AN INSITU XRD INVESTIGATION OF SINGLY AND DOUBLY PROMOTED MANGANESE OXIDE METHANE COUPLING CATALYSTS

In situ X-ray diffraction (XRD) and concurrent measurements of catalytic performance have been used to characterise the solid phases present during various stages in the history of working methane coupling catalytic systems. Three such systems were studied: unpromoted, K-promoted, and KCl-promoted manganese oxide. In each case the effect of pulses of CHCl3 on the activity, selectivity, and catalyst structure was determined. Depending on the conditions, various oxides or mixed oxides of Mn were present. Of these, Mn3O4 is an unselective total oxidation catalyst; K-promotion stabilises this phase, drastically reducing CO2 formation and raising C2 activity. Chloride ions (added as KCI) prevent the formation of highly unselective potassium manganese oxides. Chlorine introduced from the gas phase has an additional effect: C2 activity is further enhanced leaving CO2 production unaffected. This form of chlorine promotion is greatly augmented by the presence of KCl. There are strong indications that the effects of chlorine involved substantial modification of the surface chemistry and are not purely due to Cl atom-induced homogeneous chemistry. © 1992.