Reactions in Heterogeneous Systems

With the exception of the discussion of reactions on surfaces in Chapter 3 and some references to fixed-bed reactors in Chapters 5 and 6, we have largely been concerned with reactions in homogeneous phases (or pretended so) and corresponding reactor models. Although those models are not eompletely restricted to reactions in homogeneous phases, there are a number of properties of reactions in heterogeneous systems, particularly when we get down to more detailed analysis, whieh are sufficiently important that they must be treated separately. 

Ordinarily two phases are involved gas/solid, gas/liquid, or liquid/solid. Other important applications can involve three phases, and some of these will be treated in Chapter 8. The most important case of heterogeneous reaction must be the gas/solid system, which is typical of most catalytic processes fortunately, the same general principles pertain to the analysis of all two-phase systems of interest here, so separate developments on a case-by-case basis are not required. 

The primary feature of heterogeneous systems is that the purely physical problem of transporting reactants and products between phases is appended to the chemical transformation. Stated alternatively, a physical rate process (transport) occurs in series with a chemical rate process (reaction). As nature would have it, often these rates are of similar magnitudes and the overall behavior of the reaction system depends on their relative magnitude, not only in terms of the net kinetics of transformation but also with regard to the product selectivity in complex reactions. 

In this chapter we shall develop the theory at a basic level for reactions in some two-phase systems, then discuss applications to reactor modeling and design some specific examples. 

Heterogeneous catalytic reactions by their nature involve a serial transport/reaction rate process, since the reaction species must be transported to and removed from the surface site where the chemical transformation occurs. Much of the theory rests on the simplified picture of the combined reaction and transport process shown in 

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CHEMILUMINESCENCE REACTIONS IN HETEROGENEOUS SYSTEMS FOR TRACE DETERMINATION OF BIOLOGICALLY... 

Present research is devoted to investigation of application of luminol reactions in heterogeneous systems. Systems of rapid consecutive reactions usable for the determination of biologically active, toxic anions have been studied. Anions were quantitatively converted into chemiluminescing solid or gaseous products detectable on solid / liquid or gas / liquid interface. Methodology developed made it possible to combine concentration of microcomponents with chemiluminescence detection and to achieve high sensitivity of determination. 

When an increased yield per unit of time of a chemical reaction in heterogeneous systems was desired, temperature, grain size, grain shape, and contact surface were altered. For catalytic reactions there was further the method of admixing guest particles to the crystal lattice or creating... 

Heterogeneous reactions involve two or more phases. Examples are gas-liquid reactions, solid catalyst-gas phase reactions and products, and reactions between two immiscible liquids. Catalytic reactions as illustrated in Chapter 1 involve a component or species that participates in various elementary reaction steps, but does not appear in the overall reaction. In heterogeneous systems, mass is transferred across the phase. 

Since, according to the preceding descriptions, the reaction space of electrolytic processes consists of an extremely thin layer in contact with the electrode—the contact surface of electrolyte and electrode—these processes can generally be regarded as reactions in heterogeneous systems. Nernst1 has proposed a theory for such systems, which has been tested experimentally by Brunner.2 The principle of this theory consists in basing the reaction velocities on the diffusion velocity. 

The use of mono- and multilayer molecular assemblies to investigate fundamental aspects of electron-transfer reactions in heterogeneous systems has been the focus of this chapter. We discussed the development, characterization and electron-transfer... 

To establish both the photonic efficiency and the qnantnm yield of a photo-stimulated reaction in heterogeneous systems, two important conditions (Emeline et al, 1998a, 2000c), which unfortunately many researchers fail to consider, must be satisfied (i) the reaction rate must scale linearly with photon flow p, and (ii) the reaction rate must be independent of the concentration [M] of reagent molecules. Otherwise, where both photonic efficiency and quantum yield depend on photon flow and reagent concentration, the photocatalytic activities described by and (j) from different heterogeneous systems and different laboratories cannot be compared. 

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