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Alanates; Decomposition; DSC; Hydrogen storage
Abstract:
Ball milling of MgCl2 and CaCl2 with NaAlH4 or LiAlH4 can be used for the preparation of magnesium, calcium and lithium–magnesium alanates in mixture with NaCl or LiCl. Using wet chemical separation methods, it was possible to obtain these alanates in nearly pure state. The alanates were characterized by X-ray diffractometry, solid-state 27Al NMR and IR spectroscopy and thermovolumetric (TV) and differential scanning calorimetry (DSC) measurements. Mg(AlH4)2 dissociates thermally in one step to MgH2, Al and hydrogen; at a higher temperature, MgH2 and Al transform to Mg–Al alloy and hydrogen. Thermal dissociations of Ca(AlH4)2 and of LiMg(AlH4)3 (in mixture with NaCl or LiCl) proceeds in several steps, of which the first two can be assigned to the formation of CaH2 and of a MgH2/LiH mixture, respectively, in addition to Al and H2. Possible intermediates of these two steps are CaAlH5 and LiMgAlH6. Higher temperature dissociations include formation of MgH2 (LiH) and Ca–Al alloys from CaH2, CaH2 and Al, respectively. Upon ball milling of MgCl2 or CaCl2 with NaAlH4 or LiAlH4 in the presence of Ti catalysts, only the thermal dissociation products of the expected alanates are obtained. This indicates that dehydrogenation discharge of earth alkali metal alanates can be catalyzed by Ti. According to DSC measurements, the thermodynamic stability of Mg(AlH4)2 (ΔH = 1.7 kJ/mol) is too low for the purpose of reversible hydrogen storage. Determination of ΔH values for the second, endothermal step of calcium and lithium–magnesium alanate dissociations gave values of around 31.6 and 13.1 kJ/mol, respectively.
