Rim Grêce Abdallah

Savita Kaliya Bitar

Perumal

Jean-Marc Grêce Abdallah

Rim Giraudon

Nathalie Bitar

Rino De Geyter

Jean-François Morent

Lamonier

Jean-Marc Zhiping Ye

Nicolas Giraudon

Grêce Nuns

Ahmed Abdallah

Rino Addad

Nathalie Morent

Jean-François De Geyter

Nathalie Giraudon

Jean- François Morent

Savita Grêce Abdallah

Perumal Kaliya

Jean-Marc Veerapandian

Nathalie Giraudon

Jean-François Morent

Nathalie Giraudon

Jean-François Morent

Lamonier Ozone

C.4 Active international conference participations

Preferential dissolution of copper from Cu-Mn oxides

Summary

The world health organization (WHO) estimates the number of deaths due to pollution to be around 9 million per year, which corresponds to 20% of deaths in the worldwide. In Europe 800000 deaths per year are due to air pollution. Poor air quality also greatly increases the death rate from Covid-19.

Among air pollutants, Volatile Organic Compounds (VOCs) are considered as major contributors for air pollution. Indeed, they propagate in the air causing dangerous impact on human health as well as on the environment. Their transformation in the atmosphere result in the formation of tropospheric ozone and photochemical smog. On the other hand, they enhance the greenhouse effect and the destruction of the stratospheric ozone. In addition, VOCs have been proven to be toxic and/or carcinogenic for human health. European legislation has imposed stricter objectives to reduce VOC emissions. The current policy consists in priority to limit the use these compounds, but if it is not possible for technical reasons, it is required to find adequate techniques for radical reduction of VOC emissions. Conventional VOCs removal techniques, such as thermal oxidation, adsorption, condensation, biological oxidation and catalytic oxidation are linked with high energy cost especially when the concentration of VOCs is low.

As a cost and energy efficient alternative, the use of non-thermal plasma (NTP) offers interesting prospects for the removal of a low concentration of pollutants from the air at high flow rates. NTP is quite different from thermal plasma generated by plasma torches and arc discharges. The main advantage of NTP is that the energy delivered to the plasma source is almost entirely consumed to accelerate electrons while the background gas remains near room temperature, so preventing feed gas heating. Another important feature is related to the operation at atmospheric pressure which excludes the use of expensive vacuum equipment. In addition, NTP is characterized by its quick start-up and compact size. Electron impacts cause dissociation, ionization of the background gas and radical production which leads to the decomposition of the targeted VOCs.

Although NTP has been proposed in literature as a possible technology for air cleaning, the formation of undesired gaseous by-products (ozone, NOx, aerosols and other VOCs) which increase the overall toxicity of the treated gas stream, remains serious roadblocks towards industrial applications. To overcome these technological issues, the combination of NTP and heterogeneous catalysis in a technique called plasma-catalysis has become a hot research topic over the last two decades. In a plasma-catalysis system there are two ways to introduce the catalyst into the plasma: In Plasma Catalysis (IPC) the catalyst is located in the plasma discharge and in Post Plasma Catalysis (PPC) the catalyst is located downstream of the plasma discharge. As well, the choice of a suitable catalyst will help to optimize the selectivity into environmentally more friendly end products.

The treatment of air pollution by a hybrid system 'NTP and heterogeneous catalysis' requires the development of more accurate experimental conditions and new catalytic systems always more efficient to reduce the energy cost. This doctoral thesis is focused on the use PPC for the abatement of trichloroethylene (TCE), a typical chlorinated volatile organic compound used mainly in industries as a degreaser, diluted in dry/humid air.

In this context, different issues have been established during this thesis work in order to optimize the PPC process. One of the key issues has been to take maximum advantage of NTP generated ozone as a potential source of active oxygen species for the complete oxidation of the NTP generated partially oxidized gaseous compounds at low temperature (100 °C). In that way, the [O3]/[TCE]0 molar ratio has been adapted to 4, in accordance with the formal reaction of TCE oxidation by O3: C2Cl3H + 4O3  2CO2+ HCl + Cl2 + 4O2 by playing on different important parameters such as air flow rate, humidity as well as specific energy input.

The choice of the appropriate catalyst has been made considering some basic requirements such as its ozone decomposition capability, oxygen mobility, high CO2 selectivity in VOC total oxidation catalysis coupled with chlorine and water tolerance. Birnessite (δ-MnO2) doped or not with Ce, CexMn (x= 0.01, 0.1, 0.2 and 0.5) were prepared from a simple, fast and inexpensive redox reaction performed at 25°C involving potassium permanganate with or without cerium nitrate and sodium lactate. Particular attention has been paid to the activation mode of the catalyst through calcination or acid treatment. These materials have been characterized by a wide variety of physico-chemical analysis techniques and tested for NTP generated ozone decomposition or formaldehyde catalytic oxidation. Afterwards, the best catalysts in each reaction have been used for TCE abatement in dry/humid air in PPC configuration.

The manuscript is divided into 7 chapters. The first chapter outlines an overview of the literature on air pollution (sources, effects on human health as well as on environment, cleaning technologies). Then, the main features of using MnOx as a catalyst allowing better performances in the reactions expected to occur in the abatement of trichloroethylene in PPC configuration have been reviewed. After that, the use of plasma as an ozoner, including the effect of different factors on ozone production are summarized and as a second step the main features of using manganese dioxides, in particular birnessite based catalyst for ozone decomposition doped or not with transition metals (Ce) and treated or not with acid, are detailed. In this respect, the structures and physico-chemical characteristics of birnessite are reviewed. Finally, in a last part, the current state of the use of birnessite as a catalyst for the oxidation of formaldehyde (HCHO) in order to give us more information related to the catalytic performances and catalyst structure is discussed. The objective, strategy and methodology of this work are given in chapter 2. The details of the experimental conditions used for the characterizations and synthesis of the different materials as well as the experimental conditions followed for the different catalytic tests are presented in chapter 3. The fourth chapter is devoted to formaldehyde catalytic oxidation in order to highlight a correlation between oxygen mobility and catalytic activity. The effects of Ce content, temperature of calcination and relative humidity on the catalytic properties have been particularly discussed. It has been shown that the MnO2 catalysts modified by low amount of Ce (x= 0.01 and 0.1) calcined at 400 °C showed the best catalytic properties towards HCHO total oxidation and good stability in humid air. Numerous Ce-O-Mn interactions as well as high Mn 3+ /Mn 4+ and low Ce 3+ /Ce 4+ ratios have been believed to enhance oxygen mobility at low temperature which may explain the better efficiency of these catalysts.

The fifth chapter investigates ozone and nitrogen oxide formations in dry/moist air using a negative DC (Direct Current) corona discharge at atmospheric pressure in a 10-pin-to-plate reactor. The effect of flow rate (Q), Energy density (ED) and relative humidity (RH) are discussed. It has been shown that ozone production increases as a function of ED and Q while it decreases as a function of RH. Chapter 6 presents the results of NTP generated ozone decomposition at low temperature (20 -40 °C) in dry/moist air over the Ce doped birnessites (Ce/Mn = 0, 0.01, 0.1, 0.2 and 0.5) treated with nitric acid (0.2M HNO3, at 50°C for 6 hours) to be compared to the fresh ones. It has been shown that the acid-treated Ce0.01MnOx catalysts is the most efficient towards O3 decomposition. The beneficial effect of acid treatment has been ascribed to high density of active sites in line with high density of oxygen vacancies and active sites.

Finally, in the last chapter TCE abatement in PPC configuration has been investigated over the two best catalysts in terms of formaldehyde oxidation and ozone decomposition.

Plasma assisted acid-treated Ce0.01MnOx catalyst shows the best CO2 yield with a minimization of gaseous chlorinated by-products as compared to Ce0.01MnOx calcined at 400°C. Such results can be linked concomitantly to the introduction of acid sites allowing a better activation of TCE and to an increase of active sites in accordance with enhanced specific surface areas.

Samenvatting

De Wereldgezondheidsorganisatie (WHO) schat het aantal sterfgevallen als gevolg van luchtverontreiniging op ongeveer 9 miljoen per jaar, wat overeenkomt met 20% van het totaal aantal sterfgevallen wereldwijd. In Europa zijn 800000 sterfgevallen per jaar te wijten aan luchtverontreiniging. Een slechte luchtkwaliteit verhoogt ook het sterftecijfer door covid-19 aanzienlijk.

Onder de luchtverontreinigende stoffen worden vluchtige organische stoffen (VOS) beschouwd als de belangrijkste veroorzakers van luchtverontreiniging. Ze verspreiden zich in de lucht en hebben een negatief effect op de menselijke gezondheid en het milieu. Hun omzetting in de atmosfeer leidt tot de vorming van troposferisch ozon en fotochemische smog.

Anderzijds versterken zij het broeikasgaseffect en zijn ze verantwoordelijk voor de vernietiging van de stratosferisch ozon. Bovendien is bewezen dat VOS toxisch en/of carcinogeen zijn voor de menselijke gezondheid. De Europese wetgeving heeft daarom strengere doelstellingen opgelegd om de VOS-emissies te verminderen. Het huidige beleid bestaat er in prioriteit te geven aan het niet gebruiken van VOS, maar indien dit om technische redenen niet mogelijk is, is het nodig om adequate technieken te vinden voor een radicale vermindering van de VOSemissies. De conventionele VOS-verwijderingstechnieken, zoals thermische oxidatie, adsorptie, condensatie, biologische oxidatie en katalytische oxidatie, gaan gepaard met hoge energiekosten, vooral wanneer de VOS concentratie te laag is.

Als kost-en energie-efficiënt alternatief biedt het gebruik van niet-thermisch plasma (NTP) interessante vooruitzichten voor de verwijdering van lage concentraties verontreinigende stoffen uit de lucht bij hoge debieten. Niet-thermisch plasma verschilt sterk van thermisch plasma dat door plasmatoortsen en boogontladingen wordt gegenereerd. NTP's, zogenaamde koude plasma's, blijken efficiënter te zijn dan de conventionele technieken. Het belangrijkste voordeel van NTP is dat de aan de plasmabron geleverde energie bijna volledig wordt verbruikt om elektronen te versnellen, terwijl het achtergrondgas dicht bij kamertemperatuur blijft, zodat verhitting van het voedingsgas voorkomen wordt. Een ander belangrijk voordeel is de werking ervan bij atmosferische druk, waardoor het gebruik van een uitgebreide vacuüminstallatie vermeden wordt. Bovendien wordt NTP gekenmerkt door zijn snelle opstart en compacte afmetingen. Ook kunnen gemengde afvalstromen worden behandeld met niet-thermisch plasma. De elektronen impact dissociatie en ionisatie van het achtergrondgas veroorzaken radicaalproductie die leidt tot de afbraak van de beoogde VOS.

Hoewel NTP voor luchtreiniging vaak in de literatuur is voorgesteld, blijft de vorming van ongewenste nevenproducten (ozon, NOx, aërosols, fosgeen en andere VOS), die de algemene toxiciteit van de behandelde gasstroom verhogen, een ernstige hinderpaal voor industriële toepassingen. Om deze problemen op te lossen, is de combinatie van NTP en heterogene katalyse een techniek die plasmakatalyse wordt genoemd, de laatste twee decennia een populair onderzoeksthema geworden. In een plasmakatalyse-systeem zijn er twee manieren om de katalysator in het plasma te brengen: In Plasma katalyse (IPC) waarbij de katalysator zich in de plasmaontlading bevindt en Post Plasma katalyse (PPC) waarbij de katalysator zich stroomafwaarts van de plasmaontladingen bevindt. Ook zal de keuze van een geschikte katalysator helpen om de selectiviteit naar milieuvriendelijke eindproducten te optimaliseren.

De behandeling van luchtvervuiling door een hybride systeem 'Niet-Thermisch Plasma (NTP) en heterogene katalyse' vereist de optimalisatie van de experimentele condities en de ontwikkeling van nieuwe katalytische systemen die steeds efficiënter zijn om de energiekost te reduceren. Dit proefschrift richt zich in het bijzonder op de koppeling van plasmakatalysetechnologieën, voor de reductie van trichloorethyleen (TCE), een gechloreerde vluchtige organische verbinding die voornamelijk in de industrie als ontvetter wordt gebruikt.

In het kader van de vermindering van trichloorethyleen door Post-Plasma Katalyse werden tijdens dit thesiswerk verschillende parameters onderzocht om het PPC-proces te optimaliseren. Een van de belangrijkste parameters was het benutten van de door de NTP uitgestoten ozon als een potentiële bron van actieve zuurstofsoorten voor de verdere oxidatie bij zeer lage temperatuur (100 °C) van onbehandeld trichloorethyleen en van de potentiële gasvormige gevaarlijke bijproducten van de NTP. Aldus werd de molaire verhouding [O3] /

[TCE]0 aangepast tot 4, in overeenstemming met de formele reactie van TCE-oxidatie door O3: C2Cl3H + 4O3  2CO2+ HCl+ Cl2 door het variëren van de verschillende sleutelparameters zoals energiedichtheid, debiet en vochtigheid van de NTP. Bij de keuze van de katalysator werd rekening gehouden met een aantal basisvereisten zoals ozonafbraakvermogen, zuurstofmobiliteit, en hoge CO2-selectiviteit gekoppeld aan chloor-en watertolerantie. Birnessiet (δ-MnO2) al dan niet gedoteerd met Ce, CexMn (x= 0.01, 0.1, 0.2 en 0.5) werd gesynthetiseerd via een eenvoudige, snelle en goedkope redoxreactie van kaliumpermanganaat en natriumlactaat bij omgevingstemperatuur met toevoeging van ceriumnitraat. Deze reacties werden uitgevoerd in een overmaat KMnO4 (69 %). Bijzondere aandacht werd besteed aan de activeringsmodus van de katalysator door de invloed van de calcinatietemperatuur na te gaan. De katalysator werd gecalcineerd bij 200, 300 en 400°C. Ook werd een zuurbehandeling uitgevoerd om de tolerantie voor chloor en water te verhogen. Deze verschillende materialen werden gekarakteriseerd door middel van diverse analysetechnieken en getest als katalysator voor de TCE-reductie in een PPC-configuratie, voor ozonafbraak gebruikmakende van plasma als ozonisator en ook voor de formaldehyde katalytische oxidatie.

Het proefschrift is onderverdeeld in 7 hoofdstukken. In het eerste hoofdstuk wordt een overzicht gegeven van de literatuur omtrent luchtverontreiniging (bronnen, effect op de menselijke gezondheid en op het milieu, zuiveringstechnologieën). Vervolgens worden de belangrijkste kenmerken van het gebruik van MnOx als katalysator bij de bestrijding van trichloorethyleen in een PPC-configuratie besproken. Daarna wordt een overzicht gegeven van het gebruik van plasma als ozonisator, inclusief het effect van verschillende factoren op de ozonproductie. In een tweede stap worden de belangrijkste kenmerken van het gebruik van mangaandioxide, met name katalysatoren op basis van birnessiet voor de ontleding van ozon, al dan niet gedoteerd met overgangsmetalen (Ce) en al dan niet behandeld met zuur, in detail besproken. In dit verband worden de structuren en kenmerken van birnessiet onder de loep genomen. Tenslotte wordt in een laatste deel de huidige stand van zaken van het gebruik van birnessiet als katalysator voor de oxidatie van formaldehyde (HCHO) besproken om ons meer informatie te verschaffen met betrekking tot de katalytische prestaties en de katalysatorstructuur. De doelstrategie en methodologie van dit werk worden besproken in hoofdstuk 2. De technieken die werden gebruikt voor de karakterisering en de synthese van de verschillende materialen, evenals de experimentele katalytische condities, worden gepresenteerd in hoofdstuk 3. Het vierde hoofdstuk is gewijd aan het bespreken van de resultaten rond de formaldehyde katalytische oxidatie, om een correlatie tussen de hoeveelheid zuurstof vacancies en de katalytische activiteit te benadrukken. Verschillende parameters worden besproken, zoals het effect van het Ce-gehalte, de calcinatietemperatuur en de relatieve vochtigheid. Er werd aangetoond dat katalysatoren met een laag Ce-gehalte (x= 0,01 en 0,1), gecalcineerd bij 400 °C, het meest efficiënt zijn door een verbeterde lokale omgeving van de actieve sites, gekoppeld aan talrijke Ce-O-Mn interacties, een hoge Mn 3+ /Mn 4+ atoomverhouding, een lagere Ce 3+ /Ce 4+ ratio, en een hoge mobiliteit van zuurstof bij lage temperatuur. Het vijfde hoofdstuk presenteert de vorming van ozon en gelijktijdige stikstofoxiden door negatieve DC (gelijkstroom) corona-ontlading bij atmosferische druk met behulp van een 10-pins-to-plate reactor. Het effect van het debiet (Q), de energiedichtheid (ED) en de relatieve vochtigheid worden besproken. Er werd aangetoond dat de ozonproductie toeneemt in functie van de energiedichtheid en het debiet, terwijl deze afneemt in functie van de relatieve vochtigheid. In hoofdstuk 6 wordt de afbraak van ozon door Ce-gedoteerd birnessiet, al dan niet behandeld met salpeterzuur, onderzocht bij lage temperatuur (20 -40 °C).

Het effect van het Ce-gehalte (Ce/Mn = 0, 0.01, 0.1, 0.2 en 0.5), de behandeling met salpeterzuur (0.2M HNO3, bij 50 °C gedurende 6 uur), en de stikstofhoudende co-polluenten en water worden besproken. Hieruit bleek dat de met zuur behandelde lage Ce-katalysatoren het meest efficiënt zijn door de aanwezigheid van een grote hoeveelheid zuurstof vacancies in lijn met de aanwezigheid van een grote hoeveelheid zuur sites. Tenslotte wordt in het laatste hoofdstuk de TCE reductie in het PPC proces besproken. De twee beste katalysatoren voor formaldehyde-oxidatie en ozonafbraak werden gekozen. De plasma-geassisteerde Ce0.01Mn katalysator behandeld met zuur geeft de beste CO2 opbrengst met een minimale vorming van gasvormige gechloreerde bijproducten in vergelijking met Ce0.01Mn gecalcineerd bij 400°C.

Beide katalysatoren vertonen een goede stabiliteit ten opzichte van chloor en water in functie van de tijd. Deze resultaten worden toegeschreven aan een hoog gehalte aan zuurstof vacancies met een hoger gehalte aan Mn 3+ , aan een verbeterd specifiek oppervlak en aan een sterke zuurtegraad van het oppervlak.

Résumé

L'optimisation du traitement d'oxydation de COVs par un système hybride PPC (Post-Plasma Catalyse) nécessite de travailler dans des conditions expérimentales appropriées en utilisant des systèmes catalytiques toujours plus efficaces pour optimiser les rendements en CO2 et réduire les coûts énergétiques. Pour ce faire, on a étudié l'oxydation catalytique du trichloroéthylène (TCE), un composé organique chloré volatil, par PPC en s'attachant à bénéficier au mieux des effets bénéfiques de l'ozone généré par le plasma en sortie de réacteur pour le décomposer en oxygènes actifs devant permettre une meilleure minéralisation du COV chloré.

L'un des points clés était de bénéficier de l'ozone émis par le NTP comme source potentielle d'espèces réactives de l'oxygène pour une oxydation à très basse température (100°C) du trichloréthylène non traité par le plasma et des sous-produits gazeux potentiellement dangereux du NTP. Ainsi, le rapport molaire [O3]/[TCE]0 a été adapté à 4, conformément à la réaction d'oxydation du TCE par O3, en jouant sur différents paramètres clés tels que la densité énergétique, le débit et l'humidité du NTP. De plus, le choix du catalyseur a été fait en tenant compte de sa capacité à décomposer l'ozone, sa mobilité de l'oxygène et sa sélectivité élevée en CO2 en oxydation complète de COVs couplée à une bonne tolérance au chlore et à l'eau. Dans ce travail, des birnessites (δ-MnO2) dopées ou non avec le Ce, CexMn (x= 0.01, 0.1, 0.2 et 0.5) ont été préparées à partir d'une réaction redox simple, rapide et peu coûteuse, utilisant du permanganate de potassium et du lactate de sodium à température ambiante avec ajout de nitrate de cérium. Une attention particulière a été portée sur le mode d'activation du catalyseur par modulation de la température de calcination. Un traitement acide a été également effectué pour améliorer la tolérance à l'eau et au chlore. En termes d'activité catalytique, trois approches ont été adoptées. Dans une première partie, ces catalyseurs ont été testés pour l'oxydation catalytique du formaldéhyde afin de mettre en évidence une corrélation entre l'activité catalytique et la mobilité d'oxygène. Il a été montré que les catalyseurs à faible teneur en Ce (x= 0,01 et 0,1) calcinés à 400°C sont les plus efficaces en raison d'un meilleur environnement local des sites actifs lié à de nombreuses interactions Ce-O-Mn, d'un rapport atomique élevé Mn 3+ /Mn 4+ , d'un rapport Ce 3+ /Ce 4+ plus faible et d'une forte mobilité de l'oxygène à basse température. Dans une deuxième partie, il a été montré l'efficacité des catalyseurs à faible teneur en Ce traités à l'acide en termes de décomposition de l'ozone en présence de co-polluants azotés produits par le PNT en utilisant l'air comme flux gazeux. L'efficacité de ces catalyseurs a été expliquée par la présence concomitante d'une grande quantité de lacunes d'oxygène en interaction avec des sites acides générés par le traitement acide. Enfin, les deux meilleurs catalyseurs en termes d'oxydation du formaldéhyde et de la décomposition de l'ozone générée par PNT ont été choisis pour l'oxydation complète du TCE sous air en configuration PPC à 100°C. Le catalyseur Ce0.01Mn traité à l'acide présente un meilleur rendement en CO2 avec une minimisation de la formation de sous-produits organiques chlorés gazeux par comparaison avec le catalyseur Ce0.01Mn calciné à 400°C. Ces deux catalyseurs présentent néanmoins une stabilité relative vis-à-vis du chlore et de l'eau en fonction du temps. Une surface spécifique plus importante, une forte densité de lacunes d'oxygène et de sites acide, une valeur élevée du rapport Mn 3+ /Mn 4+ permettent de rendre compte du meilleur comportement du catalyseur Ce0.01Mn traité à l'acide Pour l'oxydation du TCE en configuration PPC.

Acknowledgements

As Helen Keller once said "Alone we can do so little. Together we can do so much". This doctoral dissertation is the result of the collective effort of various researchers, scientists, mentors and a team of wonderful people. I would like to take this opportunity to sincerely thank everyone who has helped and supported me over the past four years to make my dream a reality.

First of all, I would like to express my sincere gratitude towards, Prof. Dr. Rino Morent, Prof. Dr. Jean-François Lamonier and Assistant Professor Dr. Jean Marc Giraudon for offering me the opportunity to carry out this doctoral dissertation in Ghent and Lille Universities.

I would like to express my appreciation to Assistant Professor Dr. Jean Marc Giraudon, with whom I had the opportunity to benefit from his experience. Your enthusiasm and willingness in helping me plan my experiments and those stimulating discussions really inspired me to give my best. Without your enormous expertise I would never be able to succeed to grasp the complexity of the plasma-catalytic processes. I also want to say thank you to Prof. Dr. Jean-François Lamonier for all the effort you put in my work, for teaching me, for trusting me and encouraging me, for being patient and listening me, for transmitting me your knowledge. In addition, you two have helped me through some difficult times in my life. A few sentences are obviously not enough to describe what you have done for me in the past few years.

I would like to thank Prof. Rino Morent for the fruitful collaborations, all the scientific discussions, technical inputs, insights and also, for the guidance during my research stay in RUPT, UGhent. Also, I would like to express my gratitude to Prof. Nathalie De Geyter for all her help and support during my PhD.

I would like to thank the members of the examination committee for taking interest in my work and providing the valuable suggestions to improve this thesis. Thanks to Prof. Dr. Hoping not to omit anyone, my thanks also go to the whole team of the UCCS ULILLE, in particular to Olivier GARDOLL, Martine TRENTESAUX, Joëlle THURIOT-ROUKOS, Laurence BURYLO, Christine LANCELOT, Nora DJELAL, Pardis SIMONE, Jean-Charles MORIN, Nicolas NUNS, for their help, their explanations and their enthusiasm concerning the various physico-chemical characterizations and also I would like to thank the secretariat of the UCCS for their help in the administrative papers, in particular Sandrine BERTON and Barbara DECLERCK-BOULANGER. I would also like to thank the whole team of RUPT UGent, in particular Tim POELMAN and Joris PEELMAN who were always ready to help with the design and modification of the reactors and the troubleshooting of technical problems with the plasma equipment.

I warmly thank all my colleagues at UCCS and RUPT for creating the best possible working environment. These last years were not only about doing a PhD, but also about making real friends with whom I shared the ups and downs, the laughs and the tears, not to mention the coffee breaks. In particular Guillaume, Carmen, Bertrand, Herman, Maya, Zhiping, Victor, Yuanshuang, Shuo, Tanushree, Helori, Parnian, Dayan, Nicolas, Ibrahim, Bertha, Mhammad, Balsam, Bakytzhan,Wafa, Noura, Xiu, Paola, Julien, Shreya, Nassim in France and Savita, Sharmin, Rim, Rouba, Elise, Parisa, Moazameh, Tim in Belgium. A special thank you to Shilpa, my colleague in France and Belgium, "the chance made us colleagues but the fun and laughter made us friends".

My thanks would not be complete without expressing my deep gratitude and appreciation to all my friends in Lebanon and in France, for all the support and pleasure they have brought to my life. Thanks to Joumana, Dayan, Jenny, Carla, Gaëlle, Tonine, Marie Therese, Sahar, Eliane, Sara, Bachar, Yara, Charbel, Joy, Ralph, Emile Khawla, Adelina, Rachad, and Rita. A special thanks to Mirna for always being there for me in moments of sadness as well as in moments of joy, "because of you I laugh a little more, I cry a little less and I smile a lot more". I also thank my family without whom this work would never have been executed, especially my parents Abdallah and Ghada, my brothers Georges and Elie and my sisters Faten, Leila and Denise who, in all circumstances, have always been able to offer me their support, their understanding, their encouragement, their patience and their affection throughout my studies. I also warmly thank my second mother, my aunt Rosette. I also thank my cousins Joseph and Jennifer "Born as cousins, because parents couldn't handle us as brothers and sisters".

Finally, I would like to thank Jesus-Christ, my savior, without whom nothing would be.

So thank you to all these people, for this very enriching adventure on a professional and personal level.

3-1-1-Synthesis of birnessite and cerium doped birnessite

3-2-1-Thermal Gravimetric Analysis (TGA) and Differential

Thermal Analysis (DTA) .........

3-2-2-X-Ray

3-2-3-Transmission Electron

3-2-4-Inductively coupled plasma-optical emission spectrometry (ICP-OES

3-2-5-Textural properties analysis via Brunauer-Emett-Teller (BET) and

3-4-1-Description of the plasma Set-up for

5-3-1-Effect of energy density (ED)

List of figures

List of tables

1-1-Air pollution

Air pollution is a major problem that has been recognized a 100 years ago. We hear about the various forms of pollution and read about it through the mass media. Air pollution is one such form that refers to the contamination of the air, irrespective of indoor or outdoor. A physical, biological or chemical alteration of the air in the atmosphere can be termed as pollution. It occurs when any harmful gases, dust, smoke enter into the atmosphere and makes it difficult for plants, animals and humans to survive as the air becomes dirty. Air pollution has become one of the main environmental health risks in the world. According to the world health organization (WHO) report, 7 million people die prematurely from pollution worldwide (2012), including 3.3 million due to indoor air pollution and 2.6 million due to outdoor air pollution, equivalent to one death every 2.7 seconds [1,2]. Air pollutants could be discharged to the atmosphere through natural sources such as plants and volcanic eruptions or by several human activities. But the greatest contributions to air pollution today are those resulting of human impact. These are largely the result of human reliance on fossil fuels and heavy industry but can also be due to the accumulation of waste, modern agriculture, and other man-made processes. The main primary pollutants monitored in air are: Carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx), particulate matter (PM), volatile organic compounds (VOCs)…

1-2-Volatile organic compounds (VOC)

1-2-1-Definition

A volatile organic compound is defined as an organic substance which, excluding methane, is composed of carbon and hydrogen in the gaseous state under normal pressure and temperature conditions. The molecule may also contain other heteroatoms such as halogens, sulfur, oxygen, phosphorus, with the exception of carbon oxides and carbonates [3]. Although, there is no general quantitative definition of what VOCs are, a more restrictive definition of VOCs has been given in Article 2 of the European Directive 1999/13 / EC (VOC): "A volatile organic compound is defined as any compound having a vapor pressure of more than 10.3 Pa at normal pressure and temperature". Their high volatility causes them to rapidly evaporate and enter the earth's atmosphere [4][5][6].

1-2-2-Sources

Emission sources of VOC can be divided into biogenic or anthropogenic depending on whether they are released to the environment by biological life or humans respectively [7,8].



Natural sources: Globally, VOC emissions are mainly of natural origin and account for nearly 90% of total emissions. There is no real solution to prevent this kind of emission, it has significant effect on the chemical composition and physical characteristics of the atmosphere. The natural sources that emit NMVOCs (methane excepted) are the Plants that emit a very wide variety of VOCs and Geological areas that contain coal or natural gas.

However, natural sources that are sources of methane which is the major biogenic VOCs are swamps and animals. Therefore, biogenic sources become less important in the industrialized regions where anthropogenic sources produce large amount of VOCs.

 Anthropogenic sources: Are divided into two categories. On one hand, indoor pollution that comes mainly from household products such as insulation materials, solvents, cleaning products, insecticides, aerosols, printers, restaurants and domestic kitchens, paints and varnishes. On the other hand, external pollution that comes from transportation and industrial processes such as chemical plants, electronic components factories, pharmaceutical, refineries, car manufacturers, food manufacturers, textile manufacturers.

1-2-3-Effect on human health and environment

VOCs have long-term adverse effects on human health and the environment.

Therefore, their incorporation, most often by inhalation, sometimes by digestion or skin penetration, can be at the origin of a multitude of pathologies of unequal gravities. The penetration of VOCs in the body is likely to produce, by reaction with proteins or nucleic acids, various toxic metabolites that spread in the organs. On the other hand, it is established that VOCs harm health indirectly, by favoring complex chemical reactions taking place in the atmosphere (photo-oxidation). Air pollution emitted by biogenic or anthropogenic sources is released to the troposphere. The tropospheric chemistry is essential for their chemical transformation such as emission, transport, exchange, distribution and deposition.

Moreover, from one VOC to another, the danger varies according to the molecule concerned and its concentration. Some are carcinogenic, others can cause respiratory diseases. The danger is further amplified by the "cocktail effect" resulting from the uncontrolled combination of these molecules with each other. Besides, the age at which a person is exposed to these substances also plays a role. In addition, after chemical reactions, chlorinated volatile organic compounds (CVOCs) cause a number of indirect pollution problems such as ozone and smog formation in the troposphere, and depletion of the ozone layer in the stratosphere.

1-3-Conventional VOC abatement technologies

To limit the release of VOCs, it is necessary in the first step to try to minimize their emissions by replacing them with other substances by using less harmful VOCs. If no preventive solution is technically or economically feasible, it remains to treat the effluents loaded with VOCs. There are different methods, but each one has its limitations depending on the compound and its concentration, the source of the emission, the possibility or not to valorize the recovered products. These methods are divided into two categories well separated [9][10][11]12].

 Recovery methods: condensation, adsorption, absorption and membrane technique.

 Methods of destruction: thermal oxidation, biological oxidation and catalytic oxidation

1-3-1-Thermal oxidation

Thermal oxidation is the oldest method and is still the most widely used due to its simplicity of installation. VOCs are oxidized to CO2 and H2O at very high temperatures (700-1000°C). The major disadvantages of this technique are that it requires high operating costs and can generate harmful by-products such as dioxins and nitrogen oxides (NOx) [12].

1-3-2-Catalytic oxidation

Catalytic oxidation is one of the most effective and currently developed solutions for VOC abatement. Indeed, unlike thermal oxidation, catalytic oxidation operates at lower temperatures which offers considerable savings in costs thus leading to lower energy consumption. The catalytic oxidation causes a decrease in the production of harmful byproducts. The addition of a catalyst accelerates the rate of VOC oxidation into CO2 and H2O when considering a CxHyOz organic VOC. Investigations on adsorption and desorption of gas molecules at a catalytically active surface, and interaction of these molecules with each other on this surface, as well as with the surface itself might lead to various reaction mechanisms such as Langmuir-Hinshelwood, Eley -Rideal or Mars Van Krevelen. It has to be mentioned that the operating temperature range differs from the nature of the VOCS and is of 200-350°C, 370-540°C and 200-500°C for heavy, light and halogenated VOCs, respectively. Generally, the catalysts for VOC abatement can be ranked into 2 categories: those relative to noble metal based catalysts which are generally known as being the more efficient for VOC abatement. However, their high cost and limited availability have led researchers to try to replace them with transition metal oxides (TMO) which are cheaper and more abundant. Copper, cobalt and manganese oxides are the most commonly TMO studied in total oxidation reactions of VOCs. Moreover, one major drawback of this method is that it becomes cost inefficient (high energy consumption) when low VOC concentrations need to be treated (< 1000 ppmv). The design of a catalytic oxidation system should be carefully considered, as its sensitivity to VOC inlet stream, flow conditions and catalyst deactivation, limits its applicability for many industrial processes [13][14][15][16][17].

