An atomistic MD simulation and pair-distribution-function study of disorder and reactivity of α-AlF3 nanoparticles

Cubic nanoparticles of α-AlF3 containing 864 and 2048 atoms were investigated by using molecular dynamics simulations. Significant structural rearrangements of these particles occurred, primarily at the edges and corners of the particles, and 3 and 5 membered (Al–F–)n ring structures were observed in addition to the 4-membered rings seen in bulk α-AlF3. These 3 and 5 membered ring structures are, however, present in other metastable forms of AlF3, which are formed at low temperatures from high surface area precursors. The surfaces of the nanoparticles were very dynamic on the timescale of the MD run, Al–F bonds being continually broken and formed, resulting in the movement of the low coordinate Lewis acid Al sites on the surfaces of the particles. The Lewis acid sites, which represent the catalytically active sites for F/Cl exchange reactions, are largely present at the corners and edges of the particles. The particles show larger rhombohedral distortions than present in the bulk phase and do not undergo a rhombohedral to cubic phase transition at elevated temperatures. The results are compared with pair distribution function (PDF) analysis results from fluorinated γ-Al2O3, nanoparticles of AlF3 prepared by plasma routes and α- and β-AlF3. Broad peaks between 3.3 and 4.5 Å in the PDF plots of the fluorinated Al2O3 and the nanoparticles indicate a distribution of Al–F distances arising from Al and F atoms in connected AlF6 octahedra; this is consistent with the presence of ring structures other than those found in α-AlF3.