2. Importance of adsorption in solid catalysis
• As discussed in the previous section, heterogeneous solid
catalysis is strongly associated with adsorption phenomenon.
• In solid catalysis, at least one of the reactant needs to be
adsorbed on the surface of the catalyst.
• Adsorption of a component ‘A’ on surface of material ‘B’ is
defined as preferential accumulation of the component ‘A’ on
the surface of the material ‘B’.
• The component ‘A’ is called adsorbate and surface ‘B’ is
called adsorbent.
• The surfaces include both external surface and internal
surface due to the pores.
3. Importance of adsorption in solid catalysis
• For highly porous material, the internal surface area due to
the pores is much higher than the external surface area.
• The pores in a solid material can be of different
dimensions. Pores with diameter less than 2nm (20 A0) are
called micropores, pores in the range of 2- 50 nm (20 –
500 A0) are called mesopores, whereas pores greater than
50 nm are called macropores.
• The pore size distribution of a catalyst is affected by
preparation condition and amount of loading of active
component.
• Usually a wide pore size distribution exists in a catalyst.
4. Importance of adsorption in solid catalysis
• However, catalyst can also be designed to have a very
narrow pore size distribution.
• Fig 1 shows the schematic representation of a typical
porous solid catalyst particle having both the
mesopores and micropores.
• The active sites are dispersed throughout the porous
matrix.
• Under suitable conditions of temperature and
pressure, a gas can gradually adsorb on the solid
surface and finally lead to its complete coverage.
6. Why adsorption takes place on solid surface
• In the bulk of the adsorbent, the molecules are associated
with their neighbours equally in all direction and the
molecular forces are therefore balanced.
• However on the adsorbent surface, the molecules are
bounded to the inner molecules at one side leaving
unbalanced molecular forces on the other side.
• These unbalanced molecular forces on adsorbent surfaces
create the attractive force for the adsorbate molecules
approaching the surface.
• These molecular forces are weak in nature and called van
der Waals attraction forces.
7. Types of adsorption
• Depending on the nature of interaction, the adsorption can be of two
types:
1. Physisorption
2. Chemisorption
• The phenomenon of adsorbate molecules attaching themselves to
adsorbent surface under the influence of van der Waals forces is called
physisorption or physical adsorption.
• The van der Waals forces mainly consist of dipole –dipole interactions.
• This is an exothermic process with a low enthalpy change known as
heat of adsorption.
• This process resembles liquefaction and heat of adsorption for
physisorption is also known as heat of liquefaction.
8. Types of adsorption
• At higher temperature, the adsorbed molecules can undergo
electronic rearrangement with the surface molecules.
• This phenomenon is called chemisorption.
• The electronic rearrangement may include formation and breaking
of chemical bonds.
• The electronic rearrangement occurs only when there is significant
interaction between adsorbate and the adsorbent molecules.
• Hence all adsorbate will not be chemisorbed on all adsorbent
surfaces.
• Chemisorption process is selective and an adsorbate molecule will
chemisorbed only on selected adsorbent.
• The adsorption processes are shown in Fig 2.
9. Types of adsorption
• The Fig. 2(a) depicts the situation when the adsorbate molecule
approach any adsorbent surface under the influence of attractive
forces created by the unbalanced molecular forces on adsorbent
surfaces.
• The Fig. 2(b) represents the phenomenon, when any molecule is
physisorbed on surface by van der Waals forces.
• No bond formation occurs in this situation. A chemisorption
situation is represented in Fig. 2(c) when there is a weak bond
formation between adsorbate and adsorbent molecule.
• As discussed above, the adsorbate molecule will be
chemisorbed only on selected adsorbent surface with which it
can interact significantly.
12. Physisorption versus Chemisorption
1. Since physisorption involves only weak
molecular interaction, the associated enthalpy
changes are small (in the range of 10 to 40
kJ /mol) . On the other hand, in case of
chemisorption, enthalpy change is quite large
and can range from 80-400 kJ /mol.
