Full Papers
tivation of Cl2. Bending of the bond (strained CÀC bonds) re-
duces the activation energy, allowing coordination and rela-
tively strong chemisorption, although still in a reversible mo-
lecular form [Eq. (2a)]. A higher bending degree (highly strain-
ed CÀC sites), as present in more disordered onion-type mate-
rials and fullerene-type carbons, further decreases the activa-
tion energy, leading to dissociative chemisorption [Eq. (2b)].
plays a key role in the concentric growth of the spherical
carbon.[48] As for the macroshape, the nano-order in this mate-
rial resembles that of nano-onion, but with a great disorder.
The presence of sp3–sp2 mixed coordination, with the curva-
ture process leading to nanospheres with a size of ꢀ200 nm,
favors the formation of disordered stacking planes with various
terminations and defects (sp3-type carbon atoms). Thermal an-
nealing, however, rearranges these layers into materials or-
dered at the nanoscopic level, according to the description
outlined above.
*
*
*
*
*
C¼C þ Cl2 ! CÀC ðÁ Á ÁCl2Þ
ð2aÞ
ð2bÞ
*
*
*
C¼C þ Cl2 ! ðClÞ CÀC ðClÞ
Formation of HCl occurs either consecutively to the forma-
Conclusions
tion of dissociatively chemisorbed Cl2 [Eq. (2b)] by reaction
with carbon sites with terminating defects (*CÀH) [Eq. (3a)] or
alternatively by direct heterolytic chemisorption on these sites
[Eq. (3b)]. We believe that Equation (3a) is more likely and is in
better agreement with the experimental results, in particular
with Raman data (Figure 6), however, this is an aspect deserv-
ing of further studies.
A novel nanostructured carbon was synthesized by a combina-
tion of thermal decomposition in an aqueous environment
and subsequent thermal annealing. This carbon material shows
a unique strain on the graphene sheets, inducing carbon sites
for strong chemisorption of Cl2.
In the parent carbon material, the high degree of bending in
CÀC sp2-type bonds induces irreversible dissociative chemi-
sorption of Cl2, and the high concentration of defect carbon
atoms (sp3-type, to which H atoms are coordinated to saturate
the valence) favors further reaction of chemisorbed Cl species
to generate HCl. Thermal annealing forms nano-ordered hemi-
spherical graphene shell structures (hemi-fullerene type) with
a decrease of the strain in CÀC sp2-type bonds, as well as a re-
duction of defect (sp3-type) carbon atoms. The reorganization
transforms sites able to irreversibly (dissociatively) chemisorb
Cl2 to sites able to strongly, but reversibly and in a molecular
form, coordinate Cl2. These sites react with CO to catalytically
generate COCl2, and also promote CH4 chlorination. Reorgani-
zation also reduces the concentration of carbon defects (sp3-
type), thus decreasing HCl formation and catalyst deactivation.
During interaction with these sites, the formation of [*CÀCl2]
adducts with radical character is enabled. By using CO and CH4
as reactive probe molecules, it was possible to show that this
is the only species able to contribute to COCl2 synthesis and
CH4ÀnCln formation. The properties and catalytic potential of
this new material are currently being further explored.
*
*
*
*
*
*
ðClÞ CÀC ðClÞ þ CÀH ! ðClÞ CÀC þ HCl þ C
ð3aÞ
ð3bÞ
*
*
CÀH þ Cl2 ! CÀCl þ HCl
Generation of the active sites
The results show how two carbon samples, with almost identi-
cal round-shaped macroscopic morphology and similar surface
area and porosity, change their reactivity towards Cl2 activation
and further catalytic CO and CH4 chlorination by variation of
their structural order.
By using thermal annealing, onion-type carbon nanospheres,
with hemi-fullerene-type ordered nanoshells on the surface,
have been prepared. The nanoshells are obtained by thermal
reordering of the precursor, highly defective, carbon material,
but possibly already have the suitable structure to form this
specific surface nanostructure upon annealing. The catalyst ob-
tained after the thermal treatment, with respect to the parent
carbon material, has sp2-carbon sites with unusually high
strength for the adsorption of Cl2 molecularly (in a reversible
manner), leading to high catalytic activity in phosgene genera-
tion, significantly higher than for many other type of carbon
materials (although not shown here). The annealing process
lowers the amount of sp3-type defect carbon sites, responsible
for Cl2 conversion to HCl. Possibly, the thermal ordering pro-
cess, indicated by the increase of GI, also causes modifications
of highly strained sp2-carbon sites responsible for irreversible
Cl2 chemisorption, although there is no definitive evidence on
this aspect.
Experimental Section
Experimental set-up and safety issues
One major advantage of the experimental setup designed for Cl2
activation studies is the possibility to use minimal amounts of toxic
and corrosive gases. A six-port valve connected to an online mass
spectrometer (MS) was used for the automated dosage of Cl2. All
the gaseous residues were neutralized in two interconnected con-
centrated KOH and Cu2+ solution-containing bottles. An active
carbon-containing filter accounts of any remaining toxic gaseous
intermediates.
This special carbon nanostructure requires, to be created by
thermal treatment, a suitable and specific carbon arrangement
in the (quasi-amorphous) precursor. The synthesis of these pre-
cursor nanospheres depends not only on the precursors and
the heat-treatment conditions, but also on the environment in
which the carbon spheres are generated. In the case of CHT
synthesis, the solid–liquid interface between the carbon source
and the surrounding water during the thermal decomposition
Two different types of Cl2 and CO detectors were placed under the
fume hood to constantly monitor the Cl2 and CO concentration at
the work station. To avoid contamination with CO, two additional
filters for CO entrapment were installed at the outlet of the six-
port valve and of the reactor. The maximum amount of COCl2 that
could theoretically be formed in the pulse reactor (0.025 ppm) is
ChemCatChem 2015, 7, 3036 – 3046
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