Surface Oxide-Support Interaction (SOSI) for Surface Redox Sites

Recent studies of supported vanadium oxide catalysts have revealed that the vanadium oxide component is present as a twodimensional metal oxide overlayer on oxide supports (i.e., A1203, TiO2) (1-9). These surface vanadium oxide species are generally more selective and active than bulk, crystalline V205 for the partial oxidation of hydrocarbons (3, 5, 10, 11). The molecular structures of these surface vanadium oxide species have recently been characterized under in situ conditions with Raman spectroscopy (4, 12-15), infrared spectroscopy (13), solid-state 51V NMR spectroscopy (9), and X-ray absorption near edge structure (XANES) spectroscopy (16). The characterization results uniformly agree that under in situ conditions the surface vanadium oxide species appears to be four coordinated. In addition, the Raman and infrared vibrational spectroscopy measurements reveal that two different surface vanadium oxide species can be present on the oxide supports. The in situ Raman spectra exhibit a sharp band at 1030 cm1 and a broad band at -900 cm~, and the relative intensities of these two Raman bands varies with surface vanadium oxide coverage on the oxide support. At low surface coverages the sharp Raman band at -1030 cm -~ usually dominates, and the broad Raman band at -900 cm-~ generally grows in at higher surface coverages (with the exception of the V2Os/SiO 2 system which does not possess the -900 cm-1). The surface vanadium oxide responsible for the sharp Raman band at 1030 cml has been assigned to an isolated vanadate species possessing one terminal V--O bond and three bridging V--O-support bonds (9, 12-15, 17), and the broad