Mechanism of CO ₂ Electrolysis with Heterogenized Molecular Iridium Catalysts Deciphered Using Operando Spectroscopy

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.