Catalysis & Catalysts
Example Heterogeneous Catalytic Reaction Process
 The long journey for reactant molecules to
j. travel within gas phase
k. cross gas-liquid phase boundary
l. travel within liquid phase/stagnant layer
m. cross liquid-solid phase boundary
n. reach outer surface of solid
o. diffuse within pore
p. arrive at reaction site
q. be adsorbed on the site and activated
r. react with other reactant molecules, either
being adsorbed on the same/neighbour
sites or approaching from surface above
 Product molecules must follow the same track
in the reverse direction to return to gas phase
gas phase
reactant molecule
j
k
l
gas phase
liquid phase /
stagnant layer
mn
o
porous
solid
pore
pq r
 Heat transfer follows similar track
1
Catalysis & Catalysts
Solid Catalysts
 Some

common solid support / carrier materials
Alumina


Inexpensive
 Surface area: 1 ~ 700 m2/g
 Acidic

Silica

Inexpensive
 Surface area: 100 ~ 800 m2/g
 Acidic

Other supports





Active carbon (S.A. up to 1000 m2/g)
Titania (S.A. 10 ~ 50 m2/g)
Zirconia (S.A. 10 ~ 100 m2/g)
Magnesia (S.A. 10 m2/g)
Lanthana (S.A. 10 m2/g)
Active site
Zeolite

mixture of alumina and silica,
 often exchanged metal ion present
 shape selective
 acidic
porous
solid
pore
2
Catalysis & Catalysts
Adsorption on Solid Surface
 Adsorption

Adsorption is a process in which molecules from gas (or liquid) phase land
on, interact with and attach to solid surfaces.

The reverse process of adsorption, i.e. the process in which adsorbed
molecules escape from solid surfaces, is called Desorption.

Molecules can attach to surfaces in two different ways because of the
different forces involved. These are Physisorption (Physical adsorption) &
Chemisorption (Chemical adsorption)
Physisorption
Chemisorption
force
van der Waals
chemical bond
number of adsorbed layers
multi
only one layer
adsorption heat
low (10-40 kJ/mol)
high ( > 40 kJ/mol)
selectivity
low
high
temperature to occur
low
high
3
Catalysis & Catalysts
Adsorption on Solid Surface
 Adsorption
process
Adsorbent and adsorbate

Adsorbent (also called substrate) - The solid that provides surface for adsorption

high surface area with proper pore structure and size distribution is essential
 good mechanical strength and thermal stability are necessary

Adsorbate - The gas or liquid substances which are to be adsorbed on solid
Surface coverage, q
The solid surface may be completely or partially covered by adsorbed molecules
define
number of adsorption sites occupied
q=
number of adsorption sites available
q = 0~1
Adsorption heat

Adsorption is usually exothermic (in special cases dissociated adsorption can be
endothermic)

The heat of chemisorption is in the same order of magnitude of reaction heat;
the heat of physisorption is in the same order of magnitude of condensation heat.
4
Catalysis & Catalysts
Adsorption on Solid Surface
 Applications
of adsorption process

Adsorption is a very important step in solid catalysed reaction processes

Adsorption in itself is a common process used in industry for various purposes

Purification (removing impurities from a gas / liquid stream)

De-pollution, de-colour, de-odour

Solvent recovery, trace compound enrichment

etc…

Usually adsorption is only applied for a process dealing with small capacity

The operation is usually batch type and required regeneration of saturated adsorbent
Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina.

Physisorption is an useful technique for determining the surface area, the pore
shape, pore sizes and size distribution of porous solid materials (BET surface area)
5
Activated Carbon
Surface area ~ 1000 m2/g
6
Catalysis & Catalysts
Solid Catalysts

Catalyst composition

Active phase


Where the reaction occurs (mostly metal/metal oxide)
Promoter

Textual promoter (e.g. Al - Fe for NH3 production)
 Electric or Structural modifier
 Poison resistant promoters

