Abstract

The photocatalytic reduction of carbon dioxide with water to fuels and chemicals is a longstanding challenge. This article focuses on the effects of cocatalysts and reaction modes on photocatalytic behaviors of TiO2 with an emphasis on the selectivity of photogenerated electrons for CO2 reduction in the presence of H2O, which has been overlooked in most of the published papers. Our results clarified that the reaction using H2O vapor exhibited significantly higher selectivity for CO2 reduction than that by immersing the photocatalyst into liquid H2O. We examined the effect of noble metal cocatalysts and found that the rate of CH4 formation increased in the sequence of Ag < Rh < Au < Pd < Pt, corresponding well to the increase in the efficiency of electron–hole separation. This indicates that Pt is the most effective cocatalyst to extract photogenerated electrons for CO2 reduction. The selectivity of CH4 in CO2 reduction was also enhanced by Pt. The size and loading amount of Pt affected the activity; a smaller mean size of Pt particles and an appropriate loading amount favored the formation of reduction products. The reduction of H2O to H2 was accelerated more than the reduction of CO2 in the presence of Pt, leading to a lower selectivity for CO2 reduction and limited increases in CH4 formation rate. We demonstrated that the addition of MgO into the Pt–TiO2 catalyst could further enhance the formation of CH4. The formation of H2 was suppressed simultaneously, suggesting the increase in the selectivity for CO2 reduction in the presence of MgO. Furthermore, the MgO- and Pt-modified TiO2 catalyst exhibited a higher CH4 selectivity in CO2 reduction.