Download Probing vibrational ladder-excitation in CO2 microwave plasma with a free electron laser to develop a route to efficient solar fuels

22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Probing vibrational ladder-excitation in CO 2 microwave plasma with a free
electron laser to develop a route to efficient solar fuels
D.C.M. van den Bekerom1, G. Berden2, A. Berthelot3, W.A. Bongers1, C.A. Douat4, R. Engeln4, N. den Harder1,
T. Minea1, M.C.M. van de Sanden1,4 and G.J. van Rooij1
1
Dutch Institute for Fundamental Energy Research (DIFFER), Solar Fuels group, De Zaale 20, 5612 AJ Eindhoven,
the Netherlands
2
FELIX-facility Radboud University Nijmegen, Nijmegen, the Netherlands
3
Research group PLASMANT, Department of Chemistry, University of Antwerp, 2610 Antwerpen-Wilrijk, Belgium
4
Faculty of Applied Physics, Eindhoven University of Technology, Eindhoven, the Netherlands
Abstract: Plasmolytic dissociation of CO 2 has been shown to be an efficient way to
produce CO as a first step in the synthesis of chemical fuels. The proposed mechanism for
this efficient reaction is vibrational ladder-climbing; CO 2 molecules are promoted to higher
vibrational levels through inter-molecular collisions. The dynamics of this mechanism are
investigated with the use of the free electron laser FELIX. High-power infrared pulses
(20 mJ per 5 µs pulse) are generated that are resonant with the first vibrational band of CO 2
(2350 cm-1) and fired through the plasma in order to excite the CO 2 and momentarily
increase the CO production. A gated spectrometer measured the CO production by looking
at spectral emissions of CO (b3Σ+ – a3Π (0,0) at 283 nm & b3Σ+ – a3Π (0,1) at 293 nm) and
CO 2 + (B2Σ+ – X2Π at 288 nm). A scan of the gate delay was recorded and shows a small
(a few ppm) increase at a delay of 0.1 ms.
Keywords: solar fuels, vibrational excitation, plasmolysis, CO 2 -dissociation, free electron
laser, optical emission spectroscopy
1. Introduction
Efficient energy storage is an important aspect of large
scale implementation of renewable energy sources. We
focus on the plasma-chemical conversion of CO 2 to CO,
as a first step in fuel synthesis. Microwave plasma at
pressures in the mbar range. CO 2 -dissociation can be very
efficient in a plasma (up to 90%[1]), which is higher than
the maximum thermodynamical efficiency. The prevailing
model for such efficient dissociation is that of vibrational
ladder-climbing. CO 2 -molecules are excited to the first
few vibrational levels by collisions with plasma electrons.
The molecules are excited to higher levels by
intermolecular collisions, up to the point where the
molecule dissociates. In contrast to e.g. dissociative
excitation by electron impact, no energy is wasted in this
process because at no point the molecule has a higher
energy than the dissociation energy.
To gain more insight in the dynamics of vibrational
ladder-climbing, the Free Electron Laser called FELIX is
tuned to wavelengths that are resonant with vibrational
excitation of the first level of CO 2 , and is pulsed to
perturb the intrinsic vibrational distribution of the steadystate plasma. The FELIX pulse has energies up to 20mJ
deposited in a 5us pulse, equivalent to a 4kW power for
the duration of the pulse. This excitation peak induced by
FELIX will similarly traverse through the ladder,
resulting in a slight increase in CO-production after a
characteristic time delay. In this contribution, we report
on measurements of the increase in CO by absorption due
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to the FELIX pulse. A gate-delay scan relative to the
FELIX pulses is performed to determine the time constant
for the excitation-ladder. So far, two measurement
campaigns have been undertaken using 1. a gated UVspectrometer for OES and 2. a IR-QCL for absorption
measurements of CO concentration.
2. Plasma and FELIX setup
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FELIX radiation was sent through the plasma axially, in
the direction of the gas flow (co-propagating) and scanned
over a range of 2400 to 2200 cm-1 as depicted in Fig 2.
This interval includes the wavenumber of 2349 cm-1 that
corresponds with the excitation of the first asymmetric
stretch vibrational level of CO 2 . The absorption spectrum
thus made resembles the FTIR spectra in Fig 3, that were
recorded in-situ under the same conditions, also
comparing plasma ‘‘ON’’ and ‘‘OFF’’ using a resolution
of 2 cm-1 Comparing the FELIX-absorption for plasma
“ON” and “OFF”, we see an average of 15 percentage
point more absorption with the plasma on in the range of
2300 cm-1 to 2200 cm-1. The maximum absorption,
around 2300 cm-1, is just short of 50%. We stress that this
value is the actual fraction of the FELIX power absorbed,
which corresponded to an instantaneous additional power
input of 2 kW compared to the ~0.5kW plasma power,
which is essential for a significant CO production as a
result of FELIX.
Fig. 1. Experimental setup showing the
FELIX beam going through the plasma.
The plasma setup consists of a microwave power source
at 2.45 GHz that can be operated at a maximum power of
830W, but is typically operated between 40~60% of that.
A triple stub tuner is used to match the plasma impedance
in order to reduce the power that is reflected. The
microwaves are absorbed in CO 2 gas, flowing at 1.5 slm
in a quartz tube with an outer diameter of 20 mm, inner
diameter of 16mm and 40 cm optical path length through
the reactor. The plasma was stabilized by a ‘vortex flow’
to reduce contact with the walls[2]. The windows on both
ends of the tube are CaF 2 which is transparent for the
FELIX wavelengths that were used. The pressure was
scanned between 5 to 50 mbar, but since the pressure
gauge is very sensitive to the gas composition at higher
pressures, these values are to be taken as estimates.
