Adsorption on solid surfaces  

Importance of adsorption in solid catalysis

As discussed in the previous section, heterogeneous solid catalysis is strongly associated with adsorption phenomenon. In solid catalysis, at least one of the reactant needs to be adsorbed on the surface of the catalyst. Adsorption of a component ‘A' on surface of material ‘B' is defined as preferential accumulation of the component ‘A' on the surface of the material ‘B'. The component ‘A' is called adsorbate and surface ‘B' is called adsorbent. The surfaces include both external surface and internal surface due to the pores. For highly porous material, the internal surface area due to the pores is much higher than the external surface area. The pores in a solid material can be of different dimensions. Pores with diameter less than 2nm (20 A⁰) are called micropores, pores in the range of 2- 50 nm (20 – 500 A⁰) are called mesopores, whereas pores greater than 50 nm are called macropores. The pore size distribution of a catalyst is affected by preparation condition and amount of loading of active component. Usually a wide pore size distribution exists in a catalyst. However, catalyst can also be designed to have a very narrow pore size distribution. Fig 1 shows the schematic representation of a typical porous solid catalyst particle having both the mesopores and micropores. The active sites are dispersed throughout the porous matrix. Under suitable conditions of temperature and pressure, a gas can gradually adsorb on the solid surface and finally lead to its complete coverage.

 

 

Fig. 1. Schematic representation of typical porous support impregnated with active component