Enhanced photocatalytic reduction of CO₂ into CH₄ over N, Eu co-doped TiO₂: Insight into the synergistic effect of N and Eu

The synthesis of photocatalysts with good activity and strong product selectivity is of strategic importance for photocatalytic reduction of carbon dioxide (CO₂). In this study, an improved sol-gel method was employed for the synthesis of N-doped TiO₂, Eu-doped TiO₂, and N, Eu co-doped TiO₂ photocatalysts. The phase structures, optical and electrical properties of the catalysts were thoroughly investigated by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller Surface area analysis, Ultraviolet-visible diffuse reflectance spectroscopy, Photoluminescence spectroscopy, and electrochemical measurements. The experimental results of photocatalytic CO₂ reduction indicated that both N and Eu doping could effectively improve the photoresponse of TiO₂, but the synergistic effect of N and Eu made the N, Eu co-doped TiO₂ exhibit superior photocatalytic activity and product selectivity, the formation rate of methane (CH₄) by photocatalytic reduction of CO₂ using N-Eu/TiO₂ as the catalyst could reach 13.48 μmol·g_cat⁻¹·h⁻¹, which was about 12 times higher than that of pure TiO₂, and the selectivity reached nearly 100%. Finally, this synergistic effect was explained using DFT calculation and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and the possible mechanism of photocatalytic CO₂ reduction over N-Eu/TiO₂ was elucidated.