Biomedical Engineering Reference
In-Depth Information
a context, in case of isotropic graphite [51, 53], two TPD peaks
have close characteristics; apparently, the peaks are of type I (see
Fig. 2.5, desorption curve 1) and they may also occur in carbon
nanostructures.
We note that in the case of nondissipative physical adsorption
of hydrogen molecules by carbon materials [29-35, 69], there are no
internal dissociative-associative and chemical stages (2.11), (2.12),
and (2.12a), characterizing formally non dissociative chemisorption
of hydrogen molecules (processes I and II).
Moreover, there is also to consider the scarcely studied factor
of the catalytic effect [27, 28, 55] of metallic nanoparticles, soot,
and amorphous carbon, which are present in carbon materials on
the dissociative stages of the chemisorption processes I-IV at room
temperature and below. Obviously, this is not a decisive factor in
physical adsorption.
In the following section, we describe a novel method for
processing TPD spectra (a modified Kissinger method) with the aim
to identify the nature of nondissociative adsorption processes, i.e.,
to distinguish between type-I chemisorption processes and physical
adsorption processes, having similar energy characteristics of
diffusion and sorption.
2.2.3 Some Aspects of Determining Sorption
Characteristics from the Temperature-
Programmed Desorption Spectra: Identifying
the Nature of Sorption
In a number of studies [26, 54, 61-64, 70] regarding the temperature-
programmed desorption of hydrogen from carbon nanomaterials,
the desorption-activation energy (
des
E
) has been determined from
a
the dependence of temperature
T
of the TPD peak maximum on the
m
material heating rate (
), applying the Kissinger method (Figs. 2.9 and
2.10). The basic assumption is that the process is a first-order reaction,
for which the Polanyi-Wigner transport equation yields the following
expression [70]:
β
2
des
T
E
m
a
ln
B
,
(2.19)
b
RT
m
where
R
is the molar gas constant and
B
is a dimensionless
constant.
