Chemistry Reference
In-Depth Information
PAHs and soot formation it cannot be excluded that a fraction of them,
once formed, are converted into PAHs and soot during the arcing operation.
8.3.4 T
HE
R
OLE
P
LAYED BY THE
N
ATURE OF THE
E
LECTRODES IN
P
OLYYNES
F
ORMATION
Once the role of the graphite electrodes in the production of the polyynes
had been clarified, two crucial tests were carried out by replacing the graph-
ite electrodes with titanium electrodes. The first study was conducted in
n-hexane by arcing with titanium electrodes. Surprisingly, and in contrast to
all the partial conclusions of the preceding sections, C
6
H
2,
C
8
H
2
, and C
10
H
2
polyynes where found in the hexane solution arced between titanium
electrodes [38]. Five other products were found in comparable concen-
trations to the two polyynes mentioned. All were PAHs and three were
identified as naphthalene, acenaphthalene, and indene while the remaining
two presumed PAHs remained unidentified. The formation of polyynes
observed by arcing hexane with titanium electrodes seems to contradict
many conclusions of the preceding sections. However, it must be noticed
that arcing in hexane produced a significant amount of carbon black or
pyrocarbon. Since no carbon electrodes were used, it appears clear that
all the carbon black was derived from the pyrolysis and carbonization of
n-hexane. To explain the polyyne formation, it is possible to consider that
the pyrocarbon particles formed from hexane plasmalysis entered into the
plasma ball of the arc or coated the surface of the titanium electrodes as
a thin layer. Under these conditions the pyrocarbon acted as a source of
carbon vapor in the plasma phase for the production of the polyynes.
As shown in
Table 8.1
, although the relative concentration of the poly-
ynes detected in n-hexane arced with Ti electrodes appears comparable to
the distribution of polyynes in n-hexane prepared with graphite electrodes,
the overall polyyne concentration in the solution prepared with graphite
electrodes exceeds by two order of magnitudes the total concentration of the
polyynes in n-hexane prepared with Ti electrodes under similar conditions
(same current intensity and arcing time). Thus, the fundamental contribution
from the graphite electrodes in supplying elemental carbon for the polyyne
production is clear. Furthermore, it is remarkable that with graphite elec-
trodes long-chain polyynes, C
12
H
2
,C
14
H
2
, and C
16
H
2
, were also formed and
detected in appreciable amounts, while with Ti electrodes the longest
detectable chain was C
10
H
2
(see Table 8.1).
Approximately similar results were observed when n-hexane was replaced
with benzene (Table 8.1): graphite electrodes produced a higher concentra-
tion of polyynes with a wider distribution of detectable chains in compari-
son to titanium electrodes. A striking peculiarity of benzene is the formation
of a plethora of HPLC-DAD detectable PAHs as reported in Table 8.1
(plus others separated by HPLC but not identified on the basis of the
electronic absorption spectra), the distribution of the PAHs was richer and
Search WWH ::
Custom Search