1-3-3-Non-thermal plasma (NTP)

Among the different methods for the abatement of dilute VOCs from waste gas streams and indoor air, Non-Thermal Plasma (NTP) has attracted attention about 30 years ago.

NTP was previously used for the removal of NOx and SOx from flue gas [18][19][20]. Plasma is normally referred to the fourth state of matter, due to its distinctive physical and chemical properties. The term plasma was first introduced by Langmuir in 1928. Langmuir wrote:

"Except near the electrodes, where there are sheets containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the plasma to describe this region containing balanced charges of ions and electrons". There are two sources to produce plasma. Natural source such as solar corona, solar wind lightening and Aurora Borealis, or artificial through the injection of a sufficient amount of energy to cause gas excitation. The energy is in different form: by heating, photoionization, exposing neutral gas to beam of charged ions or electrons or by electrical discharge. Non-Thermal Plasma can be classified from several standpoints. One of these standpoints deals with an equilibrium between the temperature of ions and electrons. When the temperature (≈10,000 K) of ions and electrons are the same we speak about Thermal Plasma (TP). High power density e.g. from 100 W cm -3 to 10 kW cm -3 is required to obtain TP. On the other hand, when the temperature of electrons (i.e. 10,000 K to 20,000 K) is much higher than the temperature of ions which remain at ambient temperature we speak about Non-Thermal Plasma (NTP). The NTP energy electron is between 1 and 20 eV, the collision between this electrons and neutral gas produce excited gas molecules (N2 * , O2 * ) which lose their excess energy by emitting photons or heat. Next to excitation, other reactions like ionization, dissociation and electron attachment occur in the discharge zone. Unstable reactive species like ions and free radicals are formed through these channels of reactions. In NTP, ionizations and chemical reactions are directly determined by the electron temperature and are not so sensitive to thermal process and the gas temperature. Many elementary processes lead to NTP generation which can be divided into primary and secondary processes. The primary processes include charge transfer, ionization, excitation and dissociation reactions with typical timescales between 10 -15 and 10 -8

seconds. The secondary processes are relative to the subsequent chemical reactions between the products of primary processes (electrons, radicals, ions and excited molecules). Additional radicals and reactive molecules (O3, HO2 and H2O2) are also formed by radical and recombination reactions. The timescale of the secondary processes is around 10 -3 seconds. The generation of NTP at atmospheric pressure and room temperature have been the subject of many research papers in the last two decades. This type of plasma has found wide applications in the industrial, environmental and medical fields. The first environmental application of NTP is the generation of ozone for water treatment and it is still one of the most important applications [21]. Some of the NTP applications are: polymer processing, the hydrogenation of CO2, the sterilization and disinfection of contaminated surfaces including living tissues, the removal of air contaminants such as SO2, NOx, VOC and ozone generation [15,[22][23][24][25][26][27][28]. The use of NTP for air pollution control has several advantageous features resulting from its operating conditions such as: quick start-up, rapid response to changes in the composition of the waste gas and operation at ambient conditions which excludes the use of expensive vacuum systems [29][30][31][32].

However, the key advantage of NTP is the directed energy consumption. NTPs are attractive in such a way that almost no energy is dissipate in heating up the entire gas flow. On the other hand, the energy delivered to the system is efficiently used to create an intense electric field in which electrons are selectively accelerated, hence, multiple chemical process such as excitation, ionization and dissociation are initiated by collisions between background molecules (N2, O2, H2O) and these highly energetic electrons. This lead to the formation of active, unstable species (radicals, ions, photons) which are responsible for the decomposition of VOCs. Due to the previously enounced advantages NTP has been used for the removal of VOCs from dilute atmospheric pressure gas streams. But one major drawback of the NTP removal technique for VOC removal abatement in polluted air is the possible formation of undesired by-products such as O3, NOx, aerosols, plasma generated VOCs even more toxic than the VOC to be treated.

Since 2000, the combination of NTP with other techniques has been an active area of research for better VOC abatement efficiency.

1-3-4-Plasma-Catalysis

In these last decades, the combination of NTP with heterogeneous catalysis referred to as plasma-catalysis, has gained increased interest. The integration of a catalyst in the process may accelerate the chemicals reactions by providing alternative reaction pathways with a lower energy barrier to get the most environmental friendly final products such as CO2 and H2O.

Plasma-catalysis a priori is able to enhance the performance of NTP alone in VOC abatement through the concomitant improvement of the three main weaknesses of NTP [33,34]:

Incomplete oxidation with emission of harmful compounds, poor energy efficiency and low mineralization degree (non-selective process). Depending on the location of the catalyst with respect to the NTP, plasma-catalysis can be divided into two categories: In-Plasma Catalysis (IPC) and Post Plasma Catalysis (PPC).

In-Plasma Catalysis

The configuration where the catalyst is integrated in the NTP reactor refers to In-plasma catalysis. In that case all the highly reactive plasma generated species can play a role at the surface of the catalyst [35]. Furthermore, the possibility to get adsorbed species on the catalyst surface leads to an increase of the retention time promoting the interactions between the short lived oxygen based entities with the adsorbed VOC and adsorbed related degradation intermediates enabling a more complete oxidation in line with the textural properties of the support. Additionally, it has been shown that the presence of the catalyst in the discharge zone can also affect the plasma discharge type resulting in an enhanced production of reactive species through an increase of the oxidizing power of the plasma discharge. Besides, the NTP can also have beneficial effects on the catalyst properties [19].

Post-Plasma Catalysis

Post-plasma catalysis is a two stage arrangement with two reactors in series: A NTP reactor and a catalytic reactor where the catalyst is located downstream of the NTP reactor. This configuration precludes direct interaction of the NTP with the catalyst surface and it is found that only O3 and nitrogen oxides as reactive oxygen species which can reach the catalytic surface [35]. The concept of PPC is to take advantage of the catalytic ozone dissociation to produce very active oxygen species promoting the oxidation of the plasma generated byproducts and of the remaining initial VOC. PPC has a simpler configuration compared to IPC, which facilitates the replacements of the used catalyst in practical applications.

1-4-Trichloroethylene (TCE)

The aim of this work is to remove TCE with the lowest possible energy cost in line with a good selectivity towards CO2, HCl and H2O using a post plasma catalysis process.

1-4-1-Physico-chemical properties, Production and Use of TCE

Trichloroethylene is a partially chlorinated C2-Hydrocarbon with the formula C2HCl3.

It is classified as chlorinated volatile organic compound (ClVOC). TCE is a colorless liquid, with a sweet smell reminiscent of chloroform and detectable from 50 to 100 ppm of substance in the air (with risk of rapid olfactory fatigue). It is practically insoluble in water (0.1 g/100 mL of water at 20°C) but miscible with most organic solvents (acetone, ethanol, diethyl ether, chloroform, carbon disulphide…); it is completely miscible with glacial acetic acid. Further chemical and physical properties are listed in Table 1-1 [36,37].

TCE was first prepared in 1864 by Emil Fisher in experiments on the reduction of hexachloroethane with hydrogen. In 1920 commercial production of TCE began in Germany and in 1925 in USA [37]. It is an excellent solvent for a wide range of applications due to favorable properties such as high solvency, low flammability, non-corrosiveness, high stability, low specific heat, low boiling point, low heat of vaporization and low production cost.

Therefore, it is widely used as an industrial solvent to clean and degrease metal and semiconductor parts. TCE has however numerous other uses including heat transfer medium, extraction agent for greases, oils and fats, chemical intermediate in the production of chlorofluorocarbons and other organic chemicals and as an ingredient in many products for industrial and consumer use (Figure 1-1). It is for example used in dyes, printing inks, paint removers, adhesives, type writer correction fluids as well as spot removers [37][38][39]. Figure 1-1 present the % of TCE used in the different processes. However, according to Registration, Evaluation, Authorization and restriction of Chemical substances (REACH) registration information which is available on European Chemical Agency's (ECHA'), TCE is manufactured and/or imported in the European Economic area in 10,000 to 100,000 tonnes/year [40]. Conversion factor 1 ppm = 5.37 mg.m -3 (25C°)

Upper explosive limit in air 41 vol,% Lower explosive limit in air 11 vol,%

1-4-2-Health effect and legislation

TCE is detected in all media but mostly in air due to its volatility. Release of TCE at ambient to outdoor and indoor air can occur during degreasing operations and consumption of related products [38]. Contamination of groundwater and water supplies is possible during production and disposal processes, and through leaching from landfills or underground storage tanks due to its moderate solubility in water, widespread use and persistency in the environment.

Human exposure to TCE may therefore occur through inhalation, ingestion and dermal contact.

Inhalation can affect the central nervous system and is known to cause acute symptoms including sleepiness, fatigue, dizziness, headache, confusion, facial numbness and feelings of euphoria [38]. Toxicologic studies have also reported human health impacts such as speech and hearing impairments, disturbed vision, liver problems, skin rashes, kidney diseases, urinary tract, cardiac arrhythmias and blood disorders [41,42]. Furthermore, TCE has been extensively studied for its potential carcinogenicity.

The Scientific Committee of Occupational Exposure Limits (SCOEL) reports TCE being well absorbed via all major routes of exposure in humans, with quantitative data available showing an uptake between 28 and 80% by inhalation, with absolute uptake increasing with increasing physical exercise. Although the International Agency for Research on Cancer (IARC) listed TCE as potentially carcinogenic (Group 2A), recent evaluation based on new data has however proven that there is sufficient evidence to conclude that TCE is carcinogenic to humans (Group 1) [43]. It causes cancer of the kidney, liver, cervix and lymphatic system. in case no action is taken regarding carcinogen pollutants. Among them, exposure to TCE which potentially concerns 74,076 workers in Europe is expected to cause 4,800 cases of cancers and 3,300 deaths in 2069, with estimated health costs between € 1,582m-5,657m.

1-4-3-TCE abatement

Development and optimization of different processes for TCE treatment are crucial issues for the improvement of air quality.

Thermal oxidation

The first method used is the thermal oxidation. It has been proved to successfully decompose TCE but it has major drawbacks. First, temperature must be superior to 1000 °C for the complete pollutant destruction making it an uneconomical process. In addition, the high temperature reached in this process generates a higher quantity of NOx and chlorinated byproducts and finally, this technique is not very effective with low pollutant concentrations streams [44].

Catalytic oxidation

The catalytic oxidation of TCE represents an interesting alternative that permits to operate at lower temperature (250 -550 °C) with a drastic reduction of energetic costs compared with the thermal process and can be efficiently operated in dilute pollutants that cannot be thermally combusted without additional fuel [45]. Furthermore, a specific catalyst design can favor the reaction selectivity in the direction of desired products (CO2, H2O and HCl), limiting the by-product formation of NOx. In addition, the desired objective for the catalytic oxidation is to achieve complete conversion of TCE [46]. Noble metal based catalysts show a high activity for TCE abatement but due to high and fluctuating cost and scarcity, their application is restricted. Solid acid catalysts such as zeolites and alumina based materials have also been proposed to catalyze the decomposition of TCE, because Brӧnsted acidity is believed to play a key role in activities [44]. Modified zeolites by other metals (FexOy, Cr2O3, CeO2) show an improvement in catalytic activity due to interaction between zeolite and metal. Additionally, zeolite modified by MnOx has been reported as a good catalyst for ClVOC abatement such as dichloromethane and trichloroethylene [47]. Also bulk TMOs have been used for TCE oxidation such as CeO2, Mn doped CeO2 microspheres. CeO2 has been reported as an efficient oxidation catalyst because CeO2 has unique redox property and high oxygen storage capacity [48,49]. Li et al. [45] demonstrated that Mn doped CeO2 has better activity compared with pure ceria, as its specific surface area increases, the amount of oxygen vacancies as well as the mobility of oxygen increase, which are responsible for the high catalytic performance. The major drawback for the catalytic oxidation is the catalyst deactivation caused by the strong adsorption of chlorine species produced from TCE decomposition leading to the blockage of the active sites and the decrease of the surface active oxygen [44,45].

Non thermal plasma

The destruction of TCE with NTP takes place through many possible pathways [18,50].

Electron attachment of TCE leading to the formation of C2HCl2 and a chlorine anion is one of the possible initial step for TCE decomposition. Metastable nitrogen molecules are also considered as dominant dissociation species occurring in a NTP destruction process of VOC polluted air streams [18]. Overall, the initial steps of TCE decomposition may occur from electron impact dissociative attachment or reactions with metastable molecules (N2*, O2*).

Vandenbroucke et al. [51] investigate the decomposition of TCE using 5 pins to plate NTP reactor using a DC negative corona discharge and identified along CO and CO2 the formation

Post-Plasma catalysis

Since these two last decades NTP technology has partly regained interest through the combination with heterogeneous catalysis. Vandenbroucke et al. have reviewed the efficiency of different plasma catalytic processes for TCE abatement in air [52]. Although several studies have however shown enhanced performance of different catalysts in PPC rather than IPC [53].

However, in IPC, both catalyst and plasma simultaneously affect each other, resulting in a complex interaction. In a two stage process such as PPC the operating parameters can be separately optimized enhancing the feasibility of adopting this two-stage configuration in commercialized processes. In PPC the TCE contaminated gas stream to be treated passes first through the plasma reactor and then through the catalytic reactor . After NTP treatment the nature of the gaseous effluent changes in line with the high reactivity of the plasma towards Based on this previous scheme of TCE degradation which is complex, it turns out that the catalyst has to be very active for NTP generated ozone degradation allowing to get a relevant production of active oxygen species to oxidize the polychlorinated C2 by-products into CO2.

Additionally, the effect of the presence of oxygenated nitrogen species (not mention in the scheme) as well as water leaving the NTP reactor has to be taken into account as these species can compete for the active sites of O3 decomposition. The stability of the catalyst has also to be taken into account as a possible partial chlorination of the catalyst in the course of the reaction may induce catalytic chlorination reactions in line with the detection of CCl4 and HCCl3 at the exit of the catalytic reactor (Path 2 in Figure 1 -1). Hence the catalyst must be also resistant to mineral chorine (Cl2, HCl) and to chlorinated organic species which can poison the catalyst as well.

Taking into account the activity for O3 decomposition as the main criteria for the catalyst design, MnOx based catalysts have been chosen to be tested for TCE oxidation in PPC configuration based on previous work of T. Oyama et al. [80] which showed MnO2 as the most promising catalyst for O3 decomposition among different TMOs. With that respect, the performances of various Mn based oxides have been studied by our team in PPC configuration for TCE abatement in dry and/or moist air using a multi-pin-to-plate NTP reactor working in a DC negative corona discharge. The obtained results are summed up in Table 1-2. Regarding the different catalysts, we have a commercial MnO2 [9], a perovskite, LaMnO3 synthetized by the citrate method [54] and modified manganese oxides with (i) Ce (a poorly crystallized Ce-(MnOx) synthetized by a redox-precipitation route) [55], with (ii) Fe (a cryptomelane (Fe-K- Catalyst surface OMS-2) synthetized by a co-precipitation method) [56] and with Cu (a crystallized Cu:Mn oxide, namely CuMn2O4 synthetized by co-precipitation) [57].

Unfortunately, a direct comparison between the different PPC processes in terms of COx (x =1 and 2; YCOx) and CO2 (YCO2) yields is not possible due to different experimental conditions. In that way it must be stated that the NTP reactor configuration has been optimized in the last set of experiments in doubling the number of pins to increase the residence time in the reactor.

Additionally, the initial concentration of TCE, the weight of catalyst as well as the temperature of catalytic bed have been decreased over time to reduce the costs of the process. Irrespective of the catalyst formulation, high TCE removal efficiency (up to 97%) can be reached in line with the high reactivity of NTP. Additionally, a decrease of the initial TCE concentration accompanied with a decrease of the supply air flow rate (higher catalyst contact time) allows to improve CO2 yield (up to 62% but for just short periods ( 15 min) with NTP assisted Fe-K-OMS2). Nonetheless, the production of CO is always important highlighting the fact that the oxidation of CO into CO2 has to be promoted. On purpose, the beneficial role of Cu in CuMn2O4

has been highlighted as it is for this catalyst that the YCOx/YCO2 value of 1.34 is the lowest with the highest YCO2 of 50% after 5h of treatment. In any case, O3 is not detected at the exit of the catalytic reactor which shows that the catalysts are highly efficient for O3 decomposition.

However, the formation of C1 chlorinated compounds shows that the partial chlorination of the catalyst surface whatever the catalyst under concern as evidenced by the presence of a Cl 2p3/2 component at a BE of 198.5  0.5 eV which plays an important role in the selectivity of the process despite the addition of water in the gas feed to be treated which can help for the removal of chlorinated organic ad-species. The production of NTP generated by-products, namely PG, DCAC and TCAD, is strongly decreased after passing through the NTP assisted catalyst owing:

LaMnO3 > Ce-(MnOx) > CuMn2O4 -P4 >> Fe-K-OMS-2.
However, the concomitant production of chlorinated C1 products, namely CCl4 and CHCl3, and the presence of chlorinated ad-species which strongly interact with the active sites precludes to get higher CO2 yields.

1-5-Plasma as ozoner and ozone decomposition 1-5-1-NTP and ozone generation

Usually, ozone is generated by ultraviolet rays (UV lamp) or by Non-thermal plasma (NTP). UV ozone generator is a simple construction, with low cost, low nitric oxide production and is not affected by the presence of humidity in air. The main disadvantage is that it does not produce high concentrations of ozone (2 mg/h) and as a result air needs to be exposed to the UV lamps for a longer time. By comparison, NTP technology is able to produce high concentrations of ozone from air instantaneously accompanied with the formation of nitrogen oxides. The production of ozone is a function of different factors such as discharge type, discharge polarity and plasma reactor configuration.

The main types of electrical discharges to produce ozone by NTP at atmospheric pressure are dielectric barrier discharge and corona discharge. Comparing these two electrical discharges, corona discharge is an easy method for generate NTP with high flow rates. The main disadvantage is the active volume of the ionization region which is very much smaller than the total discharge volume confined between the two electrodes at atmospheric pressure [9,27,[58][59][60]. The corona discharge is initiated by acceleration of free electrons and subsequent electron collision processes. A series of gas-phase reactions controlled by the energetic electrons created in the corona discharge leads to ozone production. The initial step in O3 formation results from electron impact dissociations of molecular O2 and N2 according to the following equations [9,[61][62][63][64][65].

e -+ O2 -> O + O + e - (1) e
-+ N2 -> N + N + e - (2)
The oxygen atom can further react with O2 and a third collision partner (O2, N2) to produce O3 as:

O + O2 + M -> O3 + M (3)
The NTP discharges are subdivided by the type of the power supply such as DC, AC or pulsed depending on the temporal behavior of the sustaining electric field. Taking into consideration cost-performance, DC corona discharge is superior to the other discharges [66]. Furthermore, DC corona processing efficiency for ozone generation is different when using different discharge polarities. A positive DC corona discharge is formed when the electrode with the strongest curvature is connected to the positive output of the power supply. On the other hand, when the sharp electrode is connected to the negative output a negative DC corona discharge is generated. In the negative corona plasma, the total number of electrons is more than 50 times that produced in the positive corona as observed by Zhang et al. [66]. This can be explained by the fact that the plasma size of the plasma region (where electron-impact reactions are significant) is greater in a negative discharge. In fact, the plasma region in positive corona discharge coincides with the active ionization region but in the negative corona, the plasma region extends beyond the ionization region. Due to the significant difference in the number of electrons in positive and negative coronas, it's reasonable to assume the production of ozone to be far greater in the negative corona than in the positive corona. In contrast, the mechanism for ozone production is independent in the discharge polarity [60,62,[67][68][69]. However, NTP in DC discharge are created in closed discharge vessels using interior electrodes. Depending on the current and discharge voltage, different type of discharge can be obtained. The formula between the discharge current and the inter electrode voltage is: 𝐼 = 𝐶𝑉(𝑉 -𝑉 0 ), where C is a constant depending on the geometry [70]. According to this formula, three regimes of DC discharge are observed as shown in Figure 1231234. At a voltage V0, the corona discharge is initiated, and the corona regime increases linearly with increasing the voltage. In the glow discharge, intermediate regime, the relationship is exponential and nonlinear. The third regime is when the corona transfers to spark regime [9,27,59]. The transition to glow discharge, is marked by a decrease in the voltage and increase in the current and also the active volume of the ionization region is the bigger one compared with the other regimes leading to more ozone production. To maintain this NTP regime of "glow discharge", a multi-pin-to-plate reactor is used, with a specific geometry of electrodes. This type of reactor was first proposed by Akishev et al. [71,72]. The electrodes consist of crenellated pins (cathode) which allow the discharge to ignite at different points contributing to a better diffusion of the current. The anode is a plate containing hemi-spherical surfaces facing the pins. This type of reactor is used in our work with 10 pin-to-plates.

1-5-2-Ozone catalytic decomposition

Various techniques such as thermal decomposition [73], photocatalytic decomposition [74], adsorption [75] and catalytic decomposition were investigated and used for ozone decomposition. Among them, catalytic decomposition was the most effective method for gaseous ozone decomposition because it was a safe, economical and efficient method. Noble metal catalysts (such as Ag [76], Au [77], Pd [78]) showed excellent activity for ozone decomposition but their price limit their practical application. In that way, various transition metal based catalysts were investigated as noble metal-free substitutes for ozone decomposition (Ni, Co, Ce, Cu and Mn [79]) and it was found that p-type semi-conductors were the most active catalysts for ozone decomposition [80,81]. Dhandapani and Oyama [82] reported previously that among the different TMOs, MnO2 exhibited the best activity for ozone decomposition. The high ozone decomposition of manganese oxides was attributed to its multiple and changeable Mn valence and oxygen vacancies. However, MnO2 showed several different crystalline structures such as α, γ, β, λ and δ-MnO2, where the MnO6 octahedral were linked in different ways which could result in different catalytic properties. With that respect, Jia et al. investigated the ozone decomposition over three different tunnel structures of MnO2 (α, β and γ-MnO2) and reported that α-MnO2, a manganese oxide with a one-dimensional tunnel structure with inside dimensions of 0.46 × 0.46 nm 2 that is composed of MnO6 octahedra sharing common edges (2 × 2) and common vertices, possessed the highest activity, which was ascribed to its highest content of oxygen vacancies and largest specific surface area [83]. Catalytic decomposition of an increase in surface area, (ii) a decrease in crystallinity, and (iii) a larger amount of surface oxygen vacancies [88]. It was believed that the oxygen transfer between Ce 4 + and Ce 3+ species on the catalyst surface allowed abundant oxygen species storage and leads to superior redox properties compared to those obtained with simple manganese oxides [91,92]. Overall, these results indicated that Ce was an excellent additive for MnO2 to improve O3 decomposition performances.

1-5-3-Mechanisms for ozone decomposition in dry and humid air

The mechanism and the kinetics of the elementary reactions involved in the ozone decomposition where investigated in numerous studies on transition metal oxide (TMO) [81,82,87,90]. In general, the mechanism for ozone decomposition followed four steps which were illustrated in Figure 1-5-a which gave the mechanism for ozone decomposition on -MnO2 [85]. After the surface oxygen vacancy formed, the oxygen coordination number around Manganese site reduced from six to five, where the mononuclear five-oxygen coordinated Manganese site was regarded as the active center of ozone decomposition. Firstly, an ozone molecule was adsorbed on an oxygen vacancy (state 2) to form a dioxygen molecule and an oxygen species (O2 -, state 3) by electron transfer. Then, the active oxygen species O2 -could react with another ozone molecule to form a peroxide species (O2 2-, state 5). Finally, the peroxide species (O2 2-) transformed into O2 and the oxygen vacancy was restored.

Hence, oxygen vacancy was beneficial for ozone adsorption and decomposition on the catalyst surface. Furthermore, in the presence of humidity another pathway (Figure 1-5 b) for ozone decomposition was proposed in which water adsorbed on oxygen vacancy to produce surface-OH2 + . Under the influence of electrostatic forces and hydrogen bonding, a six-membered ring was formed with the O3 molecule [85]. This ring was unstable and transformed into a surface hydroxyl group with the release of HO3 • . At the same time, the ozone molecule reacted with the surface hydroxyl group and formed a five-membered ring (state 4). Then the five-membered ring transformed into a surface hydroperoxyl anion (HO2 -) with the release of O2. Then O2 -was formed by reaction between the surfaces HO2 -with O3. Finally, O2 -was released as an oxygen molecule from the surface to regenerate the oxygen vacancy on the surface. If RH was too high, water adsorption increased and blocked the surface active sites.

1-5-4-Deactivation mechanism

Although MnOx catalysts were widely studied due to their excellent properties, they were prone to deactivation under both wet and dry atmospheres. After reacting with ozone, manganese oxides adsorbed oxygen species leading concomitantly to a higher Mn valence state and a decrease of oxygen vacancies which were the main causes for catalyst deactivation in dry and in humid air. However, under humid atmosphere, in addition to the deactivation due to the accumulation of oxygen intermediates, the causes of deactivation were more complex and varied due to the presence of water molecules. The deactivation of MnOx under humid atmosphere was mainly caused by competitive adsorption of water molecules and O3 molecules.

The adsorbed H2O molecule was difficult to desorb from the surface oxygen vacancy, resulting in a rapid decrease in the ozone removal rate.

These two deactivation mechanisms did not take into account the deactivation due to related nitrogen oxides species. Indeed, once ozone was generated using air as a carrier gas, it was accompanied by NOx, especially when the plasma was the source of ozone. Additionally the amount of nitrogen oxides generated by the plasma was higher compared to UV lamp.

Mehandjev et al. [93] showed that the main reason of α-FeO2 deactivation for O3 decomposition by producing ozone through an ozone generator in dry air was due to the adsorption of some nitrogen oxides species transforming to nitrate ions on the catalyst surface. To support this result, Touati et al. [94] reported recently on the deactivation for ozone decomposition of Pd/Al2O3 catalyst due to NOx poisoning.

1-5-5-Acid treatment to increase MnO2 hydrophobicity

Additionally, the hydrophobicity of MnO2 was investigated in order to increase the catalytic performance for ozone decomposition. The addition of noble metals such as Ag [95] was one method to enhance the hydrophobicity. However, ammonium or acid treatments were the most encountered routes. T. Gopi et al. investigated for the first time the ozone decomposition over a layered structure of MnO2 (birnessite), a nanostructured K-δ-MnO2

(synthetized by hydrothermal method) and H-δ-MnO2. They found that the exchange of K + by H + when using a HNO3 (1M at 70°C for 6 hours) treatment, led to higher surface area, lower Mn average oxidation state (AOS) and increase in the surface oxygen vacancies. These modifications result in a better activity and stability towards ozone decomposition in dry and humid air [96]. Y. Liu et al. [97], reported that a birnessite synthesized by a redox method using KMnO4 and CH3OH and subsequently treated with HNO3 (0.2M at 50°C for 6 hours) possessed high activity for ozone decomposition due to the fact that after acid treatment the surface area of the catalyst increases together with the formation of a large amount of acid sites and oxygen vacancies. Furthermore, ozone molecules reacted as Lewis bases on Lewis sites [80,97,98].

Acid-treated [97] or ammonium-treated [99] birnessite investigated as two approaches to increase the water-resistant property in other term to increase the material hydrophobicity.

1-5-6-Mechanism for ozone decomposition over birnessite treated with nitric acid

After acid treatment, Liu et al. [97] showed by DFT calculation that the formation energy for oxygen vacancy was two times lower as compared with the as-synthesized birnessite, which indicated that oxygen vacancies could be formed much more easily over the acid treated birnessite. Additionally, a high amount of acid sites were formed on the catalyst surface. Based on these results, Liu et al. [97] proposed a reaction mechanism for gaseous ozone decomposition which was given in In order to meet most of the criteria discussed above, a higher amount of oxygen vacancy, resistance to water and chlorine undoped or Ce-doped birnessite materials, treated or not with nitric acid, were chosen as defective structures for ozone decomposition and TCE oxidation enabling to work at low temperatures.

1-6-Birnessite

The birnessite is chosen as a powerful catalyst which has been recognized to decompose different VOCs at low temperature mainly due to its redox properties as explained in more detail in the following section. Birnessite is a manganese dioxide known as δ-MnO2. It has been first discovered by Jones and Milne (1956) in the Birness region of Scotland, who gave it the formula Na0.7Ca0.3Mn7O14, 2.8H2O. This mineral is omnipresent in soils and sediments and is one of the most common Mn oxides [100][101][102]. In the natural environment, birnessite is characterized by low crystallinity [102][103][104]. It is a mixed valence manganese oxide Mn 3+ /Mn 4+ . Birnessite is a 2D layered manganese oxide material with edge shared MnO6 octahedra forming the layer with an interlayer separation of about 0.7 nm. It has a significant amount of water and stabilizing cations such as Na + and K + between the sheets. This mineral, whose sheets are negatively charged (~ 0.2-0.35 charge per Mn) has a high capacity for adsorption and ion exchange. In addition, the presence of heterovalent cations in the layers (Mn 2 + , Mn 3 + , Mn 4 + ) confers redox properties [105] which are involved in the adsorption mechanisms of certain cations: As, Cr, Co for example [104,106].

1-6-1-Birnessite crystalline structure

Birnessite is defined according to two criteria: its layers are rich in manganese and the interfoliar distance is close to 7Å. This very succinct definition does not account for very large structural variations existing among the different varieties of birnessite known to date. The first structural studies carried out on natural birnessite collected in marine or terrestrial sediments have identified a hexagonal or pseudo-hexagonal structure [102,107]. Another study conducted by Tan et al. (2010) [108] states the existence of triclinic symmetry in addition to hexagonal.

The structural study of synthetic birnessite by X-ray diffraction and Rietveld refinement was performed for the first time by Post and Veblen, (1990) [109] studying the structure of a birnessite synthesized by oxidation of Mn (II) in basic medium [55,84,86,[110][111][112]. The The triclinic birnessite studied by Post and Veblen (1990) [109] has sodium cations in the interlayer space, its chemical formula has been described as Na0.58 (Mn 4 + 1.42 Mn 3 + 0.58) O4 • 1.5H2O. The structure of this type of Birnessite is shown in Figure 1234567. It is formed of nonlacunar layers of thickness 2 Å with an equidistance of 7Å. The layers consist of MnO6-sharing octahedra containing Mn 3+ / Mn 4+ mixed valence cations [109,114]. The layers have been described as being an alternation of an octahedral chain of Mn (III) O6 and two octahedral Mn (IV) O6 chains distributed along the b axis [103,104,115]. In the layers of birnessite, manganese is in octahedral symmetry. The electronic configuration of Mn (III) being [Ar] 3d 4 , it suffers by Jahn-Teller effect, a distortion of its symmetry (non-regular octahedron). This is not the case of Mn (IV) of electronic configuration [Ar] 3d 3 . Therefore, a layer formed only by Mn (IV) O6 octahedra would have hexagonal symmetry, but the presence of Mn (III) O6 decreases symmetry due to the Jahn-Teller effect [121] The substitution of the Mn (IV) cations by Mn (III) creates a negative charge of the sheets which is compensated by the presence of hydrated interlayer cations (eg, Na + (H2O) n) which occupy the space between the sheets [114].

Interlayer sodium can be exchanged by a variety of cations: Li + , K + , Cs + , Be 2 + , Sr 2 + , Ba 2 + without affecting the structure of birnessite [116][117][118]. The exchange by Ca 2 + , Mg 2 + or Ni 2 + causes hydration of birnessite and its conversion to buserite [116]. The hydration energy of the intercalated cation, related to its size and its charge, controls the binding force between the latter and the water. Hydration degree varies with the type of interfoliar cation [119] . interlayer cations are H + and Mn 2+ /Mn 3+ which are positioned below and above the deficiencies [115] In the birnessite of hexagonal symmetry, manganese is mainly in its oxidation state (IV).

The presence of octahedral deficiencies is the major cause of layer slip deficiency [115] (Figure 12345678). This deficit is offset by the presence of protons and interlayer Mn (II) / Mn (III) that lie below and above the deficiencies [104]. The interlayer distance is 7 Å. The formula proposed by Lanson and al. (2000) for the birnessite hexagonal is H + 0.33Mn 3 + 0.111Mn 2 + 0.055 (Mn 4 + 0.722Mn 3 + 0.111☐0.167) O2 where ☐ represents the gaps in the layers. The sheets have about 72% Mn (IV), 11% Mn (III) and 17% deficiencies [121].

