Discovery of A New Al Species in Hydrogen Reactions of NaAlH 4

The NaAlH 4 system remains the archetype of hydrogen storage solids. However, the detailed mechanisms of the hydrogen reactions of NaAlH 4 remain unknown. We report 27 Al in situ nuclear magnetic resonance (NMR) spectroscopic data revealing an Al-bearing mobile species that could provide the large scale metal-atom transport needed for rehydriding. This new species forms under reaction conditions but can be captured at ambient temperature using excess H 2 pressure. The NMR measurements demonstrate that the species is highly mobile (at 20 °C) and carries both Al and H atoms. On the basis of the 27 Al shift (close to NaAlH 4 ) and the disorder evident in X-ray diffraction, the species is identified as highly defective NaAlH 4, likely having a large AlH 3 vacancy concentration.