble metal promoters on the activity and selectivity of a 25%Co/Al2O3 catalyst. Appl. Catal. Gen. 2012, 437–438, 1– 9,  DOI: 10.1016/j.apcata.2012.05.037
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    Fischer-Tropsch synthesis: Effect of Pd, Pt, Re, and Ru noble metal promoters on the activity and selectivity of a 25%Co/Al2O3 catalyst
    Ma, Wenping; Jacobs, Gary; Keogh, Robert A.; Bukur, Dragomir B.; Davis, Burtron H.
    Applied Catalysis, A: General (2012), 437-438 (), 1-9CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    The effect of noble metal promoters (at. ratio of promoter to Co = 1/170) on the activity and selectivity of a 25%Co/Al2O3 catalyst was studied at a similar CO conversion level of 50% at 493 K, 2.2 MPa and H2/CO = 2.1 using a 1-L continuously stirred tank reactor (CSTR). All promoted catalysts exhibited markedly higher initial CO conversion rates on a per g catalyst basis than the unpromoted one, which was ascribed to increased Co site d. when the promoters were present. This is because the Re, Ru, Pt and unpromoted Co catalysts give essentially the same initial Co TOF values of 0.092-0.105/s (based on hydrogen-chemisorption). However, the initial Co TOF value for the Pd-Co catalyst was ∼40% lower, which might be caused by Pd atoms segregating on the Co surface and partially blocking Co sites. At 50% CO conversion, Re and Ru promoters decreased CH4 selectivity and increased C5+ selectivity by nearly the same extent, whereas the opposite effect was obsd. for Pd and Pt promoters. The Re and Ru promoters had less of an impact on C2-C4 olefin selectivity (7.5-60%), but suppressed the secondary reaction of 1-C4 olefin (from 14.3 to 9%) compared to the unpromoted one; however, the addn. of Pd and Pt promoters resulted in lower olefin selectivity (4.4-55%) but higher 2-C4 olefin selectivity (14.3 to 27-31%). Pt promotion had a negligible effect on C4 olefin isomerization. The selectivity results were reproducible. Both Pt and Pd promoters slightly increased WGS activity, whereas Re and Ru promoters had a negligible effect. The Pd and Pt promoters were obsd. to slightly enhance oxygenate formation, while Re and Ru slightly decreased it.
  26. 26
    Moradi, G. R.; Basir, M. M.; Taeb, A.; Kiennemann, A. Promotion of Co/SiO2 Fischer–Tropsch catalysts with zirconium. Catal. Commun. 2003, 4, 27– 32,  DOI: 10.1016/S1566-7367(02)00243-1
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    Promotion of Co/SiO2 Fischer-Tropsch catalysts with zirconium
    Moradi, G. R.; Basir, M. M.; Taeb, A.; Kiennemann, A.
    Catalysis Communications (2003), 4 (1), 27-32CODEN: CCAOAC; ISSN:1566-7367. (Elsevier Science B.V.)
    The effect of zirconia addn. at various loading ratios on the performance of 10% Co/SiO2 catalysts for the so-called reaction of Fischer-Tropsch synthesis was studied. The catalysts were prepd. through a new pseudo sol-gel method which permits an uniform distribution of the incorporated components and a low deviation from theor. compn. By increasing zirconia, Co-SiO2 interaction decreases and is replaced by Co-Zr interaction which favors redn. of the catalysts at lower temps. The activity and selectivity toward higher hydrocarbons of the promoted catalysts increase with increasing zirconium loading ratios. No appreciable decrease in activity was obsd. when all catalysts were employed under H2/CO = 2 at 230° and 8 bar for 240 h.
  27. 27
    Johnson, G. R.; Bell, A. T. Role of ZrO2 in Promoting the Activity and Selectivity of Co-Based Fischer–Tropsch Synthesis Catalysts. ACS Catal. 2016, 6, 100– 114,  DOI: 10.1021/acscatal.5b02205
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    Role of ZrO2 in Promoting the Activity and Selectivity of Co-Based Fischer-Tropsch Synthesis Catalysts
    Johnson, Gregory R.; Bell, Alexis T.
    ACS Catalysis (2016), 6 (1), 100-114CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The effects of Zr promotion on the structure and performance of Co-based Fischer-Tropsch synthesis (FTS) catalysts were investigated. Inclusion of Zr in the catalysts was found to increase the FTS turnover frequency and the selectivity to C5+ hydrocarbons and to decrease the selectivity to methane under most operating conditions. These improvements to the catalytic performance are a function of Zr loading up to an at. ratio of Zr/Co = 1.0, above which the product selectivity is insensitive to higher concns. of the promoter. Characterization of the Co nanoparticles by different methods demonstrated that the optimal Zr loading corresponds to half monolayer coverage of the Co surface by the promoter. Measurements of the rate of FTS at different pressures and temps. established that the kinetics data for both the Zr-promoted and unpromoted catalysts are described by a two-parameter Langmuir-Hinshelwood expression. The parameters used to fit this rate law to the exptl. data indicate that the apparent rate coeff. and the CO adsorption const. for the Zr-promoted catalysts are higher than those for the unpromoted catalyst. Elemental mapping by STEM-EDS provided evidence that Zr is highly dispersed over the catalyst surface and has limited preference for assocn. with the Co nanoparticles. In situ x-ray absorption spectroscopy confirmed the absence of mixing between the Zr and Co in the nanoparticles. These results suggest that Zr exists as a partial layer of ZrO2 on the surface of the Co metal nanoparticles. Accordingly, Zr promotion effects originate from sites of enhanced activity at the interface between Co and ZrO2. The possibility that ZrO2 acts as a Lewis acid to assist in CO dissocn. as well as to increase the ratio of CO to H adsorbed on the catalyst surface is discussed.
  28. 28
    Piao, Y.; Jiang, Q.; Li, H.; Matsumoto, H.; Liang, J.; Liu, W.; Pham-Huu, C.; Liu, Y.; Wang, F. Identify Zr Promotion Effects in Atomic Scale for Co-Based Catalysts in Fischer–Tropsch Synthesis. ACS Catal. 2020, 10, 7894– 7906,  DOI: 10.1021/acscatal.0c01874
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    Identify Zr Promotion Effects in Atomic Scale for Co-Based Catalysts in Fischer-Tropsch Synthesis
    Piao, Yuang; Jiang, Qian; Li, Hao; Matsumoto, Hiroaki; Liang, Jinsheng; Liu, Wei; Pham-Huu, Cuong; Liu, Yuefeng; Wang, Fei
    ACS Catalysis (2020), 10 (14), 7894-7906CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Introducing promoters on cobalt-based catalysts for Fischer-Tropsch synthesis (FTS) have been found efficient for adjusting their performance in converting syngas into long-chain hydrocarbons. Details of the promotion mechanism established on at. precise identification upon the active sites structure as well as the electronic status is seldomly reported yet. In the present work, we report the direct identification of valence status and coordination configuration of ZrO2 promoter additive over the cobalt-based FTS catalysts under a wide range of Zr/Co molar ratios from 0.12 to 1.5. Evidences from multiple technologies, including in situ/ex situ at. resoln. STEM imaging, EDS elemental mapping, electron energy loss spectroscopy (EELS), as well as physicochem. analyses (in situ XRD, CO chemisorption, CO temp.-programmed surface reaction, etc.) disclose that the ZrO2 promoter presents as single-site dispersion on the surface of Co nanoparticles (NPs) and the SiC support at low content, plays as the real active site to promote CO dissocn. at the Co-ZrO2 interface. While at high ZrO2 content (Zr/Co molar ratio of 1.0), Zr species on the SiC support turns to nucleate to form an amorphous coating, whereas those on the Co NPs surface maintain monodispersion. When further increasing the ZrO2 content (Zr/Co molar ratio up to 1.5), cobalt NPs start to be encapsulated by ZrO2 coating, leading to the decline of FTS activity. The in situ EELS anal. and d. functional theory calcns. disclose that the Zr atom tends to bind at the Co NPs surface rather than embed into the lattice meanwhile a charge transfer from Zr species to Co NPs occurs, which facilitates the stronger interaction between Zr and cobalt and thus enhances the adsorption with the H2 mol. as well as the CO dissocn. It thus offers enhanced catalytic activity and long-chain hydrocarbon selectivity during the FTS reaction.
  29. 29
    Martínez, A.; López, C.; Márquez, F.; Díaz, I. Fischer–Tropsch synthesis of hydrocarbons over mesoporous Co/SBA-15 catalysts: the influence of metal loading, cobalt precursor, and promoters. J. Catal. 2003, 220, 486– 499,  DOI: 10.1016/S0021-9517(03)00289-6
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    Dinse, A.; Aigner, M.; Ulbrich, M.; Johnson, G. R.; Bell, A. T. Effects of Mn promotion on the activity and selectivity of Co/SiO2 for Fischer–Tropsch Synthesis. J. Catal. 2012, 288, 104– 114,  DOI: 10.1016/j.jcat.2012.01.008
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    Effects of Mn promotion on the activity and selectivity of Co/SiO2 for Fischer-Tropsch Synthesis
    Dinse, Arne; Aigner, Max; Ulbrich, Markus; Johnson, Gregory R.; Bell, Alexis T.
    Journal of Catalysis (2012), 288 (), 104-114CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    An investigation has been carried out of the effects of Mn promotion on the activity and product selectivity of Co/SiO2 for Fischer-Tropsch Synthesis (FTS). All expts. were conducted with an H2/CO feed ratio of 2.0 at either 1 atm or 10 atm and a temp. of 493 K. At 1 atm, the rate of CO consumption decreased with CO conversion, whereas the selectivities to all products were unaffected. However, the olefin to paraffin (O/P) ratio of the C2-4 fraction decreased with increasing CO conversion due to increased hydrogenation of olefins with increasing space time. Mn promotion increased the rate of CO consumption at Mn/Co ratios of 0.05, decreased the selectivity to methane, and increased the selectivity to C5+ products, but had no effect on the selectivity to C2-4 products. Raising the pressure to 10 atm increased the CO consumption rates for both unpromoted and Mn-promoted Co/SiO2, but the rate of reaction was lower for the Mn-promoted catalyst due to a higher level of CO inhibition. CO conversion at 10 atm had no effect on the rate of CO consumption for Co/SiO2 and caused a slight increase in the rate for Mn-promoted Co/SiO2. The intrinsic O/P ratio was higher at 10 atm than at 1 atm. In contrast to what was obsd. at 1 atm, at 10 atm, C2-4 of the C2 -C4 fraction incorporated into growing hydrocarbon chain, leading to a decrease in C1-4 selectivities and an increase in C5+ selectivities with increasing CO conversion. The obsd. effects of Mn promotion on catalyst activity and product selectivity are discussed in terms of the mechanisms for CO hydrogenation and hydrocarbon chain growth.