Birnessite symmetry transformation from one type to another is possible. This can be the result of a change in the pH of the reaction medium [103] or the presence of a cation that causes redox reactions with the manganese contained in the birnessite [104,120]. Lanson et al.

[121] reported the transformation of triclinic Na-Birnessite into H-Birnessite in acidic medium (pH 2-5). They found that hexagonal phase is only possible at pH ≤ 3. This study shows that, when a triclinic Na birnessite is placed in acidic medium, a Na + /H + exchanges takes place in the interfoliar space and a dismutation reaction of Mn (III) into Mn (IV) which remains in the layer, and into Mn (II) which migrates into the interfolaire distance takes place. The dismutation reaction reduces the amount of Mn (III) and leads to the formation of gaps. However, hexagonal birnessite maintained in alkaline solution NaOH can transform into Na triclinic birnessite as reported by Zhao et al. [120].

Vernadite: Another variety of birnessite is the vernadite structure. Vernadite has been first described by McMurdie (1948) [122] who assumes the material to be a poorly crystallized variety of cryptomelane, while other studies conducted at the same time concluded that this synthetic compound had a structure similar to pyrochroite [123]. More recently, using synchrotron techniques (XANES), Manceau et al. (1992) hypothesized that vernadite had a tunnel-like structure composed of chains of varying sizes and lengths [121]. In fact, a number of studies have studied the behavior of vernadite under the assumption that they have been dealing with a hydrous manganese oxide. Vernadite is also commonly referred to as "hydrous manganese oxides -H.M.O", a denomination that assumes that the oxide is, again, amorphous [124]. However, the true nature of vernadite, and its relationship to birnessite were discovered by R. Giovanoli in the 1970s. He was the first to describe vernadite as a turbostratic form (without three-dimensional order, which is to say with translations and/or random rotations between two successive sheets) of birnessite. This discovery engaged then strong discussions with a Russian team led by F.V. Chukhrov [107]. This team thought that birnessite and vernadite were two independent minerals. Their hypothesis was based in particular on the fact that the visible basal reflections on diffractograms of birnessite at 7.2Å (001) and 3.6Å (002)

were totally absent in the diffractograms of the vernadite. Whereas, when Jones and Milne (1956) first described birnessite, they noted only a series of four diffraction peaks corresponding to basal distances of 7.27 Å, 3.60 Å, 2.44 Å and 1.412 Å, which could indicate a low crystallinity level. However, this lack of basal reflections is explained by the nanometric nature of the vernadite. Villalobos [114] showed that vernadite is nanometric in size in all three dimensions of space, and that the number of stacked sheets along the c* axis is on average about

1-6-2-Birnessite synthesis methods

There are several chemical routes to obtain birnessite. The symmetry as well as the chemical composition of birnessite depends on the synthesis methods. A synthesis carried out in an acidic medium results in a hexagonal symmetry, whereas a synthesis carried out in a basic medium results in a triclinic symmetry. It is possible to classify the synthesis methods in four groups according to the reaction conditions.

I-Oxidation of Mn (II) by oxygen in an alkaline solution: This is the oldest method, based on the oxidation of pyrochroїte Mn(OH)2 by oxygen in the presence of alkaline solution. The birnessite obtained has a triclinic symmetry with K + or Na + as interlayer cations, depending on the alkaline solution used KOH or NaOH [125,126].

II-Reduction of Mn (VII) in acidic medium. The oldest process in the Mckenzie procedure [113], where the reduction of KMnO4 is carried out using a concentrated acid HCl, which makes this method restrictive. It remains quite reactive because it allows to obtain hexagonal symmetry. Different other reducing agents have been used such as alcohols [127], ethylene glycol [128], sugars [164] and more recently lactate [130].

III-Transformation of another manganese oxide: this method is not often used. Cornell and Giovanali [101] investigated successively the direct conversion of hausmannite to birnessite in a highly alkaline environment.

IV-Synthesis using the couple MnO4 -/Mn 2+ . This synthesis is based on a redox reaction between MnO4 -and Mn 2+ in alkaline medium. It generally involves the use of three reagents:

A salt of Mn (II) (MnCl2, MnSO4 or Mn(NO3)2), a permanganate solution (KMnO4 or NaMnO4) and a base (KOH or NaOH). This method is intensively studied because of its ease of use and contributes to the formation of triclinic birnessite [131][132][133].

1-6-3-Variability in the composition of sheet and interfoliare

The layers of birnessite are mainly composed of Mn (IV). The leaflet may also contain a number of octahedral sites whose manganese is vacant, so we shall speak of gaps. In parallel with the presence of gaps, the leaflet may include a number of Mn IV substitutions by Mn III.

In the case where the amounts of Mn III in the layer are small, and / or the Mn III is not organized in rows, the symmetry of the layers is hexagonal. In the case where the Mn III is sufficiently abundant and organized, the layer loses its hexagonal symmetry and becomes triclinic. It should be noted that the replacement of manganese by other cations, of size close to Mn III, IV, in the birnessite layer has already been observed, as for example nickel or cobalt [106]. The modeling of the structure of the layer is a fundamental parameter for understanding the reactivity of the birnessite. Indeed, if the stoichiometric structure of the layer of a phyllomanganate such as birnessite is theoretically MnO2, the various studies made on this mineral and its synthetic analogues have shown that, practically, the chemical composition was of the form:

H + a Interb Mn 2+/3+ c H2Od [Mee Mn 3+ f Mn 4+ g □ h] O2 In this formula, the species in brackets make up the layers and the species to the left of the brackets are in the interlayer distance. Inter represents any interstitial cation, H2O water molecules related to these cations and free water. Me represent any cation presents in the leaflet, other than manganese, and finally □ represents the gaps in the leaflet.

The charge deficit of the layers can thus be induced either by the presence of vacant sites or by the substitution of a Mn 4 + of the layers, by Mn 3 + more often. Depending on the origin of the charge deficit, the reactivity will be strongly different. Indeed, a site vacancy will create a locally high load deficit (4 charges), while substituting a Mn 4 + for a Mn 3 + will only create a single charge. According to the different studies conducted on the birnessite, a charge deficit created by the substitution of Mn 4 + by Mn 3 + will be compensated by cations situated relatively far from the layers, most commonly in the middle of the interlayer distance, according to a mode called adsorption in outer sphere complex, a vacant site will most often lead to the adsorption of cations just above or below these vacancies; we shall then speak of an internal sphere complex. However, the inner-sphere complexes can be located at different  Triple corner sharing: the cation is located at the bottom of a gap, and is coordinated to three oxygens of the sheet. The cation is also coordinated to water molecules in the interlayer, but this number is variable. For example, zinc (Zn) can be in octahedral coordination (coordinated to three water molecules), but when adsorbed on a birnessite with low coverage, it coordinates tetrahedrally (coordinate to only one water molecule in the interlayer) with the coordination sphere forming a pyramid with a triangular base formed by three sheet oxygen atoms [115,134,135]  Triple edge sharing (TE): The cation shares three edges with the sheet above which it is adsorbed overhanging a tridentate cavity. This position is much less frequent than the TC position. This crystallographic position, a priori less stable than the TC position, can however be explained by a cation bound to two un-saturated oxygen of an octahedral gap.

Or in the case where the sheet contains for example Mn 3+ , the third oxygen to which the cation is bound is also un-saturated. This position could be favored when a cation is absorbed on the other side of the vacant octahedral site. The number of water molecules coordinating the cation in the interfoliar distance is not fixed and is dependent on the cation as with TCS [115,134,135] .

 Double corner sharing (DC): The cation, located at the edge of the sheet, is coordinated to two oxygens of the sheet and the rest of coordination ensured by water molecules, one of which is located in the plane of the sheet oxygen atoms [115,134,135].

1-6-4-Birnessite properties (amount of alkaline metals) and capacity of ion exchange

The various synthesis procedures, precursors and reaction conditions, including reagent concentration, aging time and reaction temperature, constitute the main difficulty in determining a correct chemical formula for birnessite summarized in Table 1-3. The negative charge of the birnessite layers is compensated by alkali metals and / or protons from the reaction medium. These cations can be adsorbed on the surface or intercalated in the interlayer surface.

However, layer cation vacancies are associated with interlayer cations. On other way, the vacancy problem is related to the problem of the valence of Mn cations. Under ideal conditions, the amount of alkali metals should be equal to the amount of Mn3+ in the layers, the latter is estimated to be 1/3 [103,136]. The best known are birnessite Na and K, the following section will focus on the amount of Na and K observed in the literature as interlayer cations in birnessite.

The K/Mn ratio varies between 0.06 and 0.32 [103,125,[137][138][139][140][141][142][143], while the Na/Mn ratio varies between 0.09 and 0.58 [137,138,140,141]. Prieto et al. [144] has studied the variation of alkali/Mn ratio, and they found that variation in the experimental molar ratio influences the amount of K and Na measured in the interlayer distance, but for K the amount is limited to 0.31 which means that the maximum of K + that can be in the interlayer distance is 0.31. Varying the two ratios influences the average manganese oxidation state (AOS); increasing or decreasing the alkali metal/Mn ratio results in a decrease in the AOS of Mn [136]. However, the typical value for birnessite AOS is between 3.6 -3.8 [137]. Kulish et al. [138] have synthesized Na-Birnessite with Na/Mn = 0.22 and K-Birnessite with K/Mn = 0.6 as the result Mn AOS is 3.78 and 3.37 respectively, a higher alkali cation content in the interlayer space leads to a larger proportion of Mn 3+ . Moreover, Zhu et al. [91], have been found a collapse of the birnessite structure with a K/Mn =0.06 into an amorphous manganese structure. For Nabirnessite the amount of Na should be betzeen 0.17-0.33, which corresponds well to the Mn AOS (3.6-3.8) [103,115,125,[137][138][139][140]145]. Birkner et al. [141], has synthesize sodium birnessite by keeping the amount Na/Mn 0.09 as a results they obtained Mn AOS= 3.56. Zhang et al. [145], have reported Na-birnessite with a high amount (Na/Mn = 0.58) of Na in the interlayer distance resulting on a Mn AOS = 3.42. However, washing can affect the amount of the alkali metals. It has been reported that birnessite is washed by different solvents such as water alcohol and acetone. Feng et al. [146] has found that washing with alcohol or acetone resulted in a finer powder as compared to washing with water. The washing is needful as reported by Prieto et al. [144] to remove the excess of alkali metal. Moreover, Kulish et al.

[138] has done the washing with water, intensively more for Na-birnessite (20 times) over K-Birnessite (3 times) (the volume of water is not given, typically they may be the same amount of water for each washing) due to its less hygroscopic nature. Capacity of ions exchange: On this part we will focus on Na-Birnessite because it is more studied in the literature. The interlayer cations of hexagonal and triclinic birnessite are exchangeable with a wide variety of metal or organic cations. Na birnessite has a cation exchange capacity of about 240 meq/100g [130]. Studies on the substitution of sodium interfoliar with monovalent cations: K + , Li + , Cs + and divalent cations: Ca 2+ , Ba 2+ , Be 2+ and Sr 2+

have been performed [103,116,118,119]. In the studies carried out at pH 4 on Na + /M 2+ exchange (M 2+ = Pb 2+ , Cd 2+ and Zn 2+ ), [103,106,115] showed the formation of vacancies in the layers after reaction with H+ and the adsorption of divalent cations onto these vacancies. Similarly, a more recent study established the selectivity of hexagonal birnessite towards divalent metals and the following classification was obtained: Pb 2+ >Cu 2+ >Zn 2+ >Co 2+ >Cd 2+ [178]. Another study was interested on the selectivity of Na birnessite with respect to monovalent cations by analyzing sodium released in solution. The selectivity thus deduced decreases in the following orders: Cs + >K + ≥ NH 4+ >Na + >Li + . This selectivity depends on the ionic radius of the intercalated species and its degree of hydration [156].

1-7-Formaldehyde

Formaldehyde or methanal or formal aldehyde is an organic compound of the aldehyde family, with the chemical formula HCHO (or H2CO). It's a colorless gas under normal conditions of temperature and pressure with a strong, pungent, suffocating and highly irritating odor. HCHO is a highly reactive product that can both polymerize or form explosive mixtures in air [157]. Furthermore, formaldehyde is very toxic and dangerous for human health.

Formaldehyde is a toxic and dangerous volatile organic compounds, it has been classified in 2015 as a group 1 (definite human carcinogen) carcinogen by the International Agency for

Research on Cancer (IARC). However, the Scientific Committee on occupational exposure limits (SCOEL) recommends 0.2 ppm (0.25 mg m -3 ) as a limit exposure value for 8 hours and 0.4 ppm (0.5 mg m -3 ) as limit exposure value for short term (15 min) [START_REF]Environmental Toxicants: Human Exposures and Their Health Effects, Fourth edition[END_REF]. Formaldehyde, is widely used material and present in industry, it should also be considered as a pollutant in indoor air. It's established that HCHO is one of the most major pollutants in indoor air and there are several sources of HCHO emission in our indoor air such as plywood coatings, wallpaper, paint is considered HCHO emitting material. Indeed, many other indoor products are likely to emit HCHO in indoor air such as, cigarettes, wood, oils, plastics and some household and cosmetics products [START_REF] Salthammer | [END_REF]. However, the catalytic oxidation of this molecule is choosen in our work as an approach to validate the density of oxygen vacancies on our material which will be discussed in details in the following part.

1-7-1-Formaldehyde catalytic decomposition

Several transition metal mainly in oxide form have been used for HCHO oxidation.

However, Sekine et al. [160], in 2001, study the catalytic decomposition of HCHO over different metal oxides such as: Ag2O, PdO, CuO, CoO, ZnO, Fe2O3, TiO2, La2O3, WO3, CeO2, V2O5 and MnO2. Manganese dioxide was found to be the most active catalyst among aforementioned series. Furthermore, MnO2 has advantages to be cost-effective, environmentally friendly and easy to get. In terms of MnO2 crystal structure, birnessite type MnO2 was recognized to be the most active catalyst in HCHO oxidation [161]. Birnessite type MnO2 is a two-dimensional layered structure, composed of alternating stacked Mn-O layers (edge-shared MnO6 octahedra) and hydrated alkali cations (Na + , K + , etc.) restricted in the interlayer region [24][25][26][27]. Wang et al. [163] reported on the beneficial role of interspacing water to promote the formaldehyde decomposition [163]. Wang et al. [164] investigated the nature of the intercalated ion and found that K-birnessite achieved the best CO2 yield at ambient. K + was believed to enhance the surface oxygen activity facilitating the regeneration of surface hydroxyls by activating H2O. In order to promote the catalytic performances of such materials significant experimental researches were recently focused on defective birnessite materials with proper surface electronic properties [29][30][31]. Wang et al. [168] reported on the introduction of manganese vacancies (VMn) in K-birnessite obtained from a redox-precipitation route whose relative content was tuned by affecting the oxidant/reductant atomic ratio. The MnO2 with higher content of VMn showed a better performance for formaldehyde decomposition. The promotion effect of K + in HCHO oxidation was herein supposed to be accomplished through activation of surface oxygen species. In addition, Rong et al. [169] reported about the existence of an optimal K + content to promote the activity in Mn vacancy-rich birnessite-type. A too high K + content increased the amount and strength of surface basic sites making the CO2 desorption more difficult and in that way partially inhibited the catalyst. Taking into account these considerations, Li et al. [170] reported on K-birnessite with poor crystallinity with a medium K + concentration for obtaining a highly active birnessite in HCHO oxidation.

Element incorporation was also another followed strategy to obtain a more active catalyst. In that way, incorporation of suitable transition metals (TM) or lanthanide cations (Ln cations) was investigated to promote activity by seeking to decrease energy formation of oxygen vacancy [171]. On purpose, Tang et al. [172] investigated MnOx-CeO2 oxide catalysts for complete oxidation of formaldehyde through the effect of preparation method and calcination temperature taking advantage of the redox properties of nano sized CeO2 for oxygen storage and release. A catalyst (Ce / Ce+Mn=0.5) prepared by a modified co-precipitation method and calcined at 500 °C exhibited much higher catalytic activity toward complete oxidation of formaldehyde where CeO2 and -MnO2 phases segregated. A high oxidation state of Mn 4+ and huge amount of surface lattice oxygen was claimed to play a role in the reaction.

On contrary, a synergistic effect on the catalytic activity for 20 ppm HCHO oxidation performed at high space velocity was ascribed to high Mn 3+ / Mn 4+ and Oads/Olatt surface atomic ratios on a MnO2(x)-CeO2 (x= Ce/Mn = 0.66) catalyst [173]. Liu et al. [174] investigated the performances of MnOx-CeO2 catalysts with different Mn/Ce ratios in HCHO oxidation. The enhancement of reactivity in HCHO oxidation was ascribed to the activation of lattice oxygen species in MnOx due to CeO2 addition. Recently, following a strategy of rational design of defects to enhance catalytic activity, Zhu et al. [91] reported on the performances for HCHO oxidation of Ce modified-type MnO2 with different Ce / Mn ratios obtained from a redoxprecipitation method followed by a drying step at 105 °C for 12 h. Ce-MnO2 (1:10) was shown to be the most performant catalyst. Higher content of oxygen vacancies resulting from a close contact between CeO2 and MnO2 nanoparticles was postulated to be responsible for its high activity in HCHO oxidation.

Since then, different investigations were carried out in HCHO oxidation over MnO2 [175][176][177][178][179]. The activity of manganese oxide involves oxygen activation and oxygen transfer through redox cycle between manganese species (Mn 4+ /Mn 3+ ). The surface structure of the catalyst, crystallinity, valence state of manganese, morphology and the surface defects all may have effect on the catalytic activity. Thus, it is supposed that, surface modification to form unsaturated sites could be greatly improved the catalytic performance of manganese oxides [175,177,180].

1-7-2-Mechanism for HCHO oxidation

Selvakumar et al. [178] propose a mechanism for the catalytic oxidation of HCHO by These considerations hold if the catalytic formulation is adapted to quantitatively degrade O3 in the course of the experiment. This also implies the catalyst to be stable with time, in other terms to be water tolerant and resistant to chlorine with time.

To address these issues, the adopted strategy and related methodology are the following: A HNO3 treatment is also performed to enhance the water tolerance and ozonation performances because it's well-known that acid treatment increase the number of active sites.

3-Validation approaches:

 First the catalytic performances are tested towards formaldehyde oxidation (Figure 2 The Ce-doped MnO2 (x: Ce/Mn atomic ratio of 0.01, 0.1, 0.2 and 0.5) were prepared based on the same procedure except that Ce (NO3)3.6H2O (Alfa Aesar; ≥ 99.5 %) was dissolved concomitantly with KMnO4. The as-prepared samples were labelled CexMn.

The main reactions occurring during the synthesis of birnessite were the following:

Reduction reaction:

(K + , MnO4 -)+ 4H + + 3e - KxMnO2 +2H2O + (1-x) K + (R1)
Oxidation reactions:

(Na + ,CH3-CHOH-COO -)  (NayCH3-CO-COO -)+H + + 2 e -+(1-y) Na + (R2) CH3-CHOH-COO - CH3-COO - (R3) CH3-CHOH-COO - H-COO - (R4) CH3-CHOH-COO - -OOC-COO - (R5)
According to equation 1 and 2, the balance sheet equation is:

Na + + 2(K + , MnO4 -) + (3+x+y ) CH3-CHOH-COO - 2KxNayMnO2 + (3+x+y) CH3-CO- COO -+ 4H2O + 2(1-x)K + + 2(1-y)Na + + 2(1+x+y)H + (R6)
Where, x and y are the amounts of Na and K that are intercalated in the lamellar structure

3-1-2-Calcination procedure

The dried samples were calcined at Tc = 400 °C for 5 h in flowing dry air (1.0 g; 2 L/h; 2 °C/min) (Figure 3-1). Ce0.1Mn was calcined also at 200 °C and 300 °C and Ce0.01Mn at 300 °C in similar conditions to those at 400 °C. The suffix X00 was added to the labeling of the samples.

3-1-3-Acid treatment

The dried samples were also treated with 0.2 M HNO3 solution (VWR 68%), with a ratio mcatalyst (g)/ VHNO3 (mL) = 1/20, for 6 hours at 50°C under a vigorous stirring (500 rpm). Then, the black suspension was filtered and washed continuously with distilled water until the pH of the eluate reached ~7. After washing, the resulting black solid was dried at 105°C for 18 h. The suffix AT was added to the labelling of the acid treated samples.

3-2-Catalyst characterizations

3-2-1-Thermal Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA)

Gravimetric and differential thermal analyzes are two techniques that measure the variation of the mass of a sample as a function of temperature and the exchange of heat between the sample and the medium as a function of temperature simultaneously. The choice of gas sent to the sample is very important because it determines the type of reaction that can occur: oxidation, reduction, adsorption, desorption etc. ... The experiment can be performed under air or inert atmosphere.

In the laboratory, a thermal analysis apparatus is used which simultaneously performs thermogravimetric and differential thermal analysis experiments. The instrument consists of a waterproof enclosure that controls the atmosphere of the sample, a microbalance, an oven to manage the temperature, a thermocouple to measure the temperature and a computer to control the instrument and record the data. At the end of the experiment, the ATG and ATD curves are obtained on the same thermogramme.

TGA analyses were performed on sample weight of about 10 mg (5 mg for the acid treated samples) in air flow (100 mL/min) with a heating rate of 10°C/min up to 800°C using a TA instruments TGA-SDT 2960.

3-2-2-X-Ray Diffraction (XRD)

X-ray diffraction is a physico-chemical, non-destructive technique. It is the most widespread in the field of solid-state chemistry since it is particularly suitable for the characterization of crystalline materials. The determination of the crystallographic composition of the analyzed sample provides access to physical information about the crystals, including their size and orientation.

Its principle is based on the interaction of X-rays with the electrons of the atoms in the sample. In our case, we use the powder method. This method consists of bombarding the sample with a monochromatic and parallel X-ray beam and collecting the angles of diffraction which it emits. The sample is spread in powder form on a glass slide that rotates uniformly about an axis in its plane, thus increasing the number of possible orientations of the reticular planes and avoiding preferential orientations. by mass and varies according to the elements contained in the sample to be analyzed.

The diffractometer used is Bruker AXS D8 Advance type equipped with a CuKα source, a Lynxeye type linear fast detector and a copper anticathode, in θ / θ geometry. The recordings were made with a measurement step of 0.02 ⁰ in a 2θ domain, ranging from 5 to 80 ⁰ with an integration time of 1 s.

3-2-3-Transmission Electron Microscopy (TEM)

Transmission microscopy is a particularly important instrument that allows to highlight the micromorphology of the studied solids and the possible presence of crystalline defects, amorphous phase deformations, etc... The microscope consists of three main components: an electron-optical column, a vacuum system and the electronic elements necessary for its operation (lenses, etc.). The column comprises a succession of electromagnetic lenses which influence the trajectory of the electrons. Two categories of images are formed in TEM: The high-resolution image that results from the interference of the different beams coming from the object and the conventional image which is obtained after the selection of a single beam. The first image allows the observation of the sample at very high magnification, which gives us access to the measurement of the size of the particles, the visualization of the grain boundaries, the atomic planes and columns of the crystallized phases, etc...

Transmission electron microscopy (TEM) characterization was performed using a

TECNAI TEM operated at 200 kV. The prepared powders were deposited onto a carbon-coated copper grid for TEM observation Chapter 3: Material and methods

3-2-4-Inductively coupled plasma-optical emission spectrometry (ICP-OES)

The elementary analyses allow to determine the quantity of each species present in the solid. The results are expressed in mass percentage. The elemental analysis was performed by inductively coupled plasma-optic emission spectroscopy 720-ES ICP-OES (Agilent) with axially viewing and simultaneous CCD detection. ICP-OES operates using argon plasma into which an atomized liquid sample is injected. Inductive plasma can be generated by directing the energy from a high frequency generator to a suitable gas, usually argon, the sample is ionized in the plasma and the ions emit light at different characteristic wavelengths which are then measured. The quantitative determination of metal content in the catalysts was made based on the analysis of certificated standard solution. The ICP Expert TM software (version 2.0.4)

provides the concentration of metal in sample allowing estimating the weight percentage of Ce, Cu, K, Na, Mn. The operating parameters of the instrument were continuously monitored to ensure the maximum performance and reliability of the ICP-OES results. All the analyses were performed 40 min after the spectrometer was turned on to achieve a stable plasma as well as constant and reproducible sample introduction.

The sample preparation was made by dissolving 10 mg of dried and ground samples catalyst in concentrated aqua regia solution (HNO3:HCl) (1:3, v:v). The obtained solution was heated up to 110°C in the Vulcan autodigestor for 2 h. Then, the solution was diluted up to 20 ml by ultrapure water before being analyzed by ICP-OES.

3-2-5-Textural properties analysis via Brunauer-Emett-Teller (BET) and Barret-Joyner-Halenda (BJH)

The textural properties of the samples, were studied via N2 adsorption desorption measurements at liquid nitrogen temperature using Tristar II 3020 from micrometrics. BET theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for the measurement of specific surface areas. BJH analysis is a pore size distribution determination method, which is typically applied to nitrogen desorption data measured on sample at 77K.

The mass used in our study is in the order of 500 mg. A degassing process is carried out in advance to eliminate any impurities that may alter the experiment. For this purpose, the sample is heated at 100 ⁰C under vacuum for four hours. The tube is weighed again to determine the exact sample mass. The adsorption process occurs when the tube is immersed in a Dewar filled with liquid nitrogen. Desorption occurs when the sample is returned to room temperature.

These processes are monitored by a catharometric detector (C T D) to measure the variation in the thermal conductivity of gas streams, resulting from changes in adsorbate concentration. The area of the peaks obtained is proportional to the amount of nitrogen retained at the surface and then desorbed when the sample temperature rises. Knowing that in the case of nitrogen physisorption at the temperature of liquid nitrogen (-196 ⁰C), the surface occupied by a nitrogen molecule is equal to 16.2 . 10 -20 m 2 , the number of adsorbed molecules can be evaluated to determine the total specific area..

3-2-6-Hydrogen-Temperature Programmed Reduction (H2-TPR)

Temperature programmed reduction is a technique that makes it possible to monitor the behavior of a solid sample under a reducing atmosphere. This technique is often used in the field of heterogeneous catalysis; it consists in heating the oxide material under a flow of reducing mixture.

The monitoring of the quantity of hydrogen consumed as a function of temperature makes it possible to draw a TPR profile and the evolution of reducibility (the quantity of reducible species present in the material and the temperature at which the reduction phenomena take place) of the sample analysis. The quantification of hydrogen consumption also provides access to information such as the average oxidation degree of the active phase.

The TPR analyses were performed on a Micrometrics Autochem II device (2920). The device consists of a thermal conductivity detector TCD (which detects and quantifies the amount of hydrogen), a gas injection system consisting of mass flow meters and several threeway and six-way valves (to accurately control the flow and circulation of the gas stream),a furnace equipped with a thermocouple (to regulate the temperature rise of the sample), a saturator equipped with a heating jacket, a water trap made of a mixture of liquid isopropanol and liquid nitrogen (to protect the detector from the water formed during reduction) and a station.

The sample mass is the function of the quantity of reducible species present in the solid.

In order to obtain results with better resolution, two parameters of sensitivity: shape K and resolution P have to be calculated as follows: K= 𝑆 0 𝑉.𝑐 0 (Eq.1)

P= 𝐾. 𝐷 (Eq.2)

With S0: initial quantity of species reducible to mol, V . C0: molar flow of active gas in mol/s and D: rate of temperature rises in °C/s.

For convincing results, it is recommended that K and P have values such as 55<K<150 and 20<P<50 [2,3]. These values are respected in all our measurements.

In our study, 50 mg mass of sample is placed in a U-shaped quartz tube. The analysis begins with degassing with argon for 1 hour at 150°C followed by cooling down to room temperature. Then the temperature was allowed to increase from 25 °C to 800 °C with a heating rate of 10 °C/min under reducing gas which is a mixture of 5 vol.% H2/Ar at a total flow rate of 50 mL/min.

The surface calibration of the peaks obtained by the TCD is carried out as follows: 5

pulses of 5% H2/Ar (50 mL/min) without passing through the solid studied, were performed and their average surface is calculated. The volume of the injection loop of the device being 0.4 mL, the amount of hydrogen responsible for the average of the peak areas of the 5 pulses will be evolved and then the amount of hydrogen consumed during our experiments is determined.

3-2-7-Temperature programed desorption (O2-TPD)

Similar to H2-TPR, thermo-desorption of oxygen provides valuable information on the RedOx properties of a metal oxide material. The key principle is to heat the material in an inert atmosphere and record spectrum of exhaling gases (O2, CO2, CO, H2O) desorbed by mass spectrometer. It is thus possible to quantify and characterize the different types of oxygen from the material as a function of temperature.

Temperature-programmed Desorption of O2 (O2-TPD) tests were carried out with 0.05g of catalyst under 50 mL/min of helium using Micrometrics AutoChem 2920 and the signal for O2, CO2 and H2O were detected using mass spectrometer. After pretreated at 105°C for 0.5 h in 50 mL/min of helium, the catalyst was exposed to 50 mL/min of 5 vol % O2/He for 0.5 h at room temperature, then purging with 50 mL/min of helium for 0.5 h to remove the physisorbed O2. Then the sample temperature was increased from room temperature to 1000°C with a heating rate of 5 °C/min. The flow is analyzed at the output of the assembly via a CTD detector and a mass spectrometer Pfeifeer Vaccum, Omsistar, Quadripole (m/z = 32, O2; m/z = 18, H2O; m/z = 44, CO2, m/z = 28, CO).

Chapter 3: Material and methods

3-2-8-X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is a surface analysis technique with an analysis depth of 10 nm. It provides qualitative information about the elemental composition, the chemical environment of the elements and their oxidation state within the sample under study and semi-quantitative by determining the relative proportions of the elements constituting the surface of the materials.

XPS spectra are obtained by irradiating a material with X-ray beam and simultaneously measuring the kinetic energy of the electrons and their number emitted from the surface layer of the analyzed material. XPS is a technique based on the photoelectric effect, as enunciated by Einstein in 1905, which consists of bombarding with electromagnetic radiation of hυ energy in the X-ray range. The surface atoms of this sample are then ionized and core electrons are emitted by photoemission with a Kinetic energy Ec. The kinetic energy of the ejected electrons is characteristic of the element, the energy level from which is derived and the chemical environment of the atom. The use of photons whose wavelength is in the X-ray range (between 10 -11 and 10 -8 m) gives rise to x-ray induced photoelectron spectroscopy, which makes it possible to reach the core and valence levels of the elements under consideration.

The energy balance of the photoemission of an electron is written as follows: hυ = Eb +Ek+ φ (Eq.3)

Where hυ is the energy of the incident photons, Eb is the binding energy of the ejected electron which is the mandatory energy to bring the electron from its initial state to vacuum level, Ek is the kinetic energy of the electron ejected and φ is the extraction potential required for the electrons to be ejected in vacuum. During the analysis of weak or non-conductive samples, the ejection of photoelectrons induces a positive charge on the sample surface which decreases the apparent kinetic energy of the photoelectrons. This results in a shift of the peaks towards higher binding energies. The low mean free path of photoelectrons in solids in the kinetic energy range from 300 eV to 1400 eV makes XPS a surface analysis technique.

The XPS analyses were performed on a DLD Kratos Axis Ultra instrument equipped with a monochromatic Al charge neutralization source Kα (hυ = 1486.6 eV) under ultra-high vacuum conditions (10 -9 mbar). The instrument is calibrated to the Au 4f7/2 level of metallic gold with a binding energy of 83.96 eV. The high-resolution spectra of the different regions are collected using a bandwidth of 20 eV. The spectra are then processed using CasaXPS software.

Chapter 3: Material and methods

The atomic concentrations are calculated from the intensity of the photopics using the different sensitivity factors provided by the software. The effect of static charge is evaluated using the electron coming from the C-(C, H) component of C 1s at 284.8 eV as a reference.

Associated calculation

 Calculation of atomic concentration

The stoichiometry of the sample surface can be estimated from the area ratio of XPS peaks. The general formula for the XPS peak area is:

I = n F λ σ T sin Ө (Eq.4)
Where I is the intensity of XPS peak, n is the atomic concentration of the elements, F is the X-ray flux, σ is the photo electronic cross section for the atomic orbital of interest, Ө is the angular efficiency factor for the instrumental arrangement, λ is the mean free path of the photoelectrons in the samples, T is the detection efficiency for electrons emitted from the sample.

The atomic composition, nA/nB between A and B atoms is the following:

 Calculation of average oxidation state

The binding energies of the electrons of the different elements have been calculated. In the specific case of the analysis of manganese, the 3s level of Mn was more particularly studied.

