ovskite materials like SrFe x Al 1-x O 3-δ . They assumed this difference being due to the low concentration of oxygen vacancies in the octahedral layers of CaFeO 2.5 .

I am very grateful to the jury members who accepted to review and evaluate this work. I am especially thankful to the two opponents Prof. Silvia Gross and Prof. Henny Bouwmeester for their careful and precise comments as well as for their thorough evaluation of this work. I also want to thanks the other jury members Dr. Simone Mascotto, Dr. Philippe Papet and Dr. Celine Delwaulle for their precise, interesting and constructive questions during the defence of this work.

I am deeply honoured that Dr. Helena Kaper and Prof. Werner Paulus the two leaders of this projects accepted me as a candidate to conduct this extremely interesting research project. I personally thank Prof. Paulus for in gentle supervision and his kindness toward me when I was discovering the world of research. His deep knowledge of material science and inorganic chemistry where true assets during this work. I am extremely grateful to Dr. Helena Kaper who placed her trust in me three years ago. And even if my way of dealing with the project may be different from the expected one, she always supported my curiousness. For her permanent support everyday I am permanently grateful.

I now want to thank the people who helped me during these three years. First I want to thank the people of the lab in Saint-Gobain, beginning with its director Dr. Caroline Tardivat and Valérie Tinchon who both helped me to deal with the administrative part of this work. Then I want to thank Dr. Sylvain Deville, Dr. Adam Stevenson, Dr. Michaela Klotz and Stéphane Richaud for their kindness everyday and the very interesting discussion we may have shared. A special thank to Daniel Aubert for his help in the lab everyday, he was my guide for everything regarding partical work. Another special thank goes to Daniel Marinha who was a true example for me and who I can now count among my friends, thank you! I want to give a warm thank to the people who turned out being friends: Mangesh Avhad, Rémi Grosjean, Amin M'Barki, Corinne Salles, Lionel Magaud, Hassan Saad, Sydanth Tyagi, Anthony Pez and Laura Reynaud. You guys are great, never give up and keep going! I hope we will stay in touch.

I want to give a very deep thank to Nicolas Bion because without him I would not be here today. You placed your trust in me five years ago and I do hope that I didn't deceive you.

I want to thank my family for their constant support all along the last three years. I think that until the defence they had no clue about what is a PhD but they were supportive anyway, for this I am eternally grateful.

My final and deepest thank is going to Paula Anguita. You were always supportive even in the most difficult period. Your kindness is a true example for and your optimism was huge source of motivation for me. Now I just want to say that I may have missed to thank people so I just want to thank all the people that I've met during these last three years. You were all great and supportive, thank you! I now let you with the discovery of my work hoping that it will be helpful to you in your research, enjoy!

CONTENTS

Chapter 1 Bibliography / Introduction 1.1 Context

During the industrial era of the last century, human civilisation developed new technologies to achieve higher standard of life. Many innovations resulted in huge improvement of life quality like the evolution of medicine and most of them resulted in an easier life. But few innovations were industrialised without care taking of environmental consequences. This behaviour led us to face environmental problems nowadays such as global warming. In the early 1970's, the club of Rome produced a very criticised/controversial report named The Limits of Growth. The main purpose of the report was to make people realise the uncertainty of our future. The report was at first qualified of alarmist before being positively accepted in the beginning of 2000's. During the same period of time a lot of people tried to alarm people about the environmental situation. More specificaly, an NGO activist named Rafe Pomerance was worried about the situation. He contacted a geophysicist Gordon McDonald and together started to warn US government. But they did not manage to gather people around the situation before being hidden by the media by the election of Ronald Reagan [START_REF] Rich | Losing Earth: The Decade We Almost Stopped Climate Change[END_REF].

Environmental issues are strongly related with the emmission of green house gas, one of the main cause of emission of these gases is the transportation sector. As an example, automobile prevails around the globe as the most popular and important mode of transportation in our daily life. About 80 000 000 new cars are yearly produced while 1.2 billions are used worldwide. As this widely used mode of transportation is a huge source of pollution (12% of green house gas emission come from automobile in Europe), people have to find a solution. In early 1970s, a massive photo-chemical smog 6 CHAPTER 1. BIBLIOGRAPHY / INTRODUCTION in California forced the implementation of automotive exhaust regulation in US and Japan. Among the different technologies evaluated (including engine modification), catalytic systems for exhaust gas treatment was recognised as the only method able to meet the regulations. Oxidation catalysts were first used in 1974 for carbon monoxide (CO) and hydrocarbons (HC) treatment until the development of three-way catalyst (TWC) in 1977. This system was designed for the simultaneous treatment of CO, HC and nitrogen oxides (NO x ).

So it has been chosen to oblige automobile constructors to reduce the pollution of their vehicles by the mean of compulsory regulations. In Europe, the first regulation EURO I was implemented in 1993 and since then evolved to be even more restrictive until the last one EURO VI in 2014. Table 1.1 is describing the evolution of the regulation from EURO I to EURO IV over gasoline and diesel engines taken from [3]. Table 1.1: Evolution of European standard regulation of atmospheric pollutants for gasoline and diesel engine Since the regulation became more and more stringent with each new standards it was more and more difficult to comply with those regulations. The TWC system has been improved to meet the new-coming regulations.

Importance of oxygen conductors

In a engine, the composition, temperature and flow amount of exhaust gas are varying with the driving conditions. The system works with an alternation of rich and lean phases defined by the Air/Fuel ratio. The problem is that a specific catalyst can be efficient in the rich or lean phase but it was difficult to develop a catalyst efficient in both phases. To overcome this problem, scientist got interested in supports acting like oxygen buffers. This peculiarity allows the support to limit the oxygen variation between rich and lean phases by acting as an oxygen buffer; storing excess of oxygen [4]; examples 3; 4 & 5 are from [START_REF] Rossignol | Effect of the preparation method on the properties of zirconia â ceria materials[END_REF]; examples 7 & 8 are from [START_REF] Kacimi | Oxygen storage capacity of promoted Rh/CeC2 catalysts. Exceptional behavior of RhCu/CeO2[END_REF] during rich phase and releasing it during lean phase. One of the most efficient material for this property is the cerium oxide (with the general formula CeO 2 also called ceria). Thanks to the versatility of the cerium oxidation degree to shift from +III to +IV as needed, this material is able to uptake oxygen in the rich phase and release it during the lean phase. This ability to allow oxygen ions to move within the material lattice and to participate in the catalytic reaction as needed can be seen as oxygen mobility. Regarding the updated transport restrictions, part of the research is focusing on improving ceria based catalyst to meet the required performances. The aim of this project is not about manufacturing more efficient exhaust gas treatment catalysts. It is rather focused on developing alternative materials able to show ceria like properties (in the frame of oxygen mobility) taking into account environmental concerns.

Since the first utilisation of cerium oxide as a TWC the composition has been improved to increase its properties. Their oxygen mobility property has been identified as crucial so efforts were given to increase the latter. One way of quantifying the oxygen mobility is the oxygen storage capacity (OSC), it is basically the quantification of the uptake and release of oxygen from the material during an alternation of oxygen rich and lean atmospheres. Evolution of ceria based compositions towards their OSC are detailed in Fig. 1.2 taken from [3] Table 1.2, shows that the scientific community tried to increase the OSC of the ma-CHAPTER 1. BIBLIOGRAPHY / INTRODUCTION terials using doping/substitution strategies. Unfortunately, the base of those materials e.g. cerium oxide is facing supply issues which are the subject of the next section.

Geopolitical/Supply chain issues

Since its first utilisation in the 1970's as a catalyst and later with a noble metal impregnated on its surface, ceria imposed itself at one the most efficient catalyst support for exhaust gas treatment. But many material, cerium oxide is a limited resource on Earth. Cerium is the 30 th element on Earth regarding weight percent representing 0.004% of the planet's weight. But one of the problem is that the allocation of cerium over Earth is not homogeneous. Moreover the exploitation of cerium deposits is not ongoing for every countries that possess some of them. The current situation of global suppliers of a list of materials identified as critical raw materials is detailed on Fig. 1.1 taken from the Report on Critical Raw Materials and the Circular Economy from the European Commission [START_REF]Report on Critical Raw Materials and the Circular Economy[END_REF]. From this map, China is supplying 95% of ceria nowadays. According to this, the European union described the European reliance rate towards cerium importation being 1.1. CONTEXT Table 1.3: Energy and water consumption in production of metals from scrap and ores (range given is high to low grade) 100%. The exploitation of cerium deposits also have an environmental cost. This ore exploitation is costly, the following table 1.3 also taken from the EU report [START_REF]Report on Critical Raw Materials and the Circular Economy[END_REF] describes the cost of cerium exploitation in terms of energy and water consumption.

Cerium is part of the Rare Earths elements section and not represented alone. It is possible to observe in this table that production of cerium among other rare earths is environmentally expensive.

Besides production of cerium oxide, an alternative lies in recycling. But currently only very few (about 1% [START_REF]Report on Critical Raw Materials and the Circular Economy[END_REF]) of cerium oxide is recycled. Actually, from the engine's exhaust gas treatment lines, noble metals such as platinum are recycled but almost no cerium oxide is.

The preoccupying situation of cerium oxide is not an isolated case, so the European Union commission made a list of critical raw materials that require an alternative to. Obviously cerium oxide is among those materials in the "Light Rare Earth" category.

CaFeCat project

Looking at the presents situation and the different findings expressed, the scientific community decided to act. This is the context where this work lies. The project of this work is based on finding innovative catalytic supports materials based on non critical elements. To tackle this problem, we first identified what is making ceria such an interesting material for catalytic applications: its ability of storing oxygen in rich phase and releasing it during lean phase. Oxide materials with specific crystallographic structure are able to reproduce similar behaviour based on oxygen vacancies mechanisms. One can cite spinel (AB 2 O 4 ), ruddlesden-popper (A n+1 B n O 3n+1 ), brownmillerite (A 2 B 2 O 5 ) CHAPTER 1. BIBLIOGRAPHY / INTRODUCTION or perovskite (ABO 3-δ ) as crystallographic structures showing this property. Among those structures we focus in this work on the brownmillerite and perovskite structures.

The different features of these two structures will be described in the following sections. To synthesise those structures we chose elements taking in account the supply issues previously described. So the chosen low-risk elements for this projects are calcium, strontium, titanium and iron. Besides the supply problematic, other criteria led us to this choices. These include structural considerations, element properties and our own experience. These parameters will also be described in the sections dedicated to each structure.

In this project, we also chose to compare different synthesis methods. We first used lab-scale methods (complexation) to produce, characterise and evaluate the materials but we also used another synthesis method. Saint-Gobain is our industrial partner in the project and has a lot of experience in ceramic manufacturing. One of their expertise is the electric-arc furnace technology. The principle and description of this specific furnace will be detailed in the experimental section (see page 43 f). Regarding the feasibility, characteristic and performance of electric-arc furnace materials, this technology can be considered as an up-scaling strategy compared to lab-scale complexation methods.

Oxygen mobility and catalysis 1.2.1 From inert to active supports

Basically a catalyst is anything able to increase the rate of a process. The term of catalysis was first employed by Elizabeth Fulhame in 1794 in a book about her oxidoreduction reactions studies. Jöns Jakob Berzelius used the term catalysis in 1835 to describe reactions that are accelerated by a substance unaltered by the reaction. Later, Wilhelm Ostwald was studying reactions catalysed by acids and bases. He then proved that chemical reactions were following a finite rate and that he could use it to describe acids/bases strength. He was warded by the chemistry Nobel prize in 1909 for this work [START_REF] Nobelprize | No Title The Nobel Prize in Chemistry[END_REF]. The general concept of catalysis lies in lowering the energy required to achieve a specific reactions by the means of a specific species called catalyst which should be unaffected by the process.

Catalysis can be divided in two different parts, homogeneous catalysis where reactants, products and catalysts are in the same phase and heterogeneous catalysis where reactants, products and catalysts are in different phases. This work is only related to 1.2. OXYGEN MOBILITY AND CATALYSIS 11 heterogeneous catalysis with reactants and products in the gaseous phase and catalysts in the solid phase. Physically most of modern catalytic system are composed of a material called catalyst impregnated on the surface of another material called support. The support material is chosen to increase the performance of the catalyst or to reduce the cost of the system or both. The catalyst itself is an active species with specific characteristics enable to lower the energy required for a specific reaction. In the frame of oxidation reactions, the most used materials belong to the platinum group metals (PGM) such as platinum, palladium, rhodium. The expression catalyst is commonly abused to refer to the catalytic system composed of the noble metal and the support together. To avoid the consequent confusion, later in this manuscript we will use platinum to refer to the catalyst, support to refer to the support and catalytic system to refer to both together. It is very common that supported catalyst and the supporting material have interaction. But to describe the general principle, we will stick to inert support meaning without interactions. Fig. 1.2 is showing the simplified case of an oxidation reaction (carbon monoxide oxidation as an example) in the presence of a catalytic system composed of PGM supported on an inert support.

CHAPTER 1. BIBLIOGRAPHY / INTRODUCTION

The general steps occurring during an oxidation reaction are represented by the orange numbers on Fig. 1.2 and detailed in the following list:

1) Diffusion of reactants to the noble metal surface: Is the approach of the reactants species to the catalyst surroundings. It is governed by diffusion laws e.g. Maxwell-Stefan's equations which are describing the concentrations gradient of the reactants and products around the catalyst.

2) Adsorption on the adsorption site at the noble metal surface: It describes the physical (van der Waals) or chemical (covalent/ionic) creation of bonds between the catalyst and the reactant molecules. The adsorption can be associative or dissociative regarding the reactant molecules and the catalyst surface properties. The dissociative adsorption can possibly be divided in two different sub-steps. The adsorption of the reactant molecule and the dissociation of the molecule at the catalyst surface. The order of these two steps can also happen reversely: first the dissociation and then the adsorption. But the reactant molecule has to be close to the catalyst surface to get dissociated. It is very difficult to get information about the order of these two steps. The adsorption is a crucial step of the catalytic process as the dissociated/adsorbed species should be more reactive then the original molecule. These reactive species are commonly called reaction intermediates.

3) Chemical reaction at the catalyst surface: This is the chemical reaction itself. Actually it can be more complex than just the reactant molecules interacting together. Indeed reactants may be turned in an activated species called intermediates prior to the reaction itself. Therefore the reaction can be seen as the reactions between intermediates. The intermediates species may have a huge impact and could possibly explain why some catalysts are displaying higher performances than others.

4) Desorption of the products on the desorption site:

This step possibly includes the recombination of products reaction intermediates. Followed by the desorption of the final products in the surrounding environment of the catalyst.

5) Outgoing diffusion of the products:

This last step describes the retro-diffusion of the products from the near catalyst surface into the whole volume of the media around the catalyst. It is also driven by diffusion laws.

Each of these steps can be limiting. Meaning that each step can by described by the energy required to achieve the latter. The most energy costly step is identified as the limiting step of the overall reaction. It can be possible to tune a catalytic system properties focusing on improving a specific step. In the literature one can find three general type of catalytic mechanism describing these steps. The first description of catalytic processes was given by Langmuir in 1921 and further developed by Hinshelwood in 1926, therefore called Langmuir-Hinshelwood (L-H) mechanism [START_REF] Norman Hinshelwood | kinetics of chemical change in gaseous systems[END_REF] [START_REF] Michel | Kinetics of Heterogeneous Catalytic Reactions[END_REF]. Fig. 1.3 a) is a schematic representation of the L-H mechanism. In this mechanism, the reactant species adsorb on the catalyst surface and react over it. Then the products desorb but there is no involvement of the catalyst (or support) atoms.

The second type of mechanism was first proposed by Eley and Rideal in 1938. Basically, one of the reactants adsorbs on the catalyst surface while the other remains in the gaseous phase see Fig. 1.3 b). Catalytic reactions corresponding to this kind of interactions are belonging to the so-called Eley-Rideal (E-R) mechanism [START_REF] Michel | Kinetics of Heterogeneous Catalytic Reactions[END_REF] [START_REF] Cheng | Atomic Hydrogen-Driven Halogen Extraction from Si(100): Eley-Rideal Surface Kinetics[END_REF] [START_REF] Weinberg | Eley-Rideal Surface Chemistry: Direct Reactivity of Gas Phase Atomic Hydrogen with Adsorbed Species[END_REF].

At the same time as the Eley-Rideal mechanism was proposed, the idea that the catalyst lattice components can appear in the reaction products emerged. But it was not really recognized until the publication of successful application of the idea in the kinetics by Mars and Van Krevelen in 1954 [START_REF] Mars | Oxidations carried out by means of vanadium oxide catalysts[END_REF]. In the mechanism, the reactants adsorb on the catalyst surface, then one of both can spill over the support and interact with the support components as illustrated on Fig. 1.3. Eventually they migrate back to the catalyst surface and desorb from there. After Mars and Van Krevelen publications, the Mars -van Krevelen mechanism (MvK) was observed in many reactions [START_REF] Anderson | Catalysis Science and Technology -chapter 2 Catalytic Activation of Dioxygen[END_REF] [START_REF] Doornkamp | The universal character of the Mars and Van Krevelen mechanism[END_REF]. The Mars-Van Krevelen mechanism deserves specific attention as it involves interactions with the support components. Care should be taken when considering this mechanism as there are possible specific support-reactant interaction sites. In terms of examples, one can think about crystallographic planes, grain boundaries or even three phases (catalyst/support/gaseous phase) interface. If a support is providing mobility of its own components, the support can be seen as a reservoir of reactants or as an interfacing environment to separate two species. Materials showing oxygen mobility can thus help solving technological issues. One can see that considering the support as a reservoir is directly connected to OSC. This kind of materials are therefore perfectly suitable for TwC application where conduction properties are required.

In this work we aim at manufacturing innovative supports able to act in the reactions, so one of the objectives is spotting MvK catalytic mechanism. One of the most efficient and reliable way of spotting MvK mechanism is by the mean of labelled species Figure 1.3: Representation of the three main catalytic mechanisms in heterogeneous catalysis. a) the Langmuir-Hinshelwood mechanism with 1. adsorption of a reactant; 2. adsorption of the other reactant and 3. adsorbed reactants react together and desorb; b) the Eley-Rideal mechanism with 1. adsorption of one reactant; 2. reaction of the adsorbed reactant with the other one remaining in the gaseous phase followed by desorption; and c) the Mars-van Krevelen mechanism with 1a. adsorption of reactant A followed by reaction of reactant A and a support component; 1b. Parallel adsorption of reactant B to regenerate the support and 2. Desorption of the product made of reactant A + a support component. Taken from [START_REF] Penlkala | Low Temperature Oxygen Mobility Applied to Catalysis[END_REF] such as isotopes.

Utilisation of isotopic oxygen

Isotopes are by definition nuclei having the same number of protons (same element) but a different number of neutrons, e.g. 16 8 O and 18 8 O are two isotopes of the same element: Oxygen. Thus, isotopes properties are almost identical (similar properties) but their magnetic moment is slightly different. There exist both radioactive and radio-inactive isotopes. Every isotope of an element do not have the same natural occurrence. So their production requires the ability to separate them from the other states. Several techniques can be used for this purpose:

Production with reactors: radio-isotopes can be produced with a neutron reactors by exposing a material target to the neutron beam during a specific time.

Production with accelerators: Beam of accelerated charged nuclei is used to bombard targets leading to the production of a wide range of isotopes.

Chemical separation:

Those are methods exploiting the mass difference between a specific isotope and other state of the same element. Including distillation, cen-

OXYGEN MOBILITY AND CATALYSIS

trifugation or method using a calibrated laser to raise a specific isotope in a excited state followed by different mechanism to sweep away other states of the element.

Electromagnetic purification: This method also uses the mass difference of the isotopes to separate them using their different deflection in a magnetic field.

Isotopes are very suitable to study all kind of process, as an example they are used in medicine to follow the drugs path in living organisms. Utilisation of isotopes to study chemical processes started with the discovery of isotopes by JJ. Thomson in 1913 who separated ions of ionised neon using magnetic and electric fields. This was the first example of mass spectrometry, a technique that was be further developed by Aston in 1919. Among the first to use isotopes to probe chemical reactions, G. Von Hervesy received a Nobel prize for studying utilisation of radioactive isotopes as isotopic markers [START_REF] Nobelprize | NoThe Nobel Prize in Chemistry[END_REF]. After him many researchers used isotopes as markers to investigate chemical reactions. After world war two, production of isotopes was easier, so the number of research based on isotopes utilisation ( 2 H, 3 H, 13 C, 14 Cc, 15 N, 18 O and 35 S) increased. Those studies were done using different techniques such as Fourier transformed infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), electron para-magnetic resonance (EPR), mass spectrometry (MS) or radiation detectors.

In the 1960's, E.R.S. Winter and G.K. Boreskov performed isotopic exchange over numerous oxides [START_REF] Winter | Exchange reactions of solid oxides. Part V. Dissociation of molecular oxygen at oxide surfaces : magnesium oxide and zinc oxide[END_REF][START_REF] Winter | Exchange reactions of solid oxides. Part VII. Chronic oxide, nickel oxide, and ferric oxide[END_REF][START_REF] Winter | Exchange reactions of oxides. Part IX[END_REF][START_REF] Boreskov | The Catalysis of Isotopic Exchange in Molecular Oxygen[END_REF][START_REF] Boreskov | Catalytic Activation of Dioxygen[END_REF] spotting lattice oxygen involvement in the chemical reactions. Later on, Novakova published a review over isotopic studies in 1971 [START_REF] Novakova | Isotopic Exchange of Oxygen 18O Between the Gaseous Phase and Oxide Catalysts[END_REF]. In 1978, K. Hirota [START_REF] Hirota | Carbon monoxide oxidation with an oxygen tracer over a vanadium pentoxide catalyst[END_REF] and his team reinforced the idea of lattice oxygen participation performing CO oxidation over V 2 O 5 in the presence of 18 O. H. Kakioka, V. Ducarme and S. J. Teichner confirmed also this idea but showed that in an excess of oxygen, chemisorbed oxygen also act in CO oxidation [START_REF] Kakioka | I. Échange isotopique de 18 O du gaz carbonique avec les ions oxygène de l'hémipentoxyde de vanadium[END_REF][START_REF] Kakioka | III. Étude par traçage isotopique de la réaction d'oxydation catalytique de CO sur l'hémipentoxyde de vanadium pur ou dopé par l'ion molybdène Mo 6+[END_REF].

More recently, but following the idea of Winter and Boreskov; D. Martin and D. Duprez studied oxygen mobility over numerous oxides with isotopic exchange [START_REF] Martin | Mobilité de surface des espèces oxygénées et hydrogénées en catalyse métallique[END_REF][START_REF] Martin | Mobility of Surface Species on Oxides. 1. Isotopic Exchange of 18 O 2 with 16 O of SiO 2 , Al 2 O 3 , ZrO 2 , MgO, CeO 2 , and CeO 2 -Al 2 O 3 . Activation by Noble Metals. Correlation with Oxide Basicity â[END_REF][START_REF] Madier | Oxygen Mobility in CeO 2 and Ce x Zr (1 -x ) O 2 Compounds: Study by CO Transient Oxidation and 18 O/ 16 O Isotopic Exchange[END_REF][START_REF] Richard | ISO-TOPIQUE A L'ÉTUDE DE SYSTEMES CATALYTIQUES INNOVANTS[END_REF][START_REF] Richard | Investigation of Methane Oxidation Reactions Over a Dual-Bed Catalyst System using18O Labelled DRIFTS coupling[END_REF][START_REF] Giroir-Fendler | Synthesis of oxide supported LaMnO<inf>3</inf> perovskites to enhance yields in toluene combustion[END_REF][START_REF] Richard | Remarkable Enhancement of O 2 Activation on Yttrium-Stabilized Zirconia Surface in a Dual Catalyst Bed[END_REF][START_REF] Repecaud | Enhancement of Oxygen Activation and Mobility in CaTi x Fe 1 À x O 3 À d Oxides[END_REF].

Recent studies are correlating oxygen mobility with CO oxidation [START_REF] Taha | Exchange and oxidation of C16O on18O-predosed Rh-Al2O3and Rh-CeO2catalysts[END_REF][START_REF] Gamboa-Rosales | /inf> catalysts for CO oxidation: An <sup>18</sup>O/<sup>16</sup>O isotopic exchange study[END_REF] or with OSC in TWC [START_REF] Dong | Characterization of the dynamic oxygen migration over Pt/CeO2-ZrO2catalysts by18O/16O isotopic exchange reaction[END_REF][START_REF] Bedrane | 16O/18O isotopic exchange: A powerful tool to investigate oxygen activation on M/CexZr1-xO2catalysts[END_REF][START_REF] Galdikas | Oxygen Isotopic Exchange On Three-Way Catalysts : A Dynamic Kinetic Model[END_REF].

Active supports as a perspective for the future

To summarise what have been discussed so far: * Ceria is one the most used catalyst for exhaust gas treatment Based on this observations, the defined objective of this work is to develop innovative active supports with non-critical materials. As a reminder the selected list of elements is: Calcium, Strontium, Titanium and Iron.

To develop supports able to show oxygen mobility, the strategy is to use oxides with specific crystallographic structures able to show oxygen mobility at low temperature. The two chosen structures studied in this work are the brownmillerite structure ABO 2.5 and the perovskite structure ABO 3-δ . The features of these two structures will be detailed in the next section.

Last but not least, when a driver starts a car, the engine is cold and so is the exhaust gas stream. But the heat from the gas stream is what is bringing heat to the catalyst in order to reach its working temperature. Therefore this step (cold engine start) as been identified as one of the most pollutant. To tackle this issue, there is a need to develop catalyst working efficiently at low temperature. Therefore this project focus on new support able to work efficiently at low temperature.

Oxygen conductors: Perovskite and Brownmillerite

As detailed earlier, oxygen mobility is a key feature in the development of innovative supports as an alternative to cerium oxide. Investigation of the mechanisms enabling this property in ceria highlighted that the versatility of the cerium oxidation between +IV and +III is the key. The strategy develop in this project is based on different mechanism leading also to oxygen mobility. Based of crystallographic defects, two structures were chosen due to their already proven low temperature oxygen mobility: the perovskite structure ABO 3-δ and the brownmillerite structure ABO 2.5 .

The perovskite structure

The perovskite structure family of compositions is named after a Russian mineralogist, Count Lev Aleksevich von Perovski, and was discovered and named by Gustav Rose in 1839 from samples of CaTiO 3 found in the Ural Mountains. The perovskite structure was first described by Goldsmith in his work about tolerance factors [START_REF] Goldschmidt | Die Gesetze der Krystallochemie[END_REF] until being later published in 1945 from X-ray diffraction data on barium titanate by H.D. Megaw [START_REF] Dick | Crystal Structure of Barium Titanate[END_REF]. The perovskite structure is described with the general formula ABO 3 where A and B are being two cations and O an oxygen anion. Since first catalytic test in the 1970s reporting interesting catalytic activity for NO reduction and oxidation reactions, the perovskite represent probably the most studied mixed-oxide system in heterogeneous catalysis [START_REF] Libby | Promising catalyst for auto exhaust[END_REF][START_REF] Voorhoeve | Rare-earth oxides of manganese and cobalt rival platinum for the treatment of carbon monoxide in auto exhaust[END_REF][START_REF] Voorhoeve | Rare-Earth Manganites: Catalysts with Low Ammonia Yield in the Reduction of Nitrogen Oxides[END_REF][START_REF] Gallagher | Studies of some supported perovskite oxidation catalysts[END_REF]. They can be prepared through many synthesis routes regarding the properties required for a specific application. Table . 1.3.1 taken from [START_REF] Royer | Perovskites as substitutes of noble metals for heterogeneous catalysis: Dream or reality[END_REF] is reporting examples of these methods: Table 1.4: Examples of perovskite materials and their different synthesis methods. Example one is from [START_REF] Voorhoeve | Defect chemistry and catalysis in oxidation and reduction over perovskite-type oxides[END_REF]; example two is from [START_REF] Yamazoe | Oxygen permeation through pervoskite-type oxides[END_REF]; example three is from [START_REF] Gallagher | Studies of some supported perovskite oxidation catalysts[END_REF]; example four is from [START_REF] Wachowski | Influence of the method of preparation on the porous structure of perovskite oxides[END_REF]; example five is from [START_REF] Baythoun | Production of strontium-substituted lanthanum manganite perovskite powder by the amorphous citrate process[END_REF]; example six is from [START_REF] Taguchi | Surface characterization of LaCoO3synthesized using citric acid[END_REF]; example seven is from [START_REF] Gallagher | The activity of La0.7Sr0.3MnO3without Pt and La0.7Pb0.3MnO3with varying Pt contents for the catalytic oxidation of CO[END_REF][START_REF] Gallagher | Effects of the Pt content of La0.7Pb0.3MnO3on its catalytic activity for the oxidation of CO in the presence of SO2[END_REF]; example eight is from [START_REF] Wachowski | Influence of the method of preparation on the porous structure of perovskite oxides[END_REF] and example nine is from [START_REF] Ng Lee | Hydrogen peroxide decomposition over Ln1-xAxMnO3(Ln = La or Nd and A = K or Sr) perovskites[END_REF]; Taken from [START_REF] Royer | Perovskites as substitutes of noble metals for heterogeneous catalysis: Dream or reality[END_REF] The scientific interest towards perovskite is due to their extraordinary property to accommodate a wide range of substitution and doping elements allowing tailoring of their properties for a wide range of application. Indeed, this peculiarity is related to defect chemistry in the perovskite lattice detailed in the next section.

CHAPTER 1. BIBLIOGRAPHY / INTRODUCTION

The perfect perovskite lattice

The perovskite structure ABO 3 can be composed either with trivalent cations for both A and B sites (e.g. A +3 B +3 O 3 LaAlO 3 ), for with the most symmetric structure observed is rhombohedral R-3c. Or it can be a combination of divalent cation in the A site and tetravalent cations in the B site (e.g. CaTiO 3 ). The latter case corresponds to the perfect perovskite lattice which exhibits a cubic structure in the P m3m spacegroup where small B cations are inside oxygen octahedra and large A cations XII-fold coordinated with oxygen. A schematic representation is given on Fig. 1.4 and Table 1.5 gives the corresponding positions of A and B sites cations and oxygen anions (Taken from [START_REF] Mark R Levy | Crystal structure and defect property predictions in ceramic materials[END_REF]): Figure 1.4: Cubic perovskite unit cell where blue spheres represent the A cations, yellow spheres the B cations and red spheres are oxygen anions (space-group is P m3m). Taken from [START_REF] Mark R Levy | Crystal structure and defect property predictions in ceramic materials[END_REF].

Substituted perovskite

The properties of the perovskite depend of the elements constituting the structure but the structure itself relies on the packing of the ions. The perovskite structure can be either cubic, orthorhombic or even hexagonal depending on the ionic radii of A and B site elements. It other words, changing the elements in the A and B sites (e.g. changing the ionic radii) will lead to different symmetries. As an illustration, when decreasing the A cation size, there will be a point where the cation will be too small to remain

Sites Locations Coordinates

A cations (2a) (0,0,0)

B cations (2a) ( 1 2 , 1 2 , 1 2 ) O anions (6b) ( 1 2 , 1 2 , 0)( 1 2 , 0, 1 2 )(0, 1 2 , 1 2
)
in contact with anions in the cubic structure. As a consequence the B-O-B bonds will bend slightly, leading to a tilt in the BO 6 octahedra bringing anions in contact with A cations. Fig. 1.5 taken from [START_REF] Mark R Levy | Crystal structure and defect property predictions in ceramic materials[END_REF] illustrates this shift. This distortion will induce a change in the symmetry (e.g. orthorhombic) thus the space-group is Pnma. Fig. 1.6 gives a schematic representation of the orthorhombic structure and table 1.6 gives the relative coordinates of the atoms in the structure.

