Surface reactions Rideal-Eley steps

For economic reasons the oxidative dimerization of CH at about 750°C was extensively investigated. Li+/MgO was the most studied catalyst, but a wide variety of oxides are effective, none lamentably with enough selectivity to be interesting commercially. The reaction which probably involves oxidation at the surface with liberation of CH3 to the gas phase where it dimerizes is, like the Rideal-Eley step, an exception to the earlier statement about mechanism. 

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... 

For some steps the apparent activation energy is to be used in Eq. (10), and in others, the true activation energy. See text. (2) Where relevant, it is assumed that the symmetry number approximates unity it is also assumed that (Ijs) a 0.5, where s is the number of sites adjacent to a given site in a surface bimolecular reaction. (3) Both Cj, gas concentration in molecules cm", and P, gas pressure in atmospheres are used in this work. For an ideal gas, c, = 7.34 x 1q2i pij< 4 Except where otherwise noted, ft a 1. (5) An adsorption reaction is a Rideal-Eley reaction a surface reaction is a "Langrauir-Hinshelwood reaction. 

Detailed microkinetic models are available for CO, H2 and HC oxidation on noble metal(s) (NM)/y-Al203-based catalysts (cf., e.g. Chatterjee et al., 2001 Harmsen et al., 2000, 2001 Nibbelke et al., 1998). The model for CO oxidation on Pt sites includes both Langmuir-Hinshelwood and Eley-Rideal pathways (cf., e.g., Froment and Bischoff, 1990). Microkinetic description of the hydrocarbons oxidation is more complicated, particularly due to a large number of different reaction intermediates formed on the catalytic surface. Simplified mechanisms, using just one or two formal surface reaction steps,... 

The fundamental reactions occurring during gasification can be described by the Langmuir-Hinshelwood and Rideal-Eley mechanisms. The Langmuir-Hinshelwood mechanism involves three steps (1) adsorption of the gas onto the solid surface (2) surface migration/reaction and (3) desorption of the products from the solid surface. In the Rideal-Eley mechanism, the basic steps are (1) reaction between gas molecules and surface atoms by direct collision and (2) desorption of products. 

Many catalyzed surface reactions can be treated as a two-step process with an adsorption equilibrium followed by one rate-determining step (diffusion, surface reaction, or desorption). The surface reaction kinetics are usually discussed in terms of two limiting mechanisms, the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. In the LH mechanism, reaction takes place directly between species which are chemically bonded (chemisorbed) on the surface. For a bimolecular LH surface reaction. Aawith competitive chemisorption of the reactants, the rate of reaction is given by the following expression ... 

The initial reaction rates of esterification between acetic add and benzyl alcohol were measured at various conditions, and the Eley-Rideal (ER) model was used to correlate the data and showed a high degree of fit, indicating that the surface reaction between adsorbed alcohol and add in the bulk is the rate-limiting step during the initial stage of the reaction. The ER model of this esterification can be described as follow. 

In the Rideal-Eley mechanism, on the other hand, it is assumed that the chemisorption of A is fast and basically at equilibrium, while the irreversible surface reaction that occurs between adsorbed A and gaseous B is the rate-determining step (rds) ... 

The quasi-equilibrium assumption is frequently used in the heterogeneous catalysis, since the surface reaction steps are often rate-Hmiting, while the adsorption steps are rapid. This is not necessarily true for large molecules. Here we consider the application of the quasi-equilibrium hypothesis on two kinds of reaction mechanisms, an Eley-Rideal mechanism and a Langmuir-Hinshelwood mechanism. The rate expressions obtained with this approach are referred to as Langmuir-Hinshelwood-Hougen-Watson (LHHW) equations in the literature, in honor of the pioneering researchers. 

Yamashita and Vannice have investigated N2O decomposition on Mn203 in detail and, in addition, N2O chemisorption was measured at temperatures between 273 and 353 K [6]. The Mn203 had a surface area of 31.8 m g after heating at 773 K in He. Rates were determined at five different temperatures from 598 to 648 K under differential reaction conditions (conv. 0.10) as a function of both N2O and O2 partial pressures. The Mn203 retained its stoichiometry under reaction conditions, as verified by both XRD measurements on the used catalyst and a constant N2/O2 ratio of 2 in the effluent stream during reaction. A Rideal-Eley mechanism in which the O2 desorption step was replaced by the reaction N2O + O N2 + O2 + was found to be inconsistent with the kinetic behavior [6]. Consequently, the proposed reaction sequence was again the L-H model previously described by steps 7.11 - 7.13, i.e.. 

It is very difficult to connect the catalyst structure with reaction kinetics, however, and this remains a challenge for the XXI century. There are two main reactivity models the Langmuir-Hinselwood mechanism that involves reaction by interaction between two surface-bonded substrate atoms in the rate-determining step, and the Rideal-Eley mechanism that involves interaction between a surface-bonded substrate atom and a molecule in the gas phase in the rate-determining step. Most reactions have kinetics that are consistent with the first model. ... 

Closure. After reading this chapter, the reader should be able to discuss the steps in a heterogeneous reaction (adsorption, surface reaction, and desorption) and describe what is meant by a rate-limiting step. The differences between molecular adsorption and dissociated adsorption should be explained by the reader, as should the different types of surface reactions (single site, dual site, and Eley-Rideal). Given heterogeneous reaction rate data, the reader should be able to analyze the data and to develop a rale law for Langmuir-Hinshelwood kinetics. The reader should be able to discrimi-... 

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. 

Steps 1 through 9 constitute a model for heterogeneous catalysis in a fixed-bed reactor. There are many variations, particularly for Steps 4 through 6. For example, the Eley-Rideal mechanism described in Problem 10.4 envisions an adsorbed molecule reacting directly with a molecule in the gas phase. Other models contemplate a mixture of surface sites that can have different catalytic activity. For example, the platinum and the alumina used for hydrocarbon reforming may catalyze different reactions. Alternative models lead to rate expressions that differ in the details, but the functional forms for the rate expressions are usually similar. 

Eley-Rideal mechanisms If the mechanism involves a direct reaction between a gas-phase species and an adsorbed intermediate (Eley-Rideal step, reaction 8.4-5), the competition between the reactants for surface sites does not occur. From equations 8.4-6 and -21, since one reactant does not have to adsorb on a site in order to react,... 

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