22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Synthesis of nanocarbon materials by PECVD: challenges to direct synthesis via
CO 2 reduction using plasma-SOEC hybrid reactor
S. Mori, N. Matsuura, L.L. Tun and M. Suzuki
Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo, Japan
Abstract: Synthesis of carbon nanotubes from pure carbon dioxide was studied using
hybrid reactor of DBD and Solid Oxide Electrolyser Cell (SOEC). The removal of oxygen
from the plasma region suppresses the regeneration of CO 2 from CO and 100% conversion
could be obtained. The deactivation of catalyst by oxygen was also suppressed by the
hybrid reactor and aligned carbon nanotubes were able to be synthesized from pure CO 2 .
Keywords: carbon nanotubes, CO 2 reforming, solid oxide electrolyser cell, DBD
1. Introduction
So far the reforming of carbon dioxide has been
intensively studied by various methods. When we
consider utilizing the plasma process for reforming of
CO 2 , the high cost due to the electricity consumption
hinders the feasibility of the process and the reforming
process to produce only valuable products can remain as
the practical interest. Carbon nanotubes (CNTs) are well
recognized high-value added materials and the price is
still high. Therefore, CNTs can be a potential target to be
reformed from CO 2 using plasma processing and there
has been no report to synthesize CNTs from only CO 2 by
plasma processing. On the other hand, the synthesis of
CNTs from CO has been intensively studied and, among
them, the high pressure carbon monoxide process, HiPco,
was successfully commercialized [1]. In the process for
the synthesis of CNTs from CO, it was reported that the
formation of CO 2 deactivates catalyst and terminates the
growth of CNTs [2]. Therefore, in order to synthesize
CNTs from CO 2 , high CO 2 conversion is essential as well
as the removal of produced oxygen which is more
severely deactivates the catalyst for the CNTs synthesis.
However, it is difficult to obtain very high CO 2
conversion by conventional plasma because intensive
reverse reaction is unavoidable. In order to suppress the
reverse reaction which reproduces CO 2 from CO with
reactive oxygen species, it is highly effective to remove
oxygen from the plasma reaction region. Therefore, in
this study, by the hybridization of oxygen separator and
plasma reactor, we try to increase the CO 2 conversion and
to synthesize CNTs from CO 2 .
In this study, solid oxide electrolyzer cell (SOEC) is
utilized to separate oxygen. At higher temperature, SOEC
electrochemically
but
can
decompose
CO 2
disproportionation of CO becomes thermodynamically
difficult and requires very high pressure for the
substantial reaction rates. In order to develop much milder
process in which feed gas is only CO 2 and the
temperature is less than 700 °C in atmospheric pressure,
the hybrid reactor of DBD and SOEC is fabricated in this
study.
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2. Experimental
Fig. 1 shows the schematic diagram of the experimental
set-up used in this study. The hybrid reactor of dielectric
barrier discharge (DBD) and SOEC was fabricated. SOEC
is composed by the yttria-stabilized zirconia (YSZ) tube
(Nikatto YSZ-8. o.d. is 15 mm and thickness is 2 mm).
Both sides of YSZ tube is coated with lanthanum
strontium manganite (LSM) thin films electrodes. LSM
thin films are wrapped with chromel wires which work as
current collector. LSM electrode on the outer surface of
YSZ tube is connected to the ground and inner LSM
electrode is connected to the DC power supply (0-15 V).
In the hybrid reactor, SOEC is inserted into the quartz
tube of 18 mm inner diameter, 22 mm outer diameter, and
500 mm length. The outer surface of the quartz tube was
wrapped by the SUS mesh which was connected to the
high voltage supply (max. AC voltage: 11 kV, frequency:
18 kHz). Carbon dioxide was fed into the space between
the quartz tube and SOEC where the dielectric barrier
discharge was formed. The plasma input power was
measured by the Lissajous curve and was about 11 W in
the following conditions. The composition of product gas
was analysed by the gas chromatography (Shimadzu
GC-8A. column: activated carbon). The inside of SOEC
was evacuated by the rotary pump in order to enhance
desorption of permeated oxygen.
