Heterogeneous catalytic reactions texts

Rate equations for simple reversible reactions are often developed from mechanistic models on the assumption that the kinetics of elementary steps can be described in terms of rate constants and surface concentrations of intermediates. An application of the Langmuir adsorption theory for such development was described in the classic text by Hougen and Watson (/ ), and was used for constructing rate equations for a number of heterogeneous catalytic reactions. In their treatment it was assumed that one step would be rate-controlling for a unique mechanism with the other steps at equilibrium. 

These questions, of course, are strongly related. The following surface chemical techniques are described and exemplified very much with the first of these questions in mind, including subsidiary questions such as to (a) the nature of an active site and (b) the role of catalyst promoters. For an account of heterogeneous catalytic reaction mechanisms, the reader is referred to modern texts on reaction kinetics/catalysis. 

The rate at which each of these steps occurs ultimately determines the distribution of the participating species (reactants and products) in the system in addition, it plays a major role in determining the over-all rate of heterogeneous catalytic reactions. The factors and effects influencing each of these rates is considered in the next four sections. The final section is concerned with analyzing and determining the controlling step or steps for the reaction in question. This topic will be reviewed qualitatively since much of this material is both difficult and complex a qualitative presentation is beyond the scope of this text. 

Heterogeneous catalytic oxidation is a well studied and industrially useful process. Industrial catalytic oxidation of vapors and gases is a very broad field and is dealt with in several texts and review articles. Catalytic oxidation, both partial and complete, is an important process for such reactions as the partial oxidation of ethene and propene, ammoxidation of propene to acrylonitrile, maleic anhydride production, production of sulfuric acid, and oxidation of hydrocarbons in automotive exhaust catalysts. By far, the majority of oxidation catalysts and catalytic oxidation processes have been developed for these industrially important partially oxidized products. However, there are important differences between the commercial processes and the complete catalytic oxidation of VOCs at trace concentrations in air. For instance, in partial oxidation, complete oxidation to CO2 and H2O is an undesirable reaction occurring in parallel or in series to the one of interest. Other differences include the reactant concentration and temperature, the type of catalyst used, and the chemical nature of the oxidizable compound. Approximate ranges of the major independent variables of interest in this review are shown in Table 1. 

Researchers in the area of heterogeneous catalysis have recently focussed considerable attention to the relationships among catalytic activity, product selectivity and the size and shape of metal particles for reactions catalyzed by metals (15). Reactions that are influenced by the size and shape of metal particles or electronic interactions of the metal particles with the support are known as structure sensitive reactions. Theoretical calculations of various crystallographic structures (16) have shown that the number of specific type of surface atoms (face, corner, edge) change as a function of particle size. For example, for a face centered cubic system, the number of face atoms decreases as particle size decreases. If, therefore, a reaction is catalyzed on a face and there are a substantial number of face atoms necessary for catalysis to occur, then as particle size decreases catalytic activity will decrease. This idea often runs counter to principles discussed in general science texts (17). 

These texts cover different catalytic principles and disciplines along with their application to industrial practice. Collectively they span a wide range of material including basic concepts in heterogeneous catalyst synthesis, characterization, kinetics, reaction engineering and their application to industrial catalytic systems. 

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