Catalysis & Catalysts
Adsorption On Solid Surface
 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 de Waal
chemcal bond
number 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
1
Catalysis & Catalysts
Solid Catalysts

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
Catalyst
Support

Increase mechanical strength
 Increase surface area (98% surface area is supplied within the porous structure)
 may or may not be catalytically active
2
Catalysis & Catalysts
Solid Catalysts
 Some

common solid support / carrier materials
Alumina


Inexpensive
 Surface area: 1 ~ 700 m2/g
 Acidic

Silica

Inexpensive
 Surface area: 100 ~ 800 m2/g
 Acidic

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)
Active site
Zeolite

mixture of alumina and silica,
 often exchanged metal ion present
 shape selective
 acidic
porous
solid
pore
3
Pores of Porous Solids
 Pore
sizes

micro pores dp <20-50 nm

meso-pores 20nm <dp<200nm

macro pores dp >200 nm

Pores can be uniform (e.g. polymers) or non-uniform (most metal oxides)
 Pore


size distribution
Typical curves to characterise pore size:

Cumulative curve

Frequency curve
Uniform size distribution (a) &
non-uniform size distribution (b)
dw
dd
wt
Dwt
a
b
Dd
a
b
d
Cumulative curve
d
Frequency curve
4
Catalysis & Catalysts
Adsorption On Solid Surface
 Adsorption
process
Adsorbent 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, q
The solid surface may be completely or partially covered by adsorbed molecules
define
number of adsorption sites occupied
q=
number of adsorption sites available
q = 0~1
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.
5
Catalysis & Catalysts
Adsorption On Solid Surface
 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)
6
Adsorption On Solid Surface
Characterisation of adsorption system

Adsorption isotherm - most commonly used, especially to catalytic reaction system, T=const.
The amount of adsorption as a function of pressure at set temperature

Adsorption isobar - (usage related to industrial applications)
The amount of adsorption as a function of temperature at set pressure

Adsorption Isostere - (usage related to industrial applications)
Adsorption pressure as a function of temperature at set volume
V3>V2
V2>V1
T2 >T1
T3 >T2
T4 >T3
P3>P2
P2>P1
P1
V4>V3
V1
Pressure
T1
Vol. adsorbed
P4>P3
Vol. adsorbed

T5 >T4
Pressure
Adsorption Isotherm
Temperature
Adsorption Isobar
Temperature
Adsorption Isostere
7
Adsorption On Solid Surface
 The

Langmuir adsorption isotherm
Basic assumptions
surface uniform (DHads does not vary with coverage)
 monolayer adsorption, and
 no interaction between adsorbed molecules and adsorbed molecules immobile


Case I - single molecule adsorption
A
when adsorption is in a dynamic equilibrium
A(g) + M(surface site) D AM
the rate of adsorption
rads = kads (1-q) P
the rate of desorption
rdes = kdes q
at equilibrium
rearrange it for q
let
case I
rads = rdes  kads (1-q) P = kdes q
q
(kads / kdes ) P
1  (kads / kdes ) B0 P
B0 
kads
kdes
 q
Cs
BP
 0
C 1  B0 P
B0 is adsorption coefficient
8
Adsorption On Solid Surface
 The

Langmuir adsorption isotherm (cont’d)
Case II - single molecule adsorbed dissociatively on one site
A-B(g) + M(surface site) D A-M-B
the rate of A-B adsorption rads=kads (1-qA )(1-qB)PAB=kads (1-q )2PAB
q=qA=qB
the rate of A-B desorption rdes=kdesqAqB =kdesq2
at equilibrium rads = rdes 
rearrange it for q
Let.
q
kads (1-q )2PAB=
q2
A
A
B
B
case II
kdes
(k ads / k des ) PAB
1  (k ads / k des ) PAB
k
B0  ads
kdes

Cs
( B0 PAB )1/2
q

C 1  ( B0 PAB )1/2
9
Adsorption On Solid Surface
 The

Langmuir adsorption isotherm (cont’d)
Case III - two molecules adsorbed on two sites
A(g) + B(g) + 2M(surface site) D A-M + B-M
the rate of A adsorption
rads,A = kads,A (1- qA- qB) PA
the rate of B adsorption
rads,B = kads,B (1- qA- qB) PB
the rate of A desorption
rdes,A = kdes,A qA
the rate of B desorption
rdes,B = kdes,B qB
at equilibrium

rearrange it for q
where
rads ,A = rdes ,A
A B
case III
and  rads ,B = rdes ,B
kads,A(1-qA-qB)PA=kdes,AqA and kads,B(1-qA-qB)PB=kdes,BqB
qA 
Cs ,A
B0 ,A PA

