By John B. Butt
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Extra resources for Activation, Deactivation, and Poisoning of Catalysts
Simple models are chosen for two reasons: first, a general mathematical framework for rep resenting these ideas would be unnecessarily complex and cumbersome to follow; second, the purpose of these examples is to reveal general principles of the implications of deactivation processes rather than the more familiar mechanistic considerations and this is more easily accomplished by using simple models. Let us first consider the case of parallel fouling reactions represented by the stoichiometric equations (2-9) A«±W where A is the reactant, B the product, and W a surface residue.
The task of the catalytic chemist interested in deactivation phenomena is to relate a and a. This is where we must become more fussy in our treatment of the concept of activity than we were in Chapter 1. At first, one might be tempted to argue that they are proportional, and in many cases this is true. But, as we shall show in the next section, a can be proportional to a2 or even a 3 . Still later we shall show that no simple relationship may exist and indeed, because of heterogeneity of the surface, a single parameter such as a is insufficient to characterize the system.
For example, sometimes a is empirically related to time directly. Lastly, all examples assume separability. A. Deactivation Rate Equations The rate of the main reaction is, as before, given by 0lt = a0lo (2-75) where, according to Eq. (2-7), Mt=f2(a)Mci9T) (2-76) »o=/2(l)/3(c„T) (2-77) and for separable forms. In terms of the concentration of poison or foulant on the catalyst surface, C p , and the ultimate capacity of the catalyst for poison or foulant, C p 0 , we can write an expression for the fraction of poison or foulant, f, on the catalyst surface: /=(Cp0-Cp)/Cp0 (2-78) In many cases / corresponds to the parameter a discussed earlier in this chapter.