Most abundant surface intermediate

Boudart (1972) introduced the assumption of the most abundant surface intermediate (masi). This assumption suggests that the sites occupied by all species except the most abundant surface intermediate is regarded as negligible compared to those filled by the most abundant intermediate and to those which are empty. 

From Equation 3-343, the concentration of the most abundant surface intermediate is... 

The term reaction centre may be used to include both vacant and occupied catalytic sites. The sum of the surface concentrations of reaction centres on the surface of a catalyst is a constant L. Thus, if species m at a surface concentration Lm is the most abundant surface intermediate, Lm + Lv a L, where Lv is the surface concentration of vacant reaction centres. 

The fraction of the platinum surface that is free of adsorbed species, 0+) is equal to 0.084 under these conditions. The most abundant surface intermediates are adsorbed II and C4II indicating that the only significant kinetic parameters in the expression for a, are Kjeq and q. It is clear from these values of zt that steps 2-4 are quasi-equilibrated (z,- 1 or At 0) and step 1 is rate determining (zi ztot or A 4totai) under these reaction conditions. 

The concept of a predominant cycle member was first introduced in heterogeneous catalysis by Boudart [42], who coined the term "most abundant surface intermediate," abbreviated masi (see Section 8.10). 

The concept of a most abundant catalyst-containing species (macs in this book) was originally introduced by Boudart for heterogeneous catalysis under the name of masi, for "most abundant surface intermediate" [42]. 

In the above we have deliberately stayed close to Vannice s model, and changed only the nature of the rate-determining step from H-assisted C-O bond breaking (15) into (irreversible) hydrogenation of surface carbon (28). Consequently, the overall rate equation (30) differs from Eq. (17) only in that it contains the term 0c instead of Ochoh- When assuming further that Cads instead of CHOHajs is the most abundant surface intermediate, the model can be made formally identical to that of Vannice, including the explanations it offers for the volcano plot and the compensation effect. 

The formate, formed by oxidative dehydrogenation of the acid, is quite stable and doesn t decompose until 480 K. This decomposition is a classical first-order case with a decomposition activation energy of 130 kJ mol-1 and a normal value pre-exponential of 1013 s-1. The great ability of the TPD technique is the separation of the individual steps in the reaction in temperature. It is clear that the step proceeding over the highest barrier in this case is the formate decomposition, and that in a catalytic oxidation of formic acid the most abundant surface intermediate is likely to be the formate with its decomposition being rate determining. 

In our discussion of the influence of structure on the turnover rate our understanding is frequently hampered by lack of information on the ratedetermining step and the most abundant surface intermediate. It would be logical to consider the structure sensitivity of the rate of an elementary step, such as the desorption of a chemisorbed gas. Results on temperature programmed desorption as a function of particle size might be simpler to interpret than those of global reactions consisting of a sequence of steps. However, few such data are available. 

A concept of a most abundant catalyst-containing species macs) or most abundant surface intermediate (masi) is often used, which could reduce the complex mutistep reaction to two-step sequence. 

For simplification it can be supposed that adsorbed A is the most abundant surface intermediate, hence the surface coverage of species other then A can be neglected. The changes in coke coverage as a function of time-on-stream are given by... 

As explained earlier so many patents were filed on the Co-Mo catalysts for WGS reaction by so many researchers. After several years Hakkarainen and Salmi [9] performed transient kinetics over non-sulphided commercial Co-M0/AI2O3 catalyst and published in open Uterature. The experiments show that the commercial catalyst acts in the WGS reaction mainly as an oxide catalyst, if sulphur is absent in the feed. The most abundant surface intermediates are adsorbed water and its cleavage products as well as CO2 formed on the surface during the shift reaction. The formation rate of H2 was always higher than the formation rate of CO2 during the transient period of the kinetic experiments. The proposed reaction mechanism is as follows ... 

The Most Abundant Surface Intermediate (MASI) Approximation Catalytic transformations may include the formation of many intermediates on the catalyst surface, which are difficult to identify. In these cases, it is impossible to formulate a kinetic model based on all elementary steps. Often, one of the intermediates adsorbs much more strongly in comparison to the other surface species, thus occupying nearly all active sites. This intermediate is called the most abundant surface intermediate masi [24]. For a simple monomolecular reaction, Aj A2, the situation can be illustrated with the following scheme ... 

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]. 

For many real reactions, a mechanistically rigorous L-H formulation can become quite complex due to the involvement of several surface intermediates in the elementary steps. The resulting kinetics can be so complex as to lose much of their utility. Many such cases can be usefully treated based on the two-step kinetic model proposed by Boudart (1972). Two simplifying assumptions for the model are that (1) one step is the rate-determining step, and (2) one surface intermediate is dominant and thus all the other intermediates are present in relatively insignificant amounts. Under the model, it is not necessary to assume details of the overall mechanism. For a given reaction, the formulation depends on the assumptions made as to the rate-determining step and the most abundant surface intermediate. 

The D-isotope effect is identical to the thermodynamic isotope effect if the most abundant surface intermediate is N or NH [693]. This is consistent with the dissociative mechanisms where H is not involved in the rate-limiting step. 

The surface coverages by reaction intermediates could also be calculated by the model. It was found that N- is by far the most abundant surface intermediate under typical synthesis conditions, but the sum of the coverages by other intermediates is also larger than the coverage by free sites. This fact is seen by the authors to be the cause for the complicated kinetics deduced by Ozaki et al. [56], rather than adsorbate-adsorbate interaction or surface heterogeneity. 

Catalysis by transition metals can often be represented by reaction mechanisms, which correspond to the case of sin e catalytic cycles. Only one route typically exists, containing several steps. If the reaction mechanism can be simplified to only one intermediate besides the free catalyst form (the most abundant surface intermediate or catalytic species), while other intermediates, even if present, are in inferior quantities, then the mechanism corresponds to the two-step sequence described in detail in Section 4.3. As an example, oxidation with hydrogen peroxide can be considered (Fig. 5.10). The catalyst can exist in two forms with different oxidation states and the catalytic cycle consists efiectively of oxidation-reduction steps. The reaction rate in such a case is descrihed hy the kinetic equation (4.88). 

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