To describe the arrangement of the ions and in order to predict the symmetry, Goldsmith introduced the concept of tolerance factor. This factor is based on the ionic radii along the diagonal of the perovskite lattice and is given in equation 1.1:

t = r A + r O √ 2 * (r B + r O ) (1.1)

Sites Locations Coordinates

A cations (4c)

±[(u, v, 1 4 )( 1 2 -u, v + 1 2 , 1 4 )] B cations (4b) ( 1 2 , 0, 0)( 1 2 , 1 2 , 0)(0, 0, 1 2 )(0, 1 2 , 1 2 ) O (1) anions (4c) [(m, n, 1 4 )( 1 2 -m, n + 1 2 , 1 4 )] O (2) anions (8d) [(x, y, z)( 1 2 -x, y + 1 2 , 1 2 -z)(-x, -y, z + 1 2 )(x + 1 2 , 1 2 -y, -z)] u, v, m,
n are dependent on the particular structure under consideration.

Where r A is the atomic radius of cation at the A site; r B is the atomic radius of cation at the B site and r O the atomic radius of oxygen anion. Regarding the value of t one can predict the symmetry of the perovskite lattice: Hines et al. [START_REF] Ian | Atomistic simulation and ab initio studies of polar solids[END_REF] proposed that the ideal condition to get a cubic perovskite is when ∼ 0.9 ≤ t ≤ 1. When t > 1 the symmetry will be hexagonal. When ∼ 0.75 ≤ t ≤∼ 0.9 the perovskite will be 1.3. OXYGEN CONDUCTORS: PEROVSKITE AND BROWNMILLERITE 21 orthorhombic and if t value drops below 0.75 it is assumed to get an hexagonal ilmenite structure (FeTiO 3 ). Note that the symmetry can also change with temperature and oxygen partial pressure.

It is possible to purposely change the ionic radii in the A and B sites by substitution or doping leading to the general formula: A 1-x A x B 1-y B y O 3 . Performing partial substitution on the A site (or B or both) by an element having an ionic radius different than the initial element in the A site will lead to local intrinsic defects. This work will only focus on substitution on the B site of CaTiO 3 e.g. partial substitution of Ti +IV by Fe +III giving the general formula: CaTi 1-x Fe x O 3-δ . The associated formula of t is thus given by equation 1.2:

t = r Ca + r O √ 2 * ( (1-x) * (r T i )+x * (r F e ) 2 + r O ) (1.2)
With the exact atomic radii of Ca 2+ , Ti 4+ and Fe 3+ taken from Shannon et al. [START_REF] Shannon | Revised Effective Ionic Radii and Systematic Studies of Interatomie Distances in Halides and Chaleogenides[END_REF] accordingly to the coordination number of each perovskite site.

Substitution on the B site with an atom having a different oxidation degree will generate defects that will most of the time lead to the creation of oxygen vacancies with respect to the electro-neutrality (via local electrostatic compensation mechanism). This leads to the general formula: CaTi 1-x Fe x O 3-δ . Note that in the latter equation oxygen stoichiometry is no more 3 but 3 -δ. This oxygen vacancies creation can be described by equation 1.3 using the Kröger-Vink notation:

Fe 2 O 3 CaT iO 3 ← ---→ 2Fe T i + V •• O + 3O X O (1.3)
According to this equation, one can evaluate the amount of oxygen vacancies created by the substitution with respect to the electro-neutrality in the case that all iron ions are +III giving δ = x/2 as the substitution of two Ti 4+ by two Fe 3+ leads to the creation of one oxygen vacancy.

Oxygen vacancies are the key to oxygen mobility in the perovskite structures because the mobility is ascribed to the hopping process. Basically the hopping process is an oxygen anion "jumping" into an oxygen vacancy as described by Fig 1 .7. At long range this process can be assimilated to oxygen mobility.

Several people were interested in describing oxygen conduction process in ternary oxides related to the perovskite structure. In 1976, J-C Grenier et al. [START_REF] Jean | Etude par diffraction X et microscopie electronique du système CaTiO3Ca2Fe2O5[END_REF] focused on the system CaTi 1-x Fe x O 3-δ between CaTiO 3 to Ca 2 Fe 2 O 5 . Using XRD and TEM, they revealed an organisation of the oxygen vacancies increasing with the iron amount. When x < 0.2 they describe a statistic distribution of oxygen vacancies. When x > 0.2 a long range ordering along the (0k0) plan (of the parent orthorhombic phase Pnma) is revealed by the appearance of oriented tetrahedron towards the [START_REF] Rodríguez-Carvajal | Recent developments of the program fullprof. Commission on powder diffraction[END_REF] axis. The perovskite structure is a stacking of octahedral layers but in the same study when x > 0.5, Grenier spotted an alternative stacking of octahedral and tetrahedral layers whose frequency depend on the iron amount. More precisely they detail the formation of two ordered phases: one with a tetrahedral layer every two octahedral layers for x = 0,67 (CaTi 0.33 Fe 0.67 O 3-δ ) and one with one tetrahedral layer every octahedral layer at x = 1 (CaFeO 2.5 ). The latter corresponds to the Brownmillerite defined structure and will be further described in the next section. In 1988 Iwahara et al. [START_REF] Iwahara | Mixed conduction and oxygen permeation in the substituted oxides for CaTiO3[END_REF] reported a correlation between oxygen conductivity and iron content when 0 < x < 0.5. They reported CaTi 1-x Fe x O 3-δ with x = 0.2 having the highest oxygen conductivity. Oxygen conductivity cannot happen without electronic conduction, so one can have interest in knowing where the electrons are localised. The different possibilities listed by [START_REF] F M Figueiredo | On the relationships between structure , oxygen stoichiometry and[END_REF] are that electrons can be trapped by iron ions or by defects such as oxygen vacancies or they can even be free in the conduction bands. Using coulometric titration that they compared to Mössbauer spectroscopy with oxygen non-stoichiometry data they assume that electrons should be trapped on iron ions. [START_REF] Becerro | Oxygen vacancy ordering in CaTiO3-CaFeO2.5 perovskites: From isolated defects to infinite sheets[END_REF] This phase diagram describes the phase transition observed with temperature and compositions in the CaTiO 3 -CaFeO 2.5 system. Here the diagram starts at 900 • C whereas the synthesis temperature in this work is 800 • C. Nonetheless, the general information (phase boundaries, partial ordered domain and compositions of ordered structures) from this diagram can be extrapolated to this work.

CHAPTER 1. BIBLIOGRAPHY / INTRODUCTION

The basic consideration is that increasing the iron amount when substituting Ti by Fe in CaTiO 3 induces phase transitions that are accompanied by an increasing order. Starting from cubic CaTiO 3 , two domains of disordered structures exist until reaching a partially ordered domain when x is about 0.12. The partially ordered domain remains until x = 0.5. After that there exist three ordered structures namely TOOO, TOO and TO at x = 0.5 ; 0,67 and 1 respectively. In these ordered structures T stands for a tetrahedral layer and O for an octahedral layer. So TOOO, TOO and TO detail the sequence of the layers in these ordered structures. One can consider that the domains between two ordered phases is a weighted mix of the two ordered phases.

OXYGEN CONDUCTORS: PEROVSKITE AND BROWNMILLERITE

25

The three ordered phases are described in the literature: Hovmöller et al. [66] reported the TOOO structure at x = 0.5 with the general formula Ca 4 Fe 2 Ti 2 O 11 . They described it as "an ordered inter-growth between two perovskite unit cells and one-half brownmillerite unit cell". They determined the space group to be Pnma and added a schematic representation over a high-resolution electron microscope picture presented on Fig. 1.9. Note that the orientation is different for every tetrahedral layer.

Iron can have a tetrahedral coordination but titanium cannot, so the atoms in the tetrahedron have to be iron. Therefore, half the iron are the tetrahedron whereas the other half and all of titanium atoms are in the octahedron. Calcium position is not defined in the paper but they expect it to be in the spaces between the tetrahedron and between the octahedron (Ca interstitial). The following table 1.7 gives the coordinates of iron and titanium atoms: Taken from [66] The second TOO structure has been described by Rodriguez-Carvajalet al [START_REF] Rodríguez-Carvajal | Perovskite threefold superlattices: A structure determination of the A3M3O8phase[END_REF] as "an ordered intergrowth of the perovskite and the brownmillerite structures, that is [...] a threefold superstructure of the perovskite subcell with cell constants: √ 2a c * 3a c * √ 2a c ". The general formula of the compound is Ca 3 Fe 2 TiO 8 (corresponding to x = 0.67 in CaTi 1-x Fe x O 3-δ ) and the space group is Pcm 21 in accordance with table 1.8 giving the refined atom positions and lattice parameters from the same publication: Taken from [START_REF] Rodríguez-Carvajal | Perovskite threefold superlattices: A structure determination of the A3M3O8phase[END_REF] The TO structures corresponds to the brownmillerite structure that will be further described in the next section, so here only a primitive description is given. Brownmillerite structure was first reported in 1897 but the first paper to claim a brownmillerite refined structure was published by Colville [START_REF] Colville | The crystal structure of Ca2Fe2O5 and its relation to the nuclear electric field gradient at the iron sites[END_REF] in 1970. Working on CaFeO 2.5 (corresponding to x = 1 in CaTi 1-x Fe x O 3-δ ), Colville described the structure as "perovskite-like layers of oxygen octahedra connected along the b axis by single chains of tetrahedra centred at b = 1 4 " (in this description axis correspond to those of the parent perovskite As one can observe on the representation, in the brownmillerite structure the orientation is different every tetrahedral layers. Alternative orientation of the tetrahedral is one of the features of low temperature oxygen mobility in brownmillerite but this concept will be detailed in the section dedicated to the brownmillerite structure. The replacement of two Ti 4+ by two Fe 3+ results in the formation of two pentaco-Figure 1.12: Projection of the cubic structure onto the (0k0) plane showing the growth of an oxygen vacancy chain along [START_REF] Rodríguez-Carvajal | Recent developments of the program fullprof. Commission on powder diffraction[END_REF] . Taken from [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] after [START_REF] Becerro | Oxygen vacancy ordering in CaTiO3-CaFeO2.5 perovskites: From isolated defects to infinite sheets[END_REF] ordinated sites (occupied by Ti and one Fe 3+ ) and a single oxygen vacancy. The other Fe 3+ is replacing an Ti 4+ in an octahedral site. This isolated oxygen vacancy is called a monomer by McCammon.

Substitution of two more Fe atoms is forming a second oxygen vacancy plus a tetrahedral site and two original pentacoordinated sites. This second oxygen vacancy can either be isolated forming the monomer or can be associated to a monomer forming a dimer. In the latter, one Fe would probably be associated to the oxygen vacancy (the tetrahedral site in the case of the illustration Fig. 1.12), while the other would replace Ti on an octahedral site.

Substituting two more iron atoms will form a third oxygen vacancy that can be isolated or associated with other oxygen vacancies. If associated with a dimer, two possibilities arise. Either the forming chain is straight or kinked. The straight configuration is more favourable to the oxygen mobility because the kinked one will result in very low coordination for the A site ( [START_REF] Komornicki | Distribution des lacunes anioniques dans quelques perovskites non-stoechiometriques AMO3y[END_REF]). The straight chain of three oxygen vacancies forming a trimer is associated with an iron atom in the newly formed tetrahedral site while the other iron will replace titanium in an octahedral site.

Addition of further oxygen vacancies will increase the number of tetrahedral sites while the number of pentacoordinated site would remain the same (one at each vacancy Taken from [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] Consequently, properties of CaTi 1-x Fe x O 3-δ are not only related to the number of vacancies but also to the length of the vacancies chains. McCammon [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] interpretation of this concept is: "ionic conductivity values increase with increasing x due to the higher concentration of oxygen vacancies, but this effect is offset by the increasing length of vacancy chains which enhances the tendency to trap vacancies and reduce their mobility".

Fe-based perovskite in catalysis

Perovskite as catalytic support is among the mostly studied class of oxides. Numerous studies can be found but this work will focus on iron-based perovskite. Even though cobalt and manganese perovskites were reported [START_REF] Royer | Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides[END_REF] to be more active in CO oxidation than ferrite perovskite, the latter keeps constant interest because of low price and high availability of iron precursors. LaFeO 3 may be the most studied system. Yakovleva [START_REF] Yakovleva | Forms of Oxygen in La 1 â x Ca x MnO 3 + d ( x = 0 â 1 ) Perovskites and Their Reactivities in Oxidation Reactions[END_REF] substituted La by Sr (La 1-x Sr x FeO 3 ) after annealing at 1100 • C. Best activity was reported when x = 0.3 with the most active materials being two distinct phases with different La/Sr ratios. Isupova [START_REF] La Isupova | Oxygen States in Oxides with a Perovskite Structure and Their Catalytic Activity in Complete Oxidation Reactions: System La 1 â x Ca x FeO 3 â y (x = 0â1)[END_REF] substituted La by Ca (La 1-x Ca x FeO 3 ) and reported x = 0.6 showing the best activity. B site substitution in the system LaFe 1-x Mg x O 3 was investigated by Porta [START_REF] Ciambelli | AFeO 3 ( A = La , Nd , Sm ) and LaFe 1 â x Mg x O 3 perovskites as methane combustion and CO oxidation catalysts : structural , redox and catalytic properties[END_REF] and Ciambelli [START_REF] Ciambelli | La, Ca and Fe oxide perovskites: Preparation, characterization and catalytic properties for methane combustion[END_REF] and reported increased specific catalytic activity at low Mg content. They explained it by the increase of SSA of Mg-containing sample but highlighted that Mg does not increase the intrinsic activity of LaFeO 3 per m 2 . LaAl 1-x Fe x O 3 was also studied by Ciambelli [START_REF] Ciambelli | \ce{CO} oxidation and methane combustion on \ce{LaAl_{$1x$}}\ce{Fe_{$x$}O3} perovskite solid solutions[END_REF] but comprehension of catalytic result were complex due to the formation of Fe 2 O 3 active in CO oxidation. When working with SmFeO 3 perovskites, Mori et al. [START_REF] Mori | Effect of preparation routes on the catalytic activity over SmFeO3oxide[END_REF] reported carbonates strongly held at the surface, partially covering iron species thus decreasing CO oxidation activity. Recently, Yang et al. [START_REF] Yang | Activity of perovskite-type mixed oxides for the low-temperature CO oxidation: Evidence of oxygen species participation from the solid[END_REF] explored LaFeO 3 substituted by Cu and Pd. They reported increase activity for Cu containing samples while Pd, normally very active for CO oxidation was unable to promote activity at low temperature. 16 O/ 18 O isotopic exchange studies revealed bulk oxygen participation on Cu-substituted materials that display very high oxygen mobility. Similar results were obtained over LaMnO 3 and the CaTi 1-x Fe x O 3-δ [START_REF] Richard | ISO-TOPIQUE A L'ÉTUDE DE SYSTEMES CATALYTIQUES INNOVANTS[END_REF][START_REF] Richard | Investigation of Methane Oxidation Reactions Over a Dual-Bed Catalyst System using18O Labelled DRIFTS coupling[END_REF][START_REF] Richard | Remarkable Enhancement of O 2 Activation on Yttrium-Stabilized Zirconia Surface in a Dual Catalyst Bed[END_REF][START_REF] Repecaud | Enhancement of Oxygen Activation and Mobility in CaTi x Fe 1 À x O 3 À d Oxides[END_REF].

The brownmillerite structure

The brownmillerite structure is an oxygen deficient perovskite belonging to the perovskite supergroup with the general formula ABO 2.5 [START_REF] Bay | Nomenclature of the perovskite supergroup : A hierarchical system of classification based on crystal structure and composition[END_REF] (with A: an alkaline earth and B a transition metal). It was first described as one of the major constituent of Portland cement by Törnebohm in 1897. This mineral was then redefined as a solid solution of 2 CaO and Fe 2 O 3 by Hanson, Bogue and Brownmiller and was named after the latter. The basic crystal structure consisting of layers of corner-sharing FeO 6 octahedra alternating with layers of FeO 4 tetrahedra was first suggested by Büssem in 1937-1938. The structure was eventually solved by Bertaut, Blum and Sagnieres in 1959 until a refinement was performed by Colville in 1970 [START_REF] Colville | The crystal structure of Ca2Fe2O5 and its relation to the nuclear electric field gradient at the iron sites[END_REF]. The lattice parameters can be √ 2a p * 4a p * √ 2a p with a p being the cubic perovskite parameters. The brownmillerite can be ascribed to three different space-groups depending on the tetrahedral layers arrangement [START_REF] Artem | Compositionally induced phase transition in the Ca2MnGa1-xAlxO5 solid solutions Ordering of tetrahedral chains in brownmillerite structure[END_REF]. Imma defines a statistical random presence of L and R orientations (R: right / L: left) while the two other are ordered: I2mb and Pnma due to regular alternation of the tetrahedral layers R-L as represented in Fig. 1.14. In this work two brownmillerite compositions will be under investigation: CaFeO 2.5 and SrFeO 2.5 . Figure 1.14: Representation of the brownmillerite structure in the Pnma space-group with alternative orientation of L and R tetrahedron layers. Taken from [START_REF] Artem | Compositionally induced phase transition in the Ca2MnGa1-xAlxO5 solid solutions Ordering of tetrahedral chains in brownmillerite structure[END_REF] Low temperature oxygen mobility Low temperature oxygen mobility was not studied until oxygen oxygen intercalation was performed in LaCuO 4 by Rudolf et al. [START_REF] Rudolf | Electron/proton transfer reactions and superconductivity: The role of hydrogen in lanthanum copper oxide[END_REF]. This concept was extended in the 1990s when using similar method to prepare SrFeO 3 from SrFeO 2.5 the latter being a brownmillerite. Numerous studies report reversible oxygen ion intercalation at ambient temperature in the brownmillerite framework AMO 2.5 (with A = Sr; Ca and M = Fe; Co) [START_REF] Artem | Compositionally induced phase transition in the Ca2MnGa1-xAlxO5 solid solutions Ordering of tetrahedral chains in brownmillerite structure[END_REF][START_REF] Nemudry | Room Temperature Electrochemical Redox Reactions of the Defect Perovskite Sr-FeO <sub>2.5+</sub>[END_REF][START_REF] Nemudry | Topotactic electrochemical redox reactions of the defect perovskite SrCoO2. 5+ x[END_REF][START_REF] Grenier | The electrochemical oxidation: A new way for preparing highly oxidized ferrites[END_REF][START_REF] Ronan Le Toquin | Time-resolved in situ studies of oxygen intercalation into SrCoO2.5, performed by neutron diffraction and X-ray absorption spectroscopy[END_REF][START_REF] Wattiaux | A novel preparation method of the SrFeO3cubic perovskite by electrochemical means[END_REF][START_REF] Inoue | Anisotropic oxygen diffusion at low temperature in perovskite-structure iron oxides[END_REF]. Classical explanation through hopping process requires high tem-1.3. OXYGEN CONDUCTORS: PEROVSKITE AND BROWNMILLERITE 35 perature therefore studies were driven in order to find another mechanism explaining oxygen mobility at room temperature. Paulus et al. [START_REF] Paulus | Lattice dynamics to trigger low temperature oxygen mobility in solid oxide ion conductors[END_REF] described such mechanism using density functional theory in combination with inelastic neutron scattering. First, the specific configuration of the brownmillerite framework displaying the presence of 1D ordered oxygen vacancy channels in the tetrahedral layers associated with the specific alternative orientation of the tetrahedral layers are key features. Based on this, Paulus managed to prove that the permanent orientation switching mode of the tetrahedral layers acts like a gate for the apical oxygen atoms. This "gate" opens and closes with the switching frequency and is "pushing" the oxygen atoms through. If an apical oxygen is pushed far enough from its equilibrium position, it as been calculated (using Density Functional theory) that this oxygen atom can escape in the 1D vacancy channels of the tetrahedral layers. Once escaped, the apical oxygen will leave behind a square pyramid and a reoriented tetrahedron, see Fig. The second brownmillerite composition under study is SrFeO 2.5 . It is first necessary to specify that SrFeO x system is not stoichiometric contrary to the previously described CaFeO 2.5 structure. The consequence is that there exist meta-stable phases which make synthesis and characterisations of SrFeO x compounds truly difficult. SrFeO x system has been investigated by many groups [START_REF] Tofield | The SrFeO2.5| SrFeO3.0system. Evidence of a new phase Sr4Fe4O11(SrFeO2.75)[END_REF][START_REF] Greaves | A Powder Neutron Diffraction Investigation of the Nuclear and Magnetic Structure of ShFezOs[END_REF][START_REF] Gibb | Magnetic exchange interactions in perovskite solid solutions. Part 5. The unusual defect structure of SrFeO3 ?y[END_REF][START_REF] Takeda | Phase relation in the oxygen nonstoichiometric system, SrFeOx (2.5 â x â 3.0)[END_REF] using X-ray diffraction and Mössbauer spectroscopy in order to identify iron oxidation degree and oxygen vacancies. From the literature, four structures with different oxygen stoichiometries are identified in the system SrFeO x with 0 ≤ δ ≤ 0.5: SrFeO 3 ; SrFeO 2.875 ; SrFeO 2.75 and SrFeO 2.5 . In 2000, Hodges et al. [START_REF] Hodges | Evolution of oxygen-vacancy ordered crystal structures in the perovskite series Sr(n)Fe(n)O(3n-1) (n = 2, 4, 8, and â), and the relationship to electronic and magnetic properties[END_REF] made the first study identifying the oxygen vacancies ordering for all the synthesised phases by high temperature synthesis using neutron diffraction.

SrFeO 3 is the fully oxidised member of this system adopting the ideal cubic perovskite structure in the P m3m space-group. One possible representation of the system is to consider that other members of the systems derive from SrFeO 3 with higher amount of ordered oxygen vacancies. The number of oxygen vacancies being higher and higher for each members (regarding their amount of oxygen vacancies: SrFeO 2.875 < SrFeO 2.75 < SrFeO 2.5 ). Indeed, the two intermediate members SrFeO 2.875 and SrFeO 2.75 have one oxygen vacancy every 8 and 4 perovskite cells respectively, along the [START_REF] Rodríguez-Carvajal | Recent developments of the program fullprof. Commission on powder diffraction[END_REF] plan of the perovskite cell. The last member of the SrFeO x system SrFeO 2.5 is the only one adopting the brownmillerite structure. From the parent perovskite structure, one out of six oxygen is removed. These vacancies are also ordered along the [START_REF] Rodríguez-Carvajal | Recent developments of the program fullprof. Commission on powder diffraction[END_REF] plan of the perovskite parent cell leading to a layered structure alternating FeO 6 octahedral layers and FeO 4 tetrahedral layers. The description of the structure is still under discussion in the literature [START_REF] Avishek | Exploring oxygen diffusion mechanisms in SrFeO 3 â x and Pr 2 NiO 4 + δ , followed up on single crystals by in situ synchrotron diffraction[END_REF] but as proposed by Greaves et al. [START_REF] Greaves | A Powder Neutron Diffraction Investigation of the Nuclear and Magnetic Structure of ShFezOs[END_REF] it can be ascribed to the Icmm space-group. This space-groups described the randomness of the tetrahedral layers orientations (L or R). Fig. 1.16 gives a schematic representation of the members of the SrFeO x system in association with the lattice parameters evolution regarding a p the parent perovskite parameter.

Table 1.11 is referencing the space-groups and lattice parameters for SrFeO x system as reported by Hodges et al. [START_REF] Hodges | Evolution of oxygen-vacancy ordered crystal structures in the perovskite series Sr(n)Fe(n)O(3n-1) (n = 2, 4, 8, and â), and the relationship to electronic and magnetic properties[END_REF] obtained from neutron powder diffraction. , the Fe-O biding strength is lower. Therefore, the oxygen mobility is supposed to be facilitated. In CaFeO 2.5 the orientation of the tetrahedral layers is alternative: L-R-L-R-L. Whereas the orientation of the tetrahedral layers in SrFeO 2.5 is random, this specificity may also have an impact on the oxygen mobility. The brownmillerite specific structural architecture is a key parameters allowing low temperature oxygen mobility. But the alternative orientation of the tetrahedral layers as well as the Fe-O binding strength have an influence over the oxygen mobility mechanism.

Brownmillerite as catalytic support

Numerous studies report utilisation of perovskite materials as catalytic support as reported previously at page 32. But less studies report utilisation of brownmillerite materials as catalytic supports. Nonetheless, Shin et al. [START_REF] Shin | Catalytic decomposition of NO over brownmillerite-like compounds, Ca2Fe2O5 and Sr2Fe2O5[END_REF] reported utilisation of CaFeO 2.5 and SrFeO 2.5 as support for NO reduction. They reported remarkable NO decomposition for SrFeO 2.5 between 700 and 900 • C whereas no activity was observed for CaFeO 2.5 . Hirabayashi et al. [START_REF] Hirabayashi | Formation of brownmillerite type calcium ferrite (Ca2Fe2O5) and catalytic properties in propylene combustion[END_REF] showed catalytic activity for propylene combustion when using CaFeO 2.5 as support in the temperature range 250-450 The interest of perovskite and brownmillerite materials lies in their specific oxygen vacancies configuration. The latter impacts the catalytic actvity but also oxygen conductivity. Therefore it is interesting to compare oxygen conductivity measured on similar perovskite and brownmillerite materials. Unfortunately it is difficult to provide comparative results for oxygen conduction measurements. Nonetheless, Shaula et al. [START_REF] Shaula | Ionic conductivity of brownmillerite-type calcium ferrite under oxidizing conditions[END_REF] reported an higher oxygen conduction in CaFeO 2.5 than in CaFe 0.5 Al 0.5 O 2.5 . But the values reported remain at least 10 times lower than those obtained for per-Chapter 2

Experimental methods

Introduction

The chapter aim at describe the experimental methods used during this thesis work. The synthesis routes (for perovskite and brownmillerite materials) are going to be described followed by the platinum impregnation method. The methods used for 18 O enrichment and study of oxygen mobility are going to be briefly introduced but as they as a whole chapter is dedicated to them, more information can be found in chapter 3. The characterisations techniques (thermogravimetric analysis followed by mass spectrometry (TG-MS); X-ray diffraction (XRD); N 2 -physisorption; Transmission electron microscope (TEM); temperature programmed reduction (TPR)) are also described, measurements parameters and specific conditions will be given. Finally, the experiments aimed at evaluating the catalytic performances of the materials will be detailed.

CHAPTER 2. EXPERIMENTAL METHODS

Synthesis

Synthesis of perovskite materials

CaTi 1-x Fe x O 3-δ materials were synthesised using a modified Pechini method [START_REF] Pechini | PECHINI PATENT[END_REF]. In a typical synthesis, proper amounts of the precursors, metal nitrates salts Ca(NO 3 ) 2 • 4 H 2 O (Sigma Aldrich, >99%) and Fe(NO 3 ) 3 • 9 H 2 O (Alfa Aesar, 98%), were dissolved in distilled water (20 mL). After the addition of 20 g of citric acid C 6 H 8 O 7 (Alfa Aesar, 99%), pH was checked to be ≤ 1 and then titanium propoxide TiC 12 H 28 O 4 (Sigma Aldrich, 98%) was added. The solution was stirred (500 rpm) overnight to ensure dissolution of all the metal precursors. The solution was then heated at 120 • C by using a hot plate until the volume was less than 50 mL. 20 mL of ethylene glycol C 2 H 6 O 2 (Alfa Aesar, 99%) were added to the solution. The obtained gel was dried in a heat chamber at 120 • C overnight. once dried, the mixture was manually crushed and calcined following a three-steps temperature program: 4 hours at 130 • C (ramp rate is 4 • C.min -1 ), then 4 hours at 400 • C (ramp rate is 1.5 • C.min -1 ), and finally 2 hours at 800 • C (ramp rate is 1.66 • C.min -1 ). The obtained powders were eventually manually crushed in a mortar.

Synthesis of brownmillerite materials

Synthesis by complexation

The CaFeO 2.5 and SrFeO x powders were prepared by a method close to the citrate route, hereby called complexation method. Stoichiometric amounts of calcium (II) nitrate tetrahydrate (Ca(NO 3 ) 2 • 4 H 2 O Sigma Aldrich, >99%) or strontium (II) nitrate (Sr(NO 3 ) 2 Sigma Aldrich >99%) and iron (III) nitrate nonahydrate (Fe(NO 3 ) 3 • 9 H 2 O Alfa Aesar, 98%) were dissolved in water with citric acid (C 6 H 8 O 7 Sigma-Aldrich > 99.5%) molar ratio of citric acid towards total amount of cations = 1. The solution was stirred for 2 hours (ensuring complete dissolution of nitrates salts). While stirring, ammonia solution 28% in water (Merck) was added drop wisely until pH = 3 followed by the addition of ethylene-glycol (HOCH 2 CH 2 OH Sigma Aldrich > 99.8%) (ratio is 1 mL per gram of final material) and the solution was evaporated at 90 • C until gelification. The resulting gel was then quickly fired at 400 • C in air to obtain a black powder. The as-obtained powder was converted to brownmillerite by heating under atmosphere (synthetic air for CaFeO 2.5 and pure argon for SrFeO x ) at 2 • C.min -1 up 2.2. SYNTHESIS to 600 • C maintained there for 6 hours. The powders were finally crushed in a mortar by hand resulting in brown powder for CaFeO 2.5 and black powder for SrFeO x .

Synthesis by electric arc furnace

To synthesise bulk powders of Brownmillerite, an Electric Arc Furnace with graphite electrodes was used. Fig. 2.1 presents a schematic representation of the electric arc furnace. The general principle of such furnace is to melt a mix precursors as powders using an electric arc. The electric arc is produced by applying a difference of potential between two graphite electrodes. The precursor materials used for these synthesis were CaCO In the furnace stoichiometric amounts of CaCO 3 powder or SrCO 3 powder were mixed with Fe 2 O 3 powder. Then a difference of potential was introduced between the two graphite electrodes, creating an electric arc. Controlling intensity and voltage, the furnace reached the temperature required to obtain the brownmillerite phase. The atmosphere used was air. In our case, the electrodes were not dipped into the powders so the electric arc remained at the surface. The liquid mix was then quenched in air giving cm size agglomerates. These agglomerates were firstly crushed during 72 hours in a low energy ball-mill. Using laser scattering, particle size analysis we measured the mean particle size of 12.5 µm for CaFeO 2.5 and SrFeO x . Afterwards the powders were grinded again in isopropanol during 1 hour at high energy (1000 rpm). The agglomerates mean size were then 1.3 µm for CaFeO 2.5 and around 1 µm SrFeO x .

Platinum impregnation

The obtained powders were impregnated with 1 wt.% of platinum using wet impregnation method. A suspension of bare powders into isopropanol (Sigma-Aldrich > 98%) was placed under stirring while the proper amount of Pt(NO 3 ) 2 (Heraeus >99%) was added. After 30 minutes in an ultra-sound bath the suspension was placed in a rotavapor (Heidolph) 75 rpm, 130 mbar, 47 • C until obtaining a dry powder. The resulting powder was then calcined at 500 • C during 2 hours in air (10 • C.min -1 ).

O was used to allow mass spectrometry study of oxygen mobility. The two possibilities are either to use 18 O in the gaseous phase or to replace 16 O in the solid phase by 18 O enrichment. In order to use a continuous gas flow during the oxygen mobility study, and to make an economic use of 18 O the second possibility was chosen. The general principle of the isotopic enrichment is to perform successive steps of vacuum and pure

18

O in a close vessel containing the powders. A whole chapter of this manuscript is dedicated to the ILPOR technique and the first section of the later is dedicated to the enrichment process at page 55 ff, where further explanation can be found.

