Theoretical investigation of hydrogen adsorption in all-metal aromatic clusters

Molecular hydrogen adsorption in binary all-metal aromatic systems has been explored using ab initio quantum chemical calculations. For this study, we have considered different classes of bimetallic clusters, viz. Be3M2, Mg3M2 and Al4M2 (M = Li, Na and K). In all these bimetallic clusters, the interaction energies of the alkali metal ion with the base metal surface are quite high and the alkali metal sites are found to carry partial positive charges which enhance the adsorption enthalpies of molecular hydrogen on them. Among the three classes considered here, Mg3M2 are found to have poor hydrogen adsorption enthalpies as compared to the other two classes due to less charge on the alkali metals. Although the charge developed on K is more than that developed on Li and Na, the hydrogen adsorption in Be3K2 and Al4K2 is found to be weak in comparison to their Li and Na doped counterparts. In the case of Be3Li2 and Be3Na2, the hydrogen adsorption energies are found to be quite comparable to the optimum adsorption energy proposed for ambient temperature hydrogen storage and the gravimetric density of hydrogen is found to be 22.64 and 14.12 wt% respectively, with three H2 molecules adsorbed per alkali metal atom. In the case of Al4M2, the positive charges on the alkali metal atoms as well as the hydrogen adsorption energies are found to be higher as compared to those in Be3M2 clusters. The gravimetric densities of hydrogen in hydrogenated Al4Li2 and Al4Na2 are found to be respectively 11.59 and 9.4 wt% with four H2 molecules adsorbed per alkali metal atom.