22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
CO 2 conversion in a dielectric barrier discharge upon addition of argon or
helium
M. Ramakers, I. Michielsen, R. Aerts, V. Meynen and A. Bogaerts
Research group PLASMANT, Department of Chemistry, University of Antwerp, BE-2610 Antwerpen-Wilrijk, Belgium
Abstract: n this work, we systematically have investigated the effect of Ar and He
addition over a broad concentration range on the CO 2 conversion and on the energy
efficiency in a dielectric barrier discharge (DBD). The observed effects are explained by
detailed electrical characterization of the plasma, i.e., by studying in detail the Lissajous
plots and the current profiles from which we can deduce the breakdown voltage. Moreover,
the rates of electron impact excitation-dissociation are deduced via the estimated electron
densities and mean electron energies.
Keywords: argon, CO 2 splitting, conversion, dielectric barrier discharge (DBD), helium
1. Introduction
In the past years, there is an increased interest in the
splitting of CO 2 by plasma to produce CO and O 2 [1-16].
Different types of plasmas have been applied for CO 2
conversion, but most research is carried out with either
dielectric barrier discharges (DBDs) [2-10], microwave
plasmas [11-13] and gliding arc discharges [14-16].
Although the latter are more promising in terms of energy
efficiency, DBD plasmas have several advantages like
operating at atmospheric pressure, a simple design and
easy upscaling capabilities [17].
2. Experimental set-up
The experiments were performed with a cylindrical
DBD reactor, illustrated in Fig. 1. This reactor consists of
a stainless-steel rod with a diameter of about 12.88 mm
and length of 200 mm, which acts as inner electrode and
is grounded. It is surrounded by a dielectric tube, made of
Al 2 O 3 , with (more or less) the same length, and an outer
and inner diameter of 22 and 16.54 mm. The discharge
gap, i.e., the distance between the inner electrode and the
dielectric tube, is 1.83 mm. The dielectric tube is
surrounded by the outer electrode, made of nickel foil,
powered by a high voltage supply. The length of the
outer electrode is 90 mm, which defines the length of the
plasma.
The high voltage is supplied by a generator and
transformer (AFS). A high voltage probe (Tek P6015A) is
used to measure the applied voltage, while the total
current is measured with a Rogowski coil (Pearson 4100).
Moreover, the voltage on an external capacitor (10 nF) is
measured to obtain the generated charges (Q) in the
plasma. When plotting Q as a function of the applied
voltage (U), a Q-U Lissajous plot can be obtained.
Finally, all the electrical signals are recorded by an
oscilloscope (PicoScope 6402A).
The input gas flow of CO 2 , Ar and He is controlled by
thermal mass flow controllers (Bronkhorst), and the gas at
the outlet is analyzed by a compact GC (Interscience).
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Fig. 1. Left: schematic diagram of the cylindrical DBD
reactor, with 1 = inner electrode, 2 = discharge gap,
3 = dielectric tube, 4 = outer electrode. Right: picture of
the DBD reactor.
The CO 2 , Ar and He fractions are varied between 5 % and
95 %, while the total gas flow rate is always kept constant
at 300 mL/min. Furthermore, measurements in pure CO 2
are also carried out. Ten samples are taken for each GC
measurement, of which only the last 5 are used to ensure
stabilization of the gas composition.
First, blank
measurements are taken, i.e., without plasma.
Subsequently, a power of 80 W is applied with a
frequency of 23.5 kHz, and after half an hour, i.e., when
the measured voltage is more or less constant,
GC measurements are taken to calculate the CO 2
conversion.
3. Results and discussion
First of all we will discuss the effect of Ar and He on
the CO 2 conversion, which is shown in Fig. 2 as a
function of CO 2 fraction in the gas mixture. The
conversion is lowest in the pure CO 2 plasma, and
increases drastically upon addition of either Ar or He. Up
to Ar or He fractions of 70 %, the conversion is very
similar, with the He addition giving slightly higher
conversion. However, above Ar or He fractions of 70 %,
the effect becomes most pronounced for Ar addition. In
the 5/95 CO 2 /Ar gas mixture a conversion of 41 % is
reached, whereas in the 5/95 CO 2 /He gas mixture, the
1
conversion is around 25 %.
splitting, whereas at the lower CO 2 concentrations a
significant fraction of the energy is also consumed by
ionization/excitation of the Ar or He gas. Although some
of this energy will be indirectly used for CO 2
dissociation, through the Ar or He ions or excited atoms,
as will be elaborated below, still a considerable fraction of
this energy does not lead to CO 2 splitting.
