Spectating the proton migration on catalyst with noninnocent ligand in aqueous electrochemical CO2 reduction

It is of great significance to investigate aqueous CO2 reduction on catalysts with redox-noninnocent ligands as they effectively facilitate CO2 reduction but circumvent the H-2 evolution. By developing the methodology of rapid-scan FT-IR in external reflection mode, the pathway of CO2 reduction on iron porphyrin was in-situ monitored. It's uncovered that coupled with first-electron reduction of [Fe(II)TAPP](0), the porphyrin nitrogen is protonated as the observation of NH+ ammonium band in IR spectra, which avoids the formation of Fe-H structure for proton reduction. Moreover, an inverse kinetic isotope effect (KIE = 0.59) in formation of *COOH indicates the protonated porphyrin-N would also act as a proton relay for the efficient inner-sphere proton transfer to the activated CO2 molecules. This work experimentally identified the proton migration pathway via the reduced porphyrin ligand as proton shuttle for the first time and should benefit the design of the catalyst of high CO2 reduction selectivity.