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Transcript of Lang Mu Irs
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Adsorption and Catalysis
Dr. King Lun Yeung
Department of Chemical Engineering
Hong Kong University of Science and Technology
CENG 511Lecture 3
Adsorption versus Absorption
Adsorption Absorption
H H H H H H H H H
H H H H H H H H H
H2 adsorption on
palladium
H
H
HH
H
HHH
H
H
H
H
HH
H
H
H H
H2 absorption palladium hydride
Surface process bulk process
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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, waterAdsorption: 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 H H H H Hadsorbate
adsorbent
coverage = fraction of surface sites occupied
Types of Adsorption Modes
Physical adsorption or
physisorption
Chemical adsorption or
chemisorption
Bonding between molecules and
surface is by weak van der Waals
forces.
Chemical bond is formed betweenmolecules and surface.
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Characteristics of Chemi- and Physisorptions
Chemisorption
virtually unlimited range
wide range (40-800 kJmol-1)
marked difference for
between crystal planes
often dissociative and
irreversible in many cases
limited to a monolayer
activated process
Physisorption
near or below Tbp of adsorbate(Xe < 100 K, CO2 < 200 K)
heat of liquifaction
(5-40 kJmol-1)
independent of surface
geometry
non-dissociative and
reversible
multilayer occurs often
fast, non-activated process
Properties
Adsorption temperature
Adsorption enthalpy
Crystallographic
specificity
Nature of adsorption
Saturation
Adsorption kinetic
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
Near-IR: 4000 14000 cm-1
Mid-IR: 500 4000 cm-1
Far-IR: 5 500 cm-1
http://infrared.als.lbl.gov/FTIRinfo.html
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I.R. Measurement
I.R. Spectrum of CO2
Symmetric Stretch
Assymmetric Stretch
Bending mode
O C O
A dipole moment = charge imbalance in the molecule
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I.R. Spectrum of NO on Pt
Tem
peratureincreases
Adsorption decreases
Molecular conformation
changes
I.R. Spectrum of HCN on Pt
0.15 L HCN, 100 K
weak chemisorption
1.5 L HCN, 100 K
physisorption
30 L HCN, 200 K
dissociative chemisorption
H-C
N
Pt
(H-CN)2(HCN)(HCN)
Pt
H-C
N
H-C
N
(CN)
C
N
Pt
(a) (b) (c)
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Adsorption Rate
Rads = k Cx
x - kinetic order
k - rate constant
C - gas phase concentration
Rads = k Px
x - kinetic order
k - rate constant
P - partial pressure of molecule
Rads = A Cx exp (-Ea/RT)
Activation energyFrequency factor
Temperature dependency
of adsorption processes
Molecular level event
Adsorption Rate
Rads = S F = f() P/(2mkT)0.5 exp(-Ea/RT)
Sticking coefficient
S = f() exp(-Ea/RT)
where 0 < S < 1
Flux (Hertz-Knudsen)
F = P/(2mkT)0.5
where P = gas pressure (N m-2)
m = mass of one molecule (Kg)
T = temperature (K)
(molecules m-2 s-1)
Note: f() is a function of surface coverage
special case of Langmuir adsorption f() = 1-
E(), the activation energy is also affected by surface coverage
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Sticking Coefficient
S = f() exp(-Ea/RT)where 0 < S < 1
S also depends oncrystal planes and may
be influenced by surface
reconstruction.
Tungsten
Sticking Coefficient
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Sticking Coefficient
Steering Effects
Surface Coverage ()Estimation based on gas exposure
Rads = dNads/dt = S F
Nads S F tExposure time
Molecules adsorbed per
unit surface area
Nearly independentof coverage for most
situations
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Adsorption Energetics
d
surface
adsorbate
Potential energy (E) for adsorption is only dependent on distance
between molecule and 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
Physisorption versus chemisorption
Adsorption Energetics
surface
E(ads) E(ads) < E(ads)Physisorption Chemisorption
small minima large minima
weak Van der Waal formation of surface
attraction force chemical bonds
repulsive force
attractive forces
Chemisorption
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Physical Adsorption
d
metal surface
nitrogen
Van der Waal forces
E(d)0.3 nm
Note: there is no activation
barrier for physisorption
fast process
Applications:
surface area measurement
pore size and volume determination
pore size distribution
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 = P0C is a constant defined as:
H1 and HL are the adsorption enthalpy of first
and subsequent layers
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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
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Surface Area Determination
BET surface area by N2 physisorption
- adsorption
- desorption
Plot P/n(P0-P) versus P/P0calculate 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 = 69.25
nm = 4.2 x 10-3 mol
Area = 511 m2/g
c = 87.09
nm = 3.9 x 10-3 mol
Area = 480 m2/g
Chemical Adsorption
d
Pt surface
CO
E(d)
re
Note: there is no activation barrierfor adsorption fast process,there us an activation barrier for
desorption slow process.
