Jp. Dunn et al., INTERACTIONS BETWEEN SURFACE VANADATE AND SURFACE SULFATE SPECIES ON METAL-OXIDE CATALYSTS, JOURNAL OF PHYSICAL CHEMISTRY B, 102(32), 1998, pp. 6212-6218
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.