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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



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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


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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physisorption/

desorptionchemisorption

CO

d

E(d)

physisorption




direct chemisorption

CO

d

E(d)

physisorption



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Energy barrier

Ea(ads) ~ 0

Ea(ads) > 0


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


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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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


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


Applications:

active surface area

measurements





= H(ads)

Ea(ads) = 0

Ea(des) = - H(ads)


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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


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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on Ni(100) after successive


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)



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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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


Applications:

active surface area

measurements





= H(ads)

Ea(ads) = 0

Ea(des) = - H(ads)


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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


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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on Ni(100) after successive


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


H2/Rh(110) O2/Rh(110)


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2 TPD from Rh(110)



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

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