Kinetics of heterogeneous catalytic reactions

If rate of reaction is to have an unambiguous meaning, it should be defined as the rate of increase of the extent of reaction 

To facilitate the comparison of the results of different investigators, the rates of heterogeneous catalytic reactions should be suitably expressed and the conditions under which they have been measured should be specified in sufficient detail. If the rate of the uncatalysed reaction is negligible, the rate of the catalysed reaction may be given as 

The volume should be that of the catalyst granules excluding the intergranular space. If Q is in area, 

The turnover frequency, N, (commonly called the turnover number) defined, as in enzyme catalysis, as molecules reacting per active site in unit time, can be a useful concept if employed with care. In view of the problems in measuring the number of active sites discussed in 1.2.4, it is important to specify exactly the means used to express Q in terms of active sites. A realistic measure of such sites may be the number of surface metal atoms on a supported catalyst but in other cases estimation on the basis of a BET surface area may be the only readily available method. Of course, turnover numbers (like rates) must be reported at specified conditions of temperature, initial concentration or initial partial pressures, and extent of reaction. 

In comparing various ca talysts for a given reaction or in comparing various reactions on a given catalyst, it may be inconvenient or impracticable to compare rates at a specified temperature since rates must be measured at temperatures at which they have convenient values. Therefore, it may be expedient to compare the temperatures at which the rates have a specified value. 

Transport of reactants A, B... from the main stream to the catalyst pellet surface. 

Surface chemical reaction between adsorbed atoms or molecules. 

Transport of the products in the catalyst pores back to the particle surface. 

Transport of products from the particle surface back to the main fluid stream. 

Steps 1, 3, 4, 5, and 7 are strictly consecutive processes and can be studied separately and then combined into an overall rate, somewhat analogous to a series of resistances in heat transfer through a wall. However, steps 2 and 6 cannot be entirely separated active centers are spread all over the pore walls so that the distance the molecules have to travel and therefore the resistance they encounter, is not the same for all of them. This chapter concentrates on steps 3,4, and S and ignores the complications induced by the transport phenomena, which is treated in detail in Chapter 3. 

The kinetics of catalytic reactions on nonuniform surfaces have been discussed by Roginskii (330,331) certain general features of his discussion will be presented here. The rate of a complex multistage heterogeneous catalytic reaction is controlled by the rate of the slowest step. The slowest step may be the adsorption of the reactants, the chemical reactions on the surface, desorption of the products or diffusion of reactants or products through the gaseous phase near the surface of the catalyst. 

In the case where the reaction velocity is determined by the activated adsorption of the reactants and no poisoning by the products occurs, the action of a nonuniform surface simulates the behavior of a uniform surface. A similar situation occurs if there is redistribution on the heterogeneous surface after adsorption, assuming that the rate of surface migration is much greater than the rate of adsorption. 

If the reaction products are irreversibly adsorbed on the surface of the catalyst they may act as very effective poisons by blocking the active centers for the forward reaction. In this case Roginskil found that the kinetic equation obtained had a form which was characteristic of an activated adsorption process. 

These examples by no means exhaust the multiplicities and diversities of the kinetics of reactions on heterogeneous surface. The poisoning of catalytic reactions by means of foreign substances will be reviewed later in a section devoted to a discussion of catalyst modification. 

The adsorption coefficients of the substances present during the dehydration of ethyl alcohol (i.e., water, ethylene, and ethanol) over alumina were determined kinetically and from the adsorption isotherms. The values of these adsorption coefficients are listed in Table IV. The values of the adsorption coefficients determined from the adsorption isotherms greatly exceed those calculated from the kinetic data thus the catalytically active centers are characterized by a smaller adsorption capacity for the reaction product, water, than other sections of the surface. Antipina and Frost, therefore, assert that the dehydration of ethyl alcohol occurs on sections possessing a smaller adsorption for water than those upon which water vapor alone is adsorbed. 

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M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Wnceton, NJ, 1984. 

Boudart M and Djega-Mariadassou G 1984 Kinetics of Heterogeneous Catalytic Reactions (Prinoeton, NJ Prinoeton University Press)... 

Boudart, M., and Diega-Mariadassou, G. (1984). Kinetics of Heterogenous Catalytic Reactions. Princeton, NJ Princeton University Press. 

The third example (Fig. 4.3-27) is a loop reactor with internal recycle, developed by G. Lull. This reactor can advantageously be used to study kinetics of heterogenous catalytic reactions at pressures up to 40 MPa and temperatures to 500°C. The internal recycle... 

S.L. Kiperman, Introduction in the Kinetics of Heterogeneous Catalytic Reactions, Nauka, Moscow, 1964 (in Russian). 

Difficulties of the interpretation of experimental data on the kinetics of heterogeneous catalytic reactions are conditioned by the following. 

Kinetics of Heterogeneous Catalytic Reactions Analysis of Reaction Schemes... 

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