Visible-light CO₂ reduction over a ruthenium(ii)-complex/C₃N₄ hybrid photocatalyst: The promotional effect of silver species

Hybrid photocatalysts constructed with a mononuclear Ru(ii)-complex (RuP), silver nanoparticles, and carbon nitride nanosheets (NS-C₃N₄) photocatalyze CO₂ reduction to selectively form formate under visible light. The structure of the nanoparticulate silver species, which worked as promoters for the reaction, was characterized by X-ray diffraction, UV-VIS diffuse reflectance spectroscopy, high-resolution transmission microscopy, and X-ray absorption fine-structure spectroscopy. The silver promoters were loaded on the surface of NS-C₃N₄ by an impregnation method from an aqueous solution containing AgNO₃ or an in situ photodeposition method. Impregnation of NS-C₃N₄ with 2.0 wt% Ag followed by reduction with H₂ at 473 K (further modified with RuP) resulted in the highest photocatalytic activity, giving a turnover number of 5700 (based on RuP), which was the greatest value among the formate-generating hybrid systems with a mononuclear complex. While the optimized photocatalyst contained highly dispersed Ag₂O-like nanoclusters as the major silver species, experimental results suggested that highly dispersed Ag⁰ species are more important for enhancing CO₂ reduction activity, that is, the obtained experimental results led us to conclude that there are two major factors affecting activity: one is the feature size of silver species (smaller is better), and the other is the oxidation state of silver (metallic is better).