C 1  B0 ,A PA  B0 ,B PB
B0 ,A
kads ,A
kads ,B

and B0 ,B 
kdes ,A
kdes ,B
qB 
Cs ,B
B0 ,B PB

C 1  B0 ,A PA  B0 ,B PB
are adsorption coefficients of A & B.
10
Adsorption On Solid Surface
 The
Langmuir adsorption isotherm (cont’d)
A
A
case I
C
BP
q s  0
C 1  B0 P
k
B0  ads
kdes
B
qA 
Cs
( B0 PAB )1/2
q

C 1  ( B0 PAB )1/2
kads
kdes
B0 ,A 
Adsorption
Adsorption
Strong kads>> kdes
B0>>1 q  Cs  1
C
Weak kads<< kdes
B0<<1 q 
A, B both strong
kads>> kdes
B0>>1 q 
case III
B0 ,A PA
Cs ,A

C 1  B0 ,A PA  B0 ,B PB
C
B0 ,B PB
q B  s ,B 
C 1  B0 ,A PA  B0 ,B PB
case II
B0 
A B
Cs
1
C
kads<< kdes
Cs
C
 B0 P B0<<1 q  s  ( B0 P)1/2
C
C
A strong, B weak
A weak, B weak
kads ,A
k
and B0 ,B  ads ,B
kdes ,A
kdes ,B
Cs ,A
B0 ,A PA

C
B0 ,A PA  B0 ,B PB
C
B0 ,B PB
q B  s ,B 
C
B0 ,A PA  B0 ,B PB
qA 
q A  C s , A / C  1
q B  Cs ,B / C  ( B0 ,B / B0 ,A )
q A  Cs ,A / C  B0 ,A PA
q B  Cs ,B / C  B0 ,B PB
PB
PA
11
Adsorption On Solid Surface
case I
case II
Case III
q
adsorption isotherm
Cs
BP
 0
C 1  B0 P
Cs
( B0 PAB )1/2
q

C 1  ( B0 PAB )1/2
C
B0 ,A PA
q A  s ,A 
C 1  B0 ,A PA  B0 ,B PB
C
B0 ,B PB
q B  s ,B 
C 1  B0 ,A PA  B0 ,B PB
Strong adsorption
Weak adsorption
Amount adsorbed
 Langmuir
kads>> kdes q  Cs  1
kads<< kdes
C
C
q  s  B0 P
C
mono-layer
large B0 (strong adsorp.)
moderate B0
small B0 (weak adsorp.)
Pressure
 Langmuir
 It
adsorption isotherm established a logic picture of adsorption process
fits many adsorption systems but not at all
 The
assumptions made by Langmuir do not hold in all situation, that causing error
Solid surface is heterogeneous thus the heat of adsorption is not a constant at different q
 Physisorption of gas molecules on a solid surface can be more than one layer

12
Adsorption On Solid Surface

Five types of physisorption isotherms are found over all solids
I
amount adsorbed
II
III
IV
V
1.0
relative pres. P/P0

Type I

Type II for non-porous materials

Type III porous materials with cohesive force between adsorbate
molecules greater than the adhesive force between adsorbate
molecules and adsorbent

Type IV staged adsorption (first monolayer then build up of additional
layers)

Type V porous materials with cohesive force between adsorbate
molecules and adsorbent being greater than that between
adsorbate molecules
is found for porous materials with small pores e.g. charcoal.
It is clearly Langmuir monolayer type, but the other 4 are not
13
Adsorption On Solid Surface

Other adsorption isotherms
Many other isotherms are proposed in order to explain the observations
The Temkin (or Slygin-Frumkin) isotherm

Assuming the adsorption enthalpy DH decreases linearly with surface coverage
From ads-des equilibrium, ads. rate  des. rate
rads=kads(1-q)P  rdes=kdesq
B0 P
b1eQs / RT P
q
 qs 
1  B0 P
1  b1eQs / RT P
DH of ads

Langmuir
Temkin
where Qs is the heat of adsorption. When Qs is a linear function of qi. Qs=Q0-iS (Q0 is a
constant, i is the number and S represents the surface site),
the overall coverage
q

[b1eQs / RT P
RT 
1  b1P

q   q s dS  
dS

ln
 1  b P exp(- i  
0
0 (1 b eQs / RT P
i
1
1
RT 

1
1
When b1P >>1 and b1Pexp(-i/RT) <<1, we have q =c1ln(c2P), where c1 & c2 are constants

