The BET theory was developed by Stephen Brunauer, Paul Emmett, and Edward Teller in 1938. The first letter of each publisher’s surname was taken to name this theory. The BET theory was an extension of the Langmuir theory, developed by Irving Langmuir. The specific surface area of a powder is determined by the physical adsorption of a gas on the solid's surface and by the calculating the amount of adsorbate gas corresponding to a monomolecular layer on the surface. The physical adsorption result from relatively weak van der Waals forces between the adsorbate gas molecules and the adsorbent surface area of the test powder. Usually, the determination is performed at the temperature of liquid nitrogen. Typically the amount of gas adsorbed is measured by a volumetric or continuous flow procedure.

1. Surface Area Analysis Using the
BrunauerEmmett-Teller (BET) Method
By:
Neveen Atallah

2. Adsorption
 Is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or
dissolved solids to a surface.
This process creates a film of the adsorbate (the molecules or atoms being
accumulated) on the surface of the adsorbent.
The adsorption process is generally classified as physisorption(characteristic of weak
Van der Waals force) or chemisorption(characteristic of covalent bonding)
 Desorption is the reverse process of adsorption.

3. Is the amount of adsorbate on the adsorbent as a function of its pressure(if gas) or
concentration (if liquid) at constant temperature.
 Freundlich adsorption isotherm
 The first mathematical fit to an isotherm was published by Freundlich and Kuster (1906)
and is a purely empirical formula for gaseous adsorbates:
X/m =KP1/n
 where x is the quantity adsorbed, m is the mass of the adsorbent, P is the pressure of
adsorbate and k and n are empirical constants for each adsorbent–adsorbate
pair at a given temperature.
Adsorption Isotherm

4. Langmuir adsorption isotherm
It is based on four assumptions:
1. All of the adsorption sites are equivalent, and each site can only accommodate one
molecule.
2. The surface is energetically homogeneous, and adsorbed molecules do not interact.
3. All adsorbtion occurs through the same mechanism.
4. At the maximum adsorption, only a monolayer is formed. Adsorption only occurs on localized sites on the
surface, not with other adsorbates.
 Limitation of Langmuir adsorption isotherm
 These four assumptions are seldom all true:
1. There are always imperfections on the surface.
2. Adsorbed molecules are not necessarily inert.
3. The mechanism is clearly not the same for the very first molecules to adsorb to a surface as for the last.
4. Not only monolayer is formed.

5.  Langmuir suggested that adsorption takes place through this mechanism:
 where A is a gas molecule, and S is an adsorption site.
 BET adsorption isotherm
BET : The method of Brunauer, Emmet, and Teller is employed to determine surface area on a model of
adsorption which incorporates multilayer coverage.
 They modified Langmuir's mechanism as follows:

6. BET theory
The data are treated according to the Brunauer,
Emmettand Teller (BET) adsorption isotherm equation:

8.  Type of Adsorption Isotherm
 There are six different types of adsorption
isotherms as shown in Figure.
 First the adsorption volume quickly increases at
low relative pressures due to contact of the
adsorbate molecules with the higher energetic
section followed by the interaction with less
energetic section.
 When the monolayer formation of the adsorbed
molecules is complete, multilayer formation
starts to take place corresponding to the “sharp
knee” of the isotherms.
 As the relative pressure approaches unity a
sudden rise shows the bulk condensation of
adsorbate gas to liquid.

9. 1. Type one (I) adsorption isotherm is for very
small pores or microporous adsorbents. In
this case adsorption occurs by filling of the
micropores. The adsorbate adsorption rate
depends on the available micropore volume
instead total interior surface area.
2. Type two (II) and type four (IV) adsorption
isotherms were detected for non-porous or
macroporous adsorbents with unlimited
monolayer-multilayer adsorption.
3. Finally, type three (III) and type five (V)
isotherms do not have the “sharp knee”
shape indicating that a stronger adsorbate-
adsorbate interactions than adsorbate-
adsorbent interaction.

10.  Brunauer-Emmett-Teller (BET) model is
one of the most widely used isotherm to
measure the specific surface area including
the pore size distribution.
 BET is an extension of the Langmuir theory
where the Langmuir theory related to the
monolayer adsorption, while BET related to
the multilayer adsorption.
 In BET surface area analysis, nitrogen gas is
usually used due to its strong interaction with
most solid and its availability in high purity.
INTRODUCTION

11.  Apparatus
1. Liquid nitrogen
2. Helium gas (99.9% pure)
3. Nitrogen gas (99.999% pure)
4. BET surface area analysis instrument
5. including: Vacuum system (10-4 Torr),
6. Heating apparatus, Pressure gauge Sample
7. holder (bulb) of known volume
METHODOLOGY

12.  Multilayer formation
How it works?

14.  Gas adsorption provides a rapid and quantitative technique for specific surface area
and to determine other textural properties of a solid as pore size, total pore volume,
and pore volume distribution
Pore Volume - Volume of pores accessible to condensed adsorbate
Pore size classification
1. Micropores - Less than 2 nm
2. Mesopores - Between 2 and 50 nm
3. Macropores - Greater than 50 nm

15. RESULTS

16. BET surface area plot

17. Thank You