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    Paterson, J.; Peacock, M.; Purves, R.; Partington, R.; Sullivan, K.; Sunley, G.; Wilson, J. Manipulation of Fischer–Tropsch Synthesis for Production of Higher Alcohols Using Manganese Promoters. ChemCatChem 2018, 10, 5154– 5163,  DOI: 10.1002/cctc.201800883
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    Paterson, J.; Partington, R.; Peacock, M.; Sullivan, K.; Wilson, J.; Xu, Z. Elucidating the Role of Bifunctional Cobalt-Manganese Catalyst Interactions for Higher Alcohol Synthesis. Eur. J. Inorg. Chem. 2020, 2020, 2312– 2324,  DOI: 10.1002/ejic.202000397
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    Pedersen, E. Ø.; Svenum, I.-H.; Blekkan, E. A. Mn promoted Co catalysts for Fischer–Tropsch production of light olefins – An experimental and theoretical study. J. Catal. 2018, 361, 23– 32,  DOI: 10.1016/j.jcat.2018.02.011
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    Mn promoted Co catalysts for Fischer-Tropsch production of light olefins - An experimental and theoretical study
    Pedersen, Eirik Oestbye; Svenum, Ingeborg-Helene; Blekkan, Edd A.
    Journal of Catalysis (2018), 361 (), 23-32CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Three different CoMn/γ-Al2O3 catalysts were prepd. by the incipient wetness impregnation route and compared to a Co/γ-Al2O3 catalyst. The effect of co-impregnation vs. sequential impregnation as well as the order of component addn. was investigated. All catalysts were characterized by TPR, H2-chemisorption, XRD and XPS and their activity and selectivity in the Fischer-Tropsch reaction was investigated. Complementary, self-consistent DFT calcns. were performed to further address the obsd. promotion effects. All Mn promoted catalysts displayed heightened intrinsic activity, heightened selectivity to light olefins and C5+ species and lowered selectivity to CH4 compared to Co. The promotion effects on selectivity and intrinsic activity were found to be independent on catalyst prepn. method. The catalysts undergo a restructuring during operation, in which an excess of Mn sats. the catalytically relevant sites causing the similar behavior. The Co-specific activity differed between the Mn promoted catalysts. This was attributed to varying degrees of Mn incorporation in the Co3O4 particles, causing different degrees of redn. limiting the available metallic Co surface area. The DFT calcns. suggested that the binding energy for all investigated species increases on Co in the presence of Mn, facilitating CO dissocn. which can explain the higher intrinsic activity. The affected selectivities for olefins, C5+ and CH4 can all be attributed to an inhibited hydrogenation activity demonstrated by the increased barriers for CH3 and CH4 formation.
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    Yang, K.; Zhou, G. Hydrogen evolution/spillover effect of single cobalt atom on anatase TiO2 from first-principles calculations. Appl. Surf. Sci. 2021, 536, 147831,  DOI: 10.1016/j.apsusc.2020.147831
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    Hydrogen evolution/spillover effect of single cobalt atom on anatase TiO2 from first-principles calculations
    Yang, Kang; Zhou, Gang
    Applied Surface Science (2021), 536 (), 147831CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    In spite of progress, there is a long way to go in the use of non-precious metals instead of precious metals as catalysts in chem. reactions. Here we report an anatase TiO2-supported single-atom (SA) Co system for hydrogen evolution and also study its hydrogen spillover effect using first-principles calcns. Two stable forms of SA Co on the anatase TiO2(101) surface, achieved by adsorption and substitution, induce different confinement effects. The SA Co in the interstices of the surface exhibits better hydrogen evolution activity than bulk counterpart. The hydrogen evolution reaction proceeds on the partially hydrogenated surface of Co1/TiO2, where SA Co and adjacent O are active sites. The substitution of Co for Ti promotes the formation of surface O vacancies and the redn. of Ti4+ to Ti3+ in the H2 atmosphere, indicative of an enhanced hydrogen spillover effect. The possible catalytic mechanisms of SA catalysts in the two forms are proposed by the calcn. of reaction kinetics. The present work highlights the complexity and diversity of the confinement effect of transition metal SA in oxides, and broadens their applications in catalysis and of defect engineering.
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    Kalantari, L.; Tran, F.; Blaha, P. Density Functional Theory Study of Metal and Metal-Oxide Nucleation and Growth on the Anatase TiO2(101) Surface. Computation 2021, 9, 125,  DOI: 10.3390/computation9110125
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    Liu, J.J. Advanced Electron Microscopy of Metal–Support Interactions in Supported Metal Catalysts. ChemCatChem 2011, 3, 934– 948,  DOI: 10.1002/cctc.201100090
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    Advanced electron microscopy of metal-support interactions in supported metal catalysts
    Liu, Jingyue
    ChemCatChem (2011), 3 (6), 934-948CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Electron microscopy and its assocd. techniques have contributed significantly to the characterization of heterogeneous catalysts, esp. supported metal catalysts, and can provide nano- or at.-scale information on the structure, morphol., compn., and electronic state of the area of interest. With the recent advancement in aberration corrections to achieve an image resoln. below 0.1 nm and the rapid development of in situ techniques, advanced electron microscopy is poised to probe fundamental questions of heterogeneous catalysis and catalysts. Understanding the nature of metal-support interactions becomes crit. if we want to achieve the goal of designing and fabricating nanoarchitectured catalysts with desired performances. To reliably probe the fundamental mechanisms of charge transfer, which is the core of catalysis, between individual nanoscale components and under a realistic gas environment and temp. is still a formidable challenge. This review discusses the recent contribution of advanced electron microscopy to the study of metal-support interactions, as well as the challenges and opportunities of applying aberration-cor. electron microscopy techniques to the investigation of at.-scale structures of complex heterogeneous catalysts.
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    Yaguchi, T.; Suzuki, M.; Watabe, A.; Nagakubo, Y.; Ueda, K.; Kamino, T. Development of a high temperature-atmospheric pressure environmental cell for high-resolution TEM. J. Electron Microsc. (Tokyo) 2011, 60, 217– 225,  DOI: 10.1093/jmicro/dfr011
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    Development of a high temperature-atmospheric pressure environmental cell for high-resolution TEM
    Yaguchi, Toshie; Suzuki, Makoto; Watabe, Akira; Nagakubo, Yasuhira; Ueda, Kota; Kamino, Takeo
    Journal of Electron Microscopy (2011), 60 (3), 217-225CODEN: JELJA7; ISSN:0022-0744. (Oxford University Press)
    An environmental cell for high-temp., high-resoln. transmission electron microscopy of nanomaterials in near atm. pressures is developed. The developed environmental cell is a side-entry type with built-in specimen-heating element and micropressure gauge. The relationship between the cell condition and the quality of the transmission electron microscopic (TEM) image and the diffraction pattern was examd. exptl. and theor. By using the cell consisting of two electron-transparent silicon nitride thin films as the window material, the gas pressure inside the environmental cell is continuously controlled from 10-5 Pa to the atm. pressure in a high-vacuum TEM specimen chamber. TEM image resolns. of 0.23 and 0.31 nm were obtained using 15-nm-thick silicon nitride film windows with the pressure inside the cell being around 5 × 10-5 and 1 × 104 Pa, resp.
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    Allard, L. F.; Overbury, S. H.; Bigelow, W. C.; Katz, M. B.; Nackashi, D. P.; Damiano, J. Novel MEMS-Based Gas-Cell/Heating Specimen Holder Provides Advanced Imaging Capabilities for In Situ Reaction Studies. Microsc. Microanal. 2012, 18, 656– 666,  DOI: 10.1017/S1431927612001249
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    Novel MEMS-Based Gas-Cell/Heating Specimen Holder Provides Advanced Imaging Capabilities for In Situ Reaction Studies
    Allard, Lawrence F.; Overbury, Steven H.; Bigelow, Wilbur C.; Katz, Michael B.; Nackashi, David P.; Damiano, John
    Microscopy and Microanalysis (2012), 18 (4), 656-666CODEN: MIMIF7; ISSN:1431-9276. (Cambridge University Press)
    In prior research, specimen holders that employ a novel MEMS-based heating technol. (AduroTM) provided by Protochips Inc. (Raleigh, NC, USA) have been shown to permit sub-Ångstroem imaging at elevated temps. up to 1,000°C during in situ heating expts. in modern aberration-cor. electron microscopes. The Aduro heating devices permit precise control of temp. and have the unique feature of providing both heating and cooling rates of 106°C/s. In the present work, we describe the recent development of a new specimen holder that incorporates the Aduro heating device into a "closed-cell" configuration, designed to function within the narrow (2 mm) objective lens pole piece gap of an aberration-cor. JEOL 2200FS STEM/TEM, and capable of exposing specimens to gases at pressures up to 1 atm. We show the early results of tests of this specimen holder demonstrating imaging at elevated temps. and at pressures up to a full atm., while retaining the at. resoln. performance of the microscope in high-angle annular dark-field and bright-field imaging modes.
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    Ciobîcă, I.; Van Santen, R. A.; Van Berge, P. J.; Van De Loosdrecht, J. Adsorbate induced reconstruction of cobalt surfaces. Surf. Sci. 2008, 602, 17– 27,  DOI: 10.1016/j.susc.2007.09.060
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    Adsorbate induced reconstruction of cobalt surfaces
    Ciobica, I. M.; van Santen, R. A.; van Berge, P. J.; van de Loosdrecht, J.
    Surface Science (2008), 602 (1), 17-27CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    Coverage dependence adsorption of intermediates typical for syngas conversion is studied theor. on FCC-cobalt surfaces. The structure is relevant for cobalt particles active in this reaction. Emphasis on the anal. is on the thermodn. of surface reconstruction as a function of surface adsorbate and surface coverage. An important result is the finding that only adsorbed carbon induced reconstruction of FCC-Co(1 1 1) to FCC-Co(1 0 0) as well as the clock reconstruction for the two surfaces.
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    Hansen, P. L.; Wagner, J. B.; Helveg, S.; Rostrup-Nielsen, J. R.; Clausen, B. S.; Topsøe, H. Atom-Resolved Imaging of Dynamic Shape Changes in Supported Copper Nanocrystals. Science 2002, 295, 2053– 2055,  DOI: 10.1126/science.1069325
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    Atom-resolved imaging of dynamic shape changes in supported copper nanocrystals
    Hansen, Poul L.; Wagner, Jakob B.; Helveg, Stig; Rostrup-Nielsen, Jens R.; Clausen, Bjerne S.; Topsoe, Henrik
    Science (Washington, DC, United States) (2002), 295 (5562), 2053-2055CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    In situ TEM is used to obtain atom-resolved images of Cu nanocrystals on different supports. These are catalysts for MeOH synthesis and hydrocarbon conversion processes for fuel cells. The nanocrystals undergo dynamic reversible shape changes in response to changes in the gaseous environment. For Zn oxide-supported samples, the changes are caused both by adsorbate-induced changes in surface energies and by changes in the interfacial energy. For Cu nanocrystals supported on SiO2, the support has negligible influence on the structure. Nanoparticle dynamics must be included in the description of catalytic and other properties of nanomaterials. In situ microscopy offers possibilities for obtaining the relevant at.-scale insight.
