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.