Fig. 2. CO 2 conversion as a function of CO 2 fraction in
CO 2 /Ar and CO 2 /He gas mixtures, at an applied power of
80 W and a frequency of 23.5 kHz.
It is important to note that the effective amount of CO 2
that is converted, shown in Fig 3, will drop from about
5.5 % (in pure CO 2 ) to 2 % upon addition of 95 % Ar,
and even to 1.2 % in the case of adding 95 % He. This
can be easily explained because there is less CO 2 present
in the gas mixture that can be converted. We can also
calculate the energy efficiency of this process, using the
effective amount of CO 2 converted, the reaction enthalpy
of CO 2 splitting (i.e., 279.8 kJ/mol), the flow rate and the
power coupled into the plasma, i.e., the so-called plasma
power.
This value was obtained for each gas
composition, by means of the Lissajous figures (see
below). The values varied between 37 and 43 W for
CO 2 /Ar and between 38 and 44 W for CO 2 /He.
Fig. 3. Effective amount of CO 2 converted as a function
of CO 2 fraction in CO 2 /Ar and CO 2 /He gas mixtures, at
the same conditions as in Fig. 2.
The energy efficiency of CO 2 splitting is plotted in
Fig. 4 as a function of CO 2 fraction in the gas mixture. It
shows that the energy efficiency rises from 2-3 % at 5 %
CO 2 in the CO 2 /Ar or CO 2 /He mixture, respectively, to
about 9 % above 80 % CO 2 in the gas mixtures. The
reason for the higher energy efficiency when more CO 2 is
present is because the effective CO 2 conversion is higher.
Hence the energy is more effectively used for CO 2
2
Fig. 4. Energy efficiency of CO 2 splitting as a function of
CO 2 fraction in CO 2 /Ar and CO 2 /He gas mixtures, at the
same conditions as in Fig. 2.
The higher CO 2 conversion upon addition of Ar or He
can be explained by analysing the Lissajous figures and
current profiles. In Fig. 5 the Lissajous plots for pure
CO 2 and the two gas mixtures with 5 % CO 2 are shown.
Several characteristics can be derived from these figures,
such as the total capacity of the reactor without plasma
(C cell ), the effective capacity of the plasma reactor (C eff )
and the minimum voltage (U min ). The minimum voltage
can be used to calculate the breakdown voltage (U B ). The
obtained values for both pure CO 2 and for the CO 2 /Ar
and CO 2 /He gas mixtures are listed in Table 1.
Fig. 5. Lissajous plots of the pure CO 2 plasma, and the
plasma in a CO 2 /Ar and CO 2 /He mixture with 5% CO 2 .
The other conditions are the same as in Fig. 2.
U min drops upon addition of Ar or He, which also leads
to a drop in U B . The voltage needed to initiate the plasma
will thus be lower in the CO 2 /Ar and CO 2 /He mixtures
than in the pure CO 2 plasma. The drop in U B can
partially be explained by the Townsend ionization
coefficient α, which is estimated to be significantly larger
for Ar (1488) and He (1602) than for CO 2 (142), at the
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conditions under study.
This explains the lower
breakdown voltage for Ar and He, as a larger value for α
yields more electron production per
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3
Table 1. Electrical characteristics of the pure CO 2 plasma
and the CO 2 /Ar and CO 2 /He gas mixtures with 5% CO 2 ,
as deduced from the Lissajous plots in Fig. 5.
