In Situ Vibrational Spectroscopy Studies of Supported Niobium Oxide Catalysts

The IR spectra of bulk Nb2O5‚nH2O and niobia supported on SiO2, Al2O3, ZrO2, and TiO2 were recorded in the fundamental and overtone NbdO regions, as 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 Raman fundamental NbdO frequency in Nb2O5/Al2O3, Nb2O5/ZrO2, and Nb2O5/TiO2 provides the strongest evidence that the NbOx surface species (NbdO 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 Nb2O5/ZrO2 and Nb2O5/TiO2 and the presence of the most intense overtone at 1966 cm-1 in Nb2O5/Al2O3. Lower frequency IR fundamentals at 880 cm-1 (low loading) and 935 cm-1 (high loading) are also seen in Nb2O5/ZrO2, and the IR overtone region of Nb2O5/Al2O3 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 NbOx surface species on alumina, zirconia, and titania, are assigned to an NbsOsNb stretching mode, νs([-OsNbsO-]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 SiO2, Al2O3, ZrO2, and TiO2 supports are generally titrated preferentially as niobia loading is increased. Also, in Nb2O5/ZrO2 and Nb2O5/ TiO2 a new (nonacidic or weakly acidic) NbsOH or NbsOHsZr (NbsOHsTi) surface hydroxyl group is created at 3710-3730 cm-1 that is very similar in frequency to the NbsOH band observed in bulk Nb2O5‚ nH2O at 3702 cm-1.