Structures of vanadium oxide supported on TiO2 have been investigated by using the rectangular pulse technique coupled with various physicochemical measurements including X-ray diffraction and IR, ESR, and UV-visible spectroscopy. In situ IR spectra of adsorbed ammonia have also been measured to investigate the acid properties of the catalysts. It has been found that modification of the TiO2 support - anatase, rutile, or a mixture of anatase with rutile - does not affect significantly the structure of the supported vanadium oxide catalyst. The V=O species or the (010) face of V2O5 has been selectively exposed on the surface irrespective of the kind of TiO2 support. When the V2O5 content increases to 5 mol %, the surface of TiO2 is covered by 1-3 layers of V2O5 lamellae; however, the TiO2 surface is not completely covered by V2O5 but it is exposed to the catalyst surface. There are both Bronsted and Lewis acid sites on the TiO2 surface, while only the Bronsted acid site is formed on the unsupported V2O5 catalyst. As the V2O5 content increases to 5 mol %, the amount of the Bronsted acid site increases and the amount of the Lewis acid site decreases. When the V2O5 content is 10 mol %, the TiO2 surface is fully covered with 5-8 layers of V2O5 lamellae, and almost all of the catalyst surface (ca. 90%) is occupied by the (010) face of V2O5. When the V2O5 content increases further, the layers of V2O5 lamellae on TiO2 become thicker, which results in a decreased fraction of the (010) face of V2O5 on the catalyst surface. Correspondingly, there are only Bronsted acid sites on the V2O5/TiO2 catalysts with the V2O5 content greater than 10 mol %. According to the results of IR and UV-visible spectra of the catalysts, the coordination of oxygens around a vanadium ion in the supported catalysts is almost the same as that in the unsupported V2O5 catalyst; neither the wavenumber of the V=O stretching vibration nor the edge of the O2--V5+ charge-transfer band has shifted markedly by supporting V2O5 on TiO2. On the basis of these results, structures for the V2O5/TiO2 catalysts have been determined.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry