Adsorption and dissociation of gas-phase HCl molecules on Al (q = -2 - +3) ions.

Al clusters exhibit apparent changes in curvature, which resemble macroscopic metal tips. Here, we show, using the density functional theory method, how surface charges of Al (q = -2  to +3) ions affect the adsorption and dissociation behavior of HCl molecules. Geometries, adsorption energies, vibrational frequencies, Mulliken population analysis and transition states of (AlHCl) (q = -2  to +3) adsorption complexes were studied. The results revealed that HCl molecules tend to locate on tip sites of the Al (q = -2  to  +3) ions. Anionic adsorption complexes are prone to H affinity adsorption, whereas cationic adsorption complexes favor Cl-affinity adsorptions. These adsorption behaviors look quite like macroscopic tip effects. H-Cl bonds of the adsorption complexes weaken with an increase in either positive or negative charge. Dissociation barriers of the H-Cl bonds exhibit binding energies that are 2 orders of magnitude smaller than those of an isolated HCl molecule. Considering adsorption energies and dissociation barriers comprehensively, HCl molecules should dissociate spontaneously for all the models considered. Generally, the more negative charges the clusters carry, the more energy the reaction will release. Graphical abstract Dissociation barriers of the H-Cl bonds in Al (q = -2 - +3) cluster ions exhibit energy barriers ~2 orders of magnitude smaller than isolated HCI molecules.