Linking Operando Spectroscopy with Microkinetic Modeling to Unravel the Catalytic Mechanism for CH₄ Oxidation on IrO₂(110)

Abstract

Operando surface spectroscopy can play a key role in developing first-principles microkinetic models that accurately represent the mechanism of heterogeneous catalytic reactions and identify the roles of various surface species in catalytic chemistry. The kinetics of the catalytic oxidation of CH 4 on the IrO 2 (110) surface was investigated both experimentally and using a first-principles microkinetic model that was revised to incorporate pathways for the formation of CHO 2 surface species observed under catalytic conditions using high-resolution, ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Our results show that including pathways for surface CHO 2 formation is necessary for reproducing observations of a transition from CH 4 promotion to inhibition of the catalytic oxidation rate on IrO 2 (110) as the reactant mixture becomes enriched in CH 4 as well as the formation of high coverages of surface CHO 2 and HO species during the catalytic reaction. Analysis of the microkinetic simulations reveals that the catalytic oxidation of CH 4 on IrO 2 (110) occurs through a pathway, in which CH 4 oxidizes to adsorbed CO prior to CO 2 and H 2 O formation, while surface CHO 2 species inhibit this pathway by occupying active surface sites. Only modest destabilization of the dominant CHO 2 surface species relative to density functional theory energies is needed to significantly improve agreement between the simulated and experimentally determined kinetics and species coverages, demonstrating the critical role of CHO 2 in influencing catalytic CH 4 oxidation on IrO 2 (110). This study shows how operando AP-XPS measurements and molecular simulations can be used cooperatively to develop a mechanistic description of a complex heterogeneous catalytic reaction, in which the roles of various surface species are accurately determined.