Applications:
active surface area
measurements
surface site energetics
catalytic site determination
= strength of surface bonding
= equilibrium bond distance
= H(ads)
Ea(ads) = 0
Ea(des) = - H(ads)
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Chemical Adsorption Processes
Physisorption + molecular chemisorption
d
E(d) physisorption
chemisorption
CO
Chemical Adsorption Processes
Physisorption + dissociative chemisorption
d
E(d)dissociation
chemisorption
H2H2 2 H
physisorption
atomic chemisorption
Note: this is an energy prohibitive process
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Chemical Adsorption Processes
Physisorption + molecular chemisorption
physisorption/
desorptionchemisorption
CO
d
E(d)
physisorption
atomic chemisorption
Chemical Adsorption Processes
Physisorption + molecular chemisorption
direct chemisorption
CO
d
E(d)
physisorption
atomic chemisorption
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Chemical Adsorption Processes
Energy barrier
Ea(ads) ~ 0
Ea(ads) > 0
Chemical Adsorption Processes
Energy barrier
~ -H(ads)
- Eades
= -E(ads)
Chemical Adsorption is usually
an energy activated process.
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Formation of Ordered Adlayer
Ea(surface diffusion) < kT
activated carbon CH4
Krypton
Formation of Ordered Adlayer
Chlorine on chromium surface
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Adsorbate Geometries on Metals
Hydrogen and halogens
Hydrogen
1-H atom per 1-metal atom
H-H
H-HH
H
2-D atomic gas
Halogens
high electronegativity dissociative chemisorption
Halogen atom tend to occupy high co-ordination
sites:
X-X
X-XX
X
ionic bonding
(111) (100)
X
X
compound
Adsorbate Geometries on Metals
Oxygen and Nitrogen
(111) (100)
Oxygen
both molecular and dissociative
chemisorption occurs.
molecular chemisorption -donor or-acceptor interactions.
dissociative chemisorption occupyhighest co-ordinated surface sites, also
causes surface distorsion.
O=O
O=O
OO
Nitrogen
molecular chemisorption -donor or-acceptor interactions.
NN
NN
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Adsorbate Geometries on Metals
Carbon monoxide
Carbon monoxide
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
CO COTerminal
(Linear)
all surface
Bridging
(2f site)
all surface
Bridging
(3f hollow)
(111) surface
C
C
metal carbide
Adsorbate Geometries on Metals
Ammonia and unsaturated hydrocarbons
Ammonia
NH3
NH2 (ads) + H (ads) NH (ads) + 2 H (ads) N (ads) + 3 H (ads)
Ethene
2HC=CH2
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Active Surface Area Measurement
ost common chemisorption gases:hydrogen, oxygen and carbon monoxide
Pulse H2, O2or CO gases
exhaustcarrier gas
helium or argon
thermal conductivity
cell (TCD)
furnace
catalyst
Catalyst Surface Area and Dispersion Calculation
Pulse H2 then
titrate with O2
exhaustcarrier gas
helium or argon
thermal conductivity
cell (TCD)
furnace
1 g 0.10 wt. % Pt/-Al2O3
T = 423 K, P = 1 bar(STP)
3.75 peaks (H2)
4.50 peaks (O2)
100 l
Avogrados number: 6.022 x 1023
Pt lattice constant: a = 3.92 (FCC) Calculate surface area ofPt and its dispersion.
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Isotherms
Langmuir isotherm
S - * + A(g) S-A
surface sites
Adsorbed molecules
H(ads) is independent ofthe process is reversible and is at equilibrium
[S-M]
[S - *] [A]
K =
[S-M] is proportional to ,[S-*] is proportional to 1-,[A] is proportional to partial pressure of A
Isotherms
Langmuir isotherm
(1-) Pb =
Where b depends only on the temperature
bP
1+ bP =
Molecular chemisorption
Where b depends only on the temperature
(bP)0.5
1+ (bP)0.5 =
Dissociative chemisorption
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Variation of as function of T and P bP at low pressure 1 at high pressure
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
P P
b T
b when T b when H(ads)
Determination ofH(ads)
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
P
InP
T Ti
1/T
(P1, T1) (P2, T2)
InP( (ads)R1/T
) =const
=
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Adsorption Isotherms
Henrys Adsorption Isotherm
Special case of Langmuir isotherm
bP
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The Freundlich Isotherm
Adsorption sites are distributed exponentially with H(ads)
H(ads)
i(1-i)biP =
iNi Ni
=
RA
In = InP + B
kP1/n =Valid for low partial pressure
most frequently used for describing
pollutant adsorption on activated
carbons
The Temkin Isotherm
H(ads) decreases with
A InBP =H(ads)
Valid at low to medium coverage
gas chemisorption on clean metal
surfaces
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Thermal Desorption Spectroscopy
Thermal desorption spectra of CO
on Pd(100) after successive
exposure to CO gases
0.2 - 50 L
Chemical Adsorption
d
Pt surface
CO
E(d)
re
Note: there is no activation barrierfor adsorption fast process,there us an activation barrier for
desorption slow process.