Valid for some adsorption systems.
14
Adsorption On Solid Surface
The Freundlich isotherm

assuming logarithmic change of adsorption enthalpy DH with surface coverage
From ads-des equilibrium, ads. rate  des. rate
DH of ads

rads=kads(1-q)P  rdes=kdesq
B0 P
b1eQi / RT P
q
 qi 
1  B0 P
1  b1eQi / RT P
Langmuir
Freundlich
q
where Qi is the heat of adsorption which is a function of qi. If there are Ni types of surface
sites, each can be expressed as Ni=aexp(-Q/Q0) (a and Q0 are constants), corresponding to a
fractional coverage qi,
i qi Ni 0 [b1eQ / RT P / (1 b1eQ / RT P)] aeQ/Q0 dQ
the overall coverage q 


Q/Q0
N
 i
a
e
dQ

i
0
the solution for this integration expression at small q is:
lnq=(RT/Q0)lnP+constant, or
as is the Freundlich equation normally written,

q  c1 p1/ C where c1=constant, 1/c2=RT/Q0
2
Freundlich isotherm fits, not all, but many adsorption systems.
15
Adsorption On Solid Surface

BET (Brunauer-Emmett-Teller) isotherm

Many physical adsorption isotherms were found, such as the types II and III, that the
adsorption does not complete the first layer (monolayer) before it continues to stack
on the subsequent layer (thus the S-shape of types II and III isotherms)

Basic assumptions

the same assumptions as that of Langmuir but allow multi-layer adsorption
 the heat of ads. of additional layer equals to the latent heat of condensation
 based on the rate of adsorption=the rate of desorption for each layer of ads.
the following BET equation was derived
P / P0
1 c -1


( P / P0 )
V ( 1 - P / P0 ) cVm cVm
Where
P - equilibrium pressure
P0 - saturate vapour pressure of the adsorbed gas at the temperature
P/P0 is called relative pressure
V - volume of adsorbed gas per kg adsorbent
Vm -volume of monolayer adsorbed gas per kg adsorbent
c - constant associated with adsorption heat and condensation heat
Note: for many adsorption systems c=exp[(H1-HL)/RT], where H1 is adsorption heat of 1st layer &
HL is liquefaction heat, so that the adsorption heat can be determined from constant c.
16
Adsorption On Solid Surface

Comment on the BET isotherm

BET equation fits reasonably well all known adsorption isotherms observed so far
(types I to V) for various types of solid, although there is fundamental defect in the
theory because of the assumptions made (no interaction between adsorbed
molecules, surface homogeneity and liquefaction heat for all subsequent layers
being equal).

BET isotherm, as well as all other isotherms, gives accurate account of adsorption
isotherm only within restricted pressure range. At very low (P/P0<0.05) and high
relative pressure (P/P0>0.35) it becomes less applicable.

The most significant contribution of BET isotherm to the surface science is that the
theory provided the first applicable means of accurate determination of the surface
area of a solid (since in 1945).

Many new development in relation to the theory of adsorption isotherm, most of them
are accurate for a specific system under specific conditions.
17
Adsorption On Solid Surface

Use of BET isotherm to determine the surface area of a solid

At low relative pressure P/P0 = 0.05~0.35 it is found that
P / P0
1 c -1


( P / P0 )  ( P / P0 )
V ( 1 - P / P0 ) cVm cVm
Y
 The
= a +b
P / P0
V (1- P / P0 )
P/P0
X
principle of surface area determination by BET method:
A plot of
P / P0
V (1- P / P0 )
against P/P0 will yield a straight line with slope of equal to (c-1)/(cVm)
and intersect 1/(cVm).
For a given adsorption system, c and Vm are constant values, the surface area of a solid
material can be determined by measuring the amount of a particular gas adsorbed on the
surface with known molecular cross-section area Am,
V
As  Am N m  Am m  6.022  1023
VT , P
Vm - volume of monolayer adsorbed gas molecules calculated from the plot, L
VT,P - molar volume of the adsorbed gas, L/mol
Am - cross-section area of a single gas molecule, m2
* In practice, measurement of BET surface area of a solid is carried out by N2 physisorption
at liquid N2 temperature; for N2, Am = 16.2 x 10-20 m2
18
Adsorption On Solid Surface

Summary of adsorption isotherms
Name
Isotherm equation
Cs
BP
 0
C 1  B0 P
Application
Note
Chemisorption and
physisorption
Useful in analysis of
reaction mechanism
q =c1ln(c2P)
Chemisorption
Chemisorption
Freundlich
q  c1 p1/ C2
Chemisorption and
physisorption
Easy to fit adsorption
data
BET
P / P0
1 c -1


( P / P0 )
V ( 1 - P / P0 ) cVm cVm
Multilayer physisorption
Useful in surface area
determination
Langmuir
q
Temkin
19
The BET isotherm

Theoretical development based on several
assumptions:


OT fig1.3


multimolecular adsorption
1st layer with fixed heat of adsorption H1
following layers with heat of adsorption
constant (= latent heat of condensation)
constant surface (i.e. no capillary
condensation) gives
p
1
C -1 p



v a (p 0 - p  v m  C v m  C p 0
or
p
p
 I s
v a (p 0 - p 
p0
20
The BET isotherm, cont.

