Hydride-forming intermetallic compound

Table 5.4 The most important families of hydride forming intermetallic compounds including the prototype and the structure.

Tarasov, B.P., and Shilkin, S.P. (1995) About an opportunity of isolation and storage of high-purity hydrogen of with the help of hydride-forming intermetallic compounds. Zh. Prikl. Khim., 68, No. 1, 21-26 (in Russian). 

The hydrogenation properties of the hydride-forming intermetallic compound was also used as a driving force in alkene, alkane or alcohol dehydrogenation reactions which are thermodynamically unfavorable (Chetina and Lunin 1994). The intermetallic compound can be used both as a catalyst and hydrogen acceptor, but its activity and thermal stability can be enhanced by adding some Rh, Ru or Pt salts at the surface. 

Hydrogen Storage Materials (Solid) for Fuel Cell Vehicles, Table 1 The most important families of hydride forming intermetallic compounds including the prototype and the structure. A is an element with a high affinity to hydrogen, and B is an element with a low affinity to hydrogen... 

The very attractive H storage properties of many intermetallic compounds stimulated research on surface and bulk properties of H-metal systems. Contrary to elemental metals, which are easily passivated or poisoned, most hydride-forming intermetallic compounds react readily with gaseous at room temperature even after having been exposed to air. 

A surface segregation model (Schlapbach et al, 1980) based on the analysis of surface properties by means of photoelectron spectroscopy and magnetic susceptibility measurements, very successfully explains the great reactivity of hydride-forming intermetallic compounds AB (e.g, LaNi ). Selective oxidation and lower surface energy of the electropositive component A (La) induces a surface segregation (Fig.12). 

A hydride formed in the reaction of a binary solid-solution alloy with hydrogen can be considered as a solid solution of two binary hydrides and will have properties related to the properties of the constituent binary hydrides. An intermetallic-compound hydride, however, formed in accordance with Reaction... 

The crystal structures of all complex aluminum hydrides are built up by [AlH4] tetrahedra or [AlHg] octahedral units. These building units can be either isolated, as for example in NaAlH4, or they can form more complex structures such as chainlike structures, as for CaAlHs. The decomposition of alkaline earth aluminum hydrides proceeds via hydrides to intermetallic compounds whereas alkali metal alanates decompose via an intermediate hexahydride structure to the corresponding hydride. Table 5.2 summarizes the physical data of selected complex aluminum hydrides. 

Meanshile, surface segregation was found on very many hydride-froming intermetallic compounds (Jacob and Polak, 1981 Schlapbach, 1982 Smith and Wallace, 1986), On most compounds it already occurs at room temperature. The compound FeTi is an exception in the sense that it has to be activated at 700 K for H absorption. Indeed, surface segregation is very weak at room temperature and becomes strong above 600 K. In addition to TiO and Fe other near surface species can be formed according to temperature and partial pressure of oxygen and H. The reaction H 2H can proceed on the near-surface precipitates of Fe or on the metallic subsurface of FeTi (Schlapbach and Riesterer, 1983 Khatamian and Manchester, 1985). Pederson et al. (1983) conclude from volumetric adsorption measurements that at 80 K dissociation occurs on a non-oxidized Ti surface. 

In order for an intermetallic compound to react directly and reversibly with hydrogen to form a distinct hydride phase, it is necessary that at least one of the metal components be capable of reacting directly and reversibly with hydrogen to form a stable binary hydride. 

Metals, intermetallic compounds, and alloys generally react with hydrogen and form mainly solid metal-hydrogen compounds (MH ). Hydrides exist as ionic, polymeric covalent, volatile covalent and metallic hydrides. Hydrogen reacts at elevated temperatrrres with many transition metals and their alloys to form hydrides. Many of the MH show large deviations from ideal stoichiometry (n= 1, 2, 3) and can exist as multiphase systems. 

Figure 3.32 shows XRD patterns of (MgH -i-LiAlH ) composites after DSC testing up to 500°C. The primary phases present are Mg and Al. Peaks of MgO and (LiOH) HjO arise from the exposure of Mg and Li (or possibly even some retained LiH) to the environment during XRD tests. Apparently, XRD phase analysis indicates that a nearly full decomposition of original MgH and LiAlH hydride phases has occurred to the elements during a DSC experiment. In addition, no diffraction peaks of any intermetallic compound are observed in those XRD patterns. That means that no intermetallic compound was formed upon thermal decomposition of composites in DSC. Therefore, the mechanism of destabilization through the formation of an intermediate intermetallic phases proposed by Vajo et al. [196-198] and discussed in the beginning of this section seems to be ruled out of hand. 

Metals, intermetallic compounds and alloys generally react with hydrogen and form mainly solid metal-hydrogen compounds. Hydrides exist as ionic, polymeric covalent, volatile covalent and metallic hydrides. 

Many metals, alloys and intermetallic compounds (Me) react reversibly with gaseous H2 to form a metal hydride, MeHx, at practical temperatures and pressures. This simple reaction, neglecting the solid solution phase, may be written as ... 

AB5 alloys are intermetallic compounds with hexagonal crystalline lattice. Constituting compounds are inter alia rare earth metals. These compounds are capable to form hydrides as the dissolve hydrogen. 

Titanium iron hydrides are among the materials which, at the present time, appear to have potential for practical applications as an energy-storage medium (7). The formation and properties of titanium iron hydride have been studied by Reilly and Wiswall (3), who found that the reaction proceeds in two steps as indicated by Reactions 5 and 6. Both hydrides have dissociation pressures above 1 atm at room temperature in contrast to TiH2 which is very stable. Titanium iron is representative of intermetallic compounds that consist of an element (titanium) capable of forming a stable hydride and another element (iron) that is not a hydride former or at best, forms a hydride with great difficulty. Iron presumably plays a role in destabilizing the hydrides. Titanium also forms a 1 1 compound with copper (there are other intermetallic compounds in the titanium-copper system) and this fact, coupled with the observation that copper... 

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