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Catalysis and Catalysts - Physical Adsorption
Physical Adsorption
Texture and morphology pore size pore shape pore-size distribution (same size or various
sizes?) pore volume specific surface area of catalyst
-
Catalysis and Catalysts - Physical Adsorption
Internal Diffusion
Types of diffusion Molecular Knudsen Surface (also called configurational diffusion)
Knudsen number: Kn = /l = molecular free path lengthl = characteristic pore diameterKn > 1 Knudsen diffusion
-
Catalysis and Catalysts - Physical Adsorption
Pore Diameters, Shapes??
Pore diameters micropores (< 2 nm) mesopores (2 - 50 nm) macropores (> 50 nm)
Experimental techniques capillary condensation Hg intrusion microscopy
Shapes cylinder, slit, ink-bottle, wedge, ...
-
Catalysis and Catalysts - Physical Adsorption
Pore Size and Diffusion Regimes
Configurational diffusion
Surface migration
-
Catalysis and Catalysts - Physical Adsorption
Pore Diameters and Measurement Techniques
1 10 100 1000 10000
Pore diameter (nm)
Micro Meso Macro2 50
N2 capillary condensation
Hg porosimetry
-
Catalysis and Catalysts - Physical Adsorption
Shape Selectivity
Reactant selectivity
+
Product selectivity
CH3OH +
Restricted transition-state selectivity
-
Catalysis and Catalysts - Physical Adsorption
Pore Shapes
Slit
Ink-bottle
Cylindrical
Wedge
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Catalysis and Catalysts - Physical Adsorption
Pore Structures of Zeolites
a b
ZSM-5 Mordenite
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Catalysis and Catalysts - Physical Adsorption
Barometric Adsorption Measurement
N2 (77.3 K)
Ar, He, CH4, CO2, Kr
adsorbate
adsorbent
pressuregauge
P V1
V2
high vacuum
Conditions??
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Catalysis and Catalysts - Physical Adsorption
Adsorption Isotherms
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1p/p 0
nad
(mm
ol/g
) 1
Adsorption
Desorption
Interpretation??
AluminaN2, 77 K
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Catalysis and Catalysts - Physical Adsorption
Adsorption Isotherms
I
n ad
p/p0 p/p
III
n ad
p/p0
VI
n ad
p/p0
V
n ad
p/p0
II
n ad
0
B
IV
n ad
p/p0B
-
Catalysis and Catalysts - Physical Adsorption
Langmuir Adsorption Isotherm (Type I)
I
nad
p /p 0
Assumptions:
homogeneous surface
(all adsorption sites energetically identical)
monolayer adsorption (so no multilayer adsorption)
no interaction between adsorbed molecules
pKpKnnn mmad +
==1
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Catalysis and Catalysts - Physical Adsorption
Type II and IV Isotherms
II
n ad
0
B
IV
n ad
p/p0B
Multilayer adsorption (starting at B)
Common for pore-free materials
same with
pore condensation at high p
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Catalysis and Catalysts - Physical Adsorption
Type III and V Isotherms
III
n ad
p/p0
V
n ad
p/p0
Strong cohesion force between adsorbed molecules, e.g. when water adsorbs on hydrophobic activated carbon
Similar to III at low p
Pore condensation at high p
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Catalysis and Catalysts - Physical Adsorption
Surface Area & Monolayer Capacity
S = nmAmN
monolayercapacity (mol/g)
specific surface area (m2/g)
area occupied by one molecule (m2/molecule)
Avogadros number (molecules/mol)
BET model: SBET
t model: St
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Catalysis and Catalysts - Physical Adsorption
Properties of Adsorbates for Physisorption Measurements
Adsorbate Boiling Point (K) Am (nm2/molecule)
N2 77.3 0.162
Ar 87.4 0.142
CO2 194.5 0.17
Kr 120.8 0.152
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Catalysis and Catalysts - Physical Adsorption
N2 Adsorption Isotherm in ZSM-5
0
1
2
3
4
5
6
0 0.2 0.4 0.6 0.8 1
p/p0
nad (
mm
ol/g
) 1
Langmuir Adsorption?
