Reaction Models with a RDS - Unimolecular Surface Reactions

1 Reaction Models with a RDS - Unimolecular Surface Reactions 

The simplest reaction model can be depicted by the following sequence of elementary steps which describes a reversible isomerization reaction of A to B, where S is an active site  

The quasi-equilibrated (QE) steps and the RDS step are designated. This reaction model represents a series of elementary steps comprising a closed sequence and a catalytic cycle. In its reversible form, from the law of mass action the net rate is (Chapter 2.7)  

Substitution of equations 7.2 and 7.3 into equation 7.1 gives the most general form of this rate expression  

If Step 7.2 is irreversible (k s k for example), the second term in the numerator disappears and, further, if [B—S] is very low because Pb is very low (as in a differential reactor) or Kb is very small, then [A—S] is the MARI and the last term in equation 7.4 can be neglected. The rate then simplifies to  

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Finally, to summarize, a variety of the traditional rate expressions for reactions on an ideal surface has been examined, and many of their derivations have been discussed in detail. These include L-H and R-E models describing unimolecular and bimolecular reactions on surfaces with either one type of active site or two types of active sites. If a RDS other than that for a surface reaction is proposed, i.e., either an adsorption or a desorption step, then a H-W rate expression is derived. These standard rate laws, which assume a RDS exists, are frequently referred to and utilized, and they are summarized in Table 7.10. Many other forms of a rate expression, which do not assume a RDS and utilize the SSA, can be derived based on the reaction sequence proposed. 

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