Hydrogen desorption, hydrides

Indirect methods used can profit by the thermodynamic data of a particular metal-hydrogen system. The determination of the H/Me ratio after complete desorption of hydrogen from a sample, despite an apparent simplicity of the method, gives adequate results only when the bulk metal sample was entirely saturated with hydrogen, and that is a very rare case. The metal catalyst crystallites can be saturated in a nonuniform way, not through their whole thickness. The surface of this polycrystalline sample varies to such extent in its behavior toward interaction with hydrogen that hydride forms only in patches on its surface. A sample surface becomes a mosaique of /3-hydride and a-phase areas (85). 

As far as hydride decomposition is concerned, the relations are reversed. The larger the metal crystals are the slower their hydride decomposes (62). Moreover some deposits situated on the exit points of dislocations, for example on the surface of a nickel hydride crystal, inhibit hydrogen desorption and result in prolonging the hydride existence in the crystal (87). 

Hydrogenation is sensitive to the surface modifications. For example, the ball-milled powders require less activation compared to the conventional powders. For nanocrystalline hydrides, the grain boundary does not dramatically affect the PCI, which describes the thermodynamic aspects of hydride formation. However, the pressure for hydrogen desorption of the unmilled MgH2 is lower than that of the milled one as seen in Figure 11.7 [66]. 

Metal- hydrogen system Hydride Theoretical maximum gravimetric H, capacity) (wt%) Theoretical reversible gravimetric capacity (wt%) Approx, desorption temperature range (°C)... 

The hydrogen desorption or absorption PCT curves of various hydrides can be evaluated using a Sieverts-type apparatus. The scheme of typical Sieverts-type apparatus is shown in Fig. 1.31. 

S. Dal Toe, S. Lo Russo, A. Maddalena, G. Principi, A. Saber, S. Sartori, T. Spataru, Hydrogen desorption from magnesium hydride-graphite nanocomposites produced by ball milling, Mater. Sci. Eng. B 108 (2004) 24-27. 

F.C. Gennari, F.J. Castro, G. Urretavizcaya, Hydrogen desorption behavior from magnesium hydrides synthesized by reactive mechanical aUoying, J. Alloys Compd. 321 (2001) 46-53. 

R.A. Varin, T. Czujko, E.B. Wasmund, Z. Wronski, Catalytic effects of various forms of nickel on the synthesis rate and hydrogen desorption properties of nanocrystalhne magnesium hydride (MgH ) synthesized by controUed reactive mechanical milhng (CRMM), J. Alloys Compd. 432 (2007) 217-231. 

R.A. Varin, Ch. Chiu, T. Czujko, Z. Wronski, Mechano-chemical activation synthesis (MCAS) of nanocrystalline magnesium alanate hydride [Mg(AlH ) ] and its hydrogen desorption properties , J. Alloys Compd. 439 (2007) 302-311. 

T. Czujko, R.A. Vatin, Z. Wronski, Z. Zaranski, T. Durejko, Synthesis and hydrogen desorption properties of nanocomposite magnesium hydride with sodium borohydride (MgH + NaBH ) , J. Alloys Compd. All (2007) 291-299. 

Recently it has been shown that the hydrogen desorption reaction is reversible and the end products lithium hydride and boron absorb hydrogen at 690 ° C and 200 bar to... 

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