Adsorption of HCl on single-crystal α-Al2O3 (0001) surface:: A DFT study

First principles calculations based on density functional theory (DFT) and the pseudopotential method have been used to study the adsorption of HCl on the basal plane of an alpha-Al2O3 crystal. The calculations accurately reproduce the energetic and structural properties of bulk alumina and of the alpha-Al2O3 (0001) surface. A 2 x 2 supercell slab model was used to study both the molecular and dissociative adsorption of HCl on the alpha-Al2O3 (0001) surface. Our calculations indicate that the dissociative configurations have adsorption energies that are at least 28 kcal/mol greater than the molecular configurations on the surface. Several ionic adsorption configurations have been investigated in which the proton is adsorbed on a nearest neighbor surface O-ion site (1-2 adsorption), or a next nearest neighbor surface O-site (1-4 adsorption). We have found that the highest binding energy corresponds to 1-2 adsorption. Analysis of the surface coverage effects shows that by increasing the coverage of 1-2 adsorbed HCl molecules to a full monolayer, the adsorption energy of each HCl decreases by about 10 kcal/mol as a result of repulsions between neighboring molecules. Implications of HCl binding to particles of alpha-Al2O3 released in the exhaust of the space shuttle booster rockets on the active chlorine-producing reaction in the stratosphere are discussed.