Biomedical Engineering Reference
In-Depth Information
8.5.2.2
CNTs
As steam methane reforming of natural gas is the most common
method of producing commercial bulk hydrogen, the interest in
the CH
(and other hydrocarbons) adsorption has grown rapidly.
However chemical functionalization of CNTs with hydrocarbons is
relatively difficult due to the low reactivity of their exterior surface;
thus accelerated methane beams have been delivered on CNTs to
obtain the adsorption of radicals through collision events. In this
context, classical molecular dynamics and
4
calculations
have been employed to model the collision of energetic methane
molecules with an armchair SWCNT, for incident energy ranging from
5 to 100 eV [66]. In this way, methane molecules cracking occurs
and the obtained radicals (CH
ab initio
, and CH) are adsorbed with
different processes depending on the incident energy. Of course, the
collision of an energetic methane molecule with the SWCNT can also
break the tube wall and form structural defects that, however, can be
healed through annealing at high temperature (2000 K), provided
the incident energy is lower than 70 eV. The collected data on the
dissociation events seem to indicate that, among the investigated
SWCNTs, the one with the larger radius shows a lower reactivity. The
calculated DOS shows a localized energy state in the gap for both
the CH
, CH
3
2
and the CH adsorption and no substantial changes in the
3
CH
case; these local states are very important, since they modify
the SWCNT electronic and transport properties through decoration.
Zhao and co-workers [121] have predicted weak binding between
methane molecule and a SWCNT at zero temperature but very recent
calculations performed with tight binding molecular dynamics
at room temperature, have shown that the dissociation reaction
proceeds with low activation energy, provided the thermal energy
exceeds a rate limiting barrier [6].
Doping with metallic particles is a possible way to enhance
homolytic dissociation of the H-CH
2
bond; indeed DFT calculations
have recently suggested that a zig-zag nanotube, decorated with
an interstitial C and Mo, can decrease the energy barrier for CH
3
4
dissociation [42].
As for methane, also the simplest alkene (C
H
) and alkyne
2
4
(C
) have been proposed for catalytic hydrogenation on Pt-
doped armchair nanotubes and studied by DFT [115]. The ethylene
interaction with CNTs has revealed to be relatively weak, despite
the significant charge transfer in the case of doped SWCNT. In the
acetylene case, instead, the interaction is stronger and is presumably
H
2
2
