High Hv

Voltage

Transform Spectroscopy DRM, Dry Reforming of Methane E/N, Reduced Electric Field

Keywords: CAPEX, Capital Expenditures CCD, Charge Coupled Device DFT, Density Functional Theory DI water, Deionized water DRIFT, Diffuse Reflectance Infrared Fourier ECR, Electrocatalytic CO2 Reduction EDX, Energy Dispersive X-ray EE, Energy Efficiency FFT, Fast Fourier Transform FM, Filamentary Discharge FTS, Fischer-Tropsch Synthesis GCMS, Gas Chromatography Mass Spectrometry GHGs, Greenhouse Gases OES, Optical Emission Spectra PDC generator, Pulsed Direct Current generator PDF, Powder Diffraction File, PECVD, Plasma Enhanced Chemical Vapor Deposition PRF, Pulse Frequency PSR, Perfectly Stirred Reactors PVD, physical vapor deposition PX, Para-xylene Rt-Molsieve, fused silica PLOT column, RWGS, Reverse Water Gas Shift Reaction SEM, Scanning Electron Microscope SIE, Specific Input Energy SIW, Surface Ionization Wave STEM, Scanning Transmission Electron Microscopy TCD, Thermal Conductivity Detectors IX ` TEM, Transmission Electron Microscopy TEOS, Tetraethyl orthosilicate TGA, Thermogravimetric TPD, Temperature Programmed Desorption U/S ratio, Unsaturated to saturated hydrocarbons ratio VOCs, Volatile Organic Compounds XPS, X-ray Photoelectron Spectroscopy XRD, X-ray Diffraction μ-GC, Micro-gas Chromatography

Symbols

dg, thicknesses of the gas dc, thicknesses of catalysts Ug, gas voltage Ugap is the voltage across the gap, εg, relative dielectric constant of gas reactants, εc, relative dielectric constant of the packed catalysts. F0, electron energy distribution function n, electron number density E, electric field, m, electron mass, f, electron distribution, e, elementary charge, v, velocity coordinates, γ = 2e/m 1/2 , a constant, ε, electron energy in electron-volts, C[f], change rate, z, position along the direction f0, isotropic part of f. F0, energy distribution function 𝑊 ̃, flow velocity, 𝐷 ̃, diffusion coefficient d[Am]/dt, rate of change for the concentrations of species A for the m th reaction km, rate constants for the m th reaction [Am] and [Bm], concentrations of reactants am, preexponential factor T, temperature XI ` Eam, activation energy for the m th reaction rn, reaction rates of the n th species αnm f , stoichiometric coefficient in the m th formation reaction, αnm c , stoichiometric coefficient in the m th consumption reaction. Ea, energy barrier rp, reaction rate of plasma assisted dry reforming A, Arrhenius rate coefficients F, total flow rate of feed gas Pp, DBD discharge power Tg, gas temperature Ti, ion temperature Tr, rotational temperature Tv, vibrational temperature Te, electron temperature XII ` XIII `

RESUME DETAILLE EN FRANÇAIS Chapitre I. Introduction

La grande quantité d'émissions de gaz à effet de serre (GES) résultant de la consommation de combustibles fossiles menace l'humanité. Face à une telle préoccupation environnementale mondiale, de nombreuses technologies durables sont en cours de développement afin à la fois de remplacer les énergies fossiles traditionnelles et de réduire les émissions de CO2. Dans ce cadre, le CO2 peut être transformé en molécules de plus grande valeur par différentes stratégies, notamment CO2 splitting, reformage à sec, hydrogénation du CO2 ou encore photosynthèse artificielle. L'utilisation du CO2 pour la réaction de reformage à sec du CH4, molécule abondante et potentiellement renouvelable, suscite un énorme intérêt économique et environnemental car ces gaz sont les deux principaux GES actuels et parce qu'une chimie de synthèse organique renouvelable peut en découler.

Les plasmas non thermiques (NTP), plasmas générés sous de forts champs électriques, sont capables de produire des espèces hautement réactives de types ioniques, radicalaires ou excitées dans des conditions quasi-ambiantes de pression et température de gaz. Parmi ces espèces, les états excités vibrationnels et/ou électroniques jouent un rôle important car leur production permet de réduire le coût énergétique d'une réaction globale. La température électronique des

NTP varie en général de 1 à 10 eV offrant ainsi la possibilité de dissocier des molécules thermodynamiquement stables comme le CO2 (7,5 eV) et le CH4 (4,5 eV). Utilisés comme sources d'enthalpies en chimie catalytique, les NTP pourraient permettre d'améliorer la sélectivité grâce à l'activation électronique pilotée et de réduire les coûts liés aux procédés grâce à un fonctionnement en conditions physiques quasi-ambiantes.

Les NTP sont de ce fait de plus en plus étudiés en association avec de la catalyse hétérogène pour effectuer à plus faibles coûts par rapport à la thermocatalyse des réactions thermodynamiquement limitées, comme celles mises en oeuvre pour la conversion des GES en molécules à valeur industrielle. La conversion plasmocatalytique des mélanges CO2/CH4 suscite ainsi un intérêt croissant de la communauté scientifique. Elle conduit la plupart du temps à la production d'un mélange composé d'une part de gaz de synthèse et d'autre part de molécules C1+ oxygénées et C2+ non oxygénées. Les hydrocarbures gazeux typiques obtenus vont de C2 à C4. Il est à noter que la sélectivité des hydrocarbures saturés (ex. C2H6) est pour le moment beaucoup plus élevée que celle des hydrocarbures insaturés (ex. C2H4 et C2H2). Cette XIV ` différence de sélectivité pourrait être attribuée à la forte réactivité réductrice du plasma. Mais certaines pistes sont actuellement explorées pour modifier ce rapport des sélectivités.

Concernant les produits oxygénés liquides coproduits, nous retrouvons dans la plupart des études, les acides formique et acétique (HCOOH et CH3COOH), les alcools méthanol et éthanol (CH3OH et C2H5OH) et le formaldéhyde (HCHO). Les composés oxygénés à longue chaîne sont plus rarement rapportés.

Le travail de thèse qui est ici présenté s'inscrit dans une série d'études menées sur le sujet à Mines Paris -PSL depuis près de dix ans. Son point de départ prend appui sur le point final du travail de thèse qui l'a directement précédé (thèse de Di LI). Dans ce dernier, des oxygénés liquides ont été directement synthétisés par conversion plasmocatalytique de CO2/CH4 en utilisant deux catalyseurs supportés sur aérogel de silice : Co/SiO2 et Fe/SiO2. D'importants taux de conversion, compris entre 40% et 50%, ont été obtenus dans des conditions de pression et température quasi-ambiantes, renforçant l'intérêt de cette solution de conversion. L'une des principales conclusions de ce travail a été que la diminution du rapport CO2/CH4 réduisait la sélectivité en gaz de synthèse au profit des sélectivités en acide acétique et en hydrocarbures, à énergies équivalentes. Parmi les hydrocarbures obtenus, les hydrocarbures saturés, en particulier l'éthane, sont majoritaires tandis que les hydrocarbures insaturés plus précieux sont minoritaires. Cette distribution des produits limite évidemment la valeur de la conversion pour le moment et il est important de chercher à la modifier. Rappelons par exemple que les oléfines légères comme l'éthène et le propène sont les produits les plus importants de la pétrochimie car ce sont les composés de départ pour la fabrication des plastiques, des caoutchoucs, des huiles synthétiques, des solvants et autres produits avancés de l'industrie chimique. La recherche d'amélioration de la fraction d'hydrocarbures insaturés dans les hydrocarbures produits par conversion plasmocatalytique des mélanges CO2/CH4 est donc une nécessité pour aller au-delà de l'intérêt purement scientifique et lui donner un intérêt économique.

Si l'on regarde du côté de la thermocatalyse, les catalyseurs à base de fer sont les catalyseurs les plus courants pour la production d'oléfines car à la fois efficaces et économiques, ils peuvent être associés à des métaux alcalins servant de promoteurs d'insaturation par modification des propriétés électroniques et basiques de la phase hétérogène. Les catalyseurs à base de fer sont ainsi actuellement étudiés dans l'hydrogénation du CO2, leur bonne activité dans la réaction Water-Gas-Shift (WGS) mais aussi dans les réactions de synthèse Fischer-Tropsch (FTS) le justifiant. Plusieurs équipes de recherche ont récemment réussi à produire des hydrocarbures C2+ riches en oléfines à partir de l'hydrogénation thermocatalytique du CO2 en utilisant des XV ` catalyseurs bimétalliques Fe-Cu dopés en potassium (K). L'augmentation de la sélectivité des oléfines en présence de K à la surface catalytique s'explique par le fait que K contribue à empêcher l'adsorption de H2 au profit de celle du CO, réduisant ainsi le rapport H2/CO en phase hétérogène, ce qui conduit à une limitation du degré d'hydrogénation. Par ailleurs, le carbure de fer est connu pour avoir une forte action catalytique dans les réactions de FTS et les oxydes de fer se transforment en général en carbures de fer lors de l'hydrogénation du CO2 sous atmosphère H2/CO2.

A partir de cela, cette thèse développe une série de catalyseurs à base de Fe plus ou moins dopés en K pour la synthèse plasmocatalytique directe d'oléfines à partir de CO2 et CH4 dont la formulation a été aidée d'un modèle de cinétique chimique développé à cet effet sous CHEMKIN. Les performances de ces catalyseurs sont ensuite testées en conditions réactives dans un réacteur à plasma de type décharge à barrière diélectrique. Cette étude se veut pionnières sur la synthèse plasmocatalytique sélective d'oléfines. Les effets des espèces Fe et K sur les performances catalytiques sont évalués et discutés en détail. L'existence d'une synergie plasmocatalytique est démontrée en analysant de façon combinée les propriétés physiques du plasma et l'adsorption chimique d'espèces en surface. Cela passe notamment par une caractérisation électrique fine pour l'analyse énergétique mais aussi une caractérisation optique du plasma pour l'identification des espèces de la phase homogène. La stabilité du catalyseur est également regardée de près. L'étude expérimentale est ici accompagnée d'une étude numérique pour mieux comprendre, anticiper et orienter le travail. Le processus plasmocatalytique est pour cela modélisé sur le plan chimique sur une impulsion plasma grâce à un modèle cinétique 0D numérique élaboré sous CHEMKIN combiné à BOLSIG+, nous permettant d'estimer la densité et le taux de production des principaux intermédiaires et produits formés. Le modèle inclut à la fois les réactions homogènes et les réactions hétérogènes se susceptibles de se produire en surface catalytique. Nous parvenons par ce biais à évaluer et comparer les effets des réactions hétérogènes en fonction de la composition de la surface catalytique et à calculer numériquement les barrières de réaction associées. En combinant les résultats numériques et les résultats expérimentaux, une méthode d'estimation consolidée de la performance des catalyseurs est proposée pour la conversion plasmocatalytique de mélanges CO2/CH4 en hydrocarbures insaturés. Cette méthode pourrait ensuite être utilisée comme outil prédictif pour les études suivantes. XVI `

Chapitre Ⅱ. Méthode expérimentale, modèle cinétique numérique

Dans la première partie de ce chapitre, nous présentons une méthode de pyrolyse sol-gel pour la préparation de catalyseurs nanostructurés à base de Fe capable de donner une dispersion satisfaisante de la charge sur le support d'aérogel de silice en regard de la relative simplicité de la méthode. Cette méthode s'inspire de l'approche de synthèse des MOF et consiste, en ce qui concerne la formule Fe/C-SiO2, à dissoudre des cristaux d'acide citrique et du chlorure de fer (III) dans de l'eau déionisée (DI) en conditions ambiantes et à y ajouter de l'orthosilicate de tétraéthyle (TEOS) goutte-à-goutte. La solution est ensuite agitée pendant plusieurs heures à l'air libre puis séchée au four à 333 K pour obtenir le gel. Il est ensuite calciné à 673 K sous atmosphère d'azote pendant plusieurs heures pour former le catalyseur dans sa forme finale. En ce qui concerne la formule K-Fe/C-SiO2, catalyseur à base de Fe dopé en potassium (K), la méthode appliquée est similaire excepté l'ajout de carbonate de potassium dans la solution initiale d'acide citrique et de chlorure de fer (III) pour l'apport de potassium. Pour obtenir les formules Fe-SiO2 et K-Fe-SiO2, les catalyseurs Fe/C-SiO2 et K-Fe/C-SiO2 sont chauffés à 873 K sous air afin d'éliminer le carbone puis réduits à 873 K sous atmosphère de 5%H2/N2. Tous les catalyseurs préparés dans ce travail comportent une charge massique nominale de 20 % en Fe.

Les paramètres électriques du plasma tels que les valeurs du champ électrostatique moyen, du champ électrique réduit (E0/N), sont calculés à partir d'un modèle électrocinétique de réacteur DBD rempli. L'influence des grains de catalyseur sur les caractéristiques physiques de la décharge est relativement complexe et ne peut être entièrement pris en compte dans le modèle à ce stade. Malgré la relative simplicité du modèle, celui-ci permet de déterminer avec une bonne approximation la tension aux bornes du gaz et les différentes permittivités électriques relatives (εi) du dispositif dont celles du lit catalytique. La constante diélectrique de chaque catalyseur est estimée sur la base de la fraction massique des différentes espèces qui le composent. Les catalyseurs sont ainsi constitués de SiO2 (ε=4,42), FeO (ε=14,2), KCl (ε=4, [START_REF] Guo | Detection of hydroxyl radical in plasma reaction on toluene removal[END_REF] et Fe/C (ε=307). Le gaz non ionisé et le diélectrique sont considérés de manière équivalente comme des condensateurs plans dans le modèle.

Les paramètres électriques servent ensuite dans un modèle de cinétique chimique 0D élaboré sous Chemkin et associé au solveur de l'équation de Boltzmann BOLSIG+. Les paramètres cinétiques du modèle proviennent des bases de données UMIST, NIST Chemistry WebBook et XVII ` LXcat. Le modèle cinétique chimique contient 19 molécules, 38 espèces chargées, 29 radicaux et les électrons. Ces espèces génèrent entre elles une grande quantité de réactions élémentaires.

Sur la base de la chimie du plasma développée par De Bie, Zheng, Tosi et et al., nous avons mis en place un ensemble de 868 réactions élémentaires, comprenant les réactions d'impact électronique, les réactions de recombinaison électron-ion, les réactions ion-neutre, les réactions ion-ion et les réactions neutre-neutre. Dans le modèle, la constante de vitesse des réactions d'impact électronique est supposée être une fonction de l'énergie moyenne des électrons, c'està-dire k=f(Ee). Cette fonction peut être déterminée à partir de la base de données LXcat et lorsqu'elle est interpolée en fonction exponentielle permet d'obtenir les coefficients cinétiques d'Arrhenius pour les réactions d'impact électronique. Pour les autres types de réaction, celles impliquant des ions et des radicaux, les coefficients de d'Arrhenius proviennent des bases de données NIST Chemistry WebBook (pour les réactions radicalaires) et UMIST (pour les réactions ioniques). Nous considérons enfin la chimie hétérogène par les cinétiques d'adsorption, de désorption et de réactions hétérogènes. 13 espèces adsorbées et 42 réactions de surface composent notre chimie hétérogène.

Chapitre Ⅲ. Synthèse directe de produits à valeur ajoutée par DRM plasmocatalytique

Le chapitre 3 présente ensuite la caractérisation physico-chimique des catalyseurs synthétisés puis évalue leurs performances en réacteur plasmocatalytique à décharge à barrière diélectrique (DBD). La caractérisation des catalyseurs avant et après utilisation plasmocatalytique se base successivement sur des mesures d'adsorption-désorption de N2, de diffraction de rayons X (XRD), de spectroscopie photo-électronique de rayons X (XPS), de microscopie électronique à balayage (MEB), de microscopie électronique à transmission (MET) et de H2-TPD. Les mesures d'adsorption-désorption de N2 ont mis en évidence des surfaces spécifiques de Brunauer-Emmett-Teller (BET) élevées de nos catalyseurs et ont également permis d'en déterminer la taille moyenne et la distribution en diamètre des pores. Les mesures XRD ontelles montré que l'espèce ionique principal dans Fe-SiO2 et K-Fe-SiO2 est 2(FeO).SiO2, identifiée selon la base du centre international de données de diffraction (ICDD) sous le numéro 00-011-0262. Les pics larges et de faible intensité suggèrent que le fer est bien dispersé et présente une faible cristallinité. De même pour les catalyseurs Fe/C-SiO2 et K-Fe/C-SiO2 ou les XVIII ` diagrammes XRD suggèrent une faible cristallinité, une petite taille de particules et, là aussi, une bonne dispersion du fer.

La micrographie MEB montre pour les catalyseurs Fe-SiO2 de belles structures mésoporeuses avec peu de macropores, et pour les catalyseurs et K-Fe-SiO2 des structures mésoporeuses endommagées et une macroporosité plus élevée en raison de la présence de K ce qui en fait chuter leur surface spécifique. La comparaison des images MEB des catalyseurs Fe/C-SiO2 et K-Fe/C-SiO2 montrent aussi que les structures de surface sont altérées par la présence de K, néanmoins, les structures mésoporeuses sont ici bien conservées. Les propriétés de surface sont ensuite examinées par XPS. Outre la présence de nanoparticules bien dispersées, on observe également par endroit des ilots d'agrégation de potassium. Il est bien connu que les grands clusters conduisent à la modification substantielle de la surface et influencent les performances catalytiques en conséquence. Cela étant, aucun gros amas d'agrégation n'est observé, ce qui indique la relative bonne dispersion des promoteurs dans les catalyseurs, qu'ils soient carbonés ou non.

Ensuite, les effets des espèces Fe et K sur la sélectivité des produits mais aussi sur le transfert d'énergie sont étudiés en réacteur DBD rempli. En plus de la modification chimique qu'apporte le catalyseur dans le système, une modification physique est constatée, impactant de fait l'activité catalytique elle-même. Il y a, en effet, une réciprocité d'influence, le plasma influence les réactions catalytiques mais les catalyseurs influencent aussi les propriétés physiques du plasma. La puissance d'entrée est ainsi modifiée à même tension crête à crête appliquée. Pour conserver la puissance, il faut alors ajuster la tension appliquée, dans notre cas entre 5 kV et 6 kV selon le matériau pour rester dans la plage 2,4 -2,6 W.

La nature du catalyseur ajuste considérablement les sélectivités de H2, CO et des co-produits tandis qu'elle ne joue que très peu sur le taux de conversion plasmocatalytique des réactants.

Comparé au plasma seul, la plasmocatalyse avec K-Fe/C-SiO2 augmente la sélectivité des composés oxygénés liquides de 4,22 % à 21,76 % et supprime la formation de CO. Le rapport H/C du gaz de synthèse produit augmente à environ 2, ce qui est la composition la plus souhaitable pour la FTS.

Outre le rapport H/C, la sélectivité des hydrocarbures insaturés (par exemple, l'éthylène) est améliorée avec l'ajout du promoteur K. En effet, les produits hydrocarbonés insaturés sont quasiment absents en plasma seul tandis qu'ils deviennent de véritables co-produits du procédé lorsque des catalyseurs dopés en K sont utilisés. Pour les catalyseurs Fe-SiO2 et Fe/C-SiO2, le XIX ` rapport U/S (insaturés / saturés) est respectivement de 0,19 et 0,07, alors qu'il monte à 0,35 et 0,90 en utilisant K-Fe-SiO2 et K-Fe/C-SiO2 respectivement.

Nous constatons également que le lit catalytique a une influence directe sur l'intensité du champ électrique auquel le gaz de CO2(ε=1,6)/CH4(ε=1,06) est soumis, selon notamment la valeur de la constante diélectrique du catalyseur. De plus, une courbure locale élevée en surface catalytique, une accumulation de charges et des effets de polarisation peuvent aussi contribuer à la modification du champ électrique. Un catalyseur à constante diélectrique élevée accentue le caractère filamentaire de la décharge, créant localement des espèces plus excitées aux pieds des filaments par renforcement de champ. L'hydrogénation/déshydrogénation du catalyseur est analysée par H2-TPD. Les résultats indiquent que la capacité d'adsorption de H2 est nettement diminuée en présence de K, ce qui est conforme à l'analyse de chimisorption de H2 rapportée dans la littérature. Globalement, le dopage K diminue fortement l'adsorption de H2 et augmente la chimisorption de CO, ce qui veut aussi dire que ce dopage est moins favorable à l'hydrogénation des produits mais plus favorable à la conversion du CO. Ce résultat est en soi très important car le dopage K se présente alors comme une solution pour limiter l'hydrogénation et augmenter la fraction de produits insaturés.

En tenant compte des espèces observées par OES, du champ électrique modifié par le lit catalytique et les molécules adsorbées en surface, nous proposons également dans ce chapitre une voie de conversion propre à ce procédé. Au début du processus, les électrons libres acquèrent de l'énergie sous l'effet du champ électrique statique, excitant ainsi par impact le CO2 et le CH4 dont une fraction se dissocie. La dissociation du CO2 produit des radicaux CO, O et des composés oxygénés secondaires, tandis que la dissociation du CH4 produit préférentiellement des radicaux CHx. Ces espèces intermédiaires sont ensuite, à leur tour, converties en phase homogène ou en phase hétérogène. C'est dans les filaments que le transfert d'énergie est le plus important et où le taux de dissociation moléculaire est le plus important. C'est à l'interface dans les zones qui ont été soumises à un spot « chaud » qu'ont lieu les recombinaisons les plus intéressantes et sur lesquelles nous allons pouvoir agir. Le plasma et les promoteurs vont tous deux influencer l'adsorption des espèces et le taux des réactions surfaciques et parmi les paramètres importants, l'affinité de la surface joue un rôle prépondérant dans la chimie à l'interface. Il est connu qu'en chimie de synthèse catalytique l'hydrogénation est un processus clé pour la détermination des produits finaux. Une surface ayant une affinité XX ` avec l'hydrogène permettra une hydrogénation complète des produits jusqu'à la formation d'alcanes. Au contraire, une faible affinité limitera l'hydrogénation et favorisera la production d'alcènes et d'alcynes. Nous savons grâce à des études antérieures que le radical C2H5 se forme en phase gaz dans nos conditions. Il est donc important d'adsorber cette molécule tout en empêchant son hydrogénation, plus encore en favorisant sa déshydrogénation. Notre étude montre que la présence de K en surface ralentit la chimisorption de l'hydrogène par couverture partielle des sites actifs pour l'hydrogène sans altérer pour autant les autres sites. De plus, l'alcalinité de K permet d'augmenter le champ électrique en proportion gardée de la quantité incorporée, ce qui améliore les taux de conversion des réactifs. Ainsi, le dopage K est une solution doublement avantageuse permettant d'augmenter la sélectivité des hydrocarbures insaturés tout en élevant légèrement les taux de conversion des réactifs.

Chapitre Ⅳ. Etude numérique du DRM plasmocatalytique

Le chapitre 4 est consacrée à l'étude numérique du procédé à l'aide du modèle présenté au chapitre 2. En sortie de modèle, nous suivons les densités des réactifs, des produits et des principaux intermédiaires en fonction du temps. En premier lieu, les simulations montrent que la dissociation moléculaire (i.e. CO2 et CH4) est en décalage temporel avec l'excitation électrique du mélange gazeux. Cela est attribué au temps nécessaire à l'atteinte par une fraction significative d'électrons d'une énergie suffisante. Le résultat d'une simulation type se décompose en trois périodes distinctes : l'excitation (6.0×10 -08 s), la désexcitation (1.0×10 -07 s) et la rémanence (1.0×10 -05 s). Chacune de ces périodes est analysée en suivant les taux de réaction de 10 réactions élémentaires clés sélectionnées pour chaque composé. La température et la densité des électrons augmentent progressivement dans le temps durant l'excitation selon deux phases successives, rapide au début, plus lente à la fin. L'ionisation et la dissociation des réactifs sont uniquement dues aux réactions électroniques durant la phase rapide (première phase) de l'excitation. En ce qui concerne le CO2, deux voies de dissociation peuvent être suivies : dissociation directe de la molécule ou processus en deux étapes qui consiste en l'ionisation du CO2 en CO2+ puis dissociation de l'ion. En ce qui concerne le CH4, la dissociation directe de la molécule est la voie dominante. Les résultats de simulation montrent en toute cohérence que la dissociation du CO2 est plus lente que celle du CH4 en phase homogène.

XXI `

Les densités de gaz de synthèse, d'hydrocarbures C2 et de composés oxygénés sont également suivies en fonction du temps pendant cette période d'excitation. Le gaz de synthèse est sans surprise le produit dominant. Les densités de H2 et CO obtenues par simulation avec et sans catalyseur sont de même ordre de grandeur que celles obtenues par expériences. Le rapport H2/CO qui est l'indicateur principal de la qualité du gaz de synthèse produit est, lui, ajustable en jouant sur la composition exacte du mélange réactif. Il semble que la réponse en produits à la variation compositionnelle du mélange de départ soit plus sensible que celle des procédés traditionnels (reformages sec et humide, oxydation partielle) ce qui est un avantage.

Les coproduits non-oxygénés les plus intéressants sont les hydrocarbures C2 insaturés, molécules à haute valeur pour l'industrie chimique. Aussi, les évolutions temporelles des densités des hydrocarbures C2 (C2H6, C2H4 et C2H2) sont simulées et comparées. Nous remarquons que pendant l'excitation, le taux de formation de C2H4 est le plus rapide, et donc le C2 dominant en phase homogène. Cependant, pendant la désexcitation, la densité de C2H4 diminue légèrement en raison de la réaction d'hydrogénation, tandis que la densité de C2H6 continue d'augmenter. En fin de simulation, le C2H6 est le principal C2, la sélectivité des insaturés est alors limitée par le processus d'hydrogénation. Le rapport des insaturés sur saturés (U/S) diminue progressivement du début de la désexcitation jusqu'à la fin de la rémanence pour atteindre finalement une valeur plateau de 0,31606 sans catalyseur. L'analyse des résultats présentée en détail dans ce chapitre identifie les principales voies de formation et de perte des hydrocarbures C2 dans le gaz, ce qui permet de dresser un schéma réactionnel complet dans la phase homogène. Les produits majoritaires communs de dissociation du CH4 et des ions hydrocarbonés étant les radicaux CH3 et CH2, la production d'hydrocarbures supérieurs se fait à partir d'eux et donne comme coproduits secondaires du H2/ H, ce qui correspond là encore à de la déshydrogénation.

Concernant les oxygénés, nous suivons essentiellement le méthanol, l'éthanol, l'acide formique, l'acide acétique et l'acide propionique qui sont les principaux coproduits oxygénés obtenus par expériences. Leurs intermédiaires sont analysées de près au fil des simulations. Le méthanol, produit oxygéné majeur, atteint un pic de densité jusqu'à 2,5813 cm -3 pendant l'excitation sans catalyse mais faiblit au cours de la désexcitation et rémanence. L'éthanol présente un comportement différent, sa densité plus faible n'atteint pas de maximum, elle monotone croissante après l'excitation. Cela peut être attribué aux réactions entre les radicaux rémanents après l'excitation. Un phénomène similaire est également observé pour la production des acides carboxyliques. Les principaux intermédiaires en jeu dans ce processus après excitation sont XXII ` COOH, CH3, H, CO, OH dont la durée de vie dans nos conditions peut être relativement longue.

Il convient de mentionner que la densité numérique de l'acide acétique est supérieure à celle de l'acide formique, ce qui est conforme au résultat de nos expériences.

Outre le suivi temporel des espèces : réactifs, produits et intermédiaires, nous avons également calculé l'énergie d'activation du DRM, paramètre cinétique global essentiel, et l'avons comparée pour différents catalyseurs afin de démontrer le rôle positif des catalyseurs en plasmocatalyse.

Cela a pu se faire grâce à la prise en compte de la chimie en phase hétérogène en fonction de la nature du catalyseur, comme présenté en chapitre 2. Premier résultat important à retenir est que le procédé plasma sans catalyse présente un comportement typique d'Arrhenius et l'énergie

ABSTRACT

This thesis develops a series of Fe-based catalysts for direct synthesis of value-added products using plasma-catalytic dry reforming and further builds a chemical kinetics model by CHEMKIN software. In the first part, highly dispersed Fe-based nano-catalysts are synthesized by sol-gel pyrolysis method which is evolved from reported MOF mediated synthesis approach.

Subsequently, chapter 3 analyzes the physicochemical properties and structures of synthesized catalysts followed by evaluating the performance of catalysts in a dielectric barrier discharge (DBD) plasma catalytic reactor. The effect of Fe species and K promoters on products selectivity and electric-field enhancement are investigated in detail. By calculating plasma parameters around packed catalysts and characterizing chemical environment of catalysts' surface, we propose a persuasive explanation for the advantage of optimized catalyst. Taking account of species observed by OES, enhanced electric field and surface adsorbed molecules, we propose a reaction pathway for plasma-catalytic dry reforming as well. Chapter 4 is devoted to the numerical study of the process using the model presented in Chapter 2. At the output of the model, we follow the densities of the reactants, products and main intermediates as a function of time. The result of a typical simulation breaks down into three distinct periods: excitation (6.0×10 -08 s), de-excitation (1.0×10 -07 s) and remanence (1.0×10 -05 s). Also, the temporal evolutions of the densities of C2 hydrocarbons (C2H6, C2H4 and C2H2) are simulated and compared. Concerning oxygenates, we mainly follow methanol, ethanol, formic acid, acetic acid and propionic acid which are the main oxygenated co-products obtained by experiments.

As an opening, we propose in the last part of this chapter, on the basis of previous work, to combine an experimental study and a numerical study under the same plasmacatalytic conditions. Eventually, future perspectives and outlooks are proposed for improving catalysts and kinetic model.

1

CHAPTER I. INTRODUCTION 1.1 Background of greenhouses gases valorization

In recent decades, energy consumption kept on rising and the raise of global energy demand in 2018 was thought to be nearly 2.3% which was devoted to be the highest increase in this decade.

Moreover, between the years 2000 and 2016, the electricity consumption globally increased significantly from nearly 12670 TWh to 20864 TWh [START_REF] Siddiqui | Development of a novel renewable energy system integrated with biomass gasification combined cycle for cleaner production purposes[END_REF], and more and more fossil energy were found and consumed due to advances in petrochemical technology. Consequently, climate change is becoming a concerning problem due to the increasing consumption of such unrenewable fossil energy in recent decades, and several dreadful consequences, such as global warming, sea level rise and etc., are threatening human life. Notably the increasing emission of carbon dioxide, a result of expanding usage of coal, oil and natural gas resources, contribute tremendously to such a climate change [START_REF]What is Climate Change and Global Heating and How Does it Affect Us, Job One for Humanity[END_REF][START_REF] Thuiller | Climate change and the ecologist[END_REF][START_REF] Manabe | Role of greenhouse gas in climate change**[END_REF][START_REF] Ledley | Climate change and greenhouse gases[END_REF][START_REF] Hansen | Sun and dust versus greenhouse gases: an assessment of their relative roles in global climate change[END_REF][START_REF] Azadi | Transparency on greenhouse gas emissions from mining to enable climate change mitigation[END_REF].

Table 1-1. The curve of carbon increasing in the atmosphere and global temperature [START_REF]What is Climate Change and Global Heating and How Does it Affect Us, Job One for Humanity[END_REF].

In order to satisfy the energy demand and reduce environmental pollution at the same time, several sustainable techniques, including solar energy, wind energy, nuclear energy, hydro energy, geothermal energy, biomass energy and CO2 transformation were developed. However, some of these techniques can only provide electricity, while few of them could produce valuable chemicals and replace fossil energy. As an alternative energy to unrenewable energy, biomass energy could be applied in producing heat, electricity, or transport fuels. It accounted for approximately 11% (44.3 EJ) of the world's total primary energy supply [START_REF] Proskurina | Global biomass trade for energy-Part 2: Production and trade streams of wood pellets, liquid biofuels, charcoal, industrial roundwood and emerging energy biomass[END_REF]. However, it is still difficult to be applied in the industrial because of low energy and economic efficiency. Another state-of-art sustainable energy is the utilization of CO2, which is a promising technique because it could not only reduce CO2 in the atmosphere but also provides clean fuels and chemicals at the same time. Figure 1-1 shows that the basic strategy is to utilize CO2 in a carbon cycle in order to keep a balance in the atmosphere.

Carbon dioxide and methane, the so-called greenhouse gases (GHGs), play an undeniable role in global warming, and catalytic transformation of CO2 to valuable chemicals may help to create a sustainable low carbon economy. As shown in Figure 1-1, CO2 in the atmosphere could be converted into fuel, plastic materials using catalytic technology using clean electricity provide by nuclear energy, solar energy and wind energy. Then these fuel and material are consumed, and the CO2 is released into the atmosphere again. Recently, some promising strategies of CO2 utilization, such as CO2 splitting (A1), dry reforming of CO2 and CH4 (A2), CO2 hydrogenation (A3) and artificial photo synthesis (A4), have been widely investigated to provide sustainable hydrocarbons and achieve so-called global zero carbon emission campaigns. It is possible to directly produce CO and oxygen by means of thermal CO2 splitting which is a highly endothermic reaction. The use of semipermeable membranes to extract oxygen or other effective techniques for separating CO and O2 at high temperatures are required in this process [START_REF] Snoeckx | Plasma technologya novel solution for CO 2 conversion?[END_REF]. The combined conversion of CO2 and CH4, also called dry reforming of methane (DRM), has been studied since 1920s by Fischer and Tropsch [START_REF] Pakhare | A review of dry (CO2) reforming of methane over noble metal catalysts[END_REF]. Traditionally, this process could convert two important greenhouse gases and need to be conducted at high temperatures over Co, Ni, metallic carbides or precious metals catalysts.

Regarding the global warming and energy crisis, it has a bright outlook, whereas, DRM is not widely applied in industrial scale due to the efficiency of catalysts and energy. Another commercially interesting process is so call hydrogenation of CO2 which is also known as the Sabatier reaction or the methanation of CO2. This exothermic reaction and deep reduction of CO2 requires efficient catalyst with high rates and selectivity. Additionally, artificial photosynthesis is known as an ideal approach to utilize CO2.

CO2(g) → CO(g) + ½ O2(g) ΔH o = +283 kJ mol -1 (A1) CH4(g) + CO2(g) → 2 CO(g) + 2 H2(g) ΔH o = +247 kJ mol -1 (A2) CO2(g) + 4 H2(g) → CH4(g) + 2 H2O(g) ΔH o = -165.3 kJ mol -1 (A3) CO2(g) + H2O(g) → CO(g) + H2(g) + ½ O2(g) ΔH o = +525 kJ mol -1 (A4)
Above all, CO2 is a highly stable molecular in which the carbon-oxygen bonds are relatively strong (783 kJ mol -1 ) and activation process requires overcome huge energy barrier, which means numerous energy input is needed [START_REF] Rayne | Thermal Carbon Dioxide Splitting: A Summary of the Peer-Reviewed Scientific Literature[END_REF]. Taking this into consideration, a large number of advanced methods, including thermo catalytic conversion [START_REF] Jin | Dry reforming of methane over trimetallic NiFeCu alloy catalysts[END_REF][START_REF] Gao | Recent progress in anti-coking Ni catalysts for thermo-catalytic conversion of greenhouse gases[END_REF][START_REF] Chen | Mechanochemical in-situ incorporation of Ni on MgO/MgH2 surface for the selective O-/C-terminal catalytic hydrogenation of CO2 to CH4[END_REF], photo catalytic conversion [START_REF] Tasbihi | On the selectivity of CO2 photoreduction towards CH4 using Pt/TiO2 catalysts supported on mesoporous silica[END_REF][START_REF] Tu | Photocatalytic Conversion of CO2 into Renewable Hydrocarbon Fuels: State-of-the-Art Accomplishment, Challenges, and Prospects[END_REF][START_REF] Wang | Photocatalytic conversion of CO2 in water over Ag-modified La2Ti2O7[END_REF][START_REF] Hao | Utilizing new metal phase nanocomposites deep photocatalytic conversion of CO2 to C2H4[END_REF], electro catalytic conversion [START_REF] Pérez-Cadenas | Metal-doped carbon xerogels for the electro-catalytic conversion of CO2 to hydrocarbons[END_REF][START_REF] Ding | Metalbased electrocatalytic conversion of CO2 to formic acid/formate[END_REF][START_REF] Das | Core-shell structured catalysts for thermocatalytic, photocatalytic, and electrocatalytic conversion of CO 2[END_REF][START_REF] Genovese | Electrocatalytic conversion of CO2 to liquid fuels using nanocarbon-based electrodes[END_REF], plasma assisted conversion [START_REF] Nizio | Hybrid plasma-catalytic methanation of CO2 at low temperature over ceria zirconia supported Ni catalysts[END_REF][START_REF] Guo | Mechanochemical Synthesis of Ni-Y/CeO2 Catalyst for Nonthermal Plasma Catalytic CO2 Methanation[END_REF][START_REF] Ashford | Plasma-catalytic CO2 hydrogenation to ethane in a dielectric barrier discharge reactor[END_REF][START_REF] Cui | Plasma-Catalytic Methanol Synthesis from CO2 Hydrogenation over a Supported Cu Cluster Catalyst: Insights into the Reaction Mechanism[END_REF] and etc., have been reported in order to utilize CO2 which will be discussed later in this chapter.

