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 H 2 O and CO 2 reforming of CH 4 on supported Pt clusters and to demonstrate a rigorous kinetic and mechanistic equivalence for CO 2 and H 2 O reforming, CH 4 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 (H 2 O, CO 2 ). Kinetic isotopic effects are consistent with the sole kinetic relevance of C−H bond activation ( k H / k D = 1.58−1.77 at 873 K); these isotope effects and measured activation energies are similar for H 2 O reforming, CO 2 reforming, and CH 4 decomposition reactions. CH 4 /CD 4 cross exchange rates are much smaller than the rate of methane chemical conversion in CO 2 and H 2 O 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 12 CH 4 / 12 CO 2 / 13 CO mixtures led to identical 13 C fractions in CO and CO 2, indicating that CO 2 activation is quasi-equilibrated and kinetically irrelevant. Binomial water and dihydrogen isotopomer distributions during reactions of CH 4 /CO 2 /D 2 mixtures indicate that these products form in quasi-equilibrated steps. Turnover rates for H 2 O and CO 2 reforming and CH 4 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 (ZrO 2, γ -Al 2 O 3, ZrO 2 −CeO 2 ). Similar CO oxidation rates were measured before and after CH 4 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 CH 4 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 CH 4 on materials and at conditions relevant to industrial practice.