Heating Rate-Dependent Dehydrogenation in the Thermal Decomposition Process of Mg(BH4)2·6NH3

Abstract

The detailed mechanism of thermal decomposition of Mg(BH4)2·6NH3 synthesized via a mechanochemical reaction between Mg(BH4)2 and NH3 at room temperature was investigated for the first time. A six-step decomposition process, which involves several parallel and interrelated reactions, was elucidated through a series of structural examinations and property evaluations. First, the thermal decomposition of Mg(BH4)2·6NH3 evolves 3 equiv of NH3 and forms Mg(BH4)2·3NH3. Subsequently, Mg(BH4)2·3NH3 decomposes to release an additional 1 equiv of NH3 and 3 equiv of H2 to produce the [MgNBHNH3][BH4] polymer. And then, [MgNBHNH3][BH4] further desorbs 3 equiv of H2 through a three-step reaction to give rise to the formation of the polymer intermediates of [MgNBHNH2][BH4], MgNBHNH2BH2, and MgNBNHBH, respectively. Finally, an additional 1 equiv of H2 is liberated from MgNBNHBH to yield Mg and BN as the resultant solid products. In total, about 7 equiv of H2 and 4 equiv of NH3 are released together from Mg(BH4)2·6NH3 upon heating. Moreover, there is a strong dependence of the gas compositions released from Mg(BH4)2·6NH3 on the heating rate because the decomposition reaction of Mg(BH4)2·3NH3 is sensitive to the heating rate, as the faster heating rate induces a lower ammonia evolution. The finding in this work provides us with insights into the dehydrogenation mechanisms of the metal borohydride ammoniates as hydrogen storage media.