Bimolecular reactions on surfaces

Two molecules A and B can react on a surface if they occupy neighboring sites of the surface. Let 9 and 0 be the fractions of the surface sites covered by A and B, respectively, and let 0 be the fraction of sites that are vacant 0 = 1 — 0 — 0. We represent the reaction by 

Since 0 = 1 — 6 — these two equations can be solved for 0 and 0. We will consider only the case for which k is very small if we set k = 0, these equations reduce to 

Case 1. Both A and B are weakly adsorbed the surface is sparsely covered. In this case, K Ca 1 and 2 b 1- The denominator of Eq. (34.16) is about equal to unity and the rate law is 

The reaction is second order overall, and is first order with respect to both A and B. 

Case 2. Onereactant,A,morestronglyadsorbedthantheother.Inthiscase, iCa the denominator is about equal to 1 - - K Ca, and Eq. (34.16) takes the form 

---

V.P Zhdanov. Lattice-Gas Model of Bimolecular Reaction on Surface. Surf. Sci. 102 L35 (1981). 

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. 

---