Decomposition of lithium magnesium aluminum hydride

摘要

The quaternary aluminum hydride LiMg{AlH_4)_3 contains 9.7 wt% hydrogen, of which 7.2 wt% can be released in a two-step decomposition reaction via first formation of LiMgAlH_6 and then the binary hydrides MgH_2 and LiH. In-situ synchrotron radiation powder X-ray diffraction and thermal desorption spectroscopy measurements were performed to analyze the product distributions formed during the thermal decomposition of LiMg(AlD4)3. The first decomposition step occurs at about 120 ℃ and the second at about 160 ℃ for the as-milled sample, while for a purified sample of LiMg(AlD_4)_3, the decomposition temperatures involving release of hydrogen increase to 140 and 190 ℃, respectively, suggesting that pure samples of LiMg(AlD_4)_3 are kinetically stabilized. Studies of the purified LiMg(AlD_4)_3 also showed that the second decomposition step can be divided into two reactions: 3IiMgAlD_6 → LisAlD_6 + 3MgD_2 + 2Al + 3D_2 and Li_3AlD_6→3LiD + Al + 3/2D_2. Addition of TiCl_3 to LiMg(AlD_4)_3 under a variety of ball milling conditions consistently led to decomposition of LiMg(AlD_4)_3 during milling. Correspondingly, all attempts to rehydrogenate the (completely or partially) decomposed samples at up to 200 bar hydrogen pressure failed. Decomposition of MgD_2 was observed at relatively low temperatures. This is ascribed to thermodynamic destabilization due to the formation of different Al_xMg_y phases, and to kinetic destabilization by addition of TiCl_3. A thermodynamic assessment was established for the calculation of phase stability and decomposition reaction relationships within the Li-Mg-Al-H system. The calculations confirmed the metastability of the LiMg(AlH_4)_3 phase and the irreversibility of the Li-Mg alanate phase decomposition reactions. The Li-Mg alanate decomposition pathways followed experimentally could be explained by the endothermicity of the calculated decomposition enthalpies, in that an impure or catalyzed LiMgAlH_6 intermediate phase could more directly access an endothermic decomposition reaction at lower temperatures, while a kinetically-hindered, purified LiMgAlH_6 would require higher temperatures to initiate the two-step decomposition through an exothermic reaction.