Micromeritics Flowsorb II 2300 Instructions For Use page 2

FORMULA FOR THE SURFACE:
 

273.2 atm press .

  
S V

 
Room temp 760
BET Theory
The concept of the theory is an extension of the Langmuir theory, which is a theory for monolayer molecular
adsorption, to multilayer adsorption with the following hypotheses:
(a) gas molecules physically adsorb on a solid in layers infinitely;
(b) there is no interaction between each adsorption layer; and
(c) the Langmuir theory can be applied to each layer.
The resulting BET equation is expressed by (1):


1 c 1
 

 
 
 

P / P 1 c P

0 m
P and P are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption,
0
the adsorbed gas quantity (for example, in volume units), and
is the BET constant, which is expressed by (2):

 E E 

c exp  1 L 
 
RT
E is the heat of adsorption for the first layer, and
1
E is that for the second and higher layers and is
L
equal to the heat of liquefaction.
Equation (1) is an adsorption isotherm and can be
plotted as a straight line with 1 / v[(P
the y-axis and φ = P / P
to experimental results. This plot is called a BET
plot. The linear relationship of this equation is
maintained only in the range of 0.05 < P / P
0.35. The value of the slope A and the y-intercept I
of the line are used to calculate the monolayer
adsorbed gas quantity v
The following equations can be used:
1


m 
A I
A
 1 
c
I
The BET method is widely used in surface science for the calculation of surface areas of solids by physical
adsorption of gas molecules. A total surface area S
following equations:

Ns

m
S
BET , total
V
S

S total
BET
a
Adsorption of Gases in Multimolecular Layers, Stephen Brunauer, P. H. Emmett, and Edward Teller, J. Am.
Chem. Soc.; 1938; 60(2) pp 309 - 319





 
6.023 16.2
 

 
1
  
22.414 





P 1
 


c

0 m
/ P) − 1] on
0
on the x-axis according
0
and the BET constant c.
m


 
% N

 
atm press . 
2

 
100




atm press . 15 





is the monolayer adsorbed gas quantity. c
m
<
0
(3)
(4)
and a specific surface area S are evaluated by the
total
N: Avogadro's number,
(5)
s: adsorption cross section,
V: molar volume of adsorbent gas
(6)
a: molar weight of adsorbed species




 
atm press .
  

 
1.565 1

 
room press 




BET plot


 
% N

 
atm press . 
2

 
100




atm press . 15 




(1)

is
(2)