Physicochemical and catalytic properties of vanadia/titania catalysts. I. Structural properties

Titania gels T-a and T-b were precipitated from TiCl4 at pH 3.0 and 9.0, re spectively, using ammonia solution. Calcination products were obtained by t he thermal treatment of the gels at 300, 400, 600, 750 and 1000 degreesC, r espectively. Thermal analysis, X-ray diffraction and FT-IR spectroscopy dem onstrated that the anatase --> rutile transformation commenced when T-a was calcined at 400 degreesC, with the rutile form predominating when calcinat ion was Conducted at 600 degreesC. Calcination at 750 degreesC or 1000 degr eesC was associated with complete anatase --> rutile conversion. The calcin ation product of T-b at 400 degreesC was pure anatase, transformation to ru tile taking place above this temperature and predominating at 600 degreesC. Rutile was the sole phase present when T-b was calcined at 750 degreesC. V anadia/titania catalysts were obtained by impregnating T-a and T-b with an aqueous solution of NH4VO4 of concentration sufficient to obtain samples co ntaining 4.0, 6.0, 8.0 or 12.0 wt% V2O5. The calcination products at 400 de greesC and 600 degreesC were characterized by thermal analysis, X-ray diffr action, FT-IR spectroscopy, electronic spectral analysis and magnetic susce ptibility measurements. The presence of vanadium enhanced the anatase --> r utile transformation. The measured X-ray diffraction lines and FT-IR peaks indicated the existence of vanadium as a separate oxide, mainly V2O5. Howev er, the electronic spectra and magnetic susceptibility measurements predict ed the possible existence of a V4+ species.