Eley-Rideal kinetic expression

The rate of the DeNO, reaction is first order in respect to NO concentration and essentially independent of NH3 concentration when ammonia is in excess. However, in SCR industrial applications a substoichiometric feed ratio (a = NH3/NO < 1) is employed in order to minimize the slip of unconverted ammonia and the formation of ammonium sulfates. A kinetic dependence on ammonia is apparent when NH3 becomes the limiting reactant. Several authors have proposed kinetic expressions for the SCR reaction that account for the observed dependences [31-37]. In particular, the simplest expressions are based on Eley-Rideal kinetics in line with the mechanistic studies, they assume that the reaction occurs between strongly adsorbed ammonia and gas-phase NO. Beckman and Hegedus [36] have proposed and fitted to experimental data obtained over commercial SCR catalysts the following kinetic expression ... 

Previously, for 2-methyl pentane cracking we have used Eley-Rideal kinetic rate expressions to describe the inhibition and poisoning effects of species in the feed, as well as intermediate and product species. In order to utilise such kinetic expressions values for the adsorption equilibrium constants are required. A method for estimating adsorption equilibrium constants has been proposed that uses an integrated form of van t Hoff equation. The heats of adsorption have been calculated using proton affinities and heats of condensation. The entropy of adsorption has been calculated using the Sackur-Tetrode expression. 

Modified Kinetics (MB)—A modified Eley-Rideal rate expression... 

Mechanistic kinetic expressions are often used to represent the rate data obtained in laboratory studies, and to explain quantitatively the effects observed in the field. Several types of mechanisms have been proposed. These differ primarily in complexity, and on whether the mechanism assumes that one compound that is adsorbed on the catalyst surface reacts with the other compound in the gas phase, eg, the Eley-Rideal mechanism (23) or that both compounds are adsorbed on the catalyst surface before they react, eg, the Langmuir-Hinshelwood mechanism (25). 

Jakdetchai and Nakajima/Wang and coworkers—theoretical models favor redox mechanism. Beginning in 2002, a number of theoretical models were published in Theochem studying the water-gas shift reaction over Cu(110), Cu(lll), and Cu(100) surfaces. Perhaps the first was by Jakdetchai and Nakajima,325 relying on the AMI method. The main goal of the study was (1) to determine whether or not theoretical calculations are consistent with a redox or associative (e.g., formate) mechanism and (2) whether the kinetics are described best by a Langmuir-Hinshel-wood expression or an Eley-Rideal expression. That is, in the case of a redox model, does the adsorbed O adatom react with adsorbed CO or directly with gas phase CO Their approximate A//a[Pg.205]

In spite of the redox mechanism discussed above, the following Eley-Rideal (ER) kinetic expression has been proposed in the literature ... 

Deviations from this simple expression have been attributed to mechanistic complexity For example, detailed kinetic studies have evaluated the relative importance of the Langmuir-Hinshelwood mechanism in which the reaction is proposed to occur entirely on the surface with adsorbed species and the Eley-Rideal route in which the reaction proceeds via collision of a dissolved reactant with surface-bound intermediates 5 . Such kinetic descriptions allow for the delineation of the nature of the adsorption sites. For example, trichloroethylene is thought to adsorb at Ti sites by a pi interaction, whereas dichloroacetaldehyde, an intermediate proposed in the photo-catalyzed decomposition of trichloroethylene, has been suggested to be dissociatively chemisorbed by attachment of the alpha-hydrogen to a surface site... 

To derive the corresponding kinetic expressions for a bimolecular-unimolecular reversible reaction proceeding via an Eley-Rideal mechanism (adsorbed A reacts with gaseous or physically adsorbed B), the term K Pt should be omitted from the adsorption term. When the surface reaction controls the rate the adsorption term is not squared and the term KgKg is omitted. 

To be able to quantitatively describe and predict the aforementioned phenomena and to be able to relate catalyst properties to unit operation performance, a more detailed description of the species involved as well as a better representation of the fundamental processes that are occurring between the bulk fluid and the catalyst surface than that which is currently employed in pseudo-component, lumped parameter, power law models is required. This more fundamental approach to kinetic modelling has been achieved in many other systems where there are only a few components and reactions by using Langmuir-Hinshelwood and/or Eley-Rideal type rate expressions such expressions are usually developed by considering the... 

