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Towards Mechanochemical Preparation of AlH3

Although the properties of aluminum hydride (alane), AlH3, namely its high hydrogen content (10 wt.%) and low decomposition temperature (~80oC) are well-suited for hydrogen storage, convenient methods to prepare this hydride at reasonable hydrogen pressures are yet to be found. Recently, electrochemical (Savannah River) and supercritical fluid (Universities of Hawaii and New Brunswick) approaches offered some hope despite low yields, which were less than 1%. In another recent development, synthesis of a number of Al-H clusters has been reported in a nonequilibrium process using a pulsed ion source. These clusters, analogous to boranes, open a new chapter in alane chemistry, and the materials, once synthesized in bulk quantities, have the potential to store large amounts of hydrogen. Our theoretical work demonstrated how the nature of bonding between H and Al can be changed by adjusting the stoichiometry of the clusters. For example, we showed that Al4H6 is an unusually stable species. This theoretical discovery supports the idea that direct synthesis of alane is possible under reasonably low hydrogen pressures, if a process is carried far from thermodynamic equilibrium of the overall (Al + 1½H2 = AlH3) hydrogenation reaction. Mechanochemical treatment is a nonequilibrium process and, therefore, direct synthesis of alane in a high pressure ball milling vial has been attempted. As a result, we were able to detect formation of as much as 2.4 % of AlH3 according to the solid-state NMR and gas volumetric analyses. In addition to showing the feasibility of the direct synthesis of alane, in another experiment we found that mechanochemical solid-state reaction between stoichiometric amounts of LiAlH4 and FeCl2 (20 min at room temperature) results in the formation of LiCl, Fe, x-ray amorphous AlH3, and hydrogen. In this process, AlH3 forms in a 96 % yield, which has been confirmed by thermo-gravimetric and gas-volumetric analyses.

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