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    Creemer, J. F.; Helveg, S.; Kooyman, P. J.; Molenbroek, A. M.; Zandbergen, H. W.; Sarro, P. M. A MEMS Reactor for Atomic-Scale Microscopy of Nanomaterials Under Industrially Relevant Conditions. J. Microelectromechanical Syst. 2010, 19, 254– 264,  DOI: 10.1109/JMEMS.2010.2041190
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    A MEMS reactor for atomic-scale microscopy of nanomaterials under industrially relevant conditions
    Creemer, J. Fredrik; Helveg, Stig; Kooyman, Patricia J.; Molenbroek, Alfons M.; Zandbergen, Henny W.; Sarro, Pasqualina M.
    Journal of Microelectromechanical Systems (2010), 19 (2), 254-264CODEN: JMIYET; ISSN:1057-7157. (Institute of Electrical and Electronics Engineers)
    We present a microelectromech. systems (MEMS) nanoreactor that enables high-resoln. transmission electron microscopy (TEM) (HRTEM) of nanostructured materials with at.-scale resoln. during exposure to reactive gases at 1 atm of pressure. This pressure exceeds that of existing HRTEM systems by a factor of 100, thereby entering a pressure range that is relevant to industrial purposes. The nanoreactor integrates a shallow flow channel (35 μm high) with a microheater and with an array of electron transparent windows of silicon nitride. The windows are only 10 nm thick but are mech. robust. The heater has the geometry of a microhotplate and is made of Pt embedded in a silicon nitride membrane. To interface the nanoreactor, a dedicated TEM specimen holder has been developed. The performance is demonstrated by the live formation of Cu nanoparticles in a catalyst for the prodn. of methanol. At 120 kPa and for temps. of up to 500 °C, the formation of these nanoparticles can be obsd. clearly and with an exceptionally low thermal drift. HRTEM images of the nanoparticles show at. lattice fringes with spacings down to 0.18 nm.
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    Dembélé, K.; Bahri, M.; Hirlimann, C.; Moldovan, S.; Berliet, A.; Maury, S.; Gay, A.; Ersen, O. Operando Electron Microscopy Study of Cobalt-based Fischer–Tropsch Nanocatalysts. ChemCatChem 2021, 13, 1920– 1930,  DOI: 10.1002/cctc.202001074
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    Operando Electron Microscopy Study of Cobalt-based Fischer-Tropsch Nanocatalysts
    Dembele, Kassioge; Bahri, Mounib; Hirlimann, Charles; Moldovan, Simona; Berliet, Adrien; Maury, Sylvie; Gay, Anne-Sophie; Ersen, Ovidiu
    ChemCatChem (2021), 13 (8), 1920-1930CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
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  1. Jay M. Pritchard, Matthew Lindley, Danial Farooq, Urvashi Vyas, Sarah J. Haigh, James Paterson, Mark Peacock, Andrew M. Beale. Examining the effect of manganese distribution on alcohol production in CoMn/TiO 2 FTS catalysts. RSC Sustainability 2025, 3 (3) , 1376-1387. https://doi.org/10.1039/D4SU00746H
  2. Kende Attila Béres, Zoltán Homonnay, László Kótai. Review on Synthesis and Catalytic Properties of Cobalt Manganese Oxide Spinels (CoxMn3−xO4, 0 < x < 3). Catalysts 2025, 15 (1) , 82. https://doi.org/10.3390/catal15010082
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  • Abstract

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    Figure 1

    Figure 1. Catalytic testing of the 10 wt % Co/TiO2 catalyst with and without 5 wt % Mn. (a) The addition of Mn induces a selectivity shift from long-chain (C5+) products to those with shorter chains (C2–C4) as well as a slight increase in methane production. (b) The yield in alcohol selectivity for the C5+ products are significantly enhanced with Mn addition.

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    Figure 2

    Figure 2. Ex situ structural and chemical characterization of the TiO2-supported Co/Mn catalysts after calcination (prior to activation). (a,b) HAADF-STEM imaging and STEM-EDS elemental mapping of the (a) Co-0Mn and (b) Co-5Mn catalysts. (c) Corresponding EDS spectra from the imaged regions in (a,b). (d,e) High-resolution HAADF-STEM imaging of the supported Co clusters in (d) Co-0Mn and (e) Co-5Mn. Fourier transforms of the images are shown as insets. Co-0Mn is indexed to the Co3O4 spinel viewed along [111], while Co-5Mn is indexed to CoO viewed along [110]. (f) XRD analysis highlighting the presence of Co3O4 in the Co-0Mn catalyst and the disappearance of any discernible cobalt oxide structure in the Co-5Mn catalyst. (g–i) Summed STEM-EELS edges from the (g) Co-0Mn and (h,i) Co-5Mn catalysts. (j,k) XPS spectra of the cobalt peaks from the (j) Co-0Mn and (k) Co-5Mn catalysts. The appropriate experimentally measured spectra taken from standard reference materials (Co3O4 spinel, CoO, metallic cobalt, Mn3O4 spinel, Mn2O3, MnO2, and MnO) are included in (g–k) for comparison.

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    Figure 3

    Figure 3. In situ structural and chemical characterization of the TiO2-supported Co/Mn catalyst dispersion during activation. (a,b) HAADF-STEM imaging and STEM-EDS elemental mapping of the (a) Co-0Mn and (b) Co-5Mn catalysts. Temperatures are shown inset on the corresponding HAADF images. (c,d) NAP-XPS analysis of the in situ activation of (c) Co-0Mn and (d) Co-5Mn in hydrogen. The presence of 5 wt % Mn induces a transition from a three-stage reduction to a two-stage process, where the presence of Co3+ indicates a spinel structure, Co3O4, which transitions to Co2+, indicative of CoO rock salt, in the promoted (as-calcined) catalyst and lowers the Co reduction temperature during activation. The reference data for the Co standards are presented in Figures 2i,j and S7d,e. (e,f) Histograms illustrating the Co particle size distributions for both catalysts at (e) 250 and (f) 350 °C (see Section S1.5 and Figure S16 for details).

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    Figure 4

    Figure 4. Most stable configurations for diatomic Co and Mn clusters. (a) Plot of the mixing energy, Emix against diatomic composition for CoxMn2–x clusters. Emix of the most stable mixed CoMn cluster is −0.14 eV, i.e., more stable than the homonuclear diatomic clusters. The most stable configuration is indicated by the red dots, while other configurations are shown in blue. (b–d) Illustrations demonstrating the most stable configuration for (b) Mn2, (c) CoMn, and (d) Co2 on the anatase (101) TiO2 surface. Colors for Co, Mn, Ti, and O in the schematics are yellow, purple, blue, and red, respectively.

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      Bezemer, G. Leendert; Bitter, Johannes H.; Kuipers, Herman P. C. E.; Oosterbeek, Heiko; Holewijn, Johannes E.; Xu, Xiaoding; Kapteijn, Freek; van Dillen, A. Jos; de Jong, Krijn P.
      Journal of the American Chemical Society (2006), 128 (12), 3956-3964CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis was investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ redn. treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 × 10-3 to 1.4 × 10-3/s, while the C5+ selectivity decreased from 85 to 51 wt. % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by exptl. evidence from x-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination no. under reaction conditions, which points to reconstruction of the cobalt particles. The cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.
    10. 10
      Den Breejen, J. P.; Radstake, P. B.; Bezemer, G. L.; Bitter, J. H.; Frøseth, V.; Holmen, A.; De Jong, K. P. On the Origin of the Cobalt Particle Size Effects in Fischer–Tropsch Catalysis. J. Am. Chem. Soc. 2009, 131, 7197– 7203,  DOI: 10.1021/ja901006x
      10
      On the Origin of the Cobalt Particle Size Effects in Fischer-Tropsch Catalysis
      den Breejen, J. P.; Radstake, P. B.; Bezemer, G. L.; Bitter, J. H.; Froeseth, V.; Holmen, A.; de Jong, K. P.
      Journal of the American Chemical Society (2009), 131 (20), 7197-7203CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The effects of metal particle size in catalysis are of prime scientific and industrial importance and call for a better understanding. In this paper the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis was studied. Steady-State Isotopic Transient Kinetic Anal. (SSITKA) was applied to provide surface residence times and coverages of reaction intermediates as a function of Co particle size (2.6-16 nm). For carbon nanofiber supported cobalt catalysts at 210 °C and H2/CO = 10 vol./vol., it appeared that the surface residence times of reversibly bonded CHx and OHx intermediates increased, whereas that of CO decreased for small (<6 nm) Co particles. A higher coverage of irreversibly bonded CO was found for small Co particles that was ascribed to a larger fraction of low-coordinated surface sites. The coverages and residence times obtained from SSITKA were used to describe the surface-specific activity (TOF) quant. and the CH4 selectivity qual. as a function of Co particle size for the FT reaction (220 °C, H2/CO = 2). The lower TOF of Co particles <6 nm is caused by both blocking of edge/corner sites and a lower intrinsic activity at the small terraces. The higher methane selectivity of small Co particles is mainly brought about by their higher hydrogen coverages.
    11. 11
      Moodley, D.; Claeys, M.; Van Steen, E.; Van Helden, P.; Kistamurthy, D.; Weststrate, K.-J.; Niemantsverdriet, H.; Saib, A.; Erasmus, W.; Van De Loosdrecht, J. Sintering of cobalt during FTS: Insights from industrial and model systems. Catal. Today 2020, 342, 59– 70,  DOI: 10.1016/j.cattod.2019.03.059
      11
      Sintering of cobalt during FTS: Insights from industrial and model systems
      Moodley, Denzil; Claeys, Michael; van Steen, Eric; van Helden, Pieter; Kistamurthy, Deshen; Weststrate, Kees-Jan; Niemantsverdriet, Hans; Saib, Abdool; Erasmus, Willem; van de Loosdrecht, Jan
      Catalysis Today (2020), 342 (), 59-70CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      A review. Sintering of supported cobalt catalysts is an important deactivation mechanism at play during realistic Fischer-Tropsch synthesis conditions. The traditional ways of studying the mechanism of sintering by looking at cobalt particle size distributions derived from transmission electron microscope images on porous supported catalysts, does have its drawbacks. In this mini-review, we show collaborative work between Sasol and its academic partners at the University of Cape Town and Eindhoven University of Technol., in which a two-pronged approach was used to study the topic of sintering. This included the use of model systems (model catalysts and theory) as well as in-situ monitoring of real (industrial) systems for understanding this practical problem in industry. We also show with new data, that the understanding generated, can be used in the mitigation of sintering by using either process conditions, catalyst design or catalyst regeneration.
    12. 12
      Storsater, S.; Totdal, B.; Walmsley, J.; Tanem, B.; Holmen, A. Characterization of alumina-silica-and titania-supported cobalt Fischer–Tropsch catalysts. J. Catal. 2005, 236, 139– 152,  DOI: 10.1016/j.jcat.2005.09.021
      There is no corresponding record for this reference.
