Biomedical Engineering Reference
In-Depth Information
constant. In each instance, it is important that sufficient time is
allowed for the sample to reach equilibrium. For the physisorption
of hydrogen on microporous carbons, this is typically a very rapid
process; for example, Zhao
[16] found that for hydrogen
adsorption by a series of microporous activated carbons, measured
using isobaric gravimetric apparatus, equilibrium was reached
within 2 min. For the physisorption of other species, however, the
required equilibration times can vary considerably [17-19]. If
chemisorption occurs, during a hydrogen sorption measurement,
the equilibration time could be considerably longer, but it is crucial
to ensure that any process occurring over a longer timescale is
not the result of some form of impurity adsorption or reaction. In
hydrogen physisorption measurement, the latter is often seen as
very slow uptake superimposed on the typically rapid hydrogen
physisorption process, as well as a failure to reach equilibrium [20].
Temperature excursions are also possible following the application
of pressure changes, either due to isenthalpic expansion of the gas
or the typically exothermic nature of the adsorption process, or
both. It is therefore necessary to allow sufficient time for thermal
equilibration at each step. The likely occurrence of either effect will
depend to a certain extent on the experimental configuration, the
pressure step size and the size of the sample. Prior to a thermal
hydrogen desorption measurement, the sample must also be loaded
with hydrogen. The sample will first be taken to the temperature
at which hydrogen adsorption process is expected to occur. In the
case of TPD, the hydrogenation process will typically be performed
in a single step. Therefore, the sample is exposed to a pressure of
hydrogen and allowed to equilibrate. Thermal equilibration is not
as crucial as it is for isotherm determination, but the adsorption of
impurities must obviously also be avoided.
et al.
1..
Gas Removal
Following the completion of an adsorption isotherm measurement,
the reverse process is performed to measure the desorption isotherm.
The gas pressure in the system is reduced and the approach to
equilibrium monitored at each step. It is important to measure the
desorption isotherm to ensure that the process is fully reversible,
which is indicative of pure physisorption. If chemisorption occurs,
