The Oxygen Artifact in Visible-Light-Driven CO2 Reduction Catalyzed by Molecular Catalysts

Abstract

Visible-light-driven CO2 reduction using homogeneous molecular systems has attracted considerable attention as a strategy for converting greenhouse gases into value-added fuels. In this study, we investigated the influence of oxygen on a representative photocatalytic system composed of an iron porphyrin catalyst, a photosensitizer, triethylamine (TEA) as a sacrificial electron donor, and trifluoroethanol (TFE) as a proton donor. Contrary to previous reports, no detectable production of CO, CH4, or H2 was observed under visible-light irradiation in strictly anaerobic experiments, regardless of the photosensitizer employed, including [Ir(ppy)3], [Ru(bpy)3]2+, phenoxazine, and 9-cyanoanthracene. However, when O2 was intentionally introduced, significant amounts of CO and CH4 were observed even in the absence of CO2, suggesting that these products originate from the photodegradation of TEA rather than from CO2 reduction. These results are predictable since none of the studied photocatalytic systems are expected to be operational, given the thermodynamic and kinetic constraints. Control experiments further showed that only TEA, O2, and a photosensitizer are required to generate these gaseous products. Emission quenching analyses indicate that O2 efficiently quenches the excited states of the photosensitizers, probably initiating pathways that promote the TEA degradation. Alternatively, when TEA quenches the photosensitizer’s excited state, O2 can be reduced, leading to reduced oxygen species that may trigger TEA degradation. These findings highlight the critical importance of rigorous anaerobic conditions in photocatalytic CO2 reduction studies and reveal that oxygen contamination can lead to false-positive assignments of catalytic CO2 to fuel conversion.