Eley-Rideal

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.

Rettner C T 1994 Reaction of an H-atom beam with Cl/Au(111)—dynamics of concurrent Eley-Rideal and Langmuir-Hinshelwood mechanisms J. Chem. Phys. 101 1529... 

Shalashilin D V, Jackson B and Persson M 1999 Eley-Rideal and hot atom reactions of H(D) atoms with D(H)-covered Cu(111) surfaces quasiclassical studies J. Chem. Phys. 110 11 038... 

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... 

Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) kinetic models... 

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... 

Experimental probes of Born-Oppenheimer breakdown under conditions where large amplitude vibrational motion can occur are now becoming available. One approach to this problem is to compare theoretical predictions and experimental observations for reactive properties that are sensitive to the Born-Oppenheimer potential energy surface. Particularly useful for this endeavor are recombinative desorption and Eley-Rideal reactions. In both cases, gas-phase reaction products may be probed by modern state-specific detection methods, providing detailed characterization of the product reaction dynamics. Theoretical predictions based on Born-Oppenheimer potential energy surfaces should be capable of reproducing experiment. Observed deviations between experiment and theory may be attributed to Born-Oppenheimer breakdown. 

An interesting body of work merits the reader s attention concerning the Eley-Rideal reaction of H-atom abstraction of chlorine atoms adsorbed on gold38-41 ... 

The energetics of the Eley-Rideal reaction (A E —230 kJ mol-1) are well established.42 Here, the highly exoergic reaction forming gas-phase HC1 was probed by time-of-flight velocity measurements,39,41 scattering angular distributions,39,41 and state-selective laser spectroscopy.39-41... 

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