Elucidation of reaction pathways and identification of active sites in the oxidative coupling of methane

This dissertation details an investigation of the oxidative coupling of methane over MgO, Li/MgO, and cubic and monoclinic Sm{dollar}sb2{dollar}O{dollar}sb3{dollar} catalysts using steady-state isotopic transient kinetic analysis (SSITKA). Oxygen exchange experiments in the absence of methane resulted in a quantification of the lattice oxygen diffusivity and total oxygen uptake. The catalyst had three more or less distinct regions: the physical surface at which exchange between the gas phase and the solid occurred, several sub-surface atomic layers readily available for exchange, and the bulk oxide.; Using isotopic switches of oxygen and methane during steady-state reaction, the active sites along the carbon and oxygen reaction pathways were quantified. Lattice oxygen was found to play a significant role in the oxidation process under steady-state reaction. CO and CO{dollar}sb2{dollar} appeared to be formed via a multistep surface oxidation pathway while ethane was formed via surface generated intermediates along a parallel pathway. Sites involved with the generation of intermediates for selective coupling were found to have a lower activity than sites active for the generation of nonselective intermediates.; Isotope switches of carbon during steady-state reaction with CO{dollar}sb2{dollar}, CO, and C{dollar}sb2{dollar}H{dollar}sb6{dollar} added to the reactant stream showed that all Li/MgO active sites can adsorb CO{dollar}sb2{dollar}, CO, and C{dollar}sb2{dollar}H{dollar}sb6{dollar} with equal probability with none of the relative site activities being affected. The results for Li/MgO suggested that all sites were equally active for both selective and nonselective oxidation. No adsorption of CO{dollar}sb2{dollar} and CO on Sm{dollar}sb2{dollar}O{dollar}sb3{dollar} active sites was seen.; Surface coverages determined from transient curves produced by a step change in the concentration of one of the reactants showed such curves should not be used to quantify surface coverages of reactive intermediates existing under steady-state reaction conditions. The concentrations of intermediates leading to products (CO, CO{dollar}sb2{dollar}, C{dollar}sb2{dollar}H{dollar}sb6{dollar}) were grossly miscounted.