SM2O3/AL2O3 CATALYSTS FOR METHANE COUPLING - INFLUENCE OF THE STRUCTURE OF SURFACE SM-AL-O PHASES ON THE REACTIVITY

The oxidative dimerization of methane has been studied over samarium oxide supported on alumina as a function of the samarium loading and the calcination temperature. The structure of the supported phase has been determined by EXAFS. The EXAFS results state that all the samarium ions are present at the catalyst surface for Sm2O3 contents lower than 20% w/w. At the higher loadings tried, crystalline phases containing Sm appear, their structure depending upon the calcination temperature. The activity/selectivity toward C2 per exposed samarium ion is higher for samples containing less than a monolayer of samarium. In this case, the coordination polyhedron around samarium ions points to a nonordered structure that blocks highly reactive sites on the alumina surface. On increasing the samarium loading, two phases develop depending upon the calcination temperature; at low temperatures an oxide-like structure grows over the alumina support while at higher temperatures a perovskite-like phase evolves. For the same samarium loading, the oxide-like structure shows better selectivity toward C2 than the SmAlO3 phase.