Density Functional Theory Study of Active Oxygen at the Perimeter of Au/TiO2 Catalysts

In spite of numerous efforts, the nature of active oxygen generated at the perimeter of Au/TiO2 catalysts remains elusive; e.g., it is unclear whether the active species is atomic or molecular oxygen. Using plane-wave density functional theory (DFT) and rutile (110)-supported Au rod models, we have examined various oxygen species for their stability and reactivity to CO. Our calculation finds that O-2 attached endon to a pentacoordinate Ti site (Ti-5c) as well as to the Au perimeter (Ti-OO-Au) is a transient state, turning into states with both O attached to Ti-5c. The latter are quite stable (< -1.5 eV) without O vacancies and bonding with the Au perimeter. The stability of O-2 on Ti-5c improves with increasing Au/oxide contact area per O-2 because the contact is the origin of electrons used in adsorbing and activating O-2. Even O-O bond dissociation between two Ti-5c sites is probable at low O-2 concentration. Both O and O-2 on Ti-5c readily react with CO, although the adatom is rather unreactive to CO(g). The active oxygen is thus dissociated or undissociated O-2 on Ti-5c, depending on O-2 population relative to the contact area