Mechanistic Study of CO2 Photoreduction with H2O on Cu/TiO2 Nanocomposites by in Situ X-ray Absorption and Infrared Spectroscopies

Cu/TiO₂ composites are extensively studied for photocatalytic reduction of CO₂ with H₂O, but the roles of Cu species (Cu²⁺, Cu⁺, or Cu⁰) is not well understood, and the photocatalyst deactivation mechanism is seldom addressed. In this work, we have employed in situ techniques, i.e., X-ray absorption spectroscopy (XAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), to explore the surface chemistry of Cu/TiO₂ composites under CO₂ photoreduction environment. We found that the air-calcined Cu/TiO₂ (Cu/Ti(air)) surface was dominated by isolated Cu²⁺ sites, while the one post-treated with H₂ at 200 °C (Cu/Ti(H₂)) was rich in Cu⁺ and oxygen vacancy (VO). Cu/Ti(H₂) showed more than 50% higher activity than Cu/Ti(air) for CO₂ photoreduction to CO, mainly resulting from the synergy of Cu⁺, OH groups, and VO that could scavenge holes to enhance electron transfer, provide CO₂ adsorption sites, and facilitate the activation and conversion of the adsorbed CO₂ (HCO₃– and CO₂–). Meanwhile, the consumption of OH groups and Cu⁺ active sites by holes may result in the deactivation of Cu/Ti(H₂). Moreover, in situ XAS results directly demonstrated that (1) the photoinduced oxidation of Cu⁺ to Cu²⁺ changed the surrounding environments of Cu by increasing the coordination number; (2) thermal treatment by H₂ could not fully recover the OH and Cu⁺ sites to their original states; and (3) adding hole scavengers (e.g., methanol) maintained or even increased the more active Cu⁺ species from the photoreduction of Cu²⁺, thus leading to a higher and more stable CO₂ reduction activity. Findings in this work and the application of in situ XAS technique will help develop a more efficient photocatalyst for CO₂ photoreduction and advance the understanding of the reaction mechanism and surface chemistry.