Effects of sol–gel procedures on the photocatalysis of Cu/TiO2 in CO2 photoreduction

Copper-loaded titania (Cu/TiO2) was synthesized via an improved modified sol–gel process. Photocatalysts were applied to the CO2 photocatalytic reduction and the yield of the major product, methanol, was used to evaluate the photocatalytic performance. Copper precursors and the adding time with sol as well as posttreatments were studied to explore the relationships between the characteristics and the activity of the photocatalysts. The results revealed that Cu/TiO2 prepared from copper chloride and added in the early sol–gel stage was more photoactive than that from copper acetate. Additional H2 reduction of calcined catalysts before the photoreduction CO2 decreased the yield of methanol due to the change of copper dispersion and oxidation state. TPR, XPS, and XAS measurements verified the oxidation state of Cu on Cu/TiO2 catalysts. The results indicated that the primary Cu(I) served as an active site. The zeta potentials of catalysts were measured and compared, showing that a higher positive zeta potential at pH 7 would lead to higher activity. Under 30-h UVC (254 nm) irradiation, the best catalyst gave a methanol yield above 600 μmol/gcat. Switching to UVA (365 nm) resulted in a significant decrease of methanol yield in the range of 10 μmol/gcat.