Most abundant reaction intermediate

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 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 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 may be tempting to speculate that the most abundant reaction intermediate should be either the product of the first reaction step or one of the reactants for the rate limiting step. This is not the case. 

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 ... 

This expression is consistent with the experimental observations. For this example, the reaction equilibrium represented by step 3 is never used to solve the problem since the most abundant reaction intermediate is assumed to be H (accounted for in the equilibrated step 2.) Thus, a complex set of elementary steps is reduced to two kinetically significant reactions. 

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 ... 

A microkinetic analysis of ammonia synthesis over transition metals is presented in Section 7.3. Use the results of that analysis to explain how adsorbed nitrogen atoms (N ) can be the most abundant reaction intermediate on iron catalysts even though dissociative chemisorption of N2 is considered the rate-determining step. 

Derive a rate expression for the hydrogenation of ethylene on Pt assuming steps 1, 2, and 3 are quasi-equilibrated, step 4 is virtually irreversible, and C2H5 is the most abundant reaction intermediate covering almost the entire surface ([ ]o [ C2H5]). Discuss why the rate expression cannot properly account for the experimentally observed half order dependence in H2 and zero-order dependence in ethylene. Could the observed reaction orders be explained if adsorbed ethylene ( C2H4 ) were the most abundant reaction intermediate Explain your answer. 

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). 

Sabatier-type volcano plots have been constructed for a number of different commercially relevant systemsl l. A simple kinetic expression that simulates the Sabatier result is found when one realizes that the decomposition of molecules requires a vacant site for molecular fragments to adsorb on. For instance, in the N2O decomposition reaction, the dominant surface species (most abundant reaction intermediate) is atomic oxygen (O), which is in equilibrium with the gas phase. When the slow step in the reaction is dissociative adsorption of N2O, the mean-field kinetic rate expression for N2O decomposition, normalized per unit surface area of catalyst, becomes ... 

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 ... 

For the sake of simplicity, the reaction will be assumed to proceed far from equilibrium so that the reverse rate can be neglected. According to general equilibrium method, it must find out the expression of all reaction intermediates, and then the expression of reaction rate can be derived. Obviously, the processes are very complicated. If assuming that there both exist rate determining step and the most abundant reaction intermediate, reaction sequence can be reduced to two-step sequence. There are three cases as follows ... 

Dissociated adsorption of N2 is the rds with NH as the most abundant reaction intermediate. The sequence considered is ... 

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. 

The overall reaction is A = B with the species B being the most abundant reaction intermediate, and A requires n adjacent surface atoms in the rate-determining adsorption step. The rate is then ... 

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]. 

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