Deciphering the Selectivity of the Electrochemical CO 2 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 CO 2 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 CO 2 , 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 CO 2 RR is the nucleophilic attack on CO 2 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.