Catalysis Applications of Size-Selected Cluster Deposition - ACS

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Catalysis Applications of Size-Selected Cluster Deposition Stefan Vajda†,‡,§,¶ and Michael G. White*,∥,⊥ †

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Nanoscience and Technology Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States ¶ Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, United States ∥ Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States ⊥ Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States ‡

ABSTRACT: In this Perspective, we review recent studies of size-selected cluster deposition for catalysis applications performed at the U.S. DOE National Laboratories, with emphasis on work at Argonne National Laboratory (ANL) and Brookhaven National Laboratory (BNL). The focus is on the preparation of model supported catalysts in which the number of atoms in the deposited clusters is precisely controlled using a combination of gas-phase cluster ion sources, mass spectrometry, and soft-landing techniques. This approach is particularly effective for investigations of small nanoclusters, 0.5−2 nm ( Ti3O6, which is roughly consistent with the known bulk oxide work functions. This trend also shows that the more reducible oxides (Ti, Nb) are associated with lower electron transfer. The surface dipoles for the “reduced,” i.e., substoichiometric, clusters (Mo3O6, W3O6, Nb4O8, Nb3O5, Ti3O5, and Ti4O7) are also smaller than their fully oxidized counterparts. The latter is attributed to the decreased ability of the reduced cations (lower oxidation states) to accommodate charge from the Cu surface. DFT calculations of the oxide clusters bound to the Cu(111) surface were also performed in order to assess the relative contributions of interfacial electron transfer and geometrydependent cluster dipole moments to the calculated work function shifts. Cluster structures and a comparison of the measured surface dipoles and calculated Bader charges, are shown in Figure 16 for a few Mo, Ti, and Nb oxide clusters. For fully oxidized clusters such as Mo3O9 and W3O9 clusters, Bader charge analyses and surface potential calculations show that the dominant contributions to the work function shifts result from electron transfer from the Cu(111) surface to the clusters. The calculated Bader electron transfer is smaller for the oxidized

experiments were able to assign specific cluster masses with a unique structure through comparisons of experimental vibrational frequencies with those obtained from DFT calculations.133 Hence, a combination of characteristic mass distributions and gas-phase experimental/theoretical structural studies generally allow the determination of cluster stoichiometry in cases of near mass degeneracy between the metal and an integer number of nonmetal atoms. IV. A. Deposition of MxOy (Mo, W, Ti, Nb) Clusters: Interfacial Charge Transfer. Charge-transfer at the metal− support interface can play an important role in determining the chemical properties of supported catalysts, especially for reducible oxides which have the capacity to accept or donate charge through changes in cation oxidation state (e.g., Ti4 → Ti3+) and formation of oxygen vacancies. Recent work at BNL has focused on this issue by looking at the interfacial electronic structure of “inverse” catalysts prepared by size-selected deposition of different transition metal oxide clusters MxOy (M = Ti, Mo, Nb, W) on a metallic Cu(111) and model oxide Cu2O/Cu(111) surface. The specific systems for study where motivated by experimental and theoretical work at BNL on the water−gas-shift reaction (WGSR), CO + H2O →CO2 + H2, which is an important industrial process for purifying hydrogen in the reforming of hydrocarbon feedstocks. These studies have shown that both conventional catalysts in which Cu is deposited on reducible oxides (TiO2, CeO2) and “inverse” catalysts where CeOx islands are deposited on Cu(111) have very high activity for WGSR.107−109,134−138 The high activity is associated with the formation of oxygen vacancies in the oxide, which are stabilized by Cu → oxide electron transfer.108,109,139 Part of the BNL effort in cluster deposition described below centers on the development of an experimental probe of interfacial electron transfer based on measurements of local work function shifts which are sensitive to the electrostatic potential at the surface.24,140 Investigations of oxide−metal electron transfer were investigated on “inverse” model catalysts by depositing sizeselected metal oxide clusters (MxOy; M = Ti, Mo, W, Nb) on a Cu(111) surface.24,25 Both fully “oxidized” and “reduced” clusters were studied, for example, M3O9 and M3O6 (M = Mo, W), respectively. Electron transfer was probed by two-photon photoemission (2PPE) measurements of the surface work function (Φ) as a function of cluster coverage from which the surface dipole could be extracted. These measurements take advantage of the non-uniform distribution of the clusters 7166

DOI: 10.1021/acscatal.5b01816 ACS Catal. 2015, 5, 7152−7176

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Figure 16. Top: comparison of experimentally derived surface dipoles and DFT-calculated Bader charges on a representative group of Mo, Ti, and Nb oxide clusters deposited on Cu(111). The clusters with the prefix “r” refer to substoichiometric or “reduced” clusters with fewer oxygen atoms. Bottom: DFT-calculated lowest energy adsorption structures on Cu(111) for the Mo, Ti, and Nb clusters listed in the Table. Adapted with permission from refs 24 and 25. Copyright 2014 and 2015 American Chemical Society. Figure 17. DFT-calculated potential energy along the surface normal for (a) Mo3O9 and (b) W3O6 adsorbed on Cu(111). The change in surface potential is partitioned into contributions from electron transfer (ΔVcharge) and cluster dipole (ΔVdipole) where the work function shift is given by ΔΦ = ΔVcharge + ΔVdipole. Reprinted with permission from ref 24. Copyright 2014 American Chemical Society.

