RAMAN STUDIES OF THE STRUCTURE OF NB2O5/TIO2

The structure of titania-supported niobia has been investigated by means of Raman spectroscopy. Well-dispersed monomeric and oligomeric species are observed at low Nb loadings corresponding to 0.13 of a theoretical Nb2O5 monolayer, whereas amorphous islands of Nb2O5 are the dominant species at Nb loadings corresponding to >0.5 of a theoretical Nb2O5 monolayer. Water vapor coordinates reversibly with Nb=O groups at the surface of the dispersed niobia, through the formation of Lewis acid-base adducts. The Nb-O-Nb bonds in the oligomeric species appear to undergo hydrolysis upon H2O exposure unlike the Nb-O-Nb bonds in the amorphous Nb2O5 islands, most of which are in the bulk of the oxide and, hence, less accessible to interaction with H2O. Calcination between 500 and 700-degrees-C does not affect the structure of the supported Nb2O5 although some of the amorphous Nb2O5 is transformed into T-Nb2O5. It is proposed that the amorphous Nb2O5 is stabilized on the surface of TiO2 by the formation of Ti-O-Nb linkages. The dispersed Nb2O5 is found to stabilize the transformation of the TiO2 support from anatase to rutile and the coincident loss in surface area of the support. Further calcination to 800 and 950-degrees-C results in the formation of TiNb2O7.