Intermetallic compounds, hydrides based

More general is the rule of reversed stability (Miedema model) the more stable an intermetallic compound, the less stable is the corresponding hydride, and the other tvay around [36]. This model is based on the fact that hydrogen can only participate on a bond with a neighboring metal atom if the bonds between the metal atoms are at least partially broken (Figures 5.25 and 5.26). 

In the case of ZrNi and ZrCo intermetallics, the preformed hydrides are seen to loose hydrogen at open circuit in acid solutions, after which the intermetallic matrix dissolves [554]. A relatively higher stability of the hydride of ZrCo has been attributed to the presence of higher Co oxides which are more resistant to acids. Studies of this kind have relevance to the possible open circuit behaviour of cathodes based on intermetallic compounds. 

Lead-acid, nickel-iron (Ni-Fe), nickel-cadmium (NiCd), and nickel-metal hydride (NiMH) batteries are the most important examples of batteries with aqueous electrolytes. In lead-acid batteries, the overall electrochemical reaction upon discharge consists of a comproportionation of Pb° and Pb4+ to Pb2+. All nickel-containing battery reactions are based on the same cathodic reduction of Ni3+ to Ni2+, but utilize different anodic reactions providing the electrons. Owing to toxicity and environmental concerns, the formerly widely used Cd°/Cd2+ couple (NiCd cells) has been almost entirely replaced by H/H+, with the hydrogen being stored in a special intermetallic compound (NiMH). 

Sandrock, G. D. Reilly, J. J. and Johnson, J. R., Interrelations Between Phase Diagrams and Hydriding Properties for Alloys Based on the Intermetallic Compound FeTi, "Applications of Phase Diagrams in Metallurgy and Ceramics", National Bureau of Standards SP-496, 1977, 483. 

Sandrock, G. D., "The Interrelations Among Composition Microstructure, and Hydriding Behavior for Alloys Based on the Intermetallic Compound FeTi", International Nickel Co., Suffern, NY, 10901, Final Report for Contract BNL 352410S, June 30, 1976. 

The first intermetallic compound that could reversibly absorb hydrogen (ZrNi) was reported by Libowitz et al. [60]. A magnesium-based hydride with relatively high capacity (Mg2Ni) was reported by Reilly and Wiswall in 1968 [61]. Later, room-temperature hydrides such as the ternary hydrides TiFeH2 and LaNisHfi were... 

Such hydrides based on intermetallic compounds are not only attractive for stationary hydrogen storage applications, but also for electrochemical hydrogen storage in rechargeable metal hydride electrodes, reaching capacities of up to 400 mAh g. They are produced and sold in more than a billion metal hydride... 

In contrast to binary hydrides, relatively little is known of the structural and magnetic properties of hydrides of An-based intermetallic compounds. 

Multicomponent metallic hydrogenation catalysts, based on intermetallic compounds (IMC) of rare-earth elements with nickel, copper, cobalt, and other bimetallic systems. Most studies were devoted to two structural systems LnMs and LnMs, where Ln = La, Sm, Gd, Ce, Pr, and Nd and M = Ni (see Klabunovskii, Konenko s group 183,251,252 Compaiison of LnNis catalysts with Ni catalysts supported on oxides of Ln, show higher activities of the IMC s and their hydrides in hydrogenation of propene (100°C, 1 bar), where LaNis proved to be the most active catalyst... 

Starodubtseva, E.V., Konenko, I.R., Fedorovskaya, E.A., Klabunovskii, E.I., and Mordovin, V.P. (1986) Dissymmetric properties of hydrides of intermetallic compounds based on rare earth metals, Ni and Co, modified with (i .R)-(+)-tartaric acid, Izv. AN SSSR, Ser. khim. 1960-1965, Chem. Abstr. 1987,106, 175463n. 

Another very interesting approach to use the hydride formation ability of the rare earth based intermetallic compounds was demonstrated by using them as hydrogen absorbers in dehydrogenation reactions. In this way, the intermetallic compound is transformed into a stable hydride during the reaction. Propane can be cracked and dehydrogenated into methane and carbon over LaNig and... 

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). 

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