Elementary step quasi-equilibrated

If desorption of B from the surface is rate-determining, then all elementary steps prior to desorption are assumed to be quasi-equilibrated ... 

It must be emphasized that step 2 is not an elementary step, but a sum of all of the quasi-equilibrated steps that must occur after dinitrogen adsorption. According to this abbreviated sequence, the only species on the surface of the catalyst of any kinetic relevance is N. Even though the other species (H, NH, etc.) may also be present, according to the assumptions in this example only N contributes to the site balance ... 

In this case the remaining elementary reactions may be considered at quasiequilibrium and may be lumped into a single, intermediate, quasi-equilibrated reaction [8]. This is similar to the case of the two-step mechanism of Boudart [28]. 

Boudart has discussed in detail the fact that the rate law derived from a complex catalytic cycle comprised of a number of elementary steps can frequently be represented by only two kinetically significant steps if the assumptions of a RDS and a MARI are invoked however, ambiguities can develop which prevent one from distinguishing among different reaction models [11,26]. In similar fashion, but with perhaps less dramatic results, a L-H-type or H-W-type model [27] invoking more than one elementary surface reaction step can be greatly simplified by the presence of quasi-equilibrated steps which precede the RDS or, if a RDS does not exist, the series of slow steps on the surface. The SSA may also be required in the latter case to eliminate all unknown surface reaction intermediates from the rate law. Significant simplification is achieved with the assumption of a RDS. 

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) ... 

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... 

To account for the kinetic behavior that has just been described, numerous reaction mechanisms were considered and rejected because they were inconsistent with some or all of the trends observed. However, a L-H-type reaction sequence was proposed that contained the addition of the last H atom to form adsorbed cyclohexane (Cy) as the RDS, with a series of quasi-equilibrated (QE), H atom-addition steps preceding the RDS. Each QE step in such a series prior to the RDS involving a reactant increases the reaction order on that reactant, and because each elementary step is quasi-equilibrated, these steps can be added to give a single overall quasi-equilibrated reaction, thus the reaction model can be represented by ... 

It is worth noting at this point that a H-W model invoking product desorption as the RDS could also give equation 7.30 for values of n = 1 or 2, provided the fraction of vacant sites is very low and the fraction of sites covered by the two reactants is very high. It may also be possible to propose a more realistic sequence of steps comprised of reversible and irreversible elementary steps (perhaps even including some quasi-equilibrated steps) that could result in a rate expression of the Mars-van Krevelen form. Consequently, the Mars-van Krevelen rate expression should be considered to be only a mathematical fitting equation with no theoretical basis. 

A detailed and rather complex sequence of elementary steps has been proposed to describe NO reduction by CH4 in the presence of O2 that is consistent with both NO decomposition and NO reduction by CH4 in the absence of O2 as well as with homogeneous free radical chemistry [4]. The details of this catalytic cycle certainly have not been proven however, as mentioned previously, the assumption of a RDS and the presence of only a limited number of significant surface reaction intermediates simplifies the derived rate expression enormously. With the proposal of a RDS, the catalytic cycle can be reduced to the following steps, with S as an active site, the convention of the arrows denotes whether a step is the RDS or is quasi-equilibrated (See Chapter 2.7), and the stoichiometric number for the elementary step in the cycle is given outside the brackets [47] ... 

Many dual-substrate reactions can be represented by a sequence of elementary steps involving a ternary complex comprised of the enzyme and the two reactants, and application of the SSA to these systems can be quite complicated [10]. If a RDS is assumed to exist, then the derivation of a rate expression can be markedly simplified, as shown next. Assume that substrates A and B interact with an enzyme E to form a product P according to the following series of elementary steps, where the first four steps are quasi-equilibrated and the last step is the RDS ... 

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