Fig. 1. Schematic diagram of experimental set-up.
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3. Reduction of carbon dioxide
Fig. 2 shows the carbon dioxide conversion as a
function of residence time at 400 °C. At this relatively
low temperature conditions, the synergistic effect of
hybrid reactor is apparent; the conversion of hybrid
reactor is much higher than the sum of individual
conversions. The CO 2 conversion by single SOEC
reactor is not significant but the oxygen separation rate by
SOEC would be high enough to suppress the reverse
reaction.
Fig. 3. CO 2 conversions with residence time at 700 °C
(SOEC applied voltage: 10 V, SOEC current:
150-180 mA, CO 2 flow rate: 0.3-9 sccm, pressure:
10 kPa).
Fig. 2. CO 2 conversions with residence time at 400 °C
(SOEC applied voltage: 10 V, SOEC current: 60-90 mA,
CO 2 flow rate: 0.15-9 sccm, pressure: 10 kPa).
Fig. 3 shows the carbon dioxide conversion as a
function of residence time at 700 °C. This high
temperature case, the CO 2 conversion by the single DBD
is very low and the synergistic feature of hybrid reactor is
weaker. Higher temperature would be preferable to
convert CO 2 into CO but the formation of CNT from the
CO becomes more difficult. This is because CO is
thermodynamically unstable at low temperature but
becomes stable at such high temperature.
Fig. 4 shows carbon dioxide conversion as a function of
reactor temperature at atmospheric pressure with longer
residence time of 150 s. In this longer residence time
conditions, we obtained 100% conversion by the hybrid
reactor at 700 °C. With this condition in which we could
obtain 100% conversion, we have mainly performed the
synthesis of CNTs in the next chapter.
4. Synthesis of CNTs by CO 2 reduction
In this study, we used the supported metal catalysts to
synthesize CNTs. We have also performed the floating
metal catalysts using ferrocene as a metal catalyst source.
We could synthesize CNTs by the floating catalyst
method using hybrid reactor from pure CO 2 . However,
the metal catalyst deposited onto the SOEC surface and
degraded the SOEC performance. Another problem to
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Fig. 4. CO 2 conversions as a function of reactor
temperature (SOEC applied voltage: 10 V, SOEC current:
40-200 mA, , CO 2 flow rate 0.6 sccm, pressure: 100 kPa).
use the ferrocene for the catalyst source is that the
ferrocene can be the carbon source as well because it
contains a lot of carbon in its structure (C 10 H 10 Fe). Thus,
CNTs can be synthesized by only pure ferrocene without
any other carbon sources [3]. In this study, we wanted to
show the clear evidence to synthesize CNTs from pure
CO 2 . Therefore, in this report, we will focus on the
process using only the supported catalyst. For the
industrial applications, floating catalyst method is still
attractive and configuration of hybrid reactor should be
modified to avoid the deposition of floating catalyst onto
the SOEC surface.
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The supported metal catalysts are prepared by the wet
coating with Co and Mo acetate solutions. The solutions
were prepared by dissolving Co and Mo acetate into
ethanol, individually, to both Co and Mo concentration of
0.3 wt%. Then 0.001 ml of the Mo solution was dropped
onto the substrate and dried in the atmosphere then
calcined at 400 °C for 5 min in atmosphere. Then
0.001 ml of the Co solution was dropped onto the
substrate and again dried in the atmosphere then calcined
at 400 °C for 5 min in atmosphere. Then the substrates
are inserted into the hybrid reactor and catalyst layer was
annealed using an H 2 flow (pressure: 0.4 atm, flow rate:
50 sccm, temperature is same as CVD condition) for
20 minutes before CNTs synthesis, and then a pure CO 2
is fed into the hybrid reactor for the CNTs synthesis. The
space inside the hybrid reactor is rather small; therefore,
we used the quartz capillary tube (o.d.: 0.5 mm) as
substrates for CNTs deposition. The area of the substrate
in the hybrid reactor is divided into two parts. Upstream
part of the substrate is located in the DBD plasma region
and the other part of substrate is in the downstream region
of DBD plasma.
