Donor Speciation Rather than Intrinsic Barriers Controls Product Selectivity in Aqueous Photocatalytic CO2 Reduction...

Ru(II)-Re(I) supramolecular photocatalysts exhibit high selectivity toward formic acid (HCOOH) in aqueous CO2 reduction; however, the origin of this selectivity remains unresolved. Here, density functional theory (DFT) calculations explicitly incorporating ascorbic acid (AH2) and its redox/protonation derivatives (AH-, AH0) reveal the mechanistic origin of product selectivity. Hydride formation at the Re center via proton-coupled electron transfer (PCET) from AH- is identified as the rate-determining step for both HCOOH and H2 pathways. Despite nearly identical intrinsic activation barriers for subsequent HCOO- and H2 formation, the product selectivity is shown to be dictated by the relative concentrations of CO2 and AH2. In contrast, CO formation is kinetically suppressed due to the high barrier associated with the RuRe(COOH) intermediate. These findings demonstrate that, in aqueous photocatalytic systems, product selectivity is governed not by intrinsic reaction barriers but by donor speciation under reaction conditions. This work establishes a general mechanistic framework for controlling selectivity through protonation and redox equilibria, providing design principles for efficient and selective CO2 reduction.