Applications:
active surface area
measurements
surface site energetics
catalytic site determination
= strength of surface bonding
= equilibrium bond distance
= H(ads)
Ea(ads) = 0
Ea(des) = - H(ads)
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Thermal Desorption Spectroscopy
Thermal desorption spectra of CO
on Pd(100) after successive
exposure to CO gases
Desorption Rate
{-dNa
dT
dT
dt } =Nam
kexp(-E
dRT )
Linear heating rate
T = T0 + t
dT
dt=
Assuming kand Ed are independent of coverage
and m = 1 (i.e., first order desorption)
0.2 - 50 L-dNa
dT
d -dNa
dTdT[ ]
EdRTp
2 = exp(-EdRT
)k
Thermal Desorption Spectroscopy
Determination of Edes using different
heating rates ()
EdRTp
2 = exp(-EdRT
)k
slope, m Ea TPD provides important informationon adsorption/desorption energeticsand adsorbate-surface interactions.
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Thermal Desorption Spectroscopy
Thermal desorption spectra of CO
on Ni(100) after successive
exposure to CO gases
0.2 - 50 L Assuming kand Ed are independent of coverageand m = 2 (i.e., first order desorption)-dNa
dT
d -dNa
dTdT[ ] Second order desorption
EdRTp
2 = exp(-EdRT
)k
2(Na)p
Characterized by a shift in the peak maxima
toward lower temperature as the coverage
increases
Activation Energies for CO Desorption
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Influence of Surface Overlayer
Catalyst poison, strong adsorbates and coke
Sulfur-treatedcatalyst
Clean catalyst
CO desorption
Ordered Adsorbate layer
H2/Rh(110) O2/Rh(110)
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2 TPD from Rh(110)
Thermal Desorption Spectroscopy
Ordered Adsorbate layer
benzene/ZnO(1010)
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Kelvin Probe
Measures the change in work function ()
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 is the least amount of energy neededfor an electron to escape from metal tovacuum.
is sensitive optical, electrical andmechanical properties of materials
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The Freundlich Isotherm
Adsorption sites are distributed exponentially with H(ads)
H(ads)
i(1-i)b
iP =
iNi Ni
=
R
AIn = InP + B
kP1/n =
Valid for low partial pressure
most frequently used for describing
pollutant adsorption on activated
carbons
The Temkin Isotherm
H(ads) decreases with
A InBP =H(ads)
Valid at low to medium coverage
gas chemisorption on clean metal
surfaces
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Thermal Desorption Spectroscopy
Thermal desorption spectra of CO
on Pd(100) after successive
exposure to CO gases
0.2 - 50 L
Chemical Adsorption
d
Pt surface
CO
E(d)
re
Note: there is no activation barrierfor adsorption fast process,
there us an activation barrier for
desorption slow process.
Applications:
active surface area
measurements
surface site energetics
catalytic site determination
= strength of surface bonding
= equilibrium bond distance
= H(ads)
Ea(ads) = 0
Ea(des) = - H(ads)
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Thermal Desorption Spectroscopy
Thermal desorption spectra of CO
on Pd(100) after successive
exposure to CO gases
Desorption Rate
{-dNa
dT
dT
dt } = Nam
kexp(-E
dRT )
Linear heating rate
T = T0 + t
dT
dt=
Assuming kand Ed are independent of coverage
and m = 1 (i.e., first order desorption)
0.2 - 50 L-dNa
dT
d -dNa
dTdT[ ]
EdRTp
2 = exp(-EdRT
)k
Thermal Desorption Spectroscopy
Determination of Edes using different
heating rates ()
EdRTp
2 = exp(-EdRT
)k
slope, m EaTPD provides important informationon adsorption/desorption energetics
and adsorbate-surface interactions.
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Thermal Desorption Spectroscopy
Thermal desorption spectra of CO
on Ni(100) after successive
exposure to CO gases
0.2 - 50 L Assuming kand Ed are independent of coverageand m = 2 (i.e., first order desorption)-dNa
dT
d -dNa
dTdT[ ] Second order desorption
EdRTp
2 = exp(-EdRT
)k
2(N
a)p
Characterized by a shift in the peak maxima
toward lower temperature as the coverage
increases
Activation Energies for CO Desorption
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Influence of Surface Overlayer
Catalyst poison, strong adsorbates and coke
Sulfur-treatedcatalyst
Clean catalyst
CO desorption
Ordered Adsorbate layer
H2/Rh(110) O2/Rh(110)
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2 TPD from Rh(110)
Thermal Desorption Spectroscopy
Ordered Adsorbate layer
benzene/ZnO(1010)
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Kelvin Probe
Measures the change in work function ()
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 is the least amount of energy needed
for an electron to escape from metal to