Competition in Surface Reactions

If we pursue the model of the catalytic surface as a collection of a fixed number of sites S capable of forming chemical bonds with adsorbed species, then we may expect to find that substances which can compete for the catalytic sites may inhibit the reaction. Furthermore, the effectiveness of the inhibition will depend on the relative pressures of the two adsorbates as well as their sorption constants on the surface. Let us consider as an example the simple Langmuir-type sorption of two sorbates A and B on a surface of S° sites  

In a mechanism for which the rate-determining process is the decomposition of sorbed A, B would act as an inhibitor, or if its attachment to the catalyst surface was sufficiently strong, a poison. For the first-order decomposition of sorbed A 

A rate law of this type was found by Constable for the dehydrogenation 

If the rate-determining step in the surface reaction involves the second-order reaction of sorbed A and R, or the transition state involves both sorbed A and B, the rate law will have the form 

At fixed pressure of A, the rate of reaction will pass through a maximum with increasing pressure of B, the latter being given by 

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