The Effects of the MgO Support and Alkali Doping on the CO

J. Phys. Chem. C 2009, 113, 7329–7335

7329

The Effects of the MgO Support and Alkali Doping on the CO Interaction with Au Giannis Mpourmpakis†,‡ and Dionisios G. Vlachos*,† Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716, and Institute of Electronic Structure and Laser, FORTH, Heraklion 71110, Crete, Greece ReceiVed: January 8, 2009; ReVised Manuscript ReceiVed: March 10, 2009

First-principle calculations have been performed to examine the CO interaction with unsupported and supported Au clusters on a (001) MgO surface. It is found that the support plays a minor role on the CO chemisorption on the edge and corners of the supported clusters, induces minor structural changes, and transfers a small charge to the clusters. Linear relationships for the CO binding energy vs the Au atom coordination number are found. The support strengthens the physisorption of CO on some planar sites of the clusters. It is found that small supported Au clusters behave like molecules, and classical concepts (e.g., the expansive strain of metals) may not be appropriate to describe adsorption. Alkali doping of the structures is also investigated. We found that the presence of two alkali metals deforms completely the gold clusters. Upon alkali doping, no linear relationships for the CO binding energy vs the Au coordination number are found. Introduction CO is a reactant or a product in several catalytic reactions. For instance, CO is oxidized in catalytic converters to control car emissions.1 In proton exchange membrane (PEM) fuel-cells, minute fractions of CO poison the fuel-cell catalyst. Reforming of natural gas produces syngas that may undergo the water-gas shift reaction2,3 to convert CO to CO2 and produce hydrogen. To minimize the remaining CO content in the H2 stream in order to avoid poisoning of the fuel-cell catalyst, preferential oxidation of CO can be carried out. It becomes apparent that the CO binding energy on catalysts is of great interest, since it determines the CO coverage, its residence time on the catalyst surface, and whether CO poisons a catalyst. Moreover, the ratio of desorption to oxidation of CO controls the carbon selectivity of the transformation of natural gas to syngas. Au is one of the most commonly studied metal catalysts in recent years. While bulk gold is a chemically inert material,4 Haruta’s pioneering work5 showed exceptional reactivity of Au nanoparticles of 2-5 nm in diameter. The low temperature CO oxidation6-12 on Au has created intense interest. Model catalysts, which consist of metal oxide surfaces onto which metal particles are deposited, have been used in experiments.13-17 Reducible supports result in the highest activity for CO oxidation on Au, whereas irreducible supports are not as active.13 TiO4 was found to be an extremely active support. For this reason, Au nanoparticles on the (110) TiO2 have been a prototype catalyst.6,14,18 Despite the large volume of work on the CO oxidation on Au, the system is far from being fully understood,19 since there is still much debate on the reaction mechanism. A consensus about the active sites of Au is emerging.20 Experimentally, it has been suggested that active sites are along the interface between Au particles and the metal oxide support.21-24 CO oxidation experiments25,26 on MgO supported Au nanoparticles combined with spectroscopic characterization of the catalyst revealed that the catalytically active species consist of zerovalent Au interacting with cationic Au, present at the support-particle * Corresponding author. E-mail: [email protected].; Telephone: (001) 302-831-2830. † University of Delaware. ‡ Institute of Electronic Structure and Laser.

