The Importance of Adsorption in Heterogeneous Catalysis

First we must distinguish between physical adsorption (physisorption) and chemical adsorption (chemisorption). 

One might be tempted to believe that highly effective adsorbents are also good catalysts, but in reality the situation is not so simple, because catalytic reactions proceed highly specifically. Today it is known that adsorption is a necessary but not sufficient prerquisite for molecules to react with one another under die influence of a solid surface. Furthermore, it is important that a distinction be made between the amount of adsorded substance and the rate of adsorption. 

Since both types of adsorption are exothermic, raising die temperature generally decreases the equilibrium quantity of adsorbate. Physisorption is fast, and equilibrium is rapidly reached, even at low temperature. Chemisorption generally requires high activation energies. The rate of adsorption is low at low temperatures, but the process can be rapid at higher temperatures. 

The rate of both types of adsorption is strongly dependent on pressure. Chemisorption leads only to a monolayer, whereas in physisorption multilayers can form. Table 5-2 compares the two types of adsorption. 

Cause van der Waals forces, no electron transfer covalent/electrostatic forces, electron transfer 

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The potential energy diagram (Figure 2.22) demonstrates the importance of adsorption in heterogeneous catalysis, which could be related to some extent to effect by approximation in enzymatic catalysis. 

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