Hydrogen production from solid reactions between MAlH4 and NH4Cl

Solid reactions between alkali aluminum hydrides (MAlH4, M = Li or Na) and NH4Cl (at mole ratio 1:1) at 170 °C were investigated quantitatively using temperature programmed reaction (TPR), thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC) and x-ray diffraction (XRD). The release of 3 mol of H2 from per mole of MAlH4 was measured, corresponding to 5.6 wt.% H2 capacity for the NaAlH4/NH4Cl system and 6.6 wt.% for LiAlH4/NH4Cl, respectively. By ball milling of the precursor compounds prior to the mixing, the reaction proceeded fast and NH3 production as the by-product could be avoided. The quick solid reactions may be attributed to the low melting temperatures of MAlH4 and the exothermic nature of the reactions. The reaction mechanism was also discussed.