Hydrides hydrogen/hydride equilibria

Similarly, condensation of a mixture of CsHe and C3D6 with nickel vapor resulted in isotopic scrambling. The intraligand isomerizations may be explained in terms of a 1,3-hydrogen shift via a nf-allyl nickel hydride complex in equilibrium with a ir-propene complex, e.g.,... 

Keywords solar-to-hydrogen conversion, photoanode, metal hydride, kinetic characteristics, equilibrium potential... 

If, as shown in Figure 5.13, hyperconjugation results in the formation of species possessing both double-bond character and associated hydrogen ions, equilibrium-controlled migration of the associated hydrogen ion can be expected. This transformation, shown in Scheme 5.8, is known as a 1,2-hydride shift and results in the migration of a proton from carbon 1 to carbon 2. 

The van t Hoff plot shows the reason why NaAlH4 is one of the most promising candidates for reversible hydrogen storage. NaAlH4 as a typical low-temperature metal hydride exhibits an equilibrium pressure of 0.1 MPa at 35 °C and a storage capacity of 3.7 wt.% for the first decomposition step. This is twice the amount stored in interstitial metal hydrides. 

The H2/hydride equilibrium gives an unprecedented opportunity to study the most fundamental bond-breaking/forming process at a metal site. It appears that the H2 ligand has a chemistry of its own and that complete H - H mpture at a metal center may not be a necessary initial step in catalytic hydrogenation, since deprotonation of H2 ligands has been demonstrated. This new perspective of o-bond activation leads to a new vision of organometallic chemisoy, the outcome of which must await further study. 

FIGURE 1 Pressure-composition isotherms left) and van t Hoff plot right) tor the reaction of a metal or intermetallic compound with hydrogen. ph2. hydrogen pressure Ch. hydrogen concentration in the solid phase T, absolute temperature a, solid solution phase of hydrogen in the metal or Intermetallic compound metal hydride phase Peq, equilibrium pressure of metal hydride formation To, critical temperature AH, enthalpy of hydride formation. [From Schlapbach, L., Meli, R, and Zuttel, A. (1995). Intermetallic hydrides and their applications. In Intermetallic Compounds Principles and Practice (J. H. Westbrook and R. L. Fleischer, eds.), Vol. 2, pp. 475-488. Reproduced with permission from John Wiley Sons, New York.]... 

AG Varias, JL Feng, Simulation of hydride induced steady-state crack growth in metals - Part I Growth near hydrogen chemical equilibrium. Computational Mechanics, 2004, 34, 339-356. 

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 ... 

In order to fully understand the electrochemical behaviour of AB, hydrides, a knowledge of their chemical properties is required. Van Vucht et al. [25] were the first to prepare LaNi5 hydride and it is arguably the most thoroughly investigated H—storage compound. It reacts rapidly with hydrogen at room temperature at a pressure of several atmospheres above the equilibrium plateau pressure. PC isotherms for this system are shown in Fig. 3. 

The nickel-hydrogen system has not been studied in such detail. The isotherm at 25°C is presented in Fig. 3 on the basis of the results obtained by Baranowski and Bochenska (11a). The /3-phase of nickel hydride appears when H/Ni exceeds 0.04 at an equilibrium pressure of 3400 atm. The characteristic H/Ni ratio in the /3-phase then amounts to 0.6. 

Assuming the composition of the hydride to be expressed by PdjH2 (which corresponds to PdH0. 7) and bearing in mind the interstitial positioning of the hydrogen in the palladium lattice, the authors postulate the existence of the following equilibrium at the surface of the j8-hydride phase... 

Nevertheless it does not change the principle of the mechanism proposed by Scholten and Konvalinka, i.e. the ability to act catalytically of only the superficial palladium centers released from the vicinity of the interstitial hydrogen. Bearing in mind the dynamic character of the equilibrium in a palladium-hydrogen system as a whole is to regard such centers as being mobile in the surface layer of the hydride. 

Hydrogen may also be determined by both electrochemical and diffusion meters. The electrochemical meter is a hydride-activated concentration cell that employs an electrolyte consisting of a CaH2-CaCl2 mixture. The diffusion meter is based on the equilibrium pressures attained on either side of a thin membrane, usually nickel. 

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