Direct synthesis of valuable molecules

Derived from CO2, valuable organic products could be synthesized avoiding the usage of petrol resources. This section will introduce those renewable chemicals (including syngas, hydrocarbons and oxygenates) which are produced in thesis.

Syngas

Syngas, a mixture of hydrogen and carbon monoxide, could be used for power generation, chemical production, fuel cell and etc., depending on the H2/CO ratio of the mixture. It is produced by steam reforming [START_REF] Ren | Promoting syngas production from steam reforming of toluene using a highly stable Ni/(Mg, Al)Ox catalyst[END_REF][START_REF] Antolini | High hydrogen content syngas for biofuels production from biomass air gasification: Experimental evaluation of a char-catalyzed steam reforming unit[END_REF][START_REF] Sun | A novel system of biomass for the generation of inherently separated syngas by combining chemical looping CO2-gasification and steam reforming process[END_REF], partial oxidation of natural gas [START_REF] Siang | A review on state-ofthe-art catalysts for methane partial oxidation to syngas production[END_REF][START_REF] Zhang | Partial oxidation of n-pentane to syngas and oxygenates in a dielectric barrier discharge reactor[END_REF][START_REF] Zhang | Coupling water splitting and partial oxidation of methane (POM) in Ag modified La0.8Ca0.2Fe0.94O3δ hollow fiber membrane reactors for co-production of H2 and syngas[END_REF], coal gasification [START_REF] Matamba | A progress insight of the formation of hydrogen rich syngas from coal gasification[END_REF][START_REF] He | A novel hydrogen production system based on the threestep coal gasification technology thermally coupled with the chemical looping combustion process[END_REF][START_REF] Lee | A study on the effect of the CO2/steam mixtures and the addition of natural minerals on the reactivity of Adaro coal gasification[END_REF], pyrolysis of biomass [START_REF] Zhu | Catalytic pyrolysis of biomass with thermal treatment products of spent lithium-ion batteries for the upgrading of bio-oil and syngas[END_REF][START_REF] Yang | High-purity syngas production by cascaded catalytic reforming of biomass pyrolysis vapors[END_REF][START_REF] Kwon | Beneficial use of Fe-impregnated bentonite as a catalyst for pyrolysis of grass cut into syngas, bio-oil and biochar[END_REF]. Regarding the CO2 utilization, the chemical reduction of CO2 for syngas via DRM and artificial photo synthesis have achieved great concerning not only owning to its abundance in the products but also to the flexible H2/CO ratio of the mixture.

Depending on the reactor configuration and catalyst composition, H2/CO ratio could be tuned by adjusting the production of H2 and CO, subsequently, the syngas could be used in power generation, production of fertilizers, chemicals, plastics, solvents and etc., during the downstream processing. [START_REF] Hernández | Syngas production from electrochemical reduction of CO2: current status and prospective implementation[END_REF] As shown in Figure 1-2, the syngas with high H2/CO ratio would be widely used in gas turbine, internal combustion engine, fuel cells, refinery hydrotreating and etc., which require high concentration of hydrogen. On the other hand, Fischer-Tropsch synthesis (FTS) requires a lower and flexible for the synthesis of different products. A H2/CO ratio less than 1.0 is in favor of the production of ethanol while hydrocarbons, including wax, diesel, gasoline and naphtha, are produced from syngas with a H2/CO ratio larger than 2.0.

Methanol is considered to be an important intermediate in FTS which would be further converted to various organic chemicals, even fuels and plastics, via methanol-to-olefin, methanol-to-propylene and other processes. The production of methanol requires a moderate H2/CO ratio (around 2.0).

Figure 1-2. Syngas derivatives with reference to its H2/CO molar ratio composition [START_REF] Hernández | Syngas production from electrochemical reduction of CO2: current status and prospective implementation[END_REF].

Hydrocarbons

Hydrocarbons are often reported in the DRM and CO2 hydrogenation which have potential to replace the traditional petroleum refining and produce vital raw material from renewable sources rather than fossil source. In current research, light olefins (ethylene, propylene) [START_REF] Wang | Recent advances in catalytic hydrogenation of carbon dioxide[END_REF],

paraffins (ethane, propane) [START_REF] Wang | Recent advances in catalytic hydrogenation of carbon dioxide[END_REF] and aromatics (benzene, toluene, xylene) [START_REF] Wang | ZnZrOx integrated with chain-like nanocrystal HZSM-5 as efficient catalysts for aromatics synthesis from CO2 hydrogenation[END_REF] are the most comment products. Among all of the products, light olefins and para-xylene (PX) are considered to be more valuable. Ethylene and propylene are important chemical raw materials because it could be used in producing polymers while PX could be oxidized to terephthalic acid (TPA) which is one of the indispensable monomers to produce polyesters (e.g. polyethylene terephthalate). Olefins (alkenes), especially light olefins (C2 = -C4 = ), are one of the most important raw material produced on the largest scale industrially and considered to be building blocks of plastics, solvents, rubbers and fuels. Nowadays, olefins are mainly produced from oil refinery industry, alternatively, some sustainable process could also provide such renewable olefins building blocks by means of CO2 hydrogenation, DRM reaction, FT synthesis and biomass refinery. However, CO2, a highly stable molecular, its activation process requires complex catalysts and needs to overcome huge energy barrier, in other words, expensive catalysts and numerous energy input limit its industrial application.

Oxygenates

Last but not least, organic oxygenates are also meaningful because they are building blocks in the chemical industry and show advantages in energy density, transportation and storage comparing with gas products. The main liquid organic products consist of carboxylates, aldehydes, alcohols and ketones. The first report on the CO2 hydrogenation for the synthesis of formic acid were carried out Farlow and Adkins by using Raney-Ni catalysts in 1935 [START_REF] Aresta | Catalysis for the Valorization of Exhaust Carbon: from CO 2 to Chemicals, Materials, and Fuels. Technological Use of CO 2[END_REF]. This process reobtained people's interest owing to the mild reaction condition and energy usage.

Additionally, it is also identified as an attractive liquid hydrogen carrier. It is quite interesting to mention that acetic acid could be produced by a reaction with 100% atom economy (CO2 + CH4 → CH3COOH) in DRM. However, this ideal process is thermodynamically unfavourable which significantly restrict the application, whereupon an indirect process and high energy input are required in the conventional catalysis. Regarding the potential application on a commercial scale, more efficient utilization approaches are needed to be developed urgently.

Direct synthesis of alcohols (e.g. methanol, ethanol and etc.) are also widely studied so as to replace the usage of fossil fuels in the future owning to their high volumetric and gravimetric energy density. [START_REF] Bhardwaj | Integrated catalytic insights into methanol production: Sustainable framework for CO2 conversion[END_REF] Besides the synthetic advantages introduced previously, methanol could be used as an energy source, fuel cell, wastewater treatment and electric power plant as well due to the high hydrogen to carbon (H/C) ratio.

Utilization approaches

To produce renewable chemicals introduced in previous section, various CO2 utilization approaches were developed to achieve zero-carbon-emission campaigns for the climate action.

This section will present highly-efficient method for CO2 utilization in reported works.

Thermal catalysis

The study of CO2 utilization and recovery via thermal catalysis is an old topic which could go back to 1800s. In the last few decades, a tremendous amount of works was reported for a higher efficient process. Driven by heat and catalyst, carbon dioxide would be activated in which C=O bond would be activated, break and form new C-C bonds and valuable products via CO2 hydrogenation and DRM. This process will involve some reactions, such as FTS, Reverse Water Gas Shift Reaction (RWGS), Methanol to Olefins (MTO) and etc. Besides the experimental researches, theoretical mechanisms for producing organics with high selectivity over special catalysts are also well investigated to guide the design of catalysts. The details of catalysts structures and design are introduced in section 1.2.4.

Figure 1-3. Overview of reactions in the direct synthesis of organic products from CO2 conversion via thermal catalysis.

Photo catalysis

An alternative photo catalytic technology, which could convert CO2 and avoid extreme reaction conditions (high temperature and pressure), is promising in producing CO, formic acid, ethanol, methane, formaldehyde and etc. A series of metal oxide-based semiconductors (e.g. TiO2, CuxO and etc.) and non-oxide semiconductors (e.g. CdS, g-C3N4 and etc.) were developed to utilize solar energy using the certain wavelength of light irradiation. Lots of attempts have been reported to elucidate the micro-structures and performance of catalysts, increase the activity of photo catalysts. For instance, Cu/TiO2 catalyst were investigated in the photocatalytic reduction of CO2 and showed a superior catalytic performance in producing light olefins [START_REF] Ge | Photocatalytic conversion of CO2 into light olefins over TiO2 nanotube confined Cu clusters with high ratio of Cu+[END_REF]. Highly dispersed Cu clusters were found to show high activity and high ratio of surface Cu + species (75.7%) lead to the formation of light olefins. The researches indicated that Cu + active sites were benefit to the C-C coupling. However, in the application of such ideal energy source, there is still a long journey for fully industrial application in which a 3 order of magnitude improvement of turnover frequencies and catalytic activities are need.

Electro catalysis

Another approach to convert CO2 under mild condition (room temperature, pressure and neutral pH) is electrocatalytic CO2 reduction (ECR) which is also possible to tune products selectivity.

ECR process involves chemisorption of CO2, electron transfer, proton migration, cleavage of chemical bonds and etc., which are affected by the properties of electrode, electrolyte and their interface. Similar to the thermal catalysis and photo catalysis, ethylene and liquid oxygenates, including methanol, ethanol, formaldehyde and organic acid, could be directly synthesized with high selectivity. It is reported that the selectivity for C2H4 and C2H5OH were increased from 38.8% to 64.7% by using interconnected CuO and CeO2 nano-catalyst in the electrochemical system [START_REF] Lee | Metal-Oxide Interfaces for Selective Electrochemical C-C Coupling Reactions[END_REF]. Although it is the most potential technology to utilize greenhouse gases and the closest competitors to other approach, several challenges, including low solubility of CO2 in aqueous solution, formation of side products, catalysts deactivation, instability of electrode, low Faradaic efficiencies, expense of materials and etc., remain to be solved before more widely application.

Others

Several special converting approaches using biological chemistry, sonic chemistry, enzyme catalysis, microwave radiation, solar thermochemistry and etc., are attractive as well.

Microalgae could be used to capture CO2 from atmosphere, industrial and power plants by which has been evolved over billions of years. It is considered to be an alternative source for biodiesel. Ma et al. [START_REF] Cai | Cell-free chemoenzymatic starch synthesis from carbon dioxide[END_REF] reported a cell-free system for the starch synthesis from carbon dioxide and hydrogen via a chemical-biochemical hybrid pathway. After 11 core reactions catalysed by enzyme, they converted CO2 to starch at a rate of 22 nanomoles nano-moles of CO2 per minute per milligram of total catalyst which was about 8.5-fold higher than synthetic process in maize.

Another example for efficient utilization of CO2 is solar thermochemical conversion which could direct use the solar energy without energy-loss in redundant energy transform process.

At high temperatures (> 2500 K) heated by concentrated solar radiation, CO2 would be dissociated thermally without catalysts. Such converting process would be initiated at > 1500 K via a two-step thermochemical cycle by using metal-oxides catalysts.

Herein, Table 1-2 summarizes and compares different CO2 conversion strategies, introduced in this section, with plasma catalysis, whereas, there still are numerous challenges in those processes for the future applications. In thermal catalysis, high temperature and high pressure are required, which leads to high operating cost and poor flexibility. In other words, thermocatalysis is supposed to be a non-turnkey process and cannot be switched on and off easily to follow the supply, while others could since no need of pre-heating or long stabilization times.

Photo-catalysis and electro-catalysis require complex and expensive catalysts with low conversion [START_REF] Snoeckx | Plasma technologya novel solution for CO 2 conversion?[END_REF]. Alternatively, plasma-catalysis, investigated in this PhD thesis, could be a superior technique in CO2 valorization due to its special properties which will be further discussed in section 1.3. [START_REF] Wang | Fe-Cu Bimetallic Catalysts for Selective CO2 Hydrogenation to Olefin-Rich C2+ Hydrocarbons[END_REF] Compared to monometallic Fe catalyst, the Fe-Cu and Fe-Co bimetallic catalyst significantly increase C2-C7 hydrocarbons selectivity and decrease the fraction of undesired CH4 (Figure 5). Mg and Al composite oxides with dispersed active Fe species was investigated for the FTS, and the existence of MgO improved CO conversion, the formation of light hydrocarbons and olefin/paraffin ratio due to surface alkalinity modification. The addition of MgO also benefit to the complete crystallization of the spinel-like structure further enhancing the thermal stability of catalysts and benefit to the electronic effect between the active phase and the support. [START_REF] Wang | Effective Iron Catalysts Supported on Mixed MgO-Al2O3 for Fischer-Tropsch Synthesis to Olefins[END_REF] A ZnO-ZrO2 solid solution was reported to show considerable selectivity of methanol which could be further transformed into lower olefins over a Zn-modified SAPO zeolite via MTO reaction. More particularly, the thermodynamic and kinetic coupling between tandem reactions and the stability of those catalysts enable potential industrial application. [START_REF] Li | Highly Selective Conversion of Carbon Dioxide to Lower Olefins[END_REF] Support Supports also play an important role in catalytic process by influencing the physical and chemical properties of catalysts due to their components and complex structures. The performance of different catalysts incorporated with various supports were studied and reveal the mechanism based on the adsorption capability of CO2. Alkaline Al2O3 was found to be optimized supports with the best catalytic performance for CO2 hydrogenation to hydrocarbons due to the strongest affinity with CO2 adsorption. Numpilaia et al. [START_REF] Numpilai | Structure-activity relationships of Fe-Co/K-Al2O3 catalysts calcined at different temperatures for CO2 hydrogenation to light olefins[END_REF] investigated the impacts of different calcination temperatures on metal oxides crystallite size, the interaction between Fe and supports and the transformation of metal phases. They found that catalysts calcined at 400 °C achieved the highest CO2 conversion due to stronger interactions between Fe2O3 and other metal oxides while catalysts with the highest olefins to paraffins (O/P) ratio were calcined at 400 °C. This result attributed to the formation of K2O at high calcination temperatures and the stronger interaction between Fe2O3 and K2O. ZSM-5 zeolites is an industrial catalyst and important support due to the acidity, porosity and stability [52][START_REF] Kulkarni | Studies in the synthesis of ZSM-5 zeolites[END_REF][START_REF] Kitagawa | Transformation of propane into aromatic hydrocarbons over ZSM-5 zeolites[END_REF][START_REF] Védrine | Catalytic and physical properties of phosphorus-modified ZSM-5 zeolite[END_REF], hence, modified ZSM-5

was also widely applied in the hydrogenation of CO2. [START_REF] Dokania | Acidity modification of ZSM-5 for enhanced production of light olefins from CO2[END_REF] The presence of Ca enhanced the incorporation of CO onto the surface of zeolite, giving rise to surface acetate species, which contributed to higher ethylene/propylene/butylene production compared with the stand-alone Fe catalyst. SAPO-34 also shows superior capacity in light olefins producing and partially reduced Zn and Zr oxides could activate CO2, CO and H2, subsequently initiate C-C coupling in the confined acidic pores. [START_REF] Li | Highly Selective Conversion of Carbon Dioxide to Lower Olefins[END_REF][START_REF] Jiao | Selective conversion of syngas to light olefins[END_REF]

Promoters

Besides the active sites and supports of catalysts, the existence of promoters also is of great importance in turning the products selectivity. In the latest research, the role of promoters in enhancing products selectivity gained significant interests. A strong synergetic effect on the formation of C2+ hydrocarbons synthesis was observed over Fe-Cu bimetallic catalysts. The addition of K in Fe-Cu bimetallic catalysts enhanced olefins and C2+ hydrocarbons formation due to the modification of adsorption property, surface H/C ratio and C-C bond formation. [START_REF] Wang | Fe-Cu Bimetallic Catalysts for Selective CO2 Hydrogenation to Olefin-Rich C2+ Hydrocarbons[END_REF] The research of time (TOS) on stream indicated that the promoters migrated into the surface of Fe5C2 during the reaction. [START_REF] Liu | Direct Transformation of Carbon Dioxide to Value-Added Hydrocarbons by Physical Mixtures of Fe5C2 and K-Modified Al2O3[END_REF] The selectivity of desired light olefins (C2= -C4=) increased with TOS from 13.9 to 21.8% gradually.

Figure 1-4. Schematic of typical catalysts for enhancing the selection of olefins and C5+ hydrocarbons [START_REF] Wang | Fe-Cu Bimetallic Catalysts for Selective CO2 Hydrogenation to Olefin-Rich C2+ Hydrocarbons[END_REF] Mg, another type of promoters, is supposed to be able to improve the carbon efficiency in FTS due to the formation of the WGS-active magnetite phase and lower the selectivity to light olefins which could be amended by subsequent decoration with K. [START_REF] Cheng | Mg and K dual-decorated Fe-on-reduced graphene oxide for selective catalyzing CO hydrogenation to light olefins with mitigated CO2 emission and enhanced activity[END_REF] In the typical dualdecorated FeMgK/rGo catalyst, the selectivity to light olefins reached up to 65.0% and the selectivity to CO2 was appreciably cut down to 40.8%. Another cheaper alkali promoter (Na) has been reported considering its advantages in accelerating the generation of active Fe5C2 phase and applications in industrial scale. It is found that Na phase in catalysts not only enhanced the adsorption of CO2 but also facilitates the formation and stability of active species Fe5C2, lead to inhibition of the secondary hydrogenation of alkenes. [START_REF] Liang | Effect of Na Promoter on Fe-Based Catalyst for CO2 Hydrogenation to Alkenes[END_REF] Based on the FT synthesis route in CO2 hydrogenation, Na and Mn promoted the formation of light α-olefins.

[61] The synergy effect of Na and Mn further enhanced the selectivity of light olefins because of the combination of spatial hindrance by Mn and electron donation by Na. Such effect suppressed the chain growth which lead to the increase of surface short alkyl-metal intermediates.

Fe5C2

Fe3O4 Na

The addition of Na enhances the adsorption of CO2,

Facilitates the formation and stability of active species Fe5C2, Inhibits the secondary hydrogenation of alkenes.

[60] Fe5C2 Fe3O4 Na-Mn Mn promotes the reduction by the presence of oxygen vacancy, Na remarkably shifts the selectivity to α-olefins.

[61] Fe Fe3O4 Na-Mn High O/P ratio: 7.5,

Mössbauer spectra indicate that the addition of Mn facilitates the formation of active Fe5C2 species.

[62]

a . olefin/paraffin ratio

Reaction mechanism

In order to enhance the comprehension of catalytic process, researches on the mechanistic insight of hydrocarbons formation further revealed catalytic surface reactions on the microscale.

FTS is considered to be a vital process involved in CO2 hydrogenation and dry reforming, correspondingly, a large amount works devoted to the study of surface reactions and several sound reaction mechanisms (such as CO insertion mechanism, surface carbide mechanism, hydroxycarbene mechanism and etc.) were developed since 1940s. CH4 formation and is more selective to C2H4 due to the higher hydrogenation barrier of CH2* species. In contrast, CH4 formation is kinetically more favored due to different pathway over monometallic Fe [START_REF] Khoja | Kinetic study of dry reforming of methane using hybrid DBD plasma reactor over La2O3 co-supported Ni/MgAl2O4 catalyst[END_REF] surface. The evolution of active sites and mechanism of chain growth on the catalysts are still not clear even for the precise structures, some efforts have been reported

to demonstrated possible elementary reaction in microscale. For Fe-based catalysts, the active sites undergo an evolution (from Fe to Fe5C2) during the reaction. Zhang et al. [START_REF] Wei | The evolutions of carbon and iron species modified by Na and their tuning effect on the hydrogenation of CO2 to olefins[END_REF] revealed that the addition of Na on Fe5C2 surface suppress the hydrogenation of Fe5C2 in H2 atmosphere, which means Fe5C2 is much more stable, though in situ technologies. Furthermore, in situ Raman spectra and temperature programed characterization indicated that Na facilitated the carbon deposits by dissociation of CO and inhibited the reduction of surface carbon species. As a result, olefins were favored due to less hydrogenation of intermediate carbon species.

Inspiration for this thesis

Based on the literature above, we intended to design a series of Fe-based catalyst to achieve high-efficient utilization of GHGs somehow. For active phases in iron-based catalyst, Fe precursor need to be treated in high temperatures first, then reduced to iron oxides and metallic iron by hydrogen before reaction and finally form the iron carbides which is essential in chain growth during reaction. In order to simplify the process and obtain highly efficient catalysts, novel methods of catalysts' preparation are demonstrated. One of alternative way is to treat iron oxalate dihydrate thermally under CO/N2 flow, and catalysts with superior performance is synthesized without hydrogen reduction which is much more efficient and environmentally friendly. [START_REF] Liu | Direct Transformation of Carbon Dioxide to Value-Added Hydrocarbons by Physical Mixtures of Fe5C2 and K-Modified Al2O3[END_REF] Auto-combustion is another method to synthesize CO2 conversion catalysts without H2 treatment before reaction. The reducing environment including H2 and CO is generated from the exothermic decomposition of organic acid and metal nitrates mixture.

Consequently, the precursor will be reduced and carbonize to active sites. [START_REF] Pengnarapat | Active Fischer-Tropsch synthesis Fe-Cu-K/SiO2 catalysts prepared by autocombustion method without a reduction step[END_REF] In the preparing process, the metal is partially reduced and carbonized depending on various amounts of citric acid and metal nitrate mixture while treated in N2 or Ar atmosphere at high temperatures.

Hence, it is clear that surface properties not only influence the adsorption and desorption of different intermediates, but also activation energy barriers, reaction energies and rate constants of transition state on the active sites. Although many efforts have been devoted to the development of CO2 conversion to olefins in thermal catalysis and photo-electro catalysis, few researches focused on enhancing products selectivity though advanced catalysts, especially for olefins, in plasma catalysis. Hence, in this PhD thesis, some attempts will be made to achieve efficient utilization of CO2 and CH4 with high selectivity using well-designed catalysts.

Considering the specially properties of plasma (introduced in section 1.3), the interactions and synergetic effects of plasma and catalysts would be significantly interesting.

Plasma and plasma catalysis

What is plasma

Chemist Irving Langmuir first introduced plasma, the forth state of the matter, in the 1920s. By applying strong electromagnetic field or increasing temperature, plasma could be generated in which the gaseous substance (ionized or partially ionized) becomes highly electrically conductive to the point so that long-range electric and magnetic fields could dominate the behavior of the matter. The energetic free electrons are produced and collides subsequently with an atom, creating a positive ion and another free electron. Whereupon, electrons, charged particles, excited molecules, radicals, atoms, molecules, photons, and etc. (as shown in Figure 1-6), exist in the aggregated collection which is considered to be electrically conductive and internally interactive on a microscopic scale. The energies of those particles result in the gas temperature (Tg), ion temperature (Ti), rotational temperature (Tr), vibrational temperature (Tv) and electron temperature (Te), which are vital for describing plasma. On the other hand, equal density of electrons and charged particles make the plasma neutral on a global scale. Such quasineutral characteristics is distinguished from regular gas and contribute to special applications consequently. Plasma is widely found in the nature and applied and in the laboratory/industry as well. It is considered to not only be the most abundant form in the observable cosmos, but also constitute lighting and aurora. According to the degree of ionization and electron temperature, plasma could be categorized into two types: thermal plasma (TP) and non-thermal plasma (NTP). Thermal plasma is also known as equilibrium plasma in which gas temperature, ion temperature, rotational temperature, vibrational temperature and electron temperature are uniform (Tg ≈ Ti ≈ Tr ≈ Tv ≈ Te ≤ 2.0×10 4 K) due to entirely ionization. On the contrary, weakly ionization effect leads to the non-thermal plasma (Tg ≈ Ti ≈ Tr < Tv << Te ≤ 1.0×10 5 K) in which the gas temperature (~ room temperature, 300-2000K) is far less than electron temperature (~ 1eV, 11600K). In this nonequilibrium plasma, the electron density is not as high as that in equilibrium plasma because of lower ionization degree. A series of promising application of thermal and non-thermal plasma are developed according to their unique characteristics.

Applications of the plasma

This section will briefly introduce some application of plasma technology in laboratory and industrial. Based on the generation types, various of plasma, including glow discharge plasma, corona discharge plasma, dielectric barrier discharge plasma, radio frequency discharge plasma, microwave discharge plasma and etc., are developed and chosen depending on the desired processes. Taking the advantages of plasma, it is widely applied in material science, process engineering, medical science, and etc [START_REF] Snoeckx | Plasma technologya novel solution for CO 2 conversion?[END_REF][START_REF] Neyts | Plasma Catalysis: Synergistic Effects at the Nanoscale[END_REF][START_REF] De Bie | The Dominant Pathways for the Conversion of Methane into Oxygenates and Syngas in an Atmospheric Pressure Dielectric Barrier Discharge[END_REF].

In the material science, etching is used in microfabrication to chemically remove layers from the surface of a wafer and shows great advantages in preparing desired pattern with anisotropic and directional structures when compared with traditional wet etching (using chemicals).

Plasma enhanced chemical vapor deposition (PECVD) is used to produce high-quality and high-performance solid materials in the semiconductor industry. Besides, physical vapor deposition (PVD), implantation and etc., are also important in modifying the surface structures and characteristics. Plasma polymerization provides a unique way to activate monomer molecules and initiate polymerization. By using plasma process, highly branched and highly cross-linked polymers, which are hardly found in conventional polymers, are synthesized.

Furthermore, plasma torch could efficiently convert, including but not limited to biomass, waste plastics, methane and etc., into valuable products at extremely high temperatures.

In the topic of this thesis, we investigated so-called chemical production/destruction by using NTP which is able to convert GHGs (CO2 and CH4 in this thesis) and Volatile Organic Compounds (VOCs) with low energy cost and temperature since the molecules are excited in its non-equilibrium environment. Among various types of plasma, we chose dielectric barrier discharge (DBD) plasma in this study considering the DBD plasma reactor configuration (Figure 1234567). The catalysts in the plasma surroundings will cause more interesting synergetic effects and interactions in the plasma catalytic process.

Plasma catalysis and synergism

The general reaction processes around plasma and catalysts are illustrated in Figure 12345678. It not only consists of homogeneous reactions in the plasma flowing gas but also includes the heterogeneous (surface) diffusions and reactions on the catalyst. First, stable molecules would be activated in the homogeneous gas phase and create reactive precursors (discharged particles, radicals and etc.). Those precursors would diffuse in the gas phase and adsorbed on the surface.

Then, the adsorbed species would go through the surface diffusion, surface reaction and desorption along the surface of catalysts. It should be pointed the plasma around the catalyst beads would significantly affect its behavior which will be described later in detail. Eventually, reactants are converted into products with the help of the interactions of plasma with the surface which is different from neutral gases. As the introduction in previous section, plasma assisted DRM (co-conversion of CO2 and CH4), which is conducted under strong electric fields, create reactive species (ions, excited species, radicals) near ambient temperature [START_REF] Snoeckx | Plasma technologya novel solution for CO 2 conversion?[END_REF][START_REF] Wang | One-Step Reforming of CO2 and CH4 into High-Value Liquid Chemicals and Fuels at Room Temperature by Plasma-Driven Catalysis[END_REF][START_REF] Mizushima | Tubular membrane-like catalyst for reactor with dielectric-barrier-discharge plasma and its performance in ammonia synthesis[END_REF][START_REF] Mizushima | Catalytic Effects of Metal-loaded Membrane-like Alumina Tubes on Ammonia Synthesis in Atmospheric Pressure Plasma by Dielectric Barrier Discharge[END_REF][START_REF] Wallis | Plasma-assisted catalysis for the destruction of CFC-12 in atmospheric pressure gas streams using TiO2[END_REF][START_REF] Wang | Atmospheric Pressure and Room Temperature Synthesis of Methanol through Plasma-Catalytic Hydrogenation of CO2[END_REF]. Among the reactive species, the excited species (i.e. vibrationally or electronically excited species) play an important role because they lower the activation energy barrier and lead to more efficient global reactions. Hence, in NTP, the reactions are mainly initiated by an electronic way via free electrons which the temperature is actually much higher than the gas one. The electrons temperature commonly varies from 1 to more than 10 eV. So, activation of the highly thermodynamically stable molecules: CO2 (5.5 eV) and CH4 (4.5 eV), could be achieved at low gas temperature [START_REF] Li | Direct conversion of CO2 and CH4 into liquid chemicals by plasma-catalysis[END_REF].

The advantages of plasma assisted DRM are also attributed to the reactive species, including charged species, radicals, excited specie and etc., For instance, several reports indicated that energy efficiency in CO2 splitting is related a lot to different dissociation pathways, in other words, excitation types in plasma environment depending on electron temperature. Ionization, electron impact electronic excitation and vibrational excitation are predominant dissociation routes of CO2 plasma. Notably, vibrational excitation is considered to be the most efficient way for CO2 dissociation, [START_REF] Kozák | Splitting of CO2 by vibrational excitation in non-equilibrium plasmas: a reaction kinetics model[END_REF] and high selectivity of vibrational levels is related to fast electronvibration (e-V) energy exchange, fast vibrational-vibrational (V-V) transfer and relatively slow vibrational-translation (V-T) relaxation processes. Due to different plasma conditions, vibrational excitation level contributes variously to CO2 dissociation, which result in different energy efficient. Warm plasma is a promising plasma source, in which the gas temperatures is larger than 1000K, for the synthesis of a range of fuels and chemicals. For CO2 excitation, the electron temperature of Gliding arc (GA) is around 1-2 eV and that is effective for populating low energy vibrational levels of CO2. That results in lower energy requirement CO2 dissociation than electronic excitation pathway to dissociation. However, some drawbacks of electrode geometry should be overcome for its industrial applications. For example, a fluidized-bed GA reactor would be quite attractive to improve catalytic performance.

As shown in Figure 1-9, plasma and catalyst could affect each other on the gas composition, surface chemistry, plasma properties and electron energy distributions considering their interactions in the plasma.

Effects of catalysts on plasma

On the one hand, catalyst could possibly enhance the electric field and change the discharge type on the catalysts surface [START_REF] Neyts | Understanding plasma catalysis through modelling and simulation-a review[END_REF]. Polarization effects and the accumulation of charges on the surface would contribute to the enhancement due to the contact angle, the curvature and the dielectric constant of the catalysts. Consequently, the modified electric field further affects the electron energy distribution function, the electron impact dissociation and ionization rates [START_REF] Bal | Effect of plasma-induced surface charging on catalytic processes: application to CO2 activation[END_REF][START_REF] Van Laer | Influence of Gap Size and Dielectric Constant of the Packing Material on the Plasma Behaviour in a Packed Bed DBD Reactor: A Fluid Modelling Study: Influence of Gap Size and Dielectric Constant…[END_REF]. Secondly, Regarding the porous structures in the catalyst, some micro-discharges could form in pores which greatly affect plasma behavior [START_REF] Zhang | Formation of microdischarges inside a mesoporous catalyst in dielectric barrier discharge plasmas[END_REF][START_REF] Zhang | Can plasma be formed in catalyst pores? A modeling investigation[END_REF][START_REF] Zhang | Enhancement of plasma generation in catalyst pores with different shapes[END_REF]. The formation and stability of those micro-discharges are related to materials and the size of porous structure. Besides microdischarges in pores, more intensive discharge (compared to volumetric streamers) around the contact points would form and it could lead to higher electron impact ionization and dissociation rates [START_REF] Bal | Effect of plasma-induced surface charging on catalytic processes: application to CO2 activation[END_REF][START_REF] Wang | Streamer propagation in a packed bed plasma reactor for plasma catalysis applications[END_REF].

Effects of plasma on catalysts

On the other hand, plasma would in turn influence catalysts on the surface physiochemical properties, such as the changes of adsorption probability, surface area, oxidation state, coke formation, catalyst work function and etc. Hot spots, the regions where the temperature is locally higher, could form due to the formation of strong micro discharges near the structures with high curvature [START_REF] Kim | Effect of different catalysts on the decomposition of VOCs using flow-type plasma-driven catalysis[END_REF]. It contributes to the enhancement of catalytic activity in plasma catalysis as well. Considering the photons in plasma, photo catalytic effects would be activated by the UV-Vis irradiation around semi-conductive material [START_REF] Guaitella | C2H2 oxidation by plasma/TiO2 combination: Influence of the porosity, and photocatalytic mechanisms under plasma exposure[END_REF][START_REF] Kim | Decomposition of gas-phase benzene using plasma-driven catalyst (PDC) reactor packed with Ag/TiO2 catalyst[END_REF]. Last but not least, as the fundamental function of catalysts, surface reaction pathways inevitably influenced by plasma [START_REF] Guo | Detection of hydroxyl radical in plasma reaction on toluene removal[END_REF][START_REF] Demidyuk | Influence of Temperature on Gas-Phase Toluene Decomposition in Plasma-Catalytic System[END_REF]. Some potential intermediates and transient states would be modified, and consequently, activation barrier would also decrease owning to special reaction channels.

Research progress in plasma catalytic CO2 and CH4 conversion

This section will review the research progress of experimental work on plasma catalytic DRM but also the theoretical studies. Besides the experimental attempts for developing an efficient process, kinetic model and simulations are conducted as well for a comprehensive understanding. By taking the breakthrough and limitations of current studies both into account, it would be more persuasive to demonstrate the motivation of this work.

Experimental researches

Recently, plenty of experimental works on plasma catalytic CO2 and CH4 conversion were conducted for a novel approach of utilizing GHGs using renewable electricity (Figure 1-11).

To begin with, the production of syngas was first investigated in plasma reactor [START_REF] Li | Experimental study on plasma-catalytic synthesis of hydrocarbons from syngas[END_REF].

Meanwhile, driven by temperature in the plasma reactor, wax and carbon deposition were correspondingly found in methane enriched feedstocks. Then, more complex products, such as methanol, formic acid, acetic acid, ethane and etc., were synthesized directly via plasma assisted DRM. Kogelschatz et. al., [START_REF] Eliasson | Direct Conversion of Methane and Carbon Dioxide to Higher Hydrocarbons Using Catalytic Dielectric-Barrier Discharges with Zeolites[END_REF] produced alkanes, alkenes, oxygenates, and syngas by a DBD plasma reactor operated at ambient conditions. The products distribution was found to depend on the operating conditions (the pressure, the input power, the flow rate, the CH4/CO2 feed ratio, and the catalyst used). The cogeneration of syngas, CH3OH and higher hydrocarbons were achieved with an optimized CH4/CO2 ratio in the feed.

As an ideal reaction with 100% atom economy, the production of CH3COOH shows great potential. Tu et. al., [START_REF] Wang | One-Step Reforming of CO2 and CH4 into High-Value Liquid Chemicals and Fuels at Room Temperature by Plasma-Driven Catalysis[END_REF] achieved a total 50-60% selectivity to oxygenates with a major component of acetic acid (40.2% selectivity) at room temperature (30 o C) and atmospheric pressure. Yang et. al., [START_REF] Li | Synthesis of Oxygenates and Higher Hydrocarbons Directly from Methane and Carbon Dioxide Using Dielectric-Barrier Discharges: Product Distribution[END_REF] reported that a wider discharge gas (1.8mm) could be more favored for the production of ethanol while a narrower discharge gap (1.1mm) is benefit to the formation of acetic acid. The reactor configurations, especially the existence of after-glow zone in the reactor, contributed tremendously to the conversion and production distribution.