Ti3O6, Ti4O8, and Nb3O7 clusters consistent with the trends in experimental surface dipoles. The calculated electron transfers for the reduced counterparts of these clusters are generally even smaller, but for the reduced clusters W3O6, Nb4O7, and Nb3O5, the electron transfers are actually in the opposite direction (i.e., oxide → Cu). Here, the overall surface dipole is most likely determined by the dipole moment of the cluster which reflects its structure on the Cu(111) surface. In general, the DFT Bader charge transfer analyses are qualitatively consistent with the trends observed in the experimental surface dipoles, but they also show that the intrinsic dipole of the adsorbed cluster (structure dependent) can be the more important contribution in cases where electron transfer is small. This is highlighted in Figure 17, which compares the calculated surface potentials for Mo3O9 (17a) and W3O6 (17b) on Cu(111) with the separate contributions from electron transfer and cluster dipole explicitly shown. For Mo3O9, electron transfer dominates the change in surface potential energy (or change in work function), whereas the larger inherent dipole moment of the reduced W3O6 cluster dominates the very small electron transfer contribution. These results highlight the fact that for small clusters, both the stoichiometry and adsorbed structure strongly influence the electronic interactions with the support. IV. B. Deposition of MxOy (Mo, W, Ti, Nb) Clusters: Water Dissociation. As noted in the preceding section, oxidesupported Cu catalysts are highly active for the WGSR with water dissociation being a key step in the mechanism. Water dissociation is thought to occur on the oxide support followed by CO oxidation at the Cu−oxide interface. This bifunctional mechanism was theoretically explored by Yang et al. at BNL on the inverse catalyst system Ce6O13/Cu(111) system where oxygen “vacancies” in the cluster reduced the activation energy for the water dissociation step.109 In this redox process, reduced ceria is oxidized by water dissociation and reduced by CO oxidation. Vidal and Liu at BNL have also explored correlations of different 1D and 3D oxide clusters on Cu(111) and found that the oxide morphology strongly modifies activity and the energy for water dissociation on the cluster correlates with WGSR activity.142 As shown in Figure 18, the DFT calculations

Figure 18. Correlations of the DFT-calculated water dissociation energy versus the experimentally measured WGS activity of various metal oxide clusters supported on Cu(111). Adapted with permission from ref 142. Copyright 2012 PCCP Owner Societies.

predicted that Ti3O6 clusters supported on Cu(111) would be much more active for WGSR than Mo3O9 or W3O9. This predicted trend also suggests that higher activity for WGSR and water dissociation correlates with smaller Cu → cluster charge transfer (see Figures 15b and 16). Measurements of water dissociation on various size-selected metal oxide clusters deposited on Cu(111) were performed at BNL using temperature-programmed reaction (TPR). To avoid interference with background water in the chamber, D2O was dosed onto the samples, and detection of D2 was used as an indicator for water dissociation. No evidence of water dissociation was observed for the Mo3O9 and W3O9 clusters 7167


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Figure 19. (a) D2 signal from D2O TPR experiments for niobium oxide clusters supported on Cu(111). All of the clusters show activity for water dissociation. (b) D2 signal from three consecutive D2O TPR runs for Nb3O7 on Cu(111). Adapted with permission from ref 25. Copyright 2015 American Chemical Society.

