Catalysis 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... 

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. 

In the case of photogenerated catalysis, two different but equivalent models are worth considering the Langmuir-Hinshelwood photocatalytic process and the Eley-Rideal photocatalytic process. The former is described by Mechanism II, in which the reaction occurs at a photochemically active surface when light is absorbed by the catalyst and leads to the generation of surface electrons (e ) and holes (h" ). 

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