Aluminium clusters for molecular hydrogen storage and the corresponding alanes as fuel alternatives: A structural and energetic analysis

Bare and hydrogenated aluminium clusters Al_n (n = 5–7) are studied using density functional theory in order to evaluate the ability to store molecular hydrogen and to estimate the energy release upon combustion with the aim to understand these species as alternative fuel resources. Six sequential molecular hydrogenations are considered and are shown to occur without or very low activation barriers. The H₂ molecule uses its occupied σ orbital to react with an appropriate electron-deficient site of the cluster; this is generally followed by H migration leading to the favoured geometry. The combustion process produces alumina (Al₂O₃), water and a significant quantity of energy. The exothermicity seems to be largely independent of cluster size but it depends instead on the stored hydrogen quantity.