20 research outputs found

    Catalytic effect of nano-particle 3d-transition metals on hydrogen storage properties in magnesium hydride MgH2 prepared by mechanical milling

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    We examined the catalytic effect of nano-particle 3d-transition metals on hydrogen desorption (HD) properties of MgH2 prepared by mechanical ball milling method. All the MgH2 composites prepared by adding a small amount of nano-particle Fenano, Conano, Ninano and Cunano metals and by ball milling for 2h showed much better HD properties than the pure ball-milled MgH2 itself. Especially, the 2 mol% Ninano-doped MgH2 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 (~6.5 wt.%) is desorbed in the temperature range from 150 to 250 ºC at a heating rate of 5 ºC /min under He gas flow with no partial pressure of hydrogen. The EDX micrographs corresponding to Mg and Ni elemental profiles indicated that nano-particle Ni metals as catalyst homogeneously dispersed on the surface of MgH2. In addition, it was confirmed that the product revealed good reversible hydriding/dehydriding cycles even at 150 ºC. The hydrogen desorption kinetics of catalyzed and non-catalyzed MgH2 could be understood by a modified first order reaction model, in which the surface condition was taken into account

    Remarkable improvement of hydrogen sorption kinetics in magnesium catalyzed with Nb2O5

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    Kinetics of hydrogen absorption and desorption reactions was investigated on the MgH2 composite doped with 1 mol% Nb2O5 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 sec and finally its capacity reached more than 5 mass%. On the other hand, the catalyzed MgH2 after rehydrogenation desorbed ~6 mass% hydrogen at 160 ºC 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/molH2, indicating the product was significantly activated due to the catalytic effect of Nb2O5

    Lithium nitride for reversible hydrogen storage

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    In this paper, we examined the basic properties in the 1:1 mixture of lithium amide LiNH2 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 H2 is released in temperature range from 180 to 400 ℃ with emitting a considerable amount of ammonia NH3. On the other hand, the ball milled mixture containing a small amount of TiCl3 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 TiCl3 (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

    SEM and TEM Characterization of Magnesium Hydride Catalyzed with Ni Nano-Particle or Nb2O5

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    The microstructures of MgH2 catalyzed with Ni nano-particle or Nb2O5 mesoporous powders are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. For MgH2 catalyzed with Ni, the Ni particles with the diameter smaller than 1 μm were detected on the MgH2 particles with the diameter smaller than 5 μm by the back scattering electron (BSE) microscopy. In details, the TEM micrograph indicates that the Ni particles distribute ~20 nm in diameter on MgH2 uniformly, which was the same size as the additive doped in MgH2 before milling. On the other hand, for MgH2 catalyzed with Nb2O5, the additive particles could not be found anywhere in the BSE image. Even in the TEM micrograph by much larger magnification than the SEM micrograph, the particles corresponding to the additive can not be observed at all. Furthermore, an energy dispersive X-ray (EDX) analysis in spots with a diameter of 20 nm indicated that the existing ratio of Mg to Nb was evaluated to 98:2, being the same as the starting ratio before milling. Therefore, the metal oxide Nb2O5 becomes extremely small particle that could not be observed by the present work after milling compared to metal Ninano

    Synthesis and decomposition reactions of metal amides in metal-N-H hydrogen storage system

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    After some metal amides M(NH2)x such as LiNH2, NaNH2, Mg(NH2)2 and Ca(NH2)2 were synthesized by ball milling the corresponding metal hydrides MHx 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(NH2)2 and Ca(NH2)2 decompose and emit NH3 at lower temperature than LiNH2
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