Adsorption, Oxidation State, and Diffusion of Pt Atoms on the CeO2(111) Surface

The interaction of atomic Pt with the regular CeO2(111) surface has been studied by periodic density functional calculations using exchange-correlation potentials with the inclusion of the on-site Hubbard correction (LDA+U and GGA+U) or employing the hybrid HSE06 functional. Various starting geometries have been explored, and two types of electronic configurations-with either no or two unpaired electrons in the unit cell-have been considered. All methods consistently predict that the most favorable interaction corresponds to adsorption on top of a site bridging two nearest-neighbor surface 0 atoms (02 site) in a closed-shell electronic configuration with an essentially neutral or slightly positively charged Pt. However, for this site another slightly higher energy solution is found with two unpaired electrons in the unit cell. This other local minimum on the potential energy surface represents a completely different physical picture, where Pt has been oxidized to Pt+ with the concomitant reduction of a single Ce4+ to Ce3+. Calculations have also been carried out to estimate the energy barrier corresponding to diffusion of Pt atoms over the surface. The calculated values indicate that the energy cost required for this process is very small. This means that adsorbed Pt adatoms can easily nucleate into larger particles in Pt/CeO2 systems.