Theoretical investigation of hydrogen adsorption in all-metal aromatic clusters

Abstract

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.