Density functional theory study of hydrogen adsorption on Al₁₂ cages

In this paper we present results of DFT calculations on bare and hydrogenated Al₁₂ cluster with icosahedral symmetry, an atomic “cage” obtained by removing the central Al ion from the most stable Al₁₃ cluster. We discuss similarities and differences between structural, energetic, and electronic properties of bare Al₁₂ 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 Al₁₂ cages with the corresponding Al₁₃ 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.