Metal hydride absorption

All methyl-metal hydride absorptions lie at higher frequencies than those for compounds with longer alkyl substituents, which themselves show only small differences among their frequencies. Furthermore, the frequency difference between germanium and tin compounds is almost twice the amount ( 190) between silicon and germanium or tin and lead compounds. With this fact in mind the pairs Si/Ge and Sn/Pb can be expected... 

A number of metals have the ability to absorb hydrogen, which may be taken into solid solution or form a metallic hydride, and this absorption can provide an alternative reaction path to the desorption of H,. as gas. In the case of iron and iron alloys, both hydrogen adsorption and absorption occur simultaneously, and the latter thus gives rise to another equilibrium involving the transfer of H,<,s across the interface to form interstitial H atoms just beneath the surface ... 

Switendick was the first to apply modem electronic band theory to metal hydrides [5]. He compared the measured density of electronic states with theoretical results derived from energy band calculations in binary and pseudo-binary systems. Recently, the band structures of intermetallic hydrides including LaNi5Ht and FeTiH v have been summarized in a review article by Gupta and Schlapbach [6], All exhibit certain common features upon the absorption of hydrogen and formation of a distinct hydride phase. They are ... 

The UV-vis spectral analysis confirms the appearance of a new charge-transfer absorption band of the complexes of colorless a-donors (R3MH) and the n-acceptor (TCNE). In accord with Mulliken theory, the absorption maxima (Act) of the [R3MH, TCNE] complexes shift toward blue with increasing ionization potential of the metal hydrides (i.e., tin > germanium > silicon) as listed in Table 8. 

Thermal reaction. The charge-transfer absorption (or color) of the [R3MH, TCNE] complex fades in the dark, and a concomitant addition of the metal hydride to the TCNE double bond is observed188 (equation 50). 

Apparently an ion-exchange resin will allow the absorption of the metal hydride species onto its surface by protonation and subsequent elimination of hydrogen (Equation 2.11). This hydrogen elimination is a reversible reaction. The metal species remains as a labile species that can be desorbed by hydrogen in a fluid-stripping medium. 

Equation 2.11. Absorption of a metal hydride onto an ion exchange resin... 

Hydrogen Weight Alloy Weight Fig. 5.7 Hydrogen absorption capacity of metal hydride alloy... 

Fernandez and Sanchez [18] investigated the kinetics of hydrogen absorption and desorption by activated magnesium powder (several cycles of hydrogen absorp-tion/desorption at 375°C) using a volumetric technique. They pointed out that for-mation/decomposition of metal hydrides comprises a number of steps taking place in series transport to the surface, dissociation, H chemisorption, surface-... 

The plateau region on the PCX cnrves in Fig. 2.43a is very flat, showing only a minimal slope (Sect. 1.4.1). However, each absorption-desorption pair of the PCX curves clearly exhibits a pressure hysteresis. This means that the pressure needed for absorption (hydride formation), is always greater than that of hydride decomposition, p. The cause of pressure hysteresis in metal hydride systems is not fnlly nnderstood. A number of models that attempted to explain this phenomenon... 

K.S. Jung, E.Y. Lee, K.S. Lee, Catalytic effects of metal oxide on hydrogen absorption of magnesium metal hydride, J. Alloys Compd. 421 (2006) 179-184. 

Sorption. The word sorption will be used for the total amount of gas taken up by a given solid. This includes adsorption on the surface as well as absorption into the interior of the structure. The latter may be true endothermic solution or may be exothermic solution, which infers compound formation such as the formation of metal hydrides when hydrogen is involved. 

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