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Example Heterogeneous Catalytic Reaction Process The long journey for reactant molecules to. travel within gas phase . cross gas-liquid phase boundary. travel within liquid phase/stagnant layer. cross liquid-solid phase boundary. reach outer surface of solid. diffuse within pore. arrive at reaction site. be adsorbed on the site and activated. 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
Heat transfer follows similar track
gas phase
poreporous solid
liquid phase /stagnant layer
gas phasereactant molecule
Catalysis & Catalysts
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Some common solid support / carrier materials
Alumina Inexpensive Surface area: 1 ~ 700 m2/g Acidic
Silica Inexpensive Surface area: 100 ~ 800 m2/g Acidic
Zeolite mixture of alumina and silica, often exchanged metal ion present shape selective acidic
Solid CatalystsCatalysis & Catalysts
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)
poreporous solid
Active site
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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 bondnumber 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
Adsorption on Solid SurfaceCatalysis & Catalysts
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Adsorption processAdsorbent 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, The solid surface may be completely or partially covered by adsorbed molecules
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.
Adsorption on Solid SurfaceCatalysis & Catalysts
define = = 0~1number of adsorption sites occupiednumber of adsorption sites available
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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)
Adsorption on Solid SurfaceCatalysis & Catalysts
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Activated Carbon
Surface area ~ 1000 m2/g
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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 Increase mechanical strength Increase surface area (98% surface area is supplied within the porous structure) may or may not be catalytically active
Solid CatalystsCatalysis & Catalysts
CatalystActiv
e ph
ase
Support
Promoter
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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
HHHH
HH HH
HH
H
HH
HH
H H
H2 absorption palladium hydride
Surface process bulk process
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NomenclatureSubstrate 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
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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.
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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 liquefaction (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
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Analytical Methods for Establishing Surface Bonds
Infrared Spectroscopy
Atoms vibrate 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
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I.R. Measurement
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I.R. Spectrum of CO2
A dipole moment = charge imbalance in the molecule
Asymmetric stretch
Vertical and horizontal bend
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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
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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 on crystal planes and may be influenced by surface reconstruction.
Tungsten
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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
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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
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Physisorption versus chemisorption
Adsorption Energetics
surface
E(ads) E(ads) < E(ads)Physisorption Chemisorption
small minima large minimaweak Van der Waals formation of surfaceattraction force chemical bonds
repulsive force
attractive forces
Chemisorption
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Physical Adsorption
d
metal surface
nitrogen
Van der Waals 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
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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).
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Adsorption IsothermIUPAC Classification
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Adsorption IsothermIUPAC 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).
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Adsorption IsothermCapillary Condensation• Mesopores Capillary condensation Hysteresis occurs
• Different hysteresis Different network structureNarrow distribution of uniform pores Type IVaComplex structure made up of interconnected networks of different pore sizes and shapes Type IVb
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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)
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IsothermsLangmuir isotherm
S - * + A(g) S-A
surface sitesAdsorbed molecules
H(ads) is independent of the process is reversible and is at equilibrium
[S-A] [S - *] [A]K =
S-A] is proportional to [S-*] is proportional to 1-[A] is proportional to partial pressure of A
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IsothermsLangmuir isotherm
(1-) Pb =
Where b depends only on the temperature
bP 1+ bP =
Molecular chemisorption or physisorption
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 10
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)
bP 1+ bP =
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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
lnP
T Ti
1/T
(P1, T1) (P2, T2)
lnP( ads
R1/T ) =const
=
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Adsorption Isotherms
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Henry’s Adsorption Isotherm
Special case of Langmuir isotherm
bP << 1
= bP
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The Freundlich IsothermAdsorption sites are distributed exponentially with H(ads)
H(ads)
i
(1-i)biP =
iNi Ni
=
R Aln = lnP + B
kP1/n = Valid for low partial pressuremost frequently used for describing pollutant adsorption on activated carbons
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The Temkin Isotherm
H(ads) decreases with
A lnBP = H(ads)
Valid at low to medium coveragegas chemisorption on clean metal surfaces
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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
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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 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.
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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)
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Chemical Adsorption ProcessesPhysisorption + molecular chemisorption
d
E(d) physisorption
chemisorption
CO
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Chemical Adsorption ProcessesPhysisorption + dissociative chemisorption
d
E(d)
dissociation
chemisorption
H2
H2 2 H
physisorption
atomic chemisorption
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Chemical Adsorption Processes
Physisorption + molecular chemisorption
physisorption/desorption chemisorption
CO
d
E(d)
physisorption
atomic chemisorption
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Chemical Adsorption ProcessesPhysisorption + molecular chemisorption
direct chemisorption
CO
d
E(d)
physisorption
atomic chemisorption
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Chemical Adsorption Processes
Energy barrier
~ -H(ads)
- Eades = -E(ads)
Chemical Adsorption is usuallyan energy activated process
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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