Heterogeneous catalytic theory  

In general, it is believed that the entire surface of the solid catalyst is not responsible for catalyzing any reaction. Only certain sites on the catalyst surface actually participate in the reaction and these sites are called active sites on the catalysts. These sites may be the unsaturated atoms resulting from surface irregularities or atoms with chemical properties that enable the interaction with the adsorbed reactant atoms or molecules. Activity of the catalyst is directly proportional to the number of these active sites available on the surface and is often expressed in term s of turnover frequency. Turnover frequency is defined as the number of molecules reacting per active site per second at the condition of experiments.

A solid catalytic reaction A → B goes through the following steps. The steps are illustrated in Fig 3.

1. 1.  Transport of reactant (A) from bulk fluid to pore mouth on the external surface of catalysts pellets

2. Diffusion of the reactant (A) from the pore mouth through the catalyst pores to the immediate vicinity of internal catalytic surface

3.  Adsorption of reactant (A) onto the catalyst surface

4.  Reaction of (A) on the catalyst surface producing product (B)

5.  Desorption of the product (B) from the surface

6.  Diffusion of the product (B) from interior part of the pores to the pore mouth on the external surface

7.  Transfer of the product (B) from pore mouth on the external surface to the bulk fluid

 

 

Fig. 3. Steps in solid catalytic reactions.