Photoinduced activation of CO2 on TiO2 surfaces: Quantum chemical modeling of CO2 adsorption on oxygen vacancies

Chemical processes that utilize CO2 emissions from coal-fired power plants will be required as the world progresses towards reducing CO2 emissions. The conversion of CO2 using light energy (CO2 photoreduction) has the potential to produce useful fuels or valuable chemicals while decreasing CO2 emissions from the use of fossil fuels such as coal. Computational studies on the initial steps of photoinduced CO2 activation on TiO2 surfaces, necessary to develop a mechanistic understanding of CO2 photoreduction are a focus of this article.The results from previous quantum mechanical modeling studies conducted by the authors indicated that stoichiometric TiO2 surfaces likely do not promote electron transfer to CO2. Therefore, the role of oxygen vacancies in promoting the light-induced conversion of CO2 (CO2 photoreduction) on TiO2 surfaces was examined in this study. 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 photoexcitation of these bent-CO2 species were investigated with density functional theory calculations. Consistent with CO2 adsorption and photodesorption on other n-type metal oxides such as ZrO2, the results suggest that the bent-CO2 species do not gain further charge from the TiO2 surface under illumination and are likely photodesorbed as neutral species. Additionally, although the formation of species such as CO and HCHO is thermodynamically possible, the energy needed to regenerate the oxygen vacancy on TiO2 surfaces (~ 7 eV) is greater than that available through band-gap illumination (3.2 eV). Therefore, CO2 reactions with water on irradiated anatase TiO2 surfaces are likely to be stoichiometric.