interface. More significantly, the CO oxidation rate was found to be dependent on the partial pressure of CO and independent of O2, and there was no evidence of any cluster size effect on the catalytic activity.26 Among theoretical studies on metal-oxide supported gold clusters,27-36 many support the idea of an active metal particle/ support interface.28,29,31-36 Calculations on unsupported Au (particles, surfaces) indicate that low coordination sites are more active.34,37-43 A common result of the theoretical studies is that CO preferentially interacts with low coordinated Au atoms.20 Mavrikakis and co-workers37,39,44 showed that expansive strain can increase the reactivity of metals, a behavior that can occur on supported particles due to their lattice mismatch with the support. Regarding the MgO supported gold clusters,28,29,31,45,46 it has been proposed that the MgO surface can transfer a relatively small amount of electron density to the cluster and activate it. Increased reactivity is observed when the surface has an oxygen vacancy.28,46 In addition, it has been theoretically and experimentally shown that CO oxidation strongly depends on the charge state of the gold clusters.46-49 Prestianni et al.50 demonstrated that O2 interacts stronger with neutral clusters by acting as an electron acceptor, whereas CO preferentially interacts with positively charged species by acting as an electron donor.51 O2 dissociation on Au exhibits large energy barriers.37,38 The CO oxidation on Au occurs via the following steps: Gaseous O2 reacts with adsorbed CO and forms CO2 and an O atom, via the formation of a metastable CO-O2 intermediate (CO* + O2 f CO2 + O*; a slow reaction). CO then reacts with atomic O to form CO2 (CO* + O* f CO2; a fast reaction). The barrier of the first step reaction is calculated to be approximately 9-10 kcal/mol (9.2 kcal/mol on Au10 cluster34 and 10.6 kcal/mol on Au steps38), whereas the barrier of the second reaction step is approximately 4-6 kcal/mol (3.7 on the Au10 cluster34 and 5.8 on Au steps38). It becomes apparent that the binding energy of the adsorbates, especially of CO, on different coordinated Au atoms of the metal cluster, is of importance since it considerable affects the CO oxidation reaction rates. Moreover, the charge state of the atoms on the supported Au cluster is expected to affect the CO binding strength.

10.1021/jp900198m CCC: $40.75  2009 American Chemical Society Published on Web 04/03/2009

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Mpourmpakis and Vlachos

Density functional theory (DFT) studies are either limited to very small clusters, when using molecular orbital methods due to the sheer computational cost, or, apply periodicity to simulate extended systems (periodic slabs). In this work, we examine the CO interaction with unsupported Au clusters and Au clusters supported on a MgO surface, by performing massive cluster type DFT calculations (use of atomic basis sets). We attempt to separate electronic from strain effects induced by the support to elucidate key parameters affecting the CO binding. We retrieve linear relations between the binding energy (BE) of CO and the coordination number (CN) of Au atoms. Most importantly we address whether small Au clusters supported on a metal oxide behave as molecules or as bulk metals. Alkali metals are commonly used as promoters30,45,52 to increase the reactivity of metals. For this purpose, we investigated also the role of potassium doping of unsupported and MgO supported Au clusters in CO binding.

Figure 1. CO interaction with all different sites of unsupported Aun (n ) 6, 12, 20) clusters. The numbers reflect binding energies in kcal/ mol.

Computational Methods We examined the CO binding energy on all possible sites of the unsupported and MgO supported Au6, Au12, and Au20 clusters. In addition, we investigated the effect of alkali doping with potassium atoms on the CO interaction. Doping with two potassium metals rather than one allowed us to systematically investigate closed shell structures and explore the effect of more than one dopants.45 All calculations were carried out using the Becke-Perdew86 (BP86) functional,53,54 the resolution of the identity (RI)55 approximation to the Coulombic energy and the sv(p)56 basis set (default ECPs were included for the Au atoms) as implemented in the Turbomole 5.9.0 program.57 This combination of method and basis set has been used with success to investigate the structure and stability of small gold clusters.58,59 The calculated binding energy results should be viewed qualitatively (trends) under this basis set. The support was modeled with a Mg56O56 cluster. We chose the MgO support since it does not consist of transition metals and allows us to model a large cluster. This MgO cluster size is the maximum affordable size for our calculations, accounting also for the presence of Au cluster (up to Au20) on it. All Au clusters and the Mg56O56 support were fully optimized without any symmetry constraints. For the supported gold clusters, the Mg56O56 coordinates were kept frozen during optimization (relaxation of gold clusters and CO). To decouple strain from electronic effects, we removed the supported gold clusters from the support while freezing their geometry to that of the fully relaxed supported clusters. We term these as strained-unsupported clusters. Then we examined their interaction with CO (optimization of CO coordinates only). The CO binding energy (BE) on unsupported, MgO supported and strained-unsupported Au clusters were calculated respectively:

BECO ) EAu-CO - EAu - ECO

(1)

