The Kinetics of Heterogeneous Reactions

It is becoming recognized that something fundamental underlies the almost universal tendency of gaseous changes to take place in contact with a solid surface more readily than in the gas phase, and that the wall-effect is not to be dismissed as merely a disturbing factor . But before this difficult question can be approached some preliminary matters relating to the mode of action of the solid surface must be considered. 

The molecules of a gas are not, except perhaps in very special instances, brought into a chemically active condition by mere mechanical impact with the solid surface. The highly specific action of different surfaces, illustrated for example by the hundreds of examples quoted in Sabatier s work La Catalyse en Chimie Organique, at once rules out so simple an hypothesis. 

Intermediate compound formation between the molecules of the gas and those of the catalysing surface is not usually a helpful hypothesis. The assumptions which have to be made about the existence of compounds nearly always do violence to the principles of general inorganic chemistry. It is undesirable to postulate for ad hoc purposes the formation of compounds the separate existence of which is known to be unlikely or impossible. 

All the advantages of the intermediate compound theory without most of the disadvantages are possessed by what may be called the- chemical adsorption theory. 

Faraday went to the root of the matter and stated at the outset that the films of gas known to be adsorbed by surfaces were the seat of the chemical changes. This idea has long been used in the interpretation of catalytic phenomena, and, without further specific assumptions about the nature of the films, has been accepted for many years. 

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The flow reactor is used primarily in the study of the kinetics of heterogeneous reactions. Planning of experiments and interpretation of data obtained in flow reactors are considered in later chapters. 

The foregoing methods are certainly not exclusive, and many other techniques such as cloud chambers (e.g., see Miller et al., 1987) and fluidized bed reactors have also been applied to following the kinetics of heterogeneous reactions relevant to the atmosphere. However, due to space limitations, these will not be treated in detail here. 

Chapter 10 is devoted to the kinetics of interfaces. Nevertheless it may be appropriate in the context of heterogeneous reactions to outline the influence of the phase boundaries on the kinetics of heterogeneous reactions. This will be done in this section, whereas the detailed discussion of the interface kinetics proper is postponed until Chapter 10. 

These equations show that the A and B fluxes are composed of both a drift term and a reaction term. The drift term stems from the electric field. The reaction term was already deduced in the kinetics of heterogeneous reactions. From Eqns. (8.78) and (8.79), we obtain the reaction product s rate of thickness increase to be... 

A variety of laboratory reactors have been developed for the determination of the kinetics of heterogeneous reactions, all with specific advantages and disadvantages. Several reviews of laboratory reactors are available [28-33]. The evaluations of the available methods in these reviews are different because of the variation of chemical reactions and catalysts investigated and the different viewpoints of the authors. It is impossible to choose a best kinetic reactor because too many conflicting requirements need to be satisfied simultaneously. Berty [34] discussing an ideal kinetic reactor, collected 20 requirements as set forward by different authors. From these requirements it is easy to conclude, that the ideal reactor, that can handie all reactions under all conditions, does not exist For individual reactions, or for a group of similar reactions, not all requirements are equally important. In such cases it should be possible to select a reactor that exhibits most of the important attributes. 

Interpretations of the kinetics of heterogeneous reactions are complicated by the adsorption of reactants and intermediates. The reaction order, R, is normally defined by... 

D Me-S surface alloy and/or 3D Me-S bulk alloy formation and dissolution (eq. (3.83)) is considered as either a heterogeneous chemical reaction (site exchange) or a mass transport process (solid state mutual diffusion of Me and S). In site exchange models, the usual rate equations for the kinetics of heterogeneous reactions of first order (with respect to the species Me in Meads and Me t-S>>) are applied. In solid state diffusion models, Pick s second law and defined boundary conditions must be solved using Laplace transformation. 

A wide variety of methods have been used to study the kinetics of heterogeneous reactions. The emphasis in this chapter is on those methods which have been particularly developed for the study of such reactions, and as a consequence, the study of kinetics by conventional measurements, e.g. chemical analysis, pressure changes etc., is only briefly mentioned. 

Then the use of rate equations for studying the kinetics of reactions is illustrated for simple systems and some complex ones. Only homogeneous reactions, devoid of any physical resistances, are considered here. The kinetics of heterogeneous reactions is taken up in Chaps. 8 and 9. 

In heterogeneous reactions, the important variable is the surface concentration. This is because at least one reactant will be adsorbed in order for the catalytic reaction to take place. Since such surface concentrations usually cannot be measured, the kinetics of heterogeneous reactions are far less accessible than the kinetics of homogeneous reactions. Also the effective order of heterogeneous reactions may be sensitive to temperature. 

When studying the kinetics of heterogeneous reactions or when designing a large catalytic reactor, there are more factors to consider than when dealing with homogeneous reactions. For a solid-catalyzed reaction, the rate depends on the reactant concentrations at the catalyst surface, but these are not the same as the bulk concentrations, because some driving force is needed for mass transfer to the surface. If the catalyst is porous, as is usually the case, there are further differences in the concentration between the fluid at the external surface and the fluid in the catalyst pores. Models must be developed to predict the surface concentrations as functions of the partial pressures or concentration in the gas or liquid, and the rate expression can then be written in terms of the fluid concentrations. 

Mechanical extension of the generally accepted concepts of the absence of equilibrium in homogeneous gaseous reactions, in particular, in monomolec-ular reactions in the low-pressure field [27], to the kinetics of heterogeneous reactions. 

C. Popescu (1996). Integral method to analyze the kinetics of heterogeneous reactions under non-isothermal conditions A variant on the Ozawa-Flynn-Wall method. Thermochimica Acta 285 (2) 309-323. 

The kinetics of homogenous and heterogenous reactions is discussed later in this chapter with a special attention on mass and heat transfer influence on the kinetics of heterogenous reactions. 

Nearly all of the information available on the kinetics of heterogeneous reactions with lanthanide oxides concerns the C-type sesquioxides or the fluorite-related higher oxides. As stated in the section above, in these materials oxygen mobility is very high, whereas, metal-atom movement is extremely limited below 1200°C. Table 19 suggests the type of experiments that were done and the phenomenological mechanisms proposed before 1980 (Eyring 1979). 

Kiperman SL. Introduction to the Kinetics of Heterogeneous Reaction. Moscow 1964. 

The detailed treatment of the adsorption-desorption process as a chemical reaction reveals a few major concepts that are used in developing the kinetics of heterogeneous reactions from a sequence of several surface reactions such as the one in Eq. 2.1. For the purpose of writing the kinetics of each step, each surface reaction can be treated as an elementary step as in homogeneous reactions. The treatment also shows an individual step as a separate entity independent of the other steps, eventually leading to the concept of a rate-limiting (or rate-controlling) step. 

The kinetics of heterogeneous reactions are much more complex to interpret than homogeneous reactions. We notice, however, that within the scope of pseudosteadiness, the validity of the relation of (relation [14.3]) is fundamental. It has allowed us to decouple the modeling and to understand a mechanism considering... 

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