Electrochemical studies of oxygen activity during the catalytic oxidative coupling of methane

The oxidative coupling of methane over a Li-promoted MgO catalyst was studied with the aid of solid electrolyte potentiometry. This technique allows the measurement of the thermodynamic activity of oxygen on a catalyst surface under reaction conditions. This additional measurement may be used to establish what types of reactions must be involved in the formation of the coupling products. It was found that the catalytic reaction lowered the steady state amount of surface oxygen from its equilibrium value. This derivation from gas-surface equilibrium will be correlated with the selectivity of the reaction for the production of ethylene and ethane compared to the deep oxidation products such as carbon dioxide. These studies suggest that the formation of the coupling products, ethylene and ethane, must involve a depletion of mobile oxygen from the oxide catalyst surface. A reaction mechanism consistent with these observations will be presented.