Adsorption Langmuir-Hinshelwood-type mechanism

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 ... 

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)... 

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. 

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... 

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... 

Amadeo and Laborde [51] analyzed five kinetic expressions for low-temperature catalysts two representing redox mechanism and three representing Langmuir-Hinshelwood model. Model I is proposed by Shchibrya et al. [2b]. Model II was a redox type model. Models III and IV were Langmuir-Hinshelwood type models that considered adsorption of four species (CO, H2O, CO2 and H2) and the final model only considered the adsorption of CO and CO2. The results of these authors indicated that only model HI described the reaction behaviour in the conditions investigated. 

Ref. 205). The two mechanisms may sometimes be distinguished on the basis of the expected rate law (see Section XVni-8) one or the other may be ruled out if unreasonable adsorption entropies are implied (see Ref. 206). Molecular beam studies, which can determine the residence time of an adsorbed species, have permitted an experimental decision as to which type of mechanism applies (Langmuir-Hinshelwood in the case of CO + O2 on Pt(lll)—note Problem XVIII-26) [207,208]. 

The model is intrinsically irreversible. It is assumed that both dissociation of the dimer and reaction between a pair of adjacent species of different type are instantaneous. The ZGB model basically retains the adsorption-desorption selectivity rules of the Langmuir-Hinshelwood mechanism, it has no energy parameters, and the only independent parameter is Fa. Obviously, these crude assumptions imply that, for example, diffusion of adsorbed species is neglected, desorption of the reactants is not considered, lateral interactions are ignored, adsorbate-induced reconstructions of the surface are not considered, etc. Efforts to overcome these shortcomings will be briefly discussed below. 

There are only a few recent publications. Anshits et al. [29,30] have carried out adsorption studies with various Cu—O phases and determined kinetics at low pressure in a static system. One of their conclusions is that the kinetics of partial and complete oxidation are very different. The mechanism of the latter is supposed to be of the associative type, contrary to the redox mechanism of the partial oxidation. A kinetic study with a continuously stirred vessel (375—400°C, 1 atm) was carried out by Laksh-manan and Rouleau [185]. In contrast to the redox mechanism, a singlesite Langmuir—Hinshelwood model is proposed, for which the k values and activation energies are determined. 

The preceding treatment is, undoubtedly, an oversimplification. For example, many diatomic molecules dissociate upon adsorption (e.g., H2, SiH, GeH). Each atom from the dissociated molecule then occupies its own distinct surface site and this naturally changes the rate law expression. When these types of details are accounted for, the Langmuir-Hinshelwood mechanism has been very successful at explaining the growth rates of a number of thin-film chemical vapor deposition (CVD) processes. However, more important, our treatment served to illustrate how crystal growth from the vapor phase can be related to macroscopic observables namely, the partial pressures of the reacting species. 

The simplest mechanism for interpreting critical phenomena in heterogeneous catalysis is the Langmuir adsorption mechanism, also referred to as the Langmuir-Hinshelwood mechanism. This mechanism includes three elementary steps (1) adsorption of one type of gas molecule on a catalyst active site (2) adsorption of a different type of gas molecule on another active site (3) reaction between these two adsorbed species. For the oxidation of carbon monoxide on platinum, this mechanism can be written as follows ... 

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