10.2.4 Surface Reaction

The rate of adsorption of species A onto a solid surface,

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is given by

10-14

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After a reactant has been adsorbed onto the surface, i.e., A · S, it is capable of reacting in a number of ways to form the reaction product. Three of these ways are:

Surface reaction models

  1. Single site. The surface reaction may be a single-site mechanism in which only the site on which the reactant is adsorbed is involved in the reaction. For example, an adsorbed molecule of A may isomerize (or perhaps decompose) directly on the site to which it is attached, such as

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    The pentane isomerization can be written in generic form as

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    Each step in the reaction mechanism is elementary, so the surface reaction rate law is

    10-15

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    where KS is the surface reaction equilibrium constant KS = kS/k–S

    Single Site

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    Ks = (dimensionless)

  2. Dual site. The surface reaction may be a dual-site mechanism in which the adsorbed reactant interacts with another site (either unoccupied or occupied) to form the product.

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    For example, adsorbed A may react with an adjacent vacant site to yield a vacant site and a site on which the product is adsorbed, or as in the case of the dehydration of butanol, the products may adsorb on two adjacent sites.

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    For the generic reaction

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    the corresponding surface reaction rate law is

    10-16

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    Dual Site

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    Ks = (dimensionless)

    A second dual-site mechanism is the reaction between two adsorbed species, such as the reaction of CO with O.

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    For the generic reaction

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    the corresponding surface reaction rate law is

    10-17

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    A third dual-site mechanism is the reaction of two species adsorbed on different types of sites S and S′, such as the reaction of CO with O.

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    For the generic reaction

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    the corresponding surface reaction rate law is

    10-18

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    Langmuir–Hinshelwood kinetics

    Reactions involving either single- or dual-site mechanisms, which were described earlier, are sometimes referred to as following Langmuir–Hinshelwood kinetics.

  3. Eley–Rideal. A third mechanism is the reaction between an adsorbed molecule and a molecule in the gas phase, such as the reaction of propylene and benzene

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    For the generic reaction

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    the corresponding surface reaction rate law is

    10-19

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

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