Influence of electronic properties of Na2O/CaO catalysts on their catalytic characteristics for the oxidative coupling of methane

For Na2O/CaO catalysts of different sodium content the adsorption of oxygen and their electrical properties were studied by transient experiments and measurements of contact potential differences (CPD) as well as electrical c onductivity. CPD results show a change of the mechanism of oxygen activatio n with increasing sodium concentration due to changing the type of ionic co nductivity from cationic to anionic. Anion vacancies are formed by incorpor ation of sodium into the CaO lattice. As CPDs show, the cation conductivity promotes an accumulation of oxygen species on the catalyst surface resulti ng in a decrease of C-2 product selectivity for the catalyzed oxidative cou pling of methane. The anion conductivity favors a dissociation of molecular adsorbed oxygen and a subsequent incorporation into the oxide lattice, her eby, decreasing its concentration on the catalyst surface which favors in t erm selective formation of ethane and ethylene.