With the development of plasma catalysts [START_REF] Li | Synthesis of Oxygenates and Higher Hydrocarbons Directly from Methane and Carbon Dioxide Using Dielectric-Barrier Discharges: Product Distribution[END_REF][START_REF] Zeng | Plasma-catalytic dry reforming of methane over γ-Al2O3 supported metal catalysts[END_REF][START_REF] Zheng | Plasma-assisted catalytic dry reforming of methane: Highly catalytic performance of nickel ferrite nanoparticles embedded in silica[END_REF][START_REF] Krawczyk | Methane conversion with carbon dioxide in plasma-catalytic system[END_REF][START_REF] Pham | Activation of methane and carbon dioxide in a dielectric-barrier discharge-plasma reactor to produce hydrocarbons-Influence of La2O3/γ-Al2O3 catalyst[END_REF], more and more efficient processes were developed over La, Fe, Co, Cu, Ni, Ce and noble metal catalysts loaded on various supporting materials (e.g. SiO2, Al3O2, BaTiO3, Zeolites, LDHs and etc.). Batiot-Dupeyrat et. al., [START_REF] Pham | Activation of methane and carbon dioxide in a dielectric-barrier discharge-plasma reactor to produce hydrocarbons-Influence of La2O3/γ-Al2O3 catalyst[END_REF] reported the effects of La2O3-LaAlO3-γ-Al2O3 catalysts in the plasma zone. Their results

showed that the activation of carbon dioxide was significantly increased over 200 o C while the conversion of methane remained stable. That indicated that the formation of oxycarbonate species at the surface of the catalyst play a critical role on the plasma assisted conversion of carbon dioxide. Ni-based and spinel nickel ferrite nanoparticles embedded in silica were compared [START_REF] Zheng | Plasma-assisted catalytic dry reforming of methane: Highly catalytic performance of nickel ferrite nanoparticles embedded in silica[END_REF]. The results indicated the synergetic effect between the non-thermal plasma and catalyst which benefits to the conversion of CO2 and CH4. The aggregation of Ni-Fe alloy was also restricted and suppressed the carbon formation in the reforming process. Noble metals showed special effects on the reaction. [START_REF] Wang | One-Step Reforming of CO2 and CH4 into High-Value Liquid Chemicals and Fuels at Room Temperature by Plasma-Driven Catalysis[END_REF]. Compared with others, HCHO was formed on Au/γ-Al3O2 and Pt/γ-Al3O2 catalysts and Pt/γ-Al3O2 catalyst showed the highest selectivity. Tu Recently, direct synthesis of longer chain hydrocarbons, alcohols and acid, except for syngas and light products, are gradually gaining people's interest. Tosi et.al., [START_REF] Martini | Oxidation of CH4 by CO2 in a dielectric barrier discharge[END_REF][START_REF] Scapinello | CO2 Hydrogenation by CH4 in a Dielectric Barrier Discharge: Catalytic Effects of Nickel and Copper[END_REF] reported the production of carboxylic acids (mainly formic acid) using nickel and copper catalysts, and heavier compounds, in particular acetic acid (C2), propionic acid (C3), and butyric acid, 2methyl-propionic acid (C4), were also detected in the products. They claimed that the synthesis of oxygenates increased with power discharge and was affected by the electrode surface. Their calculations showed that the key intermediates could be carboxyl COOH and the formyl HCOO radicals, both of them could be further hydrogenated to formic and other carboxylic acids. Di et. al., [START_REF] Li | Direct conversion of CO2 and CH4 into liquid chemicals by plasma-catalysis[END_REF] packed SiO2 aerogel in the plasma reactor in which significant abatement in discharge volume and an alteration of the discharge behavior could be avoided. The synergetic effects of Co catalysts enhanced the formation of the liquid chemicals up to a total liquid selectivity of 40% while the selectivity towards liquid products was negligible without catalyst.

Possible explanation to the selectivity of syngas and C2 to C5 gaseous hydrocarbons, other than liquid products, are the decomposition of CHxO radicals in the plasma surroundings. It is exciting to mention that long chain organic products (up to C5 oxygenates) were produced over Co catalyst. Moreover, C5+ hydrocarbons (pentane and heptane) and C6 ester were favored to form with higher ratio of CH4 in the feedstock owning to enhanced C-C coupling.

Kinetic study

Even though people achieved a considerable success in converting GHGs into valuable products via various approaches, the in-depth understanding of such special technology is not clear.

Besides the development of catalysts in plasma processes, profound understanding of kinetics parameters, including rate constant, reaction orders, activation energy and etc., of CO2 conversion in macroscopic is also of great importance. Due to the adsorption, diffusion, dissociation and desorption of reactants, intermediates and products, catalysts promote the reaction in plasma and the catalytic-performance varies under different conditions. In chemical engineering, these kinetics parameters play an essential role in the proposition of mechanism, catalysts and industrial process designing. Furthermore, a thorough kinetics model could help to explain the reaction mechanism and predict the experiment result. In current studies, three kinetic models for CO2 conversion via plasma catalysis are developed and they will be discussed in this section.

Global kinetic model

A simplified global kinetic model of the CO2 and CH4 reforming reaction was established with the He dilution, based on the free radical processes' mechanism [START_REF] Goujard | Carbon Dioxide Reforming of Methane Using a Dielectric Barrier Discharge Reactor: Effect of Helium Dilution and Kinetic Model[END_REF]. In this model, several modifications and assumptions were made due to the complex environment of plasma. It is supposed that, in the plasma discharges, the reactant molecules (CO2 and CH4) were attacked by the active species which would be affected by the plasma power. Their model allows to foresee the conversion rates according to the energy transfer to the gas (P * τ) while the model took the helium dilution value in the feed gas in to consideration. An equation was proposed to well predict the conversion of reactants according to the energy transfer to the gas. In the catalytic dry reforming of methane over LaNiO3, the global kinetic model also could give a forecast of the conversion of CO2 and CH4 [START_REF] Zheng | Experimental and kinetic investigation of the plasma catalytic dry reforming of methane over perovskite LaNiO3 nanoparticles[END_REF]. They predicted the plasma catalysis dry reforming of methane over LaNiO3 nanoparticles with argon dilution. The deviation of predicted conversion rate in their model were reported to be 4.5% when compared with experimental data. Zhu et al. [START_REF] Zhu | Kinetics study on carbon dioxide reforming of methane in kilohertz spark-discharge plasma[END_REF] reported a simplified macroscopic kinetic model by which kinetic rate equation and rate constant were established and calculated to describe the plasmacatalytic reaction. The results showed that the rate constants of CO2 and CH4, which were calculated by using equations as follows where kCH4 and kCO2 were the rate constant of CH4 and CO2 conversion, respectively.

X CH 4 1 -X CH 4 = 𝑘 CH 4 × (𝑝𝑑) × SEI X CO 2 1 -X CO 2 = 𝑘 CO 2 × (𝑝𝑑) × SEI
The reliability of those rate equation was verified by comparing the experimental results with calculated data. The value of rate constants of CO2 and CH4 increased from 0.78 and 0.53 mol.MJ -1 .bar -1 .mm -1 to 1.04 and 0.63 mol.MJ -1 .bar -1 .mm -1 respectively when frequency of the plasma discharges increased.

Power law kinetic model

Power law kinetic model is another simple way to estimate and resolve complex reactions in DBD plasma reactors. Recently, Khoja et al., took DBD reactor important geometric parameters into consideration and developed a modified power low kinetic model [START_REF] Khoja | Kinetic study of dry reforming of methane using hybrid DBD plasma reactor over La2O3 co-supported Ni/MgAl2O4 catalyst[END_REF]. They studied the effects of specific input energy, discharge volume and feed flow rate corresponding to GHSV in the plasma modellings. The results indicated that the rate constant of CO2 and CH4 were linearly enhanced by specific input energy and GHSV while discharge volume had a nonlinear influence on the reactants conversions and products selective. The apparent activation energy of CH4 and CO2 were calculated to be 32.6 kJ mol -1 and 35.2 kJ mol -1 by applying modified power low kinetic model and steric factor.

𝑘 CH 4 (J -1 -atm -1 ) = [ X CH 4 1 -X CH 4 × 1 (SIE) • 𝑉 𝐷 • 𝑝 ] 𝑘 CO 2 (J -1 -atm -1 ) = [ X CO 2 1 -X CO 2 × 1 (SIE) • 𝑉 𝐷 • 𝑝 ]

Semi-empirical power law kinetic model

In order to reveal more details of plasma-catalysis system, semi-empirical power law kinetic model was introduced to describe the kinetics behavior of catalysts for plasma reactions and estimate apparent rate constants and reaction orders. [START_REF] Zheng | Silica-coated LaNiO3 nanoparticles for non-thermal plasma assisted dry reforming of methane: Experimental and kinetic studies[END_REF] Complex reactions, including dry reforming, coke formation and water gas shift reaction in the plasma system might broke the equilibrium between CH4 conversion and CO2 conversion. It still remains impossible to elucidate the reaction mechanism in the plasma-catalysis thoroughly due to its complexity.

Some process details of plasma-catalysis system, such as reactants conversion rates, is able to be expressed by semi-empirical power law kinetic model:

R CO 2 = 𝑘 1 [CH 4 ] 𝛼 1 [CO 2 ] 𝛽 1 R CH 4 = 𝑘 2 [CH 4 ] 𝛼 2 [CO 2 ] 𝛽 2

Kinetic parameters

Table 1-4 compared the activation barrier of reactants over different plasma catalyst. As shown in Table 1-4, the apparent activation energy of CO2 and CH4 over Ni-La2O3@SiO2 were estimated base on Arrhenius equation, and the activation energy of CH4 (24.73 kJ mol -1 ) is lower than that of CO2 (29.50 kJ mol -1 ) due to lower dissociation energy. [START_REF] Zheng | Silica-coated LaNiO3 nanoparticles for non-thermal plasma assisted dry reforming of methane: Experimental and kinetic studies[END_REF][START_REF] Kim | Synergistic effects of plasma-catalyst interactions for CH4 activation[END_REF] Due to convoluted components of plasma, Khoja et al., took the collision of the gas molecules into consideration and modified Arrhenius equation with collision factor [START_REF] Pakhare | A review of dry (CO2) reforming of methane over noble metal catalysts[END_REF].

ln(𝑘) = - E a R 1 T + lnZ 0 • S f
Their calculation shows that the activation energy of CH4 and CO2 were 32.6 kJ mol -1 and 35.2 kJ mol -1 respectively over Ni/La2O3-MgAl2O4. Besides in the dry reforming reaction, activation energy of CO2 in hydrogenation reaction over various catalysts were also shown in Table 1-4.

We could find that the activation energy of CO2 in plasma-catalysis is significantly lower than in thermo-catalysis. [START_REF] Xu | Mechanistic study of non-thermal plasma assisted CO2 hydrogenation over Ru supported on MgAl layered double hydroxide[END_REF][START_REF] Chen | Nonthermal plasma (NTP) activated metal-organic frameworks (MOFs) catalyst for catalytic CO2 hydrogenation[END_REF][START_REF] Mu | Kinetic Study of Nonthermal Plasma Activated Catalytic CO2 Hydrogenation over Ni Supported on Silica Catalyst[END_REF] From the comparation of activation barrier in plasma-catalysis and thermo-catalysis, plasma is further proven to be a superior CO2 valorization technique due to activity nonequilibrium statues. Regarding the complicity of plasma catalysis, there are several limitations in experimental works which couldn't fully interpret such process. Attractively, computational technology provides us a power tool to supplement the comprehension of plasma catalysis from another point of view. In current computational studies, 0D and low dimension simulation (1D and 2D) are mainly reported in which the electric field distribution, gas temperature distribution, gas flow behavior, species density, transportation phenomenon and etc., need to be partially ignored [START_REF] Khoja | Recent developments in non-thermal catalytic DBD plasma reactor for dry reforming of methane[END_REF]. By means the numerical simulation on 0D, 1D and 2D scale, the nature of plasma could be simulated and evaluated directly, whereupon some researches on the effects of catalysts, which are infeasible in experimental studies, could provide more insight of plasma catalysis. The plasma simulations include the research on the effects of packed particles, surface structures, pore size, discharge gas gap, adsorption behaviour and etc. In this section, several numerical studies and the simulation of some major concerning will be briefly introduced. the capacitance of gas gap (Cgi i = 1n) and the combined capacitance between gas gap (Ccomb), was developed. Herein, the average electric field could be estimated. Different configuration of catalyst-bed was proved to be able to greatly affect the average electric field. The electric field were found to be enhanced by some form of roughness or geometric distortion in the reactor in the simulation works. This might result from the presence of e.g. pellets, fibres or granulates in the discharge volume of the packed bed reactor, or simply due to the roughness and porosity of the catalyst surface. The polarization effects and the accumulation of charges on the surface also contribute to the enhancement of the electric field in a packed bed reactor. The development of discharges on the surface of pellets was simulated in a 2D modelling and the interaction mechanisms between plasma and discharges are elucidated on the surface of plasma-catalysts. Gao et al. [START_REF] Gao | Mode Transition of Filaments in Packed-Bed Dielectric Barrier Discharges[END_REF] investigated the properties of plasma in a packed DBD reactor by 2D modelling. Their simulations revealed that hybrid discharges transferred to surface discharges, further enhancing the tangential component of the electric field along the beads surface, due to the increase of voltage. Consequently, surface ionization waves took place near the bead surface, generated reactive species and enhanced the surface activity. The influence of metal particles on generation of discharges was also observed with a microscope-ICCD camera.

Particles

Surface

[110] The result showed that metal nanoparticle supported zeolite was benefit to the expanding of discharge plasma over a wide area. As shown in Figure 1-13, the discharge region was expanded by the presence of insulating surface. This is attributed to the surface along which the streamer could propagate (surface flashover) and result in more intensive discharges on the contact points. Compared with filamentary micro-discharge (FM), such surface discharges (SIW) would result in a higher electron impact ionization, dissociation rates and, consequently, a higher concentration of reactive species [START_REF] Cheng | The plasma enhanced surface reactions in a packed bed dielectric barrier discharge reactor[END_REF].

Porous structure

Another vital effect of catalysts in plasma-catalytic system is the formation and propagation of micro-discharge in the pores as it is related to the enhancement of electric field. For catalysts, a large amount of porous structure is on nm-scale and μm-scale which influence plasma properties greatly. Consequently, the plasma formation and propagation in these scales is of great importance on plasma catalysis as these determine the reaction process. Zhang et al. [START_REF] Zhang | Formation of microdischarges inside a mesoporous catalyst in dielectric barrier discharge plasmas[END_REF] revealed the features of electric field, ionization rate and electron density in mesoporous catalyst using 2D modelling. For the 4nm to 10nm pores, the maximum electric field and electron density increased with the raise of pores size, and for 100nm pores, those were lower because of less pronounced surface discharge (Figure 1234567891011121314). Whereas the ionization rate was highest in the 100 nm pores because there was larger effective ionization region. In order to obtain a more profound mechanisms of plasma catalysis, propagation of a plasma streamer along the pore sidewall was studied by modelling. For 50nm pores, plasma could only propagate to a small extent in the pores due to its diameter which was larger than Debye length, while plasma could propagate and lead to further discharge enhancement in pores (500nm-1000nm), whose diameter was larger than Debye length (a physical scale of the transition from plasma collectivity to individual particle behavior), in a larger extent. [START_REF] Zhang | Propagation of a plasma streamer in catalyst pores[END_REF] The result indicated that pore size (diameter and depth) was critical in plasma catalysis because it determined the formation and propagation of discharges. The reduction of Debye length, in turn, could also modify the discharges properties and electric field in pores.

Figure 1-14. Distributions of electron impact ionization rate (m -3 s -1 ) in different pores [START_REF] Zhang | Formation of microdischarges inside a mesoporous catalyst in dielectric barrier discharge plasmas[END_REF].

Zhang et al. [START_REF] Zhang | Influence of the Material Dielectric Constant on Plasma Generation inside Catalyst Pores[END_REF] investigated the properties of micro-discharges, which was influenced by the dielectric constant and various pore sizes of catalysts, inside catalyst pores by using a twodimensional fluid model. The simulations suggested that the increase of dielectric constant could decrease the ionization rate inside the pores and increase that in the sheath. The result was attributed to more effectively polarization of larger dielectric constants which could give a stronger local electric field in the sheath. They also found that ionization rate and electron temperature decreased monotonically with dielectric constant for a 10μm pore which was different with a 30μm pore. In the modelling of He-plasma, [START_REF] Zhang | Can plasma be formed in catalyst pores? A modeling investigation[END_REF] for pore size above 10μm, plasma species could be formed in the pore structure. In contrast, little ionization was formed for pore size less than 10μm, while plasma species could diffuse and migrate into pores.

Furthermore, distribution of electric potential also changed with pore size variation. Zhang et al. [START_REF] Zhang | Enhancement of plasma generation in catalyst pores with different shapes[END_REF] also reported that different shapes of pores could influence the electric field enhancement and the plasma production. The strongest electric field and ionization rate appeared in different place of the pores, and that might have great significance on understanding and improving the catalyst performance. The packing materials would influence the gas gap size in DBD reactor. Bogaerts et al., [START_REF] Van Laer | Influence of Gap Size and Dielectric Constant of the Packing Material on the Plasma Behaviour in a Packed Bed DBD Reactor: A Fluid Modelling Study: Influence of Gap Size and Dielectric Constant…[END_REF] investigated the effects of mm-gap and a so-called micro-gap in packed plasma reactor by establishing a 2D axisymmetric fluid model. Their simulation results indicated that discharges in the mm-gap reactor is characterized by a few current peaks in a half cycle while there were much more current peaks per half cycle in the micro-gap reactor (shown in Figure 123456789101112131415).

Gas gap size

Because of the small dimension, the electric field strength would be enhanced and more often reached to initiate a breakdown. Furthermore, the small size of the gas gap could cause the plasma to lose its quasi-neutrality because the electrons were more likely to be absorbed at the walls and the packing beads. On the other hand, the ion density was found to be lower in microgap reactor and plasma could hardly flow through the channels between the voids.

Adsorption behavior

Besides the modification of plasma, electric field and discharges could in turn influence the behavior of catalysts, adsorption, desorption, reactants excitation, surface reaction and etc. [START_REF] Shirazi | DFT study of Ni-catalyzed plasma dry reforming of methane[END_REF] For CO2 plasma conversion, plasma environment could increase the fraction of vibrational excited species, further lowering the activation barrier of CO2. Whereupon, density functional theory calculations (DFT) were performed to study the CO2 plasma-activation over metal surface. Several researches suggested that negative surface charge could improve catalysts activity and selectivity significantly, promoting the splitting of CO2.

[77] Jafarzadeh et al. [START_REF] Jafarzadeh | CO2 Activation on TiO2-Supported Cu5 and Ni5 Nanoclusters: Effect of Plasma-Induced Surface Charging[END_REF] elucidate the effect of surface charging over Titania-supported Cu5 and Ni5 clusters in plasma catalysis.

Notably, excess electron in plasma system further stabilized the bent CO2 structure and dissociation of CO2 on charged clusters, which was more thermodynamically favorable. They attributed these effects to the adsorption process of CO2 due to the shift of the antibonding states, and considered that it may be a possible reason for the synergy in plasma catalysis. Jafarzadeh et al. [START_REF] Jafarzadeh | Activation of CO2 on Copper Surfaces: The Synergy between Electric Field, Surface Morphology, and Excess Electrons[END_REF] reported that adsorption energy raised and the physisorption of CO2 switched to chemisorption while an external negative electric field was induced, and excess electrons could strengthen this effect (Figure 12345678910111213141516). Their calculations also indicated that electric field elongated C-O bond in CO2 and partial charges, and that might be helpful to optimize CO2 plasma-activation.

Objective of this thesis

Remaining limitations

Based on the literature reviews, the researches of plasma assisted conversion of CO2 and CH4 are still in the initial stage. It remains a long journey to be fully applied in chemical industrial.

First of all, the energy efficiency of plasma processes needs to be improved before it is applied in the industrial. Another concerning in the industrial process is products selectivity.

Unfortunately, the product is widely distributed and shows no target selectivity for single products in most of reported work which is not conducive to industrial utilization. Hence, separation and purification units, in which extra cost of energy and investment, are required in the potential industrial applications. Last but not least, the reaction mechanism for plasma catalysis is still unclear. Although a number of numerical works are devoted to describe plasma process from nanosecond to second scale, industrial dynamics simulation and more reliable theoretical models are hardly reported.

Why promoters?

Reviewing the production of added-value hydrocarbons beyond the plasma catalysis, ironbased catalysts are the most commonly catalysts used for olefin production via thermal catalysis while alkali-metal could serve as promoters by changing electronic and basic properties.

As an example, iron-based catalysts are widely investigated in CO2 Hydrogenation through the modified Fischer-Tropsch synthesis (FTS) route owing to their activity in the Water-Gas-Shift-Reaction (WGSR) and Fischer-Tropsch synthesis [START_REF] Ronda-Lloret | A Critical Look at Direct Catalytic Hydrogenation of Carbon Dioxide to Olefins[END_REF]. Promoters play an important role in improving the formation of long-chain hydrocarbons and tuning the selectivity to value-added unsaturated hydrocarbons. Besides, iron carbide is gradually identified to be the active phase for FTS other than iron oxides, and the iron oxides transform into carbides during the CO2 hydrogenation under a H2/CO2 atmosphere [START_REF] Chang | Relationship between Iron Carbide Phases (ε-Fe2C, Fe7C3, and χ-Fe5C2) and Catalytic Performances of Fe/SiO2 Fischer-Tropsch Catalysts[END_REF][START_REF] Li | Iron Carbides: Control Synthesis and Catalytic Applications in COx Hydrogenation and Electrochemical HER[END_REF]. Song et al., reported the production of olefin-rich C2+ hydrocarbons from CO2 hydrogenation using potassium (K) promoted Fe-Cu bimetallic catalysts [START_REF] Wang | Fe-Cu Bimetallic Catalysts for Selective CO2 Hydrogenation to Olefin-Rich C2+ Hydrocarbons[END_REF]. The increase of olefins' selectivity over K promoted catalysts is attributed to the change of surface properties. K decreases the surface H2/CO ratio by suppressing the H2 adsorption and greatly improving the CO adsorption, consequently, further decreases the hydrogenation activity [START_REF] Zhang | Adsorption and reaction of CO and hydrogen on iron-based Fischer-Tropsch synthesis catalysts[END_REF].

The function of promoters in non-equilibrium plasma is more complicated owning to the reactive species in plasma phase. On the other hand, this also could show possible advantages in conversion of CO2 and CH4. Nevertheless, to our best knowledge, there is few of relevant studies focusing on improving the selectivity of unsaturated hydrocarbons by using promoted plasma catalyst, not mention the deep understanding of the reaction insight experimentally and numerically. Thus, it is worth to evaluate that in the plasma surroundings and maximize the synergetic effects.

Objectives

According to current research background and limitations, we proposed following objectives which will be investigated in this thesis to achieve more efficient utilization of GHGs.

1) Preparing high activity, high selectivity and stable plasma catalysts.

2) Converting CO2 and CH4 and achieving direct synthesis of high value-added organic products in a packed DBD plasma reactor (as shown in reaction-A5).

x CH4 + y CO2 → z CO + m H2 + n hydrocarbons + l oxygenates + w H2O (A5)

3) Characterizing plasma catalysts for better understanding of catalyst structure and properties and explaining the relationship between its structure and performance.

Experimental section

Description of plasma reactor

As shown in Figure 2-1, plasma-catalytic conversion of CO2 and CH4 was performed on a fixed bed DBD reactor described in previous literature [START_REF] Li | Direct conversion of CO2 and CH4 into liquid chemicals by plasma-catalysis[END_REF]. It mainly consisted of a quartz tubular reactor, a power supply, a cold trap condenser and products analysis system. The method used in this work is developed according to the reported MOF mediated synthesis approach [START_REF] Wezendonk | Elucidating the Nature of Fe Species during Pyrolysis of the Fe-BTC MOF into Highly Active and Stable Fischer-Tropsch Catalysts[END_REF][START_REF] Santos | Metal organic framework-mediated synthesis of highly active and stable Fischer-Tropsch catalysts[END_REF][START_REF] Wezendonk | Controlled formation of iron carbides and their performance in Fischer-Tropsch synthesis[END_REF]. The Fe-based catalysts were prepared using sol-gel method. In a typical synthesis, 4.068 g citric acid, and 5.792 g iron (III) chloride were dissolved in 40 g deionized water (DI water) at 293 K, and 20.8 g tetraethyl orthosilicate (TEOS) was added to that solution drop by drop in 20 minutes. The combined solution was stirred at 293 K overnight and then sealed and aged at 333 K for 5 h to form the transparent gel. The precursor was dried in oven at 333 K and calcined at 673 K in nitrogen atmosphere for 8 h. The synthesized catalyst was named as Fe/C-SiO2. The potassium (K) modified Fe-based catalysts were synthesized using similar method. Potassium carbonate, citric acid, and iron (III) chloride were first dissolved in 40 g DI water. Then TEOS was added to the solution and treated under the same conditions.

The obtained catalyst was named as K-Fe/C-SiO2 after calcined at 673 K in nitrogen atmosphere.

This approach provides an economical way to produce metal nano-catalysts when it is compared with reported MOF mediated synthesis approach. Metal nanoparticles are embedded in the carbon matrix, as a result, well dispersed nanoparticle-carbon composites are maintained.

This special nano structure could improve catalyst's performance. On the other hand, the absence of porous carbon matrix could cause particle aggregation, which would further affect catalysis. Thus, we prepared iron catalyst by following a sequential synthesis route (e.g.

Precursor → K-Fe/C-SiO2 → K-Fe-SiO2) in order to avoid or minimize unexpected positive/negative effects of preparation method.

For preparation of Fe-SiO2 and K-Fe-SiO2, the obtained catalyst precursors (Fe/C-SiO2 and K-Fe/C-SiO2) were treated at 873 K under air for 6 h in order to remove the carbon and reduced at 873 K in 5% H2/N2 for 10 h. The catalyst was named as Fe-SiO2 and K-Fe-SiO2 respectively.

For all the catalysts we prepared in this work, the nominal Fe loading amount is around 20 wt%.

Undoubtedly, there are other feasible preparation methods (e.g. impregnation), not included in this work, which are quite interesting for the future study.

Catalyst characterization

X-ray diffraction (XRD)

Catalysts were characterized by X-ray diffraction (XRD) with a X'Pert pro, Philips

N2 adsorption-desorption characterizations

The catalysts texture properties, including the specific surface areas, pore volumes, and average pore sizes, were determined by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method using an ASAP2020 (Micromeritics Instrument Corporation) and nitrogen physisorption at 77 K. The samples are pre-treated at 573 K and held for 12 h before testing.

The BET specific surface areas were determined from the adsorption isotherms at the relative pressure range of 0.1 -1.0, while the BJH pore volumes were calculated from the desorption isotherms at the relative pressure range of 0.1 -1.0. The micropore volume and micropore surface area were then determined with the t-plot method. The pore sizes and pore size distributions were calculated through density functional theory (DFT) method and BJH method respectively.

Transmission electron microscope (TEM)

Transmission electron microscopy (TEM) observations and analysis were carried out by using a FEI Tecnai F20-ST operating at 200 kV and equipped with scanning transmission electron microscopy (STEM) system, high angle annular dark-field (HAADF) detector, Gatan Imaging Filter (GIF) and energy dispersive X-ray (EDX) device. The samples were first ground and dispersed in absolute ethanol. Then the suspension was treated with sonication and deposited on a copper grid. We first acquired conventional TEM images at 200 kV on a high angle CCD camera using bright field imaging mode. Then, high resolution TEM (HRTEM) images were acquired using a CCD slow-scan camera after the diffraction patterns were acquired.

Subsequently, local area fast Fourier transform (FFT) diffractograms, equivalent to electron diffraction patterns, were exploited to determine structural and crystallographic characteristics of observed samples. Furthermore, EDX microanalysis was combined with STEM-HAADF images (Z-contrast images) to perform chemical analyses and determine the elements dispersion on studied samples.

Scanning electron microscope (SEM)

Scanning electron microscope (SEM) are carried out on a Carl Zeiss AG electronic microscope.

The samples were first coated with a 7nm platinum layer using Quorum (Q150 TES) before testing. The SEM images were obtained by a CCD camera with a high efficiency in lens detector or Everhart Thornley secondary electron detector.

X-ray photoelectron spectroscopy (XPS)

The X-ray photoelectron spectroscopy (XPS) were carried out on a Thermo Scientific K-Alpha spectrometer to determine the surface composition and valence state of the elements. Samples are test below 2 × 10 9 Torr using an Al Kα (hυ = 1486.6 eV) monochromator X-ray source at 3 kV. XPS binding energy is calibrated internally for surface charging by the C 1s peak (BE = 284.6 eV). The data are acquired and processed using the AVANTAGE 5.98 software.

Thermogravimetric analyses (TGA)

Thermogravimetric analyses (TGA) were carried out using a SETARAM Instrumentation LABSYS evo. The samples were firstly stabilized under air flow (60 mL/min) at 298 K for 1 h, then heated up to 1173 K at a heating rate of 10 K/min, and finally kept at 1173 for 0.5h.

In-situ optical emission spectra (OES)

In-situ optical emission spectra (OES) were carried out on an FHR 640 spectrometers (HORIBA) equipped with a high-sensitive CCD detector (Jobin-Yvon: Synapse CCD). The fixed-bed plasma reactor was installed in the dark room and an optical fiber was placed near the quartz tube. The emission spectral data of plasma was recorded using Synerjy software.

Temperature-programmed desorption (TPD)

Temperature-programmed desorption (TPD) tests were carried out on a home-made apparatus reported in the literatures [START_REF] Ye | Iron-chromium mixed metal oxides catalyse the oxidative dehydrogenation of propane using carbon dioxide[END_REF][START_REF] Chen | The reaction pathways of 5-hydroxymethylfurfural conversion in a continuous flow reactor using copper catalysts[END_REF]. The catalyst (100 mg) was pretreated in He flow at 673 for 1 h and cooled to 323K. Then the sample was exposed to H2 (30 mL/min) for 1 h and flushed in He (30 mL/min) until the baseline signal leveled off in order to remove physically adsorbed molecules. Finally, the TPD spectra was recorded at the temperature range of 323K to 873K with a ramp rate of 10 K/min using Ar as the carrier gas.

Catalyst evaluation

In order to calculate the discharge power, a capacitor (3.9 nF) was connected in series with the reactor. Two voltage probes were applied to measure the voltage of the plasma reactor and the capacitor which will be used to calculate the input energy by means of Lissajous method (Equation 1) firstly introduced by Manley in 1943 [START_REF] Manley | The Electric Characteristics of the Ozonator Discharge[END_REF].

𝑃 = 𝑓 * 𝐶 𝑆 * ∑ ( 𝑉 𝐷 (𝑘+1) + 𝑉 𝐷 (𝑘+1) 2 ) + (𝑉 𝐶 (𝑘 + 1) -𝑉 𝐶 (𝑘)) 𝑛 𝑘=1 (1)
Where VD is the voltage on the reactor, VC is the voltage on the capacitor, CS is the value of the capacitor, f is the frequency.

The applied voltage, temperature and pressure data were detected via a digital oscilloscope (HP Hewlett Packard 54615B), a type K thermocouple and a pressure probe respectively, then recorded on a computer via a Keysight 34970A Data Acquisition unit. The liquid products were condensed in a cold trap, separated by a SUPEL Q PLOT (Inside diameter  Length: 0.32mm  30 m) column and analyzed by gas chromatography-mass spectrometer/flame ionization detector (GCMS/FID, GCMS-QP2010, Shimadzu) offline while the gas products were analyzed online by a micro-gas chromatography (Micro-GC Fusion). The GCMS was calibrated using a standard liquid sample (20 vol% of methanol, ethanol, acetic acid, acetone, and water) to quantify the main products [START_REF] Li | Direct conversion of CO2 and CH4 into liquid chemicals by plasma-catalysis[END_REF]. The Micro-GC Fusion equipped with two thermal conductivity detectors (TCD) in two channels. In the first channel, H2, CH4 and CO are and the mean values are taken. After the reaction, conversion rate of the reactants and the products selectivity would be calculated as follows [START_REF] Li | Direct conversion of CO2 and CH4 into liquid chemicals by plasma-catalysis[END_REF]: As the carbon deposition is negligible, carbon deposition is estimated by TGA analysis of fresh and used catalysts after the study of TOS behavior. Due to the off-line analysis of liquid, a large error may occur in the calculation of the oxygenates selectivity. To avoid this, we adapted the method in the literature which makes the value of carbon balance is meaningless except for pure CH4 conversion.

X CH 4 =
The selectivity of liquid oxygenates is calculated as:

S oxygenates = 1 -S CO -S C x H y -2 % Carbon (7) 𝑆 𝐶 𝑥 𝐻 𝑦 𝑂 𝑧 = 𝑥 × 𝑚𝑜𝑙𝑒 𝑜𝑓 𝐶 𝑥 𝐻 𝑦 𝑂 𝑧 𝑖𝑛 𝑡ℎ𝑒 𝑙𝑖𝑞𝑢𝑖𝑑 × 𝑆 𝑜𝑥𝑦𝑔𝑒𝑛𝑎𝑡𝑒𝑠 (8)
where x is the number of carbon atoms in the C x H y O z molecule.

Two important ratios of gaseous products are defined as:

H/C=
The specific input energy (SIE) is calculated as the following formula:

SIE= power of discharges Flow rate ×60 (𝑘𝐽/𝐿) ( 12
)
The energy efficiency (EE) of CO2 and CH4 conversion (mmol/kJ) are defined as:

EE CO 2 = mole of CO 2 consumed 𝐼𝑛𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 ( 13
)
EE CH 4 = mole of CH 4 consumed 𝐼𝑛𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 (14)

Numerical calculation and simulation method

Plasma parameters calculation

The plasma parameters, including the average electric field strength, the reduced electric field (E/N) and the mean electron energy, are calculated by using an equivalent electrical circuit model of the DBD reactor described in Mei et al. paper [START_REF] Mei | Plasma-assisted conversion of CO 2 in a dielectric barrier discharge reactor: understanding the effect of packing materials[END_REF]. The packed catalysts have a great influence on the physical characteristics of the discharge and those effects are for the moment too complex to be fully described. The packed catalysts consist of SiO2 (ε=4.42), FeO (ε=14.2), KCl (ε=4.86) and Fe/C (ε=307) [START_REF] Young | Compilation of the Static Dielectric Constant of Inorganic Solids[END_REF][START_REF] Grünewald | Handbook of Chemistry and Physics[END_REF]. Cd, Cc, Cg, represent the capacitance of the quartz tube, catalysts and gas respectively.

Ur is the voltage across the reactor while Ugap is the voltage across the catalysts and gas. The distorted gas phase and discrete solid region are equivalently considered as two plate capacitors in this model. So, according to the equivalent electrical circuit and the equivalent gap thicknesses of the gas (dg) and catalysts (dc), voltage across the gas (Ug) could be calculated by the equation reported in [START_REF] Mei | Plasma-assisted conversion of CO 2 in a dielectric barrier discharge reactor: understanding the effect of packing materials[END_REF][START_REF] Tu | Dry reforming of methane over a Ni/Al 2 O 3 catalyst in a coaxial dielectric barrier discharge reactor[END_REF]:

d g = 2V d 𝛼 𝜋 (𝐷 + 𝑑)𝑙 (15)
d c = d gap -d g (16)
C g =

ε g ε 0 S d g (17)
C c = ε c ε 0 S d c (18)
U g = Q 𝐶 𝑔 = 𝑈 𝑐 𝐶 𝑐 𝐶 𝑔 = (𝑈 𝑔𝑎𝑝 -𝑈 𝑔 )𝐶 𝑐 𝐶 𝑔 (19)
U g = U gap ( 𝜀 𝑔 𝜀 𝑐 )  ( 𝑑 𝑐 𝑑 𝑔 ) + 1 ( 20
)
where D is the outer diameter of the inner electrode, d is the inner diameter of the quartz tube, α is the volume ratio of catalyst to reactor, Q is the charge, l is the discharge length, ε0 is the dielectric constant of vacuum, the Ugap is the voltage across the gap which includes the gas and packed catalysts, εg is the relative dielectric constant of gas reactants, εc is the relative dielectric constant of the packed catalysts. More details on the derivation of formula could be found in published literature [START_REF] Mei | Plasma-assisted conversion of CO 2 in a dielectric barrier discharge reactor: understanding the effect of packing materials[END_REF][START_REF] Tu | Dry reforming of methane over a Ni/Al 2 O 3 catalyst in a coaxial dielectric barrier discharge reactor[END_REF].

The electron energy distribution function (F0) of CO2/CH4 discharge is calculated with BOLSIG+ according to the method developed in [START_REF] Matamba | A progress insight of the formation of hydrogen rich syngas from coal gasification[END_REF] and described in the following paragraph.

In the plasma simulation, Boltzmann Equation (BE) is solved for electrons thanks to a solver named BOLSIG+ [START_REF] Manabe | Role of greenhouse gas in climate change**[END_REF]. The electron distribution satisfies BE as:

𝜕𝑓 𝜕𝑡 + 𝑣 • 𝛻𝑓 - 𝑒 𝑚 𝐸 • 𝛻 𝑉 𝑓 = 𝐶[𝑓] (21)
Where f is normalized as:

∭ 𝑓𝑑 3 𝑣 = 4𝜋 ∫ 𝑓 0 𝑣 2 𝑑𝑣 = 𝑛 ∞ 0 (22)
n being the electron number density.

