Heterogeneous catalytic processes kinetic models

Yu.S. Snagovskii and G.M. Ostrovskii, Kinetic Modelling of Heterogeneous Catalytic Processes, Khimiya, Moscow, 1976 (in Russian). 

To conclude the discussion of some technological aspects of the theory of DS, we shall touch upon the question of its role in the catalytic reaction kinetics. Since Langmuir s time, the kinetic laws of a heterogeneous catalytic process have been described exclusively by models involving ordinary differential equation sets. Our results indicate also that under experimental conditions, the researcher is most likely to run into the stratification phenomena, the domain structure formation in a kinetic reactor (stationary. 

It should be noted that the detailed modelling of heterogeneous catalytic reactions faces some specific difficulties. Compared with homogeneous systems, the limits of the field wherein the law of mass action analog (the surface-action law) can be correctly applied are less distinct. Still less reliable are the elementary step constants. Nevertheless, we believe that, despite the complexity of "real kinetics , the importance of studying the models fitting the law of mass action cannot be undervalued. These models describe the chemical components of a complex catalytic process properly and, on the other hand, they are a necessary step that can be treated as a first approximation. Our study is devoted to the analysis of just these models. 

A different model [11] that can be used to obtain the kinetics equation for a pyrolytic reaction is adapted from the theory developed for the kinetics of heterogeneous catalytic reactions. This theory is described in literature for various cases regarding the determining step of the reaction rate. The case that can be adapted for a pyrolytic process in solid state is that of a heterogeneous catalytic reaction with the ratedetermining step consisting of a first-order unimolecular surface reaction. For the catalytic reaction of a gas, this case can be written as follows ... 

The observation of oscillations in heterogeneous catalytic reactions is an indication of the complexity of catalyst kinetics and makes considerable demands on the theories of the rates of surface processes. In experimental studies the observed fluctuations may be in catalyst temperature, surface species concentrations, or most commonly because of its accessibility, in the time variation of the concentrations of reactants and products in contact with the catalyst. It is now clear that spontaneous oscillations are primarily due to non-linearities associated with the rates of surface reactions as influenced by adsorbed reactants and products, and the large number of experimental studies of the last decade have stimulated a considerable amount of theoretical kinetic modelling to attempt to account for the wide range of oscillatory behaviour observed. 

This simplification (assumption (1) in Section 4.2.1) assumes that the concentration of at least one speeies is mueh larger than that of others. For example, in models of many reactions in aqueous solutions, the water concentration is considered to be constant. Similarly, in reactions in which precipitation occurs, the concentration of the solid phase is taken as the constant. These constant values are incorporated into the kinetic coefficients. In typical heterogeneous gas-solid reactions, the amount of reacting gas molecules is assumed to be much larger than the total amount of active catalyst sites. In this case, the concentration of the abundant gaseous species is included in the reaction rate coefficient as a constant (apparent kinetic coefficient). This simplifies the reaction model and often results in a linear model. Thus, for a surface catalytic process at steady state, the rate of adsorption for the reaction A+Z—> AZ can be expressed as... 

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