Biomedical Engineering Reference
In-Depth Information
too crude for measurements on carbon nanomaterials and are used
more commonly in metal hydride research, particularly as they
are suited better to higher temperature measurements in which
the desorbed hydrogen will not undergo such a large density
change due to the temperature difference between the low sample
temperature and the higher temperature of the apparatus. TGA
is also better suited to higher temperature measurements and
measurements performed using a TCD tend to be used more often,
for example, in the determination of active metal surface areas in
catalysis research. The measurement of the temperature-induced
desorption of physisorbed molecular hydrogen from carbon
nanomaterials requires low temperature operation of the order of
20 K. Panella
[14] presented a system suitable for operation at
low temperatures and so we will briefly describe the features and
operation of this apparatus below.
The essential stages of a TPD or TDS measurement, in general,
are the initial dosing of the sample with hydrogen at an appropriate
temperature, cooling, if required, of the sample to a temperature at
which the adsorbed hydrogen cannot readily be desorbed, removal
or evacuation of the gas phase hydrogen and, finally, the thermal
desorption of the hydrogen using an applied temperature ramp. The
total desorbed quantity of hydrogen will then be given by,
et al.
t
t
(1.4)
N
1
n t
( )
dt
tot
0
where
N
is the number of moles of desorbed hydrogen,
n
(
t
) is the
tot
molar hydrogen desorption rate as a function of time,
t
, and
t
and
t
0
1
are the start and end times, respectively, of the thermal desorption
procedure [13].
The TDS system presented by Panella
[14] consists of a
copper sample holder, surrounded by a resistive heater, which can
be connected to the cold finger of a liquid helium cryostat. The cold
finger allows the sample to be cooled to a temperature of 20 K. The
heater allows the sample temperature to be increased in a controlled
manner up to a temperature of 500 K. The sample chamber is
connected to a hydrogen supply and to a vacuum pump system,
with a mass spectrometer situated above the sample to detect the
desorbed hydrogen. High-vacuum operation is required for the
degassing of the sample and the hydrogen supply allows gas to be
dosed into the chamber prior to a thermal desorption experiment.
et al.
