Reaction Models with no RDS

This author is unaware of any unequivocal, proven R-E reaction mechanisms in the literature and is therefore hesitant to use it however, this mechanism is routinely referred to in texts on kinetics and for completeness it is represented below  

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. 

As mentioned at the beginning of this chapter, there are reactions which, under certain conditions, may not have a RDS if so, the sequence of elementary steps in the catalytic cycle must reflect this. These sequences may be comprised of two or more slow steps along with one or more 

Adsorption Step 1 r = Surface Reaction Step 2 r = Desorption Step 4 (or 3) r = 

Lk3KiK2PAPB - Lk 3PcPp/K4K5 Lk3KAKB(PAPB - PcPp/K) 

---

All three reaction sequences provided in this section have represented simple L-H models with a single elementary step on the surface representing the RDS. In many, if not most, cases the catalytic reaction on the surface may consist of a sequence of elementary steps, one or all of which may represent the slow step in the catalytic cycle. If it is just a single step, then this is the RDS and all other steps can be assumed to be quasi-equilibrated. If a series of two or more steps represents slow surface reactions, then there is no... 

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.. 

Fig. 4.3 shows a comparison of Models 1-4 with data from an RD experiment, which is typical of the results from the study discussed here other examples can be found elsewhere [4]. All model results are predictions in the sense that no adjustments to RD data were made. The predictions from the stage models, which take into account reaction kinetics (Models 2 and 3), are good and do not differ largely. 

There are, however, a number of important constraints to this technique. The reaction must be suitable for the technique, usually meaning that one of the products must be the most volatile component, and the reaction and catalyst must also be active at the set of conditions (temperature, pressure) dictated by the distillation. A further challenge currently is that modelling such systems is proving difficult due to the interplay of chemical and physical phenomena. Modelling is necessary as pilot-scale trials are expensive. Scale-up is a problem for RD associated with the separation of functions , as there is no technique at present for scaling-up the combined unit operation of reaction and distillation. 

---