Catalytic activation and reforming of methane on supported palladium clusters

The effects of reactant and product concentrations on turnover rates and isotopic tracing and kinetic isotope effects have led to a sequence of elementary steps for CH4 reactions with CO2 and H2O on supported Pd catalysts. Rate constants for kinetically-relevant C-H bond activation steps are much larger on Pd than on other metals (Ni, Ru, Rh, Ir, Pt). As a result, these steps become reversible during catalysis, because the products of CH4 dissociation rapidly deplete the required oxygen co-reactant formed from CO2 or H2O and co-reactant activation, and water-gas shift reactions remain irreversible in the time scale required for CH4 conversion. H-2 and CO products inhibit CH4 reactions via their respective effects on CH4 and CO dissociation steps. These mechanistic conclusions are consistent with the kinetic effects of reactants and products on turnover rates, with the similar and normal CH4/CD4 kinetic isotope effects measured with H2O and CO2 co-reactants, with the absence of H2O/D2O isotope effects, and with the rate of isotopic scrambling between CH4 and CD4, (CO)-C-12-O-16 and (CO)-C-13-O-18, and (CO)-C-13 and (CO)-C-12 during CH4 reforming catalysis. This catalytic sequence, but not the reversibility of its elementary steps, is identical to that reported on other Group VIII metals. Turnover rates are similar on Pd clusters on various supports (Al2O3, ZrO2, ZrO2-La2O3) and independent of Pd dispersion over the narrow range accessible at reforming conditions, because kinetically-relevant C-H bond activation steps occur predominantly on Pd surfaces. ZrO2 and ZrO2-La2O3 supports, with detectable reactivity for CO2 and H2O activation, can reverse the infrequent formation of carbon overlayers and inhibit deactivation, but do not contribute to steady-state catalytic reforming rates. The high reactivity of Pd surfaces in C-H bond activation reflects their strong binding for C and H and the concomitant stabilization of the transition state for kinetically-relevant C-H activation steps and causes the observed kinetic inhibition by chemisorbed carbon species formed in CH4 and CO dissociation steps. (C) 2010 Elsevier Inc. All rights reserved.