The aim of our present work was to investigate more finely the situation by measuring the composition at a smaller scale, i.e. at the nanoscale rather than micrometer level. As ammonia temperature programmed desorption (TPD) did not show clear correlations with the EDS data, the present article had to be restricted to the latter. This aspect will be shortly discussed later.
For the present study, we used two series of sol-gel silica-alumina catalysts, prepared in different ways: (i) laboratory prepared samples and (ii) silica-aluminas prepared in industry according to ``sol-gel techniques'', some of which are probably very different from the original alkoxide-based method. A large range of compositions was covered in both cases. One of our aims was to compare the amplitude of the local Si/Al ratio variation for specimens of similar global chemical compositions, but resulting from two different preparation procedures. To our knowledge,
such an investigation has not yet been performed. In general, there is a strong feeling that, in spite of the wide industrial application of the sol-gel processing method, there is still a lack of data concerning the detailed properties of the final products when they contain two or several metallic elements . This is a general remark, not limited to silica-aluminas. The aim of the work was not to rank samples according to their homogeneity. The commercial interests are too important for a university laboratory to influence them. For this reason, the industrial samples are not identified in this work. Another precaution we took is to report only data on samples
produced before 1991.
2.1. The samples
Two kinds of sol±gel silica±alumina powders have
been investigated: (i) samples selected from the complete
series of silica±aluminas carefully prepared in
our laboratory starting from alkoxides (see [1,2]) and
(ii) a number of industrial products prepared by
``industrial sol±gel methods'', very often based on
other precursors than alkoxides. Our laboratory prepared
materials  were made from distillation-puriÆed
aluminium isopropylate and ethyl silicate, which
were introduced progressively into a 1 N solution of
acetic acid (10 ml/g oxide to produce). The mixture
was reØuxed at 808C for 6 h, then kept overnight at
room temperature. After addition of distilled water
(1 ml/g oxide), this was reØuxed again for 8 h under
vigorous stirring. Stirring was prolonged for 1 h. The
next day, the pH was raised progressively to 7 by an
ammonia solution and kept at that value for 1.5 h. The
material became viscous but stirring was maintained.
This was followed by reØuxing for 2 h, centrifugation
and washing three times (3 ml water/g oxide). The
recovered solid was heated (heating rate: 308C/min) to
5008C and maintained at this temperature for 24 h.
The global chemical composition of the laboratory
prepared powders is accurately known, whereas the
global composition of the industrial specimens, as
indicated by the producer, probably Øuctuates somewhat
during industrial production. Sometimes the
indicated silica/alumina global composition of the
industrial samples corresponds to a large range rather
than to a precise value.
Table 1 shows the global chemical composition of
the investigated samples. Comparable samples are
shown on the same row, but the chemical compositions
may not be exactly the same. Comparison is sometimes
possible with more than one sample for a given
2.2. The microanalysis measurements
The specimens for X-ray microanalysis were prepared
by grinding the initial sol±gel silica±alumina
powder, ultrasonically dispersing the resulting powder
in water and depositing the suspension on electron
microscopy grids covered with a continuous carbon
layer, according to the conventional ``droplet'' technique.
The microanalysis was performed in a JEM-
100-C-TEMSCAN analytical electron microscope
(JEOL) equipped with a KEVEX-5100 X-ray EDS
spectrometer. The accelerating voltage was always
100 kV, our constant preoccupation being to accurately
match the thin Ælm conditions required for
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