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 examined the catalytic effect of nanoparticle 3d-transition metals on hydrogen desorption (HD) properties of MgH(2) prepared by mechanical ball milling method. All the MgH(2) composites prepared by adding a small amount of nanoparticle Fe(nano), Co(nano), Ni(nano), and Cu(nano) metals and by ball milling for 2 h showed much better HD properties than the pure ball-milled MgH(2) itself. In particular, the 2 mol % Ni(nano)-doped MgH(2) composite prepared by soft milling for a short milling time of 15 min under a slow milling revolution speed of 200 rpm shows the most superior hydrogen storage properties: A large amount of hydrogen ( approximately 6.5 wt %) is desorbed in the temperature range from 150 to 250 degrees C at a heating rate of 5 degrees C/min under He gas flow with no partial pressure of hydrogen. The EDX micrographs corresponding to Mg and Ni elemental profiles indicated that nanoparticle Ni metals as catalyst homogeneously dispersed on the surface of MgH(2). In addition, it was confirmed that the product revealed good reversible hydriding/dehydriding cycles even at 150 degrees C. The hydrogen desorption kinetics of catalyzed and noncatalyzed MgH(2) could be understood by a modified first-order reaction model, in which the surface condition was taken into account.

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.

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Kinetics of hydrogen absorption and desorption reactions was investigated on the MgH 2 composite doped with 1 mol% Nb 2 O 5 as a catalyst by ballmilling. The composite after dehydrogenation at 200 • C absorbed gaseous hydrogen of ∼4.5 mass% even at room temperature under lower pressure than 1 MPa within 15 s and finally its capacity reached more than 5 mass%. On the other hand, the catalyzed MgH 2 after rehydrogenation desorbed ∼6 mass% hydrogen at 160 • C under purified He flow, which followed the first order reaction. From the Kissinger plot, the activation energy for hydrogen desorption was estimated to be ∼71 kJ/mol H 2 , indicating the product was significantly activated due to the catalytic effect of Nb 2 O 5 .

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