Characterization of catalysts for methane-coupling by means of temperature programmed desorption

The technique of temperature programmed desorption has been employed to study the adsorptive behaviour of CaO and Na2O/CaO towards the reactants and products of the methane-coupling reaction. Two forms of adsorbed O2 can exist on CaO, and they desorb at intermediate temperatures: the first corresponds to the sites for adsorption and desorption without reaction of CH4, while the second to sites for desorption with total oxidation of CH4 itself. At the temperatures of the activity runs for methane coupling (750–800 °C), no adsorbed species is therefore stable on the surface of CaO. 7% Na2O/CaO shows different features, because the desorption of O2 produces only a large broad peak at very high temperatures, corresponding to sites for adsorption and desorption with total oxidation of CH4. (On the other hand a desorption peak of CH4 at intermediates temperatures is also present, which seems to be due to a species, adsorbed undissociatively and desorbed without reaction). At the temperatures of the activity experiments, adsorbed species are therefore stable on the surface of Na2O/CaO. A conclusion is therefore drawn on the necessity for the intervention of the gas phase O2 during the methane coupling reaction in order to produce C2-hydrocarbons, and on the greater importance of the surface reactions in the presence of Na2O, in comparison to the case of CaO.