In Situ Vibrational Spectroscopy Studies of Supported Niobium Oxide Catalysts

The IR spectra of bulk Nb 2 O 5 · n H 2 O and niobia supported on SiO 2, Al 2 O 3, ZrO 2, and TiO 2 were recorded in the fundamental and overtone Nb O regions, as well as in the hydroxyl region, to develop a better understanding of the structural models of surface NbO x species. The coincidence of the IR and Raman fundamental Nb O frequency in Nb 2 O 5 /Al 2 O 3, Nb 2 O 5 /ZrO 2, and Nb 2 O 5 /TiO 2 provides the strongest evidence that the NbO x surface species (Nb O fundamental at 980 cm -1 ) is present as a mono-oxo moiety. The IR and Raman band positions would not be coincident for a di-oxo species. This conclusion is further supported by the presence of only a single overtone at ∼1960 cm -1 (2 × 980 cm -1 ) in Nb 2 O 5 /ZrO 2 and Nb 2 O 5 /TiO 2 and the presence of the most intense overtone at 1966 cm -1 in Nb 2 O 5 /Al 2 O 3 . Lower frequency IR fundamentals at 880 cm -1 (low loading) and 935 cm -1 (high loading) are also seen in Nb 2 O 5 /ZrO 2, and the IR overtone region of Nb 2 O 5 /Al 2 O 3 exhibited weak bands at ∼1870 cm -1 (935 cm -1 overtone) and 1914 cm -1 (935 + 980 cm -1 combination). These fundamental niobia bands at 880 and 935 cm -1, which are also observed in Raman for NbO x surface species on alumina, zirconia, and titania, are assigned to an Nb O Nb stretching mode, ν s ([−O Nb O−] n ), that shifts from 880 to 935 cm -1 with increased loading. Finally, observation of the hydroxyl region indicates that the higher frequency surface hydroxyls on the SiO 2, Al 2 O 3, ZrO 2, and TiO 2 supports are generally titrated preferentially as niobia loading is increased. Also, in Nb 2 O 5 /ZrO 2 and Nb 2 O 5 /TiO 2 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 that is very similar in frequency to the Nb OH band observed in bulk Nb 2 O 5 · n H 2 O at 3702 cm -1 .