Isotope labelling pulses oxidation reaction

ILPOR standing for Isotope labelling pulses oxidation reaction is a pulsed method designed to study in-situ oxygen mobility during carbon monoxide oxidation. This quite complex technique was first developed in Penkala PhD thesis [START_REF] Penlkala | Low Temperature Oxygen Mobility Applied to Catalysis[END_REF] and has been further improved during this work. Therefore, a whole chapter is dedicated to explain in detail the method. One can find complete description of the method at page 55 ff. The optimised parameters of the technique are here just reminded:

Sample weight: 200 mg

Loop volume: 1 mL

Waiting time between pulses: 2 min

Gases: 1%C 16 O/He interrupted by pulses of pure 16 O 2

The total gas flow is 32 mL.min -1 . The temperature program starts at 40

Characterisations

Thermo-gravimetry followed by mass spectrometry

To identify the temperature upon which pure isotopic exchange occurs, and its mechanism, TG-MS measurements were performed. 20 mg of powder were placed in the thermo-balance (TG96, SETARAM) which was heated at 5 • C.min -1 under a stream of air (50 mL.min -1 ). The outgoing gases where identified with a mass spectrometer (Omnistar, PFEIFFER VACUUM) during the whole temperature program. The same setup was used to perform the thermo-gravimetric analysis of the polymer foam of CaTi 0.9 Fe 0.1 O 3-δ but the analysis was not coupled with a mass-spectrometer that time.

X-ray diffraction

Room temperature XRD analysis were performed on a PANanalytical X'Pert with an X'Celerator detector working in Bragg-Bentano geometry (θ -2θ) and with CuKα (45 kV and 40 mA) radiation. A graphite mono-chromator was placed after the sample to remove the fluorescence due to the high amount of iron in the samples. The diffractograms were recorded from 10 to 80 • with 0.017 • steps and 100 seconds per step. Phase identification was performed using EVA software (BRÜCKER) and the ICDD 2016 database. The refinement were performed using Fullprof [START_REF] Rodríguez-Carvajal | Recent developments of the program fullprof. Commission on powder diffraction[END_REF] software and crystallographic information data-file from the inorganic crystal structure database. The diffractograms in the section about the optimisation of SrFeO x synthesis (see page 80 ff) are presenting a halo in the region 12-25 • . This amorphous halo is due to a protective dome placed on the sample-holder to prevent any spillage of the fluffy powders.

Temperature programmed

The temperature programmed XRD measurement presented page 124, has also been recorded with PANanalytical X'Pert with an X'Celerator detector working in Bragg-Bentano geometry (θ -2θ) and with CuKα (45 kV and 40 mA) radiation. The sample was placed in an Anton Paar HTK2000 reactor chamber. The diffractograms were recorded from 10 to 80 • but with 0.03 • step and 100 seconds per step this time. The step are larger than the room temperature in order to reduce the recording time as a too long recording time will not be representative to a continuous temperature

X-ray fluorescence

Semi-quantitative analysis of the compositions with X-ray fluorescence were performed with a MagiX PRO fluorescence from PANanalytical. The powders were weighted and mixed with a proper amount of LiB 4 and melted. The obtained pearl was used to perform the analysis itself. Semi-Quantitative measurement were carried out using the SuperQ software.

N 2 -physisorption

Specific surface area were measured by N 2 physisorption using the BET [START_REF] Brunauer | Adsorption of Gases in Multimolecular Layers[END_REF] method. Each samples were degassed under vacuum for one hour at 180 • C. The measurement itself was performed by adsorbing and desorbing nitrogen at 78 K in a ASAP 2020 system (Micromeritics). The cells were containing between 5 and 50 m 2 • g -1 ensuring a good measurement.

Transmission electron microscope imaging

In order to quantify the platinum impregnation in term of platinum dispersion, transmission electron microscope (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) pictures were taken. The TEM pictures were taken using a TEM Jeol 2010 and the HAADF-STEM with a FEI TITAN ETEM at CLYM, IRCE Lyon.

Temperature programmed reduction

To study the reducibility of the materials, temperature programmed reduction (TPR) experiments were performed. For this, an automated catalyst characterization system Autochem 2920 (Micromeritics) was used. 200 mg of each sample were introduced on quartz wool in a U-shaped quartz reactor. In the first part of the experiment (e.g. oxidative pre-treatment) the reactor was heated up to 800 • C (10 • C.min -1 ) for one hour under pure oxygen (30 mL.min -1 ) and then cooled down to room temperature under helium flow. The pre-treatment was followed by the reductive step, during which the sample was heated under 3H 2 /97% Argon (30 mL.min -1 ) at 10 • C.min -1 up to 1 000 • C and kept during one hour under the same condition. The sample H 2 intake was measured with a thermal conductivity detector (TCD). The TCD is comparing the thermal conductivity of two gas lines. In these two lines 3H 2 /97% Argon is flowing by one is bypassing the sample (called reference) and the other one is flowing through the sample (carrier line). When hydrogen is consumed by the sample reduction the thermal conductivity of the gas in the carrier line is changing. The TCD is compensating the difference in the thermal conductivity using bridged resistance. Therefore the results are presenting the intensity as a function of the temperature.

CATALYTIC OXIDATION OF CARBON MONOXIDE

Catalytic oxidation of carbon monoxide

Catalytic performances of samples (with of without platinum impregnated) were measured using oxidation of carbon monoxide as model reaction see equation 2.1.

CO + 1 2 O 2 → CO 2 (2.1)
A U-shaped quartz reactor was filled with 200 mg of samples for temperature programmed experiments and with 100 mg of sample for isotherm experiments. The grain size of the powders was kept between 125-250 µm by consecutive steps of pressing and sieving. Prior to the catalytic reaction itself, each sample was subject to a reducive pre-treatment at 300 or 200 • C during 2 hours under 40% H 2 /60% Helium (10 L.h -1 ). Thereafter, a mix of 6 000 ppm of CO and 10 000 ppm of O 2 (diluted in helium to reach 10 L. h -1 flow) was sent through the catalytic bed which was heated from room temperature to 500 • C at 2 • C.min -1 . The gas products were detected and quantified with a micro-gas chromatography (SRA instruments). From the chromatography quantification, we are able to define the CO conversion towards CO disappearance using equation 2.2:

Conversion(%) = CO i -CO 0 CO 0 * 100 (2.2)
Where CO i is the CO concentration at a specific time and CO 0 is the intial CO concetration (e.g. 6000 ppm). The catalytic performances of each samples was done three (two for the CaTi 1-x Fe x O 3-δ compositions) times in a row, the sample was just flushed with helium while the temperature was decreasing.

Chapter 3

Isotope Labelling Pulses Oxygen Reaction: Optimization

Introduction

In the field of catalysis, the understanding of mechanism in necessary to the development of more reactive catalysts. The goal is to be able to tune the properties of the catalyst either to reach better activity or to protect it from undesired deactivation. Indeed, if one knows the catalyst deactivation mechanism, then one can think of a solution to solve the deactivation. But being able to understand a catalytic mechanism is not easy. In the development of active catalytic support, it is very important to be able to spot if a support is active or not. An active support can be defined as a material able to speed up a reaction or to quantitatively increase a reaction rate thanks to the ability of being able to make part of its constituents taking part into the reaction. But being able to spot this ability, requires being able to know where each element is coming from. It could be very straightforward if the elements are different from one another. But in the frame of catalytic oxidation, one has to make the difference between oxygen atoms coming from the support and those coming from the gaseous phase a reactants.

To tackle the problem of where does a specific element is coming from, G. von Hevesy studied the utilization of radioactive isotopes as isotopic markers and received the Nobel prize for this in 1943 [START_REF] Nobelprize | NoThe Nobel Prize in Chemistry[END_REF]. Since then many authors used isotopes to probe oxygen mobility in oxides catalysts and Novakova published a compilation [START_REF] Novakova | Isotopic Exchange of Oxygen 18O Between the Gaseous Phase and Oxide Catalysts[END_REF] of numerous results in 1971. More recently, research scientists are still using similar techniques to 52CHAPTER 3. ISOTOPE LABELLING PULSES OXYGEN REACTION: OPTIMIZATION study oxygen mobility [START_REF] Descorme | Oxygen surface mobility and isotopic exchange on oxides: Role of the nature and the structure of metal particles[END_REF] [START_REF] Zhang-Steenwinkel | Step response and transient isotopic labelling studies into the mechanism of CO oxidation over La0.8Ce0.2MnO3perovskite[END_REF].

Techniques used to study oxygen transport processes using isotopic oxygen can be separated in two categories. First are the ex-situ techniques among which are secondary ions mass spectrometry (SIMS) [START_REF] Chater | Development of a novel SIMS technique for oxygen self-diffusion and surface exchange coefficient measurements in oxides of high diffusivity[END_REF] [106] and nuclear reaction analysis (NRA) [START_REF] Maria | Role of lattice oxygen in the partial oxidation of methane over Rh/zirconia-doped ceria. Isotopic studies[END_REF]. Then there are the in-situ techniques. In-situ techniques are also separated in two categories, those not based on pulse chemistry methods like the monitoring of time dependent equilibrium of isotopic oxygen species in the gas phase in a close vessel using a gas mass spectrometer. [START_REF] Dong | Characterization of the dynamic oxygen migration over Pt/CeO2-ZrO2catalysts by18O/16O isotopic exchange reaction[END_REF] [109] [START_REF] Richard | Remarkable Enhancement of O 2 Activation on Yttrium-Stabilized Zirconia Surface in a Dual Catalyst Bed[END_REF] And the techniques based on pulse chemisorption methods such as the steady-state isotopic transient kinetic analysis technique (SSITKA) [110] [111], the one developped by Bouwmeester et al. [START_REF] Henny | A novel pulse isotopic exchange technique for rapid determination of the oxygen surface exchange rate of oxide ion conductors[END_REF] or the technique isotope labelling pulses oxydation reaction (ILPOR) already develop in the lab during Penkala's work [START_REF] Penkala | The role of lattice oxygen in CO oxidation over Ce18O2-based catalysts revealed under operando conditions[END_REF]. All those techniques naturally have different advantages and drawbacks. Ex-situ techniques are very useful to determine oxygen transportation within the materials using depth profile analysis. In-situ techniques allow in-time observation and measurement of oxygen species by recording them in the gaseous phase with a mass spectrometer leading to mechanistic information. But they might require several pre-heating and cooling steps under different oxygen partial pressures. As these steps may impact the material, one has to take it into account when analysing materials behaviour. However, it is necessary to precise that for pulse isotopic exchange technique developed by Bouwmeester, a pretreatment is not mandatory. All those techniques may have different isotopic oxygen consumptions and one has to think about the consequent economical cost when building or using such techniques. This chapter is dedicated to the comprehensive description of ILPOR experimental setup and the improvements done during this study. A first part describes the isotopic enrichment, then a description of our experimental setup. And finally a description of the improvements illustrated by examples.

Isotopic enrichment

Using isotopes as probe molecules requires some anticipation. All isotopes of a specific element do not have the same natural relative abundance. Table 3.1 is showing the relative abundance of the three oxygen isotopes taken from Hargreaves [START_REF] Hargreaves | of Catalytic Science Series[END_REF]. Natural abundance of each oxygen isotopes is of crucial importance as it is related to the economic prize of the associated isotope. Techniques based on centrifugation are usually used to refine a specific isotope and consequently the more rare an isotope is, the more difficult it is to refine. This explains why 18 O is more used than 17 O to probe oxygen mobility. But 17 O also find application thanks to its nuclear spin or when two probe molecules are required. In this work, we are using 18 O provided by Eurisotop (purity 97.2%). As we aim to probe the oxygen mobility in reaction conditions ("continuous" flow of reactant) we made the decision of enriching each samples with 18 O instead of pulsing the 18 O in order to limit the consumption. Enrichment is performed in a close vessel connected to the 18 O cannister, a pressure sensor and a primary vacuum pump. The vessel is maintained at 400 • C by a furnace during the whole enrichment. The enrichment consists in three steps, with each is made of a time under vacuum followed by a time under 18 O atmosphere. Some assumptions have to be made, one is that the temperature (400 • C) is sufficient to allow oxygen exchange between atmosphere and solid. And the other is that the time for 16 O/ 18 O exchange is long enough to reach the equilibrium. During each step under 18 O atmosphere, the 16 O atoms from the material lattice will be exchanged with the 18 O from the atmosphere until the equilibrium between 18 O and 16 O is reached, both in the solid and gaseous phase. In other words, the equilibrium is reached when 18 O concentration is the same in the lattice and the atmosphere. So the parameters over 54CHAPTER 3. ISOTOPE LABELLING PULSES OXYGEN REACTION: OPTIMIZATION it are the number of 16 O inside the vessel (which is related to the sample mass), the pressure inside the vessel and the volume of the vessel. Each equilibrium will increase the 18 O concentration in the sample. Assuming that the 18 O sample concentration is 0 prior to the enrichment, the 18 O sample concentration ([C 0 ]) at the first equilibrium is defined by equation 3.1:

C 0 (%) = n18 O n18 O + n16 O * 100 (3.1)
Where n18 O is the number of 18 O atoms is the system defined by equation 3.2 following the perfect gas law:

n18 O (mol) = P vessel * V vessel R * T vessel (3.2)
Where P vessel is the pressure inside the vessel in pascal (usually about 120 000 P), V vessel is the volume of the vessel in cubic meter (80.10 -5 m 3 ) and T vessel is the vessel temperature in Kelvin (673.15 K). Also, n16 O is the number of 16 O atoms in the system, (coming only from the sample for the first step) defined by equation 3. With this set of equations, we area able to theoretically define the 18 O concentration in the sample after the first enrichment step. But the exchanged yield is linked to the quantity of oxygen in both gaseous and solid phases. As we want to ensure that bulk oxygen are exchanged, we chose to increase the enrichment yield and so we repeated the steps previously described three times. Therefore we can determine the 18 O sample concentration (C i (%)) for the step i using equation 3.4:

C i (%) = n18 O + n16 O * C i-1 n18 O + n16 O * 100 (3.4)
Thanks to the equation set 3.1-3.4 the theoretical concentration of 18 O in the samples lattice can be determined. The enrichment yield will be controlled afterwards by a thermo-gravimetric analysis coupled with a mass-spectrometer (see page 90).

ILPOR GENERALITIES

55

ILPOR generalities

Experimental set-up

The ILPOR experimental set-up uses an autochem 2920 from Micromeritics connected to a GSD 301 C1 Omnistar mass-spectrometer from Pfeiffer Vaccum. This experimental set-up has been build to be similar to the classic catalytic test conditions. But the major difference comes from the utilization of an automated loop to introduce pulses of 16 O 2(g) . The purpose of this, is to allow a time resolution control by mastering the pluses duration and frequency. And also to purposely limit the amount of 16 O 2(g) as it is limited by the loop volume. By respect to its purpose, this technique is called Isotope Labeling Pulse Oxidation Reaction (ILPOR). The apparatus described in Fig. 3.1 a) consists of a gas feed section, a reactor 56CHAPTER 3. ISOTOPE LABELLING PULSES OXYGEN REACTION: OPTIMIZATION placer in a vertical clamshell furnace and an on-line gas analysis section. The gas feeding section equipped with mass flow controllers contains an automated loop and a gas mixing system allowing the generation of tunable gas streams. The sample is placed in a U-shaped quartz reactor on a quartz wool bed. The reactor is placed in a clamshell furnace operating from room temperature to 1100 • C. The gas analysis systems is an on line quadrupole mass-spectrometer (Pfeiffer Vacuum GSD 301 C1 Omnistar) able to make one measure every three seconds. The loop of the gas feeding section is a six way valve with two ways connected to each other as described on Fig.

Experimental procedure

The experiments described in this work were performed in the temperature range 40

• C-500 • C. Different parameters were changed and evaluated during this work. The following table 3.2 describes the evolution of the different parameters performed along this work. Each parameters subject to variation is represented associated with its different values.

The total gas flow is 32 mL.min -1 and will be kept constant if the gas concentration is subject to change. The temperature program starts at 40 • C then increases by steps of 2 • C until 500 • C. At each 2 • C steps there is a small isotherm allowing the time to perform one pulse of 16 O 2(g) . The duration of this small isotherm is mastered by the pulse duration. The quadrupole mass spectrometer measures the different m/z: 2; 16; 17 ; 18; 19; 20; 28; 30; 32; 34; 36; 44;46; 48. Not all those masses will be represented for every example in the following sections as they are not always relevant and sometimes have been recorded only to control that everything went right with the experiment.

The temperature program as well as the m/z recorded by the mass spectrometer and the total gas flow will remain unchanged during this study. Besides one experiment which is not made with a temperature program but consists of two isothermal steps, see page 70. As numerous parameters can change from one example to another, the parameters corresponding to each test will be reminded at the beginning of each example for a clarity purpose.

ILPOR: a tunable technique

The following section will firstly describe the results obtained on a empty cell with the initial parameters. It will then describe the advantages and consequences of modifying the ILPOR parameters. It will also explain the philosophy of those changes, and so their impact such as highlighting a specific feature of catalytic materials.

Empty cell

This first example is the starting point of this study, it is the result of an ILPOR experiment with all the original parameters (before any optimization). This test is performed on an empty cell and can be considered as a reference regarding to other tests either with samples or when changing any parameters. The parameters for this first test are:

Sample weight: empty cell

Gas flow: 32 mL.min -1

Loop volume: 0.5 mL

Waiting time between pulses: 0 min

Gases: 1%C 16 O/He interrupted by pulses of 1% 16 O 2

The waiting time between pulses is a short amount of time mastering the loop injection. It correspond to awaiting time beginning once a pulse is finished and ending with the beginning of another pulse. It is necessary to precise that the pulse detection is followed by a Thermal Conductivity Detector (TCD) placed in parallel to the mass spectrometer but this TCD is absolutely not used to analyse the experiment. This TCD is detecting the pulses thanks to already defined threshold parameters (slope speed and slope acceleration regarding time). Once the TCD signal is back to baseline, a pulse is considered finished. So the time between two pulses actually is the time to perform the pulse itself (from the injection + the return to the TCD baseline), the waiting time between two pulses and the time for the temperature increase of 2 • C (the temperature ramp is of 2 • C so the time for the temperature increase is about one minute) . The temperature programme as the dark green plot, even looking like a constant temperature increase, is actually made of several small 2 • C increase steps between each there is an isothermal time corresponding to a unique pulse time.

As this experiment is performed with the initial parameters, it corresponds to a reference regarding the evolution of the different parameters. There is a measurement artefact at 14200 seconds, the cause of this artefact has not been identified so far, but it is no threat for the results analysis.

Modification of the C 16 O concentration

The first parameters that has been changed was the C 16 O concentration. The intensities of the different signals seem to be high enough for the detection the different species. But if the reactions leads to low amount of C 18 O 2 or 16 O 18 O, it could be difficult to identify and observe these species. To avoid this, we increase the C 16 O constant flow concentration from one to two percent, in order to increase the signal of the associated products. The first observation is that the two figures are similar. The MS scale bar has been added on purpose, so one can observe that the initial value for the C 16 O is around 1.3 E -11 on Fig. 3.3 a) and is about 8 E -11 on Fig. 3.3 b). It is normal that the intensity is higher on b) but it is 6 times higher whereas the concentration as only been increased by a factor of two. The general impact of this modification is that the intensity of all the others signals have been also increased. Even for 16 O 2 which should not been have increased as the concentration and the loop volume are the same. The detection and identification of the different species is therefore easier. But a problematic consequence arises. As the quantity of available C 16 O is higher, the oxidation is faster (the corresponding MS signal does not have the time to fully return to baseline between two pulses. Especially in the case of 16 O 2 which is the limiting reactant at each pulse. This is problematic because the pulses are also giving information of the materials performances. Another problem is the lack of oxygen. Indeed, with the constant flow of C 16 O compared to the pulses of 16 O 2 , makes that 16 O 2 is limiting reactant. See on Fig. 3.3 b) where quantity of 16 O 2 is zero after 11000 seconds. This is a problem as the aim of this experiment is to follow the mobility of oxygen. And when oxygen is lacking, it is more difficult to observe the latter.

Modification of the loop volume, the 16 O 2 and the loop frequency

In the latter parameters (e.g. Fig. 3.3 b) the problem was the limitation of the amount of 16 O 2 . Indeed, its consumption was so fast that the TCD could not detect the changes and so the pulse frequency was too high. Also as 16 O 2 was the lacking reactant, especially at high temperature, it is more difficult to observe the oxygen containing species.

ILPOR: A TUNABLE TECHNIQUE

63

The loop volume and the oxygen concentrations were thus increased, so we replaced the loop by another one with an twice higher volume (from 0.5 mL to 1 mL) and the 16 O 2 concentration is now 100%. (red plot). But the variation in the intensity due to the 16 O 2 pulses is higher than before allowing an easier observation of the pulses. Also there are two signals corresponding to 16 O 2 . The dark blue plot associated to m/z=16 and the light blue plot associated 64CHAPTER 3. ISOTOPE LABELLING PULSES OXYGEN REACTION: OPTIMIZATION to m/z=32. The intensity of m/z=32 is so high that it is difficult to represent on the figure. So hereafter, the signal 16 O 2 will be represented by m/z=16. It is necessary to precise that the correspondence between m/z=32 and m/z=16 is due to the strength of the quadrupole splitting but the two signals are describing the same 16 O 2 . Still, this could be identified as a source of error as any gases containing an oxygen atom could be split by the quadrupole. This "isolated" oxygen atom would also have a signal at m/z=16 without being 16 O 2 . But this only happens in a so small quantity that the impact has not actually been observed so far.

The loop frequency has also been changed from zero ton one between this experiment and the latter. It was necessary because with a waiting time of zero between each pulses, the MS signals were not able to return to baseline. The analysis of the pulses was therefore difficult. By increasing the time between each pulse to one minute, the MS signals were able to return to baseline, and it was possible to separate the pulses.

It also necessary to mention that changing the 16 O 2 concentration in the loop fro 1% to 100% is also changing the general parameters of the experiment. On Fig. 3.4 one can see that the m/z = 32 signal does not go back to baseline contrary the previously described experiments. Regarding general conditions on the experiments, the conditions were mostly reductive prior tot the oxygen concentration augmentation and are mostly oxidative after. Here the word "mostly" is used to describe the fact that it is not that simple. Indeed, the oxygen concentration variation due to pulses utilisation (transient system) induce a constant shift between oxidative/reductive conditions. One consequent (but interesting) feature of increase the oxygen concentration is that shifting to "mostly" oxidative conditions allow ILPOR experiment to be closer to the oxidative conditions of the catalytic tests see page 49.

Experiment over silicon carbide

Thanks to the loop volume and 16 O 2 concentration modifications, the signals variations due the 16 O 2 pulses is now easily observable. In this subsection, the usual sample is replaced by 200 mg of silicon carbide. silicon carbide is supposed to be inactive regarding catalytic oxidation processes. But by adding it into the cell allows the observation of the signals as if they went through the sample but without any chemical modification (isotopic exchange or catalytic conversion). Fig. 3.5 shows the results obtained in the same conditions as the last subsection but the cell is filled with silicon carbide. The parameters of the experiment are referred So, even if SiC should be catalytically inactive, the conversion happens at lower temperature. It is also possible that the conversion is happening at lower temperature because the gases are better mixed (increasing their interactions). Indeed, with an empty cell, the gases are just flowing through a "pipe". But with SiC, the gases have to flow through the quartz wool and the SiC grains. This could impact their mixing and so their co-interactions. This is the subject of the next section.

Mixing of the gases

The point of replacing a sample by silicon carbide and perform an ILPOR experiment is to study the mixing of the gases. Indeed, when the loop is performing the pulses there O 2 between two C 16 O volumes". Therefore, if there is absolutely no mixing between the gases, the pulses could have a "square" shape. On the opposite, if the gases were perfectly mixed, the pulses should look like two perfectly opposed Gaussian. The point of this comparison is to see whether the gases are mixed during the experiment. And if they are mixed, are they already mixed before reaching the sample or are they mixed when going through the powder or even after the sample? On Fig. 3.6 a) the pulses are not square shaped, meaning that the gases are at least partially mixed. The two arrows are pointing two sections that should be in perfect opposition if the gases were perfectly mixed. So, at this point the gases appear to be mixed but not perfectly as there is a small time shift between C 16 O maxima and 16 O 2 minima. At this point, the remaining question is: are the gases already mixed before reaching the sample? To answer the latter, one can see on Fig. 3.6 b) that when performing the pulses with silicon carbide in the cell, the pulses are similar to those observed with an empty cell. The pulses are not square shaped and the small time shift between C 16 O maxima and 16 O 2 minima is present in both cases. In conclusion, when reaching the sample, the gases are mixed but not perfectly. It is important to know that the gases are mixed, even partially. Otherwise, it would have been difficult to make any mechanism prediction using this technique. Actually they could also get mixed after the sample (because of the pipe length between sample and mass spectrometer). But no catalytic effect is possible there, so the high CO 2 concentration (regarding empty cell experiment) ensures the partial mix of the gas before the sample.

Control of the sample grain size

Grain size may have an influence over the catalytic properties. Basically, controlling the grain size allows a better experimental reproducibility by controlling the gas dispersion through the sample. But being able to control the grain size means performing different steps of pressing / crushing and sieving the materials. Potentially, those steps may also have an impact over the materials properties and/or performances. In order to study the influence of the grain size over the gas diffusion, two experiments were performed. The first one is performed on a sieved SrFeO x (eaf) (125 µm < grainsize < 250 µm) and the second on not-sieved SrFeO x (eaf) . The parameters of those experiments are: . The two experiments are very similar but the H 16 2 O signal is higher on the sieved SrFeO x (eaf) . Actually the baseline of the signal is higher but not the peak. The highest intensity of the water signal baseline is mainly due to a not completed clean vacuum chamber inside the mass spectrometer prior to the experiment and is not due to the material. Therefore it is possible to assume that sieving a powder to perform an ILPOR experiment has no direct influence on the obtained results.

ILPOR: A TUNABLE TECHNIQUE

69

Nonetheless, sieving a material is useful to avoid any instrument pollution during the future experiments (it is possible that very tiny particles can go through the quartz wool by the help of the gas flow).

Quantitative limitation of the catalytic conversion

The ILPOR technique has been developed in the philosophy of being able to study the mechanisms and the behaviours of catalysts. As previously explained, the temperature program is not a continuous increase but several isothermal steps every two degree Celsius. This section is showing that with ILPOR it is possible to identify when the thermal energy is sufficient enough for the catalytic oxidation of C 16 O to sustain itself (shift from chemically to thermally controlled). But moreover, it is possible to perform an isothermal experiments before and after the shift. And also to perform quantitative oxidation of C 16 O by using the loop to add a known quantity of one reactant ( 16 O 2 in this study).

The parameters of these two experiments are: . In these conditions, it is possible to quantitatively study the conversion in isothermal conditions, just by mastering the amount of 16 O 2 thanks to the loop volume. Even if it has not been done yet, this could be really helpful to perform accurate kinetic studies or in situ study of the Oxygen Storage Capacity (OSC) of oxides catalyst.

Mechanism Identification

In order to make accurate identification of oxidation mechanisms, it is required to study if C In this section the interest is the mechanism identification. More specifically, it is to study if the isotopic exchange between adsorbed C 16 Fig. 3.9 presents the results of this experiments. From the instrumentation point of view, this experiment ensures that the coupling is possible. But there are some limitations regarding the gases concentration in the bottles and the mass flow controllers precisions, but the point is, it is possible to couple ILPOR with our gas mixing system. The most interesting part of this experiment is between 5000 and 10000 seconds, before the catalytic conversion start. In this region, one can see that the signal of C 16 O 2 is decreasing while the signal of C 16 O remains stable. Meanwhile, signals of C 16 O 18 O and C 18 O 2 are increasing. This means that there exists an isotopic exchange between adsorbed C 16 O 2 and 18 O from the lattice. This exchange occurs in parallel of the spotted Mars-van Krevelen mechanism but it has to be taken into account.

The best way to ensure that the catalytic mechanism is of Mars -van Krevelen type would be to spot the attendance of C 18 O. On Fig. 3.9 the signal is too weak but 72CHAPTER 3. ISOTOPE LABELLING PULSES OXYGEN REACTION: OPTIMIZATION it is observed when performing ILPOR experiment over 1wt.%Pt/CaFeO 2.5 (eaf) in the brownmillerite chapter see page 116.

ILPOR without pulses

This section is more a proof of concept. One of the advantages of the ILPOR is to control the amount of a specific reactant thanks to the loop volume. But we want to know if it is possible to follow the catalytic reaction in a more classical way, e.g. without the pulses. The parameters of this experiment are the same as the general one, but the t i m e ( s ) There is a break on the intensity axe because the signal of the pure C 16 O 2 is so high that it would have been irrelevant to present it without the break (other signals would have been barely observable). On the figure, one can see the catalytic conversion as if it would have been observed on any catalytic conversion bench (continuous flow). But the utilisation of the mass spectrometer allows the differentiation of the 18 O containing species from those not containing it (with respect to the molecule formulae). What is very interesting is the jump in the C 16 O 2 ; C 16 O 18 O and C 18 O 2 signals occurring around 8000 seconds. This jump demonstrates the crossing of the thermal energy limitation. It is also possible to observe 16 O 2 / 18 O isotopic exchange from the appearance of 16 O 18 O in the region 12500-17500 seconds. This observation indicates that the latter isotopic exchange occurs in parallel of the catalytic conversion. In other words, the oxygen atoms from the lattice are not fully dedicated to the catalytic conversion. Nonetheless, it is necessary to precise that during this experiment the conditions are strongly oxidative due to the high oxygen concentration in the flow (pure oxygen canisters). It was not the case in most of the other ILPOR experiments, explaining why 16 O 18 O signal was not always observed. 74CHAPTER 3. ISOTOPE LABELLING PULSES OXYGEN REACTION: OPTIMIZATION

1 6 O 2 C 1 6 O C 1 6 O 2 C 1 6 O 1 8 O 1 6 O 1 8 O H 2 1 6 O C 1 8 O 2

Conclusion

The development of the ILPOR technique described in this chapter has proven the versatility of the technique. Starting from the as build instrument the different instrument modifications and their subsequent impact were described. The modification of the C 16 O concentration from one to two percent has impacted the signals intensities. As it also has an impact on the catalytic reaction, one concentration or the other can be picked regarding the needs of the experiment/sample, but this section proved the feasibility of the ILPOR with both concentration. The modification of the loop volume from 0.5 to 1 mL coupled with the modification of the 16 O 2 concentration from 1 to 100% allowed a better observation of the signals. With the signals easily observable, mechanism identification was easier thanks to the increase amount of information given by the signals. Increasing the loop volume and the 16 O 2 concentration shifted the experimental conditions from reductive to oxidative as 16 O 2 was no longer the limiting reactant. This allowed us to be closer to the test conditions in our catalytic CO oxidation set-up, the latter working with an excess of oxygen (oxidative conditions).

The experiment performed with the silicon carbide as an inert material, ensures that there are almost no instrumentation impact over the experiment results. It also ensures us that the gas were partially mixed in the set-up. This parameter is very important because if miss-interpreted, the signals could lead to a false explanation. Checking that the sample grain size has no impact over the experiments results allows us skiping this time consuming part. But one has to keep in mind that sieving a material usually ensured better reproducibility and generally avoid instrument pollution.

