Physical and Chemical Adsorption
Adsorption can be divided into two main categories – physical and chemical adsorption.
It is the common type of adsorption. The basic feature of physical adsorption is that the adsorbate molecules are held at the surface of the adsorbent by weak van der Waals forces. These are the forces that exist between particles of all matter. Because of their universal nature, these forces would operate between any adsorbent and adsorbate pair. Therefore, the physical adsorption is observed on surface of any solid. Only, the extent of adsorption varies according to the nature of the adsorbent and adsorbate as discussed earlier.
Physical adsorption is characterized by low enthalpy of adsorption, that is about 10 – 40 kJ mol–1.
Another feature of the physical adsorption of a gas by a solid is that it is reversible in nature and an equilibrium is established between the adsorbent and the adsorbate as discussed earlier. Increase of pressure increases the adsorption and the release of pressure desorbs the gas. When temperature is increased, the physical adsorption decreases and when it is lowered, the adsorption increases. In physical adsorption, several layers of adsorbate are adsorbed one over the other.
We have seen earlier that some unsaturated valancies exist on the surface of a solid. Whenever a chemical combination takes place between the adsorbent and the adsorbate the adsorption becomes very strong. This type of adsorption caused by forces similar to chemical bonds between the adsorbent and the adsorbate is called chemisorption or chemical adsorption.
The enthalpy of chemisorption is as high as that of chemical bonds (bond enthalpies) and is in the range of 40 – 400 kJ mol–1. Chemisorption is highly specific and is possible between a specific adsorbent – adsorbate pair. Like most of the chemical changes it is irreversible. Attempts to release the adsorbed gas gives the gas and some amount of a definite compound. For example, oxygen gas is chemisorbed on tungsten. It is released from the surface of tungsten as a mixture of oxygen and tungsten oxide. Unlike physical adsorption, chemisorption first increases and then decreases with rise in temperature [Fig. 17.4 (b)]. This shows that chemisorption has an energy of activation*. During chemisorption, only one layer of adsorbate molecules is adsorbed. The main distinctions between physical adsorption and chemisorption are summarized in Table 17.1.
Table 17.1 : Physical Adsorption and Chemisorption