BECO ) EMgOAu-CO - EMgOAu - ECO

(2)

BECO ) E(f)Au-CO - EAu - ECO

(3)

Here E is the total energy (total electronic energy; thermal and zero-point energy corrections are not included) of each system (full optimization to minima) and E(f) is the total energy of the system when freezing the Au cluster’s geometry to that of the MgO supported clusters. CO Interaction with Unsupported and MgO Supported Au Clusters. We examined the CO binding on all binding sites of the Au6, Au12, and Au20 clusters. The first two clusters are planar and have a distorted C3V symmetry, whereas Au20 has an

Figure 2. CO interaction with different sites of the Aun (n ) 6, 12, 20) clusters supported on the MgO (001) surface. Binding energies are in kcal/mol.

icosahedral symmetry. Au clusters are known to be planar up to Au1360. The Au6, Au12, and Au20 clusters are known to be very stable.60-62 In particular, the Au20 icosahedron62 was found to be extremely stable in experiments and calculations due to a large HOMO-LUMO energy gap. CO binds the metals through a formally single M-CO bond known as the linear form. In this form, CO is held by a “push-pull” bond in which charge is transferred from the 5σ orbital of CO into the metal’s d-band, and charge is back-donated from the top of the metal’s d-band into the vacant 2π* antibonding orbital of CO20. The fully optimized structures and the corresponding CO binding energies are presented in Figure 1. Only binding on nondegenerate sites is shown. After relaxing the gold clusters on the MgO (001) surface, we investigated the CO interaction with the same sites, presented in Figure 1 (plus a few additional ones, due to lack of degeneracy). The relaxed structures as well as the binding energies of CO are presented in Figure 2. Overall the binding energy of CO on supported Au structures changes slightly compared to the unsupported ones. The CO binding energy on the planar sites of the clusters increases in the Au6 and Au12 case, whereas it is not affected from the support in the Au20 case. In particular, CO does not bind on the planar site of the unsupported Au6 cluster, whereas on the planar

CO Interaction with Au Clusters on a MgO Surface

Figure 3. CO interaction with different sites of the strained from the support Aun (n ) 6, 12, 20) clusters. Binding energies are in kcal/mol.

site of the supported cluster it physisorbs with -0.6 kcal/mol binding energy (upper right structure of Figure 2). In both cases, the initial structural configuration of CO was the same (interaction with the Au plane in a vertical position). Moreover CO was weakly physisorbed on the planar site of the unsupported Au12 cluster, with a binding energy of -0.8 kcal/mol. This (absolute) value increases to -4.6 kcal/mol on the planar site of the supported cluster. The CO binding energy decreases slightly on the corners of the supported clusters. The unsupported Au6 corner binds CO with -20 kcal/mol which decreases to -19.4 kcal/mol (mean value of -20.4 and -18.3) in the supported cluster. The same behavior is observed for the Au12 and Au20 clusters. The CO binding energy decreases from -20.2 kcal/mol to -17.5 kcal/mol in the corners of the Au12 and from -15.6 kcal/mol to -13.3 kcal/mol (mean value of -13.5 and -13.1) in the corners of the Au20 cluster in the presence of the support. In contrast, the binding energy change on the edges of the clusters in the presence of the support appears to depend on the cluster size. For instance, the support increases the CO binding energy on the Au6 edge (from -10.7 kcal/mol to -14.3 kcal/mol), does not affect the binding energy on the Au12 edge, whereas it slightly decreases the binding energy on the Au20 edge (from -9.2 kcal/mol to -8.6 kcal/mol mean value). By removing the optimized gold clusters from the support with frozen coordinates and calculating the binding energy of the CO (optimization of the CO coordinates only), we assess the strain effect on this energy change. These results are presented in Figure 3. In every case, the CO binding energy on the strained clusters decreases, compared with the gas-phase clusters. This effect is rationalized below. The CO binding energy on the (fully relaxed) unsupported gold clusters is linearly related to the coordination number of the gold atom that it is bound to, in accordance with other theoretical studies.43 That is, the less coordinated gold atoms bind CO stronger. This behavior is clearly depicted in Figure 4 (back 0). Figure 4 also compiles the data of Figures 2 and 3, excluding the planar sites that weakly bind CO (physisorption), since such sites are not candidate sites for catalytic reactions under many conditions. We comment on physisorption later on since some systematic trends are observed (for these calculations, electron correlation methods have to be applied combined with large basis sets). The calculated binding energy of CO on an atom of coordination number 6 of MgO supported Au clusters (red b) is approximately -10 kcal/mol. Our cluster model result

