Vanadia–silica mixed oxides. Influence of vanadia precursor, drying method and calcination temperature on structural and chemical properties
1997
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
CO
2
at 313 K (low-temperature aerogels) or high-temperature
supercritically dried (high-temperature aerogels). The mixed oxides were
characterized by N
2
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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