Mixed RuₓIr₁–ₓO₂ Oxide Catalyst with Well-Defined and Varying Composition Applied to CO Oxidation

The Pechini method allows for compositional and structural control of mixed ruthenium–iridium powder samples. Extensive characterization reveals that the calcination step leads to agglomerates with a metallic core that is encapsulated by the mixed RuₓIr₁–ₓO₂ oxides. In the composition range of 21 up to 74 mol % ruthenium, the metal core reveals a miscibility gap with an Ir-rich fcc structure and a Ru-enriched hcp structure. Quite in contrast, the mixed oxide particles form a well-defined RuₓIr₁–ₓO₂ solid solution throughout the entire composition range. Catalytic activity tests of the mixed RuₓIr₁–ₓO₂ oxide catalysts are conducted with the prototypical CO oxidation reaction under oxidizing and under both stoichiometric reaction conditions; note that the metallic core is buried and does therefore not contribute to the activity. The least active catalyst is pure IrO₂. At moderate reaction temperatures above 100 °C, the most active oxidation catalyst is identified with the Ir₀.₁₂₅Ru₀.₈₇₅O₂ mixed oxide. All mixed RuₓIr₁–ₓO₂ oxide catalysts are bulk-stable under the reaction conditions considered. However, upon CO oxidation reaction, the Ir⁴⁺ concentration at the mixed oxide surface is significantly enhanced with respect to its bulk composition. This information may be important for other catalytic oxidation reactions as well, such as the anodic oxygen evolution reaction in the electrocatalytic water splitting.