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7331

Figure 4. CO binding energy on differently coordinated gold atoms when interacting with gold clusters (black 0, linear fit relation: BE ) -26.1 + 3.0 · CN, R2 ) 0.72) MgO supported gold clusters (red b, linear fit relation: BE ) -23.2 + 2.4 · CN, R2 ) 0.84) and strained gold clusters (blue 2, linear fit relation: BE ) -22.7 + 3.4 · CN, R2 ) 0.74).

is in excellent agreement with periodic DFT calculations on MgO supported Au34 nanoparticle,32 which gave the CO binding Au atoms of coordination number 6 of -10 to -11 kcal/mol. In addition, our extrapolated binding energy for a coordination number of 9 is in the range of 0 to -2 kcal/mol (Figure 4, depending on which relation of the three is used), in good agreement with the binding energy of ∼-4 kcal/mol on Au (111) (see review63). Figure 4 indicates that the MgO support (red b, red line) plays a minor role in the CO chemisorption (with respect to the unsupported ones), and the coordination number of the Au atoms appears to be the major descriptor for the Au-CO interaction. This can be attributed to the fact that, overall, the MgO support interacts weakly with the Au clusters, so that the clusters practically do not change their gas-phase geometry.64 In order to further understand this point, we calculated the Au6, Au12, and Au20 interaction energy with the MgO support to be -36.3, -64.5, and -77.5 kcal/mol, respectively. Equivalently, this binding energy (BE) corresponds to -6.0, -5.4, and -7.8 kcal/mol per Au atom that interacts with the support (BE/n, where n is 6, 12, and 10 for the Au6, Au12, and Au20 clusters, respectively). Furthermore, the change in their bond lengths due to the support is small. Specifically, the unsupported Au6, Au12, and Au20 clusters have mean bond lengths of 2.74, 2.78, and 2.84 Å that increase to 2.76, 2.80, and 2.88 Å, respectively, for the supported clusters (the bond length changes in the Au20 cluster were calculated for the planar atoms interacting with the support). This corresponds to ∼1% strain effect. The strained Au clusters do not shift their d-band center; rather, they have identical density of states (DOS) with the gas-phase clusters, as shown in Figure 5. The lack of significant changes in the d-band center (DOS) is attributed to having clusters instead of extended metal surfaces and consequently to the discrete nature of the molecular orbital energy levels (see also below). Subtraction of eq 1 (CO binding energy on unsupported clusters) from eq 3 (CO binding energy on strained clusters) gives the difference in the CO binding energies due to strain:

∆BE ) BE(eq3) - BE(eq1 ) ) E(f)Au-CO - EAu-CO (4) The term EAu-CO is always lower than E(f)Au-CO since the strained clusters are close but not in the minimum geometry of the fully optimized ones (gas-phase structures). As a result, the ∆BE term is positive, so that the CO binding on the strained

7332 J. Phys. Chem. C, Vol. 113, No. 17, 2009

Figure 5. Density of states (DOS) plots of the unsupported (solid lines) and strained (dashed lines) Au6, Au12, and Au20 clusters. Zero is the Fermi level.