Support / carrier
Catalyst
Support

Increase mechanical strength
 Increase surface area (98% surface area is supplied within the porous structure)
 may or may not be catalytically active
7
Adsorption versus Absorption
H
H H
H
H
H H
H
H
H2 adsorption on
palladium
Adsorption
Surface process
Absorption
bulk process
H
H
H H
H HH H
H
H H
H
H
H
H
H H
H H
H
H
H
H
H
H
H
H
H2 absorption 
palladium hydride
8
Nomenclature
Substrate or adsorbent: surface onto which adsorption can occur.
example: catalyst surface, activated carbon, alumina
Adsorbate: molecules or atoms that adsorb onto the substrate.
example: nitrogen, hydrogen, carbon monoxide, water
Adsorption: the process by which a molecule or atom adsorb onto a surface of
substrate.
Coverage: a measure of the extent of adsorption of a specie onto a surface
Exposure: a measure of the amount of gas the surface had been exposed to
( 1 Langmuir = 10-6 torr s)
H
H H
H
H
H H
H
H
adsorbate
coverage q = fraction of surface sites occupied
H
H
H
H
H
adsorbent
9
Types of Adsorption Modes
Physical adsorption or
physisorption
Bonding between molecules and
surface is by weak van der Waals
forces.
Chemical bond is formed between
molecules and surface.
Chemical adsorption or
chemisorption
10
Characteristics of Chemi- and Physisorptions
Properties
Chemisorption
Physisorption
Adsorption temperature
virtually unlimited range
near or below Tbp of adsorbate
(Xe < 100 K, CO2 < 200 K)
Adsorption enthalpy
wide range (40-800 kJmol-1)
heat of liquefaction
(5-40 kJmol-1)
Crystallographic
specificity
marked difference for
between crystal planes
independent of surface
geometry
Nature of adsorption
often dissociative and
irreversible in many cases
non-dissociative and
reversible
Saturation
limited to a monolayer
multilayer occurs often
Adsorption kinetic
activated process
fast, non-activated process
11
Analytical Methods for Establishing Surface Bonds
Atoms vibrate in the I.R. range
Infrared Spectroscopy
http://infrared.als.lbl.gov/FTIRinfo.html
• chemical analysis (molecular fingerprinting)
• structural information
• electronic information (optical conductivity)
IR units: wavenumbers (cm-1),
10 micron wavelength = 1000 cm-1
Near-IR: 4000 – 14000 cm-1
Mid-IR: 500 – 4000 cm-1
Far-IR: 5 – 500 cm-1
12
I.R. Measurement
13
I.R. Spectrum of CO2
Vertical and
horizontal bend
Asymmetric
stretch
A dipole moment = charge imbalance in the molecule
14
Adsorption Rate
Rads = k C x
Rads = k’ P x
x - kinetic order
k - rate constant
C - gas phase concentration
x - kinetic order
k’ - rate constant
P - partial pressure of molecule
Rads = A C x exp (-Ea/RT)
Frequency factor
Temperature dependency
of adsorption processes
Activation energy
15
Adsorption Rate
Molecular level event
Rads = S • F = f(q) P/(2pmkT)0.5 exp(-Ea/RT)
(molecules m-2 s-1)
Sticking coefficient
Flux (Hertz-Knudsen)
S = f(q) exp(-Ea/RT)
F = P/(2pmkT)0.5
where 0 < S < 1
where P = gas pressure (N m-2)
m = mass of one molecule (Kg)
T = temperature (K)
Note: f(q) is a function of surface coverage
special case of Langmuir adsorption f(q) = 1-q
E(q), the activation energy is also affected by surface coverage
16
Sticking Coefficient
S = f(q) exp(-Ea/RT)
where 0 < S < 1
Tungsten
S also depends on
crystal planes and may
be influenced by surface
reconstruction.
17
Surface Coverage (q)
Estimation based on gas exposure
Rads = dNads/dt = S • F
Nads  S • F • t
Nearly independent
of coverage for most
situations
Exposure time
Molecules adsorbed per
unit surface area
18
Adsorption Energetics
Potential energy (E) for adsorption is only dependent on distance
between molecule and surface
adsorbate
d
surface
P.E. is assumed to be independent of:
• angular orientation of molecule
• changes in internal bond angles and lengths
• position of the molecule along the surface
19
Adsorption Energetics
Physisorption versus chemisorption
repulsive force
DE(ads)
Chemisorption
surface
DE(ads)
<
Physisorption
DE(ads)
Chemisorption
small minima
large minima
weak Van der Waals formation of surface
attraction force
chemical bonds
attractive forces
20
Physical Adsorption
Applications:
• surface area measurement
• pore size and volume determination
• pore size distribution
0.3 nm
E(d)
Van der Waals forces
d
nitrogen
Note: there is no activation
barrier for physisorption
 fast process
metal surface
21
Adsorption Isotherm
• Adsorption Isotherm:
– The equilibrium relationship between the amount adsorbed
and the pressure or concentration at constant temperature
(Rouquerol et al., 1999).
• Importance of Classification
– Providing an efficient and systematic way for theoretical
