Biomedical Engineering Reference
In-Depth Information
In the second interpretation of the data in Refs. [26, 70], the
thermal desorption of hydrogen (peak A) from single-wall nanotubes
and from samples of activated carbon (Fig. 2.9a) is limited not by
diffusion but by physical desorption (a first-order reaction) with
the activation energy
des
= 19.2 ± 1.2 kJ mol
) and the pre-
exponential (frequency, or entropy) factor of the rate constant in the
Polanyi-Wigner transport equation [70]
E
(H
-1
a
2
9
−1
.
Using Eq. (2.26), we obtain the experimental value of the physical
adsorption enthalpy ∆
K
(1 ± 0.2)
⋅
10
s
=
0
des
), which
agrees, as an order of magnitude, with some theoretical estimates
(modified DFT) of the van der Waals interaction energy [69].
The above discussion shows that the physical nature and
the energy characteristics ∆
H
ads
= -
E
= −19.2 ± 1.2 kJ mol
−1
(H
a
2
des
of the sorption process
corresponding to TPD peak A may be close to those of chemisorption
process I, in other words, they may correspond to a chemical-like
physical adsorption of hydrogen. At the same time, process A, as
well as sorption processes corresponding to TPD peak B in single-
wall nanotube samples [26, 70], may correspond to a physical-
like chemical adsorption of hydrogen. This agrees with the data
in Ref. [70] on the manifestation of TPD peaks A and B in single-
wall nanotube samples saturated with hydrogen at an increased
temperature (873 K), characteristic to chemisorption processes.
As already discussed in a previous section, the question is open,
but it is worth noticing that many researchers have concluded in
favor of the possibility of a weak chemisorption mechanism that is
intermediate in relation to physical sorption and chemisorption. In
such a context, it is useful to examine the TPD data on multiwall [62]
and single-wall [63] nanotubes with the concentration of nanotubes
and samples of the material higher than those used in Ref. [26].
The samples were saturated with hydrogen at room temperature
and at high pressures (up to 4 and 9 MPa, respectively). The thermal
desorption activation energy for hydrogen
ads
H
= -
E
a
des
determined from
Eq. (2.19) is practically coincident, within the calculation error, with
the
E
a
value reported in Table 2.1.
The assessment, via Eq. (2.25), of the diffusion characteristic size
for multiwall nanotube samples [52] with a mass of approximately
2 mg and with
Q
I
D
corresponding to the TPD peak I (Table 2.1),
I
yielded a value of
L
≈ 130-200 µm [10] close, as order of magnitude,
to the value
≈ 40-70 µm obtained above (see Table 2.2) for the
single-wall nanotube samples with a mass of about 1 mg [26].
L