clusters decreases compared to the unsupported ones. Mavrikakis et al.44 showed that an expansive strain of 1% on the lattice constant of a Ru(0001) surface affects O’s binding but has a negligible effect on the CO’s binding (it increases by approximately only 0.3 kcal/mol). In our case, the binding energy of CO decreases (Figure 4) due to the fact that the strained Au clusters are not in their ground-state geometry (gas-phase structures). This difference between extended surfaces and clusters raises the question of whether strain (a concept at the large size limit) is an appropriate concept for small catalyst clusters (see below). A natural bond orbital (NBO) population analysis of the MgO supported gold systems revealed that the support transfers a small electron density to the Au clusters that is proportional to the cluster size (Aun) with q/n ) -0.03 for all n. The total charge transferred to the clusters, q(Au6) ) -0.2, q(Au12) ) -0.4, q(Au20) ) -0.6 |e|, is in good agreement with the 0.5 |e| transferred from the MgO to the Au8 cluster computed by Sanchez et al.28 Less stable gold structures (comparing the energy gap between the HOMO and the LUMO levels) are more affected from the support regarding CO physisorption. The stability of the clusters according to the calculated HOMO-LUMO gaps is Au12 ) 0.68 < Au6 ) 2.27, Au20 ) 2.00 eV. These values are in good agreement with the calculated HOMO-LUMO gaps reported in other theoretical works: Au6 ) 2.06,60 Au12 ) 0.9760 and Au20 ) 1.8062 eV. The energetic stability appears to be correlated with the change in the physisorption energy of CO on supported clusters:

∆BE(CO): Au12 ) 4 > Au6 ) 0.6, Au20 ) 0 kcal ⁄ mol The more stable a gold cluster is, the less is structurally and electronically affected from the MgO support and less affects the physisorption energy of CO. Our results are in agreement with those of Molina and Hammer, who found the Au20 cluster to be extremely robust against distortions when supported on a MgO surface and its activity to be practically unchanged compared to that of the unsupported cluster.45 Given that the strain decreases the CO binding energy on the clusters and that the supported clusters hold a small negative charge on their surfaces (CO preferentially interacts with positive charged clusters50 acting as an electron donor molecule), one would expect that the MgO support decreases significantly the CO binding on the Au clusters. However, such an approach fails to describe the CO adsorption behavior on MgO supported Au clusters, according to Figure 4. Strain does not appear to

Mpourmpakis and Vlachos

Figure 6. HOMO, LUMO, and LUMO+1 orbitals of the gas-phase Au12 cluster and the corresponding molecular orbitals when the cluster is supported on MgO.

be a suitable concept for small supported clusters because the clusters’ bond lengths are subjected to changes eVen when charge is transferred through the support-cluster interface (not only due to lattice mismatch between the cluster and the support). The CO adsorption on Au supported clusters depends on the electron density of the clusters. The latter may be similar to that of the gas-phase clusters (since there are no significant changes in the clusters’ geometry), but perturbed, due to the support-cluster interactions (hybridization of support’s molecular orbitals with the ones of the Au clusters). This hybridization will result in changing the coefficients of the molecular orbitals, which contribute to the CO-Au bonds and the CO binding energy compared with the unsupported and strained clusters. In Figure 6, we show the HOMO and LUMO orbitals of the gas-phase Au12 cluster and supported Au12 cluster. It is clear that the HOMO orbital of the gas-phase Au12 cluster is perturbed by the presence of the support in the MgO supported Au12 cluster (similar behavior is observed for the LUMO orbital located at the corners of the Au12). Our results clearly show that in order to understand the factors controlling CO adsorption on small Au clusters when supported on metal oxides, we need to consider the clusters as molecules and not as bulk metals. This conclusion is underscored in a recent review paper.65 In particular, the authors proposed rules regarding the binding of electron donor molecules (including CO). These rules can be applied to our supported clusters. More specifically, an electron donor molecule binds strongest at a site of the cluster where the LUMO orbital protrudes, and the next strongest binding site is where the LUMO+1 (second lowest in energy unoccupied molecular orbital) protrudes. Figure 6 shows that the LUMO orbital is localized at the corners of both the unsupported and MgO supported Au12 cluster, whereas the LUMO+1 at the edges. These localizations are consistent with the binding energies in Figures 1 and 2. By comparing the LUMO and LUMO+1 orbitals of the unsupported with the MgO supported Au12 cluster (Figure 6), it is seen that the LUMO+1 orbital is completely unaffected by the presence of the support, whereas the LUMO orbital slightly changes its directionality (at the corners of the cluster) in the presence of the MgO. The latter is responsible for the CO binding energy decrease (from -20.2 kcal/mol to -17.5 kcal/mol) on the corners of the Au12 cluster by the support; the former for the lack of change on its edges (from -13.3 kcal/mol changed to -13.6 kcal/mol). CO Interaction with Alkali Doped Unsupported and Supported Au Clusters. Next, we investigate doping effects on the CO binding energy by relaxing unsupported and supported Au6, Au12 and Au20 in the presence of two potassium atoms. We have chosen two potassium atoms to avoid ending

