USE OF METAL HYDRIDES

This group of reagents is conunercially available in large quantities some of its members-notably lithium aluminium hydride (LiAlH4), calcium iQ dride (CaH2), sodium borohydride (NaBH4) and potassiirm borohydride (KBH4)-have foimd widespread use in the pirrification of chemicals. 

This powerful drying agent is suitable for use with hydrogen, argon, hehum, nitrogen, hydrocarbons, chlorinated hydrocarbons, esters and higher alcohols. 

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Electrode corrosion is the critical problem associated with the use of metal hydride anodes in batteries. The extent of corrosion is essentially determined by two factors alloy expansion and contraction in the charge-discharge cycle, and chemical surface passivation by the formation of corrosion—resistant oxides or hydroxides. 

This great affinity for water gives rise to the use of metal hydrides as drying agents for removing the last traces of water from organic liquids (that do not contain OH bonds). Calcium hydride, CaH2, is commonly used for this purpose. 

The simple, salt elimination reaction of Equation (8.1) has been employed for amides of all the group 13 metals. In addition, it is currently the only well-established route to M(I) metal amides where M = Ga or Tl. The alkane elimination route of eqn. (8.2) is generally employed only for M = Al or Ga. This synthetic approach is also used for the metal imides (RMNR )n where a primary amine H2NR is the reactant. The use of metal hydrides, of which Equation (8.3) is but one example, is limited mainly to aluminium and, to a lesser extent, gallium because of the decreased stability of the heavier metal hydrides. 

For successful use of metal hydrides as chemical heat pumps, alloy systems should allow for changes in the free energies of formation continuously over a wide range. Given a particular hydride, it would be possible to specify AH and AS of a second hydride to reach a given Th and Tj. Recent work in our laboratory has shown that LaNi5 xAlx alloys provide a close approach to such a system (27). 

Composite graphite electrodes based on metal hydrides (YH3, LaH3, CeH3 and GdH3) were for the first time used in the synthesis of EMFs containing soot. The use of metal hydrides instead of metal sward afforded electrodes with metals more homogeneously distributed in a graphite rod that provided more stable electric arc... 

Note A safe storage method for hydrogen for possible use as automotive fuel involves the use of metal hydrides from which the hydrogen is released at specified temperatures. Iron titanium hydride has been found the most satisfactory. 

Due to the energy intensive nature and the cost associated with hydrogen distribution by high-pressure cylinders and tube trailers, this method of distribution has a range limited to approximately 200 km. The use of metallic hydrides seems to be one of the most promising modes of delivering. However, it is more expensive than hydrogen compression in cylinders. 

The MG A energy per analysis requirements may threaten the compactness of a battery-operated unit, even if the ambitious goal of 1 J per analysis is met, because a lithium battery pack capable of 24 h of analyses repeated every 4 s would occupy a volume of 50.7 cm equivalent to a 3.7-cm cube [14], but would still be acceptable for hand-held analyzers. A volumetric reduction by factor of 4—6 may be achievable by the use of metal hydride sources of Hj for fuel cells [15]. 

The use of metal-hydride as the cathode and GaAs modified by Pt as the photoanode in the photoelectrochemical cell for water splitting under action of sunlight permits to conduct the reaction more effectively and to obtain hydrogen in the suitable technological form. 

The planned use of metal hydride gas gap thermal switches in the compressor beds of the Planck sorption cryocoolers [81,86,88] was briefly mentioned in Section... 

Prior to the widespread use of metal hydrides, it was common to reduce (or attempt to reduce) esters with active metals (e.g., Na, Zn) in suitable solvents. Thus,... 

An alternative mechanism involves the use of metal hydrides (Scheme 11.76). Palladium(O) complexes can oxidatively add to acetic acid to generate such a hydride. Rather than an oxidative cyclization occurring, a sequence of insertion into the palladium-hydrogen bond, followed by insertion into the palladium-carbon bond forms the new bicyclic complex 11.230. P-Hydride elimination then releases the product 11.227 and regenerates the acetoxypalladium hydride. ... 

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