Indeed, this photopic has two components which can be explained by the coupling of the unionized 3s electron with the 3d electrons of the valence band. The energy gap between these two peaks gives information on the average oxidation state of Mn. The energy gap (ΔE) and the AOS of Mn can be related as follows: AOS = 8.956-1.126*ΔE [4][5][6] (Eq.8)

Mn AOS was also calculated by decomposition of Mn2p core level. The most intense Mn2p3/2 signal was considered for decomposition. Mn2p3/2 has been decomposed considering a mixture of Mn (III) and Mn (IV) which can be simulated by a mixture of MnO2 and Mn2O3 in accordance with the fit of M. Biesinger [7]. For MnO2 a set of multiplet peaks (6 peaks) have been used, with a fixed relative intensity, the full width at half maximum (FWHM) is fixed at the same value for the 6 peaks of each sample which is adapted for the passing energy used herein, and differing in BE each other by a fixed difference. The only parameter left to modify is the position in BE (± 0.2 eV) of the peak located at the lowest BE. For Mn2O3 instead of the multiplet an envelope Mn2O3 (Mn (III)) was used. The BE of Mn (III) was set at 1 eV of difference with the peak of Mn (IV) located at the lowest BE.

3-2-9-Infrared spectroscopy (IR)

This technique makes it possible to study the details of the molecular structure of the sample. An infrared spectrum is obtained by passing an infrared beam through IR-active samples and measuring the fraction of this incident radiation absorbed by the solid.

For a molecule to have infrared absorption bands, it must have a permanent dipole moment that interacts with the oscillating electric field of the incident infrared radiation. Thus, for a molecule to be active in IR, there must be a change in its dipole moment during the period of vibration.

The term Fourier Transform (FT) refers to a mathematical development in which data is collected and converted from an interference model to an infrared absorption spectrum that is like a molecular "fingerprint".

The various infrared spectra were obtained using a Thermo scientific iS50 total reflection attenuation infrared spectrometer (IR-ATR). The spectra are recorded using a pyroelectric triglycine sulphate detector (DTGS). Samples are pressed directly between the detector window and a diamond tip. For each spectrum, 256 scans are recorded in the range 4000 to 200 cm -1 with a resolution of 2 cm -1 .

3-2-10-Pyridine adsorption followed by infrared spectroscopy (IR pyridine)

Pyridine is considered as basic molecule (pKa = 5.2), because it process as lone pair of electrons at the nitrogen available for donation to other species during bonding, and because it can accept a proton from other species. It has been used to probe for the presence and the nature of surface acid sites. It can bond to the surface in three different mode: First modes involves transfer of proton from surface hydroxyl group to the adsorbate (Pyridinium) and occurs with the surface acting as Brӧnsted sites (PyH + ). Second mode involves coordination of the adsorbate through the electron lone pair to the metal of the oxide and take place with the solid acting as Lewis acid sites (PyL) and the third mode is hydrogen bonding (PyH) as shown in Figure 3-2.

The discrimination of the different adsorption modes can be carried out by an analysis of the vibration modes of the pyridine cycle (8a, 8b, 19a, 19b), the position of the bands varying according to the adsorption mode. The strength of the acidic sites can be estimated by the evolution of the quantity of PyL or PyH+ sites as the spectra recording temperature increases.

The stronger the acidic sites, the higher the pyridine desorption temperature will be. mbar) for 30 min and then the spectra acquisition was performed. All recorded spectra were recalculated to a normalized wafer of 10 mg.

3-2-11-Raman spectroscopy

Raman spectroscopy is used in the laboratory for structure analysis and chemical identification of molecules; it provides a wealth of information on molecular and crystalline The Laser Raman spectra of Mn based catalysts were performed on a Xplora (Horiba)

spectrometer with red laser (785 nm) with 1 % laser intensity and acquisition time of 1h. The laser power on the sample is controlled via a software-controlled density filter wheel. A resolution of about 4 cm -1 . A ×50 objective was used for the selection of the particles to be analyzed and the recording of the spectra. LabSpec6 software allows data acquisition and processing.

3-2-12-Time of Flight Secondary ion Mass spectrometry (ToF-SIMS)

Time of Flight Secondary Ion Mass Spectrometry (TOF SIMS) is a technique for analyzing the extreme surface (about 1mm) with very high sensitivity. This technique consists in bombarding the sample surface (<4 monolayers) by a pulsed source of primary ions such as Bi3 + , Cs + , Ga + , Au + , etc. Once extracted from the surface, these are then accelerated by an electric field to the time-of-flight analyzer and their masses are determined by measuring their time of flight to reach the detector. These ions have the same kinetic energy.

This method of analysis makes it possible to obtain a very good resolution in mass (> 10000), a study of the unlimited mass range and a 'parallel' detection of all the secondary ions emitted. The mass spectra obtained are described for secondary ions as a function of the m / z ratio and the isotope fragments can be detected.

In our work ToF-SIMS data were acquired using ToF-SIMS5 spectrometer (ION-TOF GmbH Germany) equipped with bismuth liquid metal ion gun 5LIMG. The powders were crushed using an agate mortar and pestle and the standard tablets were prepared using a press machine. The samples were bombarded with a pulsed Bi 3+ primary ion beam (25 keV, 0.25 pA) rastered over 100 m x 100 m surface area (128 x 128 pixels and 100 scans). The Total fluence did not amount up to 1012 ions/cm 2 ensuring static conditions. Charge effects were compensated by means of a 20 eV pulsed electron gun. With a cycle time of 200 µs, data were collected over a mass range m/z = 0-3500 for both positive and negative secondary ions. The fragments were identified by their exact mass, coupled with the appropriate intensities for the expected isotope pattern. The mass resolution m/m at m/z = 55 for Mn + was 2500.

3-3-Determination of catalytic properties of materials for the total oxidation of formaldehyde

3-3-1-Description of the set-up

A schematic representation of the set-up is given in equilibrium between the solid phase or liquid phase and the gas phase (formaldehyde gas) is established in these inert tubes. The permeation rate of the tube at the chamber temperature is the parameter determining the amount of formaldehyde gas emitted into the permeation chamber through the polytetrafluoroethylene (PTFE) membrane. This parameter, which depends on the length of the tube and its thickness, is given by the producer at the time of purchase. The permeation rate of the tubes that have been used is in the range of ~10100 ng/min at 100°C.

In order to achieve the desired concentration of HCHO (100 ppmv), the temperature of the permeation chamber must be 105 °C. The calculations performed to correlate the concentration and temperature of the permeation chamber are as follows: Humidity is generated by a bronkhorst's CEM (controlled evaporation and mixing) system. This system consists of dispersing a very fine water droplet in an air stream. The CEM is composed from two mass flow controllers, one controlling the air and the second controlling the water flow, and a CEM mixer dispersing the fine water droplets into the air flow. The entire system is thermostatically controlled at 42°C.

c) Catalytic reactor

The catalytic test reactor used is made of pyrex, with a fixed bed reactor and 10 mm as diameter. A 100 mg of catalyst is introduced into it. Then, the reactor is introduced into a tubular furnace equipped with a temperature control thermocouple and a reading thermocouple.

d) Analytical system

Total oxidation of formaldehyde produces carbon dioxide CO2 and water H2O. With incomplete oxidation, products such as carbon monoxide CO, formic acid HCOOH and methanol CH3OH may be formed. To detect and quantify all these products online, a Varian gas chromatography system (model CP4900) equipped with two analysis modules was used.

Each module is equipped with an injector, a column and a TCD type detector. The carrier gas used for both modules is helium. Module 1 is equipped with a column of type COx (1m) column for the analysis of formaldehyde and carbon dioxide while module 2 is equipped with a CP-Sil 5CB (8m) type column for the analysis of incomplete oxidation products of formaldehyde. The micro gas chromatograph is equipped with a Genie Filter membrane connected in series just before the injection pump to avoid the introduction of liquid or solid particles into the system.

A gas micro chromatography analysis is carried out under the conditions reported in Table 3-3.

3-3-2-HCHO and CO2 calibration

To calibrate HCHO, the temperature of the permeation chamber is set to a desired value and injections (6 injections) are made with micro chromatography (by pass). Then the specific air of the HCHO peak is integrated. The selected temperatures are 85, 90, 95, 100, 103 and 105°C. Then the calibration curve is drawn: formaldehyde concentration versus specific air and the equation of the linear regression is y= 0.4213 x (x specific air of the HCHO peak, and y is the HCHO concentration) with R 2 = 0.9943.

In order to quantify the CO2 concentration, a gas flow containing different concentrations of HCHO (depending on the temperature of the permeation chamber) is sent over a (manganese and/or platinum) catalyst heated to 300°C, (for each concentration, 6 chromatograms are recorded). HCHO is thus completely converted into CO2 and H2O. So according to the HCHO response factor obtained previously, we can calculate the CO2 concentration, by plotting the calibration curve: HCHO concentration versus CO2 specific air peak considering that the carbon balance is 100%. The equation of the linear regression obtained is y = 0.4488 x (x specific air of CO2 peak, and y is the CO2 concentration with R 2 = 0.9796.

3-3-3-Experimental procedure of HCHO oxidation

Light off curves

Oxidation of formaldehyde was carried out in a fixed bed reactor loaded with 100 mg of catalyst. Catalyst activation was carried out over the calcined CexMn pretreated in situ at 200

°C for 1 h in 20 vol% O2/N2 (100 mL/min). Afterwards, the stabilized reactive flow consisting of 100 ppmv of HCHO mixed with 20 vol% O2/N2 was sent towards the fixed bed reactor for 1 h. At that time the temperature was allowed to decrease from 200 °C to 25 °C at a rate of 0.2 °C/min. The total flow rate of 100 mL/h allowed to get a gas hourly space velocity (GHSV) of mixture of Cu oxides ans Mn oxides with a surface are 225 m 2 /g) the sample was pretreated at 300 °C in similar conditions as above.

The HCHO elimination corresponding to the HCHO removal efficiency from the gas phase was expressed by the HCHO conversion (eq.10) while the HCHO oxidation in CO2 was expressed from the CO2 yield (eq.11) as follows:

HCHO elimination (%) =

Effect of moisture

Catalyst activation was carried out over the calcined CexMn-400 adopting a similar procedure as given above. The reactor was then cooled down to 50 °C in flowing dry air.

Afterwards, the stabilized reactive flow (100 ppmv of HCHO diluted in 20 vol% O2/N2) was sent towards the fixed bed reactor and RH was allowed to increase with time. The duration and RH% of the successive steps were specified in the appropriate figures in the text. At the end of the test the reactor was cooled down in static air. In the case of commercial Hopcalite, the stability test was performed after performing the light-off curve. The temperature was allowed to increase up to 75 °C (2 °C/min). At the end of the test the reactor was cooled down in static air. The suffix SF (stability formaldehyde oxidation test) was added for the labelling of the used catalysts.

Adsorption and thermo-desorption of formaldehyde

In order to assess the possible participation of catalyst oxygen in HCHO oxidation, experiments were performed in oxygen free atmosphere. Before each test, catalysts were pretreated for 2h at the temperature of calcination in air flow of 100 mL/min (2°C/min). After cooling down to 25 °C, the catalyst was submitted to gaseous formaldehyde (100 ppmv) / air mixture (100 mL/min) until a steady state was achieved. At that stage catalyst performances were evaluated from 25°C to 480°C with heating rate of 1°C/min under helium at a GHSV of 60 L/(gcat.h).

3-4-Determination off catalytic properties of materials for ozone decomposition

For ozone decomposition, a 10 pin-to-plate negative corona-glow non-thermal plasma (NTP) discharge reactor was used as ozone generator. For TCE abatement discussed in the next the same plasma discharge and reactor are used coupled with catalyst downstream of the plasma reactor is used

3-4-1-Description of the plasma Set-up for Ozone generation and TCE abatement

A schematic representation of the experimental set-up is given in Figure 3-5. It consists of three main parts: The gas supply system, the reactor system and the gas analysis system. In the following sections, each of these parts will be presented in more detail. As can be seen from Figure 3-9, the plasma voltage was calculated as follows:

𝑈 𝑝𝑙 = 𝑈 𝑡𝑜𝑡 -𝑈 𝑏 -𝑈 𝑚 (Eq.12)

𝑈 𝑝𝑙 = 𝑈 𝑡𝑜𝑡 - 𝑅 𝑏 𝐼 𝑁 -𝑅𝐼 (Eq.13)
The radius and depth of coverture for the spherical surface segment was 17, 5 mm and 5 mm respectively. Stable and uniform glow discharge operation was ensured and the gas flow ballasting each cathode pin with a 1.5 MΩ resistor Rb. The fraction of the total electrical power that was dissipated in these resistors amounted to 10 % at most. This ratio forms the basis for the calculation of the ozone concentration. Ozone monitor is used only to calculate the ozone calibration factor described below.

The Beer-Lambert equation, shown below, calculates the concentration of ozone from the ratio of light intensities.

𝑐 𝑂 3 = -

3-4-2-Experimental procedure for using plasma as ozoner:

The 10-pin-to-plate reactor was connected to both the high voltage DC power supply and the gas flow system. Dry or humid air (RH = 15%) was flushed to the NTP reactor at a fixed flow rate (0.5; 1.0; 1.5 L/min) until the RH reach the desired value. In the case of dry air, the RH is equivalent to 0.6-0.7%, which cannot be lower and corresponds to the residual water present in the gas stream. Afterward, the DC power supply was turned on. The nature of the NTP processed gaseous species effluent gases were investigated using FT-IR spectroscopy. The effluent gas was sampled 10 times at intervals of 30s. The ED was allowed to increase between

3-4-3-Experimental procedure for catalytic ozone decomposition

In dry air 0.1 g of the as-synthesized samples mixed with 1g of SiC (EMB 45053 Prolabo, 0.150 mm was activated under air flow (flow rate = 200 mL/min) at 150 °C (heating rate = 1°C/min) for 8 hours. However, for acid treated samples, 0.05 g catalyst was mixed with 1g of SiC, and the catalyst activity tests were carried out without any activation.

The catalytic activity test was carried out at atmospheric pressure and room temperature (20°C).

1-First a background was taken.

2-Then ozone was generated by flowing 1L/min of dry air (RH = 0.6 / 0.7%) through NTP reactor, by adjusting the applied voltage in order to obtain a certain energy density and thus the desired initial concentration of ozone.

3-In order to estimate the initial ozone concentration, the reactive flow was initially pass through by-pass of the catalyst reactor (close valves 1 and 2). This reactive gas was samples 10 times at intervals of 30 s, where the number of scans for each spectra is 10, using FTIR spectrometer and these FTIR are labeled as plasma alone in chapter 6.

4-Then, the reactive flow was diverted through the catalyst reactor (open valves 1 and 2), and the effluent gas was sampled 60 times at intervals of 30 s, and then 135 times at intervals of 120 s, where the number of scans for each spectra is 10. During the test, the catalyst was maintained at room temperature (20°C). At the end of the experiment, the ozone concentration is again measured in order to control the initial concentration of ozone during the experiment by passing the effluent gas through by-pass of the catalyst reactor.

Effect of humidity 0.05 g catalyst was mixed with 1g of SiC, and the catalyst activity tests were carried out without any activation and the catalytic activity test was performed at atmospheric pressure and 30°C. After the same three first steps given previously in dry air were followed.

4-Then plasma was turned off and the water is introduced after the plasma reactor 5-A background in presence of water has been taken 6-The reactive humid flow was pass through by-pass of the catalyst reactor (close valves 1 and

2). This reactive gas was sampled 10 times at intervals of 30s, where the number of scans for each spectra is 10, using FTIR spectrometer and these FTIR are labeled as Inlet CR in chapter 6.

7-Then, the reactive humid flow was diverted through the catalyst reactor (open valves 1 and

2), and the effluent gas was sampled 60 times at intervals of 30s, and then 135 times at intervals of 120s, where the number of scans for each spectra is 10. During the test, the catalyst was maintained at room temperature (20°C). At the end of the experiment, the ozone concentration is again measured in order to control the initial concentration of ozone during the experiment by passing the effluent gas through by-pass of the catalyst reactor.

Experiment under 5% O2/Ar

In order to study the effect of nitrogen oxides generated by NTP on the catalyst activity, experiment has been performed using 0.05 mg of Ce0.01Mn at atmospheric pressure and 20 °C under 1L/min of 5% O2/Ar as carrier gas. The catalytic activity test procedure is the same as given before using dry air.

Evaluation of the ozone removal efficiency:

The ozone conversion efficiency was calculated using the following equation.

O3 conversion = [𝑂 3 ]𝑖𝑛-[𝑂 3 ]𝑜𝑢𝑡

[𝑂 3 ]𝑖𝑛 ×100 (Eq.17)

Where, [O3]in and [O3]out were the concentration introduced in the reactor and that measured at the exit of the catalytic reactor, respectively. When ozone is decomposed no ozone characteristic band is observed by the FTIR.

In order to know if the ozone concentration is stable at a fix flow rate and energy density, we study the ozone generation with flow rate 1L/min and energy density 150J/L for 5 hours. We found that the ozone generation is stable as a function of time and according to this experiment we calculate the error by calculating the variance and then the standard deviation. The variance is calculated according to the following formula: S 2 = ∑(𝑥-𝑥̅ ) 2

𝑛

and the standard deviation is the square root of the variance. The standard deviation is about 3%

Chapter 3: Material and methods

3-5-Determination of catalytic properties of materials for TCE abatement

3-5-1-Calibration FTIR

As discussed before the Set-up description is the same. Only a new calibration for CO, CO2 and TCE has been performed. Therefore the FT-IR spectrometer was calibrated with calibration mixtures delivered by Air liquid. The mixture was flushed through the gas cell, with a flow rate 1L/min (identical to the flow rate applied during the experiment) until a steady state was reached. After 2 spectra were recorded each minute to obtain the required surface area of the adsorption peaks under consideration. The peak areas between 916 -966 cm -1 (TCE), 2388

-2287 cm -1 (CO2) and 2140 -2071 cm -1 (CO) were integrated. Linear regression was applied to the integration area values and the slopes K1, K2 and K3 were used to calculate the concentration of TCE, CO2 and CO respectively. It should note that by using plasma alone the CO amount is qualitative not quantitative due to the fact that CO band overlaps with ozone and with N2O band K1, K2 and K3 were defined as following:

𝐾1 = 𝐶 𝑇𝐶𝐸 𝑆 𝑇𝐶𝐸 = 334

3-5-2-Experimental procedure for trichloroethylene in PPC process

Trichloroethylene (TCE, 99.99% purity, ACROS) was evaporated by bubbling dry or moist air (RH = 15%) into a saturator and introduced into the plasma reactor after diluting with air. The total flow rate and TCE concentration were maintained at 1 L/min and 150 ppmv respectively, using mass flow controllers (MFC, El-Flow, Bronkhorst). A 10-pin-to-plate plasma reactor configuration was used working in a negative DC corona discharge. 0.1g of catalyst diluted in 1g of SiC (EMB 45053 Prolabo, 0.150 mm) placed in a quartz reactor was activated for 1h in dry air (0.2 L/min) at 100°C.

Evaluation of the TCE removal efficiency and COx (x = 1; 2)

The TCE abatement (conversion) efficiency was calculated from:

TCE conversion = [𝑇𝐶𝐸]𝑖𝑛-[𝑇𝐶𝐸]𝑜𝑢𝑡 [𝑇𝐶𝐸]𝑖𝑛 ×100 (Eq.21)
Where [TCE]in and [TCE]out were the inlet and outlet concentrations, respectively.

The CO2, CO and COx yields were calculated as follows:

YCO2 (%) = [𝐶𝑂 2 ]𝑜𝑢𝑡 2𝑥[𝑇𝐶𝐸]𝑖𝑛 ×100 (Eq.22) YCO (%) = [𝐶𝑂]𝑜𝑢𝑡 2𝑥[𝑇𝐶𝐸]𝑖𝑛 ×100 (Eq.23)
YCOx (%) = YCO2 (%) + YCO (%) (Eq.24)

Chapter 4:

CERIUM DOPED (K, Na) -BIRNESSITE LIKE MnO2 FOR LOW TEMPERATURE OXIDATION OF FORMALDEHYDE: EFFECT OF CALCINATION TEMPERATURE

4-1-Introduction

In order to address our issues, the first validation approach was to study the catalytic oxidation of formaldehyde (HCHO) since this reaction is well known to be correlated with the density of oxygen vacancies in the materials [1][2][3].

HCHO classified as a volatile organic compounds, is considered as one of the most hazardous pollutants especially in an indoor environment. It is mainly emitted from construction and decoration materials and can cause serious illnesses to human health and is also very toxic to the environment. Among the different approaches proposed for HCHO removal, heterogeneous catalytic oxidation has been recognized to be the most efficient technology.

Noble metal based catalysts exhibit high activity allowing total HCHO oxidation into CO2 and H2O at room temperature [4,5]. Rochard et al. [6], have been investigated that the oxygen vacancies played a key role in enhancing the HCHO catalytic activity over Au/Copromoted CeO2 catalysts. However, their high costs and lack of resources impede their practical applications [7][8][9]. In 2002, Sekine et al. [10], found MnO2 as the most active noble-metal-free catalyst in HCHO oxidation among various commercial transition metal oxides (TMO). Among the different MnO2 crystal structure, birnessite type MnO2 was recognized to be the most active catalyst in HCHO oxidation [8]. In order to promote the catalytic performances of such materials significant experimental researches were recently focused on defective birnessite materials (oxygen and manganese vacancies [11]) with proper surface electronic properties [29][30][31]. Element incorporation was also another followed strategy to obtain a more active catalyst.

In that way, incorporation of suitable transition metals (TM) or alkali cations was investigated to promote activity by seeking to decrease energy formation of oxygen vacancy [15]. In particular, Ce doping in the birnessite structure, among the other different transition metals, was

shown to be the most performant catalyst. Higher content of oxygen vacancies resulting from a close contact between CeO2 and MnO2 nanoparticles was postulated to be responsible for its high activity in HCHO oxidation.

In this chapter, formaldehyde oxidation over Ce-doped (K, Na) birnessites has been investigated. Particular emphasis was placed herein on the effect of calcination temperature over the as-synthesized catalysts to be discussed with the catalytic performances in HCHO oxidation in dry and moist air.

4-2-Experimental conditions

4-3-Results and discussion

4-3-1-Characterizations of the fresh samples

The X-ray diffraction pattern of Mn-B exhibits four broad diffraction peaks centered at 12.2 °, 24.5 °, 37.0 °, and 66.2 °, which are the main peaks of birnessite structure (Figure 4-1).

Their positions and intensities closely resemble those reported by Drits et al. [16], for turbostratic stacking of the birnessite with the alternation of one-layer hexagonal (JCPDS No.

04-017-0609) and one-layer monoclinic (JCPDS No. . The two peaks in the basal reflections at 12.3 ° and 24.5 ° are common for both symmetries and arises respectively from 001 and 002 reflections. The two peaks observed at higher angles (37.0 °, and 66.2 °) correspond to the two dimentional diffraction peaks having (100) and ( 110) indices, respectively [16]. The interlayer spacing of 7.2 Å, calculated from the diffraction peak located at 12.2 °, is similar to the one obtained with Händel et al. [17]. The poorly resolved (001) and (002) lattice diffraction peaks can be explained by a decrease of the crystallite size or/and to randomly stacked layers along the c axis composed of edge-sharing MnO6 octahedra [46][47]. However, the d spacing ratio for the peaks at high angles is approximately 1.71, close to 3 1/2 characteristic for hexagonal symmetry which implies that the monoclinic symmetry doesn't have a strong distortion for the hexagonal symmetry [16].

With low Ce doping (Ce0.01Mn), the XRD pattern reduces to three diffraction peaks at 24.5 °, 37 ° and 66.2 ° (Figure 4-1). As the Ce content increased, (Ce0.1Mn) the XRD pattern reduces to two weak diffraction peaks at 37.0 ° and 66.2 ° indicating a poorly crystallized nano-Chapter 4: Cerium doped (K, Na) -birnessite like MnO2 for low temperature oxidation of formaldehyde: effect of calcination temperature 106 mineral (Figure 4-1). The d-spacing ratio of the two peaks remains close to 3 1/2 , suggesting pseudo-hexagonal symmetry in the manganese octahedral layer [48][49]. The absorption bands at 3200-3300 cm -1 (H-O-H stretching mode) and 1620 cm -1 (H-O-H bending mode) were linked to adsorbed water and water in the interlayer spacing of the birnessite [17]. The lowest bands at 400 cm -1 and 480 cm -1 relate to vibrational modes of MnO6 octahedra [21]. The broadening of these structural modes may be related to grain size and local differences in bonding environments [22]. Moreover, some additional bands located around 1500-1510 cm -1 , 1405-1417 cm -1 , 1313-1303 cm -1 and 1060-1050 cm -1 have been associated to residual CO3 2-and HCO3 2-species. 4-2. The N2-sorption isotherms of the assynthesized samples belong to a type IV isotherm with a H3 (Mn-B, Ce0.1Mn) and H2 hysteresis (Ce0.01Mn) loop. Vp increases and Dp decreases after doping with Ce. The BET surface area of the parent birnessite of 44 m 2 /g increased markedly to 127 m 2 /g and to 243 m 2 /g with increasing Ce content. Taking into account a similar evolution, replacement of manganese ions by cerium ions in the lattice has been previously postulated [3]. The morphology and structure of the as-prepared CexMn have been investigated by TEM (Figure 4 -4). The birnessites appear as aggregates of nanosized 2D crystals. The lattice fringes of layers are mostly curved and curled. The curling aspect is indicative of a structure with high-density defects [20] ascribed to the presence of vacancies, of Mn x+ substitutions by Ce x+ and of the presence of different intercalated cations. High-resolution TEM images show lattice fringes of 0.71 nm spacing assigned to the (100) planes of the birnessite. A lattice spacing of 0.31 nm has also been observed on both Ce-doped samples even at low doping ratio (001). This spacing arises from the (111) lattice planes of CeO2. As a consequence, detection of CeO2 nanocrystals even at low doping ratio, allows a close contact between CeO2 and MnO2 nanocrystals. Such grain boundaries have been invoked to play a role in oxygen mobility and activation of oxygen [54][55][56][57][58][59]. ICP-OES results show that the doping Ce/Mn ratios are consistent with the nominal ones (Table 4-3). For the as-synthesized samples, the K/Mn and Na/Mn ratios of 0.35 ± 0.01 and 0.06 ± 0.01 respectively, compared to the ratio obtained by Händel et al. [17] 0.13 and 0.04, K/Mn is substantially higher. Different initial reactant ratios and washing procedures account for such discrepancies. These results thus leave open the possiblity of mixed (Na, K)-birnessites and thermodynamic studies support this possibility [29]. Additionally, intercalated cation exchange in birnessite monitored by real-time XRD analysis has been shown to involve continuous shifts in atomic positions rather than the co-existence of end-member configurations [30] which is in contrast to the results of Putnis et al. [31] who postulate that exchange processes occur through dissolution of one phase and precipitation of another. qualitatively observed to be a slight Ce 3+ /Ce 4+ decrease with increasing Ce content [32]. As explained in the experimental section Mn 2p3/2 has been decomposed as shown in Fig. 45, the ratio Mn 3+ /Mn 4+ was calculated by their peak areas and the results are shown in Table 4-4.

The ratio Mn 3+ /Mn 4+ decreases after doping with cerium from 0.24 to 0.17. This result is The O 1s core level of the as-synthesized samples shown in Figure 45, can be decomposed in three contributions at positions P1 = 529.7-529.4 eV, P2 = 530.9-531.4 eV and P3 = 533.4-533.6 eV. OI, OII and OIII have been assigned to surface lattice oxygen, surface adsorbed oxygen species (such as O2 -, O2 2-, O -, OH -/CO3 2-groups) and adsorbed H2O on the surface, respectively [33]. The OII/OI of 0.32 for Mn-B increasing to 0.59 (CexMn) means much more adsorbed oxygen species and especially OH -when Ce is added (Table 4-4). This high level of relative Oadsorbed species is important as it has been generally recognized to play a key role in HCHO oxidation [4,[34][35][36][37] The reducibility of the as-synthesized samples has been investigated by H2-TPR to differanciate the activity of surface oxygen. As shown in Figure 456, the H2-TPR curves could be fitted into four principal peaks. The α peak at the lowest temperature represents the consumption of surface adsorbed oxygen species while, β, γ, and δ peaks correspond to the successive reductions : NaxKyMnO2 -> Mn2O3 (Mn3O4) -> MnO [38][39][40]. The maximum temperature of the four peaks for the as synthesized samples are present in Table 4-6. For CexMn, the four reduction peaks shift to lower temperature as comparing with Mn-B. However, with increasing the Ce content, α remain shifting to low temperature (LT), but γ, and δ shift to higher temperature. By cerium doping the reduction onset temperature decreases significantly. This indicates that cerium improves the reducibility of the material leading to higher mobility of oxygen [41,42]. of 10.7 wt% has been ascribed to the successive release of adsorbed and interlayer water [43] while the second one of 5.1 wt% corresponds to dehydroxylation [29] followed by oxygen release [43,44] of the MnO6 layers in relation to the partial reduction of Mn 4+ into Mn 3+ up to the transformation of the birnessite into cryptomelane. The Ce-doped birnessites follow globally a similar multi-step weight loss except that a small weight gain due to O2 uptake is seen as usually observed for birnessite-cryptomelane transformation in air [45,46].

Interestingly, this transformation is delayed from 136 °C when increasing the Ce amount. This clearly shows that Ce doping promotes the thermal stability of the birnessite like material. Ce is either incorporated in the layer structure or it will be present on particles edges, as a consequence it can be assumed that Ce substitution of Mn reduces the content of Mn 3+ in Ce containing layered precursors and slow triggering the transformation into tunnel structure. In contrast when the Ce content is high it can be supposed that Ce will be present on particles edges rather than incorporated into the structure and that the hydrolysis and polymerization of Ce (hydr)oxide fragments in birnessite interlayers is favored over the migration of Mn 3+ from the layer into the interlayer leading to birnessite transformation into cryptomelane [47,48].

However, the exothermic peak around 200 °C for the as-synthesized samples could be ascribed to the release of residual organic compound (detected also by FTIR-ATR discussed above) and the additional DSC peak observed at 686 °C for Ce0.01Mn can be ascribed to the appearance of a cristallized K-rich birnessite as evidenced by XRD (see below). The XRD patterns recorded after TG (Figure 45678) show the presence of a cryptomelane-type structure for all samples. However, for Ce0.01Mn, the relative high intensity of the peak at approximately In order to obtain more information on the species released as a function of temperature and on the thermal behavior of birnessite, a TGA analysis under helium followed by MS was performed on Ce0.1Mn. The signal m/z = 44 characterized for CO2 shows a peak at 200 °C which can be correlated with the exothermic DSC peak indicating the release of residual organic compound which is not the case for the signal m/z = 28 (also characteristic for CO2) indicating that this release of CO2 is not very important (Figure 4 corresponding to the release of O2, this indicates that it can be more correlated to the hydroxylation process. Moreover, the peak at 644°C observed with the O2 signal is correlated to the phase transformation.

