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

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 reaction. Reactions of 12CH4/12CO2/13CO mixtures led to identical 13C fractions in CO and CO2, indicating that CO2 activation is quasi-equilibrated and kinetically irrelevant. Binomial water and dihydrogen isotopomer distributions during reactions of CH4/CO2/D2 mixtures indicate that these products form in quasi-equilibrated steps. Turnover rates for H2O and CO2 reforming and CH4 decomposition increased with increasing Pt dispersion, suggesting that coordinative unsaturated surface Pt atoms, prevalent in small crystallites, are more reactive than Pt atoms in low-index surfaces for C−H bond activation. Pt dispersion but not turnover rates were influenced by the identity of the support (ZrO2, γ-Al2O3, ZrO2−CeO2). Similar CO oxidation rates were measured before and after CH4 reactions, indicating that Pt dispersion is not affected by unreactive deposits or sintering during catalysis. These mechanistic conclusions and metal dispersion effects appear to apply generally to CH4 reactions on Group VIII metals, but the surface reactivity of Pt clusters in C−H bond activation reactions is greater than for similar size clusters of other metals. Turnover rates are compared here, for the first time, for most catalytically important noble metals (Rh, Ir, Pt, Ru) as a function of their metal dispersion on several supports. These turnover rates rigorously exclude transport and thermodynamic artifacts and provide a direct comparison of the reactivity of noble metal clusters for catalytic reactions of CH4 on materials and at conditions relevant to industrial practice.