1. Characterization Techniques
Gas Adsorption: Study of the porosity of materials
Bartłomiej Gaweł,
Norwegian University of Science and Technology (NTNU)

2. Adsorption: Terminology
Adsorption  adhesion (binding) of a species (adsorptive) to an interface (adsorbent)
Gas phase
Adsorptive
Desorption
Adsorption
Adsorbate
Adsorbent
Adsorbent: Substance (usually solid) with an interface where the adsorption occurs
Adsorptive: Gas or Liquid compound that can adsorb at the interface of the solid
Adsorbate: Substance trapped at the interface

3. Physical adsorption vs. chemical adsorption

4. Physical adsorption vs. chemical adsorption
(Physisorption)
- no barrier (not activated)
- fast
- vdW/dipole interactions
- weak (<0.4 eV)
- reversible
- surface symmetry insensitive
- may form multilayers
Chemical Adsorption
(Chemisorption)
- may have barrier
- variable uptake kinetics
- covalent/metallic/ionic
- strong (>0.4 eV)
- may be dissociative
- often irreversible
- surface symmetry specific
limited to monolayer

5. Physical adsorption Adsorption Isotherm Isotherm: T=const.
Isobar: p=const.
Isochore:
=const. p T
p/po T
Adsorption Isotherm
Plot of the adsorption isotherm : Measure of the amount of gas adsorbed for different increasing values of p/p0 until saturation (p/p0=1).

6. Determination of the monolayer volume
Isotherm models:
Langmuir Model 1916
Determination of the monolayer volume
Assumptions and limitations
Adsorption does not proceed beyond monolayer coverage
All adsorption sites are equivalent, and the surface is completely uniform
Adsorption to a given adsorption site is independent of the surface coverage (No lateral interactions between adsorbate molecules)
No surface diffusion among localized adsorbate molecules
Completely reversible adsorption

7. Langmuir Model Rate of adsorption Adsorbate Adsorbent
Pressure
The rate of adsorption rads is then:
rads = kads (1-θ) p
with

8. Langmuir Model Rate of desorption Adsorbate Adsorbent
The rate of desorption rdes is then:
rdes = kdes θ
with
What happens at equilibrium?

9. Langmuir Model The Langmuir isotherm
At equilibrium the two rates are equal:
rads = r des
Adsorbate
kads (1-θ) p = kdes θ
Yielding:
Adsorbent
If we define and,
after transfmation we get:
The Langmuir isotherm

10. Isotherm models: BET Model: 1938 Assumptions and limitations
Stephen Brunauer
BET Model: 1938
Edward Teller
Paul H. Emmett
Assumptions and limitations
All adsorption sites have the same energy (homogeneous surface).
No lateral interactions between adsorbate molecules.
The evaporation rate on a layer is equal to the condensation rate on the previous layer.
The surface is covered by 0, 1, 2,…, i, n layers; molecules occupy a fraction of surface θ0, θ1, θ2,…, θi, θn.

11. BET Model The surface is covered by 0, 1, 2, I, n layers
Molecules occupy a surface fraction θ0, θ1, θ2, θi, θn θn θi
θi-1 θ3 θ2 θ1 θ0
BET equation :

12. Isotherm models:
Determination of the morphology of a material : Characterization of its surface
related to its extension → specific area
related to its form → porosity, pores volume…
The specific surface area is the accessible internal and external surface to an adsorptive for 1 gram of solid.
m2.g-1
The measure of the total pore volume gives the volume of the accessible pores from the adsorbed quantity for 1 gram of solid.
cm3.g-1

13. Physisorption measures surface areas
Very low temperature
(77 K), vacuum, injection of known doses of inert gas
Surface area is measured by counting the number of molecules deposed in a monolayer
Pore size is determined by gas condensation pressure into the pores

14. Divided Solids : lots of small separated particles
Various porosity aspects :
Divided solids
Divided Solids : lots of small separated particles
Grains : 1/10 mm (sand aspect); μm (flour aspect); a few nm (fume aspect)

15. Various porosity aspects : Porous solids
We can classify the pores according to their availability to an external fluid
Influence
Bulk density
Mechanical strength
Thermal conductivity
Inactive
Fluid flow
Adsorption of gases
Closed pores : a
Ink-bottle shaped : b
Cylindrical : f
One side open
Two open sides
Cylindrical : c
Funnel shaped : d
Through pores : e
Open pores
Roughness of the external surface : g

16. Diverse porosity aspects : Porous solids
P. Behrens, Adv. Mater. 1993, 5, 127

17. Volumetric methods (manometry)
Techniques
Volumetric methods (manometry)
Gravimetric methods
Measure of the amount of gas that came from the gas phase
Measure of mass variation of the adsorbent

18. Adsorption Manometry
In a closed system, with a known volume, at a fixed temperature:
A determined amount of solid is put under an initial gas pressure. At the equilibrium, the pressure difference allows for determination of the amount of gas adsorbed by the sample.

