Types of Adsorption

Depending on the nature of the forces holding the adsorbate to the adsorbent, adsorption is classified into two distinct kinds: physical adsorption (physisorption) and chemical adsorption (chemisorption).

In chemisorption, gas molecules are held to the surface by the formation of genuine chemical bonds between adsorbate and adsorbent. Because a real bond is being formed, chemisorption releases a substantial amount of heat — nearly 400 kJ/mole — as its heat of adsorption. Typical examples include the adsorption of $\text{O}_2$ on tungsten, $\text{H}_2$ on nickel, and ethyl alcohol vapour on nickel; in each case the adsorbed species is chemically bonded to specific surface atoms rather than merely resting on top of them.

In physisorption, the adsorbate is instead held by comparatively weak physical forces — van der Waals forces of attraction, dipole–dipole interactions, and dispersion forces. Because these intermolecular forces are much weaker than a chemical bond, the heat of adsorption released is correspondingly low, and physisorption occurs preferentially at LOW temperatures, where the adsorbate's thermal kinetic energy is not enough to overcome the weak attractive forces. Typical examples include the adsorption of $\text{N}_2$ on mica and the adsorption of various gases on charcoal.

The two types differ across almost every measurable property, summarised in Table 10.1: chemisorption is slow and highly specific to the particular adsorbent–adsorbate pair, whereas physisorption is essentially instantaneous and non-specific. Chemisorption's rate increases quickly with pressure but the total AMOUNT adsorbed cannot be increased much this way (since it is limited by the fixed number of chemical bonding sites), while in physisorption the extent of adsorption genuinely increases with increasing pressure, since more adsorbate molecules can simply pile up in additional physical layers. With temperature, chemisorption shows a characteristic rise-then-fall pattern — first increasing as heat supplies the activation energy needed to form the adsorbent–adsorbate bond, then decreasing at higher temperature as desorption takes over — while physisorption decreases monotonically as temperature rises, since heat simply supplies the adsorbate molecules enough kinetic energy to escape the weak physical attraction. Chemisorption necessarily involves a genuine transfer of electrons between adsorbent and adsorbate (that is what a chemical bond is), whereas physisorption involves no electron transfer at all. The heat of adsorption reflects this directly: chemisorption releases 40–400 kJ/mole, roughly an order of magnitude more than physisorption's typical 40 kJ/mole. Structurally, chemisorption is confined to a single monolayer of adsorbate — once every bonding site on the surface is occupied, no further chemisorption can occur there — while physisorption can build up a multilayer, since van der Waals attraction can act even between adsorbate molecules already sitting on the surface and fresh molecules arriving from the gas phase. Chemisorption occurs only at …