In Situ Vibrational Spectroscopy Studies of Supported Niobium Oxide Catalysts

Abstract

The IR spectra of bulk Nb2O5·nH2O and niobia supported on SiO2, Al2O3, ZrO2, and TiO2 were recorded in the fundamental and overtone NbO 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 NbO frequency in Nb2O5/Al2O3, Nb2O5/ZrO2, and Nb2O5/TiO2 provides the strongest evidence that the NbOx surface species (NbO 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...