O2 evolution on a clean partially reduced rutile TiO2(110) surface and on the same surface precovered with Au1 and Au2: the importance of spin conservation.

We have used spin-polarized density functional theory (DFT) to study O(2) evolution on a clean partially reduced rutile TiO(2)(110) surface (i.e., a surface having oxygen vacancies) and its interaction with Au(1) or Au(2) cluster adsorbed on it. We assume that the total spin of the electronic wave function is related to the number of unpaired spins (N(s)) and calculate the binding and the activation energies involved in O(2) evolution for fixed values of N(s). In addition to keeping N(s) constant, we assume that reactions in which the N(s) of the reactants differs from that of the products are very slow. The potential energy surfaces obtained in this way depend strongly on N(s). For example, O(2) dissociation at the vacancy site on a clean partially reduced TiO(2)(110) surface is exothermic by 0.85 eV in the triplet state and the highest activation energy in the chain of reactions leading to the O(2) dissociation is 0.67 eV. In the singlet state, O(2) dissociation is endothermic by 0.11 eV and the activation energy leading to dissociation is 1.30 eV. These observations are in qualitative agreement with scanning tunneling microscopy experiment in which O(2) dissociation on a partially reduced rutile TiO(2)(110) surface is observed at temperature as low as 120 K. In contrast, O(2) dissociation is predicted to be endothermic and is prevented by an activation barrier larger than 1 eV in all the previous DFT calculations, in which the DFT program varies N(s) to get the lowest energy state. We find that on a partially reduced rutile TiO(2)(110) with Au(1) and Au(2) preadsorbed on its surface, O(2) dissociates at the vacancy site: One oxygen atom fills the oxygen vacancy and the other becomes available for oxidation chemistry. This means that Au(1) and Au(2) supported on a partially reduced TiO(2)(110) surface is not an oxidation catalyst since the presence of oxygen turns it into a stoichiometric Au(n)/TiO(2)(110) surface. Finally, we find that the evolution of oxygen on Au(1) and Au(2) in the gas phase is very different from the evolution on the same clusters supported on the partially reduced TiO(2)(110) surface. For example, the molecular adsorption of O(2) is favored in the gas phase (except on Au(1) (-) and Au(2) (-) in the quartet state), while the dissociative adsorption is favored by more than 1 eV when Au(1) and Au(2) are supported on the partially reduced TiO(2)(110). Furthermore, the activation energies associated with O(2) dissociation in the gas phase (DeltaE(act)>2.4 eV) are reduced by at least a factor of 2 when the clusters are supported on TiO(2)(110).

Enhanced adsorption energy of Au1 and O2 on the stoichiometric TiO2(110) surface by coadsorption with other molecules.

During heterogeneous catalysis the surface is simultaneously covered by several adsorbed molecules. The manner in which the presence of one kind of molecule affects the adsorption of a molecule of another kind has been of interest for a long time. In most cases the presence of one adsorbate does not change substantially the binding energy of another adsorbate. The calculations presented here show that the stoichiometric rutile TiO(2)(110) surface, on which one of the compounds -OH, Au(3), Au(5), Au(7), Na, K, or Cs or two different gold strips was preadsorbed, behaves differently: the binding energy of Au(1) or O(2) to such a surface is much stronger than the binding to the clean stoichiometric TiO(2)(110) surface. Moreover, the binding energy of Au(1) or O(2) and the amount of charge they take from the surface when they adsorb are the same, regardless of which of the above species is preadsorbed. The preadsorbed species donate electrons to the conduction band of the oxide, and these electrons are used by Au(1) or O(2) to make stronger bonds with the surface. This suggests that adding an electron to the conduction band of the clean stoichiometric TiO(2)(110) slab used in the calculation will affect similarly the adsorption energy of Au(1) or O(2). Our calculations show that it does. We have also studied how the preadsorption of Au(4) or Au(6) affects the binding of Au(1) or O(2) to the surface. These two gold clusters do not donate electrons to the surface when they bind to it and therefore should not influence substantially the binding energy of Au(1) or O(2) to the surface. However, adsorbing O(2) or Au(1) on the surface forces the clusters to change their structure into that of isomers that donate charge to the oxide. This charge is used by Au(1) or O(2) to bind to the surface and the energy of this bond exceeds the isomerization energy. As a result the surface with the isomerized cluster is the lowest energy state of the system. We believe that these results can be generalized as follows. The molecules that we coadsorbed with Au(1) or O(2) donate electrons to the oxide and are Lewis bases. By giving the surface high energy electrons, they turn it into a Lewis base and this increases its ability to bind strong Lewis acids such as Au(1) and O(2). We speculate that this kind of interaction is general and may be observed for other oxides and for other coadsorbed Lewis base-Lewis acid pairs.

