PNNL

Abstract

In this study we show that molecular oxygen reacts with bridging OH (OHbr) groups that are formed as a result of water dissociation at oxygen vacancy defects on the surface of rutile TiO2 (110). The electronic structure of an oxygen vacancy defect on TiO2 (110) is essentially the same as that of electron trap states detected on photoexcited or sensitized TiO2 photocatalysts, being Ti3? in nature. Electron energy loss spectroscopy (EELS) measurements, in agreement with valence band photoemission results in the literature, indicate that water dissociation at oxygen vacancy sites has little or no impact on the electronic structure of these sites. Temperature programmed desorption (TPD) measurements show that O2 adsorbed at 120 K reacts with near unity reaction probability with OHbr groups on TiO2 (110) to form an unidentified intermediate that decomposes to generate terminal OH groups at non-defect sites. Commensurate with this process, the electronic defect associated with the original oxygen vacancy defect (Ti3?) is oxidized. Both vibrational and electronic EELS results indicate that the reaction between O2and OHbr occurs at about 230 K. Detailed TPD experiments in which the precoverage of water was varied indicate that O2 need not chemisorb to cation sites on the TiO2 (110) surface in order for the reaction between O2 and OHbr to occur, which implies a direct interaction between weakly bound O2 and the OHbr groups. In agreement with this conclusion, we find that second layer water, which selectively hydrogen-bonds to bridging O and OH groups, blocks the reaction of O2with OHbr groups and prevents oxidation of the vacancy-related Ti3? electronic state.