in qualitative agreement with the theoretical predictions in Figure 18. The TixOy clusters (x/y = 3/5, 3/6, 4/7, 4/8) were found to have detectable activity but were also found to be sensitive to thermal treatment, suggesting that either changes in cluster morphology or cluster aggregation occurred during TPR heating cycles. Substantially more water dissociation activity was observed for niobium oxide clusters (NbxOy; x/y = 3/5, 3/7. 4/7, 4/10) on the Cu(111) surface.25 Niobium oxide is also a readily reducible oxide with known forms Nb2O5 (Nb5+), NbO2 (Nb4+), and NbO (Nb2+) and has been widely used as a promoter in catalysis applications.143−145 Surprisingly, the fully oxidized (x/y = 3/7, 4/10) clusters were found to be more active for water dissociation than their reduced counterparts (x/y = 3/5, 4/8), and both sets of clusters exhibited increased activity with repeated TPR reaction cycles (see Figure 19). Insight into the mechanism and active site on the NbxOy clusters was obtained from XPS measurements on the Nb 3d core region. Specifically, the activity of the NbxOy clusters roughly followed the percentage of Nb5+ cations as determined from fits of the XPS data. Moreover, the XPS spectra were essentially unchanged after multiple TPR cycles, suggesting that the clusters were not oxidized as a result of water dissociation as might be expected for a redox process. The proposed mechanism consistent with these data is that Nb5+ cations in the clusters act as Lewis acid sites for water adsorption and subsequent dissociation. From previous studies, such Lewis acid sites are associated with NbO oxo groups that are converted to O−Nb−O(H) structures following water adsorption/ dissociation. The mechanism is illustrated in the DFTcalculated reaction sequence shown in Figure 20a for the Nb3O7/Cu(111) surface where water dissociation leads to the loss of NbO oxo group and the formation of a terminal and bridging hydroxyl groups. The fact that the activity increased with repeated TPR runs is not related to the Lewis acid sites that are lost in the first reaction cycle but due to the bridging hydroxyl (Nb−OH−Nb) groups that are thermally stable over the temperature ranges used in the TPR measurements. The bridging hydroxyls are strong Brønsted acids, and it is proposed that these induce water dissociation of other water molecules at the oxide−Cu interface. Additional experiments were performed by depositing the NbxOy clusters on a Cu2O thin film oxide surface prepared by oxidation of the Cu(111) surface in UHV. Unlike the results on metallic Cu, only the reduced Nb3O5 and Nb4O7 clusters showed activity on the Cu2O/Cu(111) oxide surface. The XPS

Figure 20. DFT-calculated reaction pathways for water dissociation on Nb3O7 on (a) Cu(111) and (b) Cu2O(111). IS = initial state; TS = transition state; FS = final state; Ea = activation energy. The energies in parentheses are relative to the optimized cluster-support and an isolated water molecule. The color scheme is as follows: blue-green, Nb; red, O of cluster; purple, O of substrate; and blue, Cu. Reprinted with permission from ref 25. Copyright 2015 American Chemical Society.

shows that the stoichiometric Nb3O7 and Nb4O10 clusters become reduced when deposited on the oxide film (i.e., less Nb5+ and more Nb4+ character), whereas the reduced clusters gain Nb5+ character and are thereby more oxidized. As noted above, the ability of niobium clusters to dissociate water depends on the presence of NbO (Nb5+) sites, and hence, the reduced clusters are still able to induce water dissociation on the Cu2O surface where the fully stoichiometric are not. This is again illustrated in the DFT results in Figure 20b, where the Nb3O7 cluster on a model Cu2O(111) surface does not have a free NbO group and where water dissociation has a relatively high activation energy. The results presented in the sections above show that in many cases electron transfer at the oxide−metal interface can be quantified through experimental measurements of work function shifts as a function of cluster coverage. Moreover, the oxides containing the more oxophilic cations of Mo(+6) and W(+6) induce large interfacial charge transfer when deposited on Cu(111) but do not readily dissociate water. By contrast, the more reducible TixOy clusters induce much smaller charge transfer and are able to dissociate water via a redox mechanism involving undercoordinated Ti(+4/+3) cations. The NbxOy clusters are unique in that they display both reducibilty and Lewis acid (NbO) and Brønsted (Nb−OH−Nb) acid sites, 7168


