Elucidating the mechanism of photocatalytic reduction of bicarbonate (aqueous CO₂) into formate and other organics

The photocatalytic reduction of CO₂ under solar irradiation is an ideal approach to mitigating global warming, and reducing aqueous forms of CO₂ that interact strongly with a catalyst (e.g., HCO₃⁻) is a promising strategy to expedite such reductions. This study uses Pt-deposited graphene oxide dots as a model photocatalyst to elucidate the mechanism of HCO₃⁻ reduction. The photocatalyst steadily catalyzes the reduction of an HCO₃⁻ solution (at pH = 9) containing an electron donor under 1-sun illumination over a period of 60 h to produce H₂ and organic compounds (formate, methanol, and acetate). H₂ is derived from solution-contained H₂O, which undergoes photocatalytic cleavage to produce •H atoms. Isotopic analysis reveals that all of the organics formed via interactions between HCO₃⁻ and •H. This study proposes mechanistic steps, which are governed by the reacting behavior of the •H, to correlate the electron transfer steps and product formation of this photocatalysis. This photocatalysis achieves overall apparent quantum efficiency of 27% in the formation of reaction products under monochromatic irradiation at 420 nm. This study demonstrates the effectiveness of aqueous-phase photocatalysis in converting aqueous CO₂ into valuable chemicals and the importance of H₂O-derived •H in governing the product selectivity and formation kinetics.