Structural requirements and reaction pathways in methane activation and chemical conversion catalyzed by rhodium

Kinetic and isotopic tracer methods led to a simple and unifying mechanistic proposal for reactions of CH4 with CO2 and H2O, for its decomposition on Rh clusters, and for water–gas shift reactions. Kinetic rates for forward reactions were measured by correcting net rates for approach to equilibrium and by eliminating transport artifacts. These rates were proportional to CH4 pressure (5–450 kPa) and independent of CO2 or H2O pressures (5–450 kPa) on all supported Rh catalysts; the resulting first-order rate constants were identical for H2O and CO2 reforming and for CH4 decomposition. Kinetic isotope effects (kCH4/kCD4 = 1.54–1.60) were also independent of the concentration or identity of the co-reactant, consistent with the sole kinetic relevance of C–H bond activation steps. These data indicate that co-reactant activation and its kinetic coupling with CH4 activation via scavenging of chemisorbed carbon intermediates are fast steps and lead to Rh surfaces essentially uncovered by reactive intermediates during H2O and CO2 reforming. CO oxidation rates before and after reforming reactions showed that Rh surfaces remain uncovered by unreactive species during reforming catalysis under conditions relevant to industrial practice. CH4 conversion rates for CH4/CD4/CO2 reactant mixtures are much faster than CH4−xDx formation rates, indicating that C–H bond activation elementary steps are irreversible. CH4/CO2/D2 reactant mixtures led to binomial isotopomer distributions in dihydrogen and water at all reactant conversions. Their D contents were identical and corresponded to equilibration between all H atoms in reacted CH4 and all D2 in the inlet stream. Thus, recombinative desorption steps of H atoms and OH groups to form H2 or H2O are quasi-equilibrated during CH4 reforming. CH4/ CO2/ 13CO mixtures led to identical 13C contents in CO and CO2, as expected from quasi-equilibrated CO2 activation steps. The quasi-equilibrated nature of all these steps requires that water–gas shift reactions also be at equilibrium during CH4 reforming, as found experimentally. CH4 reforming turnover rates increased as the size of Rh clusters supported on Al2O3 or ZrO2 decreased, suggesting that coordinatively unsaturated Rh surface atoms prevalent in smaller clusters activate C–H bonds more effectively than atoms on lower-index surfaces, as also found on single-crystal surfaces. Turnover rates do not depend on the identity of the support; any involvement of the support in the activation of co-reactants is not kinetically relevant.  2004 Elsevier Inc. All rights reserved.