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

Abstract

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 CoII -porphyrin (CoII -L) undertakes a ligand-based reduction to generate the active species CoII -L⋅- , where the CoII center antiferromagnetically interacts with the ligand radical anion. CoII -L⋅- then performs a nucleophilic attack on CO2 , followed by protonation and a reduction to give CoII -L-COOH. An intermolecular proton transfer leads to the heterolytic cleavage of the C-O bond, producing intermediate CoII -L-CO. Subsequently, CO is released from CoII -L-CO, and CoII -L is regenerated to catalyze the next cycle. The rate-determining step of this CO2 RR is the nucleophilic attack on CO2 by CoII -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.