Enhancement of methanation of carbon dioxide using dielectric barrier discharge on a ruthenium catalyst at atmospheric conditions

Article abstract of DOI:10.1016/j.cattod.2017.01.022

This paper describes the enhancement of CO₂ methanation using dielectric barrier discharge (DBD) on Ru catalyst under atmospheric conditions. The DBD plasma leads to increased deoxygenation of CO₂, which is decomposed into CO. Subseq

Full text of DOI:10.1016/j.cattod.2017.01.022

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Enhancement of methanation of carbon dioxide using dielectric
barrier discharge on a ruthenium catalyst at atmospheric conditions
Chung Jun Leea, Dae Hoon Leeb, Taegyu Kima,∗
a Department of Aerospace Engineering, College of Engineering, Chosun University 309 Pilmun-daero, Dong-gu, Gwangju 61452, Republic of Korea
b Industrial Plasma Laboratory, Korea Institute of Machinery and Materials, 156 Gajeonbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper describes the enhancement of CO2 methanation using dielectric barrier discharge (DBD) on Ru
catalyst under atmospheric conditions. The DBD plasma leads to increased deoxygenation of CO2, which
is decomposed into CO. Subsequently, the Ru catalyst activates the methanation in the DBD plasma at
atmospheric conditions. The effect of the discharge frequency of the DBD plasma and the H2/CO2 mixture
ratio on the CO2 conversion and CH4 selectivity is investigated. The CO2 conversion and CH4 selectivity
rapidly increase at a discharge frequency above 2.5 kHz, and reach 23.20% and 95.02%, respectively, when
the H2/CO2 molar ratio is 7. CO2 methanation and deoxygenation are simultaneously enhanced by adding
Ar. Optical emission spectroscopy is used for the plasma diagnostics in the CO2 methanation process. The
optical emission spectra of the ␥-Al2O3 and Ru/␥-Al2O3 catalysts are measured to investigate the effect
of the presence of Ru during plasma generation. Finally, the excited and ionized hydrogen atoms are
confirmed as the primary accelerators in the plasma-assisted CO2 methanation process.
Received 23 August 2016
Received in revised form 2 January 2017
Accepted 14 January 2017
Available online xxx
Keywords:
Carbon dioxide
Methanation
Power to gas
Dielectric barrier discharge plasma
Ruthenium
Optical emission spectroscopy
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
CO2 into useful products, such as CH4 and H2O, according to the
process shown in Eq. (1),
Recently, methanation of CO2 has become an important chal-
lenge. CO2 methanation is important for two reasons: the first is
CO2-related environmental issues and the second is the potential of
the “power to gas (P2G)” process [1,2]. Since the industrial revolu-
tion, atmospheric concentration of CO2 has continuously increased;
the increased CO2 concentration causes climate change and global
warming [3,4]. The major source of CO2 emissions is from the com-
bustion of fossil fuels, such as in an internal combustion engine.
However, human beings cannot stop using fossil fuels to prevent
CO2 emissions because the modern human lifestyle is highly depen-
dent on the fossil fuel-based economy. For this reason, reasonable
and alternative solutions to reduce CO2 emissions involve recycling
or utilizing the emitted CO2.
CO2 + 4H2 → CH4 + 2H2O(ꢀH = −165.0 kJ/mol)
(1)
The CO2 methanation process has a great potential for environ-
mental, industrial, and economical applications because fuels or
fuel sources can be regenerated from the waste CO2 gas. Based on
the renewable energy growth, CO2 methanation can be used for the
P2G process that converts electrical power to a gas fuel. It means
that the excess power can be utilized and stored in a storable gas
state, such as CH4.
For this reason, the CO2 methanation process has been widely
studied. Typically, CO2 methanation has been achieved for group
VIII–based metal catalysts, such as Ni and Ru [5–9]. However, the
catalytic methanation process using the Sabatier reaction is usu-
ally activated above 300 ◦C and 20 bar [10–12]. The temperature
and pressure requirements make the process complex because a
heater, pressurization device, and pressurized reactor are required.
