Cu/TiO2 Photocatalysts for CO2 Reduction: Structure and Evolution of the Cocatalyst Active Form

Extensive work on a Cu-modified TiO2 photocatalyst for CO2 reduction under visible light irradiation was conducted. The structure of the copper cocatalyst was established using UV–vis diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. It was found that copper exists in different states (Cu0, Cu+, and Cu2+), the content of which depends on the TiO2 calcination temperature and copper loading. The optimum composition of the cocatalyst has a photocatalyst based on TiO2 calcined at 700 °C and modified with 5 wt% copper, the activity of which is 22 µmol/(h·gcat) (409 nm). Analysis of the photocatalysts after the photocatalytic reaction disclosed that the copper metal on the surface of the calcined TiO2 was gradually converted into Cu2O during the photocatalytic reaction. Meanwhile, the metallic copper on the surface of the noncalcined TiO2 did not undergo any transformation during the reaction.