Biomedical Engineering Reference
In-Depth Information
It can be useful to analyse the data of TPD measurements for
the single-wall nanotubes in details. Analysis of the data in Table 2.2
suggests that it is acceptable to interpret TPD peak B in single-
wall nanotubes [26, 70] as the manifestation of a physical-like
chemisorption process of type I. It has been shown that the thermal
adsorption (peak B) from single-wall nanotube samples [26, 70]
is limited by hydrogen diffusion to their external surface, which
formally manifests itself as a first-order reaction. The characteristic
diffusion path
for peak B (Table 2.2) corresponds to the total
thickness of the samples, while the diffusivity is described by
the equation and characteristics (
L
B
D
, Q
,
and
D
) corresponding to
0I
I
I
chemisorption process I (Table 2.1).
As for the TPD peak A in single-wall nanotube samples [26, 70],
two interpretations are possible: a physical-like chemisorption
process of type I (Table 2.1) or a chemical-like physical adsorption.
In the first interpretation, thermal desorption (peak A) for single-
wall nanotubes [26, 70] is limited by diffusion (a first-order reaction)
of hydrogen from the inner regions of the bundles of single-wall
nanotubes (and/or from the inner regions of individual nanotubes)
to the interbundle or interface, surfaces (and/or to the outer surfaces
of the nanotubes). The characteristic size
for peak A (Table 2)
corresponds to the cross-sectional size (diameter) of the bundle (see
Figs. 2.1 and 2.2) or the distance to the closest exit from a nanotube,
while the diffusivity is described by the equation and characteristics
(
L
A
) corresponding to chemisorption process I. The
process of the subsequent diffusion mass transfer of hydrogen (A)
to the outer surface of single-wall nanotube samples, i.e., hydrogen
transfer over distances in the order of
D
,
Q
, and
D
,
D
0I
I
I
value (Table 2.2), is then not
limiting because it proceeds with a considerably higher diffusivity
L
B
s
D
(Table 2.2), characteristic of the van der Waals interaction, i.e., of a
physical adsorption. It can be also assumed that the chemisorption
centers on outer surfaces, the interfaces of bundles or the outer
surfaces of nanotubes, are not decelerating traps for hydrogen
diffusing to the outer surface of single-wall nanotubes (hydrogen A),
because in the given conditions they are filled by hydrogen B to the
limit, i.e., they are “frozen.”
We note that with this interpretation of the data in Refs. [26]
and [70], the adsorbed hydrogen corresponding to TPD peak A
(hydrogen A) is localized primarily in the inner regions of single-
wall nanotube bunches and/or in the inner regions (surfaces) of
individual nanotubes, while hydrogen B is localized on the outer
surfaces of bundles or nanotubes.
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