Chemical reactions heterogeneous

Generally, gasification kinetics research includes the study of rates and mechanisms of gasification reactions and the effect of factors on the reaction rates. Reactions in coal gasification are mainly heterogeneous. Besides the chemical reaction (heterogeneous and homogeneous), some physical processes such as adsorption, diffusion, heat transfer, and fluid mechanics also play important roles in this complicated process. 

In the last decade, in situ NMR spectroscopy has found an increasing application for the investigation of chemical reactions heterogeneously catalyzed by acidic and basic zeolites. Using this method, it is possible to study the formation of adsorbate complexes, intermediates, and reaction products inside the zeolite crystals. An excellent review of in situ NMR spectroscopic studies of chemical reactions in zeolites is given by Haw [335]. Furthermore, the reader is referred to papers published by Kfinowski [54, 58], Ivanova and Derouane [336], and Pfeifer and Ernst [4]. 

Studies of surfaces and surface properties can be traced to the early 1800s [1]. Processes that involved surfaces and surface chemistry, such as heterogeneous catalysis and Daguerre photography, were first discovered at that time. Since then, there has been a continual interest in catalysis, corrosion and other chemical reactions that involve surfaces. The modem era of surface science began in the late 1950s, when instmmentation that could be used to investigate surface processes on the molecular level started to become available. 

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... 

Modelling plasma chemical systems is a complex task, because these system are far from thennodynamical equilibrium. A complete model includes the external electric circuit, the various physical volume and surface reactions, the space charges and the internal electric fields, the electron kinetics, the homogeneous chemical reactions in the plasma volume as well as the heterogeneous reactions at the walls or electrodes. These reactions are initiated primarily by the electrons. In most cases, plasma chemical reactors work with a flowing gas so that the flow conditions, laminar or turbulent, must be taken into account. As discussed before, the electron gas is not in thennodynamic equilibrium... 

Sonochemistry can be roughly divided into categories based on the nature of the cavitation event homogeneous sonochemistry of hquids, heterogeneous sonochemistry of hquid—hquid or hquid—sohd systems, and sonocatalysis (which overlaps the first two) (12—15). In some cases, ultrasonic irradiation can increase reactivity by nearly a million-fold (16). Because cavitation can only occur in hquids, chemical reactions are not generaUy seen in the ultrasonic irradiation of sohds or sohd-gas systems. 

Consider a closed PVT system, either homogeneous or heterogeneous, of uniform T and P, which is in thermal and mechanical equilibrium with its surroundings, but which is not initially at internal equilibrium with respect to mass transfer or with respect to chemical reaction. Changes occurring in the system are then irreversible, and must necessarily bring the system closer to an equihbrium state. The first and second laws written for the entire system are... 

That this is not always the case should be expected. In fact, if it was not for heterogeneous localization of some flow phenomena, it would be very diflicult to initiate secondary explosives, or to effect shock-induced chemical reactions in solids. Heterogeneous shear deformation in metals has also been invoked as an explanation for a reduction in shear strength in shock compression as compared to quasi-isentropic loading. We present here a brief discussion of some aspects of heterogeneous deformation in shock-loaded solids. 

Chemical reactions obey the rules of chemical kinetics (see Chapter 2) and chemical thermodynamics, if they occur slowly and do not exhibit a significant heat of reaction in the homogeneous system (microkinetics). Thermodynamics, as reviewed in Chapter 3, has an essential role in the scale-up of reactors. It shows the form that rate equations must take in the limiting case where a reaction has attained equilibrium. Consistency is required thermodynamically before a rate equation achieves success over tlie entire range of conversion. Generally, chemical reactions do not depend on the theory of similarity rules. However, most industrial reactions occur under heterogeneous systems (e.g., liquid/solid, gas/solid, liquid/gas, and liquid/liquid), thereby generating enormous heat of reaction. Therefore, mass and heat transfer processes (macrokinetics) that are scale-dependent often accompany the chemical reaction. The path of such chemical reactions will be... 

The chemical engineer is concerned with the industrial application of processes. This involves the chemical and microbiological conversion of material with the transport of mass, heat and momentum. These processes are scale-dependent (i.e., they may behave differently in small and large-scale systems) and include heterogeneous chemical reactions and most unit operations. Tlie heterogeneous chemical reactions (liquid-liquid, liquid-gas, liquid-solid, gas-solid, solid-solid) generate or consume a considerable amount of heat. However, the course of... 

Considering the factors shown above and the heterogeneous, transient nature of the mechanical, kinetic, and thermal components, the most favorable time for chemical reaction is during the loading pulse, not during the post-shock thermal-cooling period. 

Horie and his coworkers [90K01] have developed a simplified mathematical model that is useful for study of the heterogeneous nature of powder mixtures. The model considers a heterogeneous mixture of voids, inert species, and reactant species in pressure equilibrium, but not in thermal equilibrium. The concept of the Horie VIR model is shown in Fig. 6.3. As shown in the figure, the temperatures in the inert and reactive species are permitted to be different and heat flow can occur from the reactive (usually hot) species to the inert species. When chemical reaction occurs the inert species acts to ther-... 

Hardness on the Mohs scale is often above 8 and sometimes approaches 10 (diamond). These properties commend nitrides for use as crucibles, high-temperature reaction vessels, thermocouple sheaths and related applications. Several metal nitrides are also used as heterogeneous catalysts, notably the iron nitrides in the Fischer-Tropsch hydriding of carbonyls. Few chemical reactions of metal nitrides have been studied the most characteristic (often extremely slow but occasionally rapid) is hydrolysis to give ammonia or nitrogen ... 

The oxidation methods described previously are heterogeneous in nature since they involve chemical reactions between substances located partly in an organic phase and partly in an aqueous phase. Such reactions are usually slow, suffer from mixing problems, and often result in inhomogeneous reaction mixtures. On the other hand, using polar, aprotic solvents to achieve homogeneous solutions increases both cost and procedural difficulties. Recently, a technique that is commonly referred to as phase-transfer catalysis has come into prominence. This technique provides a powerful alternative to the usual methods for conducting these kinds of reactions. 

As a first approach to the principles which govern the behaviour of metals in specific environments it is preferable for simplicity to disregard the detailed structure of the metal and to consider corrosion as a heterogeneous chemical reaction which occurs at a metal/non-metal interface and which involves the metal itself as one of the reactants (cf. catalysis). Corrosion can be expressed, therefore, by the simple chemical reaction ... 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes, on the kinetics of heterogeneous electron-transfer reactions, and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media upon the redox process. 

Generally, the more intense the available beam source, the shorter the time scales, the weaker the heterogeneities, and the longer the distances that can be probed by a scattering method. Hence, there is a strong drive to utilize high-powered lasers, synchrotrons, and intense neutron somces in research on surfaces, interfaces, and microstmctures. This is particularly tme in the study of liquid materials and of systems that undergo rapid physical transformations or chemical reactions. 

As is well known chemical reactions are accelerated by increasing the temperature. This also applies to heterogeneously catalyzed reactions taking place on the surface of polar sorbents such as aluminium oxide or silica gel (Tables 2.1 and 2.2). Such reactions have also been reported on the moderately polar NH2 layers. Alperin et al. have described the activation of cellulose to yield specific information concerning the substances chromatographed [1]. 

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