adsorbsi katalis
TRANSCRIPT
Adsorption and Catalysis
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 onpalladium
H
H
HH
H
HH H
H
H
HH
HH
H
H
H H
H2 absorption palladium hydride
Surface process bulk process
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 H H H H Hadsorbate
adsorbent
coverage fraction of surface sites occupied
Types of Adsorption Modes
Physical adsorption orphysisorption
Chemical adsorption orchemisorption
Bonding between molecules andsurface is by weak van der Waalsforces.
Chemical bond is formed betweenmolecules and surface.
Characteristics of Chemi- and Physisorptions
Chemisorption
virtually unlimited range
wide range (40-800 kJmol-1)
marked difference forbetween crystal planes
often dissociative andirreversible 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 andreversible
multilayer occurs often
fast, non-activated process
Properties
Adsorption temperature
Adsorption enthalpy
Crystallographicspecificity
Nature of adsorption
Saturation
Adsorption kinetic
Analytical Methods for Establishing Surface BondsInfrared 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-1Mid-IR: 500 – 4000 cm-1Far-IR: 5 – 500 cm-1
http://infrared.als.lbl.gov/FTIRinfo.html
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
I.R. Spectrum of NO on Pt
Tem
pera
ture
incr
ease
s
Adsorption decreases
Molecular conformationchanges
I.R. Spectrum of HCN on Pt
0.15 L HCN, 100 Kweak chemisorption
1.5 L HCN, 100 Kphysisorption
30 L HCN, 200 Kdissociative chemisorption
H- C
N
Pt
(H-CN)(HCN)(HCN)
Pt
H- C
N
H
- C
N
(CN)
C
N
Pt
(a) (b) (c)
Adsorption Rate
Rads = k C x
x - kinetic order k - rate constantC - gas phase concentration
Rads = k’ P x
x - kinetic order k’ - rate constantP - partial pressure of molecule
Rads = A C x exp (-Ea/RT)
Activation energyFrequency factor
Temperature dependencyof 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
Sticking Coefficient
S = f() exp(-Ea/RT)where 0 < S < 1
S also depends on crystal planes and may be influenced by surface reconstruction.
Tungsten
Sticking Coefficient
Sticking CoefficientSteering Effects
Surface Coverage ()
Estimation based on gas exposure
Rads = dNads/dt = S • F
Nads S • F • tExposure time
Molecules adsorbed perunit surface area
Nearly independentof coverage for mostsituations
Adsorption Energetics
d
surface
adsorbate
Potential energy (E) for adsorption is only dependent on distancebetween 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 minimaweak Van der Waal formation of surfaceattraction force chemical bonds
repulsive force
attractive forces
Chemisorption
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 = 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
Plot P/n(P0-P) versus P/P0
calculate c and nm from the slope (c-1/ nmc) andintercept (1/nmc) of the isothermmeasurements usually obtained for P/P0 < 0.2
c = 69.25nm = 4.2 x 10-3 molArea = 511 m2/g
c = 87.09nm = 3.9 x 10-3 molArea = 480 m2/g
BET Measurements
DegassingDegassing
Pure gas introduces into supply Pure gas introduces into supply chamber chamber constant P constant P11 T T11 are are
recorded recorded V V11
Gas flows into adsorption cellGas flows into adsorption cell
PP22 and T and T22 are recorded when are recorded when
equilibrium is reached equilibrium is reached V V22
P 1 T 1 P 2 T 2
To vacuum
Gas cylinder
Gas SupplyChamber
AdsorptionCell
Volumetric Method
BET Measurements
Dynamic Method
DegassingDegassingFlow carrier gas (He)Flow carrier gas (He)
Pulse NPulse N22/He into adsorption cell /He into adsorption cell at a given Pat a given PN2N2
Record the amount of nitrogen Record the amount of nitrogen adsorbed using TCD adsorbed using TCD Calculate surface areaCalculate surface area
(Rouquerol, 1999)
BET Measurements
Gravimetric Method
DegassingDegassingRecord initial weight of adsorbent Record initial weight of adsorbent MM11
Introduce pure gas into Introduce pure gas into adsorption celladsorption cellRecord the adsorbent equilibrium Record the adsorbent equilibrium weight Mweight M22
Record the equilibrium pressureRecord the equilibrium pressure
(Rouquerol, 1999)
Adsorption Isotherm
Adsorption Isotherm:Adsorption Isotherm:– The equilibrium relationship between the amount adsorbed The equilibrium relationship between the amount adsorbed
and the pressure or concentration at constant temperature and the pressure or concentration at constant temperature (Rouquerol et al., 1999).(Rouquerol et al., 1999).
Importance of ClassificationImportance of Classification– Providing an efficient and systematic way for theoretical Providing an efficient and systematic way for theoretical
modeling of adsorption and adsorbent characteristics modeling of adsorption and adsorbent characteristics determinationdetermination
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
Type I(Activated Carbon,
Zeolites)
Micropores
(< 2 nm)
Type III(Bromine onsilica gel)*
Type V(Water oncharcoal)*
Weakinteraction
Type II(Clay, pigments,
cements)
Type IV(oxide gels,
zeolites)
Stronginteraction
Macropores
(> 50 nm)
Mesopores
(2 – 50 nm)
* Do, D. D., Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London (1998).