Gas
mixture
C cell (pF)
C eff (pF)
U min (kV)
U B (kV)
100 % CO 2
21
73
2.31
2.06
5 % CO 2 –
95 % Ar
23
161
1.32
1.18
5 % CO 2 –
95 % He
23
160
1.18
1.05
unit length, so that a lower voltage can be sufficient to
initiate the plasma. Another possible explanation is the
lower probability for inelastic collisions. Indeed, in Ar
and He, electron impact excitation and ionization are the
only possible inelastic collisions, and they are
characterized by rather high threshold energies. On the
other hand, for CO 2 , the threshold for inelastic collisions
is much lower. Furthermore, as many vibrational levels
can be excited, there are many more possibilities for the
electrons to participate in inelastic collisions in CO 2 than
in Ar or He. Moreover, the probability for recombination
of electrons with Ar+ or He+ ions is also much lower than
for recombination with CO 2 + ions, because of the absence
of dissociative recombination. In conclusion, the lower
probability of the electrons for inelastic collisions with Ar
or He, on one hand, and for recombination with Ar+ or
He+ ions, on the other hand, results in a longer mean free
path of the electrons in the Ar or He plasma. Hence, the
electrons have more time to become accelerated in the
applied electric field, so that lower voltages are sufficient
for electrical breakdown. Since the plasma power in our
experiments is more or less the same for the pure CO 2
plasma and the CO 2 /Ar and CO 2 /He mixtures, this means
that more power can be used for dissociation of CO 2 , as
less power will be dissipated for the gas breakdown. We
believe this is one of the reasons for the higher CO 2
conversion upon addition of Ar or He.
From the Lissajous plots, we can also deduce the
capacities of the different gas mixtures with and without
plasma. In Table 1 we can see that C cell is almost the
same for the different cases. The effective capacity of the
plasma, on the other hand, rises significantly upon
addition of either Ar or He. This indicates that the
discharge gap will be more “filled with plasma”, i.e.,
either as a homogenous plasma, like in the case of He, or
with a higher density of microdischarge filaments, like in
the case of Ar. This also explains why the dissociation of
CO 2 will be higher in the CO 2 /Ar and CO 2 /He gas
mixtures compared to pure CO 2 . This is also visible in
Fig. 6, which illustrates the current profiles for pure CO 2
and the CO 2 /Ar and CO 2 /He gas mixtures with 5 % CO 2 .
From these current profiles, a rough estimation of the
electron density can be made. The electron densities for
the pure CO 2 plasma and the CO 2 /Ar and CO 2 /He
mixtures, obtained in this way, are given in Table 2. We
see that the electron density is more than a factor two
higher in the CO 2 /Ar mixture, but it is a factor three lower
4
in the CO 2 /He
Fig. 6. Electrical current profiles in the pure CO 2 plasma,
and in the CO 2 /Ar and CO 2 /He plasmas with 5% CO 2 ,
for the same conditions as in Fig. 2.
mixture, compared to the pure CO 2 plasma. The mean
electron energy and the rate constants for electron impact
excitation of CO 2 , leading to dissociation, are calculated
with Bolsig+ for the three cases. Table 2 summarizes,
besides the values of the electron density, also the mean
electron energy, the rate constant for this CO 2 excitationdissociation process, and the resulting rate of this
reaction, calculated for E/n = 200 Td, which is a typical
value for a DBD plasma [1, 10], for the pure CO 2 plasma
and the CO 2 /Ar and CO 2 /He mixtures.
Table 2. Electron density, obtained from the current
profiles of Fig. 6, mean electron energy and rate constant
for CO 2 excitation, leading to dissociation, calculated
with Bolsig+ for a reduced electric field of 200 Td, and
corresponding rate of this reaction, for the pure CO 2
plasma and the CO 2 /Ar and CO 2 /He plasmas with
5% CO 2 .
Gas mixture
Electron
density (m-3)
Mean
electron
energy (eV)
Rate
constant
(m3/s)
Reaction rate
(s-1)
100
%
CO 2
6.52 × 1018
5 % CO 2 –
95 % Ar
1.64 × 1019
5 % CO 2 –
95 % He
2.05 × 1018
6.61
7.83
15.43
1.8 × 10-16
2.7 × 10-16
1.8 × 10-15
1200
4500
3700
The mean electron energy increases upon addition of
Ar, and more drastically upon addition of He, and the
same is of course true for the rate constant of CO 2
excitation-dissociation, because it depends on the mean
electron energy. The product of this rate constant with
the electron density gives us the electron impact
excitation-dissociation rate, which is indeed higher in the
CO 2 /Ar and CO 2 /He mixtures than in the pure CO 2
plasma, explaining the higher CO 2 conversion, shown in
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Fig. 2 above. Moreover, the rate is higher in the CO 2 /Ar
mixture than in the CO 2 /He mixture, explaining also why
the CO 2 conversion is higher upon addition of Ar than
upon addition of He, at least for Ar and He fractions
above 70%.
