Au cours de ce travail de thèse, nous avons développé un procédé plasma-catalyse d'élimination de l'acétaldéhyde en utilisant un processus diphasique couplant un catalyseur nano-structuré et a plasma à la pression atmosphérique. L’élaboration du catalyseur nanostructuré a été d'abord étudiée. Puis la performance de dégradation du polluant a été étudiée. Les nanoparticules monodispersées (titane-oxo-alcoxy) sont générées dans le réacteur de sol-gel avec micro-mélange turbulent et déposés sur des plaques de verre ou des billes de verre comme monocouches nanostructurées. Le dépôt de l'argent et de l'or est réalisé par la réduction des ions sous l’irradiation de UV-A. La cinétique de croissance photocatalytiques et de la morphologie des nanoparticules sont étudiés expérimentalement par les méthodes MET, MEB et AFM. Il est également intéressant de discuter du mécanisme de la formation des nanoparticules et d'évaluer son efficacité quantique. Les conclusions expérimentales sont supportées théoriquement par le calcul des spectres d'absorption. Ensuite l'efficacité du processus de couplage d'une décharge à barrière diélectrique et d’un lit fluidisé d'argent et d’or nanostructurés, pour la dégradation d'un polluant modèle (acétaldéhyde CH₃CHO), est étudiée. Dans la première partie, l'efficacité du procédé plasma seul est discutée, en termes de dégradation des polluants et de production de CO et CO₂. Dans la deuxième partie, la dégradation de CH₃CHO ainsi que la production COx sont étudié en fonction du temps de réduction photocatalytique d’Ag+ et d’Au³⁺ ions, qui est liée à la masse d'argent et d’or déposée. Les voies de dégradation des polluants, notamment la chimie homogène dans la phase de plasma et la chimie hétérogène sur la surface, sont discutées. Enfin, la production des sous-produits principaux est présentées et comparées entre les catalyseurs Ag et Au. / During this Phd work, we have developed a plasma-catalytic process of acetaldehyde removal using a diphasic process coupling a nano-structured catalyst and an atmospheric pressure plasma. The elaboration of the nanoparticulate catalyst has been firstly studied. Then its performance coupling with plasma has been investigated. The monodispersed titanium-oxo-alkoxy nanoparticles are generated in the sol-gel reactor with turbulent micromixing and deposited onto glass plates or glass balls as monolayer nanocoatings. The silver and gold deposition is achieved by the ions reduction at UV-A light illumination. The photocatalytic growth kinetics and nanoparticle morphology are studied experimentally by the TEM, SEM and AFM methods. It’s also interesting to discuss the mechanism of the nanoparticles formation and evaluate its quantum efficiency. The drawn conclusions are supported theoretically through the calculation of the absorption spectra. Then the efficiency of the process coupling a dielectric barrier discharge and a fluidized nanostructured silver and gold based bed for the degradation of a model pollutant (acetaldehyde CH₃ CHO) is studied. In the first part, the efficiency of the plasma alone process is discussed, in terms of pollutant removal and CO and CO₂ production. In the second part, CH₃ CHO removal as well as COx production is studied as a function of the photocatalytic reduction time of Ag⁺ and Au³⁺ ions, which is related to the deposited silver and gold mass. The pollutant removal pathways, including homogeneous chemistry in the plasma phase and heterogeneous chemistry on the surface, are discussed. Finally, the production of main by-products is presented and compared between Ag and Au catalysts.

