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.

ATaO 3 (A = Li, Na, K) compound oxides exhibit photocatalytic activity for the reduction of CO 2 in the presence of H 2. Only CO gas was generated over all samples under photoirradiation. The photocatalytic activity was higher in the order corresponding to KTaO 3 , NaTaO 3 and LiTaO 3 (LiTaO 3 > NaTaO 3 > KTaO 3). The order of the photocatalytic activities was consistent with that of the E g (optical gap) values. After 24 h of photoirradiation, the amount of evolved CO reached 0.42 µmol•g-1 over LiTaO 3. TPD experiments indicated that the broad peak which is assigned to chemisorbed CO 2 gas was observed at 573 K in the case of LiTaO 3. On the contrary, there was no peak in the spectra of NaTaO 3 and KTaO 3. The amount of evolved CO gas almost strongly depends on amount of chemisorbed CO 2 in the case of ATaO 3 (A = Li, Na, K). In addition, the photocatalytic activity increased with increasing the calcination temperature of LiTaO 3. This means that a smooth charge separation in a LiTaO 3 photocatalyst and chemisorption of CO 2 on the surface contribute to effective reduction of CO 2 in the presence of H 2. Teramura et al. Photocatalytic reduction of CO 2 over ATaO 3 4 / 21

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In photocatalytic aerobic oxidation of cyclohexene catalyzed by 5,10,15,20-tetrakis(pentafluorophenyl)porphyriniron(III) chloride [Fe(TPFPP)Cl] under visible light irradiation, the coexistence of solid bases was found to enhance the selectivity to cyclohexene oxide drastically. In the absence of solid bases, however, only the allylic oxidation of cyclohexene was catalyzed by the porphyrin complex acting as a sensitizer. Among the various metal oxides investigated, hydrotalcite exhibited the highest conversion of cyclohexene and selectivity to cyclohexene oxide.