    13. 13
      Gholami, Z.; Tišler, Z.; Rubáš, V. Recent advances in Fischer–Tropsch synthesis using cobalt-based catalysts: a review on supports, promoters, and reactors. Catal. Rev. 2021, 63, 512– 595,  DOI: 10.1080/01614940.2020.1762367
      13
      Recent advances in Fischer-Tropsch synthesis using cobalt-based catalysts: a review on supports, promoters, and reactors
      Gholami, Zahra; Tisler, Zdenek; Rubas, Vlastimil
      Catalysis Reviews: Science and Engineering (2021), 63 (3), 512-595CODEN: CRSEC9; ISSN:0161-4940. (Taylor & Francis, Inc.)
      Fischer-Tropsch (FT) process is a promising method for producing liq. fuels and other valuable chems. through CO hydrogenation. The catalyst activity and product selectivity can be strongly affected by different parameters such as support and promoters. The physicochem. and textural properties of the support affect the metal-support interaction, crystallite size, metal dispersion, mass transfer of reactants/products, mech. strength, and thermal stability of the catalyst. Promoters can also be used as structural, textural, electronic modifier, stabilizers, and catalyst-poison-resistant, which can improve the catalytic performance. According to the parameters mentioned above, this paper reviews the brief history of the FT process, the effect of different supports and promoters on the catalytic performance of cobalt-based catalysts. In addn. to the catalyst properties, the reactor must also be designed appropriately to handle the heat of this highly exothermic reaction. The reactor types have also been reviewed and compared as a crucial part of the catalytic reactions.
    14. 14
      Khodakov, A. Y.; Chu, W.; Fongarland, P. Advances in the Development of Novel Cobalt Fischer–Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels. Chem. Rev. 2007, 107, 1692– 1744,  DOI: 10.1021/cr050972v
      14
      Advances in the Development of Novel Cobalt Fischer-Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels
      Khodakov, Andrei Y.; Chu, Wei; Fongarland, Pascal
      Chemical Reviews (Washington, DC, United States) (2007), 107 (5), 1692-1744CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Advances in the development of Novel Cobalt Fischer-Tropsch Catalysts for synthesis of long-chain Hydrocarbons and Clean Fuels are reviewed. Synthesis methods of catalysts, supports, and promoters are included, as well as pretreatments. Optical and magnetic characterization methods were reviewed. Various aspects of the Fischer-Tropsch process reactors and operation parameters and limitations were also included.
    15. 15
      Xu, L.; Ma, Y.; Zhang, Y.; Jiang, Z.; Huang, W. Direct Evidence for the Interfacial Oxidation of CO with Hydroxyls Catalyzed by Pt/Oxide Nanocatalysts. J. Am. Chem. Soc. 2009, 131, 16366– 16367,  DOI: 10.1021/ja908081s
      15
      Direct Evidence for the Interfacial Oxidation of CO with Hydroxyls Catalyzed by Pt/Oxide Nanocatalysts
      Xu, Lingshun; Ma, Yunsheng; Zhang, Yulin; Jiang, Zhiquan; Huang, Weixin
      Journal of the American Chemical Society (2009), 131 (45), 16366-16367CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      By rational design of an FeO(111)/Pt(111) inverse model catalyst and control expts., this work reports, for the first time, direct exptl. evidence for the interfacial COads + OHads reaction, producing CO2 at the Pt-oxide interface at low temps. and providing deep insight into the reaction mechanism and active site of the important low-temp., water-gas shift and preferential CO oxidn. reactions catalyzed by Pt/oxide nano-catalysts at the mol. level.
    16. 16
      Rodriguez, J. A.; Liu, P.; Stacchiola, D. J.; Senanayake, S. D.; White, M. G.; Chen, J. G. Hydrogenation of CO2 to Methanol: Importance of Metal–Oxide and Metal–Carbide Interfaces in the Activation of CO2. ACS Catal. 2015, 5, 6696– 6706,  DOI: 10.1021/acscatal.5b01755
      16
      Hydrogenation of CO2 to Methanol: Importance of Metal-Oxide and Metal-Carbide Interfaces in the Activation of CO2
      Rodriguez, Jose A.; Liu, Ping; Stacchiola, Dario J.; Senanayake, Sanjaya D.; White, Michael G.; Chen, Jingguang G.
      ACS Catalysis (2015), 5 (11), 6696-6706CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A review. The high thermochem. stability of CO2 makes it very difficult to achieve the catalytic conversion of the mol. into alcs. or other hydrocarbon compds., which can be used as fuels or the starting point for the generation of fine chems. Pure metals and bimetallic systems used for the CO2 → CH3OH conversion usually bind CO2 too weakly and, thus, show low catalytic activity. Here, a series of recent studies are discussed that illustrate the advantages of metal-oxide and metal-carbide interfaces when aiming at the conversion of CO2 into methanol. CeOx/Cu(111), Cu/CeOx/TiO2(110), and Au/CeOx/TiO2(110) exhibit an activity for the CO2 → CH3OH conversion that is 2-3 orders of magnitude higher than that of a benchmark Cu(111) catalyst. In the Cu-ceria and Au-ceria interfaces, the multifunctional combination of metal and oxide centers leads to complementary chem. properties that open active reaction pathways for methanol synthesis. Efficient catalysts are also generated after depositing Cu and Au on TiC(001). In these cases, strong metal-support interactions modify the electronic properties of the admetals and make them active for the binding of CO2 and its subsequent transformation into CH3OH at the metal-carbide interfaces.
    17. 17
      Bertella, F.; Concepción, P.; Martínez, A. The impact of support surface area on the SMSI decoration effect and catalytic performance for Fischer–Tropsch synthesis of Co-Ru/TiO2-anatase catalysts. Catal. Today 2017, 296, 170– 180,  DOI: 10.1016/j.cattod.2017.05.001
      There is no corresponding record for this reference.
    18. 18
      Hernández Mejía, C.; Van Deelen, T. W.; De Jong, K. P. Activity enhancement of cobalt catalysts by tuning metal-support interactions. Nat. Commun. 2018, 9, 4459,  DOI: 10.1038/s41467-018-06903-w
      18
      Activity enhancement of cobalt catalysts by tuning metal-support interactions
      Hernandez Mejia Carlos; van Deelen Tom W; de Jong Krijn P
      Nature communications (2018), 9 (1), 4459 ISSN:.
      Interactions between metal nanoparticles and support materials can strongly influence the performance of catalysts. In particular, reducible oxidic supports can form suboxides that can decorate metal nanoparticles and enhance catalytic performance or block active sites. Therefore, tuning this metal-support interaction is essential for catalyst design. Here, we investigate reduction-oxidation-reduction (ROR) treatments as a method to affect metal-support interactions and related catalytic performance. Controlled oxidation of pre-reduced cobalt on reducible (TiO2 and Nb2O5) and irreducible (α-Al2O3) supports leads to the formation of hollow cobalt oxide particles. The second reduction results in a twofold increase in cobalt surface area only on reducible oxides and proportionally enhances the cobalt-based catalytic activity during Fischer-Tropsch synthesis at industrially relevant conditions. Such activities are usually only obtained by noble metal promotion of cobalt catalysts. ROR proves an effective approach to tune the interaction between metallic nanoparticles and reducible oxidic supports, leading to improved catalytic performance.
    19. 19
      Kliewer, C. E.; Soled, S. L.; Kiss, G. Morphological transformations during Fischer–Tropsch synthesis on a titania-supported cobalt catalyst. Catal. Today 2019, 323, 233– 256,  DOI: 10.1016/j.cattod.2018.05.021
      19
      Morphological transformations during Fischer-Tropsch synthesis on a titania-supported cobalt catalyst
      Kliewer, C. E.; Soled, S. L.; Kiss, G.
      Catalysis Today (2019), 323 (), 233-256CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      Deactivation mechanisms for a rhenium-promoted titania-supported cobalt catalyst are investigated during Fischer-Tropsch (FT) synthesis. Bench-scale reactor tests, chemisorption studies, thermogravimetric analyses (TG), and TEM are used to probe environmental effects on catalytic activity. Bench-scale reactor studies show a steady decrease in activity with time. While a fraction of this loss can be recovered with a low temp. redn. (rejuvenation) inside the reactor, multiple data indicate this phenomenon is primarily attributable to water-induced oxidn. of small cobalt particles. Data from long-term FT runs indicate the presence of three non-rejuvenable deactivation mechanisms: metal agglomeration, strong metal-support interaction (SMSI), and mixed metal oxide formation. High conversion studies implicate byproduct water in the agglomeration process, and ex-situ TEM data conclusively reveals that growth occurs via a coalescence mechanism. Combined kinetic and chemisorption studies reveal that SMSI results from the gradual buildup of titania decoration on the surface of the active cobalt and is exacerbated with multiple rejuvenation cycles. TGA data indicate that mixed metal oxide formation occurs in long-duration, pilot plant runs. In all cases, the aggregate of reactor kinetic, chemisorption, TG, and TEM results point to chem.-assisted deactivation phenomena attributable to the byproduct water.
    20. 20
      Morales, F.; Desmit, E.; Degroot, F.; Visser, T.; Weckhuysen, B. Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt Fischer–Tropsch catalysts. J. Catal. 2007, 246, 91– 99,  DOI: 10.1016/j.jcat.2006.11.014
      20
      Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt Fischer-Tropsch catalysts
      Morales, Fernando; De Smit, Emiel; De Groot, Frank M. F.; Visser, Tom; Weckhuysen, Bert M.
      Journal of Catalysis (2007), 246 (1), 91-99CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Ltd.)
      The adsorption properties of manganese-promoted Co/TiO2 Fischer-Tropsch (FT) catalysts were investigated by diffuse reflectance IR spectroscopy (DRIFTS) using CO and H2 as probe mols. Manganese was found to be closely assocd. to the FT active Co0 sites at the surface of the catalysts. With increased MnO loading, CO preferentially bound linearly to surface metal sites. Manganese also decreased the extent of Co-TiO2 interactions, increasing the Co0 dispersion, resulting in higher H2 chemisorption uptake. Furthermore, with increasing MnO loading, FT catalytic tests at 1 bar and 220° revealed an increase in C5+ selectivity and olefinic products. These findings suggest that MnO species induce both structural and electronic promotion effects, resulting in higher metal dispersions and lower hydrogenation activity of the catalyst, ultimately enhancing the overall FT catalytic performance. The findings also suggest that MnO catalyzes the water-gas shift reaction, thereby changing the syngas feed compn. and affecting overall catalyst performance.
    21. 21
      Thiessen, J.; Rose, A.; Meyer, J.; Jess, A.; Curulla-Ferré, D. Effects of manganese and reduction promoters on carbon nanotube supported cobalt catalysts in Fischer–Tropsch synthesis. Microporous Mesoporous Mater. 2012, 164, 199– 206,  DOI: 10.1016/j.micromeso.2012.05.013
      21
      Effects of manganese and reduction promoters on carbon nanotube supported cobalt catalysts in Fischer-Tropsch synthesis
      Thiessen, J.; Rose, A.; Meyer, J.; Jess, A.; Curulla-Ferre, D.
