Binary Metal-Hydrogen Systems

In contrast to f c.c. Pd, striking discrepancies were found for zl(Q) for hydrogen in the a-phases of b.c.c. metals, in particular in Nb where hydrogen occupies tetrahedral sites. For hydrogen in the a-phases of b.c.c. metals it was observed by Rowe 

Here P(r,t) and U(i,t) denote the probabilities of finding a hydrogen in the occupied and unoccupied layers, respectively r and are the jump rates within the occupied and unoccupied layers, respectively (both along the jump vectors Sj to S4), and Tq and are the change-over rates from the occupied to the empty layer and vice versa, respectively (both along the jump vectors S to S ). This jump model yields a 2x2 jump matrix and subsequently an incoherent scattering function consisting of two Lorentzians. 

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Extensive compilations of experimental data on hydrogen diffusion coefficients in binary metal-hydrogen systems have been published by Volkl and Alefeld [9] and Wipf [42]. These reviews can be referred to as sources of information on H diffusivities in different M-H systems. In this section we shall discuss some general features of H diffusivity in metals resulting from numerous experimental studies. 

To actually calculate the pressure-composition isotherms of a binary metal-hydrogen system we require at least the following additional ingredients ... 

Flanagan and Oates have extensively reviewed the thermodynamics of intermetallic hydrides [1] also recommended are the classic work of Libowitz [2] and the comprehensive text of Muller, Blackledge, and Libowitz [3] which treats the properties of binary hydrides. The properties of a metal-hydrogen system can be conveniently summarized by a pressure-temperature-composition (PTC) diagram of which an idealized version is shown in Figures 9.1 and 9.2. The former is... 

This chapter commences with a review of a limited number of ternary hydride systems that have two common features. First, at least one of the two metal constituents is an alkali or alkaline earth element which independently forms a binary hydride with a metal hydrogen bond that is characterized as saline or ionic. The second metal, for the most part, is near the end of the d-electron series and with the exception of palladium, is not known to form binary hydrides that are stable at room temperature. This review stems from our own more specific interest in preparing and characterizing ternary hydrides where one of the metals is europium or ytterbium and the other is a rarer platinum metal. The similarity between the crystal chemistry of these di-valent rare earths and Ca2+ and Sr2+ is well known so that in our systems, europium and ytterbium in their di-valent oxidation states are viewed as pseudoalkaline earth elements. 

Congruent melting points are commonly encountered both in metallic and organic systems. The former is exemplified by the binary Zn/Mg system (with MgZn2 compound), whereas the latter is exemplified by phenol/aniline mixtures (with 1 1 hydrogen-bonded complex). 

Some solid-state metal hydrides are commercially (and in some cases potentially) very important because they are a safe and efficient way to store highly flammable hydrogen gas (for example, in nickel-metal hydride (NiMH) batteries). However, from a structural and theoretical point of view many aspects of metal-hydrogen bonding are still not well understood, and it is hoped that the accurate analysis of H positions in the various interstitial sites of the previously described covalent, molecular metal hydride cluster complexes will serve as models for H atoms in binary or more complex solid state hydride systems. For example, we can speculate that the octahedral cavities are more spacious in which H atoms can rattle around , while tetrahedral sites have less space and may even have to experience some expansion to accommodate a H atom. 

It has been demonstrated that sequential precipitation in a moderately acid pH range for the binary metal systems Cu/Al, Cu/Cr, Cu/Fe and Nl/Fe in the presence of oxalic acid with aluminum or ferric hydroxide as the first stage and adsorption/precipitation of copper or nickel as the second stage provides metal oxides (after 250-350° C air calcine) with considerable enhancement in dispersion and in catalytic activity, notable for Cu/Al, for the room temperature decomposition of hydrogen peroxide and benzaldehyde oxidation by hydrogen peroxide. 

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 ... 

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