modeling of adsorption and adsorbent characteristics
determination
Rouquerol, F., J., Rouquerol and K., Sing, Adsorption by Powders and Porous Solids:
Principles, Methodology and Applications, Academic Press, London (1999).
22
Adsorption Isotherm
IUPAC Classification
23
Adsorption Isotherm
IUPAC Classification
Micropores
(< 2 nm)
Type I
Strong
interaction (Activated Carbon,
Zeolites)
Weak
interaction
Mesopores
(2 – 50 nm)
Macropores
(> 50 nm)
Type IV
Type II
(oxide gels,
zeolites)
(Clay, pigments,
cements)
Type V
Type III
(Water on
charcoal)*
(Bromine on
silica gel)*
•Do, D. D., Adsorption Analysis: Equilibria and Kinetics, Imperial College Press,
London (1998).
24
Adsorption Isotherm
Capillary Condensation
• Mesopores
 Capillary condensation
 Hysteresis occurs
• Different hysteresis  Different network structure
Narrow distribution of uniform pores  Type IVa
Complex structure made up of interconnected
networks of different pore sizes and shapes  Type
IVb
25
Adsorption Isotherm
Type VI Isotherm
• Highly uniform surface
 Layer by layer adsorption
 Stepped isotherm
Example:
• Adsorption of simple
molecules on uniform surfaces
(e.g. basal plane of graphite)
26
Isotherms
Langmuir isotherm
S - * + A(g)  S-A
Adsorbed molecules
surface sites
DH(ads) is independent of q
the process is reversible and is at equilibrium
K=
[S-A]
[S - *] [A]
[S-A] is proportional to q,
[S-*] is proportional to 1-q,
[A] is proportional to partial pressure of A
27
Isotherms
Langmuir isotherm
Molecular chemisorption or physisorption
b=
q
(1-q) P
q=
bP
1+ bP
Where b depends only on the temperature
Dissociative chemisorption
q=
(bP)0.5
1+ (bP)0.5
Where b depends only on the temperature
28
Variation of q as function of T and P
q  bP at low pressure
q  1 at high pressure
bP
1+ bP
b  when T 
b  when DH(ads) 
1
q
0.8
0.8
0.6
0.6
q
q=
1
T
b
0.4
0.4
0.2
0.2
0
0
0
0.2
0.4
0.6
0.8
P1
0
0.2
0.4
0.6
0.8
P 1
29
Determination of DH(ads)
(
q
lnP
1/T
)
q =const
=
1
DH(ads)
R
lnP
0.8
0.6
T
T
qi
0.4
(P2, T2)
(P1, T1)
0.2
0
0
0.2
0.4
0.6
0.8
P1
1/T
30
Adsorption Isotherms
31
Henry’s Adsorption Isotherm
Special case of Langmuir isotherm
bP << 1
q = bP
32
The Freundlich Isotherm
Adsorption sites are distributed exponentially with DH(ads)
qi
b iP =
(1-qi)
q=
-DH(ads)
 qiNi
 Ni
q
RT
lnq =
lnP + B
A
q = kP1/n
Valid for low partial pressure
most frequently used for
describing pollutant adsorption
on activated carbons
33
The Temkin Isotherm
DH(ads) decreases with q
q = A lnBP
-DH(ads)
q
Valid at low to medium
coverage
gas chemisorption on clean
metal surfaces
34
The Brunauer-Emmett-Teller Isotherm
BET isotherm
where: n is the amount of gas adsorbed at P
nm is the amount of gas in a monolayer
P0 is the saturation pressure
n   at P = P0
C is a constant defined as:
H1 and HL are the adsorption enthalpy of first
and subsequent layers
35
BET Isotherm
Assumptions
• adsorption takes place on the lattice and molecules stay put,
• first monolayer is adsorbed onto the solid surface and each layer
can start before another is finished,
• except for the first layer, a molecule can be adsorbed on a given
site in a layer (n) if the same site also exists in (n-1) layer,
• at saturation pressure (P0), the number of adsorbed layers is
infinite (i.e., condensation),
• except for the first layer, the adsorption enthalpy (HL) is identical
for each layers.
36
Chemical Adsorption
E(d)
Applications:
• active surface area
measurements
• surface site energetics
• catalytic site determination
re = equilibrium bond distance
Ea(ads) = 0
d
Ea(des) = - DH(ads)
= strength of surface bonding
= DH(ads)
CO
Pt surface
Note: there is no activation barrier
for adsorption  fast process,
there us an activation barrier for
desorption  slow process.
37
Chemical Adsorption Processes
Physisorption + molecular chemisorption
CO
E(d)
physisorption
chemisorption
d
38
Chemical Adsorption Processes
Physisorption + dissociative chemisorption
H2  2 H
E(d)
H2
dissociation
chemisorption
physisorption
atomic chemisorption
d
39
Chemical Adsorption Processes
Physisorption + molecular chemisorption
E(d)
CO
physisorption/
desorption
chemisorption
physisorption
atomic chemisorption
d
40
Chemical Adsorption Processes
Physisorption + molecular chemisorption
E(d)
CO
direct chemisorption
physisorption
atomic chemisorption
d
41
Chemical Adsorption Processes
Chemical Adsorption is usually
an energy activated process
Energy barrier
des
- Ea
= -DE(ads)
~ -DH(ads)
42
Adsorbate Geometries on Metals
Ammonia and unsaturated hydrocarbons
Ammonia
NH3
NH2 (ads) + H (ads)  NH (ads) + 2 H (ads)  N (ads) + 3 H (ads)
Ethene
2HC=CH2
43