Most Abundant Reaction Intermediate MARI

The Most Abundant Reaction Intermediate (MARI) approximation is a further development of the quasi-equilibrium approximation. Often one of the intermediates adsorbs so strongly in comparison to the other participants that it completely dominates the surface. This intermediate is called the MARI. In this case Eq. (156) reduces to... 

In many reaction mechanisms there are several intermediates, but frequently the concentration of one of the intermediates is much larger than the concentration of all other intermediates. This intermediate is then called the most abundant reaction intermediate (MARI). 

It is possible to generalize the treatment of single-path reactions when a most abundant reaction intermediate (mari) can be assumed. According to M. Boudart and G. Djega-Mariadassou Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Princeton, 1984, p. 104) three rules can be formulated ... 

The steady-state approach generally yields complex rate expressions. A simplification is obtained by the introduction of one or several rate-determining step(s) and ( wasi-equilibrium steps, and further by the initial reaction rate approach. For complex reaction schemes, identifying the most abundant reaction intermediates ("mari") and making use of the site balance can simplify the kinetic models and rate expressions. 

MASI) or most abundant reaction intermediate (MARI) are terms to denote the (reactive) intermediate, which is present in the highest concentration on the surface. If the concentration of the MASI/MARI sufficiently exceeds that of the other surface species, simplified rate expressions can be obtained.)(13,14,23). As a result of this microscopic vuiderstanding of the macroscopic phenomena, that is, the observed catalytic activity and selectivity, a more rational design of new catalysts becomes possible, provided that relations can be established between the catalyst synthesis procedure and the surface phenomena on the catalyst (see Fig. 8). 

At low temperatures, the only products that form are N2O and N2. In situ spectroscopic studies of working Cu and Ag catalysts show that apart from adsorbed oxygen, there is a high surface coverage of nitrite and nitrate speciesl . Hence, on these metals at low temperature, N2 and N2O production is likely the result of consecutive reactions of NOj, the most abundant reaction intermediate (MARI), with NH3. N2 is formed by the reaction of nitrite with NH3, whereas N2O can also form via reaction of nitrate with ammonia (see also Section 6.4.1). 

The so-called two-step sequence method is that the derivation of reaction rate expression only requires to consider two key steps for a reaction involving multielementary steps. Only the rate constants or equilibrium constants of the two key steps appear in rate expression, which are of clear physical meaning. In order to determine the key steps, a concept of most abundant reaction intermediate (Mari) must be introduced. Mari is an intermediate of maximum concentration among all reactive intermediates invovled in the reaction, and the concentration of other intermediates can be ignored. Based on the concepts of both rate determining step and most abundant reaction intermediate, the mechanisms of many catalytic reactions can be simplified to two-step sequences for the derivation of kinetic equations. In order to explain the rules for the treatment of heterogeneous catalytic reaction kinetics by simplest two-step sequences method, two examples are given as follows ... 

In conclusion, the above two examples allow us to generalize the treatment of single-path reactions as follows. Two major simplifications are introduced as a result of the assumption of a most abundant reaction intermediate, Mari, if possible, of a rate determining step, rds. Three rules can then be formulated. 

If, under reaction conditions, one of the adsorbed species dominates on the surface and the fractional coverage of this intermediate on the catalytic sites is much greater than any other species, then it is said to be the most abundant reaction intermediate (MARI). Technically, it may not be the most abundant surface intermediate (MASI) because some adsorbed species may not be participating in the reaction sequence [2], although these two terms tend to be used interchangeably [1]. 

Use of a Rate Determining Step (RDS) and/or a Most Abundant Reaction Intermediate (MARI)... 

TDI and TDTS are new terms that only have sense in catalysis. In noncatalytic reactions, it is more accurate to speak about rate-determining intermediate (RDI) and rate-determining intermediate transition state (RDTS) (or collectively, RDStates) [37, 54, 55]. However, the concept is not new (as always), and has been called by other names. The determining intermediate is also known as the resting state or the most abundant reaction intermediate MARI) [2, 22-24, 56]. Unfortunately, the kinetic importance of the TDI has been disregarded, rarely... 

In solving the kinetics of a catalytic reaction, what is the difference between the complete solution, the steady-state approximation, and the quasi-equilibrium approximation What is the MARI (most abundant reaction intermediate species) approximation ... 

Propose a mechanism where the rate-limiting step is the recombination of adsorbed carbon C and adsorbed oxygen O and write up an equation for the rate. In the following we assume that only one adsorbate dominates the surface. The so-called MARI for the most abundant reaction intermediate. Here we assume that it is oxygen O. Is that reasonable ... 

In a case such as this one where only one species is present in appreciable concentration on the surface, that species is often referred to as the most abundant reaction intermediate, or mari. The overall rate of reaction can be expressed as the rate of dissociative adsorption of N2 ... 

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