13. Physisorption versus Chemisorption
2. The Fig. 3 compares the volume of gas adsorbed as a function of
temperature under physisorbed and chemisorbed conditions.In
physisorption, the molecules are adsorbed on surface by weak
interaction. With increase in temperature, adsorbed molecules gain
excess energy and their tendency to escape from the surface
increases. Hence volume of gas adsorbed on the surface decreases
with temperature as shown in Fig. 3. However, the chemisorption
involves higher interaction energy between adsorbate and adsorbent
molecules and hence is favored by temperature rise. Hence at low
temperature range volume of adsorbed gas increases with
temperature. However, at higher temperature range as the adsorbed
molecules gains excess energy, rate of desorption becomes higher
resulting in decrease in adsorbed gas volume as shown in Fig. 3.
15. Physisorption versus Chemisorption
3. In case of chemisorption, since there is electronic interaction
between adsorbate and adsorbent molecules, only a monolayer of
adsorbate can be formed on the adsorbent surface. In case of
physisorption, the first monolayer formed on the surface of the
adsorbent can act as adsorbing surface for formation of next layer
of adsorbate and so on. This phenomenon is called multilayer
adsorption. The formation of monolayer and multilayers of the
adsorbed molecules on a surface is shown in Fig. 4. For the
physisorption, volume of gas adsorbed increases with pressure due
to increase in concentration of adsorbate and formation of
multilayers. However for chemisorption process which corresponds
to monolayer formation, the effect of pressure is not significant.
17. Physisorption versus Chemisorption
4. Chemisorption is specific for adsorbate and adsorbent
pair. Specific solid adsorbent can undergo electronic
interaction only with specific adsorbate gas molecule.
5. Physisorption is highly reversible while chemisorption
can be irreversible.
6. Physisorption is important for estimating the total
surface area. It also provides a basis for estimating the
pore volume and pore size distributions. On the other
hand, chemisorption is important in estimation of
area of catalytic active sites as well as its dispersion.
18. Examples
• Physisorption : Adsorption of nitrogen on
carbon or alumina.
• Chemisorption : Adsorption of hydrogen on
active platinum sites on any support.
19. Potential energy diagram of approaching
molecule towards a solid surface
• The potential energy variation of a molecular system as it
approaches a solid surface can be depicted by the potential
energy diagram, where the potential energy is plotted as a
function of distance of the approaching molecule from
adsorbent surface.
• When the molecule approaches the surface, at first it becomes
attracted by a weak attractive force resulting in relatively flat
potential minimum corresponding to non – dissociative physical
adsorption.
• Then depending on extent of interaction it can be carried to
non-dissociative chemisorbed state and finally to stable
dissociated state.
20. Potential energy diagram of approaching
molecule towards a solid surface
• When the extent of interaction is less, the adsorbate
molecules are only physically adsorb on the adsorbent
surface or may occur in non-dissociative chemisorbed state.
• If the interaction is only van der Waals type then the
adsorbates will be in physisorbed state.
• In case of stronger electronic interaction the process may be
directly carried on to dissociative chemisorption.
• If the crossing points are below the line of zero potential
energy as shown in Fig. 5, then the overall process is non-
activated. If they are above, the overall process requires
activation.
22. Book References
• J.J. Carberry , Chemical and catalytic reaction Engineering,
Dover Publications, 2001
• J. M. Thomas & W. J. Thomas, Principles and Practice of
Heterogeneous Catalysis, VCH, 1997
• J. M. Smith , Chemical Engineering Kinetics, McGraw-Hill
Book Company, 1981
• R. J. Farrauto & C. H. Bartholomew, Fundamentals of
Industrial catalytic Processes, Blackie Academic &
Professional, 1997
• D.M. Ruthven, Principle of adsorption & adsorption
processes, John Wiley & sons, 1984.
Editor's Notes
#5:Fig. 1. Schematic representation of typical porous support impregnated with active component
#10:Fig. 2. Schematic representation of different adsorption processes
#11:Fig. 2. Schematic representation of different adsorption processes CONT’D
#14:Fig. 3. Volume of gas adsorbed as a function of temperature for physisorption and chemisorption processes
#16:Fig. 4 . Monolayer and multilayer formation of the adsorbed molecules on a surface
#21:Fig. 5. Potential energy diagram for non activated dissociative chemisorption
