CO2 conversion for clean synthetic fuel production through
plasma- assisted catalysis.
S. Welzel1, S. Ponduri1, F. Brehmer1, M. Creatore1, M.C.M. van de Sanden1,2, R. Engeln1
1
Eindhoven University of Technology, Applied Physics (PMP-group), Eindhoven, The Netherlands
2
FOM Institute for Plasma Physics Rijnhuizen, Nieuwegein, The Netherlands
Abstract: We report on plasma-assisted conversion of CO2 to hydrocarbon
molecules in an argon-hydrogen plasma expansion. More particular, we focus on
the efficiency of CO2 depletion and selectivity of CH4 production in a lowtemperature plasma expansion. The plasma is created from mixtures of argon and
hydrogen while CO2 is injected into the expansion part where the dissociation
mechanism might be radical- and/or ion-driven. Results on measurements of the
(steady state) gas composition obtained by mid-infrared tuneable diode laser
absorption spectroscopy will be reported.
Keywords: CO2 conversion, plasma-assisted gas conversion
1. Introduction
The emission of carbon dioxide to the
atmosphere, leading to global warming, is
widely regarded as one of the most severe
environmental issues [1]. Therefore there is a
clear demand to find ways (i) to drastically
decrease the net CO2 production, e.g. using
renewable energy sources such as solar fuels
and (ii) to capture or even recycle the remaining
CO2 emissions. Direct conversion of CO2 and
water into hydrocarbon fuels using sunlight
shows extremely challenging prospects. It could
reduce atmospheric CO2 concentrations, while
at the same time provide on a renewable basis
fuels that can directly be supplied to our present
energy infrastructure. The idea is based on the
following closed-loop cycle: when hydrocarbon
fuel is burned CO2 is produced; the CO2 is
collected and passed through a photocatalytic
sieve; in the sieve the CO2 is exposed to sunlight
and with the addition of water converted back
into a hydrocarbon fuel. In order to be viable,
the efficiency of the photocatalytic materials to
convert CO2 to fuel under sunlight exposure is
of
critical
importance.
Recently,
the
photocatalytic reduction of CO2 with H2 and/or
H2O with titania as catalyst has been reported to
show promising results. Titania is ideally suited
for this purpose, as it has strong oxidizing
power and long-term photostability, and is
nontoxic. However, due to its large band gap,
the efficiency of pure TiO2 as photocatalyst for
the CO2 reduction with H2O is too low to be of
practical use. Therefore, most of the research
efforts have been put in developing TiO2 based
photocatalytic materials with improved activity.
The doping of titania with nitrogen, for
example, showed an increased photocatalytic
conversion efficiency of CO2 reduction to CH4
under solar light exposure. The increase was
ascribed to the improved use of the solar energy
spectrum, due the lowering of the band gap in
TiO2. The addition of cocatalysts like copper
and platinum and the use of multi wall carbon
nanotubes as supporting catalyst structure have
also been investigated. However, even with the
most sophisticated TiO2-based catalysts, the
photocatalytic conversion efficiency of CO2 and
water to hydrocarbon fuels is too low to be
economically viable. One of the main reasons
for this low efficiency is the high stability of the
CO2 molecule. At least 8 photons are necessary
in the photocatalytic conversion of CO2 and
water to CH4, and even more for higher
hydrocarbon fuels [2].
from 1 eV at the start of the expansion, to 0.2 ...
0.3 eV after the stationary shock.
We report on a study of the CO2 conversion to
hydrocarbons
using
a
low-temperature
expanding plasma instead of a photocatalyst. In
this contribution we focus on CO2 depletion and
CH4 production in plasma expansions created
from mixtures of H2 and Ar.
The CO2 gas is injected directly into the reactor
and mixes efficiently with the expansion. Due to
the low electron temperature, direct electronimpact ionization or dissociation reactions are
not efficient. In the expansion, the feedstock
gases are dissociated by charge transfer and
subsequent dissociative recombination, in case
of a pure argon expansion. When hydrogen is
added to the gas mixture flowing through the arc
channel, the electron density decreases rapidly
and the chemistry in the plasma expansion
becomes mainly radical-driven. The radicals,
produced in the plasma expansion, travel
through the reactor, and before they are pumped
away, they recombine into stable molecules
mainly at the walls of the reactor.
2. Experimental setup and diagnostics
The experiments discussed in this paper have
been performed on the PLEXIS setup
(http://plexis.nl). In this setup an subatmospheric thermal plasma, produced in a
cascaded arc, expands into a low pressure vessel
[3]. The cascaded arc plasma source consists of
a stack of four copper plates with a central bore
of four mm, through which the working gas is
directed. A discharge is maintained between
cathode tip and anode plate, at a current setting
of 45 A, which results in ionization of a
significant fraction of the working gas (10 to
15%) in case pure argon is used, while at
already small amounts of hydrogen molecules
added to the argon flow, the source mainly
produces atomic hydrogen radicals [4]. The
copper plates are electrically insulated from
each other, which stabilizes the discharge.