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NH3 readily bind at all the exposed Mo-atom sites of these clusters.29 Two of the clusters, Mo4S6 and Mo6S8, are highly symmetric cage structures with tetrahedral Mo4 and octahedral Mo6 metal cores, respectively, and relatively large HOMO− LUMO gaps that suggest that they are especially stable (“magic”).165,166 The Mo6S8 cluster is also interesting because it is known to be the basic structural element in the Chevrel phase of molybdenum sulfide168,169 and can be prepared as a ligated complex in solution.170 Larger platelet-like clusters have been suggested for tungsten sulfide (similar in properties to molybdenum sulfide) from a pulsed arc ion cluster source with abundant clusters identified as W15S42 and W21S56.171 The structural and chemical properties of isolated MoxSy clusters are likely to be modified when deposited on a substrate, yet the ability to vary cluster size, structure, and metal-to-sulfur ratio allows cluster deposition to explore a wide range of potentially active Mo−S sites for catalysis applications. V.A. MoxSy Clusters Deposited on Au(111). The BNL group investigated the deposition of small MoxSy clusters (x/y = 3/5, 4/6, 5/7, 6/8, 7/10, 8/12) on a Au(111) surface by a combination of electron spectroscopy (Auger, XPS, UPS), TPD, and theoretical modeling using DFT.172,173 The choice of the Au(111) substrate was based on previous studies that showed that MoS2 nanoclusters grown on a Au(111) substrate were reasonable models for exploring desulfurization activity.160,174,175 Moreover DFT calculations by Siefert and coworkers predicted that strong Au−S interactions could result in highly ordered cluster arrays on the Au(111) surface but that the cluster structures should remain nearly unperturbed relative to the unbound (i.e., gas-phase) cluster.166,176,177 Thermal desorption measurements of probe molecules such as CO and NH3 showed that exposed Mo-atom sites remain on the deposited clusters, but they become mobile at temperatures above 550 K and appear to form 2D islands with reduced adsorption sites. In the case of Mo4S6/Au(111), experiment and DFT calculations show that charge transfer from the Au surface modifies the adsorption behavior relative to the gasphase cluster.172 By contrast, the Mo6S8 cluster experiences almost no charge transfer from the Au surface.176 The binding strength of CO shows significant variations as a function of cluster size and structure (see Figure 22). In addition to having the highest CO binding energy, the Mo6S8/Au(111) surface exhibits the most available Mo-sites (at the same cluster coverage) and smallest range of CO binding energies.

which promote water dissociation but not via a redox process. The results for the Nb clusters also show that state of reduction and substrate (i.e., metal vs oxide) can strongly influence reactivity by inducing changes in metal coordination and cluster structure.

V. TRANSITION METAL SULFIDES Molybdenum sulfide (MoS2) is widely used in industry as a heterogeneous catalyst for hydrogenation (HYD), hydrodesulfurization (HDS), and hydrodenitrogenation (HDN) processes in oil refining.146−148 Typical HDS catalysts consists of nanoparticles of MoS2 (“platelets”) with Co or Ni promoters that are dispersed on a high surface area support, typically γAl2O3. It is generally accepted that the sulfur-terminated basal planes of the MoS2 platelets are unreactive and that the active sites for reaction are associated with metal sites (Mo or Ni/Co promoter atoms) located at the edges of the particles.149−151 Technical HDS catalysts are difficult to characterize at the atomic level due to their inherent structural and chemical heterogeneity, so that the preparation of model supported MoS2 catalysts with control over surface quality, termination structure, and particle size distributions are an attractive alternative. Besenbacher and co-workers have extensively studied MoSx nanostructures deposited on different supports (Au(111), TiO2(110), HOPG) using Mo PVD in the presence of H2S gas at low pressure.152−160 STM measurements show that the deposited MoS2 particles are composed of single S− Mo−S layers that exhibit metallic-like edge (“brim”) states.160,161 These brim states were shown to be associated with Mo-terminated edges that are capped by S atom dimers and also identified as active sites for H atom adsorption and thiophene reaction. Other studies suggest that vacancies in the sulfur-capped, basal planes of MoS2 are also reactive.162,163 Gas-phase studies and associated DFT calculations show that small MoxSy (x ≤ 10) clusters are typically substoichiometric (x/y < 2) and have three-dimensional structures with exposed Mo atoms at the edges (see Figure 21).29,30,164−167 Gas-phase collision studies have also shown that adsorbates like CO or

Figure 21. Mass spectrum of Mo sulfide clusters produced by reactive magnetron sputtering of a Mo target with a mixture of 4% H2S in Ar gas. Cluster structures obtained from DFT calculations using DFT with the DMol3 code (yellow: sulfur atoms, blue: molybdenum atoms). Adapted with permission from ref 30. Copyright 2008 American Chemical Society.

Figure 22. Strength of CO binding energies as a function of MoxSy cluster size deposited on Au(111). The vertical bars represent the width of the CO TPD peak. The coverage was 0.2 ML for all the clusters (yellow: sulfur atoms, blue: molybdenum atoms). 7169


