Photocatalytic reduction of CO2 over a hybrid photocatalyst composed of WO3 and graphitic carbon nitride (g-C3N4) under visible light

Abstract

Graphitic carbon nitride (g-C3N4) has attracted much attention as a metal-free semiconductor having visible light absorption and relatively high chemical stability. g-C3N4 can reduce CO2 to organic fuels such as methanol (CH3OH), formic acid (HCO2H), and methane (CH4) under visible light irradiation. However, oxidation potential of g-C3N4 is not enough for water oxidation. Therefore, we focused on hybridization of g-C3N4 and tungsten(VI) oxide (WO3) which has high oxidation potential for water oxidation. In this study, we examined CO2 reduction by composite photocatalyst of g-C3N4 and WO3, which was prepared by three methods (mixture using an agate mortar, impregnation and planetary mill). As a result, composite photocatalyst prepared with planetary mill showed the highest photocatalytic activity.Photodeposition of silver or gold nanoparticles only on g-C3N4 of the hybrid photocatalyst induced an increase in CH3OH because the loaded metal nanoparticles play an important role in multi-electron reduction of CO2. Photocatalytic activity of the Au-loaded hybrid photocatalyst composed of g-C3N4 and WO3 was 1.7-times higher than that of the hybrid photocatalyst without Au loading.In addition, we investigated photocatalytic reaction mechanism of composite photocatalyst by double-beam photoacoustic spectroscopy. This result revealed Z-scheme reaction proceeds in the composite photocatalyst to maintain high oxidation ability of WO3 and high reduction ability of g-C3N4, resulting in high photocatalytic activity.