STRUCTURE OF UNPROMOTED AND ALKALI-METAL PROMOTED MNOX-BASED CATALYSTS FOR OXIDATIVE COUPLING OF METHANE

Bulk manganese oxide, a catalyst for total combustion, becomes moderately selective for the oxidative coupling of methane when supported on α-alumina, its activity being dramatically reduced. Its selectivity can be further increased by doping with potassium chloride. In order to explain these effects in terms of catalyst structure, samples of manganese oxide catalysts, both supported and unsupported, with or without potassium chloride promoter, have been characterized by means of temperature-programmed reduction, X-ray diffractometry and X-ray photoelectron spectroscopy. The results revealed that supported promoted manganese oxide catalyst has a complex structure. The main effect of the support was to modify the reducibility of manganese oxide, reducing the lability of its oxygen atoms. The action of the alkali metal halide takes place in three different ways: modifying the MnOx active centers, as a source of active chlorine radical species and giving rise to new active centers through interaction of potassium with the alumina support. © 1990.