Then the technique consists in separation of the energy-dependence of f from one side and its space-dependence and time-dependence by assuming:

𝑓 0 (𝜀, 𝑧, 𝑡) = 1 2𝜋𝛾 3 𝐹 0 (𝜀)𝑛(𝑧, 𝑡) (23)
Where E is the electric field, m is the electron mass, f is the electron distribution, e is the elementary charge, v is the velocity coordinates, γ = (2e/m) 1/2 is a constant, ε is the electron energy in electron-volts, C[f] represents the change rate, z is the position along this direction and f0 is the isotropic part of f. The energy-dependence of f (i.e. F0) is normalized as:

∫ 𝜀 1/2 𝐹 0 𝑑 𝜀 ∞ 0 = 1 (24)
After some development, the energy distribution function F0 becomes the solution of the following equation written in energy space:

𝜕 𝜕𝜀 (𝑊 ̃𝐹0 -𝐷 ̃𝜕𝐹 0 𝜕𝜀 ) = 𝑆 ̃ (25)
Where 𝑊 ̃ is the flow velocity, 𝐷 ̃ is the diffusion coefficient and 𝑆 ̃ a source term indicating that the equation is a no ordinary differential equation. This equation is then solved with the BOLSIG+ solver.

CHEMKIN simulation

Species

We established a 0D kinetic model in which the species and density distribution are not considered and then, it is solved by CHEMKIN [START_REF] Zheng | Kinetic Study of Nonequilibrium Plasma-Assisted Methane Steam Reforming[END_REF].

Electrons

Kinetic model

To begin with, the 0D kinetic model is first used by ignoring the configuration of plasma reactor and the effects of catalysts (as shown in Figure 2345). Several simplifying assumptions are made in the model. The rate of conversion of reactants to products is controlled by chemical reaction rates and not by mixing processes. Mass transport to the reactor walls is assumed to be infinitely fast. Natural convection and heat transfer in the reactor are ignored.

In this model, stable molecules are first initiated by energetic electrons, creating excited particles, positive and negative ions, accompanied by the electron-ion recombination reactions in plasma environment which are also meaningful for the formation of the intermediates. In the gas phase, various products are formed from active intermediates via electron-neutral, electronion, ion-ion, ion-neutral and neutral-neutral reactions. For a general elementary reaction in proposed mechanism:

A m +B m ↔ C m +D m (26)
The reaction rate of species A for the m th reaction is defined as rm =d[Am]/dt.

r m = -d[A m ] dt =k m [A m ][B m ] (m = 1, 2, 3, … … M) (27)
The rate constants for the m th reaction are expressed as km and the concentrations of reactants are written as [Am] and [Bm].

k m =a m ( T 298 ) β exp (- E am RT ) (m = 1, 2, 3, … … M) ( 28
)
Where am is the preexponential factor which is a temperature-independent constant, T is refers to the electron temperature or gas temperature in electron-impact reaction or other reaction respectively, Eam is the activation energy for the m th reaction. Consequently, the reaction rates of the n th species (rn) are defined as:

r n = ∑ r m α nm f -∑ r m α nm c M m=1 , (n = 1, 2, 3, … … N) M m=1 (29)
Where the symbols of αnm f represent n th species' stoichiometric coefficient in the m th formation reaction, while αnm c represent its stoichiometric coefficient in the m th consumption reaction.

Based on the electron collision cross-section from the online LXcat database, the rate coefficient of electron impact reaction is obtained as a function of the electron temperature. The relationship between them is established by the following energy balance equation [START_REF] Zheng | Kinetic Study of Nonequilibrium Plasma-Assisted Methane Steam Reforming[END_REF]:

𝑑 𝑑 𝑡 ( 3 2 𝑛 𝑒 𝑘 𝐵 𝑇 𝑒 ) = 𝑄 𝑠𝑜𝑢 -𝑄 𝑒𝑙𝑠 -𝑄 𝑖𝑛𝑒 (30)
Where, in the term at the left-hand side of this equation, ne is the electron density, kB is Boltzmann's constant, Te is the electron temperature. At the right-hand side of this equation, the first term represents the energy gain due to Joule heating which is an input of the model. It can be chosen under the form of a time dependent function Qsou(t), as we will see in section 3.

The second and third terms denote the energy loss due to elastic and inelastic collisions, respectively.

𝑄 𝑒𝑙𝑠 = ∑ 3 2 𝑛 𝑒 𝑣 𝑚𝑖 ( 2𝑚 𝑒 𝑀 𝑖 ) 𝑘 𝐵 (𝑇 𝑒 -𝑇 𝑖 ) (31)
𝑄 𝑖𝑛𝑒 = ∑ 𝑛 𝑒 𝑘 𝑙 𝑁 𝑙 ∆𝜀 𝑙 (32)
In the equation of elastic collisions, Ti and Mi are the temperature and mass of species i, me is the electron mass, νmi is the electron momentum transfer collision frequency with species i. In the equation of inelastic collisions, Nl is the density of the gas phase collision partner, kl is the reaction rate coefficient for the l th electron impact process and ∆εi is the corresponding change in the electron energy.

Thus, we first use BOLSIG+ for solution of the above equation in order to obtain the rate coefficients of the electron-impact reactions. Then, Arrhenius coefficients (for CHENKIN input data) are obtained by curve fitting since rate coefficients are assumed to be a function of the mean electron energy [START_REF] Zheng | Kinetic Study of Nonequilibrium Plasma-Assisted Methane Steam Reforming[END_REF]. Besides the electron impacted reaction, the reactions involving ions, radicals and molecules contribute greatly to the plasma chemistry and pulses. The Arrhenius rate coefficients and activation energy of the radical reactions are obtained from NIST Chemistry WebBook while the kinetic parameters are acquired from the UMIST Database for the ion involved reactions [START_REF] Manion | NIST Chemical Kinetics Database[END_REF][START_REF] Mcelroy | The UMIST database for astrochemistry[END_REF].

According to the method presented in the literature [START_REF] Kim | Synergistic effects of plasma-catalyst interactions for CH4 activation[END_REF][START_REF] Xu | Mechanistic study of non-thermal plasma assisted CO2 hydrogenation over Ru supported on MgAl layered double hydroxide[END_REF], energy barrier (Ea, kJ mol -1 ) of the NTP-catalysis was obtained by using a modified Arrhenius law:

𝑟 𝑝 = 𝐴 𝑝 × e F * 𝐸𝑎 𝑝 𝑃 𝑝 ( 33
)
where rp is the reaction rate of plasma assisted dry reforming, Ap is Arrhenius rate coefficients, F is the total flow rate of feed gas (50 mL min -1 ), while Pp is the plasma power (W). In this work, the conversion rate of CO2 and CH4 are calculated by using a plasma Perfectly Stirred Reactor (PSR) reactor and the DBD discharge power is fixed between 2 and 3 W. The activation energy of plasma-catalytic dry reforming was also obtained by using a modified Arrhenius law:

𝑟 𝑝𝑐 = 𝐴 𝑝𝑐 × e F * 𝐸𝑎 𝑝𝑐 𝑃 𝑝 (34)
Now, what we aimed to see were the differences between Ap and Apc from one side, and Eap and Eapc from another side.

For both cases, ln (ri) and the inverse of plasma power (1/Pp) are plotted, then the slopes are obtained by linear fitting of them according to:

ln(𝑟 𝑖 ) = 𝐸𝑎 𝑖 × F × 1 P p + ln(𝐴 𝑖 ) (35)
We neglected the contribution of thermal-catalytic processes since gas temperature in this model is fixed to 300K, Relying on species thermal-dynamic data, gas-phase kinetics file and reactor physical properties introduced above, Chemkin could allow us to perform a quickly calculation of reaction rates for each reaction at different time in the reactor. Consequently, number density of all of the species would be given in the out-put files.

Surface mechanism

We also consider the surface reaction kinetics (Table S3 in Supporting Information.) in which the rate constants of adsorption, desorption and surface reactions are presented in our model.

The surface reaction mechanism consists of 13 surface species and 42 surface reactions [START_REF] Karakaya | Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts[END_REF].

The surface site density of the catalyst is 2.66 × 10 -9 mol/cm 2 (given by Deutschmann) The reactants firstly adsorb on the catalyst surface:

CO2 + Rh(s) → CO2(s) (RS-04) CH4 + Rh(s) → CH4(s) (RS-06)
The activation processes of CO2 and CH4 occur stepwise on the Rh sites after the adsorption:

CO2(s) + Rh(s) → CO(s) + O(s) (RS-22) CH4(s) + Rh(s) → CH3(s) + H(s) (RS-33)
Dissociated with other adsorbed species:

CH4(s) + O(s) → CH3(s) + OH(s) (RS-41)
and desorption of products and reactants:

H(s) + H(s) → H2 + Rh(s) (RS-07) CO2(s) → CO2 + Rh(s) (RS-11)
For each adsorbed species, the thermodynamic data used in evaluation of thermodynamic properties (entropy, enthalpy, heat capacity) and reverse reaction rate constants are adopted from Deutschmann's work [START_REF] Karakaya | Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts[END_REF][START_REF] Delgado | Surface Reaction Kinetics of Steam-and CO2-Reforming as Well as Oxidation of Methane over Nickel-Based Catalysts[END_REF][START_REF] Janardhanan | CFD analysis of a solid oxide fuel cell with internal reforming: Coupled interactions of transport, heterogeneous catalysis and electrochemical processes[END_REF][START_REF] Maier | Steam Reforming of Methane Over Nickel: Development of a Multi-Step Surface Reaction Mechanism[END_REF].

It should be noted that, at the current stage, only adsorption of reactants, critical intermediates (e.g. CH3, H and etc.) and products (e.g. CO, H2O and etc.) are involved in our model which could partially simulate plasma-catalytic coupling. The surface mechanism adopted in our model takes adsorbed H(s), OH(s), C(s), CH3(s), CH2(s), CH(s), O(s) and COOH(s) into consideration while more complex intermediates are found in the gas phase. This limitation is attributed to complexity of plasma-catalysis and lack of kinetic data. Full description of surface reactions and model would be achieved after having a better comprehension of the interactions

Introduction

Inspired by the role of catalysts and promoters discussed in the first chapter, plasma catalytic direct synthesis based on iron carbides catalyst and alkaline promoters might have promising performance. The major concern is the mutual influence of catalyst and plasma. Catalyst would affect plasma properties which, in turn, could affect solid surface properties. Among the parameters being able to play an influencing role, roughness and porosity of the catalyst could lead to an enhancement of the local electrical field which partially determine the energy of the resulting micro-discharges. Intensity and density of micro-discharges then drive local surface temperature by production of hotspots at the interface and change surface kinetics. On so-called hotspots, the temperature of local regions stands in sharp contrast to that of plasma where nonthermal plasma is operated near or slightly above room conditions. Local plasma chemistry would be modified accompanying the initiation of thermal-catalytic effects. Thus, each material will have its own plasma-surface interaction depending on many parameters. Although the plasma catalysis association exhibits a lot of potential advantages for the selective synthesis, the function of plasma, that of catalyst as well as the nature of the interactions between them (physical or chemical) still remain unclear for the moment.

Herein, this chapter is focusing on the direct synthesis of such high added-value hydrocarbons (e.g. ethene and ethyne) in plasma surroundings. We prepared a series of iron-based catalysts and pioneered the selective synthesis of high added-value chemicals in DBD plasma-catalytic reactor. The effects of Fe species and K promoter on the catalytic performance are evaluated in detail. The plasma-catalytic process is also demonstrated by analyzing and the plasma physical parameters and chemical adsorptions on the surface. Furthermore, the catalyst's stability is illustrated to show the possible industrial application. In addition, optical emission spectra (OES)

profile further exhibits the excited species, radicals, molecules and atoms in the plasma phase.

Structures of the catalysts

Physical properties of catalysts

First, we investigated the basic physical properties of catalysts in this section. By using N2 adsorption-desorption characterizations, the Brunauer-Emmett-Teller (BET) surface areas, average pore diameter distribution and pore volume of catalysts are compared in Table 3-1, and specific surface areas of SiO2 calculated from the linear part of the plot reach 631.09 m 2 g -1 .

The N2 adsorption-desorption isotherms (Figure 3-1) of SiO2, classified according to IUPAC, is type IV -H2 isotherms with a hysteresis loop, which indicates the mesoporous structure (2-50nm) of the catalysts. The average pore diameter distributions of all samples are calculated by using the Barrett-Joyner-Halenda (BJH) method and the desorption branch of the nitrogen isotherms. The modification of iron species lowers the BET surface area to 510.08 m 2 g -1 and the existence of K promoter further decrease the BET surface area. K modification would alert the uniformity of the catalysts which further leads to the changes of hysteresis loop and porous structures. The effect of K in Fe/C catalyst is less substantial and it indicates that the surface structure could be maintained by carbon matrix to some extent.

X-ray diffraction analysis

The X-ray diffraction (XRD) patterns of as-synthesized iron-based catalysts are presented in for K-Fe-SiO2 is observed at 710.8 eV which indicate the Fe 2+ in FeO+SiO2 species (fayalite) [START_REF] Guo | Good lithium storage performance of Fe2SiO4 as an anode material for secondary lithium ion batteries[END_REF][START_REF] Wang | Formation mechanism of zinc-doped fayalite (Fe2-xZnxSiO4) slag during copper smelting[END_REF][START_REF] Mekki | An XPS study of iron sodium silicate glass surfaces[END_REF]. This result is in agreement with species clarified by XRD data. According to the peaks area after the deconvolution, Fe 2+ /Fe 3+ ratio in K-Fe/C-SiO2 is estimated to be 0.77 which is lower than that ratio (4.29) in K-Fe-SiO2. It is reasonable because the reducing ability of CA pyrolysis gas is weaker than that of hydrogen. Another interesting finding extracted from this semi-quantitative analysis is the ratio of Fe-C/(Fe 2+ + Fe 3+ ). The ratio (1.59) indicates that Fe-C is an important phase in the catalyst and it is formed by combining Fe with residual carbons after pyrolysis

SEM analysis

N2 adsorption-desorption characterizations show high surface areas and special pore structures in iron-based and modified catalysts. However, characterization of the K-Fe-SiO2 catalyst suggests that there are specific processes in the synthetic procedure that make surface structures 53 of this material unique. The dramatically lower surface area of the K-Fe-SiO2 catalyst compared to all the other materials and the prominent KCl peaks in the XRD of this material suggest that K is not well distributed throughout the material and is actually agglomerated in large clusters on the material, unlike the analogous K-Fe/C-SiO2. Hence, SEM images of catalysts and their multi-element EDS mapping spectra are obtained to elucidate the surface structures. nanoparticles is assigned to be epsilon carbides (ε′-Fe2.2C). The pyrolysis of Fe precursor is not easy and the formed species are highly depended on the temperatures [START_REF] Wezendonk | Elucidating the Nature of Fe Species during Pyrolysis of the Fe-BTC MOF into Highly Active and Stable Fischer-Tropsch Catalysts[END_REF]. From 300 to 900 °C, the Fe3O4, FeO, α-Fe, epsilon carbides (ε′-Fe2.2C), Hägg carbides (χ-Fe5C2) and cementite (θ-Fe3C) occupied the predominant position successively. At the typical preparation temperature (400 °C), highly dispersed ε′-Fe2.2C are the main carbide phase. Compared with them, we manage to produce Fe-based catalysts with a similar particle size and avoid the using of complex and expensive reagents. Due to the poor crystallinity, small particle size and well dispersity of the iron specie, the lattice fringes of related crystals are inconspicuous. The TEM result further confirmed the high dispersion of iron species and small particle size. The HAADF-STEM image and EDX elemental mapping of K-Fe-SiO2 catalyst further exhibits the atomic distribution in modified catalysts. As shown in Figure 3-10, iron and K species is not well dispersed not only in low magnification which is identified by SEM test, but also exhibit heterogeneous characteristics in larger magnification. The Fe distribution of K-Fe-SiO2 catalyst is not uniform in SiO2 matrix, whereas, the elemental distribution is more homogeneous in K-Fe/C-SiO2 catalyst.

Plasma-catalytic conversion of CO2 and CH4

Effect of catalysts

Before the test of plasma-catalytic conversion of CO2 and CH4, we first carried out the experiment of pure CH4 conversion, then pure CO2 conversion in plasma surroundings using optimized K-Fe/C-SiO2 catalysts to prove that carbons in the catalyst matrix didn't participate in the formation of CO and CxHy.

As shown in Figure 3 The carbon balance for pure CH4 conversion is 100.9% which is a little bit over-calculated due to instrument-induced measurement error. As one of the foremost parts of plasma system, the presence of catalyst could lead to the change of physical parameters and chemical properties in the system, which will have a great impact on the catalytic activity. Accordingly, in the first section, we evaluated the conversion of CO2 and CH4 without catalyst in a fixed bed DBD reactor. After the test in an empty reactor, we tested SiO2 grains filling the plasma reactor under the same CO2/CH4 treatment capability. As a result, the energy efficiencies of CO2 and CH4 fluctuate around 0.15 mmol/kJ and 0.40 mmol/kJ respectively. Plasma inevitably heats the reactor due to enhanced electric field.

Nevertheless, the temperature of the gas in all of the tests is around 323K -333K which is insufficient to initiate the reaction of CO2 and CH4 thermally. Hence, we consider that the potential thermo-catalytic effect does not contribute to the conversion of CO2 and CH4. As shown in Figure 3-15, catalysts exhibit synergistic effects during plasma-catalytic process and considerably tune the selectivity of H2, CO and liquid oxygenates while decrease the conversion rate slightly. The conversion rate is influenced by many parameters such as the intensity of electric field, the spatial distribution of discharges, the specific surface area of the catalytic material, the residence time and all the coupling parameters. As an example, the plasma volume which plays the role of intermediate species reservoir in the process is reduced by the presence of packed catalysts. Under the same GHG treatment capacity (30 mL/min), we pack catalysts, including inert SiO2, in the plasma reactor. Even though SiO2 shorten the residence time of reactants, we observe a slight increase of conversion rate which could be attributed to electric characteristic change (e.g. electric filed strengthen). Similarly, the conversion rate over iron-catalysts also affected by the type transition of discharges contribute.

On the other hand, hydrogen adsorption capabilities (clarified in the following paragraph) on iron species could improve CO2 conversion. So, an optimum should be found in order to produce enough intermediates and to have a large number of catalytic sites. More in-depth studies are required to elucidate these points.

Compared to SiO2 packed process, K-Fe/C-SiO2 catalyst raises the selectivity of liquid oxygenates from 4.22 % to 21.76 %. This optimized catalyst also decreases the selectivity of CO (34.64%). Consequently, H/C ratio (represents the molar ratio of H2 to CO) increases to around 2.0 which is the most desirable gas composition in F-T synthesis (Figure 3 It is essential to mention that Fe/C-SiO2 catalyst produced more CO and less HC than that of Fe-SiO2. A possible explanation could be the enhancement of CO2 conversion compared to CH4 conversion over the Fe species clarified by XPS. The increase of CO2 conversion is attributed to the surface defects over iron carbide (in Fe/C-SiO2 catalyst) where C-O bonds in CO2 become weaker [START_REF] Aresta | Catalysis for the Valorization of Exhaust Carbon: from CO 2 to Chemicals, Materials, and Fuels. Technological Use of CO 2[END_REF][START_REF] Bhardwaj | Integrated catalytic insights into methanol production: Sustainable framework for CO2 conversion[END_REF]. On the other hand, high H2 adsorption capabilities also leads to the increase of CO2 conversion by promoting the hydrogenation reactions. H2 adsorption is inhibited by K promoter in K-Fe/C-SiO2 catalyst, leading to a decrease of CO selectivity.

One interesting thing to mention is the significant influence of the promoter on the products selectivity when the catalyst contains carbon (Fe/C-SiO2 vs K-Fe/C-SiO2 catalysts), while it is not the case when the catalyst does not contain carbon. The K addition in Fe-SiO2 seems to have a negligible effect on the products selectivity. This is attributed to the poor dispersion of the potassium, larger mesopore and low surface area of K-Fe-SiO2 catalyst, probably due to the preparation process operating at high temperature. Indeed, EDS and XRD reveal an aggregation of clusters at the surface of the material. This poor dispersion explains the weak effect of promoter addition on the selectivity tuning with carbon free materials prepared by high temperature process. The role of the catalysts in plasma process remains unclear in current research stage. Plasma physical parameters and chemical adsorptions are supposed to be altered around the catalysts, so, both of them are investigated to reveal the catalysts' function in plasma-catalysis. Before all else, the average electric field strength, the reduced electric field (E/N) and the mean electron energy are calculated by using an equivalent electrical circuit model [START_REF] Mei | Plasma-assisted conversion of CO2 in a dielectric barrier discharge reactor: understanding the effect of packing materials[END_REF].

The effect of packed catalysts on those plasma parameters are summarized in Table 3-3.

Compared to plasma process, the packed catalysts enhance the electric field strength of the CO2 (ε=1.6)/CH4 (ε=1.06) gas mixture discharge. More notable enhancement of the electric field strength is observed when catalyst having a higher dielectric constant is packed. The result is in line with a previous reported work [START_REF] Mei | Plasma-assisted conversion of CO2 in a dielectric barrier discharge reactor: understanding the effect of packing materials[END_REF]. High local curvature, accumulation of charges and polarization effects could contribute to the field enhancement when porous catalysts are packed [START_REF] Neyts | Plasma Catalysis: Synergistic Effects at the Nanoscale[END_REF]140]. Packing catalyst with high dielectric constant is in favor of the formation of local filamentary discharges between the packed beads which could create more reactive species.

The local electric field strength near the catalyst surface, especially near the contact points, are strengthened [START_REF] Rodriguez-Gomez | Fe-MOF Materials as Precursors for the Catalytic Dehydrogenation of Isobutane[END_REF]. The addition of promoters also could suppress the adsorption strength and amount of H2. Figure 3-21 indicates that H2 adsorption capacity decreased markedly after the modification of K which is in line with the previous literatures. The desorption peak shift to a lower temperature on the K modified surface. According to the reported H2 chemisorption analysis [START_REF] Ramirez | Metal Organic Framework-Derived Iron Catalysts for the Direct Hydrogenation of CO2 to Short Chain Olefins[END_REF][START_REF] Han | Interfacing with Carbonaceous Potassium Promoters Boosts Catalytic CO2 Hydrogenation of Iron[END_REF] the amount of H2 adsorption decreased sharply after K promotion while the chemisorption of CO is improved. Owning to less surface H adsorbed on the catalysts, the value-added unsaturated hydrocarbons, formed in the plasma surrounding, would be less converted into the saturated hydrocarbons via the hydrogenation [START_REF] Wang | Fe-Cu Bimetallic Catalysts for Selective CO2 Hydrogenation to Olefin-Rich C2+ Hydrocarbons[END_REF]. Besides physical properties in plasma environment, chemical adsorptions properties also contribute to the hydrogenation and dehydrogenation process, consequently, they are analyzed by H2-TPD. Figure 3-21 and Figure 3-22 compare H2 adsorption capacities of catalysts. The result indicates that H2 adsorption capacity decreased markedly after the modification of K which is in line with the reported H2 chemisorption analysis in previous literatures [START_REF] Ramirez | Metal Organic Framework-Derived Iron Catalysts for the Direct Hydrogenation of CO2 to Short Chain Olefins[END_REF]. Two hydrogen adsorption peaks indicate different types of absorbing sites over catalyst. Weak H physisorption at lower temperature is ascribed to the H species adsorbed on the metallic iron surface while H2 chemisorption on carburized catalysts occurred at higher temperature. The weak and the strong hydrogen adsorption sites are also observed on carburized iron catalyst surface at lower (167 ℃) and higher (506 ℃) temperature respectively [START_REF] Tang | Study of K/Mn-MgO Supported Fe Catalysts with Fe(CO)5 and Fe(NO3)3 as Precursors for CO Hydrogenation to Light Alkenes[END_REF]. Brucker et. al., [START_REF] Brucker | Chemisorption and reaction of acetylene and ethylene on the α-Fe(100) clean iron surface[END_REF] claimed that the adsorption of hydrogen on a 'carbonated' Fe (100) surface led to the formation of CH species which was considered to be the most stable surface species on carburized catalysts. Due to the decomposition of surface CH, Zhang et. al., [START_REF] Zhang | Adsorption and reaction of CO and hydrogen on iron-based Fischer-Tropsch synthesis catalysts[END_REF] reported H2 desorption peaks on carburized catalysts appear at range of 250-500 ℃. The amount of H2 adsorption decreased sharply after K promotion while the chemisorption of CO is improved.

We surmise that the surface environment of modified catalysts with a lower H2 coverage is less conducive to hydrogenation but benefits to the CO conversion. Thus, due to the coverage of activity sites on the catalyst, K could tune the adsorption properties on the surface which restricts the hydrogenation of unsaturated hydrocarbons [START_REF] Wang | Fe-Cu Bimetallic Catalysts for Selective CO2 Hydrogenation to Olefin-Rich C2+ Hydrocarbons[END_REF]. Hence, under the coordination of surface K promoter which might restrict the hydrogenation rate, the fractions of unsaturated hydrocarbons are significantly increased. In a short summary, compared with SiO2, iron-based catalysts, notably alkali metal promoted catalysts, exhibit superior catalytic activity and selectivity in plasma environment. K-Fe/C-SiO2

is the optimized catalysts to produce added-value hydrocarbons and liquid oxygenates in plasma-catalytic conversion of CO2 and CH4. The addition of K is beneficial for the formation of unsaturated hydrocarbons and suppresses paraffins selectivity. Although the conversion for CH4 and CO2 are around 15% and 5% respectively, they could be improved by increasing input energy, optimizing catalysts and choosing an appropriate plasma. Those efforts are currently investigated from an industrial point of view. Considering the wide application of multi-stage reactor in chemical industry, it is possible to extend it to the plasma-catalytic technique. Hence, a multi-stage plasma-catalytic reactor would be a useful configuration in which unreacted feedstock from previous stage would feed to the next stage [START_REF] Sreethawong | Partial oxidation of methane with air for synthesis gas production in a multistage gliding arc discharge system[END_REF]. Each stage would be maintained at suitable conditions in order to produce added-value chemicals with high selectivity and the conversion rates reach the desired values after the last stage. The characteristics of packed catalysts, including their components, dielectric constants and their granule size, affect the performance of plasma-catalysts by changing electric field, types of discharge, physicochemical properties and the formation of hot spots in DBD reactor [START_REF] Liu | Review of Plasma-Assisted Catalysis for Selective Generation of Oxygenates from CO2 and CH4[END_REF].

Effect of granule size

Bogaerts et al, [START_REF] Wang | Streamer propagation in a packed bed plasma reactor for plasma catalysis applications[END_REF] reported that there were more contact points around smaller spheres and local electric field was also enhanced. Therefore, that resulted in more reactive plasma. As In order to interpret the connection of catalyst granule size and its performance, subsequently, the variation of plasma physical parameters caused by different particle size was elucidated using equivalent electrical circuit model. For comparing different granule sizes, the equivalent gap thicknesses of gas and catalysts are determined by the volume fractions after packing. The volume fraction of catalyst decreases from 0.69 to 0.56 when catalyst with a large size (2 mm) is packed, and such decreased catalyst volume results in the increasing of equivalent gap thicknesses of the gas. As shown in Table 3-3 and Figure 3-26, the mean electron energy and the electric field around the catalyst is strengthen when smaller catalyst is loaded. It results in an increase of unsaturated hydrocarbons selectivity due to a higher dehydrogenation rate. The catalyst with smaller size also provides more interface between catalyst and plasma which is beneficial for surface reactions.

Time-on-stream (TOS)

The stability and persistent activity of the catalysts make future industrial applications possible.

The chemical activity of the plasma environment for producing the desired intermediates

Plasma environment is at the same time favorable to production of intermediate active species playing an important role in heterogeneous chemistry but also to co-production of some powerful oxidants or reductants able to affect chemically or physically the catalytic surface. For this consideration, we carried out the catalyst's life-time test over 9 hours to ensure that catalyst keeps reactive when exposed to the plasma surrounding. We characterized these characteristics and demonstrated the promising application prospect of the selected catalyst by testing the timeon-stream (TOS) behavior. TOS behavior (Figure 3-27) shows that K-Fe/C-SiO2 catalyst exhibits decent catalytic stability in the plasma-catalytic conversion of CO2 and CH4 over 9

hours. The characterization of used catalysts exhibits that the structures and properties of catalysts are negligibly changed after plasma-catalytic conversion of CO2 and CH4. Catalyst keeps stable at typical reaction conditions with the CO2 conversion rate and CH4 conversion rate maintaining steadily between 4.08 -5.85% and 12.18 -14.32% respectively. However, the selectivity of H2, CO and hydrocarbons exhibits a variation with the time on stream. At the initial stage, the H/C ratio and U/S ratio in the gaseous products raise from 1.46 and 0.31 to 2.09 and 0.90 respectively in half an hour. Then, at the steady stage, they are gradually remained around 0.87 and 2.18 respectively. Based on the catalyst's TOS behavior and characterization, it is proved that K-Fe/C-SiO2 catalyst is beneficial for syngas production with desired H/C ratio and unsaturated hydrocarbons with considerable activity and stability.

Comparison

As shown in Table 3-4, the performance of plasma-catalytic conversion of CO2 and CH4 are compared (focusing on input power, conversion and products selectivity) to previous literatures.

In the most of reported works, it is essential to note that plasma-catalytic process seems not to be in favor of ethylene and added-value syngas having a suitable H/C ratio for FTS. But K promoted catalysts as explored in this work is able to directly synthesize unsaturated hydrocarbons, to increase H2/CO ratio and to enhance the formation of long-chain oxygenates which are more valuable and conducive to industrial applications. An improvement of C2H2/C2H4 distribution in the products was also achieved by plasmaassisted dry reforming of methane by using UiO-67 catalyst in [START_REF] Vakili | Plasma-assisted catalytic dry reforming of methane (DRM) over metal-organic frameworks (MOFs)-based catalysts[END_REF]. In this interesting work, the excited Argon initiates the dissociation of the reactants while UiO-67 catalyst tunes the products distribution by promotion of dehydrogenation reactions. The ratio of unsaturated hydrocarbons over saturated hydrocarbons went up to 1.29. Nonetheless, the utilization of Argon diluent and low concentration of reactants could more or less restrict the industrial applications.

In our work, a significant improvement of C2H2 and C2H4 selectivity is achieved without using expensive diluent and complex catalysts. Even though the result is not capable to be applied in the chemical industrial immediately, it does provide an attracting approach for future studying.

Reaction pathway

OES analysis

OES is used to elucidate the spectral characteristics of the homogenous reactions taking place in the plasma and the heterogeneous reactions taking place on the catalytic surface. Figure 3-28 shows that the main numerous activated species in the emission spectra of plasma-catalytic conversion of CO2 and CH4 includes CO2 + , CO, CH, C2 and Hα. As ground electrode is in air atmosphere, pattern (a) displays the emission spectra of air without catalyst and reactant at 6.0kV. In the background, N2 molecular emission is in the range of 300 -400 nm and there are no emission signals of CO2 and CH4. CO2 + (A 2 Π → X 2 Π) transition peaks observed at 353.2 nm indicate that CO2 is efficiently excited in non-thermal plasma. Notably, CO (b 3 Σ → a 3 Π)

and CO (B 1 Σ → A 1 Π) Angstrom bands are in high intensity, proving that CO radicals is one of the key intermediates in DBD plasma reactor which results in high selectivity of CO (Figure 3-15). Except for CO Angstrom bands, CH (A 2 ∆ → X 2 Π), C2 swan bands and Hα atomic lines driven from CH4 dissociation also are detected and further confirm the highly reactive plasma environment. A small peak centered around 777.5 nm is O atomic line due to the splitting of CO2. Whereas, the peaks, compared to plasma-catalytic conversion of CO2 and CH4 without catalysts, are less intense when K-Fe/C-SiO2 catalysts is packed and the spectrum becomes too weak to be measured when catalyst with 1mm diameter is packed. This phenomenon is consistent with previous works. It should be pointed out that reaction pathways are not only determined by homogeneous reactions in the plasma phase and heterogeneous reactions on the catalytic surface considered separately, but also by the plasma-catalyst synergy itself which impose the state of the interface, particularly presence of hotspots. The formation of hotspots (local high temperature surface regions) plays a role in the determination of products selectivity. Indeed, the thermal influence on the reaction paths over iron carbide and K has been widely demonstrated in thermal catalysis [START_REF] Chang | Relationship between Iron Carbide Phases (ε-Fe2C, Fe7C3, and χ-Fe5C2) and Catalytic Performances of Fe/SiO2 Fischer-Tropsch Catalysts[END_REF][START_REF] Li | Iron Carbides: Control Synthesis and Catalytic Applications in COx Hydrogenation and Electrochemical HER[END_REF][START_REF] Santos | Metal organic framework-mediated synthesis of highly active and stable Fischer-Tropsch catalysts[END_REF][START_REF] Ramirez | Metal Organic Framework-Derived Iron Catalysts for the Direct Hydrogenation of CO 2 to Short Chain Olefins[END_REF] and certainly plays a role on the products distribution tuning in non-thermal plasma catalysis. High temperature being generally favorable to high U/S products in thermocatalysis, the hotspots density of the medium could act on the nature of the major products.

From catalytic side, it is obvious that addition of promoters can modify adsorption rates and heterogeneous reactions, so changing the role playing by the catalyst in the reaction chain. In the plasma phase, free electrons obtain energy from the intense electric field, then excite and dissociate CO2 and CH4, initiating reactions which will end at the plasma-catalyst interface [START_REF] Li | Direct conversion of CO2 and CH4 into liquid chemicals by plasma-catalysis[END_REF].

Dissociation of CO2 will produce CO and O while CH4 will be dehydrogenated and will create H and CHx radicals. Then, the conversion can continue either in plasma phase or in adsorbed phase at the catalytic surface. For example, C2 dehydrogenation (ex. C2H6 + e → C2H4 + H2 + e via C2H5, and C2H4 + e → C2H2 + H2 + e) is promoted in gas phase [START_REF] Vakili | Plasma-assisted catalytic dry reforming of methane (DRM) over metal-organic frameworks (MOFs)-based catalysts[END_REF]. But it was also shown in a previous work that formation of C2H5 could serve as a key process for the formation of C3H8 and C2H6 by heterogeneous reactions [START_REF] Wang | Non-thermal plasma-enhanced dry reforming of methane and CO2 over Ce-promoted Ni/C catalysts[END_REF]. K-modified surface could decrease the chemisorption of H2 and so slow-down alkene and alkyne hydrogenation [START_REF] Ramirez | Metal Organic Framework-Derived Iron Catalysts for the Direct Hydrogenation of CO2 to Short Chain Olefins[END_REF]. In this case, the inhibition of the back reaction from C2H4 into C2H5 over the catalyst possibly leads to the increase of unsaturated hydrocarbons selectivity in the process. Hence, we propose two possible reasons which can explain the selective synthesis of unsaturated hydrocarbons: (i) formation of hotspots by local strengthening of electric field due to the influence of catalyst's surface state on the plasma properties which can boost relevant homogeneous reactions (e.g. dehydrogenation) thanks to a more intense energy input, (ii) competitive adsorption disfavoring the re-hydrogenation of the intermediate products at the surface of the K-modified catalyst.

For acetic acid, two possible mechanisms are widely studied [START_REF] Wang | One-Step Reforming of CO2 and CH4 into High-Value Liquid Chemicals and Fuels at Room Temperature by Plasma-Driven Catalysis[END_REF]. An acetyl radical (CH3CO) is formed by the combination of CO and CH3 radicals. Then it is further combined with OH to form CH3COOH. Meanwhile, the reaction of COOH and CH3 provides another route for the formation of acetic acid. CH3 radicals also contribute to the carbon prolongation of oxygenates.

That explains why the selectivity of organic acids increases with the CH4 content in reactant gas, suggesting that the CHx species are vital in the prolongation of C-C bond.

Challenges and future opportunities

In this first trial of promoted catalyst in plasma-catalysis, surface species proportion and its interactions with plasma remain incompletely described. In this thesis, characterization provided a preliminary indication of the valence and possible structure of iron on the catalyst surface. However, the technique used in this work cannot provide more comprehensive surface information, such as metal dispersion, influence of promoter on electronic structure, interface information of promoter with active phase, surface reaction, etc. Some reported works claim that promoter could act as an electron donor to increase the reducibility of Fe species and in turn favor the formation of Fe carbides. Such a behavior in plasma-catalytic process should be carefully studied in following works. We are also wondering, on the interface of promoter and active site, which reactions particularly dominate or contribute to the formation of value-added products together with plasma. In-situ Fourier transform infrared spectroscopy (FTIR) might play an important role in our future studies in order to investigate the insight of micro-reactions.