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moment,30,165,166,184 it was expected that the surface dipoles reflect the direction of electron transfer at the cluster−alumina interface. The negative surface dipoles therefore imply electron transfer from the alumina surface to the MoxSy clusters. This is opposite to what would be expected for Type I bonding, where Mo−O bonding would lead to electron transfer from the cluster to the alumina film. By analogy with recent studies of electronic charging of Au deposits (adatoms and chains) on metal-supported ultrathin oxide films such as MgO/ Ag(100),22,185−192 the observed increase in work function was attributed instead to electron tunneling from the NiAl(110) support to the MoxSy clusters. Electron tunneling through the ultrathin alumina film, or “charging,” is favored by the large electron affinities of the MoxSy clusters165,166,184 and relatively low NiAl work function induced by the presence of the alumina film.193,194 As a result, the MoxSy clusters deposited on the Al2O3/NiAl(110) surface are electronically very different from those supported on bulk alumina (or thick films). This may explain the low reactivity of the MoxSy/Al2O3/NiAl(110) surface. Particularly intriguing is the possibility of modifying the reactivity properties of the MoxSy clusters by manipulating electron transfer at the interface by controlling the oxide thin film thickness.195 V. C. Theoretical Studies of Catalysis on MoxSy Clusters. DFT calculations by Liu and co-workers at BNL have also been used to investigate the possibility of using MoxSy nanoclusters for the conversion of syngas to C1−C2 oxygenates.196,197 Previous studies have shown that unsupported MoS2 catalyzes the conversion of syngas primarily to methane and other hydrocarbons.198,199 However, the addition of electrondonating promoters (e.g., K and Cs) shifts the products toward ethanol and other linear C2+ alcohols.200−202 Overall, the activity of promoted MoS2 is less than modified methanol catalysts, but its selectivity for ethanol and C2+ alcohol production can be significantly higher (up to 50% of total yield).148 Offsetting the lower activity of MoS2 catalysts are its low cost and its tolerance to carbon and sulfur contamination which eliminates the need for energy intensive preprocessing of the syngas feed. Like copper, MoS2 is effective in catalyzing the RWGS reaction so that both the formate and RWGS + COHydro pathways are viable mechanisms for methanol formation.203,204 DFT calculations of methanol formation from the hydrogenation of CO2 were performed on the unsupported, highly symmetric Mo6S8 cluster.197 The cluster exhibits generally moderate interactions with adsorbates and intermediates involved in methanol synthesis (i.e., CO2, CO, H, and CHxO), and C−O bond cleavage is suppressed. By contrast, calculations for a stoichiometric MoS2 cluster show that edgesites promote C−O scission of the HxCO intermediates, which ultimately leads to the formation of methane (or higher hydrocarbons). The reaction sequence on Mo6S8 shown in Figure 24 involves the conversion of CO2 to CO via the RWGS reaction and the HOCO intermediate. The CO is then hydrogenated to the HCO radical and ultimately to methanol. The rate-limiting step is CO hydrogenation to HCO. The corresponding barrier is ∼1 eV, which is lower than that on the Cu29 nanoparticle.205 Both Mo and S atoms actively participate in the reaction with Mo-sites adsorbing CO2, CO, and CHxO, as well as S-sites facilitating H2 dissociation by stabilizing the resulting H atoms. A more recent DFT study examined methanol synthesis on metal (M = K, Ti, Co, Rh, Ni, and Cu)-modified model Mo6S8

V.B. MoxSy Clusters Deposited on Al2O3/NiAl(110). As noted above, γ-Al2O3 is typically used as the support for commercial MoS2 catalysts for HDS. On bulk alumina surfaces the bonding of the MoS2 particles involve strong Mo−O bond linkages (Type I) or weak van der Waals interactions (Type II) between the S-terminated basal planes of the MoS2 particles and the oxygen terminated alumina surface.178 The Mo−O (Type I) linkages generally increase the Mo oxidation state via charge transfer to the support, whereas the weaker interactions involve no charge transfer. The charge transfer associated with the Mo−O linkages polarizes the nearby Mo−S bonds and decreases the tendency to form S-vacancies, which is correlated with lower HDS activity.179,180 The alumina plays no specific role in the HDS chemistry but is structurally stable and provides high dispersion for the MoS2 nanoplatelets under the harsh reaction conditions of HDS.148 To investigate the electronic and chemical properties of the MoSx-alumina interface the BNL group deposited MoxSy clusters onto an alumina thin film prepared by oxidation of a NiAl(110) susbstrate.140 Previous studies had shown that the Al2O3 film prepared by oxidation of the NiAl(110) surface is atomically flat with a thickness of approximately 5 Å, corresponding to two Al−O layers.181−183 TPD experiments using CO as a probe molecule show that the MoxSy clusters are more thermally stable against agglomeration than on Au(111), which is consistent with the stronger Mo−O bonds expected for binding at the alumina surface. Surprisingly, the MoxSy clusters deposited on the alumina thin film exhibited essentially no reactivity in TPR experiments with sulfur compounds such as OCS and thiophene. Bonding interactions at the MoxSy−alumina interface were probed by measurements of surface dipoles extracted from coverage-dependent work function measurements using 2PPE as described in Section IV. A.140 As shown in Figure 23, the MoxSy clusters induced an increase in the work function relative to the bare alumina film (i.e., negative surface dipoles). Because the gas-phase Mo2S6, Mo4S6 and Mo6S8 clusters are predicted to have symmetric structures with no permanent dipole

Figure 23. Measured work functions versus local cluster coverage for different molybdenum sulfide clusters deposited on a Al2O3/ NiAl(110) surface. The solid lines are least-squares linear fits. Insets: structures for the Mo2S6, Mo4S6, and Mo6S8 clusters based on previous DFT calculations. Yellow: sulfur atoms; Blue: molybdenum atoms. Reproduced with permission from ref 140. Copyright 2012 PCCP Owner Societies. 7170


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Figure 25. Variation of the methanol synthesis rate versus the difference in HOCO and HCOO binding energies for various M− Mo6S8 clusters (yellow: sulfur atoms, blue: molybdenum atoms, purple: M-atom). Reprinted with permission from ref 196. Copyright 2014 American Chemical Society.

