Photocatalytic reduction of CO2 with H2O on mesoporous silica supported Cu/TiO2 catalysts

Photoreduction of CO2 to hydrocarbons is a sustainable energy technology which not only mitigates emissions but also provides alternative fuels. However, one of the largest challenges is to increase the overall CO2 photo-conversion efficiency when water is used as the reducing reagent. In this work, mesoporous silica supported Cu/TiO2 nanocomposites were synthesized through a one-pot sol–gel method, and the photoreduction experiments were carried out in a continuous-flow reactor using CO2 and water vapor as the reactants under the irradiation of a Xe lamp. The high surface area mesoporous silica substrate (>300 m2/g) greatly enhanced CO2 photoreduction, possibly due to improved TiO2 dispersion and increased adsorption of CO2 and H2O on the catalyst. CO was found to be the primary product of CO2 reduction for TiO2–SiO2 catalysts without Cu. The addition of Cu species, which was identified to be Cu2O by the XPS, markedly increased the overall CO2 conversion efficiency as well as the selectivity to CH4, by suppressing the electron–hole recombination and enhancing multi-electron reactions. A synergistic effect was observed by combining the porous SiO2 support and the deposition of Cu on TiO2. The peak production rates of CO and CH4 reached 60 and 10 μmol g-cat−1 h−1, respectively, for the 0.5%Cu/TiO2–SiO2 composite that has the optimum Cu concentration; the peak quantum yield was calculated to be 1.41%. Deactivation and regeneration of the catalyst was observed and the mechanism was discussed. Desorption of the reaction intermediates from the active sites may be the rate limiting step.