Summary

This pioneering work focused on the effect of K-promoter use in iron-based plasma-catalytic conversion of CO2 and CH4 by comparing experimental results obtained with and without Kpromoter to show the difference in terms of products and selectivities. For that, a non-thermal DBD-type plasma was generated in a fixed iron-based catalytic bed reactor. Syngas, C2 + hydrocarbons and C1 + oxygenates were obtained under soft conditions (i.e. near ambient conditions). The influence of operating parameters on the plasma-catalytic conversion was investigated. Moreover, a TOS study allowed to demonstrate the catalytic stability and an insitu OES analysis allowed to identify species generated in the plasma.

Comparing to SiO2 packed process, the formation of hydrocarbons and liquid oxygenates are indisputably enhanced by the use of iron-based catalysts, while the introduction of K-promoter clearly increases the selectivity of unsaturated hydrocarbons and liquid oxygenates because of the changes in structure and physicochemical properties. Notably, the unsaturated over saturated hydrocarbons ratio raises up to almost 1 over K-Fe/C-SiO2 catalyst which is a great advance compared to most of the related works published in the literature until now. Moreover, with this catalyst, the H2/CO ratio of the co-produced syngas reaches 2 which is a great value for its use in the Fischer-Tropsch process. To finish, the selectivity of liquid oxygenates reaches 21.76 % which is an encouraging result for future developments.

It is also important to mention that the catalysts keep stable over 9 hours of TOS, no decrease in conversion rate and products selectivity was observed. Architectures of catalysts and granule sizes significantly alter the discharge behavior and consequently affect the catalytic performance. So, an optimal applied voltage must be found to limit that. With the K-Fe/C-SiO2 catalyst in our reactor configuration, the optimum value is equal to 6.0 kV considering the conversion rate and selectivity variations. Last but not least, the variation of excited species concentration, adjusted by playing on the CO2 over CH4 ratio, changes a lot the distribution of products, as expected. At this stage, the contribution of hotspots is not clear in this plasmacatalytic process which globally operates near ambient conditions but the results of this work showed that their density could be a lever to orientate the process towards higher U/S products.

CHAPTER IV. NUMERICAL STUDY OF PLASMA CATALYTIC DRY REFORMAING Abstract

This chapter delegates to describe elementary reactions in plasma catalysis using a chemical

Introduction

The goal of this Chapter is to focus on the plasma chemistry during the power pulse and its afterglow from a numerical point of view. We propose an approach to understand micro kinetic mechanism for plasma-assisted dry reforming process which also has potential to be further applied in fluid simulations and industrial process simulations. In this study, we simulated a 0D-plasma reactor by using two softwares: BOLSIG+ and CHEMKIN. First, the density and production rate of key intermediates and products are analyzed. Subsequently, critical reaction channels are investigated to describe the reaction mechanisms on a micro time scale.

Additionally, we also investigate the role of catalysts by inducing the surface reaction mechanism into the model. We manage to compare the effect of surface reactions and calculate the reaction barriers via a numerical route. At current stage, unfortunately, it is hard to obtain a reliable activation energy using our experimental approach. Whereas the amount of research on the activation energy for plasma-catalysis is also limited.

Description and validation of the model

Table 4-1 shows the critical input information and values of our simulations. First, we simulate the plasma kinetics in gas phase in order to validate the model, elucidate the micro mechanism and plasma chemistry. In this case, a transient problem (0.001 s) is solved in the closed plasma reactor (1 cm 3 ) at atmospheric pressure and 300 K. The initial reactant fraction is set as 1/1 and the power deposition is shown in Figure 4-2. Subsequently, the activation energies of the reactants are calculated by using a 0D plasma PSR. For this simulation, considering the reality of the plasma reactor, the flow rate and power deposition are set as 50 mL/min and 2-3 W respectively while the other parameters remain the same value. Typically, two surface mechanisms on the Ni based catalysts are evaluated so that we could investigate the effects of catalysts in plasma surroundings. Dry reforming over Rh based catalyst is also studied especially.

INFORMATION NTP alone NTP alone

Ni Plasmacatalysis according to mechanism of [START_REF] Elsayed | Low temperature dry reforming of methane over Pt-Ni-Mg/ceria-zirconia catalysts[END_REF] Ni Plasmacatalysis according to mechanism of [START_REF] Delgado | Surface Reaction Kinetics of Steam-and CO2-Reforming as Well as Oxidation of Methane over Nickel-Based Catalysts[END_REF] Rh Plasmacatalysis according to mechanism of [START_REF] Karakaya | Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts[END_REF] Key A large number of independent micro-discharge filaments with a duration in the nanosecond range are reported in a published paper and the authors proposed a triangular pulse power deposition (one micro-discharge pulse) for their modelling of plasma with a sinusoidal waveform [START_REF] Snoeckx | Plasma-Based Dry Reforming: A Computational Study Ranging from the Nanoseconds to Seconds Time Scale[END_REF]. However, they simply assumed that the duration of triangular pulse is 30 ns.

Hence, according to the duration of current (60-80 ns), value of voltage (15 kV) and current ( 25A) from plasma experiment (Figure 4-1), a triangular pulse with a maximum power deposition of 3.0×10 5 Watts is applied in our simulation. In our modelling, the electron temperature dose not exhibit a similar triangular pulse because the heating rate of electrons in strong electric field is nonlinear. The electron temperature gradually increases in the plasma pulse and sharply decreases as soon as the input energy ends.

For typical cases, the maximum electron temperature could reach up to 3-4 eV. This value corresponds with that in experimental and numerical studies. Due to the delay of energy transformation to the electrons, molecules, radicals and other particles, the behavior of plasma catalysis in our modelling is complicated which will be discussed in the following sections.

Although only a single power pulse is simulated in current work, it is the foundation to investigated the cumulative effects for the future study. and etc.) are found to be the major oxygenates in our simulations. The CH3 radicals can also be further dissociated in which more H and CH2 species are generated.

Density of critical species

This process explains why H is so abundant in the plasma surroundings from another side.

Subsequently, those primary species participate in the formation of products or other intermediates. C2H3 and C2H5 are found to play a critical role in the simulations which will be discussed in detail in the following sections. Last but not least, H is able to combine with O, CO, CO2 and etc. to form OH, COOH and etc., whose density is not as high as primary species.

CHxO (x=1, 2, 3) radicals are considered to be key transient states which provide reaction path to produce oxygenates, water and syngas. Taking this into consideration, the number density of them are shown in Figure 4-4. The details of their density and formation pathways will be analyzed in the following sections. In the first 1.0×10 -10 s of plasma initiation, the electron temperature gradually increases but is below 1 eV, which is not enough to rapidly dissociate the reactants. At 1.5×10 -08 s, the electron temperature reaches the maximum, at this time the reaction speed of each element increases and their dissociation accelerates. At this stage, the main reaction type involved in the reaction is electron involved reactions. Interestingly, when the plasma stops, they rise first, which may be caused by the sudden drop in electron temperature. The decrease in electron temperature leads to a substantial decrease in the rate of CO2 and CH4 dissociation with electron participation. Then neutral reactions involving free radicals dominate, resulting in a slight decrease in the number density of reactants. Finally, due to the recombination reaction of many free radicals (CH3, H, CO, OH and etc.,), CO2 and CH4 are produced, and their number density increases and tends to be balance. Overall, the conversion of carbon dioxide is slightly lower than that of methane, which is consistent with the experimental data. The detailed generation and consumption channels of CO2 and CH4 at each stage will be described in the following section.

To better understand the micro process, as illustrated in Figure 4 coupling could happen, attribute to the formation of higher hydrocarbons. Besides, the combination of intermediates also contributes to the production of higher hydrocarbons which will be discussed in detail in the following parts. rates of critical reaction channels for the production of (a) CO2, (b) CH4 at 6.0×10 -08 s, 1.0×10 -07 s and 1.0×10 -05 s.

CH4 + CH → C2H4 + H (R-083) CH2 + CO2 → CH2O + CO (R-212) CH + CO2 → HCO + CO (R-225) CO2 + + CH4 → CH4 + + CO2 (R-600) CH4 + HCO2 + → CO2 + CH5 + (R-813) CH4 + + CO2 → HCO2 + + CH3 (R-818) CH5 + + CO2 → HCO2 + + CH4 (R-820)
After the termination of the discharge, the neutral-neutral reactions and the ion-neutral reactions gradually dominate due to the short life time of electrons and the aggregation of derivative intermediates. CO2 is activated and decomposed in the form of collision with free radicals or positive ions (e.g. R-212, R-225, R-818 and etc.), while CH4 also could be dissociated and form hydrocarbons through those routes (e.g. R-083, R-813, R-818 and etc.). Those reactions cause the formation of more intermediates, but at 1.0×10 -05 s, only neutral-neutral reactions are observed to exist in the simulated plasma process due to the decreased number density of ions.

This suggests that ions species are converted back to the neutral species.

Syngas production

Syngas is supposed to be the major product in the most of reported experimental works. The number density of H2 and CO as a function of time during the plasma pulse are illustrated in Among all of products, H2 and CO have the highest number density, reaching 6.9×10 15 cm -3 and 2.2×10 15 cm -3 , respectively, which are consistent with the experimental results. Snoecks et.al, [START_REF] Snoeckx | Plasma-Based Dry Reforming: A Computational Study Ranging from the Nanoseconds to Seconds Time Scale[END_REF] reported that the H2/CO ratio decreased from ~4 to ~1.4 since the termination of the plasma. This ratio change is attributed to the continued dissociation of CO2. The H2/CO ratio is flexibly adapted in DBD plasma by altering reactant ratio in the inlet gas mixture [START_REF] Bogaerts | Plasma based CO 2 and CH 4 conversion: A modeling perspective[END_REF]. This exhibits great advantages compared with traditional processes (such as CO2 reforming, partial oxidation and steam reforming) which produce syngas with H2/CO ratio ranging from 0.5-5.

Besides the reactant ratio, the presence of dilution gas, including N2 and Ar, clearly alert the products selectivity. The metastable nitrogen molecules (mainly N2(a′Σu-) and N2(a′Σu-))

increase the CO2 and CH4 conversion due to the collision between them while it also leads to produce more H2 in syngas slightly [START_REF] Wang | Streamer propagation in a packed bed plasma reactor for plasma catalysis applications[END_REF][START_REF] Wang | Modeling Plasma-based CO 2 and CH 4 Conversion in Mixtures with N 2 , O 2 , and H 2 O: The Bigger Plasma Chemistry Picture[END_REF][START_REF] Zhang | Dry reforming of methane in a nanosecond repetitively pulsed discharge: chemical kinetics modeling[END_REF]. The dissociation rate of CH4 is higher than that of CO2 when it collides with metastable nitrogen molecules in the plasma. The proper H2/CO ratio is benefit to producing certain product (e.g. a H2/CO ratio of 2.0 is appropriate for methanol synthesis) in subsequent Fisher-Tropsh synthesis [START_REF] Karakaya | Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts[END_REF].

The H2/CO ratio is simulated to be around 3.0, which might be overestimated, while it is larger than optimal ratio of Fisher-Tropsh synthesis [START_REF] Karakaya | Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts[END_REF]. The possible explanation is the insufficient further dissociation of CO2 in the afterglow due to the simulation environment.

In our simulations, syngas could be generated from various elementary reactions including the addition of CH3 and CO. This process and structure show great importance in the formation of higher oxygenates. More details of organic acid and alcohol producing will be introduced in section 4.3.4. It is important to note that CO + OH addition play a more important role in producing COOH radicals rather than CO2 + H addition due to its high reaction barrier (calculated to be 108 kJ/mol) [START_REF] Martini | Oxidation of CH4 by CO2 in a dielectric barrier discharge[END_REF]. The isomerization formyl radical-carboxyl radical could also contribute to the formation of COOH (HCO2 → COOH) whereas the barrier is calculated to be 79 kJ/mol. Hence, that pathway is also limited in generating COOH radicals and R-370 is considered to be the major formation way of COOH radical which play a role in further production of organic acid and other oxygen containing products.

CH3 + H2 → CH4 + H (R-127) C2H2 + H2 → C2H4 (R-141) CH3 + CO → CH3CO (R-197) H + CO → CHO (R-280) CO + OH → COOH (R-370)
Ion reactions are not negligible in the formation processes for H2 and CO. Similar to the electron impact dissociation of charged reactant (CH4 + ), the reactions of charged intermediates (such as CH3 + , CH2 + , CO + and etc.,) would provide vital formation channels for syngas. Charged H + species would transfer the energy to hydrocarbons (e.g. R-532, R-538 and etc.) and form charged transient states (such as C2H5 + , C2H3 + ) which play an important role in addition.

Together with the dissociation of CO + , charged hydrocarbons and CO are generated additionally.

e + CH3 + → CH + H2 (R-041) We notice that in the plasma pulse, the rate of C2H4 formation is the fastest, and it is the dominant product of C2 hydrocarbons in the plasma environment. However, after the power deposition, the number density of C2H4 decreases slightly around 1.0×10 -05 s due to hydrogenation reaction, while the number density of C2H6 keeps increasing. In the C2 hydrocarbons, saturated C2H6 is the main product while the selectivity of unsaturated hydrocarbons with higher value decreases. It is worth noting that the ratio of unsaturated hydrocarbon to saturated hydrocarbon (U/S) would gradually decrease in afterglow and finally reach 0.31606 in our modelling.

H + + C2H6 → C2H5 + + H2 (R-532) H + + C2H4 → C2H3 + + H2 (R-538) e + CO → 2 e + CO + (R-026) CO + + CH4 → CH4 + + CO (R-607) CO + + CH2 → CH2 + + CO (R-
Our modelling illustrates detailed formation and loss reactions of C2 hydrocarbons. As CH3 and CH2 radicals are mainly formed by the dissociation of CH4 and charged species, the attachment of them lead to higher hydrocarbons and produce H2 / H radical as side products (e.g. R-83, R-90, R-91, and etc.). De Bie et al., [START_REF] De Bie | Fluid Modeling of the Conversion of Methane into Higher Hydrocarbons in an Atmospheric Pressure Dielectric Barrier Discharge: Fluid Modeling of the Conversion of Methane …[END_REF] reported that the formation of C3H8 attributes to the recombination reactions of those radicals in addition.

CH4 + CH → C2H4 + H (R-083) CH3 + CH3 → C2H6 (R-090) CH3 + CH3 → C2H4 + H2 (R-091) CH3 + CH2 → C2H4 + H (R-092)
Besides, charged ions (e.g. CH3 + , CH2 + and etc.) should be point out because they are important in plasma reactions. As represent in following elementary reactions, charged CHx + could combine with another hydrocarbon species which correspondingly leads to the generation of charged higher species (e.g. C2H2 + , C2H4 + and etc.) and hydrogen products and intermediates in the meantime.

CH2 + + CH4 → C2H4 + + H2 (R-422) CH2 + + CH4 → C2H2 + + 2H2 (R-424) CH + + CH4 → C2H4 + + H (R-427)
Among various species in plasma surroundings, C2 intermediates, including C2H5, C2H3 and etc., attract our attention. As shown in Figure 4-10, it clearly indicates that time delay does exist between different species. First, the total production rates of C2H4, C2H6 and C2H2 increase in succession, then, their production terminate in the afterglow. Exceptionally, an extra peak of C2H6 production rate draws our attention which might indicate that C2H6 is formed via some neutral reaction channels during the afterglow. Between those two peaks, C2H5 production rate is found to exhibit a broad peak (1.0×10 -07 s) spanning the plasma pulse and afterglow. The possible explanation is that part of C2H4 is consumed via the hydrogenation and recombination which contribute to the formation of C2H5 specie and higher hydrocarbons [START_REF] Bogaerts | Plasma based CO 2 and CH 4 conversion: A modeling perspective[END_REF]. Subsequently, C2H5 specie will be further reduced to C2H6 which corresponds to the production peak in the afterglow. The processes (C2H4 → C2H5 → C2H6) restrict the selectivity of unsaturated hydrocarbons which is regarded to be more valuable. It is in good agreement with the results in literature. [START_REF] Bogaerts | Plasma based CO 2 and CH 4 conversion: A modeling perspective[END_REF][START_REF] De Bie | Fluid Modeling of the Conversion of Methane into Higher Hydrocarbons in an Atmospheric Pressure Dielectric Barrier Discharge: Fluid Modeling of the Conversion of Methane …[END_REF][START_REF] Aerts | Influence of Vibrational States on CO 2 Splitting by Dielectric Barrier Discharges[END_REF] Hence, some approaches, including the design of special catalysts, optimization of plasma input., could be investigated to achieve higher selectivity of unsaturated hydrocarbons by inhibiting the process mentioned. C2H5 could be produced both from radicals and charged species via electron impact reactions and neutral reactions (e.g. R-048, R-089, R-393, and etc.). Subsequently, two C2H5 radicals are converted into C2H4 and C2H6 via R-122 reaction. Such consumption route causes about 83% C2H6 formation according to De Bie's et al simulation [START_REF] De Bie | Fluid Modeling of the Conversion of Methane into Higher Hydrocarbons in an Atmospheric Pressure Dielectric Barrier Discharge: Fluid Modeling of the Conversion of Methane …[END_REF]. It is the dominant production process when compared with R-090 (recombination of two CH3 radicals) which is only responsible for around 11% of the C2H6 production. The explanation is that the overall averaged number density of CH3 radicals is significantly (three orders of magnitude) lower than the number density of C2H5 (about 7.0×10 20 m -3 ) in the plasma surroundings. Whereas, the rate coefficient of CH3 conversion reaction is about 5 orders of magnitude higher than that of C2H5 conversion reaction. Considering the square of the concentrations when the reaction rate of them are calculated, it is clear to point out that the formation rate via R-122 is higher than the reaction rate of R-090.

e + C2H6 + → C2H5 + H (R-048)

CH3 + CH3 → C2H5 + H (R-089) 2 C2H5 → C2H6 + C2H4 (R-122) CH5 + + C2H4 → C2H5 + + CH4 (R-393) C2H5 + + H → C2H4 + + H2 (R-464)
C2H2 + H → C2H3 (R-096) 2 C2H3 → C2H2 + C2H4 (R-133) C2H3 + H2 → C2H4 + H (R-138) CH5 + + C2H2 → C2H3 + + CH4 (R-394) C2H3 + + C2H6 → C2H5 + + C2H4 (R-470)
To investigate the insight of hydrocarbons formation, we subsequently analyze the detailed reaction progress of C2H4, C2H5 and C2H6 at different stages. The main reaction channels differ significantly in terms of time scale. In the plasma pulse, the combination between the derivative radicals not only dominates the formation reaction of C2H4 but also contribute to the formation of C2H6 (e.g. R-090, R-091, R-092 and etc.) [START_REF] De Bie | Fluid Modeling of the Conversion of Methane into Higher Hydrocarbons in an Atmospheric Pressure Dielectric Barrier Discharge: Fluid Modeling of the Conversion of Methane …[END_REF]. rates of critical reaction channels for the production of (a) C2H4, (b) C2H5 and (c) C2H6 at 6.0×10 -08 s, 1.0×10 -07 s and 1.0×10 -05 s.

After the power pulse, parts of those reaction rates are significantly reduced while others become more dominant. For instance, the reactions involving C2H4, the C2H4 hydrogenation becomes the dominant reaction at 1.0×10 -05 s and consequently a large amount of C2H5 radicals are produced in this duration. The crucial product, C2H4, are converted via neutral reactions (e.g. R-109, R-132 and etc.) rather than electron reactions (e.g. R-018 and etc.). This indicates that concentration of unsaturated hydrocarbons would shift to the chemical equilibrium instantly out of the plasma environment. Among them, the H attachment to C2H4 occupy the main route. Those hydrogen attachment (R-096 and R-132) reactions are in line with the analysis in the work reported by Bogaerts et, al [START_REF] Van Laer | Influence of Gap Size and Dielectric Constant of the Packing Material on the Plasma Behaviour in a Packed Bed DBD Reactor: A Fluid Modelling Study: Influence of Gap Size and Dielectric Constant…[END_REF] who claim that it is important for the loss processes for C2H2 and C2H4 leading to derived C2H3 and C2H5 radicals, respectively.

e + C2H4 → e + C2H3 + H (R-018)

CH2 + C2H4 → C3H6 (R-109) C2H4 + H → C2H5 (R-132)
Correspondingly, C2H5 further hydrogenation (e.g. R-127, R-128, R-130 and etc.) gradually becomes a major reaction path for the formation of C2H6 after the end of pulse and finally occupies a dominant position in the afterglow.

C2H5 + CH4 → C2H6 + CH3 (R-084) C2H5 + H2 → C2H6 + H (R-127) C2H5 + H → CH3 + CH3 (R-128) C2H5 + H → C2H6 (R-130)

Oxygenates production

Another vital kind of products which cannot be ignored is oxygenates. We mainly include the methanol, ethanol, formic acid, acetic acid, propionic acid, key intermediates and etc., in our modelling according to the findings in experimental studies. Figure 4-13 shows the number density of oxygenates as a function of time in one plasma pulse. Methanol is the major product and the number density of that reaches up to 2.58×10 13 cm -3 . The number density of ethanol exhibits a different behavior with that of methanol in which the density keeps on increasing even after the termination of the plasma. We assume that could be attributed to the neutral reactions of radicals remained in the afterglow. Similar phenomenon is also observed for the production of organic acid. The combination of COOH, CH3, H, CO, OH and etc., contribute to the final number density. The oxygenate radicals and ions, including O, CH3O, CH2O, CHO, CO2 + , H3O + , O -, OH -and etc., are formed in the plasma pulse and have a considerable lifetime in the afterglow. Snoeckx et al., [START_REF] Snoeckx | Plasma-Based Dry Reforming: A Computational Study Ranging from the Nanoseconds to Seconds Time Scale[END_REF] reported that, after the immediate formation in the plasma pulse, the number density of CH5 + , C2H5 + and OH -ions reached a steady state in a short time due to the competition of consumption and formation processed. As shown in Figure 4-4, the number density of CHO is significantly higher than CH3O due to the continuous reaction of H (e.g. R-278, R-280, R-387 and etc.,).

CH + CO2 → CHO + CO (R-225) H2 + CHO → CH2O + H (R-278) H + CH2O → CH3O (R-387)
According to the discussion in section 4.3.2, COOH radicals are obtained as a product of syngas.

It is worth mentioning the decomposing reaction (R-372) which would significantly influence the formation of formic acid, acetic acid, propanoic acid and even higher organic acids. The formation of formic acid is found to greatly rely on COOH radicals. The reaction rate of H and carboxyl radicals (R-371) is calculated to be 10 -10 molec -1 s -1 cm 3 [START_REF] Martini | Oxidation of CH4 by CO2 in a dielectric barrier discharge[END_REF]. In our model, we further include one more formation route (R-793) for formic acid.

H + COOH → HCOOH (R-371) COOH → OH + CO (R-372) OH + CH2O → HCOOH + H (R-793)
As reported by Martini et al., [START_REF] Martini | Oxidation of CH4 by CO2 in a dielectric barrier discharge[END_REF] three mechanisms attribute to the formation of acetic acid.

First, carbon monoxide (CO) could attach to a methyl radical (CH3) producing acetyl radical (CH3CO) which would further combine with a hydroxyl (OH). A small reaction barrier is required for the production of acetyl radical (R-197) while less energy is needed for the attachment of CH3CO and OH (R-375). Another formation pathway of acetic acid is corresponding to the addition of CH3 and CO2 and it requires higher reaction barrier (calculated to be 80.3 kJ/mol) when it is compared with the first route (calculated to be 23.8 kJ/mol)., Furthermore, acetoxyl CH3CO2 is considered to be unstable which limits the importance in the production of acetic acid. Last but not least, the coupling of methyl and carboxyl radicals could also lead to acetic acid. Nevertheless, according to the reported results, the energy barrier of this process is laborious to calculate and the order of the rate constant is estimated to be 10 -10 molec -1 s -1 cm 3 [START_REF] Martini | Oxidation of CH4 by CO2 in a dielectric barrier discharge[END_REF].

CH3 + CO → CH3CO (R-197) CH3 + COOH → CH3COOH (R-374) CH3CO + OH → CH3COOH (R-375)
For higher organic acid, to the best of our knowledge, there is few reports focusing on the propanoic acid formation in plasma catalysis. By analogy to the acetic acid formation, propanoic acid is also produced by three possible reaction pathways. CO could combine with, besides CH3, ethyl (C2H5) radical which is produced and essential in the process of hydrocarbons formation as discussed in previous section. The theoretical rate constant of this process is estimated to be 3.0×10 -16 molec -1 s -1 cm 3 . Direct addition of CO2 to C2H5 is also unfavorable for the production of propanoic acid. The reaction of C2H5 and COOH (R-377) occurs in the propanoic acid formation which requires low free energy barrier.

C2H5 + COOH → C2H5COOH (R-377) C2H5 + CO → C2H5CO (R-378) C2H5CO + OH → C2H5COOH (R-380)
We also noticed that, although acetic acid is predominant in the organic acid, methanol is the major product in the modelling. Those unrelated results could be possibly caused by single micro-discharge pulse, simplification of power, incomplete kinetics and etc. Obtaining the results which are consistent with experimental results and fully describing the reaction process are the goals of numerical simulation that we should pursuit.

Effects of plasma parameters

In this section, we investigate the effect of plasma parameters (the peak power of the pulse and the duration of the pulse) on the modelling results. Those parameters are supposed to affect the energy of electrons and electrical field strength in the plasma surrounding which consequently contribute to the conversion rates and products selectivity.

Effects of peak power of the pulse

As electron energy is found to significantly affect the products selectivity in the experimental work [START_REF] Snoeckx | Plasma-Based Dry Reforming: A Computational Study Ranging from the Nanoseconds to Seconds Time Scale[END_REF][START_REF] Bogaerts | Plasma based CO 2 and CH 4 conversion: A modeling perspective[END_REF][START_REF] Wang | Modeling Plasma-based CO 2 and CH 4 Conversion in Mixtures with N 2 , O 2 , and H 2 O: The Bigger Plasma Chemistry Picture[END_REF][START_REF] Zhang | Dry reforming of methane in a nanosecond repetitively pulsed discharge: chemical kinetics modeling[END_REF][START_REF] De Bie | Fluid Modeling of the Conversion of Methane into Higher Hydrocarbons in an Atmospheric Pressure Dielectric Barrier Discharge: Fluid Modeling of the Conversion of Methane …[END_REF][START_REF] Aerts | Influence of Vibrational States on CO 2 Splitting by Dielectric Barrier Discharges[END_REF][START_REF] Heijkers | Nanosecond Pulsed Discharge for CO 2 Conversion: Kinetic Modeling To Elucidate the Chemistry and Improve the Performance[END_REF], it is essential to evaluate the effects of power deposition, which could somehow determine the electron energy, on the products distribution. In this numerical work, different peak power and duration of the pulse are studied so that we could focus on analyzing the kinetics of C2 hydrocarbons production in the plasma pulse (e.g. transient U/S ratio are compared in Figure 4-14a). It is clear to claim that U/S ratio exhibits a higher peak when we apply a greater power during the pulse. This indicates that higher electron temperature (shown in Figure 4-14b) promotes the formation of C2H4 and C2H2 due to enhanced dehydrogenations (e.g. C2H6 + e → C2H4 + H2 + e and C2H4 + e → C2H2 + H2 + e) in stronger plasma discharges. From this point of view, it is reasonable to conclude that enhanced plasma discharges (described in section 3.1.2) around the packed catalysts indeed play a critical role in the formation of more valuable unsaturated hydrocarbons. U/S ratio sharply decreased as soon as the termination of the plasma, when the non-equilibrium plasma environment disappears, owing to the chemical equilibrium shift. The value of U/S ratio is around 0.3 at 1.0×10 -05 s.

Although the disparity of this value is ignorable, it is necessary to emphasize that a single plasma pulse cannot exhaustively elucidate the performance of the plasma catalysis, whereas, accumulation effects should also be carefully considered. We plot the number density of reactants with different duration of the pulse power deposition as a function of time in Figure 4-18. It is clear to find that the dissociation of CO2 and CH4 are more and more pronounced when longer duration of the plasma pulse is applied, which could be attributed to a more continuous and efficient electron impact dissociation of reactants during the plasma pules. In the afterglow, the dissociated reactants form again due to the recombination reactions which are discussed in previous section. CO2 number in the afterglow density continues to decrease owning to high electron temperature in the beginning of afterglow. For the syngas production, we analyze the effects of plasma pulse duration on the number density of CO and H2 in Figure 4-18. General number density of CO and H2 are increased with a more continuous and efficient electron impact dissociation of reactants in longer plasma pulse

(a) (b) (c) (d) (e) (f)
duration. Interestingly, the number density of CO increases more drastically than H2 number density due to the continues dissociation of CO2 in the afterglow. On the other hand, this is also attributed to the electron temperatures during the afterglow.

Figure 4-20 presents the H2/CO ratio during the plasma pulse and afterglow. The H2/CO ratio is estimated to be 15 -18 during the plasma pulse when the duration of the pulse power deposition is set as 40 ns -200 ns. Nevertheless, the ratio decreases as soon as plasma discharges terminated due to further CO2 dissociation and CO formation during the afterglow. The final ratios drop significantly during the afterglow due to production of CO.

Finally, Figure 4-21 illustrates the number density of hydrocarbons and the mole ratio of unsaturated hydrocarbons to saturated hydrocarbons in the plasma pulse and afterglow. The number density of C2 hydrocarbons in all of the cases are similar (Number density: C2H6 > C2H4 > C2H2) as a function of time while the pulse duration has significant influence on the C2H6 density. This could be attribute to the delay of neutral reactions in the afterglow. In Figure 4-21a, the formation pathway of C2H6 (via neutral reactions) remain negligible after the end of the pulse (40 ns) which leads to temporarily decrease after the termination of the plasma pulse.

This decrease becomes indistinctive when longer discharges duration is applied. As shown in Figure 4-21e, the neutral reactions dominate the formation of C2H6 in the plasma afterglow as soon as plasma pulse stop (200 ns).

Surface mechanism

According to the calculated enthalpy of formation and gas phase thermochemistry data from NIST Chemistry WebBook, the enthalpy of formation of key intermediates, including C2H5 (119 kJ mol -1 ), C2H3 (299 kJ mol -1 ), CH3 (146 kJ mol -1 ), O (249 kJ mol -1 ), are higher than that of products, such as CO (-110 kJ mol -1 ), C2H6 (-84 kJ mol -1 ), C2H4 (52 kJ mol -1 ). Thus, the formation of those transient structures requires higher energy input which would be supplied by energetic electrons in non-equilibrium plasma.

The activation energy of the plasma-assisted dry reforming without catalyst and over Rh-based catalyst are compared in Figure 4-22. The plasma process exhibits a typical Arrhenius behavior and the activation energy of CO2 and CH4 are estimated to be 55.41 kJ mol -1 and 58.65 kJ mol - 1 . It is critical to mention that the activation energy decreases after introducing the surface reactions on the Rh sites. The activation energy of CO2 and CH4 decrease to 42.97 kJ mol -1 and 48.65 kJ mol -1 respectively due to the interactions of plasma and catalysts. Compared with plasma process, the activation energy decreases up to 22.45% and 16.90% respectively. The catalysts provide an alternative route in which less activation energy is required (Figure 45678910111213141516171819202122).

The adsorbed species, such as CO2(s) and CH4(s), are activated by other species adsorbed on active sites [START_REF] Ding | Metalbased electrocatalytic conversion of CO2 to formic acid/formate[END_REF]:

CO2(s) + H(s) → COOH(s) (RS-26) CO2(s) + H(s) → CO(s) + OH(s) (RS-30) CH4(s) + O(s) → CH3(s) + OH(s) (RS-41)
As a result, new transition species and states, such as COOH(s), CH3(s) and etc., are produced via this novel approach which require less energy. Chain decompositions of CH4 exist and produce adsorbed intermediates:

CH3(s) + Rh(s) → CH2(s) + H(s) (RS-35) CH2(s) + Rh(s) → CH(s) + H(s) (RS-37)
We noticed that, besides the direct dissociation of CO2(s), combination of CO2(s) and H(s) also is significant while formed COOH(s) will further decompose.

COOH(s) + Rh(s) → CO(s) + OH(s) (RS-24)

COOH(s) + H(s) → CO(s) + H2O(s) (RS-27)

This species could also participate in the formation of organic acid, which is not described in our kinetic model, under certain conditions.

In dry reforming, the CH4 chemisorption (RS-06) is considered to be a rate determining step.

[108] It is reported that the activation energy would decrease from 91 kJ/mol to 44.7 kJ/mol (in 100 kHz DBD) due to the state-specific reactivity of vibrationally excited CH4. In their plasma assisted CH4 reforming, vibrationally excited CH4 in streamers contribute a lot to the enhancement of CH4 chemisorption which is considered to be the rate-determining step [START_REF] Ashford | Plasma-catalytic CO2 hydrogenation to ethane in a dielectric barrier discharge reactor[END_REF].

RS-06 and RS-33 cooperatively simulate such a dissociative chemisorption of CH4 (CH4 → CH3* + H*), which is corresponding to the increase of efficiency. Besides, the rate of adsorption, desorption, surface diffusion processes and elementary reactions on different surfaces also affects the apparent activation energy. Comparing different surface mechanisms on Rh and Ni sites, it is worthy to mention that the parameters in the Arrhenius equations for CO2 and CH4 adsorption processes (RS-06 and RS-04) show great difference which further affects the reaction progress. The sensitivity analysis of methane dry reforming shows that methane adsorption (RS-06) and desorption (RS-12) reactions are the most sensitive reactions [START_REF] Delgado | Surface Reaction Kinetics of Steam-and CO2-Reforming as Well as Oxidation of Methane over Nickel-Based Catalysts[END_REF]. There is another discussion in thermal-catalytic modelling. Eley-Rideal reactions play an important role in products formation. Nakamura et al., [START_REF] Nakamura | Role of support in reforming of CH4 with CO2 over Rh catalysts[END_REF] reported that H2O reacts with adsorbed CH2 species and forms adsorbed CO and H2 through RER-01. On the other side, gas phase CO2 could also collide with an adsorbed H and form CO and adsorbed OH species (RER-02) [START_REF] Erdohelyi | Activation of CH4 and Its Reaction with CO2 over Supported Rh Catalysts[END_REF]. Mark and Maier further proposed an alternative path for the formation of CO in which gas phase CO2 reacts with surface carbon species (RER-03) [START_REF] Mark | CO2-Reforming of Methane on Supported Rh and Ir Catalysts[END_REF]. With the help of fundamental study of surface reactions (to obtain relevant kinetic data), we will understand those Eley-Rideal reactions more globally in the future modelling of plasma-catalysis. The ratio of adsorbed H(s) to CO(s) is estimated to be 3.55. On the other hand, the higher adsorption amount of CH3(s) also attribute to the high stability compared with adsorbed CH2(s) or absorbed CH(s) [START_REF] Pan | Numerical modeling and mechanism investigation of nanosecond-pulsed DBD plasma-catalytic CH$\less$sub$\greater$4$\less$/sub$\greater$ dry reforming[END_REF]. Adsorbed O(s) is one of the dominant species on the surface of the catalysts and is able to participate in the formation of other oxygenates (e.g.

OH(s)

). H2O(s), the most abundant side-products, is formed via the further combination of OH(s) and H(s). Hence, deeply understanding of the surface reactions could benefit to the designing of catalysts, suppressing the side reactions and increasing the atomic utilization rate. Meanwhile, H(s) would also participate in other useful reactions except for the formation of water. Similar to the gas phase reaction, the combination of adsorbed CO2(s) and H(s) leads to the production of COOH(s) on the active sites. This also explains why the formation of formic acid and acetic acid, which are formed via the reactions of COOH, H, CH3 and etc., are enhanced when using metallic catalysts as packed materials.

The reaction pathway (at 6.0×10 -08 s) of surface species are generated by CKEMKIN-pro post analyzer and shown in Figure 4

Combination of numerical and experimental studies

In the last, we were wondering whether the model we have established can have more interesting applications in practical applications, such as predicting, at least roughly, the change of product (hydrocarbons) distribution caused by catalysts in plasma catalysis. Therefore, in this section, we use the previously developed model to first obtain the influence of the catalyst on the product distribution through theoretical simulation and then verify the results of the numerical simulation by combining the experimental results.

We chose SAPO-34 as the catalyst in the simulation. The reason why we chose SAPO-34 is that Fe-based catalysts, in general, contain other components (such as promoters, supports). The complexity of catalysts' structure and synthetic methods collaboratively result in a numerous of diverse kinetics. On the other hand, SAPO-34 has a determined framework structure and it is a widely applied catalyst in producing C2H4 so that we can find reliable surface reaction mechanism and kinetic data from literature. The contribution of surface reactions (in another word, catalysts) to product formation, is described by varying the specific surface area in the calculation input data. Then, predicted products selectivity over catalysts is tested by using our plasma bench (described in Chapter II).

