Biomedical Engineering Reference
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the samples and the relaxation time of process
≈ 70
h. Assuming that the characteristic diffusion path for both processes
is equal to the thickness of single-wall nanotube samples (
β
amounted to
τ β
≈ 0.1
mm), we can use Eqs. (2.21) and (2.22) to obtain the diffusivity value
D α
L
at 293 K, i.e., corresponding to
chemisorption process I, and the value
for a process
α
close to that of
D
I
D β
=
D
for the process
β
,
I*
which is smaller than
by three orders of magnitude but is larger
than the diffusivity value
D
I
corresponding to chemisorption
process II by a factor of 109 (Table 2.1). As a consequence, the
process
D
II
α
is comparable to chemisorption process I, and the
process
is comparable to a chemisorption process of type I (or I*)
with somewhat higher values of the effective diffusion activation
energy
β
Examining the data in Ref. [29] on thermal adsorption
for single-wall nanotube samples saturated with hydrogen at room
temperature and at pressure of 2 MPa, the TPD peak at
Q
I*.
T
≈ 290 K
m
can be considered corresponding to process
α
(I), and the TPD peak
at
.
The authors of [73] have studied the isotherms of adsorption and
desorption of hydrogen by single-wall carbon nanotubes, untreated
with
T
≈ 800 K corresponding to process
β
(I*)
m
exp
2
−1
exp*
S
≈ 420 m
g
, cleared by the metallic catalyst with
S
2
−1
1670 m
and carbon nanofibers, activated a high-purity carbon at
about 290 K and pressured up to 2 MPa.
For the clean single-wall nanotubes, adsorption and desorption
isotherms almost coincide (Fig. 2.13). As in Ref. [31], equilibrium
had enough time to set in, due to the fairly fast diffusion kinetics
characteristic of the physical sorption or a type-I chemisorption
process. Only the initial (close to linear) section of the Henry-
Langmuir isotherm is evident, with the deviation from the linear
behavior not exceeding 10%, while the adsorbate concentration at
2 MPa reaches (H
g
−3
/C)
*
≈ 8 × 10
(0.13 wt%). Hence, using Eq. (2.34),
2
*
−2
we obtain (H
(1.25 wt%), which corresponds to
the averaged value of the maximum concentration of adsorbate
on the total (internal and external) surface (
/C)
≈ 7.6 × 10
m
2
th
) of the nanotubes
[9, 29]. Assuming that the adsorbate is localized on the specific
surface area of clean single-wall nanotubes (
S
tot
exp*
S
), Eq. (2.35) can be
s
*
used to obtain the value of (H
≈ 0.12 for the clean single-wall
nanotubes. Taking the data in Fig. 2.14 into account, which imply
that only 25% of the single-wall nanotubes surface, cleared by the
metallic catalyst, is sorption-active, we obtain a revised value of the
/C
)
exp
2
m
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