Biomedical Engineering Reference
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process I. In this case, therefore, we can speak of a diffusion
manifestation of the physical-like chemisorption mechanism.
The sorption capability of approximately 7.3 wt% (without
saturation) has been achieved [77] for single-wall nanotube samples
at 77 K and at pressure of about 12 MPa (see Fig. 2.16), i.e., at much
higher pressures than those used in Ref. [87]. It can be noted that the
value of maximum sorption capacity of single-wall nanotube samples
obtained in Ref. [87] (about 6.4 wt% (saturation) at 77 K and about
0.2 MPa) is comparable with the data in Ref. [72]: about 2.4 wt%
(but without saturation) at 77 K and about 0.1 MPa (Fig. 2.15b). The
values of the sorption capacity and ∆
ads
obtained by Ref. [87] and
also the nature of the adsorption isotherm and the pressure range are
comparable with the characteristics of the sorption process studied
in Ref. [26], corresponding to TPD peak A (Fig. 2.9a and Table 2.1),
as discussed in the previous section.
The Raman spectra studied in Refs. [42, 85-87], for a number
of carbon materials interacting with hydrogen at 85 K and at
pressures of 0.4 or 0.8 MPa, contain information about the sorption
carbon centers on the graphene surface for highly oriented pyrolytic
graphite, fullerite C
H
, and single-wall carbon nanotubes. It is useful
to consider this information when examining the nature of the
interaction of these materials with hydrogen.
However, some doubts can be expressed regarding the
assumption made by the authors in Refs. [42, 85-87] that one of the
Q
60
) which
is detected for all these materials, being the only component in
the spectrum of pyrolytic graphite and the main component in the
spectrum of single-wall carbon nanotubes at 0.8 MPa, is caused by
the hypothetical surface gaseous phase of H
-components of the spectrum (at 4161.3 and 4155.4 cm
-1
. Based on the results
discussed in the previous sections, it seems reasonable to make
instead the hypothesis that the physical-like chemisorption (type-I
process, Table 2.1) contributes to this situation.
Raman spectroscopy has been also used in Ref. [88] to study
the mechanism of hydrogen adsorption by carbon materials at the
hydrogen saturation pressures in the range 0.2-6.5 MPa and at
temperatures in the range 20-300 K. In Refs. [42, 85-87] it has been
assumed that the physical adsorption dominates, on the basis of the
observed small shifts of the peaks, corresponding to a very small
charge transfer during the hydrogen sorption. However, the authors
have not considered the possibility of physical-like chemisorption,
for which the expected charge transfer is negligible.
2
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