Surface heterogeneity of zirconia-supported V2O5 catalysts. The link between structure and catalytic properties in oxidative dehydrogenation of propane

Undoped and alkali-doped zirconia-supported vanadia catalysts for the oxida tive dehydrogenation of propane were prepared by impregnation and character ized by various techniques. The chemical state of surface and bulk vanadium was investigated as a function of the calcination temperature, V2O5 loadin g, and the nature and content of alkali-metal additives. It is found that o n the surface vanadium is present in the form of isolated vanadyl species o r oligomeric vanadates, or as nanocrystalline V2O5 and that V5+ and V4+ ion s coexist in octahedral and tetrahedral coordination, while within the bulk of zirconia matrix, V4+ ions are stabilized in a VxZr1-xO2 solid solution. Presence of the alkali-metal additives and water changes the dispersion of surface vanadium species favoring, in both cases, formation of mononuclear vanadyl surface complexes. Surface heterogeneity plays a vital role for th e catalytic performance of V2O5/ZrO2 catalysts in oxidative dehydrogenation of propane. Catalytic properties are related to the nature of VOx surface species and correlates well with their reducibility. The maximum of catalyt ic activity was observed for catalysts with vanadia content between 3 and 5 mol &, for which octahedral polyvanadate surface species are dominant. It is proposed that the catalytic activity is affected by the nucleophilicity of bridging oxygen in V-O-V entities, modified by the adjacent alkali catio ns.