Materials kinetics heterogeneous

Welbes, L. L. Searrow, R. C. Borovik, A. S. (2004) Development of porous materials for heterogeneous eatalysis Kinetic resolution of epoxides, Chem. Commun., 2544-2545. 

There are two main aspects that distinguish materials kinetics (1) the involvement of at least one solid phase and (2) the (frequent) presence of heterogeneity. 

The second point essentially arises directly from the first point. Because materials kinetics focuses on the solid state, the systems we encounter almost always involve heterogeneity. What is meant by heterogeneity The simplest definition of a heterogeneous kinetic process is one in which more than one phase is involved. Thus, any kinetic process involving both a solid and a liquid or a solid and a gas is necessarily heterogeneous. The metal oxidation process discussed in Figure 1.1, for example, is... 

Essentially all of the quirks and imperfections that make solid-state systems interesting— point defects, dislocations, grain boundaries, inclusions, voids, surfaces—fall within the scope of materials kinetics. This focus on solid-state processes and heterogeneity—what many would call microstructural development—is what makes materials kinetics unique. In order to tackle this topic, we will need to borrow a lot of concepts from chemical reaction kinetics, which we will cover in Chapter 3 of this textbook, but we will also learn many other concepts that are not usually covered in traditional chemical-based treatments of kinetics. In particular, we will spend a lot of time on solid-state diffusion and transport (Chapter 4). Compared to the gas and liquid phases, transport of matter in the solid phase tends to be slower and more difficult thus, atomic transport processes such as diffusion become much more important in determining kinetic behavior in solid-state systems. 

A homogeneous kinetic process is one which occurs in a single phase, while a heterogeneous kinetic process involves several (two or more) distinct regions or phases. Almost all solid-state kinetic processes are heterogeneous because almost all solid-state systems manifest heterogeneity. Therefore, the field of materials kinetics mainly confronts heterogeneous kinetic processes. 

In turn, results of the chemical kinetics compose the scientific foundation for the synthetic chemistry and chemical technology. The methods for affecting the reaction developed in the kinetics are used for controlling the chemical process and creation of kinetic methods for the selective preparation of chemical compounds. The methods for retardation (inhibition) of chemical processes are used to stablize substances and materials. Kinetic simulation is ised for the prognostication of terms of the operation of items. The kinetic parameters of reactions of substances ccmtained in the atmosphere are used for prognosis of processs that occur in it, in particular, ozone formation and decomposition (problem of the ozone layer). The kinetics is an important part of photochemistry, electrochemistry, biochemistry, radiation chemistry, and heterogeneous catalysis. 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

We can anticipate that the highly defective lattice and heterogeneities within which the transformations are nucleated and grow will play a dominant role. We expect that nucleation will occur at localized defect sites. If the nucleation site density is high (which we expect) the bulk sample will transform rapidly. Furthermore, as Dremin and Breusov have pointed out [68D01], the relative material motion of lattice defects and nucleation sites provides an environment in which material is mechanically forced to the nucleus at high velocity. Such behavior was termed a roller model and is depicted in Fig. 2.14. In these catastrophic shock situations, the transformation kinetics and perhaps structure must be controlled by the defective solid considerations. In this case perhaps the best published succinct statement... 

In this chapter we will discuss the results of the studies of the kinetics of some systems of consecutive, parallel or parallel-consecutive heterogeneous catalytic reactions performed in our laboratory. As the catalytic transformations of such types (and, in general, all the stoichiometrically not simple reactions) are frequently encountered in chemical practice, they were the subject of investigation from a variety of aspects. Many studies have not been aimed, however, at investigating the kinetics of these transformations at all, while a number of others present only the more or less accurately measured concentration-time or concentration-concentration curves, without any detailed analysis or quantitative kinetic interpretation. The major effort in the quantitative description of the kinetics of coupled catalytic reactions is associated with the pioneer work of Jungers and his school, based on their extensive experimental material 17-20, 87, 48, 59-61). At present, there are so many studies in the field of stoichiometrically not simple reactions that it is not possible, or even reasonable, to present their full account in this article. We will therefore mention only a limited number in order for the reader to obtain at least some brief information on the relevant literature. Some of these studies were already discussed in Section II from the point of view of the approach to kinetic analysis. Here we would like to present instead the types of reaction systems the kinetics of which were studied experimentally. 

Many authoritative accounts of both general and specific aspects of the reactions of solids and related topics appear as plenary lectures and research papers in the series of International Symposia on the Reactivity of Solids [82—86]. The material presented at these meetings reflects contemporary interests in a diverse and developing field, so that changes in emphasis are to be discerned in the content of the successive symposia held at four-yearly intervals. Reference can also be made here to the conference on Reaction Kinetics in Heterogeneous Chemical Systems in which useful review material is found [87],... 

The first of these factors pertains to the complications introduced in the rate equation. Since more than one phase is involved, the movement of material from phase to phase must be considered in the rate equation. Thus the rate expression, in general, will incorporate mass transfer terms in addition to the usual chemical kinetics terms. These mass transfer terms are different in type and number in different kinds of heterogeneous systems. This implies that no single rate expression has a general applicability. 

The material on catalysis and heterogeneous reactions in Chapters 6, 1%, and 13 is a useful framework for an intermediate level graduate course in catalysis and chemical reactor design. In the latter course emphasis is placed on developing the student s ability to analyze critically actual kinetic data obtained from the literature in order to acquaint him with many of the traps into which the unwary may fall. Some of the problems in Chapter 12 and the illustrative case studies in Chapter 1 3 have evolved from this course. 

It has also been recognized that micro-structured components, because of their low mass and thermal inertia, are able to offer short response times for unsteady state periodic operations. Micro-reactors have been used successfully for fluorination, oxidations and both heterogeneous [63-65] and homogeneous hydrogenations [66]. A review on gas-liquid micro-structured reactors has been published [67]. The very small material inventory when using micro devices offers another advantage, notably as a laboratory tool for screening applications, kinetics determination and process data acquisition, where the main concern is... 

The types of explosions that may occur depend on the confinement of the reactive material, its energy content, its kinetic parameters, and the mode of ignition (self-heating or induced by external energy input). Explosions are characterized as physical or chemical explosions, and as homogeneous or heterogeneous as described in Figure 2.2. 

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