Nucleation mechanism, autocatalytic

In this autocatalytic mechanism the olefin hydrogenation step is considered faster than the nucleation and growth steps. When the olefin hydrogenation is a rapid process, the equations can be deduced in terms of the two constants, ki and lc2, the values of which can be obtained from kinetic Equation 15.1 ... 

The decomposition mechanisms are difficult to understand because (i) the surface is not homogeneous with respect to its morphology and chemical composition and (ii) these features evolve continuously during the deposition process. Moreover, as has been clearly demonstrated for noble metals, autocatalytic phenomena can occur, dramatically increasing the growth rate while decreasing the nucleation rate. 

Figure 6.15. Schematic of the four-step mechanism for transition metal (e.g., Pt) nanocluster formation. Shown are (i) nucleation to a desired cluster size (ii) autocatalytic growth onto the cluster surface (hi) diffusive agglomerative growth of two nanoclusters and (iv) autocatalytic agglomeration into bulk metal particulates. Reproduced with permission from Besson, C. Finney, E. E. Einke, R. G. J. Am. Chem. Soc. 2005,127, an9. Copyright 2005 American Chemical Society.

Kinetic analyses in the absence of denaturants provided evidence that the conversion process can be separated into two stages, first the binding of PrP-sen to PrP and then a slower conversion of the bound PrP-sen to PrP-res (Bessen et al, 1997 DebBurman et al, 1997 Horiuchi et al, 1999). These observations, and the formation of amyloid fibril polymers by PrP-res, are consistent with an autocatalytic or templated nucleated polymerization mechanism. However, the fact that PrP-res usually induces the conversion of only substoichiometric quantities of PrP-sen in current cell-free reactions makes the reaction less continuous than typical nucleated polymerizations of proteins or peptides. This may be a technical problem rather than a fundamental limitation of the reaction mechanism. On the other hand, since PrP-res forms continuously for long periods of time in vivo, there may be important elements of the mechanism, such as cofactors or microenvironments, that remain to be elucidated (DebBurman et al, 1997 Saborio etal, 1999). 

Autocatalytic mechanism of pit growth 7.3.2.1 Formation of nucleated pits... 

Mathematical models to describe the crystallization process in batch systems, assuming uniform distribution of the autocatalytic nuclei in the amorphous gel particles, were developed and solved. However, it was shown recently [36] that the model predicts that nucleation should continue much longer in the process than has been observed in several studies previously (e.g., [28,30-32]). It was shown to be more reaHstic, therefore, to assume that the dormant nuclei were located preferentially near the outer edges of the gel particles, and, thus, became activated much earlier in the process. Alternatively, there may be some other mechanism by which dormant nuclei are activated as the process evolves, which will be discussed below. 

The formation of the nanoparticles from the first two methods apparently follows the autocatalytic mechanism (Besson et al, 2005 Besson et al., 2005) that basically involves two steps nucleation and surface growth. In various cases these ionic liquids colloidal mixtures can be used directly as catalysts or they may be isolated and used as powders in solventless conditions (the substrates/ products are per definition the solvent) or re-dispersed in the ionic liquids. 

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