In this paper, we examined the basic properties in the 1:1 mixture of lithium amide LiNH 2 and lithium hydride LiH as a candidate of reversible hydrogen storage materials.The thermal desorption mass spectra of the ball milled mixture without any catalysts indicated that hydrogen H 2 is released in temperature range from 180 to 400 ℃ with emitting a considerable amount of ammonia NH 3 . On the other hand, the ball milled mixture containing a small amount of TiCl 3 as a catalyst showed the most superior hydrogen storage properties among the 1:1 mixtures with a small amount of catalysts, Ni, Fe, Co metals and TiCl 3 (1 mol.%). That is, the product desorbs a large amount of hydrogen (~5.5 wt.%) in the temperature from 150 to 250 ℃ under the condition of a heating rate of 5 ℃/min, but it does not desorb ammonia at all within our experimental accuracy. In addition, we confirmed that the product shows an excellent cycle retention with an effective hydrogen capacity of more than 5 wt.% and a high reaction rate until at least 3 cycles.
We have investigated the hydrogen storage properties of a ball-milled mixture of 3Mg(NH 2 ) 2 and 8LiH after first synthesizing Mg(NH 2 ) 2 by ball milling MgH 2 under an atmosphere of NH 3 gas at room temperature. The thermal desorption mass spectra of the mixture without any catalysts indicated that a large amount of hydrogen (∼7 wt %) was desorbed from 140 °C, and the desorption peaked at ∼190 °C under a heating rate of 5 °C/min with almost no ammonia emission. Moreover, the reversibility of the hydrogen absorption/desorption reactions was confirmed to be complete. The above results indicate that this system is one of the promising metal-N-H systems for hydrogen storage.
The mechanism of the hydrogen desorption (HD) reaction from the 1:1 mixture of lithium amide (LiNH2)
and lithium hydride (LiH) to lithium imide (Li2NH) and hydrogen (H2) has been proposed on the basis of our
experimental results in this paper. The proposed model is constituted by 2 kinds of elementary reactions: the
one is that 2LiNH2 decomposes to Li2NH and ammonia (NH3), the other is that the emitted NH3 reacts with
LiH and transforms into LiNH2 and H2. Since the former and the latter reactions are, respectively, endothermic
and exothermic, the HD reaction corresponding to the latter reaction occurs as soon as LiNH2 has decomposed
into Li2NH and NH3. Therefore, the HD reaction can be understood by the following processes: at the first
step, LiNH2 decomposes into Li2NH/2 + NH3/2, and then the emitted NH3/2 quickly reacts with LiH/2,
transforming into LiNH2/2 + H2/2; at the second one, the produced LiNH2/2 decomposes to Li2NH/4 +
NH3/4, and then NH3/4 + LiH/4 transform to LiNH2/4 + H2/4, and such successive steps continue until
LiNH2 and LiH completely transform into Li2NH and H2, even at low temperatures, by the catalytic effect of
TiCl3.
After some metal amides M(NH 2) x such as LiNH 2 , NaNH 2 , Mg(NH 2) 2 and Ca(NH 2) 2 were synthesized by ball milling the corresponding metal hydrides MH x under ammonia atmosphere at room temperature, their thermal decomposition properties were examined, which play important roles for designing a new family of novel Metal-N-H systems. The results indicate that the kinetics of their synthesizing reactions are faster in the order of Na amide > Li amide > Ca amide > Mg amide, while both Mg(NH 2) 2 and Ca(NH 2) 2 decompose and emit NH 3 at lower temperature than LiNH 2 .
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