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.