Metal-N-H system

H.Y Leng, T. Ichikawa, S. Isobe, S. Hino, N. Hanada, H. Fujii, Desorption behaviors from metal-N-H systems synthesized by baU milling , J. Alloys Compd. 404 06 (2005) 443 47. 

Proposed Mechanism ofthe Hydrogen Storage Reaction in the Metal-N-H Systems... 

As indicated above, for metal-N-H systems the disproportionation takes place during the hydrogenation, whereas the hydrogen desorbing reaction is carried out by a solid-solid reaction. After Chen s report [6], a number of studies contributed to our understanding of these reactions. In this section, the proposed mechanisms of hydrogen storage reactions in the metal-N-H systems will be introduced. 

Because the structures of some imides have not yet been determined, a great effort should be directed towards this. With respect to hydrogen desorption from the metal-N-H systems, two representative reaction models have been proposed - the direct solid-solid reaction model and the ammonia-mediated model. Understanding the reaction mechanism is still a pending issue. Therefore, research on the metal-N-H system should focus on these fundamental aspects. 

TABLE 13.5 Summary of Hydrogen Storage Properties of Metal-N H Systems [95] ... 

In order to improve the kinetics of the Li-N-H system, Xie et al. [96] prepared Li2NH hollow nanospheres by plasma metal reaction based on the Kirkendall effect. The special nanostructure showed significantly improved hydrogen storage kinetics compared to that of the Li2NH micrometer particles. The absorption temperature decreased markedly, and the absorption rate was enhanced dramatically because... 

Change in the selectivity patterns of transition metal ion/H+ systems has been encountered with the amorphous and anatase types of hydrous titanium oxides with different crystallinities [24]. Potassium titanate, KjO nXi02 (n = 2-4), in particular, exhibits a layered structure. Fibrous titanic acid, H2Ti409 nHjO, is obtained by acid treatment of fibrous K2Ti409 nH20 and shows higher selectivity for K, Rb and Cs than the amorphous titanic acid [206]. 

The products of the reaction were ethylene and ethane. In the case of Ni, formation of n-butane was also observed in the late stages of the reaction. The reaction pathway to ethane was established by making additions of [14-C]ethylene to reaction mixtures. The results showed that, with each catalyst, the yield of ethane from the further hydrogenation of ethylene is small the major route to ethane formation is by direct hydrogenation of acetylene. Thus, it was concluded that the origins of the selectivity in the reaction, are to be found in the ability or otherwise of the metal, or the metal/C/H system, to catalyse the direct hydrogenation of acetylene to ethane. Similar conclusions have been reached by Guczi et in studies... 

The M-N-H systems are the composites of alkali or alkaline-earth based metal hydrides (LiH, NaH, MgHj, and CaHj) and amides (LiNH2, NaNHj, Mg(NH2)2, and Ca(NH2>2) [99-126]. Chen et al. reported on hydrogen absorption of a lithium nitride LisN as hydrogen storage material [99], in which reversible reactions of hydrogen absorption and desorption reactions proceed with the following two steps ... 

As example, the Li-Mg-N-H system was studied by differential scanning calorimetry by Gross s group [53]. The major issue for metal-N-H storage systems is the formation of NH3, that takes place in parallel with H2 release, and that acts as a... 

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