During operation of the plasma source the
power input reaches values between 2 kW
during pure Ar operation, up to 6.5 kW during
pure H2 operation The mixture of atoms, ions
and electrons flows through a nozzle and
expands supersonically into the vessel.
Due to the fast expansion into the reactor, the
gas temperature and the electron temperature
drop rapidly. The gas temperature drops from
approximately 5000 K at the start of the
expansion, down to 450 K in the background of
the reactor. The electron temperature decreases
The setup is equipped with a mid-infrared
tunable diode laser absorption spectrometer and
quadrupole mass spectrometer (QMS).
In the experiments discussed in this extended
abstract, the cascaded arc is operated on
mixtures of hydrogen and argon with a total
flow rate of 3000 standard cubic centimeters per
minute, sccm. Also an axial magnetic field of 40
mT is applied, which is able to (partially)
magnetize the expanding plasma (depending on
the gas mixture). The feedstock gas CO2 is
injected directly into the reactor at a flow rate of
360 sccm. The walls of the reactor are
constructed from stainless steel.
3. Results
Results on measurements of the (steady state)
gas composition obtained by mid-infrared
tuneable diode laser absorption spectroscopy are
shown in Figure 1. The Figure shows the ratio
of the partial pressure of a molecule to the total
pressure as function of the relative H2-flow, i.e.
the ratio of the H2-flow to the total (Ar+H2)flow through the arc. The grey line in Figure 1
at 0.107 corresponds to the mixing ratio for the
injected CO2 and can be calculated from the
injected flow rates, i.e. 360 sccm CO2 and 3000
sccm total (Ar+H2)-flow. We distinguish three
regions 1. relative H2-flow below 0.05, 2.
relative H2-flow between 0.05 and 0.8, and 3.
relative H2-flow higher than 0.8. In region 1 an
efficient CO2 dissociation can be observed.
Under these conditions no CH4 is measured and
most of the CO2 is converted into CO. Most of
the hydrogen is found back into water. In the
second region much less CO2 is converted, and
at (slightly) higher relative H2 flows, some of
the hydrogen is found back in CH4. In the third
region, even less CO2 is converted, but at the
same time a steep increase in CH4 production
can be observed.
Under all conditions CO and H2O were the main
stable products while higher hydrocarbons
(C2Hy) were absent (not observed with mass
spectrometry).
Mixing Ratio
0.1
0.01
1E-3
1E-4
0.0
0.2
0.4
0.6
0.8
1.0
H2/Ar+H2 Flow
Figure 1. Mixing ratios of different molecules formed during
CO2 conversion in a plasma expansion generated from mixtures
of hydrogen and argon. ：●: CO2, ○: CO, ▼: H2O, ■:
CH4 , grey line: input of CO2
4. Discussion and Conclusions
From previous studies we know that when a few
percent of H2 is added to an argon flow through
the arc the flow of Ar-ions into the vessel is
drastically reduced. The main reason is the very
efficient loss channel for Ar-ions, i.e. the
combined charge exchange(CE)/dissociative
recombination(DR) processes with H2. This
process is much faster than the loss of Ar-ions
via three body recombination in a pure Ar
expansion. This means that at very low relative
H2 flows the CO2 chemistry is probably driven
by charge exchange reactions with Ar-ions,
forming CO and O in the subsequent
dissociative recombination step. Almost 50% of
the CO2 is converted into CO under these
conditions. Adding more hydrogen to the argon
flow will shift the ion-driven chemistry to a
more radical driven chemistry. Clearly less CO
is formed, and the atomic hydrogen and most of
the atomic oxygen is converted to H2O,
probably via recombination of H and OH on the
walls of the reactor vessel. By adding even more
hydrogen, the total amount of atomic hydrogen
increases and the CH4 production becomes
measurable. One reason might be that at this
stage the flow of Ar-ions has become so low,
that the destruction rate of CH4 through CE and
DR reactions becomes much lower. But, also
possible, the production rate increases due to the
increase of the H-flux. At even higher relative
H2 flows, the effect of the magnetic field on the
transport of the hydrogen ions might start to
play a role. But also, the size of the supersonic
part of the expansion and the shock region are
affected by the effective mass of the expanding
gas mixture. To better understand and further
explore the possibilities of the plasma
conversion route of CO2 experiments are
planned to measure molecules like CH2O and
methanol, which are expected to be present in
CO2/CO/H2 plasma.
References
[1] ‘Understanding and responding to climate
change, 2008 edition’, The National Academies,
National Academy of Sciences
[2] S.C. Roy, O.K. Varghese, M. Paulose, C.A.
Grimes, Toward solar fuels: Photocatalytic
conversion of carbon dioxide to hydrocarbons, ACS
Nano 3, 1259 (2010)
[3] M. C. M. van de Sanden, J. M. de Regt, G. M.
Janssen, J. A.M. van der Mullen, B. van der Sijde,
and D. C. Schram, Rev. Sci. Instrum. 63, 3369
(1992)
[4] R. F. G. Meulenbroeks, D. C. Schram, L. J. M.
Jaegers, and M. C. M. van de Sanden, Phys. Rev.
Lett. 69, 1379 (1992)