Page 302 - Gas Adsorption Equilibria

288 Chapter 6

processes to check the actual state of a sorbent material and to detect for
example preadsorbed dirt molecules, unusual CO-production or hot spots
within an adsorption reactor.

Combining the dielectric measurements with either manometric,
gravimetric or oscillometric measurements of gas adsorption equilibria states,
one gets calibration curves allowing one the determination of Gibbs excess
adsorbed masses by purely electric measurements which normally can be
performed fairly quickly and on site in industrial situations.

It also should be possible to detect single components of a
multicomponent adsorbate or molecules at different adsorption places within
an energetically heterogeneous sorbent material by impedance measurements
using appropriate (high) frequency electric fields leading to resonance
phenomena of the respective molecules. Unfortunately, these frequencies
often are – for example for water adsorbed on zeolite – in the GHz- and even
THz-region requiring sophisticated impedance analysing technology.
Therefore, presently they do not seem to be suited for industrial
measurements. Also, the pore spectrum of a sorbent material, i. e. micropores
(d < 2 nm) or mesopores (2 nm < d < 50 nm) cannot easily be detected by
dielectric permittivity measurements as these would require high frequency
electric fields at corresponding to heat radiation or near infrared
radiation fields which normally are strongly absorbed by the material [6.1].

Dielectric measurements of gas adsorption systems can be performed
fairly quickly, typically within a few seconds [6.3]. Hence the kinetics of
adsorption processes being “slow” on this time scale can be observed. Indeed
these processes are sometimes “invisible” to purely manometric or even
gravimetric measurements. As examples we mention internal diffusion,
reorientation or catalytically induced chemical reaction processes of
admolecules within a sorbent material. The mass of the adsorbed phase
normally is constant during processes of this type, whereas the dipole moment
of the admolecules and hence their polarization changes, cp. Sect. 3.2.

We here restrict to consider only gas adsorption systems exposed to weak
electric fields, these being considered as sensors for the system without
changing its macroscopic properties. However, it should be mentioned that in
principle the adsorption properties of a sorbent material are changed by the
electric field. This so-called electro-adsorptive effect is important in
microsystems as used, for example, in advanced gas sensing devices [6.4].