Adsorption and Catalysis
Dr. King Lun Yeung
Department of Chemical Engineering
Hong Kong University of Science and Technology
CENG 511
Lecture 3
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 H H H
H
H
H HH H
H H
H
H H
H
H
H
H
H
H
H
H
H2 absorption 
palladium hydride
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
adsorbent
coverage q = fraction of surface sites occupied
H
H H
H
H
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
Characteristics of Chemi- and Physisorptions
Properties
Chemisorption
Adsorption temperature virtually unlimited range
Physisorption
near or below Tbp of adsorbate
(Xe < 100 K, CO2 < 200 K)
Adsorption enthalpy
wide range (40-800 kJmol-1) heat of liquifaction
(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
Analytical Methods for Establishing Surface Bonds
Infrared Spectroscopy
Atoms vibrates in the I.R. range
• chemical analysis (molecular fingerprinting)
• structural information
• electronic information (optical conductivity)
IR units: wavenumbers (cm-1),
10 micron wavelength = 1000 cm-1
http://infrared.als.lbl.gov/FTIRinfo.html
Near-IR: 4000 – 14000 cm-1
Mid-IR: 500 – 4000 cm-1
Far-IR: 5 – 500 cm-1
I.R. Measurement
I.R. Spectrum of CO2
O
C
O
Symmetric Stretch
Assymmetric Stretch
A dipole moment = charge imbalance in the molecule
Bending mode
I.R. Spectrum of NO on Pt
Temperature increases
Adsorption decreases
Molecular conformation
changes
I.R. Spectrum of HCN on Pt
0.15 L HCN, 100 K
weak chemisorption
CN
H- C  N
H- C  N
1.5 L HCN, 100 K
physisorption
Pt
Pt
Pt
(a)
(b)
(c)
30 L HCN, 200 K
dissociative chemisorption
d(HCN)
2d(HCN)
n(CN)
n(H-CN)
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
Activation energy
Temperature dependency
of adsorption processes
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
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.
Sticking Coefficient
Sticking Coefficient
Steering Effects
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
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
Adsorption Energetics
Physisorption versus chemisorption
repulsive force
DE(ads)
Chemisorption
surface
attractive forces
DE(ads)
<
Physisorption
DE(ads)
Chemisorption
small minima
weak Van der Waal
attraction force
large minima
formation of surface
chemical bonds
Physical Adsorption
Applications:
• surface area measurement
• pore size and volume determination
• pore size distribution
0.3 nm
E(d)
Van der Waal forces
d
nitrogen
Note: there is no activation
barrier for physisorption
 fast process
metal surface
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
BET Isotherm
Assumptions
• adsorption takes place on the lattice and molecules stay put,
• first monolayer is adsorbed onto the solid surface and each layers 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.
Activated Carbon
Surface area ~ 1000 m2/g
Surface Area Determination
BET surface area by N2 physisorption
 - adsorption
- desorption
c
= 69.25
nm = 4.2 x 10-3 mol
Area = 511 m2/g
Plot P/n(P0-P) versus P/P0
calculate c and nm from the slope (c-1/ nmc) and
intercept (1/nmc) of the isotherm