4-3-2-Characterization of the calcined samples

XRD patterns of all calcined samples (Figure 4-10-a) have overall similar features than those of the as-synthesized samples except Ce0.01Mn, after calcination at 300 and 400°C, the first peak around 12° appears again. These results suggest that the birnessite structure is stable as a function of temperature. However, it is observed that the interlayer spacing decreases from However, for Ce0.1Mn, the total weight loss decreases as a function of Tc, while the interlayered water decreases until Tc = 300°C, and then increases with Tc = 400 °C, whereas an increase in the weight loss is observed in the dehydroxylation and oxygen release step when comparing the as-synthesized samples to Ce0.1Mn-200 and then this loss decreases as the Tc increases. However, when increasing the Ce content, the interlayered water released decreases while the hydroxyl group and O from MnO6 octaedra released increases. These observation hold our hypothesis that when increasing the Ce content, Ce is rather present on particle edges and that the hydrolysis and polymerization of Ce (Hydr)oxide are promoted. The calcined Mn-B and Ce0.01Mn samples exhibit a type IV isotherm profile with a H3 hysteresis loop as previously observed for the the as-synthesized samples (Figure 45678910111213). The BET surface area increases by a factor 3.4 for Mn-B after calcination. The BET surface area of 127 m 2 /g for Ce0.01Mn increases to 165 m 2 /g and then decreases to 132 m 2 /g when increasing Tc from 300 °C to 400 °C, respectively. The Vp of 0.32 cm 3 /g decreases to 0.22 cm 3 /g after calcination (Table 4-9; Figures 45678910111213). The calcined Ce0.1Mn samples exhibit a similar type IV isotherm profile with a H2 hysteresis loop as previously observed for the as-synthesized sample. The lower closure point is at P/P0  0.42 and the upper closure point increases from 0.73 to 0.87 when increasing Tc from 200 °C to 300 °C. The BET surface area of 243 m 2 /g for Ce0.1Mn decreases to 213 m 2 /g to futher increases to 256 m 2 /g and finally decreases to 196 m 2 /g when increasing Tc. Vp shows a similar evolution going from 0.20 cm 3 /g to 0.25 cm 3 /g through a maximum of 30 cm 3 /g while Dp monotonically increases from 4 to 6 nm.. indicate that the amount of Mn and Ce increases while the amount of K and Na decreases. The K/Mn, Na/Mn and Ce/Mn ratios remain unchanged as a function of Tc in accordance with water removal because these counterions are present in the birnessite structure but in a specific hydration state [51]. The H2-TPR traces of the calcined samples shown in CexMn shift also to lower temperature with increasing Tc and the amount of the relative LT H2

consumption is now increased. The α peak corresponding to the consumption of H2 at low temperature became larger as the calcination temperature increase. Additionally, the consumed hydrogen amounts of the calcined samples are much higher than those of the as-synthesized catalysts, and show a maximum for Ce0.01Mn-400 (9.28 mmol/g) (Table 4567891011). In any case, the calcination step induces a decrease of the reduction onset temperature, an increase of the LT H2 consumption and a downward shift in temperature of the peaks. Hence, the calcination step allows to improve the reducibility of the material which imply higher mobility of oxygen leading to the formation of more oxygen vacancies. Raman features emerged at 573-575 cm -1 (in-plane symmetric stretching of Mn-O) and 642-645 cm -1 (out-of-plane symmetric stretching of Mn-O of MnO6 groups) accompanied with two other contributions at 510 cm -1 and 732 cm -1 [52] for the ex situ analysis which not shift as a function of temperature. A red shift of 10 cm -1 and of 13 cm -1 is observed regarding the in-plane and out-of-plane symmetric stretching frequency respectively, when the analysis is performed in-situ and the catalyst was heated at 400°C. These shifts indicate the formation of defects in the structure due to the calcination (dehydration) [40]. However, when coming back to the room temperature, the in plane and out-of -plane stretching come back to their initial positions which can be explained by the rehydration process. This behavior is also observed by Johnson et al. [53]. They study the effect of temperature on the dehydration process of the birnessite by insitu FTIR, and they found that by increasing the temperature, the water is lost and when water is totally lost they have collapse of the birnessite structure. Thus, they attest that after being fully (or almost fully) dehydrated during heating, a rehydration is observed when coming back to room temperature. The rehydration capacity depend on the type of the interlayer cations, related to its charge and size. ascribed to the release of chemisorbed oxygen which was formed through the conversion of surface lattice oxygen around vacancy [54]. The onset temperature (Tonset) of O2 desorption for the calcined samples are present in Table 4-12. T onset moved to lower temperature after Ce doping. The O2 desorbed between 500 and 700 ⁰C can be ascribed to the subsurface lattice oxygen [54]. The O2-desorption behavior in this temperature range for Ce0.1Mn calcined at 400 ⁰C is different than Mn-B, and a new weak peak around 700 °C is observed, this phenomena is also observed with Liu et al. [55], which indicate the replacement of the lattice Mn by Ce ions would weaken the metal oxygen bonds, making the lattice oxygen easier to release [55,56].

However, the intense peak at 820 °C observed for Mn-B is attributed to the release of oxygen lattice [54], which shifts to low temperature (677 ⁰C) as the Ce content increase. The desorbed O2 content (Table 4-12) of CexMn is higher than parent birnessite, representing better oxygen desorption ability this behavior is also observed with Guan et al. [56]. Combined with the above results of hydrogen consumption, it can be concluded that the highly mobile lattice oxygen species were related to the active oxygen vacancies (which increase after Ce doping) and would enhance the mobility of lattice oxygen [55][56][57]. The XPS results for the calcined samples are present in Tables 4-13 and 4-14. As a result, if the relative proportion of Mn 2+ is assumed to be negligible, the calculated Mn 3+ /Mn 4+ ratio increases after calcination for Mn-B as well as for CexMn (Table 4-13, Figure 4-17-b) which is also in line with the Mn AOS calculated from the Mn3s splitting except for Ce0.1Mn-400

(Table 4567891011121314). As discussed in the literature, the higher Mn 3+ /Mn 4+ ratio implies a higher content of oxygen vacancies, because once Mn 3+ appears in the manganese dioxides, oxygen vacancies will be generated to maintain the electrostatic balance. The OII/OI atomic ratio also decreases after calcination (Table 4- Static ToF-SIMS has been performed on as-synthesized CexMn (x = 0.1 ; 0.01) and calcined CexMn-X00 (X=4 for x = 0.01; X=2,3,4 for x=0.1) samples to get surface chemistry information ( 3 nm depth) [58]. For example, secondary ions related to Ce and Mn, detected in (+) and (-) polarities from all ToF-SIMS spectra, have been reported in Table 4-16.

Irrespective of the Ce/Mn doping ratio, it is observed an increase of the secondary ion intensity Ce + /Mn + ratio after calcination which indicates a cerium enrichment (Table 4- The secondary ions at m/z = 211 and 212 are assigned to CeMnO + and CeMnOH + , respectively.

Additionally, several secondary ions CexMnyOzHw +/-have also been detected (Table 45678910111213141516attesting of the presence of Ce-O-Mn interactions. By comparing the secondary ion intensity CeOMn + / (Ce + +Mn + ) ratios it is shown that the amount of Ce-O-Mn interactions increases after calcination while the variations with Tc are less sensitive. ions are observed at m/z = 140 and 156 respectively. Interestingly, the secondary ion intensity CeO + /Ce + ratio decreases strongly after calcination at 200 °C and keeps rather stable when increasing further Tc. For Ce0.01Mn, the CeO + /Ce + ratio is approximately 16 times lower after calcination at 400 °C (Table 456789101112131415). These results indicate a significant lowering of the Ce 3+ /Ce 4+ atomic ratio after calcination at 400 °C for low Ce doping. The secondary ions CeO4 + , Ce2O2 + , Ce2O3 + are also detected for the Ce0.1Mn derived samples as expected for CeO2 containing samples. The absence of Ce2Ox + ions for Ce0.01Mn-400 may indicate the high dispersion of Ce in the birnessite, which is in accordance with the TEM results. In addition, the identification of KNa + as well as K2NaMnO + and K2NaMnOH + as secondary ions provide direct evidence of a close interaction between potassium and sodium as interspacing cations in a mixed (Na, K)-birnessite. In addition, in order to assess the possible participation of the catalyst oxygen in HCHO oxidation, reaction without oxygen in the feed were performed over Ce0.01Mn-300, Ce0.01Mn-

-19).

As a result, the catalyst calcined at 400 show a higher mobility of oxygen which is in good agreement with the improvement of catalytic activity observed above as a function of Tc. To sum up Ce doped birnessite as well as the calcination step enhance the catalytic oxidation of HCHO. Low doping of Ce lead to an improvement in the textural properties in line with decreasing the material crystallinity. However, the stacked curled layers observed by TEM for CexMn ensured high defective structures, and the presence of ceria nanocrystals, allows a close contact between CeO2 and MnO2. Thus doping with cerium increase the number of oxygen vacancies. However, As the calcination increases the Mn 3+ /Mn 4+ ratio increases as evidenced by XPS analysis, and the H2-TPR analysis show that the reducibility of the material increases which mean that the mobility of oxygen increases. In addition, the formation of Mn x+ -O-Ce y+ bonds increases and concomitantly decrease the Ce 3+ /Ce 4+ atomic ratio. The local charge imbalance between the two cations is well known to promote the reactivity of oxygen in these asymmetric active sites [66][67][68][69]. However, by increasing the temperature, the phase transformation of birnessite into cryptomelane will occur. This transformation is well known that it's consistent with the high content of Mn 3+ (almost 1/3 per layer octahedron) and then the migration of these Mn 3+ from layer into interlayer to release the strains related to Jahn-teller distortion [47,[70][71][72][73]. In our work the calcination step was performed in a condition that no phase transformation it will ensue, which mean they have migration of Mn 3+ from layer into the interlayer but not reaching the maximum value. The migration of Mn 3+ led to a vacant site in the layer structure. However, it is prove that the energy of the formation of oxygen vacancy will decrease if it is near to manganese vacancy. This energy will decreases also if K interlayer cation will shift near to the Mn vacancy to compensate the charge. Thus, the effect of calcination is herein to increases number of manganese and oxygen vacancies which led to a higher mobility of oxygen species may explain the higher activity of the catalyst towards HCHO oxidation. In the case of Ce0.01Mn-400, the presence of numerous Mn 4+ -O-Ce 3+ interactions leading to asymmetric oxygen/vacancy sites are believed to improve in that way the catalytic performance in HCHO oxidation.

To better apprehend the scheme of HCHO oxidation, infrared spectroscopic measurements have been performed after stability tests. ATR-FTIR spectra recorded after the HCHO catalytic oxidation tests are shown in Figure 4-23. For used Hopcalite, two intense bands at 1570 cm -1 and 1320 cm -1 with a shoulder at 1370 cm -1 are found. The bands at 1570 cm -1 and 1370 cm -1 belong to formate species νs(COO) and νas(COO) [74] and the energy difference of 200 cm -1 is herein indicative of a bidentate formate [1]. The band at 1320 cm -1 is ascribed to carbonate species νs(COO) [60]. For the used CexMn catalysts overlapping bands are observed in the 1650-1250 cm -1 region which are less intense than those relative to Hopcalite. In addition, the bands at 1500 cm -1 and 1480 cm -1 already ascribed to carbonate and hydrogenocarbonate species at the surface of the fresh catalysts are again detected but with a lower intensity. These results indicate that the oxidation of HCHO generates formate species in small amounts. Less intermediates generated over CexMn due to the high content of vacancies may explain their higher activity. intermediates. An adsorbed oxygen will be consumed for HCHO oxidation and an oxygen vacancy (V0) will occur. Then dioxygen molecule in air would be adsorbed on V0 and dissociated: O2+ V0  O2 -+O -. Due to the existence of manganese vacancy, potassium ion compensates its charge facilitating the formation of surface active oxygen, which in turn acts as the active site for HCHO oxidation. In the presence of water the catalytic activity is more stable than in dry air and that could be explained by the fact the surface hydroxyl group is replenished via the reaction with water (O2-+ H2O  2OH)

4-4-Conclusion

Cerium-doped (K, Na) birnessite type MnO2 with low doping ratios were prepared from a simple, fast and inexpensive redox reaction using permangante potassium and sodium lactate at ambient with addition of cerium nitrate. At these doping ratios the particle growth was inhibited and the textural properties were improved. Imbricated assemblies consisting of a few stacked curled layers ensured high defective stuctures and the presence of nano-ceria particles allowed an improved CeO2/MnO2 interface. Enhancement of the catalytic performances with increasing calcination temperature are due to improved local environment of the active sites linked to numerous Ce-O-Mn interactions, high atomic ratio Mn 3+ /Mn 4+ , lower Ce 3+ /Ce 4+ balance, and high mobility of oxygen at low temperature. Ce0.01Mn calcined at 400 °C, which exhibited the best activity, showed a T50 of 50 °C for the abatement of 100 ppmv of HCHO with a GHSV of 60 Lgcat -1 h -1 and outperformed commercial Hopcalite (T50: 89 °C). High oxygen mobility of the catalyst to participate in the oxidation of HCHO in an oxygen-free atmosphere. In addition, excellent stability of Ce0.01Mn-400 in the presence of water, even at high RH (75 %), confirms the potential of this catalyst for indoor depollution processes.

5-1-Introduction

Non-Thermal Plasma (NTP) technology has been investigated since several decades for the abatement of diluted (< 1000 ppmv) Volatile Organic Compounds (VOCs) present in waste gas and indoor air [1]. The use of atmospheric pressure NTP working at 20 °C has several advantages for air pollution control resulting from its operating conditions, quick start-up and low cost technology. NTP is an attractive technology in such a way that the energy delivered to the plasma allows to get very energetic electrons while the temperature of the gas keeps at ambient. These highly accelerated electrons trigger multiple chemical processes such as excitation, ionization and dissociation of background molecules (N2/O2) enabling an efficient decomposition of the VOC. However, this high reactivity towards VOC removal is tempered by a low selectivity into CO2/H2O and the production of toxic by-products such as nitrogen oxides, O3 and other gaseous harmful VOCs cannot be discarded [2,3]. Since these two last decades NTP technology has partly regained interest through Post-Plasma catalysis (PPC) which results from the combination of NTP technology and heterogeneous catalysis. Two reactors are connected in series. In that configuration the catalytic reactor is located downstream of the plasma reactor. This resulting hybrid technology can overpass NTP and total oxidation catalysis in terms of selectivity, efficiency and energy cost [4,5]. In this configuration catalytic reactions can take advantages of the NTP emitted O3 as a potential source of active oxygen species enabling further oxidation at ambient or very low temperatures of the NPT non processed parent VOC and potential NTP gaseous hazardous by-products [4][5][6].

Consequently, in this chapter, the functioning of plasma as ozoner is investigated. The ozone and concomitant nitrogen oxide formations by negative DC (Direct Current) corona discharge at atmospheric pressure using a 10-pin-to-plate reactor in dry/humid air stream have been discussed. The effects of operating parameters such as energy density (ED), flow rate (F) and relative humidity (RH) on the ozone and nitrogen oxide production have been investigated.

A reaction scheme is proposed to account for the N2O5 and HNO3 formations based on the NOx oxidation by ozone.

5-2-Experimental conditions

The detail information can be seen in chapter 3. Briefly, dry (RH -0.7 %) or humid air (RH =15%) are flushed into the plasma reactor at a fix flow rate (Q = 0.5; 1; 1.5 L/min). The energy density was allowed to increase between 50 and 250 J/L. Using a DC negative corona discharge, it is found that O2 is partly converted into ozone and N2 into N2O, N2O5 and HNO3. The ozone formation can be initiated by the electron impact dissociation of molecular O2 and N2 owing to the reactions (1) and ( 2). The produced atomic oxygen reacts with O2 and a third collision partner (M = O2, N2) through reaction (3) producing ozone as already reported [7]:

5-3-Experimental results

e -+ O2  O + O + e - (1)
e -+ N2  N + N + e -

O + O2 + M  O3 + M (2)
The formation of N2O5 can be explained through the postulated generation of small amounts of NO in line with the reactions (4) and ( 5) given below and NO can then be oxidized by O3 into NO2. Considering the production of N2O, it has been previously reported [9] that the most important reaction can be the following:

N2 • + O2  N2O + O (6)

5-3-1-Effect of energy density (ED)

Figure 5 -3, shows the typical effect of the energy density of plasma on the production of ozone. A linear increase of the production is observed at low ED (100 J/L) while a downward departure from linearity is observed afterwards. A maximum of about 840 ppmv can be achieved for an ED equal to 250 J/L in dry air and F = 1L/min.

The ED is specific to the current which is itself proportional to the production of ozone.

Increasing current lead to an increase in the number and density of electrons and so more ozone can be formed. In this regards, the linear increase of ozone concentration with ED in dry air may be due to the increase of electron density with appropriate energy [10].

5-3-2-Influence of Flow rate (Q)

The air flow rate inside the reactor was allowed to vary from 0.5 L/min to 1.5 L/min, with a corresponding residence time varying from 10.8 s to 3.6 s.

5-3-3-Influence of Relative Humidity (RH):

Figure 5-8, shows the evolution of the ozone concentration as a function of ED in dry and humid air (RH = 15%). In the presence of humidity, the ozone concentration drops significantly as compared to that obtained in dry air. This relative decrease in ozone concentration is enhanced with ED. In that respect, the ozone concentration is about four times lower than that obtained in dry air for ED equal to 250 J/L.

In the presence of 15% of relative humidity, N2O, N2O5 and HNO3 were detected by FT-IR but their relative contributions appear very low as compared to those obtained in dry air as shown in Figure 56789. The decrease of ozone concentration in the presence of humidity can be ascribed to the changes induced in the plasma content due to the initial presence water through OH and H radicals. Hence, H2O may be decomposed into OH and H radicals as reported elsewhere [8,11] following the formal reactions ( 7) -( 9):

H2O + e - OH • + H • + e -

H2O

+ N2  N2 + OH • + H • (8) H2O + O  2OH • (9)
Such OH radicals may react with ozone to give water and dioxygen following reaction (10):

OH • + O3  H2O + O2 (10

5-4-Conclusion

In this chapter ozone production using negative DC corona discharge in the presence of concomitant nitrogen oxides was investigated as a function of the plasma key parameters.

Ozone production increases with increasing the energy density (ED) and flow rate (Q). A partial destruction of ozone due to nitrogen oxides production was observed. N2O5 and HNO3 show an apparent maximum at 200 J/L as increasing the ED while N2O increases linearly. However, N2O and HNO3 productions linearly increases as a function of flow rate while N2O5 shows a maximum with 1L/min as Flow rate. Furthermore a negative effect of humidity on ozone production as well as on N2O5 and HNO3 productions.

6-1-Introduction

As seen in chapter 2, the second validation approach is to investigate the efficiency of the catalyst in terms of ozone degradation in the presence of other pollutants such as nitrogen oxides species in dry and humid air using NTP as an ozoner.

Briefly as discussed in chapter 1, transition metal oxides, especially manganese-based oxides, have been widely studied for catalytic ozone decomposition because of their low cost and high efficiency due to the presence of large amounts of surface oxygen vacancies [1,2]. However, their efficiency is reduced in the course of the reaction and a deactivation over time is observed. This deactivation is generally explained by an adsorption competition between ozone and water molecules (also present in the gas stream) which leads to a blockage of the active sites [3] . Nitric acid-treated birnessite-type MnO2 has been demonstrated to be an efficient and hydrophobic catalyst for humid ozone decomposition [4]. The addition of cerium element to manganese oxides could be also an efficient way in preventing the deactivation of the catalysts under high humidity conditions [5]. Furthermore, the addition of cerium in MnOx may contribute to a significant improvement of the catalytic performance in the ozone decomposition reaction owing to (i) an increase in surface area, (ii) a decrease in crystallinity, and (iii) a larger amount of surface oxygen vacancies [6]. It is believed that the oxygen transfer between Ce 4+ and Ce 3+ species on the surface of the catalysts allows abundant oxygen species storage and leads to superior redox properties compared to those obtained with simple manganese oxides [7,8]. The introduction of Ce also leads to an electron transfer between manganese and cerium which improves the redox capacity of the material and consequently its catalytic performance [7].

Moreover, the development of novel catalysts for the ozone decomposition reaction that are highly active and stable at low temperatures in the presence of water but also in the presence of other molecules is required. It is particularly important to estimate the sensitivity of these new catalysts to nitrogen oxides [9]. An inhibiting effect of NOx on ozone decomposition is observed in the presence several catalyst compositions such as Ni-Mn spinel [10] and activated carbon [11]. Very recently, ozone decomposition has been investigated in the presence of NOx at low temperature over a commercial Pd/Al2O3. NOx species have been shown to significantly poison the catalyst, resulting in a significant decrease of ozone conversion over time on stream [12].

It is worthy to note that a significant enhancement of the specific surface area occurs in most cases after acid-treatment (Table 6-2) due to the formation of enhanced amorphous zone and low crystallinity of the layered structure observed by TEM analysis.

6-3-3-Gas phase characterization by FTIR

To assess the involvement of related nitrogen containing species in the activity decrease for O3 decomposition the formation of gaseous species in the plasma discharge (PD) in the presence of air with low humidity (RH = 0.7 %) was evalated by FTIR and their temporal evolutions have been monitored at the outlet of the catalyst reactor in the course of the reaction for Ce0.01Mn and Ce0.01Mn-AT as shown in Figure 6-11, respectively. A typical FTIR spectrum at the inlet of the catalyst reactor (Inlet CR) shows bands at 2223 cm -1 , 1720 cm -1 and 1325 cm - 1 assigned to N2O, N2O5 and HNO3, respectively, along with those at 2111 cm -1 and 1056 cm -1 attributed to O3. To sum up, the NTP processed air allows the formation of N2O, N2O5 and HNO3 as nitrogen gaseous pollutants along with the formation of O3 in the PD. The temporal evolutions of N2O5 and HNO3 differ by the near non-observance of the characteristic bands relative to these species in the first 3 h for Ce0.01Mn (Figure 6-11(a)) unlike Ce0.01Mn-AT (Figure 6-11(b)) for which slight variations in band intensity decrease occur. In contrast, the relative content of N2O is unaffected by post-plasma catalysis. Therefore, the uptake of N2O5 and HNO3 in the first hours of reaction is clearly observed in the plasma-catalysis experiment when using Ce0.01Mn as catalyst while this gaseous uptake seems to be lower for Ce0.01Mn-AT. In the presence of humidity, the FTIR spectrum of the inlet gaseous stream is significantly affected (Figure 6789101112). N2O5 is no longer observed while the band intensity ascribed to HNO3 is enhanced. Additionally, some broad bands at 1409 cm -1 and 1477 cm -1 are now observed whose assignations are currently unclear. Besides, the strong band at 1627 cm -1 has been assigned to water. N2O5 reacts with H2O to give HNO3. Interestingly, the release of CO2 in the early stages of the reactions can be linked to the destruction of surface carbonate species at the catalyst surface of Ce0.01Mn-AT. Consequently, the activation period of the catalyst can be partly explained by a progressive recovery of active sites poisoned by carbonate like species. It is worthwhile to note that the distribution of the nitrogen containing species keeps rather unchanged in the course of the reaction.

6-3-4-Characterizations studies after O3 testing

X-ray photoelectron spectroscopy has been used to evidence adsorbed nitrogen related products on the non-treated CexMn catalyst surfaces as a consequence of the uptake of N2O5 and HNO3 in the gas phase at the early stages of O3 decomposition. The high-resolution spectra of the N 1s region shown in Figure 6-13 highlight the presence of adsorbed nitrogen containing species that differ in the oxidation state of nitrogen atom and the pertinent data have been given in Table 6-6. Considering the non-treated catalyst (Figure 6-13(a)) a predominant peak at BE of  406.8 eV and small contributions at  403.2 eV and about 400.0 eV have been assigned to adsorbed nitrate (NO3 -) [32], nitrite (NO2 -) [32] and to reduced N such as NO - [33]. It has to be noted that the N/Mn atomic ratio of 0.08 on Mn-B-O3 increases slightly for low Ce/Mn ratios but increases significantly for higher ratios to reach a value of 0.24 for Ce0.5M-O3 (see Table

6-6).

Formation of predominantly adsorbed nitrate is observed upon uptake of gaseous HNO3 and N2O5. The formation of NO3 -(ads) species can occur owing to: Inlet CR difference between the N/Mn atomic ratios of the tested and fresh catalysts has been calculated.

It has a value of 0.09 for Mn-B-AT and for the cerium-based catalysts it increases from 0.04 to 0.25 with increasing Ce content. Besides, it is remarkable to note that this ratio correlates with O3 conversion in agreement with a poisoning of the catalytic sites (Figure 6 Consequently, water enhances both the presence of reduced surface-bound nitrogen related products and decreases substantially the amount of nitrogen adsorbed species. Considering the non-treated catalysts after testing (Figure 6-15 (a)) with Ce/Mn ratios lower than 0.2 the band which appears at highest frequency positioned at 1617 cm -1 has been principally assigned to the bending mode of water [27]. There are at least three different bands in the medium frequency range (1400-1200 cm -1 ) around 1380, 1340 and 1250 cm -1 . By comparison with reference spectra of alkali metal nitrates (Figure 6-15 (c)) the two intense bands located at about 1380 cm -1 and 1340 cm -1 have been assigned to adsorbed ionic nitrate in interaction with K + and Na + , respectively, in line with the presence of a band at about 815 cm - 1 attributed to the out-of-plane rocking mode. Considering the FTIR spectra of Mn-B-O3 the remaining distinct three bands at 1511 cm -1 , 1248 cm -1 and 1042 cm -1 can be ascribed to bidentate nitrato ligands [34] linked to manganese cations in accordance with the works of D.

Pozdnyakov et al. [37]. In the presence of low cerium content, it is observed that the relative intensity of these bands is decreasing indicating that this mode of interaction decreases with Ce content. For higher Ce/Mn ratio, although the bands are larger and overlap, the above general conclusions hold on. In (Figure 6-15 (b)) are represented the FTIR-ATR spectra of the acid treated Ce0.01Mn catalyst after testing in nearly dry and moist air at 30 °C and 40 °C. It turns out that the spectrum of Ce0.01Mn-AT-O3 is rather similar than that of Ce0.01Mn-O3 except that the most intense band is now shifted from 1344 cm -1 to 1310 cm -1 . In moist air (RH = 30%) the number of bands increases and all the more that temperature decreases. At 30°C the band characteristic of water at 1617 cm -1 is the most intense. A new band appears at about 1500 cm - 1 attributed again to bidentate NO3 -. However, the presence of such nitrato species does not seem to affect greatly the activity of the catalyst. The effect of temperature on the adsorbed nitrogen containing species has also been investigated in the case of Ce0.2Mn. correlates well with the onset temperature of reduction of the catalysts (Figure 6 -17) which may signify that the oxygen vacancies play an important role. In addition, the acidity resulting from the acid treatment must also be taken into account because Ce0.1Mn-AT, which is more active than Ce0.2Mn-AT, has more active sites. Additionally, the high activity of CexMn-AT (x = 0.01;0.1) in moist air (RH = 30 %) in the presence of nitrogen containing co-pollutants can be envisioned considering the mechanism for O3 decomposition in humid air involving acid sites and oxygen vacancies proposed by Liu. et al. [15]. More water acid sites in interaction with oxygen vacancy results in a better hydrophobic catalyst that can facilitate adsorption and subsequent decomposition of gaseous ozone.

Furthermore, in the presence of water, the contribution of N-adspecies is significantly reduced and the distribution of reduced N-based adspecies is much larger, suggesting a catalytic reduction of NO 3-adsorbed species.

6-4-Conclusion

A pioneer series of experiments were performed to study the decomposition of ozone in the presence of nitrogen pollutants and humidity on acid-treated Ce modified birnessites. The main findings could be summarized as follows: The performances of ozone decomposition were closely related to the presence of nitrogen pollutants. At very low humidity, the acid treated catalysts showed a minimization of deactivation while the non-treated catalysts deactivate rapidly over time on stream due to nitrate poisoning. At higher RH, after an induction period, the acid treated catalysts were effective for the elimination of ozone and stable during 5 h of reaction. Higher amounts of oxygen vacancies and acid sites were invoked to improve water resistance properties and to decrease nitrate interaction. The generation of such efficient catalysts open new perspectives for ozone decomposition from polluted air.

7-1-Introduction

The third validation approach as seen in chapter 2, was to investigate the abatement of TCE in dry and humid gas stream in PPC process, by following all the above required conditions: (i) the flow rate and the energy density have been choosen according to Trichloroethylene (TCE), is a chlorinated VOC commonly used in industry as a solvent and acts as a degreaser in the semiconductor and metal industry [1][2][3]. Due to its adverse impacts on environmental and human health (carcinogenic Group 1), TCE removal from waste gas streams is an issue of major concern. Conventional techniques, such as adsorption, thermal incineration, or catalytic oxidation can been used for chlorinated VOC removal. However, such techniques are not energy efficient for the removal of diluted VOCs (<1000 ppmv) from the waste gas [4][5][6][7][8]. In these last decades, post plasma catalysis (PPC) resulting from the coupling of a Non-Thermal Plasma (NTP) reactor with a catalytic reactor positioned downstream was recognized as an advanced oxidation process for diluted VOCs removal in waste air [9,10].

Although NTP displays a high reactivity towards VOC removal at 20°C, the selectivity of the process into COx (CO and CO2) is poor and gaseous byproducts hazardous are formed. In PPC configuration catalytic reactions can take advantages of the NTP emitted O3 as a potential source of active oxygen species enabling further oxidation at ambient or very low temperatures of the NTP non-degraded parent VOC and NTP gaseous hazardous by-products. Therefore, this resulting hybrid technology can overpass NTP and total oxidation catalysis in terms of selectivity, efficiency and energy cost.

The catalyst positioned after the plasma reactor must have specific characteristics such as high efficiency in plasma generated ozone decomposition, high oxygen mobility, high CO2 selectivity, coupled with an elevated resistance to chlorine. MnO2 based catalysts have been particularly studied in PPC configuration for TCE removal because of their high ability to decompose ozone to provide surface active oxygen species for the decomposition of plasmagenerated polychlorinated by-products [11,12]. However, the plasma-generated by-products are not fully decomposed into CO2 and the catalysts deactivate over time due to chorine induced deactivation. In order to promote the efficiency of PPC process towards chlorinated VOC abatement, the catalyst efficiency should be improved.

physisorption hysteresis loops appear in different shapes. Ce0.01Mn-400 sample presents a H3 loop while Ce0.01Mn-AT presents a H2 loop. This change in hysteresis loop from H3 to H2 can be ascribed to the change in pore shape [18] caused by the acid treatment. Both surface area and total pore volume are significantly increased after acid treatment. Such textural properties modification can be explained by the H + exchange during the acid treatment which can destabilize the layered structure. The change in surface acidity has been evaluated using pyridine as infrared probe molecule over dried Ce0.01Mn-and Ce0.01Mn-AT samples. FTIR spectra of the pyridine region obtained after pyridine adsorption at 150°C for dried Ce0. It should be noted, however, that it was not possible to assess the proportion of Ce 3+ /Ce 4+ in the samples due to Ce 3d and Auger Mn LMN peaks overlap.

7-3-2-O3 decomposition over Ce0.01Mn-400 and Ce0.01Mn-AT samples using non-thermal plasma (NTP) as ozoner

Figure 7-5 presents the ozone decomposition at room temperature as a function of time with Ce0.01Mn-400 and Ce0.01Mn-AT catalysts. Ce0.01Mn-400 maintains high ozone conversion (> 97%) for 140 minutes but then shows a gradual decrease in ozone conversion to 73 % after 5 hours. Ce0.01Mn-AT exhibits different behavior with a low decrease in ozone conversion after 30 minutes of testing from 100 % to 94 % after 120 minutes. Thereafter Ce0.01Mn-AT is able to maintain this high conversion over time. It has been reported earlier that oxygen vacancies play a key role in ozone decomposition on manganese oxides. Therefore, the better catalytic performance of Ce0.01Mn-AT compared to that of Ce0.01Mn-400 is consistent with its more abundant oxygen vacancies in agreement with a higher amount of surface Mn 3+ species (XPS). This result confirms the important role of oxygen vacancies in the catalytic ozone decomposition as reported in the literature [30][31][32][33]. In addition, the larger specific surface area of Ce0.01Mn-AT may contribute to a wider surface exposure of oxygen vacancies. This could % and 15 %, respectively (Figure 7-6). Interestingly, in comparison with results obtained with the same plasma discharge and plasma reactor (COx yield of 15% in humid air (RH = 15%) for ED of 120 J.L -1 ), the mineralization of TCE is then much higher, probably as a result of [O3]/[TCE]0 optimized ratio of 4 instead of less than 1 in previous studies [11,12,42,43]. formation such (oxi) chlorinated oxidation by products have already been observed and discussed elsewhere [36][37][38]. With almost no water in the feed (RH = 0.7%), TCE molecules react with ClO radicals to produce DCAC, then DCAC molecules further decompose by attack of Cl radicals to produce CCl4, CHCl3 and COCl2 molecules [36,44]. However, with RH of 15%, ClO radicals are certainly quenched by OH radicals (coming from H2O) leading to less amount of DCAC (Figure 7).

7-3-4-TCE abatement by Post plasma catalysis (PPC)

In PPC configuration with RH = 0.7%, TCE is fully converted regardless of the catalysts (Ce0.01Mn-AT and Ce0.01Mn-400) used. Figure 8 shows the COx and CO2 yields as a function of time with the two catalysts. As compared with NTP, COx yield increased and especially CO2 yield is significantly improved from 4 % to 58 % at t = 0. Among the two PPC configurations, that with Ce0.01Mn-AT downstream of the NTP outperforms the one with Ce0.01Mn-400 for the 5-hour test. For both configurations, COx and CO2 yields decreased with time of reaction.

However, for Ce0.01Mn-400 the decay is more pronounced during the first 30 minutes. After

7-3-5-TCE abatement in post plasma catalysis (PPC) configuration RH = 15%

The effect of relative humidity on PPC process for TCE decomposition has been studied in the presence of Ce0.01Mn-AT as the best catalyst.

