Characterization of the Active Surface Species Responsible for UV-Induced Desorption of O2 from the Rutile TiO2(110) Surface

We have examined the chemical and photochemical properties of molecular oxygen on the (110) surface of rutile TiO2 at 100 K using electron energy loss spectroscopy (EELS), photon stimulated desorption (PSD), and scanning tunneling microscopy (STM). Oxygen chemisorbs on the TiO2(110) surface at 100 K through charge transfer from surface Ti3+ sites. The charge-transfer process is evident in EELS by a decrease in the intensity of the Ti3+ d-to-d transition at similar to 0.9 eV and formation of a new loss at similar to 2.8 eV. On the basis of comparisons with the available homogeneous and heterogeneous literature for complexed/adsorbed O-2, the species responsible for the 2.8 eV peak can be assigned to a surface peroxo (O-2(2-)) state of O-2. This species was identified as the active form of adsorbed O-2 on TiO2(110) for PSD. The adsorption site of this peroxo species was assigned to that of a regular five-coordinated Ti4+. (Ti-Sc) site based on comparisons between the UV exposure-dependent behavior of O-2 in STM, PSD, and EELS data. Assignment of the active form of adsorbed O-2 to a peroxo species at normal Ti-Sc, sites necessitates reevaluation of the simple mechanism in which a single valence band hole neutralizes a singly charged O-2 species (superoxo or O-2-), leading to desorption of O-2 from a physisorbed potential energy surface.