Biomedical Engineering Reference
In-Depth Information
We therefore believe reliable the data in Ref. [26] on the pressure
dependence of the relative adsorbate concentration, i.e., the
increase, when the sorption centers on the surface of the samples
of single-wall nanotubes are filled with hydrogen molecules or, in
other words, the increase of the filling factor
from 0.3 to 0.1 as the
hydrogen pressure grows from roughly 3.3 to 40 kPa (Fig. 2.9b).
As the surface filling factor increases, the position of the TPD
peak B in Fig. 2.9b at
θ
β
≈ 1 K/s shifts from
T
≈ 307 K to
T
≈ 276 K,
m
m
at
≈ 1, respectively. According to Eqs. (2.23) and
(2.24), it corresponds to a decrease in the activation energy
θ
≈ 0.3 and
θ
Q
≈
Q
I
≈ (
Q
− ∆
H
) of the process by approximately 1 kJ mol
(H
). This
−1
s
(13)I
2
apparently indicates that the dependence of ∆
H
and, therefore,
(13)I
∆
for the sorption
centers on the surface of single-wall nanotubes is weak.
Using the data in Fig. 2.9b on the dependence of the monolayer
filling factor
H
(Eq. (2.18) as applied to process I) on
θ
(12a)I
on the hydrogen pressure, and Eq. (2.14), we
can determine the equilibrium constant
θ
=
X/X
m
K
of the hydrogen saturation
of single-wall nanotube samples [26] at
= 133 K. By substituting
the result in Eq. (2.15) and assuming the adsorption enthalpy ∆
T
ads
H
≈ ∆
H
(Table 2.1) and
T
= 133 K, we obtain the adsorption entropy
(13)I
ads
value ∆
value from Table 2.1.
Thus, the study of the experimental data reported in Ref. [26] shows
that a type-I physical-like chemisorption process may occur in
single-wall nanotubes.
In Ref. [70] are reported thermal desorption studies of single-
wall nanotube samples (12-15 wt%, the rest being soot, amorphous
carbon, and a metallic catalyst) with a mass of about 100 mg,
saturated with hydrogen for 2 h at 298 K and 2 MPa and cooled to
77 K, with evacuation of the residual gas. The Kissinger method has
been used to determine the hydrogen desorption-activation energy
E
S
≈ −17.5 R, which is close to ∆
S
(13)I
des
= 19.2 ± 1.2 kJ mol
(H
), which is close to
Q
(Table 2.1), and the
−1
a
2
I
×
9
-1
pre-exponential factor of the rate constant
, Eq.
(2.24), for the process corresponding to the TPD peak A in Fig. 2.10.
The TPD peak A has been also observed [26] in samples of
multiwall nanotubes and activated carbon, which were saturated
with hydrogen in a similar way. We should note that in single-wall and
multiwall nanotubes, in contrast to activated carbon [70], TPD peak B
appears in addition to peak A (Fig. 2.10).
The evaluation [26], from Eqs. (2.27)-(2.31), of the desorption-
activation energies for processes corresponding to TPD peaks A and
B in single-wall nanotubes yielded values of
K
= (1 ± 0.2)
10
s
0
E
≈
E
≈
Q
, close to the
A
B
I
diffusion-activation energy
Q
for chemisorption process I (Table 2.1).
I
