Adsorbed Species of CO2 and H2 on Ga2O3 for the Photocatalytic Reduction of CO2

Photocatalytic reduction of CO2 in the presence of H2 as the reductant was performed over MgO, CaO, ZrO2, Ga2O3, and Al2O3. Ga2O3 exhibited the highest photocatalytic activity in this process, and CO gas was selectively generated at room temperature and atmospheric pressure. The amount of CO gas evolved depended not only on the amount of CO2 but also on the amount of H2 adsorbed on Ga2O3. The Fourier transform infrared (FT-IR) spectra showed symmetric CO3 stretching vibration bands from bidentate and monodentate bicarbonate species in the presence of CO2 and OH and Ga−H stretching vibration bands in the presence of H2. The chemisorbed CO2 species involved in the photocatalytic reduction of CO2 over Ga2O3 was not the bidentate bicarbonate species but the monodentate bicarbonate species. The dissociatively adsorbed hydrogen on Ga2O3 reduced the monodentate bicarbonate to the bidentate formate under photoirradiation. The bidentate formate, which was an intermediate in the photocatalytic reduction, decomposed to CO. We propose that photocatalytic reduction of CO2 over Ga2O3 takes place via a Langmuir−Hinshelwood-type mechanism, which is different from the previously reported mechanism for photocatalytic reduction using ZrO2 or MgO.