      Microporous and Mesoporous Materials (2012), 164 (), 199-206CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier Inc.)
      Carbon nanotube (CNT) supported Co catalysts were promoted with manganese and noble metals (Pt, Ru) and tested in Fischer-Tropsch synthesis at temps. between 200 and 250° and pressures of 1 and 30 bar. A significant decrease in methane selectivity and with that an increase in C5+ selectivity was found due to promotion with manganese. In addn., the activity (syngas conversion) slightly increases and the olefin selectivity drastically increases. The promotion with noble metals (Pt, Ru) resulted in a higher degree of catalyst redn., yet otherwise only had small effects. The double promotion of Co with Mn and Pt or Ru lead to a higher activity and activation energy in the case of Pt, and in both cases to a lower olefin selectivity compared to the catalyst promoted with manganese only. For a Mn promoted and an unpromoted Co/CNT catalyst, an increase of total pressure from 1 to 30 bar leads to a lower methane selectivity and higher chain growth probability. The rate of syngas consumption was similar at 30 bar compared to 1 bar.
    22. 22
      Tucker, C. L.; Ragoo, Y.; Mathe, S.; Macheli, L.; Bordoloi, A.; Rocha, T. C. R.; Govender, S.; Kooyman, P. J.; Van Steen, E. Manganese promotion of a cobalt Fischer–Tropsch catalyst to improve operation at high conversion. J. Catal. 2022, 411, 97– 108,  DOI: 10.1016/j.jcat.2022.05.006
      There is no corresponding record for this reference.
    23. 23
      Vada, S.; Hoff, A.; Ådnanes, E.; Schanke, D.; Holmen, A. Fischer–Tropsch synthesis on supported cobalt catalysts promoted by platinum and rhenium. Top. Catal. 1995, 2, 155– 162,  DOI: 10.1007/BF01491963
      23
      Fischer-Tropsch synthesis on supported cobalt catalysts promoted by platinum and rhenium
      Vada, S.; Hoff, A.; Aadnanes, E.; Schanke, D.; Holmen, A.
      Topics in Catalysis (1995), 2 (1-4, Fischer-Tropsch and Methanol Synthesis), 155-62CODEN: TOCAFI; ISSN:1022-5528. (Baltzer)
      An investigation of the CO hydrogenation on Pt- or Re-promoted 8.7 wt.% Co/Al2O3 (1.0 wt.% Pt or 1.0 wt.% Re) has been carried out at two different conditions: 473 K, 5 bar, H2/CO = 2 and 493 K, 1 bar, H2/CO = 7.3. The addn. of Pt or Re significantly increased the CO hydrogenation rate (based on wt. of Co), but the selectivity was not changed. The results show that the obsd. increases in the reaction rates are caused by increased reducibility and increased no. of surface exposed Co-atoms. Steady-state isotopic transient kinetic anal. (SSITKA) with carbon tracing was used to decouple the effects of the concn. of active surface intermediates and the av. site reactivity of intermediates during steady-state CO hydrogenation. The SSITKA results show that the concn. of active surface intermediates leading to CH4 increased as a result of the addn. of a noble metal promoter. However, the av. site activity was not significantly affected upon Re or Pt addn.
    24. 24
      Bertole, C. J.; Mims, C. A.; Kiss, G. Support and rhenium effects on the intrinsic site activity and methane selectivity of cobalt Fischer–Tropsch catalysts. J. Catal. 2004, 221, 191– 203,  DOI: 10.1016/j.jcat.2003.08.006
      There is no corresponding record for this reference.
    25. 25
      Ma, W.; Jacobs, G.; Keogh, R. A.; Bukur, D. B.; Davis, B. H. Fischer–Tropsch synthesis: Effect of Pd, Pt, Re, and Ru noble metal promoters on the activity and selectivity of a 25%Co/Al2O3 catalyst. Appl. Catal. Gen. 2012, 437–438, 1– 9,  DOI: 10.1016/j.apcata.2012.05.037
      25
      Fischer-Tropsch synthesis: Effect of Pd, Pt, Re, and Ru noble metal promoters on the activity and selectivity of a 25%Co/Al2O3 catalyst
      Ma, Wenping; Jacobs, Gary; Keogh, Robert A.; Bukur, Dragomir B.; Davis, Burtron H.
      Applied Catalysis, A: General (2012), 437-438 (), 1-9CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      The effect of noble metal promoters (at. ratio of promoter to Co = 1/170) on the activity and selectivity of a 25%Co/Al2O3 catalyst was studied at a similar CO conversion level of 50% at 493 K, 2.2 MPa and H2/CO = 2.1 using a 1-L continuously stirred tank reactor (CSTR). All promoted catalysts exhibited markedly higher initial CO conversion rates on a per g catalyst basis than the unpromoted one, which was ascribed to increased Co site d. when the promoters were present. This is because the Re, Ru, Pt and unpromoted Co catalysts give essentially the same initial Co TOF values of 0.092-0.105/s (based on hydrogen-chemisorption). However, the initial Co TOF value for the Pd-Co catalyst was ∼40% lower, which might be caused by Pd atoms segregating on the Co surface and partially blocking Co sites. At 50% CO conversion, Re and Ru promoters decreased CH4 selectivity and increased C5+ selectivity by nearly the same extent, whereas the opposite effect was obsd. for Pd and Pt promoters. The Re and Ru promoters had less of an impact on C2-C4 olefin selectivity (7.5-60%), but suppressed the secondary reaction of 1-C4 olefin (from 14.3 to 9%) compared to the unpromoted one; however, the addn. of Pd and Pt promoters resulted in lower olefin selectivity (4.4-55%) but higher 2-C4 olefin selectivity (14.3 to 27-31%). Pt promotion had a negligible effect on C4 olefin isomerization. The selectivity results were reproducible. Both Pt and Pd promoters slightly increased WGS activity, whereas Re and Ru promoters had a negligible effect. The Pd and Pt promoters were obsd. to slightly enhance oxygenate formation, while Re and Ru slightly decreased it.
    26. 26
      Moradi, G. R.; Basir, M. M.; Taeb, A.; Kiennemann, A. Promotion of Co/SiO2 Fischer–Tropsch catalysts with zirconium. Catal. Commun. 2003, 4, 27– 32,  DOI: 10.1016/S1566-7367(02)00243-1
      26
      Promotion of Co/SiO2 Fischer-Tropsch catalysts with zirconium
      Moradi, G. R.; Basir, M. M.; Taeb, A.; Kiennemann, A.
      Catalysis Communications (2003), 4 (1), 27-32CODEN: CCAOAC; ISSN:1566-7367. (Elsevier Science B.V.)
      The effect of zirconia addn. at various loading ratios on the performance of 10% Co/SiO2 catalysts for the so-called reaction of Fischer-Tropsch synthesis was studied. The catalysts were prepd. through a new pseudo sol-gel method which permits an uniform distribution of the incorporated components and a low deviation from theor. compn. By increasing zirconia, Co-SiO2 interaction decreases and is replaced by Co-Zr interaction which favors redn. of the catalysts at lower temps. The activity and selectivity toward higher hydrocarbons of the promoted catalysts increase with increasing zirconium loading ratios. No appreciable decrease in activity was obsd. when all catalysts were employed under H2/CO = 2 at 230° and 8 bar for 240 h.
    27. 27
      Johnson, G. R.; Bell, A. T. Role of ZrO2 in Promoting the Activity and Selectivity of Co-Based Fischer–Tropsch Synthesis Catalysts. ACS Catal. 2016, 6, 100– 114,  DOI: 10.1021/acscatal.5b02205
      27
      Role of ZrO2 in Promoting the Activity and Selectivity of Co-Based Fischer-Tropsch Synthesis Catalysts
      Johnson, Gregory R.; Bell, Alexis T.
      ACS Catalysis (2016), 6 (1), 100-114CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The effects of Zr promotion on the structure and performance of Co-based Fischer-Tropsch synthesis (FTS) catalysts were investigated. Inclusion of Zr in the catalysts was found to increase the FTS turnover frequency and the selectivity to C5+ hydrocarbons and to decrease the selectivity to methane under most operating conditions. These improvements to the catalytic performance are a function of Zr loading up to an at. ratio of Zr/Co = 1.0, above which the product selectivity is insensitive to higher concns. of the promoter. Characterization of the Co nanoparticles by different methods demonstrated that the optimal Zr loading corresponds to half monolayer coverage of the Co surface by the promoter. Measurements of the rate of FTS at different pressures and temps. established that the kinetics data for both the Zr-promoted and unpromoted catalysts are described by a two-parameter Langmuir-Hinshelwood expression. The parameters used to fit this rate law to the exptl. data indicate that the apparent rate coeff. and the CO adsorption const. for the Zr-promoted catalysts are higher than those for the unpromoted catalyst. Elemental mapping by STEM-EDS provided evidence that Zr is highly dispersed over the catalyst surface and has limited preference for assocn. with the Co nanoparticles. In situ x-ray absorption spectroscopy confirmed the absence of mixing between the Zr and Co in the nanoparticles. These results suggest that Zr exists as a partial layer of ZrO2 on the surface of the Co metal nanoparticles. Accordingly, Zr promotion effects originate from sites of enhanced activity at the interface between Co and ZrO2. The possibility that ZrO2 acts as a Lewis acid to assist in CO dissocn. as well as to increase the ratio of CO to H adsorbed on the catalyst surface is discussed.
    28. 28
      Piao, Y.; Jiang, Q.; Li, H.; Matsumoto, H.; Liang, J.; Liu, W.; Pham-Huu, C.; Liu, Y.; Wang, F. Identify Zr Promotion Effects in Atomic Scale for Co-Based Catalysts in Fischer–Tropsch Synthesis. ACS Catal. 2020, 10, 7894– 7906,  DOI: 10.1021/acscatal.0c01874
      28
      Identify Zr Promotion Effects in Atomic Scale for Co-Based Catalysts in Fischer-Tropsch Synthesis
      Piao, Yuang; Jiang, Qian; Li, Hao; Matsumoto, Hiroaki; Liang, Jinsheng; Liu, Wei; Pham-Huu, Cuong; Liu, Yuefeng; Wang, Fei
      ACS Catalysis (2020), 10 (14), 7894-7906CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Introducing promoters on cobalt-based catalysts for Fischer-Tropsch synthesis (FTS) have been found efficient for adjusting their performance in converting syngas into long-chain hydrocarbons. Details of the promotion mechanism established on at. precise identification upon the active sites structure as well as the electronic status is seldomly reported yet. In the present work, we report the direct identification of valence status and coordination configuration of ZrO2 promoter additive over the cobalt-based FTS catalysts under a wide range of Zr/Co molar ratios from 0.12 to 1.5. Evidences from multiple technologies, including in situ/ex situ at. resoln. STEM imaging, EDS elemental mapping, electron energy loss spectroscopy (EELS), as well as physicochem. analyses (in situ XRD, CO chemisorption, CO temp.-programmed surface reaction, etc.) disclose that the ZrO2 promoter presents as single-site dispersion on the surface of Co nanoparticles (NPs) and the SiC support at low content, plays as the real active site to promote CO dissocn. at the Co-ZrO2 interface. While at high ZrO2 content (Zr/Co molar ratio of 1.0), Zr species on the SiC support turns to nucleate to form an amorphous coating, whereas those on the Co NPs surface maintain monodispersion. When further increasing the ZrO2 content (Zr/Co molar ratio up to 1.5), cobalt NPs start to be encapsulated by ZrO2 coating, leading to the decline of FTS activity. The in situ EELS anal. and d. functional theory calcns. disclose that the Zr atom tends to bind at the Co NPs surface rather than embed into the lattice meanwhile a charge transfer from Zr species to Co NPs occurs, which facilitates the stronger interaction between Zr and cobalt and thus enhances the adsorption with the H2 mol. as well as the CO dissocn. It thus offers enhanced catalytic activity and long-chain hydrocarbon selectivity during the FTS reaction.
