Kinetics of Heterogeneously Catalyzed Reactions

Various shaped solid catalysts. BASF, Germany, from Flagen, 1999. 

For a reaction to take place on the surface of a solid catalyst, the formation of a surface complex is essential, and the kinetic equation must account for this. In addition, mass and heat transport processes are involved, since the reactants, products, and heat of reaction must be transported to and removed from the surface site, where the reaction between adsorbed atoms or molecules takes place. This may lead to a decrease of the effective rate compared to the absence of any transport limitations. 

An overview of the spectrum of factors influencing the rate of heterogeneously catalyzed reactions is given in Section 4.5.1. We will then study in detail the  

For a basic understanding of heterogeneously catalyzed reactions, start with the Sections 4.5.1 and 4.5.2, where the kinetics will be treated without the influence of mass and heat transfer. Then study the influence of external and internal mass transfer (Sections 4.5.3.1 and 4.5.4.1), and the situation when both phenomena occur simultaneously (Section 4.5.5). 

Heat and mass transfer to and within a porous particle are in many cases coupled processes. Consequently, advanced learners should also study the additional influence of heat transfer on the effective rate of a heterogeneously catalyzed reaction (Sections 4.5.3.2 and 4.5.4.2). 

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A quite analogous treatment may be applied to the kinetics of heterogeneously catalyzed reactions. Consider a surface S that contains so active sites to which reactant A, a gaseous or solute species, may bind reversibly with an equilibrium constant KA ... 

The study of the kinetics of heterogeneous catalyzed reactions consists of at least three rather different aspects. 

An interesting further development in describing the kinetics of heterogeneously catalyzed reactions is the so-called microkinetics approach, whereby independent information about adsorbed species from temperature programmed desorption and spectroscopic studies are used to predetermine rate and equilibrium constants of elementary processes, thus enabling the prediction of the overall rate. Especially for metal catalyzed reactions this gives good results [9]. More information about reaction kinetics related to catalysis can be found in Refs. [10-13]. 

Fig. 3.1.3-1 Laboratory reactors for determining the kinetics of heterogeneously catalyzed reactions [Luft 1987a, Forni 1997, Cavalli 1997] ...

Fig. 3.1.3-2 Types of difFerential circulating reactors with catalyst beds [Buzzi-Ferraris 1999, Perego 1999, Forni 1997] for determining the kinetics of heterogeneously catalyzed reactions, a) Spinning-basket principle a rotating basket containing catalyst pellets acts as a stirrer, b) Jet principle with internal recirculation [Luft 1973b, Luft 1978, Dreyer 1982]. c) External recirculation.

In a few cases, the kinetics of heterogenous catalyzed reactions is based on a complete knowledge of the underlying reaction mechanisms. Generally, the kinetics of many commercially important reactions are derived from experimental investigations and are often based on simplified reaction models. 

The observed kinetics of heterogeneously catalyzed reactions are affected by various phenomena at different scales. For a detailed investigation of a reaction mechanism the effects of mass and heat transfer limitations at the catalyst scale should be negligible. Also no deviations from the ideal plug flow or perfectly mixed flow pattern should occur at the reactor scale. 

Many kinetic studies will involve heterogeneous catalysts, since they are so widely used commercially. The kinetics of heterogeneously catalyzed reactions always must be studied... 

This fairly complex rate expression is typical of those obtained for catalytic reactions and is valuable for further discussion of the kinetics of heterogeneously catalyzed reactions. 

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