Based on the nature of interactions between adsorbates and adsorbents, adsorption can be classified into chemical adsorption and physical adsorption.

Chemical Adsorption

Chemical adsorption refers to the adsorption process accompanied by chemical reactions.During chemical adsorption, adsorbate molecules react with the surface of adsorbents to form surface complexes, and the adsorption heat is close to the heat of chemical reactions.

A certain amount of activation energy is required for chemical adsorption to occur. Under normal conditions, both the adsorption and desorption rates of chemical adsorption are slower than those of physical adsorption.Examples: Limestone-chlorine gas adsorption, zeolite-ethylene adsorption.

Physical Adsorption

Also known as van der Waals adsorption, physical adsorption is caused by the intermolecular forces (van der Waals forces) between adsorbate molecules and the surface molecules of adsorbents. Different from internal molecules of a solid, the surface molecules have a residual surface free force field. When gas molecules collide with the solid surface, some of them are adsorbed and release adsorption heat.

Among the adsorbed molecules, only those whose thermal kinetic energy is sufficient to overcome the potential energy of the adsorbent’s gravitational field can return to the gas phase. Therefore, a large number of adsorbed molecules are always retained on the solid surface in contact with the gas.Adsorption caused by intermolecular forces features relatively low adsorption heat, which is close to the vaporization heat or condensation heat of the adsorbate. Both adsorption and desorption proceed at a fast rate, and the adsorbed gas can be desorbed from the solid surface relatively easily, making physical adsorption a reversible process. The pressure swing adsorption (PSA) process is mainly based on physical adsorption.

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Comparison Between Physical Adsorption and Chemical Adsorption