Biomedical Engineering Reference
In-Depth Information
Process I in isotropic
graphite [51, 53] (Fig.
5, TPD peak I), in
single-wall nanotubes
[26, 63, 64], and in
multiwall nanotubes
[62] (Section 5.1)
Dissociative-associative
chemisorption of H
D
H
≈
D
H
+
D
H
(13)I
dis
(12,
kJ mol
-1
in surface layers of the
material [reactions
(10)-(13) and (12a)].
Difusion of H
≈
-10
± 7
2
12a)I
(H
),
D
S
/
R
≈
-20 at
2
(13)I
X
0.5 (0.25), Eqs. of
the (14)-(16).
D
≈
Im
in these
layers with reversible
diffusant dissociation
and capture at
chemisorption centers
(Fig. 8, models G and
F);
=
D
exp(-
Q
/
RT
),
2
I
0I
I
3 × 10
-3
cm
2
s
-1
,
D
≈
0I
s
Q
≈
Q
-
D
H
≈
20
±
I
(3)I
kJ mol
-1
(H
s
2
),
Q
≈
10
2
±
8 kJ mol
-1
(H
), Eqs.
2
(17), (18)
D
H
≈
-460
(12, 12a)I
10 kJ mol
-1
(2H)
Process IV in isotropic
[51, 53] (Fig. 5, peak IV)
and in pyrolytic [60]
and nanostructured
[52, 53] (Fig. 7a, peak
IV) graphite
Dissociative
chemisorption of H
D
H
≈
1/2
D
H
+
(4)IV
dis
D
H
5 kJ
mol
-1
(H), Eqs. (5)-(7).
D
IV
≈
-140
±
2
(3)IV
in defective regions
of the graphite lattice
[reactions (1)-(4)].
Bulk diffusion of
hydrogen atoms in
defective regions
with reversible
diffusant capture by
chemisorption centers
(Fig. 8, models C and
D);
=
D
exp(-
Q
IV
/
RT
),
0IV
D
0IV
≈
6 × 10
2
cm
2
s
-1
,
Q
IV
≈
-
D
H
(3)IV
≈
365
kJ mol
-1
(H), Eqs.
(8), (9)
± 50
D
H
≈
-364
5
(13)IV
kJ mol
-1
(2H)
Note:
D
,
D
,
D
0I
,
D
are the pre-exponential (entropic) factors of hydrogen diffusivities (
D
,
0III
0II
0IV
III
D
,
D
I
,
D
IV
) in carbon materials corresponding to the respective processes.
II
If we ignore the dissociative-associative mechanism, the
overall process I, similarly to process II, manifests itself as a
nondissociative adsorption of hydrogen molecules by carbon
materials (corresponding to type-II process in Ref. [27]). The values
of the effective energy characteristics of process I (
−
∆
H
and
(13)I
Q
) are intermediate between those known [27] for bond rupture
energy characteristics of the chemical [35-40] and physical [29-35]
adsorption of hydrogen by carbon structures.
There are reasons [10, 18, 19, 36-38, 69] for believing that
the effective energy characteristics of the desorption and diffusion
of hydrogen for type-I processes may be very close to the typical
values of the interaction energy for physical sorption. In such
I
