Reaction Pathways and Site Requirements for the Activation and Chemical Conversion of Methane on Ru−Based Catalysts

Abstract

Kinetic and isotopic tracer and exchange measurements were used to determine the identity and reversibility of elementary steps required for CH 4 reforming reactions on Ru-based catalyst. These studies provide a simple mechanistic picture and a unifying kinetic treatment for CH 4 /CO 2 and CH 4 /H 2 O reforming reactions and CH 4 decomposition. Forward kinetic rates were measured from net rates by correcting for the approach to equilibrium, after ruling out transport artifacts using pellet and bed dilution tests. The kinetic processes involved are exclusively limited by C−H bond activation, and CH 4 reaction rates are unaffected by the identity or the concentration of co-reactants (H 2 O, CO 2 ). Similar normal kinetic isotopic effects ( k C - H /k C - D = 1.40−1.51) were measured for CO 2 reforming, H 2 O reforming, and CH 4 decomposition, consistent with kinetically relevant C−H bond activation steps. The ratio of CH 4 /CD 4 cross-exchange to methane chemical conversion rates during the reaction of CO 2 reforming with CH 4 −CD 4 mixtures was 0.05, suggesting that steps involving C−H bond activation are essentially irreversible. Binomial D-atom distributions in dihydrogen and water were obtained during reactions of CH 4 /CO 2 /D 2 mixtures, and their D-contents were identical to those expected from complete equilibration between D 2 and H-atoms from reacted CH 4, indicating that H−OH and H−H recombination steps are quasi-equilibrated. Reactions of 12 CH 4 / 12 CO 2 / 13 CO mixtures gave identical 13 C contents in CO and CO 2, even far away from the CO 2 reforming equilibrium; thus, CO 2 activation is reversible and quasi-equilibrated during CO 2 reforming on Ru-based catalysts, as expected from the kinetic irrelevance of co-reactant activation steps. These conclusions suggest that water-gas shift reactions are also equilibrated, as confirmed by chemical analyses of reaction products. Forward CH 4 turnover rates increased with increasing Ru dispersion, but they were essentially unaffected by the identity of the support. This behavior reflects the higher reactivity of coordinatively unsaturated surface atoms, prevalent in small Ru clusters, for C−H bond activation reactions, as previously inferred from the effect of crystal orientation on CH 4 activation rates.