In Eley-Rideal and Langmuir-Hinshelwood type of kinetic rate expressions, the effect of poisons and inhibitors on the reaction rate is accounted for by allowing part of the catalyst surface to become covered with the poisoning compound and so unavailable for desirable reactions. For example, consider the decomposition reaction of A —> B + C that occurs in the presence of an inhibitor, I. If it is assumed that the inhibitor does not participate in the reaction but that it does occupy active catalyst sites and if the surface decomposition is rate controlling then the observed rate of reaction in Langmuir-Hinshelwood terms is given by [13]... 

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 choice to work with a macrokinetic model (i.e., an empirical equation) is made to simplify the kinetic parameters to be regressed. Because the WGS reaction has been discovered, many kinetic expressions have been developed and used. In general, either the Langmuir—Hinshelwood or the Eley—Rideal models have been used. In Table 1.1, most of the macrokinetic models are reported as an extract of a review of Smith, Loganathan, Shantha (2010). 

As previously reported, an Eley-Rideal mechanism is generally accepted for the standard SCR reaction, which implies the reaction between adsorbed NH3 and gas-phase NO accordingly, a kinetic expression, which is in agreement with the observed dependencies of ammonia, NO, oxygen and water on the rate of reaction, is given by... 

The MR rate expression Eq. (10.34) differs from the Eley-Rideal rate Eq. (10.18) only in its denominator, which accounts both for the adverse kinetic effect of NH3 and for the favorable O2 dependence at low ammonia coverage (as, e.g., at high temperature), the denominator tends to unity and Eq. (10.34) formally reduces to Eq. (10.18). Indeed, this is consistent with the experimental indications discussed above the ammonia inhibiting effect is particularly evident at low temperatures, but tends to disappear in the runs performed at temperatures of 250 °C and above, where ammonia coverage becomes lower. Likewise, the oxygen dependence is reportedly most manifest at low temperatures. 

Metkar et al. [9] developed a global kinetic model from a derivation of detailed reaction steps. They used reactions described in (12.11-12.14 [11]) and combined two reactions (see Eqs. 12.15-12.16 [11]). They received the best results when assuming the Eley-Rideal step, during the NO oxidation (Eq. 12.14) as rate determining. The resulting rate expression was ... 

This mechanism can be converted to a kinetic scheme to describe the steady state or transient performances of the catalyst. Assuming (i) an Eley-Rideal mechanism (reaction between adsorbed NH3 and gas-phase NO), (ii) that ammonia and water compete for adsorption onto the active sites, and (iii) that adsorption equilibrium is established for both species, the following Rideal rate expression is eventually obtained [6] ... 

Competitive adsorption plays no role in the Eley-Rideal mechanism, but it is a key feature in Langmuir-Hinshelwood kinetics. In (6.59) the rate of reaction reaches its maximum when the surface species A and B are ideally mixed, and both surface concentrations are equal to one-half. Realize that the assumption of ideal mixing in the adsorbate layer is implicitly present in (6.61). The expression for the overall reaction rate becomes... 

Altiokka et al. (2003) obtained the kinetics data on the esterification of acetic acid with isobutanol from both homogeneously (autocatalyzed) and heterogeneously catalyzed reactions using dioxane as a solvent in a stirred batch reactor. The uncatalyzed reaction was found to be second-order reversible. In the presence of the catalyst, on the other hand, the reaction was found to occur between an adsorbed alcohol molecule and a molecule of acid in the bulk fluid (Eley-Rideal model). It was also observed that the initial reaction rate decreased with alcohol and water concentrations and linearly increased with that of acid. The temperature dependency of the constants appearing in the rate expression was also determined. 

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. 

At low temperature, the reaction rate can be expressed by a Rideal-Eley mechanism where dissociatively adsorbed oxygen is assumed to be in equilibrium with gas-phase O2 and to react with gaseous CH4. The kinetic rate expression is given by Arai et al. (1986)... 

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