Synthesis and Thermal Decomposition Behaviors of Magnesium Borohydride Ammoniates with Controllable Composition as Hydrogen Storage Materials

摘要

An ammonia-redistribution strategy for synthesizing metal borohydride ammoniates with controllable coordination number of NH_3 was proposed, and a series of magnesium borohydride ammoniates were easily synthesized by a mechanochemical reaction between Mg(BH_4)_2 and its hexaammoniate. A strong dependence of the dehydrogenation temperature and purity of the released hydrogen upon heating on the coordination number of NH_3 was elaborated for Mg(BH_4)_2·x NH_3 owing to the change in the molar ratio of H~(δ+) and H~(δ-), the charge distribution on H~(δ+) and H~(δ-), and the strength of the coordinate bond N:→Mg~(2+). The monoammoniate of magnesium borohydride (Mg (BH_4)_2·NH_3) was obtained for the first time. It can release 6.5% pure hydrogen within 50 minutes at 180℃.