Figure 24. Optimized potential energy diagram for methanol synthesis from CO2 and H2 on a Mo6S8 cluster. Thin horizontal bars represent stable reactants, products, and intermediates, and thick bars are transition states (Mo: cyan; S: yellow; C: gray; O: red; H: white). Reprinted with permission from ref 197. Copyright 2014 American Chemical Society.

for future size-selected experiments involving cluster−support interfaces in all areas of catalysis (hetero-, electro-, and photo-) and in other areas not discussed in this perspective (e.g., magnetic interfaces).12,206,207 One common theme to emerge from the ANL and BNL studies is the importance of cluster−support interactions in determining the structural, electronic, and reactivity properties in the cluster-supported system. The latter is not especially surprising given the fact that a significant fraction of the atoms in clusters of subnanometer size are in direct contact with the supporting surface. For the ANL studies of Con (n = 4, 7, 27) supported on ALD oxide films (Section III. D), a strong dependence of activity and selectivity was found for dehydrogenation of cyclohexene and C4+ hydrocarbon synthesis using different oxide supports with MgO showing superior performance over ZnO, TiO2, and Al2O3. Strong metal−metal oxide interactions were evident through the formation of a Co−Mg−O mixed oxide phase and the observation of oxidized Co (Co0, Co2+, Co3+) through GISAXS, GIXANES, and NEXAFS measurements. By comparison to the oxide supports, Con clusters deposited on the more weakly interacting ultracrystalline diamond films (UNCD) exhibited even higher activity for these reactions. The BNL studies of inverse catalysts composed of metal oxide clusters (Ti, Nb, Mo, W) supported on Cu(111) also showed strong variations in water dissociation activity that could be roughly correlated with interfacial electron transfer between the cluster and the Cu support (Sections IV. A and IV. B). Among the oxide clusters studied, those containing highly oxophilic cations (Mo6+, W6+) showed the most electron transfer from the Cu surface, but the surface was not active for water dissociation. By contrast, the more reducible oxides of Ti and Nb were characterized by low interfacial electron transfer and promoted water dissociation when deposited on the Cu surface. In the case of NbxOy clusters, both the reducibility and Lewis acid character of nearby cation sites play a role in its reactivity properties. Finally, the BNL study of molybdenum sulfide clusters supported on an ultrathin alumina film deposited on a NiAl(110) metal substrate show that electron tunneling or “charging” through the oxide film can result in cluster electronic and reactivity properties that are quite different than the same cluster deposited on the surface of the bulk oxide (Section V.B). The ability to control electron

catalysts.196 The results show that the catalytic behavior of a Mo6S8 cluster is changed significantly due to the modifiers, via electron transfer from the modifier metal to the cluster. Electron transfer induces reduction of the Mo cation (ligand effect) and the direct participation of the modifier in the reaction (ensemble effect) promotes some elementary steps. With the most positively charged modifier, the ligand effect in the case of K−Mo6S8 is the most obvious among the systems studied; however, it cannot compete with the ensemble effect, which plays a dominant role in determining activity via the electrostatic attraction which stabilizes the CHxOy species adsorbed at the Mo sites of Mo6S8. In addition, the modifiers also vary the optimal reaction pathway from the reverse water− gas shift (RWGS) + CO hydrogenation to a pathway involving a formate intermediate. The addition of K improves methanol formation the most, while Ti, Co, Ni, and Cu decrease the activity of Mo6S8. The relative stability of *HCOO versus *HOCO was identified as a descriptor of the preferred mechanism and activity (see Figure 25).

