Mechanism of CO2Electrolysis with Heterogenized Molecular Iridium Catalysts Deciphered Using Operando Spectroscopy

Abstract

Although many molecular catalysts for electrochemical reduction of carbon dioxide (CO2) to formic acid (HCO2H) have been surveyed to date, catalytic mechanisms proposed are often based on computational studies of hypothetical intermediates that lack direct experimental evidence and thus remain poorly understood. A solid understanding of the reaction mechanism is thus crucial to develop more efficient molecular catalysts. Multioperando analysis was conducted for the electrochemical reduction of CO2 using gas diffusion electrodes (GDEs), where a heterogenized molecular iridium (Ir) catalyst was coordinated with a tetradentate 6,6′-bis(phosphinomethyl)-2,2′-bipyridyl ligand. Application to a GDE system maintained plasmon-enhanced particles on the electrode, enabling time course operando measurements. Operando X-ray absorption fine structure spectroscopy and surface-enhanced Raman scattering measurements were used to track real-time changes in the electronic/intermediate structure of the catalyst, where three Ir intermediates were observed on the path to HCO2H production. The appearance times of each Ir intermediate were clearly distinguishable and showed a high correlation with the activation energy values calculated from DFT calculations. A highly accurate reaction mechanism of heterogenized catalysis was corroborated experimentally, which is typically more difficult to elucidate than homogeneous catalysis.