Physicochemical and Catalytic Properties of Vanadia/Titania Catalysts. I. Structural Properties

Abstract

Titania gels T_a and T_b were precipitated from TiCl₄ at pH 3.0 and 9.0, respectively, using ammonia solution. Calcination products were obtained by the thermal treatment of the gels at 300, 400, 600, 750 and 1000°C, respectively. Thermal analysis, X-ray diffraction and FT-IR spectroscopy demonstrated that the anatase → rutile transformation commenced when T_a was calcined at 400°C, with the rutile form predominating when calcination was conducted at 600°C. Calcination at 750°C or 1000°C was associated with complete anatase → rutile conversion. The calcination product of T_b at 400°C was pure anatase, transformation to rutile taking place above this temperature and predominating at 600°C. Rutile was the sole phase present when T^b was calcined at 750°C.

Vanadia/titania catalysts were obtained by impregnating T_a and T_b with an aqueous solution of NH₄VO₄ of concentration sufficient to obtain samples containing 4.0, 6.0, 8.0 or 12.0 wt% V₂O₅. The calcination products at 400°C and 600°C were characterized by thermal analysis, X-ray diffraction, FT-IR spectroscopy, electronic spectral analysis and magnetic susceptibility measurements. The presence of vanadium enhanced the anatase → rutile transformation. The measured X-ray diffraction lines and FT-IR peaks indicated the existence of vanadium as a separate oxide, mainly V₂O₅. However, the electronic spectra and magnetic susceptibility measurements predicted the possible existence of a V⁴⁺ species.