Mechanistic insight into electrocatalytic CO2 reduction to formate by the iron(I) porphyrin complex: A DFT study

Electrocatalytic reduction of CO2 into value-added chemicals has been considered as a promising pathway to mitigate the energy crisis and global warming. Iron porphyrins have been extensively studied for electrocatalytic CO2 reduction reaction (CO2RR), known for their ability to promote CO2-to-CO conversion. However, the mechanism of CO2-to-HCOO- conversion by Fe porphyrin remains unclear. Here, by means of density functional theory (DFT) calculations, we investigated the detailed mechanism of a novel Fe porphyrin catalyst for CO2 reduction to HCOO- in its Fe(I) state. Our results demonstrated that the reduction of CO2 to HCOO- proceeds via the C-protonation of an FeII-OCO center dot- complex, rather than through the hydrolysis of an FeIII-COOH complex or CO2 insertion in an Fe-H bond. Furthermore, the FeIII-COOH complex is found to be a unstable intermediate. Protonation of its hydroxyl group, accompanied by C-OH bond cleavage to produce CO, is both thermodynamically and kinetically unfeasible. Instead, the FeIII-COOH complex undergoes a coordination switch followed by a conformational change to form the active FeII-OCO center dot- complex, which promotes the production of HCOO- . Moreover, the single-electron reduction of FeIII-COOH gives FeII-COOH, leading to formation of CO rather than HCOO- . The insights gained from this study may contribute to design of electrocatalysts for selective CO2 reduction to formate.