Isotopic and kinetic assessment of the mechanism of methane reforming and decomposition reactions on supported iridium catalysts

Abstract

Isotopic tracer and kinetic measurements were used to determine the identity and reversibility of elementary steps required for CH₄–CO₂, CH₄–H₂O and CH₄ decomposition reactions on supported Ir clusters. The results led to a simple and rigorous mechanism that includes steps required for these reactions as well as water–gas shift reactions. All three CH₄ reactions gave similar forward rates, rate constants, activation energies, and kinetic isotopic effects, indicating that C–H bond activation is the only kinetically relevant step on Ir surfaces. CO₂ and H₂O activation is quasi-equilibrated and intermediates derived from these co-reactants are not involved in kinetically-relevant steps. Isotopic cross-exchange during CH₄/CD₄/CO₂ and CH₄/CD₄/H₂O reactions is much slower than chemical conversion, indicating that C–H bond activation is irreversible. Identical ¹³C contents in CO and CO₂ formed from ¹²CH₄/¹²CO₂/¹³CO reactions showed that CO₂ activation is reversible and quasi-equilibrated. Binomial isotopomer distributions in water and dihydrogen formed from CH₄/CO₂/D₂ and CD₄/H₂O mixtures are consistent with quasi-equilibrated hydrogen and water activation and recombinative desorption during CH₄ reforming on Ir surfaces. Taken together with the quasi-equilibrated nature of CO₂ activation steps, these data require that water–gas shift reactions must also be at equilibrium, as confirmed by analysis of the products formed in CH₄ reforming reactions.