The last part of this chapter was dedicated to the versatility of this experiment. We showed isothermal experiment with quantitative reactant limitations giving information on the occurring mechanisms. In the field of catalysis, this type of experiment is usually used to perform kinetic studies. We think that ILPOR experiment could be of great help in this direction.

In order to spot the Mars -van Krevelen mechanism, we performed an ILPOR with C 16 O 2 in the gas stream. This highlighted the parallel C 16 O 2 /C 16 O 18 O isotopic exchange. In this way, ILPOR can also be use to focus on specific interactions, especially to observe side reactions otherwise non/barely observable. We also performed ILPOR without pulses. This specific configuration hides all information brought by the time relative pulses but allows a more classical observation of catalytic results while allowing the distinction of isotopic oxygen containing species.

Finally, the following list is detailing what we consider the most interesting levels of the parameters evaluated here:

Sample weight: 200 mg

Loop volume: 1 mL

Waiting time between pulses: 2 min

Gases: 1%C 16 O/He interrupted by pulses of pure 16 O 2

The ILPOR technique is a recent technique but very versatile as demonstrated here. This work is briefly showing what could be achieved with this technique. One could expect even further development of this technique in the future, especially if one would like to adapt it to other oxidation reactions than CO oxidation.

Chapter 4

Brownmillerites as catalytic supports 4.1 Introduction

As described in the introduction, a catalytic system is made of an active species impregnated on another material called support. The supporting materials were usually showing few or no catalytic activity. Recently, a new kind of supporting materials called "active support" gained researchers interest. The general idea is that these materials were able to actively participate in the catalytic reaction therefore strengthening the catalytic process. In the frame of oxidation reactions it was shown that catalytic activity is often strongly linked to oxygen mobility process [START_REF] Liotta | Total oxidation of propene at low temperature over Co3O4-CeO2mixed oxides: Role of surface oxygen vacancies and bulk oxygen mobility in the catalytic activity[END_REF] [START_REF] Montini | Fundamentals and Catalytic Applications of CeO2-Based Materials[END_REF]. Some oxide materials were thus used as catalytic supporting materials and proved the ability to involve their lattice oxygen in the catalytic process. This specific ability allow those materials to act as oxygen buffers, using their surface oxygen and sometimes even their bulk oxygen to fulfill the need of the reaction. In order to study the impact of chemical composition or differences in the micro-structure on the oxygen mobility, this study contains comparison of two different compositions synthesized by two different methods. Citrate complexation route (cp) was used to obtain CaFeO 2.5 (cp) and SrFeO x (cp) as fine-powder and electric arc furnace synthesis (EAF) to get CaFeO 2.5 (eaf) and SrFeO x (eaf)

The differences between CaFeO 2.5 and SrFeO x have already been detailed in the bibliographic chapter see page 38. The aim here is to see if a non-stoichiometric brownmillerite (SrFeO x ) is able to show faster or quantitatively higher oxygen mobility than 78 CHAPTER 4. BROWNMILLERITES AS CATALYTIC SUPPORTS a stoichiometric one (CaFeO 2.5 ). We also use two synthesis methods (complexation and EAF) because it has already been shown in the literature [START_REF] Gupta | Scaling of extended defects in nano-sized Brownmillerite CaFeO2.5[END_REF] that grain size has an impact on oxygen mobility. Indeed, the complexation method allows the synthesis of fine-powder with superior surface area and smaller crystallite size than EAF synthesis. Using fusion as a synthesis pathway is also an interesting up-scaling strategy used in industry.

In the first part, we characterize each type of supporting materials using XRD, XRF, N 2 -physisorption and TPR as bare supports. In a second part, we use isotopic oxygen O as probe species to characterize oxygen mobility and temperature programmed reduction to evaluate the reducibility. And in a third part, we compare the catalytic activities of each supports impregnated with one weight percent of platinum using carbon monoxide oxidation as a model reaction.

Physico-chemical properties of supporting materials

The synthesis of CaFeO 2.5 (cp) and SrFeO x (cp) as fine powders under mild conditions is described in the experimental part at page 42 accordingly to the work of Penkala et al. [START_REF] Penlkala | Low Temperature Oxygen Mobility Applied to Catalysis[END_REF]. The synthesis of CaFeO 2.5 (eaf) and SrFeO x (eaf) using electric arc furnace (see page 43) is a classic industrial ceramic synthesis path but allows a totally different micro-structure than the citrate route. The following subsections are describing the results obtained by characterisation techniques allowing evaluation of physico-chemical properties.

Synthesis optimisation of SrFeO x (cp)

SrFeO x is presenting different meta-stable phases as detailed in the bibliography. The material prepared by complexation e.g. SrFeO x (cp) is showing a mix of those metastable phases. To optimize the synthesis process, the first step is a study of the atmosphere during the calcination step. Indeed this partial pressure of oxygen is one of the parameters influencing the oxygen stoichiometry and thus the fraction of each meta-stable phase. By changing the atmosphere, one can play of the oxygen partial pressure and influence the oxygen stoichiometry to obtain the suitable material. when calcined under nitrogen. When calcined under air, the obtained pattern is close to cubic, corresponding to the perovskite SrFeO 3 , showing that, the oxygen partial pressure of air is too high. When calcined under hydrogen, the general pattern is close to the model but some reflections belonging to metallic iron appear, hydrogen atmosphere is thus too reductive. When calcined under argon or nitrogen, the diffractograms are close to the model, so it was chosen to perform the calcination under inert atmosphere. As the crystallization was a bit better with argon, in the following every other SrFeO x (cp) samples were calcined under argon. It would have been interesting to change the temperature programme to obtain a SrFeO x material with an higher Brownmillerite content. But in order to be able to compare SrFeO x ma-

PHYSICO-CHEMICAL PROPERTIES OF SUPPORTING MATERIALS

81

terials with CaFeO 2.5 materials the temperature programme was kept the same as previously described (6 hours to go to to 600 • C kept 6 hours at 600 • C). That way all Brownmillerite materials have the same temperature history.

Elemental analysis

The chemical compositions of the samples are confirmed by XRF analysis and are presented in table 4.1. From these amounts, the A/B ratio (eg: Ca/Fe for CaFeO 2.5 compositions or Sr/Fe for SrFeO x compositions) was calculated: The four samples are close to the theoretical ratio (=1) and a bigger deviation was expected with the EAF because of the bigger batch size. With this elemental analysis carbonaceous species such as carbonates are not detected. Using XRD it is possible to observe carbonates (see page 4.2.5 ff) but there again only the cristalline part is observable. The amorphous one is therefore inaccessible with these techniques.

Specific surface area

The specific surface area was measured using N 2 physisorption following the method detailed in the experimental part. The obtained isothemrs are shown on Fig. 4.2. CaFeO 2.5 (cp) shows a low specific surface area of 12 m 2 • g -1 while CaFeO 2.5 (eaf) shows an even lower specific surface area inferior to 1 m 2 • g -1 . Similar results were obtained for SrFeO x : 13 m 2 • g -1 for SrFeO x (cp) and below 1 m 2 • g -1 for SrFeO x (eaf) . Both compositions have the same range of specific surface area regarding the synthesis pathway, the cause of this difference can be the temperature reached during each synthesis methods (calcination at 600 • C for the complexation compared to more than 1400 • C for the fusion method). The four different materials show the same type of isotherm i.e. type II showing no hysteresis loop, which is typical of non-porous materials.

Particles mean size

A the end of the EAF process, the melted powder is blown into spheres. The materials thus presents centimetre size agglomerates of CaFeO 2.5 (eaf) and SrFeO x (eaf) . To perform catalytic tests, these materials were crushed during a two steps project (first, a crushing in a ball-mill followed by an attrition in isopropanol) detailed in the experimental part see page 43 f. Measurement of the mean particle size presented on Fig. 4.3 shows that the materials made using Electric Arc furnace have a micro-structure made of bigger particles. But using the two steps crushing method, the mean particles size can be reduced to a more convenient size ( slightly bigger than 1 µ m)allowing lab-scale manipulations.

X-ray diffraction

Measured diffractograms and refined parameters using Rietveld equation [START_REF] Rietveld | A profile refinement method for nuclear and magnetic structures[END_REF] of each sample are presented in the Fig. 4.4 and table 4.2 contains the refined parameters. On Fig. 4.4, the good accordance of CaFeO 2.5 (cp) and CaFeO 2.5 (eaf) data points with CaFeO 2.5 brownmillerite simulated pattern allows concluding on the good purity of the samples. Regarding SrFeO x (eaf) it is necessary to use the meta-stable phases to simulate a pattern which fit the experimental data; the material is then not a pure brownmillerite phase but a mix of phases among which we are able to determine the quantity of each phase. The same observations can be done on SrFeO x (cp) except that the presence of SrCO 3 having reflections in the 25 • -30 • area is detected. Note that SrCO 3 has not been refined on the simulated pattern. Table 4.2 contains the refined parameters and the weight fraction of each phase. We observe that the b parameter of the lattice is bigger in the cases of both SrFeO x than with CaFeO 2.5 as expected from the differences between the two composition sin the bibliography (see page [START_REF] Bedrane | 16O/18O isotopic exchange: A powerful tool to investigate oxygen activation on M/CexZr1-xO2catalysts[END_REF].

Using Rietveld refinement method, we are able to determine the weight fraction of each phase presented in table 4.2. We obtain for SrFeO x (cp) : 47.3 wt.% of SrFeO of SrFeO 2.5 for CP versus 70% for EAF). This is due to the reductive conditions of the electric arc when performing EAF synthesis. As henceforth, the samples based on SrFeO 2.5 are named SrFeO x due to the presence of the meta-stable phases in significant amounts.

Table 4.3 presents the crystallite sizes calculated using Scherrer equation [START_REF] Bergeret | Characterization of Metallic Catalysts by X-Ray and Electron Microscopy Techniques[END_REF] 4.1:

t = κ * λ β * cos θ (4.1)
where t is the crystallite size in Å, κ is a dimensionless shape factor of 0.9, λ is the wavelength, β is the full width at half maximum in radian and θ the diffraction angle in radian.

For CaFeO 2.5 (cp) and CaFeO 2.5 (eaf) the crystallite size is determined using reflection at 43.45 • (corresponding to the plan: 1 6 1) and for SrFeO x (cp) and SrFeO x (eaf) the reflection at 41.5 • (corresponding to the plan 1 6 1) is used.

Results of the crystallite size determination are presented in table 4.3. CaFeO 2.5 (cp) and SrFeO x (cp) are composed of small crystallites 35 nm and 37 nm respectively. CaFeO 2.5 (eaf) is made of bigger crystallite (130 nm) but surprisingly SrFeO x (eaf) is displaying small crystallites (51 nm). One could have expected bigger crystallite size for SrFeO x (eaf) and the error could come from the difficulty of evaluating the width at half maximum of the corresponding reflection as the material is multi-phased and the reflections are very close to each others. Rietveld refinement is also able to provide the cristallite sizes of the materials, but the algorithme is using all the reflections of the patterns. In the case of SrFeO x materials the reflections of the different phases (e.g. SrFeO 2.5 , SrFeO 2.75 and SrFeO 2.875 ) are so close that the calculated cristallite sizes by Rietveld are assumed to be possibly inaccurate. For the same reasons, the reflections at 43.45 • for CaFeO 2.5 materials or 41.5 • for SrFeO x materials have been chosen for the Debye-Scherrer method because they present no over-lapping with any other reflections from other phases. To summarize, the two synthesis pathways led to two different kind of micro-structure, i.e. large agglomerates, very low SSA and big crystallites are obtained from EAF synthesis whereas sol-gel method leads to powders with small SSA and smaller crystallites. In previous studies [START_REF] Gupta | Scaling of extended defects in nano-sized Brownmillerite CaFeO2.5[END_REF] [99] [START_REF] Maity | Solid-state reactivity explored <i>in situ</i> by synchrotron radiation on single crystals: from SrFeO <sub>2.5</sub> to SrFeO <sub>3</sub> via electrochemical oxygen intercalation[END_REF] [86] , the authors have shown that oxygen mobility in the brownmillerite lattice is strongly dependent on the grain size. Indeed the grain size is limiting the length of the (FeO 4 ) ∞ tetrahedral chains because the grain boundary limits the chains growth. As explained by Paulus et al. [START_REF] Paulus | Lattice dynamics to trigger low temperature oxygen mobility in solid oxide ion conductors[END_REF], one crucial parameters is the length of the (FeO 4 ) ∞ tetrahedral chains. The oxygen mobility in the brownmillerite framework is enabled when there is an inversion of the direction of the FeO 4 tetrahedron. Therefore, shortening of the FeO 4 tetrahedron chains means more FeO 4 at the end of the chains thus enabling the inverse their direction. In other words smaller chains means more inversions and thus an higher oxygen mobility. One can imagine that each grain boundary represents the end of a FeO 4 tetrahedral chain, consequently the smaller the grains are, the more inversion of FeO 4 tetrahedron can occur. It has already been shown that micro-structure has an impact on the the oxygen mobility and on the catalytic performances in the work of Lassiri [START_REF] Laassiri | Clear microstructure-performance relationships in Mn-containing perovskite and hexaaluminate compounds prepared by activated reactive synthesis[END_REF] over perovskite materials. But, one of the remaining questions is about the impact of the composition. In other words, what is the most performing composition in agreement with the properties obtained by each synthesis method.

Temperature programmed reduction of bare supports

Study of the materials reducibility is a mean of evaluating oxygen mobility by measuring their ability to release oxygen. During the reductive (40%H 2 /He) pretreatment prior to catalytic carbon monoxide oxidation the materials are submitted to a reducive atmosphere. It is therefore essential to understand the reduction of each material and see if they are impacted by such pre-treatment. To achieve this, the reducibility of the four brownmillerite samples is evaluated during Temperature programmed Reduction (TPR) using 3 %H 2 /Ar. SrFeO x (eaf) shows the same reduction behaviour than SrFeO x (cp) but at slightly higher temperature. The first reduction area starts from 300 • C until 570 • C and the second one initiates at 750 • C and is still ongoing at 1000 • C. Both SrFeO x (cp) and SrFeO x (eaf) have the same reduction behaviour but the one of SrFeO x (eaf) occurs at higher temperature. This difference can be explained by the different micro-structural properties as it seems to require more energy to impact the bulk micro-structure of SrFeO x (eaf) than the one of SrFeO x (cp) .

Their composition is also slightly different (according to Rietveld refinement) SrFeO x (cp) average oxygen stoichiometry is higher than the one of SrFeO x (eaf) . This difference could also have an impact over the reduction temperature: first, the reduction of SrFeO 2.875 may be easier than the one of SrFeO 2.75 , itself easier than the one of SrFeO 2.5 according to the different oxygen bindings energies in the respective lattices. And secondly the reduction intensity can also be impacted as more oxygen to potentially remove means higher hydrogen consumption. The lower surface area of EAF materials can also have an impact as the interaction between gaseous hydrogen with solid supports will be limited.

To sum-up differences in the composition, lower surface area results in a more difficult (in terms of energy) reduction of SrFeO x (eaf) than SrFeO x (cp) . One also have to keep in mind the different weight content of other phases of each materials. But being able to quantify the impact of each parameter is difficult. One idea to overcome this 4.2. PHYSICO-CHEMICAL PROPERTIES OF SUPPORTING MATERIALS 89 issue could be to perform modelisation of reduction interaction. TPR experiment could in some case be able to differentiate surface reduction from bulk reduction but in the case of this study it is difficult regarding the number of possible explanations.

Isotopic Enrichment

In order to observe oxygen mobility in operando, the 16 O contained in the samples is replaced by 18 O by isotope enrichment. To access the exact amount of 18 O in each sample, we need to determine the enrichment yield by TG-MS. A general observation is that all samples contain carbonates. This is due to the fact that these materials are assumed to act as CO 2 sponges. And during synthesis and storage, carbonate formation is assumed to happen. The amount of carbonate is higher with materials prepared by complexation than with EAF materials because the supposedly very high temperature during the EAF process helps to remove those carbonates. The last observation is that SrFeO x compositions contain more than CaFeO 2.5 compositions. This can be explained by the higher stability of strontium carbonates than the one of But the presence of carbonates is not a problem as we aim at manufacturing catalyst representative of the application conditions. Ceria materials widely used in catalysis also contain surface carbonates. We assume that carbonate presence has a limited impact on the properties studied in this work but allow our catalyst to be representative of application catalysts.

The determination of the exact enrichment rate is based on the weight loss expected for a total replacement of 18 O in the lattice by 16 O from the air. Similar experience performed by Paulus [START_REF] Paulus | Lattice dynamics to trigger low temperature oxygen mobility in solid oxide ion conductors[END_REF] showed that after enrichment CaFeO 2.5 samples kept the brownmillerite stoichiometry whereas SrFeO x samples were mostly SrFeO 2.875 (changes in the oxygen contents are due to enrichment temperature coupled with total oxygen partial pressure). Based on those compositions, the theoretical weight losses are about 3.5 wt.% for CaFeO 2.5 and 3 wt.% for SrFeO x as described in the corresponding equation 4.2 and 4.3: The comparisons of the theoretical weight losses with the experimental ones (attributed to oxygen movements) give the exact enrichment rate: 180% for CaFeO 2.5 (cp) , 66% for SrFeO x (cp) , 70% for CaFeO 2.5 (eaf) and 46% for SrFeO x (eaf) . Regarding the enrichment method and parameters (bulb volume, mass of materials, pressure and purity of gas) the expected enrichment yield is about 75/80% of oxygens. Also the materials studied adsorb a lot of water which first disturb the TG and thus the quantification of weight loss related to oxygen exchange. But also the adsorbed water may be involved in the isotopic exchange (signal m/z = 20 not shown for clarification purpose is not totally flat) as a 18 O consumer. This concept can also be applied for carbonates. Indeed adsorbed CO 2 - 3 may also exchange oxygen and so induce a miss-quantification. Regarding CaFeO 2.5 (cp) the enrichment yield is greatly superior to 100%. This is due to the concomitant release of carbonates leading to a overestimation of the weight loss due to oxygen exchange. To avoid such problem, it would have been better to heat the materials to release the carbonates before performing the enrichment. bi -atomic exchange

t = M CaF
: 18 O 2(g) + 2 18 O - (s) ↔ 18 O 2(g) + 2 16 O - (s) (4.5)
For the four materials, the signal of 16 O 18 O (green plots) always has higher intensities than the signal of 18 O 2 (orange plots). This observation is typical of a monoatomic exchange mechanism described in equation 4.4. The mono-atomic mechanism was already reported for perovskite materials [START_REF] Repecaud | Enhancement of Oxygen Activation and Mobility in CaTi x Fe 1 À x O 3 À d Oxides[END_REF] whereas bi-atomic exchange has been observed for ceria based catalysts [START_REF] Gamboa-Rosales | Ceria-supported AuâCuO and AuâCo3O4 catalysts for CO oxidation: An 18O/16O isotopic exchange study[END_REF]. The mono-atomic exchange mechanism implies the dissociation of oxygen during the exchange process and can be explained by the involvement of a lattice oxygen vacancy present in the brownmillerite lattice. In this way, 16 O (ads) from the gas phase can react with 18 O (s) from the lattice leading to the production of 18 O 16 O (g) and one oxygen vacancy in the lattice. The created vacancy is automatically refilled by 16 O (ads) from the atmosphere. This mechanism explains the exchange mechanism while keeping oxygen stoichiometry constant at 2.5.

But it is necessary to mention that the production of 18

h h h h h h h h h h h h h h h h h h h synthesis methods compositions CaFeO 2.5 materials SrFeO x materials

Complexation materials 370

• C 250 • C Electric Arc Furnace materials 400 • C 350 • C
From the latter table, one can see that complexation materials have a lower onset temperature with respect to compositions: 370 • C for CaFeO 2.5 (cp) instead of 400 • C for CaFeO 2.5 (eaf) and 250 • C for SrFeO x (cp) instead of 350 for SrFeO x (eaf) . This illustrate the higher oxygen mobility for materials obtained by complexation than those obtained by electric arc furnace. The explanation lies in the textural properties because complexation materials have an higher SSA, smaller cristallites and smaller grain size than fusion materials. Regarding the compositions, SrFeO x materials show a lower exchange onset temperature than CaFeO 2.5 compositions with respect to synthesis methods: 250 • C for SrFeO x (cp) instead of 370 • C for CaFeO 2.5 (cp) and 350 • C for SrFeO x (eaf) instead of 400 • C for CaFeO 2.5 (eaf) . The lower exchange onset temperature for SrFeO x compositions is due to the weaker Fe-O bonds in SrFeO x than in CaFeO 2.5 as explained in the bibliography (see page 1.3.2 f).

Other parameters due to the materials different microstructures (SSA, cristallite sizes and percentage of meta-stable phases) and compositons (ratio of meta-stable phases or amount of adsorbed water and carbonates) can also play a role, but similarly to the TPR conclusion (see page 4.2.6 f), it is very difficult to accurately differentiate

Study of raw supports 4.3.1 Isotope Labelling Pulse temperature programmed Oxidation Reaction over raw supporting materials

To study the possible implication of lattice oxygen and its dynamics within the enriched powders (CaFe 18 O 2.5(cp) ; CaFe 18 O 2.5(EAF) ; SrFe 18 O 2.5(cp) ; and SrFe 18 O 2.5(EAF) ) we developed a specific technique called ILPOR [START_REF] Penkala | The role of lattice oxygen in CO oxidation over Ce18O2-based catalysts revealed under operando conditions[END_REF].

Our setup uses a constant flow of C 16 O (g) while 16 O 2(g) is constantly pulsed in the reaction vessel containing the enriched powders, as a function of temperature. In this way the real-time monitoring of oxygen containing species via mass spectrometry delivers valuable information about oxygen mobility, oxygen exchange and to a certain extent CO oxidation mechanism. Note that with these specific parameters, the experiment is assumed to be under oxidative condition.

The three mechanisms detailed in the introduction are occurring at the brownmillerite surface and possibly concomitantly, it seems difficult to unambiguously differentiate them. However, with the alternation of oxidative and reductive atmospheres induced by the 16 O 2(g) pulses coupled with the monitoring of oxygen containing species by mass spectrometry, we seek to have a good insight into the impact of the brownmillerite support on the catalytic reaction.

In this way, Fig. 4.7 shows ILPOR experiments performed on (a) CaFeO 2.5 (eaf) and (b) SrFeO x (eaf) followed by quadrupole mass spectrometer.

For each material, the pink curve shows the release of weakly adsorbed water starting around 60 • C, with a maximum at 100 • C and slowly decreasing until all water has desorbed.

The onset temperature of CO (g) oxidation is related to the appearance of the C 16 O 16 O (g) signal (red plots). For CaFeO 2.5 (eaf) (Fig. analysis page 92 allows knowing that the enrichment yield is 65% for CaFeO 2.5 (eaf) , so 18 O atoms are present not only inside the first atomic layers but also in the bulk of CaFeO 2.5 (eaf) . Also, the slope change at 340 • C may be due to a larger number of available 18 O, thus indicating an energy sufficiently high to reach the bulk of the material.

As the enrichment yield is not 100%, there are some 16 O atoms remaining in the brownmillerite. Hence, when C 16 O (g) interacts with surface oxygen atoms to form CO 2(g) , two reactions will be in competition: Moreover, from 350 • C on, the signal of C 18 O 2(g) starts to increase gradually. This species can be the result of the reaction of C 16 O 18 O (g) with 18 O (s) from the lattice, leading to the production of C 18 O 2(g) described in reaction 4.9 which would then follow reaction 4.8:

C 16 O (g) + 16 O (s) ↔ C 16 O 16 O (g) + V
C 16 O 18 O (g) + 18 O (s) ↔ C 18 O 2(g) + V •• O (s) (4.9)
But this assumption can be pushed aside because pulsed production of C 16 O 2(g) and C 16 O 18 O (g) are in a perfectly matched modulation (see zoom Fig. 4.7 c). However, C 18 O 2(g) may also be the product of C 16 O (g) direct oxidation by 18 O (s) coupled with an isotopic exchange 16 O/ 18 O described in reaction4.10 :

C 16 O (g) + 2 18 O (s) ↔ C 18 O 2(g) + V •• O (s) + 16 O (s) (4.10)
In the same situation than reactions 4.7 and 4.8, reactions 4.9 and 4.10 would be immediately followed by subsequent refilling of the created oxygen vacancies inside the lattice by 16 O 2(g) from the pulses. After 18 O (s) depletion, C 16 O 2(g) intensity will be around the maximal intensity and the latter is reached at 370 • C in the case of CaFeO 2.5 (eaf) .

16

O/ 18 O isotopic exchange can occur without being combined to an oxidation reaction thus without production of carbon dioxide. Indeed, after 370 • C, C 18 O (g) is present but the signal remains very small. 16 O 18 O (g) and 18 O 2(g) signals are also present after can react with dioxygen species ( 16 O 2(g) / 16 O 18 O (g) / 18 O 2(g) ) leading to the production of C 16 O 18 O (g) or C 18 O 2(g) but this side reactions are considered almost negligible due to the very low signal intensity.

To discriminate the mechanisms, it is necessary to remind the initial conditions. At the beginning of the experiment, 18 O atoms are only present inside the brownmillerite. Also, Eley-Rideal or Langmuir-Hinshelwood mechanisms describe reaction between species in the gaseous phase, therefore to produce C 16 O 18 O (g) or C 18 O 2(g) they require 18 O atoms in the gaseous phase. Although, 18 O containing species such as C 18 O (g) , 16 O 18 O (g) or 18 O 2(g) are present in the gaseous phase they are present only after 370 • C. However, signal of C 16 O 18 O (g) is already present at 190 • C allowing us to discard Eley-Rideal or Langmuir-Hinshelwood mechanism as the main mechanisms involved in this reaction. Nonetheless, the appearance of C 18 O 2(g) is a key feature ensuring MvK mechanism. Indeed, it discards that adsorbed C 16 O 2(ads) produced during the reaction can exchange its oxygen atoms through E-R or L-H as a parallel reaction. Note that this process has already been pushed aside by the discussion page 70 and will be further detailed on page 116.

Consequently, the description of this system now includes reaction 4.7; 4.8 and 4.10. Those three reactions are presumed to happen at the surface of the support while involving lattice oxygen. It is therefore possible to conclude that the reaction occurs through a Mars van Krevelen mechanism because 18 O species are involved (e.g. detected in the gaseous phase).

The results obtained with SrFeO x (eaf) are presented in Fig. 4.7 b). The red plot describes C 16 O 2(g) signal while blue plot stands for C 16 O 18 O (g) and orange plot for C 18 O 2(g) . But these oxygen containing products occur at different temperature than CaFeO 2.5 (eaf) . Indeed, production of C 16 O 2(g) starts around 120 • C when signal of C 16 O 18 O (g) begins to increase. They are rapidly followed by signal of C 18 O 2(g) starting at 150 • C. Interestingly, one can notice the presence of 18 O containing carbon dioxide species from 130 • C on, in other words already at the reaction onset temperature.

Although, in the case of 16 O/ 18 O exchange via TG-MS it has been shown that oxygen mobility starts around 250 • C but during ILPOR, C 16 O 18 O (g) signal is already present at 130 • C. The different atmospheres used during TG (air) and ILPOR (mainly CO) may be the cause of this change. But the point is that 18 O brownmillerite oxygen atoms are already taking part into the reaction at the reaction onset temperature (with the reactive conditions). Moreover, between 180 • C and 250 • C, C 16 O 18 O (g) signal becomes more intense than C 16 O 2(g) signal, meaning that oxygen mobility within the brownmillerite lattice is sufficient enough to sustain more 18 O (s) coming from the lattice to the oxidation reaction than those coming from 16 O 2(g) . Sustaining the oxidation reaction with oxygen from the lattice, might require oxygen from the bulk. But as no distinguishable slope change is occurring on the C 16 O 18 O (g) signal from the C 16 O 2(g) signal, we can assume that oxygen is already moving almost freely inside the lattice at 130 • C, the reaction onset temperature.

Signal of C 16 O 18 O (g) hits a maximum at 300 • C and then drops. The same behaviour occurs with C 18 O 2(g) but the maximum is already reached at 260 • C. This behaviour is explained by the 18 O depletion in the brownmillerite. Indeed, the number of 18 O that the brownmillerite can give to the oxidation reaction is limited by the number of 18 O inside the lattice. Once lattice oxygens (both 16 O and 18 O) are consumed by the reaction, the oxygen quantity in the lattice is replenished by 16 O 2(g) from pulses. It is necessary to keep in mind that there is no real equilibrium because of the continuous flow. So, at some point the 18 O concentration in the brownmillerite will be close to the natural occurrence. We can also assume that competition between reactions 4.8 and 4.10 is driven by 18 sities. This spikes corresponding to a brief CO 2 production might be due to interaction between adsorbed carbonate species and 16 O 2(g) . But ensuring this assumption would require further analysis like surface Infra-red or environmental TEM.

The modulation of carbon dioxide species is showing that ILPOR with SrFeO x (eaf) occurs through the same mechanism than CaFeO 2.5 (eaf) but at a much lower temperature because oxygen atoms are already mobile at the reaction onset temperature of SrFeO x (eaf) . We can notice that the reaction onset temperature during ILPOR is the same (120 • C) for both materials, only the oxygen mobility onset temperature is different. In other words, the brownmillerite studied here are able to convert CO at similar temperature but involvement of oxygen lattice is different regarding the composition as expected from discussion page 38.

Evaluation of catalytic performances of raw supports after hydrogen pretreatment at 200 • C.

CO oxidation (equation 4.11) is used as a model reaction to study the catalytic performances of the supports.

CO + 1/2O 2 → CO 2 (4.11)
CO oxidation is an exothermic reaction, meaning that once it is started it produces thermal energy at some point supporting the reaction itself; this leads to conversion plots starting slowly and then shooting up to the maximal conversion (exponential type). Because of this peculiarity, temperatures at 10% and 50% conversion have been chosen to compare performances respectively named T 10 and T 50 . This way, comparisons are easier because T 10 and T 50 are kept in a more reproducible temperature range. As a reminder, between each conversion cycle, the samples were kept under helium flow as described in the experimental section. This can have an impact on the measured performances as it is changing the oxygen partial pressure. Moreover, it could allow desorption of adsorbed CO or carbonates.

Whereas catalytic performances of platinum impregnated supports will be evaluated in the two other sections pages 109 and 114, this section will detail catalytic performances of raw supports without any platinum. Adding platinum allows comparison with literature in term of performances and construction, but this platinum can have a huge impact. Moreover, the possible synergy between support and platinum can be different from one sample to another. To investigate this, catalytic CO oxidation lightoff curves were performed on raw supports. The corresponding plots are present on Fig. 4.8: CaFeO 2.5 (cp) is showing a deactivation of 30 • C and 45 • C between first and third cycles at 10% and 50% conversion respectively. The general plot trends is changing between the first cycle and the others. These two observation shows that the materials undergo a deactivation process during the first cycle but is stable after. Indeed the second and third cycles are similar. the first cycle but are stable after.

SrFeO x (cp) is showing a third cycle occurring at a slightly lower temperature than the first one. Indeed, at 10% and 50% conversion, the first cycle happens about 13 • C higher. This is interesting because it means that this catalyst is able to show better performances one stabilised with the tests conditions. SrFeO x (eaf) is showing 10% conversion 50 • C higher and 50% conversion 63 • C higher during the third cycle than during the first. These loss of activities are the biggest reported among those four materials.

