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

Abstract

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.