Site external growth

Some limitations of optical microscopy were apparent in applying [247—249] the technique to supplement kinetic investigations of the low temperature decomposition of ammonium perchlorate (AP), a particularly extensively studied solid phase rate process [59]. The porous residue is opaque. Scanning electron microscopy showed that decomposition was initiated at active sites scattered across surfaces and reaction resulted in the formation of square holes on m-faces and rhombic holes on c-faces. These sites of nucleation were identified [211] as points of intersection of line dislocations with an external boundary face and the kinetic implications of the observed mode of nucleation and growth have been discussed [211]. 

The exponent n is Unked to the munber of steps in the formation of a nucleus (this is a zone in the soUd matrix at which the reaction occurs), ft, and the number of dimensions in which the nuclei grow, X. It can be difficult to distinguish ft and X without independent evidence, and ft can fall to zero following the consumption of external nuclei sites. Hulbert has analysed the possible values of the exponent, n, for a variety of conditions of instantaneous (/3 = 0), constant (ft = 1) and deceleratory (0 < /I < 1) nucleation and for growth in one, two and three dimensions (X = 1 - 3) [ 17]. He also considered the effects of a diffusion contribution to the reaction rate. This reduces the importance of the acceleratory process and reduces the value of n. For diffusion controlled processes, n = ft + Xjl, whereas for a phase boimdary controlled process n = ft + X. Possible values of n are summarised in Table 1. Interpretation of these values can be difficult, and a given value does not unequivocally allow the determination of the reaction mechanism. 

These interrelations are consistent with the above model of high temperature deactivation by coke formation through a reaction of coke growth with methanol. However, this mechanism needs coke seeds provided as "olefin coke" on external acidic centers. Development of ZSM5-catalysts for high temperature application with long life time thus concerns minimizing of acid sites on crystallite surfaces. 

The general analysis of the dependences D(6) without lateral interactions on the inhomogeneous surfaces has been performed [259]. As 6 grows, the random walk of the adspecies in the external field on a lattice with the chaotic sites distribution first causes their increasing and then (on further 6 growth) decreasing active displacement. If the external field is absent or the number of traps is small, this effect is not observed. The dependence D(6) has been computed for different fractions of the casually blocked regions and the variously distributed traps [259]. 

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