No:
at low p strong adsorption due to condensation in micropores
at higher p saturation due to finite (micro)pore volume
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Catalysis and Catalysts - Physical Adsorption
BET (Brunauer, Emmett, Teller) Method
Based on Langmuir isotherm Monolayer and multilayer adsorption Layers of adsorbed molecules divided in:
First layer with heat of adsorption Had,1 Second and subsequent layers with Had,2 = Hcond
BET isotherm:
BET equation does not fit entire adsorption isotherm different mechanisms play a role at low and at high p
( ) 0mm0ad11
pp
CnC
Cnppnp
+=
=
RTHHC condadexp
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Catalysis and Catalysts - Physical Adsorption
BET Model
reality model54
32 1
0
( )...321 210mad +++== nni
For every layer Langmuir model
RTH
RTH
RTH
KKK
KKcondn
ads
ee
e
0,n0,nn
0,11
=
=
( )
+
=
0
0
0m
ad
111ppC
pp
pp
Cnn
RTHH
Ccondads
e
=with
00 1
0 1 1 0 1 01a
a dd
kk p k p K pk
= = =1st layer
nth layer0
1n-1 n n n-1 n n-11
n n aa d
d
kk p k p K pk
= = =
Assume
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Catalysis and Catalysts - Physical Adsorption
Non-Porous Silica and Alumina
p/p0
n ad/n
m
(B) (A)
Low p/p0:
filling of micropores
favoured adsorption at most reactive sites (heterogeneity)
High p/p0:
capillary condensation
Range 0.05 < p/p0 < 0.3 is used to determine SBET
BET equation
-
Catalysis and Catalysts - Physical Adsorption
Texture Data of Commercial Catalysts
Material Mean dp (nm) SBET (m2/g)
Catalyst supports
Silica gel 10 200
6 400
4 800
-Al2O3 10 150
5 500
Zeolite 0.6-2 400-800
Activated carbon 2 700-1200
TiO2 400-800 2-50
Aerosil SiO2 - 50-200
Catalysts
MeOH synthesis (Cu/ZnO/Al2O3) 20 80
NH3 synthesis (Fe/Al2O3/K2O) 100 10
Reforming (Pt/Re/Al2O3) 5 250
Epoxidation (Ag/-Al2O3) 200 0.5
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Catalysis and Catalysts - Physical Adsorption
Adsorption at Pore Wall
Cylindrical pore
Ink-bottle pore Pore with shape of interstice between close-packed particles
Adsorbed layer
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Catalysis and Catalysts - Physical Adsorption
Kelvin Equation for Nitrogen
Rel
ativ
e pr
essu
re, p
/p0
rm (nm)
1.0
0.5
0.00.1 10 100 1000 10000
m0
12lnrRT
Vpp L
=
micro meso macro
VL = 34.6810-6 m3/mol
= 8.88 mN/m
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Catalysis and Catalysts - Physical Adsorption
Hysteresis Loops
HI
n ad
p/p0
H3
n ad
p/p0
H2
n ad
p/p0
Information on pore shape
-
Catalysis and Catalysts - Physical Adsorption
t-method
nm354.0m
ad=
nnt t m mS n A N=
nad
t
Proportional to St
Note:
nad is experimental result
t is calculated from correlation t versus p
9 6ad adt m t0.354 10 5.73 10
n nS A Nt t
= =
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Catalysis and Catalysts - Physical Adsorption
t-method
BET only valid in small pressure interval interpretation not very easy
thickness (t) of adsorbed layer can be calculated
plot of t versus p for non-porous materials is the same (has been checked experimentally)
t-plot helps in interpretation
0.354 nm
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Catalysis and Catalysts - Physical Adsorption
t-curves
p/p0
Thic
knes
s of
ads
orbe
d la
yer t
(nm
)
a
b
Halsey
Harkins-Jura-de Boer
( )333.0
0/ln00.5354.0
=
ppt
( )5.0
0/log034.099.131.0
=
ppt
-
Catalysis and Catalysts - Physical Adsorption
t-plot of -alumina
0
2
