In situ vibrational spectroscopy studies of supported niobium oxide catalysts

The IR spectra of bulk Nb2O5. nH(2)O and niobia supported on SiO2, Al2O3, Z rO2, and TiO2 were recorded in the fundamental and overtone Nb=O regions, a s well as in the hydroxyl region, to develop a better understanding of the structural models of surface NbOx species. The coincidence of the IR and Ra man fundamental Nb=O frequency in Nb2O5/Al2O3, Nb2O5/ZrO2, and Nb2O5/TiO2 p rovides the strongest evidence that the NbOx surface species (Nb=O fundamen tal at 980 cm(-1)) is present as a mono-ore moiety. The IR and Raman band p ositions would not be coincident for a di-oxo species. This conclusion is f urther supported by the presence of only a single overtone at similar to 19 60 cm(-1) (2 x 980 cm(-1)) in Nb2O5/ZrO2 and Nb2O5/TiO2 and the presence of the most intense overtone at 1966 cm(-1) in Nb2O5/Al2O3. Lower frequency I R fundamentals at 880 cm(-1) (low loading) and 935 cm(-1) (high loading) ar e also seen in Nb2O5/ZrO2, and the IR overtone region of Nb2O5/Al2O3 exhibi ted weak bands at similar to 1870 cm(-1) (935 cm(-1) overtone) and 1914 cm( -1) (935 + 980 cm(-1) combination). These fundamental niobia bands at 880 a nd 935 cm(-1), which are also observed in Raman for NbOx surface species on alumina, zirconia, and titania, are assigned to an Nb-O-Nb stretching mode , v(s)([-O-Nb-O-](n)), that shifts from 880 to 935 cm(-1) with increased lo ading. Finally, observation of the hydroxyl region indicates that the highe r frequency surface hydroxyls on the SiO2, Al2O3, ZrO2, and TiO2 supports a re generally titrated preferentially as niobia loading is increased. Also, in Nb2O5/ZrO2 and Nb2O5/TiO2 a new (nonacidic or weakly acidic) Nb-OH or Nb -OH-Zr (Nb-OH-Ti) surface hydroxyl group is created at 3710-3730 cm(-1) tha t is very similar in frequency to the Nb-OH band observed in bulk Nb2O5. nH (2)O at 3702 cm(-1).