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

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 IrO 2 (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 IrO 2 (110) is highly active for catalytic CH 4 oxidation in CH 4 -rich mixtures (∼90% CH 4 ) 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 CH 4 -rich mixture at temperatures up to 650 K but becomes nearly saturated with chemisorbed O atoms. According to DFT, CH 4 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 O 2 is more efficient than CH 4 dissociation on Ir(100) and generates high O coverages even in CH 4 -enriched reactant mixtures. As a result, the preferential adsorption of oxygen effectively poisons Ir(100) toward initial CH 4 activation, rendering this surface inactive for catalytic CH 4 oxidation. Our results clarify the fundamental origins of the catalytic activity of Ir(100) and IrO 2 (110) toward CH 4 oxidation and provide insights about reaction conditions that can enhance the activity of Ir catalysts for this chemistry.