Artificial photosynthesis that converts CO2 into value-added chemicals under mild conditions remains a key goal in sustainable catalysis. Hybrid photocatalysts that integrate molecular CO2 reduction cocatalysts with semiconductor light absorbers provide a versatile platform to combine molecular-level selectivity with solid-state photostability. However, their quantum efficiencies have generally remained low, partly because side reactions of the molecular component have been overlooked. Here we show that suppressing a photochemical ligand-exchange reaction of a surface-anchored Ru complex, trans(Cl)-Ru(bpy(CH2PO3H2)2)(CO)2Cl2, markedly enhances photocatalytic CO2 reduction over a well-established Ag-loaded polymeric carbon nitride hybrid. The suppression of this undesirable photochemical reaction is achievable under low-intensity visible light when the Ru complex is loaded at a high density. The optimized system achieves selective CO2-to-formate conversion with an apparent quantum yield of 27.7% at 400 nm and a formate selectivity greater than 99%. Spectroscopic analyses reveal that the suppression of photochemical ligand exchange maintains the original Ru coordination environment with large driving force for CO2 reduction, thereby stabilizing the catalytic cycle and facilitating efficient interfacial electron transfer. These results reveal an unrecognized limitation in molecule/semiconductor hybrid photocatalysts─photochemical ligand exchange of the molecular cocatalyst─and demonstrate that controlling such side reactions offers an important strategy to design high-efficiency CO2 reduction systems.
nakada et al. (Thu,) studied this question.