On the kinetics of AlH3 decomposition and the subsequent Al oxidation

Metal hydrides are used for hydrogen storage. AlH3 shows a capacity to store about 10 wt% hydrogen. Its hydrogen is split-off in the temperature interval of 400-500 K. On dehydrogenation a nano-structured Al material emerges with specific surfaces up to 15-20 m2/g. The surface areas depend on the heating rate because of a temperature dependent crystallite growth. The resulting Al oxidizes up to 20-25% weight on air access forming an alumina passivation layer of 3-4 nm thickness on all exposed surfaces. The heat released from this Al oxidation induces a high risk to this type of hydrogen storage if the containment might be destroyed, accidentally. The kinetics of the dehydrogenation and the subsequent oxidation is investigated by methods of thermal analysis. A reaction scheme is confirmed which consists of a starting Avrami-Erofeev mechanism followed by formal 1st order oxidation on unlimited air access. The kinetic parameters, activation energies and pre-exponentials are evaluated and can be used to calculate the reaction progress. Together with the heat of the Al oxidation the overall heat release and the related rate can be estimated.