It means that the system has a limitation for miniaturization and
simplification, which limits its applicability to various industrial
applications. Therefore, an alternative approach to lower the tem-
perature and pressure to activate the CO2 methanation process is
required.
Either recycling or utilizing CO2 can be achieved by the Sabatier
reaction. The Sabatier reaction is a well-known process to convert
Abbreviations: DBD, dielectric barrier discharge; OES, optical emission spec-
troscopy; P2G, power to gas; SED, specific energy density.
Recently, non-thermal plasma-assisted processes have been
proposed for various reactions, including treatment, decompo-
sition, conversion, and reforming processes. Many studies have
∗
Corresponding author.
E-mail address: taegyu@chosun.ac.kr (T. Kim).
http://dx.doi.org/10.1016/j.cattod.2017.01.022
0920-5861/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: C.J. Lee, et al., Enhancement of methanation of carbon dioxide using dielectric barrier discharge on a
ruthenium catalyst at atmospheric conditions, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.01.022

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Fig. 1. Typical configuration for dielectric barrier discharge.
reported that the non-thermal plasma-assisted process provides a
synergistic effect between the plasma and catalyst. The surfaces
of the catalysts were pretreated by the non-thermal plasma to
enhance their catalytic performance; this surface-modifying treat-
ment also improved the durability of the catalysts [13–17]. The
enhancement of the catalytic performance using the non-thermal
plasma was also reported for pollutant removal and reforming
species, such as methanol and methane [18–28].
The non-thermal plasma has many advantages in various chem-
ical processes. A few groups have reported the study of CO and CO2
hydrogenation using a non-thermal plasma [29–32]. Therefore, the
non-thermal plasma-assisted CO2 methanation process was inves-
tigated in this study. Among the non-thermal plasma techniques,
dielectric barrier discharge (DBD) was used to generate the plasma
on the catalyst. The activation and enhancement of the CO2 metha-
nation in the plasma at atmospheric conditions were verified, and
the mechanism between the plasma and catalyst was investigated
in this study.
2. Experiments
2.1. Catalyst preparation
Ru is a popular catalyst in the chemical industry. For example,
Ru-promoted cobalt catalysts have been commonly used for energy
conversion processes, such as Fischer-Tropsch synthesis and the
CO2 methanation process [33–35]. Therefore, Ru was selected as
the catalyst for the DBD plasma-assisted CO2 methanation process
in this study. The Ru chloride powder (RuCl3·3H2O, >99% purity)
and alumina spheres (␥-Al2O3, 1/16 inches) that are used as the
catalyst support were purchased from Kojima Chemicals and Strem
Chemicals, respectively. The Ru powder was dissolved in distilled
water, and the solution was added to the ␥-Al2O3 support. The
impregnated catalyst was dried at room temperature for 24 h. After
drying, the catalyst was calcined at 350 ◦C for 2 h; as a result, the
Ru/␥-Al2O3 catalyst was obtained.
In plasma catalysis, the catalyst is exposed to the plasma
streamer. Thus, the catalyst can influence the generation and prop-
agation of the plasma streamer. When the streamer is generated,
it would first be in contact with the surface layer of the catalyst.
Without sufficient catalyst loading, the probability for interaction
between the streamer and the catalyst would be reduced. The load-
ing fraction of Ru in the Ru/␥-Al2O3 catalyst was 5.369 wt%, which
is enough for the streamer to be in contact with the catalyst when
propagated.
Fig. 2. Schematic of the dielectric barrier discharge plasma reactor.
The quartz glass tube was used as the dielectric barrier because
it is not reactive with other substances and its high dielectric
constant can sustain the discharge. The configuration of the DBD
plasma-catalytic reactor is shown in Fig. 2. The reactor consists of a
quartz glass tube and two electrodes. The quartz glass has an axis-
symmetrical shape with dimensions of 402 mm (length), 13 mm
(outer diameter), and 10.9 mm (inner diameter). The stainless steel
fittings were connected on both ends of the reactor, which was used
for the gas supply.
