Langmuir-Hinshelwood reaction

The monomer-monomer (MM) model, for the reaction A -H B —> AB, assumes the following Langmuir-Hinshelwood reaction schema ... 

Figure 15.4 Schematic representation of the Langmuir-Hinshelwood reaction between two adsorbates mobile X (black) and immobile Y (white) on a stepped single-crystalline surface (a) and a facetted nanoparticle (b).

A Langmuir-Hinshelwood reaction rate model for the reaction between an adsorbed nitric oxide molecule and one adjacently adsorbed hydrogen molecule is described by ... 

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 ... 

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... 

Petukhov, A.V. (1997) Effect of molecular mobility on kinetics of an electrochemical Langmuir-Hinshelwood reaction. Chem. Phys. Lett., 277, 539. 

For a Langmuir-Hinshelwood reaction in which both reactants are thermally equilibrated on the surface, reaction is initiated by thermal activation of the adsorbate. This thermal sampling can be turned to advantage by invoking detailed balance to equate the rates of adsorption and desorption for a surface at equilibrium [66, 67]. This allows us to relate the final state distributions measured for desorption to the detailed sticking probability for each product quantum state (v, J). This approach has been applied very successfully to hydrogen adsorption/desorption [4, 68] but its use has not been widely explored for reactions of heavier molecules. 

Dissociation is often a crucial step in a catalytic reaction chain. However this is not always the case. Metals active for the O2 + H2 reaction are the noble metals of group 8-10, especially Pt, a metal with a low O-metal bond strength. The same metals are active for the CO + O2 reaction. The most active metals do not easily break the C-O bond and do not form oxides under reaction conditions. Both reactions are Langmuir-Hinshelwood reactions between adsorbed O and adsorbed H or CO. Desorption of products of the reaction intermediates here is reaction rate controlling. 

Active catalysts for the NO + CO reaction must be more active in bond breaking NO must dissociate, whereas the C-O bond must stay intact. Dissociation is followed by a Langmuir-Hinshelwood reaction of O with adsorbed CO. 

A generalization of the original Eigenberger model was later analyzed by Lynch and Wanke (278). If one considers reaction (5) not as an Eley-Rideal type of reaction but as a Langmuir-Hinshelwood reaction between adsorbed species A and adsorbed species B, one must replace reaction (5) with the following two equations ... 

Aluko. M., and H.-C. Chang. Multi-scale analysis of exotic dynamics in surface catalyzed reactions-II. Quantitative parameter space analysis of an extended Langmuir-Hinshelwood reaction scheme, Chem. Eng. ScL, 39, 51-64 (1984). 

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... 

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

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. 

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. 

Ivanov et al. (1980) modelled a class of Langmuir-Hinshelwood reactions, and by analyzing the mathematical model given in two dimensions, they obtained limit cycle oscillations and showed that the influence of adsorbed species on the catalytic reaction rate may lead to periodic oscillations. 

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. 

Consider a Langmuir-Hinshelwood reaction of the following type ... 

In the treatment above it was assumed that two adsorbed species react forming a product, the so-called case of Langmuir-Hinshelwood reaction (Figure 3.10). 

The platinum catalyzed CO oxidation of carbon monoxide was studied using an advanced transient reactor system, referred to as Multitrack. The experiments indicate, that the reaction is taking place according to the Langmuir-Hinshelwood reaction model. The desorption of CO from the catalyst surface was shown to be a kinetically relevant step in the reaction. From experiments performed using it was shown that isotopic mixing occurs on the catalyst surface due to the decomposition of CO present on the catalyst surface. 

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