Enhanced Thermo-Photocatalytic CO2 Reduction to CH4 over Cu-Modified TiO2-Based Catalysts

This study investigates the thermo-photocatalytic reduction in CO2 to methane over copper-modified TiO2 (P25) and TiO2/g-C3N4 catalysts, building upon prior works that identified two top-performing systems: 0.8%Cu-TiO2/g-C3N4 prepared via mechanochemical synthesis, and 0.2%Cu-TiO2 prepared by aqueous impregnation. Comprehensive physicochemical characterisation, including ICP-OES, XPS, FT-IR, UV–Vis DRS, UPS, and PL spectroscopy, was conducted to explain the structural, electronic, and surface properties of the catalysts. Temperature-dependent catalytic tests in a 2D-flow reactor ranging from room temperature to 70 °C under UV irradiation (368 nm) with water and ethanol vapour revealed methane production maxima at 50–60 °C, with rates of ~1010 µmol/g⋅h for 0.2%Cu-TiO2 and ~720 µmol/g⋅h for 0.8%Cu-TiO2/g-C3N4, significantly exceeding those of pristine P25. Moderate heating enhanced charge carrier mobility, ethanol functioned effectively as a hole scavenger, and CO2 reduction kinetics were accelerated. Beyond the optimal temperature, a decline in performance attributed to reactant desorption counterbalanced these improvements. Copper species contributed through providing active sites for CO2 chemisorption and charge-transfer processes, generating a synergistic effect that surpassed purely photocatalytic performance upon heating. These findings underscore the roles of the synthesis method and defect engineering in optimising catalyst performance and demonstrate the potential of combining photonic and thermal energy inputs to improve sustainable CO2 conversion technologies.