4
6
8
10
0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)
n ad (
mm
ol/g
)
St,micro= 0 m2/g
V t,micro = 0 ml/g
St = 200 m2/g
mesopores
macropores
-
Catalysis and Catalysts - Physical Adsorption
Shape of t-plots
nm354.0m
ad=
nnt
t
nad
t
nad
t
nad
Non-porous Microporous Micro- and mesoporous
St
Smesopores
p
nad
Adsorption isotherm
t = f(p)
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Catalysis and Catalysts - Physical Adsorption
t-plot of N2 Physisorption on ZSM-5
0
6
0.0 0.2 0.4 0.6 0.8
t ( nm)
nad
(m
mol
/g)
n 1
n 2
n 1 = liquid N2n 2 = solid N2
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Catalysis and Catalysts - Physical Adsorption
Pore-Size Distribution of -Alumina
0.0
0.1
0.2
0.3
0.4
0.5
1 10 100 1000d pore (nm)
dV/d
d (m
l/g/n
m)
-
Catalysis and Catalysts - Physical Adsorption
Mercury Intrusion Porosimetry
pd 14860=
Convenient method for determining pore volume versus pore size
Hg does not wet surfaces; pressure is needed to force intrusion
nm
bar
From a force balance:
-
Catalysis and Catalysts - Physical Adsorption
Mercury Intrusion Curve of -Alumina
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1 1 10 100 1000p (MPa)
V (m
l/g)
-
Catalysis and Catalysts - Physical Adsorption
Surface Areas - SHg and SBETAdsorbent SHg SBET
m2/g m2/g deg
Iron Oxide 14.3 13.3 130
Tungsten Oxide 0.11 0.10 130
Anatase 15.1 10.3 130
Hydroxy Apatite 55.2 55.0 130
Carbon Black (Spheron-6) 107.8 110.0 130
0.5 % Ru/-Al2O3 237.0 229.0 140
0.5 % Pd/-Al2O3 115.0 112.0 140
TiO2 Powder 31.0 25.0 140
Sintered Silica Pellets 20.5 5.0 140
Zeolite H-ZSM-5 39.0 375.0 140
Norit Active Carbon R1 Extra 112.0 915.0 140
-
Catalysis and Catalysts - Physical Adsorption
Discrepancy SHg and SBETfor Microporous Materials
Hg cannot penetrate small (micro)pores, N2 can Uncertainty of contact angle and surface tension values Cracking or deforming of samples
-
Catalysis and Catalysts - Physical Adsorption
Texture Properties
N2-physisorption Hg-porosimetry
SBET St Vp dp SHg Vp dpm2/g m2/g ml/g nm m2/g ml/g nm
Wide Pore Silica 78 52 0.91 47 80 0.92 54
-Alumina 196 202 0.49 10 163 0.49 10
-Alumina 9 8 0.12 112 12 0.48 150
Active Carbon 1057a 28 0.51 2 0.6 0.46 106
Raney Ni 76 - 0.14 5.80 - - -
ZSM-5 345 344 0.19 0.58 11 1.1 820b
a p/p0 range of 0.01-0.1 was used in the calculation.b intraparticle voids.
-
Catalysis and Catalysts - Physical Adsorption
N2 Adsorption Isotherms & Pore Volume Distributions
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1p/p 0
n ad (
mm
ol/g
) 1
wide-pore silica -alumina
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1p/p 0
n ad (
mm
ol/g
) 1
0.00
0.02
0.04
0.06
0.08
0.10
1 10 100 1000d pore (nm)
dV/d
d (m
l/g/n
m)
0.0
0.1
0.2
0.3
0.4
0.5
1 10 100 1000d pore (nm)
dV/d
d (m
l/g/n
m)
-
Catalysis and Catalysts - Physical Adsorption
N2 Adsorption Isotherms & Pore Volume Distributions-alumina activated carbon
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1p/p 0
n ad (
mm
ol/g
) 1
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1p/p 0
n ad (
mm
ol/g
) 1
0.000
0.002
0.004
0.006
0.008
0.010
1 10 100 1000d pore (nm)
dV/d
d (m
l/g/n
m)
0.0
0.1
0.2
0.3
0.4
0.5
1 10 100 1000d pore (nm)
dV/d
d (m
l/g/n
m)
} Tensile strength effect
-
Catalysis and Catalysts - Physical Adsorption
N2 Adsorption Isotherms & Pore Volume DistributionsRaney Ni ZSM-5
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1p/p 0
n ad (
mm
