The NaAlH4 system remains the archetype of hydrogen storage solids. However, the detailed mechanisms of the hydrogen reactions of NaAlH4 remain unknown. We report 27Al 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 H2 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 27Al shift (close to NaAlH4) and the disorder evident in X-ray diffraction, the species is identified as highly defective NaAlH4, likely having a large AlH3 vacancy concentration.

Hydrogen and 23 Na NMR were used to probe diffusive motions of the ions in several NaH powders. In three NaH samples, the H resonance is a superposition of broad and narrow components, reflecting the presence of relatively immobile and mobile H, respectively. The fraction of mobile H grows from 23 to 250 °C; this pattern has been observed previously in other ionic hydrides. By 300 °C, the formerly broad hydrogen component has itself motionally narrowed. In these samples, the observation of a smaller amount of 23 Na line narrowing by 300 °C indicates that only the H − ions are mobile at 300 °C, leaving 23 Na− 23 Na dipole interactions unaveraged. A deep minimum in the hydrogen rotating-frame relaxation time T 1ρ is observed near 325 °C, as expected from the onset of motional averaging. In a fourth sample, the entire H line is narrowed already by 150 °C. In this sample, the 23 Na is also partially narrowed at this temperature and by 175−225 °C, further narrowing of the 23 Na resonance indicates that now both ions are in rapid motion. In all the samples, the spin−lattice relaxation times T 1 for hydrogen and sodium decrease monotonically with temperature, in qualitative accord with relaxation by physical diffusion of spin magnetization to relaxation centers.

Li wide-line NMR spectroscopy incorporating a high pressure NMR apparatus has allowed the first in situ study of the solvent mediated, direct synthesis of an alanate, thus overcoming the dearth of analytical techniques available to study phenomena occurring in a pressurised slurry. In contrast to the decomposition reaction, the elucidated hydrogenation pathway does not proceed through the hexahydride intermediate.The practical utilisation of hydrogen as an energy carrier awaits the development of high-capacity, hydrogen storage materials that can be recharged under moderate conditions. A viable onboard hydrogen carrier must have: high gravimetric and volumetric hydrogen capacities; thermodynamic properties that are within rather stringent limits; and dehydrogenation and rehydrogenation kinetics that allow hydrogen cycling at moderate temperatures and pressures.1,2 Although no directly reversible hydrogen material has yet met all of these criteria, a great deal of progress as have been made towards harnessing the high storage capacity, relatively low desorption temperatures, and comparative ease of hydrogenation of sodium alanate (NaAlH 4 ) and lithium alanate (LiAlH 4 ). 3It is well established that the dehydrogenation of both undoped and Ti-doped LiAlH 4 (Al(Ti)) proceeds via Li 3 AlH 6 as an intermediate before decomposition into LiH and Al as seen in eqn (1) and (2). 4-73LiAlH 4 / Li 3 AlH 6 + 2Al + 2H 2 (1)The in situ decomposition of LiAlH 4 has been studied previously by DSC, 7,8 X-ray and neutron diffraction measurements 9 and NMR spectroscopy. 10 The direct re-hydrogenation of LiH and Al to LiAlH 4 is challenging as the reaction in eqn (1) While Ashby et al. 13 found that THF solvated LiAlH 4 could be obtained in 96% yield following 5 h or reaction at 393 K under 340 bar of H 2 .More recently, Wang et al. reported the utilisation of high pressure ball-milling to form crystalline LiAlH 4 with a desolvation step following the reaction.14 Graetz et al. subsequently demonstrated the reversibility of this material using PCT isotherms (eqn (3)).15 Hydrogenation was reported to occur at room temperature and 13 bar H 2 forming the LiAlH 4 $4THF adduct from a THF slurry of LiH and Al(Ti), with the removal of the adduct at 333 K in vacuo to form crystalline LiAlH 4 . In a nal development, the complication of a requisite side process to remove the adduct prior to dehydrogenation was eliminated by Liu et al. who reported a remarkably mild and simple process to generate LiAlH 4 from the dehydrogenation products (eqn (4)).

The direct synthesis of NaAlH 4 has been studied, for the first time, by in situ 27 Al and 23 Na wide-line NMR spectroscopy using high pressure NMR apparatus. Na 3 AlH 6 formation is observed within two minutes of hydrogenation addition, while NaAlH 4 is detected after a total of four minutes. This indicates the formation of the hexahydride does not proceed to completion before the formation of the tetrahydride ensues.The practical utilization of hydrogen as an energy carrier awaits the development of high-capacity, hydrogen storage materials that can be recharged under moderate conditions. A viable on-board hydrogen carrier must have high gravimetric and volumetric hydrogen capacities; thermodynamic properties that fall within rather stringent limits; and dehydrogenation and rehydrogenation kinetics that allow hydrogen cycling at moderate temperatures and pressures. 1,2 One of the most important breakthroughs in the development of hydrogen storage materials in the past 20 years was provided by Q4Bogdanović and Schwickardi, whose pioneering studies demonstrated that addition of selected titanium compounds to NaAlH 4 results in enhanced kinetics and reversibility under moderate conditions in the solid state. 3 These studies were prompted by earlier reports from Wiberg et al., who observed that titanium compounds catalyze the dehydrogenation of complex aluminum hydrides in solution. 4 The enigmatic extension of this catalytic effect to the solid state has been the inspiration for over 260 publications on Ti-enhanced NaAlH 4 alone, and it is safe to estimate that it prompted an equal number of studies of the effect of Ti additives on the dehydrogenation kinetics of other complex hydrides. 5

Lithium aluminium hydride (LiAlH 4 ) has long been identified as a viable hydrogen storage material, due to its high attainable theoretical gravimetric hydrogen capacity of 7.9 wt%.The main impediment to its viability for technical application is its limitation for regeneration. Recently, solvent-mediated regeneration has been achieved at room temperature using dimethyl-ether, Me 2 O, although the reaction pathway has not been determined. This in situ multinuclear NMR spectroscopy study ( 27 Al and 7 Li) has confirmed that the Me 2 O-mediated, direct synthesis of LiAlH 4 occurs by a one-step process in which LiAlH 4 ·xMe 2 O is formed, and does not involve Li 3 AlH 6 or any other intermediates.Hydrogenation has been shown to occur below ambient temperatures (at 0° C) for the first time, and the importance of solvate adducts formed during the process is demonstrated.