Finally, besides the above explanation, we have to keep
in mind that in the CO 2 /Ar and CO 2 /He mixtures, some
other reactions might be responsible for CO 2 dissociation
as well. Indeed, it is stated in literature [18, 19] that the
addition of Ar has a beneficial effect on the CO 2
dissociation, because of charge transfer of the Ar+ ions
with CO 2 , yielding CO 2 + ions (Ar+ + CO 2 → Ar + CO 2 +).
The latter can undergo dissociative electron-ion
recombination (CO 2 + + e- → CO + O), effectively
contributing to CO 2 splitting. Alternatively, as the above
charge transfer reaction is exothermic (∆H = - 2 eV), it
occurs quickly [1], and the excess energy can be coupled
into the vibrational and rotational states of the CO 2
molecule, thereby also increasing the dissociation rate.
the higher CO 2 conversion upon addition of Ar or He, and
can also explain why the CO 2 conversion is higher in the
CO 2 /Ar mixtures than in the CO 2 /He mixtures at CO 2
concentrations below 30 %.
Finally, the effective CO 2 conversion drops upon
addition of Ar or He. The same is true for the energy
efficiency, which is calculated from the effective CO 2
conversion. Indeed, at lower CO 2 fractions in the gas
mixtures, a larger fraction of the energy will be consumed
by the Ar or He gas, and only part of it (through the Ar or
He ions or excited atoms) will be finally used for CO 2
conversion.
4. Conclusions
An electrical characterization of a DBD plasma
operating in CO 2 and in CO 2 /Ar and CO 2 /He mixtures is
performed to explain why the CO 2 conversion increases
upon addition of Ar or He, and why there is a larger
increase for Ar than for He, at least for Ar or He gas
fractions above 70 %. The Lissajous plots show that the
breakdown voltage is significantly lower in the CO 2 /Ar
and CO 2 /He mixtures, which suggests that a larger
fraction of the applied power can be used effectively for
conversion of CO 2 . Moreover, the effective capacity of
the plasma is higher in the CO 2 /Ar and CO 2 /He mixtures,
which indicates that the discharge gap is more filled with
plasma, enhancing also the possibility for CO 2
conversion. Finally, the Lissajous plots illustrate that
more charges are generated in the CO 2 /Ar and CO 2 /He
mixtures, which can also be observed from the electrical
current profiles.
Electron densities are estimated from these current
profiles and the electron mean energies are calculated
with Bolsig+, as well as the rate constants for electron
impact excitation of CO 2 , which is the most important
process responsible for CO 2 dissociation in a DBD. The
product of the electron densities and the rate constants
gives us the rates of electron impact excitationdissociation of CO 2 . These rates are indeed higher,
especially in the CO 2 /Ar mixture, but also in the CO 2 /He
mixture, compared to the pure CO 2 plasma. In the
CO 2 /Ar mixture, also a charge transfer process between
Ar+ or Ar 2 + ions and CO 2 , followed by electron-ion
dissociative recombination of the CO 2 + ions, will most
probably contribute to the dissociation of CO 2 , and it
might even be the dominant process. A similar effect
could take place with He+ or He 2 + ions as well, but the
He+/He 2 + ion density is expected to be (significantly)
lower than the Ar+/Ar 2 + density, based on the higher
ionization potential, and as can be deduced from the
electron densities. All of the aforesaid effects can explain
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5. Acknowledgement
This research was carried out in the framework of the
network on Physical Chemistry of Plasma-Surface
Interactions - Interuniversity Attraction Poles, phase VII
(http://psi-iap7.ulb.ac.be/), and supported by the Belgian
Science Policy Office (BELSPO).
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