The purpose of this dissertation is to — on occasion very literally — shine a light on processes that occur in non-thermal plasmas containing CO2, mostly for CO2 conversion. In particular, the focus lies on the transient behaviour of these discharges: how do these systems evolve over time before they settle in a (non-thermal) equilibrium. In addition to that, it analyses trends in the field of plasma-catalytic CO2 conversion as a whole to evaluate the current state-of-the-art, but also presents a new platform for the community to contribute and collaborate on, to facilitate cross-comparison between disparate experiments. The first part consists of experiments performed on: (a) an atmospheric pressure nanosecond repetitively pulsed (NRP) discharge for CO2 conversion, and (b) a test bed system for a remote CCP plasma source for plasma-enhanced atomic layer deposition (PE-ALD) of trimethylaluminium (TMA). The common theme in these experiments is a focus on the application of time-resolved, in situ diagnostics to study transient behaviour in the systems under investigation. The main diagnostics employed for such measurements are optical emission spectroscopy (OES) and laser induced fluorescence (LIF), which can provide complementary results when used in conjunction. In particular, this work presents the following results: A study of the evolution of emission from an NRP discharge (using OES), establishing both electron densities (by Stark broadening of atomic oxygen and carbon lines) and gas temperatures (by the N2 second positive system) as the discharge evolves from a breakdown phase to a spark phase. It furthermore explores the changes to these properties when operating in burst mode, where a subsequent pulse experiences a memory effect from the preceding one, which has been shown to be conducive to efficient conversion in literature. A study into the effect on energy efficiency of CO2 conversion by alternating the power modulation in an NRP discharge. Crucially, using CET-LIF (collisional energy transfer LIF) and OES it is shown that while power deposition to the discharge occurs in the order of 100 ns in the discharge, CO2 dissociation occurs on a timescale beyond a microsecond. This indicates that instead of direct electron impact, molecular-excitation kinetics play an important role under these conditions for CO2 dissociation. By shortening the time between pulses in a burst (down to 33 us in the work), these mechanisms can be further enhanced, by prolonging the quasi-‘metastable’ state of the system. The application of LIF in a PE-ALD process plasma along with OES, where diffusion profiles were measured close to the substrate surface with local time-resolved measurements of the OH ground state density. These indicate that the investigated surface reactions finish on a timescale of 100 ms, faster than would be indicated by OES which effectively measures emission from the bulk plasma after diffusion of reaction products away from the surface. The second part of this work is an open access database on plasma(-catalytic) CO2 conversion that is instrumental in identifying and verifying trends in experimental data, but also stresses the importance of rigorous reporting of essential parameters in literature. The approach in literature is diverse: some studies focus more on a mechanistic understanding of the fundamental processes, whilst others already focus on process tailoring and optimization for industrial applications. Trends observed in earlier review papers are observed as well and can now be trivially reproduced. The database platform (https://db.co2pioneer.eu) is put forward as a new tool for the community to easily cross-compare and contextualize experimental outcomes and strongly encourages new contributions. Based on the 196 papers included at the time of publication, a number of observations and recommendations can already be made. Chief among those is a clear and present need in the field for a more fundamental understanding of plasma-catalysis interaction, to develop techniques and criteria that are properly suited to test the synergy of both, rather than relying on methods from e.g. traditional thermal-catalysis. Also in this instance, local, time-resolved diagnostics may play a key role, but their implementation will be challenging.

Ce travail de thèse s’intéresse aux phénomènes de surface ayant lieu lorsque l’on combine un procédé en phase homogène – contrôlé par la chimie d’un plasma non-thermique – et un procédé en phase hétérogène – contrôlé par la chimie ayant lieu à la surface d’un matériau nanostructuré Ag/TiO₂/SiO₂ – lors de la dégradation de l’acétaldéhyde, CH₃CHO.Il a été montré que le procédé diphasique permet de convertir 100 % de l’acétaldéhyde à traiter avec une SIE de 168 J.L-1 (soit une puissance de 280 mW). Dans ces conditions, CH₃CHO est converti en COx à plus de 60 %. Une telle efficacité n’est pas atteinte avec les procédés en phase homogène et en phase hétérogène seuls. Les processus se déroulant au sein du procédé diphasique mènent donc à une dégradation de CH₃CHO autrement meilleure que l’ensemble des cinétiques mises en oeuvre lors de l’utilisation des deux procédés seuls.Afin de comprendre quels processus physico-chimiques permettent d’obtenir un tel effet de synergie, l’étude de l’interaction acétaldéhyde/surface a été initiée, par spectroscopie infrarouge à réflexion diffuse (DRIFTS), et constitue le coeur de ce travail de thèse. Une attention particulière a été portée à l’étude des modes d’adsorption de l’acétaldéhyde sur Ag/TiO₂/SiO₂ en absence de plasma. Puis, l’effet de l’apport d’une source thermique et d’une espèce à fort pouvoir oxydant (l’ozone) sur l’acétaldéhyde présent en phase adsorbé a été évalué. / This thesis investigates the surface phenomena which occur when combining a homogeneous phase process – governed by the chemistry of a non-thermal plasma – and a heterogeneous phase process – controlled by the chemistry taking place on the surface of a nanostructured Ag/TiO₂/SiO₂ material – during acetaldehyde (CH₃CHO) removal.It has been shown that acetaldehyde can be removed up to 100 % with a 168 J.L-1 SIE consumption, by using the diphasic process. In these conditions, CH₃CHO is converted into 60 % of COx. Such efficiency is not achieved when using the homogeneous and heterogeneous phase processes alone. Thus, the physico-chemical phenomena occurring in the diphasic process allow a higher CH₃CHO removal compared to the whole kinetics involved in the homogeneous and heterogeneous phase processes alone. So as to understand which physico-chemical processes are involved in this synergistic effect, the study of the acetaldehyde/surface interaction has been started, by Diffuse-Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), in this thesis. The acetaldehyde adsorption modes on the Ag/TiO₂/SiO₂ surface, without plasma, have been pointed out. Moreover, the effect of bringing a thermal energy source or an oxidizing species (like ozone) on adsorbed acetaldehyde has been evaluated.

