Density functional theory study of hydrogen adsorption onAl12cages

In this paper we present results of DFT calculations on bare and hydrogenated Al-12 cluster with icosahedral symmetry, an atomic 'cage' obtained by removing the central Al ion from the most stable Al-13 cluster. We discuss similarities and differences between structural, energetic, and electronic properties of bare Al-12 and hydrogenated Al12H12 and Al12H20 clusters, where the Al12H20 cluster is of particular interest for hydrogen storage application as it has a relatively high hydrogen storage capacity. We also compare structure and properties of bare and hydrogenated Al-12 cages with the corresponding Al-13 clusters. We found that in contrast to Al13H12 cluster the Al12H12 expands upon hydrogen adsorption on its surface. This expansion can be attributed to the missing central Al atom. Due to the cluster expansion there is more surface area available for hydrogen atoms resulting in a weakly bound but nevertheless rather stable, Al12H20 cluster. The analysis of deformation electron density reveals the covalent bonding character of Al12H12 cluster and more polar bonding in Al12H20 clusters. Clusters stability against fragmentation and their vibrational spectra are discussed.