Cu/TiO2 Derived from Cu-Doped MIL-125 for Enhanced Photocatalytic CO2-to-CH4 Conversion

Photocatalytic CO2 reduction into CH4 is a promising route for solar fuel production, but its efficiency is still limited by poor charge separation, insufficient CO2 activation, and sluggish multi-electron transfer kinetics. Herein, Cu-modified TiO2 (Cu/TiO2) was prepared by calcining a Cu-modified defective MIL-125(Ti) precursor, denoted as Cu-MIL-125, through a temperature-controlled calcination strategy. The effects of calcination temperature on the structural evolution, surface chemical states, interfacial charge transport, and CO2 photoreduction performance were examined. These results indicated that the Cu/TiO2 was successfully prepared, while the crystallinity, porous structure, and interfacial electronic properties of Cu/TiO2 were strongly dependent on the calcination temperature. Among the obtained samples, the Cu/TiO2 sample obtained by calcining Cu-MIL-125 at 450 °C (450 Cu/TiO2) exhibited the highest CH4 formation rate, reaching 15.90 μmol g−1 h−1, corresponding to an approximately 9.8-fold enhancement over TiO2 calcined from defective MIL-125(Ti) at 450 °C, together with a high CH4 selectivity of 93.05%. Control experiments and 13CO2 isotope-labeling tests confirmed that the detected carbon-containing products were generated from CO2 under photocatalytic conditions. In situ diffuse reflectance infrared Fourier transform spectroscopy measurements further revealed the formation of carbonate, bicarbonate and hydrogenated carbon-containing intermediates during the reaction. This work offers a practical route for constructing metal–organic framework-derived Cu/TiO2 photocatalysts for selective CH4 production from CO2.