Climate change and global warming caused by the increasing greenhouse gases emissions (such as CO2) in the atmosphere recently attract the attention of the scientific community. These large emissions have been correlated to the Global Warming effect which has many consequences across the globe, including glacial retraction, ocean acidification and increased severity of weather events. With green technologies still in the infancy stage, it can be expected that CO2 emissions will stay this way for a long time to come. It is necessary to find an alternative way to get rid of the resulting environmentally harmful emissions. A promising solution is the use of CO2-free electrical energy produced, for example, by renewable or nuclear sources, for dissociation of CO2 or other greenhouse gases, followed by their conversion into easily storable fuels. In this context, the CO2 re-utilization to synthesize syngas, fuels or chemical compounds as well as pure CO2 dissociation into CO and O2, is of a special interest. Among the different methods to convert CO2 into added-value products (thermolysis, thermochemical cycles, electrolysis, photocatalysis, etc), the discharges sustained by microwave radiation combining high electron and low gas temperature have already demonstrated huge potential for plasma-assisted CO2 conversion. The present research work is targeted to the systematic investigation of the microwave-assisted conversion of various CO2-based gas mixtures being especially focused on plasma catalysis. The different physical effects affecting the efficiency of plasma catalysis are considered, for a better understanding of the synergistic effects between plasma and catalyst. The characterization of microwave discharges is performed by various plasma diagnostics methods, including optical spectroscopy and gas chromatography. In addition, the catalysts have been characterized by the state of art material characterization techniques, such as Transmission electron microscopy (TEM), Raman spectroscopy, etc. Such a combined characterization of both plasma and catalysts is performed for the sake of better understanding of the plasma-catalytic processes.In the first part of this study, the different dissociation pathways of the studied molecule as a function of different plasma parameters are considered by evaluating the composition with different plasma diagnostic techniques. A simple increase of Specific Energy Input (SEI) is not a promising solution since in this case the energy efficiency drops. The beneficial effects of lowering the pulse frequency for increasing CO2 conversion efficiency are observed and discussed. The obtained results are explained by the relation between the plasma pulse parameters and the rates of the relevant energy transfer mechanisms in the discharge. Simultaneous dissociation of CO2 and H2O has been investigated as well. It was clearly demonstrated that both H2 and CO productions are strongly affected by the different plasma parameters. The second part of this study deals with the effects of catalyst preparation method, nature of plasma activation gas, gas admixture, as well as NiO content and their influences on the CO2 conversion and energy efficiencies in microwave plasma. It was found throughout this work that the catalyst preparation method has a significant effect on the chemical and physical properties of the catalysts, which in turn strongly influences CO2 conversion and energy efficiencies of this process. In particular Ar plasma treatment results in a higher density of oxygen vacancies and a very favorable distribution of nickel oxide on the TiO2 surface. It is concluded that, the oxygen vacancies are the key factor explaining high catalytic activity in CO2 decomposition. The dissociative electron attachment of CO2 at the catalyst surface enhanced by the oxygen vacancies and plasma electrons can explain the observed increase of CO2 conversion efficiency as well as the energy efficiency. A mechanism explaining the observed plasma–catalyst synergy is proposed. The overall aim is to establish a model describing the interaction between highly reactive species produced in plasma discharge and supported catalyst for the conversion of CO2 into useful compounds. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

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