Isothermal Reduction of IrO2(110) Films by Methane Investigated Using In Situ X-ray Photoelectron Spectroscopy

Abstract

Continuous exposure to methane causes IrO2(110) films on Ir(100) to undergo extensive reduction at temperatures from 500 to 650 K. Measurements using in situ X-ray photoelectron spectroscopy (XPS) confirm that CH4 oxidation on IrO2(110) converts so-called bridging oxygen atoms (Obr) at the surface to HObr groups while concurrently removing oxygen from the oxide film. Reduction of the IrO2(110) film by methane is mildly activated as evidenced by an increase in the initial reduction rate as the temperature is increased from 500 to 650 K. The XPS results show that subsurface oxygen efficiently replaces Obr atoms at the IrO2(110) surface during CH4 oxidation, even after the reduction of multiple layers of the oxide film, and that metallic Ir gradually forms at the surface as well. The isothermal rate of IrO2(110) reduction by methane decreases continuously as metallic Ir replaces surface IrO2(110) domains, demonstrating that IrO2(110) is the active phase for CH4 oxidation under the conditions studied. A key finding is that the replacement of Obr atoms with oxygen from the subsurface is efficient enough to preserve IrO2(110) domains at the surface and enable CH4 to reduce the ∼10-layer IrO2(110) films nearly to completion. In agreement with these observations, density functional theory calculations predict that oxygen atoms in the subsurface layer can replace Obr atoms at rates that are comparable to or higher than the rates at which Obr atoms are abstracted during CH4 oxidation. The efficacy with which oxygen in the bulk reservoir replenishes surface oxygen atoms has implications for understanding and modeling catalytic oxidation processes promoted by IrO2(110).