Activity and adsorption behavior of oxygen on rutile TiO2(110).

The activity and adsorption behavior of oxygen on rutile TiO₂(110) (RTiO₂(110)) were investigated using the temperature programmed desorption (TPD) method with methanol (CH₃OH) as the probe molecule. By controlling the coverage of molecular O₂ on the surface via increasing or decreasing O₂ exposure, two chemisorbed O₂ species on the surface are confirmed, one at the bridging oxygen vacancy (O_V) site (O₂^2-/O_v) and the other at the five-fold coordinated titanium (Ti_5c) site (O₂^2--/Ti_5c). At low O₂ exposure, O₂^2--/O_V is the main species on the surface, which only leads to the O–H bond cleavage of CH₃OH, producing methoxy groups (CH₃O). However, after the O_V sites are nearly filled by O₂ at about 0.1 L O₂ exposure, O₂/Ti_5c species begins to appear on R-TiO₂(110) surface, resulting in the formation of formaldehyde (CH₂O) via the reaction of O₂/Ti_5c species with CH₃OH or CH₃O to break the C-H bond at low surface temperature. Moreover, the yield of CH₂O increases linearly with that of H₂O. In addition, when the 1 L O₂ covered surface is irradiated with 355 nm UV irradiation to desorb and dissociate O₂/Ti_5c species, the yield of CH₂O decreases linearly with that of H₂O. Further analysis suggests that the charge state of O₂/Ti_5c may not change as the exposure of O₂ changes on the R-TiO₂(110) surface, and O₂ is most likely to adsorb on the Ti_5c sites in the form of O₂^2-, not O₂⁻. The result not only advances our understanding on the adsorption state of O₂ on TiO₂, but also provides clues for low temperature C–H bond activation with O₂ on TiO₂. [ABSTRACT FROM AUTHOR]