Chemistry Reference
In-Depth Information
arrangements giving rise to the variety of carbon materials known now-
adays. Unfunctionalized carbon materials such as nanotubes and ful-
lerenes showed some catalytic activity in cracking and dehydrogenation
of hydrocarbons.
12,13
C
60
has been reported to catalyze the hydrogenation
of nitrobenzene to aniline at room temperature under UV light irradi-
ation.
14
However, in the absence of defects, the graphitic basal planes are
not very active as catalyst, with the only active sites being present at
unsaturated carbon atoms located at the edges or defects of the graphene
layers. As an exception, the delocalized p-electron system of pristine
graphene layers has been reported to be active for complexation reactions
in organometallic catalysis.
15
Despite the few examples mentioned above
in which pristine carbon materials exhibited some activity, the catalytic
activity of carbon can be enhanced remarkably after introducing new
active sites via doping or functionalization with heteroatoms. These
metal-free carbon catalysts have some favorable properties for chemical
synthesis, including their biocompatibility, homogeneity, high surface
area, and rich surface chemistry as well as their low cost and ease of
preparation.
9
Due to the good perspectives, the use of metal-free carbon
materials as heterogeneous catalyst has been designated under the name
of ''carbocatalysis''.
16
Carbon materials functionalised with heteroatoms open the possibility
of fundamental studies similar to those carried out using model single-
crystal catalysts, but with the added advantage of working with a practical
catalyst that could eventually be used industrially. Thus, heteroatom
doped carbon materials bridge the material gap in catalysis research. It is
possible to imitate heterogeneously the concepts of homogeneous metal-
free catalysis. The function of oxygen heteroatoms in molecular catalysts
is reproduced by defects of bent graphitic sheets. The catalytic principle
of site isolation can be realized by electronic localization of charges at the
defect sites corresponding to molecular analogues of double bonds. For
instance, the mechanism of oxidation reactions can be studied for metal-
free catalysts with greater precision than for metal oxide systems. There is
neither lattice nor structural oxygen, but only oxygen at active sites.
Carbocatalysts are free of polyvalent metal sites with complex electronic
and spin structures, allowing for a facile theoretical treatment. Finally,
the application of a heterogeneous carbocatalyst is attractive because of
favourable management of energy over a good thermal and electronic
conductor.
There are several arguments in favour of the carbon materials as
promising catalyst. Carbon, in its hybridisation state sp
2
is a semimetal
with a small number of metallic electrons which can be delocalized such
as in graphite or localized such in carbon nanotubes or other nano-
structured carbon materials. The edges of the graphitic platelets always
require the saturation of the free bonds with heteroatoms. Oxygen and
nitrogen are the atoms incorporated more frequently. These atoms give
rise to acid or basic functionalities depending on the local electronic
structure. Additionally, other species can be adsorbed on defects of the
hexagonal rings of the graphitic planes. Moreover, in contrast with other
catalytic materials, very important gas reactants such as hydrogen,
