Promoted hydrogen release from ammonia borane by mechanically milling with magnesium hydride: a new destabilizing approach

Abstract

Ammonia borane (NH₃BH₃, AB) is an intriguing molecular crystal with an extremely high hydrogen capacity and moderate thermal stability. In the present study, we show a simple but effective approach for destabilizing AB for promoted hydrogen release at moderate temperatures. It is found that mechanically milling with magnesium hydride (MgH₂) can dramatically improve the dehydrogenation properties of AB, on both the kinetic and thermochemical aspects. For the mechanically milled AB/0.5MgH₂ material, over 8 wt% hydrogen can be released from AB within 4 h at around 100 °C without undesired volatile by-products. Moreover, the dehydrogenation reaction of the AB/0.5MgH₂ sample becomes significantly less exothermic than that of neat AB. In situ X-ray diffraction results demonstrate that the MgH₂ additive well maintains its phase stability during the ball-milling and the subsequent heating processes. Meanwhile, Raman spectroscopy and in situ¹¹B NMR studies show that the MgH₂ additive exerts considerable influence on the chemical bonding state and decomposition process/products of AB. Combined phase/structure analyses results suggest that MgH₂ exerts effect via developing solid phase interaction with AB.