The effect of the oxidation state of the metal center in metalloporphyrins on the electrocatalytic CO2-to-CO conversion: A density functional theory study

Metalloporphyrins are a promising and sustainable class of molecular catalysts for the transformation of CO2 into value-added chemicals whereby the catalytic process is tuneable through the modification of the peripheral ligands and the electronic properties of the metal centre. In this work, we studied the electrochemical reduction of CO2 on metalloporphyrins using computational modelling. Density functional theory calculations were used to characterize the mechanism of 2-electron CO2-to-CO conversion on three metalloporphyrins [M-POR] catalysts, where M = Fe, Co and Ni, with the initial CO2 adsorption step taking place on the metal center in the neutral, [M-POR](0), singly reduced [M-POR](-), and doubly reduced, [M-POR](2-), oxidation states. The three alternative pathways display different energetic trends. In general, the catalytic activity for CO formation on metalloporphyrins in the neutral oxidation state, [M-POR](0), is negatively influenced by the conversion of CO2 to adsorbate formate, which is hindered by the weak *C(OH)O binding. The more favorable association of CO2 and strong stabilization of *C(OH)O occurs on the catalysts in the doubly reduced oxidation state, [M-POR](2-). Higher Faraday efficiencies for CO formation could be achieved under electrochemical conditions promoting well-reduced metal species.