Reaction Models with a RDS Bimolecular Surface Reactions

2 Reaction Models with a RDS Bimolecular Surface Reactions 

A large fraction of surface reactions are naturally bimolecular in nature. The simplest L-H model for a catalytic cycle in which a bimolecular surface reaction is the RDS is shown below for reactants A and B to produce a single product C, where S is an active site  

= 1 /Kc, where Kc is the adsorption equilibrium constant for C. In its reversible form, again from the law of mass action for an elementary step, the net rate is  

The rate expression given by equation 7.21 (or 7.22) is easily modified to accommodate dissociative adsorption to allow the surface reaction of atomic species, if required. For example, if step 7.14 were written instead as  

9a would be represented by K P /(1 -I- SKiP,) with KiPi = K P for A atoms. Keep in mind that the site balance used to obtain the denominator in the preceding sentence can contain other species, such as an inhibitor, that are not directly involved in the reaction, as discussed in Chapter 5.3.4. Using this term in the site balance (Equation 7.18 or 7.20), the reversible rate expression now becomes  

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A L-H model with a bimolecular surface reaction between two adsorbed NO molecules as the RDS was proposed as follows, where S is an active site and the stoichiometric number for an elementary step lies outside the brackets around that step, i.e.. 

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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