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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 P-II-8-31 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 1 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 P-II-8-31 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 P-II-8-31 3 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 4 P-II-8-31