CO Interaction with Au Clusters on a MgO Surface

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7333 TABLE 1: Adiabatic and Vertical Electron Affinity (AEA, VEA) and Adiabatic and Vertical Ionization Potential (AIP, VIP) Values of the MgO Support, the Unsupported Au Clusters and the Supported Ones

a

Figure 7. Aun (n ) 6, 12, 20) clusters interacting with two potassium atoms: in (a) unsupported and (b) supported on a MgO (001) surface. The potassium atoms are shown in purple.

Figure 8. CO binding energy on differently coordinated gold atoms when interacting with gold clusters (black 0, from Figure 4), potassiumdoped gold clusters (blue 2) and potassium-doped MgO supported gold clusters (red b). The black dashed line is a guide to the eye depicting the correlation of the black squares.

up in spin-polarized systems having doublet spin states, rather than singlet, which was the case of the undoped structures. The optimized structures are presented in Figure 7. Figure 7 clearly demonstrates that the two alkali metals deform the clusters. We further investigated the interaction of those systems with CO in several binding sites of the clusters and with the potassium atoms. The optimized structures together with the binding energies are presented in Figures 1S and 2S of the Supporting Information. Both chemisorption and physisorption cases were investigated. We observed that despite the fact that the structure of the Au12 cluster (and the MgO supported Au12 cluster) slightly changed when interacting with the potassium atoms, it deformed upon interacting with the CO. Moreover, the potassium atoms create new physisorption sites, since CO can interact with the alkali metals with a binding energy of -3 to -4 kcal/mol. In order to assess the alkali doping effect on the CO binding energy, we plotted in Figure 8 the chemisorption energies of CO vs the coordination number of the interacting gold atom. It becomes apparent that the presence of alkali metals totally changes the CO binding energy and no correlation exists with the Au coordination number.

System

AEA/VEAa

AIP/VIPa

Au6 Au12 Au20 MgOAu6 MgOAu12 MgOAu20 MgO

-52.6/-50.6 -78.1/-78.1 -64.6/-61.24 -56.8/-54.1 -70.9/-68.9 -50.3/-46.1 -21.3 (VEA)

193.7/194.6 172.0/172.8 170.9/171.8 132.4/132.6 133.2/133.3 132.5/132.5 129.3 (VIP)

Values are in kcal/mole.

