Elucidating the Origin of Hidden Limitations in Ru-Complex/Ag/Polymeric Carbon Nitride Hybrid Photocatalysts for Visible-Light CO ₂ Reduction

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.