Renewed insight into the promoting mechanism of magnesium hydride on ammonia borane.

Our previous study found that mechanically milling with magnesium hydride (MgH(2)) could dramatically improve the dehydrogenation property of ammonia borane (AB). Meanwhile, it appears that the MgH(2) additive maintains its phase stability in the milling and subsequent heating process. In an effort to further the mechanistic understanding of the AB/MgH(2) system, we reinvestigated the property and structure evolution in the hydrogen release process of the AB/0.5MgH(2) sample. Property examination using volumetric method and synchronous thermal analyses showed that the AB/0.5 MgH(2) sample releases approximately 13.8 wt % hydrogen after being heated at 300 degrees C. This hydrogen amount is in excess of that available from AB, indicative of the participation of a faction of MgH(2) in the dehydrogenation process of AB. Structural and chemical state analyses using Fourier transformation infrared spectroscopy and solid-state (11)B nuclear magnetic resonance techniques further showed that part of MgH(2) participates in the dehydrogenation process of AB from the first step, resulting in the formation of Mg-B-N-H intermediate species. The incorporation of Mg in AB is believed to be a crucial event leading to dehydrogenation property improvements, particularly for the release of the last equivalent of H(2) in AB at relatively moderate temperature. These findings have provided renewed insight into the promoting mechanism of MgH(2) on the hydrogen release from AB.