Vanadia–silica mixed oxides. Influence of vanadia precursor, drying method and calcination temperature on structural and chemical properties

Abstract

The effects of vanadium precursor, drying procedure and calcination temperature, decisive for the chemical, structural and textural properties of sol–gel-derived vanadia–silica mixed oxides, have been studied. Vanadium(iii) acetylacetonate and vanadium(v) oxide triisopropoxide were used as precursors with largely different reactivities. The vanadia–silica mixed oxides were preparedvia the sol–gel method, involving acid catalysis together with pre-hydrolysis and gelation forced by the addition of ammonia. The as-prepared gels were dried evaporatively (xerogels), dried supercritically by semicontinuous extraction with supercritical CO₂ at 313 K (low-temperature aerogels) or high-temperature supercritically dried (high-temperature aerogels). The mixed oxides were characterized by N₂ physisorption, XRD, FTIR, FT-Raman, XPS and diffuse reflectance UV–VIS spectroscopy. Vanadia–silica aerogels derived from vanadium(iii) acetylacetonate, which possesses much lower reactivity than vanadium(v) oxide triisopropoxide, had textural properties similar to those of silica and exhibited a low vanadia surface concentration. Higher vanadium surface concentrations were detected for samples derived from vanadium(v) oxide triisopropoxide, as indicated by XPS analysis. Supercritical drying led to mesoporous samples. Vanadia remained well dispersed in the gels dried at low temperature (low-temperature aerogel, xerogel) even after calcination in air at 873 K. In contrast, the high-temperature aerogels showed significant agglomeration and crystallisation upon calcination. The studies indicate that the choice of the vanadium precursor and drying procedure are crucial parameters for controlling segregation–agglomeration and/or crystallisation of well dispersed vanadia and thus tailoring the vanadium surface concentration.

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