    29. 29
      Martínez, A.; López, C.; Márquez, F.; Díaz, I. Fischer–Tropsch synthesis of hydrocarbons over mesoporous Co/SBA-15 catalysts: the influence of metal loading, cobalt precursor, and promoters. J. Catal. 2003, 220, 486– 499,  DOI: 10.1016/S0021-9517(03)00289-6
      There is no corresponding record for this reference.
    30. 30
      Dinse, A.; Aigner, M.; Ulbrich, M.; Johnson, G. R.; Bell, A. T. Effects of Mn promotion on the activity and selectivity of Co/SiO2 for Fischer–Tropsch Synthesis. J. Catal. 2012, 288, 104– 114,  DOI: 10.1016/j.jcat.2012.01.008
      30
      Effects of Mn promotion on the activity and selectivity of Co/SiO2 for Fischer-Tropsch Synthesis
      Dinse, Arne; Aigner, Max; Ulbrich, Markus; Johnson, Gregory R.; Bell, Alexis T.
      Journal of Catalysis (2012), 288 (), 104-114CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      An investigation has been carried out of the effects of Mn promotion on the activity and product selectivity of Co/SiO2 for Fischer-Tropsch Synthesis (FTS). All expts. were conducted with an H2/CO feed ratio of 2.0 at either 1 atm or 10 atm and a temp. of 493 K. At 1 atm, the rate of CO consumption decreased with CO conversion, whereas the selectivities to all products were unaffected. However, the olefin to paraffin (O/P) ratio of the C2-4 fraction decreased with increasing CO conversion due to increased hydrogenation of olefins with increasing space time. Mn promotion increased the rate of CO consumption at Mn/Co ratios of 0.05, decreased the selectivity to methane, and increased the selectivity to C5+ products, but had no effect on the selectivity to C2-4 products. Raising the pressure to 10 atm increased the CO consumption rates for both unpromoted and Mn-promoted Co/SiO2, but the rate of reaction was lower for the Mn-promoted catalyst due to a higher level of CO inhibition. CO conversion at 10 atm had no effect on the rate of CO consumption for Co/SiO2 and caused a slight increase in the rate for Mn-promoted Co/SiO2. The intrinsic O/P ratio was higher at 10 atm than at 1 atm. In contrast to what was obsd. at 1 atm, at 10 atm, C2-4 of the C2 -C4 fraction incorporated into growing hydrocarbon chain, leading to a decrease in C1-4 selectivities and an increase in C5+ selectivities with increasing CO conversion. The obsd. effects of Mn promotion on catalyst activity and product selectivity are discussed in terms of the mechanisms for CO hydrogenation and hydrocarbon chain growth.
    31. 31
      Paterson, J.; Peacock, M.; Purves, R.; Partington, R.; Sullivan, K.; Sunley, G.; Wilson, J. Manipulation of Fischer–Tropsch Synthesis for Production of Higher Alcohols Using Manganese Promoters. ChemCatChem 2018, 10, 5154– 5163,  DOI: 10.1002/cctc.201800883
      There is no corresponding record for this reference.
    32. 32
      Paterson, J.; Partington, R.; Peacock, M.; Sullivan, K.; Wilson, J.; Xu, Z. Elucidating the Role of Bifunctional Cobalt-Manganese Catalyst Interactions for Higher Alcohol Synthesis. Eur. J. Inorg. Chem. 2020, 2020, 2312– 2324,  DOI: 10.1002/ejic.202000397
      There is no corresponding record for this reference.
    33. 33
      Pedersen, E. Ø.; Svenum, I.-H.; Blekkan, E. A. Mn promoted Co catalysts for Fischer–Tropsch production of light olefins – An experimental and theoretical study. J. Catal. 2018, 361, 23– 32,  DOI: 10.1016/j.jcat.2018.02.011
      33
      Mn promoted Co catalysts for Fischer-Tropsch production of light olefins - An experimental and theoretical study
      Pedersen, Eirik Oestbye; Svenum, Ingeborg-Helene; Blekkan, Edd A.
      Journal of Catalysis (2018), 361 (), 23-32CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Three different CoMn/γ-Al2O3 catalysts were prepd. by the incipient wetness impregnation route and compared to a Co/γ-Al2O3 catalyst. The effect of co-impregnation vs. sequential impregnation as well as the order of component addn. was investigated. All catalysts were characterized by TPR, H2-chemisorption, XRD and XPS and their activity and selectivity in the Fischer-Tropsch reaction was investigated. Complementary, self-consistent DFT calcns. were performed to further address the obsd. promotion effects. All Mn promoted catalysts displayed heightened intrinsic activity, heightened selectivity to light olefins and C5+ species and lowered selectivity to CH4 compared to Co. The promotion effects on selectivity and intrinsic activity were found to be independent on catalyst prepn. method. The catalysts undergo a restructuring during operation, in which an excess of Mn sats. the catalytically relevant sites causing the similar behavior. The Co-specific activity differed between the Mn promoted catalysts. This was attributed to varying degrees of Mn incorporation in the Co3O4 particles, causing different degrees of redn. limiting the available metallic Co surface area. The DFT calcns. suggested that the binding energy for all investigated species increases on Co in the presence of Mn, facilitating CO dissocn. which can explain the higher intrinsic activity. The affected selectivities for olefins, C5+ and CH4 can all be attributed to an inhibited hydrogenation activity demonstrated by the increased barriers for CH3 and CH4 formation.
    34. 34
      Yang, K.; Zhou, G. Hydrogen evolution/spillover effect of single cobalt atom on anatase TiO2 from first-principles calculations. Appl. Surf. Sci. 2021, 536, 147831,  DOI: 10.1016/j.apsusc.2020.147831
      34
      Hydrogen evolution/spillover effect of single cobalt atom on anatase TiO2 from first-principles calculations
      Yang, Kang; Zhou, Gang
      Applied Surface Science (2021), 536 (), 147831CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      In spite of progress, there is a long way to go in the use of non-precious metals instead of precious metals as catalysts in chem. reactions. Here we report an anatase TiO2-supported single-atom (SA) Co system for hydrogen evolution and also study its hydrogen spillover effect using first-principles calcns. Two stable forms of SA Co on the anatase TiO2(101) surface, achieved by adsorption and substitution, induce different confinement effects. The SA Co in the interstices of the surface exhibits better hydrogen evolution activity than bulk counterpart. The hydrogen evolution reaction proceeds on the partially hydrogenated surface of Co1/TiO2, where SA Co and adjacent O are active sites. The substitution of Co for Ti promotes the formation of surface O vacancies and the redn. of Ti4+ to Ti3+ in the H2 atmosphere, indicative of an enhanced hydrogen spillover effect. The possible catalytic mechanisms of SA catalysts in the two forms are proposed by the calcn. of reaction kinetics. The present work highlights the complexity and diversity of the confinement effect of transition metal SA in oxides, and broadens their applications in catalysis and of defect engineering.
    35. 35
      Kalantari, L.; Tran, F.; Blaha, P. Density Functional Theory Study of Metal and Metal-Oxide Nucleation and Growth on the Anatase TiO2(101) Surface. Computation 2021, 9, 125,  DOI: 10.3390/computation9110125
      There is no corresponding record for this reference.
    36. 36
      Liu, J.J. Advanced Electron Microscopy of Metal–Support Interactions in Supported Metal Catalysts. ChemCatChem 2011, 3, 934– 948,  DOI: 10.1002/cctc.201100090
      36
      Advanced electron microscopy of metal-support interactions in supported metal catalysts
      Liu, Jingyue
      ChemCatChem (2011), 3 (6), 934-948CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Electron microscopy and its assocd. techniques have contributed significantly to the characterization of heterogeneous catalysts, esp. supported metal catalysts, and can provide nano- or at.-scale information on the structure, morphol., compn., and electronic state of the area of interest. With the recent advancement in aberration corrections to achieve an image resoln. below 0.1 nm and the rapid development of in situ techniques, advanced electron microscopy is poised to probe fundamental questions of heterogeneous catalysis and catalysts. Understanding the nature of metal-support interactions becomes crit. if we want to achieve the goal of designing and fabricating nanoarchitectured catalysts with desired performances. To reliably probe the fundamental mechanisms of charge transfer, which is the core of catalysis, between individual nanoscale components and under a realistic gas environment and temp. is still a formidable challenge. This review discusses the recent contribution of advanced electron microscopy to the study of metal-support interactions, as well as the challenges and opportunities of applying aberration-cor. electron microscopy techniques to the investigation of at.-scale structures of complex heterogeneous catalysts.
    37. 37
      Yaguchi, T.; Suzuki, M.; Watabe, A.; Nagakubo, Y.; Ueda, K.; Kamino, T. Development of a high temperature-atmospheric pressure environmental cell for high-resolution TEM. J. Electron Microsc. (Tokyo) 2011, 60, 217– 225,  DOI: 10.1093/jmicro/dfr011
      37
      Development of a high temperature-atmospheric pressure environmental cell for high-resolution TEM
      Yaguchi, Toshie; Suzuki, Makoto; Watabe, Akira; Nagakubo, Yasuhira; Ueda, Kota; Kamino, Takeo
      Journal of Electron Microscopy (2011), 60 (3), 217-225CODEN: JELJA7; ISSN:0022-0744. (Oxford University Press)
      An environmental cell for high-temp., high-resoln. transmission electron microscopy of nanomaterials in near atm. pressures is developed. The developed environmental cell is a side-entry type with built-in specimen-heating element and micropressure gauge. The relationship between the cell condition and the quality of the transmission electron microscopic (TEM) image and the diffraction pattern was examd. exptl. and theor. By using the cell consisting of two electron-transparent silicon nitride thin films as the window material, the gas pressure inside the environmental cell is continuously controlled from 10-5 Pa to the atm. pressure in a high-vacuum TEM specimen chamber. TEM image resolns. of 0.23 and 0.31 nm were obtained using 15-nm-thick silicon nitride film windows with the pressure inside the cell being around 5 × 10-5 and 1 × 104 Pa, resp.
