Biomedical Engineering Reference
In-Depth Information
Raman and IR spectroscopies were used in Ref. [89] to study
the configurational state of hydrogen in mechanically synthesized
nanostructured graphite. An oscillation signal corresponding to the
covalent C-H bond has been observed in the IR spectrum, mostly for
the C-H
configuration, which is comparable with model H in Fig. 2.8
for chemisorption process II. This C-H peak has not been observed
in the Raman spectrum.
Regarding the results in Ref. [89], two of the three TPD peaks
(see Fig. 2.7) have been observed in nanostructured graphite [14,
53-56]. According to the analysis in Refs. [10, 17] (see also Table 2.1),
this circumstance corresponds to the chemisorption processes II, III,
and/or IV. This suggests that such processes do not fully manifest
themselves in Raman spectra and only one of them (apparently,
process II) manifests itself in IR spectra (in contrast to processes
III and IV). This agrees with the experimental data examined in the
following section.
In such a context, the Near Edge X-ray Absorption Fine Structure
(NEXAFS) spectra for nanostructured carbon films saturated with
hydrogen under the pressure 0.12 MPa at room temperature deserve
some attention [90]. These results indicate that chemisorption
dominates in the given conditions (see also theoretical results in
Refs. [91, 92]). The reported data demonstrate that the physical
adsorption of hydrogen at room temperature plays a negligible role
in carbon nanotubes or in highly defective graphite. At the same
time, according to the theoretical data in Refs. [35, 93], the physical
hydrogen sorption may dominate under certain conditions.
2
2.2.4.3
Physical Adsorption and Chemisorption in
Single-Wall Nanotubes and GNFs Saturated with
Hydrogen at 9 GPA
In Ref. [94], single-wall nanotube samples (50-60%) and GNF
(about 90%) have been saturated with hydrogen, at a temperature of
623 K (18 h, first stage), and then of 723 K (6 h, second stage),
followed by a cooling to 133 K (third stage). The initial pressure of
9 GPa was lowered to 1 atm, and the samples were stored in liquid
nitrogen. The total hydrogen content obtained by burning the
samples in a current of oxygen reached 6.8 wt% [(H/C) ≈ 0.88] for
single-wall nanotubes and 6.3 wt% [(H/C) ≈ 0.81] for GNF.
In Ref. [95], single-wall nanotube samples (80-85%) were
saturated with hydrogen at a pressure of 5 GPa and at a temperature,
