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 conditions have been investigated with infrared and Raman spectroscopies. The surface sulfate species present on sulfated TiO 2, ZrO 2, and Al 2 O 3 supports and V 2 O 5 /TiO 2, V 2 O 5 /ZrO 2, and V 2 O 5 /Al 2 O 3 catalysts have identical molecular structures, i.e., (M−O) 3 S O, where M = Ti, Zr, or Al. Interactions between the surface vanadia and surface sulfate species do not lead to the formation of sulfate−vanadate compounds. The surface sulfate species on the V 2 O 5 /TiO 2, V 2 O 5 /ZrO 2, and V 2 O 5 /Al 2 O 3 catalysts anchor to and displace only the most basic support hydroxyls, while the surface vanadate species titrate both basic and neutral support hydroxyls forming a complete close-packed monolayer. At low V 2 O 5 loadings, the surface vanadia species on these catalysts preferentially titrate basic hydroxyls, which consumes the sites capable of sulfate adsorption. 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 CeO 2 lead to the formation of bulklike cerium−oxy−sulfur compounds. The stable monolayer of VO 4 units present on the surface of a 4% V 2 O 5 /CeO 2 catalyst was disrupted upon sulfation and lead to the formation of bulklike cerium sulfate and cerium vanadate compounds.