Catalytic activation and reforming of methane on supported palladium clusters

Pd cluster surfaces show much higher reactivity and rate constants for C–H bond activation than other Group VIII metals, irrespective of support or metal cluster size. This high reactivity leads to reversible C–H and C–O dissociation steps and to concomitant inhibition effects of H 2 and CO on CH 4 reactions with H 2O and CO 2. 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 CH 4 reactions with CO 2 and H 2O 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 CH 4 dissociation rapidly deplete the required oxygen co-reactant formed from CO 2 or H 2O and co-reactant activation, and water–gas shift reactions remain irreversible in the time scale required for CH 4 conversion. H 2 and CO products inhibit CH 4 reactions via their respective effects on CH 4 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 CH 4/CD 4 kinetic isotope effects measured with H 2O and CO 2 co-reactants, with the absence of H 2O/D 2O isotope effects, and with the rate of isotopic scrambling between CH 4 and CD 4, 12C 16O and 13C 18O, and 13CO and 12CO during CH 4 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 (Al 2O 3, ZrO 2, ZrO 2−La 2O 3) 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. ZrO 2 and ZrO 2−La 2O 3 supports, with detectable reactivity for CO 2 and H 2O 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 CH 4 and CO dissociation steps.