Infrared Spectroscopic Observation of Oxo- and Superoxo-Intermediates in the Water Oxidation Cycle of a Molecular Ir Catalyst

Abstract

Molecular Ir catalysts have emerged as an important class of model catalysts for understanding structure–activity relationships in water oxidation, a reaction that is central to renewable fuel synthesis. Prior efforts have mostly focused on controlling and elucidating the emergence of active species from prepared precursors. However, the development of efficient and stable molecular Ir catalysts also necessitates probing of reaction intermediates. To date, relatively little is known about the key intermediates in the cycles of the molecular Ir catalysts. Herein, we probed the catalytic cycle of a homogeneous Ir catalyst (“blue dimer”) at a Au electrode/aqueous electrolyte interface by combining surface-enhanced infrared absorption spectroscopy (SEIRAS) with phase-sensitive detection (PSD). Cyclic voltammograms (CVs) from 1.4 to 1.7 V RHE (RHE = reversible hydrogen electrode) give rise to a band at ∼818 cm –1, whereas CVs from 1.4 to ≥1.85 V RHE generate an additional band at ∼1146 cm –1 . Isotope labeling experiments indicate that the bands at ∼818 and ∼1146 cm –1 are attributable to oxo (Ir V ═O) and superoxo (Ir IV –OO • ) moieties, respectively. This study establishes PSD-SEIRAS as a sensitive tool for probing water oxidation cycles at electrode/electrolyte interfaces and demonstrates that the relative abundance of two key intermediates can be tuned by the thermodynamic driving force of the reaction.