7-4-XPS characteriations of the used catalysts after PPC process

After PPC process a slight oxidation of manganese species is observed at the surface regardless of the catalysts. Indeed, the Mn 3+ /Mn 4+ decreases (Figure 7-13(a)) in agreement with the increase of Mn AOS calculated from Mn 3s splitting (Table 7-4). All catalysts present also a surface Ce enrichment over Mn after PPC process. After reaction, chlorine element has been detected at the surface of both catalysts (Table 7-4). This chlorine detection can be explained by the incomplete degradation of TCE during the PPC process. The Cl 2p core level envelope can be simulated by considering two types of chlorine (Figure 7-13(b)). The Cl 2p3/2 photopeak at 198.5 ± 0.2 eV can be attributed to metal (M) chloride, with M = Ce and/or Mn [39,42] while the Cl 2p3/2 photopeak at 200.6 ± 0.2 eV is ascribed to (oxi)chlorinated organic species such as CHxCly(Oz) [39,42]. The organic chlorine over mineral chlorine ratio (Clorg/Clmin) has been calculated for the tested catalysts. The best catalyst in TCE degradation (Ce0.01Mn-AT) presents a lower Clorg/Clmin ratio (0.14) in comparison with the ratio obtained for Ce0.01Mn-400 (0.42), suggesting that the formation of (oxi)-chlorinated organic species at the surface is detrimental to the catalytic performance. This assumption is reinforced by comparing the ratios obtained for the catalyst tested in different RH conditions. Indeed, with RH = 15% the activity of Ce0.01Mn-AT was lower and the Clorg/Clmin ratio is higher (0.35).

General conclusion and outlook

Conclusion

In this thesis, the use of PPC for the abatement of trichloroethylene is investigated with the objective to optimize the PPC process that means, total VOC decomposition with a low energy consumption. Thus, Focus was on operating the PPC system at low energy density in combination with a catalyst at low temperature to improve the efficiency of the abatement of low TCE concentration (150 ppmv) to be as close as industrial issues. Special attention was paid to adapt the molar ratio of NTP produced O3 to the initial concentration of TCE, namely

[O3] / [TCE]0 to supply enough reactive oxygen species to quantitatively oxidize TCE into CO2.

The choice of a suitable catalyst for PPC process is based on some basic requirements such as, ozone decomposition ability, oxygen mobility, and high CO2 selectivity coupled with a chlorine and water tolerance. Ce doped birnessite type-MnO2 are chosen as a base catalysts as they are known to be highly efficient for ozone decomposition due to improved textural properties, a modification of Mn average oxidation state and an increase of structural defects e.g oxygen vacancies resulting from the formation of crystal boundaries between MnO2 and CeO2. These materials were synthesized via a redox method using KMnO4 as oxidant and NaC3H5O3 as reductant with a ratio KMnO4 / NaC3H5O3 = 1.2. In order to improve the water and chlorine tolerance a nitric acid treatment is also performed.

In a first step (chapter IV), the decomposition of HCHO using cerium doped birnessite as a catalysts has been systematically studied. Indeed, this reaction is well known to correlate the density of oxygen vacancies in the material, which are the active center for ozone decomposition and TCE oxidation. The effect of calcination temperature (Tc = 200, 300 and 400 °C), Ce content (CexMn; x= 0, 0.01, 0.1, 0.2 and 0.5) for the materials calcined at 400°C, and relative humidity (RH = 0, 50 and 75%) on formaldehyde oxidation has been evaluated. it is published that Pd/Al2O3 is a very good catalyst for ozone decomposition in presence of nitrogen oxides and water [1]. However, in real world exhaust gas, the presence of moisture is unavoidable, while in this process water has negative effect on ozone production resulting in a negative effect on TCE abatement. To further take advantage from ozone produced by NTP it could be interesting to study the effect of water by using plasma as ozoner that means to introduce TCE and water after plasma reactor. Indeed, it could also be interesting to use plasma for TCE degradation and for ozone production and study the effect of introduce only the water after the plasma reactor. In general, the typical industrial exhaust emission is rarely composed of well-defined single compounds. On one hand it is reported in the literature a positive effect such as enhanced removal efficiency, formation of reactive intermediates and reduction of the undesired by products, despite having issues of complex interacting chemistry [2][3][4]. On the other hand other researchers are reported negative effect resulting on the catalyst deactivation and that is related to VOC properties and concentration [5,6]. Thus it is also essential to investigate the performance if plasma-catalytic oxidation of binary and ternary mixtures VOC with different compositions. Furthermore it is very interesting to understand the mechanism of TCE oxidation on post plasma catalysis. Different assumptions of adsorption and desorption of intermediates species, oxidation of chlorinated by products by active oxygen species formed after decomposition of ozone on catalyst surfaces, determination of active sites and which chemical species play the important role in the oxidation process… all these assumptions is based on the observations and characterization observed separately through gas phase characterizations and catalyst surface analysis before and after test. From this point of view, development of well-designed instruments specialized in-situ measurements is highly recommended.

Filip

  DE TURCK, Prof. Dr. Patrick DA COSTA, Dr. Anton NIKIFOROV, Dr. Jean-Marc CLACENS and Dr. Cyril THOMAS.

First 1 - 2 - 1 - 1 - 2 - 2 - 1 - 2 - 3 - 1 - 3 - 1 - 3 - 1 - 1 - 3 - 2 - 1 - 3 - 3 - 1 - 4 - 2 - 1 - 4 - 3 - 1 - 5 - 1 - 5 - 1 - 1 - 5 - 2 - 1 - 5 - 3 - 1 - 5 - 4 - 1 - 5 - 5 - 1 - 6 - 2 - 1 - 6 - 3 - 1 - 6 - 4 - 1 - 7 - 1 - 7 - 1 - 1 - 7 - 2 -

 121122123131311321331421431515115215315415516216316417171172 international conference on Cold Plasma Sources and Applications (COPSA), Ypres, Belgium, 2018, 'Poster presentation' Table of contents Summary ................................................................................................................................................ Samenvatting ......................................................................................................................................... Résumé ................................................................................................................................................... Acknowledgements .............................................................................................................................. List of Publications ..............................................................................................................................Table of contents .................................................................................................................................. List of figures ....................................................................................................................................... List of tables ......................................................................................................................................... Chapter 1: ............................................................................................................................................LITERATURE OVERVIEW ............................................................................................................. 1-1-Air pollution ............................................................................................................................. 1-2-Volatile organic compounds (VOC) ....................................................................................... Definition ........................................................................................................................... Sources ............................................................................................................................... Effect on human health and environment ........................................................................ Conventional VOC abatement technologies ........................................................................... Thermal oxidation ............................................................................................................. Catalytic oxidation ............................................................................................................. Non-thermal plasma (NTP) ............................................................................................... 1-3-4-Plasma-Catalysis ................................................................................................................ 1-4-Trichloroethylene (TCE) ......................................................................................................... 1-4-1-Physico-chemical properties, Production and Use of TCE .............................................. Health effect and legislation ............................................................................................. TCE abatement .................................................................................................................. Plasma as ozoner and ozone decomposition .......................................................................... NTP and ozone generation ................................................................................................ Ozone catalytic decomposition .......................................................................................... Mechanisms for ozone decomposition in dry and humid air ........................................... Deactivation mechanism ................................................................................................... Acid treatment to increase MnO2 hydrophobicity ............................................................ 1-5-6-Mechanism for ozone decomposition over birnessite treated with nitric acid ................. 1-6-Birnessite .................................................................................................................................. 1-6-1-Birnessite crystalline structure .......................................................................................... Birnessite synthesis methods ............................................................................................. Variability in the composition of sheet and interfoliare ................................................... Birnessite properties (amount of alkaline metals) and capacity of ion exchange ........... Formaldehyde .......................................................................................................................... Formaldehyde catalytic decomposition ............................................................................. Mechanism for HCHO oxidation ...................................................................................... References ........................................................................................................................................ Chapter 2: ............................................................................................................................................ STRATEGIC METHODOLOGY ..................................................................................................... Chapter 3: ............................................................................................................................................ MATERIAL AND METHODS .......................................................................................................... 3-1-Catalyst preparation ................................................................................................................

3 - 1 - 3 - 3 - 2 -

 31332 through a redox precipitation method. .......................................................................................................................................... 3-1-2-Calcination procedure ....................................................................................................... Acid treatment .................................................................................................................... Catalyst characterizations .......................................................................................................

  Diffraction (XRD) ..................................................................................................

  Microscopy (TEM) ......................................................................

  ) ..........................

3 - 2 - 7 - 3 - 2 - 9 - 3 - 2 - 10 - 3 - 2 - 11 - 3 - 2 - 12 - 3 - 3 - 3 - 3 - 1 - 3 - 3 - 2 -

 32732932103211321233331332 Barret-Joyner-Halenda (BJH) ............................................................................................................................. 3-2-6-Hydrogen-Temperature Programmed Reduction (H2-TPR) ............................................ Temperature programed desorption (O2-TPD) ................................................................. 3-2-8-X-ray photoelectron spectroscopy (XPS) .......................................................................... Infrared spectroscopy (IR) ................................................................................................ Pyridine adsorption followed by infrared spectroscopy (IR pyridine) ........................... Raman spectroscopy ........................................................................................................ Time of Flight Secondary ion Mass spectrometry (ToF-SIMS) .................................... Determination of catalytic properties of materials for the total oxidation of formaldehyde ........................................................................................................................................................... Description of the set-up.................................................................................................... HCHO and CO2 calibration .............................................................................................. 3-3-3-Experimental procedure of HCHO oxidation .................................................................. 3-4-Determination off catalytic properties of materials for ozone decomposition ...................

3 - 4 - 2 - 3 - 4 - 3 - 3 - 5 - 3 - 5 - 1 - 3 - 5 - 2 - 4 - 1 - 4 - 2 - 4 - 4 - 4 - 5 - 5 - 1 - 5 - 2 - 5 - 3 -

 3423433535135241424445515253 Ozone generation and TCE abatement ................ Experimental procedure for using plasma as ozoner: ..................................................... Experimental procedure for catalytic ozone decomposition ............................................ Determination of catalytic properties of materials for TCE abatement ........................... Calibration FTIR ............................................................................................................. Experimental procedure for trichloroethylene in PPC process ..................................... References ...................................................................................................................................... Chapter 4: .......................................................................................................................................... CERIUM DOPED (K, Na) -BIRNESSITE LIKE MnO2 FOR LOW TEMPERATURE OXIDATION OF FORMALDEHYDE: EFFECT OF CALCINATION TEMPERATURE ..... Introduction............................................................................................................................ Experimental conditions........................................................................................................ 4-3-Results and discussion ........................................................................................................... 4-3-1-Characterizations of the fresh samples ........................................................................... 4-3-2-Characterization of the calcined samples ....................................................................... 4-3-3-Catalytic activity ............................................................................................................... Conclusion .............................................................................................................................. References ............................................................................................................................... Chapter 5: .......................................................................................................................................... OZONE AND NITROGEN OXIDE FORMATION THROUGH 10 PIN-TO-PLATE NEGATIVE DC CORONA DISCHARGE ..................................................................................... Introduction............................................................................................................................ Experimental conditions........................................................................................................ Experimental results ..............................................................................................................

5 - 3 - 2 - 5 - 3 - 3 - 5 - 4 - 6 - 1 - 6 - 2 - 6 - 3 - 6 - 3 - 3 - 6 - 3 - 4 - 6 - 3 - 5 - 6 - 4 - 6 - 5 - 7 - 1 - 7 - 2 - 7 - 3 - 7 - 3 - 4 - 7 - 3 - 5 - 7 - 4 - 7 - 5 - 7 - 6 -

 532533546162636336346356465717273734735747576 ......................................................................................... Influence of Flow rate (Q) .............................................................................................. Influence of Relative Humidity (RH): ............................................................................ Conclusion .............................................................................................................................. References ...................................................................................................................................... Chapter 6: .......................................................................................................................................... ACID TREATED Ce MODIFIED BIRNESSITE-TYPE MnO2 FOR OZONE DECOMPOSITION AT LOW TEMPERATURE: EFFECT OF NITROGEN CONTAINING CO-POLLUTANTS AND WATER ................................................................................................. Introduction............................................................................................................................ Experimental conditions........................................................................................................ Results and discussion ........................................................................................................... 6-3-1-Characterization studies of the acid-treated catalysts .................................................... 6-3-2-Catalytic tests for ozone decomposition .......................................................................... Gas phase characterization by FTIR .............................................................................. Characterizations studies after O3 testing ....................................................................... Effect of nitrogen containing co-pollutants for gaseous ozone decomposition ............ Conclusion .............................................................................................................................. References ............................................................................................................................... Chapter 7: .......................................................................................................................................... POST PLASMA CATALYSIS FOR TRICHLOROETYLENE ABATEMENT WITH Ce DOPED BIRNESSITE DOWNSTREAM DC CORONA DISCHARGE REACTOR ................ Introduction............................................................................................................................ Experimental conditions........................................................................................................ Results and discussion ........................................................................................................... 7-3-1-Main physicochemical characterizations of the fresh materials ................................... 7-3-2-O3 decomposition over Ce0.01Mn-400 and Ce0.01Mn-AT samples using non-thermal plasma (NTP) as ozoner ............................................................................................................. 7-3-3-TCE abatement by non-thermal plasma (NTP) .............................................................. TCE abatement by Post plasma catalysis (PPC) ............................................................. TCE abatement in post plasma catalysis (PPC) configuration RH = 15% ................... XPS characteriations of the used catalysts after PPC process........................................... Conclusion .............................................................................................................................. References ............................................................................................................................... General conclusion and outlook ....................................................................................................... Conclusion ...................................................................................................................................... Outlook ............................................................................................................................................ Appendix A ........................................................................................................................................

Figure 1 - 1 :

 11 Figure 1 -1: Use of TCE 'INERIS, Institut national de l'environnement industriel et des risques, Trichloréthylène, 2003' ..........................................................................................................................................

Figure 1 - 2 :

 12 Figure 1 -2: Reaction scheme for TCE abatement in PPC ...................................................................................

Figure 1 - 4 :

 14 Figure 1 -4: Different discharge regimes of a negative DC excited glow discharge: (a) corona regime; (b) glow regime; (c) spark regime ........................................................................................................................................

Figure 1 - 3 :

 13 Figure 1 -3: Current voltage characteristic of a negative DC geometry ...............................................................

Figure 1 - 5 :

 15 Figure 1 -5: Mechanism of ozone decomposition on -MnO2 in a) dry air b) humid air ....................................

Figure 1 - 6 :

 16 Figure 1 -6 : Mechanism for gaseous ozone decomposition over acid treated birnessite ....................................

Figure 1 - 7 :

 17 Figure 1 -7: Scheme of the crystal structure of a triclinic birnessite : alternation of a chain of Mn(III)O6 and two chains of Mn(IV)O6 [114] ....................................................................................................................................

Figure 1 - 8 :

 18 Figure 1 -8: Crystalline structure of hexagonal birnessite: a predominantly Mn(IV) chain sheet with gaps, interlayer cations are H + and Mn 2+ /Mn 3+ which are positioned below and above the deficiencies [115] ..............

Figure 1 - 9 :

 19 Figure 1 -9: XRD pattern for a) birnessite b) vernadite .......................................................................................

Figure 1 - 10 :

 110 Figure 1 -10: Inner sphere complexes crystallographic position: Me represents metal; E Me represents edge sharing ....................................................................................................................................................................

Figure 1 - 11 :

 111 Figure 1 -11: Relation between the concentration of oxygen vacancies and the total oxidation of HCHO .......

Figure 2 - 1 :

 21 Figure 2 -1: Concept of post plasma catalysis ......................................................................................................

Figure 2 - 2 :

 22 Figure 2 -2: Schematic of the operation of the plasma as an ozoner ....................................................................

Figure 2 - 3 :

 23 Figure 2 -3: Schematic presentation of HCHO oxidation ....................................................................................

Figure 2 - 4 :

 24 Figure 2 -4: Schematic presentation of O3 decomposition ..................................................................................

Figure 2 - 5 :

 25 Figure 2 -5: Schematic presentation of TCE abatement in PPC process ..............................................................

Figure 3 - 1 :

 31 Figure 3 -1: Scheme of the calcination conditions ...............................................................................................

Figure 3 - 2 :

 32 Figure 3 -2: Pyridine bond on the surface ............................................................................................................

Figure 3 - 3 :

 33 Figure 3 -3: Scheme of the set-up of formaldehyde catalytic oxidation ..............................................................

Figure 3 - 4 :

 34 Figure 3 -4: Dynacal permeation tubes ................................................................................................................

Figure 3 - 5 :

 35 Figure 3 -5: Experimental Set-Up ........................................................................................................................

Figure 3 - 6 :

 36 Figure 3 -6: (a) MFC, (b) Testo 445 climate measurement device with temperature/humidity probe, (c) Pressure indicator .................................................................................................................................................................

Figure 3 - 7 :

 37 Figure 3 -7: Photo of the plasma reactor with 10 electrodes pins ........................................................................

Figure 3 - 8 :

 38 Figure 3 -8: Photo of DC power source ...............................................................................................................

Figure 3 - 9 :

 39 Figure 3 -9: Electrical scheme of the 10 pin to plate ..........................................................................................

Figure 3 - 10 :

 310 Figure 3 -10: Overview of the catalytic reactor component with (a) oven, (b) temperature controller and .......

Figure 3 - 11 :

 311 Figure 3 -11: (a) Bruker Vertex 70 FT-IR spectrometer, (b) Gas cell ................................................................

Figure 3 - 12 :

 312 Figure 3 -12: Teledyne ozone monitor Model 465M ...........................................................................................

Figure 3 - 13 :

 313 Figure 3 -13: Selected FT-IR band for ozone calibration...................................................................................

Figure 4 - 1 :

 41 Figure 4 -1: XRD patterns of the as-synthesized samples ..................................................................................

Figure 4 - 2 :

 42 Figure 4 -2: FTIR-ATR spectra of the as-synthesized samples .....................................................................

Figure 4 - 3 :

 43 Figure 4 -3: (a) N2 isotherms and (b) PDS of the as-synthesized samples .........................................................

Figure 4 - 4 :

 44 Figure 4 -4: TEM of the fresh (a)-(b) Ce0.01Mn and (c)-(d) Ce0.1Mn catalysts..................................................

Figure 4 - 5 :

 45 Figure 4 -5: XPS (a) Ce 4d, (b) Mn 3s core levels (c) O1s and (d) Mn 2p3/2 for the as-synthesized samples. ...

Figure 4 - 6 :

 46 Figure 4 -6: H2-TPR traces of the as-synthesized samples ...............................................................................

Figure 4 - 7 :

 47 Figure 4 -7: (a) TGA b) DTA and (c) DSC profiles for the as-synthesized samples .......................................

Figure 4 - 8 :

 48 Figure 4 -8: XRD patterns of the samples after TG analysis ............................................................................

Figure 4 - 9 :

 49 Figure 4 -9: Temporal evolution during TGA under helium (a) of the signal m/z =17, 18, 28 and 44 and (b) of the signal m/z =32 for Ce0.1Mn. .........................................................................................................................

Figure 4 - 10 :

 410 Figure 4 -10: (a) XRD patterns for the calcined samples and (b) XRD zoom for first peak in the following analysis conditions: 10-20°, step size: 0.2°, step time: 5s ....................................................................................

Figure 4 - 11 :

 411 Figure 4 -11: FTIR-ATR spectra of the calcined samples .............................................................................

Figure 4 - 12 :

 412 Figure 4 -12: (a) TGA, (b) DTA and (c) DSC profiles for the calcined Ce0.01Mn and Ce0.1Mn (d) dehydroxylation and O release for Ce0.01Mn and Ce0.1Mn ...............................................................................

Figure 4 - 13 :

 413 Figure 4 -13: (a) N2 isotherms and (b) PDS of the calcined samples ...............................................................

Figure 4 - 14 :

 414 Figure 4 -14: H2-TPR traces of the calcined samples ......................................................................................

Figure 4 - 15 :

 415 Figure 4 -15: Raman spectra of Ce0.1Mn (a) ex situ as-synthesized and calcined at 200, 300 and 400 °C and (b) in-situ calcined at 400°C for 1h and for 3h ........................................................................................................

Figure 4 - 16 :

 416 Figure 4 -16: O2-TPD traces of the calcined samples at 400 °C ......................................................................

Figure 4 - 17 :

 417 Figure 4 -17: XPS (a) O1s and (b) Mn 2p3/2 core levels of the calcined samples ...............................................

Figure 4 - 18 :

 418 Figure 4 -18: ToF-SIMS positive ion spectra (a) and (b) in the 200-220 m/z range and (c) and (d) n the range 135-160 mz of Ce0.01Mn-400 and Ce0.1Mn-400. ................................................................................................

Figure 4 - 19 :

 419 Figure 4 -19: Light-off curves over (a) Mn-B-400, CexMn-400 and Hop-300; (b) Ce0.01Mn-X00 and (c) Ce0.1Mn-X00 catalysts. HCHO = 100 ppm, O2 = 20%, N2 balance, GHSV = 60 L/(gcat.h); (d) Arrhenius plot of the tested samples. ................................................................................................................................................

Figure 4 - 20 :

 420 Figure 4 -20: Effect of RH on HCHO elimination and HCHO conversion over (a) Ce0.01Mn-400 and (b) Ce0.1Mn -400. T = 50 °C, HCHO = 100 ppm, O2 = 20%, N2 balance, GHSV = 60 L/(gcat.h). .............................

Figure 4 - 21 :

 421 Figure 4 -21: Effect of RH on HCHO elimination and HCHO conversion over Hopcalite. T = 75 °C, HCHO = 100 ppm, O2 = 20%, N2 balance, GHSV = 60 L/(gcat.h). ....................................................................................

Figure 4 - 22 :

 422 Figure 4 -22: HCHO-TPD (a) HCHO adsorption as a function of time, (b) evolution of gaseous species (HCHO and CO2) and (c) evolution of the atomic ratio O/ (Mn+Ce) as a function of temperature in HCHO oxidation under helium of Ce0.1Mn-400, Ce0.01Mn-400 and Ce0.01Mn-300. .........................................................

Figure 4 - 23 :

 423 Figure 4 -23: FTIR-ATR spectra of the fresh and used catalysts after stability tests .........................................

Figure 4 - 24 :

 424 Figure 4 -24: Scheme of the possible reaction pathway of HCHO oxidation over Ce doped birnessite ............

Figure 5 - 1 :

 51 Figure 5 -1: FT-IR spectrum of the plasma generated gaseous species. Dry air; F = 1L/min; ED = 150J/L .....

Figure - 2 : 145 Figure - 3 :Figure - 4 : 147 Figure - 5 : 148 Figure - 6 :Figure - 7 :Figure - 8 : 150 Figure - 9 :Figure - 1 :Figure - 2 :Figure - 3 :Figure - 4 :Figure - 5 : 162 Figure - 6 : 165 Figure - 8 : 167 Figure - 9 : 167 Figure - 10 : 168 Figure 6 - 11 : 170 Figure - 12 :Figure - 13 :Figure 6 - 14 :Figure - 15 :Figure - 16 : 177 Figure 6 - 17 : 178 Figure - 1 :

 214534147514867815091234516261658167916710168611170121361415161776171781 Figure -2: Scheme of N2O5 and HNO3 production via NOx oxidation by O3 .................................................. 145 Figure -3: Ozone concentration as a function of ED. Dry air; Q = 1 L/min ................................................ 14646 Figure -4: Ix/I250 as a function of ED; Dry air; Q = 1L/min ............................................................................. 147 Figure -5: Evolution of [O3] as a function of ED. Dry air; Q = 0.5, 1 and 1.5 L/min ..................................... 148 Figure -6: Effect of Q on [O3] and rO3 ......................................................................................................... 14949 Figure -7: Evolution of the integrated area as a function of Q. Dry air; ED = 150 J/L ............................... 14950 Figure -8: Evolution of [O3] as a function of ED in dry air and humid air; Q = 0.5 L/min ............................. 150 Figure -9: FT-IR for the plasma generated gaseous species in dry air and in 15 % as relative humidity. Q = 0.5L/min; ED = 250 J/L ..................................................................................................................................... 1511

Figure 7 - 2 :

 72 Figure 7 -2: (a) N2 adsorption desorption (b) pore size distribution of Ce0.01Mn-400 and Ce0.01Mn-AT samples .................................................................................................................................................................

Figure 7 - 3 :

 73 Figure 7 -3: FTIR spectra of the Py region obtained after Py adsorption at 105°C for dried Ce0.01Mn and Ce0.01Mn-AT samples ...........................................................................................................................................

Figure 7 - 4 :

 74 Figure 7 -4: XPS Mn2p3/2 core level decomposition for Ce0.01Mn-400 and Ce0.01Mn-AT samples ...................

Figure 7 - 5 :

 75 Figure 7 -5: Ozone conversion over Ce0.01Mn-400 and Ce0.01Mn-AT ([O3] = 300 ppmv, GHSV = 1200 L/(gcata h); RH = 0.7 %) ..................................................................................................................................................

Figure 7 - 6 :

 76 Figure 7 -6: COx, CO and CO2 yields obtained in NTP TCE abatement with RH of 0.7 % and 15 % (ED = 150 J/L; Q= 1 L/min; [TCE]0 = 150 ppmv) ..............................................................................................................

Figure 7 - 7 :

 77 Figure 7 -7: FT-IR spectra in the range 2500-1700cm -1 and 1110-700cm -1 of outlet stream after NTP in 0.7 % and 15 % as relative humidity (ED = 150 J/L, Q= 1 L/min) (PG : phosgene; DCAC : dichloroacetylchloride and TCAD : trichloroacetaldehyde) ..........................................................................................................................

Figure 7 - 8 :

 78 Figure 7 -8: COx and CO2 yields obtained for TCE abatement (RH = 0.7%) in PPC configuration with Ce00.1Mn-400 and Ce0.01Mn-AT as catalysts ......................................................................................................

Figure 7 - 9 :

 79 Figure 7 -9: FT-IR spectra of the inlet TCE and outlet gaseous stream after PPC process (RH = 0.7 %) as a function of time with Ce0.01Mn-400 as catalyst (PG : phosgene; DCAC : dichloroacetylchloride ;TCAD : trichloroacetaldehyde) ........................................................................................................................................

Figure 7 - 10 :

 710 Figure 7 -10: FT-IR spectra of the inlet TCE and outlet gaseous stream after PPC process (RH = 0.7 %) as a function of time with Ce0.01Mn-AT as catalyst (PG: phosgene; DCAC: dichloroacetylchloride; TCAD: trichloroacetaldehyde) ........................................................................................................................................

Figure 7 - 11 :

 711 Figure 7 -11: COx and CO2 yields obtained for TCE abatement (RH = 15%) in PPC configuration with Ce0.01Mn-AT as catalyst .......................................................................................................................................

Figure 7 - 12 :

 712 Figure 7 -12: FT-IR spectra of the inlet TCE and outlet gaseous stream after PPC process (RH = 15 %) as a function of time with Ce0.01Mn-AT as catalyst (PG: phosgene; DCAC: dichloroacetylchloride; TCAD: trichloroacetaldehyde) ..........................................................................................................................................

Figure 7 - 13 :

 713 Figure 7 -13: XPS (a) Mn2p3/2 and (b) Cl2p core levels decomposition after PPC process ...............................

Figure A - 1 :

 1 Figure A -1: Survey XPS spectra of Mn-B ....................................................................................................

Figure 1 - 1 :

 11 Figure 1 -1: Use of TCE 'INERIS, Institut national de l'environnement industriel et des risques, Trichloréthylène, 2003'

  of polychlorinated gaseous by-products by combining the qualitative data from mass spectrometry and Fourier-transform infrared spectroscopy. Phosgene (PG), dichloroacetylchloride (DCAC) and trichloroacetaldehyde (TCAD) are identified as incomplete oxidation products. According to Vandenbroucke et al. [51], the ClO • radical oxidizes TCE leading to the formation of significant amount of DCAC by the following reaction: C2HCl3 + ClO •  CHCl2COCl + Cl • [9,18,19]. Hydrogen chloride, chlorine and ozone are also detected in the outlet stream. Overall the high reactivity towards TCE removal is tempered by a low selectivity into CO2/H2O/HCl due to the production of toxic by-products such as PG, DCAD, TCAD and O3.

  TCE and its poor CO2 and HCl selectivity. A typical reactional scheme for TCE abatement in PPC configuration shown in Figure1-2 has been reported by Vandenbroucke et al.[51] using a 5 pins to plate NTP reactor configuration working with a DC negative corona discharge and MnO2 as catalysts positioned downstream of the NTP reactor.As stated previously the main issues to address is the total oxidation of the NTP generated gaseous polychlorinated by-products such as DCAC, TCAD and COCl2 along with the elimination of O3. In PPC configuration the NTP generated O3 is decomposed on the catalyst surface producing active oxygen species enabling further oxidation at low temperatures of the NTP non processed parent VOC and NTP gaseous hazardous by-products owing to path 1. But on the other hand, the detection of polychlorinated C1 species such as CCl4 and HCl3 at the exit of the catalytic reactor can result from the C-C bond breaking of the in C2 polychlorinated compounds allowing the formation of CCl3 and HCCl2 species able to react with adsorbed chlorine (path 2). Overall, this resulting hybrid technology can overpass NTP and total oxidation catalysis in terms of TCE removal efficiency and energy cost at low temperature[35].

Figure 1 - 2 :

 12 Figure 1 -2: Reaction scheme for TCE abatement in PPC

Figure 1 - 4 :Figure 1 - 3 :

 1413 Figure 1 -4: Current voltage characteristic of a negative DC geometry

Figure 1 - 5 :

 15 Figure 1 -5: Mechanism of ozone decomposition on -MnO2 in a) dry air b) humid air

Figure 1 - 6 .

 16 The catalyst surface was composed of adjacent oxygen vacancy and acid site.O3 molecule was firstly adsorbed on a surface acid site via one terminal oxygen atom through hydrogen bond. Secondly, the other terminal oxygen atom of the O3 molecule inserted into an oxygen vacancy site, leading to the formation of an O 2-species which was released as an oxygen molecule into the gas phase resulting in the regeneration of the acid site. Third, another O3 molecule formed hydrogen bond with surface acid site and its free terminal oxygen atom interacted with the formerly formed O 2-species to produce a peroxide (O2 2-) with release of an oxygen molecule. Finally, the O2 2-decomposed was transformed into oxygen molecule which was released in the gas phase allowing the regeneration of the oxygen vacancy.

Figure 1 - 6 :

 16 Figure 1 -6 : Mechanism for gaseous ozone decomposition over acid treated birnessite

  structure obtained was described as monoclinic. Studies byDrits et al. (1997) [103] andSilvester et al. (1997) [104] extended the structural analysis and confirmed the monoclinic symmetry of birnessite synthesized under basic conditions and described a hexagonal structure for that synthesized by the reduction of Mn (VII) in an acidic medium according to the McKenzie protocol (1971)[113]. The monoclinic structure of birnessite synthesized in a basic medium has nonetheless been questioned byPost et al. (2002) [114] who demonstrated that it was more of a triclinic symmetry.

Figure 1 - 7 :

 17 Figure 1 -7: Scheme of the crystal structure of a triclinic birnessite : alternation of a chain of Mn(III)O6 and twochains of Mn(IV)O6[114]

Figure 1 - 8 :

 18 Figure 1 -8: Crystalline structure of hexagonal birnessite: a predominantly Mn(IV) chain sheet with gaps,interlayer cations are H + and Mn 2+ /Mn 3+ which are positioned below and above the deficiencies[115]

2- 3 .

 3 In fact, basal reflections are extremely attenuated, and it is possible that they have been completely masked if the diffractometers used by the F.V. Chukhrov team were less efficient than at present, and/or if the particles studied were even smaller in size. It is now accepted that vernadite are turbostratic birnessite, but the structural characterization of these minerals remains difficult, and the word "vernadite" ultimately refers only to a turbostratic and nano particulate variation of a non-stoichiometric Mn IV oxide. Due to its low crystallinity and turbostratic character, vernadite has a diffractogram in which the first two Bragg rays of birnessite are almost absent (not well defined and broad) (Figure1-9), and has only two diffraction bands in the angular region 30 -80° 2ϴ CuKα classically used in mineralogical studies.

Figure 1 - 9 Figure 1 - 9 :

 1919 Figure 1 -9 Figure 1 -9: XRD pattern for a) birnessite b) vernadite

  crystallographic positions such as triple corner sharing (TC), triple edge sharing (TE) and double corner sharing (DC) Figure 1-10.

Figure 1 - 10 :

 110 Figure 1 -10: Inner sphere complexes crystallographic position: Me represents metal; E Me represents edge sharing

  birnessite based on ToF-SIMS and XPS. First, HCHO adsorbs on the surface of manganese.Partial oxidation of adsorbed HCHO by reduction of manganese allows the formation of several reaction intermediates: formates (mono-or dentate or bridged), dioxymethylene, methoxy or carbonates. The oxidation of these reaction intermediates to CO2 and H2O is considered the limiting step in the oxidation reaction of HCHO. The oxidation is carried out by the active oxygen species from the manganese oxides. Furthermore the activity of surface oxygen species is directly related to the amount of oxygen vacancies. However, Rochard et al.[181], have been investigated that the oxygen vacancies played a double role in enhancing the HCHO catalytic activity as shown in Figure1-11 over Au/Co-promoted CeO2 catalysts.

