Heterogeneous catalytic kinetics

In Chapters 3 and 4 the Langmuir-Hinshelwood-Hougen-Watson approach to heterogeneous catalysis was discussed. Such an approach supposes that usually there is one rate determining step (adsorption, surface reaction or desorption) and that the other steps are in quasi-equilibria. 

If either adsorption or desorption steps are rate determining and the other steps are quasiequilibria then the rates respectively are 

Notice, that eq.7.2 is similar to Michaelis-Menten kinetics. In fact it is a more general case as it implies also binding of the product to the catalyst. As in enzymatic kinetics, linearization in coordinates 1/r vs 1/Pa in the framework of Lineweaver-Burk approach can be quite easily done and leads to the same conclusions for a simple case of irreversible reaction without product binding as discussed in the previous chapter on enzymatic kinetics. 

Some other cases for the rate determining surface reactions and overall irreversibility are collected in Table 7.1. 

Derivation of these rate expressions is straightforward and follows the pattern examplified below for the mechanism 

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Although computational methods are available for numerical treatment of enzymatic kinetics and estimation of parameters, still plotting methods are rather widesperad in enzymatic catalysis, while there are rather seldom used by researchers in heterogeneous catalytic kinetics. It is believed, that via plotting methods it is possible to recognized unexpected behavior and to better design experiments. 

A special case of heterogeneous catalytic kinetics, that we will briefly discuss, is electrocatalysis, which is defined as the acceleration of an electrodic reaction by a substance that which is not consumed in the overall reaction. For a catalytic reaction with the following step Sads+Hads=>SHads this is only one of the possible mechanisms for the involvement of hydrogen in the catalytic reduction. Another mechanism is the electrochemical or ionic mechanism where the adsorbed hydrogen serves only as an electron source for the reduction process. This type of reaction is formulated by the following reaction steps ... 

Denominator I like the analogous term in equation (11) is the inhibition term that characterizes the complexity of the reaction It should be stressed that both equations (11) and (14) have the Langmuir form that is well-known in heterogeneous catalytic kinetics. 

The single CSTR has been used for many years in the laboratory for the study of kinetics of liquid phase reactions, and is now increasingly being employed for the measurement of gas/solid heterogeneous catalytic kinetics as well [early developments in the latter application are described by J.J. Car berry, Ind. Eng. Chem., 56, 39 (1964) D.J. Tajbl, J.B. Simons and J.J. Carberry, Ind. Eng. Chem. Eundls., 5, 171 (1966). A number of related designs based on internal recirculation of the reaction mixture through a small fixed bed of catalysts may also be treated conceptually as... 

Bennett, C.O., 1967. A dynamic method for the study of heterogeneous catalytic kinetics. AICHE J. 13, 890-895. Bennett, C.O., 1976. The transient method and elementary steps in heterogeneous catalysis. Catal. Rev. 13,121-148. Bennett, C.O., 2000. Experiments and processes in the transient regime for heterogeneous catalysis. In Haag, W.O., Gates, B., Knozinger, H. (Eds.), Advances in Catalysis. Elsevier Sciences, Dordrecht, pp. 329-416. 

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