Page 23 - Adsorbents fundamentals and applications

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FUNDAMENTAL FACTORS

FOR DESIGNING
ADSORBENT

Selection or synthesis of adsorbents for a target adsorbate molecule is based on
the adsorption isotherm. With the availability of high-speed computing, it is now
possible to calculate the adsorption isotherms based on: (1) interaction potentials
and (2) structure/geometry of the adsorbent. Let us begin with a review of the
basic forces between the adsorbent and adsorbate, paying particular attention to
adsorbent design.

2.1. POTENTIAL ENERGIES FOR ADSORPTION
Adsorption occurs when the interaction potential energy φ is equal to the work
done to bring a gas molecule to the adsorbed state. As a first approximation, the
adsorbed state is assumed to be at the saturated vapor pressure.

P 0
P 0
−φ =− G = V dP = RT ln (2.1)
P P
where G is the free energy change and P 0 is the saturated vapor pressure.
Hence P is the pressure when adsorption occurs for the given φ (φ is actually
the sorbate–sorbate interaction energy on the liquid surface).
The total potential between the adsorbate molecules and the adsorbent is
the sum of the total adsorbate–adsorbate and the adsorbate–adsorbent interac-
tion potentials:
(2.2)
φ total = φ adsorbate–adsorbate + φ adsorbate–adsorbent
The adsorbent has only a secondary effect on the adsorbate–adsorbate
interaction. For this reason, we will focus our attention on the second term,
adsorbate–adsorbent potential, and refer to this term as φ.

Adsorbents: Fundamentals and Applications, Edited By Ralph T. Yang
ISBN 0-471-29741-0 Copyright  2003 John Wiley & Sons, Inc.

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