With these bare supports, it is relatively easy to discriminate materials performances. Materials based on SrFeO x composition (SrFeO x (cp) and SrFeO x (eaf) ) are displaying better performances than the CaFeO 2.5 composition. This confirms the assumptions made in the bibliography that SrFeO x compositions should be more active than CaFeO 2.5 compositions. Regarding their micro-structure properties: the general trend is that complexation compounds have better performances than EAF materials. This observation is matching the lower surface area reported for EAF materials. However it is difficult to identify exact deactivation process(es) and further analysis (post catalytic tests XPS/infra-red and XRD) could help to identify this process.

Study of platinum impregnated supports

In order to test the catalytic performance of the four supports, each of them is loaded with 1 weight percent (1wt.%) of platinum as described in the experimental section. Adding a noble metal on a support surface will create a reaction trigger. This will create an active site, a preferential place for the reaction to happen. The general idea is to increase the catalytic performances by lowering the activation energy [START_REF] Calla | Oxygen-exchange reactions during CO oxidation over titania-and alumina-supported Au nanoparticles[END_REF]. So noble metal impregnation is a very important step when manufacturing a catalytic system. The properties of the impregnated noble metal are defined as its dispersion and will have an impact on the overall catalytic performances. As the aim of this work is to compare the support materials properties, platinum properties have to be similar from one sample to another.

Platinum impregnation of brownmillerite

In the catalytic CO oxidation, platinum triggers the reaction as described by [START_REF] Isaifan | Metal-Support Interaction of Pt Nanoparticles with Ionically and Non-Ionically Conductive Supports for CO Oxidation[END_REF]. This trigger is therefore interacting both with the reactants in the gas phase and with the supports if the supports are involved in the reaction. Platinum will thus be at the interface of every reaction, requiring the necessity to exactly know its properties. To characterise the platinum dispersion, we took TEM and HAADF-STEM pictures (see page [START_REF] Voorhoeve | Defect chemistry and catalysis in oxidation and reduction over perovskite-type oxides[END_REF]. Some results obtained are present on Fig. 4.9 and all of them are summarised in table 4.6: Magnifications x120 000 and x150 000 have been used to measure the mean size of platinum nano-particles. More than 300 platinum particles have been counted for each supports as represented on Fig. 4.9 b) d) e) and f). The calculated mean sizes before catalysis on table 4.6 are between 1.2 nm for 1wt.%Pt/CaFeO 2.5 (cp) and 1.7 nm for 1wt.%Pt/CaFeO 2.5 (eaf) . These values are similar to those generally reported in the literature for platinum impregnation (< 1 to 5 nm). The particle size distribution is slightly broader for 1wt.%Pt/CaFeO 2.5 (eaf) than 1wt.%Pt/CaFeO 2.5 (cp) but they remain in the same range.

The determination of platinum particles mean size is the average diameter measured on the TEM pictures whereas the determination of platinum dispersion is made from a spherical model following equation4.12 taken and adapted from [START_REF] Bergeret | Characterization of Metallic Catalysts by X-Ray and Electron Microscopy Techniques[END_REF]: Where M Pt is the platinum atomic mass; d va the volume-area mean diameter; a the surface area occupied by an atom of platinum ; ρ the platinum mass density and N a the Avogadro number. This model allows noble metal dispersion determination but requires some assumptions. First, this model considers only spherical particles of diameter d i and one has to make some assumptions on the crystal nature to link dispersion and ** ** after CO oxidation (*) could not be obtained because the platinum was not easily visible. mean particle diameter. As platinum is an fcc metal, it is assumed an equal proportion of ( 111), ( 100) and [START_REF] Sadykov | Fuel-rich methane combustion: Role of the Pt dispersion and oxygen mobility in a fluorite-like complex oxide support[END_REF] planes allowing the determination of the surface area occupied by one platinum atom : a. The last parameter d va is described by equation 4.13 where n i is the number of particles of d i diameter:

d va = n i * d 3 i n i * d 2 i (4.13)
Fig. 4.9 is showing very similar dispersion and particles mean size for complexationmaterials: 74% and 80% for CaFeO 2.5 (cp) and SrFeO x (cp) before catalytic test respectively. CaFeO 2.5 (eaf) before catalytic test is showing lower dispersion (57%) whereas a dispersion of 80% is measured on SrFeO x (eaf) . It seems that specific area of the supports has no impact on the dispersion as SrFeO x (eaf) shows a dispersion similar to complexation materials before catalysis. But the dispersion of CaFeO 2.5 (eaf) is lower than any other material. It is possible that an EAF sample was badly impregnated. Performing the impregnation again seems an interesting future experiment. That way, this study would be able to show is physico-chemical properties and thus synthesis pathways have an impact on impregnation. This highlights the difficulties and also the importance of the impregnation process.

The measurements of platinum means sizes and their dispersion was also performed after catalytic tests. The data are showing a general augmentation of the particles mean sizes and so a diminution of their dispersion. This means that platinum particles have agglomerated in bigger particles. This is an expected observation as already detailed in [125] [126]. This effect is due to catalytic conditions and temperature and has an impact on the catalytic performances of the systems. The performance loss induced by the dispersion lowering is bigger when the dispersion was higher. Indeed, samples with very high dispersion faced a stronger agglomeration than those with an already lower dispersion before test. This is illustrated by the dispersion loss of 24% for SrFeO x (cp) when CaFeO 2.5 (eaf) lost only 15%. CaFeO 2.5 (cp) is showing a dispersion lowering of only 11% without any strong explanation.

In order to have a comparative study, the parameters of the platinum loading need to be comparative for each supports. Table 4.6 and Fig. 4.9 show that the four supporting materials were impregnated within the same order of magnitude for those parameters and can therefore be compared. But one has to keep in mind that CaFeO 2.5 (eaf) presents a slightly lower dispersion still.

Catalytic tests after 300 • C hydrogen pretreatment

This sections details the results obtained when performing catalytic tests over platinum impregnated brownmillerites. A first section is dedicated to the CO oxidation after reductive pretreatment at 300 • C. A second when the pretreatment is performed at 200 • C and a third is dedictated on an ILPOR experiment performed over 1wt.%Pt/CaFeO 2.5 . Fig. 4.10 displays the light-offs plotted during the 3 cycles for each materials after pretreatment under hydrogen at 300 • C. All the plots presented here belong to the exponential type described before. First oxidation cycle is the black square plot, second cycle is the red spheres plot and third is the green triangles. All catalytic systems undergo a performance loss. This loss can be very important as it is with SrFeO x (cp) or very slight in the case of SrFeO x (eaf) . Second and third cycles are very similar for all the materials meaning that they are stable. The deactivation process has reduced the performances but at the third cycle, samples are showing reproducible light-off. Table 4.7 contains the conversion results necessary to compare the supports performances.

CaFeO 2.5 (cp) shows performances decrease as conversion happens 25 and 20 • C higher between the first and third cycles at 10% and 50% conversion respectively. During first cycle 50% conversion is reached at 205 • C and once stabilized (e.g. third cycle) it is showing 50% conversion at 240 • C. CaFeO 2.5 (eaf) displays performances losses of 18 • C between first and third cycles for both 10 and 50% conversion. As the loss is similar for 10% and 50% conversion, the conversion plot is very similar but it just occurs 18 C). But the deactivation is so strong that when stabilized, it is the less efficient catalyst. But if one can identify and avoid the deactivation process, SrFeO x (cp) would be a very interesting catalyst. SrFeO x (eaf) exhibits the smallest deactivation recorded during the study. The difference is 10 • C for 10% conversion and only 4 • C at 50%. This sample is therefore the most resistant material to the deactivation occurring during CO oxidation. But its performances are also the lowest. Indeed, 50% conversion are reached at 243 and 247 • C for first and third cycles respectively. Only SrFeO x (cp) when deactivated (third cycle) is showing conversion at higher temperature. SrFeO x is a very interesting composition because the complexation sample (SrFeO x (cp) ) show the second best performances during the first cycle with 50% conversion at 205 • C. But then it gets mostly deactivated and is only reaching 50% conversion at 262 • C during its third cycle. SrFeO x (eaf) is interesting in another way because its performances are almost not changing from one cycle to another. So this material is stable but not very efficient as reaching 50% conversion at 243 • C during first cycle.

CHAPTER 4. BROWNMILLERITES AS CATALYTIC SUPPORTS

CaFeO 2.5 is also interesting because CaFeO 2.5 (cp) and CaFeO 2.5 (eaf) look similar in the way that their undergo a slight deactivation. But this small deactivation is stronger on CaFeO 2.5 (cp) than on CaFeO 2.5 (eaf) . Eventually, CaFeO 2.5 (eaf) seems to be the most efficient catalyst of this 300 • C pretreatment study because its performances are good while deactivation remains weak on this sample. Moreover CaFeO 2.5 (eaf) is showing the best performances during the third cycle with 50% conversion reached at 208 • C despite the low dispersion.

Synthesis method also have an impact on the catalytic behaviour. Indeed, both complexation materials were more deactivated than their respective bulk counterpart with respect to compositions.

Regarding the deactivation process, sintering or changes in the materials because of temperature are not considered because the materials were already heated up to 600 • C during synthesis (way above the catalytic test temperature). Deactivation could be due to different processes. One could be platinum segregation where the size of platinum particles increase and so their number diminish. Thanks to the HAADF-STEM, we can see that the platinum particles size increased during the catalytic test, confirming platinum segregation. Another deactivation explanation could be a chemical poisoning during the reaction such as CO poisoning. It happens when some CO molecules remain adsorbed on the platinum and at some point are blocking the access to it. As the amount of platinum is very low, it is very difficult to identify this process from conversion plots. But one can identify it using in-situ Diffused Reflection Infra-red Fourier Transformed Spectroscopy as CO vibration should be easily identifiable. Finally, it is also possible that the materials get impacted by the catalytic test process. To check that, XRD were performed after CO oxidation with 300 • C pretreatment. Fig. 4.11 is showing comparison between diffactograms of the four samples before (black diffractograms) and after catalysis (red diffractograms). CaFeO 2.5 (cp) was an almost pure brownmillerite before catalytic test. Whereas after, the brownmillerite structure is still present but a lot of calcium carbonates are also present. The latter phase is easily identifiable from the reflection at 29 • . Carbonates formation could be an explanation of the deactivation because carbonates covering could inhibit the gaseous species access to the platinum. It could also change the support surface properties and thus disable interaction between platinum and support. SrFeO x (cp) has a similar behaviour than CaFeO 2.5 (cp) but to a bigger extent. Indeed, SrFeO x (cp) was a mix of SrFeO 2.5 , SrFeO 2.75 and SrFeO 2.875 before catalytic test and is mostly strontium carbonates after. The reflections belonging to the strontium carbonates are easily observable at 26 • on the red plot of Fig. 4.11 b). The impact of the catalytic test over the material is so high that it is very difficult to accurately identify any phases composing the sample prior the test. To summarise, in the case of SrFeO x (cp) the support has been mostly turned into carbonates, that could explain the huge activity loss recorded when cycling the material. CaFeO 2.5 (eaf) was a pure brownmillerite before catalytic test and presents almost not changed after the three cycles. The only change is the relative intensity of the calcium carbonate phase when comparing CaFeO 2.5 (cp) and CaFeO 2.5 (eaf) after catalytic tests. The lower intensity of the calcium carbonate phase observed for CaFeO 2.5 (eaf) , testifies for a bit of carbonate formation. So the bulk micro-structure of CaFeO 2.5 (eaf) avoids carbonate formation and the activity loss associated with it. SrFeO x (eaf) is showing a complete different structure than the brownmillerite it was before catalytic test. Indeed, it is now a perovskite as it can be identified from the regular reflections testifying of the cubic structure. Further analysis shows that SrFeO x (eaf) was already turned into a perovskite during the platinum impregnation. So it should be interesting to try to test this sample kept with its brownmillerite structure, but this would require an improvement in the platinum impregnation process.

Catalytic tests after 200 • C hydrogen pretreatment

Regarding results from the previous sections, the majority of the catalysts was affected by the catalytic test process. Moreover, from the temperature programmed reduction experiments one of the sample (SrFeO x (cp) ) undergoes a beginning of reduction at 300 • C. Therefore, we chose to repeat the catalytic test study but with a pretreatment step performed at 200 • C instead of 300 • C. The same procedure (hydrogen pretreatment at 200 • C this time; followed by 3 catalytic cycles with a cooling under helium flow between each) has been carried out. Fig. 4.12 shows the results obtained.

Evaluation of performances is also consisting of selecting the temperature at 10% conversion and 50% conversion. The results are presented in table 4.8

The first observation is that for all the samples the conversion takes place at lower temperature. Indeed, after 300 • C pretreatment, recorded temperature were between 160 and 262 • C whereas after 200 • C pretreatment they are between 87 and 193 • C. Even for the less efficient sample (CaFeO 2.5 (eaf) ), 50% conversion is reached before 200 • C, so conversion takes place at lower temperature. Being able to lower the operating temperature just by lowering the pretreatment temperature is already a very interesting result.

Comparing results obtained in the same conditions without platinum with the latter results with 1wt.% platinum impregnated shows that the operating temperature range is lower with platinum. More specifically, when looking at the best catalysts performances during their third cycle, one can see a difference of about 200 • C (comparison of 50% conversion of third cycle of both complexation samples with and without platinum after 200 • C pretreatment). This illustrates the impact of the platinum during the catalytic reactions.

As already observed at page 109, after 200 • C pretreatment the conversion is still fast (as all samples display less than 40 • C between 10 and 50% conversion). CaFeO 2.5 (cp) is presenting a temperature gap of 40 • C to reach 10% conversion between first and third cycle. To reach 50% conversion the same gap is about 35 • C. So this material is still affected by the catalytic process and displays less efficient performances during its third cycle. An interesting comparison is that after 300 • C pretreatment the temperature gap between first and third cycles was about 25 • Cwhereas it is 40 • C after 200 • C pretreatment. So CaFeO 2.5 (cp) is less deactivated when performing cycles after 300 • C pretreatment. But it is necessary to take in account the operating temperature which is higher after 300 • C pretreatment (180-240 • C in comparison with 87-161 • C this time). A higher temperature possibly allows an easier desorption of adsorbed species thus inhibiting deactivation processes like CO poisoning. CaFeO 2.5 (eaf) converts 10% of CO 45 • C higher during the third cycle and 30 • C higher for 50%. similarly to CaFeO 2.5 (cp) , CaFeO 2.5 (eaf) also have a wider gap between first and third cycles after 200 • C pretreatment compared with 300 • C pretreat- C pretreatment), thus also increasing the deactivation process.

To reach 10% conversion, SrFeO x (cp) requires 40 • C more during the third cycle than during the first and 35 • C in the case of 50% conversion. Contrary to the two previous samples, SrFeO x (cp) has a smaller temperature gap between first and third cycles after 200 • C pretreatment than after 300 • C pretreatment. This observation can possibly be explained by the structural changes happening on the supports. SrFeO x (eaf) is a different sample than the one evaluated after 300 • C pretreatment. Indeed, the impregnated methods has been modified to keep the same structure before and after impregnation (a mix of the SrFeO x meta-stable phases). But the dispersion has not yet been measured by TEM imaging after this modified impregnation. But this sample behaves differently than the other samples. The light-off trend is "slower" (the plots do not shoot up to full conversion but have an inflexion point instead). It is necessary to mention that the last part of the light-off is affected by reproducibility troubles. One explanation could be the differences in the samples granulometry (sieving process). The point is that SrFeO x (eaf) is presenting almost no deactivation as the three cycles are very similar. But the conversion is better during the third cycle of this sample (SrFeO x (eaf) ) than the third cycle of CaFeO 2.5 (eaf) (except the part after the inflexion). This observation correlates with the assumption that SrFeO x compositions are supposed to be more efficient than CaFeO 2.5 compositions. But one as to keep in mind that SrFeO x (eaf) is not a pure brownmillerite phase and that this differences in the composition may have an impact over the overall catalytic performances.

The cause of the deactivation is again difficult to identify. XRD measurements after catalytic tests have not been performed yet so there is no clue about the impact over the structure yet.

Isotope Labelling Pulse temperature programmed Oxidation Reaction over 1%Pt/CaFeO 2.5 (eaf)

The same type of ILPOR experiment as previously described is performed. But this time, platinum was impregnated on the surface of CaFeO 2.5 (eaf) following the same impregnation method as detailed at page 105. The goal is to see the impact of platinum impregnation over the oxygen mobility in the ILPOR test parameters. We chose to impregnate CaFeO 2.5 (eaf) because it is the most stable structure so far (but SrFeO x (eaf) with the second impregnation method). The signals recorded during the experiment are presented on Fig. 4.13.

This experiment was performed under the same conditions as the one performed on CaFeO 2.5 (eaf) without platinum. The main focus here will be the differences between these two experiments but they have some similarities. First, the signal corresponding to the release of adsorbed water on the catalyst surface appears between room temperature and 100 • C as already seen on raw CaFeO 2.5 (eaf) . But then the signals of species behave differently. The signal of C 16 O starts to decrease from 120 • C with 1%Pt/CaFeO 2.5 (eaf) , the same temperature than without platinum (see page 96) but the disappearance rate with platinum is faster. At the same temperature (120 C lower) than without it. But there the mechanism does not involve any isotopic oxygen. This means that the mechanism occurs only with species from the gaseous phase. Therefore, the mechanism has to be different from the Mars-van Krevelen one identified in 96. It has already be proven that some materials are able to switch from one mechanism to eaf another regarding the tests conditions or when fonctionalized differently [START_REF] Liotta | Catalytic oxidation of volatile organic compounds on supported noble metals[END_REF].

Several articles already showed that carbon monoxide oxidation occurs through a Langmuir-Hinshelwood mechanism [START_REF] Deutschmann | Heterogeneous Catalysis and Solid Catalysts[END_REF] [127] [14] [START_REF] Bjørn | A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts[END_REF] when performed over a platinum containing catalyst. As it is difficult to make a difference between Langmuir-Hinshelwood and Eley-Rideal mechanism, following literature this study will comply with Langmuir-Hinshelwood mechanism.

So without any isotopic oxygen involvement, the oxidation reaction starts occurring on the platinum particles through a Langmuir-Hinshelwood mechanismm. At 200 • C the reaction rate is reaching its maximum as the signal of C 16 O 2 starts to stabilize and at this temperature there are still no sign of species containing isotopic oxygen but one. Indeed at the same temperature there is a spike of C 18 O (dark green signal). This spike can be attributed to an oxygen exchange between adsorbed 16 O on the platinum with 18 O from the support lattice. This spike was unexpected because isotopic oxygen is not involved in the reaction meaning that if it interacts with the platinum, it does not react to produce any CO 2 (neither C 16 O 2 nor C 18 O 2 ). But instead, it desorbs from the platinum in the form of C 18 O. It means that C 16 O adsorbs on the platinum then a 16 O/ 18 O oxygen exchange with 18 O (or 16 O) from the support would happen. But a question remains, because if these events happen as described, why would C 18 O Ptads desorb at this temperature.

Another possibility to explain the presence of C 18 O with 1 wt • %Pt/CaFeO 2.5 eaf is the back-spillover of C 18 O. It means that C 18 O produce at the surface of the support during the exchange process would move from the support onto the platinum to desorb during the ILPOR experiment. CConfirmation of the back-spillover process is very difficult. One possibility would be to use environmental XPS coupled with massspectrometry.

Knowing why this C 18 O release occurs, needs further information about the system behaviour, some changes happening "in parallel" of C 18 O spike could help to explain why this is happening. Before the spike, the CO conversion is still quiet low but after the spike CO conversion is almost total. This sudden change in the CO partial pressure could trigger the release of some C 18 O adsorbed on the platinum (driven by partial pressure equilibrium).

Nonetheless the presence of C 18 O is a strong clue of MvK mechanism as this indicates that adsorbed CO exchanges its oxygen with the support lattice (only source of 18 O).

From 250 • C, signal of C 16 O 18 O starts to increase and is followed by C 18 O 2 at 320 • C. Consequently, signal of C 16 O 2 decreases accordingly with the isotopic repartition when part of the CO oxidation leads to isotopic oxygen containing species. The point is, from 290 • C oxygen mobility is allowed and the mechanism is switching from Langmuir-Hinshelwood (not involving the support) to Mars-van Krevelen (involving the support).

So before 290 • C, the platinum was the site for Langmuir-Hinshelwood (so only dealing with adsorbed species and to a lesser amount with classic oxygen exchange) and after 290 • C, it will be the site for Mars-van Krevelen (dealing with both adsorbed species and lattice oxygen). Moreover, without platinum isotopic oxygen involvement was spotted from 340 • C, meaning that at this temperature, raw CaFeO 2.5 (eaf) is able to convert CO into CO 2 . So after 340 • C with or without platinum, CO oxidation occurs with lattice oxygen involvement.

Finally, a last event is happening. Indeed, the signals corresponding to 16 O 18 O occurs from 320 • C and is rapidly followed by 18 O 2 from 400 • C. These two signals can come from oxygen exchange described by equation 4.4 and equation 4.5 in the previous section 92. The appearance of these signals but also their intensities are non negligible give clues of a so strong oxygen mobility in the support. It proves that classic oxygen exchange can happen, certainly driven by a molar equilibrium between lattice isotopic proportion and atmosphere isotopic oxygen proportion.

Conclusion

Materials of the ABO 2.5 brownmillerite structure were synthesized by two different synthesis methods. The first one is a lab scale method inspired from wet chemistry routes leading to fine powders. The second one is the electric arc furnace synthesis a classic ceramic industrial route giving materials with a bulk micro-structure. For each kind of synthesis, two compositions were synthesized. The gave the following samples to study: CaFeO 2.5 (cp) , CaFeO 2.5 (eaf) , SrFeO x (cp) and SrFeO x (eaf) .

Pure brownmillerite phases were obtained with CaFeO 2.5 compositions, but for SrFeO x compositions, the two materials (SrFeO x (cp) and SrFeO x (eaf) ) were made of a mix of SrFeO 2.5 , SrFeO 2.75 and SrFeO 2.875 with different ratio regarding the synthesis pathway. The EAF synthesis is giving an higher ratio of SrFeO 2.5 thanks to the reductive conditions of the electric arc.

The structural properties of the four samples were evaluated trough different techniques ensuring knowledge of the phases. Their oxido-reduction behaviour was also studied using TPR, we observed more reduction for SrFeO x compositions than for the CaFeO 2.5 . We also observed reduction at lower temperature for SrFeO x (cp) than SrFeO x (eaf) . This difference can be due either to the surface area or the composition differences. Then oxygen mobility within the lattice of these materials were studied by ILPOR. For all materials we observed lattice oxygen involvement thanks to the presence of C 16 O 18 O and CO 18 O 2 .

Catalytic performances of raw supports were evaluated towards CO oxidation. The operating temperature range was high (200-500 • C) and a deactivation was observed. Then these materials have been impregnated with one weight percent of pure platinum and this impregnated platinum was characterised using HAADF-STEM pictures. Their catalytic performances were evaluated again after evaluation; first after 200 • C reductive pretreatment and then after 300 • C reductive pretreatment. The operating temperature range has been lower by 200 • C for complexation compositions. Even though general results are encouraging, the remaining problem is the deactivation occurring on the materials. Indeed, they are not completely stable in the catalytic test conditions. When getting cycled, one can observe an increase in the operating temperature from one cycle to the following with an overall stabilisation at the third cycle. This deactivation was hypothetically identified as CO poisoning coupled with structural changes. Indeed, after catalytic tests, CaFeO 2.5 (cp) and SrFeO x (cp) were showing the presence of carbonates in their structures. But being able to identify the exact cause of the deactivation would require further study. It could be achieve using in-situ XPS or environmental TEM but also by DRIFTS study.

The general classification of the materials is difficult because it changes from one aspect of the study to another especially regarding the catalytic tests conditions. But the general trend is that complexation materials seem more efficient at first cycle but also less resistant than their EAF relatives, possibly giving more efficient EAF materials at third cycle. There is a general trade-off between efficiency and stability but the two synthesis methods presented here are not really presenting a material with both advantages. And materials based on the SrFeO x composition seem also slightly more efficient than CaFeO 2.5 composition.

Finally, it necessary to understand that due to its specific layered structure, the brownmillerite structure is anisotropic (1D oxygen vacancies channel). So one could think that the efficiency of this structure would rely in its preferred orientation. Therefore evaluating the catalytic performances as powder may not be the more accurate. It would be very interesting to build orientated thin-films from these materials and study their performances afterwards.

Finally, it necessary to understand that due to its specific layered structure, the brownmillerite structure is anisotropic (1D oxygen vacancies channel). So one could think that the efficiency of this structure would rely in its preferred orientation. Therefore evaluating the catalytic performances as powder may not be the more accurate. It would be very interesting to build orientated thin-films from these materials and study their performances afterwards.

Introduction

As detailed in the bibliography section page 32, perovskite are of particular interest in catalysis. They represent one of the most studied class of materials as active supports so far [START_REF] Royer | Perovskites as substitutes of noble metals for heterogeneous catalysis: Dream or reality[END_REF] [START_REF] Zhu | Perovskite oxides: Preparation, characterizations, and applications in heterogeneous catalysis[END_REF]. Their inherent ability to withstand high level of substitution combined with the consequent electronic and ionic mobility turn them into good candidates for application as active supports in catalysis. CaTi 1-x Fe x O 3-δ (CTF) system has already been studied and is showing interesting properties so far [131] [34].

This chapter is made of the results obtained while studying the CaTi 1-x Fe x O 3-δ system. It involves in a first part the physico-chemical characterizations such as elemental analysis, XRD and specific surface area performed during the synthesis and on the synthesized materials. The second part will be dedicated to the study of the red-ox behaviour of the system with a focus on oxygen mobility. In the third part the catalytic properties of this system will be evaluated according to its red-ox properties. And finally, the fourth part will be assigned to the study of the catalysts stability and the comprehension of the degradation mechanisms.

Synthesis optimizations

Thermogravimetric analysis

The CaTi 1-x Fe x O 3-δ powders studied in this chapter were made following the process described in the experimental section (see page 2.2.1 f). In order to optimize the calcination program in the synthesis process, a thermo-gravimetric analysis of the polymer network has been performed on a CaTi 0.9 Fe 0.1 O 3-δ «muffin» corresponding to the polymerized gel before calcination. Fig. 4.14 shows the TG where the black plot represents the mass variation of the polymer network. The theoretically expected weight loss is about 83%, whereas here the total weight loss is of 76.2%. The difference can come from the purity of the sample and or the precursor but also from adsorbed water on the sample surface when starting the TG. This total loss can be divided in several smaller losses. The first of 3.7% occurs below 100 • C and can be assigned to the release of water, which is also the synthesis solvent. The second weight loss accounting for 9.8% is centered at 250 • C and is possibly due to the decomposition of a first part of the organic compounds (e.g. citric acid and ethylene glycol). The third and largest weight loss of 60.2% starts from 350 • C up to 500 • C and is considered as the decomposition of the main part of the organics such as nitrates, citric acid or species formed during the hydrolysis or gelation process. This is confirmed by the associated exothermic spike around 452 • C. A last weight loss of 2.18% occurs around 570 • C and is attributed to the combustion of remaining carbonates. The exothermic event above this temperature is attributed to the crystallization.

Pfaff [START_REF] Pfaff | Synthesis of Calcium Titanate Powders by the Sol-Gel Process[END_REF] performed X-Ray Diffraction on the dry gel before calcination, and after calcination at 500 • C; 700 • Cand 900 • C. He highlighted that the calcination temperature has to be at least superior to 700 • Cbecause of the stability of CaTi 2 O 5 and CaTi 4 O 9 below this temperature. He also evidenced that the specific surface area of CaTiO 3 powders were drastically reduced from 29 m 2 • g -1 at 800 • Cto 18 m 2 • g -1 at 900 • C.

SYNTHESIS OPTIMIZATIONS 123

According to these data, the optimal parameters were chosen to reach the aimed composition but also the highest specific surface area possible with this process due to the importance of this characteristic in catalytic reactions. Moreover, the next section will explain why the temperature of the higher dwell in the calcination program is 800 degree C. It seems possible to avoid formation of the secondary phases he mentioned by using other techniques or by adapting the sol-gel process. Therefore, it is possible to increase the textural properties of perovskites by using other synthesis methods allowing lower calcination temperature (such as sol-gel [START_REF] Salles | Determining the rate-limiting step during oxygen semi-permeation of CaTi0.9Fe0.1O3-∆oxygen transport membranes[END_REF] or hard-templating [START_REF] Buära | Cooperative assembly synthesis of mesoporous SrTiO 3 with enhanced photocatalytic properties[END_REF]).

Determination of calcination temperature

XRD measurements collected on CaTi 0.9 Fe 0.1 O 3-δ pre-calcined powder at 500 • C are presented in Fig. 4.15. A first diffractogram was recorded at room temperature (25 • Cblack) then one at 550 • C(red); 600 • C(blue); 700 • C(light green); 800 • C(magenta); 900 • C(orange) and finally a second one at room temperature after this thermal treatment (dark green). The small shift along the X axis is due to the temperature increase. Some small reflections (example with the shoulder at 34 • on the black diffractogram, 25

• C) disappear with temperature (by comparison with other diffractogramms such as the red, 550 • C). These reflections were due to the presence of a secondary phase (CaO; see page 132) which has been removed by the thermal treatment.

It is possible to distinguish an increase in the overall intensities coming along the temperature (easy to see on the reflection at 33 • ). This increase is the sign of an augmentation of the crystallization leading at some point to a possible phase transition. Indeed, McCammon [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] showed that CaTi 1-x Fe x O 3-δ is eventually shifting to a cubic structure with temperature. Possibly, the increase of the crystallization comes with a lowering of some properties such as SSA and if the temperature is high enough, the sintering process may happen. These observations, support our choice of choosing 800 • Cas the maximal calcination temperature. That way, the crystallization is high enough (see the 800 • C/ magenta plot on Fig. 4.15) and the SSA is not too low (around 13 m 2 • g -1 ). But as the aim is to obtain a material as pure (without any secondary phases) as possible, the disappearance of secondary phases is primordial. Such kind of disappearance can be observed with the reflection at 29 • only present on the 25 • C(black) and the 550 • C(red) diffractogramms. Synthesis of 10 different compositions in the CaTi 1-x Fe x O 3-δ with x = 0.1; 0.2; 0.3; 0.4; 0.5; 0.6; 0.7; 0.8; 0.9; have been made following this process optimization accordingly to the synthesis process proposed in the experimental section.

Physico-chemical properties

This section will be dedicated to the evaluation of chemical and physical properties of the synthesized CaTi 1-x Fe x O 3-δ with x = 0.1; 0.2; 0.3; 0.4; 0.5; 0.6; 0.7; 0.8; 0.9; materials. First, chemical compositions were analysed to see if they fit with theoretical ones, then SSA was measured because this parameter is strongly linked to the synthesis method used and has an impact over catalytic properties. Finally, X-ray diffraction patterns were recorded to check the non-presence of secondary phases and also to see the impact of substitution along the whole phase diagram over the crystalline phases.

X-Ray Fluorescence

To ensure good chemical composition of the prepared powders, X-Ray Fluorescence analysis were made on each sample. The results are presented in Table 4.9. Performing substitution in the CaTi 1-x Fe x O 3-δ gives either perovskite or ordered phases based on the perovskite structure as described by McCammon et al. [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF]. Xray fluorescence allows us knowing the molar amount of each element. To ensure the manufacturing of the perovskite phase, the relative amount of B sites elements towards A site element is crucial. Therefore values presented are the molar ratio of each B site elements (e.g. Ti or Fe) towards A site element (e.g. Ca). Values for Ca are always 100% as it is the calculation basis. For each substitution the values obtained are close to the desired one. But one can notice the differences due to the experimental error (both from analysis and/or synthesis preparation) usually inferior to 1% of substitution.

