Page 183 - Gas Adsorption Equilibria

3. Gravimetry 169

6. Kinetics
Modern microbalances do allow one to take weight data every tenth of
a second. If the kinetics of a pure gas sorption process is slow
compared to this time, it can be easily recorded. Examples have been
given in the foregoing Sects. 2.3, 4.4. Here we only want to mention
that curves depicting the mass of a sorbent / sorbate sample as
function of time do not always show a simple exponential approach to
an equilibrium state but may be much more complicated. An example
of practical importance is chemisorption of gas on activated
carbon (AC). Due to catalytic properties of the AC, can be
converted via to sulfuric acid which at near ambient
conditions periodically falls down in droplets from the carbon sample
leading thus to saw-tooth like curves in the balance’s recordings.

Also kinetic curves of ad- and desorption processes can give hints to
hysteresis phenomena as they may occur in mesoporous sorbent
materials.
Mixture adsorption processes normally cannot be detected by simple
gravimetric / manometric measurements. The reason for this is that
during such a process the sorptive gas mixture has to be circulated in
order to avoid local concentration gradients. As the gas flow inside the
adsorption vessel causes dynamic forces acting on the sorbent sample,
the balance recording is changed. According to our experience the
influence of these dynamic forces, is often irreproducible due to the
complex geometry of piled sorbent pellets and can not easily be taken
into account by calibration experiments with a non-adsorbing “dummy
sorbent”.

7. Activation
In practice, activation of a sorbent material much more easily can be
accomplished in gravimetric than in volumetric sorption instruments.
The main reason for this is that in gravimetry the mass of the sample
can be recorded even during the activation process whereas in
volumetry / manometry it normally can not. Hence in gravimetry the
initial state of a sorbens is fairly well known. Also presorption in a
new sorbent sample and remnant sorption after a desorption
experiment easily can be checked.