Activity of Ir(100) and IrO2(110) for the Catalytic Oxidation of Methane

Abstract

Identifying and characterizing active surface phases are critical for understanding the behavior of oxidation catalysts. In this study, we investigated the catalytic activity of Ir(100) and an IrO2(110) film for the complete oxidation of methane at pressures near 1 Torr and characterized the surface activity using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) measurements and density functional theory (DFT) calculations. Our results show that IrO2(110) is highly active for catalytic CH4 oxidation in CH4-rich mixtures (∼90% CH4) and moderate temperature (650 K), whereas the Ir(100) surface is inactive at the same conditions. AP-XPS shows that the Ir(100) surface remains metallic when immersed in the CH4-rich mixture at temperatures up to 650 K but becomes nearly saturated with chemisorbed O atoms. According to DFT, CH4 activation is facile on clean Ir(100), but activation via H transfer to a chemisorbed O atom has a significantly higher energy barrier. The DFT calculations further predict that the dissociative chemisorption of O2 is more efficient than CH4 dissociation on Ir(100) and generates high O coverages even in CH4-enriched reactant mixtures. As a result, the preferential adsorption of oxygen effectively poisons Ir(100) toward initial CH4 activation, rendering this surface inactive for catalytic CH4 oxidation. Our results clarify the fundamental origins of the catalytic activity of Ir(100) and IrO2(110) toward CH4 oxidation and provide insights about reaction conditions that can enhance the activity of Ir catalysts for this chemistry.