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... 

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. 

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. 

Typical catalysts for SCR include supported vanadia, and iron or copper supported on zeolite. Here the application of a model to the design and understanding of vanadia catalyst systems is presented. Over the vanadia-based catalyst system, a Rideal-Eley approach has been adopted by most workers in the field, in which the first step is ammonia adsorption on the catalyst. This stored ammonia can then either react with NOx or be desorbed. Some important contributions to the SCR modelling literature are Andersson et al. (1994), Lietti and Forzatti (1994), Dumesic et al. (1996), Lietti et al. 

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. 

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. 

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) ... 

Miranda et a/. studied the kinetics of TCE oxidation using four different models Langmuir-Hinshelwood (reaction between the two adsorbed species was the rate-controlling step), Rideal-Eley (assuming that oxygen reacts from the gas phase), Mars-van Krevelen (Eq. 4.10), and a CI2 inhibition model (Eq. 4.11). The last of these resulted in the best fit with the experimental laboratory data... 

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. ... 

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. 

The question of the role of the Eley-Rideal (ER) mechanism in the reaction is usually investigated by the reaction of preadsorbed oxygen with CO from the gas phase. The well known results of Bonzel and Ku ( 6) are shown in Fig. 12. C02 is immediately produced, lending some support to the existance of an ER step. However, Matsushima et al. (26) have used Auger electron spectros-... 

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,... 

Finally, we present the results of the case studies for Eley-Rideal and LH reaction mechanisms illustrating the practical aspects (i.e. convergence, relation to classic approximations) of application of this new form of reaction rate equation. One of surprising observations here is the fact that hypergeometric series provides the good fit to the exact solution not only in the vicinity of thermodynamic equilibrium but also far from equilibrium. Unlike classical approximations, the approximation with truncated series has non-local features. For instance, our examples show that approximation with the truncated hypergeometric series may supersede the conventional rate-limiting step equations. For thermodynamic branch, we may think of the domain of applicability of reaction rate series as the domain, in which the reaction rate is relatively small. 

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