Chemical Reactions on Solid Surfaces - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 416

TABLE 9.1 Characteristics of Physical and Chemical Adsorption

Physical adsorption Chemisorption

Low heat of adsorption. Usually around 5 kcal/mol, but
can be as high as 20 kcal/more. Always exothermic.

High heat of adsorption. Usually higher than 20 kcal/
mol, but can be less, even endothermic is possible.

Not specific. Highly specific.

Adsorption is fast with zero activation energy.
Desorption is activation energy equal to heat of
adsorption.

Adsorption is slow with activation energy, but could
be fast with aero activation energy.
Desorption activation energy is at least as high as
adsorption heat.

Monolayer or multilayer. No dissociation of adsorbed
species. Only significant at temperature close to
condensation temperature of the adsorbate, however,
Kelvin effect can occur for some porous adsorbents.

Monolayer only, but can appear to be multilayer
(in isotherms) due to interaction energy distribution
nonuniformity. Adsorbate may involve dissociation.
Possible over a wide range of temperature.

No effect on the adsorbent electric conductivity. May have an effect on the adsorbent electric

conductivity.

No electron transfer although polarization of adsorbate
may occur.

Electron transfer leading to bond formation between
adsorbate and surface.

a whole. One might say that physical adsorption does things a bit differently from chemi-
sorption, as is shown in Table 9.1 . The analysis of physical adsorption can be complicated,
even more so than for chemisorption, because attractive e repulsive interactions are involved
that may be more complex than direct chemisorptive interactions. Nevertheless, for adsorp-
tion to occur, the adsorbate molecule has to “collide” with the center where adsorption is to
take place. Therefore, Eqns (9.1a) and (9.2) apply equally well to physisorptions.
We need to examine a particular model for physical adsorption that has been extremely
useful over the years in the characterization of the porous materials that are often employed
as catalyst support structures or as catalysts in their own right. The basic question is how to
measure the internal (pore) surface area of a material that is perhaps 50% void and has an
average pore diameter on the order of 5 nm. We want to know this surface area because it
is presumably important in determination of the overall rate of reaction per unit volume
of catalyst in a given application.
The analysis of physical adsorption in general and that used to approach this particular
problem, derives from a classification later summarized by Brunauer (S. Brunauer, The Adsorp-
tion of Gases and Vapors , Princeton University Press, Princeton, NJ, 1945). He classified the
isotherms for physical adsorption of gases on surfaces into five general types, as shown in
Fig. 9.12 . Isotherms I andVare already recognizable. Isotherm I is a typical Langmuir isotherm,
while isotherm V is a typical bi-layer adsorption isotherm. Isotherms II e IV show the various
complexities of physical adsorption. Early work showed that that isotherm II was typical for
adsorption of nitrogen (at liquid nitrogen temperatures) on a large number of porous adsor-
bents. The result of that observation led to the derivation of an appropriate analytical theory
to describe this type of adsorption and to use it for physical characterization, in terms of
internal surface area of the adsorbent (S. Brunauer, P.H. Emmett and E.J. Teller, J. Amer.
Chem. Soc. , 60, 309, 1938). From the last initials of the authors comes the name “BET theory.”