Kinetics and the derivation of rate expressions

A knowledge of the kinetic parameters and, in particular, the orders of reaction of a catalysed reaction is important to the accurate definition of the reaction mechanism. However, catalytic hydrogenation reactions proceed through a series of elementary steps, only one of which may be ratedetermining. In consequence, the observed rate expressions give little or no direct information about most of the steps involved and kinetics alone are not sufficient for a precise description of the mechanism. 

The general principles of the kinetics of surface-catalysed reactions have been discussed in detail elsewhere [12,13] and in this section we shall confine our attention to a discussion of the kinetics and rate expressions which are applicable to hydrogenation reactions. 

While in homogeneous systems the reaction is occurring throughout the entire volume of the reaction vessel and the partial pressures (concentrations) of the species participating in the rate-controlling step are often directly observable, the same is not true for heterogeneous systems. Here, reaction is confined to a monomolecular layer at the surface, around 10 6 of the total volume of the reaction system, and the concentrations of 

In heterogeneous systems, the rate expressions have to be developed on the basis of (a) a relation between the rate and concentrations of the adsorbed species involved in the rate-determining step and (b) a relation between the latter and the directly observable concentrations or partial pressures in the gas phase. In consequence, to obtain adequate kinetic rate expressions it is necessary to have a knowledge of the reaction mechanism, and an accurate means of relating gas phase and surface concentrations through appropriate adsorption isotherms. The nature and types of adsorption isotherm appropriate to chemisorption processes have been discussed in detail elsewhere [16,17] and will not be discussed further except to note that, in spite of its severe theoretical limitations, the Langmuir isotherm is almost invariably used for kinetic interpretations of surface hydrogenation reactions. The appropriate equations are 

In a catalytic reaction, the overall rate depends upon the relative rates of five distinct processes, any of which may be rate-determining. These are 

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