Figure 1 - 11 :

 111 Figure 1 -11: Relation between the concentration of oxygen vacancies and the total oxidation of HCHO

Figure 2 - 1 :

 21 Figure 2 -1: Concept of post plasma catalysis

1 - 2 - 2 )

 122 [O3]/ [TCE]0 optimization: Study the performances of the NTP as an ozoner (Figure to be able to tune the O3 production as a function of the different key processing parameters. On purpose the effect of air flow rate (Q), specific energy density (SED) and humidity (RH) will be carried out. The obtained results are discussed in chapter 5.

Figure 2 - 2 :

 22 Figure 2 -2: Schematic of the operation of the plasma as an ozoner

- 3 )Figure 2 - 3 :

 323 Figure 2 -3: Schematic presentation of HCHO oxidation  The efficiency of the catalysts is investigated also in terms of ozone degradation ability in the presence of others pollutants such as nitrogen oxide species in dry and humid air using the NTP as an ozoner (Figure 2-4). The experimental conditions are chosen in order to discriminate efficiently the catalysts in terms of catalytic performances. The effect of Ce content as well as the effect of acid treatment are investigated. The obtained results are presented in chapter 6.

Figure 2 - 4 :

 24 Figure 2 -4: Schematic presentation of O3 decomposition  The catalysts are then tested in the total oxidation of TCE in dry and humid air in PPC configuration (Figure 2-5). Based on that, the performances of the processes are discussed in terms of TCE removal efficiency, COx yields, and carbon-balance and in terms of cost energy saving. These results are discussed in terms of the physicochemical characterization of the catalysts which are presented in chapter 7.

Figure 2 - 5 :

 25 Figure 2 -5: Schematic presentation of TCE abatement in PPC process

Figure 3 - 1 :

 31 Figure 3 -1: Scheme of the calcination conditions

Chapter 3 :

 3 Material and methodsThe Bragg's law governing diffraction is as follows: 2d(hkl)sinθ(hkl) = nλ; with n the order of diffraction (is an integer) and d(hkl) the distance between the reticular planes, where the hkl indices indicate the direction considered in the crystal. For a certain d(hkl) value, and monochromatic incident X-ray radiation, diffracted intensities can only be observed from welldefined 2θ (hkl) angles. The crystalline phases obtained were identified by comparison with the reference diffractograms obtained in the PDF database obtained by the International Center for Diffraction Data (ICDD), which provides access to Joint Committee of Powder Diffraction Standards (JCPDS) sheets. The disadvantage of this technique is that it does not make it possible to demonstrate the phases present in small quantities in the sample. Its sensitivity is about 1%

Figure 3 - 2 : 2 )

 322 Figure 3 -2: Pyridine bond on the surface Pyridine adsorption followed by Fourier Transform Infrared spectroscopy (FTIRpyridine) were recorded on a Thermo Nicolet 460 protégé instrument equipped with a CSi beam splitter and a MCT detector with a 2 cm -1resolution. 256 scans were collected over the spectral range extending from 4000 to 400 cm -1 . The cell was connected to a vacuum line for different treatments. All samples were ground in an agate mortar and were pressed into the form of a self-supporting wafers (1 tonne/ cm 2 , 2,01 cm 2 ) then placed in a quartz sample holder and after in a quartz cell equipped with KBr windows. Samples were activated under high vacuum (P<10 -

  nature of chemical bonds and intermolecular interactions. It is a scattering spectroscopy, as it consists in sending a monochromatic light on the sample, through a laser, and analyzing the scattered light. Like infrared spectroscopy, it is a technique allowing the observation of the vibrational states of the analyzed compound. Raman spectroscopy also allows the observation of rotational states. During a Raman spectroscopy study, it is important that the compound does not give rise to fluorescence since the spectral background becomes too high.

Figure 3 - 3 .

 33 The complete set up consists of four main parts: a) -formaldehyde generation system, b) -vapor phase water generation system, c) -catalytic reactor, d) -analytical system. The set-up was constructed using Swagelok 1/8-inch diameter stainless steel tubing (except for the catalytic reactor). The carrier gas used for catalytic test is synthetic air consisting of 20% oxygen by volume and 80% nitrogen by volume. The carrier gas used for HCHO termodesorption is He. The total gas flow rate in the catalytic test was set to 100 mL/min using a Bronkhorst mass flow meter.

Figure 3 - 3 :Chapter 3 :

 333 Figure 3 -3: Scheme of the set-up of formaldehyde catalytic oxidation a) Gaseous phase formaldehyde generation

Figure 3 - 4 :F(Eq. 9 )Chapter 3 :

 3493 Figure 3 -4: Dynacal permeation tubesThe concentration of HCHO in the carrier gas leaving the permeation chamber can be calculated

  ]out and [CO2]out were the outlet HCHO and CO2 concentrations and [HCHO]in the initial HCHO concentration (100 ppmv).

Figure 3 - 5 :

 35 Figure 3 -5: Experimental Set-Up

Figure 3 - 6 : 3 - 7 ,

 3637 Figure 3 -6: (a) MFC, (b) Testo 445 climate measurement device with temperature/humidity probe, (c) Pressure indicator

Figure 3 - 7 :Figure 3 - 8 :

 3738 Figure 3 -7: Photo of the plasma reactor with 10 electrodes pins

Figure 3 - 9 ,

 39 Figure 3-9, shows the electrical scheme of the discharge. The applied power in the reactor was measured via determination of the plasma voltage (Upl) and the discharge current. To determine Upl, a high voltage probe (Fluka 80 K-40, division ratio1/1000) was used to measure the total voltage Utot, and the discharge current I was determined by recording the voltage signal Um across a 100 Ω resistor Rm placed in series between the counter electrode and ground.

Chapter 3 :

 3 Material and methods

Figure 3 - 9 :Figure 3 - 10 ,Figure 3 - 10 :

 39310310 Figure 3 -9: Electrical scheme of the 10 pin to plate

Figure 3 - 11 :

 311 Figure 3 -11: (a) Bruker Vertex 70 FT-IR spectrometer, (b) Gas cell

Figure 3 - 1  10  4 .

 31104 Figure 3-11 (a), shows the Bruker Vertex 70 FT-IR spectrometer used. The following experimental conditions were used to measure both the background and sample spectrum. The latter measurements were recorded after steady-state condition was reached.

Figure 3 - 12 :Table 3 - 4 :Chapter 3 :Figure 3 - 13 :

 312343313 Figure 3 -12: Teledyne ozone monitor Model 465MTable 3 -4: Specifications of the Ozone monitor Measurement range 0-1000 ppm to 0-10000 ppm Sample flow rate 0.8 L/min Resolution 1 ppm Accuracy 1% of full scale

  ppm, O 2 = 20%, N 2 balance, GHSV = 60 L/(g cat .h)

Figure 4 - 1 :

 41 Figure 4 -1: XRD patterns of the as-synthesized samples FTIR-ATR spectra of the as-synthesized derived birnessites (Figure 4-2) are almost identical. The absorption bands at 3200-3300 cm -1 (H-O-H stretching mode) and 1620 cm -1 (H-

Figure 4 - 2 :Figure 4 - 3

 4243 Figure 4 -2: FTIR-ATR spectra of the as-synthesized samples

Figure 4 - 3 :

 43 Figure 4 -3: (a) N2 isotherms and (b) PDS of the as-synthesized samples

Figure 4 - 4 :

 44 Figure 4 -4: TEM of the fresh (a)-(b) Ce0.01Mn and (c)-(d) Ce0.1Mn catalysts.

XPS spectra of the as-synthesized samples are given in Figure 4 - 5 .

 45 The quantitative and qualitative results are shown in Tables 4-4 and 4-5, respectively. From the survey XPS spectra (Appendix A), Na + and K + are always detected in line with the adopted experimental redox procedure and their position are listed in Table 4-5. A slight enrichment of Ce is observed at the surface of the Cedoped samples and based on the shape of the Ce 4d XPS (Figure 4-5 a) envelope it is

Figure 4 - 5 :

 45 Figure 4 -5: XPS (a) Ce 4d, (b) Mn 3s core levels (c) O1s and (d) Mn 2p3/2 for the as-synthesized samples.

Figure 4 - 6 :Figure 4 - 7 and Table 4 - 7 )

 464747 Figure 4 -6: H2-TPR traces of the as-synthesized samples

12. 5 °Figure 4 - 7 :Figure 4 - 8 :Table 4 - 7 : 60 Chapter 4 :

 5474847604 Figure 4 -7: (a) TGA b) DTA and (c) DSC profiles for the as-synthesized samples

  -9a). The m/z = 17 and 18 signals characteristics for water confirm that the weight loss observed up to 220°C corresponds to adsorbed and interlayer water. Furthermore, as shown in Figure 4-9a this water release is continuous up to 400°C which is also the case for the signal m/z = 32 (Figure 4-9b)

Figure 4 - 9 :

 49 Figure 4 -9: Temporal evolution during TGA under helium (a) of the signal m/z =17, 18, 28 and 44 and (b) of the signal m/z =32 for Ce0.1Mn.

Chapter 4 :

 4 Cerium doped (K, Na) -birnessite like MnO2 for low temperature oxidation of formaldehyde: effect of calcination temperature 116 0.72 nm to 0.67 nm when calcining Mn-B and Ce0.01Mn at 400 °C (Figure4-10-b) in accordance with the interlayer water removal.

Figure 4 - 10 :

 410 Figure 4 -10: (a) XRD patterns for the calcined samples and (b) XRD zoom for first peak in the following analysis conditions: 10-20°, step size: 0.2°, step time: 5s

Figure 4 - 11 :

 411 Figure 4 -11: FTIR-ATR spectra of the calcined samples

Figure 4 - 12 :

 412 Figure 4 -12: (a) TGA, (b) DTA and (c) DSC profiles for the calcined Ce0.01Mn and Ce0.1Mn (d) dehydroxylation and O release for Ce0.01Mn and Ce0.1Mn

Figure 4 - 13 :

 413 Figure 4 -13: (a) N2 isotherms and (b) PDS of the calcined samples ICP-OES analysis of the calcined samples are summarized in Table 4-10. The results

Figure 4 -

 4 14 are significantly modified as compared to those of the as-synthesized samples. The global H2 consumption envelope is shift to lower temperature for Mn-B-400. The H2-TPR traces for the calcined

Figure 4 - 14 :

 414 Figure 4 -14: H2-TPR traces of the calcined samples Table 4 -11: H2-TPR of the calcined samples

Figure 4 - 15 :Chapter 4 :

 4154 Figure 4 -15: Raman spectra of Ce0.1Mn (a) ex situ as-synthesized and calcined at 200, 300 and 400 °C and (b) in-situ calcined at 400°C for 1h and for 3h

Figure 4 - 16 :

 416 Figure 4 -16: O2-TPD traces of the calcined samples at 400 °C

  13 and Figure4-17-a) except for the catalyst Mn-B-400. The enhancement of OII/OI atomic ratio of Mn-B after calcination is consistent with a marked increase in BET surface area.

Figure 4 - 17 :

 417 Figure 4 -17: XPS (a) O1s and (b) Mn 2p3/2 core levels of the calcined samples

  15).

Figure 4 -

 4 18 a and b shows positive ion ToF-SIMS spectra in the m/z 200-220 range for CexMn-400.

Figure 4 -

 4 18 c and d shows positive ion ToF-SIMS spectra in the range m/z 135-160 for CexMn-400. The Ce + and CeO + secondary

Figure 4 - 18 :

 418 Figure 4 -18: ToF-SIMS positive ion spectra (a) and (b) in the 200-220 m/z range and (c) and (d) n the range 135-160 mz of Ce0.01Mn-400 and Ce0.1Mn-400.

linearly for 5 h(Figure 4 -Figure 4 - 21 .Figure 4 - 20 :

 54421420 Figure 4 -20: Effect of RH on HCHO elimination and HCHO conversion over (a) Ce0.01Mn-400 and (b) Ce0.1Mn -400. T = 50 °C, HCHO = 100 ppm, O2 = 20%, N2 balance, GHSV = 60 L/(gcat.h).

Figure 4 - 21 :

 421 Figure 4 -21: Effect of RH on HCHO elimination and HCHO conversion over Hopcalite. T = 75 °C, HCHO = 100 ppm, O2 = 20%, N2 balance, GHSV = 60 L/(gcat.h).

  .1Mn-400. Experiments were conducted by saturating first the catalyst at 25 ⁰ C as shown in Figure 4-22 (a) , then the evolution of HCHO and CO2 were recorded as a function of temperature Figure 4-22 (b) . The amount of HCHO retained, desorbed and CO2 formed are summarized in

[ 64 ]

 64 over birnessite during HCHO-TPD. However, in the temperature range 25-180 ⁰C, CO2 concentration firstly rapidly increased, to a maximum at almost 100 ⁰ C, then decreased rapidly until temperature reach 180 ⁰ C, then CO2 concentration descended gradually to a low level.Zhang et al.[65], explained the CO2 behavior as follow: First the surface lattice oxygen of the catalyst participate in the oxidation of HCHO. After the surface oxygen is partly consumed, then the subsurface oxygen even the lattice oxygen gradually shift to the surface to participate in the oxidation. As the mobility of oxygen to shift from lattice to surface became lower, the rate of supplementary oxygen decreases which lead to a drop in the CO2 formation. This study indicates that the CO2 formed in HCHO-TPD experiments was mainly produced by the oxidation of some adsorbed HCHO or intermediates. The CO2 concentration in the temperature range is the highest with Ce0.1Mn-400 follow by Ce0.01Mn-400 and then Ce0.01Mn-300. However, it is well known that the quantity of HCHO retained, desorbed and CO2 released are proportional to the specific surface area of the material. As a results Ce0.1Mn-400 (SBET = 196 m 2 /g) show better catalytic activity compared to Ce0.01Mn-400 (SBET = 132 m 2 /g). Whereas by increasing the calcination temperature from 300 to 400 °C over the catalyst Ce0.01Mn, the mobility of oxygen at low temperature increases. In addition to get more accurate comparison about the mobility of catalyst oxygen in the oxidation of HCHO the evolution of n(O)/n(Mn+Ce) was calculated as a function of temperature (figure4-22 c and d, table 4

Figure 4 - 22 :

 422 Figure 4 -22: HCHO-TPD (a) HCHO adsorption as a function of time, (b) evolution of gaseous species (HCHO and CO2) and (c) evolution of the atomic ratio O/ (Mn+Ce) as a function of temperature in HCHO oxidation under helium of Ce0.1Mn-400, Ce0.01Mn-400 and Ce0.01Mn-300.

Figure 4 - 23 :

 423 Figure 4 -23: FTIR-ATR spectra of the fresh and used catalysts after stability tests According to the above results, the possible reaction pathway of HCHO over Ce doped birnessite is illustrated in Figure 4-24. Adsorbed HCHO would be oxidized into dioxymethylene (DOM), formate, carbonate and CO2 sequentially, via the participation of

Figure 4 - 24 :

 424 Figure 4 -24: Scheme of the possible reaction pathway of HCHO oxidation over Ce doped birnessite

Figure 5 - 1 :Figure 5 - 1 ,

 5151 Figure 5 -1: FT-IR spectrum of the plasma generated gaseous species. Dry air; F = 1L/min; ED = 150J/L Figure 5-1, shows a typical in situ IR spectrum of the plasma generated gaseous species in dry air. The presence of the IR bands located at 1042 and 2121 cm -1 evidences the presence of ozone. Furthermore, the generation of N2O, N2O5 as well as HNO3 is also observed consistent with the absorbance band positions. The characteristics band for HNO3, N2O5 and N2O was observed at 743, 1246, 1720 and 887, 1326 and 2223 cm -1 respectively.

  NO2 can be further oxidized into NO3 by O3 and the retrodismution between NO2 and NO3 can afford N2O5. The residual H2O present in the gas carrier contributes to the formation of HNO3. The reaction scheme depicting the formation of N2O5 and HNO3 is given in Figure 5-2 in accordance with the work of C. Sun et al. [8].

Figure 5 - 2 :

 52 Figure 5 -2: Scheme of N2O5 and HNO3 production via NOx oxidation by O3

Figure 5 - 3 :

 53 Figure 5 -3: Ozone concentration as a function of ED. Dry air; Q = 1 L/min

Figure 5 - 4 ,Figure 5 - 4 :

 5454 Figure 5-4, shows the evolution of the IX/I250 intensity ratio (where IX and I250 are the intensities of selected bands for ED of a given X value and ED for 250 J/L) for each nitrogen containing species as a function of ED. A linear N2O production increase with ED is observed whereas the intensity ratio traces relative to N2O5 and HNO3 productions show a maximum at 200 J/L.The ozone concentration increases linearly at low ED. The downward departure from linearity at high ED may be due to competing reactions (nitrogen oxides production) affording an ozone concentration less than that expected.

Figure 5 - 5 ,Chapter 5 : 148 Figure 5 - 5 :Figure 5 - 6 ,Figure 5 - 6 :Figure 5 - 7 :

 55514855565657 Figure 5 -5: Evolution of [O3] as a function of ED. Dry air; Q = 0.5, 1 and 1.5 L/minFigure 5 -6, shows the rate of ozone production rO3 as a function of the flow rate. It is found an increase of rO3 by a factor of 1.5 with a threefold increase of Q. Hence a decrease of the residence time has a beneficial effect on O3 production which is all the more important that ED is important.

Figure 5 - 8 :Figure 5 - 9 :

 5859 Figure 5 -8: Evolution of [O3] as a function of ED in dry air and humid air; Q = 0.5 L/min

Figure 6 - 3 :Figure 6 - 4 :

 6364 Figure 6 -3: (a) N2 adsorption-desorption isotherms and (b) pore size distribution of the fresh acid-treated samplesThe reducibility of the samples has been evaluated by H2-TPR (Figure6-4). The H2-TPR traces of the acid-treated CexMn oxides (x = 0-0.2) samples can be divided into 4 components α, β, γ and δ. While the α consumption is linked to the removal of surface oxygen species, the other components are related to the near surface and bulk oxygen removal. As compared to the undoped sample, the addition of Ce decreases the onset reduction temperature, increases the relative β consumption while the γ and δ peaks move to higher temperature. These results indicate that the reducibility of the samples is enhanced with a low cerium doping which might be related from the formation of more surface oxygen vacancies[1,4,7,[20][21][22].

Figure 6 - 11 :

 611 Figure 6 -11: Temporal evolution of the gas phase species for O3 decomposition in air (RH~0.7 %) over (a) Ce0.01Mn, (b) Ce0.01Mn-AT

Figure 6 - 12 :

 612 Figure 6 -12: Temporal evolution of the gas phase species for O3 decomposition in moist air (RH~30 %) over Ce0.01Mn-AT

  -14). Besides, in the presence of significant amount of water (RH = 30 %) (N/Mn)c drops strongly to reach 0.01 and 0.02 for Ce0.01M-O3-H and Ce0.01Mn-O3-H, respectively. Additionally, the N 1s contribution located at 403. 0 eV disappears and a new N 1s component emerges at 401.0 eV and 402.3 eV for Ce/Mn of 0.01 and 0.1, respectively as seen in Figure 6-13 (b) and (c).

Figure 6 - 14 :

 614  Correlation between O3 conversion and poisoning of active sites by N adspecies for the catalysts treated with acid

5 FTIR

 5 -ATR has been used for detecting surface nitrogen containing species. Tentative band assignments of FTIR-ATR spectra have been performed based on XPS results showing that most nitrogen containing species on CexMn catalysts are nitrates. In addition to free nitrate ion, NO3 -can be coordinated in different ways (monodentate, bidentate chelating and/or bridging)[34]. The FTIR-ATR spectra of the different series of catalysts recorded between 700 cm -1 and 1820 cm -1 are displayed in Figure6-15.

Figure 6 - 15 :

 615 Figure 6 -15: FTIR-ATR spectra of the tested catalysts (a): at RH = 0.7 % ; (b) at RH = 30 % and (c) reference FTIR-ATR spectra of metal alkali nitrates ; (d) FTIR-ATR spectra of Ce0.01Mn-AT-03-H30 and after heating at 200 °C for 4 h.

Figure 6 -

 6 15 (d)compares the FTIR-ATR spectrum of Ce0.2Mn-O3 with that obtained after heating the sample at 200 °C for 4h in an oven. It is observed that the prominent IR bands at 1390 cm -1 and 1323 cm -1 decrease strongly in intensity as compared to the other ones. This indicates clearly that the ionic nitrates interact in a less manner than the nitrato ligands.ToF-SIMS in static mode has been applied to get valuable insight on the outermost surface. In the negative ToF-SIMS spectra, the detection of secondary ions NO3 -(m/z = 62), NO2 -(m/z = 46) and NO -(m/z = 30) reveals the presence of adsorbed nitrate/nitrite species at the surface of all samples. Observance of HNO3 -at m/z = 63 indicates that nitric acid adsorbed also molecularly to some extent. Additionally, a series of MNOx -(M = Mn, Ce) secondary ions have been observed (see Table6-7) which provided direct evidence of interactions between nitrogen containing species and cerium and manganese cations on the catalyst surface.

Figure 6 - 16 : 6 - 3 - 5 -

 616635 Figure 6 -16: Temporal evolution during TPD (a) of the signal m/z =30 and (b) of the signal m/z =32 for Mn-B-AT-O3, Ce0.01Mn-AT-O3 and Ce0.1Mn-AT-O3

Figure 6 - 17 :

 617 Figure 6 -17: Correlation between O3 conversion and Tonset for the acid treated catalysts

chapter 5 in

 5 order to optimize the initial molar ratio [O3]/[TCE] = 4. (ii) the most active catalyst for HCHO oxidation (Chapter 4) and for O3 decomposition (Chapter 6) has been choosen for TCE abatement.

Figure 7 - 2 :

 72 Figure 7 -2: (a) N2 adsorption desorption (b) pore size distribution of Ce0.01Mn-400 and Ce0.01Mn-AT samples

Figure 7 - 3 .Figure 7 - 4 :

 7374 Figure 7 -4: XPS Mn2p3/2 core level decomposition for Ce0.01Mn-400 and Ce0.01Mn-AT samples

Figure 7 - 6 :

 76 Figure 7 -6: COx, CO and CO2 yields obtained in NTP TCE abatement with RH of 0.7 % and 15 % (ED = 150 J/L; Q= 1 L/min; [TCE]0 = 150 ppmv)

Figure 7 - 7 :

 77 Figure 7 -7: FT-IR spectra in the range 2500-1700cm -1 and 1110-700cm -1 of outlet stream after NTP in 0.7 % and 15 % as relative humidity (ED = 150 J/L, Q= 1 L/min) (PG : phosgene; DCAC : dichloroacetylchloride and TCAD : trichloroacetaldehyde)

Figure 7 Figure 7 - 9 :Figure 7 - 10 :

 779710 Figure 7 -9: FT-IR spectra of the inlet TCE and outlet gaseous stream after PPC process (RH = 0.7 %) as a function of time with Ce0.01Mn-400 as catalyst (PG : phosgene; DCAC : dichloroacetylchloride ;TCAD : trichloroacetaldehyde)

Figure 7 -

 7 11 shows the COx and CO2 yields obtained as a function of time in humid air (RH = 15 %). In comparison with RH = 0.7% condition, the yields are lower but rather stable during the 5 hours of experiment.

Chapter 7 :Figure 7 - 11 :

 7711 Figure 7 -11: COx and CO2 yields obtained for TCE abatement (RH = 15%) in PPC configuration with Ce0.01Mn-AT as catalyst

Figure 7 - 2 Figure 7 - 12 :

 72712 Figure 7-12 presents the FT-IR spectra of the inlet TCE and outlet gaseous stream after PPC process (RH = 15 %) as a function of time with Ce0.01Mn-AT as catalyst. As previously observed with RH = 0.7 %, neither bands for TCE nor ozone are observed after PPC process.DCAC and TCAD bands are observed in contrast to what we observed in PPC process with RH = 0.7 %. However, it is usually reported that (i) the DCAC formation comes from the reaction of TCE with ClO radicals and (ii) in the presence of water, DCAC formation is suppressed because of the quenching of ClO radicals by OH radicals according to the following reaction: ClO + OH  HCl+O2. This discrepancy could be explained by the presence of ozone in higher quantity in our experimental conditions in comparison with that found in the literature. In our experimental conditions, OH radicals could react with O3 in sufficient quantity instead of ClO radicals. This result could also explain the lowest COx yield value owing to O3 consumption by OH radicals.

Chapter 7 :Figure 7 - 13 :

 7713 Figure 7 -13: XPS (a) Mn2p3/2 and (b) Cl2p core levels decomposition after PPC process

Four important conclusions were made from this work: ( 1 )

 1 Regardless of Ce content, calcinedCexMn exhibited higher catalytic performance than calcined parent birnessite. (2) Low Ce doping ratio are beneficial for formaldehyde oxidation. (3) The catalytic activity increases as increasing the calcination temperature. These superior performance of CexMn calcined at 400 °C was attributed to improved textural properties such as high surface areas, low crystallinity and structural disorder (presence of oxygen vacancies) and to its high reducibility. Additionally, the overall characterization (H2-TPR, TEM, ToF-SIMS), can ensure the interaction between Ce and Mn even at low doping ratio which improves the content of oxygen vacancies and enhanceOutlookLast but not least, this work regarding plasma-catalytic configuration for VOCs abatement shows that there are still scientific challenges to overcome. In terms of catalytic formulation further improvement of the energy efficiency could be realized by operating catalyst at room temperature which would exclude catalyst heating and very selective towards CO2. The good results obtained with Ce modified birnessite treated with nitric acid open up new frontiers for further research. It would be very interesting to investigate for example the effect of palladium on the performances on these material in terms of TCE abatement. Recently,

Table 1 -1: Physical

 1 and Chemical properties of TCE .........................................................................................

Table 1 -2: Summarizing

 1 table of the different catalysts used in PPC for TCE abatement with the different plasma conditions ...................................................................................................................................................

Table 1 -3: Summary

 1 table of the different synthesis procedure of birnessite, K/Mn and Na/Mn ratios reported in literature .............................................................................................................................................................

Table 3 -1: The

 3 required amount of chemicals for the synthesis of Mn-B ...........................................................

Table 3 -2: Weight

 3 of potassium permanganate and cerium nitrate required for each Ce/Mn ratio. ....................

Table 3 -3: Analytical

 3 conditions in gas chromatography analysis ......................................................................

Table 3 -4:

 3 Specifications of the Ozone monitor ..................................................................................................

Table 4 -1: Experimental

 4 conditions ..................................................................................................................

Table 4 -2: Textural

 4 properties of the as-synthesized samples ...........................................................................

Table 4 -3: ICP

 4 analysis of the as-synthesized samples .....................................................................................

Table 4 -4: Quantified

 4 results by obtained XPS analysis ...................................................................................

Table 4 -5: Binding

 4 energy (eV) determined by XPS analysis ..........................................................................

Table 4 -6 : H2

 4 -TPR of the as-synthesized samples ............................................................................................

Table 4 -7: Weight

 4 loss as a function of temperature for the as-synthesized samples .......................................

Table 4 -8: Weight

 4 loss as a function of temperature for the calcined samples..................................................

Table 4 -9: Textural

 4 properties of the calcined samples .....................................................................................

Table 4 -10: ICP

 4 analysis of the calcined samples .............................................................................................

Table 4 -11: H2-TPR of the calcined samples

 4 ....................................................................................................

Table 4 -12: O2-TPD quantitative analysis of the calcined samples at

 4 400°C ....................................................

Table 4 -13: Quantified results by XPS analysis for the calcined samples

 4 ........................................................

Table 4 -14: Binding energy (eV) determined by XPS analysis for the calcined samples

 4 .................................

Table 4 -15: ToF

 4 -SIMS results ...........................................................................................................................

Table 4 -16: Secondary

 4 ions of the positive and negative ToF-SIMS of Ce0.1Mn-400 ......................................

Table 4 -17: Catalytic

 4 results .............................................................................................................................

Table 4 -18: Overview

 4 study for HCHO catalytic oxidation ..............................................................................

Table 4 -19: Adsorption

 4 and thermo-desorption of formaldehyde results ......................................................

Table 6 -1: Experimental

 6 conditions ..............................................................................................................

Table 6 -2: Chemical

 6 composition, textural and redox roperties of the fresh acid-treated samples ...............

Table 6 -3: Water

 6 loss between 25 °C and 100 °C determined from TGA measurements expressed in mg/gcat .............................................................................................................................................................

Table 6 -4: XPS

 6 reszults of the fresh acid-treated samples ............................................................................

Table 6 -5: ToF

 6 -SIMS results of the Fresh acid-treated samples ...................................................................

Table 6 -6: N1s

 6 componenets and N/Mn XPS ratios for the treated catalysts ................................................

Table 6 -7: Nitrogen

 6 containing secondary ions from ToF-SIMS spectra in polarity (-) of the tested catalysts ............................................................................................................................................................

Table 6 -8: ToF

 6 -SIMS data in positive polarity for Ce0.1Mn before and after ozone decomposition .............

Table 7 -1: Experimental

 7 conditions ..............................................................................................................

Table 7 -2: Weight

 7 composition and atomic ratio for Ce0.01Mn-400 and Ce0.01Mn-AT samples ...................

Table 7 -3: Textural

 7 properties of the Ce0.01Mn-400 and Ce0.01Mn-AT samples ............................................

Table 7 -4: Semi

 7 quantitative XPS analysis for used (PPC) catalysts ................................................................

	Conclusion

Table 1 :

 1 Summary of the important results obtained in this work...................................................

	Chapter 1:
	LITERATURE OVERVIEW

Table 1 - 1 :

 11 Physical and Chemical properties of TCE

	PROPERTY	VALUE
	Chemical formula	C2HCl3
	CAS number	79-01-6
	Molecular weight	131.39 g mol -1
	Density	1.46 g cm -3 (20°C)
	Boiling point	87.2°C
	Melting point	-73.0°C
	Vapor exposure	7.6 kPa (20°C)
	Auto ignition point	410 °C
	Solubility in water	1.28 gL -1

Table 1 -

 1

	Catalyst	NTP (pin-to-plate)*	m cata /g	[TCE] / μmol/min (ppm)	Q /L/h	T /°C	RH /%	ED /J/L	GHSV /L/g.h	Time/ min	RE /%	/ % Y COx	% CO2 / Y	ref
	MnO2	5	1	10 (600)	30	300	0	240	30	30	90	54	22	[9]
	LaMnO3	5	1	41(600)	120	150	18	460	120	30	93	75	29	[54]
	CeMnO4	5	1	30 (400)	120	150	18	240	120	30	97	57	30	[55]
	Fe-K-OMS	10	0.5	6 (300)	30	150	15	120	60	15	83	83	62	[56]
	CuMn2O4-P4	10	0.5	6 (300)	30	150	15	120	60	60	65	65	50	[57]

2: Summarizing table of the different catalysts used in PPC for TCE abatement with the different plasma conditions *: Number of pins RE: Removal efficiency

Table 1 - 3 :

 13 Summary table of the different synthesis procedure of birnessite, K/Mn and Na/Mn ratios reported in literature

	Oxydant/reductant (molar ratio)	Medium Washing	aging	K/Mn Na/Mn Ref
	KMnO4 / HCl (1/2)	Acid	water	10 min	0.20	[147]
	KMnO4 / HCl (0.2/0.36)	Acid	water	12h	0.17	[148]
	KMnO4 / NH2OH.HCl (0.02/0.006)	Acid	water		0.14	[149]
	KMnO4 / HCl (0.20/0.27)	Acid	water	overnight	0.15	[150]
	KMnO4 / HCl (0.20/0.27)	Acid	water	overnight	0.20	[151]
	KMnO4 / HCl (0.20/0.27)	Acid	water	overnight	0.22	[152]
	KMnO4 / C2H2O4 (0.001/0.0015)	Acid	Water	30 min	0.14	[153]
	KMnO4 / C2H2O4 (2/3)	Acid		30 min	0.39	[154]
	NaMnO4 / HNO3 (100/1)	Acid		4 days /170°C	0.69	[155]
	NaMnO4 / Ethanol (excess of NaMnO4)	NaOH		40 days	0.33	[140]
	NaMnO4 / Ethanol (excess of ethanol)	NaOH		40 days	0.33	[140]
	KMnO4 / Ethanol (excess of NaMnO4)	KOH		40 days	0.30	[140]
	KMnO4 / Ethanol ( excess of ethanol)	KOH		40 days	0.32	[140]

Table 3 -

 3

1, presents the required amount for the synthesis of Mn-B in excess of potassium permanganate conditions.

