Chemistry Reference
In-Depth Information
Appendix C: Gas Adsorption
on Solid Surface—Theory
Gas molecules will adsorb and desorb at the solid surface. At equilibrium, the rates
of adsorption (R
ads
) and desorption (R
des
) will be equal. The surface can be described
as consisting of
Total surface area = A
t
= A
o
+ A
m
Area of clean surface = A
o
Area covered with gas = A
m
Enthalpy of adsorption = E
ads
The following relations can be written:
R
ads
= ka p A
o
(C.1)
R
des
= kb A
m
exp(-E
ads
/RT)
(C.2)
where ka and kb are constants.
At equilibrium,
R
ads
= R
des
(C.3)
and the magnitude of A
o
is a constant.
Further we have the following:
Amount of gas adsorbed = N
s
Monolayer capacity of the solid surface = N
sm
By combining these relations
N
s
/N
sm
= A
m
= A
t
(C.4)
we get the well-known Langmuir adsorption equation (Birdi, 2008):
N
s
= N
sm
/(a p)/(1 + a p))
(C.5)
Additionally, the heat of adsorption has been investigated. For example, the amount
of Kr adsorbed on AgI increases when temperature is decreased from 79 K (0.13 cc/g)
to 77 K (0.16 cc/g). These data allow one to estimate the isosteric heat of adsorption
(Jaycock and Parfitt, 1981):
237
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