Plot of left side vs. p/p0 should give
straight line with slope s and intercept I
p
p
 I s
v a (p 0 - p 
p0
OT fig1.5

Reorganizing gives
1
Is
and C 
sI
I
Knowledge of S0 (specific area for a
volume of gas then allows the
calculation of the specific surface area
Sg:
vm 

Sg 
v m  S0
mp
where mp is the mass of the sample
21
BET cont’d

BET method useful, but has limitations

microporous materials: mono - multilayer adsorption cannot occur, (although BET surface
areas are reported routinely)
 assumption about constant packing of N2 molecules not always correct?
 theoretical development dubious (recent molecular simulation studies, statistical
mechanics) - value of C is indication o f the shape of the isotherm, but not necessarily
related to heat of adsorption
22
Simplified method

1-point method

simplefied BET assuming value of C  100 (usually the case), gives
p
1
C -1 p
p



 '
v a (p 0 - p  v m  C v m  C p 0 v m  p 0
v 'm 


v a  (p 0 - p 
p0
usually choose p/p0  0,15
method underestimates the surface area by approx. 5%.
23
Adsorbates

An adsorbate molecule covers an area , calculated assuming dense packing of the
molecules in the multilayer. The corresponding area per volume gas is S0:
Gas
Temp.
[K]
σ
[Å2/molecule]
S0
[m2/cm3 gas (STP)]
N2
Kr
Ar
H2O
C2H6
CO2
77,5
77,5
77,5
298
90
195
16,2
19,5
14,6
10,8
22,5
19,5
4,36
5,24
3,92
2,90
6,05
5,24
24
Porosity and pore size

The pore structure (porosity, pore diameter, pore shape) is important for the catalytic
properties

pore diffusion may influence rates
 pores may be too small for large molecules to diffuse into

Measurement techniques:

Hg penetration
 interpretation of the adsorption - desorption isotherms
 electron microscopy techniques
25
Hg penetration

Based on measuring the volume of a non-wetting liquid forced into the pores by
pressure (typically mercury)
 Surface tension will hinder the filling of the pores, at a given pressure an equilibrium
between the force due to pressure and the surface tension is established:
P    r 2  -2  r    cos
where P = pressure of Hg,  is surface tension and  is the angle of wetting
 Common values used:
 = 480 dyn/cm and = 140° give average pore radius
r
75000
Å
2
P[kp / cm ]
valid in the range 50 - 50000Å
26
Pore size distribution

If the Hg-volume is recorded as a function of
pressure and this curve is differentiated we can find
the pore size distribution function
V(r)=dV/dr
OT fig 2.3.
27
The Kelvin equation

If adsorbent is mesoporous we get Type
IV isotherm
 Deviation upwards is due to filling of
mesopores by capillary condensation curved liquid meniscus in narrow
_ pores
with radius rk:
- 2   V
rk 
 p
RT ln 
 p0 
V is molar volume of the liquid, minus
sign introduced since in the actual range
of measurement 0 < p/p0 <1
28
The Kelvin equation

Since capillary condensation is preceeded by multilayer adsorption on the wall the value
is corrected with t, the thickness of this layer:
Cylindrical pores: rp = rk + t
Parallell sided slits: dp = rk + 2t
Value of t determined from measurements without capillary condensation
 Practical experience, typical values give for circular pores:
rk  -

9,547
[ Å]
 p
ln 
 p0 
Values for t have been found to be a function of rk, e.g. for rk > 20Å:
t  0,429 (ln rk 
2, 61
Å
29
Adsorption-desorption hysteresis

Hysteresis is classified by IUPAC (see fig.)
 Traditionally desorption branch used for
calculation
 H1: narrow distribution of mesopores
 H2: complex pore structure, network
effects, analysis of desorption loop
misleading

Handbook
fig 2 s 431

H2: typical for activated carbons
H3 & 4: no plateau, hence no well-defined
mesopore structure, analysis difficult

H3: typical for clays
30