Eley-Rideal reaction mechanism

Many experiments (especially electrochemical studies) are performed under wet conditions, and the presence of an electrolyte may change the overall energetics of gas phase reaction mechanisms. Of course, the ORR reaction creates water molecules as final reaction products. Eley-Rideal reaction mechanisms (which will be discussed in the next Section) also rely on the electrolyte as a source of hydrogen atoms. Although results from gas-phase calculations are sometimes used to interpret experiments performed in solution, we believe that at least some treatment of the water solvent is required to obtain relevant results. 

Stability was investigated for n = I and n = 2 in both the Langmuir-Hinshelwood and the Eley-Rideal cases. The results indicate that the Eley-Rideal reaction mechanism tends to stabilize the system whereas the Langmuir-Hinshelwood mechanism easily leads to oscillatory behavior. A serious problem with this analysis, however, is that the physical interpretation of the notion of a surface temperature in the first few layers of catalyst remains unclear. 

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-... 

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. 

The resulting data are represented in Fig. 28. The conclusion might be drawn that the Eley-Rideal I mechanism best represents the equilibration data. This is not an acceptable conclusion, since this same mechanism was definitely excluded as an explanation for the exchange reaction, while, moreover, those mechanisms that do fit the exchange data will be much faster. 

Presentation of a Reaction Mechanism. In the Na0 -CO reactions, after the reaction had achieved a steady state, the reaction gas mixture was switched into either a pure helium stream or a 02-He stream. The responses of C02 and CO were then followed. The CO(dec. , 0)-CO response obtained instantaneously responded zero with no delay, indicating that there was no reversibly adsorbed CO. Furthermore, the C0(dec.,0)-C02 response obtained was not affected by the presence of 02 in the stream, suggesting the nonexistence of irreversible adsorption of CO which could react with oxygen. Thus, a model of the direct reaction of gaseous CO with adsorbed oxygen, an Eley-Rideal type mechanism, may be proposed. 

The surface chemical reaction seems to follow the Eley-Rideal (ER) mechanism, in which the adsorption of propylene is predominant over benzene, as represented by the equation ... 

For the case of supported model catalyst under surface science conditions an Eley-Rideal mechanism has so far not been unambiguously identified. However, for gas-phase clusters, an Eley-Rideal type mechanism, in which the reactant has to collide at the right position with the cluster-complex to initiate the reaction, is more likely to proceed due to the reduced size of the catalytically active unsupported cluster. 

Bernasek and Somorjai proposed that, as the temperature increased and the surface concentration of the D2(ads) species decreased, an Eley— Rideal-type mechanism became important, involving direct reaction of an incident gas phase D2 molecule with H atoms present at the stepped active sites, having (Id ) as the r.d.p. and obeying the rate expression... 

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 ... 

For the description of surface reactions two well-established approaches are used, namely (i) the Eley-Rideal (ER) mechanism and (ii) the Langmuir-Hinshelwood (LH) 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. 

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... 

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. 

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... 

The same reaction mechanism operated on the surfaces of both kinds of catalyst on metals and their hydrides as well. The reaction proceeded according to the Rideal-Eley mechanism and was of first order with respect to the atomic hydrogen concentration in the gas phase. 

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. 

In the Rideal-Eley (RE) alternative mechanism, reaction occurs between an adsorbed active species and a gas phase molecule (T). The rate is now proportional to ... 

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