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32 2. Adsorption, Ion Exchange, and Catalysis

Table 2.1

Physical versus chemical sorption
Chemisorption Physisorption
Temperature range over Virtually unlimited; however, Near or belo w the
which adsorption occurs a given molecule may be condensation point of the gas
effectively adsorbed only (e.g. CO 2 200 K)
over a small range
y Adsorption enthalp Wide range, related to the Related to factors lik e
chemical bond strength— molecular mass and polarity
typically 40–800 kJ/mol but typically 5–40 kJ/mol
(i.e. heat of liquef action)
Nature of adsorption Often dissociative and may be Nondissociative and reversible
irreversible
e Saturation uptak Limited to one monolayer Multilayer uptake is possible
Kinetics of adsorption Very variable; often is an Fast, because it is a
activated process nonactivated process

When the species of the adsorbate trael between the atoms, ions, or the molecules of
v
the adsorbent, the phenomenon of “absorption” takes place and this discriminates absorp-
tion from the main phenomenon of adsorption that takes place on the interf ace.
The adsorption of various substances from solids is due to the increased free surf ace
ace.
energy of the solids due to their ee surfAccording to the second law of ther-
xtensi
v
modynamics, this energy has to be reduced. This is achieed by reducing the surface ten- v
sion via the capture of extrinsic substances.
Consider a molecule above a surface with the distance from the surface being normal to
ace.
the surfThere are two competitie types of influence occuring: (a) repulsion between
v
the cloud of electrons in the atoms that form the surface and those of the molecule and (b)
van der Waals nuclear attraction force. The nuclear attraction has a much shorter radius of
influence and as a result of the balance of these two forces, there is a “well” in the poten-
tial energy curve at a short distance from the surf as shown in Figure 2.1. Molecules
ace,
or atoms that reach this “well” are trapped or “adsorbed” by this potential energy “well”
and cannot escape, unless they obtain enough kinetic energy to be desorbed.
The surface can be characterized either as external when it involves bulges or cavities with
width greater than the depth, or as internal when it involves pores and cavities that have depth
greater than the width (Gregg and Sing, 1967). All surf y aces are not really smooth and the
exhibit valleys and peaks at a microscopic level. These areas are sensitive to force fields. In
these areas, the atoms of the solid can attract atoms or molecules from a fluid nearby.
The most important property of adsorbent materials, the property that is decisi v e for the
adsorbent’s usage, is the pore structure. The total number of pores, their shape, and size
determine the adsorption capacity and een the dynamic adsorption rate of the material.
v
Generally, pores are divided into macro-, meso- and micropores. According to IUP A C,
able 2.2. wn in pores are classified as shoT
Porosity is a property of solids that is attributed to their structure and is evident by the pres-
ence of pores between internal supermolecular structures (Tager, 1978). It is not considered