Eley-Rideal type mechanism

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. 

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... 

A hydrogen molecule from the gas phase forms an activated complex with an adsorbed D atom situated next to an empty site, the activated complex occupying two sites. This mechanism is another variation of the Eley-Rideal type, we will denote it as Eley-Rideal II, or mechanism II ... 

In order to explain the mechanism of the Fisher-Tropsch reaction, some authors derived the Langmuir-Hinshelwood or Eley-Rideal types of rate expressions for the reactant consumption, where in the majority of cases the rate-determining step is supposed to be the formation of the building block or monomer, methylene [134],... 

One reaction model that explains these results has been proposed [165]. The hydrogen atom is transferred to the ethylene molecule that is weakly adsorbed on top of the ethylidyne and in the second layer perhaps by forming an ethyl idene intermediate. This model of hydrogen transfer from hydrocarbons to ethylene was first proposed by Thomson and Webb [175]. This mechanism is of the Eley-Rideal type and is characterized by low activation energy and structure insensitivity. 

In 1949 Kulkova and Temkin [2a] proposed oxidation reduction mechanism for the first time. They proposed regenerative mechanism of the Eley-Rideal type in which water is dissociated on the surface of the catalyst to produce hydrogen. The cycle is completed by the reduction of the surface of the catalyst by the CO that produces CO2. 

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

Island Formation and Segregation. Island formation affects the kinetics because often only the adsorbates at the edges of islands can react with other molecules. (Exceptions are adsorbates that form islands and that can react with each other, and if there is an Eley-Rideal mechanism.) To form islands it seems necessary to have an attractive interaction that keeps adsorbates together. Indeed, if there is just one type of adsorbate an attractive interaction leads to island formation at low temperatures, whereas a repulsive one does not. If there is more than one type of adsorbate, then one can even have island formation if all lateral interactions are repulsive. The term segregation may be more appropriate for such a situation. 

This mechanism might be considered as a variation of a type advocated by Eley and Rideal (55). We will refer to it either as Eley-Rideal I, or as mechanism I. 

Within the frameworks of the lattice gas model it is reasonable to classify the elementary processes by the number of sites m, which a given process occurs on, i.e., one- and two-site cases. In the first case the changing parameter is the occupancy state of one site. The processes such as these include isomerization associated with changes in the internal degrees of freedom of the adspecies (ZA- ZB, i.e., transition of the adspecies from state A to state B), adsorption-desorption of the atoms and nondissociating molecules (A + Z- ZA), reaction according to the collision mechanism (A + ZB ->ZD + C, Eley-Rideal s-type mechanism). It should be remembered that ZA, Z and A denote adspecies A, empty lattice site and species A in the gaseous phase, respectively. 

This type of mechanism is referred to as an Eley-Rideal mechanism. 

It is assumed that this is also a rate determining step for the overall reaction. The activation energy of reaction (4) and the site density of oxygen active centers were the only adjustable parameters of the model. In general, a C-H bond scission for reactants and products of the methane dimerization process occurs by an Eley-Rideal (E-R) type mechanism to form a gas-phase alkyl radical and a hydroxyl surface site (HO ) ... 

Two types of mechanisms are proposed in the literature the Langmuir-Hinshelwood and the Eley-Rideal mechanisms. Takagi et al. [92] presented a study of the SCR reaction over V2O5 on Y-AI2O3. It was suggested that the reaction proceeds via the adsorbed species of NO2 and through a Langmuir-... 

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