Adsorption Isotherm
Capillary Condensation
Mesopores Mesopores
Capillary condensationCapillary condensation
Hysteresis occursHysteresis occurs
Different hysteresis Different hysteresis Different network structure Different network structure
Narrow distribution of uniform pores Narrow distribution of uniform pores Type IVa Type IVa
Complex structure made up of interconnected networks of Complex structure made up of interconnected networks of different pore sizes and shapes different pore sizes and shapes Type IVb Type IVb
Adsorption Isotherm
Type VI Isotherm
Highly uniform surfaceHighly uniform surface
Layer by layer adsorptionLayer by layer adsorption
Stepped isothermStepped isotherm
Example:Example:
Adsorption of simple non-porous Adsorption of simple non-porous molecules on uniform surfaces molecules on uniform surfaces (e.g. basal plane of graphite)(e.g. basal plane of graphite)
Adsorption Isotherm
Composite Isotherm
N2 adsorption in (a) micropores and (c) micropores and mesopores
Type I Type I & IV
(Rouquerol, 1999)
Chemical Adsorption
d
Pt surface
CO
E(d)
re
Note: there is no activation barrier for 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)
Chemical Adsorption ProcessesPhysisorption + molecular chemisorption
d
E(d) physisorption
chemisorption
CO
Chemical Adsorption ProcessesPhysisorption + dissociative chemisorption
d
E(d)dissociation
chemisorption
H2H2 2 H
physisorption
atomic chemisorption
Note: this is an energy prohibitive process
Chemical Adsorption ProcessesPhysisorption + molecular chemisorption
physisorption/desorption chemisorption
CO
d
E(d)
physisorption
atomic chemisorption
Chemical Adsorption ProcessesPhysisorption + molecular chemisorption
direct chemisorption
CO
d
E(d)
physisorption
atomic chemisorption
Chemical Adsorption ProcessesEnergy barrier
Ea(ads) ~ 0
Ea(ads) > 0
Chemical Adsorption ProcessesEnergy barrier
~ -H(ads)
- Eades = -E(ads)
Chemical Adsorption is usuallyan energy activated process.
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
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-ordinationsites:
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 dissociativechemisorption occurs.molecular chemisorption -donor or-acceptor interactions.
dissociative chemisorption occupy highest co-ordinated surface sites, alsocauses surface distorsion.
O=O
O=O
OO
Nitrogen
molecular chemisorption -donor or-acceptor interactions.
NN
NN
Adsorbate Geometries on Metals
Carbon monoxide
Carbon monoxide
forms metal carbides with metals locatedat the left-hand side of the periodic table.
molecular chemisorption occurs on d-blockmetals (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
Active Surface Area Measurement
ost common chemisorption gases: hydrogen, oxygen and carbon monoxide
Pulse H2, O2
or CO gases
exhaustcarrier gashelium or argon
thermal conductivity cell (TCD)
furnace
catalyst
Catalyst Surface Area and Dispersion Calculation
Pulse H2 thentitrate with O2
exhaustcarrier gashelium 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
Avogrado’s number: 6.022 x 1023
Pt lattice constant: a = 3.92 (FCC) Calculate surface area of Pt and its dispersion.
Isotherms
Langmuir isotherm
S - * + A(g) S-A
surface sites
Adsorbed molecules
H(ads) is independent of the 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-) P
b =
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
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 of H(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
=
Adsorption Isotherms
Henry’s Adsorption Isotherm
Special case of Langmuir isotherm
bP << 1
= bPV = k’P where k’ = bV
The Freundlich Isotherm
Adsorption sites are distributed exponentially with H(ads)
H(ads)
i
(1-i)biP =
iNi Ni
=
R A
In = InP + B
kP1/n =Valid for low partial pressuremost 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 coveragegas chemisorption on clean metal surfaces
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 barrier for 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)
Thermal Desorption Spectroscopy
Thermal desorption spectra of CO on Pd(100) after successive exposure to CO gases
Desorption Rate
{-dNadT
dTdt } = Na
mk exp( -Ed
RT )
Linear heating rate
T = T0 + t
dTdt
=
Assuming k and Ed are independent of coverageand m = 1 (i.e., first order desorption)
0.2 - 50 L-dNa
dT
d -dNadTdT
[ ]
Ed
RTp2 = exp( -Ed
RT ) k
Thermal Desorption Spectroscopy
Determination of Edes using different heating rates ()
Ed
RTp2 = exp( -Ed
RT ) k
slope, m EaTPD provides important informationon adsorption/desorption energeticsand adsorbate-surface interactions.
Thermal Desorption Spectroscopy
Thermal desorption spectra of CO on Ni(100) after successive exposure to CO gases
0.2 - 50 L Assuming k and Ed are independent of coverageand m = 2 (i.e., first order desorption)-dNa
dT
d -dNadTdT
[ ] Second order desorption
Ed
RTp2 = exp( -Ed
RT ) k
2(Na)p
Characterized by a shift in the peak maximatoward lower temperature as the coverageincreases
Activation Energies for CO Desorption
Influence of Surface Overlayer
Catalyst poison, strong adsorbates and coke
Sulfur-treatedcatalyst
Clean catalystCO desorption
Ordered Adsorbate layer
H2/Rh(110) O2/Rh(110)
TPD from Rh(110)
Thermal Desorption Spectroscopy
Ordered Adsorbate layer
benzene/ZnO(1010)
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 electricalfield microscopy
Kelvin Probe
Basic principle
Vibrating capacitor measures is the least amount of energy needed for an electron to escape from metal to vacuum.is sensitive optical, electrical and mechanical properties of materials
refref
Dr. King Lun YeungDr. King Lun Yeung
Department of Chemical EngineeringDepartment of Chemical Engineering
Hong Kong University of Science and Hong Kong University of Science and TechnologyTechnology
CENG 511 Lecture 3