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

Photocatalytic reduction of CO 2 in the presence of H 2 as the reductant was performed over MgO, CaO, ZrO 2, Ga 2 O 3, and Al 2 O 3 . Ga 2 O 3 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 CO 2 but also on the amount of H 2 adsorbed on Ga 2 O 3 . The Fourier transform infrared (FT-IR) spectra showed symmetric CO 3 stretching vibration bands from bidentate and monodentate bicarbonate species in the presence of CO 2 and OH and Ga−H stretching vibration bands in the presence of H 2 . The chemisorbed CO 2 species involved in the photocatalytic reduction of CO 2 over Ga 2 O 3 was not the bidentate bicarbonate species but the monodentate bicarbonate species. The dissociatively adsorbed hydrogen on Ga 2 O 3 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 CO 2 over Ga 2 O 3 takes place via a Langmuir−Hinshelwood-type mechanism, which is different from the previously reported mechanism for photocatalytic reduction using ZrO 2 or MgO.