VI. SUMMARY AND OUTLOOK As we have seen in the examples discussed above, size-selected deposition is very effective for preparing model supported catalysts that can be used to explore fundamental aspects of size-effects, cluster−support interactions, and reaction mechanisms of cluster materials (metals, oxides, sulfides) and surface chemistry (e.g., dissociation, reduction/oxidation reactions) that is relevant to heterogeneous catalysis. By providing welldefined surfaces, size-selected experiments have also taken full advantage of modern electronic structure theory, which can essentially model the exact experimental system (i.e., cluster and support) used in the laboratory. As a result, theory provides essential information on cluster structure versus size and stoichiometry as well as mechanistic information on site-specific (undercoordinated atoms, cluster-support edge sites, Lewis acid cation sites) reactions which cannot be extracted directly from experiment. This powerful combination of atomically precise measurements and theory will continue to provide the impetus 7171


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NSLS-II at BNL are capable of operating at several Torr and could provide detailed electronic characterization of supported clusters via core level and valence band spectra.208,209 Similarly, new AP-STM instruments capable of operation with up to 10 Torr of background gases and temperatures up to 600 K would allow detailed studies of cluster size distributions, binding sites, atomic structure, and cluster density of states (scanning tunneling spectroscopy-STS) that are not accessible to GISAXS.210,211 Transport of cluster-based catalysts to standalone STM or synchrotron beamline instruments is best done without exposure to air to prevent carbon contamination and/ or oxidation. The use of actively pumped vacuum “suitcases”, load locks and common sample holders among instruments can make such transfers possible without colocating cluster deposition instruments at individual facilities. In addition to the implementation of new characterization techniques, cluster source development is another area that could broaden the types and mass ranges of cluster materials used to develop model supported catalysts. In this area, electrospray ionization (ESI) sources offer the opportunity to couple solution-based synthesis methods with gas-phase mass selection for deposition of species that are not accessible by sputtering or plasma techniques, for example, quantum dots, metal organic complexes. At PNNL, Laskin and co-workers have recently used an ESI source and mass-selection to deposit ligand-stabilized gold clusters onto supports.212−217 Especially, stable Au11L53+ (L = 1,3-bis(diphenylphosphino)-propane) clusters were found to retain different charge states (+1, 2, 3) when deposited onto self-assembled monolayers with different terminal groups.216,217 Collision-induced dissociation (CID) of the Au11L53+ cluster within the source was also used to generate Au10L42+ and Au8L42+ clusters,213 whose Aun cores have been previously studied by size-selected deposition of the bare Aun clusters.6,82,86,91 The ability of ligand-protected metal clusters to promote reactivity was recently reviewed by Li and Jin who showed that clusters such as Au25(SR)18 (SR = thiol-bonded ligand) are thermally stable up to ∼200 °C and have a Au13@Au12 core−shell structure which allows access to the outer shell of Au atoms.218 The Au25(SR)18 cluster was found to promote CO oxidation when supported on CeO2, especially after heating pretreatments to ∼150 °C in the presence of O2, but the authors claim that the ligands were still present on the cluster. For other reactions (e.g., styrene epoxidation), the ligands were removed by heating to 300 °C before activity was observed. In this regard, ESI sources may be especially useful in tailoring the reactivity and selectivity of ligand-stabilized clusters by controlling the number of ligands prior to deposition (e.g., by CID) so that heating pretreatments can be avoided. Despite the inherent challenges, ESI sources promise to introduce a much wider range of cluster materials with potential catalysis applications than currently available. Other developments which could impact cluster assembled catalysts include improved computational methods for predicting cluster structure on supports. Except in rare cases where STM can directly image the structure of a supported cluster (see, for example, ref 117), structural information will continue to come from computationally based structural optimizations using periodic DFT codes such as VASP. The starting point for such optimizations are typically the gas-phase cluster structures, if known, followed by manual searches of surface binding sites; for more rigid clusters like oxides and sulfides described in Sections IV and V, possible orientations of the cluster with respect to the surface must also be tried. In