Methods

In this simulation, the surface mechanism on SAPO- (RS-67)

The simulation methods are described exhaustively in section 2.2 and section 4.2. In order to evaluate the role of catalysts and the effects of surface reactions on products selectivity, we introduce reaction mechanism on SAPO-34 surface into our model. The variation of surface area of catalyst in this modelling represents the amount of catalysts and their contribution to overall reaction processes. Hence, as shown in Table 4-4, 10 cases of simulations (with different surface area range from 60 cm -2 to 600 cm -2 ) are conducted while other simulation parameters are the same as that in plasma alone simulations in Table 4-

Experimental verification

Figure 4-26 shows the ratio of unsaturated hydrocarbons to saturated hydrocarbons produced over SAPO-34 catalyst and in plasma-only process. In the plasma only process, the selectivity of C2H6 is found to be predominant, at least one order of magnitude higher than others, in gaseous products. U/S ratio is calculated to be around 0.05 which is not affected by variation of input power. Strongly reductive atmosphere and existence of H2 and H radicals in plasma are benefit to C-C bonds formation.

Attractively, that ratio becomes strongly dependant on the plasma power when catalyst is applied. For one reason, catalyst greatly influence the formation and propagation of plasma in the gas phase and on the surface. This indicates the importance of surface ionization wave and micro discharges. On the catalyst side, surface reactions, especially MTO reactions on the active sites and pores, also benefit to increase the number of unsaturated hydrocarbons. MTO reaction is considered to be one of the most critical approach for olefins production and hydrocarbons pool mechanism is a widely accepted pathway for that reaction [START_REF] Wang | Mechanistic investigations of the methanol-to-olefin (MTO) process on acidic zeolite catalysts by in situ solid-state NMR spectroscopy[END_REF][START_REF] Wei | Direct observation of induction period of MTO process with consecutive pulse reaction system[END_REF][START_REF] Dahl | On the reaction mechanism for propene formation in the MTO reaction over SAPO-34[END_REF][START_REF] Sun | On reaction pathways in the conversion of methanol to hydrocarbons on HZSM-5[END_REF]. Extra C2H4 and C2H2 are produced and remained in the products when compared with plasma only process. With higher plasma power, in other words stronger plasma effect on total process, more unsaturated hydrocarbons are converted into saturated hydrocarbons. U/S ratio drops from 0.25 to less than 0.05 when applying 210 -270 W. This phenomenon also indicates that two conflict effects together lead to products selectivity in plasma catalysis.

Summary

In this numerical work, we investigated the kinetic aspect of plasma-catalytic dry reforming via a zero-dimension plasma-catalytic model in a batch reactor. A kinetic method for evaluating the role of the electrons and the catalyst in this plasma-catalytic process is proposed. The number densities of reactants, products and key intermediates were calculated during one plasma pulse. For C2 hydrocarbons, in agreement with previous published experimental results [START_REF] Li | Direct conversion of CO2 and CH4 into liquid chemicals by plasma-catalysis[END_REF], C2H6 was found to be the main product. The ratio of unsaturated over saturated hydrocarbons first increases and then decreases in the plasma pulse, and finally returns to an equilibrium value. This could explain the limitation of this ratio observed in the experimental studies. It may be related to two successive processes: a dehydrogenation during the plasma phase followed by a re-hydrogenation after plasma extinction. In this frame, we observed that C2H5, an intermediate, plays an important role in the plasma-catalytic reduction of C2H4 to C2H6.

The detailed reaction pathway was investigated via time scale analysis of reaction rates of important elementary reactions. owing to the MTO reactions on catalysts surface. On parallel, experimental results showed that the ratio of unsaturated hydrocarbons to saturated hydrocarbons produced over SAPO-34 catalyst was calculated to be around 0.25 which was affected by variation of input power.

However, this first complete study is based on several approximations to simplify the model.

The main one is the consideration of a partial plasma-catalytic coupling due to the lack of experimental data. In future, the model will be extended to a full plasma-catalytic coupling supported by new experimental data in order to get closer to the reality. Other crucial factors such as accumulation effect of charges and species should not be ignored either.

CHAPTER V. CONCLUSIONS AND PERSPECTIVES

Conclusions

Direct synthesis of value-added products

This work developed a feasible approach for producing value-added products directly via plasma assisted dry reforming over promoted Fe-based catalysts. This section summarizes the experimental achievement of this thesis which is delegated to develop a potentially way to substitute traditional petrol resources and produce renewable chemicals. a) By using simple sol-gel method in one-pot, a series of Fe-based and modified Fe-based nanocatalysts were successfully synthesized. Citric acid and iron (III) formed a stable complex in solution which prevented the aggregation of Fe particles during calcination. Our preparation method is able to produce Fe nanoparticles (2-4 nm) and avoid using expensive MOF mediated.

Regarding the porous structures in catalysts, TEOS, a common silica source, was applied to prepare mesoporous SiO2 support with specific areas around 300-600 m 2 /g. Such high dispersed nanoparticles exhibited high activity, stability and considerable products selectivity in plasma catalysis.

The introduction of K promoters significantly changed surface properties of catalysts. For Fe-SiO2 catalyst, mesoporous structures were constructed and few of macro-pore was observed while more macro-porous structures formed due to the addition of K. As a result, the specific surface area decreased tremendously in K-Fe-SiO2 catalyst. Whereas, promoter in K-Fe/C-SiO2 catalysts didn't significantly affect surface structures and high surface area were maintained consequently. From EDS results, it was clear to find a form of aggregation in K-Fe-SiO2 catalysts and these large clusters would lead to a substantial surface modification which inevitably influenced the catalytic performance. On the contrary, no aggregated large clusters were observed which indicated the well dispersion of promoters in the catalyst. This result also explained the slight decrease of surface areas for K-Fe/C-SiO2 catalyst compared to K-Fe-SiO2 catalyst. The well dispersed promoters and well sustained mesoporous structures together lead to the special performance in plasma assisted dry reforming.

b) Optimized catalysts were applied to convert CO2 and CH4 and achieved direct synthesis of high value-added organic products in a packed DBD plasma reactor. The formation of hydrocarbons and liquid oxygenates were enhanced in plasma DBD reactor over iron-based catalysts when it was compared with SiO2 packed process. The introduction of K clearly increased the selectivity of higher hydrocarbons and liquid oxygenates due to the changes of structure and physicochemical properties. Notably, the unsaturated over saturated hydrocarbons ratio raised up to 0.9 over K-Fe/C-SiO2 catalyst which was much higher than in previous works (U/S=0.037). Moreover, H2/CO ratio in the co-produced syngas also rise from 0.82 in the previous works to 2 in this work. From its side, the selectivity of liquid oxygenates selectivity reached 21.76 % which could be an encouraging result for future developments.

Regarding the stability of catalyst, it kept stable over 9 hours of TOS without the decrease in conversion and products selectivity. Architectures of catalysts and granule sizes significantly alter the discharge behavior and affect catalytic performance consequently at the same time. So, an optimal applied voltage was found equal to 6.0 kV with K-Fe/C-SiO2 catalyst in consideration of conversion, selectivity, and efficiency. Last but not least, the variation of excited species concentration, adjusted by using reactant gas with different CO2 to CH4 ratio, particularly changed the distribution of products. c) We conducted various characterizations, including but not limited TPD, for a better understanding of catalyst structure and properties, then explained the relationship between its structure and performance. The foremost chemical environment on catalyst surface were characterized by CO2-TPD and H2-TPD. H2 adsorption capacity was tested by H2-TPD method.

The result indicated that H2 adsorption capacity decreased markedly after the modification of K which lead to a lower H2 coverage on the surface. Hence, such lower H2 adsorption amount was less conducive to hydrogenation but benefit to the CO conversion. It is reasonable to conclude that K could tune the adsorption properties on the surface which restricted the hydrogenation of unsaturated hydrocarbons. Under the coordination of surface K promoter which might restrict the hydrogenation rate, the fractions of unsaturated hydrocarbons were significantly increased. CO2-TPD result showed that the introduction of K promoters could lower the desorption peak of CO2 since K promoters covered the surface basicity of catalysts.

The decreased surface basicity of Fe/C catalysts corresponded to the slight decrease of conversion.

d) Average electric field strength, reduced electric field (E/N) and mean electron energy were calculated by using an equivalent electrical circuit model. Packed catalysts enhanced the electric field strength of the CO2 and CH4 gas mixture discharge. More notable enhancement of the electric field strength was observed when catalyst with a higher dielectric constant was packed. Packing catalyst with high dielectric constant was in favor of the formation of local filamentary discharges between the packed beads which could create more reactive species.

The local electric field strength near the catalyst surface, especially near the contact points, were strengthened. The configuration of catalyst structure, such as different granule sizes, played a vital role as well. The mean electron energy and the electric field around the catalyst was strengthen when smaller catalyst is loaded and it would also provide more interface between catalyst and plasma which is beneficial for surface reactions. This calculation enabled us to gain a deeper understanding of plasma effect on products selectivity in direct synthesis.

e) Taken the experimental and simulation results into consideration, we proposed and verified possible catalytic reaction pathways. Reaction pathways are not only determined by homogeneous reactions in the plasma phase and heterogeneous reactions on the catalytic surface considered separately, but also proved by the plasma-catalyst synergy itself which impose the state of the interface (presence of hotspots, etc.). C2H5 was recognized to be key intermediate and the inhibition of the back reaction from C2H4 into C2H5 over the catalyst possibly leads to the increase of unsaturated hydrocarbons selectivity. We hope this could provide a theoretical basis for subsequent research.

Establishment of kinetic model

The second achievement of this thesis is the establishment of kinetic model, combining reported elementary reactions and chemical kinetics simulator for plasma-catalytic DRM. introduced in validated model. The selectivity of C2H4 gradually exceeded C2H6 owing to the MTO reactions on catalysts surface. On parallel, experimental results showed that the ratio of unsaturated hydrocarbons to saturated hydrocarbons produced over SAPO-34 catalyst was calculated to be around 0.25 which was affect by variation of input power. On the contrary, in the plasma only process, the selectivity of C2H6 is found to be predominant, at least one order of magnitude higher than others, in gaseous products due to the reductive atmosphere and existence of H2 and H radicals in plasma.

In this part, we established the kinetic model of the plasma catalytic process which helps us to describe the plasma catalytic process from a computational chemistry point of view. This kinetic model also provided a theoretical tool for precise synthesis of catalysts and efficient utilization of greenhouse gases.

Numerical research

In current simulations, some critical assumptions and simplification were included so that some transportation phenomenon were ignored. Kinetics reported in this model is still imperfect and could partially simulate plasma catalysis. Nevertheless, plasma-induced highly reactive molecules are important aspects of plasma-catalysis, including the adsorption and surface reactions over active sites. To investigate their functions, not only reliable reaction mechanisms but also precise kinetic data are required. Although, useful basic information of dissociative adsorption and energy barriers are obtained on a given surface (e.g. clean Ni (100) surface) by using cutting-edge technology. Unfortunately, to our best knowledge, there is still lack of studies on the kinetic parameters (A, Ea, and etc.) of adsorption and reaction of excited particles 126 in complex systems (over catalysts' surface). Alternatively, these kinetic parameters could also be simulated by computational chemistry (e.g. DFT), however, this is not the main objective of this paper. We believe that, with further research on plasma-catalysis in the future, it is quite exciting to obtain more detailed kinetic parameters (surface behavior of hot molecules) for modelling which allows us to optimize our model.

In the future work, simulations could also be used for evaluating plasma reaction on a longtime scale which provides more realistic information. High-dimensional simulations (2D modelling and 3D modelling) will take mass transfer and energy transfer into consideration to establish a more persuasive model. Development of detailed mechanism for surface reactions will be a promising direction as well.

LIST OF FIGURES

APPENDIX

  d'activation du CO2 et du CH4 est estimée grâce à notre travail numérique à 55,41 kJ.mol -1 et 58,65 kJ.mol -1 . Second résultat important à retenir est que ce comportement cinétique est conservé en plasmocatalyse et que l'énergie d'activation diminue par inclusion de sites catalytiques (ici sites Rh). L'énergie d'activation du CO2 à 42,97 kJ.mol -1 tandis que celle du CH4 diminue à 48,65 kJ.mol -1 , ce qui correspond à une diminution respectivement de 22,45% et 16,90%. Le catalyseur joue donc un rôle essentiel pour améliorer l'énergétique de la conversion. Il est essentiel de comparer nos valeurs simulées d'énergie d'activation avec des valeurs expérimentales. Comme il n'a pas été possible de déterminer les énergies d'activation par voie expérimentale faute d'équipement adéquat, nous avons comparé nos résultats de simulation avec des énergies d'activation expérimentales disponibles dans la littérature. En DRM thermocatalytique par exemple, l'énergie d'activation est comprise selon les travaux et les catalyseurs utilisés entre 62,2 kJ.mol -1 et 108,9 kJ.mol -1 pour le CO2 et entre 65,5 kJ.mol -1 et 87,8 kJ.mol -1 pour le CH4. Les barrières de réaction diminuent remarquablement en plasmocatalyse. Elles sont en effet de 6,1 kJ.mol -1 à 29,5 kJ.mol -1 pour le CO2 et de 32,6 kJ.mol - l'ajout d'un catalyseur améliore le DRM par plasma. En guise d'ouverture, nous proposons en dernière partie de ce chapitre, sur la base des travaux précédents, de combiner une étude expérimentale et une étude numérique dans les mêmes XXIII ` conditions plasmocatalytiques. Nous faisons alors le choix d'un plasma impulsionnel nanosecondes et d'un catalyseur zéolite microporeux SAPO-34 favorable à la formation d'hydrocarbures insaturés en thermocatalyse tout comme nos catalyseurs ferreux dopés en K. En adaptant la chimie hétérogène de notre modèle à ce catalyseur dont les mécanismes réactionnels et données cinétiques sont documentés dans la littérature, nous tentons d'avancer dans la compréhension du rôle joué par le catalyseur en comparant nos résultats expérimentaux et numériques. Dans ce travail numérique, nous faisons en particulier varier la surface spécifique du catalyseur. La densité des produits principaux et des structures transitoires liées aux réactions de surface est suivie dans le temps. Parmi les résultats numériques importants, nous retiendrons que sans catalyseur l'hydrocarbure majeur est C2H6 tandis qu'avec catalyseur SAPO-34 à haute surface spécifique l'hydrocarbure majeur est C2H4. Les résultats expérimentaux vont dans le même sens confirmant cette tendance. Sans catalyseur, C2H6 est dominant avec un rapport U/S d'environ 0.05 tandis qu'avec catalyseur SAPO-34 de surface spécifique 550 m 2 /g, C2H6 et C2H4 passent aux mêmes ordres de grandeur avec un rapport U/S atteignant 0.25. Le résultat le plus intéressant de cette étude d'ouverture est sans aucun doute le fait que le rapport U/S ne dépend pas de la puissance électrique du plasma sans catalyseur mais dépend fortement d'elle avec catalyseur. Ce résultat est une preuve qu'il existe un réel effet de synergie entre le plasma et le catalyseur et que cette synergie est favorable au réglage du rapport U/S des hydrocarbures co-produits par DRM assisté par plasma. Chapitre V. Conclusion et perspectives Ce travail, dont l'objectif global est de proposer et d'étudier une voie de conversion chimique du CO2 en produits à plus haute valeur, s'est concentré sur le DRM plasmocatalytique avec pour but spécifique de trouver le moyen d'augmenter la part des hydrocarbures insaturés coproduits. Un réacteur plasmocatalytique à barrière diélectrique intégrant un lit catalytique a été mis au point et expérimentalement testé pour la conversion de CO2 et CH4. Sur la base de travaux antérieurs et d'une revue bibliographique préliminaire, le choix d'un catalyseur de formule (K-)Fe(/C)-SiO2 a été fait pour son aptitude à aider à la production, en plus du gaz de synthèse, de co-produits présentant un intérêt chimique. Il a ainsi été expérimentalement démontré au cours de ce travail que le dopage K favorisait clairement la formation d'hydrocarbures insaturés, molécules de base pour la chimie des polymères. En parallèle, un modèle numérique de cinétique chimique, incluant chimie homogène et chimie hétérogène, XXIV ` dans lequel les électrons du plasma possèdent une distribution résolue en énergie avec un solveur spécifique, a été élaboré sous Chemkin et appliqué pour simuler le DRM plasmocatalytique. Un grand nombre de simulations a été effectué permettant de mener une étude paramétrique sur notamment la puissance plasma, la composition du mélange réactif, la nature du catalyseur et la surface spécifique du catalyseur. De cette étude numérique découle un résultat important qui est l'importance du processus de déshydrogénation des molécules dans les phases homogène et hétérogène pour la détermination de la composition finale du mélange produit. Des pistes sont ainsi discutées pour la maitriser. Finalement, une meilleure compréhension du mécanisme chimique susceptible de conduire à la production d'hydrocarbures insaturés dans ces conditions est acquise par comparaison numérique/expérimentale. Pour valider le modèle, la thèse finit par un cas d'application précis du modèle avec un catalyseur SAPO-34 dont la cinétique en DRM est bien documentée dans la littérature et dont l'effet attendu est proche de celui de notre catalyseur. Des expérimentations ont alors été faites dans les mêmes conditions avec ce matériau pour comparaison. Les résultats corroborent et confirment le rôle essentiel du catalyseur pour accroitre le rapport U/S des hydrocarbures co-produits. Ce cas d'application a également permis d'apporter une preuve qu'une synergie à proprement parler se met en place entre plasma et catalyseur car si le rapport U/S ne dépend pas de la puissance du plasma sans catalyseur, il y dépend fortement avec catalyseur. Enfin, des perspectives sont proposées à l'issue de ce travail pour améliorer les catalyseurs. Si les sélectivités de C2H4 et C2H2 ne sont pas encore aussi élevés qu'en thermocatalyse, les résultats apportés ici par le dopage alcalin du catalyseur ouvrent la voie à une piste prometteuse d'amélioration de la plasmocatalyse pour la co-production d'insaturés par DRM. XXV `

Figure 1 - 1 .

 11 Figure 1-1. Schematic diagram of carbon cycle 1.2 Strategies of CO2 utilization and selective synthesis

Figure 1 -

 1 [START_REF] Ledley | Climate change and greenhouse gases[END_REF] shows surface carbide mechanism developed since 1970 with the advent of surface science instrumentation[START_REF] Davis | Fischer-Tropsch Synthesis: Reaction mechanisms for iron catalysts[END_REF].

Figure 1 - 5 .

 15 Figure 1-5. Surface carbide mechanism of chain growth in FTS. Density functional theory (DFT) calculations were carried out to investigate the reaction pathway of the C1 and C2 hydrocarbons formation in CO2 hydrogenation over Cu-Fe bimetallic catalyst. [64] The calculations illustrate that the CH*, the most essential monomeric species, formed through CO2* → HCOO* → HCOOH* → HCO* → HCOH* → CH*, followed by C-C coupling of two CH* species. They also indicated that the bimetallic surface suppresses

Figure 1 - 6

 16 Figure 1-6 Diagram of species in the aggregated collection of plasma.

Figure 1 - 7 .

 17 Figure 1-7. The configuration of (a) Fixed-bed reactor, (b) DBD plasma reactor and (c) packed DBD plasma reactor.

Figure 1 - 8 .

 18 Figure 1-8. Interaction of homogeneous and heterogeneous (surface) processes.

Figure 1 - 9 .

 19 Figure 1-9. An overview of plasma catalysis.

Figure 1 - 10 .

 110 Figure 1-10. An overview of the effects of the catalyst on the plasma and vice versa [9].

Figure 1 - 11 .

 111 Figure 1-11. Schematic diagram of plasma assisted CO2 and CH4 conversion.

  et. al., also investigated the role of Al3O2 in the plasma surrounding. Zeolites are widely applied catalysts in the petrochemical industry and some experimental works were conducted to study their potential application in plasma catalysis[START_REF] Krawczyk | Methane conversion with carbon dioxide in plasma-catalytic system[END_REF]. Schmidt-Szałowski et. al., found that Na-ZSM-5 catalyst participated in the formation of liquid aromatic hydrocarbons. The conversions of reactants are enhanced by Na-ZSM-5 and the methane conversion to C2-C4 hydrocarbons could reach up to 19.5% and 14.5%.

Figure 1 - 12 .

 112 Figure 1-12. Equivalent circuit of PB-DBD configuration. (a) Packed-bed configuration creating a complex network of capacitance. (b) A unit capacitance system before gas breakdown; (c) After gas breakdown over metal-containing dielectric pellets [108].

Figure 1 -

 1 Figure 1-12a shows the randomly packed catalysts particles which would create a complex network of local capacitance in the reactor. An equivalent circuit of DBD plasma reactor configuration containing the capacitance of quartz (Cd), the capacitance of pellet (Cpi i = 1-n),

Figure 1 - 13 .

 113 Figure 1-13. The development of discharge in packed DBD reactor shown by the ionization rate at (a) 0.6 ns, (b) 1.2 ns, (c) 1.6 ns, (d) 2.5 ns, (e) 3.0 ns, (f) 3.5 ns, (g) 7.5 ns, (h) 10 ns, (i) 11.5 ns. The inserted picture in (b) and (c) are the magnified picture corresponding to the red dash rectangle. The inserted picture in (h) is the corresponding electric field [111].

Figure 1 - 15 .

 115 Figure 1-15. Current profiles of the mm-gap (a) and micro-gap (b) packed bed DBD reactors with a zirconia packing, for one cycle of applied potential [78].

Figure 1 - 16 .

 116 Figure 1-16. Adsorption energy of CO2 on Cu as a function of electric field and excesselectrons.[START_REF] Jafarzadeh | Activation of CO2 on Copper Surfaces: The Synergy between Electric Field, Surface Morphology, and Excess Electrons[END_REF]

4 )

 4 Calculating the physicochemical properties of plasma to gain a deeper understanding of the mechanism for the improvement of products selectivity in direct synthesis. 5) Proposing and verifying possible catalytic reaction pathways to provide a theoretical basis for subsequent research.

Figure 1 - 17 .

 117 Figure 1-17. Schematic of direct synthesis of value-added products via plasma catalysis.

  A 150 mm stainless-steel coils wrapped around the quartz tube (10 mm O.D.  7 mm I.D.) was used as the ground electrode, while an aluminum rod (2 mm O.D.) placed in the quartz tube was used as the high voltage electrode. Two Teflon caps were installed on the aluminum rod to fix the catalysts in the discharge area and keep concentricity of the electrode and the tube. The power supply based on a transformation chain technology and comprised a signal function generator (HP 33120A), an audio amplifier (IMG Stageline STA-1400) and a transformer. The transformer was coupled with the audio amplifier to generate high enough peak-to-peak voltage (a sinusoidal signal) which would breakdown the gas and generate discharges in the reactor.The flow rates at the inlet before the discharge and outlet after the discharge were measured by a soap-film flowmeter.

Figure 2

 2 Figure 2-1. Experimental setup

  diffractometer PW1830 using Cu K𝛼 = 1.5405 Å radiation source. The measurement was operated at 45 kV and 30 mA with the 2θ range from 10˚ -90˚. The XRD patterns were processed via HIGHSCORE PLUS V3.05 and compared with PDF standard data base and the average crystalline sizes of synthesized catalysts were calculated by Scherrer Equation, D = Kλ/(βcosθ).

  separated by a Rt Molsieve 5 Å (Inside diameter  Length: 0.25 mm  10 m, column temperature: 323 K) column and are detected with Argon carrier gas, while CO2 and gaseous hydrocarbons are separated by a Rt Q Bond (Inside diameter  Length: 0.25 mm  12 m, column temperature: 323K) column and detected with Helium carrier gas. The optical emission spectra of plasma catalysis were collected by an optical fiber placed near reaction zone (as shown in Figure 2-2) and detected by a high-sensitive CCD detector (Settings: Integration Time: 2.000000s, Start: 216.50nm, End: 995.02nm, Front Entrance Slit: 0.50 mm, Detector Accumulations: 2, Accumulations Mode: Summed Scans, and Detector Cycles: 1).

Figure 2 - 2 .

 22 Figure 2-2. Photograph of the DBD plasma reactor and the component of in situ OES.In a typical reaction, 3.0 g catalyst (granule size: 1.0 mm) was packed in the discharge zone (5.30 mL) where the discharge length and gap were 150 mm and 3.5 mm respectively between two Teflon plugs. Prior to reaction, CO2 was introduced into the reactor first to exhaust air and then CO2 and CH4 were introduced into the fix-bed reactor with a specific molar ratio of CO2/CH4 = 1 for 2 h. The flow rate of mixed reactant gas is fixed at 30 mL/min. Then we regulate the power supply, adjust the peak-to-peak voltage to 6.0 kV and keep the frequency constant at 4.0 kHz to generate plasma in catalyst's zone. The experiments are repeated 3 times

Figure 2 - 3 .

 23 Figure 2-3. Schematic diagram of the equivalent model for the determination of the gas gap Hence, Figure 2-3 shows a simplified model for the packed DBD reactor to determine the voltage across the gas and different dielectric constant (ε) of the solid catalysts. The dielectric constant of each catalyst is estimated base on the nominal mass fraction of different species.

Figure 2 - 4 .

 24 Figure 2-4. Equivalent electrical circuit of the model.

Figure 2 - 5 .

 25 Figure 2-5. Schematic diagram of the plasma PSR reactor.

  a. Calculated based on Brunauer-Emmett-Teller (BET) method. b. Determined by Barrett-Joyner-Halenda method via N2 desorption data. c. Estimated by TEM images.

Figure 3 - 1 .

 31 Figure 3-1. (a) N2 adsorption/desorption isotherms of SiO2, Fe-SiO2 and K-Fe-SiO2, (b) N2 adsorption/desorption isotherms of Fe/C-SiO2 and K-Fe/C-SiO2.

Figure 3 - 2 .

 32 Figure 3-2. A broad diffraction peak at 2θ ≈ 20 o -30 o observed in all the samples is ascribed

Figure 3 - 2 .

 32 Figure 3-2. XRD patterns of fresh catalysts (a) SiO2, (b) Fe-SiO2, (c) K-Fe-SiO2, (d) Fe/C-SiO2, (e) K-Fe/C-SiO2 (■: amorphous silica species, ▲: FeO+SiO2 species, •: KCl species). The ion-based species in Fe-SiO2 and K-Fe-SiO2 are identified to be 2(FeO).SiO2 according to International Centre for Diffraction Data (ICDD): 00-011-0262. The broad and low intensity peaks suggest that the metal species are highly dispersed and exhibit low crystallinity. For the K promoted catalyst, the K species are identified to be KCl according to the diffraction peaks at 28.3 o and 40.5 o . The XRD patterns of Fe/C-SiO2 and K-Fe/C-SiO2 catalysts also suggest the poor crystallinity, small particle size and well dispersity of iron species. Crystallite size of FeO are estimated to be 5.83 nm and 7.25 nm by Scherrer equation on the basis of visible peaks in patterns of Fe-SiO2 and K-Fe-SiO2 respectively. Such a larger size of FeO in K-Fe-SiO2 might be attributed to KCl specie (8.90 nm) which could adjust catalyst homogeneity.

Figure 3 - 4 .

 34 Figure 3-4. SEM images of (a), (b) Fe-SiO2, (c), (d) K-Fe-SiO2 catalyst. SEM micrograph obtained from Fe-SiO2 and K-Fe-SiO2 catalysts are shown in Figure 3-4 (magnification = 25000 and 100000), in which the surface structures of the catalysts are compared. The mesoporous structures are constructed in bulk Fe-SiO2 catalyst and few of macro-pore is observed (Figure 3-4 a and b). Contrarily, mesoporous structures are damaged and more macro-porous structures form due to the addition of K (Figure 3-4 c and d). The dramatical decreasing of surface area of K-Fe-SiO2 catalyst is attributed to the low porosity of modified catalyst. To further elucidate the relation of structure and surface area, the SEM images of Fe/C-SiO2 and K-Fe/C-SiO2 catalysts are compared in Figure 3-5. It is clear to find that the surface structures are altered after the modification of K species, nevertheless, mesoporous structures are well maintained (Figure 3-5 c and d).

Figure 3 - 5 .

 35 Figure 3-5. SEM images of (a), (b) Fe/C-SiO2, (c), (d) K-Fe/C-SiO2 catalyst. The surface properties are further examined by EDS where the distribution of Si, O, Fe and K in K-Fe-SiO2 catalyst are shown in Figure 3-6. In the EDS spectrum, besides the well dispersed nanoparticles, a form of aggregation is observed meanwhile. The large clusters would lead to the substantial surface modification and influence the catalytic performance correspondingly.

Figure 3 -

 3 Figure 3-7 displays the distribution of Si, O, Fe, C and K in K-Fe/C-SiO2 catalyst. On the contrary, no aggregated large clusters are observed which indicates the well dispersion of promoters in the catalyst. This result also explains the slight decreasing of surface areas for K-Fe/C-SiO2 catalyst when compared with K-Fe-SiO2 catalyst. The well dispersed promoters and well sustained mesoporous structures together lead to the special performance of K-Fe-SiO2 catalyst in plasma assisted dry reforming.

Figure 3 - 6 .

 36 Figure 3-6. SEM image and multi-element EDS mapping spectra of K-Fe-SiO2 catalyst for Si, O, Fe and K.

Figure 3 - 7 .

 37 Figure 3-7. SEM image and multi-element EDS mapping spectra of K-Fe/C-SiO2 catalyst for Si, O, Fe, C and K.

Figure 3 - 8 .

 38 Figure 3-8. Elements content on the surface of K-Fe/C-SiO2 catalyst 3.2.5 TEM images of catalysis Figure 3-9 shows the TEM images and particle size distribution of synthesized catalysts via solgel method.According to the TEM images, the iron species are all well dispersed and exhibit a mixed phase. The nano-particles are uniform and the average size of as-prepared catalysts is estimated to be 2.65 ± 0.4 nm (calculated by counting the diameter of more than 200 random particles) which is similar to the previous research[START_REF] Wezendonk | Elucidating the Nature of Fe Species during Pyrolysis of the Fe-BTC MOF into Highly Active and Stable Fischer-Tropsch Catalysts[END_REF][START_REF] Wezendonk | Controlled formation of iron carbides and their performance in Fischer-Tropsch synthesis[END_REF]. The carbon matrix around Fe

Figure 3 - 9 .

 39 Figure 3-9. TEM image of (a1) Fe-SiO2, (b1) K-Fe-SiO2, (c1) Fe/C-SiO2, (d1) K-Fe/C-SiO2 catalyst; HRTEM image of (a2) Fe-SiO2, (b2) K-Fe-SiO2, (c2) Fe/C-SiO2, (d2) K-Fe/C-SiO2 catalyst; Particle size distribution of (a3) Fe-SiO2, (b3) K-Fe-SiO2, (c3) Fe/C-SiO2, (d3) K-Fe/C-SiO2 catalyst.

Figure 3 -

 3 Figure 3-10. HAADF-STEM images and EDX elemental mapping of K-Fe-SiO2 (left) and K-Fe/C-SiO2 (right) catalyst for Si, O, Fe, Cl and K.3.2.6 Chemical adsorption analysisFigure 3-11 display the H2-TPD results of iron-based catalyst and modified catalysts with various K loading amount. Two desorption peaks, which represent the weak adsorption and strong adsorption of H2, are detected over original Fe-SiO2 catalyst and locate at near 200 o Cand 550 o C respectively. However, the addition of K gradually diminishes the adsorption amount and shift the peaks to lower temperature which indicates the decreasing of adsorption strength. We suppose that the adsorption amount of hydrogen is decreased and the adsorption strength is weaken (shown in Figure3-12) after adopting potassium promoter. The partial coverage of active sites leads to the adsorption behavior which is further influence the catalytic performance.

Figure 3 - 11 .

 311 Figure 3-11. Effect of K on H2 adsorption.

Figure 3 - 12 .

 312 Figure 3-12. The surface modification of the catalysts.

  -13, the broad peak (Retention time = 0-20 s) is attributed to an instrumentinduced baseline. We could obtain H2 (Module A, Retention time ≈ 31.0 s) and hydrocarbons (Module B, Retention time: 25-32 s) with relatively high unsaturated hydrocarbons selectivity in the conversion of CH4 process. Initiated and activated by plasma, CH4 would be gradually converted to hydrocarbons via dehydrogenation and C-C coupling. In the module B, ethane, ethylene and acetylene are found to be the major products and it is worth mentioning a large number of unsaturated hydrocarbons (C2H2 and C2H4) are detected. It should be noted that CO peak, which can only come from catalyst matrix, is not observed in the conversion of pure CH4.

Figure 3 - 13 .

 313 Figure 3-13. Conversion and products selectivity of plasma catalytic conversion of pure CH4 over K-Fe/C-SiO2 catalyst (Module A: left; Module B: right). Reaction conditions: Peak-topeak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min. Hereafter, the feed gas was switched to CO2. Instrument-induced broad peak becomes more obvious in Module A due to low concentration of gases (weak signals). The peaks of CxHy ((Retention time > 25 s) disappeared while only a trace amount (< 0.01%) of N2/O2 (Retention time = 35-45 s), residual H2 and CH4 were detected in Module A and Module B. Another proof for the absence of CO2 splitting and surface reaction is that the trace amount of O2 remains unchanged and the ratio of N2/O2 remains to be 4. Possible explanations include low input power (2.5W) and lack of RWGS.

Figure 3 - 14 .

 314 Figure 3-14. Conversion and products selectivity of plasma catalytic conversion of pure CO2 over K-Fe/C-SiO2 catalyst (Module A: left; Module B: right). Reaction conditions: Peak-topeak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min.

Figure 3 -

 3 Figure 3-15 exhibits the conversion rate of reactants, main products selectivity, the ratio of key products and detailed products distribution which demonstrates the superiorities of catalysts.

Figure 3 -

 3 Figure 3-15. Conversion and products selectivity of plasma-catalytic conversion of CO2 and CH4 over different catalysts. Reaction conditions: Peak-to-peak voltage = 5.1 -6.0 kV, Input power = 2.4 -2.6 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min, CO2/CH4 ratio = 1/1, Catalysts granule size = 1.0 mm.

Figure 3 -

 3 Figure 3-16. Typical Lissajous figures of oscilloscope over different catalystsThe presence of catalysts would alter plasma properties (Figure3-16) and would consequently change input power when the same peak-to-peak voltage is applied. Thus, the peak-to-peak voltage is adjusted between 5.2 -6.0 kV in order to keep the same input energy. By this way, input power is fixed in the range 2.4 -2.6 W. Table 3-2 summarizes the details of energy consumption, SIE, energy efficiency of CO2 and CH4 over different catalysts. As Lissajous

  -18a). Apart from H/C ratio, the selectivity of added-value unsaturated hydrocarbons (e.g., ethylene) is enhanced due to K promoter. GC spectrum (Figure3-17) demonstrates the distribution of gaseous hydrocarbons over different catalysts. Unsaturated hydrocarbons products are negligible in the SiO2 packed process while K promoted catalysts further improve the unsaturated hydrocarbons production.For Fe-SiO2 and Fe/C-SiO2 catalysts, the U/S ratio (represent the molar ratio of ethylene and acetylene to ethane) is 0.19 and 0.07 respectively, nevertheless the U/S ratios rise to 0.35 and 0.90 using K-Fe-SiO2 and K-Fe/C-SiO2 catalysts.

Figure 3 -

 3 Figure 3-17. GC chromatograms of gaseous hydrocarbons obtained by plasma catalytic conversion of CO2 and CH4 over different catalysts. Reaction conditions: Peak-to-peak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min, CO2/CH4 ratio = 1/1.

Figure 3 - 18 . 65 W

 31865 Figure 3-18. Effect of catalysts on the plasma-catalytic conversion of CO2 and CH4. (a) Selectivity of gaseous hydrocarbons and (b) Selectivity of liquid oxygenates over different catalysts. Reaction conditions: Peak-to-peak voltage = 5.1 -6.0 kV, Input power = 2.4 -2.6

Figure 3 -

 3 Figure 3-19. GCMS result of liquid oxygenates over K-Fe/C-SiO2 catalyst. Reaction conditions: Peak-to-peak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min, CO2/CH4 ratio = 1/1, Catalysts granule size = 1.0 mm.

Figure 3 -

 3 Figure3-20 shows the electron energy distribution function as a function of the electron energy when different catalysts are packed. As one can see, the mean electron energy, which is crucial in plasma-catalysis, is increased by the presence of the packing materials in discharge. A low electron energy could promote the formation of saturated hydrocarbons while a high electron energy could be in favor of the formation of unsaturated hydrocarbons[START_REF] Vakili | Plasma-assisted catalytic dry reforming of methane (DRM) over metal-organic frameworks (MOFs)-based catalysts[END_REF]. In the K-Fe/C-

Figure 3 - 20 .

