Theoretical insights into the partial oxidation of methane over supported metal clusters

The catalytic conversion of methane via reforming, partial oxidation and combustion over supported metal catalysts occurs through similar elementary C-H and O ₂ bond activation steps which differ only by the nature of the active sites and the surface coverages that result under operating conditions. The prevailing chemistry is ultimately controlled by the the metal, the surface coverage and reactivity of chemisorbed oxygen. Experimental results for the oxidation of methane over supported transition metal clusters, for example, reveal the presence of multiple kinetic regimes which can be described by unique rate expressions that result for the conversion of methane over bare and oxygen-covered metal surfaces. First-principle density functional theoretical calculations and kinetic Monte Carlo simulation are used together with kinetic labeling and fixed-bed reactor studies to follow the elementary steps and establish the intrinsic and apparent kinetics for methane reforming, paritial oxidation and combustion. The theoretical and experimental results show that there is no direct partial oxidation route and that CO and H ₂ form instead as a result of CO ₂ or steam reforming which occurs upon the loss of O ₂ further down the reactor.