Specific Surface Area

Even though the goal of this project is to develop catalytically active supports, the catalytic activity from the support only might not be high enough to be comparable with other compounds. Thus to get a measurable, comparable and improved catalytic activity, an active phase usually a noble metal will be deposited on the supports. The deposition of such catalyst depends of the specific surface area (SSA), indeed very different SSA could lead to different active phase coverage. Moreover, our heterogeneous catalytic model reaction involves the diffusion of a gaseous phase through a powder bed.

Here also SSA plays a role over gas diffusion inside the bed.

As already detailed in 4.7.1 at page 121, SSA of supporting materials is usually achieved during the synthesis process and depends a lot of the synthesis technique used and more specifically the calcination programme. We used the citrate-gel complexation synthesis method derived from PECHINI's [START_REF] Pechini | PECHINI PATENT[END_REF] method to synthesize the perovskite materials. Measurement of SSA are presented in Table 4.10 and were made accordingly to the experimental part, Fig. 4. [START_REF] Penlkala | Low Temperature Oxygen Mobility Applied to Catalysis[END_REF] is showing examples of measured isotherms.

The two isotherms present on Fig. 4.16 are the ones measured for CaTi 0.9 Fe 0.1 O 3-δ (black curve) and CaTi 0.6 Fe 0.4 O 3-δ (red curve). Both show the same SSA of 7 m 2 • g -1 and have the same shape but the adsorbed quantity during the multi-layer domain (after the dwell) is different expliciting different textural properties. The shape of these isotherms corresponds to type II isotherm and the H3 hysteresis type after IUPAC and described by K.S.W. Sing et al. [START_REF] Sing | INTERNATIONAL UNION OF PURE COMMIS-SION ON COLLOID AND SURFACE CHEMISTRY INCLUDING CATALYSIS-Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity[END_REF]. The Type II isotherms represent unrestricted monolayer-multilayer adsorption and are typically obtained with non-porous or macroporous adsorbent. The points at the beginning of the almost linear middle section of the isotherms is often used to describe the end of the mono-layer coverage and the beginning of the multi-layer adsorption. H3 type of hysteresis is typical of non-reversible multilayer adsorption associated to the metastability of the multilayer possibly due to pore curvature and non-rigidity of aggregate structure. Data of SSA measurement by BET method for the whole CaTi 1-x Fe x O 3-δ system are presented in 4.10 . These materials reveal SSA between 4 and 8 m 2 • g -1 . These values are pretty low regarding surface that could be reached in material science even for oxides. Here CaTi 1-x Fe x O 3-δ materials remain below 10 m 2 • g -1 . As previously mentioned, SSA is usually an important parameter in catalysis, especially when studying powders. But in this study, it has been chosen to focus on the compositions so SSA require to be in the same range. This way, differences between materials should only come from their compositions or structure.

X-Ray Diffraction

Perovskite structure (ABO 3 ) has the ability to withstand a high degree of substitution.

Here, we want to see the impact of huge substitution from 0 to 100% in the perovskite B site, which can be written as ABB'O 3 . Substitution is basically replacing one atom by another in the same cristallographic site, but those atoms do not have the exact same size. The perfect perovskite structure is cubic, but increasing the substitution degree will add stress onto the structure stability. To get an evaluation of perovskite stability undergoing such high degrees of substitution, we calculated Goldschmidt tolerance factor [START_REF] Goldschmidt | Die Gesetze der Krystallochemie[END_REF] following equation 4.14:

t = r A + r O √ 2 * (r B + r O ) (4.14)
Where r A is the atomic radius of cation in A site; r B is the atomic radius of cation in B site and r O the atomic radius of oxygen anion. Goldschmidt tolerance factor t is calculated on a model where the packing/compactness of atoms is at its maximum thus the value of the factor gives indication of the possible obtention of a perovskite. The limiting values of Goldschmidt tolerance factor have been experimentally determined. Hines et al. [START_REF] Ian | Atomistic simulation and ab initio studies of polar solids[END_REF] proposed that the ideal condition to get a cubic perovskite is when ∼ 0.9 ≤ t ≤ 1. When t > 1 the symmetry will be hexagonal. When ∼ 0.75 ≤ t ≤∼ 0.9 the perovskite will be orthorhombic and if t value drops below 0.75 it's assumed to get 4.8. PHYSICO-CHEMICAL PROPERTIES 129 an hexagonal ilmenite structure (FeTiO 3 ). The exact atomic radii where taken from Shannon et al. [START_REF] Shannon | Revised Effective Ionic Radii and Systematic Studies of Interatomie Distances in Halides and Chaleogenides[END_REF] accordingly to the coordination number of each perovskite site:

• r A = r Ca 2+ = 1.34 Å when coordination number is 12

• r B = r Ti 4+ = 0.605 Å when coordination number is 6

• r B' = r Fe 3+ = 0.55 Å when coordination number is 6 For every degree of substitution of Ti 4+ by Fe 3+ the values of t we obtain are ∼ 0.9 ≤ t ≤ 1 or slightly superior to one for CaFeO 3 . This was expected because it's well known that CaTiO 3 has the orthorhombic symmetry. Thus, the next part will be dedicated to the analysis of XRD data collected on the CaTi 1-x Fe x O 3-δ system. When increasing the amount of substitution in the B site of CaTi 1-x Fe x O 3-δ system, following the PECHINI [START_REF] Pechini | PECHINI PATENT[END_REF] synthesis method, it is possible to see that the system does not follow Vegard's Law.

• r O = r O 2 -= 1.35 Å
Indeed, CaTiO 3 is a perovskite (ABO 3 ) whereas Ca 2 Fe 2 O 5 is a Brownmillerite (ABO 2.5 ) oppositely to the first assumption of this law. Hence, it is impossible to have an approximation of the lattice parameters using the composition of each sample. To simplify this system behaviour, one can follow the possible ordering of the oxygen vacancies created along the substitutions. As a guideline, the study performed by McCammon et al. [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] describes a disordered or partially ordered system up to 50% susbtitution and an ordered system containing 3 ordered phases when substitution degree is higher than 50%. Thus we have chosen to split this system in two parts, the first one including substitutions from 0 to 40% and the second containing substitutions from 50% to 100%.

Structural analysis

CaTi 0.9 Fe 0.1 O 3-δ as a model Straight after synthesis of CaTi 1-x Fe x O 3-δ by PECHINI's derived method, XRD measurement were done to ensure that the material has the perovskite structure. Fig. 4.17 is presenting the comparison between experimental measurement (red dots) with Crystallographic Information File (CIF) pattern (black line) (database code ICSD 51876). The pattern from this CIF belongs to the Pnma space-group and will be used as a reference in this chapter.

Although structural parameters were not refined neither following LeBail nor Rietveld methods, one can see that the experimental pattern is fitting with the model one. The HKL index and the position of the corresponding reflections were added to give more clarity.

As previously mentioned, secondary phases can be formed during the synthesis process. Fig. 4.18 is presenting the same pattern as Fig. 4.17 but most encountered secondary phases were added. The list of secondary phases is not complete but the most intense reflexions of the most common phases are mentioned. In this case, CaO; TiO 2 ; Fe 2 O 3 and CaCO 3 are listed but their amount remain very low compared to the CaTi 0.9 Fe 0.1 O 3-δ main phase.

Perovskite has the specificity to undergo high level of substitution and remain stable (a section will detail this latter at page 133). But manufacturing a pure perovskite is not that easy, the stability of the phase rely on the packing of the atom composing it, as described by the goldschmidt tolerance factor detailed page 129. When preparing the substituted perovskite, the smallest deviation from the theoretical ideal packing will lead to the formation of secondary phases. Among those phases, the most encountered one was CaCO 3 certainly due to an excess of calcium during the preparation. CaTiO 3 (dark blue diffractogramm) is supposed to be a perfectly cubic phase as detailed in the bibliography (see page 1.3.1 f). Some secondary phases can by present as detailed previously. Here the main one is TiO 2 anatase which main reflexions can be seen at 26 [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] ,CaTi 0.9 Fe 0.1 O 3-δ is in the partially ordered range. Thus, it is no more a perfectly cubic perovskite, and when checking literature, the space group As explained, when the substitution degree increases, the lattice is subject to higher stress. This stress has an impact over the lattice stability. To release this stress, the 4.9. TEMPERATURE PROGRAMMED REDUCTION OF THE BARE CATI 1-X FE X O 3-δ SUPPORTS1 lattice can either expel some elements thus forming a another phase or a phase transition can happen. This mechanism will allow the formation of a new material with all the desired atoms but in a different configuration. McCammon [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] study revealed the formation of three ordered phases with the following stoichiometry: CaTi 0.5 Fe 0.5 O 3-δ / CaTi 0.44 Fe0.66O 3-δ and Ca 2 Fe 2 O 5 . These three structures are described in the introduction (see page 23 ff). As a reminder, tetrahedral layers are inserted between the octahedral ones, according to this: The phase transition detailed here may have an impact over the catalytic performances when using CaTi 1-x Fe x O 3-δ compositions as catalytic supports. These influences will be detailed latter (see page 145 ff).

Temperature Programmed Reduction of the bare CaTi 1-x Fe x O 3-δ supports

Temperature Programmed Reduction (TPR) were performed on the CaTi 1-x Fe x O 3-δ system following the protocol described in the experimental section (see page 2.4.6 f).This technique allows (in the ideal case) the quantitative measurement of a material reducibility by following the hydrogen consumption while increasing the temperature. In the CaTi 1-x Fe x O 3-δ system the potential reducibility comes from reduction of iron in the lattice. In the following we identify the reduction temperatures of the possible different iron species. Then we will try to identify the iron species present in our samples according to the reduction temperature experiment performed on standard iron oxides composition e.g. F rom 320 • C to 650 • C : Fe 3+ + e -<=> Fe 2+ (4.17)

F rom 650 • C to 1000 • C : Fe 2+ + 2e -<=> Fe 0 (4.18)
With these data, we identify temperature range where each reduction takes place, but one has to know that these reactions can be competitive. Indeed, less energy is required to reduce surface iron atoms than bulk or non surface iron species, for the same reduction state. Therefore it is possible that the reduction of surface Fe 2+ starts already when reduction of Fe 3+ is not completely finished. The labelled reduction area on Fig 4 .21 should be considered as guidelines. Moreover, the crystallographic environment of iron ions certainly have an influence over reduction temperature, making exact identification of reduced species more uncertain.

The model described by McCammon et al. [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] in the bibliography (page 23 ff) is based on the identification of Fe 3+ with different coordination numbers. But unfortunately TPR technique on its own is not accurate/sensitive enough to differentiate such differences. Therefore, we will essentially use this technique to understand the red-ox potential of each CaTi 1-x Fe x O 3-δ compositions and maybe associate it with the catalytic performances. CaTi 0.9 Fe 0.1 O 3-δ is showing a first H 2 consumption between 220 • C and 420 • C which can possibly correspond to the reduction of surface Fe 3+ into Fe 2+ , even though this reduction is happening at slightly lower temperature than the one identified with the standards. The second reduction happens from 720 • C and is not totally finished when the temperature reaches the isothermal step at 1000 • C. But after the one hour long isothermal step, the signal goes back to baseline and is ascribed to the reduction of Fe 2+ into Fe 0 . Another work [START_REF] Salles | Determining the rate-limiting step during oxygen semi-permeation of CaTi0.9Fe0.1O3-∆oxygen transport membranes[END_REF] performed on similar samples revealed using XRD that after TPR (following the same protocol) Fe 0 is massively present in the sample. CaTi 0.6 Fe 0.4 O 3-δ is presenting the same behaviour than CaTi 0.7 Fe 0.3 O 3-δ but the two contiguous peaks are almost distinct. The second reduction is present at equivalent temperature but does not go back to the baseline during the isothermal step.

As the iron concentration is increased by substitution between each sample, one can expect that at some point, our TPR protocol might not be able to fully reduce samples containing a high amount of iron. It is already the case for CaTi 0.6 Fe 0.4 O 3-δ and will also happen for higher substitution level as we observe in Fig. 4.23. A second reduction is present between 600 • C and 900 • C as a shouldering of the final reduction which does not end during the isothermal step at 1000 • C. The differences between these two reductions could be due to different iron species (either regarding oxidation degree or coordination number). But it could be also attributed to 4.9. TEMPERATURE PROGRAMMED REDUCTION OF THE BARE CATI 1-X FE X O 3-δ SUPPORTS1 a difference between surface iron and bulk iron. CaTi 0.4 Fe 0.6 O 3-δ displays a similar behaviour but a new reduction area is appearing between 550 • C and 650 • C. When looking at the phase diagram of CaTi 1-x Fe x O 3-δ in [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF], one can notice that a phase transition is occurring from CaTi 0.5 Fe 0.5 O 3-δ when increasing the iron substitution degree. Indeed, CaTi 0.5 Fe 0.5 O 3-δ is an ordered phase with one tetrahedral layer every three octahedral layers. The appearance of this tetrahedral layer might be the cause of this new reduction area. First we can observe that hydrogen uptake increases with iron content, besides CaTi 0.1 Fe 0.9 O 3-δ which hydrogen consumption is a bit lower than the one of CaTi 0.2 Fe 0.8 O 3-δ . The trend of the plot is almost constant in the range (0, 1 ≤ x ≤ 0, 4), this observation is in accordance with McCammon's model [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] where CaTi 1-x Fe x O 3-δ is suppose to follow a partial order/disorder in this range. With iron content higher than x=0.4, ordered phases start to appear. CaTi 0.5 Fe 0.5 O 3-δ is the first one explain-ing the decrease in the plot's trend. The second ordered phase is CaTi 0.44 Fe 0.66 O 3-δ , possibly explaining why hydrogen uptake is equivalent between CaTi 0.4 Fe 0.6 O 3-δ and CaTi 0.3 Fe 0.7 O 3-δ . The last ordered phase is Ca 2 Fe 2 O 5 with the brownmillerite structure. TPR plot of Ca 2 Fe 2 O 5 described in the previous chapter (see page 87 f) is different than the one of CaTi 0.2 Fe 0.8 O 3-δ and CaTi 0.1 Fe 0.9 O 3-δ . In the region of CaTi 0.2 Fe 0.8 O 3-δ and CaTi 0.1 Fe 0.9 O 3-δ are supposed to be a weighted mix of TOO and TO phases. But the evolution of TPR plots in this region do not reflect this evolution. It is thus difficult to explain the differences in the plot trend in the region of CaTi 0.2 Fe 0.8 O 3-δ and CaTi 0.1 Fe 0.9 O 3-δ .

Evaluation of catalytic performances

Catalytic performances of CaTi 1-x Fe x O 3-δ system were evaluated regarding their ability to convert CO (g) into CO 2(g) . But in order to have competitive materials performances and compare them with literature, materials were impregnated with one weight percent of platinum.

Platinum Impregnation

Platinum impregnation was made following the protocol described in the experimental section (see page [START_REF] Voorhoeve | Rare-Earth Manganites: Catalysts with Low Ammonia Yield in the Reduction of Nitrogen Oxides[END_REF]. It is necessary to precise that impregnating a noble metal over our supports may have an impact over the catalytic system performances. Indeed platinum will be deposited in the form of nano particles onto the supports surfaces and will increase the catalytic activity. Different parameters will have an impact on the overall activity such as particles sizes, SSA, platinum oxidation state and other textural or physical properties. But all the platinum atoms present on the support surface will not get involved in the reaction, only those present at the particles surface.

The dispersion of the platinum is defined as the number of platinum atoms at the surface of the platinum particles over the total number of platinum atoms. This parameter is used to evaluate the noble metal proximity with the support (in our case to ensure that the influence of platinum is the same for every materials). Determination of dispersion is made following equation 4.19 adapted from work of Boudart [START_REF] Boudart | Catalysis by Supported Metals[END_REF]:

D(%) = 6 * M P t * d va * 100 a * ρ * N a (4.19)
Where M Pt is the platinum atomic mass; d va the volume-area mean diameter; a the surface area occupied by an atom of platinum ; ρ the platinum mass density and N a the Avogadro number. This model allows noble metal dispersion determination but requires some assumptions. First, this models considers only spherical particles of diameter d i and has to make some assumption of the crystal nature to link dispersion and mean particle diameter. As platinum is an fcc metal, it is assumed an equal proportion of ( 111), ( 100) and ( 110) planes allowing the determination of the surface area occupied by one platinum atom : a. The last parameter d va is described by equation 4.20 where n i is the number of particles of d i diameter:

d va = n i * d 3 i n i * d 2 i (4.20)
The particles mean diameter is determined from HAADF-STEM images analysed using ImageJ software. From the diameters measured on 1%Pt/CaTi 0.9 Fe 0.1 O 3-δ and 1%Pt/CaTi 0.6 Fe 0.4 O 3-δ using equation 4.20 we determine platinum dispersions of 79% and 77% respectively. As the mean particles diameters and dispersions are very similar for both samples we ensured that any differences in the catalytic behaviour from one sample to another in the CaTi 1-x Fe x O 3-δ system is coming from the composition and not platinum impregnation.

CO oxidation light-off plots

Screening materials of CaTi 1-x Fe x O 3-δ system towards their catalytic performances in the carbon monoxide oxidation was performed accordingly to the protocol described in the experimental chapter (see page 49 f). To evaluate materials' catalytic performances in the frame of CO oxidation, the conversion of CO (based on the CO disappearance) is determined following equation 4.22: 1wt%Pt/CaTi 0.8 Fe 0.2 O 3-δ presents a similar activity to the one of 1wt%Pt/CaTi 0.9 Fe 0.1 O 3-δ but with slightly better performances. Beginning of conver-sion is 65% at 40 • C and then drops to 40 % conversion at 48 • C. After that, conversion increases gradually before reaching full conversion at 170 • C, the same temperature required for full conversion of CO over 1wt%Pt/CaTi 0.9 Fe 0.1 O 3-δ . When comparing 1wt%Pt/CaTi 0.8 Fe 0.2 O 3-δ and 1wt%Pt/CaTi 0.9 Fe 0.1 O 3-δ , they both undergo a deactivation but 1wt%Pt/CaTi 0.8 Fe 0.2 O 3-δ drops to 40 % whereas 1wt%Pt/CaTi 0.9 Fe 0.1 O 3-δ drops to 37%. But they reach full conversion at the same temperature (170 • C). 1wt%Pt/CaTi 0.9 Fe 0.1 O 3-δ is showing a very constant conversion increase after deactivation but for 1wt%Pt/CaTi 0.8 Fe 0.2 O 3-δ , conversion increases first quite fast and then slows a bit. This may be due to a change occurring on the catalyst surface (either noble metal or support). In a next section, catalytic system stability will be further studied (see page 153). 1wt%Pt/CaTi 0.7 Fe 0.3 O 3-δ is basically showing the same kind of behaviour as 1wt%Pt/CaTi 0.9 Fe 0.1 O 3-δ and 1wt%Pt/CaTi 0.8 Fe 0.2 O 3-δ but with even better performances. Indeed at room temperature CO conversion is already at 75% then undergoes the loss of activity down to 65% at 35 • C. From this temperature, conversion starts to increase due to thermal energy until reaching full conversion at 110 are showing a slope change when conversion increases after the deactivation. These slope changes are very slight so identification of the mechanism occurring here is therefore difficult. It is nonetheless still possible to think that this slope change is due to a chemical mechanism. Indeed if it was because of thermal energy, one could expect that the conversion would have shoot up to 100% conversion.

CO conversion(%) = [CO] initial -[CO] t [CO] initial * 100
Chemical changes can occur because of the atmosphere and the temperature. And usually result in a different catalytic activity. As an example, the activity loss occurring on 1wt%Pt/CaTi 0.9 Fe 0.1 O 3-δ ; 1wt%Pt/CaTi 0.7 Fe 0.3 O 3-δ and 1wt%Pt/CaTi 0.6 Fe 0.4 O 3-δ will be further studied in the section about isothermal experiments at page 153 focusing on 1wt%Pt/CaTi 0.6 Fe 0.4 O 3-δ . also different from other compositions. They display a very low activity (below 5% conversion) at room temperature before increasing slowly to 20% conversion at 175 • C. Finally they both shoot up to 100% conversion at 205 • C for 1wt%Pt/CaTi 0.2 Fe 0.8 O 3-δ and 210 • C for 1wt%Pt/CaTi 0.1 Fe 0.9 O 3-δ .

On Fig. 4.27 a material different than 1wt%Pt/CaTi 1-x Fe x O 3-δ was added. This material is a 5wt%Pt/Al 2 O 3 commercial catalyst presented here for comparison purpose. Its performances follow a similar trend as the one of 1wt%Pt/CaTi 0.2 Fe 0.8 O 3-δ and 1wt%Pt/CaTi 0.1 Fe 0.9 O 3-δ , starting with very low activity (inferior to 5%) at room temperature and then increasing exponentially to reach full conversion at 187 • C. This material is thus more efficient than 1wt%Pt/CaTi 0.2 Fe 0.8 O 3-δ and 1wt%Pt/CaTi 0.1 Fe 0.9 O 3-δ but also contains more platinum (5% for Al 2 O 3 whereas platinum content is only about 1% for CaTi 1-x Fe x O 3-δ ). Platinum is a very active noble metal for CO oxidation explaining why activity is higher for this commercial material, but it is also very expensive.

There is an economical trade-off between performances and platinum content, as performances may be increased with an higher platinum loading but the material will be more expensive. The point is to reach the best performances possible while keeping an affordable platinum content. Here, some CaTi 1-x Fe x O 3-δ materials are showing higher performances than the commercial catalyst (5wt%Pt/Al 2 O 3 ) with a platinum loading 5 times lower. CaTi 0.7 Fe 0.3 O 3-δ and CaTi 0.6 Fe 0.4 O 3-δ as the most performing supports are very promising materials, but they undergo a deactivation process at low temperature. It could be possible to solve this issue by tuning the materials (composition / surface properties). Or one could try to regenerate the materials by alternating different atmospheres (one with the catalytic conditions and one to regenerate the catalysts). Moreover those materials are displaying a low SSA as detailed at page 128. Thus, regarding the performances reached with a low SSA, it seems very interesting in a future prospect to try to increase this parameter as it should give even more efficient materials.

Stability

Stability is a big challenge in catalysis because it most of the time is a trade-off at the cost of efficiency. The general idea is: red-ox reactions usually require mass transfer, and so mobility of oxygen ions. Mobility of those species within the catalytic system can happen thanks to different mechanisms (like the hopping process in the case of oxygen ions in CaTi 1-x Fe x O 3-δ system). In the case of active supports based on oxide materials it can be allowed because of local oxido-reduction and so inevitably mobility of electrons and counter-ions. One can understand that perfectly ordered materials (such as cubic perovskites) are very stable and a relatively big amount of energy is required to enable mobility of species within their lattice. To have more efficient catalytic materials there is a need to reduce this amount of energy. One strategy could therefore be to manufacture less ordered materials such the perovskites described page 131 ff.

By definition a catalyst lowers the amount of energy required to realise a chemical reaction without being involved in the latter. But in some cases, it is possible that the catalyst get affected by the reaction conditions (thermal treatment, chemical poisoning, metal leaching, etc...).Moreover there are different applications and as much reaction conditions. In this study, catalysts stability is tested by performing 2 catalytic cycles (thermal treatment from room temperature to full conversion temperature) under reacting gas flow between which a cooling step under helium is performed. Fig. 4.28 is presenting conversion recorded during the second catalytic cycle: CaTi 0.9 Fe 0.1 O 3-δ conversion starts around 10% at room temperature and then a small deactivation to 7% at 55 • C occurs. But after this deactivation and because temperature increases (intake of thermal energy) conversion rises. It first rises fast and then a bit slower until eventually reaching 100 % conversion at 185 • C. The deactivation happening during the second cycle is showing that CaTi 0.9 Fe 0.1 O 3-δ has not been totally deactivated during the first cycle or that the helium sweeping step between the two cycles could partially regenerate the catalyst. Overall performances recorded during a second CO oxidation cycles of CaTi 1-x Fe x O 3-δ materials are inferior to the ones of the first cycles. Nonetheless they are still interesting because a catalyst has to be efficient the longest time possible (either in continuous utilization or when cycled). CaTi 1-x Fe x O 3-δ is composed by very interesting materials able to show conversion higher than 40% at room temperature. Some of them were able to withstand the catalytic tests conditions and keep displaying good performance after deactivation. More specifically, CaTi 0.8 Fe 0.2 O 3-δ and CaTi 0.7 Fe 0.3 O 3-δ maintained a conversion close or higher than 20% even when a second deactivation is happening. And every materials show full conversion before 180 • C or at least when reaching the latter temperature.

Second CO conversion cycle of CaTi

Isothermal CO oxidation

Focusing on room temperature catalysis, isothermal experiment where performed. First in order to evaluate the catalyst deactivation (how low the conversion would drop) and if it is possible to perform in-situ regeneration of the catalyst. Only one material is evaluated here: CaTi 0.6 Fe 0.4 O 3-δ because its catalytic performances were among the more interesting during first cycle and a big activity loss occurred during the second cycle in comparison with the first one. The parameters of the isothermal tests were the same as described for the classic light-off plots but sample temperature was kept at 30 • C. After the isothermal step, temperature was increased until full conversion with the same 2 • C.min -1 as previously done. CO conversion measured for fresh 1%Pt/CaTi 0.6 Fe 0.4 O 3-δ during the isotherm study is presented on Fig. 4.29: 1%Pt/CaTi 0.6 Fe 0.4 O 3-δ CO conversion starts around 70% as it was starting at 85% on the first cycle see Fig. 4.26. So one can see that reproducibility of the first conversion measurements when coupled with a deactivation process are not that good especially at high conversion. But the deactivation process is still occurring as conversion drops with time. It first drops quiet fast and then slower and slower until gently declining to 25% after almost five hours. After these five hours, temperature was increased and above 37 • C, conversion begins to increase (see blue lines). This means that even if the catalyst was loosing its activity, only a small amount of thermal energy is enough to enable an higher CO conversion. Moreover, CO conversion keeps increasing with temperature and reaches 100 % conversion at 110 • C (see green lines). This temperature is even lower than the one recorded when reaching total conversion during the first cycle (125 • C).

There exist different deactivation processes that can happen. One could be the adsorption of carbonates on the catalyst surface. Another one would be the poisoning of the platinum by CO. Indeed, CO can adsorb on the platinum surface and block other molecules to access it. The platinum oxidation degree could also change to a less reactive state due to the tests conditions (mostly oxidative).

One can notice that full conversion is reached 15 • C lower than expected (110 • C after isotherm instead of 125 • C measured when performing classic light-off). But tests conditions (reactants concentration and temperature program) are different from previous experiments. This may explain the temperature difference when reaching full conversion.

At 17500 seconds (before the temperature starts to increase, the conversion is very close to be stabilized around 26%. The deactivation hypothesis is a coverage possibly made of carbonates. Moreover, in the case of a full covering the access to the catalyst would be impossible. As there is still 26% conversion after a long time, it seems that the coverage is not total. In order to improve the catalyst activity, one could either try to remove the carbonates (by changing the atmosphere for example) or make the catalyst resistant to carbonate formation (by doping or changing the composition for instance).

Isothermal CO oxidation + atmosphere alternation

In the previous section, the catalyst deactivation as a function of time and test conditions was presented. We showed that catalytic activity was recovered by increasing the temperature. But in "real-life" conditions or outside of lab-scale facilities, it can be very difficult (or very cost-full) to locally increase the temperature. Also it is theoretically possible to re-activate a catalyst by changing the atmosphere. The process could possibly remove the adsorbed CO or even re-oxide or reduce the catalyst. Fig. 4.30 is thus presenting the conversion of 3%Pt/CaTi 0.6 Fe 0.4 O 3-δ regarding time in an isothermal experiment at 30 • C when atmosphere is changed.

First, the platinum loading has been increased because the aim here is to study the deactivation. Usually adding more platinum leads to higher catalytic performances until a maximum, of course. So to study the deactivation, we choose to increase the platinum loading to 3wt% allowing 3%Pt/CaTi 0.6 Fe 0.4 O 3-δ to have almost full conversion at 30 • C before deactivation starts.

On Fig. 4.30, the conversion is the black plot. The atmosphere is modified and is mentioned by the colour of the rectangle: light green is the "classic" reactant mix of carbon monoxide and oxygen into helium; orange is pure helium and blue is oxygen into helium.

At the beginning of the measurement, CO conversion is already at 96% (18300 seconds) but slowly drops to 90% (24000 seconds). After that, a sweeping step under helium is performed during 7000 seconds (from 24000 to 31000 seconds). When switching back to the reactant mix, conversion starts at 92% and drops to 83% at 35500 seconds. So 2% more conversion have been achieved after the sweeping but the catalyst get deactivate again. This time, the deactivation went a bit faster (9% down in 4500 seconds whereas it was 6% in 5700 seconds).

Then a second shorter sweeping step of helium was performed during 1000 seconds. After this, conversion is 90% and drops to 74% at 40300 seconds. As the catalyst regenerate 7% in 1000 seconds, it seems that a shorter sweeping time may be sufficient to re-activate it. From the plot, one can see that the catalyst is loosing its activity even faster than after the first helium step.

A third helium sweep of 1000 seconds allows the conversion to be 89% (almost the same as after the second sweep). But conversion drops to 65% in 4000 seconds. So the conversion drop gets faster and faster after each helium sweep.

A fourth helium sweep of 1000 seconds allow the conversion to get back to 87% before dropping to 60% at 50000 seconds. Then a last 1000 seconds long helium seep was performed, conversion was regenerated to 86% and dropped to 51% in 7000 seconds. Up to that point, deactivation is increasing after each sweep and helium does not allow a full regeneration. Meaning that after every cycle of sweep and reactant mix, it gets further/faster deactivated.

From 58000 seconds, a 1000 seconds long oxygen sweep was performed. Surprisingly, conversion gets back to 91%. So oxygen sweep seems to be able to regenerate the catalyst way better than helium sweep is. But after, the conversion drops to 50% in 6000 seconds. Deactivation is here slightly faster than it was after the last helium step, but the activity recovery was higher.

A second oxygen sweep between 66000 and 67000 seconds allows regeneration to 64% only, which then dropps to 17% at 73500 seconds. This is the largest conversion loss observed and the conversion is a the point even lower than the one recorded when performing the classic isotherm with 1%Pt/CaTi 0.6 Fe 0.4 O 3-δ see Fig. 4.29.

Finally, two more 1000 seconds long oxygen sweep were performed but conversion recovering was only until to 40% for both of them. And as the deactivation was still present, the activity dropped down to 16% the two times.

In conclusion, oxygen was able to regenerate the catalyst better than helium. As only one oxygen sweep was sufficient to get back to previous activity. But longer oxygen sweep were not able to regenerate the catalyst to its best activity, and may have deactivated it even more. It is possible that 3%Pt/CaTi 0.6 Fe 0.4 O 3-δ has been oxidized during oxygen sweep. Either the platinum could be oxidized therefore decreasing its activity or the support got oxidized meaning the oxygen vacancies could have been (partially or totally) filled, disabling surface oxygen involvement from the support, but it could be both at the same time. Unfortunately, the time resolution of the analyser is too long (235 seconds for each measurement) to observe the desorption of any species during sweeping steps. Techniques with an higher time resolution would be required such as a mass spectrometry or in operando DRIFTS or XPS.

Oxygen Mobility

Study of oxygen mobility during reaction is difficult with active support because one has to make the difference between the oxygen coming from the gaseous phase as reactant from the one coming from the support lattice. To solve this problem we use 18 O enriched materials as described in the experimental part (see page 45 f) and then tested them following the "old" ILPOR procedure (see page 56 ff).

