Biomedical Engineering Reference
In-Depth Information
catalyst, amorphous carbon, and soot have been studied in Ref. [31].
The specific surface area of single-wall nanohorn samples increases by
a factor greater than three (from
exp
exp
2
−1
2
−1
),
due to the “opening” of nanotubes as a result of oxidation in oxygen at
693 K. In such a condition, the amount of the adsorbed hydrogen also
grows by a factor of almost three (in saturation with hydrogen at high
pressures up to 6.5 MPa and three temperatures, 303, 196, and 77 K).
The adsorption isotherms [31] from the initial (closed) and oxidized
(open) nanotubes (single-wall nanohorns) at the three temperatures
are described fairly well by the Henry-Langmuir model for process I
(Eqs. (2.14) and (2.15), and Table 2.1). The maximum adsorbate
concentrations has been achieved with hydrogen saturation at
77 K and at a pressure of about 3 MPa for both the initial samples
(0.7 wt%, (H
S
≈ 308 m
g
to
S
≈ 1006 m
g
ox
cl
/C)
≈ 0.04) and the oxidized samples (2.5 wt%,
2
m,cl
(H
≈ 0.15). Combining these data with the Henry-Langmuir
model, we can obtain the averages of the experimental values of the
adsorption enthalpy ∆
/C)
2
m,ox
ads
ads
ads
H
≈ -4 kJ mol
-1
(H
) and ∆
H
≈ −6 ∆
H
(H
),
ox
2
2
cl
cl
ads
which are close to the experimental values of ∆
obtained in Refs.
[78, 79] for single-wall nanotube samples, and also to the value of
∆
H
in Table 2.1.
Using the Clausius-Clapeyron equation [68], similar values for
the isosteric enthalpy of hydrogen adsorption have been obtained in
Ref. [31]: ∆
H
(13)I
ads
ads
-1
H
in the range from −4.2 to −2.2 kJ mol
(H
) and ∆
H
2
cl
ox
−1
from −5.9 to −4.9 kJ mol
.
Considering the maximum values of adsorbate content (H
/
2
C)
≈ 0.15, we can assume [31] that in initial
samples, the adsorbate is mainly localized on regions of external
surface of the tubes (
≈ 0.04 and (H
/C)
m,cl
2
m,ox
exp
S
), which amounts to about 24% of the
cl
th
theoretical value (
, [29]). In oxidized samples, approximately
70% of the adsorbate is localized on the sections of the internal
surface of the tubes (
S
ext
exp
exp
≈ 7
⋅
2
2
−1
), which amounts to
approximately 54% of the theoretical value of the internal surface
of the tubes [29]. Combining it with Eq. (2.33), it is possible to
determine the maximum local concentrations of the adsorbate on
the sorption sections of external ((H
S
−
S
10
m
g
ox
cl
s
/C
)
≈ 0.17) and internal
2
exp
m
s
((H
≈ 0.19) surfaces of single-wall nanohorn samples [31]
close to the carbohydride values (process I, Table 2.1).
It has been suggested [31] that a physical adsorption of
hydrogen is predominant in both the initial and the oxidized single-
/C
)
exp
2
m
