Langmuir-Hinshelwood step

Studies by a number of authors (8.22-27) have shown that H2 adsorbs dissociatively on Rh and that this process is reversible at the temperatures used in the present studies. As noted earlier, the atomic hydrogen formed by this means is believed to be responsible for the formation of NH3 and H2O. Consequently, these products are assumed to be formed by a sequence of Langmuir-Hinshelwood steps. While there is no independent evidence to support this hypothesis for the synthesis of NH3, recent results reported by Thiel et al. (Z7) indicate that the formation of H2O from H2 and adsorbed 0-atoms does proceed via a two step sequence such as that represented by reactions 6 and 7 in Fig. 11. 

Before A and B can react, they must both adsorb on the catalyst surface. The next event is an elementary step that proceeds through a reaction of adsorbed intermediates and is often referred to as a Langmuir-Hinshelwood step. The rate expression for the bimolecular reaction depends on the number density of adsorbed A molecules that are adjacent to adsorbed B molecules on the catalyst surface. This case is similar to the one developed previously for the recombinative desorption of diatomic gases [reverse reaction step in Equation (5.2.20)] except that two different atomic species are present on the surface. A simplified rate expression for the bimolecular reaction is ... 

Theoretically, if reactions are able to proceed through either a Rideal-Eley step or a Langmuir-Hinshelwood step, the Langmuir-Hinshelwood route is much more preferred due to the extremely short time scale (picosecond) of a gas-surface collision. The kinetics of a Rideal-Eley step, however, can become important at extreme conditions. For example, the reactions involved during plasma processing of electronic materials... 

The strategy is to propose a reasonable sequence of steps, derive a rate expression, and then evaluate the kinetic parameters from a regression analysis of the data. As a first attempt at solution, assume both CI2 and CO adsorb (nondissociatively) on the catalyst and react to form adsorbed product in a Langmuir-Hinshelwood step. This will be called Case 1. Another possible sequence involves adsorption of CI2 (nondissociatively) followed by reaction with CO to form an adsorbed product in a Rideal-Eley step. This scenario will be called Case 2. 

The isothermal models can be divided into elementary-step models wherein interaction of the elementary steps (adsorption, desorption, and reaction) can produce oscillations without additional mechanisms, and models with additional, non-Langmuir-Hinshelwood steps, such as phase transitions and oxidation-reduction cycles. The latter models are usually supported by experimental evidence obtained by the methods discussed in Section III. 

The reactions depicted by steps (9) - (11) are again the Langmuir - Hinshelwood steps, i. e. the chemisorption of A and B which then react. Step (12) is a catalyst deactivation reaction in which a site, which has chemisorbed B (presumably oxygen), is converted to a metal oxide SB. In equation (13), the oxidized site is reduced by chemisorbed reducing agent A. It seems reasonable to assume that the first set of reactions is much faster... 

If reaction XVIII-42 is the slow step, the Langmuir-Hinshelwood rate law is... 

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

The catalytic reaction of NO and CO on single crystal substrates, under ultra-high vacuum conditions, has been extensively studied. Neglecting N2O formation and CO desorption, the Langmuir-Hinshelwood mechanism of the NO + CO reaction can be described by the following sequence of steps [16,17] ... 

The reaction scheme of the ZGB-DD model is based upon the Langmuir-Hinshelwood mechanism. Thus, it is assumed that the reaction occurs according to the following steps ... 

Writing out the catalytic reaction between A and B in elementary steps according to the Langmuir-Hinshelwood mechanism, we obtain ... 

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 ... 

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).

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 ... 

If much less than 10% of the surface is covered, then the conventional Langmuir-Hinshelwood TST equation, given by Eq. (10) and the C expression for Step 7 of Table I, applies for both one- and two-reactant cases. Step 7 is second order in the intermediate coverage range, where Eq. (34) can best be used, the reaction is first order and at high coverage the reaction is zero order even when bimolecular. For a zero-order reaction. Step 5 of Table I applies. (The assumption that the entropy of activation is zero for Step 5 is not necessarily correct when the surface reaction is bimolecular.)... 

In this rate expression we have lumped C/js into the effective surface rate coefficient by defining k" — CC s- AU sohd reactions have reaction steps similar to those in catalytic reactions, and the rate expressions we need to consider are basically Langmuir-Hinshelwood kinetics, which were considered in Chapter 7. Our use of a first-order irreversible rate expression is obviously a simplification of the more complex rate expressions that can arise from these situations. 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

The Langmuir-Hinshelwood mechanisms form an important class of reactions. These mechanisms consist of the following types of steps ... 

The last type of steps cannot occur in a Langmuir-Hinshelwood mechanism. 

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