The Key Ru^V=O Intermediate of Site-Isolated Mononuclear Water Oxidation Catalyst Detected by <i>in Situ</i> X‑ray Absorption Spectroscopy

Improvement of the oxygen evolution reaction (OER) is a challenging step toward the development of sustainable energy technologies. Enhancing the OER rate and efficiency relies on understanding the water oxidation mechanism, which entails the characterization of the reaction intermediates. Very active Ru-bda type (bda is 2,2′-bipyridine-6,6′-dicarboxylate) molecular OER catalysts are proposed to operate via a transient 7-coordinate Ru^VO intermediate, which so far has never been detected due to its high reactivity. Here we prepare and characterize a well-defined supported Ru­(bda) catalyst on porous indium tin oxide (ITO) electrode. Site isolation of the catalyst molecules on the electrode surface allows trapping of the key 7-coordinate Ru^VO intermediate at potentials above 1.34 V vs NHE at pH 1, which is characterized by electron paramagnetic resonance and <i>in situ</i> X-ray absorption spectroscopies. The <i>in situ</i> extended X-ray absorption fine structure analysis shows a RuO bond distance of 1.75 ± 0.02 Å, consistent with computational results. Electrochemical studies and density functional theory calculations suggest that the water nucleophilic attack on the surface-bound Ru^VO intermediate (O–O bond formation) is the rate limiting step for OER catalysis at low pH

IrO₂‑TiO₂: A High-Surface-Area, Active, and Stable Electrocatalyst for the Oxygen Evolution Reaction

The utilization and development of efficient water electrolyzers for hydrogen production is currently limited due to the sluggish kinetics of the anodic processthe oxygen evolution reaction (OER). Moreover, state of the art OER catalysts contain high amounts of expensive and low-abundance noble metals such as Ru and Ir, limiting their large-scale industrial utilization. Therefore, the development of low-cost, highly active, and stable OER catalysts is a key requirement toward the implementation of a hydrogen-based economy. We have developed a synthetic approach to high-surface-area chlorine-free iridium oxide nanoparticles dispersed in titania (IrO₂-TiO₂), which is a highly active and stable OER catalyst in acidic media. IrO₂-TiO₂ was prepared in one step in molten NaNO₃ (Adams fusion method) and consists of ca. 1–2 nm IrO₂ particles distributed in a matrix of titania nanoparticles with an overall surface area of 245 m² g^–1. This material contains 40 mol_M % of iridium and demonstrates improved OER activity and stability in comparison to the commercial benchmark catalyst and state of the art high-surface-area IrO₂. Ex situ characterization of the catalyst indicates the presence of iridium hydroxo surface species, which were previously associated with the high OER activity. Operando X-ray absorption studies demonstrate the evolution of the surface species as a function of the applied potential, suggesting the conversion of the initial hydroxo surface layer to the oxo-terminated surface via anodic oxidation (OER regime)