Biomedical Engineering Reference
In-Depth Information
Refs. [10, 18, 19]) in graphene layers of the crystal lattice of isotropic
graphite [51, 53] (Figs. 2.5 and 2.6a, TPD peak III) and in related
carbon nanostructures with sp
2
hybridization, including GNF (Fig. 2.7,
TPD peak γ (III)) and nanostructured graphite [14, 52-56] (Figs. 2.6b
and c, TPD peak III). Chemisorptions process III
described below by
the overall reaction (2.4), can be related to different steps as
,
1
__
gas
⇔
s
2
H
H
,
(2.1)
2
s
→
l
H
H
,
(2.2)
l
ch
⇔
abs
H
+ C
(C - H)
,
(2.3)
1
__
gas
ch
abs
2
H
+ C
⇔
(C - H)
,
(2.4)
2
s
where H
are hydrogen atoms on the surface of graphite grains or
nanoregions of the material, H
l
are hydrogen atoms in the graphite
lattice (between graphene layers) outside the chemisorption
centers, C
ch
are internal carbon centers of chemisorption for
hydrogen atoms in the graphene layers corresponding to potential
C-H complexes, and (C-H)
abs
are the absorbed hydrogen atoms
on carbon chemisorption centers in the graphene layers of the
material (C-H complexes).
The analysis shows that in the first (dissociative) and third
(chemical) stages of the overall process III, the state of the hydrogen-
saturated material subjected to thermal-desorption heating is in
many cases close to equilibrium (local equilibrium, or reversibility),
and that (2.1) and (2.3) are not limiting stages. The second stage,
(2.2), may be diffusion-limited, i.e., the stage that determines the
rate of the overall process III corresponding to the TPD peak III in
experiments involving temperature-programmed desorption of
hydrogen from the material.
Process III (reaction (2.4)) is characterized [10, 18, 19] by
the experimental value [51] of the standard enthalpy of the bulk
solution, or the chemisorptions of one mole of hydrogen atoms
from the initial state H
gas
in the graphite lattice of the material
2
-1
(∆
(H)) and the experimental value [51, 52,
54, 55] of the effective enthalpy of the bulk-diffusion activation of
hydrogen atoms in the graphite lattice,
H
= -19 ± 1 kJ mol
III
(4)
Q
= 250 ± 3 kJ mol
(H).
-1
III
