Hydrogen fluoride adsorption and reaction on the α-Al2O3(0001) surface: A density functional theory study

The adsorption and reaction behaviors of HF on the alpha-Al2O3(0001) surface are systematically investigated using density functional theory method. By increasing the number of HF molecules in a p(2 x 1) alpha-Al2O3(0001) slab, we find that HF is chemically dissociated at low coverage; while both physical and dissociative adsorption occurs at a 3/2 monolayer (ML) coverage. At the same coverage (1.0 ML), diverse configurations of the dissociated HF are obtained in the p(2 x 1) model; while only one is observed in the p(1 x 1) slab due to its smaller surface area compared with the former one. Preliminary fluorination reaction study suggests that the total energy of two dissociated HF in the p(2 x 1) slab increases by 1.00 and 0.72 eV for the formation and desorption of water intermediate, respectively. The coadsorption behaviors of HF and H2O indicate that the pre-adsorbed water is unfavorable for the fluorination of Al2O3, which is well consistent with the experimental results. The calculated density of states show that the peak of sigma(H-F) disappears, while the peaks of sigma(H-O) and sigma(Al-F) are observed at -8.4 and -5 to -3 eV for the dissociated HF. Charge density difference analysis indicates that the dissociated F atom attracts electrons, while no obvious changes on electrons are observed for the surface Al atoms. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3694102]