introduction

1. SURFACE CHEMISTRY
ADSORPTION
D. JIM LIVINGSTON
Asst. Prof. of Chemistry
St. John’s College

2. Surface Chemistry
• Study of the phenomenon occurring on the
surfaces of substances.
• Reactions that occur at the interface of two
surfaces which can be solid-liquid, solid-
gas, solid-vacuum, liquid-gas, etc.

3. • Which has high surface area? Compact solid or powdered solid
Surface Area

4. Surface Area
• If a cube of edge-length 1cm is divided into small cubes of edge-length 0.001cm, then
what is the total surface area?
• S.A of Large cube : 6a2 = 6x12 = 6cm2
• S.A of small cube : 6a2 = 6x(.001)2=6x10-6cm2
• No. of small cubes can be obtained : ?
• Volume of Large cube v1 : A3 = 1cm3
• Volume of one small cube v2 : A3 = (.001)3 = 10-9cm3
• No. of small cubes can be obtained : v1/v2 = 1/10-9 cm3 = 109 cubes
• S.A. of 109 cubes : 6x10-6 x 109 = 6000 cm2
6cm2
6000cm2

5. Adsorption
• a process which involves
the accumulation of a
substance in molecular
species in higher
concentration on the
surface.
• ADSORBATE - the
substance which is
adsorbed on the surface
• ADSORBENT the
substance on whose
surface the particles are
adsorbed
Gases like O2, H2,
N2 are adsorbed at
the surface of
charcoal.

6. • Adsorption: is the accumulation of a substance at a surface or
interface
• Absorption: is the accumulation and distribution of a substance
throughout a phase.
• Desorption - a phenomenon whereby a substance is released from
or through a surface.

7. • Physisorption: It is due to weak Van der Waals forces
between adsorbate and adsorbent.
• Chemisorption. It is due to strong chemical forces of
bonding type between adsorbate and adsorbent.

8. Adsorption and Reaction at Surfaces
Adsorption
PHYSISORPTION CHEMISORPTION
WEAK, LONG RANGE BONDING
Van der Waals interactions (e.g. London dispersion,
dipole -dipole)..

9. Adsorption and Reaction at Surfaces
Adsorption
PHYSISORPTION CHEMISORPTION
STRONG, SHORT RANGE BONDING
Chemical bonding involving orbital overlap and
charge transfer.

10. Adsorption and Reaction at Surfaces
Adsorption
PHYSISORPTION CHEMISORPTION
WEAK, LONG RANGE BONDING
Van der Waals interactions (e.g. London dispersion,
dipole-dipole)..
STRONG, SHORT RANGE BONDING
Chemical bonding involving orbital overlap and charge
transfer.
NOT SURFACE SPECIFIC
Physisorption takes place between all molecules on
any surface providing the temperature is low enough.
SURFACE SPECIFIC
E.g. Chemisorption of hydrogen takes
place on transition metals but not on
gold or mercury.

11. CHEMISORPTION
STRONG, SHORT RANGE BONDING
Chemical bonding involving orbital overlap and
charge transfer.
SURFACE SPECIFIC
E.g. Chemisorption of hydrogen takes place on
transition metals but not on gold or mercury.
ads = 50 … .. 500 kJ mol-1
Can be activated, in which case equilibrium can be
slow and increasing temperature can favour
adsorption.
Surface reactions may take place:- Dissociation,
reconstruction, catalysis.
MONOLAYER ADSORPTION
Langmuir Isotherm used to model adsorption
equilibrium..
Adsorption and Reaction at Surfaces
Adsorption
potentialenergy
PHYSISORPTION
N2
0
z
WEAK, LONG RANGE BONDING
Van der Waals interactions (e.g. London dispersion,
dipole-dipole)..
NOT SURFACE SPECIFIC
Physisorption takes place between all molecules on any
surface providing the temperature is low enough.
ads = 5 … .. 50 kJ mol-1
Non activated with equilibrium achieved
relatively quickly. Increasing temperature
always reduces surface coverage.
No surface reactions.
REVERSIBLE

