INTERACTIONS BETWEEN SURFACE VANADATE AND SURFACE SULFATE SPECIES ON METAL-OXIDE CATALYSTS

The interactions between surface sulfate and surface vanadate species present on sulfated supported vanadia catalysts under dehydrated condi tions have been investigated with infrared and Raman spectroscopies. T he surface sulfate species present on sulfated TiO2, ZrO2, and Al2O3 s upports and V2O5/TiO2, V2O5/ZrO2, and V2O5/Al2O3 catalysts have identi cal molecular structures, i.e., (M-O)(3)S=O, where M = Ti, Zr, or Al. Interactions between the surface vanadia and surface sulfate species d o not lead to the formation of sulfate-vanadate compounds. The surface sulfate species on the V2O5/TiO2 V2O5/ZrO2, and V2O5/Al2O3 catalysts anchor to and displace only the most basic support hydroxyls, while th e surface vanadate species titrate both basic and neutral support hydr oxyls forming a complete close-packed monolayer. At low V2O5 loadings, the surface vanadia species on these catalysts preferentially titrate basic hydroxyls, which consumes the sites capable of sulfate adsorpti on. Thus, the amount of adsorbed surface sulfate species decreases in an exponential fashion with increasing surface coverage of the vanadia species. Strong interactions between sulfate species and CeO2 lead to the formation of bulklike cerium-oxy-sulfur compounds. The stable mon olayer of VO4 units present on the surface of a 4% V2O5/CeO2 catalyst was disrupted upon sulfation and lead to the formation of bulklike cer ium sulfate and cerium vanadate compounds.