Page 301 - Gas Adsorption Equilibria

Chapter 6

IMPEDANCE SPECTROSCOPY

Abstract The physical principles and basic experimental techniques of impedance
spectroscopy, i. e. static or frequency dependent dielectric permittivity
measurements of sorbent/sorbate systems are given. These measurements can be
used to characterize the state of a sorbent material in industrial adsorption
processes. Combined with either manometric or gravimetric measurements of
adsorption equilibria leading to calibration curves, permittivity measurements also
allow fairly simple and quick measurements of gas adsorption equilibria. Kinetic
processes and catalytic reactions inside a sorbent/sorbate system also can be
observed. Pros and cons of dielectric measurements are discussed.
List of Symbols. References.

1. INTRODUCTION

If a dielectric material or a weakly electrical conducting material like most
activated carbons and zeolites is exposed to either a static or an alternating
electric field, the molecular state of the material is changed [6.1]. The nuclei
of the atoms and molecules within the material are shifted somewhat in the
direction of the electric field whereas the electrons are displaced a little bit in
the opposite direction. Hence dipole moments are introduced in the material
either by orientation of polar molecules like etc. in the
direction of the electric field or by separating electric charges within non-
polar molecules like etc. These atomic or molecular dipole
moments are fixed within the electric field, their sum within the unit volume
being called the dielectric polarization of the material [6.1, 6.2]. The dielectric
polarization can be measured by capacitance or impedance measurements of
capacitors filled with the material, cp. Sect. 2.1, Fig. 1. Applying oscillating
electric fields of variable frequency one gets curves which
are characteristic not only for the sorbent material in vacuum but also for the
sorbent / sorbate system which is realized after exposing the sorbent to a
sorptive gas atmosphere [6.9-6.11]. Though a molecular interpretation of the
resulting curves normally is not easy, they can be used in industrial adsorption