19. Adsorption Manometry Adsorption Desorption Vacuum gauge Gas
1: Vacuum (10-6 torr) in all the instrument : Valve 1 closed, 2 and 3 opens
2: Sample at the selected temperature (for N2: 77K)
3: Gas in the apparatus : Valves 2 and 3 closed, 1 open → measure of pressure to determine the initial quantity of gas
4: Opening of valve 3: Gas in contact with the solid → When the equilibrium is reached : Measure of the pressure to determine the amount of gas in the instrument
5: Closing of valve 3 and introduction of some gas. Repetition of the measure for an other pressure. So, by several steps, saturation pressure p0 is reached leading to the adsorption isotherm
Vacuum
gauge
Gas
Liquid nitrogen
Desorption
Reducing of the pressure inside the instrument. Valves 1 and 3 closed, 2 open, then 2 is closed. Measure of the pressure and opening of 3 to obtain the equilibrium pressure.

20. The physisorption phenomenon
1st Step: For low pressures p/p0 ~ 10-6 till 10-4 →
Adsorption on the most actives sites
2nd Step: p/p0 ~ 10-4 till 10-1 →
Filling of the micropores and formation of a monolayer (BET model) B
3rd Step: If p/p0 →
Multilayer adsorption and filling of the small mesopores by capillary condensation. The big mesopores are covered by a layer of the same thickness than the elements of the external surface. Each gas pressure corresponds to only one pore size (Kelvin’s law)
4th Step:
All the pores are full, even the bigger ones. A limit is reached when p/p0 ≈ 1. There is saturation of the adsorbate gas.

21. The physisorption phenomenon
Adsorption onto preferential sites
1 step
Statistical monolayer adsorption
2 step
Multilayer adsorption and capillary condensation
3 step
Saturation
4 step

22. The physisorption phenomenon
4th step
3rd step
2nd step
Capillary condensation
in the mesopores
Formation of multilayers
Micropores
filling +
Progressive
formation of
the monolayer B
Formation condition of the
statistical monolayer
1st step
Adsorption
by sites

23. Isotherms classification
The shape of the adsorption-desorption isotherms is characteristic of the texture of the adsorbent (physical bonds).
Sing et al., Pure Appl. Chem. 1985, 57, 603

24. Type I Isotherms Big micropores
Small micropores with a narrow distribution
Plateau with a low slope on a large pressure scale
Weak external surface : Negligible adsorption
Characteristic of MICROPOROUS Materials

25. Characteristics of NON POROUS Materials
Type II and III Isotherms
Adsorption mono-multilayer on an open surface
Very weak adsorbate-adsorbent interactions
No B point
Surface impossible to determine
B point
Characteristics of NON POROUS Materials

26. Characteristic of MESOPOROUS Materials
Type IV Isotherms
Tubular pores, open at both ends
wide pores, narrow apertures
Cylindrical pores, uniform dimension, narrow distribution
Characteristic of MESOPOROUS Materials

27. Adsorption on mesoporous samples: capillary condensation

28. Type V and VI Isotherms
Very weak adsorbate-adsorbent interactions but hysteresis so presence of mesopores
Layer by layer adsorption
No B point

29. Classification proposed by IUPAC in 1985
Hysteresis Attributions
Hysteresis phenomenon between adsorption and desorption is often observed for type IV and V isotherms
Classification proposed by IUPAC in 1985
Sing et al., Pure Appl. Chem. 1985, 57, 603

30. H1 and H2 Types Wide pores Tubular pores, open at both ends
Narrow apertures called neck of bottle or ink bottle
Tubular pores, open at both ends
Narrow distribution

31. Parallel corrugated platelets (clays)
H3 and H4 Types
Parallel corrugated platelets (clays)

32. Thank you!!
This project is funded by the Norwegian Financial Mechanism. Registration number: NF-CZ07-ICP Name of the project: „Formation of research surrounding for young researchers in the field of advanced materials for catalysis and bioapplications“