(337a) Mechanistic Interpretations in Methane Activation and Chemical Conversion Catalyzed by Supported Metal Clusters

Chin, Y. (., University of California, Berkeley
Li, M., Jiangsu Polytechnic University
Wei, J., Engelhard Corporation
Iglesia, E., University of California at Berkeley

Conversion of natural gas with CO2, H2O, and O2 co-reactants over supported Ni, Pt, Rh, and Ru metal clusters are effective routes toward production of hydrogen and synthesis gas. Despite many research efforts, the fundamental kinetics associated with these reactions remains controversial and often contradictory. In this contribution, kinetic and isotopic evidence for the elementary steps and for the metal cluster size effects is provided after rigorously eliminating the inherent transport artifacts and correcting the catalytic turnover for thermodynamic equilibrium. Isotopic studies at conditions relevant to industrial practices demonstrated that the C-H bond activation elementary step is the sole kinetically relevant step for direct decomposition and reforming. CH4 turnover rates are similar for CH4/CO2, CH4/H2O and CH4 decomposition reactions, exhibit a first order dependence on the partial pressure of CH4, and are independent of the identity and concentration of the co-reactants and products. The turnover rates increase with metal dispersion on all samples, apparently because of the high reactivity of coordinatively unsaturated, exposed metal atoms prevalent on small metal clusters. The activation energies measured are similar for reactions of CH4 with CO2 and H2O and for CH4 decomposition on each catalyst, but consistently much higher compared to the CH4 activation on single crystals. These differences do not reflect the titration of very active sites by unreactive carbon, but may arise instead from minority defects on macroscopic crystals, which cannot be maintained on the near-molten surfaces of small metal clusters during catalysis. Supports did not influence turnover rates, except indirectly through their effect on dispersing the metal clusters, as expected from the sole kinetic relevance of C-H bond activation steps catalyzed exclusively by metal surfaces. In the CH4/O2 reaction under excess O2, C-H activation remains as the kinetically relevant step. Turnover rates are, however, significantly higher on these O*-O* site pairs than on the bare metal sites. At low O2 concentrations, an even more reactive vacancy-oxygen *-O* site pairs formed leading to direct partial oxidation of CH4. Competitive reaction of CO and CH4 with O2 demonstrated a CO oxidation rate that is at least three orders of magnitude higher than CH4. CO and H2 became detectable only as O2 was depleted. Even if CO and H2 formed via direct CH4 partial oxidation, they do not leave the reactor before all the O2 was consumed. It is therefore concluded that direct partial oxidation only proceeds at a narrow O2 concentration range of limited relevance in industrial practice. H2-CO mixtures form from partial oxidation reaction via a sequential combustion-reforming pathway.