Study on carbon dioxide reduction with water over metal oxide photocatalysts

Various metal oxides with 0.1 wt% Ag loaded as a cocatalyst were prepared by an impregnation method and examined their photocatalytic activity for CO 2 reduction with water. Among all the prepared Ag‐loaded metal oxides, Ga 2 O 3 , ZrO 2 , Y 2 O 3 , MgO, and La 2 O 3 showed activities for CO and H 2 productions under ultraviolet light irradiation. Thus, metal oxides involving metal cations with closed shell electronic structures such as d 0 , d 10 , and s 0 had the potential for CO 2 reduction with water. In situ Fourier transform infrared measurement revealed that the photocatalytic activity and selectivity for CO production are controlled by the amount and chemical states of CO 2 adsorbed on the catalyst surface and by the surface basicity, as summarized as follows: Ag/ZrO 2 enhanced H 2 production rather than CO production due to very little CO 2 adsorption. Ag/Ga 2 O 3 exhibited the highest activity for CO production, because adsorbed monodentate bicarbonate was effectively converted to bidentate formate being the reaction intermediates for CO production owing to its weak surface basicity. Ag/La 2 O 3 , Ag/Y 2 O 3 , and Ag/MgO having both weak and strong basic sites adsorbed larger amount of carbonate species including their ions and suppressed H 2 production. However, the adsorbed carbonate species were hardly converted to the bidentate formate.