Biomedical Engineering Reference
In-Depth Information
which can be the case for hydrogen, the desorption branch of the
isotherm will not follow the adsorption branch. The thermal and
sorption equilibration requirements are the same for desorption
as they are for adsorption, and so sufficient time must also be
allowed at each equilibrium point, as discussed above. For a TPD
measurement, the sample temperature is reduced to a temperature
below that at which significant hydrogen desorption will occur.
In the case of physisorption, low temperatures are required, as
discussed in Section 1.2.3. Following hydrogenation and cooling, the
thermal desorption process is performed at a constant heating rate.
In the case of the study by Panella
[14], thermal desorption
was complete for both the activated carbon and the purified single-
walled carbon nanotube sample below approximately 80 K.
et al.
1..
Signal Calibration
If the purpose of a TPD experiment is the quantitative determination
of the desorbed hydrogen, a method to calibrate the desorption
signal is required. For TPD using a mass spectrometer, this typically
involves the measurement of the desorption spectrum of a sample
of known hydrogen content. A number of different materials have
been used for this purpose. Fernández
et al.
[12] chose TiH
as
1.92
a calibration material, whereas von Zeppelin
[13] used three
different calibration materials, namely a hydrogenated PdGd alloy,
TiH
et al.
[13] found that the former two
were better suited to the task and that CaH
and CaH
. Von Zeppelin
et al.
2
2
should be avoided
due to its hygroscopic nature. The latter was examined because it
was chosen as a calibrant sample by Dillon
2
et al.
[21] in previous
carbon nanotube work. Von Zeppelin
[13] estimate an error
of ±5% in their quantitative calculations of desorbed hydrogen.
They used a series of TiH
et al.
samples that were in the mass range
1-6 mg, but the PdGd alloy sample was significantly larger (~1 g)
due to its lower gravimetric hydrogen content. Note that the studies
by both Fernández
2
[13] focused
on TPD from metal hydrides, although the principle of the method
is the same as that applied to carbon nanomaterials. In later work
on low-temperature TDS from microporous materials, including
nanostructured carbon, Panella
et al.
[12] and von Zeppelin
et al.
et al.
[14, 22] used PdH
as a
0.7
calibrant material.
