Eley-Rideal-mechanism

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

Adsorption from the gas-phase Desorption to the gas-phase Dissociation of molecules at the surface Reactions between adsorbed molecules Reactions between gas and adsorbed molecules. 

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

However, without further evidence we cannot conclude that the above mechanism is an Eley-Rideal mechanism. The last step may be composite and consist of the following steps 

In the Eley-Rideal (E-R) mechanism, the reaction takes place between a surface-adsorbed species and a gaseous reactant. That is, in this mechanism, only one of the reactants is bound to the surface [4,5], Consequently, in the E-R mechanism, a gas phase molecule hits the adsorbed molecule and the reaction continues as follows 

The reaction rate is then a function of the fraction of the concentration of the adsorbed molecules on the surface and the pressure of the gas molecule, as follows 

FIGURE 9.7 Schematic representation of the Eley-Rideal mechanism. 

If we now suppose that the pressure of both reactants is very high, then the surface is practically totally covered, 0A 1, and the rate reduces to 

Therefore, the rate-determining step in this case is the rate at which the gas molecules, B(g), hit the catalyst surface. If we suppose that the pressure of A is low, then 1 + Kk Pk 1 and 

It is also possible that one of the reactants, say B in the above reaction, is not adsorbed. In such a mechanism (known as the Eley-Rideal mechanism), we simple use pg or [5] for B (and not Og). While the LHHW mechanism requires the adsorption of all reactants on the surface, the Eley-Rideal mechanism proceeds with one adsorbed reactant and one gas phase species. Depending on the interaction between the adsorbate and the adsorbent, one of the species may be so weakly bound to the surface that it is essentially not adsorbed. Furthermore, some of the reactions may proceed via a nonadsorbed intermediate. In addition to catalytic reaction kinetics, the Eley-Rideal mechanism is frequently encountered during the crystal growth processes. 

Reaction of gas-phase atomic hydrogen with dissociatively chemisorbed oxygen on Ru(OOl) can be taken as a model reaction for the Eley-Rideal mechanism. The Langmuir-Hinshelwood reaction between co-adsorbed hydrogen and atoms does not occur on the Ru(OOl) surface. The reason for this is simple. The activation energy for recombi-native desorption of molecular hydrogen 

Therefore the reaction occurs between the hydrogen molecules (or atoms) in the vapour phase and the adsorbed oxygen atoms. 

Weinberg reports studies with an ordered p(lX2) oxygen adatom overlayer corresponding to a fractional area of 9q = 0.5. The metal surface was bombarded with deuterium atoms with a flux 2n=1.2Xl0 cm sec D and H atoms produce identical results within experimental uncertainties. The elementary reactions taken into consideration were 

On neglecting the terms involving lh nd based on the surface coverage of water on Ru(OOl), Weinberg reported cross-section values of (Tod = 6.8X10 cm and = 2.2X 10 cm. The product water remains adsorbed at the reaction temperature and acts as an inhibitor of the direct reaction when 0 q 0.3. 

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Similar equations were written by Eley [204] for the exchange of N2 with N2 catalyzed by Fe or W, and mechanisms such as Eq. XVIII-33 have come to be known as Eley-Rideal mechanisms. Mechanisms such as that of Eq. XVIII-32 are now most commonly called Langmuir-Hinshelwood mechanisms (see... 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

Derive the probable rate law for the reaction CO + j02 = CO2 as catalyzed by a metal surface assuming (a) an Eley-Rideal mechanism and (b) a Langmuir-Hinshelwood mechanism. 

Figure A3.9.1. Schematic illustrations of (a) the Langmuir-Hinshelwood and (b) Eley-Rideal mechanisms in gas-surface dynamics.

Xi M and Bent B E 1993 Reaction of deuterium atoms with cyclohexane on Cu(111)—hydrogen abstraction reactions by Eley-Rideal mechanisms J. Phys. Chem. 97 4167... 

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. 

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). 

The MM model with one species desorption (say B) has also been studied [90]. Due to desorption, the B-poisoned state is no longer observed and the system undergoes a second-order IPT between a reactive regime and an A-poisoned state. The behavior of the MM model with one species desorption is similar to another variant of the MM model which incorporates the Eley-Rideal mechanism [57]. 

J. Mai, W. von Niessen. The influence of physisorption and the Eley-Rideal mechanism on the surface reaction C0-(-02. Chem Phys 156 63-69, 1991. 

There are few studies in the literature on the kinetics and mechanism of oxidation over base metal oxides. Blumenthal and Nobe studied the oxidation of CO over copper oxide on alumina between 122 and 164°C. They reported that the kinetics is first order with respect to CO concentration, and the activation energy is 20 kcal/mole (77). Gravelle and Teichner studied CO oxidation on nickel oxide, and found that the kinetics is also first order with respect to CO concentration (78). They suggested that the mechanism of reaction is by the Eley-Rideal mechanism... 

However, at high temperatures and lower concentrations of CO, the Eley-Rideal mechanism may become more important... 

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 Eley-Rideal mechanism for gas-solid heterogeneous catalysis envisions reaction between a molecule adsorbed on the solid surface and one that is still in the gas phase. Consider a reaction of the form... 

Whether a catalytic reaction proceeds via a Langmuir-Hinshel vood or Eley-Rideal mechanism has significant implications for the kinetic description, as in the latter case one of the reactants does not require free sites to react. However, Eley-Rideal mechanisms are extremely rare, and we will assume Langmuir-Hinshelwood behavior throughout the remainder of this book. 

What is the essential difference between the Langmuir-Hinshelwood and Eley-Rideal mechanisms Which of the two is the more likely mechanism ... 

Perhaps the most extensive computational study of the kinetics of NO reactions on Rh and Pd surfaces has been provided by the group of Zgrablich. Their initial simulations of the NO + CO reaction on Rh(lll) corroborated the fact that the formation of N-NO intermediate is necessary for molecular nitrogen production [83], They also concluded that an Eley-Rideal mechanism is necessary to sustain a steady-state catalytic regime. Further simulations based on a lattice-gas model tested the role of the formation of... 

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,... 

Fiolitakis and Hofmann—wavefront analysis supports Eley-Rideal/redox mechanisms. In 1982 and 1983, Fiolitakis and Hofmann231,232 carried out wavefront analysis to analyze the dependence of the microkinetics of the water-gas shift reaction on the oxidation state of CuO/ZnO. They observed three important mechanisms after treatment of the catalyst surface with different H20/H2 ratios. These included two Eley-Rideal mechanisms which converted the reactants via adsorbed intermediates, and a redox mechanism that regulated the oxygen activity, as shown in Scheme 56. The authors indicated that different mechanisms could be dominating at different sections along the length of the fixed bed reactor. 

To account for the exchange and isomerization of a number of poly-methylcyclopentanes, Rooney et al. (3S) postulated that intermediates corresponding to the w-allyl structures written above were not only able to abstract hydrogen from the surface as in the classical mechanism, but also could accept an atom from molecular hydrogen according to an Eley-Rideal mechanism (Fig. 26). 

In that study, it was demonstrated that kinetic-energy-enhanced N(C2H4)3N reacts with FI on a Pt(lll) surface to form an ion. The ion leaves the surface with a translational energy that depends on the energy of the incident species. In a study of FI atoms incident on D/Cu(ll ), direct evidence that HD is formed by the Eley-Rideal mechanism was found, in part, by varying the translational energy of the H reagent. 

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