Surface Heterogeneity of Zirconia-Supported V2O5Catalysts. The Link between Structure and Catalytic Properties in Oxidative Dehydrogenation of Propane

Undoped and alkali-doped zirconia-supported vanadia catalysts for the oxidative dehydrogenation of propane were prepared by impregnation and characterized by various techniques. The chemical state of surface and bulk vanadium was investigated as a function of the calcination temperature, V 2 O 5 loading, and the nature and content of alkali-metal additives. It is found that on the surface vanadium is present in the form of isolated vanadyl species or oligomeric vanadates, or as nanocrystalline V 2 O 5 and that V 5+ and V 4+ ions coexist in octahedral and tetrahedral coordination, while within the bulk of zirconia matrix, V 4+ ions are stabilized in a V x Zr 1 - x O 2 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 the catalytic performance of V 2 O 5 /ZrO 2 catalysts in oxidative dehydrogenation of propane. Catalytic properties are related to the nature of VO x surface species and correlates well with their reducibility. The maximum of catalytic 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 cations.