Mechanistic insights into the electroreduction of CO2 by a phosphine-nitrogen-coordinated manganese carbonyl complex for CO2-to-CO conversion over H2 formation

Abstract

The electrocatalytic mechanism for carbon dioxide (CO2) reduction by {MnPNBr}, PN = {κ2-(Ph2P)NMe(NC5H4)}(CO)3, was investigated by means of density functional theory (DFT) calculations. Here, we identify a couple of mechanistic scenarios involving the CO2 reduction reaction (CO2RR) over the competing hydrogen evolution reaction (HER) to understand the electronic structure and selectivity. Our calculation results show that the coordination of the phosphorus atom increases the electron density of the Mn center, leading to the overall CO2RR and HER being thermodynamically favorable. In the presence of a water cluster, the hydrogen bonding stabilizes the transition states, which in turn makes CO2 fixation and reduction favorable. The cleavage of the MnC(O)–OH bond in the CO2RR and the cleavage of the MnHH-OH bond in the HER become the respective rate-determining step. The selectivity for CO2-to-CO conversion over H2 evolution results from a higher energy barrier for the generation of the species {MnH2} before H2 removal. Therefore, these results suggest the possibility of adjusting the electron density of the Mn center to improve the reactivity and selectivity of the CO2RR.