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Outline
• Use of mercury porosimetry and nitrogen adsorption for characterization of the porosity
• Estimation of particle size from the porosity measurements
• Application of porous materials
2
3
Porous materials
4
Types of pore
5
Measurement techniques
6http://www.micromeritics.com/
7
http://www.micromeritics.com/
8http://www.micromeritics.com/
9
http://www.micromeritics.com/
10
Mercury porosimetry
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Mercury porosimetry
Pore diameter (nm)
Vo
lum
e (m
l/g)
12
Mercury porosimetry• Total volume (Vtot): total intruded volume of mercury at Pmax
• Total pore surface area (S):
• Mean pore diameter:
∫=totV
pdVS0|cos|
1
θγ
S
Vd
totmean ×= 4
13
Limitations of mercury porosimetry
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Adsorption vs Absorption
H H H H H H H H H
H H H H H H H H H
H2 adsorption onpalladium
H
H
HH
H
HH H
H
H
HH
HH
H
H
H H
H2 absorption →palladium hydride
adsorbent
adsorbate
15
Quantachrome Instruments
16
Adsorption and Desorption Isotherms
17
www.quantachrome.com
18
Adsorption and Desorption Isotherms�As P/Po changes to 1� coverage of surface passes from monolayer to multilayer
� pressure ratio at which condensation takes place on a concave surface of radius “r” is smaller than that for a flat surface (P/Po=1)
� as condensation progresses in a pore → added layer of condensate reduce the pore radius → pressure ratio for this pore is further reduced
� any multilayer which has developed on the walls of a wider capillary tube will evaporate and condense in the narrower tubes
RTr
V
P
P
K
0 2ln
γ=
19
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Video on Gas adsorption processhttp://www.youtube.com/watch?v=ZF-sk5hYSwA
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Information from Adsorption and Desorption Isotherms
• Surface area (BET)
• Pore size distribution– Mesopore: Kelvin equation, BJH method– Micropore: Surface area of microporous by Langmuir,
t-plot, HK, SF
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Brunauer-Emmett-Teller (BET)� surface of a monolayer adsorbate of N2-molecules at boiling
temperature of N2 at Po=1 atm, 77 K
� determination of surface area of solid materials
−+=
− ommo P
P
CW
C
CWPPW
11
)1)/((
1W: weight of gas adsorbed at P/Po
Wm: weight of monolayer adsorbate
C: related to energy of adsorption in the first layer
0.05 < P/Po < 0.3MNAW
Sis
WCW
iCW
Cs
sXiY
csmtm
mm=
+=−=
+=
= ,1
,1
,1
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Pore size distribution (Mesopore)
� desorption isotherm is closer to true thermodynamic stability
� using Barrett, Joyner and Haldena (BJH) method
2/1
034.0)/log(
99.13
)/ln(
2
+=
−=
+=
PPt
trr
PPRT
Vr
o
Kp
o
mK
γ
24
Pore size distribution (Micropore)
� Langmuir equation – adsorption of single molecular layer of adsorbate, Surface area calculation
� t-method: determination of micropore volume in presence of mesopore
�HK method, SF method: pore size calculation independetly from Kelvin equation
)/(1)/(
o
o
m PPCPPC
WW
+=
25
IUPAC Isotherm classification
III
n ad
p/p0
VI
n ad
p/p0
V
n ad
p/p0
In a
d
p/p0 p/p
II
n ad
0
B
IV
n ad
p/p0
B
III
n ad
p/p0
VI
n ad
p/p0
V
n ad
p/p0
In a
d
p/p0
In a
d
p/p0 p/p
II
n ad
0
Bp/p
II
n ad
0
B
IV
n ad
p/p0
B
IV
n ad
p/p0
B
26
27
28
29
Isotherms
Assumptions:
• homogeneous surface
(all adsorption sites energetically identical)
• monolayer adsorption (so no multilayer adsorption)
pKpK
nnn mmad +⋅=θ⋅=1
n ad
p/p0
Type I Langmuir Adsorption Isotherm
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Isotherms
Multilayer adsorption (starting at B)
Common for pore-free materials
p/p
n ad
0
B
Type II
Type IV
Similar to II at low p
Pore condensation at high p
n ad
p/p0
B
31
Type III
Type V
n ad
p/p0
Weak adsorbate-adsorbent interactions, Strong cohesion force between adsorbed molecules,
n ad
p/p0
Similar to III at low p
Pore condensation at high p
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Type VI
n ad
p/p0
� Highly uniform surface� Layer by layer adsorption� Stepped isotherm
Example:� Adsorption of simple non-porous
molecules on uniform surfaces
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Isotherm: γ-Al2O3 (Mesoporous)
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BET specific surface area: γ-Al2O3
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Pore size distribution: γ-Al2O3
36
Isotherm: Activated carbon (Microporous)
37
Pore size distribution: Activated carbon
38
Limitations of N2 adsorption method�Micropores are not accessible to N2 at 77 K
�Micropores can not accommodate 2 layers of adsorptive molecules
underestimates surface area
�Ar, Kr based measurements
� samples with low surface area can be characterized by Kr gas adsorption
39
Properties of adsorbates
40
Hysteresis?
41
Application of porous materials
• Gas separation membranes• Sensors• Hydrogen storage • Catalysis