Nickel hydride systems

Nickel-cadmium rechargeable batteries are being researched. Alternatives such as cadmium-free nickel and nickel hydride systems are also being researched, but nickel-cadmium batteries are unlikely to be totally replaced. Nickel-cadmium batteries can be reprocessed to reclaim the nickel. However, currently, approximately 80% of all nickel-cadmium batteries are permanently sealed in appliances. Changing regulations may result in easier access to these nickel-cadmium batteries for recycling. 

Fig. 12.7 The spectrum of Fe in the nickel/nickel hydride system as a function of hydrogen content. The insert shows one line of the source with an iron absorber, showing the intrinsic broadening. [Ref. 94, Fig. 1 ]...

Compared with nickel-cadmium and nickel-metal hydride systems RAM cells exhibit very low self-discharge, making them ideal for intermittent or periodic use without the need to recharge before using, even in hot climates. Figure 6 shows a comparison of the temperature characteristics, for various battery systems in the form of Arrhenius diagrams. 

This review aims to present an account of the catalytic properties of palladium and nickel hydrides as compared with the metals themselves (or their a-phase solid solutions with hydrogen). The palladium or nickel alloys with the group lb metals, known to form /8-phase hydrides, will be included. Any attempts at commenting on the conclusions derived from experimental work by invoking the electronic structure of the systems studied will of necessity be limited by our as yet inadequate knowledge concerning the electronic structure of the singular alloys, which the hydrides undoubtedly are. 

A short survey of information on formation, structure, and some properties of palladium and nickel hydrides (including the alloys with group IB metals) is necessary before proceeding to the discussion of the catalytic behavior of these hydrides in various reactions of hydrogen on their surface. Knowledge of these metal-hydrogen systems is certainly helpful in the appreciation, whether the effective catalyst studied is a hydride rather than a metal, and in consequence is to be treated in a different way in a discussion of its catalytic activity. 

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. 

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

On the basis of information on the properties of the nickel-hydrogen and nickel-copper-hydrogen systems available in 1966 studies on the catalytic activity of nickel hydride as compared with nickel itself were undertaken. As test reactions the heterogeneous recombination of atomic hydrogen, the para-ortho conversion of hydrogen, and the hydrogenation of ethylene were chosen. 

The second pathway is represented by Eqs. (8)—(11). These reactions involve reduction of the Nin halide to a Ni° complex in a manner similar to the generation of Wilke s bare nickel (37, 38) which can form a C8 bis-77-alkyl nickel (17) in the presence of butadiene [Eq. (9)]. It is reasonable to assume that in the presence of excess alkyaluminum chloride, an exchange reaction [Eq. (10)] can take place between the Cl" on the aluminum and one of the chelating 7r-allyls to form a mono-77-allylic species 18. Complex 18 is functionally the same as 16 under the catalytic reaction condition and should be able to undergo additional reaction with a coordinated ethylene to begin a catalytic cycle similar to Scheme 4 of the Rh system. The result is the formation of a 1,4-diene derivative similar to 13 and the generation of a nickel hydride which then interacts with a butadiene to form the ever-important 7r-crotyl complex [Eq. (11)]. 

Other hydride systems do not have such weight penalties and include magnesium nickel alloys, non-metallic polymers, or liquid hydride systems that use engine heat to disassociate fuels like methanol into a mixture of hydrogen and carbon monoxide. 

Industrially this diene is made the same way as ethylidenenorbomene from butadiene and ethene, but now isomerisation to 2,4-hexadiene should be prevented as the polymerisation should concern the terminal alkene only. In both systems nickel or titanium hydride species react with the more reactive diene first, then undergo ethene insertion followed by (3-hydride elimination. Both diene products are useful as the diene component in EPDM rubbers (ethene, propene, diene). The nickel hydride chemistry with butadiene represents one of the early examples of organometallic reactions studied in great detail [22] (Figure 9.14). 

The nickel-based systems include the flowing systems nickel—iron (Ni/Fe), nickel—cadmium (NiCd), nickel—metal hydrides (NiMH), nickel—hydrogen (Ni/ H2), and nickel—zinc (Ni/Zn). All nickel systems are based on the use of a nickel oxide active material (undergoing one valence change from charge to discharge or vice versa). The electrodes can be pocket type, sintered type, fibrous type, foam type, pasted type, or plastic roll-bonded type. All systems use an alkaline electrolyte, KOH. 

The addition-elimination mechanism, however, is strongly preferred for monohydride systems such as [HCo(CO)4]187 and the Vaska complex193,194 promoting extensive isomerization. Hydroformylation of 2-pentenes in the presence of [HCo(CO)4], for instance, yields mainly the nonbranched aldehyde resulting from double-bond migration.195 Nickel hydride complexes are one of the most active... 

Prototype sealed button cells and larger prismatic cells have been fabricated and studied. As with the nickel-cadmium and nickel-metal hydride systems, an oxygen recombination route is necessary, but the use of membrane separators limits oxygen transport to the negative plate and... 

The regiospecificity of DCN addition using ZnCl2, and the fact that very little deuterium is found in recovered 3PN, strongly suggest that, at least in this system, the back reactions in steps 3, 6, 14, and 16 are slow and olefin isomerization is catalyzed by cationic nickel hydrides, as shown in Fig. 6. This may also occur with other Lewis acids. 

The system is halide-free and the actual catalyst is probably a nickel hydride with the extremely toxic and volatile Ni(CO)4 as precursor. No detailed mechanistic studies have been published, but a possible scheme for the reaction is in Figure 8. 

The catalysts comprise a chelating ligand such as diphenylphosphinobenzoic or-acetic acid combined with nickel sources like (j) -l,5-cyclooctadiene)2Ni (equation 18) or NiCl2/NaBH4. A nickel hydride species is presumed to be the active catalytic species. On an industrial scale, the reaction is run in a polar solvent such as 1,4-butanediol, in which the catalyst is soluble but the olefin products are insoluble. Addition of B(C6F5)3 to these systems converts them from neutral to zwitterionic see Zwitterion) with a large increase in activity (equation 19). ... 

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. 

Combining the nickel cadmium and nickel-hydrogen systems technologies has given rise to the nickel-metal hydride rechargeable battery, one of the most advanced rechargeable systems commercially available and an environmentally friendlier alternative to nickel-cadmium batteries. The cell and its reaction may be written ... 

Typical catalysts that employ this mechanism are nickel hydrides and Ru hydrides such as HRhCl[P(C6H5)3]3 (which is also a classic olefin hydrogenation catalyst) and [HNi P(C6H5)3 3], which is present in the system Ni[P(QH5)3]4/ CF3COOH according to eq. (9) [19, 20]. 

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