Rapid Self-Decomposition of g-C3N4 During Gas–Solid Photocatalytic CO2 Reduction and Its Effects on Performance Assessment

We report direct evidence of the rapid self-decomposition of graphitic carbon nitride (g-C3N4), a popular photo (electro) catalyst, during the gas–solid photocatalytic reaction. Crucially, the average rate of CO production from the light-induced self-decomposition of g-C3N4 in Ar is almost equal to that in a CO2 atmosphere, and the products of the self-decomposition include CO, CO2, NO2, and NO2–/NO3–. Using experimental and theoretical studies, we reveal that the chemical instability of g-C3N4 is related to the adsorbed hydroxyl groups (OHads) on the catalyst surface. Specifically, the electronic interactions between OHads and g-C3N4 reduce the stability of the C–N═C bonds, and photogenerated charge carriers attack the structural units of g-C3N4, leading to rapid decomposition. Theoretical calculations indicate that self-decomposition reaction is more thermodynamically favorable than CO2 reduction reaction. Overall, these findings demonstrate the importance of catalyst self-decomposition and the need to fully consider the products from catalyst instability when evaluating the gas–solid photocatalytic redox reaction performance.