Transient Studies on Reaction Steps in the Oxidative Coupling of Methane over Catalytic Surfaces of MgO and Sm2O3

Abstract

Adsorption of CH4 and O2 as well as surface reactions of CH4, CD4, and CH4-CD4 mixtures in the absence and presence of gas-phase oxygen were studied over MgO and Sm2O3 in the temperature range from 373 to 1073 K applying the temporal analysis of products (TAP) reactor. Formation of CH3. 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 Sm2O3 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. The pathways of surface-oxygen species of short lifetimes were monitored using sequential pulses of oxygen and methane having various time intervals between 0.02 and 20 s. On MgO, surface-lattice oxygen is responsible for methyl radical formation resulting in C2 hydrocarbons, while adsorbed oxygen species have very short lifetimes (<0.1 s) on the surface and take part in the reactions of total oxidation. On Sm2O3, active oxygen species formed by the interaction of gaseous O2 with the catalyst surface have lifetimes up to 20 s and are mainly responsible for methane conversion and product formation. Based on the response analysis it was assumed that direct interaction of gas-phase methane with active oxygen surface species is the first step in the oxidative coupling of methane (OCM) over Sm2O3. It was found that the type of methane activation which takes place in H-D exchange was not involved in the OCM reaction over Sm2O3. The interaction of C2H6, CO, and CO2 with the surface of Sm2O3 was also studied. C2H6 was converted to C2H4 and COx; CO was effectively oxidized to CO2, which was strongly adsorbed on Sm2O3 up to T = 1073 K.