Table 3 -1: The

 3 For the preparation of Ce doped MnO2 the amount of sodium lactate remains the same regardless of x (Ce/Mn) value, because only the Ce/Mn ratio change and the other experimental conditions are the same. However, Table3-2, presents the amount of potassium permanganate and cerium nitrate used for the preparation of CexMn samples.

		required amount of chemicals
	for the synthesis of Mn-B
		KMnO4	NaC3H5O3
	n (mol)	2.53 x 10 -2	2.254 x 10 -2
	M (g/mol)	158.03	112.06
	ρ ( Kg/L)		1.33 x 10 3
	C (mol/L)	0.0633	5.635
	m req (g)	4
	Vreq (mL)		4±0.01
	m req: Required weight
	Vreq: Required volume

Table 3 - 2 :

 32 Quantity of potassium permanganate and cerium nitrate required for each CexMn preparation.

	Name	n (Ce/Mn)	n (Mn) (mol)	n (Ce) (mol)	m (KMnO4) (g)	m (Ce(NO3)3.6H2O) (g)
	Ce 0.01 Mn	0.01	0.0247	3.10 -4	3.9033	0.1074
	Ce 0.1 Mn	0.1	0.0227	2.27.10 -3	3.5872	0.9868
	Ce 0.2 Mn	0.2	0.0208	4.2.10 -3	3.2922	1.8237
	Ce 0.5 Mn	0.5	0.0167	8.3.10 -3	2.6338	3.6041

Table 3 - 3 :

 33 Analytical conditions in gas chromatography analysis

		Module 1	Module 2
	Sampling time	30 s	30 s
	Analysis time	10 min	10 min
	Inlet bottom gas pressure	5.5 bars	5.5 bars
	Column pressure	140 kPa	200 kPa
	Column temperature	30°C	80°C

Table 4 -

 4 1, summarizes the basic experimental conditions used in this chapter. The detailed experimental conditions can be seen in chapter 3.

Table 4 -

 4

		1: Experimental conditions
	Gas source	Air liquid, or Helium, Alphagaz 1
	Flow rate	0.1 L/min
	Relative humidity (RH)	0 % / 50 %/ 75 %
	Initial HCHO concentration	100 ppmv
		Produced from para-HCHO tube incorporated
		in a permeation chamber
	Catalyst	CexMn-T
		0.1 g of catalyst
	Activation conditions	100 mL/min, 200 °C for 1 hour
	Catalyst temperature	Decreases from 200 °C to RT

Table 4 - 2 :

 42 Textural properties of the as-synthesized samples

	Catalyst	SBET (m 2 /g)	Dp max (nm)	Vp tot (cm 3 /g)
	Mn-B	44	4; 22	0.12
	Ce0.01Mn	127	4.3; 6.6; 37	0.32
	Ce0.1Mn	243	3; 4	0.23

Table 4 -

 4

3: ICP analysis of the as-synthesized samples

Table 4 -5).

 4

	Catalyst	K	Weight % Na Ce	Mn	Atomic % K/Mn Na/Mn Ce/Mn
	Mn-B	7.12 1.87	-	44.1	0.39	0.059	-
	Ce0.01Mn	6.64 2.08 1.11	46.4	0.34	0.063	0.0094
	Ce0.1Mn	6.27 1.90 9.47	38.3	0.39	0.069	0.097

Table 4 - 4 :

 44 Quantified results observed by XPS analysis

Table 4 - 5 :

 45 Binding energy (eV) determined by XPS analysis

3/2 Mn 3+

								Mn 4+
		Catalyst	O/Mn	K/Mn Na/Mn Ce/Mn	Mn 3+ /Mn 4+	OII/OI
		Mn-B	2.24		0.28	0.13	-	0.24	0.32
		Ce0.01Mn	2.13		0.24	0.12	0.016	0.18	0.48
		Ce0.1Mn	2.49		0.21	0.12	0.13	0.17	0.59
	Catalyst	OI	OII		OIII	K 2p3/2 (FHWM) K 2p1/2 (FHWM)	Na 1s	ΔE Mn3s (AOS)
	Mn-B	529.7	531.4	533.4	292.5 (1.35)		295.4 (1.39)	1070.7	4.81 (3.52)
	Ce0.01Mn	529.7	531.1	533.4	292.5 (1.43)		295.3 (1.62)	1070.8	4.85 (3.49)
	Ce0.1Mn	529.6	531.2	533.4	292.6 (1.46)		295.4 (1.52)	1070.9	4.88 (3.46)

Temperature peak (°C) n (H2) (mmol/gcat) α peak β peak γ peak δ peak λ peak

	Mn-B	250	299	388	477	516		7.55
	Ce0.01Mn	150	237	300	330	362	433	7.03
	Ce0.1Mn	170	241	320	364	420		4.93

Table 4 - 8 :

 48 Weight loss as a function of temperature for the calcined samples

	Tt*: Temperature of phase transformation

Table 4 - 9 :

 49 Textural properties of the calcined samples

	Catalyst	Total loss (%)	25-220°C (100-220°C) (%)	220°C-Tt * (%)	Weight gain (%)
	Ce0.01Mn-300	11.0	9.0 (5.3)		2.3 (525)	0.30
	Ce0.01Mn-400	7.2	5.4 (4.0)		1.9 (554)	0.12
	Ce0.1Mn-200	10.6	5.2 (3.9)		5.6 (676)	0.23
	Ce0.1Mn-300	7.8	4.3 (2.8)		3.9 (670)	0.49
	Ce0.1Mn-400	7.4	5 (3.1)		2.9 (674)	0.52
		Catalyst	SBET (m 2 /g)	Dp max (nm)	Vp tot (cm 3 /g)
		Mn-B-400	148	4; 6.7; 30	0.29
		Ce0.01Mn-300	165	4; 5	0.22
		Ce0.01Mn-400	132	4; 5	0.22
		Ce0.1Mn-200	213	4	0.20
		Ce0.1Mn-300	256	5	0.30
		Ce0.1Mn-400	196	6	0.25

Table 4 - 10 :

 410 ICP analysis of the calcined samples

Table 4 - 12 :

 412 O2-TPD quantitative analysis of the calcined samples at 400°C

	Catalyst	Tonset * (°C)	n (O2) (mmol/gcat)	n (O/Ce+Mn)
	Mn-B-400	174	1.70	0.32
	Ce0.01Mn-400	154	2.20	0.49
	Ce0.1Mn-400	158	2.11	0.52

Table 4 - 13 :

 413 Quantified results by XPS analysis for the calcined samples

Table 4 - 14 :

 414 Binding energy (eV) determined by XPS analysis for the calcined samples

Table 4 -15: ToF-SIMS results Catalyst Ce + /Mn + CeO + /Ce + CeOMn + /(Ce + + Mn + ) ×10 3

 4

	/counts	K 2 Mn 2 O +	Mn 3 O 3 +
	Intensity	CeMnOH + CeMnO +	Mn 3 O 3 H +
		202 204 206 208 210 212 214 216 218 220
		m/z
		Ce0.01Mn		0.002	5.995	0.867
		Ce0.01Mn-400	0.023	0.367	1.432
		Ce0.1Mn		0.015	13.191	7.087
		Ce0.1Mn-200	0.057	3.069	10.321
		Ce0.1Mn-300	0.029	4.750	9.262
		Ce0.1Mn-400	0.042	3.359	10.889

Table 4 -17: Catalytic results

 4

	Catalyst	T10 (°C)	T50 (°C)	T90 (°C)	Ea (kJ/mol)
	Mn-B-400	69	101	124	61
	Ce0.01Mn-300	-	56	80	32
	Ce0.01Mn-400	-	50	73	22
	Ce0.1Mn-200	37	78	107	33
	Ce0.1Mn-300	25	68	96	28
	Ce0.1Mn-400	-	56	90	26
	Hopcalite-300	66	89	118	130

Table 4 - 18 :

 418 Overview study for HCHO catalytic oxidation

	Catalyst	Test conditions	T 50	T 90	Ref.

Table 4 -

 4 19.The HCHO retained over Ce0.1Mn-400 is the highest, while comparing Ce0.01Mn calcined at 300 and 400 is the same. Irrespective of the calcination temperature and Ce content, HCHO desorption occurs between 30 ⁰C and 100 ⁰C with a maximum value around 35 ⁰C. This value is lower than 50 ⁰C obtained bySelvakumar et al.

Table 4 - 19 :

 419 Adsorption and thermo-desorption of formaldehyde results

	Catalyst	n (HCHO)retained (µmol/g)	n (HCHO) desorbed (µmol/g)	n (CO2) formed (µmol/g)	nO/n(Mn+Ce)
	Ce0.01Mn-300	97.34	18.5	83.1	0.18
	Ce0.01Mn-400	97.7	17.9	71.3	0.15
	Ce0.1Mn-400	131.7	23	98	0.25

  )

						O 3 (2121)
					HNO 3	HNO 3
					(1326)	(887)
	Absorbance /a.u.		(2121) O 3 N 2 O (2223)	N 2 O 5 (1720)	N 2 O 5 (1246)	N 2 O 5 (743)
	3000	2600	2200	1800	1400	1000	600
			Wavenumber/ cm -1

Chapter 5 :

 5 Ozone and nitrogen oxide formation through 10 Pin-to plate negative DC corona discharge

		Chapter 6:
	ACID TREATED Ce MODIFIED BIRNESSITE-TYPE MnO2
	FOR OZONE DECOMPOSITION AT LOW TEMPERATURE:
	EFFECT OF NITROGEN CONTAINING CO-POLLUTANTS
		AND WATER
	0 3			0 2
		Catalytic
		decomposition
	100%	94%	94%	96%
	19%	19%
	+NO x	+acidic treatment		Effect
	Part of this chapter has been published as an article in the following International journal:

Grêce Abdallah, Rim Bitar, Savita Kaliya Perumal Veerapandian, Jean-Marc Giraudon, Nathalie De Geyter, Rino Morent, Jean-François Lamonier," Acid treated Ce modified birnessite-type MnO2 for ozone decomposition at low temperature: effect of nitrogen containing co-pollutants and water", Applied Surface Science, 2021, 151240.

O 3 conversion +H 2 O Ce doped birnessite type MnO 2

Table 6 - 6 :

 66 N1s components and N/Mn XPS ratios for the treated catalysts

	Catalyst	N (1)	Position (eV) N (2)	N (3)	N (1)	% atom N (2)	N (3)	N/Mn (N/Mn)c
	Mn-B-O3	399.8	403.3	406.9	21.3	15.5	63.1	0.08	-
	Ce0.01Mn-O3	399.5	403.2	406.7	16.7	12.7	70.6	0.11	-
	Ce0.1Mn-O3	399.8	403.3	406.8	12.9	14.1	73.0	0.10	-
	Ce0.2Mn-O3	399.7	403.3	406.9	2.2	14.3	83.4	0.16	-
	Ce0.5Mn-O3	399.7	403.4	407.0	12.5	18.1	69.4	0.24	-
	Mn-B-AT-O3	399.5	401.5	406.8	18.7	18.1	63.2	0.12	0.09
	Ce0.01Mn-AT-O3	399.5	403.0	406.8	26.3	6.3	67.4	0.09	0.04
	Ce0.01Mn-AT-O3-H	399.3	401.0	406.8	31.1	38.7	30.2	0.06	0.01
	Ce0.1Mn-AT-O3	399.7	403.0	407.0	54.2	25.3	20.5	0.15	0.09
	Ce0.1Mn-AT-O3-H	399.5	402.3	406.8	43.6	20.5	35.8	0.08	0.02
	Ce0.2Mn-AT-O3	399.7	403.0	407.0	22.5	11.0	66.5	0.16	0.11
	Ce0.5Mn-AT-O3	399.6	403.4	407.0	17.9	14.3	67.7	0.32	0.25

Table 7 - 3 :

 73 Textural properties of the Ce0.01Mn-400 and Ce0.01Mn-AT samples

Sample SBET / m 2 /g Dp max/ nm Vp tot / cm 3 /g

	Ce0.01Mn-400	132	4.0	0.22
	Ce0.01Mn-AT	385	5.3	0.37

Table 7 - 4 :

 74 Semi quantitative XPS analysis for used (PPC) catalysts

	a.u.
	Intensity/
	650	645	640	635

Binding energy/ eV Ce 0.01 Mn-400- PPC Ce 0.01 Mn-AT- PPC Ce 0.01 Mn-AT- PPC-15%

	(a)	Mn 2p 3/2
			a.u.
			Intensity/
		Mn 4+	Mn 3+
			210	206	202	198	194

Binding energy/ eV Ce 0.01 Mn-400- PPC Ce 0.01 Mn-AT- PPC Ce 0.01 Mn-AT- PPC-15% Cl 2p

		(b)	3/2
			1/2
			organic	Mineral
	Catalyst	O/Mn K/

Mn Na/Mn Ce/Mn Cl/Mn Mn 3+ /Mn 4+

								Mn AOS
								from Mn 3s
								splitting
	Ce0.01Mn-400	2.07	0.18	0.05	0.015		0.34	3.44
	Ce0.01Mn-400-PPC	2.34	0.26	0.05	0.024	0.18	0.21	3.56
	Ce0.01Mn-AT	1.97	0.06	-	0.018		0.38	3.32
	Ce0.01Mn-AT-PPC	1.91	0.04	-	0.028	0.16	0.29	3.42
	Ce0.01Mn-AT-PPC-15%	1.81	0.04	-	0.025	0.22	0.28	3.41

List of Publications

MATERIAL AND METHODS

Chapter 3: Material and methods Throughout this chapter, the experimental part concerning the abatement of trichloroethylene (TCE) in Post plasma catalysis process is presented in details. However, the use of non-thermal plasma as ozoner in combination with catalyst to decompose ozone in order to improve the VOCs decomposition in the hybrid system plasma catalysis is also described.

The way in which the experiments of formaldehyde (HCHO) catalytic oxidation will be conducted is also explained in this chapter. Thus this chapter is devised into 5 parts.

The first part consists to the description of the preparation of the catalysts and the second part to the presentation of the different techniques used for the characterization of the materials. Synthesis and characterization were conducted at Lille University.

The third part is dedicated to the catalytic oxidation of formaldehyde. All experiments described in this section were conducted at Lille University.

The fourth part, concerns the use of plasma as ozoner, and the catalytic decomposition of ozone. All experiments described in this section were conducted at Ghent University.

The fifth part consists to use the Post-Plasma-Catalysis process for the abatement of trichloroethylene. All the experiments described in this section were conducted at Ghent University.

3-1-Catalyst preparation

3-1-1-Synthesis of birnessite and cerium doped birnessite through a redox precipitation method.

Birnessite was prepared following a redox method inspired from that of Hӓndel et al. [1].

Typically, 4.0 g (25.3 mmol) of KMnO4 (Fluka, ≥ 99 %) was dissolved into 400 mL of distilled water under stirring (350 rpm). 4 mL (22.54 mmol) of NaC3H5O3 (50 % w/w) (Fisher Chemical, solution 60% w/w) was added dropwise to the aqueous KMnO4 solution. Thus the synthesis was performed with a ratio KMnO4/ NaC3H5O3 = 1.2. The resulting brown suspension was stirred for 2 h. After centrifugation (4000 rpm for 20 min) the collected brown precipitate was washed 2 times (25 mL of distilled water each time) and finally dried in an oven at 40 °C for

4-3-3-Catalytic activity

Figure 4-19a shows the HCHO conversion into CO2 as a function of temperature over the catalysts calcined at 400 °C and commercial Hopcalite at a HCHO concentration of 100 ppmv and a GHSV of 60 L/(gcat.h). T10, T50 and T90 (corresponding to 10 %, 50 % and 90 % HCHO conversion) summarized in Table 4-17 have been used to compare the catalyst activity.

As expected, the catalytic activity for HCHO oxidation using Ce-doped samples is better than that of the parent birnessite and even better than that of the commercial Hopcalite. Ce0.01Mn-400 exhibits the best catalytic activity whith T50 and T90 of 50 °C and 73 °C, respectively (Figure 45678910111213141516171819). The T50 of 50 °C is close to values reported in literature for the most active MnO2 based catalysts as shown in Table 4-18 [3,11,56,[59][60][61][62][63]. The effect of Tc on activity has also been investigated on the Ce-doped catalysts (Figure 4-19 b andc). Irrespective of the Ce/Mn ratio, the catalytic activity increases with increasing Tc until 400°C. Further increases of Tc lead to reduce the catalytic activity. In addition, to get a more accurate comparison, the apparent activation energy (Ea) has also been calculated according to the Arrhenius plot of the catalysts for HCHO oxidation (Figure 45678910111213141516171819). Ce0.01Mn-400 exhibits an Ea value of 22 kJ.mol - 1 which is less than the value of 27 kJ.mol -1 reported for the very active K-birnessite-type MnO2 with moderate K + content [61].

6-2-Experimental conditions

6-3-Results and discussion

6-3-1-Characterization studies of the acid-treated catalysts

The X-ray diffraction pattern of Mn-B-AT which exhibits three broad peaks of low intensity centered at 12.2°, 37.0 ° and 66.2 ° (Figure 6-1) can be consistent with the formation of a related disordered birnessite [13]. These results suggest the formation of a layer structure with coexisting amorphous state domains. Besides, the diffraction peaks become even less resolved as cerium is added. At the highest doping ratio of 0.5 only a very broad peak of low intensity at about 30 ° is observed due to cubic CeO2 phase (JCPDS No.00-034-0394) which implies an extensive destruction of the crystallized layer structure. The results of elemental analysis performed by ICP-EOS are given in Table 6-2. For Mn-B-AT it is observed a K/Mn atomic ratio of 0.065 consistent with the value of 0.05 obtained by Liu et al. [4] for an acid-treated birnessite MnO2 which is substantially lower than for a conventional birnessite ( 0.26). The loss of K + is consistent with K + /H + exchange at the interlayer position as already pointed out [2]. While this K/Mn ratio is poorly affected by a low loading of cerium, it markedly decreases with increasing cerium nominal content. This can be explained by an increase amount of leached K + resulting from partial substitution of K + by H + and/or Ce 4+ . Besides, the Ce/Mn ratio is consistent with the nominal one for the lowest doping ratio while a two-fold decrease is observed for higher doping ratio (Ce/Mn: 0.1, 0.2). Such higher release of cerium could be correlated with different speciations of Ce in the material. The HRTEM images of the Ce0.1Mn-AT sample are given in is assigned to pyridine adsorbed on both L-sites and B-sites. The two remaining bands at 1573 cm -1 and 1471 cm -1 have been ascribed to pyridine physisorption (P) [21,27]. B acid sites can originate from acidic hydroxyl groups attached to metal atoms (Mn, Ce) while L acid sites are regarded as empty orbitals of metal ions in the oxides (Mn 4+/3+ ; Ce 4+/3+ ). From Figure 6-5, it can be shown that no acidity can be detected on the pristine samples. In contrast, L and B sites can be observed on the acid treated samples. The L sites dominated over the B sites when cerium content increases in the sample. As discussed previously, a low doping amount of Ce enhances the surface area throughout an increase of the number of defects and especially of the number of oxygen vacancies. However, some oxygen vacancies may be occupied by water or its dissociative species like hydroxyl groups, which act as Brønsted acid sites. In order to investigate the effect of acid treatment on the hydrophobicity of the acidtreated samples the amount of adsorbed water per BET specific surface area has been determined from TGA measurements (Table 6-3) and has been compared to those obtained on the fresh non-treated catalysts as seen in Based on the results, it is shown that the acid-treated catalysts are more resistant to water than the non-treated counterparts in agreement with previous results [15]. precludes a direct correlation between these two ratios. However the high simultaneous Mn 3+ /Mn 4+ ratios, whose values increase with Ce content and high adsorbed oxygen concentration, correspond to high content of oxygen vacancies [23,29,30].

Considering both the values of Mn 3+ /Mn 4+ ratios and those of the Ce/Mn ICP ratio the highest density of oxygen vacancies can be expected on the Ce0.01Mn-AT and Ce0.1Mn-AT samples based on charge balance process. It has also to be mentioned that the direct comparison of the Mn 3+ /Mn 4+ ratio determined from the Mn 2p3/2 core level to the Mn AOS determined from the Mn 3s splitting (figure 6 Static ToF-SIMS has also been performed in order to extract surface molecular changes induced by the acid treatment and the results are reported in Table 6-5. In any case, the mixed oxide formation can be easily evidenced by ToF-SIMS through the detection of mixed oxide fragments CexMnyOzHw +/-. The enhancement of the CeMnOH + /CeOMn + secondary ion ratio after acid treatment suggests an increase of the Brønsted acidity in line with the results of acidic surface site probing by pyridine. Furthermore, comparison of the MnOH + /MnO + secondary ion ratio and CeOH + /CeO + ratio suggests that Mn is more affected than Ce in surface OH related

6-3-2-Catalytic tests for ozone decomposition

The decomposition of 300 ppmv of O3 generated from air at 20 °C as a function of time over the pristine and acid-treated catalysts is shown in Besides, if ozone is generated from O2 diluted in Ar, 100 % O3 conversion is observed over time on Ce0.01Mn (Figure 6789). Nitrates can react with Mn/Ce cationic sites and the protons on oxygen sites [34,35].

Formation of adsorbed nitrate ions has been previously observed after exposure of hematite (-Fe203) surface to gaseous HNO3 [32]. Regarding N2O5, it can autoionize to NO2 + and NO3 -and react with water owing to:

NO2 + + NO3 -+ H2O = 2 HNO3 [36].

The presence of NO2 -and NO -are believed to occur from the partial reduction of nitrate into nitrite on the catalysts. Considering now the acid treated catalysts, it has to be mentioned that a small but significant contribution of N 1s at about 400.0 eV is always detected for the fresh acid treated catalysts indicating the presence of reduced nitrogen species induced by the acid treatment.

Taking this N 1s contribution into account, a corrected N/Mn atomic ratio (N/Mn)c which is the Besides, the effect of ozone decomposition has been investigated in polarity (+) by ToF-SIMS for the Ce0.1Mn sample as shown in Table 6-8. A significant enrichment of K + and Na + is noticed in line with the presence of ionic NO3 -previously postulated from FTIR-ATR results. A decrease in the NO desorption onset and peak temperature is observed with increasing Ce content which indicates that addition of cerium decreases the interaction of the NOx -adsorbates with the catalyst. Additionally, the relative amount of desorbed NO increases with SBET. It is suggested that the wide temperature range of NO desorption can be linked to various forms of nitrate/nitrite interaction with M 4+ /M 3+ (M = Mn, Ce) cations of the cerium modified birnessite. • High CO 2 yield (58%)

•

In this paper the TCE abatement is investigated in PPC configuration in dry and moist air (15 %) with NTP using a 10 pin-to plate negative DC corona discharge and with PPC using Ce0.01Mn as catalyst calcined at 400 °C (Ce0.01Mn-400) or treated with nitric acid (Ce0.01Mn-AT). In order to increase the efficiency of the NTP assisted catalyst for TCE total oxidation, particular attention has been paid to regulate the ratio of NTP generated ozone concentration to that of inlet TCE. The effect of humidity towards TCE abatement in plasma alone and in PPC process has been also investigated. Finally, the performance of the plasma-assisted catalysts for TCE abatement have been correlated with the physico-chemical properties of the tested catalysts.

7-2-Experimental conditions

7-3-Results and discussion

7-3-1-Main physicochemical characterizations of the fresh materials

The powder XRD patterns of samples are shown in Figure 7-1. Ce0.01Mn-400 sample is poorly crystallized and exhibits four broad peaks centered at 12.2 ° (001), 24.5° (002), 37.0° (110) and 66.2° (110) which are characteristics for turbostratic birnessite structure [13]. The XRD pattern of Ce0.01Mn-AT sample is ill-defined and presents a diffraction peaks centered at 37.0 ° and 66.2 °. However, the two diffraction peaks (001) and (002) are no more visible in the powder XRD pattern of Ce0.01Mn-AT sample. This could be related to the diminution of the number of layers stacked in the crystal after the acidic treatment. Indeed, when the coherent scattering domain size decreases, the intensity of the peaks decreases and their full width at half maximum increase [14,15]. The elemental compositions of the material after calcination and acid treatment are given in Table 7-2. The Ce/Mn atomic ratio for Ce0.01Mn-400 is close to the expected value (0.01) and the value is slightly modified after acid treatment. After acid treatment the Na and K contents decreased drastically: the Na/Mn as well the K/Mn ratio significantly dropped. This result can be partly explained by Na + -H + and K + -H + ion exchange reactions [16,17]. The N2 adsorption/desorption isotherms and pore size distribution of Ce0.01Mn-400 and Ce0.01Mn-AT samples are shown in Figure 7-2 and the data coming from these isotherms, BET surface area (SBET), pore diameter (Dp) and total pore volume (Vp), are gathered in Table 7-3.

The two samples exhibit a type IV isotherm, given by mesoporous materials. However, the two 1540 and 1635 cm -1 owing to pyridinium ions coming from pyridine protonation on Brønsted acid sites, (ii) two bands at 1450 and 1610 cm -1 attributed to pyridine coordinated to Lewis acid sites and (iii) overlapping bands of pyridine on Lewis and Brønsted acid sites at 1487 cm -1 [16,[19][20][21][22]. Moreover the band at 1573 cm -1 is attributed to the pyridine physisorption (P) [23] while the band at 1471 cm -1 could result from the interaction between the adsorbed pyridine and manganese oxides [24]. Mn 3+ and Mn 4+ species, as coordinatively unsaturated metal atom which can accept an electron pair, can be considered as Lewis acid sites while Brønsted acid sites can be identified as hydroxyl groups attached to the metal oxides (Mn-OH) [25]. The surface speciation of manganese species in both samples was assessed by analyzing the 2p photopeak of Mn. Mn 2p3/2 signal has been decomposed considering a mixture of Mn (III) and Mn (IV) as shown in Figure 7-4 [26]. The obtained Mn 3+ /Mn 4+ ratio is higher for the acid-treated sample (0.38) compared to the calcined one (0.33). In addition to curve-fitting of the Mn 2p3/2 signal, the Mn 3s photopeak can also provide useful insight into the Mn average oxidation state (Mn AOS) [27,28]. Indeed, the Mn 3s photopeak is split into two signals and the value of the splitting is an indication of the average oxidation state of Mn. Mn AOS values calculated from the Mn 3s splitting are 3.44 and 3.32 for Ce0.01Mn-400 and Ce0.01Mn-AT samples, respectively. These results agree with the aforementioned Mn 3+ /Mn 4+ ratio. As expected the acid treatment generated additional oxygen vacancies to maintain electronic balance, and more Mn 3+ species are present at the surface of the Ce0.01Mn-AT sample [17,29]. explain the maintain of high ozone conversion in the presence Ce0.01Mn-AT, while avoiding the saturation of the oxygen vacancies sites, probably at the origin of the observed deactivation in the presence of Ce0.01Mn-400 [34,35].

7-3-3-TCE abatement by non-thermal plasma (NTP)

The initial TCE concentration ([TCE]0) was set at 150 ppmv based on the O3 production through the plasma reactor. Indeed, without TCE and H2O in the feed, the ozone concentration at the exit of the plasma reactor was 600 ppmv. Thus the [O3]/[TCE]0 ratio equal to the stoichiometric one of 4 considering the total transformation of initial amount of C into CO2 as follow:

Regardless of the water content in the feed, RH = 0.7% or RH = 15%, TCE molecules are not fully degraded, since TCE conversion reaches 85 % and 59 %, respectively. Moreover, O3 molecules are still present at the outlet of the plasma reactor, 315 and 190 ppmv. The lowest O3 concentration at the outlet of the plasma reactor for the NTP-RH15% test despite the lowest obtained TCE conversion can be ascribed to the reaction of OH radicals, coming from the H2O decomposition in plasma reactor [36,37], which can react with O3 in the plasma [38][39][40][41].

Additionally, the COx yields are 46 % and 34 % for the NTP TCE abatement with RH of 0.7 cm -1 and 1700-1800 cm -1 , of the inlet TCE and the outlet gaseous stream after post plasma treatment using Ce0.01Mn-400 and Ce0.01Mn-AT with RH= 0.7%. In PPC configurations, as compared with NTP, band at 945 cm -1 corresponding to TCE is not observed while the intensity of the bands corresponding to CO2 vibration increases whatever the catalyst under concern.

Similarly, the ozone band disappears. Interestingly, the characteristic bands of DCAC and TCAC decreased in intensity when Ce0.01Mn-400 is used as catalyst and even disappeared with

7-5-Conclusion

In this work TCE abatement was investigated at 100 °C in nearly dry (RH = 0.7 %) and moist air (RH = 15 %) in PPC configuration using a 10 pin-to plate negative DC corona discharge NTP reactor and cerium modified birnessites calcined at 400 °C or treated with nitric acid as catalysts. The plasma conditions were chosen in order to treat a low concentration of TCE (150 ppmv) to be as close as industrial issue in terms of VOC concentration abatement. A stoichiometric ratio [O3]/[TCE]0 of 4 was adopted in order to take advantage of the efficient properties of the catalysts for O3 decomposition.

In these conditions, the TCE removal efficiency was 85 % but the COx yield was only 46 % for the NTP process alone. Adding a catalyst downstream of the NTP reactor enhanced the performances of the process in dry and humid air in terms of TCE removal efficiency and CO2 selectivity due to the ability of the catalyst to totally decompose ozone to generate surface active oxygen species able to oxidize the unreacted TCE and part of the polychlorinated byproducts. Plasma assisted Ce0.01Mn-AT catalyst presents the best CO2 yield in dry air (58 %) at the beginning of the experiment to reach after 5 hours 40 % with no production of DCAC and TCAD in hard experimental conditions such as high GHSV of 600 L/g.h, with only 0.1g of catalyst. In this configuration the formation of gaseous chlorinated by-products was minimized but not totally suppressed. Both catalysts exhibited a good stability for TCE abatement which mean they were chlorine tolerant. Ce0.01Mn-AT was tested in moist air and was also stable over time which means also its water tolerance.

the oxygen mobility of CexMn surface, which contributes to the enhancement of catalytic activity as well as reaction rate for HCHO removal since less intermediates was formed over CexMn. The high extent of Ce reduced state for Ce0.01Mn calcined at 400 °C accounted for its improved performances for HCHO oxidation in dry and moist air (RH: 75 %). ( 4) Catalytic experiments performed in a free oxygen atmosphere showed the high mobility of catalyst oxygen to participate in HCHO oxygen which increases as increasing the calcination temperature (Tc). In the final experimental chapter, trichloroethylene (TCE) abatement was investigated in dry and moist air (15%) with NTP using a 10-pin-to-plate negative DC corona discharge and with PPC using Ce0.01Mn as catalysts calcined at 400°C (Ce0.01Mn-400) or treated with nitric acid (Ce0.01Mn-AT). In comparison with NTP process, PPC was found to be more efficient.

Total conversion of TCE was obtained in PPC process irrespective of the catalyst used with catalyst temperature at 100°C in dry and moist air. A remarkable increase of CO2 yield has been observed in PPC configuration and explained by the facile oxidation of polychlorinated byproducts (adsorbed at the surface) by active oxygen species coming from the catalytic decomposition of ozone. Plasma assisted Ce0.01Mn-AT catalyst (100°C) presents the best CO2 yield in dry air with minimization of the formation of gaseous chlorinated by-products in hard experimental conditions such as high GHSV of 600 L/g.h, with only 0.1g of catalyst. This result is attributed to high content of oxygen vacancies with a higher amount of Mn 3+ , to improved specific surface area and to strong surface acidity. These features allow also a promoting ozone decomposition efficiency. Both catalysts exhibited a good stability towards chlorine. Ce0.01Mn-AT tested in moist air (RH= 15 %) showed a good stability as a function of time indicating also a good water tolerance.

Conclusion Table 1: Summary of the important results obtained in this work

HCHO catalytic oxidation

Ce0.01Mn-400 exhibited the best activity, showed a T50 of 50 °C for the abatement of 100 ppmv of HCHO with a GHSV of 60 Lgcat -1 h -1 in dry air with 0.2g of catalyst. Excellent stability vs time of Ce0.01Mn-400 in the presence of water (RH= 75%). 50% HCHO conversion into CO2 at 50°C for 13 h O3 catalytic decomposition Ce0.01Mn-AT exhibited the best activity, showed 94 % and 96 % conversion of 300 ppmv of O3 with a GHSV of 1200 Lgcat -1 h -1 in dry air at RT and moist air (RH = 30 %) at 40°C respectively with 0.05g of catalyst in the presence of nitrogen containing co-pollutants.

TCE abatement in PPC process

These two best previous catalyst (Ce0.01Mn-400 and Ce0.01Mn-AT) are used for TCE abatement. Ce0.01Mn-AT catalyst (100°C) presents the best CO2 yield in dry air with of minimization of the formation of gaseous chlorinated by-products in hard experimental conditions such as high GHSV of 600 L/g.h, with only 0.1g of catalyst

Appendix A