measurements usually obtained for P/P0 < 0.2
c
= 87.09
nm = 3.9 x 10-3 mol
Area = 480 m2/g
BET Measurements
Volumetric Method
To vacuum
Gas Supply
Chamber
P1 T1
Gas cylinder
• Degassing
• Pure gas introduces into supply chamber
 constant P1 T1 are recorded  V1
• Gas flows into adsorption cell
Adsorption
• P2 and T2 are recorded when
Cell
equilibrium is reached  V2
P2 T2
BET Measurements
Dynamic Method
• Degassing
• Flow carrier gas (He)
• Pulse N2/He into adsorption cell at
given PN2
• Record the amount of nitrogen
adsorbed using TCD
• Calculate surface area
(Rouquerol, 1999)
BET Measurements
Gravimetric Method
• Degassing
• Record initial weight of adsorbent
M1
• Introduce pure gas into adsorption
cell
• Record the adsorbent equilibrium
weight M2
• Record the equilibrium pressure
(Rouquerol, 1999)
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
Rouqerol, F., J., Rouquerol and K., Sing, Adsorption by Powders and Porous Solids: Principles,
Methodology and Applications, Academic Press, London (1999).
Adsorption Isotherm
IUPAC Classification
Adsorption Isotherm
IUPAC Classification
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).
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
Adsorption Isotherm
Type VI Isotherm
• Highly uniform surface
 Layer by layer adsorption
 Stepped isotherm
Example:
• Adsorption of simple non-porous
molecules on uniform surfaces (e.g.
basal plane of graphite)
Adsorption Isotherm
Composite Isotherm
Type I
Type I & IV
N2 adsorption in (a) micropores and (c) micropores and
mesopores
(Rouquerol, 1999)
Chemical Adsorption
re = equilibrium bond distance
E(d)
Ea(ads) = 0
Applications:
• active surface area
measurements
• surface site energetics
• catalytic site determination
Ea(des) = - DH(ads)
= strength of surface bonding
= DH(ads)
d
Pt surface
CO
Note: there is no activation barrier
for adsorption  fast process,
there us an activation barrier for
desorption  slow process.
Chemical Adsorption Processes
Physisorption + molecular chemisorption
CO
physisorption
E(d)
chemisorption
d
Chemical Adsorption Processes
Physisorption + dissociative chemisorption
H2  2 H
E(d)
H2
dissociation
chemisorption
physisorption
atomic chemisorption
d
Note: this is an energy prohibitive process
Chemical Adsorption Processes
Physisorption + molecular chemisorption
CO
E(d)
physisorption/
desorption
chemisorption
physisorption
atomic chemisorption
d
Chemical Adsorption Processes
Physisorption + molecular chemisorption
CO
E(d)
direct chemisorption
physisorption
atomic chemisorption
d
Chemical Adsorption Processes
Energy barrier
Ea(ads) ~ 0
Ea(ads) > 0
Chemical Adsorption Processes
Energy barrier
Chemical Adsorption is usually
an energy activated process.
des
- Ea
~ -DH(ads)
= -DE(ads)
Formation of Ordered Adlayer
Ea(surface diffusion) < kT
activated carbon
CH4
Krypton
Formation of Ordered Adlayer
Chlorine on chromium surface
Adsorbate Geometries on Metals
Hydrogen and halogens
Halogens
high electronegativity  dissociative chemisorption
Hydrogen
X-X
H-H
ionic bonding compound
2-D atomic gas
H-H
1-H atom per 1-metal atom
HH