Density functional study of the interaction between small Au clusters, Au(n) (n=1-7) and the rutile TiO(2) surface. I. Adsorption on the stoichiometric surface.

This is the first paper in a series of four dealing with the adsorption site, electronic structure, and chemistry of small Au clusters, Au(n) (n=1-7), supported on stoichiometric, partially reduced, or partially hydroxylated rutile TiO(2)(110) surfaces. Analysis of the electronic structure reveals that the main contribution to the binding energy is the overlap between the highest occupied molecular orbitals of Au clusters and the Kohn-Sham orbitals localized on the bridging and the in-plane oxygen of the rutile TiO(2)(110) surface. The structure of adsorbed Au(n) differs from that in the gas phase mostly because the cluster wants to maximize this orbital overlap and to increase the number of Au-O bonds. For example, the equilibrium structures of Au(5) and Au(7) are planar in the gas phase, while the adsorbed Au(5) has a distorted two-dimensional structure and the adsorbed Au(7) is three-dimensional. The dissociation of an adsorbed cluster into two adsorbed fragments is endothermic, for all clusters, by at least 0.8 eV. This does not mean that the gas-phase clusters hitting the surface with kinetic energy greater than 0.8 eV will fragment. To place enough energy in the reaction coordinate for fragmentation, the impact kinetic energy needs to be substantially higher than 0.8 eV. We have also calculated the interaction energy between all pairs of Au clusters. These interactions are small except when a Au monomer is coadsorbed with a Au(n) with odd n. In this case the interaction energy is of the order of 0.7 eV and the two clusters interact through the support even when they are fairly far apart. This happens because the adsorption of a Au(n) cluster places electrons in the states of the bottom of the conduction band and these electrons help the Au monomer to bind to the five-coordinated Ti atoms on the surface.

Density functional study of the charge on Aun clusters (n=1-7) supported on a partially reduced rutile TiO2(110): are all clusters negatively charged?

It is widely believed that small gold clusters supported on an oxide surface and adsorbed at the site of an oxygen vacancy are negatively charged. It has been suggested that this negative charge helps a gold cluster adsorb oxygen and weakens the O-O bond to make oxidation reactions more efficient. Given the fact that an oxygen vacancy is electron rich and that Au is a very electronegative element, the assumption that the Au cluster will take electron density from the vacancy is plausible. However, the density functional calculations presented here show that the situation is more complicated. The authors have used the Bader method to examine the charge redistribution when a Aun cluster (n=1-7) binds next to or at an oxygen vacancy on rutile TiO2(110). For the lowest energy isomers they find that Au1 and Au3 are negatively charged, Au5 and Au7 are positively charged, and Au2, Au4, and Au6 exchange practically no charge. The behavior of the Aun isomers having the second-lowest energy is also unexpected. Au2, Au3, Au5, and Au7 are negatively charged upon adsorption and very little charge is transferred when Au4 and Au6 are adsorbed. These observations can be explained in terms of the overlap between the frontier molecular orbitals of the gold cluster and the eigenstates of the support. Aun with even n becomes negatively charged when the lowest unoccupied molecular orbital has a lobe pointing in the direction of the oxygen vacancy or towards a fivefold coordinated Ti (5c-Ti) located in the surface layer; otherwise it stays neutral. Aun with odd n becomes negatively charged when the singly occupied molecular orbital has a lobe pointing in the direction of a 5c-Ti located at the vacancy site or in the surface layer, otherwise it donates electron density into the conduction band of rutile TiO2(110) becoming positively charged.

Density functional study of the interaction between small Au clusters, Au(n) (n=1-7) and the rutile TiO2 surface. II. Adsorption on a partially reduced surface.

