1 example heterogeneous catalytic reaction process the long journey for reactant molecules to...

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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