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Theses

Investigation of fundamental mechanisms of CO2 plasmas

Abstract : The use of non thermal plasmas is one of the most promising paths to efficiently recycle CO2 into more complex organic molecules, such as energy-dense hydrocarbon fuels, and it is compatible with the use of intermittent renewable energy sources. To obtain satisfactory energy yields, it is necessary to properly control the energy transfer processes, including the vibrational energy of the CO2 believed to be beneficial for the CO2 conversion, or the energy stored in electronically excited species. Recombination processes producing CO2 from the dissociation products (the so-called back reaction) must also be prevented. However, despite the extensive literature in the fields of CO2 lasers, atmospheric entry plasmas or CO2 conversion, many of the basic mechanisms essential for the description of CO2 plasmas are still very poorly understood. The objective of this thesis is therefore to perform experiments under sufficiently well controlled conditions to identify and study some of these fundamental mechanisms. Two types of plasma sources, a DC "glow" discharge and a radio frequency (RF) discharge were studied at low pressures (27-1000 Pa) to slow down characteristic times of various processes. Advanced optical diagnostic techniques were used in situ and time-resolved to obtain all the relevant parameters for a complete description of the plasma. The densities and vibrational temperatures of CO2 and CO were measured by infrared absorption spectroscopy (FTIR), giving also insight in back reaction mechanisms. The density and loss frequencies of oxygen atoms were obtained with High Resolution Two photon Absorption Laser Induced Fluorescence (HR-TALIF), actinometry and Cavity Ring Down Spectroscopy (CRDS), while isotopic exchange measurements provided information on the role of O(1D). Most of these techniques were also used to determine the gas temperature showing simultaneously the consistency and accuracy of the different techniques.The experimental results made possible, for instance, the identification of the most accurate cross section for CO2 dissociation by electronic impact or the quantification of the vibratory de-excitation of CO2 by oxygen atoms. The obtained data were also used to validate a 0D kinetic model developed at IST Lisbon, which allowed the validation of the rates for vibration-vibration or vibration-translation energy transfer processes for the low vibrational levels of CO2.Another important part of the work focused on the role of the surfaces on the CO2 plasma kinetics. The O atoms loss processes were found to be dominated by surface recombination, dependent on the temperature of the O atoms near the surface, similarly to a pure O2 plasma. However, it was found that CO2 plasma can passivate SiO2 surfaces, reducing the recombination probability of oxygen atoms at the walls, and making it identical under plasma exposure and in post-discharge, unlike what is observed in O2 plasma. A preliminary comparison with a Monte-Carlo surface model, provides a valuable insight in the surface mechanisms involved. Large specific SiO2 surfaces were found to induce CO2 formation in the surface under high O atom flux regimes, limiting dissociation efficiency, whereas the use of carbon-based surfaces showed an enormous potential to use the oxygen atoms to enhance the final CO2 conversion, demonstrating the key role of the surfaces in the efficiency of the CO2 conversion and the importance of a proper handling of the oxygen atoms. These results are therefore very valuable to understand which materials would be relevant to be used as catalysts to improve CO2 conversion efficiency by plasma.The thesis provides a detailed view on the fundamental mechanisms controlling the kinetics of CO2 plasmas, and the results presented are therefore useful not only for developing more efficient CO2 conversion processes, with or without catalysts, but they are also relevant in fields such as surface treatment using O2/CO2-containing plasmas.
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https://hal-polytechnique.archives-ouvertes.fr/tel-03014566
Contributor : Aurélien Arnoux <>
Submitted on : Thursday, November 19, 2020 - 2:47:15 PM
Last modification on : Saturday, November 21, 2020 - 3:29:25 AM

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  • HAL Id : tel-03014566, version 1

Citation

Ana-Sofia Morillo-Candas. Investigation of fundamental mechanisms of CO2 plasmas. Plasma Physics [physics.plasm-ph]. Université Paris-Saclay, 2019. English. ⟨NNT : 2019SACLX091⟩. ⟨tel-03014566⟩

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