The reactor has two electrodes: one is a stainless steel rod and
the other is a helical steel wire. The stainless steel rod was installed
inside the reactor to align with the center axis of the reactor, and the
steel wire was installed around the outside of the reactor. The steel
wire electrode was 45 mm long in the axial direction of the quartz
tube. The plasma was generated in the space between the two elec-
trodes. Thus, the prepared catalysts were filled in this space, which
was matched with the plasma discharge volume. The plasma was
generated in the same volume in which the catalysts of 3 g were
packed. Glass wool was used to fix the catalysts in the predeter-
mined location. The reactor was installed vertically with respect to
the ground to remove water gravitationally from the plasma dis-
2.2. DBD plasma-catalytic reactor
DBD is a method for plasma generation that requires a dielectric
barrier material between the electrodes, as shown in Fig. 1 [36].
Please cite this article in press as: C.J. Lee, et al., Enhancement of methanation of carbon dioxide using dielectric barrier discharge on a
ruthenium catalyst at atmospheric conditions, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.01.022

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Fig. 3. Schematic diagrams of the experimental apparatus for (a) the methanation experiment and (b) optical emission spectroscopy experiment.
Table 1
Effect of the presence of the Ru/␥-Al2O3 catalyst on CO2 methanation under the dielectric barrier discharge (DBD) plasma at 3 kHz and 9 kV.
CO2
Conversion (%)
Ru/␥-Al2O3
CH4
Selectivity (%)
Ru/␥-Al2O3
CO
Selectivity (%)
Ru/␥-Al2O3
No Catalyst
No Catalyst
No Catalyst
DBD Off
DBD On
0.03
8.21
0.01
12.80
0.15
1.42
0.10
73.30
0.13
43.84
0.01
5.12
charge area and catalyst bed, and a gas downflow configuration was
used.
The moisture in the product gases was filtered through a dehy-
dration chamber before being supplied into the gas chromatograph
(YL 6100 GC, Samarth Instruments). The composition of the prod-
uct gas was analyzed by gas chromatography to detect H2, N2, O2,
CO, CO2, CH4, C2H2, C2H4, C2H6, and C3H8. The definitions for con-
version and selectivity are expressed in Eqs. (2)–(4); the average
values of three experiments are presented, and the deviation for all
values was less than 1.5%.
2.3. Experimental apparatus
The schematic diagrams of the experimental apparatus are
shown in Fig. 3, including the plasma reactor setup to perform
the CO2 methanation under plasma generation (Fig. 3(a)), and the
optical emission spectroscopy (OES) setup to analyze the plasma
characteristics during the plasma-catalytic reaction (Fig. 3(b)). An
identical reactor was used for CO2 methanation and OES analysis
of the plasma generation.
Converted CO2
Supplied CO2
CO2 conversion (%) =
CH4 selectivity (%) =
CO selectivity (%) =
× 100
(2)
(3)
(4)
Produced CH4
Converted CO2
× 100
Produced CO
Converted CO2
A high voltage was applied to the reactor to generate the DBD
plasma on the catalyst. The voltage waveform was applied using a
function generator (Agilent 33220A), and the applied voltage was
amplified using a high-voltage amplifier (Trek Model 20/20C). The
electrical discharge characteristics, such as the onset voltage and
frequency of the discharge, were measured by a high-voltage probe
(Tektronix P6015A) and digital oscilloscope (LeCroy WaveSurfer
424). The capacitor (1024 pF) was connected to the gap capaci-
tance between the electrodes in series to analyze the discharge
power. The discharge power was calculated using the V-Q Lissajous
method. Fig. 4 shows a V-Q Lissajous diagram of the DBD plasma
discharged on Ru/␥-Al2O3 at 9 kV and 3 kHz. The preliminary tests
confirmed that the onset voltage of the discharge was 9 kV and that
the arc transition occurred as the discharge frequency increased
above 2 kHz at a discharge voltage above 9 kV. The change of the
discharge frequency was a more dominant factor to determine the
performance than the discharge voltage. Therefore, the discharge
voltage was fixed at 9 kV for all experiments.
