Mechanism and Selectivity of Electrochemical Reduction of CO2 on Metalloporphyrin Catalysts from DFT Studies

Electrochemical reduction of CO2 to value-added chemicals has been hindered by poor product selectivity and competition from hydrogen evolution reactions. This study aims to unravel the origin of the product selectivity and competitive hydrogen evolution reaction on [MP](0) catalysts (M = Fe, Co, Rh and Ir; P is porphyrin ligand) by analyzing the mechanism of CO2 reduction and H-2 formation based on the results of density functional theory calculations. Reduction of CO2 to CO and HCOO- proceeds via the formation of carboxylate adduct ([MP-COOH](0) and ([MP-COOH](-)) and metal-hydride [MP-H](-), respectively. Competing proton reduction to gaseous hydrogen shares the [MP-H](-) intermediate. Our results show that the pK(a) of [MP-H](0) can be used as an indicator of the CO or HCOO-/H-2 preference. Furthermore, an ergoneutral pH has been determined and used to determine the minimum pH at which selective CO2 reduction to HCOO- becomes favorable over the H-2 production. These analyses allow us to understand the product selectivity of CO2 reduction on [FeP](0), [CoP](0), [RhP](0) and [IrP](0); [FeP](0) and [CoP](0) are selective for CO whereas [RhP](0) and [IrP](0) are selective for HCOO- while suppressing H-2 formation. These descriptors should be applicable to other catalysts in an aqueous medium.