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 TiO2(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 TiO2(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 TiO2(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 TiO2(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 TiO2(110) surface is not an oxidation catalyst since the presence of oxygen turns it into a stoichiometric Au-n/TiO2(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 TiO2(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 TiO2(110). Furthermore, the activation energies associated with O-2 dissociation in the gas phase (Delta E-act>2.4 eV) are reduced by at least a factor of 2 when the clusters are supported on TiO2(110). (C) 2008 American Institute of Physics.