Adsorption of Gases on Solids
The extent of adsorption of a gas on a solid surface is affected by the following factors:
1) Nature of the gas
2) Nature of adsorbent
3) Specific area of the adsorbent
4) Effect of temperature
5) Effect of pressure
6) Activation of adsorbent.
Nature of the gas (or adsorbate)
The adsorption depends upon the nature of the gas adsorbed. The physical adsorption is non-specific in nature and therefore, every gas gets adsorbed on the surface of any solid to a lesser or a greater extent. However, under given conditions of temperature and pressure, the easily liquifiable gases such as CO2, HCl, NH3, Cl2 are adsorbed more than the permanent gases such as H2, N2 and O2.
The ease of liquefication of a gas depends upon its critical temperature (Tc).The critical temperature of a gas is the minimum temperature above which a gas cannot be liquefied however, high the pressure may be applied.
The higher the critical temperature, the more easily a gas is liquefied and hence more readily it will be adsorbed.
Nature of adsorbent
The extent of adsorption of a gas depends upon the nature of adsorbent. Activated charcoal can adsorb gases which are easily liquefied. Many poisonous gases are adsorbed by charcoal. Therefore, it is used in gas masks for adsorbing these poisonous gases. Gases are generally adsorbed on finely divided transition metals e.g. Ni and Co.
Specific area of the adsorbent
The specific area of the adsorbent is the surface area available for adsorption per gram of the adsorbent. The larger the surface area of the solid, the greater would be its adsorbing capacity. Therefore, the porous and finely divided forms of adsorbents adsorb large quantities of adsorbate. However, the pores of the adsorbent should be large enough to allow the gas molecules to enter them.
Effect of temperature
The process of adsorption is an exothermic process and is invariable accompanied by evolution of heat. Adsorption is a process involving a true equilibrium. The two opposing processes involved are condensation (i.e., adsorption) of the gas molecules on the surface of the adsorbent solid and evaporation (i.e., desorption) of the gas adsorbed molecules from the solid surface into gaseous phase. Since the process of condensation is exothermic, the reverse process of evaporation is endothermic.
The equilibrium may be expressed as:
Gas (adsorbate) +Solid (adsorbent) ⇔ Gas adsorbed on solid + Heat
According to Le-Chatelier’s principle, the increase in temperature will favour the reverse process of desorption. Therefore, adsorption will decrease with increase in temperature and adsorption will be favoured by decrease in temperature.
Effect of Pressure
At a constant temperature the adsorption of a gas increases with increase in pressure. Adsorption of a gas leads to decrease in pressure and therefore, according to Le-Chatelier’s principle, the magnitude of adsorption increases with increase in pressure. The variation of adsorption with pressure at different constant temperatures.
Decrease of temperature and increase of pressure both tend to increase the extent of adsorption of a gas on a solid.
Activation of adsorbent
Activation of adsorbent means the increasing of the adsorbing power of the adsorbent. It is very necessary to increase the rate of adsorption.
This can be done by the following methods :
(1) Metallic adsorbents are activated by mechanical rubbing or by subjecting them to some chemical reactions or by depositing finely dispersed metals on the surface of adsorbent by electroplating.
(ii) To increase the adsorbing power of adsorbents, they are sub-divided into smaller pieces. As a result, the surface area increases and therefore, the adsorbing power increases.
(iii) Some adsorbents are activated by strong heating in contact with superheated steam or in vacuum at a temperature of 623-1273 K so that gases already adsorbed are removed.