Adsorption Adsorption versus absorption, Desorption Types of adsorption: Physisorption and Chemisorption Factors affecting adsorption Adsorption isotherms: Freundlich and Langmuir Gibbs adsorption isotherm Bet equation and its use in surface area determination Applications ADSORPTION Adsorption is the process in which matter is extracted from one phase and concentrated at the surface of a second phase. (Interface accumulation). This is a surface phenomenon as opposed to absorption where matter changes solution phase, e.g. gas transfer. This is demonstrated in the following schematic.

1. Adsorption
BY- SANCHIT DHANKHAR

2. Contents
 Adsorption
 Adsorption versus absorption, Desorption
 Types of adsorption: Physisorption and Chemisorption
 Factors affecting adsorption
 Adsorption isotherms: Freundlich and Langmuir
 Gibbs adsorption isotherm
 Bet equation and its use in surface area determination
 Applications
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3. ADSORPTION
Adsorption is the process in which matter is extracted from one phase
and concentrated at the surface of a second phase. (Interface
accumulation). This is a surface phenomenon as opposed to absorption
where matter changes solution phase, e.g. gas transfer. This is
demonstrated in the following schematic.
3

4. d
4

5. Adsorption
The phenomenon of higher concentration of any molecular
species at the surface than in the bulk
5

6. Adsorbent
The substance on the surface of which adsorption takes place is called
adsorbent
Adsorbate
The substance which is being adsorbed on the surface of another
substance.
6

7. • Activated Carbon
• Activated Alumina
• Silica Gel
• Molecular Sieves (Zeolites)
• Polar and Non-polar adsorbents
Adsorbent Materials
7

8. • Made from nutshells, wood, and petroleum, bituminous coal by
heating in the absence of oxygen to dehydrate and carbonize (remove
volatile components),
• "Activation" is the process that produces the porous structure essential
for effective adsorption by oxidation of carbon with water vapor or
CO2.
• Activated carbon attracts non-polar molecules such as hydrocarbons.
• Typical surface areas are 300 to 1500 m2/g.
Activated carbon
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9. Adsorption vs Absorption
9

10. Types of Adsorption
Positive adsorption occurs when the concentration of adsorbate is higher on
the surface of adsorbent than in the bulk.
Negative adsorption occurs when the concentration of adsorbate is less on
the surface of adsorbent than in the bulk.
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11. Types of Adsorption
S. No. Physical adsorption Chemical adsorption
1 Caused by intermolecular van der
Waals' forces
Caused by chemical bond formation
2 It is not specific It is highly specific
3 It is reversible It is irreversible
4 Heat of adsorption is low (20-40 kJ/mol) High heat of adsorption (80-240 kJ/mol)
5 Low temperature is favourable Increases with high temperature
6 Results multilayer adsorption Results unimolecular layer
Comparison between physisorption and chemisorption
1. Physical adsorption
2. Chemical adsorption
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12. Factors Affecting Adsorption
Effect of adsorbate: The easily liquifiable gases like NH3, HCl,
CO2 etc. are adsorbed to a greater extent than the permanent
gases such as H2 ,O2, N2, etc.
Effect of specific area of the absorbent: The greater the
specific area of the solid, the greater would be its adsorbing
capacity.
Effect of temperature:adsorption decreases with increase in
temperature.
Effect of pressure: An increase in pressure causes an increase
in the magnitude of adsorption of an adsorbent.
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13. Adsorption On Solid Surface
 Five types of physisorption isotherms are found over all solids
 Type I is found for porous materials with small pores e.g.
charcoal. It is clearly Langmuir monolayer type, but the other
are not
 Type II for non-porous materials
 Type III porous materials with cohesive force between adsorbate
molecules greater than the adhesive force between
adsorbate molecules and adsorbent
 Type IV staged adsorption (first monolayer then build up of
additional layers)
 Type V porous materials with cohesive force between adsorbate
molecules and adsorbent being greater than that between
adsorbate molecules
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I
II
III
IV
V
relative pres. P/P0
1.0
amount
adsorbed

14. 1/n
x
K p
m
 
Over a narrow range of p
Freundlich Isotherm
A graph between the amount (x/m) adsorbed by an
adsorbent and the equilibrium pressure of the adsorbate at
constant temperature is called adsorption isotherm
At low pressure the graph is nearly straight line
At high pressure x/m becomes
independent of p
0
x
p
m

1
x
p
m

14

15. Freundlich Isotherm
15

16. Rate of adsorption a a
r =k p(1- f)
Rate of desorption d d
r =k f
At equilibrium, ra = rd; a
a d
k p
f = .........(i)
k p+k
Mono-layer coverage '
a
x
=k f .........(2)
m
m: mass of adsorbate adsorbed per
unit mass of adsorbent
f: fraction of surface area covered
f
1-f
p: partial pressure of the adsorbate
Langmuir Adsorption Isotherm
16

