The results from previous quantum mechanical modelling studies had indicated that stoichiometric TiO2 surfaces likely did not promote electron transfer to CO2. Therefore, the role of oxygen vacancies in promoting the light-induced conversion of CO2 (CO2 photo-reduction) on TiO2 surfaces was examined here. Two different side-on bonded bent-CO2 (bridging Ti–CO2δ•−–Ti species) were formed on the reduced rutile (110) and anatase (010), (001) surfaces, indicating charge transfer from the reduced surface to CO2. Further steps in the photo-excitation of these bent-CO2 species were investigated with density functional theory calculations. Consistent with CO2 adsorption and photo-desorption on other n-type metal oxides such as Zero, the results suggested that the bent-CO2 species did not gain further charge from the TiO2 surface under illumination and were likely photo-desorbed as neutral species. Additionally, although the formation of species such as CO and H.CHOO was thermodynamically possible, the energy needed to regenerate the oxygen vacancy on TiO2 surfaces (~7eV) was greater than that available through band-gap illumination (3.2eV). Therefore, CO2 reactions with water on irradiated anatase TiO2 surfaces were likely to be stoichiometric.

Photoinduced Activation of CO2 on TiO2 Surfaces: Quantum Chemical Modeling of CO2 Adsorption on Oxygen Vacancies. V.P.Indrakanti, J.D.Kubicki, H.H.Schobert: Fuel Processing Technology, 2011, 92[4], 805-11