Reactions rate-limited by an interface process

There have been few satisfactory demonstrations that decompositions of hydrides, carbides and nitrides proceed by interface reactions, i.e. either nucleation and growth or contracting volume mechanisms. Kinetic studies have not usually been supplemented by microscopic observations and this approach is not easily applied to carbides, where the product is not volatile. The existence of a sigmoid a—time relation is not, by itself, a proof of the occurrence of a nucleation and growth process since an initial slow, or very slow, process may represent the generation of an active surface, e.g. poison removal, or the production of an equilibrium concentration of adsorbed intermediate. The reactions included below are, therefore, tentative classifications based on kinetic indications of interface-type processes, though in most instances this mechanistic interpretation would benefit from more direct experimental support. 

Kinetic data for the decompositions of several metal hydrides are summarized in Table 12 to which the following information can be added. The acceleratory period in the decomposition of BeH2 (a 0.35) is ascribed [673] to the random formation of metal nuclei followed by linear growth. The increase in rate consequent upon exposure to X-irradia-tion is attributed to enhanced nucleation. Grinding similarly increased the 

Summary of kinetic characteristics for decompositions of some metal hydrides (interface reactions) 

Reactant Kinetic characteristics Temperature range (K) E (kj mole-1 Ref. l) 

Silver acetylide decomposition was studied [679] by X-ray diffraction and microscopic measurements and, although the a—time relationship was not established, comparisons of intensities of diffraction lines enabled the value of E to be estimated (170 kj mole 1). The rate-limiting step is believed to involve electron transfer and explosive properties of this compound are attributed to accumulation of solid products which catalyze the decomposition (rather than to thermal deflagration). 

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