Aluminium fluoride (AlF3) is an industrially important material because of its use in the production of hydrofluorocarbons (HFCs), as strong Lewis acid catalysts in chlorine-fluorine exchange reactions. Little is known about the atomic scale structure of AlF3 surfaces, however, the knowledge is important to design the material for the above purpose. Most of the experimental studies of surface electronic structure are performed under ultra high vacuum (UHV) but catalytic reactions are carried out in very different environments. It is possible that a surface structure stable under UHV is not the stable structure in these conditions. In the current study we have employed surface thermodynamics calculations based on hybrid-exchange density functional theory (DFT) to predict the composition and structures of AlF3 surfaces in contact with gaseous environment of HF and H2O. The electronic structure of the surface has been calculated in both gaseous and in UHV environment to understand the effects of environment on its surface stability and compare with experiment.
We have modelled both (0001) and (01-12) surfaces of α-AlF3. The phase stability has been determined for different OH/F ratios on the surface as a function of temperature and HF and H2O partial pressures. The five coordinated exposed Al ions on the stable stoichiometric AlF3 (01-12) surface has been identified as active Lewis acidic site. Our calculation shows that this Al ion can be identified by 1eV shift in Al-2p core level and the single co-ordinated F ions by 3.5eV shift in F-1s core level binding energy. Under most reaction conditions, however, this site is unavailable with respect to the adsorption of hydroxyl ions which is consistent with the experimental observations.
We have compared successfully the calculated electronic structure with the available XAFS and XPS experiments and the results of the vibrational analysis with IR and Raman spectrum.
References:
1. A. Wander, C. L. Bailey, S. Mukhopadhyay, B. G. Searle and N. M.Harrison, J. Mat. Chem., 16 1906 (2006) .
2. A.Wander, C.L.Bailey, B.G. Searle, S.Mukhopadhyay and N.M. Harrison, Phys. Chem.Chem. Phys., 7 3989 (2005).
|