This document discusses nitrogen gas adsorption techniques for measuring the specific surface area and pore size distribution of porous materials. It describes the concepts of porosity, pore classification, gas adsorption theories including physisorption, chemisorption and adsorption isotherms. Common applications of porous materials as catalysts, adsorbents, membranes and electrodes are highlighted. Procedures for BET surface area and pore size analysis using the Kelvin equation are provided.

1. BET surface area and pore size measurement of
porous materials by N2-sorption technique
Assoc. Prof. Dr. Thongthai Witoon
Department of Chemical Engineering,
Faculty of Engineering,
Kasetsart University
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2. 2
What is porosity ?
Non-porous solid
 Low specific surface area
 Low specific pore volume
Porous solid
 High specific surface area
 High specific pore volume
 Porous materials have highly developed internal surface area that can be used to
perform specific function.
 Almost all solids are porous except for ceramics fired at extremely high
temperatures.
F. Rouquerol, J. Rouquerol, K.S.W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999.

3. 3
Concept of porosity: Open vs. Closed pores
 Accessibility: Open pores/Closed pores
- Open pores > accessible to external fluid/Gas adsorption.
- Closed pores > not accessible but affect the bulk density.
 Open pores: Through pores/Blind pores
- Through pores > open channel at 1 location, extend into particle and re-emerge at
different location (e.g. c-e-c, c-e-d).
- Blind pores > Dead end pores (e.g. b and f)

4. 4
Pore classification: By size (IUPAC definition)

5. 5
Pore classification: By shape

6. 6
Application of porous materials: Catalysts
Catalyst (ตัวเร่งปฏิกิริยา)
 Catalysts are materials that increase/decrease the rate of reaction without taking part in the reaction.
As catalysis takes place on the surface of materials, increase in specific surface increases the catalytic
performance.
 In catalytic converter, by creating a structure that exposes the maximum surface area of catalyst to the
exhaust stream, polluting gases are oxidized/reduced to non-polluting gases.

7. 7
Application of porous materials: Adsorbents
Air and Water Filters
 Activated carbon and ceramics based filters are commonly used for air and
water filter.

8. 8
Application of porous materials: Membranes
Desalination
 Desalination is a process of separating salt from sea water.

9. 9
Application of porous materials: Batteries and
Supercapacitors
Electrodes for Batteries and Supercapacitors
 Electrodes in batteries and supercapacitors store ions in pores. To increase
efficiency of storage, porous material with high surface area and suitable
pore size to fit the ions of different sizes.

10. 10
Theory of gas adsorption
Adsorption Process
Adsorptive – adsorbate before being adsorbed on the surface
Adsorbate – the gas adsorbed on the
surface of solids
Adsorbent – the solid where adsorption takes place
Adsorption is brought by the forces acting between the solid and the
molecules of the gas. These forces are of two kinds: physical (physisorption)
and chemical (chemisorption).

11. 11
Theory of gas adsorption
Physisorption vs Chemisorption

12. 12
Theory of gas adsorption

13. 13
Theory of gas adsorption
adsorbent.
and
adsorbate
between
n
interactio
re;
temperatu
adsorbate;
of
pressure
adsorbent;
of
weight
adsorbed;
gas
of
volume
)
,
,
,
(
=
=
=
=
=
=
I
T
P
W
V
where
P
I
T
W
f
V
a
a
adsorptive
of
pressure
Saturated
adsorbate
of
Pressure
0
0
=








=
P
P
where
P
P
f
Va
:
as
written
be
can
equation
above
the
constant,
made
are
I
and
T,
W,
If
Equation of adsorption
isotherm
Adsorbate
Adsorbent

14. 14
Gas adsorption vs Isotherm
adsorptive
of
pressure
Saturated
adsorbate
of
Pressure
0
0
=








=
P
P
where
P
P
f
Va
 Isotherm is a measure of the volume
of gas adsorbed at a constant
temperature as a function of gas
pressure.
 Isotherms can be grouped into six
classes.

15. 15
Gas adsorption vs Isotherm
 Concave to the P/P0 axis
 Exhibited by microporous solids (<2 nm)
 Exhibited by nonporous or macroporous
solids (>50 nm)
 Unrestricted monolayer-multilayer
adsorption
 Point B indicates the relative pressure at
which monolayer coverage is complete.

16. 16
Gas adsorption vs Isotherm
 Convex to the P/P0 axis
 Exhibited by nonporous solids
 Exhibited by mesoporous solids
 Initial part of the type IV follows the
same path as the type II
 Hysteresis loop

17. 17
Gas adsorption vs Isotherm
 Highly uncommon
 Exhibited by mesoporous solids
 Exhibited by nonporous solids with an
almost completely uniform surface

18. 18
Hysteresis
 Hysteresis indicates the presence of mesopores.
 Hysteresis gives information regarding pore shapes.
 Types I,II and III isotherms are generally reversible but type I can have a
hysteresis. Type IV exhibits hysteresis.

19. 19
Hysteresis and pore shapes
Cylindrical
Slits
Ink bottle
Slits

20. 20
Adsorption Theory: BET surface area
 The BET equation was used to determine the monolayer absorbed gas volume (Vm).
Adsorbate
Adsorbent

21. 21
Adsorption Theory: BET surface area

22. 22
Adsorption Theory: BET surface area calculation

23. 23
Adsorption Theory: Pore size: Kelvin Equation
phase
condensed
and
solid
e
between th
angle
contact
re;
temperatu
constant;
gas
real
meniscus;
liquid
the
of
cuvature
of
radius
mean
adsorbate;
condensed
of
me
molar volu
tension;
surface
liquid
;
adsorbate
of
pressure
saturated
adsorbate
of
pressure
cos
2
ln
0
0
=
=
=
=
=
=








=








=








θ
γ
θ
γ
T
R
r
V
P
P
where
RT
r
V
P
P
k
k
 Multilayer formation occurs in parallel to
capillary condensation.
 Capillary condensation is described by the
Kelvin equation.

24. 24
 BJH (Barrett, Joyner and Halenda)
method
 DH (Dollimore Heal) method
 Dubinin-Astakhov method
 HK (Horvath-Kawazoe) method
 Saito-Foley method
 NLDFT (Non local density
functional theory) and monte carlo
simulation method
Microporous solids
Mesoporous solids
Microporous solids
Mesoporous solids
Methods for calculation of Pore Size Distribution

25. 25
General Process Flow for Gas adsorption Analysis

26. 26
Analysis of N2-sorption results
Questions:
1) Please arrange the samples from high-to-low surface area.
2) Please arrange the samples from large-to-small pore sizes.
3) Please indicate a pore shape of the samples.

27. Thank you for your kind attention!
Assoc. Prof. Dr. Thongthai WITOON
Department of Chemical Engineering,
Faculty of Engineering, Kasetsart University
E-mail: fengttwi@ku.ac.th