× 100
The OES system consists of a monochromator (Monora500i,
Dongwoo Optron) and optical CCD detectors (Andor iDus DV420A-
OE). The in-situ optical emission spectra were recorded using the
Andor Solis software.
3. Results and discussion
3.1. Effect of the DBD plasma on the CO2 conversion and CH4
selectivity
DBD plasma-assisted CO2 methanation was performed at atmo-
spheric temperature and pressure. The discharge voltage and
frequency were 9 kV and 3 kHz, respectively. The CO2 conversion
and CH4 selectivity were measured when the plasma was applied in
the empty reactor, when the catalyst was used without the plasma,
and when both the plasma and catalyst were used simultaneously
(Fig. 5 and Table 1). The CO2 conversion and CH4 and CO selectivity
were essentially zero in the absence of the plasma. The Ru/␥-Al2O3
catalyst also had no reactivity in the absence of the plasma. In con-
trast, a CO2 conversion of 8.21% and CO selectivity of 43.84% were
obtained using only the plasma, but the CH4 selectivity decreased
by 1.42%. However, the CO2 conversion and CH4 selectivity were
12.80% and 73.30%, respectively; the CO selectivity decreased when
the DBD plasma was applied to the Ru/␥-Al2O3 catalyst.
A mass flow controller (MKP TSC-210) and readout unit (Sehwa
KRO-4001) were used to adjust and maintain the gas supply rates.
Ultrahigh purity (99.999%) gases, including N2, H2, CO2, and Ar,
were used for all experiments. The N2 gas was used as a carrier gas
and played a role as a major discharge media. The space−v1eloci−ty1
based on the unit weight of the catalyst was 18,625 mL h gcat.
at a total flow rate of 50 mL/min without Ar addition, whereas it
−1
−1
at 80 mL/min with Ar addition.
was 29,800 mL h
g
cat.
Please cite this article in press as: C.J. Lee, et al., Enhancement of methanation of carbon dioxide using dielectric barrier discharge on a
ruthenium catalyst at atmospheric conditions, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.01.022

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Fig. 4. Lissajous diagram of the dielectric barrier discharge (DBD) plasma discharged on Ru/␥-Al2O3 at 9 kV and 3 kHz.
Fig. 6. Effect of the dielectric barrier discharge frequency for CO2 conversion and
CH4 selectivity within 1.0–3.0 kHz (1 atm, 25 ◦C).
Fig. 5. Comparison of the CO2 conversion and CH4 selectivity at dielectric barrier
discharge (DBD) plasma conditions (1 atm, 25 ◦C).
3.2. Effect of the discharge frequency on the activation of
methanation
The effect of the discharge frequency on the CO2 conversion and
CH4 selectivity is shown in Fig. 6. The discharge frequency increased
by 0.5 kHz, while the discharge voltage was maintained at 9 kV
because it was higher than the onset voltage of discharge. At 1 kHz,
a small amount of CO2 was converted to CH4 even though the DBD
plasma was generated on the Ru/␥-Al2O3 catalyst. However, the
CO2 conversion and CH4 selectivity gradually increased with the
increase of the discharge frequency.
The CO2 conversion and CH4 selectivity were directly propor-
tional to the discharge frequency. In particular, the CH4 selectivity
suddenly increased when the discharge frequency increased from
2.5 to 3.0 kHz. The discharge frequency is a determining factor
The CO2 conversion and CO selectivity were high but the CH4
selectivity was low when only the plasma was applied. In contrast,
the CO2 conversion was nearly zero when only the Ru/␥-Al2O3 cata-
lyst was used because the catalyst was not activated at atmospheric
temperature. The CO2 conversion and CH4 selectivity were both
high when the plasma and the catalyst were used simultaneously,
but the CO selectivity significantly decreased. The results indicate
that the plasma increases the deoxygenation of CO2 and thereby
enhances the activity of methanation through interactions with the
Ru/␥-Al2O3 catalyst.
