Chemistry Reference
In-Depth Information
coordinated to four or five F ions. We have characterized the reactivity of the under
coordinated Al ions on several surfaces via the calculation of their NH
3
binding energies.
Our results show that fourfold Al ions are the least reactive type of under coordinated
Al ions. It is suggested that this is because the coordination geometries exert a strong
influence on the reactivity of the Al ions. The fourfold Al ions form stable tetrahedral
structures, while the fivefold Al ions are in a distorted and truncated octahedral environ-
ment. The most reactive surface Al ions were bound to five bidentate F ions. The surface
displaying this type of Al site is not predicted to be exposed on crystalline
-AlF
3
samples;
however, it is predicted to occur in small quantities on
crystallites. It is speculated that
such sites occur in higher quantities on high surface area materials. This result may explain
the different reactivity of
-,
-and HS-AlF
3
.
The structure and composition of the (011
2) and (0001) terminations of
-AlF
3
were
calculated as a function of HF and H
2
O chemical potentials. The phase diagrams for the
two surfaces showed many similarities. Under standard atmospheric conditions the sur-
faces were predicted to adsorb water above under coordinated Al ions. To expose the
under coordinated Al ions it was shown that the surfaces must be heated up and put under
conditions of low H
2
O partial pressures and high HF partial pressures. Although phase
diagrams for the
-AlF
3
surfaces were not calculated, it is predicted that they would be
similar to the phase diagrams of the
surfaces.
The phase diagram for the (011
2) termination contains phase boundaries between the
structures derived from the (1
1) and the (
p
2
p
2) surfaces. The (1
1) 3F termi-
nation consists of very strong Lewis acid sites, however it is only thermodynamically
stable when its Lewis acid sites are saturated by HF or H
2
O. This suggests that to obtain
catalytically active AlF
3
it is necessary to desorb these molecules at a temperature
below that at which the surface reconstructs to form the inactive (
p
2
p
2) phase. The sol-
gel process used to obtain catalytically active HS-AlF
3
satisfies this condition [32, 33].
The strength of Lewis acid sites on clean and hydroxylated AlF
3
surfaces were char-
acterized from calculations of CO adsorption. This study supported the results obtained
from calculations of NH
3
adsorption; the strongest Lewis acid sites consist of Al ions
bound to five bidentate F ions. We show, furthermore, that partial hydroxylation of the
surface significantly weakens the Lewis acidity of the under coordinated Al ions. The
strength of the strongest type of Al site is reduced to the strength of the majority of sites
found on crystalline
-and
-AlF
3
surfaces.
Analysis of the adsorption of NH
3
to the surfaces of AlF
3
reveals that the binding energy
is predominately due to the interaction of the molecules with the large electrostatic
potential above the under coordinated Al ions. Lewis acidity is commonly associated
with the donation of electrons from the base to the acid. This result shows that this is not
always an accurate description of the interaction. Our detailed understanding of AlF
3
surfaces finally enabled us to propose a mechanism by which the dismutation of CCl
2
F
2
occurs on
-AlF
3
.
Our study of AlF
3
materials from ab initio calculations has made significant progress
towards development of a new conceptual framework for understanding the chemical
reactivity of high surface area AlF
3
structures. This conceptual framework will underpin
efforts to design better AlF
3
based catalysts. The work also provides a firm basis for the
investigation of a wide variety of other ionic catalysts.
