Biomedical Engineering Reference
In-Depth Information
nondissociative chemical adsorption of hydrogen (process II) from
the initial state of the molecular gas (H
gas
) to the intergranular or
defective (surface) regions in isotropic graphite [51, 53] (see Figs.
2.5 and 2.7a; TPD peak II) and related carbon nanostructures,
including GNF [12] (TPD peak
2
(II) in Fig. 2.6), nanostructured
graphite [14, 53, 54] (TPD peak II in Figs. 7b and c), single-wall
nanotubes deformed in a ball mill [61] (see Fig. 2.4; Hirscher
β
et al.
)
,
and defective multiwall nanotubes [62].
Chemisorption process II (overall reaction (2.13)) may be
related [10, 18] to the reaction stages
gas
def
,
→
H
H
(2.10)
2
2
def
def
H
⇔ 2
H
,
(2.11)
2
def
def
def
2
H
+ C
⇔
(C = H)
,
(2.12)
ch
ch
gas
def
def
→
H
+ C
(C = 2H)
,
(2.13)
2
ch
ch
def
def
where H
are hydrogen molecules or atoms in the
respective intergranular or defective regions of the material (C
,
and H
2
def
)
def
outside the carbon chemisorption centers and (C=2H)
denotes
adsorbed pairs of hydrogen atoms on carbon chemisorption
centers (C
ch
def
) or dangling C-C σ-bonds in zigzag edge positions
(see Fig. 2.8; model H, sp
ch
3
hybridization) localized in intergranular
or defective regions of the material.
As in the case of process III, the analysis shows that at the
dissociation (2.11) and chemical (2.12) stages of overall process II,
the state of the hydrogen-saturated material subjected to thermal-
desorption heating is close, in many cases, to a local equilibrium,
approaching a condition of reversibility. It means that these stages
are not the limiting ones. The first stage (reaction (2.10)) may be
diffusion-limited, i.e., determining the rate of the overall process
(2.13) corresponding to TPD peak II.
Process II can be characterized through the standard enthalpy
of the chemisorption of one mole of hydrogen molecules from
the initial state of the molecular gas (H
gas
) to the intergranular or
2
−1
defective regions of the material, ∆
) and by
the experimental value of the effective enthalpy of the bulk diffusion
H
≈ -110 kJ mol
(H
(13)II
2
