In situ X-ray Absorption Spectroscopy in Homogeneous Conditions Reveals Interactions Between CO2 and a Doubly and Triply Reduced Iron(III) Porphyrin, then Leading to Catalysis

Iron porphyrins are attractive catalysts for the electrochemical reduction of carbon dioxide (CO2), owing to their high activity and selectivity while being tunable through ligand functionalization. Iron tetraphenyl porphyrin (FeTPP) is the simplest of them, and its catalytic behavior toward CO2 has been studied for decades. Although kinetic information is available, spectroscopic signatures are lacking to describe intermediate species along the catalytic cycle. In situ UV-Visible and X-ray absorption near edge spectroscopy (XANES) were used to monitor the local and electronic structure of FeTPP homogeneously dissolved in dimethyl formamide (DMF) under reductive potentials. The Fe(III) starting species was identified, together with its one, two and three electron-reduced counterparts under both argon and CO2 atmospheres. Under argon, the second and third reductions lead to species with electronic density shared between the metal and the porphyrin backbone. In the presence of CO2 and with a low amount of protons, the doubly and triply reduced species interact with CO2 at the metallic site. In light of these results, an electronic structure for a key intermediate along the catalytic cycle of the CO2-to-CO reduction reaction is proposed.