Multicomponent Hydride Systems

Another attractive alternative to tailor the reaction enthalpies of already known single component hydrides MH is to mix them with suitable reactants (A) which react exothermically with the hydrides during heating whereby hydrogen is released and one or many stable compounds are formed according to the scheme  

The stability of these compounds in the desorbed state has to be in the right range to allow reversibility under moderate conditions. Furthermore, the stability of these compounds formed upon desorption directly determines the change in reaction enthalpy of the multicomponent hydride system in comparison to the singlecomponent hydride. 

This approach goes back to Reilly and Wiswall [79] who were the first to prove that this concept indeed allows the modification of the thermodynamics of hydrogena-tion/dehydrogenation reactions. 

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This chapter will first explain the theory of destabilisation and then move on to look at specific examples starting with the destabilisation of complex hydrides with separate sections devoted to different classes of destabilisation agents first metal hydrides, then non-hydride systems (where either an element or alloy which does not hydride or an inorganic salt is added as the destabilisation agent) and finally attempts to use another complex hydride to form a destabilised multicomponent system. The penultimate section summarises the work reported on other multicomponent hydrogen storage systems, predominantly the destabilisation of binary hydrides by other elements, and the chapter is concluded with an outlook of potential future research. 

The same research group has further performed radical carbonylation reactions on the same microreactor system [36]. First, alkyl halides were initiated and effectively reacted with pressurized carbon monoxide to form carbonyl compounds. The principle was subsequently successfully extrapolated to the multicomponent coupling reactions. 1-Iodooctane, carbon monoxide and methyl vinyl ketone were reacted in the presence of 2,2 -azobis(2,4-dimethylvaleronitrile) (V-65) as an initiator and tributyltin hydride or tris(trimethylsilyl)silane (TTMSS) as catalyst (Scheme 15). 

P.I. Gold, Chemical species and chemical reactions of importance in nonequilibrium propellant performance calculations, NASA Accession No N66-33714, Rept No NASA-CR-65442, avail CFSTI, SciTechAerosp Rept 4 (19), 3722 (1966) CA 67, 4484 (1967) 47) S.S. Cherry L.J. van Nice, Pyrodynamics 6 (3-4), 275 (1969) CA 70, 98394 (1969) 48) R.E. Lo, Theoretical performance of the multicomponent rocket propellant system (oxygen, fluorine/ beryllium, lithium hydride)/hydrogen, Deutsche Versuchsanst Luft- und Raumfehri Rept11968, DLR-Mitt-68-21 (Ger), avail CFSTI, SciTech Aerosp Rept 7 (1), 161 (1969) CA 71,... 

As more sophisticated metal hydrides are developed (nanocrystalline, multicomponent systems, composites and nanocomposites, graphite/metals or similar hybrid systems, clusters, etc.), it is important to be a vare that, for practical applications, a large volume of material should be processed in a fast, inexpensive and reliable vay, for example casting. Techniques such as cold vapor deposition may be impossible to scale up but this does not mean they should be discarded as a means of studying new metal hydrides. On the contrary, laboratory techniques allow much better control of the end product and permit the elaboration of new compounds. Once an attractive compound is found then another challenge w ill have to be faced scaling up the synthesis. In this respect, it is important for the community of metal hydrides researchers to also study large-scale production techniques in order to make the transition from laboratory to industrial scale easier. 

Another important research for dehydrogenation and rehydrogenation is for multicomponent (or mixed) system. For example, the dehydrogenation and rehydrogenation reactions of complex hydride-metal hydride e.g. M(BH4) -M Hm have been recently investigated ... 

A reversible destabilised complex hydride-complex hydride multicomponent system has yet to be published. It would appear that although the starting phases are two complex hydrides, either these two phases react together to form a single phase complex hydride or the system upon rehydrogenation does not regenerate the starting phases. One such example is the LiBHj-... 

Another promising multicomponent system is lithium alanate-lithium amide, this was originally investigated not as a destabilised multicomponent system, but as a means of utilising the lithium amide to destabilise the lithium hydride formed from the decomposition of lithium alanate (i.e. following the reactions for the Li-N-H system as reported by Chen et al. (2002)) and hence release all the hydrogen contained within the alanate (Lu and Fang, 2005) ... 

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... 

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