Influence of Water on the Catalytic Oxidation of Ethane on IrO2(110)

Gaseous H2O can strongly influence the performance of solid catalysts in applications of alkane oxidation. In the present study, we investigated the influence of H2O on the catalytic oxidation of C2H6 on IrO2(110) thin films using measurements of reaction rates and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) performed with synchrotron radiation. We find that the IrO2(110) surface is highly active and selective for catalyzing the complete oxidation of C2H6 at temperatures between 550 and 650 K and that adding H2O in small quantities (<1%) to the gaseous reactant mixtures significantly lowers the catalytic activity without altering the selectivity for CO2 production. AP-XPS measurements performed in about 0.10 Torr of H2O and temperatures between 500 and 600 K show that H2O adsorption on IrO2(110) produces high-coverage mixtures of H2O and HO in which HO groups are the majority species. We present evidence that H2O molecules preferentially incorporate into HO-H2O aggregates on IrO2(110) when HO groups are abundant. AP-XPS further shows that high coverages of HO and COx surface species are generated on IrO2(110) during the catalytic oxidation of C2H6. Introducing H2O into the gas phase causes the coverage of COx surface species to significantly decrease, while the coverages of H2O and HO increase. Based on this behavior, we conclude that H2O suppresses the catalytic oxidation of C2H6 on IrO2(110) at 550-650 K by outcompeting C2H6 and O-2 for Ir adsorption sites in addition to deactivating surface O atoms toward C2H6 dehydrogenation by converting them to HO groups. Our results provide molecular-level insights for understanding the adsorption of H2O on IrO2(110) as well as its influence on both the selectivity and activity of IrO2(110) in catalyzing the oxidation of light alkanes.