Visible-light-driven CO₂ reduction on a hybrid photocatalyst consisting of a Ru(ii) binuclear complex and a Ag-loaded TaON in aqueous solutions

A hybrid photocatalyst consisting of a Ru(ii) binuclear complex and a Ag-loaded TaON reduced CO₂ by visible light even in aqueous solution. The distribution of the reduction products was strongly affected by the pH of the reaction solution. HCOOH was selectively produced in neutral conditions, whereas the formation of HCOOH competed with H₂ evolution in acidic conditions. Detailed mechanistic studies revealed that the photocatalytic CO₂ reduction proceeded via 'Z-schematic' electron transfer with step-by-step photoexcitation of TaON and the photosensitizer unit in the Ru(ii) binuclear complex. The maximum turnover number for HCOOH formation was 750 based on the Ru(ii) binuclear complex under visible-light irradiation, and the optimum external quantum efficiency of the HCOOH formation was 0.48% using 400 nm monochromic light with ethylenediaminetetraacetic acid disodium salt as a sacrificial reductant. Even in aqueous solution, the hybrid could also convert visible-light energy into chemical energy (ΔG⁰ = +83 kJ mol⁻¹) by the reduction of CO₂ to HCOOH with methanol oxidation.