INFLUENCE OF SURFACE-COMPOSITION OF PEROVSKITE-TYPE COMPLEX OXIDES ON METHANE OXIDATIVE COUPLING

Alkaline earth metal titanates used as catalysts for methane oxidative coupling have been prepared by (1) solid state reaction and (2) spray combustion methods. It has been established that the bulk phase of all the samples prepared have a perovskite-like structure and that the surface composition and hence the catalytic performance for methane oxidative coupling of the samples differ considerably and are dependent on the preparation method. The relation between the catalytic performance and the composition of the catalyst surface has been studied by means of X-ray diffraction, CO2 temperature-programmed desorption, O2 temperature-programmed desorption, X-ray photoelectron spectroscopy, and laser Raman spectroscopy techniques. A high surface concentration of Ti4+ cations leads to the complete oxidation of methane, and a high surface concentration of alkaline earth metal cations facilitates the formation of C2 hydrocarbons. Surface basicity is necessary for methane oxidative coupling and the oxygen adsorbed on the basic site is also indispensable; in fact, the adsorbed electron deficient species, i.e. O- and O2-, on the surface play a crucial role in methane conversion and C2 selectivity. The adsorption of carbon dioxide could compete for the adsorption site of oxygen, and exerts an unfavorable influence in methane oxidative coupling.