Types of Adsorption

Adsorption of gases on solids falls mainly into two types, distinguished by the kind of force holding the gas.

• Physical adsorption (physisorption): the gas is held by weak van der Waals' forces.
• Chemical adsorption (chemisorption): the gas is bound to the surface by actual chemical bonds (covalent or ionic). Because it needs a high energy of activation, it is also called activated adsorption.

The two can overlap, and physisorption at low temperature may convert to chemisorption as temperature rises. For example, dihydrogen first sticks to nickel by van der Waals' forces; the $H_2$ molecules then dissociate into H atoms that are chemisorbed.

Characteristics of physisorption

• Lack of specificity: van der Waals' forces are universal, so a surface shows no preference for any particular gas.
• Nature of the adsorbate: more easily liquefiable gases (higher critical temperature) are adsorbed more readily. Thus 1 g of activated charcoal adsorbs more $SO_2$ (critical temp. 630 K) than $CH_4$ (190 K), which in turn exceeds $H_2$ (33 K).
• Reversible: the process $Solid + Gas \rightleftharpoons Gas/Solid + Heat$ shifts with conditions — higher pressure increases adsorption, and being exothermic it is favoured at low temperature (Le Chatelier's principle).
• Surface area: greater surface area gives greater adsorption; finely divided and porous solids are best.
• Enthalpy of adsorption: low, about 20–40 kJ mol⁻¹, reflecting the weak forces.

Characteristics of chemisorption

• High specificity: occurs only where chemical bonding is possible (e.g. $O_2$ on metals via oxide formation, $H_2$ on transition metals via hydride formation).
• Irreversibility: it forms a surface compound, so it is largely irreversible; being activated, it is slow at low temperature and usually increases with rising temperature, and high pressure favours it.
• Surface area: like physisorption, it increases with surface area.
• Enthalpy of adsorption: high, about 80–240 kJ mol⁻¹, because chemical bonds form.

Comparison of physisorption and chemisorption