Eley-Rideal -type reaction

Now, we consider a few other types of surfaces processes, but not in the same detail as dissociative chemisorption. The first is the Eley-Rideal type reaction, A(g) + B(a)-I-SAB(g) + S. This is the inverse of the exchange process mentioned above. All cases have considered systems in which exceeds the A—S and B—S interactions ... 

The kinetics of the ethylene oxidation are rather complicated as they depend not only on ethylene and oxygen pressure but also on the concentration of the reaction products. These influence the rate by adsorption competition with the reactants. Moreover, different forms of adsorbed oxygen may occur on the catalyst surface. Consequently, the rate equations proposed in the literature consist of either Langmuir—Hinshelwood and Eley—Rideal types or power rate models with non-integer coefficients. Power rate models are less appropriate as their coefficients inevitably depend on the reaction conditions. 

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],... 

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. 

A generalization of the original Eigenberger model was later analyzed by Lynch and Wanke (278). If one considers reaction (5) not as an Eley-Rideal type of reaction but as a Langmuir-Hinshelwood reaction between adsorbed species A and adsorbed species B, one must replace reaction (5) with the following two equations ... 

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... 

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... 

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. 

Noble metals may follow either a Langmuir-Hinshelwood type of mechanism (reaction between adsorbed oxygen and an adsorbed reactant) or an Eley-Rideal mechanism (reaction between adsorbed oxygen and a gas phase reactant molecule). In the case of nucleophilic organic reactants (e.g., CO or alkenes), both the oxygen and reactant are adsorbed and react on the surface. 

The problem whether the periodical operation of a technical reactor with input concentrations which change periodically provides higher selectivities and yields than stationary operating was examined with the example of benzene oxidation into malein anhydride [103], A rather complex example was the oxide-hydrogenation of isobutyric aldehyde to methacrolein [100], Based on dynamic experiments a reaction scheme is proposed and estimation of kinetic parameters of the main reaction using an Eley-Rideal type rate equation was carried out. The examples revealed that the wave-front analysis provides valuable qualitative and quantitative kinetic information of heterogeneous catalytic reactions. 

If the reaction occurs through an Eley-Rideal type of process, i.e., from the gas-phase to the transition-state, then we have, according to the classical transition state theory, an assumption of equilibrium between the gas-phase and transition state concentrations, that is,... 

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

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. 

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-... 

Both reactions are therefore seen to be kinetically of Eley-Rideal or noncompetitive Langmuir-Hinshelwood type, with a change from NH3 to O2 as the strongly adsorbed intermediate as temperature or O2/NH3 ratio increase. The active sites are most likely to be steps in the (111) surface. 

H2/D2 exchange involves type (/) species, but their exact role is still under debate. A kinetic isotope effect operates such that HD is produced — 1.5 times faster from a 2H2 D2 mixture than from a H2 2D2 mixture. Naito et al favour an Eley-Rideal mechanism at 200 K involving a gas-phase molecule and a type (/) chemisorbed atom. Kokes et a/. initially favoured a similar mechanism involving a type Hi) chemisorbed molecule instead of one in the gas phase. However, in a later paper they show that type Hi) species are only important in allotropic ortho-para) conversion and that the exchange reaction involves a Bonhoeffer-Farkas mechanism, using type (/) adatoms, at all temperatures. Richard et al., on the basis of the variation of the first-order rate constant with pressure, deduced that the mechanism must involve atomic H on surface pair sites that most probably were adjacent ion pairs thus, a Rideal type of mechanism was favoured. 

---