Heterogeneous mechanism

Both homogeneous and heterogeneous mechanisms requite relatively high supersaturation, and they exhibit a high order dependence on supersaturation. These factors often lead to production of excessive fines ia systems where primary aucleatioa mechanisms are important. The classical theoretical treatment of primary nucleation results ia the expressioa (5) ... 

In both 4.1.1. and Table 4-1, S, L G refer to solid, liquid and gas, respectively. Note that we have also classified these heterogeneous mechanisms in terms of the same PHYSICAL CHANGES given above for homogeneous transformations. For the most part, the initial material will be a solid while the nature of the final product will vary according to the type of material undergoing solid state reaction. 

Abstract Heterogeneous chemical reactions at the surface of ice and other stratospheric aerosols are now appreciated to play a critical role in atmospheric ozone depletion. A brief summary of our theoretical work on the reaction of chlorine nitrate and hydrogen chloride on ice is given to highlight the characteristics of such heterogeneous mechanisms and to emphasize the special challenges involved in the realistic theoretical treatment of such reactions. 

In the work concerning the mechanism of solid-catalysed aldol reactions, the analogy between the homogeneous and heterogeneous mechanisms is usually assumed [370,372—375]. The mechanism of base-catalysed condensations, which has received much attention (cf. ref. 371), may be pictured in general as... 

The homogeneous acid-catalysed esterification has been thoroughly studied and a lot of information on its mechanism has been accumulated. The assumed close relation to the heterogeneous mechanism can, therefore, be made use of in mechanistic considerations about the esterification catalysed by ion exchangers. 

Although the reaction mixture will become heterogeneous, mechanical stirring is unnecessary on this scale. 

In several recent studies an assumption is made concerning the homogeneous-heterogeneous mechanisms of oxidation reactions as a reason for critical effects, in particular in the oxidation of cyclohexane over zeolites [131] and of CO over Pd [132-134] and V [135] catalysts. Berman and Elinek [131] have established in their experiments that cyclohexane oxidation over zeolites follows a mixed homogeneous-heterogeneous mechanism. Studies of the mathematical reaction model written down in accordance with the law of mass action showed that the system can have from one to three steady states. When the steady state is unique, there exists a region of parameters... 

The nature and mechanism of the antiknock effect is still little understood. Considerable effort has been devoted to a study of the mechanism since the time tetraethyllead first became a commercial antiknock agent but direct proof of the mechanism is still lacking. Very possibly, both homogeneous and heterogeneous mechanisms maybe competing in antiknock systems. Reviews of the various theories that have been proposed to explain antiknock action have been made byBaruschandMacpherson 31>, Lewis and von Elbe 206), Ross and Rifkin 267>, Chamberlain, Hoare and Walsh 88>, and Beatty and Edgar 44>. 

Fig. 12.9. A metal can corrode (a) by a heterogeneous mechanism if it contains areas of different electrodic properties or (b) by a homogeneous one if the surface is uniform.

A number of authors investigated the process of foam breakdown by antifoams introduced in the foaming system or the foam such as O/W emulsion and solutions in water-soluble organic solvents. Under such defoaming conditions the mechanism of antifoaming action becomes more complex and a destabilisation of the films is possible to occur by an entirely different (heterogeneous) mechanism (see below). 

On the basis of the heterogeneous mechanism of foam inhibition Kruglyakov and Koretskaya [56,91] have given an explanation of the reasons for inversion of the defoaming ability within a homologous series of alkyl alcohols with the increase in surfactant concentration and the maximum effectiveness of the intermediate members of the series at intermediate surfactant concentrations. 

In virtually all food systems, there are numerous small particulates (dust, etc.) or surfaces (vessel walls, etc.) that catalyze formation of nuclei by providing a portion of the energy needed to form the interface. Thus, in most situations relevant to food manufacturing, homogeneous nucleation does not occur and nucleation based on a heterogeneous mechanism dominates. 

Typical values of 6 lie between 0.5 and 2.5. These values are much lower than the t5q)ical ones for primary nucleation by either homogeneous or heterogeneous mechanisms, where b values between 6 and 12 are more common. The importance of is first order (i.e.,y = 1), suggesting that contact of the crystals with the walls and impeller is the important phenomenon. However, some systems (i.e., K2SO4 [19] and KCl [20]) have much lower values of j 0.4. Typical values of h range from 0 to 8 but most fall between 2 and 4, which are expected from semi-theoretical models [21]. 

Fig. 6.8-19 shows the reaction scheme as well as the Raman spectra of the a-DSP monomer and its polymer at 100 K in the spectral region between 1100 and 1700 cm . The significant difference between the spectra leads to the conclusion that n-DSP has undergone a chemical transformation. Fig. 6.8-20 exhibits phonon spectra of a-DSP, recorded during polymerization. Clearly, the phonon spectrum of the partially polymerized material results from superposition of monomer and polymer bands. As the reaction proceeds, the intensity of the monomer peaks decreases while the intensity of the polymer bands increases. The authors concluded that the solid state polymerization of a-DSP proceeds by a heterogeneous mechanism throughout the entire conversion. 

Reactions 53. 54, and 58 are not well established, and the rate constants given in Table AI are somewhat speculative. For example. Reaction 53 is presumably a termolecular gas phase reaction of NO, NO2, and H2O (62), but the possibility of a heterogeneous mechanism cannot be entirely eliminated. The photo-dissociation rate for Reaction 58 (63, 64) is probably an upper limit, based on available experimental data. In any case, with the rate constants given in Table AI, exclusion of... 

Homogeneous nucleation, however, rarely occurs under commercially important conditions. In practice, nucleation is usually dominated by a heterogeneous mechanism, where a foreign surface serves to reduce the energy barrier to nucleation. 

The approach of combustion modeling used in the present work is similar to that of Chelliah et al. [6], In particular, the gaseous combustion is described by the detailed reaction mechanism, while the char combustion is described by the semi-global heterogeneous mechanism. The following reactions were originally taken into account for solid carbon ... 

This also has analogies with some proposed heterogeneous mechanisms (14),... 

It seems therefore that little or no stability is to be expected for the point defect aggregates which provide the necessary shear-plane precursors in the homogeneous shear-plane formation mechanisms. These homogeneous nucleation mechanisms are therefore unlikely to operate, and we turn our attention now to a heterogeneous mechanism, in which point defects aggregate at pre-existing planar-defect sites. 

The most obvious heterogeneous mechanism follows from our discussion in Section 2 where we showed that shear planes could be related to metal interstitial defects, but that a pre-existing anti-phase boundary (APB) is required. Thus shear-plane formation may occur by metal interstitial capture at pre-exist ng APBs (Bursill et More particularly, it is proposed that metal interstitial ions produced by... 

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