    38. 38
      Allard, L. F.; Overbury, S. H.; Bigelow, W. C.; Katz, M. B.; Nackashi, D. P.; Damiano, J. Novel MEMS-Based Gas-Cell/Heating Specimen Holder Provides Advanced Imaging Capabilities for In Situ Reaction Studies. Microsc. Microanal. 2012, 18, 656– 666,  DOI: 10.1017/S1431927612001249
      38
      Novel MEMS-Based Gas-Cell/Heating Specimen Holder Provides Advanced Imaging Capabilities for In Situ Reaction Studies
      Allard, Lawrence F.; Overbury, Steven H.; Bigelow, Wilbur C.; Katz, Michael B.; Nackashi, David P.; Damiano, John
      Microscopy and Microanalysis (2012), 18 (4), 656-666CODEN: MIMIF7; ISSN:1431-9276. (Cambridge University Press)
      In prior research, specimen holders that employ a novel MEMS-based heating technol. (AduroTM) provided by Protochips Inc. (Raleigh, NC, USA) have been shown to permit sub-Ångstroem imaging at elevated temps. up to 1,000°C during in situ heating expts. in modern aberration-cor. electron microscopes. The Aduro heating devices permit precise control of temp. and have the unique feature of providing both heating and cooling rates of 106°C/s. In the present work, we describe the recent development of a new specimen holder that incorporates the Aduro heating device into a "closed-cell" configuration, designed to function within the narrow (2 mm) objective lens pole piece gap of an aberration-cor. JEOL 2200FS STEM/TEM, and capable of exposing specimens to gases at pressures up to 1 atm. We show the early results of tests of this specimen holder demonstrating imaging at elevated temps. and at pressures up to a full atm., while retaining the at. resoln. performance of the microscope in high-angle annular dark-field and bright-field imaging modes.
    39. 39
      Ciobîcă, I.; Van Santen, R. A.; Van Berge, P. J.; Van De Loosdrecht, J. Adsorbate induced reconstruction of cobalt surfaces. Surf. Sci. 2008, 602, 17– 27,  DOI: 10.1016/j.susc.2007.09.060
      39
      Adsorbate induced reconstruction of cobalt surfaces
      Ciobica, I. M.; van Santen, R. A.; van Berge, P. J.; van de Loosdrecht, J.
      Surface Science (2008), 602 (1), 17-27CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      Coverage dependence adsorption of intermediates typical for syngas conversion is studied theor. on FCC-cobalt surfaces. The structure is relevant for cobalt particles active in this reaction. Emphasis on the anal. is on the thermodn. of surface reconstruction as a function of surface adsorbate and surface coverage. An important result is the finding that only adsorbed carbon induced reconstruction of FCC-Co(1 1 1) to FCC-Co(1 0 0) as well as the clock reconstruction for the two surfaces.
    40. 40
      Hansen, P. L.; Wagner, J. B.; Helveg, S.; Rostrup-Nielsen, J. R.; Clausen, B. S.; Topsøe, H. Atom-Resolved Imaging of Dynamic Shape Changes in Supported Copper Nanocrystals. Science 2002, 295, 2053– 2055,  DOI: 10.1126/science.1069325
      40
      Atom-resolved imaging of dynamic shape changes in supported copper nanocrystals
      Hansen, Poul L.; Wagner, Jakob B.; Helveg, Stig; Rostrup-Nielsen, Jens R.; Clausen, Bjerne S.; Topsoe, Henrik
      Science (Washington, DC, United States) (2002), 295 (5562), 2053-2055CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      In situ TEM is used to obtain atom-resolved images of Cu nanocrystals on different supports. These are catalysts for MeOH synthesis and hydrocarbon conversion processes for fuel cells. The nanocrystals undergo dynamic reversible shape changes in response to changes in the gaseous environment. For Zn oxide-supported samples, the changes are caused both by adsorbate-induced changes in surface energies and by changes in the interfacial energy. For Cu nanocrystals supported on SiO2, the support has negligible influence on the structure. Nanoparticle dynamics must be included in the description of catalytic and other properties of nanomaterials. In situ microscopy offers possibilities for obtaining the relevant at.-scale insight.
    41. 41
      Creemer, J. F.; Helveg, S.; Kooyman, P. J.; Molenbroek, A. M.; Zandbergen, H. W.; Sarro, P. M. A MEMS Reactor for Atomic-Scale Microscopy of Nanomaterials Under Industrially Relevant Conditions. J. Microelectromechanical Syst. 2010, 19, 254– 264,  DOI: 10.1109/JMEMS.2010.2041190
      41
      A MEMS reactor for atomic-scale microscopy of nanomaterials under industrially relevant conditions
      Creemer, J. Fredrik; Helveg, Stig; Kooyman, Patricia J.; Molenbroek, Alfons M.; Zandbergen, Henny W.; Sarro, Pasqualina M.
      Journal of Microelectromechanical Systems (2010), 19 (2), 254-264CODEN: JMIYET; ISSN:1057-7157. (Institute of Electrical and Electronics Engineers)
      We present a microelectromech. systems (MEMS) nanoreactor that enables high-resoln. transmission electron microscopy (TEM) (HRTEM) of nanostructured materials with at.-scale resoln. during exposure to reactive gases at 1 atm of pressure. This pressure exceeds that of existing HRTEM systems by a factor of 100, thereby entering a pressure range that is relevant to industrial purposes. The nanoreactor integrates a shallow flow channel (35 μm high) with a microheater and with an array of electron transparent windows of silicon nitride. The windows are only 10 nm thick but are mech. robust. The heater has the geometry of a microhotplate and is made of Pt embedded in a silicon nitride membrane. To interface the nanoreactor, a dedicated TEM specimen holder has been developed. The performance is demonstrated by the live formation of Cu nanoparticles in a catalyst for the prodn. of methanol. At 120 kPa and for temps. of up to 500 °C, the formation of these nanoparticles can be obsd. clearly and with an exceptionally low thermal drift. HRTEM images of the nanoparticles show at. lattice fringes with spacings down to 0.18 nm.
    42. 42
      Dembélé, K.; Bahri, M.; Hirlimann, C.; Moldovan, S.; Berliet, A.; Maury, S.; Gay, A.; Ersen, O. Operando Electron Microscopy Study of Cobalt-based Fischer–Tropsch Nanocatalysts. ChemCatChem 2021, 13, 1920– 1930,  DOI: 10.1002/cctc.202001074
      42
      Operando Electron Microscopy Study of Cobalt-based Fischer-Tropsch Nanocatalysts
      Dembele, Kassioge; Bahri, Mounib; Hirlimann, Charles; Moldovan, Simona; Berliet, Adrien; Maury, Sylvie; Gay, Anne-Sophie; Ersen, Ovidiu
      ChemCatChem (2021), 13 (8), 1920-1930CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Thanks to their stability and selectivity for long-chains hydrocarbons, supported Co nanoparticles are the most commonly used catalysts in the Fischer-Tropsch synthesis reaction. We report here on the use of in situ transmission electron microscopy (TEM) to address the real-time evolution of cobalt-based catalysts during their redn. under relevant industrial activation condition (105 Pa, 430°C), and their operation in syngas (H2/CO=2, 105 Pa, 220°C). To do so, we chose Co3O4-Pt nanoparticles supported on silica or alumina that can be directly compared to some industrial catalysts. By analyzing the real space information contained in the TEM images, we have monitored the fragmentation of cobalt aggregates, the disappearance of cavities within the particles, their shape changes, the particle diffusion and coalescence processes, as well as the effect of the support (silica or alumina) on the behavior of the Co phase. An easier redn. of cobalt catalysts supported on silica as compared to the same catalyst supported on alumina was also obsd. During the catalyst operation under syngas, we have noticed the stability of the general shape of the particles. Simultaneously, using a residual gas analyzer connected to the TEM holder, the main gas products of the Fischer-Tropsch reaction were systematically analyzed. Our findings underline the benefit of the operando TEM to study the dynamical evolution of catalysts, at the nanoparticle level, under operation conditions.
    43. 43
      Morales, F.; Grandjean, D.; Mens, A.; De Groot, F. M. F.; Weckhuysen, B. M. X-ray Absorption Spectroscopy of Mn/Co/TiO2 Fischer–Tropsch Catalysts: Relationships between Preparation Method, Molecular Structure, and Catalyst Performance. J. Phys. Chem. B 2006, 110, 8626– 8639,  DOI: 10.1021/jp0565958
      43
      X-ray absorption spectroscopy of Mn/Co/TiO2 Fischer-Tropsch catalysts: relationships between preparation method, molecular structure, and catalyst performance
      Morales, Fernando; Grandjean, Didier; Mens, Ad; de Groot, Frank M. F.; Weckhuysen, Bert M.
      Journal of Physical Chemistry B (2006), 110 (17), 8626-8639CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The effects of the addn. of manganese to TiO2-supported cobalt Fischer-Tropsch (FT) catalysts prepd. by different methods were studied by a combination of XRD, temp.-programmed redn. (TPR), TEM, and in situ x-ray absorption fine structure (XAFS) spectroscopy at the Co and Mn K-edges. After calcination, the catalysts were generally composed of large Co3O4 clusters in the range 15-35 nm and a MnO2-type phase, which existed either dispersed on the TiO2 surface or covering the Co3O4 particles. Manganese was also found to coexist with the Co3O4 as Co3-xMnxO4 solns., as revealed by XRD and XAFS. Characterization of the catalysts after H2 redn. at 350° by XAFS and TEM showed mostly the formation of very small Co0 particles (around 2-6 nm), indicating that the cobalt phase tends to redisperse during the redn. process from Co3O4 to Co0. The presence of manganese was found to hamper the cobalt reducibility, with this effect being more severe when Co3-xMnxO4 solns. were initially present in the catalyst precursors. Also, the presence of manganese generally gave larger cobalt agglomerates (∼8-15 nm) upon redn., probably as a consequence of the decrease in cobalt reducibility. The XAFS results revealed that all reduced catalysts contained manganese entirely in a Mn2+ state, and two well-distinguished compds. could be identified: (1) a highly dispersed Ti2MnO4-type phase located at the TiO2 surface and (2) a less dispersed MnO phase being in the proximity of the cobalt particles. Also, the MnO was also found to exist partially mixed with a CoO phase as rock-salt Mn1-xCoxO-type solid solns. The existence of the later solns. was further confirmed by scanning TEM with EELS (STEM-EELS) for a Mn-rich sample. Finally, the cobalt active site compn. in the catalysts after redn. at 300° and 350° was linked to the catalytic performances obtained under reaction conditions of 220°, 1 bar, and H2/CO = 2. The catalysts with larger Co0 particles (approx. >5 nm) and lower Co redn. extents displayed a higher intrinsic hydrogenation activity and a longer catalyst lifetime. The MnO and Mn1-xCoxO species effectively promoted these larger Co0 particles by increasing the C5+ selectivity and decreasing the CH4 prodn., while they did not significantly influence the selectivity of the catalysts contg. very small Co0 particles.
