Heterogeneously Eley-Rideal mechanism

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... 

Direct reaction between an adsorbed species A(s) and a gas-phase molecule B is sometimes proposed. This reaction pathway is called the Eley-Rideal mechanism. Although such a mechanism may seem as reasonable as the Langmuir-Hinshelwood model discussed above, very few heterogeneous reactions are still thought to occur by the Eley-Rideal mechanism. (An exception seems to be when species B is a very reactive radical species, e.g., a gas-phase H-atom reacting with an adsorbed species, as is discussed in Problem 11.10, in which an Eley-Rideal pathway initiates the growth process.)... 

Several examples of analysis of simple kinetic schemes that are linear in respect to catalytic intermediates but whose initial or final reaction groups include several "external" reactants are considered following. Obviously, according to such schemes, some of the catalytic intermediates are interacting directly with the external reactants. In this situation, the catalytic heterogeneous reaction is usually said to follow the Eley-Rideal mechanism. 

C0(ads)+0(ads)— C02(ads)+, 4.C02(ads) — C02(gas)+. The probabilities of steps 1 and 2 are between 0 and 1, while probabilities of other steps are P(3) = 1, P 4) = 1, P(-l)= 0 P(-2)= 0, P(-4)=0. The ZGB-model shows the effect of heterogeneity in the adlayer because of the infinitely fast formation of C02, there is a segregation of the reactants in CO and oxygen islands. The original model has later been extended and modified by numerous people to include desorption of the reactants, diffusion, an Eley Rideal mechanism for the oxidation step, physisorption of the reactants, lateral interactions, an oxidation step with a finite rate constant, surface reconstruction and additional poisoning adsorbates. 

Figure4.11.5 (Pre)selection ofthe appropriate kinetic equation influence of total pressure and partial pressure of one reactant on initial rate of a heterogeneously catalyzed bimolecular gas reaction of A and B (a) Langmuir-Hinshelwood mechanism for variation of total pressure p (b) Eley-Rideal mechanism for variation ofp (c) Langmuir-Hinshelwood for variation of partial pressure of one ofthe adsorbed reactants (e.g., Pa) (d) Eley-Rideal mechanism for variation of partial pressure of adsorbed reactantA (p ) (e) Eley-Rideal mechanism for variation of partial pressure of reactant B (not adsorbed).

The quasi-equilibrium assumption is frequently used in the heterogeneous catalysis, since the surface reaction steps are often rate-Hmiting, while the adsorption steps are rapid. This is not necessarily true for large molecules. Here we consider the application of the quasi-equilibrium hypothesis on two kinds of reaction mechanisms, an Eley-Rideal mechanism and a Langmuir-Hinshelwood mechanism. The rate expressions obtained with this approach are referred to as Langmuir-Hinshelwood-Hougen-Watson (LHHW) equations in the literature, in honor of the pioneering researchers. 

As an example, let us consider a heterogeneous catalytic reaction A2 + 2B = 2AB following the Eley-Rideal mechanism ... 

The Eley-Rideal heterogeneous photochemical process may be observed when the catalyst is photoexcited and no preadsorption of M occurs. This is described by Mechanism III. 

The heterogeneously catalyzed reaction of starting materials A and B can follow different mechanisms [Claus 1996, Ertl 1990]. The models of Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen are widely used in practice. 

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