Surface heterogeneity homogeneous

Nucleation in solids is very similar to nucleation in liquids. Because solids usually contain high-energy defects (like dislocations, grain boundaries and surfaces) new phases usually nucleate heterogeneously homogeneous nucleation, which occurs in defect-free regions, is rare. Figure 7.5 summarises the various ways in which nucleation can take place in a typical polycrystalline solid and Problems 7.2 and 7.3 illustrate how nucleation theory can be applied to a solid-state situation. 

The effects due to the finite size of crystallites (in both lateral directions) and the resulting effects due to boundary fields have been studied by Patrykiejew [57], with help of Monte Carlo simulation. A solid surface has been modeled as a collection of finite, two-dimensional, homogeneous regions and each region has been assumed to be a square lattice of the size Lx L (measured in lattice constants). Patches of different size contribute to the total surface with different weights described by a certain size distribution function C L). Following the basic assumption of the patchwise model of surface heterogeneity [6], the patches have been assumed to be independent one of another. 

As explained in Chapter 1, catalytic reactions occur when the reacting species are associated with the catalyst. In heterogeneous catalysis this happens at a surface, in homogeneous catalysis in a complex formed with the catalyst molecule. In terms of kinetics, the catalyst must be included as a participating species that leaves the reaction unaltered, as indicated schematically in Fig. 2.7, which shows the simplest conceivable catalytic cycle. We will investigate the kinetics of this simple two-step mech-... 

The final one is to nucleate particles heterogeneously only on the support surfaces, not homogeneously independently from the surfaces, as shown in Figure 4. 

The majority of heterogeneous chemical and physical-chemical processes lead to formation of the intermediate particles - free atoms and radicals as well as electron- and oscillation-excited molecules. These particles are formed on the surface of solids. Their lifetime in the adsorbed state Ta is determined by the properties of the environment, adsorbed layer, and temperature. In many cases Ta of different particles essentially affects the rate and selectivity of heterogeneous and heterogeneous-homogeneous physical and chemical processes. Therefore, it is highly informative to detect active particles deposited on surface, determine their properties and their concentration on the surface of different catalysts and adsorbents. 

Eranen, K., Lindfors, L.E., Klingdted, F. et al. (2003) Continuous reduction of NO with octane over a silver/alumina catalyst in oxygen-rich exhaust gases combined heterogeneous and surface-mediated homogeneous reactions, J. Catal. 219, 25. 

There are two kinds of catalysts. Heterogeneous catalysts are insoluble in the medium in which the reaction is taking place so that reactions of gaseous or liquid reagents occur at the surface, whilst homogeneous catalysts are dissolved in the reaction medium and hence all catalytic sites are available for reaction. Some of the properties of catalysts are collected in Table 1.1, where heterogeneous and homogeneous catalysts are compared. 

It is possible that the species Red generated at the electrode surface may be unstable and tend to decompose. It may also be involved in chemical reactions with other species present in solution while it is moving towards the mass of the solution (homogeneous chemical reactions) or while it is still adsorbed on the electrode surface (heterogeneous chemical reactions). Furthermore, the new species formed during such reactions may be electroactive. These kind of reactions are called following chemical reactions (following, obviously, the electron transfer). 

Free radicals are atoms or groups of atoms possessing an odd (unpaired) electron. Radical recombination occurs when active flame propagating species (O , H and OH) recombine (heterogeneously) on particle surfaces or (homogeneously) as a result of gas phase reactions catalysed by alkali metal atoms in the flame, e.g. 

Moreover, as neither the concept of surface initiated homogeneous-heterogeneous reaction (11) can be invoked to explain our results, it can be stated that the methane partial oxidation reaction proceeds via a surface catalysed process which likely involves specific catalyst requirements. However, by comparing the HCHO productivity of the different catalytic systems previously proposed (9) with that of our 5% V205/Si02 catalyst, it emerges that our findings constitute a relevant advancement in this area (23),... 

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

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