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 α-Al 2 O 3 crystal. The calculations accurately reproduce the energetic and structural properties of bulk alumina and of the α-Al 2 O 3 (0001) surface. A 2 × 2 supercell slab model was used to study both the molecular and dissociative adsorption of HCl on the α-Al 2 O 3 (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 α-Al 2 O 3 released in the exhaust of the space shuttle booster rockets on the active chlorine-producing reaction in the stratosphere are discussed.