    44. 44
      Pennycook, S. J. Z-Contrast Transmission Electron Microscopy: Direct Atomic Imaging of Materials. Annu. Rev. Mater. Sci. 1992, 22, 171– 195,  DOI: 10.1146/annurev.ms.22.080192.001131
      44
      Z-contrast transmission electron microscopy: direct atomic imaging of materials
      Pennycook, S. J.
      Annual Review of Materials Science (1992), 22 (), 171-95CODEN: ARMSCX; ISSN:0084-6600.
      A review with 41 refs. on the imaging process, with examples on the growth mechanisms and properties of semiconducting and superconducting materials.
    45. 45
      Frost, D. C.; McDowell, C. A.; Woolsey, I. S. X-ray photoelectron spectra of cobalt compounds. Mol. Phys. 1974, 27, 1473– 1489,  DOI: 10.1080/00268977400101251
      45
      X-ray photoelectron spectra of cobalt compounds
      Frost, D. C.; McDowell, C. A.; Woolsey, I. S.
      Molecular Physics (1974), 27 (6), 1473-89CODEN: MOPHAM; ISSN:0026-8976.
      Satellite lines for the 2p, 3s, and 3p peaks in the photoelectron spectra of high spin Co(II) complexes were examd. Satellites were absent in the spectra of low spin Co(III) complexes. The 2p satellites were due to shake-up processes and the 3s and 3p satellites due to multiplet splitting. The chem. shifts for the Co 2p levels were plotted against those of the 3s and 3p level.
    46. 46
      Paterson, J.; Brown, D.; Haigh, S. J.; Landon, P.; Li, Q.; Lindley, M.; Peacock, M.; Van Rensburg, H.; Xu, Z. Controlling cobalt Fischer–Tropsch stability and selectivity through manganese titanate formation. Catal. Sci. Technol. 2023, 13, 3818– 3827,  DOI: 10.1039/D3CY00030C
      There is no corresponding record for this reference.
    47. 47
      Prestat, E.; Kulzick, M. A.; Dietrich, P. J.; Smith, M. M.; Tien, M. E.; Burke, M. G.; Haigh, S. J.; Zaluzec, N. J. In Situ Industrial Bimetallic Catalyst Characterization using Scanning Transmission Electron Microscopy and X-ray Absorption Spectroscopy at One Atmosphere and Elevated Temperature. ChemPhysChem 2017, 18, 2151– 2156,  DOI: 10.1002/cphc.201700425
      47
      In Situ Industrial Bimetallic Catalyst Characterization using Scanning Transmission Electron Microscopy and X-ray Absorption Spectroscopy at One Atmosphere and Elevated Temperature
      Prestat, Eric; Kulzick, Matthew A.; Dietrich, Paul J.; Smith, Matthew; Tien, Eu-Pin; Burke, M. Grace; Haigh, Sarah J.; Zaluzec, Nestor J.
      ChemPhysChem (2017), 18 (16), 2151-2156CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We have developed a new exptl. platform for in situ scanning transmission electron microscope (STEM) energy dispersive X-ray spectroscopy (EDS) which allows real time, nanoscale, elemental and structural changes to be studied at elevated temp. (up to 1000 °C) and pressure (up to 1 atm). Here we demonstrate the first application of this approach to understand complex structural changes occurring during redn. of a bimetallic catalyst, PdCu supported on TiO2, synthesized by wet impregnation. We reveal a heterogeneous evolution of nanoparticle size, distribution, and compn. with large differences in redn. behavior for the two metals. We show that the data obtained is complementary to in situ STEM electron energy loss spectroscopy (EELS) and when combined with in situ X-ray absorption spectroscopy (XAS) allows correlation of bulk chem. state with nanoscale changes in elemental distribution during redn., facilitating new understanding of the catalytic behavior for this important class of materials.
    48. 48
      Tang, M.; De Jongh, P. E.; De Jong, K. P. In Situ Transmission Electron Microscopy to Study the Location and Distribution Effect of Pt on the Reduction of Co 3 O 4 – SiO 2. Small 2024, 20, 2304683,  DOI: 10.1002/smll.202304683
      There is no corresponding record for this reference.
    49. 49
      Van Koppen, L. M.; Iulian Dugulan, A.; Leendert Bezemer, G.; Hensen, E. J. M. Elucidating deactivation of titania-supported cobalt Fischer–Tropsch catalysts under simulated high conversion conditions. J. Catal. 2023, 420, 44– 57,  DOI: 10.1016/j.jcat.2023.02.019
      There is no corresponding record for this reference.
    50. 50
      Kistamurthy, D.; Saib, A. M.; Moodley, D. J.; Niemantsverdriet, J. W.; Weststrate, C. J. Ostwald ripening on a planar Co/SiO2 catalyst exposed to model Fischer–Tropsch synthesis conditions. J. Catal. 2015, 328, 123– 129,  DOI: 10.1016/j.jcat.2015.02.017
      50
      Ostwald ripening on a planar Co/SiO2 catalyst exposed to model Fischer-Tropsch synthesis conditions
      Kistamurthy, D.; Saib, A. M.; Moodley, D. J.; Niemantsverdriet, J. W.; Weststrate, C. J.
      Journal of Catalysis (2015), 328 (), 123-129CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Catalyst deactivation is an important topic for industrial catalyst development. Sintering of small cobalt crystallites is one of the deactivation mechanisms of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. This study investigates the mechanism of cobalt sintering at low-conversion FTS conditions. A Co/SiO2/Si(1 0 0) model catalyst is exposed to 20 bar dry synthesis gas (H2/CO: 2/1) at 230 °C for 10 h. Cobalt nanoparticles were characterized before and after treatment using transmission electron microscopy (TEM) and XPS. TEM images of identical locations on the model catalyst showed a loss of some small crystallites and decrease in size of some crystallites. Sintering is dominated by an Ostwald ripening mechanism using our model catalyst under the present conditions. Complementary XPS measurements confirm the loss of Co dispersion. Therefore, the loss of small Co nanoparticles causes a rapid loss of metal surface area when exposed to model FTS conditions.
    51. 51
      Rahmati, M.; Safdari, M.-S.; Fletcher, T. H.; Argyle, M. D.; Bartholomew, C. H. Chemical and Thermal Sintering of Supported Metals with Emphasis on Cobalt Catalysts During Fischer–Tropsch Synthesis. Chem. Rev. 2020, 120, 4455– 4533,  DOI: 10.1021/acs.chemrev.9b00417
      51
      Chemical and Thermal Sintering of Supported Metals with Emphasis on Cobalt Catalysts During Fischer-Tropsch Synthesis
      Rahmati, Mahmood; Safdari, Mohammad-Saeed; Fletcher, Thomas H.; Argyle, Morris D.; Bartholomew, Calvin H.
      Chemical Reviews (Washington, DC, United States) (2020), 120 (10), 4455-4533CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. This comprehensive crit. review combines, for the first time, recent advances in nanoscale surface chem., surface science, DFT, adsorption calorimetry, and in situ XRD and TEM to provide new insights into catalyst sintering. This work provides qual. and quant. ests. of the extent and rate of sintering as functions of nanocrystal (NC) size, temp., and atm. This review is unique in that besides summarizing important, useful data from previous studies, it also advances the field through addn. of (i) improved or new models, (ii) new data summarized in original tables and figures, and (iii) new fundamental perspectives into sintering of supported metals and particularly of chem. sintering of supported Co during Fischer-Tropsch synthesis. We demonstrate how the two widely accepted sintering mechanisms are largely sequential with some overlap and highly NC-size dependent, i.e., generally, small NCs sinter rapidly by Ostwald ripening, while larger NCs sinter slowly by crystallite migration and coalescence. In addn., we demonstrate how accumulated knowledge, principles, and recent advances, discussed in this review, can be utilized in the design of supported metal NCs highly resistant to sintering. Recommendations for improving the design of sintering expts. and for new research are addressed.
    52. 52
      Labat, F.; Baranek, P.; Adamo, C. Structural and Electronic Properties of Selected Rutile and Anatase TiO2 Surfaces: An ab Initio Investigation. J. Chem. Theory Comput. 2008, 4, 341– 352,  DOI: 10.1021/ct700221w
      52
      Structural and Electronic Properties of Selected Rutile and Anatase TiO2 Surfaces: An ab Initio Investigation
      Labat, Frederic; Baranek, Philippe; Adamo, Carlo
      Journal of Chemical Theory and Computation (2008), 4 (2), 341-352CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      Five low-index stoichiometric TiO2 rutile and anatase surfaces, i.e., rutile (110), (100), and (001) as well as anatase (101) and (100), have been investigated using different Hamiltonians with all-electron Gaussian basis sets, within a periodic approach. Full-relaxations of the aforementioned surfaces have been essentially carried out at the Hartree-Fock (HF) level, but selected surfaces were treated also using pure and hybrid D. Functional Theory (DFT) models. Mulliken charges, band structures, and total and projected-densities of states have been computed both at the HF and the hybrid DFT (B3LYP and PBE0) levels. As regards DFT, the local d. (LDA) and generalized gradient approxns. (GGA) have been used. No matter which Hamiltonian is considered, as long as sufficiently thick slabs are taken into account, computed at. relaxations show an overall excellent agreement with the most recent exptl. reports. This is esp. true when using hybrid functionals which enable the clarification of some conflicting results. Moreover, both at the LDA and HF levels, we were able to classify the surface relative energies in the following sequence: anatase (101) < rutile (110) < anatase (100) < rutile (100) « rutile (001). Instead, when using PBE, B3LYP, or PBE0, the two most stable surfaces are reversed.
    53. 53
      Ohtani, B.; Prieto-Mahaney, O. O.; Li, D.; Abe, R. What is Degussa (Evonik) P25? Crystalline composition analysis, reconstruction from isolated pure particles and photocatalytic activity test. J. Photochem. Photobiol. Chem. 2010, 216, 179– 182,  DOI: 10.1016/j.jphotochem.2010.07.024
      53
      What is Degussa (Evonic) P25? Crystalline composition analysis, reconstruction from isolated pure particles and photocatalytic activity test
      Ohtani, B.; Prieto-Mahaney, O. O.; Li, D.; Abe, R.
      Journal of Photochemistry and Photobiology, A: Chemistry (2010), 216 (2-3), 179-182CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      Anatase and rutile crystallites were isolated from Degussa (Evonic) P25 by selective dissoln. with a hydrogen peroxide-ammonia mixt. and dild. hydrofluoric acid, resp., and used as std. samples for calibration curves of X-ray diffraction analyses. The results showed that P25 contains more than 70% anatase with a minor amt. of rutile and a small amt. of metallic glasses phase. The compn. anatase/rutile/metallic glasses could be detd. by anal. of P25 mixed with an internal std., nickel(II) oxide. However, it was also found that the compn. of P25 used in this study was inhomogeneous and changed depending on the position of sampling from the same package. Comparison of activities of original P25 and reconstructed P25 with those of isolated anatase and rutile particles suggested a less-probable synergetic effect of the co-presence of anatase and rutile.

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