ol/g
) 1
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1p/p 0
n ad (
mm
ol/g
) 1
0.00
0.02
0.04
0.06
0.08
0.10
1 10 100 1000d pore (nm)
dV/d
d (m
l/g/n
m)
0
2
4
6
8
10
0.0 0.5 1.0 1.5 2.0d pore (nm)
dV/d
d (m
l/g/n
m)
-
Catalysis and Catalysts - Physical Adsorption
Hg Intrusion Curves & Pore Volume Distributionswide-pore silica -alumina
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1 1 10 100 1000p (MPa)
V (m
l/g)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1 1 10 100 1000p (MPa)
V (m
l/g)
0
0.02
0.04
0.06
0.08
1 10 100 1000 10000
d pore (nm)
dV/d
d (m
l/g/n
m)
0.0
0.1
0.2
0.3
0.4
0.5
1 10 100 1000 10000d pore (nm)
dV/d
d (m
l/g/n
m)
-
Catalysis and Catalysts - Physical Adsorption
Hg Intrusion Curves & Pore Volume Distributions-alumina activated carbon
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1 1 10 100 1000p (MPa)
V (m
l/g)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1 1 10 100 1000p (MPa)
V (m
l/g)
0.000
0.001
0.002
0.003
0.004
0.005
1 10 100 1000 10000d pore (nm)
dV/d
d (m
l/g/n
m)
0.000
0.002
0.004
0.006
0.008
0.010
1 10 100 1000 10000d pore (nm)
dV/d
d (m
l/g/n
m)
-
Catalysis and Catalysts - Physical Adsorption
Hg Intrusion Curves & Pore Volume DistributionsRaney Ni ZSM-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1 1 10 100 1000
p (MPa)
V (m
l/g)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1 1 10 100 1000p (MPa)
V (m
l/g)
0.00
0.02
0.04
0.06
0.08
0.10
1 10 100 1000 10000
d pore (nm)
dV/d
d (m
l/g/n
m)
0
0.001
0.002
0.003
0.004
0.005
1 10 100 1000 10000 100000d pore (nm)
dV/d
d (m
l/g/n
m)
-
Catalysis and Catalysts - Physical Adsorption
BET- & t-plotswide-pore silica -alumina
0.0
0.1
0.2
0.3
0.4
0.5
0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0
p/[
nad
(p0 -
p)]
(g/
mm
ol)
S BET = 78 m2/g
C = 146
0.0
0.1
0.2
0.3
0.4
0.5
0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0
p/[n
ad(p
0 -p
)] (
g/m
mol
)
S BET = 196 m2/g
C = 97
0.0
0.5
1.0
1.5
2.0
2.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)
nad
(mm
ol/g
)
S t,micro=28 m2/g
V t,micro = 0.013 ml/g
0
2
4
6
8
10
0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)
nad
(mm
ol/g
)
S t,micro= 0 m2/g
V t,micro = 0 ml/g
-
Catalysis and Catalysts - Physical Adsorption
BET- & t-plots-alumina activated carbon
0.0
0.1
0.2
0.3
0.4
0.5
0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0
p/[n
ad(p
0 -p)
] (g
/mm
ol)
S BET = 9.3 m2/g
C = 142
0.0
0.1
0.2
0.3
0.4
0.5
0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0
p/[n
ad(p
0 -p)]
(g/
mm
ol)
S BET = 1057 m2/g
C = 1057p/p 0 = 0.01 - 0.1
0.00
0.05
0.10
0.15
0.20
0.25
0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)
nad
(mm
ol/g
)
S t, micro= 1.4 m2/g
V t,mcro = 0.001 ml/g
0
5
10
15
0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)
nad
(mm
ol/g
)
S t,micro = 856 m2/g
V t,micro = 0.42 ml/g
-
Catalysis and Catalysts - Physical Adsorption
BET- & t-plotsRaney Ni ZSM-5
0.0
0.1
0.2
0.3
0.4
0.5
0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0
p/[n
ad(p
0 -p
)] (
g/m
mol
)
S BET = 76 m2/g
C = 46
0.0
0.1
0.2
0.3
0.4
0.5
0.00 0.05 0.10 0.15 0.20 0.25 0.30p/p 0
p/[n
ad(p
0 -p)]
(g/
mm
ol)
S BET = 345 m2/g
C = -245
p/p 0 : 0.01 -0.1
0
1
2
3
4
5
0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)
n ad
(mm
ol/g
)
St,micro = 0 m2/g
Vt,micro = 0 ml/g
0
2
4
6
0.0 0.2 0.4 0.6 0.8 1.0 1.2t ( nm)
n ad (
mm
ol/g
)
St ,micro= 344 m2/g
Vt,micro = 0.18 ml/g