It is necessary to mention that contrary to the previous sections, these experiments have been performed on raw supports without any noble metal. To study oxygen mobility over the CaTi 1-x Fe x O 3-δ system, we have chosen to focus on CaTi 0.7 Fe 0.3 O 3-δ and CaTi 0.6 Fe 0.4 O 3-δ . These materials were enriched with 18 O prior to the experiment. Indeed, the enrichment yield is not 100% (although it was not measured by thermogravimetry coupled with mass-spectrometry) It is necessary to mention that if the mechanism was Langmuir-Hinshelwood, both species would react as adsorbed species oxygen mobility in the lattice: because some isotopic oxygen will be present at the surface and may react but once the reacted only equation 4.23 will happen. But if oxygen mobility is enabled, 18 O from the bulk can move to the surface and react following equation 4.24.

availability of 16 O 2(g) : In the specific case where 16 O 2(g) couldn't reach the surface to react, the support could very locally reduce itself and "give" either 16 O (s) or O (s) to the reaction. This behavior is described by the oxygen storage capacity of materials acting like oxygen buffers.

The previous equations (e.g. 4.23 and 4.24) and the last item of the previous list involves a very local reduction of the support (as one of its oxygen is taken out of the lattice by reaction). In order to compensate this, the support will be almost instantly re-oxidized by oxygen from the gaseous phase (filling of easily accessible vacancies). This behaviour was already oserved with ceria by Montiniet al. [START_REF] Montini | Fundamentals and Catalytic Applications of CeO2-Based Materials[END_REF] (note that ceria is one of the mostly studied active support) and is detailed by equation 4. Reactions involving lattice oxygen are following the Mars-Van Krevelen mechanism. Therefore the presence of 18 O isotopic oxygen in gaseous species ensures that CO oxidation over CaTi 0.7 Fe 0.3 O 3-δ is following (at least partially) Mars-Van Krevelen mechanism.

From 9000 seconds corresponding to 315 • C, C 16 O 18 O (g) intensity is higher than C 16 O 2(g) . So at higher temperature than 315 • C, oxygen mobility within the lattice seems to be sufficient enough so that equation 4.24 becomes the main one involved in the CO oxidation. This is a very interesting result because it means that the support is involved in the reaction (active) but also that it can give some of its oxygen to fulfill the reaction's need. In other words this is a proof that CaTi 0.7 Fe 0.3 O 3-δ displays oxygen storage capacity, a property which was mainly attributed to ceria as explained by Trovarelli [START_REF] Trovarelli | Catalysis by Ceria and related materials[END_REF] (and pretty unusual in other structures).

The signal of C 18 O 2 is increasing from 6500 seconds (or 240 O (g) seems to stabilize and signal of C 18 O 2(g) which was already stable start to decrease. Also, the quantity of 18 O (g) inside the lattice is related to the enrichment process and parameters defining its yield. But as this amount is limited, it is normal that when reacting, at some point 18 O concentration start to deplete. This depletion explains the signal decrease of isotopic oxygen containing species. However, this depletion should not have an impact on the ongoing mechanisms. ; is providing a zoom on the time frame 10500 -11750 seconds. Intensity scale was also adjusted to provide good information. First, it is necessary to remind that the 16 O 2(g) pulses are produced by a 6-way valve. This valve can be seen as a plug-flow system. Making the hypothesis that gases are not mixed, this system can be seen as a quantity of pure oxygen between two quantities of pure carbon monoxide (both diluted in helium). Keeping that in mind, it is normal that 16 O 2(g) pluses are anti-phased with C 16 O (g) pulses as it is on Fig. 4.31 c). Also, pulses of C 16 O 2(g) ; C 16 O 18 O (g) and C 18 O 2(g) are in phase with 16 O 2(g) pulses. Moreover, the perfect phase matching of the 3 carbon dioxide species means that they are all produced by the same mechanism (or that our set-up is not accurate enough to allow discrimination between the mechanisms). If they were anti-phased it would mean that the reaction path of one compound is slower than the other because of adsorption or dissociation steps. Assuming that they are all produced following the same mechanism, and because isotopic oxygen in present in CO 2(g) , one can assume that the main mechanism is the one described by equations 4.24-4.27.

The peak shape of C 16 O (g) is also changing from something irregular before 10875 seconds (378 • C) to a reproducible shape with a shouldering after. This shouldering appears because carbon monoxide is reacting with oxygen although oxygen quantity is diminishing after each pulse. Once oxygen has completely disappeared, the signal is normally increasing (getting back to normal intensity) until the new pulse comes. Then C 16 O (g) will react, decreasing the signal intensity until oxygen will start to lack. 18 O quantity is limited. Therefore, the signal containing 1 8O will make a plateau, before decreasing. Moreover, if no reactant is lacking and if temperature allows it, CO oxidation will continue. Signal of C 16 O 2(g) will keep increasing until, a reactant is missing and that every isotopic oxygen from the lattice is replaced by a 16 O coming from the pulses. Here also, the peaks of all CO 2(g) species are perfectly phased together, and anti-phased with CO (g) . As C 16 O 2(g) and C 16 O 18 O (g) peaks are matching, they may be produced by the same mechanism. And from the attendance of isotopic oxygen containing species, one could think the main mechanism is equation 4.24. But then signal of C 16 O 2(g) should have been lower. Explanation of the high intensity of C 16 O 2(g) is that enrichment yield was not sufficient to provide enough isotopic oxygen and thus equation 4.23 explains the high level of C 16 O 2(g) .

The general peak shape is also changing at 13500 seconds (385 • C) from something irregular before to more reproducible peak shape after. This might be the sign of a stabilization in the reaction mechanisms.

In conclusion, the chemical equations involved in the oxidation of carbon monoxide into carbon dioxide are the same for CaTi 0.7 Fe 0.3 O 3-δ and CaTi 0.6 Fe 0.4 O 3-δ . What is interesting is that they highlight a Mars-van Krevelen mechanism. Moreover these two materials show local oxido-reduction properties. They seem to be able to act as oxygen buffers, a property very specific and useful in the development of active catalytic supports.

Conclusion

Materials belonging to the CaTi 1-x Fe x O 3-δ system were all made following a facile synthesis pathways close to PECHINI's. Their structures are in accordance with Mc-Cammon's previous results although the synthesis method is different. They display relatively low surface area but very interesting other properties. Such as being able to reduce and oxidise themselves to give part of their oxygen. Acting like oxygen buffer allow these materials to still be efficient in lean regime and get regenerated during rich regime, and thus are good candidates for three way catalysts application.

They had to be impregnated with platinum to be catalytic competitive. Control of the platinum impregnation revealed very high noble metal dispersion around 80%. These materials are able to fully convert carbon monoxide into carbon dioxide at temperature below 200 • C even during a second cycle. Some of them such as CaTi 0.7 Fe 0.3 O 3-δ and CaTi 0.6 Fe 0.4 O 3-δ are showing room temperature conversion around 80%. But these materials undergo a rapid deactivation. This deactivation is not completely studied but first clues tends to lead to CO poisoning of the platinum. Nonetheless ensuring this would require further studies using XPS and diffused infrared analysis. But whatever is the cause of the deactivation, being able to stop or reverse it will not be easy. Because changing the atmosphere, didn't allow a full regeneration of the catalyst. But just a small increase of the temperature was able to re-activate the catalyst until full conversion at temperature just above to 100 • C. Last, the mechanism occurring during carbon monoxide oxidation over CaTi 0.7 Fe 0.3 O 3-δ and CaTi 0.6 Fe 0.4 O 3-δ was proven to be a Mars-van Krevelen. CaTi 1-x Fe x O 3-δ type of support is thus able to involve its lattice oxygen into the oxidation reactions.

The perovskite materials studied in this chapter are showing encouraging catalytic performances with low textural properties such as < 10 m 2 • g -1 SSA. Recent studies [START_REF] Sanjeev | Nanostructured MIEC Ba0.5Sr0.5Co0.6Fe0.4O3-δ(BSCF5564) cathode for IT-SOFC by nitric acid aided EDTA-citric acid complexing process (NECC)[END_REF] [140] [START_REF] Feldhoff | The solgel synthesis of perovskites by an EDTA/citrate complexing method involves nanoscale solid state reactions[END_REF] are focusing on increasing this property as it should increase the overall catalytic performances.

As an opening, it would be very interesting to focus on the structure of CaTi 1-x Fe x O 3-δ . Especially the quantification of the oxygen vacancies could help to better understand their catalytic impact. Testing these materials in other reactions would broaden their possible applications as perovskite materials are suitable for other applications [START_REF] Royer | Perovskites as substitutes of noble metals for heterogeneous catalysis: Dream or reality[END_REF]. More specifically testing them as catalyst for the partial oxidation of methane sounds CHAPTER 4. BROWNMILLERITES AS CATALYTIC SUPPORTS interesting as they already reveal oxygen mobility within their lattice.

Chapter 5

Conclusion and perspectives

The principal aim of this work was to develop and study novel and unconventional catalytic supports materials. For this purpose we observed what is among the most used and efficient catalytic support nowadays, the cerium oxide. The main feature of cerium oxide over other kind of support is its ability to transfer its lattice oxygen to the catalytic reaction need. This process is allowed by the oxidation degree versatility of the cerium ions. They can easily shift from +III to +IV as needed thus enabling oxygen mobility within the ceria lattice. We believe this property to be crucial to the development of active and efficient supports. So the innovative supports that have been studied in this work are require to show oxygen mobility as-well.

In order to manufacture novel and interesting supports materials with respect to their potential oxygen mobility property , two crystallographic structures were chosen: the perovskite ABO 3 and the brownmillerite ABO 2.5 . Regarding their oxygen mobility properties, both systems have already been intensively investigated in the literature during the last decade. Thus their oxygen mobility properties were proven to work with very different mechanism. Basically the brownmillerite system is able to show oxygen mobility thanks to the rotation of the FeO 4 tetrahedron. This specific mechanism is able to show oxygen mobility even at room temperature for some compositions e.g; SrCoO 2.5 . We chose to study two brownmillerite compositions: CaFeO 2.5 and SrFeO x . While CaFeO 2.5 is fully ordered, the FeO 4 layer of SrFeO x is disordered. This difference has an impact over their lattice dynamics and thus over their catalytic properties. The perovskite system is showing oxygen mobility thanks to the hopping process. This process is based on the presence of oxygen vacancies in the material structure and oxygen ions can jump from their position into the oxygen vacancy. Thus there is a 166 CHAPTER 5. CONCLUSION AND PERSPECTIVES need of creating those oxygen vacancies in the system. For this purpose we based our strategy on defect chemistry brought by substitution for perovskite materials.

To develop those systems we smartly took in account environmental and geopolitical problems. According to this, the European Union produced a list a raw materials that scientific community has to find an alternative to. Therefore, we decided to use only easily available and cheap elements to develop the materials of this work. This in mind lead us to chose calcium, strontium, iron and titanium as cations elements to develop our materials. The materials we prepared are thus the brownmillerites CaFeO 2.5 and SrFeO x and the perosvkite system CaTi 1-x Fe x O 3-δ with 0.1 < x < 0.9.

Isotope Pulses Labelling Oxygen Reaction: Optimization

Being able to study oxygen mobility from a catalytic point of view requires specific tools. Consequently the first part of this manuscript was dedicated to the improvement of one specific technique called isotope labelling pulse oxidation reaction (ILPOR). The ILPOR technique was developed by our team in the early stage of this project. This technique is basically an in-situ observation of oxygen mobility by the use of 18 O and a mass spectrometer. The supports materials were enriched with 18 O prior to the ILPOR itself. During the ILPOR, a flow of C 16 O interrupted by pulses of 16 O 2 is sent to the pre-enriched materials and the gaseous species involved in the reaction are recorded by a mass spectrometer. The ILPOR technique revealed itself being a very useful way to observe oxygen mobility during catalytic reactions but we aimed to improve it even further. To this end, we studied the impact of different parameters and have chosen their values according to our needs. The evaluated parameters and their most interesting values are summarised in the following list: During this study we also want to prove the versatility of the technique. We manage to differentiate the instrumental impact from the material one on the measured results. We showed almost no impact of the grain-size over the results. We are able to perform isotherm experiments with quantitative limitation by the pulse volume of reactants, which we think can be very useful for kinetic studies. And we were able to ensure mechanism or to observe side reactions by adding C 16 O 2 as reactant to cross-check our assumptions.

The development of ILPOR was very fruitful to observe oxygen mobility on our samples. The technique revealed its versatility during its improvement. And we hope it can lead to even further development. Especially if one can adapt the technique to other reaction than CO oxidation such as methane oxidation of NO x reduction the time resolution brought by the pulses of ILPOR could help to resolve mechanism issues.

Brownmillerites

The second part of this manuscript was dedicated to the study of the CaFeO 2.5 and SrFeO x brownmillerites. These materials were synthesised by two synthesis methods. On one hand we manufactured them by a complexation / combustion method. We obtained powders with specific surface area (SSA) of 12 m 2 • g -1 for CaFeO 2.5 (cp) and of 13 m 2 • g -1 for SrFeO x (cp) . One the other hand we synthesized the same compositions using an electric arc furnace (EAF); the two materials CaFeO 2.5 (eaf) and SrFeO x (eaf) display SSA below 1 m 2 • g -1 . These four materials were characterised after synthesis. Both CaFeO 2.5 (cp) and CaFeO 2.5 (eaf) were almost pure brownmillerite powders whereas SrFeO x (cp) and SrFeO x (eaf) were a mix of SrFeO 2.5 , SrFeO 2.75 and SrFeO 2.875 . It is known in the literature that SrFeO 2.75 and SrFeO 2.875 are two meta-stable phases making the synthesis of pure SrFeO 2.5 very difficult. XRD analysis revealed the presence of carbonates forCaFeO 2.5 (cp) and SrFeO x (cp) .

The materials were enriched with 18 O to study their oxygen mobility. We observed oxygen mobility at lower temperature for materials made by complexation than those made by EAF. We also observed oxygen mobility at lower temperature for SrFeO x compositions than for the CaFeO 2.5 thanks to TG-MS measurements. We thus obtained the following onset temperature classification: SrFeO x (cp) < CaFeO 2.5 (cp) < SrFeO x (eaf) < CaFeO 2.5 (eaf) . The enriched EAF samples were tested in ILPOR. They both revealed involvement of their lattice oxygen in the CO reaction spotted by the presence of C 18 O 16 O and C 18 O 2 . But SrFeO x (eaf) showed the presence of C 18 O 16 O concomitantly with C 16 O 2 at 120 • C whereas 190 • Cis required for the same observation in the case of CaFeO 2.5 (eaf) . By the presence of C 18 O 16 O and C 18 O 2 we assume the materials with poor textural properties especially a very low SSA. As the brownmillerite structure is 1D anisotropic, we think than evaluation of performance as powder is not suitable to reveal its true potential. It could be very interesting to build a thin film made of CaFeO 2.5 and SrFeO x brownmillerite and then evaluating its catalytic performances as it has been published with SrCoO 2.5 . They might not be sufficient to be generally competitive but could play a real role in anisotropic applications such as ctalytic memebrane reactor.

On the opposite, the CaTi 1-x Fe x O 3-δ materials especially with 0.1 ≤ x ≤ 0.4 are showing competitive performances. Their biggest issue lies in their instability and the deactivation occurring during the catalytic process. To solve this, one could try doping these structures with elements bringing stability such as lanthanum is doing with BaLaFeO 3 . Moreover, the specific surface area of our samples was low, and this parameters can be improved following the strategy of Mascotto [START_REF] Buära | Cooperative assembly synthesis of mesoporous SrTiO 3 with enhanced photocatalytic properties[END_REF] who is using glycerol as the synthesis complexing agent reaching SSA higher than 60 m 2 • g -1 .

In general, there are still a lot of work left to get the best out of these innovative supporting materials based on defect chemistry.

Figure 1 . 1 :

 11 Figure 1.1: Contribution of primary global suppliers of critical raw materials, average from 2010-2014.

Figure 1 . 2 :

 12 Figure 1.2: Scheme representing the simplified steps of an oxidation reaction. Species in red are reactants, in green are the products and in blue are possibly already present species.

CHAPTER 1 .

 1 BIBLIOGRAPHY / INTRODUCTION* Ceria is facing supply issues so there is a need to find an alternative * Oxygen mobility and oxygen storage capacity are key properties to develop efficient exhaust gas treatment catalyst * Supports showing oxygen mobility may follow the Mars-van Krevelen mechanism

Figure 1 . 5 :

 15 Figure 1.5: Distortion of the perovskite lattice from a) cubic to b) orthorhombic.Taken from[START_REF] Mark R Levy | Crystal structure and defect property predictions in ceramic materials[END_REF]

Figure 1 . 6 :

 16 Figure 1.6: Orthorhombic perovskite unit cell where blue spheres represent the A cations, yellow spheres the B cations and red spheres are oxygen anions (space-group Pnma). Taken from [55].

Figure 1 . 7 :

 17 Figure 1.7: Schematic representation of the hopping process where an oxygen anion (orange sphere) jumps into an oxygen vacancy.

1. 3 .Figure 1 . 8 :

 318 Figure 1.8: Phase diagram for the system CaTiO 3 -CaFeO 2.5 as a function of temperature and composition. Symbols TO, TOO and TOO indicates the alternation sequence of octahedral and tetrahedral layers in the ordered phases. The broad phase boundary between the cubic and partially ordered domain indicates that the nature of the phase transition is still undefined. Taken from[START_REF] Becerro | Oxygen vacancy ordering in CaTiO3-CaFeO2.5 perovskites: From isolated defects to infinite sheets[END_REF]

Figure 1 . 9 :

 19 Figure 1.9: The structure of Ca 4 Fe 2 Ti 2 O 11 projected down the a-axis, after applying crystallographic image processing on the high resolution electron microscope image .Taken from[66]

CHAPTER 1 .Figure 1 . 10 :

 1110 Figure 1.10: The structure of Ca 3 Fe 2 Ti 1 O 8 .Taken from[START_REF] Rodríguez-Carvajal | Perovskite threefold superlattices: A structure determination of the A3M3O8phase[END_REF]

CHAPTER 1 .Figure 1 . 11 :

 1111 Figure 1.11: The structure of CFO. Taken from[START_REF] Rodríguez-Carvajal | Perovskite threefold superlattices: A structure determination of the A3M3O8phase[END_REF]

CHAPTER 1 .

 1 BIBLIOGRAPHY / INTRODUCTION chain end). Using this model, McCammon plotted the average number of vacancies per chain as a function of the substitution presented on Fig. 1.13.

Figure 1 . 13 :

 113 Figure 1.13: Average number of vacancies per chain as a function of x in CaTi 1-x Fe x O 3-δ calculated from site abundance assuming Ti distributed over both octahedral and pentacoordinated sites. Symbols relate to annealing temperature: 900 • C (crosses);1000 • C (diamonds); 1100 • C (solid circles); 1200 • C (open circles) and 1320 • C (triangles).Taken from[START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF]

  1.15.

Figure 1 . 15 :

 115 Figure 1.15: Representation of the oxygen mobility mechanism within the brownmillerite framework. Left figure represents the brownmillerite structure with the red arrow indicating the movement of the apical oxygen. Right figure shows the intermediate state after the apical oxygen left the 1D vacancy channel generating FeO 5 square pyramid and the newly formed FeO 4 tetrahedron. Taken from[START_REF] Paulus | Lattice dynamics to trigger low temperature oxygen mobility in solid oxide ion conductors[END_REF]

CHAPTER 1 .

 1 Figure 1.16: Representation of the SrFeO x system members. a) SrfeO 3 , b) SrFeO 2.875 , c) SrFeO 2.75 and d) SrFeO 2.5 . Taken from[START_REF] Avishek | Exploring oxygen diffusion mechanisms in SrFeO 3 â x and Pr 2 NiO 4 + δ , followed up on single crystals by in situ synchrotron diffraction[END_REF]

  3 and Fe 2 O 3 for CaFeO 2.5 and SrCO 3 and Fe 2 O 3 for SrFeO x .

Figure 2 . 1 :

 21 Figure 2.1: Schematic representation of the electric arc furnace used for the synthesis of bulk brownmillerites

2. 4 .

 4 CHARACTERISATIONSprogrammed heating. A first diffractogram was recorded at room temperature (25 • C) then at 550 • C, 600 • C, 700 • C, 800 • C, 900 • C and finally at room temperature again.

4 :

 4 n16 O (mol) = oxygen stoichiometry * n sample = oxygen stoichiometry * m sample * M sample (3.3) Where the oxygen stoichiometry is the ratio of oxygen atoms regarding the sample chemical formula; as an example it is 2.5 for CaFeO 2.5 , m sample is the sample mass and M sample is the sample molecular weight.

2 Figure 3 . 1 :

 231 Figure 3.1: Scheme of the ILPOR apparatus with a) the general description, b) the loop in the loading stance and c) the loop in the pulse stance.

  3.1 b) and c). The loop has two different stances. The first is the loading stance see Fig. 3.1 b), during which C 16 O (g) is flowing continuously to the reactor and 16 O 2(g) is flowing through the loop before going to the vent. The second stance is the pulse stance during which 16 O 2(g) is flowing directly to the vent while C 16 O (g) is flowing to the loop (pushing the 16 O 2(g) trapped in the loop) and going to the reactor.

Table 3 . 2 : 1 Loop volume: 0. 5 mL 2 3. 4 .

 321524 The different parameters evaluated during this study and their associated valuesat the beginning of the study are described hereafter: Sample weight: empty cell Gas flow: 32 mL.min -Waiting time between pulses: 0 min Gases: 1%C 16 O/He interrupted by pulses of 1% 16 O 2 The last parameters, leading to the most interesting configuration in general are: Sample weight: 200 mg 58CHAPTER 3. ISOTOPE LABELLING PULSES OXYGEN REACTION: OPTIMIZATION Loop volume: 1 mL Waiting time between pulses: 2 min Gases: 1%C 16 O/He interrupted by pulses of pure 16 O ILPOR: A TUNABLE TECHNIQUE 59

Fig. 3 . 2 Figure 3 . 2 :

 3232 Fig. 3.2 is showing the results obtained when performing an ILPOR experiment with the initial parameters. The black plot is showing the continuous flow of C 16 O

Fig. 3 .Figure 3

 33 Fig. 3.3 is showing two experiments. The first one on Fig. 3.3 a) is an ILPOR performed in the exact initial conditions (see the following list) as a proof of what can be reached on a sample in the initial conditions.

Figure 3 . 4 :

 34 Figure 3.4: ILPOR experiment performed on an empty cell with a twice larger loop volume and pure oxygen pulses.

Fig. 3 .

 3 Fig. 3.4 is showing the results obtained when keeping the same parameters as before but with a loop volume twice as big and pure oxygen pulses. The parameters of the experiment are referred in the following list:

2 Fig. 3 .

 23 Figure 3.5: ILPOR experiment performed on silicon carbide with pulses of pure oxygen using 1 mL loop.

66CHAPTER 3 .

 3 ISOTOPE LABELLING PULSES OXYGEN REACTION: OPTIMIZATION is an interruption of the C 16 O flow by the 16 O 2 pulses. One can see it as "a volume of 16

Figure 3 . 6 :

 36 Figure 3.6: Zoom on the time axe of a) the experiment on an empty cell and b) the experiment on SiC.

Fig. 3 .

 3 Fig. 3.6 a) is a zoom between 22500 and 23000 seconds allowing a better observation of the pulses versus the time during the experiment performed with an empty cell described page 63. And Fig. 3.6 b) is the exact same zoom during the experiment with the Silicon Carbide see page 65.

Figure 3

 3 Figure 3.7: a) ILPOR performed over sieved SrFeO x (eaf) and b) ILPOR over not sieved SrFeO x (eaf) .

Fig. 3 .

 3 Fig. 3.7 is showing the two experiments performed on a) sieved SrFeO x (eaf) and b) not sieved SrFeO x (eaf). The two experiments are very similar but the H16 2 O signal is higher on the sieved SrFeO x (eaf) . Actually the baseline of the signal is higher but not the peak. The highest intensity of the water signal baseline is mainly due to a not completed clean vacuum chamber inside the mass spectrometer prior to the experiment and is not due to the material. Therefore it is possible to assume that sieving a powder to perform an ILPOR experiment has no direct influence on the obtained results.

2 Fig. 3 .

 23 Fig. 3.8 a) is showing an ILPOR experiment performed on SrFeO x (cp) . We observe that around 275 • C, there is a jump in the oxidation reaction. The intensities of C 16 O 2 / C 18 O 16 O and C 18 O 2 signals are suddenly increasing. This is the indication that the energy limitation has been crossed (a typical observation for a light-off). ILPOR technique allows study of this specific event in more details. Indeed, Fig. 3.8 b) is showing an isothermal ILPOR over the same material (SrFeO x (cp) ) just before (268 • C) and straight after (278 • C) the catalytic jump. An intensity break has been added for presentation purpose (it allows an easier observation of the weak signals of C 18 O 16 O and C 18 O 2 ). In these conditions, it is possible to quantitatively study the conversion in isothermal conditions, just by mastering the amount of16 O 2 thanks to the loop volume. Even if it has not been done yet, this could be really helpful to perform accurate kinetic studies or in situ study of the Oxygen Storage Capacity (OSC) of oxides catalyst.

70CHAPTER 3 .Figure 3

 33 Figure 3.8: a) ILPOR performed over SrFeO x (cp) and b) isothermal ILPOR over SrFeO x (eaf) before and after the shift from chemically to thermally driven.

2 5

 2 O 2 and 18 O from the lattice is possible. So, C 16 O 2 was introduced in the continuous flow of C 16 O. The experiment parameters are therefore: Sample weight: 200 mg of SrFeO x (cp) Gas flow: 32 mL.min -1 Loop volume: 1 mL Waiting time between pulses: 2 min Gases: 2%C 16 O/He and 6000 ppm C 16 O 2 /He interrupted by pulses of pure 16 O

16 O 2 2 5

 1622 is no more pulsed but in the continuous flow with C 16 O and C 16 O 2 . Therefore, the parameters are: Sample weight: 200 mg of SrFeO x (cp) Gas flow: 32 mL.min -1 Loop volume: 1 mL Waiting time between pulses: 2 min Gases: 2%C 16 O/He / 6000 ppm C 16 O 2 /He and 16 O

Figure 3 .

 3 Figure 3.10: ILPOR experiment over SrFeO x (cp) without pluses, instead C 16 O 2 and 16 O 2 in the reactant flow.

Fig. 3 .

 3 Fig.3.10 is showing the results obtained with the latter parameters. There is a break on the intensity axe because the signal of the pure C 16 O 2 is so high that it would have been irrelevant to present it without the break (other signals would have been barely observable). On the figure, one can see the catalytic conversion as if it would have been observed on any catalytic conversion bench (continuous flow). But the utilisation of the mass spectrometer allows the differentiation of the18 O containing species from those not containing it (with respect to the molecule formulae). What is very interesting is the jump in the C 16 O 2 ; C 16 O 18 O and C 18 O 2 signals occurring around 8000 seconds. This jump demonstrates the crossing of the thermal energy limitation. It is also possible to observe16 O 2 / 18 O isotopic exchange from the appearance of16 O 18 O in the region 12500-17500 seconds. This observation indicates that the latter isotopic exchange occurs in parallel of the catalytic conversion. In other words, the oxygen atoms from the lattice are not fully dedicated to the catalytic conversion. Nonetheless, it is necessary to precise that during this experiment the conditions are strongly oxidative due to the high oxygen concentration in the flow (pure oxygen canisters). It was not the case in most of the other ILPOR experiments, explaining why 16 O 18 O signal was not always observed.

18

 18

Fig. 4 .

 4 Fig. 4.1 a) is showing the diffraction pattern of SrFeO 2.5 from CIF data 249006 ICSD. Then Fig. 4.1 b) is the diffractogram of SrFeO x (cp) when calcined under hydrogen; c) is the diffractogram of SrFeO x (cp) when calcined under air; d) is the diffractogram of SrFeO x (cp) when calcined under argon and e) is the diffractogram of SrFeO x (cp)when calcined under nitrogen. When calcined under air, the obtained pattern is close to cubic, corresponding to the perovskite SrFeO 3 , showing that, the oxygen partial pressure of air is too high. When calcined under hydrogen, the general pattern is close to the model but some reflections belonging to metallic iron appear, hydrogen atmosphere is thus too reductive. When calcined under argon or nitrogen, the diffractograms are close to the model, so it was chosen to perform the calcination under inert atmosphere. As the crystallization was a bit better with argon, in the following every other SrFeO x (cp) samples were calcined under argon. It would have been interesting to change the temperature programme to obtain a SrFeO x material with an higher Brownmillerite content. But in order to be able to compare SrFeO x ma-

Figure 4 . 1 :

 41 Figure 4.1: Diffraction data of SrFeO x (cp) calcined under different atmosphere. a) SrFeO 2.5 diffraction pattern from CIF, b) SrFeO x (cp) calcined under hydrogen, c) SrFeO x (cp) calcined under air, d) SrFeO x (cp) calcined under argon and e) SrFeO x (cp) calcined under nitrogen.

Figure 4 . 2 :

 42 Figure 4.2: N 2 physisorption isotherms of brownmillerites

Figure 4 . 3 :

 43 Figure 4.3: Laser Scattering Particle size distribution of (a) CaFeO 2.5 (eaf) and (b) SrFeO x (eaf) before (red plots) and after (black plots) IPA grinding

Figure 4 . 4 :

 44 Figure 4.4: Rietveld refinements of the diffraction data for a) CaFeO 2.5 (cp) / b) CaFeO 2.5 (eaf) / c) SrFeO x (cp) / d) SrFeO x (eaf)

Figure 4 . 5 : 3 %H 2 /

 4532 Figure 4.5: 3 %H 2 /Ar (g) -TPR obtained with CaFeO 2.5 (cp) (blue plot), CaFeO 2.5 (eaf) (red dashed plot), SrFeO x (cp) (red plot), SrFeO x (eaf) (blue dashed plot)

Fig. 4 .

 4 6 is presenting TG-MS experiments performed. CaFe 18 O 2.5(cp) is presenting three weight loss. The first one of 4.4 wt.% between 50 • C and 390 • C corresponds to the release of water. The second one of 6.4 wt.% between 390 and 680 • C corresponds to oxygen exchange and calcium carbonates release. The third and last weight loss above 680 • C of 4.4 wt% is due to carbonate release and a bit of oxygen exchange. Oxygen exchange induces a weight loss because the 18 O (s) present in the solid phase is replaced by the 16 O (g) from the atmosphere. The release of carbonates (observable by the 44 MS signal in red) happens in two times which may be explained by first the removal of surface carbonates and then the remaining bulky carbonates. SrFe 18 O x(cp) is also presenting three different weight loss. The first one between 50 and 310 • Cis about 3.5 wt% and corresponds to the release of water. The release happens in two times possibly due to first the removal of weakly adsorbed water and then the remaining water. The second weight loss of 2 wt% between 390 and 620 • C correspond to the oxygen exchange. The last weight loss of 8.2 wt% is the release of strontium carbonates above 600 • C. CaFe 18 O 2.5(eaf) is presenting two weight loss. Between 370 • C and 650 • Cthe first one of 2.5 wt% is the oxygen exchange. The second and last weight loss of 1.5 wt% happens above 650 • C corresponding to release of carbonates.. SrFe 18 O x(EAF) is presenting three weight loss. The release of water is the first weight loss between 50 and 360 • C and corresponding to 1.3 wt%. The second loss of 1.4 wt% is the oxygen exchange between 360 and 640 • C. The last loss of 4.8 wt.% happening above 640 • C is the removal of carbonates.

