Adsorption and Diffusion of Pt and Au on the Stoichiometric and Reduced TiO$_2$ Rutile (110) Surfaces

A comparative first principles pseudopotential study of the adsorption and migration profiles of single Pt and Au atoms on the stoichiometric and reduced TiO$_2$ rutile (110)surfaces is presented. Pt and Au behave similarly with respect to (i) most favorable adsorption sites, which are found to be the hollow and substitutional sites on the stoichiometric and reduced surfaces, respectively, and (ii) the large increase in their binding energy (by $\sim$ 2 eV) when the surface is reduced. Pt, on the other hand, binds more strongly (by $\sim$ 2 eV) to both the stoichiometric and reduced surfaces. The migration profiles of the two metals also display interesting similarities and differences. In particular, although the energy barrier for both Pt and Au diffusion on the stoichiometric surface is rather low (0.1 - 0.2 eV), Pt displays a one-dimensional migration pattern, while Au migration is two-dimensional. In contrast, the energy barrier on the reduced surface is significantly higher ($\sim$ 0.5 eV) for Pt than Au. These results for Pt,Au/TiO$_2$ are discussed in connection with the activity of the two metals and the origin of the strong-metal-support-interaction phenomenon.