22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Spectroscopic studies on medium-pressure microwave plasmas for CO 2
conversion
S. Welzel1,2, W.A. Bongers1, G.F.W.M. Frissen1, M.F. Graswinckel1, B. van Heemert1 and M.C.M. van de Sanden1,2
1
Dutch Institute for Fundamental Energy Research (DIFFER), De Zaale 20, 5612 AJ Eindhoven, the Netherlands
2
Eindhoven University of Technology, P.O. Box 513, NL-5600 MB, Eindhoven, the Netherlands
Abstract: Gas-phase conversion of CO 2 in microwave (MW) plasmas is considered as
promising solution to efficiently overcome the initial energy barrier in CO 2 processing.
Therefore a MW plasma source was studied with respect to its CO production and emission
characteristics within a wide pressure (50 - 1000 mbar) and flow range. Elevated pressures
typically lead to a constriction of the active zone along with a decrease in CO production.
Keywords: microwave plasma, CO 2 conversion, plasma diagnostics, OES
1. General
Plasma-assisted gas conversion techniques are widely
considered as emerging building blocks in a future, CO 2 neutral energy infrastructure which will be based on
intermittent, renewable electricity sources. A CO 2 -neutral
process chain will form a closed loop for CO 2 which is
first converted into CO and further hydrogenated to valueadded hydrocarbons. In this way, renewable electricity
will be stored in chemical bonds. The initial CO 2
dissociation is typically the most-energy demanding step
which usually reduces the viability of entire carbon
capture and utilisation process chains for the production
of CO 2 -neutral fuels.
CO 2 dissociation in high-frequency, more specifically
in microwave (MW), plasmas is of increasing interest as
the energy efficiency in these type of discharges can be
clearly higher than 50 % [1]. More recently, some of these
results have been confirmed [2], thereby strongly
indicating that specifically designed power injection and
gas flow regimes are required [3]. Moreover, the most
promising results have been achieved so far at reduced
pressures (around 150 mbar) at specific injected energy
levels of less than 1 eV/molecule (~ 100 kJ/mol) [1-3].
This contribution is concerned with the CO production
and emission characteristics of CO 2 MW plasmas across a
broad pressure range. A relatively straightforward MW
source design was used here while different reactor
configurations and gas flow regimes to achieve high
energy efficiencies are discussed elsewhere [3].
means of fibre-coupled compact spectrometers throughout
the entire ultra-violett (UV), visible (VIS) and nearinfrared range (NIR) (200-2500 nm). A simplified
schematic diagram of the setup encompassing the
diagnostic tools is shown in Fig. 1.
2. Experimental
A MW plasma setup (2.45 GHz, max. 1 kW injected
power) was operated in undiluted CO 2 . The gas injection
aimed at generating a vortex that inherently stabilises the
discharge within the entire pressure range of 501000 mbar (flow rate was 7.3 slm (standard litres per
minute)). The CO production was quantified by mass
spectrometry (MS) and complementary tuneable diode
laser (TDL) absorption spectroscopy downstream the
plasma. The emission from the plasma was recorded by
3. Results &Discussion
The emission spectrum from the plasma in the UV-VIS
range (Fig. 2) is typically characterised by a few lines of
electronically excited atomic species (C, O) as well as
bands of CO (250-400 nm) and C 2 (435-670 nm). NIR
spectra (not shown) are dominated by emission of
vibrationally excited CO 2 (combination bands). Fig. 2
shows an example of the emission obtained from the
brightest part of the discharge, i.e. from the centre of the
waveguide. The most distinctive CO* emission bands are
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Fig. 1. MW plasma setup and diagnostic tools used in this
study. CO 2 is injected from the top of a quartz tube. A
fraction of the exhaust is analysed in a MS and by means
of TDL absorption spectroscopy in a gas cell of 100 cm
absorption path. The emission from the plasma is
collected by moveable optics through a slot in the
waveguide and finally analysed in a fibre-coupled
spectrometer.
1
observed from CO(b3Σ - a3Π, 3rd positive system) with an
excitation threshold of ~10.4 eV. Swan bands C 2 (A3Π g X3Π u ) are also frequently observed at elevated pressures.
Given the relatively low excitation energy of the upper
levels (~2.3 eV), the emission may not only be caused by
electron-impact: As described in [4], C 2 may be a
transient species formed by C 2 +CO(ν>10) and C 2 O+C,
respectively (C 2 O in turn would be a product of CO*+CO
recombination).
Fig. 2. Survey spectrum of the emission recorded from a
CO 2 plasma at the centre of the waveguide.
Fig. 3 (top) shows the emission of CO* collected from
the centre of the waveguide while varying the total
pressure in the reactor at constant injected power and gas
flow rate (1.7 eV/molecule). By contrast to an increasing
CO* emission as function of pressure, the dissociation
degree (Fig. 3, bottom), considerably decreases from ~
22 % (at ~125 mbar) to~ 5 % (at 1000 mbar). This
pressure dependence is even more pronounced at higher
specific injected energies and flattens for values
< 1eV/molecule.
Fig. 3. CO 2 dissociation degree (bottom, MS - mass
spectrometry/full circle, TDL - tuneable diode laser/open
circle) determined from CO(X) as measured downstream
the plasma. Results from CO(X) may be considered as
upper (MS) and lower (TDL) limit. Emission from
CO*(b-a) (top, 0-2 band of the 3+ system/diamonds).
A decrease in conversion degree is usually observed in
conjunction with an intensification of the bright discharge
phase, as e.g. shown by the increase in CO* (Fig. 3, top).
2
A similar behaviour is also recognised for other excited
species such as C 2 * or O* (Fig. 4) . The radial intensity
profiles in Fig. 4 clearly reveal a constriction of the
discharge from ~ 15 mm diameter (150 mbar) to ~ 7 mm
(550 mbar). This 'edge' is also visible in certain profiles of
NIR features (e.g. CO 2 (ν*)).
Fig. 4. Emission intensities of CO* (circle), O* (triangle
up) and C 2 * (triangle down) measured in the centre of the
waveguide along the radius of the quartz tube. Negative
positions (symbols filled left) correspond to 150 mbar
total pressure; positive positions (symbols filled right)
are chosen for 550 mbar total pressure. Dashed lines
show the boundary of the bright/hot plasma core.
Using the (oversimplified) assumption that I em (CO) ~
n e n CO k(EEDF) and that the excitation cross section k
varies slowly (e.g. only weak changes in the EEDF), an
increase in emission intensity at increasing pressure is
accompanied by an increase in electron density.
Considering the decrease in n CO , the increase in n e must
be superlinear. Additionally, a non-negligible gas heating
(up to several 1000 K as estimated from the not entirely
resolved rotational structure of C 2 *) is detected. Both
effects may enhance undesirable, i.e. less efficient, CO 2
dissociation channels such as thermal dissociation and
direct electron impact, respectively. The corresponding
decrease in CO production was indeed observed (Fig. 3).
It should be noted that the assumption of weak changes in
k(EEDF) is very likely too straightforward: the drop in
CO* in Fig. 3 around 250 mbar is caused by an
adjustment of the matching network and hence of the E/n
value which apparently also influences the emission
intensity through k(EEDF).
4. References
[1] A. Fridman, Plasma Chemistry (Cambridge
University Press, Cambridge, 2008).
[2] A.P.H. Goede et al., EPJ Web of Conferences 79
(2014) 01005.
[3] W.A. Bongers et al., at this conference.
[4] E.A.H. Timmermans, Thesis (Eindhoven University of
Technology, 1999).
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