Heating Rate-Dependent Dehydrogenation in the Thermal
Decomposition Process of Mg(BH<sub>4</sub>)<sub>2</sub>·6NH<sub>3</sub>
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Abstract
The
detailed mechanism of thermal decomposition of Mg(BH<sub>4</sub>)<sub>2</sub>·6NH<sub>3</sub> synthesized via a mechanochemical
reaction between Mg(BH<sub>4</sub>)<sub>2</sub> and NH<sub>3</sub> 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(BH<sub>4</sub>)<sub>2</sub>·6NH<sub>3</sub> evolves 3 equiv of NH<sub>3</sub> and forms Mg(BH<sub>4</sub>)<sub>2</sub>·3NH<sub>3</sub>. Subsequently, Mg(BH<sub>4</sub>)<sub>2</sub>·3NH<sub>3</sub> decomposes to release an additional 1 equiv of NH<sub>3</sub> and
3 equiv of H<sub>2</sub> to produce the [MgNBHNH<sub>3</sub>][BH<sub>4</sub>] polymer. And then, [MgNBHNH<sub>3</sub>][BH<sub>4</sub>]
further desorbs 3 equiv of H<sub>2</sub> through a three-step reaction
to give rise to the formation of the polymer intermediates of [MgNBHNH<sub>2</sub>][BH<sub>4</sub>], MgNBHNH<sub>2</sub>BH<sub>2</sub>, and
MgNBNHBH, respectively. Finally, an additional 1 equiv of H<sub>2</sub> is liberated from MgNBNHBH to yield Mg and BN as the resultant solid
products. In total, about 7 equiv of H<sub>2</sub> and 4 equiv of
NH<sub>3</sub> are released together from Mg(BH<sub>4</sub>)<sub>2</sub>·6NH<sub>3</sub> upon heating. Moreover, there is a strong dependence
of the gas compositions released
from Mg(BH<sub>4</sub>)<sub>2</sub>·6NH<sub>3</sub> on the heating
rate because the decomposition reaction of Mg(BH<sub>4</sub>)<sub>2</sub>·3NH<sub>3</sub> 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
