METHANE OXIDATIVE COUPLING OVER NONSTOICHIOMETRIC BISMUTH-TIN PYROCHLORE CATALYSTS

A series of expanded pyrochlore oxides Bi2Sn2-xBixO7-x/2 (0 less than or equal to x less than or equal to 0.86) was synthesized and the infl uence of their composition on their performance as methane coupling ca talysts was examined. A trend to higher selectivity and lower activity accompanies increases in x. However, analysis of the kinetic data by a simple procedure which separates the catalyst activity and selectivi ty shows that all the catalysts have similar intrinsic surface selecti vities, independent of composition. The trend in observed selectivity is an indirect effect of variations in activity. The similarity in sur face selectivity is attributed to the formation of a bismuthoxide-rich surface layer in all materials upon heating to reaction temperatures. (C) 1995 Academic Press, Inc.