Langmuir-Hinshelwood reaction mechanism

The reaction mechanism of the SMR reaction strongly depends on the nature of the catalytically active metal and the support (the detailed discussion is provided in the review [14]). The kinetics and mechanism of the SMR reaction over Ni-based catalysts have been extensively studied by several research groups worldwide. For example, Xu and Froment [16] investigated the intrinsic kinetics of the reforming reaction over Ni/MgAl204 catalyst. They arrived at the reaction model based on the Langmuir-Hinshelwood reaction mechanism, which includes several reaction steps as follows ... 

Basic Mechanisms of Catalytic Reactions Langmuir Hinshelwood Reaction Mechanism... 

Sales et al. (1982) presented a simple physical model to explain oscillatory oxidation of carbon monoxide over Pt, Pd, and Ir catalysts. The model is based on a kinetic model incorporating a Langmuir-Hinshelwood reaction mechanism and the alternate oxidation and reduction of the catalyst. Simulation results of these three coupled differential equations (of oxidation of CO) model are shown to fit experimental observations. 

In this study, in order to overcome these difficult problems, a novel reaction mechanism. Microscopic Sequential Reaction mechanism (MSR mechanism), was considered. The MSR mechanism was built from the view point that the essential property of the reaction on the solid catalyst must be non-linear phenomena involving microscopically sequential reaction processes, which are different from the conventional Langmuir-Hinshelwood reaction mechanism based on linear phenomena. To realize the MSR mechanism and achieve the NO decomposition, novel metal containing zeolite catalysts were prepared and the reaction conditions were investigated. 

Bandeira and Naccache [79] proposed a duel aeid-base meehanism in which CH OH reacts on a Bronsted acid site and another methanol molecule reacts at an adjacent 02- site. Thus a bimolecular Langmuir-Hinshelwood reaction mechanism is proposed according to the following reaction steps ... 

There are two distinctly different mechanisms for a surface reaction between two species [8], for example toluene (T) and an active surface species ( ). In the Langmuir-Hinshelwood (LH) mechanism, reaction occurs between toluene emd the active surface species when both are adsorbed on the catalyst surface. If this step is the slow initiation step, the rate is proportional to the product of the coverages of toluene and the active site species ... 

The Langmuir-Hinshelwood reaction between adsorbed CO and O atoms is well established as the dominant reaction mechanism for conditions where CO is the primary surface species . This mechanism has been confirmed by numerous UHV studies of the coadsorption of the transient kinetic... 

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... 

It is surprising that complicated dynamic behaviour proved to be characteristic of the simplest and quite ordinary kinetic models of catalytic reactions, namely of the Langmuir Hinshelwood adsorption mechanism. We are possibly at the initial stage of interpreting the kinetics of complex reactions and the "Sturm und Drang period has not yet been completed. 

Zhdanov and Kasemo (2000) considered the reaction 2A + B2->2AB occurring via the standard Langmuir-Hinshelwood (LH) mechanism,... 

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. 

The validity of a Langmuir-Hinshelwood s mechanism is shown for the whole range of operating conditions. It is also proven that the behaviour of the catalyst active sites in the gaseous phase polymerization is similar either in the main reaction or in the deactivation, to the behaviour in the rest of the catalytic reactions where the mechanism has been widely established. 

There is clear evidence that adsorbate and alloyed metal atoms on platinum surface promote CO electro-oxidation. The reduced overpotential is primarily a result of the promotion of the activation of water. The subsequent kinetics are determined by the details of a Langmuir-Hinshelwood reaction between the adsorbed oxidant (OH) and adsorbed CO. Evidence is also presented that relates this promotion (or poisoning) of CO electro-oxidation to tolerate CO in hydrogen feeds in the hydrogen electro-oxidation reaction. An alternative mechanism that may operate at low potentials [79,113] may be that the reduction in CO adsorption energy on platinum induced by Ru [86,113,114] results in a higher equilibrium concentration of nonpoisoned sites. The relative importance of these mechanisms is a function... 

Very poor correlation coefficients were obtained with all Langmuir-Hinshelwood reaction test plots. Figure 4 illustrates a sample test plot for a Mars-van Krevelen (MvK) reaction mechanism, with an associative oxygen adsorption step. 

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 ... 

Postulate a Langmuir-Hinshelwood heterogeneous mechanism for the chemical reaction and develop the corresponding Hougen-Watson kinetic rate law when five-site reaction on the catalytic surface is the slowest step. 

For the description of surface reactions two well-established approaches are used, namely (i) the Eley-Rideal (ER) mechanism and (ii) the Langmuir-Hinshelwood (LH) mechanism. 

The Langmuir-Hinshelwood (LH) mechanism assumes that both A and B are adsorbed, and that the surface reaction of both adsorbed species is the ratedetermining step, which leads to the following equation for the initial reaction rate if adsorption of products is negligible ... 

The kinetics of H2 formation (and other surface reactions) via the Langmuir-Hinshelwood (diffusive) mechanism can be treated by rate equations, as in Eq. (1.52), or by stochastic methods.There are two main objections to the former approach it does not handle random-walk correctly and it fails in the limit of small numbers of reactive species. The latter objection is a far more serious one in the interstellar medium because dust particles are small, and the number of reactive atoms and radicals on their surfaces can be, on average, less than unity. Nevertheless, with rare exceptions, the few large models of interstellar chemistry that include surface processes as well as gas-phase chemistry do so via the rate equation approach, so we discuss it here. In the treatment below, we do not use the ordinary units of surface chemistry — areal concentrations or mono-layers — but instead refer to nmnbers of species on the mantle of an individual but average grain. Numbers can be converted to bulk concentrations, as used in Eq. (1.52), by multiplication by the grain number density n. ... 

The conversion of CO and O2 into CO2 in the gas phase has a free enthalpy of -283kJ/mol and is therefore thermodynamically favored [1, 2], However, in order to initiate this reaction, the activation energy for the dissociation of O2 has to be overcome, lowered e.g. by a heterogeneous catalyst. The reaction occurs on Pt (and other group VIII metals) surfaces via a Langmuir-Hinshelwood (LH) mechanism [3-8] with the following steps (Eqs.2.1-2.4, represent surface adsorption sites). 

Reaction of gas-phase atomic hydrogen with dissociatively chemisorbed oxygen on Ru(OOl) can be taken as a model reaction for the Eley-Rideal mechanism. The Langmuir-Hinshelwood reaction between co-adsorbed hydrogen and atoms does not occur on the Ru(OOl) surface. The reason for this is simple. The activation energy for recombi-native desorption of molecular hydrogen... 

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