XX

X-X
XX

Halogen atom tend to occupy high co-ordination
sites:
(111)
(100)
Adsorbate Geometries on Metals
Oxygen and Nitrogen
Oxygen
Nitrogen
OO

O=O
NN
O=O
both molecular and dissociative
chemisorption occurs.
molecular chemisorption  s-donor or
p-acceptor interactions.
NN
molecular chemisorption  s-donor or
p-acceptor interactions.
dissociative chemisorption  occupy
highest co-ordinated surface sites, also
causes surface distorsion.
(111)
(100)
Adsorbate Geometries on Metals
Carbon monoxide
Carbon monoxide
CO
CO
CC
metal carbide

forms metal carbides with metals located
at the left-hand side of the periodic table.
molecular chemisorption occurs on d-block
metals (e.g., Cu, Ag) and transition metals
Terminal
(Linear)
all surface
Bridging
(2f site)
all surface
Bridging
(3f hollow)
(111) surface
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
Active Surface Area Measurement
Most common chemisorption gases:
hydrogen, oxygen and carbon monoxide
furnace
catalyst
Pulse H2, O2
or CO gases
thermal conductivity
cell (TCD)
exhaust
carrier gas
helium or argon
Catalyst Surface Area and Dispersion Calculation
furnace
1 g 0.10 wt. % Pt/g-Al2O3
T = 423 K, P = 1 bar
100 ml (STP)
Pulse H2 then
titrate with O2
3.75 peaks (H2)
4.50 peaks (O2)
thermal conductivity
cell (TCD)
exhaust
Avogrado’s number: 6.022 x 1023
Pt lattice constant: a = 3.92 (FCC)
carrier gas
helium or argon
Calculate surface area of
Pt and its dispersion.
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-M]
[S - *] [A]
[S-M] is proportional to q,
[S-*] is proportional to 1-q,
[A] is proportional to partial pressure of A
Isotherms
Langmuir isotherm
Molecular chemisorption
b=
q
(1-q) P
Where b depends only on the temperature
Dissociative chemisorption
q=
(bP)0.5
1+ (bP)0.5
Where b depends only on the temperature
q=
bP
1+ bP
Variation of q as function of T and P
q  bP at low pressure
q  1 at high pressure
b  when T 
b  when DH(ads) 
1
1
q
q
0.8
0.8
0.6
0.6
b
0.4
0.2
0.2
0
0
0
0.2
0.4
0.6
T
0.4
0.8
P1
0
0.2
0.4
0.6
0.8
P1
Determination of DH(ads)
(
InP
1/T
)
q =const
DH(ads)
R
=
1
q
InP
0.8
0.6
0.4
qi
T
T
(P2, T2)
(P1, T1)
0.2
0
0
0.2
0.4
0.6
0.8
P1
1/T
Adsorption Isotherms
Henry’s Adsorption Isotherm
Special case of Langmuir isotherm
bP << 1
q = bP
V = k’P
where k’ = bV
The Freundlich Isotherm
Adsorption sites are distributed exponentially with DH(ads)
biP =
q=
qi
(1-qi)
-DH(ads)
 qiNi
 Ni
RT
Inq =
InP + B
A
q = kP1/n
q
Valid for low partial pressure
most frequently used for describing
pollutant adsorption on activated
carbons
The Temkin Isotherm
DH(ads) decreases with q
q = A InBP
-DH(ads)
q
Valid at low to medium coverage
gas chemisorption on clean metal
surfaces
Thermal Desorption Spectroscopy
0.2 - 50 L
Thermal desorption spectra of CO
on Pd(100) after successive
exposure to CO gases
Chemical Adsorption
re = equilibrium bond distance
E(d)
Ea(ads) = 0
Applications:
• active surface area
measurements
• surface site energetics
• catalytic site determination
Ea(des) = - DH(ads)
= strength of surface bonding
= DH(ads)
d
Pt surface
CO
Note: there is no activation barrier
for adsorption  fast process,
there us an activation barrier for
desorption  slow process.
Thermal Desorption Spectroscopy
Desorption Rate
[ ]
d -dNa
dT dT
-dNa
dT
0.2 - 50 L
-dNa dT
{
dT dt
} = Nam k exp ( -Ed )
RT
Linear heating rate
T = T0 + bt
dT
= b
dt
Assuming k and Ed are independent of coverage
and m = 1 (i.e., first order desorption)
Thermal desorption spectra of CO
on Pd(100) after successive
exposure to CO gases
Ed
2
RTp
=
k exp -Ed
( RT )
b
Thermal Desorption Spectroscopy
Ed
2
RTp
slope, m  Ea
Determination of Edes using different
heating rates (b)
=
k exp -Ed
( RT )
b
TPD provides important information
on adsorption/desorption energetics
and adsorbate-surface interactions.
Thermal Desorption Spectroscopy
Second order desorption
[ ]
d -dNa
dT dT
-dNa
dT
0.2 - 50 L
Assuming k and Ed are independent of coverage
and m = 2 (i.e., first order desorption)
Ed
2
RTp
= 2(Na)p
k exp -Ed
( RT )
b
Characterized by a shift in the peak maxima
toward lower temperature as the coverage
increases
Thermal desorption spectra of CO
on Ni(100) after successive
exposure to CO gases
Activation Energies for CO Desorption
Influence of Surface Overlayer
Catalyst poison, strong adsorbates and coke
CO desorption
Sulfur-treated
catalyst
Clean catalyst
Ordered Adsorbate layer
H2/Rh(110)
O2/Rh(110)
Thermal Desorption Spectroscopy
O2 TPD from Rh(110)
Ordered Adsorbate layer
benzene/ZnO(1010)
Kelvin Probe
Measures the change in work function (Df)
Typical Kelvin probe for adsorption
studies
Scanning Kelvin probe for surface work
function (i.e., elemental and
compositional) imaging
also known as scanning electrical
field microscopy
Kelvin Probe
Basic principle
Vibrating capacitor measures f
f is the least amount of energy needed
for an electron to escape from metal to
vacuum.
f is sensitive optical, electrical and
mechanical properties of materials