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Fig. 7. Specific energy density as a function of the discharge frequency from 1.0 to
3.0 kHz.
Fig. 8. Effect of the H2/CO2 mixture ratio on the CO2 conversion and CH4 selectivity
for Ru/␥-Al2O3 with 3 kHz and 9 kV dielectric barrier discharge (DBD) (1 atm, 25 ◦C).
for the energy deposition to activate methanation. The specific
energy density (SED) is a function of the discharge frequency, as
shown in Fig. 7. Similarly, with the tendency for CH4 selectivity,
the SED increased rapidly after a discharge frequency of 2.0 kHz
was applied. CO2 methanation was enhanced when a sufficient dis-
charge energy was applied to the catalyst. CO2 deoxygenation was
dominant and increased the CO selectivity if either the discharge
energy was lacking or the catalyst was absent.
A minimum discharge frequency is required to activate the
Ru/␥-Al2O3 catalyst so that it can interact with the DBD plasma.
The discharge frequency is an important parameter because ion-
ization, excitation, and radical generation can be influenced by the
discharge frequency [37–39]. This discharge frequency might also
be related to the aforementioned activation condition between
the plasma and catalyst because a higher discharge frequency
can provide more abundant ionized radicals and excited species.
CO2 deoxygenation was improved by only using the plasma, as
summarized in Table 1. Thus, the plasma alone did not activate
methanation. Applying the plasma to the catalyst could also satisfy
the discharge condition.
Fig. 9. Effect of N2 and Ar added to N2 on the dielectric barrier discharge (DBD) over
Ru/␥-Al2O3 with 3 kHz and 9 kV DBD (1 atm, 25 ◦C).
atoms and excited hydrogen molecules can actively react with CO2
and CO, which increases the carbon balance and CH4 selectivity.
CO2 can be dissociated to C, CO, O, and O2 by electron collisions
[40,41]. At a low H2/CO2 mixture ratio, C and O2 were generated
from CO2 dissociation, but some carbon was deposited onto the cat-
alyst surface because there was not enough chance to bond with
ionized hydrogen atoms and excited hydrogen molecules. How-
ever, the plasma can generate abundantly ionized hydrogen atoms
and excited hydrogen molecules, which can provide more chances
for bonding with the decomposed carbons.
3.3. Effect of the H2/CO2 mixture ratio
The effect of the H2/CO2 mixture ratio on the CO2 conversion
and CH4 selectivity is shown in Fig. 8. N2, which is a primary dis-
charge media, was supplied at 30 mL/min, and the H2/CO2 mixture
ratio was varied by adjusting the H2 and CO2 flow rates at a constant
total flow rate (50 mL/min). The CO2 conversion and CH4 selectivity
increased the H2/CO2 mixture ratio. In particular, the CO2 conver-
sion increased from 12.80% to 23.20% when the H2/CO2 mixture
ratio increased from 3 to 7. At the same conditions, the CH4 selec-
tivity and carbon balance increased from 73.30% to 95.02%, and
from 78.93% to 97.38%, respectively. However, the CO selectivity
decreased as the H2/CO2 mixture ratio increased.
3.4. Effect of the addition of Ar
This result can be explained by the relation of deoxygena-
tion and methanation in the H2-rich environment provided by the
high H2/CO2 mixture ratio. The carbon balance and CH4 selectivity
decreased but the CO selectivity increased at a low H2/CO2 mixture
ratio. In contrast, the carbon balance and CH4 selectivity increased
at a high H2/CO2 mixture ratio. The carbon balance and CH4 selec-
tivity increased because more carbon atoms were bonded with
either four hydrogen atoms or two hydrogen molecules. H2 and CO2
can be dissociated, ionized, or excited by electron collisions from
the plasma. Therefore, at a high H2/CO2 mixture ratio, hydrogen can
be ionized and excited by the plasma so that the ionized hydrogen
Ar has been widely used in various plasma applications because
it is relatively easy to ionize. It has been widely used as a primary or
secondary gas with N2 because the electric discharge of the N2/Ar
mixture can increase the concentration of plasma-active species by
the Penning effect [42,43]. In addition, Ar can be recycled after the
methanation process because it is inert, i.e., it has not participated
in the reaction, and is simply separated from product gases using
either the membrane or the pressure swing adsorption process.
