Limiting Phase Segregation in Complex Systems

Another effect is that confinement in pores might influence the phases formed for complex systems. Although not completely understood this is found experimentally, for instance, by Autrey ef al. for the confinement of ammonia borane in mesoporous Si02 and carbon (see Section 10.6.1) [74]. For complex materials often not only thermodynamic considerations but also kinetics have an import influence on which phases are formed upon hydriding or dehydriding. For instance, on decomposition of UBH4 the thermodynamically most favorable reaction is  

Nevertheless, in the decomposition ofboronates, the formation of B2H6 and other boron hydride species is also often observed. 

This probably proceeds via the formation of BH3 for kinetic reasons. Another vell-known example is the decomposition of LiNH2, vhich usually results in the formation of significant amounts of ammonia  

This reaction path changes completely, and the formation of ammonia is suppressed if the LiNH2 is in close contact with an equimolar amount of LiH [71]  

In these examples the formation of unwanted gaseous products can only be prevented if the kinetics for the desired reaction path are favorable, and intimate contact between the relevant reactants is ensured. Confining the material in nanopores might be very useful for increasing the effective contact area and time. 

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