Biomedical Engineering Reference
In-Depth Information
saturation (about 5.0 wt%) has been reached at pressures ≥ 0.6
MPa. The obtained anomalous (for monolayer adsorption) values of
(H
s
suggest that the multilayer adsorption is possible.
For the remaining samples, either initial or treated, the sorption
capacity at 77 K and at pressures up to 1.6 MPa is in the range
1.0-3.3 wt%, i.e., there is no significant correlation between the
adsorbed hydrogen content and the specific surface size (
/C
)
exp
2
m
exp
S
and
i
exp
S
) determined from the BET isotherms for N
at 77 K. It may
c
2
correspond to the multilayer adsorption.
The isosteric hydrogen adsorption enthalpy determined in
Ref. [87] for the most defective single-wall nanotube samples with
nanoholes in tube walls (
exp
≈ 250 m
2
-1
ads
S
g
) amounts to ∆
H
≈ -12 ± 2
c
-1
kJ mol
). In Ref. [87] none adsorbate concentration specification
is reported). For instance, using the sorption data gathered in Ref.
[87] and related to a concentration of about 4 wt%, we find out a
value ∆
(H
2
ads
-1
ads
are about
10 times higher than the hydrogen liquefaction enthalpy ∆
H
≈ -8 kJ mol
. The absolute values of ∆
H
liq
[76]
and close to the energy characteristics of the sorption process
[26, 70], corresponding to TPD peak A (Fig 2.9a and 2.10, Table 2.2),
as discussed in the previous section.
In the Refs. [42, 85-87], high experimental values of the
adsorption enthalpy have been found, assuming that the physical
adsorption is predominant, on the basis of analysis of the data
gathered by various methods, such as Raman spectroscopy, thermal
gravimetry and measurements of the thermal electromotive force
and electric resistivity. At the same time, in Ref. [42] it is not excluded
the possibility to interpret the data on thermal electromotive force
and electric resistivity of hydrogen-saturated single-wall nanotube
samples on the basis of the physical adsorption mechanisms and
chemisorption with interactions weaker than that in the common
covalent C-H bonds.
The kinetics of hydrogen saturation of single-wall nanotube
samples in form of “mats” with a thickness
H
≈ 1 mm at 500 K and
0.1 MPa, and the kinetics of isothermal hydrogen desorption from
samples in a vacuum at 500 K have been studied in Ref. [85]. Both
processes ran as first-order reactions with a relaxation time
L
≈ 1 h.
Assuming that the limiting stage is the diffusion of hydrogen
molecules from the surface or in the sample surface direction [10],
we obtain the diffusivity
τ
D
≈
L
2
/
τ
corresponding to chemisorption
I
