Biomedical Engineering Reference
In-Depth Information
Panella
[14] dosed hydrogen at a pressure of 25 mbar (2.5 kPa)
at ambient temperature before cooling the sample slowly to 20 K.
After an equilibration time of 30 min, the gas phase hydrogen was
evacuated from the sample chamber and a heating rate of 0.1 K s
et al.
-1
was applied. The mass spectrometer signal was calibrated using
a thermal desorption measurement performed with palladium
hydride. The operation of the apparatus was demonstrated using an
activated carbon and a single-walled carbon nanotube sample.
1.
Experimental Methodology
The exact experimental method or procedure used for a gas sorption
measurement must be considered carefully. In this section, we
shall discuss the most important aspects. The measurement of a
gas adsorption and desorption isotherm typically involves four
stages: (i) sample degassing or activation; (ii) thermal equilibration
at the measurement temperature; (iii) stepwise gas dosage; (iv)
stepwise gas removal. A typical hydrogen TPD experiment involves
the following five stages: (i) sample degassing or activation; (ii)
thermal equilibration at the hydrogenation, or hydrogen loading,
temperature; (iii) hydrogenation of the sample; (iv) cooling and
thermal equilibration of the sample at the base temperature;
(v) thermal desorption at a constant temperature ramp rate. A
quantitative TPD determination requires a subsequent hydrogen
signal calibration measurement. We shall now discuss each of these
stages in turn.
1..1
Sample Degassing or Activation
Prior to every gas adsorption measurement, the sample must be
degassed. This process is also known as
, and its purpose is
to remove any pre-adsorbed environmental contaminants from the
pore network and to clean the surface in order to allow gas to adsorb.
It is a crucial stage in the preparation of microporous solids for the
gas adsorption process and normally requires the exposure of the
sample to high vacuum at an elevated temperature for a significant
period of time. In gravimetric instrumentation the process can be
observed, to a certain extent, by monitoring the weight loss of the
sample as a function of time. The degassing process is considered
activation
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