Simultaneous modulation of Ni single atoms and NiO_x clusters on TiO₂ for solar-driven CO₂ and H₂O conversion to CH₄

Construction of the photocatalysts with synergistic active sites holds great significance in enhancing the direct CO₂ reduction coupled with H₂O oxidation under solar irradiation. This work demonstrates the fabrication of a dual-active-site catalyst (Ni_SA-NiO_x/TiO₂) through in-situ formation and simultaneous modulation of Ni single atoms (Ni_SA) and NiO_x clusters on porous TiO₂. Both Ni_SA and NiO_x are characterized by X-ray absorption fine structure (XAFS) analyses and diffuse reflectance infrared Fourier transform spectroscopy using CO as a probe molecule (CO-DRIFTS). The optimized Ni_SA-NiO_x/TiO₂ photocatalyst exhibits enhanced performance in the reduction of CO₂ to CH₄ using H₂O as a hydrogen source. The deliberate modulation of Ni_SA and NiO_x on TiO₂ provides active sites for efficient activation of CO₂ and H₂O, synchronously promoting the two half-reactions of CO₂ reduction and H₂O oxidation. The photocatalytic mechanism is elucidated based on a series of control experiments and in situ characterizations. The results reveal that Ni_SA sites play a crucial role in CO₂ adsorption and activation, while NiO_x clusters facilitate H₂O oxidation for proton provision. The synergistic effect of Ni_SA and NiO_x greatly enhances the photocatalytic conversion of CO₂ and H₂O to CH₄. This work showcases the rational design of semiconductor photocatalysts featuring synergistic active sites for efficient conversion of CO₂ and H₂O into hydrocarbons by utilizing solar energy.