Surface Basicity of Metal@TiO₂ to Enhance Photocatalytic Efficiency for CO₂ Reduction

Photocatalytic reduction of CO₂ to valuable chemical fuels is of broad interest, given its potential to activate stable greenhouse CO₂ using renewable energy input. We report how to choose the right metal cocatalysts in combination with the surface basicity of TiO₂ to enhance their photocatalytic efficiency for CO₂ photoreduction. Uniform ligand-free metal nanoparticles (NPs) of Ag, Cu, Au, Pd, and Pt, supported on TiO₂, are active for CO₂ photoreduction using water as an electron donor. The group XI metals show a high selectivity to CO and Ag/TiO₂ is most active to produce CO at a rate of 5.2 μmol g^-1 h^-1. The group X metals, e.g., Pd and Pt, mainly generate hydrocarbons including methane and ethane, and Pd/TiO₂ is slightly more active in methane production at a rate of 2.4 μmol g^-1 h^-1. The activity of these photocatalysts can be enhanced by varying the surface basicity of TiO₂ with primary amines. However, proton reduction selectivity is greatly enhanced in the presence of amine except amine-modified Ag/TiO₂, which shows an activity enhancement by 2.4 times solely for CO₂ photoreduction as compared to that without amines without switching its selectivity to proton reduction. Using in situ infrared spectroscopy and CO stripping voltammetry, we demonstrate that the improvement of electron density and the low proton affinity of metal cocatalysts are of key importance in CO₂ photoreduction. As a systematic study, our results provide a guideline on the right choice of metals in combination of the surface functionality to tune the photocatalytic efficiency of supported metal NPs on TiO₂ for selective CO₂ photoreduction.