We use density functional theory to examine the electronic structure of small Au(n) (n=1-7) clusters, supported on a rutile TiO(2)(110) surface having oxygen vacancies on the surface (a partially reduced surface). Except for the monomer, the binding energy of all Au clusters to the partially reduced surface is larger by approximately 0.25 eV than the binding energy to a stoichiometric surface. The bonding site and the orientation of the cluster are controlled by the shape of the highest occupied molecular orbitals (HOMOs) of the free cluster (free cluster means a gas-phase cluster with the same geometry as the supported one). The bond is strong when the lobes of the HOMOs overlap with those of the high-energy states of the clean oxide surface (i.e., with no gold) that have lobes on the bridging and the in-plane oxygen atoms. In other words, the cluster takes a shape and a location that optimizes the contact of its HOMOs with the oxygen atoms. Fivefold coordinated Ti atoms located at a defect site (5c-Ti(*)) participate in the binding only when a protruding lobe of the singly occupied molecular orbital (for odd n) or the lowest unoccupied molecular orbital (for even n) of the free Au(n) cluster points toward a 5c-Ti(*) atom. The oxygen vacancy influences the binding energy of the clusters (except for Au(1)) only when they are in direct contact with the defect. The desorption energy and the total charge on clusters that are close to, but do not overlap with, the vacancy differ little from the values they have when the cluster is adsorbed on a stoichiometric surface. The behavior of Au(1) is rather remarkable. The atom prefers to bind directly to the vacancy site with a binding energy of 1.81 eV. However, it also makes a strong bond (1.21 eV) with any 5c-Ti atom even if that atom is far from the vacancy site. In contrast, the binding of a Au monomer to the 5c-Ti atom of a surface without vacancies is weak (0.45 eV). The presence of the vacancy activates the 5c-Ti atoms by populating states at the bottom of the conduction band. These states are delocalized and have lobes protruding out of the surface at the location of the 5c-Ti atoms. It is the overlap of these lobes with the highest orbital of the Au atom that is the major reason for the bonding to the 5c-Ti atom, no matter how far the latter is from the vacancy. The energy for breaking an adsorbed cluster into two adsorbed fragments is smaller than the kinetic energy of the mass-selected clusters deposited on the surface in experiments. However, this is not sufficient for breaking the cluster upon impact with the surface, since only a fraction of the available energy will go into the reaction coordinate for breakup.

Pinning mass-selected Agn clusters on the TiO2(110)-1x1 surface via deposition at high kinetic energy.

We present the first scanning tunneling microscopy (STM) study of the deposition of mass-selected silver clusters (Ag(n),n=1, 2, 3) on a rutile TiO(2)(110)-1x1 surface at room temperature under hard-landing conditions. Under hard-landing conditions, only small features are observed on the surface in all cases without sintering or surface damage. This suggests that the high impact energy of the clusters mainly dissipates as thermal energy in the substrate, resulting in the recovery of any initial impact-induced surface damage and the formation of bound clusters on the surface near the impact point. STM images indicate that Ag(1) binds on the bridging oxygen rows twice as often as on the Ti rows. Density-functional Theory (DFT) calculations are consistent with Ag(1) binding at either bridging oxygen vacancies or with two adjacent bridging oxygen atoms in the same bridging oxygen row. STM images of Ag(2) and Ag(3) depositions indicate almost exclusive binding centered on the Ti-atom rows. DFT calculations suggest that the Ag(2) and Ag(3) clusters are bound between two bridging oxygen rows, which is consistent with the STM observations.

Where does the planar-to-nonplanar turnover occur in small gold clusters?

Several levels of theory, including both Gaussian-based and plane wave density functional theory (DFT), second-order perturbation theory (MP2), and coupled cluster methods (CCSD(T)), are employed to study Au6 and Au8 clusters. All methods predict that the lowest energy isomer of Au6 is planar. For Au8, both DFT methods predict that the two lowest isomers are planar. In contrast, both MP2 and CCSD(T) predict the lowest Au8 isomers to be nonplanar.

Binding of propene on small gold clusters and on Au(111): simple rules for binding sites and relative binding energies.

We use density functional theory (DFT) to investigate the bonding of propene to small gas-phase gold clusters and to a Au(111) surface. The desorption energy trends and the geometry of the binding sites are consistent with the following set of rules. (1) The bond of propene to gold is formed by donation of electron density from the highest occupied molecular orbital (HOMO) of propene to one of the low-lying empty orbitals [denoted by LUMO1, LUMO2, em leader (LUMO-lowest unoccupied molecular orbital)] of the gold cluster. (2) Propene binds to a site on the Au cluster where one of the low-lying LUMOs protrudes in the vacuum. Different isomers (same cluster, but different binding sites for propene) correspond to sites where different low-lying LUMOs protrude in space. (3) The desorption energy of the lowest energy isomer correlates with the energy of the lowest empty orbital of the cluster; the lower the energy of that LUMO, the higher the desorption energy. (4) If the lowest-lying LUMO protrudes into space at two nonequivalent sites at the edge of a cluster, propene binds more strongly to the site with the lowest coordination. These rules are consistent with the calculated bond energies and geometries for [Au(n)(C(3)H(6))](q), for n=1-5 and n=8 and q=-1, 0, +1. Based on them we have made a number of predictions that have been confirmed by DFT calculations. The bond of propene to gold is strengthened as the net charge of the cluster varies from -1, to zero, to +1. Compared to a gas-phase cluster, a cluster on a support binds propene more strongly if the support takes electron density from the cluster (e.g., a Au cluster on a gold surface) and more weakly if the support donates electron density to the cluster (e.g., a Au cluster on an oxygen vacancy on an oxide surface).