Deciphering the Selectivity of the Electrochemical CO2 Reduction to CO by a Cobalt Porphyrin Catalyst in Neutral Aqueous Solution: Insights from DFT Calculations

Density functional theory (DFT) calculations were conducted to investigate the cobalt porphyrin-catalyzed electro-reduction of CO2 to CO in an aqueous solution. The results suggest that Co-II-porphyrin (Co-II-L) undertakes a ligand-based reduction to generate the active species Co-II-L.(-), where the Co-II center antiferromagnetically interacts with the ligand radical anion. Co-II-L.(-) then performs a nucleophilic attack on CO2, followed by protonation and a reduction to give Co-II-L-COOH. An intermolecular proton transfer leads to the heterolytic cleavage of the C-O bond, producing intermediate Co-II-L-CO. Subsequently, CO is released from Co-II-L-CO, and Co-II-L is regenerated to catalyze the next cycle. The rate-determining step of this CO2RR is the nucleophilic attack on CO2 by Co-II-L.(-), with a total barrier of 20.7 kcal mol(-1). The competing hydrogen evolution reaction is associated with a higher total barrier. A computational investigation regarding the substituent effects of the catalyst indicates that the CoPor-R3 complex is likely to display the highest activity and selectivity as a molecular catalyst.