12. PHYSISORPTION
WEAK, LONG RANGE BONDING
Van der Waals interactions (e.g. London dispersion,
dipole-dipole)..
NOT SURFACE SPECIFIC
Physisorption takes place between all molecules on
any surface providing the temperature is low enough.
ads = 5 … .. 50 kJ mol-1
Non activated with equilibrium achieved relatively
quickly. Increasing temperature always reduces
surface coverage.
No surface reactions.
MULTILAYER ADSORPTION
BET Isotherm used to model adsorption equilibrium.
Adsorption and Reaction at Surfaces
Adsorption
CHEMISORPTION
2N STRONG, SHORT RANGE BONDING
Chemical bonding involving orbital overlap and
charge transfer.
0
N2
z
SURFACE SPECIFIC
E.g. Chemisorption of hydrogen takes place on transition
metals but not on gold or mercury.
ads = 50 … .. 500 kJ mol-1
Can be activated, in which case equilibrium can
be slow and increasing temperature can favour
adsorption.
Surface reactions may take place:
IRREVERSIBLE
potentialenergy

13. CHEMISORPTION
STRONG, SHORT RANGE BONDING
Chemical bonding involving orbital overlap and
charge transfer.
SURFACE SPECIFIC
E.g. Chemisorption of hydrogen takes place on
transition metals but not on gold or mercury.
ads = 50 … .. 500 kJ mol-1
Can be activated, in which case equilibrium can be
slow and increasing temperature can favour
adsorption.
Surface reactions may take place:- Dissociation,
reconstruction, catalysis.
MONOLAYER ADSORPTION
Langmuir Isotherm used to model adsorption
equilibrium..
Adsorption and Reaction at Surfaces
Adsorption
PHYSISORPTION
WEAK, LONG RANGE BONDING
Van der Waals interactions (e.g. London dispersion,
dipole-dipole)..
NOT SURFACE SPECIFIC
Physisorption takes place between all molecules on any
surface providing the temperature is low enough.
ads = 5 … .. 50 kJ mol-1
Non activated with equilibrium achieved relatively
quickly. Increasing temperature always reduces surface
coverage.
No surface reactions.
MULTILAYER ADSORPTION
BET Isotherm used to model adsorption equilibrium.

14. PHYSISORPTION
WEAK, LONG RANGE BONDING
Van der Waals interactions (e.g. London dispersion,
dipole-dipole)..
NOT SURFACE SPECIFIC
Physisorption takes place between all molecules on
any surface providing the temperature is low enough.
ads = 5 … .. 50 kJ mol-1
Non activated with equilibrium achieved relatively
quickly. Increasing temperature always reduces
surface coverage.
No surface reactions.
MULTILAYER ADSORPTION
BET Isotherm used to model adsorption equilibrium.
Adsorption and Reaction at Surfaces
Adsorption
CHEMISORPTION
STRONG, SHORT RANGE BONDING
Chemical bonding involving orbital overlap and charge
transfer.
SURFACE SPECIFIC
E.g. Chemisorption of hydrogen takes place on transition
metals but not on gold or mercury.
ads = 50 … .. 500 kJ mol-1
Can be activated, in which case equilibrium can be
slow and increasing temperature can favour adsorption.
Surface reactions may take place:- Dissociation,
reconstruction, catalysis.
MONOLAYER ADSORPTION
Langmuir Isotherm used to model adsorption
equilibrium..

15. ADSORPTION
OF
GASES BY SOLIDS

17. Un-balanced
force
Un-balanced force

18. The extent of
adsorption increases
with the increase of
surface area
Mechanism of Adsorption
decrease in residual
forces of the surface,
i.e., there is decrease in
surface energy which
appears as heat.
decrease in the entropy
of the gas after
adsorption,