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Strong Lewis acid catalysts are used in a variety of industrial processes. High surface area AlF3 has great potential as a heterogeneous catalyst for such applications. However, little is known about the atomic structure of its surfaces. The current study employs state of the art thermodynamics calculations based on hybrid-exchange density functional theory (DFT) to investigate the Lewis acid sites on the â-AlF3 (100) surface.
Calculations of the clean surface structure allow us to identify a number of possible terminations that may exist (or co-exist) on the surface under real catalytic conditions [1,2]. We show that a number of different under-coordinated aluminium ions are likely to exist at the surface of this material. These sites are predicted to be Lewis acidic and are thought to closely resemble the active sites on the highly catalytic high surface area material. In order to characterise the Lewis acidity of these surfaces, the geometries, binding energies and vibrational spectra of adsorbed NH3 and CO above these Lewis acid sites have been calculated. Comparisons with experimental Temperature Programmed Desorption (TPD) and IR spectra are made.
It is well known that â-AlF3 catalyses many Cl/F exchange reactions. One such example is the dismutation reaction 2CCl2F2 → CCl23F + CClF3. We have studied the absorption of these molecules on the surface of â-AlF3 and have made some preliminary investigations to the possible reaction pathways. Understanding the mechanisms by which â-AlF3 catalyses this reaction is essential to understanding the general catalytic properties of both â-AlF3 and the high surface area material' which is known to have a catalytic activity similar to SbF.
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