Langmuir-Hinshelwood -type mechanism

The electrooxidation of CO to C02 is, similar to its electroless counterpart in gas-phase catalysis, one of the most widely studied electrochemical reaction processes [131,152]. It is generally assumed that the electrooxidation of adsorbed CO proceeds primarily via a Langmuir-Hinshelwood type mechanism involving either adsorbed water molecules or, at higher electrode potentials, adsorbed surface hydroxides (see Figure 6.25) according to... 

The EOTR between the organic compound (R) and the hydroxyl radicals take place at the electrode surface (both adsorbed) according to a Langmuir-Hinshelwood type mechanism. This process has been extensively studied mainly for fuel cell applications. However, as it has. been reported in Sect. 1.2, it is limited for simple Ci organic compounds (methanol, formic acid). Furthermore, there are problems with electrode deactivation due to CO chemisorption on the electrode active sites. 

The kinetics and the mechanism of the gas phase acetoxylation of ethylene on palladium catalyst has been the subject of many studies. These studies are based on a Langmuir-Hinshelwood type mechanism in which all reacting species are chemisorbed on the Pd surface and reaction occurs between chemisorbed species. Although a complete description is still pending a commonly accepted proposal is shown in Figure 30. 

In summary, the use of isotopic tracers has greatly increased our understanding of the WGS reaction. The data permit the elimination of some mechanisms, such as the Temkin oxidation-reduction mechanism, widely accepted at one time. Likewise, a Rideal-Eley-type and a Langmuir-Hinshelwood-type mechanism are not consistent with a stoichiometric number of 2. However, while the data generated to date permit the elimination of some mechanisms, one is still left with a number of possibilities. The current view that the rate-controlling step probably changes as the reaction approaches equilibrium suggests that the situation is much more complicated than was the view of the mechanism a few years ago. 

The most important characteristic of this problem is that the Hougen-Watson kinetic model contains molar densities of more than one reactive species. A similar problem arises if 5 mPappl Hw = 2CaCb because it is necessary to relate the molar densities of reactants A and B via stoichiometry and the mass balance with diffusion and chemical reaction. When adsorption terms appear in the denominator of the rate law, one must use stoichiometry and the mass balance to relate molar densities of reactants and products to the molar density of key reactant A. The actual form of the Hougen-Watson model depends on details of the Langmuir-Hinshelwood-type mechanism and the rate-limiting step. For example, consider the following mechanism ... 

Rideal type mechanism where adsorbed ammonia reacts with NOx from the gas phase and that a Langmuir-Hinshelwood type mechanism seems to be of minor importance. 

If the rate of both single reactions is expressed separately, e.g. by means of the equations of Langmuir-Hinshelwood type (when written in a more general way and if the mechanism of both reactions with common reagent X will be the same), we obtain... 

Ihe HVDg exchange reaction. This reaction vras studied vd.th both sur-faces in the wide tenperature range from 100 K to 600 K details are ocnrnunicated in a separate article (25). The H./D- exchange is known to be of the Langmuir-Hinshelwood type and proceed via the following mechanism ... 

Langmuir-Hinshelwood mechanism, 19 78 Langmuir-Hinshelwood type rate expressions, 25 192... 

A Langmuir—Hinshelwood type of mechanism is also supported by the work of Klugherz and Harriott [178] and Spath and Handel [300], These authors, and several others, advocate 02 (and O2-) as the active oxygen species. Carberry et al. [74] and Forzatti et al. [118] contributed to the evidence for Oj with their results on the effect of promotors. 7-Irradiation of a calcium-promoted silver catalyst enhanced the yield of ethylene oxide. It could be shown that, during irradiation, calcium migrated to the surface, increasing the 02 concentration there. 

Kinetic redox models, as formulated by Mars and van Krevelen [204], have not been considered in any recent work. Although the combined dependence on both propene and oxygen pressures does arise in certain investigations, the authors seem to ignore redox mechanisms completely and correlate their data with Langmuir—Hinshelwood type models. 

It is obvious (see Figure 9.19) that the catalyst is stabilized by water addition to the feed. It is well known that the mechanism of formation of diethyl ether is of the Eley-Rydeal type [148] (see Section 9.6), and the mechanism of formation of ethylene is of the Langmuir-Hinshelwood type [145], Figure 9.20 shows the fitting of the experimental data obtained during the kinetic study... 

Bonn6 [1] studied reaction kinetics of tetraphenylporphyrin model compounds over several catalysts and concluded that hydrogenation and hydrogenolysis occur on different sites. Therefore the reaction mechanism of VO-TPP hydrodemetallisation is described by a two-site model. It was also found that a Langmuir-Hinshelwood type of kinetics applies with small inhibition by hydrogen. 

Two reaction mechanisms were proposed [602] a) the catalyst acts as a simple adsorbent on which the reactants react on the surface (Langmuir-Hinshelwood type) b) the active catalyst sites are successively oxidized and reduced by the adsorbed reactants (Rideal-Eley type). Evaluation of the vast amount of experimental material leads to the conclusion that the HT shift (and also the LT shift) proceeds via mechanism (b), which schematically may be formulated as shown in Figure 62. 

The kinetic model is an application for polymerization reactions, of the deduction of kinetic equations of deactivation of Langmuir Hinshelwood type, equations that fit the real data in a large number of reactions (7, 8). Deactivation mechanism 1. Adsorption and activation step (Initiation)... 

Analysis of such statistical correlations may reveal the significant variables and interactions and may suggest potential mechanisms and kinetic models, say, of the Langmuir-Hinshelwood type, that could be analyzed in greater detail by a regression process. The variables x could be various parameters of heterogeneous processes as well as concentrations. An application of this method to isomerization of n-pentane is given by Kittrel and Erjavec [Ind. Eng. Chem. Proc. Des. Dev. 7 321 (1968)]. 

From adsorption measurements during reaction one can also examine whether the reaction rate is correlated with the amounts of adsorbed reactants or with the pressure of the reactants, i.e., whether the mechanism is of Langmuir-Hinshelwood type or of the Eley-Rideal variety. 

Fig. 32. Data for CO oxidation on Rh, showing the maximum in CO2 production as a function of time which is characteristic of a Langmuir-Hinshelwood type of mechanism. From Bowkcr...

The effect of partial pressure of hydrogen and concentration of p-isobutyl acetophenone on the initial rate of reaction is shown in Fig. (3) at different temperatures. It can be observed that the reaction is first order tending to zero order with respect to partial pressure of hydrogen and close to first order with respect to p-isobutyl acetophenone concentration. This clearly indicates that the reaction proceeds through a Langmuir-Hinshelwood type of mechanism. Moreover, from the typical concentration profile shown in Fig. (2) it can be concluded that the adsorption of the liquid phase components may not be important as the formation of different liquid phase... 

Capsule membrane PTC systems are more amenable to a mechanistic analysis than typical triphase systems where the mechanism of interaction between the aqueous and organic phases with the catalytic sites is complex and not understood. A mechanism for capsule membrane PTC involving mass transfer and smface reaction for both PTC and IPTC reactions has been developed by Yadav and Mehta(1993), Yadav and Mistry (1995). A Langmuir-Hinshelwood type model with the anchored quaternary-nucleophile complex as the active site was assumed to govern the overall rate of reaction... 

The mechanism of the hydrogenation reaction for hydrogen supplied by the gas phase could be described in terms of a Langmuir-Hinshelwood type of kinetic equation, assuming a fast dissociative adsorption of hydrogen and cyclohexene on two different sites. Following Wallace et al. (1979) and Johnson et al. (1992) it was assumed that these... 

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