Operando Spectroelectrochemistry Unravels the Mechanism of CO₂ Electrocatalytic Reduction by an Fe Porphyrin

Iron porphyrins are molecular catalysts recognized for their ability to electrochemically and photochemically reduce carbon dioxide (CO₂). The main reduction product is carbon monoxide (CO). CO holds significant industrial importance as it serves as a precursor for various valuable chemical products containing either a single carbon atom (C1), like methanol or methane, or multiple carbon atoms (Cn), such as ethanol or ethylene. Despite the long-established efficiency of these catalysts, optimizing their catalytic activity and stability and comprehending the intricate reaction mechanisms remain a significant challenge. This article presents a comprehensive investigation of the mechanistic aspects of the selective electroreduction of CO₂ to CO using an iron porphyrin substituted with four trimethylammonium groups in the para position [(pTMA)Fe^III-Cl]⁴⁺. By employing infrared and UV/Visible spectroelectrochemistry, changes in the electronic structure and coordination environment of the iron center can be observed in real-time as the electrochemical potential is adjusted, offering new insights into the reaction mechanisms. Catalytic species were identified, and evidence of a secondary reaction pathway was uncovered, potentially prompting a re-evaluation of the nature of the catalytically active species.