Density functional theory study of hydrogen adsorption onAl12cages

Abstract

In this paper we present results of DFT calculations on bare and hydrogenated ${\mathrm{Al}}_{12}$ cluster with icosahedral symmetry, an atomic ``cage'' obtained by removing the central Al ion from the most stable ${\mathrm{Al}}_{13}$ cluster. We discuss similarities and differences between structural, energetic, and electronic properties of bare ${\mathrm{Al}}_{12}$ and hydrogenated ${\mathrm{Al}}_{12}{\mathrm{H}}_{12}$ and ${\mathrm{Al}}_{12}{\mathrm{H}}_{20}$ clusters, where the ${\mathrm{Al}}_{12}{\mathrm{H}}_{20}$ 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 ${\mathrm{Al}}_{12}$ cages with the corresponding ${\mathrm{Al}}_{13}$ clusters. We found that in contrast to ${\mathrm{Al}}_{13}{\mathrm{H}}_{12}$ cluster the ${\mathrm{Al}}_{12}{\mathrm{H}}_{12}$ 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, ${\mathrm{Al}}_{12}{\mathrm{H}}_{20}$ cluster. The analysis of deformation electron density reveals the covalent bonding character of ${\mathrm{Al}}_{12}{\mathrm{H}}_{12}$ cluster and more polar bonding in ${\mathrm{Al}}_{12}{\mathrm{H}}_{20}$ clusters. Clusters stability against fragmentation and their vibrational spectra are discussed.