(43d) Kinetics and Pathways for the Catalytic Oxidation of Methane on IrO2(110) Thin Films

The exceptional activity of IrO₂(110) for CH₄ C-H cleavage as well as its ability to stabilize CH_x groups over a wide temperature range provide substantial motivation for investigating methane oxidation on IrO₂(110) under catalytic conditions. In this talk I will discuss recent kinetic studies as well as operando x-ray photoelectron spectroscopy (XPS) measurements of the catalytic oxidation of methane on IrO­₂(110) thin films and the relation between these results and the chemical behavior identified from ultrahigh vacuum (UHV) surface science experiments and molecular simulations. We find that IrO₂(110) films remain stable and are highly active for the catalytic combustion of CH₄ at moderate temperatures (~500-650 K) and CH₄-rich conditions, with activities comparable to that reported for the most active PdO catalysts. Measurements using near-ambient pressure, synchrotron XPS show that high coverages of OH groups and CH_yO₂ species form on IrO₂(110) during catalytic CH₄ oxidation at all conditions studied. The conversion of CH₄ to the CH_yO₂ surface species becomes optimal at an intermediate composition of the reactant mixture (∼90% CH₄), with this condition coinciding with the maximum CH₄ oxidation activity measured for IrO₂(110). Using density functional theory (DFT), we identified two predominant pathways for CH₄ oxidation to CO₂ involving either an adsorbed CO or a CHO₂ intermediate. A microkinetic model developed from our DFT calculations reproduces key aspects of the catalytic kinetics and the coverages of adsorbed intermediates measured experimentally over a wide range of conditions. Our study provides molecular-level insights about the dominant reaction pathways and the roles of various adsorbed species in mediating catalytic CH₄ oxidation on IrO₂(110) and highlights the significant utility that operando surface spectroscopy can play in validating first principles models of catalytic reaction kinetics.