Rate limiting step, heterogeneous catalysis

In any catalyst selection procedure the first step will be the search for an active phase, be it a. solid or complexes in a. solution. For heterogeneous catalysis the. second step is also deeisive for the success of process development the choice of the optimal particle morphology. The choice of catalyst morphology (size, shape, porous texture, activity distribution, etc.) depends on intrinsic reaction kinetics as well as on diffusion rates of reactants and products. The catalyst cannot be cho.sen independently of the reactor type, because different reactor types place different demands on the catalyst. For instance, fixed-bed reactors require relatively large particles to minimize the pressure drop, while in fluidized-bed reactors relatively small particles must be used. However, an optimal choice is possible within the limits set by the reactor type. 

The first term of this series is the rate of rate-limiting step calculated at the equilibrium of the rest of reactions. This first term approximation is widely applied in heterogeneous catalysis. We have derived the following explicit formula (in the assumptions of the Basic case-, see Lazman and Yablonskii, 1988). 

Why adsorption, ion exchange and heterogeneous catalysis in one book The basic similarity between these phenomena is that they all are heterogeneous fluid-solid operations. Second, they are all driven by diffusion in the solid phase. Thus, mass transfer and solid-phase diffusion, rate-limiting steps, and other related phenomena are common. Third, the many aspects of the operations design of some reactors are essentially the same or at least similar, for example, the hydraulic analysis and scale-up. Furthermore, they all have important environmental applications, and more specifically they are all applied in gas and/or water treatment. 

All these steps can influence the overall reaction rate. The reactor models of Chapter 9 are used to predict the bulk, gas-phase concentrations of reactants and products at point (r, z) in the reactor. They directly model only Steps 1 and 9, and the effects of Steps 2 through 8 are lumped into the pseudohomoge-neous rate expression, a, b,. ..), where a,b,. .. are the bulk, gas-phase concentrations. The overall reaction mechanism is complex, and the rate expression is necessarily empirical. Heterogeneous catalysis remains an experimental science. The techniques of this chapter are useful to interpret experimental results. Their predictive value is limited. 

Discuss the various steps involved in heterogeneous catalysis. Derive an expression for the rate constant and discuss limiting cases of rate equation. 

It plays the same role as the effectiveness factor in heterogeneous catalysis and is a measure of the film thickness uniformity. It represents the ratio of the total reaction rate on each pair of wafers to that we would obtain if the concentration in the cell formed by the two wafers were equal to the bulk concentration everywhere. Thus, if the surface reaction is the rate controlling step, n = 1, whereas if the diffusion between the wafers controls, n < 1. In the limit of strong diffusion resistance the deposition is confined to a narrow outer band of the wafers and a strongly nonuniform film results. 

While it is often possible to demonstrate that a surface process is rate limiting, identification of the step concerned is not always so readily achieved (as in heterogeneous catalysis which involve comparable mechanistic steps). Reaction rates are determined by reactant areas and are slow compared with the rate of diffusive transport of material to the appropriate boundaries. Surface limited reactions are also sensitive to the ease of removal of volatile products, which may be hampered by the presence of an inert gas. Readsorption may influence the effective concentrations of participating surface intermediates. As in catalytic heterogeneous reactions, the sequence of changes which precede product evolution may involve several interlinked steps, and the parameters which determine the overall progress of reaction are not always readily identified. 

A quantitative description of relations between structural parameters of solid catalysts or substrates on one side and reaction rates or adsorption equilibria on the other side, even if valid only in limited areas, may form an important step in the development of a general theory of catalysis. Some years ago, Boudart 1) noticed that such correlations in heterogeneous catalysis can be divided into two broad classes. In the first type a series of catalysts is tested by means of a standard reaction and some kinetic parameter is related to a property of the solid catalyst. In correlations of the second type, the reactivity of a series of compounds is studied on a single catalyst and some kinetic parameter is related to a property of the reacting molecules. Boudart pointed out that correlations of the first type are more frequent in the literature than those of the second type. He also presented some examples of both types. Correlations between the substrate structure and its reactivity were qualitative or semiquantitative. 

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