Transient studies on reaction steps in the oxidative coupling of methane over catalytic surfaces of MgO and Sm[sub 2]O[sub 3]

Adsorption of CH[sub 4] and O[sub 2] as well as surface reactions of CH[sub 4], CD[sub 4], and CH[sub 4]-CD[sub 4] mixtures in the absence and presence of gas phase oxygen were studied over MgO and Sm[sub 2]O[sub 3] in the temperature range from 373 to 1073 K applying the temporal analysis of products (TAP) reactor. Formation of CH[sub 3][center dot] radicals was observed during surface reaction of methane in the Knudsen-diffusion regime while ethane and ethylene were detected only at increasing pulse intensity, i.e., in the molecular-diffusion regime. The reactivity of surface-lattice oxygen of MgO and Sm[sub 2]O[sub 3] was studied in the Knudsen regime with respect to the H-D exchange in methane. Surface hydroxyl groups were found to participate in this reaction, but no direct interaction of methane molecules on the catalyst surface occurred. H-D exchange proceeds via a multistep mechanism involving methane-surface interaction leading to dissociative adsorption of methane. On MgO, surface-lattice oxygen is responsible for methyl radical formation resulting in C[sub 2] hydrocarbons, while adsorbed oxygen species have very short lifetimes (