The Microkinetics of Several Coupled Elementary Surface Processes

3 THE MICROKINETICS OF SEVERAL COUPLED ELEMENTARY SURFACE PROCESSES 

We shall now utilize the microkinetic expressions for individual elementary surface processes, as described in the previons section, in order to establish a model for describing fnll catalytic cycles. A heterogeneous catalytic process always includes a number of elementary steps happening in sequence one after the other. If the reaction occurs over a surface, for example, the reactants first need to be adsorbed on the surface, then move into the vicinity of each other through diffusion processes, then react with each other forming the products, and finally desorb. Sometimes, the adsorbed reactants need to undergo various activation steps before being able to react with each other. 

In order for the process to be catalytic, the catalyst should not be consumed in the process. A process comprising a number of elementary steps can therefore be written up in such a way that the reactants have been turned into products according to an integer times the reaction stoichiometry. The surface after having taken part in a number of reactions has been reinstated in its original configuration. Such a series of elementary reaction steps is referred to as a catalytic cycle. When analyzing catalytic processes it is important always to look at the full catalytic cycle, as it is otherwise very easy to misinterpret key aspects of the process. 

Let us first focus on the simple catalytic cycle, whereby two reactant gases and B react to form the gas AB. We assume that the process proceeds in the two elementary steps 

In these expressions, describes the net rate of A removal, and describes the net rate of AB formation. Since both of the two elementary processes need to occur in order for the product to be formed and since step 1 changes the coverage that goes into step 2, the two rate expressions must be solved simultaneously in order to obtain the overall reaction rate. If we think of the net rates in terms of what surface species 

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