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

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


Asthagiri, A., The Ohio State University
Kim, M., The Ohio State University
Martin, R., University of Florida
Lee, C., University of Florida
Mehar, V., University of Florida
Jamir, J., North Carolina State University
The exceptional activity of IrO2(110) for CH4 C-H cleavage as well as its ability to stabilize CHx 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 CHyO2 species form on IrO2(110) during catalytic CH4 oxidation at all conditions studied. The conversion of CH4 to the CHyO2 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.