 320 Figure 3-20. The electron energy distribution function in CO2/CH4 (mole ratio: 1/1) gas discharges packing with different catalysts.

Figure 3 -

 3 Figure 3-21. H2-TPD profiles of the (a) Fe-SiO2 and K-Fe-SiO2.

Figure 3 -

 3 Figure 3-22. H2-TPD profiles of the Fe/C-SiO2 and K-Fe/C-SiO2 catalysts.

Figure 3 -

 3 Figure 3-23. CO2-TPD profiles of the (a) Fe-SiO2 and K-Fe-SiO2, (b) Fe/C-SiO2 and K-Fe/C-SiO2 catalysts.

Figure 3 - 24 .

 324 Figure 3-24. Conversion and products selectivity of plasma catalytic conversion of CO2 and CH4 over K-Fe/C-SiO2 catalyst with different granule sizes. Reaction conditions: Peak-topeak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min, CO2/CH4 ratio = 1/1.

shown in Figure 3 - 24 ,

 324 catalysts with different sizes (1.0 mm and 2.0 mm) result in distinct distribution of catalyst particles in the reactor and further change the performance of K-Fe/C-SiO2 catalyst. As shown in Figure 3-25, the selectivity of H2 and oxygenates are slightly decreased due to larger volume without catalyst. For the gaseous and liquid products distribution, larger contact area of packed catalyst (1 mm) enhances the formation of added-value hydrocarbons and the combination of *CHx and *COOH which contribute to higher selectivity of organic acid. Thus, packed catalysts with proper granule size, 1 mm in this work, is preferred to optimize plasma dry reforming.

Figure 3 - 25 .

 325 Figure 3-25. Conversion and products selectivity of plasma catalytic conversion of CO2 and CH4 over K-Fe/C-SiO2 catalyst with different granule sizes. (a) Selectivity of gaseous hydrocarbons and (b) Selectivity of liquid oxygenates. Reaction conditions: Peak-to-peak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min, CO2/CH4 ratio = 1/1.

Figure 3 - 26 .

 326 Figure 3-26. The electron energy distribution function in CO2/CH4 (mole ratio: 1/1) gas discharges packing with different size of catalysts.

Figure 3 -

 3 Figure 3-27. Time-on-stream profiles of plasma-catalytic conversion of CO2 and CH4 over K-Fe/C-SiO2 catalyst. Reaction conditions: Peak-to-peak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min, CO2/CH4 ratio = 1/1, Catalysts granule size = 1.0 mm.

[ 73 ]

 73 Filamentary (FM) discharge become weak while surface ionization wave (SIW) discharge is formed. Besides, the adsorption of gaseous radicals (including CO2 + , CO, CH) on the catalysts is another essential reason to dim the plasma reaction.

Figure 3 -

 3 Figure 3-28. OES spectrum of (a) air around the ground electrode, (b) plasma catalytic conversion of CO2 and CH4 without catalysts (c) plasma catalytic conversion of CO2 and CH4

Figure 3 - 29 .

 329 Figure 3-29. Reaction mechanisms proposed for plasma-catalytic conversion of CO2 and CH4.

  kinetics model from a theoretical point of view. The recent intensification of the research on plasma-catalytic dry reforming as a new promising route in synthesis chemistry encourages to increase the understanding of its underlying mechanisms. In this frame, an advanced zerodimension chemical model of plasma-catalytic dry reforming is here proposed where homogeneous gas phase reactions as well as heterogeneous catalytic reactions are considered together to establish a comprehensive kinetic model. The number density of reactants, products and critical intermediates are described as a function of time outputs. Subsequently, important reaction routes are analyzed and show the insight of products formation. Furthermore, the kinetic parameters of CO2/CH4 conversion in a nonthermal plasma are calculated. The reaction barriers in various conditions with and without catalysts are compared in order to determine the role of the surface mechanisms in plasma-catalysis. Thanks to the surface mechanisms induced put in evidence over Rh and Ni catalysts, it is shown that apparent activation energies decreased by 22.45% and 17.05% for CO2 conversion and CH4 conversion respectively.

Figure 4 -

 4 1 shows the voltage and current recorded in one typical test of our ns-pulse plasma reactor. In one cycle, the duration of voltage is around 500 ns while the duration of current is around 60-80 ns which located at the rising and falling phases of voltage. The highest value of voltage and current are around 15 kV and 25 A respectively.

Figure 4 - 1 .

 41 Figure 4-1. The measurement of voltage and current of plasma reactor.

Figure 4 - 2 .

 42 Figure 4-2. Plasma power deposition as a function of time Qsou(t) and (b) electron temperature in one plasma pulse (before the dashed line) and afterglow (after the dashed line).

Figure 4 -

 4 Figure 4-3 summarizes the number density of reactants and main products in one plasma power pulse. Besides the unreacted CO2 and CH4, the syngas (H2 and CO) is the main gas products while C2H6 is observed to have the highest number density among C2 hydrocarbons (C2H6, C2H4 and C2H2). We didn't analyze higher hydrocarbons (C3+ hydrocarbons) which is form via the recombination of C1 or C2 radicals considering less important mole fraction in the simulation and experiments. Since the combination of transient structures (such as O, H, CH3, CO, OH) derived from CO2 and CH4 activation, liquid oxygenated molecules are form in the plasma surroundings. Alcohols (e.g. CH3OH, C2H5OH and etc.) and acids (e.g. HCOOH, CH3COOH

Figure 4 - 3 .

 43 Figure 4-3. Summary of number density of reactants and main products after the plasma power pulse.

Figure 4 -

 4 Figure 4-4 presents the key radicals generated in the plasma power pulse. First, H and CHx have the highest number density because they are dissociated direct from CH4. It is worth to mention that the number density of O is significantly lower than H and CHx not only attribute to the consumption rate of CO2 in the plasma but also due to further recombination with other species.

Figure 4 - 4 .

 44 Figure 4-4. Summary of number density of key intermediates (radicals) in the plasma power pulse.

Figure 4 - 5 .

 45 Figure 4-5. Summary of number density of key intermediates (charged species) in the plasma power pulse.

Figure 4 - 4 . 3 . 1

 4431 Figure 4-5 shows another important group of intermediates, which are regarded as charged species, in our modelling. Due to different structures and electronic properties, those species could be categorized as two parts. Some positive particles are produced via the electron impact dissociation and ionization in which CH3+, CH2+, CO+, CO2+ and CH4+ are generated respectively by electrons collision. Meanwhile, oxygen-containing species could obtain electrons in the electron impact (dissociation and ionization) processes and form negative particles (e.g. O-, OH-).

Figure 4 - 6 .

 46 Figure 4-6. Number density of CO2 and CH4 as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line).

- 7 ,

 7 three critical stages in our model, including the plasma power deposition (excitation at 6.0×10 -08 s), termination of the power deposition (de-excitation at 1.0×10 -07 s) and the afterglow (remanence at 1.0×10 -05 s), are analyzed by comparing the production (consumption: for the reactants) rate of selected 10 key elementary reactions for each compounds. At the beginning of the discharge, the electron temperature and density start to increase and this indicate that the electron impacted reactions occupy the decomposition pathway of the reactants. For CH4, it is noted that CH4 is activated via ionization (e.g. R-001) and electron impacted dissociation (e.g. R-002 and R-003). The derivative products would be further consumed (e.g. R-009 and R-037) when collide with another energetic electrons. similar reaction pathways are found at the early stage of the plasma. CO2 could be activated by energetic electrons via ionization (e.g. R-025) and dissociation (e.g. R-025). On the one hand, CO and O are produced via the decomposition of CO2, on the other hand, it could loss a free electron and form CO2 + species which will play an important role in producing CO and O in the following process (e.g. R-072). e + CO2 → e + CO2 + (R-023) e + CO2 → e + CO + O (R-025) e + CO2 + → CO + O (R-072) Besides, the neutral radicals, oxygenates and cationic also participate in the conversion of CO2 and CH4. The reactions of reactants and derived transient structures (e.g. R-083), in which C-C

Figure 4 - 7 .

 47 Figure 4-7. Consumption (left-side of the figures) and production (right-side of the figures)

Figure 4

 4 Figure 4-8.

Figure 4 - 8 .

 48 Figure 4-8. Number density of H2 and CO as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line).

608) 4 . 3 . 3

 433 C2 hydrocarbons production As discussed in the previous chapter, C2 hydrocarbons, especially for the unsaturated hydrocarbons, are high value-added products in the chemical industry. Hence, the time evolutions of C2 hydrocarbons (C2H6, C2H4 and C2H2) and number density are simulated and compared in Figure 4-9.

Figure 4 - 9 .

 49 Figure 4-9. Number density of C2 hydrocarbons as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line).

Figure 4 - 10 .

 410 Figure 4-10. Total production rates of C2 hydrocarbons as a function of time.

Figure 4 - 11 .

 411 Figure 4-11. Number density of C2 intermediates as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line).Similar to C2H5, C2H3 could be produced via electron impact reactions, electron-ion recombination reactions, neutral-neutral reactions, ion-neutral and ion-ion reactions (e.g. R-056, R-096, R-394, and etc.). The recombination of two C2H3 species (R-133) results in the formation of C2H2 and C2H4. Together with the combination of CH2 radicals, the recombination of two C2H5 radicals and electron impact dissociation of C3H8, they all contribute to the production of C2H4.

Figure 4 -

 4 Figure 4-12. Consumption (left-side of the figures) and production (right-side of the figures)

Figure 4 - 13 .

 413 Figure 4-13. Number density of oxygenates as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line).

Figure 4 -

 4 Figure 4-14. Effects of power deposition (A triangular pulse with a maximum power deposition of 100-600 kW) on (a) the mole ratio of unsaturated hydrocarbons to saturated hydrocarbons and (b) the electron temperature in the plasma pulse (before the dashed line) and afterglow (after the dashed line).

Figure 4 - 15 .

 415 Figure 4-15. Effects of power deposition (A triangular pulse with a maximum power deposition of 100-600 kW) on the mole fraction of (a) H and (b) H2 in the plasma pulse (before the dashed line) and afterglow (after the dashed line).

Figure 4 - 16 .

 416 Figure 4-16. Effects of power deposition (A triangular pulse with a maximum power deposition of 100-600 kW) on the rates of critical reaction channels for the production of (a) C2H4, (b) C2H5 and (c) C2H6.It should be noted that the critical process with higher electron energy would enhance dehydrogenations. It is elucidated by further comparing the variation of H species in our modelling. As shown in Figure4-15a, the mole fraction of H significantly increased since the initiating of the plasma pulse and it declines subsequently. It is interesting to note that the concentration of H in the plasma reactor increases with the accelerated dehydrogenation of hydrocarbons. On the other hand, ions species, such as C2H6+, C2H4+ and etc., also contribute to the formation of H via reactions shown in Figure4-16 (including electron-ion recombination, ion-ion reactions) in which the reaction rate depends on the electron temperature. The mole fraction of H2 (Figure4-15b) will not decrease after the plasma pulse and the final fraction of H2 is corresponding to the electron temperature as well.

2

 2 Figure 4-17. Effects of the duration of the pulse power deposition (A triangular pulse with a maximum power deposition of 300 kW and a duration of 40-200ns) on the electron temperature in the plasma pulse (before the dashed line) and afterglow (after the dashed line). In this section, we adjust the duration of the pulse power deposition from 40 ns to 200 ns and investigate their effects on the physical properties and products selectivity. The electron temperatures are first compared in Figure 4-17. Due to the different duration of plasma discharges, the energetic electrons are sustained for various time while the maximum value of electron temperatures are similar. It is necessary to point out that a slight increasing of electron temperature exists after the termination of the plasma discharges.

Figure 4 - 18 .

 418 Figure 4-18. Effects of the duration of the pulse power deposition (A triangular pulse with a maximum power deposition of 300 kW and a duration of 40-200ns) on the number density of (a) CO2 and (b) CH4 in the plasma pulse (before the dashed line) and afterglow (after the dashed line).

Figure 4 - 19 .

 419 Figure 4-19. Effects of the duration of the pulse power deposition (A triangular pulse with a maximum power deposition of 300 kW and a duration of 40-200ns) on the number density of (a) H2 and (b) CO in the plasma pulse (before the dashed line) and afterglow (after the dashed line).

Figure 4 - 20 .

 420 Figure 4-20. Effects of the duration of the pulse power deposition (A triangular pulse with a maximum power deposition of 300 kW and a duration of 40-200ns) on the mole ratio of H2 to CO in the plasma pulse (before the dashed line) and afterglow (after the dashed line).

  maximum power deposition of 300 kW and a duration of (a) 40ns, (b) 80ns, (c) 120ns, (d) 160ns and (e) 200ns) on the number density of C2H2, C2H4, C2H6 and (f) the mole ratio of unsaturated hydrocarbons to saturated hydrocarbons in the plasma pulse (before the dashed line) and afterglow (after the dashed line).

Figure 4 -

 4 Figure 4-22. The Arrhenius plot for the plasma-assisted dry reforming (a) without catalyst and (b) over Rh-based catalyst.

Figure 4 -

 4 Figure 4-23. Number density of surface adsorbed species in one pulse of DBD plasma assisted dry reforming on Rh site (case 5).

Figure 4 -

 4 Figure 4-23 presents the number density of Rh site and surface adsorbed species in one pulse of DBD plasma assisted dry reforming. It is clear to find that adsorbed species driven directly from dissociation of reactant, such as H(s), CO(s), O(s) and CH3(s), are the most abundant adsorbed species. The ratio of adsorbed H(s) to CO(s) is estimated to be 3.55. On the other hand,

Figure 4 - 24 .

 424 Figure 4-24. Reaction path diagram of surface species.

1 .

 1 Simulation results are then postprocessed by plotting the number density of products and intermediates as a function of time and surface areas.

  Surface area (cm 2 ) 60 120 180 240 300 360 420 480 540 6004.5.2 Simulation resultsOur simulation results show that the major product in hydrocarbons is C2H6. Whereas the selectivity of C2H4 gradually exceed C2H6 owing to the function of catalysts in the plasma surroundings. The formation of C2H4 is enhanced by the surface reactions on the catalysts. As a result, the ratio of C2H4 to C2H6 raise from about 0.2 to 1.2.

Figure 4 - 25 .

 425 Figure 4-25. Effects of the surface reactions on the ratio of C2H4 and C2H6 at the end point of the simulation.

Figure 4 - 26 .

 426 Figure 4-26. Effect of catalysts on U/S ratio as a function of input power.

Figure 5 - 1 .

 51 Figure 5-1. Schematic diagram of simulations. f) 0D kinetic model was first established by ignoring the configuration of plasma reactor and the effects of catalysts (as shown in Figure 5-1). Rate constants, reaction rates and other kinetic parameters were simulated in sequence via PSR CHEMKIN model. In this model, the

123 5. 1 . 3

 12313 ComparationOur experimental result of total conversion rates as a function of SEI are plotted and compared with published data in Figure5-2 and Figure5-3. Special catalysts properties and moderate input power in our system lead to relatively low conversion rate with low SIE. A considerable energy efficiency (around 0.6 mmol/kJ) was achieved. Regarding products distribution, in most of reported works, it is essential to note that plasma-catalytic process seems not to be in favor of ethylene and added-value syngas having a suitable H/C ratio for FTS. But K promoted catalysts as explored in this work is able to directly synthesize unsaturated hydrocarbons, to increase H2/CO ratio and to enhance the formation of long-chain oxygenates which are more valuable and conducive to industrial applications.

Figure 5 - 2 .

 52 Figure 5-2. Comparation of experimental data collected from the literature for the total conversion rate as a function of the SEI in plasma conversion of CO2 with CH4.

Figure 5 - 3 .

 53 Figure 5-3. Comparation of experimental data collected from the literature for the total energy efficiency as a function of the total conversion rate in plasma conversion of CO2 with CH4.

Figure 5 - 4 .

 54 Figure 5-4. (a) Schematic and design of in situ DRIFTS, spectra of surface species on the 15Ni/UiO-66 catalyst (b) under thermal conditions, (c) under plasma conditions, and (d) corresponding MS signals collected at the exit of the DRIFTS cell as a function of time-onstream during the NTP-assisted catalytic CO2 hydrogenation under plasma off and onconditions.[START_REF] Chen | Nonthermal plasma (NTP) activated metal-organic frameworks (MOFs) catalyst for catalytic CO2 hydrogenation[END_REF]

Figure 1 - 10 .

 110 Figure 1-10. An overview of the effects of the catalyst on the plasma and vice versa [9]. ..... 17

Figure 1 - 11 . 19 Figure 1 - 12 . 24 Figure 1 - 13 . 25 Figure 1 - 14 . 26 Figure 1 - 15 . 27 Figure 1 - 16 . 28 Figure 1 - 17 . 30 Chapter 2 Figure 2 - 1 . 34 Figure 2 - 2 . 38 Figure 2 - 3 . 40 Figure 2 - 4 . 128 Figure 2 - 5 . 45 Chapter 3 Figure 3 - 1 . 50 Figure 3 - 2 . 51 Figure 3 - 3 . 52 Figure 3 - 4 . 53 Figure 3 - 5 .

 1111911224113251142611527116281173022134223823402412825453315032513352345335 Figure 1-11. Schematic diagram of plasma assisted CO2 and CH4 conversion. ...................... 19 Figure 1-12. Equivalent circuit of PB-DBD configuration. (a) Packed-bed configuration creating a complex network of capacitance. (b) A unit capacitance system before gas breakdown; (c) After gas breakdown over metal-containing dielectric pellets.[93] .................................... 24 Figure 1-13. The development of discharge in packed DBD reactor shown by the ionization rate at (a) 0.6 ns, (b) 1.2 ns, (c) 1.6 ns, (d) 2.5 ns, (e) 3.0 ns, (f) 3.5 ns, (g) 7.5 ns, (h) 10 ns, (i) 11.5 ns. The inserted picture in (b) and (c) are the magnified picture corresponding to the red dash rectangle. The inserted picture in (h) is the corresponding electric field. [96] ................ 25 Figure 1-14. Distributions of electron impact ionization rate (m -3 s -1 ) in different pores [97] 26 Figure 1-15. Current profiles of the mm-gap (a) and micro-gap (b) packed bed DBD reactors with a zirconia packing, for one cycle of applied potential. [102] ........................................... 27 Figure 1-16. Adsorption energy of CO2 on Cu as a function of electric field and excess electrons. [104] ......................................................................................................................................... 28 Figure 1-17. Schematic of direct synthesis of value-added products via plasma catalysis. ..... 30

Figure 3 - 8 .Figure 3 - 9 . 58 Figure 3 - 10 . 59 Figure 3 - 11 . 59 Figure 3 - 12 . 60 Figure 3 - 13 . 60 Figure 3 - 14 . 61 Figure 3 - 15 . 61 Figure 3 - 16 . 63 Figure 3 - 17 . 64 Figure 3 - 18 . 65 Figure 3 - 19 . 67 Figure 3 - 68 Figure 3 - 68 Figure 3 - 22 . 70 Figure 3 - 23 . 71 Figure 3 - 24 . 72 Figure 3 - 25 .

 38395831059311593126031360314613156131663317643186531967368368322703237132472325 Figure 3-8. TEM image of (a1) Fe-SiO2, (b1) K-Fe-SiO2, (c1) Fe/C-SiO2, (d1) K-Fe/C-SiO257 Figure 3-9. HAADF-STEM images and EDX elemental mapping of K-Fe-SiO2 (left) and K-Fe/C-SiO2 (right) catalyst for Si, O, Fe, Cl and K. .................................................................. 58 Figure 3-10. Effect of K on H2 adsorption. .............................................................................. 59 Figure 3-11. The surface modification of the catalysts. ........................................................... 59 Figure 3-12. Conversion and products selectivity of plasma catalytic conversion of pure CO2 over K-Fe/C-SiO2 catalyst (Module A: left; Module B: right). Reaction conditions: Peak-topeak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min.60 Figure 3-13. Conversion and products selectivity of plasma catalytic conversion of pure CH4 over K-Fe/C-SiO2 catalyst (Module A: left; Module B: right). Reaction conditions: Peak-topeak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min.60 Figure 3-14. Conversion and products selectivity of plasma-catalytic conversion of CO2 and CH4 over different catalysts. Reaction conditions: Peak-to-peak voltage = 5.1 -6.0 kV, Input power = 2.4 -2.6 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min, CO2/CH4 ratio = 1/1, Catalysts granule size = 1.0 mm. .............................................................................................. 61 Figure 3-15. Typical Lissajous figures of oscilloscope over different catalysts ...................... 61 Figure 3-16. GC chromatograms of gaseous hydrocarbons obtained by plasma catalytic conversion of CO2 and CH4 over different catalysts. Reaction conditions: Peak-to-peak voltage = 6.0 kV, Input power = 2.4 W, Frequency = 4.0 kHz, Flow rate = 30 mL/min, CO2/CH4 ratio = 1/1. ......................................................................................................................................... 63 Figure 3-17. Effect of catalysts on the plasma-catalytic conversion of CO2 and CH4. (a) Selectivity of gaseous hydrocarbons and (b) Selectivity of liquid oxygenates over different

Figure 4 - 6 . 87 Figure 4 - 7 . 89 Figure 4 - 8 . 90 Figure 4 - 9 . 94 Figure 4 - 10 . 95 Figure 4 - 11 . 96 Figure 4 - 12 . 97 Figure 4 - 13 . 99 Figure 4 - 14 . 101 Figure 4 - 15 . 102 Figure 4 - 16 . 102 Figure 4 - 17 . 103 Figure 4 - 18 . 103 Figure 4 - 19 . 104 Figure 4 - 20 . 104 Figure 4 - 21 . 105 Figure 4 - 22 . 108 Figure 4 - 23 . 111 Figure 4 - 24 . 112 Figure 4 - 25 . 115 Figure 4 - 26 . 116 Chapter 5 Figure 5 - 1 . 121 Figure 5 - 2 . 123 Figure 5 - 3 . 123 Figure 5 - 4 .

 468747894890499441095411964129741399414101415102416102417103418103419104420104421105422108423111424112425115426116551121521235312354 Figure 4-6. Number density of CO2 and CH4 as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line). ................................................ 87 Figure 4-7. Consumption (left-side of the figures) and production (right-side of the figures) rates of critical reaction channels for the production of (a) CO2, (b) CH4 at 6.0×10 -08 s, 1.0×10 - 07 s and 1.0×10 -05 s. .................................................................................................................... 89 Figure 4-8. Number density of H2 and CO as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line). ............................................................ 90 Figure 4-9. Number density of C2 hydrocarbons as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line). ................................................ 94 Figure 4-10. Total production rates of C2 hydrocarbons as a function of time. ...................... 95 Figure 4-11. Number density of C2 intermediates as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line). ................................................ 96 Figure 4-12. Consumption (left-side of the figures) and production (right-side of the figures) rates of critical reaction channels for the production of (a) C2H4, (b) C2H5 and (c) C2H6 at 6.0×10 -08 s, 1.0×10 -07 s and 1.0×10 -05 s. .................................................................................... 97 Figure 4-13. Number density of oxygenates as a function of time during the plasma pulse (before the dashed line) and afterglow (after the dashed line). ................................................ 99 Figure 4-14. Effects of power deposition (A triangular pulse with a maximum power deposition of 100-600 kW) on (a) the mole ratio of unsaturated hydrocarbons to saturated hydrocarbons and (b) the electron temperature in the plasma pulse (before the dashed line) and afterglow (after the dashed line). ............................................................................................................ 101 Figure 4-15. Effects of power deposition (A triangular pulse with a maximum power deposition of 100-600 kW) on the mole fraction of (a) H and (b) H2 in the plasma pulse (before the dashed line) and afterglow (after the dashed line).............................................................................. 102 Figure 4-16. Effects of power deposition (A triangular pulse with a maximum power deposition of 100-600 kW) on the rates of critical reaction channels for the production of (a) C2H4, (b) C2H5 and (c) C2H6. ................................................................................................................. 102 Figure 4-17. Effects of the duration of the pulse power deposition (A triangular pulse with a maximum power deposition of 300 kW and a duration of 40-200ns) on the electron temperature in the plasma pulse (before the dashed line) and afterglow (after the dashed line). .............. 103 Figure 4-18. Effects of the duration of the pulse power deposition (A triangular pulse with a maximum power deposition of 300 kW and a duration of 40-200ns) on the number density of

  O2 + + CH → CH + + O2 3.10 × 10 -10 -0.5 0.0 R-588 O2 + + C → CO + + O 5.20 × 10 -11 0.0 0.0 R-589 O2 + + C → C + + O2 5.20 × 10 -11 0.0 0.0 R-590 O2 + + C2H4 → C2H4 + + O2 6.80 × 10 -10 0OH + + C2 → C2 + + OH 4.80 × 10 -10 0.0 0.0 R-634 OH + + H2 → H2O + + H 1.01 × 10 -09 0.0 0.0 R-635 OH + + O2 → O2 + + OH 5.90 × 10 -10 0.0 0.0 R-636 OH + + O → O2 + + H 7.10 × 10 -10 0CH + CO + → HCO + + C 3.20 × 10 -10 -0.5 0.0 R-680 H + + CO2 → HCO + + O 3.50 × 10 -09 0.0 0.0 R-681 H2 + + CO → HCO + + H 2.16 × 10 -09 0.0 0.0 R-682 H2 + CO + → HCO + + H 7.50 × 10 -10 0

Table 1 -

 1 2. Comparison of different CO2 conversion strategies[START_REF] Snoeckx | Plasma technologya novel solution for CO 2 conversion?[END_REF].

		Turnkey	Conversion	Operating	Overall	Catalysts	Use of rare
		Process	Rate into HC	Cost	Flexibility	Complexity	earth metals
	Thermo-catalysis	No	High	High	Low	Medium	Yes
	Photo-catalysis	Yes	Low	Low	Low	Very high	Yes
	Electro-catalysis	No	High	Low	Medium	High	Yes
	Bio-catalysis	No	High	High	Low	High	No
	Plasma-catalysis	Yes	High	Medium	High	Medium	No
	a. Whether could be easily switched on and off to follow the supply
	b. Needs to operate at certain temperatures and sensitive to on/off fluctuations
	1.2.4 Design of the catalysts
	Catalysts play a vital role in catalytic industry because of their promotions to the CO2
	conversion and the target products selectivity, however, current studies of catalysts in plasma
	catalysis are still in the early stage. Therefore, comprehensive understanding of syngas and
	hydrocarbons production process in thermal-catalysis are of great significance for the design of
	more complicated plasma catalysts. In this section, we will introduce the active sites, supports,
	promoters of catalysts and microscopic mechanism in hydrocarbons production which might
	contribute to the development of plasma catalytic process.
	Active sites
	According to literature, CO2 hydrogenation to olefins is supposed to take place in 2 steps: (a)
	CO2 is transformed into CO by reverse water gas shift (RWSG) reaction or methanol, then (b)
	hydrocarbons form via FTS or methanol to olefins (MTO) reaction. A large number of well-
	designed catalysts are developed to improve the selectivity, conversion and energy efficiency
	of CO2 utilization. Iron, cobalt, copper, manganese and zinc-based catalysts are the most
	extensively applied elements in CO2 hydrogenation, FTS and RWGS reaction. [47] Song et al.
	reported bimetallic catalysts for synthesis of olefin-rich C2+ hydrocarbons and further improved
	the formation of C2+ hydrocarbons and olefins selectivity by introducing promoters. They
	combined Fe with Cu on alumina support and found dramatic bimetallic promotion of C2+
	hydrocarbons formation. Fe5C2, another active site in CO2 hydrogenation and FT synthesis, also
	exhibits superior activity compared with most iron-based catalysts. Cobalt and copper normally
	be combined with iron in the bimetallic catalysts to further improve the catalysts' performance
	by mean of suppressing the undesired CH4 formation and enhancing CO and C2+ hydrocarbon
	formation

Table 1 -

 1 3. Summary of catalysts in olefins production.

	Typical catalyst (5Mn-Na/Fe) exhibited a CO2 conversion of 38.6% and low
	selectivity of methane (11.8%) and CO (11.7%) while the selectivity of light olefins and
	olefin/paraffin ratio was up to 30.2% and 7.5. [62]

Table 1 -

 1

	Catalysts	Reaction system	Reaction type	Ea (CO2) kJ mol -1	Ea (CH4) kJ mol -1	Ref & Year
	Ni-CeMgAl	Thermal-catalysis CO 2 /CH 4 reforming	62.2	65.5	[106] (2017)
	Ni/La2O3-MgAl2O4 Plasma-catalysis	CO 2 /CH 4 reforming	35.2	32.6	[100] (2020)
	Ni-La2O3@SiO2	Plasma-catalysis	CO 2 /CH 4 reforming	29.5	24.7	[101] (2015)
	Ni-SiO2	Plasma-catalysis	CO 2 /CH 4 reforming	-	6.1-18.6	[102] (2017)
	Ru/MgAl	Thermal-catalysis CO 2 hydrogenation	68-112	-	[103] (2020)
	Ni/UiO-66	Thermal-catalysis CO 2 hydrogenation	70	-	[104] (2020)
	Ni@SiO2	Thermal-catalysis CO 2 hydrogenation	76-82	-	[105] (2020)
	Ru/MgAl	Plasma-catalysis	CO 2 hydrogenation	21-43	-	[103] (2020)

4. Comparison of activation barrier of reactants over different plasma catalyst

  7) Using established kinetic model and calculating the important kinetic parameters (such as activation energy, etc.) in the catalytic process. 8) Combining theory and experiments and using validated kinetic models to predict the selectivity of catalysts for specific products. Providing a theoretical tool for precise synthesis of catalysts and efficient utilization of greenhouse gases.

	CHAPTER II. EXPERIMENTAL METHOD, KINETICS
	MODEL AND NUMERICAL SIMULATION
	Abstract
	This chapter summarizes the experimental and numerical method used in this thesis. By
	improving reported MOF mediated synthesis approach, a series of highly dispersed Fe-based
	nano-catalysts are synthesized. Catalysts are characterized by various technologies in order to
	reveal the nature of active species and catalysts structures. To evaluate the performance of
	catalysts, we developed a fixed bed dielectric barrier discharge (DBD) equipped with on-line
	gas products analyzer and optical emission spectroscopy. The plasma parameters, including the

average electric field strength, the reduced electric field (E/N) and the mean electron energy, are calculated by using an equivalent electrical circuit model of the DBD reactor described in reported literature. Regarding the dynamics, we establish a 0D kinetic model which contains 19 molecules, 38 charged species, 29 radicals, and electrons. Those particles collide with each other and generate a large amount of elementary reactions correspondingly. Hence, we summarize 868 elementary reactions, including electron impact reactions, electron-ion recombination reactions, ion-neutral reactions, ion-ion reactions and neutral-neutral reactions, in our simulation work of plasma catalysis. Last but not least, surface reactions are evaluated in our model for the first trial.

  The kinetic model contains 19 molecules, 38 charged species, 29 radicals, and electrons which are listed in Table 2-1. Those particles collide with each other and generate a large amount of elementary reactions correspondingly.

Based on the plasma chemistry developed by De Bie, Zheng, Tosi

[START_REF] Martini | Oxidation of CH4 by CO2 in a dielectric barrier discharge[END_REF][START_REF] Zheng | Kinetic Study of Nonequilibrium Plasma-Assisted Methane Steam Reforming[END_REF][START_REF] De Bie | The Dominant Pathways for the Conversion of Methane into Oxygenates and Syngas in an Atmospheric Pressure Dielectric Barrier Discharge[END_REF]

, we summarize 868 elementary reactions, including electron impact reactions, electron-ion recombination reactions, ion-neutral reactions, ion-ion reactions and neutral-neutral reactions, in our simulation work of plasma catalysis.

Table 2 -

 2 1. Summary of species included in the modelling.

	Molecules		Radicals		Positive species	Negative species
	CO 2 CO H 2 O 2	C 2 O H O OH	CO 2 + CO +	H 2 + H 3 +
	H 2 O H 2 O 2 CH 4	HO 2 O 2 H C	CH	OH + H 3 O + H 2 O + O 2 +
	C 2 H 2 C 2 H 4	CH 2	CH 3 C 2 C 2 H	O + H + CH 5 + CH 4 +
	C 2 H 6 C 3 H 6 C 3 H 8 CH 3 OH HCOOH CH 3 CHO	C 2 H 3 C 2 H 5 C 3 H 7 HCO CH 2 O CH 2 OH CH 3 O	CH 3 + CH 2 + CH + O 2 H + C 2 H 6 + C 2 H5 + C 2 H 4 + C 2 H 3 + C 2 H 2 + C 2 H + C 2 +	O 2 -O -OH -CH 2 -H -
	CH 3 COOH	COOH CH 2 CO	C + HCO + CH 2 O +
	C 2 H 5 OH C 3 H 7 OH	CH 3 CO C 2 HO		CH 3 O + HCOOH +
	C 2 H 5 COOH	CH 2 CHO C 2 H 5 CO	C 2 H 5 OH + CHCO +
	C 3 H 7 COOH	C 3 H 7 CO C 2 H 5 O	CH 2 CO + CH 3 CO + HCO 2 +

Table 3 -

 3 1. Physical properties of fresh catalysts and used K-Fe/C-SiO2.

	Sample	Surface area a (m 2 /g)	Average pore diameter b (nm)	Pore volume b (cm 3 /g)	Average metal particle size c (nm)
	SiO 2	631.09	3.2	0.50	-
	Fe-SiO 2	510.08	4.2	0.54	2.69
	K-Fe-SiO 2	169.34	13.8	0.23	2.86
	Fe/C-SiO 2	421.82	4.5	0.47	2.71
	K-Fe/C-SiO 2	330.54	3.7	0.31	2.65
	Used K-Fe/C-SiO 2	303.76	3.8	0.29	2.73

Table 3

 3

			-2 Summary of energy consumption
		Peak-to-peak	Input power	SIE	EE-CO2	EE-CH4
	Catalyst
		voltage (kV)	(W)	(kJ/L)	(mmol/kJ)	(mmol/kJ)
	None	5.2	2.40	7.2	0.14	0.43
	SiO2	5.7	2.48	7.44	0.17	0.48
	Fe-SiO2	6.0	2.64	7.92	0.16	0.38
	K-Fe-SiO2	6.0	2.33	6.99	0.15	0.44
	Fe/C-SiO2	6.0	2.37	7.11	0.16	0.42
	K-Fe/C-SiO2	6.0	2.35	7.05	0.15	0.36

Table 3 -

 3

	Catalysts	Electric field strength (kV/cm)	Reduced electric field (Td)	Mean electron energy (eV)
	None	13.00	48.51	2.45
	SiO 2	24.55	91.59	3.87
	Fe-SiO 2	28.56	106.58	4.19
	K-Fe-SiO 2	28.23	105.32	4.17
	Fe/C-SiO 2	36.38	135.75	4.70
	K-Fe/C-SiO 2 (1mm)	36.14	134.84	4.71
	K-Fe/C-SiO 2 (2mm)	29.26	109.20	4.24

[START_REF] Thuiller | Climate change and the ecologist[END_REF]

. Effect of packed catalysts on the average electric field strength, the reduced electric field and the mean electron energy.

Table 3 -

 3 4. Comparison of plasma-catalytic conversion of CO2 and CH4.