Therefore, the N2/Ar mixture was supplied in the reactor and the
plasma discharge was performed to verify the effect of Ar addition
on CO2 methanation. Ar was added at a flow rate of 30 mL/min
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Fig. 10. Comparison of the optical emission spectra for the ␥-Al2O3 and Ru/␥-Al2O3 catalysts at 3 kHz and 9 kV dielectric barrier discharge (DBD) conditions from (a)
150–550 nm and (b) 450–850 nm (1 atm, 25 ◦C).
Fig. 11. Comparison of the optical emission spectra for H2 and CO2 + H2 mixture gas discharged over the Ru/␥-Al2O3 catalyst at 3 kHz and 9 kV dielectric barrier discharge
(DBD) conditions from (a) 150–550 nm and (b) 450–850 nm (1 atm, 25 ◦C).
without changing the other flow rates (the flow rates of N2, H2,
and CO2 were fixed at 30, 15, and 5 mL/min, respectively) because
N2 is a primary discharge medium and CO2 methanation can be
affected by different gas mixture ratios. Thus, the total flow rates
were different with and without Ar addition; the total flow rate
was 1.6 times higher when Ar was added. The discharge conditions
were identical to the other experiments (9 kV and 3 kHz).
The CO2 conversion and CH4 selectivity when only N2 was used
were compared with those when Ar was added to N2 (Fig. 9). The
CO2 conversion is inversely proportional to the total flow rate. How-
ever, the CO2 conversion with the N2-Ar discharge was higher even
though the total flow rate was 60% higher than for the N2 discharge.
The CO2 conversion increased by 1.68%, while the CH4 selectivity
increased by 11.60% in the N2-Ar discharge condition. In contrast,
the CO selectivity increased by 1.72%, which means that methana-
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tion and deoxygenation were simultaneously enhanced when Ar
was added.
and CH4 selectivity (34.95%) were four times and two times lower
than those with the plasma, respectively. Therefore, in addition
to dielectric heating, electron collisions are a major contributor to
the improvement of CO2 methanation because the plasma provides
electron collisions to generate highly reactive plasma species, such
as excited and ionized hydrogen species, or dissociate oxygen from
CO2.
As aforementioned, the discharge can increase the concentra-
tion of the plasma active species, which leads to an increase of the
electron collision frequency. As a result, this induced active CO2
dissociation and generated more active carbons and ionized and
excited hydrogens. The increased active carbon and hydrogen ions
can influence both CO2 methanation and deoxygenation because
the increased electron collisions with the CO2 species can increase
deoxygenation. In addition, the increased electron collisions also
induce ionization, excitation, and radical generation, which can
provide more chances for carbon to bond with hydrogen.
4. Conclusions
The DBD plasma was used to activate the Ru/␥-Al2O3 catalyst
for CO2 methanation at atmospheric conditions, and the effect of
the interaction between the plasma and catalyst on the conver-
sion and selectivity was investigated. The CO2 conversion increased
with the DBD plasma regardless of the presence of the catalyst at
atmospheric conditions. However, when the DBD plasma was used
alone, most of the CO2 conversion was induced by CO2 deoxygena-
tion because CO2 molecules were exposed to electron collisions and
decomposed to CO. In contrast, the Ru/␥-Al2O3 catalyst was not
activated without the DBD plasma. CO2 methanation was activated
when the DBD plasma was applied to the Ru/␥-Al2O3 catalyst. The
CO2 conversion and CH4 selectivity increased with the increase of
the discharge frequency and H2/CO2 ratio, and with the addition of
Ar. The maximum CO2 conversion and CH4 selectivity were 23.20%
and 97.38%, respectively, when the discharge conditions were 9 kV
and 3 kHz, and the H2/CO2 ratio was 7.
