J. Phys. Chem. 1993,97, 12178-12185
12178
Raman Studies of the Structure of NbzOs/TiOz Rusty M. Pittman and Alexis T. Bell' Chemical Sciences Division, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California, Berkeley, California 94720- 9989 Received: July 16, 1993; In Final Form: September 22, 1993"
The structure of titania-supported niobia has been investigated by means of Raman spectroscopy. Welldispersed monomeric and oligomeric species are observed at low N b loadings corresponding to 0.13 of a theoretical NbzOs monolayer, whereas amorphous islands of Nb2O5 are the dominant species at N b 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-0-Nb bonds in the oligomeric species appear to undergo hydrolysis upon HzO exposure unlike the Nb-0-Nb bonds in the amorphous Nb2O5 islands, most of which are in the bulk of the oxide and, hence, less accessible to interaction with HzO. Calcination between 500 and 700 OC does not affect the structure of the supported Nb2O5 although some of the amorphous NbzO5 is transformed into T-NbzOs. It is proposed that the amorphous NbzO5 is stabilized on the surface of Ti02 by the formation of Ti-0-Nb linkages. The dispersed NbzO5 is found to stabilize the transformation of the Ti02 support from anatase to rutile and the coincident loss in surface area of the support. Further calcination to 800 and 950 OC results in the formation of TiNbzO7.
Introduction Niobia supported on various oxides is catalytically active for a number of reactions. Silica-supported niobia promotes the dehydrogenation of ethanol to acetaldehyde,1+2the dehydration of ethanol to ethene and diethyl ether: the photooxidation of propene, and the photocatalyzed dehydrogenation of 2-proponal to acetone.3~~ Titania-supported niobia is active for the selective catalytic reduction (SCR) of N O by NH3.5 While the activity of this catalyst is lower than that of V205/Ti02, the addition of a small amount of niobia to V205/Ti02 increases the catalyst activity for SCR significantlye6 The structure of supported NbzO5 has been investigated by a number of group^.^.^-^^ The picture that has emerged from these studies is that on Si02 and A1203 niobia is present as isolated monomeric and oligomeric species at low Nb loadings but aggregates into a bulklike structure at higher loadings. A similar picture has also been suggested for Nb205/TiOz. The coordinationof Nb has been proposed to beoctahedral in the aggregated state but tetrahedral in the highly dispersed state. The latter state of coordination has been questioned, though, on the basis of theoretical considerations and model compound studies.1417 The present investigation was undertaken to obtain further information about the structure of niobia dispersed to titania and its dependence on calcination temperature. The effects of sample exposure to water vapor were also examined. Raman spectroscopy was used as the principal technique of sample characterization, complemented by X-ray diffraction (XRD). Direct subtraction of the spectral contribution of the support permitted a more detailed view of the spectrum of dispersed niobia than has been possible previously. Experimental Section High surface area anatase was prepared by slowly dripping a solution of 1:1 titanium isopropoxide (Tyzar, DuPont) and 2-propanol into an 8:l solution of 2-propanol and water cooled to 0 OC. The resulting Ti(OH)4 was washed with water several times and dried overnight in a vacuum oven at 120 OC and then calcined in pure 0 2 (100 cm3/min) at 400 O C for 12 h. Samples of NbzOJ/TiOz were prepared by incipient wetness impregnation of the support with a solution of niobium ethoxide *Abstract published in Advance ACS Absrracrs, November 1, 1993.
0022-3654/93/2097- 12178$04.00/0
(Nb(OC2H5)5,Alfa Products, 99.999%,