Reproduction of the absorption spectrum as shown in Fig.
2 indicated a pressure of around 18 mbar.
Fig. 2. Absorption of FELIX power, comparing
plasma ‘‘OFF’’ and plasma ‘‘ON’’.
2
Fig. 3. 2 cm-1 resolution in-situ FTIR spectrum
comparing plasma ‘‘OFF’’ and plasma ‘‘ON’’.
3. OES Gate-delay scans
A gated imaging spectrometer was used to
simultaneously monitor the optical emission of CO
3 +
3
3 +
3
(b Σ – a Π (0,0) @ 283nm & b Σ – a Π (0,1) @ 293nm)
+
2 +
2
and CO 2 (B Σ – X Π @ 288nm) as a relative measure for
the (change in) the CO and CO 2 + density. FELIX light
was tuned to three different wavelengths and passed
axially through the flow tube. The spectrometer covered
the FELIX path inside the waveguide, looking
perpendicular to the FELIX beam. The gating delay of the
spectrometer was scanned over a range of 0 to 500 μs.
Each spectrum consisted of 500 FELIX macro pulses, i.e.
100 s. The spectra were integrated over the spatial
coordinate to cover the entire plasma region and over the
three spectral features.
The data show large statistical variations. Fluctuations
in the plasma discharge on the time scale of the different
measurement series (i.e. FELIX laser wavelength) were
not recorded, could thus not be corrected for, and caused
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the different baselines for the different series.
Nevertheless, a peaking seems observable around a delay
of 0.1 ms for FELIX wavelengths of 2281 cm-1 and 2205
cm-1, as can be seen in Fig 4. At this point, it has to be
noted that these spontaneous emissions all involve high
energy excitations (~10 eV for the CO lines, ~20 eV for
the CO 2 + line). The consequence was that the reactor
conditions had to be tuned to high values of input energy
per molecule in order to increase the emission intensity
above the noise for an integration time of 10-50 μs. In
fact, whereas the ideal energy per molecule for optimal
effect of vibrational excitation is estimated to be ~0.3 eV,
these experiments involved ~10 eV per molecule. Under
such high power densities, rapid translational heating of
the heavy molecules will induce quenching of vibrational
excitation.
Fig. 4. Scan of the spectrometer gate-delay
with respect to a FELIX-pulse, showing the
CO2+, CO(0,1), CO(0,0) emission in top,
center and bottom graph respectively.
Absorption by different FELIX
wavelengths is shown in blue, green and
red.
4. QCL absorption measurements
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For the second campaign a Quantum Cascade Laser
(QCL) is used to measure the CO-increase as a result of
FELIX. The QCL is chirped from 2213.0 to 2212.3 cm-1,
covering the J=19 R-branch ν = 0→1 ro-vibrational COpeak, that was used to measure the CO-density. Since the
QCL has to cool down after every shot in order to
produce pulses at the same wavelength, the maximal time
between two QCL-pulses was set at 1.0ms A reference
spectrum was thus taken 1.0 ms before every FELIX
pulse, while a second spectrum was taken at a varying
delay w.r.t. the FELIX pulse going from 0.0 to 5.0 ms in
steps of 0.2 ms. FELIX and the QCL both measured
axially and co-linear, to increase the overlap of the two
beams. Using a mirror with a hole, the beams were
overlapped by minimizing the angle between them. For
later scans, the QCL was also used perpendicular to the
FELIX-pulse to decrease the steady state CO that was
measured. A Sapphire tube that is transparent for the QCL
wavelength was used for this so that measurements could
be performed inside the cavity, where the plasma is
concentrated and hence the effect of ladder climbing is
expected to be the most prominent.
Fig. 5 shows a spectrum of a QCL absorption spectrum,
measured in the direction of the gas flow. The acquired
spectrum was used to estimate the CO-production and gas
temperature by approximately fitting the CO 2 and CO
spectra respectively. The temperature was found to be
around 900K while the CO-concentration was found to be
around 25%. It should be noted that plasma was generated
roughly halfway the quartz tube, so that the actual COconcentration is closer to 50%, i.e. a CO number density
of 2∙1023 m-3. Detailed analysis and fitting of spectra at
different plasma settings will give a better view on this.
Similar delay scans as before that were studied by QCL
absorption measurements are presently being analysed.
5. Conclusions
Plasmolytic dissociation of CO 2 has been shown to be
an efficient way to produce CO as a first step in the
synthesis of chemical fuels. Using the FELIX free
electron laser, some of the CO 2 was excited to higher
vibrational modes on top of the steady state vibrational
distribution, to probe the dynamics of the ladder-climbing
mechanism. Ultimately these insights could help to break
the trade-off between energy- and conversion efficiency,
and could enable one to increase both at the same time.
By making a gate-delay scan of the CO spectrum, the
time constant of propagation through the vibrational
ladder was measured. A very small signal was measured,
indicating a peak in CO-concentration at a delay-time of
0.1 ms. These experiments were redone using a QCL for
measuring the increase in CO-concentration, and the
experimental results are currently under analysis.
6. References
[1] Alexander Fridman “Plasma chemistry” Cambridge
university press (2008)
[2] W.A. Bongers et al. “Developments in CO 2
dissociation using non-equilibrium microwave plasma
activation for solar fuels”, this conference
Fig. 5. QCL-Absorption spectrum with
overlaid CO (blue) and CO2 (red) spectra
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