Probing Oxo-and Superoxo-Intermediates in the Water Oxidation Cycle of a Molecular Ir Catalyst at the Electrode/Electrolyte Interface

Water oxidation plays a crucial role as the anodic half-reaction in various renewable fuel-formation processes, including carbon dioxide reduction, hydrogen evolution, and nitrogen activation. However, the slow kinetics of water oxidation limits the overall efficiency of renewable fuel synthesis. Molecular Ir catalysts are particularly interesting due to their high water-oxidation activity. They also serve as important model systems for elucidating structure-function relationships that inform the design of heterogeneous Ir-based electrodes. To advance this design, a deeper mechanistic understanding of Ir-mediated water oxidation catalysis is essential. However, the intermediates of Ir molecular catalysts at the electrode/electrolyte interface have not been investigated with structure-sensitive methods. In this work, we identified two reaction intermediates in the electrocatalytic water oxidation cycle of a molecular Ir catalyst known as the “Ir blue dimer” at the Au electrode/aqueous electrolyte interface. This advance was enabled by combining phase sensitive detection (PSD) with surface-enhanced infrared absorption spectroscopy (SEIRAS). By coupling these two techniques, we were able to detect infrared bands with amplitudes as low as a few of µOD. We determined that with increasing potential, the predominant intermediate changes from Ir-oxo to Ir-superoxo. We rationalize this change in intermediate population in terms of the accumulated oxidative charge on the catalyst. This study demonstrates that PSD-SEIRAS is a versatile and sensitive method for probing reaction intermediates at electrode/electrolyte interfaces.