Synchrotron-based high-resolution photoemission, thermal desorption mass spectroscopy, and first-principles density functional calculations were used to study the adsorption and reaction of S with TiO2(110). At 100 to 300K, S atoms bond much more strongly to O vacancy sites than to atoms in the Ti rows of a perfect oxide surface. The electronic states associated with Ti3+ sites favored bonding to S, but there was no substantial oxide → adsorbate charge transfer. In general, the bond between S and the Ti cations was covalent, with a small degree of ionic character. In order to dose with S at above 500K, a layer of TiSx was formed on TiO2(110). The O signal disappeared from photo-emission and Auger spectra, and the Ti 2p core levels exhibited a complete TiO2 → TiSx transformation. The O ↔ S exchange did not involve the production of SO or SO2 species. Instead, the formation of TiSx involved the migration of O vacancies from the bulk to the surface. The results illustrated the potential importance of surface and sub-surface defects in the behavior of an oxide surface.

Importance of O Vacancies in the Behavior of Oxide Surfaces - Adsorption of Sulfur on TiO2 (110). Rodriguez, J.A., Hrbek, J., Chang, Z., Dvorak, J., Jirsak, T., Maiti, A.: Physical Review B, 2002, 65[23], 235414