transfer through variations in oxide support layer thickness has the potential for “tuning” the properties of clusters for enhanced activity. The results in this review also show that size effects are often more appropriately attributable to changes in cluster structure and stoichiometry, which in turn can be significantly modified by interactions with the support and/or by reaction conditions. In the ANL electrochemical studies of the water oxidation reaction, dramatic changes in activity were observed for Pd4 versus Pd6 clusters (Figure 11), where the size effects could be attributed to the formation of different Pd4On and Pd6On oxide structures caused by immersion in aqueous solution and under positive potential conditions (see Section III. E). The size dependence of activity and/or selectivity for other metal cluster (Ag) catalyzed reactions studied by the ANL group was more modest, which in some cases was the result of cluster aggregation under higher-temperature reaction conditions (see Section III. B). Nonetheless, the intact metal clusters or small aggregates formed under reaction conditions were found to be generally much more active than their conventional counterparts consisting of larger supported metal nanoparticles prepared by benchtop chemical methods (see, for example, Sections III. A and III. C). Also, subnanometer Ag clusters exhibited surprisingly strong size-dependent performance in LiAir batteries (see Section III. F). For deposited metal oxide clusters, the BNL studies showed that changes in size are often accompanied by significant changes in cluster structure which are further modified by deposition onto the substrate (see Figure 16). Moreover, changes in stoichiometry (i.e., metal-to-oxygen ratio) can be more important than size alone in determining interfacial electronic and reactivity properties of deposited metal oxide clusters. Increases in cluster size (i.e., more metal cations) generally induce larger electron transfer to or from the support consistent with more cation sites being able to accommodate larger changes in electron density without major changes in average oxidation state. Reactivity of metal oxide clusters (e.g., water dissociation) is less sensitive to size of the cluster as it is to the presence of specific active sites (i.e., NbO Lewis acid sites of NbxOy clusters) when deposited on metal or metal oxide surfaces (see Section V.A). These can be modified or even lost by strong bonding to another metal oxide surface (see Figures 19 and 20). The advantage of the small oxide clusters versus larger, bulk-like oxide particles is that the number of such active sites can be maximized on a per-atom basis and cluster−support interactions can be “tuned” to maximize the preferred chemical property. The ANL in situ studies highlight the importance of probing chemical and structural changes of cluster catalysts under catalytic reactions conditions (i.e., elevated temperatures and pressures). For example, X-ray-based GISAXS can follow morphological changes of submonolayer coverages of clusters, whereas GIXANES and NEXAFS can probe local electronic structure during the course of reaction. The former is especially important for investigating the effects of cluster aggregation on activity/selectivity (see Figures 5 and 8) and the latter for determination of oxidation/reduction of metal clusters (see Figures 7−10). In the future, these X-ray scattering and absorption methods could be complimented by new XPS and STM instruments capable of operating at near ambient pressures (NAP). High-pressure XPS instruments such as those operating at the Advance Light Source at Lawrence Berkeley National Laboratory (LBNL) and recently installed at 7172


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Contract No. DE-SC0012704. The accompanying DFT calculations were performed using computational resources at the Center for Functional Nanomaterials which is a DOE Office of Science User Facility located at Brookhaven National Laboratory. S.V. acknowledges the support by the U.S. Department of Energy, BES-Materials Science and Engineering, under Contract DE-AC-02-06CH11357, with UChicago Argonne, LLC, the operator of Argonne National Laboratory.

cases where no information on the isolated cluster exists, and for larger clusters consisting of tens of atoms, the optimization procedure is mostly guided by intuition (“biased” search), which provides little guarantee to locate the global minimum energy structure. Developments of efficient search algorithms for global optimizations (GO) have been applied to free cluster structures using both empirical potentials and DFT calculations to determine configurational energies.219,220 Empirical potentials are generally not appropriate for describing interatomic bonding in small clusters typical of size-selected cluster deposition experiments where quantum effects dominate. For the latter, DFT is used to optimize structures in local confined regions of the potential energy surface (PES) while the search algorithms (e.g., genetic or basin hopping) introduce stochastic sampling of other regions of the PES. Even for free clusters, the DFT-based search algorithms can require hundreds of DFT calculations and thereby require considerable computer resources. To date, only a few applications of these optimization methods for small metal clusters supported on surfaces have appeared in the literature.220−225 Vilhelmsen and Hammer recently employed a genetic-DFT search to optimize the structures of metal M8 (M = Ru, Rh, Pd, Ag, Pt, Au) clusters on a TiO2(110) surface where they have used parallelization and smaller unit cells to increase efficiency and reduce computational costs.225 The authors find that only Au8 and Ru8 maintain their gas-phase structures on the surface, whereas the other metal clusters adopt very different surface structures which are driven by interactions with the titania surface; they further argue that the latter structures could not be anticipated using nonautomated searches. A somewhat different approach was taken by Negreiros et al., who developed a reactive global optimization (RGO) method in which the search criteria are driven by low activation energy (Ea) paths determined by DFT quantum calculations.224 In this approach, preferred cluster and cluster−adsorbate structures are those which are energetically accessible at room temperature (Ea < 1.2 eV). The example RGO calculation of propylene oxidation on Ag3/MgO(100) was motivated by the reactivity study of Ag3 clusters deposited on alumina described in Section III. B.45 The results of the RGO method found that strong substrate and adsorbate interactions induced structural changes in the Ag3 cluster (vertical or flat lying) which in turn influenced energy barriers for oxygen dissociation and propylene epoxidation. These two examples suggest that intelligent search algorithms coupled with quantum calculations offer a more robust approach to obtaining the lowest energy structures of small supported clusters. It can be expected that such approaches will gain more widespread use as the computer resources needed are driven down by increased efficiencies in search algorithms and optimization of codes for advanced computer architectures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work performed by M.G.W. at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under 7173


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Catalysis Applications of Size-Selected Cluster Deposition - ACS

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