Hole Scavenger Role during CO ₂ Photoreduction: Toward a New Hybrid Homogeneous/Heterogeneous Process

The photoreduction of carbon dioxide (CO2) into solar fuels and value-added chemicals is a promising strategy to address climate change and the energy crisis. However, the efficiency of heterogeneous semiconductor-based photocatalytic systems is often hindered by the rapid recombination of charge carriers (electrons and holes). This paper examines the crucial role of the sulfite ion (SO32–) as a hole scavenger during aqueous-phase photoreduction of CO2, achieving the unprecedented productivity of 58 to 67 mol/h kgcat. A new hybrid homogeneous/heterogeneous process has been developed, activated by UVA and UVC irradiation, never reported previously. Low-energy radiation is sufficient to induce the conventional photogeneration of charge carriers responsible for the well-known heterogeneous photocatalytic pathway of CO2 photoconversion to HCOOH. UVC radiation is able to activate a homogeneous direct photolysis of sodium sulfite, with the generation of solvated electrons as powerful reducing species. The synergistic action of both mechanisms demonstrated the possibility to achieve, for the first time, critically relevant amounts of HCOOH produced in a semipilot-scale reactor (ca. 1.5 L size). The effect of high-pressure operation seems also to be a determinant from a kinetic point of view to increase the concentration of available reacting species.