Interstitial metal hydrides

A FIGURE 14.3 One plane of the structure of an interstitial metallic hydride. The metal atoms (larger spheres) have a face-centered cubic structure, and the hydrogen atoms (smaller spheres) occupy interstices (holes) between the metal atoms. 

Most interstitial metal hydrides have variable composition, for example, PdHx with x < 1. The hydrogen atoms are assumed to have lost their electrons to the d... 

A striking property of many interstitial metal hydrides is the high rate of hydrogen diffusion through the solid at slightly elevated temperatures. This mobility is utilized in the ultra-purification of H2 by diffusion through a palladium-silver alloy tube. 

In a metal, certainly the transition metals, the electrons are more or less free to move in conduction bands. This fact is responsible for the high electrical conductivity of metals. When hydrogen atoms are present in the holes between the atoms, the movement of the electrons is somewhat impaired. As a result, the metal hydrides of this class are poorer conductors than the pure metals. The presence of hydrogen atoms makes the metal atoms less mobile and more restricted to particular lattice sites. Accordingly, the interstitial metal hydrides are more brittle than the parent metal. Also, the inclusion of the hydrogen atoms causes a small degree of lattice expansion so that the interstitial hydrides are less dense than the parent metal alone. 

The nature of the process of forming the interstitial metal hydrides explains why some of these compounds are accompanied by a positive heat of formation. The bond energy in H2... 

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. 

Hydrogen atoms are small enough to occupy the interstitial holes in a metal lattice and the absorption of H2 by a variety of metals (and also alloys) leads to the formation of metal hydrides in which hydrogen atoms reside in interstitial cavities, interstitial metal hydrides. For example, non-stoichiometric hydrides TiHj 7, HfHi gg and HfH2.io are formed when titanium and hafnium react with H2. Niobium forms a series of non-stoichiometric hydrides of formula... 

The US Department of Energy has proposed that manufacturers should aim for a target of 9 GJ m of H2 in a fuel ceU-powered vehicle. The chart opposite shows that interstitial metal hydrides can store arormd 12 GJ m and may be a realistic option for the storage of H2 in vehicles. 

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