CO2 methanation was activated by the interaction between the
DBD plasma and Ru/␥-Al2O3 catalyst. The OES results confirmed
that the presence of the catalyst in the plasma could influence the
characteristics of the plasma discharge, thereby generating highly
reactive plasma species, such as the excited and ionized hydrogen
species. In addition, the DBD plasma could induce dielectric heating
of the catalyst and increase the catalytic activity when enough heat-
ing is provided. Therefore, the enhancement of CO2 methanation is
expected to occur by electron collisions and dielectric heating from
the DBD plasma. However, the electron collisions were a major
contributor because the DBD plasma could provide electron col-
lisions to generate highly reactive plasma species, such as ionized
and excited hydrogens, and dissociate carbon and oxygen from CO2.
3.5. Optical emission spectroscopy
OES is a powerful, non-contact diagnostic method to moni-
tor excited and ionized species for plasma applications. When the
excited atoms and molecules are restored to low states and the
ground state, photons are released from the excited atoms and
molecules; thereby, the optical emission can be detected by OES.
The types of elements as well as the excitation and ionization states
are determined based on the optical emission spectra of the plasma
[42,44,45]. For this reasons, in-situ OES analysis was performed dur-
ing CO2 methanation using the Ru/␥-Al2O3 catalyst in the plasma.
The optical emission spectra of the ␥-Al2O3 and Ru/␥-Al2O3 cat-
alysts were measured to investigate the effect of the presence of
Ru in the plasma with H2 and CO2 flow rates of 15 mL/min and
5 mL/min, respectively, as shown in Fig. 10. The discharge condi-
tions were 9 kV and 3 kHz. Different emission peaks and intensities
were observed in both spectra. The increased intensity of the emis-
sion bands and additional peaks were observed for ␥-Al2O3. In
contrast, several emission bands were suppressed and disappeared
for the Ru/␥-Al2O3 catalyst. Thus, the discharge characteristics of
the DBD plasma were changed by ␥-Al2O3, thereby generating
excited species with different excitation states. In addition, the
presence of Ru influenced the discharge characteristics. However,
the effect of the change in the discharge characteristics on the
enhancement of CO2 methanation is not clearly identified. There-
fore, additional OES analyses were performed at the different gas
conditions.
Optical emission spectra were recorded when the H2/CO2 mix-
ture was supplied (condition for methanation) and when only H2
was supplied (Fig. 11). The Ru/␥-Al2O3 catalyst was used for both
cases. Similar spectra were observed in both cases, which indicate
that hydrogen was mainly ionized and returned to a lower state
or the ground state when the plasma was applied to the catalyst
at methanation conditions. At the same time, the emission bands
for the Balmer series, including H␣ (656.5 nm), H␤ (486.1 nm),
H␦ (410.2 nm), H (388.9 nm), and H (383.5 nm), as well as the
Acknowledgements
Funding: This research was supported by the Basic Science
Research Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Science, ICT and Future
Planning [grant number 2014R1A2A1A11054686].
References
␩
␨
Paschen series of H+ (820 nm) were also identically detected at
methanation conditions. Hydrogen radicals with different excita-
tion states existed in the reaction because these emission series and
bands are emitted from excited hydrogen atoms [46–49].
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Please cite this article in press as: C.J. Lee, et al., Enhancement of methanation of carbon dioxide using dielectric barrier discharge on a
ruthenium catalyst at atmospheric conditions, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.01.022

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Please cite this article in press as: C.J. Lee, et al., Enhancement of methanation of carbon dioxide using dielectric barrier discharge on a
ruthenium catalyst at atmospheric conditions, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.01.022