Chemistry Reference
In-Depth Information
illustrating that the observed dimensions of the crystallites are in good agreement with the
coherence domains calculated by X-ray line broadening.
Nanostructured HTB Phase As mentioned above, the surface area of the solid can be
monitored by tuning the volume ratio V
ΒΌ
V
water
/V
isopropanol
. Three different types of
samples can be obtained with various surface areas, that is, 3, 49 and 82 m
2
.g
1
and
labelled as LSA (low surface area), MSA (medium) and HSA (high).
41
When getting
towards high surface area, some X-ray peaks display some anisotropy with regards to their
FWHM. The profile matching of the HSA compound evidences that the FWHM of some
experimental peaks was smaller than the calculated one (Figure 2.7, left). This hkl-depen-
dent line broadening has been proved to arise from size effect and has been solved by the
use of anisotropic size broadening model, that is, a platelet-shape along the a axis (Figure
2.7, right). The calculated dimensions of the platelet are 8 nm along the a-axis and 34 nm in
the b, c plane. Such a model has been confirmed by HRTEM observations, with an average
particle size of 15 nm (cf. Figure 2.6). The knowledge of the morphology of a crystallite is
of great importance because it enables a map of the surface to be drawn up. The above
result suggests that the {100} surface is weakly exposed, as compared to the {001}, {011}
and {010} surfaces. Wander et al.
52
have studied the equilibrium crystal morphology of
-
AlF
3
and their results predicted that the exposed crystal surfaces are: 59 % for {010}, 38 %
for {001} and 3 % for {100}, proving that the last one is not a thermodynamically stable
surface. Although the metastability of the {100} surface is confirmed by the above
microscopy observations, the relative contribution of the {100} surface is higher than
those predicted by computational data. Since the anisotropic size effect appears to be
related to the nano-size, it can be suggested that the relative contribution of the {100}
surface type increases with decreasing the crystallite size. Such a trend should impact on
the acidity of the solid because ab initio calculations of NH
3
adsorption on the (100)
surface of
-AlF
3
reveal that this surface displays strong Lewis acidity.
53
This surface also
displays some high catalytic properties toward chlorine-fluorine exchange reactions.
54
2.3.1.2.2 Structural Features
The above sections have presented the synthesis and the textural properties of Al-based
materials prepared by microwave treatment. In the following, the structural features of the
three compounds will be presented in more details.
Comparison between the HTB and the Pyrochlore Hydroxyfluorides Description of
the networks. The pyrochlore and the hexagonal tungsten bronze frameworks are built of
corner-sharing AlF
6
x
(OH)
x
octahedra exhibiting rather similar arrangements. While the
pyrochlore-type structure displays a three dimensional hexagonal tunnel system, the HTB
form is characterized by a one-dimensional tunnel along the c-axis (Figure 2.8). The
pyrochlore structure
20
crystallizes in the cubic symmetry (Fd-3 m) leading to a random
distribution of OH/F anions in the 48 f sites. The HTB form
21
crystallizes in an orthor-
hombic system (Cmcm) with two unequivalent Al atoms and four unequivalent F atoms
(Figure 2.8A). Aluminium-based fluoride structures can be described by the connection of
building units
5
(Table 2.4). The pyrochlore network can be drawn through the linkage of
three- and six-membered rings, the latter forming the 3 D interconnected channel system.
In the HTB form, anionic crystallographic sites, labelled F3 and F4, allow connection of
the planes containing the three- and six-membered rings resulting in a four-membered ring
generating the one-dimensional hexagonal channel system along the c axis (Figure 2.8B).