	Catalyst	CO2/CH4	Power (W)	Conversion (%) CO2 CH4	H2/CO	U/S	Liquid oxygenates	Ref
	None	1:1	9.71	29.0 44.0	0.81	0.03	negligible	[74]
	Pt/γ-Al2O3	1:1	10	12.6 17.3	-	-	Acetic acid, Methanol, Ethanol, Acetone, HCHO	[69]
	NiCe1C	1:1	40	53.7 55.6	0.98	0.1	-	[153]
	Ni/Al2O3	2:3	16	19.6 27.2	1.96	≈ 0.01	-	[154]
	NiGa/NF	1:1	25	8.5	16.0	≈ 0.92 ≈ 0.06 Methanol, Ethanol, C3H8O, Formic acid, C3H6O2	[155]
	UiO-67	1:1 a	11	39.0 52.0	0.85	1.29	-	[156]
	K-Fe-SiO2 K-Fe/C-SiO2	1:1 1:1	2.37 2.35	5.17 15.34 5.09 12.65	1.48 2.39	0.35 0.90	Formic acid, Acetic acid, C3+ acid, Methanol, Ethanol	This work This work
	a. Diluted CH4 and CO2 (5000 ppm CH4 and 5000 ppm CO2 in Argon balance)

Table 4 -

 4 1. Summary of critical input data for CHEMKIN simulations.

  electron impact reactions, ion reactions and neutral reaction. It could be further converted and reduced into hydrocarbons and oxygenates via FTS in the meantime. First, H radicals are generated from electron impact dissociation of CH4 and CH4 + (e.g. R-003, R-039, R-041 and etc.) which produce H radical and C1 radicals. H2 is also produced due to the combination of H.

	in C1 or C2 intermediates (e.g. R-158, R-177 and etc.) which also lead to the formation of CO
	in the meantime.
	CH4 + H → CH3 + H2	(R-088)
	CH3 + H → CH2 + H2	(R-103)
	C2H6 + H → C2H5 + H2	(R-121)
	CH2 + O → CO + H2	(R-158)
	C2H3 + O → CO + CH3	(R-177)
	H + CO2 → CO + OH	(R-279)
	Besides, CO is able to form via the reaction R-025 and R-072.
	e + CH4 → e + CH + 2 H2	(R-003)
	e + CH4 + → CH + H + H2	(R-039)
	e + CH3 + → CH + H2	(R-041)
	Since the intermediates exist in the plasma surrounding, those radicals participated reactions
	play an important role in the formation of CO and H2. It contributes significantly to the CO and
	H2 formation, especially in the afterglow, due to the longer life time compared to the charged
	particles. In our model, produced H species further react with CH4 and other radicals in so called
	secondary reactions (e.g. R-088, R-103 and etc.), consequently, more H2 and secondary
	particles are produced. It is clear that H could also collide with products (e.g. C2H6, C2H4 and
	etc.) in which the selectivity of them are greatly influenced. For example, C2H5 transient state
	is able to be produced from the collision of C2H6 and H radicals which generates H2
	simultaneously. This process of H2 formation is similar to C. De Bie et.al, [132] who proposed
	that the electron impact dissociations of C2H6 and C3H8 are the most important formation
	channels of H2 while electron impact dissociation of CH4 contributes less.

Regarding the O radicals derived from CO2 dissociation, it is worth to mention that CO, H2 and CHx radicals are critical products in O involved reactions. O could cause the crack of C-H bond Apart from formation reactions in plasma, CO and H2 also participate in further producing hydrocarbons and oxygenates via FTS. On the one hand, H2 and H species could reduce C1 and C2 transient structures (e.g. R-127, R-141 and etc.), on the other hand, CHxO intermediates are also attributed to the reaction of CO and H (e.g. R-280 and etc.). As CO is able to combine with another carbon-containing species (such as CH3, CH2 and etc.), carbon chain prolongation, which is critical in producing higher hydrocarbons and oxygenates, is achieved via C-C coupling in plasma environment (e.g. R-127, R-141 and etc.). Particularly, in CO participated carbon chain prolongation, we notice that C2 oxygenate (CH3CO transient structure) form via

Table 4 -

 4 Rh sites are compared. The catalysts show capability to decrease the activation energy. The Ni-based catalysts have a comparable slope, in other word comparable reaction barrier, since they share a similar reaction routes, while Rh-based catalyst exhibits even lower activation energy due to their surface reactions. It is critical to mention that, for all the simulations, the activation energy of the CO2 is lower than that of CH4. This tendency could not only be attributed to the surface properties of the metal sites but also to the reverse water gas shift (RWGS) reaction. This phenomenon is consistent with the literature[START_REF] Elsayed | Low temperature dry reforming of methane over Pt-Ni-Mg/ceria-zirconia catalysts[END_REF][START_REF] Zhang | Kinetic Studies of Carbon Dioxide Reforming of Methane over Ni-Co/Al-Mg-O Bimetallic Catalyst[END_REF]170]. 2. Summary of calculated activation energy of CO2 and CH4.

	CH2(s) + H2O → CO(s) + 2H2

  -24. The thickness of lines and arrows represent the relative rate of production for elementary reaction. As shown in the reaction path diagram, the dominant CHxO), which are not involved in this modelling, also are indispensable in the surface reaction mechanism. More comprehensive numerical model should be developed to elaborate the reaction pathway by which products and intermediates form.

	H2 + Rh(s) + Rh(s) → H(s) + H(s)	(RS-01)
	CO + Rh(s) → CO(s)	(RS-05)
	O(s) + H(s) → OH(s) + Rh(s)	(RS-13)
	OH(s) + H(s) → H2O(s) + Rh(s)	(RS-15)
	CO(s) + OH(s) → COOH(s) + Rh(s)	(RS-23)
	Then, adsorbed H(s) plays a critical role in the formation of OH(s) via RS-13 which is essential
	in the production of H2O(s) and COOH(s) as well via RS-15, RS-26 and etc. Figure 4-24 also
	shows the surface losing reactions of OH(s) (RS-42, RS-44, RS-46 and etc) which indicates
	further effect of oxygenates on the production of hydrocarbons. It should be noted that another
	types of oxygenates (
	surface reactions are the adsorption processes, formation of H(s), CO(s), CH3(s) and CH4(s)
	starting from Rh(s) specie (e.g. RS-01, RS-05, RS-06, RS-33, and etc.).

Table 4 -

 4 4. Summary of surface area for CHEMKIN simulations.

  Activation barriers of plasma assisted dry reforming with or without catalysts are calculated and fitted by a modified Arrhenius law which are rarely calculated by computational method in previous study. In this study, by inducing reported surface mechanisms over Rh and Ni catalysts, it was shown that the activation energies decreased by 22.45% and 17.05% for CO2 conversion and CH4 conversion respectively. These results show by a numerical way the advantage in terms of energy of plasma-catalytic conversion face of nonthermal plasma conversion alone. We proposed in the last part of this chapter, on the basis of previous work, to combine an experimental study and a numerical study under the same plasma-catalytic conditions. we combined theoretical model and experiments to predict the selectivity of catalysts for specific products. Surface reaction mechanism of SAPO-34 were introduced in validated model. The selectivity of C2H4 gradually exceeded C2H6

  Figure 1-1. Schematic diagram of carbon cycle ......................................................................... 2 Figure 1-2. Syngas derivatives with reference to its H2/CO molar ratio composition [39]. ...... 4 Figure 1-3. Overview of reactions in the direct synthesis of organic products from CO2 conversion via thermal catalysis. ............................................................................................... 6 Figure 1-4. Schematic of typical catalysts for enhancing the selection of olefins and C5+ hydrocarbons [48] .................................................................................................................... 10 Figure 1-5. Surface carbide mechanism of chain growth in FTS. ............................................ 12 Figure 1-6 Diagram of species in the aggregated collection of plasma. .................................. 14 Figure 1-7. The configuration of (a) Fixed-bed reactor, (b) DBD plasma reactor and (c) packed DBD plasma reactor. ................................................................................................................ 15 Figure 1-8. Interaction of homogeneous and heterogeneous (surface) processes. ................... 16

	Chapter 1

Figure 1-9. An overview of plasma catalysis. .......................................................................... 17

  .................... 54 Figure 3-6. SEM image and multi-element EDS mapping spectra of K-Fe-SiO2 catalyst for Si, O, Fe and K. ............................................................................................................................. 55 Figure 3-7. SEM image and multi-element EDS mapping spectra of K-Fe/C-SiO2 catalyst for Si, O, Fe, C and K. ................................................................................................................... 55

  Catalysts granule size = 1.0 mm. .......................................................................................................................... 75 Figure 3-27. Reaction mechanisms proposed for plasma-catalytic conversion of CO2 and CH4. .................................................................................................................................................. 76 Figure 4-1. The measurement of voltage and current of plasma reactor. ................................. 84 Figure 4-3. Summary of number density of reactants and main products after the plasma power pulse. ........................................................................................................................................ 85 Figure 4-4. Summary of number density of key intermediates (radicals) in the plasma power pulse. ........................................................................................................................................ 86 Figure 4-5. Summary of number density of key intermediates (charged species) in the plasma power pulse. ............................................................................................................................. 87

	Figure 3-26. OES spectrum of (a) air around the ground electrode, (b) plasma catalytic
	conversion of CO2 and CH4 without catalysts (c) plasma catalytic conversion of CO2 and CH4
	with K-Fe/C-SiO2 catalysts (granule size: 2 mm). Reaction conditions: Input power = 2.4 -
	2.6W, Frequency = 4.0 kHz, Flow rate = 30 mL/min, CO2/CH4 ratio = 1/1, Chapter 4

.................................................................................................................................. 73 Figure 4-2. Plasma power deposition as a function of time Qsou(t) and (b) electron temperature in one plasma pulse (before the dashed line) and afterglow (after the dashed line). ............... 85

Table A1 :

 A1 Electron impact reactions, electron-ion recombination reactions, neutral-neutral reactions, ion-neutral and ion-ion reactions included in the model. The corresponding rate coefficients are adopted from[START_REF] De Bie | The Dominant Pathways for the Conversion of Methane into Oxygenates and Syngas in an Atmospheric Pressure Dielectric Barrier Discharge[END_REF][START_REF] Martini | Oxidation of CH4 by CO2 in a dielectric barrier discharge[END_REF][START_REF] Zheng | Kinetic Study of Nonequilibrium Plasma-Assisted Methane Steam Reforming[END_REF][START_REF] Manion | NIST Chemical Kinetics Database[END_REF][START_REF] Mcelroy | The UMIST database for astrochemistry[END_REF][START_REF] De Bie | Fluid Modeling of the Conversion of Methane into Higher Hydrocarbons in an Atmospheric Pressure Dielectric Barrier Discharge: Fluid Modeling of the Conversion of Methane …[END_REF][START_REF] Aerts | Influence of Vibrational States on CO 2 Splitting by Dielectric Barrier Discharges[END_REF][START_REF] Heijkers | Nanosecond Pulsed Discharge for CO 2 Conversion: Kinetic Modeling To Elucidate the Chemistry and Improve the Performance[END_REF][START_REF] Snoeckx | Plasma-Based Dry Reforming: A Computational Study Ranging from the Nanoseconds to Seconds Time Scale[END_REF].

	R-039 R-269 R-315 R-545	e + CH4 + → CH + H + H2 C3H7 + HCO → C3H8 + CO O + C2HO → CO + CO + H H + + C2 → C2 + + H	1.41 × 10 -08 1.00 × 10 -10 1.90 × 10 -10 3.10 × 10 -09	0.0 0.0 0.0 0.0	0.0 0.0 0.0 0.0
	R-040 R-270 R-316 R-546	e + CH3 + → CH2 + H C3H7 + CH3O → C3H8 + CH2O O2 + HCO → CO + HO2 H + + O → O + + H	2.25 × 10 -08 4.00 × 10 -11 5.10 × 10 -11 6.86 × 10 -10	0.0 0.0 0.0 0.3	0.0 0.0 0.0 224.3
	R-041 R-271 R-317 R-547	e + CH3 + → CH + H2 C3H7 + OH → C3H6 + H2O O2 + CH3O → CH2O + HO2 H + + O2 → O2 + + H	7.88 × 10 -09 4.00 × 10 -11 1.97 × 10 -15 2.00 × 10 -09	0.0 0.0 0.0 0.0	0.0 0.0 0.0 0.0
	R-042 R-272 R-318 R-548	e + CH3 + → CH + 2H C3H7 + H2O2 → C3H8 + HO2 O2 + CH2CHO → CH2O + CO + OH H + + OH → OH + + H	9.00 × 10 -09 5.15 × 10 -15 3.00 × 10 -14 2.10 × 10 -09	0.0 0.0 0.0 -0.5	0.0 0.0 0.0 0.0
	R-043 R-273 R-319 R-549	e + CH3 + → C + H + H2 C3H7 + CH3OH → C3H8 + CH3O O2 + C2HO → CO + CO + OH H + + H2O → H2O + + H	1.69 × 10 -08 1.12 × 10 -15 6.46 × 10 -13 6.90 × 10 -09	0.0 0.0 0.0 -0.5	0.0 0.0 0.0 0.0
	R-044 R-274 R-320 R-550	e + CH2 + → CH + H C3H7 + CH3OH → C3H8 + CH2OH O + H2O2 → HO2 + OH H + + H -→ H + H	1.00 × 10 -08 3.90 × 10 -15 1.42 × 10 -12 7.51 × 10 -08	0.0 0.0 0.0 -0.5	0.0 0.0 0.0 0.0
	R-045 R-275 R-321 R-551	e + CH2 + → C + H2 C3H7 + CH2OH → C3H8 + CH2O O + H2O2 → O2 + H2O H -+ CH3 → CH4 + e	4.82 × 10 -09 1.60 × 10 -12 2.76 × 10 -12 1.00 × 10 -09	0.0 0.0 0.0 0.0	0.0 0.0 0.0 0.0
	No. R-046 R-276 R-322 R-552	Reactions e + CH2 + → C + 2H C3H7 + CH2OH → C3H6 + CH3OH O + CH3CO → OH + CH2CO H -+ CH2 → CH3 + e	A 2.53 × 10 -08 8.00 × 10 -13 8.75 × 10 -11 1.00 × 10 -09	b 0.0 0.0 0.0 0.0	Ea 0.0 0.0 0.0 0.0
	R-001 R-047 R-277 R-323 R-553	e + CH4 → 2e + CH4 + e + CH + → C + H H2 + OH → H + H2O O + CH3CO → CO2 + CH3 H -+ CH → CH2 + e	9.34 × 10 -08 3.23 × 10 -08 7.02 × 10 -15 2.63 × 10 -10 1.00 × 10 -10	0.0 0.0 0.0 0.0 0.0	3062.2 0.0 0.0 0.0 0.0
	R-002 R-048 R-278 R-324 R-554	e + CH4 → e + CH3 + H e + C2H6 + → C2H5 + H H2 + HCO → H + CH2O O + CH3OH → OH + CH2OH H -+ C → CH + e	2.53 × 10 -08 2.19 × 10 -08 2.78 × 10 -26 3.32 × 10 -11 1.00 × 10 -09	0.0 0.0 0.0 0.0 0.0	1762.4 0.0 0.0 0.0 0.0
	R-003 R-049 R-279 R-325 R-555	e + CH4 → e + CH2 + H2 e + C2H6 + → C2H4 + 2H H + CO2 → CO + OH O + CH3OH → OH + CH3O H -+ C2H → C2H2 + e	2.60 × 10 -08 3.36 × 10 -08 1.40 × 10 -29 1.66 × 10 -11 1.00 × 10 -09	0.0 0.0 0.0 0.0 0.0	1861.0 0.0 0.0 0.0 0.0
	R-004 R-050 R-280 R-326 R-556	e + CH4 → e + CH + H2 + H e + C2H5 + → C2H4 + H H + CO → HCO O + CH2OH → CH2O + OH H -+ C2 → C2H + e	2.66 × 10 -08 7.70 × 10 -09 1.54 × 10 -34 1.00 × 10 -11 1.00 × 10 -09	0.0 0.0 0.0 0.0 0.0	1956.2 0.0 0.0 0.0 0.0
	R-005 R-051 R-281 R-327 R-557	e + CH4 → e + C + H2 + H2 e + C2H5 + → C2H3 + 2H H + H2O → H2 + OH O2 + CH2OH → CH2O + HO2 H -+ H → H2 + e	2.72 × 10 -08 1.92 × 10 -08 5.86 × 10 -26 1.66 × 10 -11 4.32 × 10 -09	0.0 0.0 0.0 0.0 -0.4	2047.9 0.0 0.0 0.0 39.4
	R-006 R-052 R-282 R-328 R-558	e + CH4 → CH3 + H -e + C2H5 + → C2H2 + H2 + H H + OH → H2 + O CO + OH → CO2 + H H -+ O → OH + e	1.74 × 10 -12 1.90 × 10 -08 1.05 × 10 -16 1.25 × 10 -13 1.00 × 10 -09	0.0 0.0 0.0 0.0 0.0	159.8 0.0 0.0 0.0 0.0
	R-007 R-053 R-283 R-329 R-559	e + CH4 → CH2 -+ H2 e + C2H5 + → C2H2+3H H + OH → H2O CO + CH3O → CO2 + CH3 H -+ OH → H2O + e	7.13 × 10 -13 8.98 × 10 -09 4.33 × 10 -30 6.56 × 10 -20 1.00 × 10 -10	0.0 0.0 0.0 0.0 0.0	242.3 0.0 0.0 0.0 0.0
	R-008 R-054 R-284 R-330 R-560	e + CH4 → 2e + CH3 + + H e + C2H5 + → CH3 + CH2 H + HO2 → H2 + O2 H2O + CH3O → CH3OH + OH H -+ H2O → OH -+ H2	9.56 × 10 -08 9.62 × 10 -09 5.90 × 10 -12 1.67 × 10 -14 4.80 × 10 -09	0.0 0.0 0.0 0.0 0.0	3456.4 0.0 0.0 0.0 0.0
	R-009 R-055 R-285 R-331 R-561	e + CH3 → 2e + CH3+ e + C2H4 + → C2H2 + H2 H + HO2 → H2O + O OH + OH → H2O + O H -+ O + → H + O	8.77 × 10 -08 3.36 × 10 -08 2.40 × 10 -12 1.47 × 10 -12 7.51 × 10 -08	0.0 0.0 0.0 0.0 -0.5	2657.7 0.0 0.0 0.0 0.0
	R-010 R-056 R-286 R-332 R-562	e + CH3 → 2e + CH2 + + H e + C2H4 + → C2H3 + H H + HO2 → OH + OH OH + OH → H2O2 H -+ H3O + → H2 + OH + H	6.05 × 10 -08 8.29 × 10 -09 7.20 × 10 -11 6.86 × 10 -31 7.51 × 10 -08	0.0 0.0 0.0 0.0 -0.5	3235.8 0.0 0.0 0.0 0.0
	R-011 R-057 R-287 R-333 R-563	e + CH2 → 2e + CH2 + e + C2H4 + → C2H2 + 2H H + CH2O → H2 + HCO OH + HO2 → H2O + O2 H -+ H3O + → H2O + H2	8.52 × 10 -08 3.43 × 10 -08 5.72 × 10 -14 1.10 × 10 -10 7.51 × 10 -08	0.0 0.0 0.0 0.0 -0.5	2661.6 0.0 0.0 0.0 0.0
	R-012 R-058 R-288 R-334 R-564	e + CH2 → 2e + CH + + H e + C2H4 + → C2H + H2 + H H + HCO → H2 + CO OH + CH2O → H2O + HCO O + + CH4 → CH4 + + O	3.79 × 10 -08 5.53 × 10 -09 1.50 × 10 -10 8.47 × 10 -12 8.90 × 10	0.0 0.0 0.0 0.0	3417.6 0.0 0.0 0.0
	R-013 R-059 R-289 R-335	e + CH → 2e + CH + e + C2H3 + → C2H2 + H H + CH3O → H2 + CH2O OH + HCO → CO + H2O	6.16 × 10 -10 1.34 × 10 -08 2.32 × 10 -11 1.70 × 10 -10	0.4 0.0 0.0 0.0	1912.3 0.0 0.0 0.0
	R-014 R-060 R-290 R-336	e + CH → e + C + H e + C2H3 + → C2H + H + H H + CH3O → CH3 + OH OH + CH3O → CH2O + H2O	9.24 × 10 -11 2.74 × 10 -08 9.93 × 10 -12 3.00 × 10 -11	0.4 0.0 0.0 0.0	2627.4 0.0 0.0 0.0
	R-015 R-061 R-291 R-337	e + C2H6 → e + CH3 + CH3 e + C2H2 + → C2H + H H + CH3CHO → H2 + CH3CO OH + CH3CHO → CH3CO + H2O	9.22 × 10 -09 1.87 × 10 -08 8.98 × 10 -14 1.49 × 10 -11	0.2 0.0 0.0 0.0	1380.2 0.0 0.0 0.0
	R-016 R-062 R-292 R-338	e + C2H6 → e + C2H5 + H e + C2H2 + → 2CH H + CH2CO → CH3 + CO OH + CH2CO → CO + CH2OH	1.81 × 10 -06 4.87 × 10 -08 1.04 × 10 -13 1.14 × 10 -11	-0.2 0.0 0.0 0.0	2394.6 0.0 0.0 0.0
	R-017 R-063 R-293 R-339	e + C2H6 → 2e + C2H6 + e + C2H + → C2 + H H + C2HO → CH2 + CO HO2 + HO2 → H2O2 + O2	3.79 × 10 -07 1.16 × 10 -07 2.50 × 10 -10 1.63 × 10 -12	0.0 0.0 0.0 0.0	3321.6 0.0 0.0 0.0
	R-018 R-064 R-294 R-340	e + C2H4 → 2e + C2H4 + e + C2H + → CH + C H + H2O2 → H2O + OH HO2 + CH2O → HCO + H2O2	2.72 × 10 -07 1.53 × 10 -07 1.66 × 10 -11 1.05 × 10 -20	0.0 0.0 0.0 0.0	3253.6 0.0 0.0 0.0
	R-019 R-065 R-295 R-341	e + C2H4 → e + C2H3 + H e + C2H + → 2C + H H + H2O2 → H2 + HO2 HO2 + HCO → OH + H + CO2	2.46 × 10 -10 1.53 × 10 -07 2.81 × 10 -12 5.00 × 10 -11	0.5 0.0 0.0 0.0	1031.0 0.0 0.0 0.0
	R-020 R-066 R-296 R-342	e + C2H4 → e + CH2 + CH2 e + C2 + → 2C H + CH3OH → CH2OH + H2 HO2 + CH3O → CH2O + H2O2	1.86 × 10 -08 3.00 × 10 -07 2.42 × 10 -12 5.00 × 10 -11	0.1 0.0 0.0 0.0	1455.0 0.0 0.0 0.0
	R-021 R-067 R-297 R-343	e + C2H2 → e + C2H + H e + H3 + → 3H H + CH3OH → CH3O + H2 OH + H2O2 → HO2 + H2O	1.90 × 10 -03 4.36 × 10 -08 6.64 × 10 -11 1.16 × 10 -11	-1.1 0.0 0.0 0.0	1238.9 0.0 0.0 0.0
	R-022 R-068 R-298 R-344	e + C2H2 → e + CH + CH e + H3 + → H2 + H H + CH2OH → CH2O + H2 OH + CH3CO → CH2CO + H2O	3.34 × 10 -06 2.34 × 10 -08 4.98 × 10 -11 2.01 × 10 -11	-0.3 0.0 0.0 0.0	1388.5 0.0 0.0 0.0
	R-023 R-069 R-299 R-345	e + CO2 → 2e + CO2 + e + H2 + → H + H H + CH2OH → CH3 + OH OH + CH3CO → CH3 + CO + OH	2.10 × 10 -07 1.60 × 10 -08 1.60 × 10 -10 7.01 × 10 -11	0.0 0.0 0.0 0.0	4025.9 0.0 0.0 0.0
	R-024 R-070 R-300 R-346	e + CO2 → O -+ CO e + O2 + → O + O H + CH2OH → CH3OH OH + CH3OH → H2O + CH2OH	4.36 × 10 -12 1.94 × 10 -20 2.89 × 10 -10 3.10 × 10 -12	0.0 0.0 0.0 0.0	63.2 0.0 0.0 0.0
	R-025 R-071 R-301 R-347	e + CO2 → e + CO + O e + O2 + + O2 → O2 + O2 O + CO → CO2 OH + CH3OH → H2O + CH3O	2.56 × 10 -10 1.00 × 10 -26 1.11 × 10 -35 1.40 × 10 -13	0.0 0.0 0.0 0.0	1771.0 0.0 0.0 0.0
	R-026 R-072 R-302 R-348	e + CO → 2e + CO + e + CO2 + → CO + O O + H2O → OH + OH OH + CH2OH → CH2O + H2O	1.82 × 10 -08 2.71 × 10 -07 4.48 × 10 -24 4.00 × 10 -11	0.0 0.0 0.0 0.0	1529.0 0.0 0.0 0.0
	R-027 R-073 R-303 R-349	e + CO → O -+ C e + CO + → C + O O + OH → H + O2 HO2 + CH3CO → CH3 + CO2 + OH	2.76 × 10 -06 2.00 × 10 -07 3.46 × 10 -11 5.00 × 10 -11	-0.8 0.0 0.0 0.0	1061.0 0.0 0.0 0.0
	R-028 R-074 R-304 R-350	e + CO → e + C + O e + OH + → O + H O + HO2 → O2 + OH HO2 + CH3OH → CH2OH + H2O2	2.79 × 10 -08 3.75 × 10 -08 5.70 × 10 -11 1.60 × 10 -13	0.0 0.0 0.0 0.0	2383.8 0.0 0.0 0.0
	R-029 R-075 R-305 R-351	e + H2O → O -+ H2 e + H2O + → OH + H O + CH2O → OH + HCO HO2 + CH2OH → CH2O + H2O2	3.34 × 10 -05 8.60 × 10 -08 1.73 × 10 -13 2.01 × 10 -11	-1.1 0.0 0.0 0.0	1197.3 0.0 0.0 0.0
	R-030 R-076 R-306 R-352	e + H2O → OH + H -e + H2O + → O + H2 O + HCO → CO + OH H2O2 + CH3CO → CH3CHO + HO2	5.36 × 10 -04 3.90 × 10 -08 5.00 × 10 -11 3.01 × 10 -13	-1.3 0.0 0.0 0.0	898.0 0.0 0.0 0.0
	R-031 R-077 R-307 R-353	e + H2O → OH -+ H e + H2O + → O + 2H O + HCO → H + CO2 H2O2 + CH2OH → CH3OH + HO2	6.99 × 10 -10 3.05 × 10 -07 5.00 × 10 -11 5.00 × 10 -15	-0.3 0.0 0.0 0.0	776.6 0.0 0.0 0.0
	R-032 R-078 R-308 R-354	e + H2O → e + H + OH e + H3O + → H2O + H O + CH3O → CH3 + O2 CH2O + CH3O → CH3OH + HCO	4.29 × 10 -09 2.45 × 10 -08 2.20 × 10 -11 1.69 × 10 -13	0.1 0.0 0.0 0.0	1588.1 0.0 0.0 0.0
	R-033 R-079 R-309 R-355	e + H2O → e + O + H2 e + H3O + → O + H2 + H O + CH3O → OH + CH2O HCO + HCO → CH2O + CO	5.01 × 10 -09 5.60 × 10 -09 3.00 × 10 -12 3.01 × 10 -11	0.1 0.0 0.0 0.0	3267.6 0.0 0.0 0.0
	R-034 R-080 R-310 R-356	e + H2 → 2e + H2 + e + H3O + → OH + H2 O + CH3CHO → OH + CH3CO HCO + CH3O → CH3OH + CO	4.80 × 10 -08 6.58 × 10 -09 4.68 × 10 -13 1.50 × 10 -10	0.0 0.0 0.0 0.0	3004.8 0.0 0.0 0.0
	R-035 R-081 R-311 R-357	e + H → 2e + H + e + H3O + → OH + H + H O + CH2CO → CH2 + CO2 CH3O + CH3O → CH2O + CH3OH	6.44 × 10 -07 4.02 × 10 -07 2.29 × 10 -13 1.00 × 10 -10	0.0 0.0 0.0 0.0	2040.3 0.0 0.0 0.0
	R-036 R-082 R-312 R-358	e + H2 → e + 2H CH4 + CH2 → CH3 + CH3 O + CH2CO → CH2O + CO CH2O + CH3CO → CH3CHO + HCO	9.91 × 10 -04 3.01 × 10 -19 7.88 × 10 -14 3.01 × 10 -13	-0.9 0.0 0.0 0.0	1920.6 0.0 0.0 0.0
	R-037 R-083 R-313 R-359	e + CH4 + → CH3 + H CH4 + CH → C2H4 + H O + CH2CO → HCO + CO + H CH2O + CH2OH → CH3OH + HCO	1.18 × 10 -08 1.70 × 10 -11 4.33 × 10 -14 7.72 × 10 -14	0.0 0.0 0.0 0.0	0.0 0.0 0.0 0.0
	R-038 R-084 R-314 R-360	e + CH4 + → CH2 + 2H CH4 + C2H5 → C2H6 + CH3 O + CH2CO → HCO + HCO HCO + H2O2 → CH2O + HO2	2.42 × 10 -08 1.83 × 10 -24 4.33 × 10 -14 1.69 × 10 -13	0.0 0.0 0.0 0.0	0.0 0.0 0.0 0.0

à 35,2 kJ.mol -1 pour le CH4 en DRM plasmocatalytique. Dans la plupart des études expérimentales, l'énergie d'activation du CO2 est inférieure à celle du CH4, ce qui est conforme à notre étude numérique. A titre de comparaison, l'énergie d'activation du CO2 pour la réaction d'hydrogénation passe de 68 kJ.mol -1 -112 kJ.mol -1 en thermocatalyse à

kJ.mol -1 -43 kJ.mol - 1 en plasmocatalyse. Ce chapitre et ses résultats numériques montre finalement à quel point

between plasma and catalysts.

This preliminary work demonstrates the advantages of using K promoted iron-based catalyst for plasma-catalytic conversion of the two main greenhouse gases at near ambient conditions and so provide a new trail for improving the production of added-value hydrocarbons and liquid oxygenates by this way to move forward the development of an innovative green sustainable chemistry solution. The products whose production rate is increased by this means are valuable molecules for many current chemical industrial applications.

CHAPTER III. SELECTIVE SYNTHESIS OF VALUE-ADDED MOLECULES VIA PLASMA CATALYSIS Abstract

This chapter introduces a pioneering research on direct synthesis of value-added products using well-designed catalyst. In the new field of plasma-catalytic synthesis, iron-based catalysts have been intensively studied in the recent years but no study was reported on the use of potassium promoter (K-promoter) although it is widely used in traditional thermo-catalytic synthesis, particularly to produce lightweight unsaturated hydrocarbons. In this frame, we study effect of K-promoter on iron-based plasma-catalytic conversion of CO2 and CH4 into higher value products, through series of experiments using iron-based catalysts with and without potassium.

As expected, K-promoter tunes the selectivity of unsaturated hydrocarbons but also that of longchain oxygenates, by changing the H-adsorption property of the catalyst. With K addition in the catalytic material, the ratio of unsaturated over saturated hydrocarbons significantly increases while the formation of long-chain oxygenates (propionic, butyric and valeric acids, and C3+ alcohols) are clearly enhanced. Thus, thanks to an experimental comparative study, this chapter proposes a first process optimization attempt for maximizing at the same time the production of liquid products and the share of unsaturated hydrocarbons. At this early stage, this first attempt allows to coproduce high quality syngas (H2/CO~2.0), C2 + hydrocarbons and C1 + oxygenates with 5.10% CO2 conversion and 12.65% CH4 conversion and 21.76% liquid selectivity.

INFORMATION Plasma alone

Ni Plasmacatalysis according to mechanism of [START_REF] Elsayed | Low temperature dry reforming of methane over Pt-Ni-Mg/ceria-zirconia catalysts[END_REF] Ni Plasmacatalysis according to mechanism of [START_REF] Delgado | Surface Reaction Kinetics of Steam-and CO2-Reforming as Well as Oxidation of Methane over Nickel-Based Catalysts[END_REF] Rh Plasmacatalysis according to mechanism of [START_REF] Karakaya | Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts[END_REF] Slope (CO2) - It is essential to compare our simulated values of activation energy with experimental values.

We compared our simulation results with experimental activation energies available in the literature. Table 4-3 shows the activation barrier of CO2 and CH4 conversion in thermal catalysis and plasma-catalysis. For the thermal catalysis, the activation energies of CO2 are reported to be between 62.2 kJ mol -1 and 108.9 kJ mol -1 while it requires 65.5 kJ mol -1 -87.8 kJ mol -1 for CH4 activation [START_REF] Bao | Kinetic study of methane reforming with carbon dioxide over NiCeMgAl bimodal pore catalyst[END_REF][START_REF] Zhang | Kinetic Studies of Carbon Dioxide Reforming of Methane over Ni-Co/Al-Mg-O Bimetallic Catalyst[END_REF]. The reaction barriers decrease remarkably in plasma assisted process. The activation energies are estimated to be 29.5 kJ mol -1 -6.1 kJ mol -1 and 35.2 kJ mol -1 -32.6 kJ mol -1 for CO2 and CH4 respectively [START_REF] Khoja | Kinetic study of dry reforming of methane using hybrid DBD plasma reactor over La2O3 co-supported Ni/MgAl2O4 catalyst[END_REF][START_REF] Kim | Synergistic effects of plasma-catalyst interactions for CH4 activation[END_REF]. In most of works, the activation energy of CO2 is lower than CH4 which is in line with our numerical study. Besides dry reforming, the activation energy of CO2 also declines sharply in CO2 plasma assisted hydrogenation (21 kJ mol -1 -43 kJ mol -1 ) when it is compared with the value in thermal catalysis (68 kJ mol -1 -112 kJ mol -1 ) [START_REF] Xu | Mechanistic study of non-thermal plasma assisted CO2 hydrogenation over Ru supported on MgAl layered double hydroxide[END_REF].

Perspectives

In this work, plasma assisted DRM achieved a considerable improvement of value-added products. Nevertheless, such improvement is not adequate for applications on an industrial scale.

The conversion rates and selectivity of C2H4/C2H2 in the gas products are not as high as that in thermal and photoelectric catalysis. One explanation is that K is under the form of KCl rather than K2O or K2CO3. Although these K species all keep stable during the preparation and reaction, the latter could further increase the catalyst surface alkalinity. Whereupon great efforts are still required to optimize current pilot study. This section would propose some future perspectives and outlooks for plasma catalysis.

Experimental approaches

Catalyst development always is attractive since it could not only affect plasma but also tune the reaction pathway on the surface. With the development of well-designed active sites, promoters, porous structures and etc., it would yield a more meaningful process toward the commercialism for direct synthesis of value-added products. On the other hand, light radiation is also a nonnegligible factor for catalytic reactions under certain conditions. The ultraviolet-visible light produced in plasma-catalysis needs to be further studied, and the potential synergistic effect should and must be explored. Reactor configuration, including discharge gap, electros properties, cooling system, products collection, power supply, operating condition need to be further investigated for higher yield of target products. In-situ OES could be better conducted to elucidate the insight of plasma discharges. The states of transient states should be carefully interpreted so that Te, Tv, and etc., could be given by quantitative analysis which would be helpful to energy efficiency improvement in plasma catalysis. Besides, chemical kinetic study in plasma catalysis is still in an early stage. It is essential to calculate key kinetic parameters by establishing precise model.

Application

LIST OF TABLES

ABSTRACT

This thesis studies the plasma-catalytic conversion of a mixture composed of methane and carbon dioxide into higher value products. To achieve this, a cold plasma reactor and a series of iron-based catalysts are developed to allow partial dry reforming capable of simultaneously co-producing oxygenated and nonoxygenated higher molecules. This experimental work is accompanied by chemical kinetic modelling work using CHEMKIN. In the first part of this thesis, we present a method for the preparation of nanostructured iron-based catalysts capable of giving a relatively good dispersion of the charge on a silica support in view of the relative simplicity of the method. A complete physic-chemical characterization of these catalysts produced by this method as well as an evaluation of their performance in a plasma catalytic reactor are carried out. We then show that promotion by K doping is an advantageous solution to increase the selectivity of unsaturated.

Based on an analysis of the species observed by OES, the modified electric field in the discharge chamber and the molecules adsorbed on the catalytic surface, we propose a global empirical chemical mechanism.

The second part of the thesis is devoted to the actual modelling of the process using a chemical model combining the numerical resolution of the gas kinetics equations and the chemical kinetics equations. In this model, both homogeneous and heterogeneous chemistries are considered. Over a series of simulations, we then follow the densities of the reactants, products and main intermediates as a function of time. Successive simulations show that the conversion process breaks down into three distinct phases: excitation (6.0E-08 s), de-excitation (1.0E-07 s) and remanence (1.0E-05 s). The temporal evolutions of the densities of C2 hydrocarbons (C2H6, C2H4 and C2H2) and oxygenates such as alcohols and acids observed experimentally are thus monitored on these three phases. As an opening, we finally propose, on the basis of previous work, to combine an experimental study and a numerical study carried out under the same plasma-catalytic conditions, validate the model, advance in the physic-chemical understanding of this conversion process and suggest prospects for future improvement.

MOTS CLÉS

Plasmocatalytique, Reconversion du CO2, Etude numérique, Plasma non-thermique RÉSUMÉ Cette thèse étudie la conversion plasmocatalytique d'un mélange composé de méthane et dioxyde de carbone en produits à plus haute valeur. Pour cela un réacteur à plasma froid et une série de catalyseurs à base de Fer sont développés pour permettre un reformage à sec partiel capable de coproduire simultanément des molécules supérieures oxygénées et non-oxygénées. Ce travail expérimental est accompagné d'un travail de modélisation de la cinétique chimique sous CHEMKIN. Dans la première partie de cette thèse, nous présentons une méthode de préparation de catalyseurs nanostructurés à base de Fer capable de donner une relative bonne dispersion de la charge sur un support d'aérogel de silice en regard de la relative simplicité de la méthode. Une caractérisation physico-chimique complète de ces catalyseurs produits par cette méthode ainsi qu'une évaluation de leurs performances en réacteur plasmocatalytique sont effectuées.