Biomedical Engineering Reference
In-Depth Information
Such two-stage sorption model resembles the model of
monolayer adsorption and the subsequent condensation of the
adsorbate, at subcritical temperatures, proposed in Ref. [30]. As
noted in Refs. [29, 30], the question about the adsorbate condensation
[72, 77] at about 80 K, a temperature much higher than the critical
temperature of 33 K, remains open. The problem has been studied
theoretically in Ref. [83] using the molecular dynamics method.
There is also a question about the small experimental values
exp
S
of the specific surface area of single-wall nanotube samples
compared with the theoretical value
th
. According to the data in
Refs. [29, 30, 84], in such cases the sorption is directly proportional
to
S
tot
exp
. The situation corresponds to the monolayer model for both
physical sorption [69] and type-I chemisorption (Table 2.1).
The analysis and comparison of the data in Refs. [72, 74, 77]
show that, in any case, the anomalous hydrogen sorption by single-
wall nanotube structures (Figs. 15b and 16) can be explained
only by a model, accounting for both a monolayer (undoubtedly a
chemisorption) stage of the process and a multilayer stage initiated
by the monolayer stage. Obviously, the multilayer stage constitutes
a special physical adsorption process of the adsorbate condensation
type that takes place at subcritical temperatures. This process
probably occurs due to the absence of a rigid “frame” in the single-
wall nanotube structures (in contrast to activated carbon) that
prevents the nanotube bundle structure from disintegrating and/or
“swelling,” caused by multilayer intercalation and/or condensation
of the adsorbate.
Special polylayer physical adsorption, in addition to
chemisorption, can occur in single-wall nanotube samples (50-70
wt%,
S
exp
≈ 270 m
exp
≈ 180-470 m
2
2
-1
) with a considerable
amount of soot and metallic catalyst, in untreated samples (
S
g
, and
S
g
−1
c
i
exp
) or
S
i
exp
in samples (
S
) chemically cleaned with oxygen and/or acids (HCl
c
or HNO
) and saturated with hydrogen at 87-77 K under pressure in
the 0.001-1.6 MPa.
The sorption capability of the most defective (treated with HNO
3
)
3
exp
≈ 250 m
2
-1
single-wall nanotube samples [87] (
), saturated with
hydrogen at 77 K, reaches a maximum hydrogen saturation of about
6.4 wt% [(H
S
g
c
≈ 0.42] at pressure ≥0.2 MPa. It corresponds to
the anomalous local concentration [(H
/C)
2
m
s
≈ 4.4 (Eq. (2.33)),
that is about 10 times higher than the carbohydride value. When the
samples have been saturated with hydrogen at 87 K, the maximum
/C
)
exp
2
m
