Structures of supported vanadium oxide catalysts. 1. V₂O₅/TiO₂ (anatase), V₂O₅/TiO₂ (rutile), and V₂O₅/TiO₂ (mixture of anatase with rutile)

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 O^2--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.