In order to explain this result, we calculated the ionization potentials (IP) and electron affinity (EA) values of the interacting systems. The results are presented in Table 1. The calculated IP of the potassium is 102.9 kcal/mol. By comparing this value with the IP of the unsupported and the supported Au clusters, we observe that IP (K) < IP (MgO supported clusters) < IP (Au clusters). This means that independent of the state of the Au cluster (unsupported or supported on MgO), when the clusters interact with K atoms, the outer s electron flows from the K atoms to the Au clusters. As a result the Au clusters are charged negatively and the potassium atoms positively (similar methodology has successfully been applied to understand the electron flow in metalcarbon nanotube contacts66 and in alkali doped carbon nanotubes67). The K-Au clusters interaction is of electrostatic character, resulting in strong K-Au binding and deformation of the Au clusters (e.g., we calculated the K-Au12 interaction to be 57.2 kcal/mol). In order to assess whether this behavior is alkali element specific, we also investigated the interaction of the unsupported Au6 and MgO-supported Au6 with Cs; the results were the same as in the potassium case. Comparing our results with the ones of Molina and Hammer,45 we notice that one alkali metal does not deform the clusters (data not shown) since one electron can be accommodated in the Au clusters. However, when two electrons are transferred to the clusters from two alkali atoms, the clusters deform, indicating that there is a maximum charge that the clusters can accommodate. The structural deformation of the Au clusters can be viewed as a two-step process. First, the two electrons that flow from the K atoms to the Au clusters destabilize the clusters electronically. Full optimization of the Au6, Au12, and Au20 in the -2 charge state showed that the clusters keep the geometry of the neutral state, but exhibit very small HOMO-LUMO gaps (an unstable electronic structure). Second, the presence of the K atoms and the strong Au-K bonds cause cluster deformation. The fact that no linear relationships of CO binding energy on Au exist when alkali metals are present (Figure 8) can partially be attributed to two competing factors. The deformed clusters have atoms with decreased coordination number that strengthens the CO binding energy. On the other hand, the negative charge of the clusters weakens the CO interaction because CO acts as an electron donor.50,51 Alkali doping is a more effective way to charge the supported cluster than the charge transferred from the (MgO) support itself. It has been shown that even the presence of oxygen vacancies (F+ centers) on the MgO support are not capable to charge the Au20 cluster and activate it for the CO oxidation reaction.45 In fact, the Au20 cluster was found to be slightly more active when interacting with a F+ center than when being unsupported and neutral. In addition, when the Au20 cluster interacts with the

7334 J. Phys. Chem. C, Vol. 113, No. 17, 2009 vacancy, small cluster distortions are observed.45 Finally, the number of defects/vacancies on alkaline-earth oxides is too small to have a large effect, and it has to be artificially increased via electron bombardment or by producing substoichiometric oxides.68 Overall, this study shows that when small clusters interact with an ideal MgO surface, it is the surface, low coordinated Au atoms that determine the CO binding and not the charge transfer from the support to the clusters. Conclusions First-principle calculations have been performed to investigate the CO interaction with unsupported and MgO (001) supported Au6, Au12 and Au20 clusters. Our results showed that the MgO support has a minor effect on the CO chemisorption energy on Au (on corner and edge sites of the clusters). The Au coordination number is the major descriptor for the CO binding on gas-phase, strained, and MgO supported Au clusters. The CO binding energy decreases with increase of the Au atom coordination number. The support strengthens physisorption on the planes of certain gold structures (Au6 and Au12). The MgO support induces minor structural changes and transfers a negligible charge to the clusters, due to the relatively weak support-cluster interactions. However, these interactions affect the Au cluster’s molecular (LUMO) orbitals (but not the LUMO+1 orbitals) that participate in CO’s bonding. Classical concepts, such as the effect of expansive strain on adsorption, may not be suitable for small, metal oxide supported Au clusters; these behave like molecules rather than extended metals. Interestingly, Au clusters that are doped with two alkali atoms deform. This behavior is attributed to the flow of electrons from the alkali metals to the gold clusters, which in turn was attributed to the electronic properties of the interacting systems (ionization potential). Very stable structures, such as the Au20 cluster, can accommodate up to one electron while retaining their gas-phase geometry. The CO binding energy vs Au coordination number in the alkali-doped structures does not show a linear relationship. Two competing factors are responsible for this behavior, the decrease of Au coordination number and the negative charge transferred to the clusters. The former increases and the latter decreases the CO binding. Acknowledgment. This research was supported by a Marie Curie International Outgoing Fellowship to G.M. within the seventh European Community Framework Programme and by the Department of Energy (DE-FG02-05ER257022). However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE. G.M. also thanks Dr. Andreas Mavrandonakis from Institute for Nanotechnology, Forschungszentrum Karlsruhe for fruitful discussions. We also acknowledge valuable discussions with Dr. A. N. Andriotis. Supporting Information Available: Figures of CO interaction with K-doped unsupported and MgO-supported Au clusters and coordinates of structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kaspar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77, 419. (2) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935. (3) Liu, Z.-P.; Jenkins, S. J.; King, D. A. Phys. ReV. Lett. 2005, 94, 196102. (4) Hammer, B.; Norskov, J. K. Nature (London) 1995, 376, 238. (5) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405.

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