Mechanism and Site Requirements for Activation and Chemical Conversion of Methane on Supported Pt Clusters and Turnover Rate Comparisons among Noble Metals

Abstract

Isotopic tracer and kinetic studies are used to probe the identity and reversibility of elementary steps required for H2O and CO2 reforming of CH4 on supported Pt clusters and to demonstrate a rigorous kinetic and mechanistic equivalence for CO2 and H2O reforming, CH4 decomposition, and water-gas shift reactions. Reforming rates are exclusively limited by C−H bond activation on essentially uncovered Pt crystallite surfaces and unaffected by the concentration or reactivity of co-reactants (H2O, CO2). Kinetic isotopic effects are consistent with the sole kinetic relevance of C−H bond activation (kH/kD = 1.58−1.77 at 873 K); these isotope effects and measured activation energies are similar for H2O reforming, CO2 reforming, and CH4 decomposition reactions. CH4/CD4 cross exchange rates are much smaller than the rate of methane chemical conversion in CO2 and H2O reforming reactions; thus, C−H bond activation steps are irreversible, except as required by the approach to equilibrium for the overall reforming rea...