Chemical kinetics of heterogeneous catalytic reactions

To complete the discussion of factors involved in the design of gas-solid heterogeneous catalytic reactors we will examine several aspects of the kinetics of chemical reactions occurring in the presence of a catalyst surface. We consider, for heuristic purposes, the equilibrium reaction  

We may conveniently define nA/S, where S is the surface area of the catalyst, as the surface concentration CA. The corresponding surface concentration, CB, of B is defined as nB/S and the total concentration, Cs, of catalyst sites as ns/S where ns is the number of available active sites. The fraction 0A of surface covered by A will be njns, which, in terms of surface concentrations becomes CA/CS. Similar substitutions may be made for any other species so the term (1 - X /) representing the 

As well as the net rate of surface reaction we must also consider the net rate of each reactant at the surface. We may think of this as the difference between the rates of adsorption and desorption of both A and B. Since the rate of adsorption is usually expressed in terms of the product of the prevailing partial pressure of 

Since P is the final product of reaction it is also necessary to consider the net rate of desorption. In an analogous way to the net rate of adsorption of A, the net rate of desorption of P may be written  

It should be noted that the partial pressures in equations 3.71 and 3.72 are those corresponding to the values at the interface between solid and gas. If there were no resistance to transport through the gas phase then, as discussed in an earlier section, the partial pressures will correspond to those in the bulk gas. 

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