Thus, upstream DBD part and
downstream after DBD part are denoted as
“Direct-PECVD” region and “Remote–PECVD” region,
respectively.
Figs. 5 and 6 shows the SEM (Hitachi S-4500) images
of substrate surface after PECVD at 600 °C and 700 °C,
respectively. At 600 °C, aligned CNTs with diameter of
10-20 nm were synthesized both direct and
remote-PECVD region. At 700 °C, much thicker CNTs
are synthesized in direct-PECVD region; but only very
short CNTs are observed in the remote-PECVD area. At
800 °C, the CNTs formation was more difficult to be
observed and very large particles were found on the
surface of the substrate after the synthesis. At such high
temperature, the reaction Gibbs energy for the
disproportionation reaction of CO becomes positive and
this can be the possible reason for the reduction of CNTs
growth rate. But from the morphology of the carbon
products formed on the substrates, the agglomeration of
the catalyst metal particle would be the other main reason
for the low growth rate of CNTs at high temperature.
Below 500 °C, CNTs growth were not observed.
In order to obtain high conversion rate, we had to set the
flow rate very low. Figs. 7 and 8 show the SEM images of
substrate surface after the PECVD with the flow rate of 0.6
and 0.9 sccm, respectively and explain the influence of
CO 2 flow rate on the formation of CNTs. With the flow
rate of 0.6 sccm, we could still synthesize CNTs; but with
0.9 sccm, only in the remote-PECVD region, formation of
CNTs was slightly observed. The difficulty to synthesize
CNTs with higher flow rate is attributed to the low ionic
current due to the very thick electrolyte. In this study, we
use the YSZ with the thickness of 2 mm. So, if we could
reduce the thickness of electrolyte of the SOEC, we will
obtain higher ionic current and could improve the
performance of SOEC and increase the flow rate of CO 2 .
Our study is now in progress in that direction.
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(a)
500 nm
(b)
500 nm
Fig. 5. SEM images of carbon deposits after PECVD
with the flow rate of 0.45 sccm at 600 °C. (a) DirectPECVD, (b) Remote-PECVD.
(a)
500 nm
(b)
500 nm
Fig. 6. SEM images of carbon deposits after PECVD
with the flow rate of 0.45 sccm at 700 °C. (a) DirectPECVD, (b) Remote-PECVD.
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Fig. 9 shows the TEM (Hitachi H-7650) images of
carbon deposits with the flow rate of 0.45 sccm at 600 °C.
Hollow structure of the CNTs are clearly seen in the TEM
images and the diameter of CNTs synthesized by Directand Remote-PECVD are both about 10-20 nm.
(a)
(a)
500 nm
(b)
(b)
50 nm
500 nm
Fig. 7. SEM images of carbon deposits after PECVD
with the flow rate of 0.6 sccm at 600 °C. (a) DirectPECVD, (b) Remote-PECVD.
(a)
50 nm
Fig. 9. TEM images of carbon deposits after PECVD
with the flow rate of 0.45 sccm at 600 °C. (a) DirectPECVD, (b) Remote-PECVD.
The CO 2 flow rate of the present system is very low;
however, we could successfully synthesize CNTs directly
from the pure CO 2 by the hybridization of DBD and
SOEC.
500 nm
(b)
5. Conclusion
Hybrid reactor of DBD and SOEC was fabricated. The
complete conversion of CO 2 into CO was achieved by the
hybrid reactor. Direct synthesis of CNTs from pure CO 2
was also successfully demonstrated by the hybrid reactor.
6. References
[1] M.J. Bronikowski, et al. J. Vac. Sci. Technol. A, 19,
1800 (2001)
[2] A.G. Nasibulin, et al. Chem. Phys. Lett., 417, 179
(2006)
[3] R. Bhatia and V. Prasad. Solid State Comm., 150,
311 (2010)
500 nm
Fig. 8. SEM images of carbon deposits after PECVD
with the flow rate of 0.9 sccm at 600 °C. (a) DirectPECVD, (b) Remote-PECVD.
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