92168.pdf

116 The solid-gas interface

Physical adsorption^2 '^3 '^6 *"^4 and chemisorption^2 •^65 ~^68

The forces involved in the adsorption of gases and vapours by solids

may be non-specific (van der Waals) forces, similar to the forces

involved in liquefaction, or stronger specific forces, such as those

which are operative in the formation of chemical bonds. The former

are responsible for physical adsorption and the latter for chemisorption.

When adsorption takes place, the gas molecules are restricted to

two-dimensional motion. Gas adsorption processes are, therefore,

accompanied by a decrease in entropy. Since adsorption also involves

a decrease in free energy, then, from the thermodynamic relationship,

AG = Atf-rAS (5.1)

it is evident that A//ads. must be negative - i.e. the adsorption of gases
and vapours on solids is always an exothermic process.* The extent of
gas adsorption (under equilibrium conditions), therefore, increases
with decreasing temperature (see Figure 5.1). Heats of adsorption
can be measured by direct calorimetric methods. Isosteric (constant
adsorption) heats of adsorption can be calculated from reversible
adsorption isotherms using the Clausius-Clapeyron equation:

dlnp} -A//ads,

dT L RT^2

(5.2)

The heats of physical adsorption of gases are usually similar to their
heats of condensation; for example, the integral heat of physical
adsorption of nitrogen on an iron surface is c. — 10 kJ mol"^1 (the heat
of liquefaction of nitrogen is — 5.7 kJ mol"^1 ). Heats of chemisorption
are, in general, much larger; for example, the integral heat of
chemisorption of nitrogen on iron is c. -150 kJ mol"^1.
The attainment of physical adsorption equilibrium is usually rapid,
since there is no activation energy involved, and (apart from
complications introduced by capillary condensation) the process is
readily reversible. Multilayer physical adsorption is possible, and at

*This argument does not necessarily hold for processes such as adsorption from
solution and micellisation, since a certain amount of destructuring (e.g. desoivation)
may be involved and the net entropy change may be positive.