MgO- and Pt-Promoted TiO2 as an Efficient Photocatalyst for the Preferential Reduction of Carbon Dioxide in the Presence of Water

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 TiO 2 with an emphasis on the selectivity of photogenerated electrons for CO 2 reduction in the presence of H 2 O, which has been overlooked in most of the published papers. Our results clarified that the reaction using H 2 O vapor exhibited significantly higher selectivity for CO 2 reduction than that by immersing the photocatalyst into liquid H 2 O. We examined the effect of noble metal cocatalysts and found that the rate of CH 4 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 CO 2 reduction. The selectivity of CH 4 in CO 2 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 H 2 O to H 2 was accelerated more than the reduction of CO 2 in the presence of Pt, leading to a lower selectivity for CO 2 reduction and limited increases in CH 4 formation rate. We demonstrated that the addition of MgO into the Pt–TiO 2 catalyst could further enhance the formation of CH 4 . The formation of H 2 was suppressed simultaneously, suggesting the increase in the selectivity for CO 2 reduction in the presence of MgO. Furthermore, the MgO- and Pt-modified TiO 2 catalyst exhibited a higher CH 4 selectivity in CO 2 reduction.