17. Langmuir adsorption isotherm:
ap
x /m
1 bp


The values of constants ‘a’ and ‘b’ depend upon
the nature of adsorbate, nature of solid
adsorbent and temperature.
a = ka x ka’/kd
b = ka/kd
Combining equations (1) and (2):
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18. Summary of adsorption isotherms
Easy to fit
adsorption data
Chemisorptions and
physisorption
Freundlich
Useful in analysis of
reaction mechanism
Chemisorption and
physisorption
Langmuir
Note
Application
Isotherm
equation
Name
1/n
x
=Kp
m
x ap
=
m 1+bp
18

19. Adsorption Isobar
Graph between the amount adsorbed(x/m) and temperature at a
constant equilibrium pressure of adsorbate gas is known as
adsorption isobar
Chemisorption isobar shows an initial increase with temperature and
then expected decrease .The initial increase is because of the fact that
the heat supplied acts as activation energy required in chemisorption.
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20. Adsorption On Solid Surface
 BET (Brunauer-Emmett-Teller) isotherm
 Many physical adsorption isotherms were found, such as the types II and III, that
the adsorption does not complete the first layer (monolayer) before it continues
to stack on the subsequent layer (thus the S-shape of types II and III isotherms)
 Basic assumptions
 the same assumptions as that of Langmuir but allow multi-layer adsorption
 the heat of ads. of additional layer equals to the latent heat of condensation
 based on the rate of adsorption=the rate of desorption for each layer of ads.
the following BET equation was derived
Where P - equilibrium pressure
P0 - saturate vapour pressure of the adsorbed gas at the temperature
P/P0 is called relative pressure
V - volume of adsorbed gas per kg adsorbent
Vm - volume of monolayer adsorbed gas per kg adsorbent
c - constant associated with adsorption heat and condensation heat
Note: for many adsorption systems c=exp[(H1-HL)/RT], where H1 is adsorption heat of 1st
layer & HL is liquefaction heat, so that the adsorption heat can be determined from
constant c. 20
)
(
1
1
1
0
0
0
P
/
P
cV
c
cV
)
P
/
P
(
V
P
/
P
m
m





21. Adsorption On Solid Surface
 Comment on the BET isotherm
 BET equation fits reasonably well all known adsorption isotherms observed so far
(types I to V) for various types of solid, although there is fundamental defect in
the theory because of the assumptions made (no interaction between adsorbed
molecules, surface homogeneity and liquefaction heat for all subsequent layers
being equal).
 BET isotherm, as well as all other isotherms, gives accurate account of
adsorption isotherm only within restricted pressure range. At very low
(P/P0<0.05) and high relative pressure (P/P0>0.35) it becomes less applicable.
 The most significant contribution of BET isotherm to the surface science is that
the theory provided the first applicable means of accurate determination of the
surface area of a solid (since in 1945).
 Many new development in relation to the theory of adsorption isotherm, most of
them are accurate for a specific system under specific conditions.
21

22. Adsorption On Solid Surface
 Use of BET isotherm to determine the surface area of a solid
 At low relative pressure P/P0 = 0.05~0.35 it is found that
Y = a + b X
 The principle of surface area determination by BET method:
A plot of against P/P0 will yield a straight line with slope of equal to (c-
1)/(cVm) and intersect 1/(cVm).
For a given adsorption system, c and Vm are constant values, the surface area of a
solid material can be determined by measuring the amount of a particular gas
adsorbed on the surface with known molecular cross-section area Am,
* In practice, measurement of BET surface area of a solid is carried out by N2
physisorption at liquid N2 temperature; for N2, Am = 16.2 x 10-20 m2
22
)
(
)
(
1
1
1
0
0
0
0
P
/
P
P
/
P
cV
c
cV
)
P
/
P
(
V
P
/
P
m
m





P P
V P P
/
( / )
0
0
1
P/P0
P P
V P P
/
( / )
0
0
1
A A N A
V
V
s m m m
m
T P
   
,
.
6022 1023
Vm - volume of monolayer adsorbed gas molecules calculated from the
plot, L
VT,P - molar volume of the adsorbed gas, L/mol
Am - cross-section area of a single gas molecule, m2

23. Application of Adsorption
 In clarification of sugar
 In gas masks
 In catalysis
 In adsorption indicators
 In chromatographic analysis
 In softening of hard water
 In preserving vacuum
 In paint industry
 In removing moisture from air in the storage of delicate
instruments 23

24. Summary
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25. THANKYOU
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