4. 2 .

 2 PHYSICO-CHEMICAL PROPERTIES OF SUPPORTING MATERIALS 91 calcium carbonates.

Figure 4

 4 Figure 4.6:TG-MS measurement of a) CaFeO 2.5 (cp) (solid lines) and SrFeO x (cp) (dashed lines); and b) CaFeO 2.5 (eaf) (solid lines) and SrFeO x (eaf) (dashed lines). Black lines correspond to the gravimetric analysis and colors signals correspond to the identify mass spectrometry signals.

4

 4

Figure 4 . 7 :

 47 Figure 4.7: ILPOR plots for a) CaFeO 2.5 (eaf) and b) SrFeO x (eaf) with black rectangles corresponding to the zooms presented at the bottom of each ILPOR plot c) zoom on CaFeO 2.5 (eaf) ILPOR experiment and d) zoom on SrFeO x (eaf) experiment

CaFeO 2 . 5 ( 102 CHAPTER 4 Figure 4 . 8 :

 25102448 Figure 4.8: CO oxidation Light-offs plots for a) CaFeO 2.5 (cp) , b) SrFeO x (cp) , c) CaFeO 2.5 (eaf) and d) SrFeO x (eaf) after pretreatment at 200 • C.

Figure 4 . 9 :

 49 Figure 4.9: HAADF-STEM pictures and platinum size statistic of the four samples: a) & b) 1wt.%Pt/CaFeO 2.5 (cp) ; c) & d) 1wt.%Pt/CaFeO 2.5 (eaf) ; e) & f) 1wt.%Pt/SrFeO x (cp) and g) & h) 1wt.%Pt/SrFeO x (eaf) .

Figure 4 . 10 :

 410 Figure 4.10: CO oxidation Light-offs plots for a) 1wt.%Pt/CaFeO 2.5 (cp) , b) 1wt.%Pt/SrFeO x (cp) , c) 1wt.%Pt/CaFeO 2.5 (eaf) and d) 1wt.%Pt/SrFeO x (eaf) after pretreatment at 300 • C.

Figure 4 .

 4 Figure 4.11: XRD comparisons of a) CaFeO 2.5 (cp) , b) SrFeO x (cp) , c) CaFeO 2.5 (eaf) and d) SrFeO x (eaf) before (black diffractograms) and after (red diffractograms) CO oxidation.

Figure 4 . 12 :

 412 Figure 4.12: CO oxidation Light-offs plots for a) 1wt.%Pt/CaFeO 2.5 (cp) , b) 1wt.%Pt/SrFeO x (cp) , c) 1wt.%Pt/CaFeO 2.5 (eaf) and d) 1wt.%Pt/SrFeO x (eaf) after pretreatment at 200 • C.

Figure 4 .

 4 Figure 4.13: ILPOR plots 1%Pt/CaFeO 2.5 (eaf)

Figure 4 .

 4 Figure 4.14: TG Analysis of CaTi 0.9 Fe 0.1 O 3-δ polymer network

Figure 4 . 15 :

 415 Figure 4.15: Temperature dependent XRD of CaTi 0.9 Fe 0.1 O 3-δ (the offset is only for presentation purpose

Figure 4 .

 4 Figure 4.16: N 2 physisorption isotherms of CaTi 0.9 Fe 0.1 O 3-δ and CaTi 0.6 Fe 0.4 O 3-δ

Figure 4 . 17 :

 417 Figure 4.17: Pattern comparison (red is the experimental pattern, black is the CIF pattern and blue are the reflections of the model pattern) of CaTi 0.9 Fe 0.1 O 3-δ after sol-gel synthesis with CIF

Figure 4 . 18 :

 418 Figure 4.18: Pattern of CaTi 0.9 Fe 0.1 O 3-δ and most encountered secondary phases

Figure 4 . 19 : 4 CaTi 0 . 6 134 CHAPTER 4 .From CaTi 0. 5

 41940613445 Figure 4.19: Diffractogramms of CaTi 1-x Fe x O 3-δ with 0 ≤ x ≤ 0.4

Figure 4 . 20 :

 420 Figure 4.20: Diffractograms of CaTi 1-x Fe x O 3-δ with 0.5 ≤ x ≤ 0.9

CaTi 0. 5

 5 Fe 0.5 O 3-δ : TOOOTOOO CaTi 0.44 Fe 0.66 O 3-δ : TOOTOO Ca 2 Fe 2 O 5 : TOTO Where T stands for a tetrahedral layer and O for an orthogonal one. The more tetrahedral layers are present, the more the structure differs from the perovskite until being a Brownmillerite for Ca 2 Fe 2 O 5 (see30 f). According to this, CaTi 0.5 Fe 0.5 O 3-δ (light blue plot) is supposed to be a cubic structure in the Im-3 space group (data 79277 ICSD). CaTi 0.4 Fe 0.6 O 3-δ and CaTi 0.3 Fe 0.7 O 3-δ have very different diffraction patterns but this was expected as their compositions are respectively before and after the phase transition happening for CaTi 0.44 Fe 0.66 O 3-δ . The latter has an orthorhombic structure and belongs to the PCM2 1 space-group. CaTi 0.2 Fe 0.8 O 3-δ and CaTi 0.1 Fe 0.9 O 3-δ are displaying patterns closer and closer to the Brownmillerite. But they are not proper Brwonmillerite as they do not have the reflections around 12 • which is characteristic of the Brownmillerite pattern.

Fe 2 O 3 ;

 23 Fe 3 O 4 and FeO presented in Fig. 4.21. Different reduction temperature regions were identified. First the oxidation state of iron for each iron oxide was defined as Fe 3+ in Fe 2 O 3 , as a mixture of Fe 3+ and Fe 2+ in Fe 3 O 4 and as Fe 2+ in FeO. The list of possible reduction of all iron species are listed with the temperature regions from Fig. 4.21: F rom room temperature to 320 • C : Fe 4+ + e -<=> Fe 3+ (4.16)

4. 9 .Figure 4 . 21 :Fig. 4 .

 94214 Figure 4.21: Temperature Programmed Reduction of iron oxides

CaTi 0. 8

 8 Fe 0.2 O 3-δ is showing a first reduction between 80 • C and 300 • C which could assigned to the reduction of a Fe 4+ into Fe 3+ , by comparison with Fig. 4.21. Two small

Figure 4 .

 4 Figure 4.22: H 2 -TPR of CaTi 1-x Fe x O 3-δ (0.1 ≤ x ≤ 0.4) with a temperature increase of 10 • C. min -1 from room temperature to 1000 • C (followed by an isothermal step of one hour at 1000 • C as indicated by the arrow after 1000 • C)

Figure 4 .

 4 Figure 4.23: H 2 -TPR of CaTi 1-x Fe x O 3-δ (0.5 ≤ x ≤ 0.9) with a temperature increase of 10 • C. min -1 from room temperature to 1000 • C (followed by an isothermal step of one hour at 1000 • C) as indicated by the arrow after 1000 • C

CaTi 0. 3

 3 Fe 0.7 O 3-δ exposes a first reduction area 230-500 • C as one unique reduction peak (with lower iron amount it was two peaks) possibly the reduction of Fe 3+ . A very slight reduction is observed between 540 • C and 600 • C similar to CaTi 0.4 Fe 0.6 O 3-δ but less pronounced. Finally a last reduction starts at 600 • C and does not end during the isothermal step. CaTi 0.44 Fe 0.66 O 3-δ is also an ordered phase with one tetrahedral layer every two octahedral layers as described by Mc Cammon[START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF]. This structural change could explain the differences between the reductions plots of CaTi 0.4 Fe 0.6 O 3-δ and CaTi 0.3 Fe 0.7 O 3-δ . Nonetheless we would have expect CaTi 0.3 Fe 0.7 O 3-δ TPR plot to be closer to the one of CaTi 0.4 Fe 0.6 O 3-δ . CaTi 0.2 Fe 0.8 O 3-δ displays a first small reduction peak in the range 415-530 • C followed by a second peak between 580 • C and 680 • C, these two peaks can also be ascribed to the reduction of Fe 3+ .These two peaks are followed by a large reduction area until 970 • C. And finally a last reduction happens during the whole isothermal step without going back to the baseline. CaTi 0.1 Fe 0.9 O 3-δ is showing a first reduction peak between 380 • C and 540 • C similarly to others CTF first reduction peaks. Then a second big peak appears in the range 560-720 • C as observed with CaTi 0.3 Fe 0.7 O 3-δ but with a higher surface. Finally a last reduction happens after 720 • C and does not reach the baseline back during the isothermal step.Keeping in mind that Ca 2 Fe 2 O 5 crystallises with a brownmillerite pattern in our synthesis parameters (see page4.2.5 ff).one can see the influence of the ordering/phase transition as the reduction peak in the range 560-720 • C is only present with CaTi 0.2 Fe 0.8 O 3-δ and CaTi 0.1 Fe 0.9 O 3-δ . None of the composition CaTi 1-x Fe x O 3-δ with (0, 5 ≤ x ≤ 0, 9) are showing a complete reduction as the signal never goes back to baseline within the end of the programme.As previously mentioned our TPR protocol might not be accurate enough to perform a reliable quantification of each reduction area/peaks separately because of reproducibility problems. Consequently we have chosen to quantify the hydrogen uptake during the whole reduction program represented on Fig.4.24.

Figure 4 . 24 :

 424 Figure 4.24: Total Hydrogen uptake of CaTi 1-x Fe x O 3-δ as obtained from TPR

Fig. 4 .

 4 [START_REF] Kakioka | I. Échange isotopique de 18 O du gaz carbonique avec les ions oxygène de l'hémipentoxyde de vanadium[END_REF] is showing two pictures a) and b) of 1%Pt/CaTi 0.9 Fe 0.1 O 3-δ and 1%Pt/CaTi 0.6 Fe 0.4 O 3-δ and the diameter distributions of a population of 330 particles for each samples c) and d) respectively. On Fig.4.25 a) and b) some particles are highlighted with a yellow label used for diameter measurements.

Figure 4 .

 4 Figure 4.25: a) HAADF-STEM pictures of CaTi 0.9 Fe 0.1 O 3-δ ; b) HAADF-STEM picture of CaTi 0.6 Fe 0.4 O 3-δ ; c) Platinum particles size distribution of CaTi 0.9 Fe 0.1 O 3-δ ; d) Platinum particles size distribution of CaTi 0.6 Fe 0.4 O 3-δ

(4. 21 )

 21 Where[CO] initial is the mean value of 8 measurements considered as the initial CO concentration in the reactants flow (instrumental calibration) and [CO] t the CO concentration in the products flow at a given time. Conversion was also determined towards the CO 2 production to see any possible adsorption. But as no adsorption of CO neither carbonates formation has been observed from CO 2 quantifications, we choose to not present it. Firstly, results obtained for CaTi 1-x Fe x O 3-δ with 0.1 ≤ x ≤ 0.4 are present in Fig.4.26:

Figure 4 .

 4 Figure 4.26: light-off plots of the first CO conversion cycle of 1wt%Platinum/CaTi 1-x Fe x O 3-δ with 0.1<x<0.4

0. 8

 8 Fe 0.2 O 3-δ is also showing a small deactivation because conversion drops from 22 % at room temperature to 19 % at 50 • C. As with CaTi 0.9 Fe 0.1 O 3-δ CaTi 0.8 Fe 0.2 O 3-δ might have also been partially regenerated by the helium sweeping. After the deactivation, the conversion increases exponentially until total conversion at 170 • C. CaTi 0.7 Fe 0.3 O 3-δ second conversion cycle is very similar to the one of CaTi 0.8 Fe 0.2 O 3-δ but with slightly better performances. Conversion is of 25 • C at room temperature before a slight drop to 22 % at 45 • C. After this, conversion increases with the usual exponential trend to reach 100 % at 150 • C. Unexpectedly CaTi 0.6 Fe 0.4 O 3-δ is only showing a conversion of 6% at room tem-

Figure 4 .

 4 Figure 4.28: light-off plots of the second CO conversion cycle of 1wt%Pt/CaTi 1-x Fe x O 3-δ

Figure 4 . 29 :

 429 Figure 4.29: Isothermal CO oxidation test with 3wt%Pt/CaTi 0.6 Fe 0.4 O 3-δ (temperature was kept at 30 • C and then increased at 2 • C.min -1 as previously done).

Figure 4 . 30 :

 430 Figure 4.30: conversion of 3%Pt/CaTi 0.6 Fe 0.4 O 3-δ regarding time and different atmospheres

Fig. 4 . 2 16O 2

 422 31 is thus presenting the results of full ILPOR program of a) CaTi 0.7 Fe 0.3 O 3-δ and b) CaTi 0.6 Fe 0.4 O 3-δ with c) and d) respective zooms. ILPOR has been developed around the use of pulses of 16 O 2 so the c) and d) zooms allow a deeper observation on the time resolution. CaTi 0.7 Fe 0.3 O 3-δ is showing CO consumption when black (C 16 O) plot starts to decrease. Also this consumption is a conversion into CO 2 by the appearance of red (C 16 O 2 ) / blue (C 16 O 18 O) and orange (C 18 O 2 ) plots. Fig.4.31 a), shows that at 6250 seconds red and blue plots start to appear and increase. At the corresponding temperature (230 • C), C 16 O 2 displays an higher intensity than C 16 O 18 O. This means that CO oxidation is favourably using oxygen from the gaseous phase as described by equation 4.22:C 16 O (g) + 1 (g) ↔ C 16 O 2(g)(4.22)This equation explains the formation of C 16 O 2(g) (red plot). Three different mechanisms can explain 4.22, it can be Langmuir-Hinshelwood or Eley-Rideal or even Mars-Van Krevelen in the specific case where the oxygen coming from the lattice is16 O (s) as described by equation 4.23: C 16 O (ads) + 16 O (s) ↔ C 16 O 2(g) (4.23)

Figure 4 .

 4 Figure 4.31: ILPOR experiments of a) CaTi 0.7 Fe 0.3 O 3-δ b) CaTi 0.6 Fe 0.4 O 3-δ c) zoom on CaTi 0.7 Fe 0.3 O 3-δ and d) zoom on CaTi 0.6 Fe 0.4 O 3-δ .

  25: 2 V •• O (s) + 16 O 2(g) ↔ 2 O O(s) (4.25)

Fig. 4 .

 4 Fig.4.31 c); is providing a zoom on the time frame 10500 -11750 seconds. Intensity scale was also adjusted to provide good information. First, it is necessary to remind that the16 O 2(g) pulses are produced by a 6-way valve. This valve can be seen as a plug-flow system. Making the hypothesis that gases are not mixed, this system can be seen as a quantity of pure oxygen between two quantities of pure carbon monoxide (both diluted in helium). Keeping that in mind, it is normal that16 O 2(g) pluses are anti-phased with C 16 O (g) pulses as it is on Fig.4.31 c). Also, pulses of C 16 O 2(g) ; C 16 O 18 O (g) and C 18 O 2(g) are in phase with16 O 2(g) pulses. Moreover, the perfect phase matching of the 3 carbon dioxide species means that they are all produced by the same mechanism (or that our set-up is not accurate enough to allow discrimination between the mechanisms). If they were anti-phased it would mean that the reaction path of one compound is slower than the other because of adsorption or dissociation steps. Assuming that they are all produced following the same mechanism, and because isotopic oxygen in present in CO 2(g) , one can assume that the main mechanism is the one described by equations 4.24-4.27.

Fig. 4 .

 4 Fig.4.31 b) is showing the result obtained when performing ILPOR over CaTi 0.6 Fe 0.4 O 3-δ . From 200• C (6700 seconds) signals of C 16 O 2(g) and C 16 O 18 O (g) are appearing and simultaneously C 16 O (g) signal start to decrease. Intensity of C 16 O 2(g) is slightly higher than the one of C 16 O 18 O (g) . Contrary to what has been observed with CaTi 0.7 Fe 0.3 O 3-δ , in the case of CaTi 0.6 Fe 0.4 O 3-δ C 16 O 2(g) signal remains higher. This indicates that equation 4.22 is still contributing to the CO oxidation. Signal of C 16 O 2(g) start also to increase from 240 • C (8100 seconds). Following the same explanations, the attendance of signal containing isotopic oxygen justify that a Mars-Van Krevelen mechanism takes places.

Fig. 4 .

 4 Fig. 4.31 d) is giving a closer look at the pulses during the ILPOR experiment over CaTi 0.6 Fe 0.4 O 3-δ .Here also, the peaks of all CO 2(g) species are perfectly phased together, and anti-phased with CO (g) . As C 16 O 2(g) and C 16 O 18 O (g) peaks are matching, they may be produced by the same mechanism. And from the attendance of isotopic oxygen containing species, one could think the main mechanism is equation 4.24. But then signal of C16 O 2(g) should have been lower. Explanation of the high intensity of C16 O 2(g) is that enrichment yield was not sufficient to provide enough isotopic oxygen and thus equation 4.23 explains the high level of C 16 O 2(g) .

Sample weight: 200 mg Loop volume: 1 mL

 1 Waiting time between pulses: 2 minGases: 1%C16 O/He interrupted by pulses of pure16 O 2

Table 1 . 2 :

 12 Examples of ceria based compositions associated with their OSC values. Examples 1 & 2 are from

Table 1 .

 1 5: Positions of ions in the cubic perovskite structure (space-group is P m3m).

	1.3. OXYGEN CONDUCTORS: PEROVSKITE AND BROWNMILLERITE	19

Table 1 . 6

 16

: Positions of ions in the orthorhombic perovskite structure (space-group Pnma).

Table 1 .

 1 7: Positions of iron and titanium atoms in Ca 4 Fe 2 Ti 2 O 11 as reported by Hovmöller et al. (space-group is Pnma).

Table 1 .

 1 8: Positions of atoms in Ca 3 Fe 2 TiO 8 as reported by Hovmöller et al. (spacegroup is Pnma). On this figure one can see that the tetrahedral layers have the same orientation. Two positions are given for calcium atoms, the first where Ca is ninefold coordinated and the second where Ca is eightfold coordinated similarly to the brownmillerite structure.

	Atom	x	y	z
	Fe	0.309(2)	0	0.050(2)
	Ti	0.258(1) 0.3303(5)	0.0
	Ca 1	0.234(2)	0.5	0.486(4)
	Ca 2	0.225(1) 0.1884(5)	0.526(3)
	O 1	0.398(6)	0	0.410(6)
	O 2	0.316(5)	0.5	-0.010(7)
	O 3	0.169(3) 0.144(2)	-0.0.18(5)
	O 4	-0.010(5) 0.353(2)	0.227(7)
	O 5	0.469(5) 0.300(2)	0.281(6)
	cell parameters are: a = 5.4761(2) / b = 11.1506(4) / 5.3603(2) Å
	A schematic representation is given in Fig. 1.10

Table 1 .

 1 CaFeO 2.5 single-cristal and refined where found to belong to the Pcmn space-group. The atomic positions and the lattice parameters are presented in table 1.9. 9: Positions of atoms in CaFeO 2.5 as reported by Colville (space-group is Pcmn).

	Atom	x	y	z
	Ca	0.0233(2) 0.1079(1)	0.4806(2)
	Fe 1	0	0	0
	Fe 2	0.9338(2)	0.25	0.9459(2)
	O 1	0.2366(7) 0.9839(2)	0.2632(8)
	O 2	0.0716(7) 0.1403(2)	0.0234(8)
	O 3	0.8746(8)	0.25	0.6005(9)

1.3. OXYGEN CONDUCTORS: PEROVSKITE AND BROWNMILLERITE 29 structure). X-ray diffraction data recorded over a cell parameters are: a = 5.559(1) / b = 14.771(2) / 5.429(1) Å A schematic representation is given on Fig. 1.11

The SrFeO x composition

	1.3. OXYGEN CONDUCTORS: PEROVSKITE AND BROWNMILLERITE
		(space groupe Pnma) taken from [88]. The atoms
	positions are detailed in table 1.10:
	Table 1.10: Positions of atoms in CaFeO 2.5 as reported by Redhammer [88] (space-group
	is Pnma).
	Atom	x	y	z
	Ca	0.04808(1) 0.10798(4) 0.0227(1)
	Fe 1	0	0	0
	Fe 2	0.9461(1)	0.25	0.93403(9)
	O 1	0.2623(3) 0.9843(1) 0.2370(3)
	O 2	0.0240(3) 0.1406(1) 0.0735(3)
	O 3	0.5991(5)	0.25	0.8741(4)

Table 1 .

 1 11: Space-groups and lattice parameters for SrFeO x system as reported by Hodges et al.[START_REF] Hodges | Evolution of oxygen-vacancy ordered crystal structures in the perovskite series Sr(n)Fe(n)O(3n-1) (n = 2, 4, 8, and â), and the relationship to electronic and magnetic properties[END_REF]

	Phase	space-group	a	b	c
	SrFeO 3	P m3m	3.851(1) 3.851(1) 3.851(1)
	SrFeO 2.875	I4/mmm	10.929(1) 10.929(1) 7.698(1)
	SrFeO 2.75	Cmmm	10.974(1) 7.702(1) 5.473(1)
	SrFeO 2.5	Icmm	5.527(1) 15.59(2) 5.672(1)

Differences between CaFeO 2.5 and SrFeO x

  CFO = 14.77 Å and c SFO = 15.85 Å) and thus on the strength of Fe-O bonds. The escape of the apical oxygen (part of the brownmillerite oxygen mobility mechanism) is conditioned by the Fe-O bond strength. As r Sr 2+ is larger than r Ca 2+

	1.4. BROWNMILLERITE AS CATALYTIC SUPPORT
	(c
	Mechanism of oxygen mobility within the brownmillerite framework has been briefly
	described at page 35. But CaFeO 2.5 and SrFeO 2.5 have differences (consequent
	to their different compositions) that can influence the efficiency if this mechanism.
	CaFeO 2.5 and SrFeO 2.5 have different compositions, more precisely they have a dif-
	ferent cations in the A site. These two cations obviously have different ionic raddi
	e.g. r Ca 2+ = 1.34 Å and r Sr 2+ = 1.44 Å. This difference in the ionic radii has an
	impact on the crystallographic structure, more precisely over the brownmillerite c axis

  Cr 0.2 O 2.5 < CaFeO 2.5 < CaFe 0.8 Mn 0.2 O 2.5 < CaFe 0.8 Co 0.2 O 2.5 < CaFe 0.8 Cu 0.2 O 2.5 . The CaFe 0.8 Cu 0.2 O 2.5 was even further investigated[START_REF] Penkala | In situ generated catalyst: copper( ii ) oxide and copper( i ) supported on Ca 2 Fe 2 O 5 for CO oxidation[END_REF]. They reported tunable catalytic activity regarding the copper content and reported the best activity when CaFe 0.8 Cu 0.2 O 2.5 is in-situ reduced prior to the CO oxidation.

	• C. More
	rencently, Penkalaet al. [97] highlighted lattice oxygen involvement from CaFeO 2.5 in
	the catalytic oxidation of CO using 18 O. Penkala [16] also evaluated CaFeO 2.5 with
	different doping in the B Site for CO oxidation and obtained the following activity
	ranking: CaFe 0.8

  • C then increases by steps of 2 • C until 500 • C (ramp rate between each step is 2 • C.min -1 ).

	At each 2 • C steps there is a small isotherm allowing the time to perform one pulse of
	16 O 2(g) . The quadrupole mass spectrometer measures the different m/z: 2; 16; 17; 18;
	19; 20; 28; 30; 32; 34; 36; 44;46; 48.

Table 3 .

 3 1: Isotopic composition of oxygen Isotopes Atomic mass Natural abundance (%) Nuclear spin

	16 O	15.99491	99.762	0
	17 O	16.9991	0.038	+5/2
	18 O	17.9992	0.200	0

  18 O 16 O and C 18 O 2 signals are due to isotopic exchange between18 O from the lattice and adsorbed C 16 O 2 or if they are produced from the Mars-van Krevelen mechanism.

Table 4 .

 4 1: Physico-chemical properties of supporting material CaFeO 2.5 (cp) CaFeO 2.5 (eaf) SrFeO x (cp) SrFeO x (eaf)

	A/B ratio (molar ratio -XRF)	0.95	1.06	0.99	1.05

  2.5 ; 41.2 wt.% of SrFeO 2.75 and 11.5 wt.% of SrFeO 2.875 and for SrFeO x (eaf) : 70.1 wt.% of SrFeO 2.5 ; 26.8 wt.% of SrFeO 2.75 and 3.1 wt.% of SrFeO 2.875 . We observe that the weight percentage obtained by the two synthesis methods are different for SrFeO x (47%

Table 4 .

 4 2: Table of refined parameters

	sample	phases	(wt.%) space group	a	b	c
	CaFeO 2.5 (cp) CaFeO 2.5	100	Pnma	5.433 (2) 14.776 (7) 5.604 (2)
	CaFeO 2.5 (eaf) CaFeO 2.5	100	Pnma	5.430 (4) 14.77 (1) 5.597 (4)
		SrFeO 2.5	47	Icmm	5.638 (6) 15.55 (2) 5.517 (5)
	SrFeO x (cp)	SrFeO 2.75	41	Cmmm	11.10 (4)	7.66 (1)	5.52 (2)
		SrFeO 2.875	12	I4/mmm	10.97 (2) 10.97 (2) 7.63 (2)
		SrFeO 2.5	70	Icmm	5.678 (2) 15.616 (5) 5.533 (2)
	SrFeO x (eaf)	SrFeO 2.75	27	Cmmm	10.99 (1) 7.727 (5) 5.499 (6)
		SrFeO 2.875	3	I4/mmm	10.974 (4) 10.974 (4) 7.712 (4)

Table 4 .

 4 3: Crystallite size of supporting material CaFeO 2.5 (cp) CaFeO 2.5 (eaf) SrFeO x (cp) SrFeO x (eaf)

	Crystallite size (nm)	35	130	37	51

  e 18 O 2.5 -M CaF e 16 O 2.5 M CaF e 16 O 2.5

			* 100 = 3.55%	(4.2)
	t =	M SrF e 18 O 2.875 -M SrF e 16 O 2.875 M SrF e 16 O 2.875	* 100 = 3.02%	(4.3)

Table 4 . 4 :

 44 O 2 is possible via a two-steps 4.2. PHYSICO-CHEMICAL PROPERTIES OF SUPPORTING MATERIALS 93 mono-atomic exchange mechanism. The 16 O 18 O produced during the mono-atomic exchange mechanism described in equation 4.4 can exchanged another oxygen leading to the possible production of 18 O 2 . This process happens via a second step of monoatomic exchange detailed in reaction 4.6. second step of mono -atomic exchange : 18 O 16 O (g) + 18 O (s) ↔ 18 O 2(g) + 16 O (s) (4.6) The 16 O/ 18 O exchange onset temperatures (spotted by the green plots) are recorded in table 4.4 for the four materials: Table of the 16 O/ 18 O exchange onset temperatures.

  At 370 • C (just after the production of C 16 O 18 O (g) increased) C 16 O 16 O (g) hits a maximum, perfectly illustrating the competition between reactions 4.7 and 4.8. These two reactions are immediately followed by the subsequent refile of the created oxygen vacancies inside the lattice by16 O 2(g) from the pulses. Equations 4.7 and 4.8 lead to the concomitant production of C 16 O 18 O (g) and C 16 O 16 O (g) until depletion of 18 O (s) leaving C 16 O 16 O (g) as the only possible product.

	4.3. STUDY OF RAW SUPPORTS	97
	C 16 O (g) + 18 O (s) ↔ C 16 O 18 O (g) + V •• O (s)	(4.8)
	•• O (s)	(4.7)

  O concentration, as C18 O 2(g) drops at lower temperature than C16 O 18 O (g) .With SrFeO x (eaf) it is also possible to observe C 18 O (g) ; 16 O 18 O (g) and 18 O 2(g) signals. But here also those signals are very weak and reactions involving those species are considered negligible beside ensuring MvK regarding the presence of C 18 O 2(g) . Nonetheless, C 16 O 18 O (g) is showing a slow increase from 425 • C, corresponding to the temperature of 16 O/ 18 O exchange observed via TG-MS. The zoom on Fig.4.7 d) is showing the pulses modulation of C 16 O (g) ; C 16 O 2(g) ; C 16 O 18 O (g) ; C 18 O 2(g) and 16 O 2(g) signals. As it was the case for CaFeO 2.5 (eaf) , with SrFeO x (eaf) we also observe a perfect modulation of C 16 O 2(g) ; C 16 O 18 O (g) and C 18 O 2(g) signals and those signals are in opposition with 16 O 2(g) signal. The signals of C 16 O 2(g) , C 16 O 18 O (g) and C 18 O 2(g) are presenting a small spike at their respective maximum inten-

	100	CHAPTER 4. BROWNMILLERITES AS CATALYTIC SUPPORTS

Table 4 .

 4 5: Temperature at 10% and 50% conversion for the three cycles of each raw materials pretreated at 200 • C

		Temperature at 10% Temperature at 50%
		conversion ( • C)	conversion ( • C)
	CaFeO 2.5 (cp) 1 st cycle	295	371
	CaFeO 2.5 (cp)	325	415
	3 rd cycle
	CaFeO 2.5 (eaf) 1 st cycle	327	422
	CaFeO 2.5 (eaf)	376	403
	3 rd cycle
	SrFeO x (cp) 1 st cycle	258	385
	SrFeO x (cp)	245	369
	3 rd cycle
	SrFeO x (eaf) 1 st cycle	250	339
	1wt.%Pt/SrFeO x (eaf)	300	402
	3 rd cycle

Table 4 .

 4

	6: Impregnated platinum mean size and distribution
		Platinum particles	Platinum dispersion
		mean size (nm)	%
	1wt.%Pt/CaFeO 2.5 (cp) before CO oxidation	1.2	74
	1wt.%Pt/CaFeO 2.5 (cp) after CO oxidation	1.5	63
	1wt.%Pt/CaFeO 2.5 (eaf) before CO oxidation	1.7	57
	1wt.%Pt/CaFeO 2.5 (eaf) after CO oxidation	2.3	42
	1wt.%Pt/SrFeO x (cp) before CO oxidation	1.1	80
	1wt.%Pt/SrFeO x (cp) after CO oxidation	1.7	56
	1wt.%Pt/SrFeO x (eaf) before CO oxidation	1.2	80
	1wt.%Pt/SrFeO x (eaf) (*)

Table 4 .

 4 • C higher. SrFeO x (cp) presents the biggest deactivation as 65 • C and 57 • C are separating first and third cycles at 10% and 50% conversion. Performance of SrFeO x (cp) first cycle is very interesting as conversion starts at the lowest temperature of the four samples 7: Temperature at 10% and 50% conversion for the three cycles of each materials with platinum and pretreated at 300 • C

		Temperature at 10% Temperature at 50%
		conversion ( • C)	conversion ( • C)
	1wt.%Pt/CaFeO 2.5 (cp) 1 st cycle	180	220
	1wt.%Pt/CaFeO 2.5 (cp) 3 rd cycle	205	240
	1wt.%Pt/CaFeO 2.5 (eaf) 1 st cycle	160	190
	1wt.%Pt/CaFeO 2.5 (eaf) 3 rd cycle	178	208
	1wt.%Pt/SrFeO x (cp) 1 st cycle	165	205
	1wt.%Pt/SrFeO x (cp) 3 rd cycle	230	262
	1wt.%Pt/SrFeO x (eaf) 1 st cycle	210	243
	1wt.%Pt/SrFeO x (eaf) 3 rd cycle	220	247
	(165

Table 4 .

 4 8: Temperature at 10% and 50% conversion for the three cycles of each materials with platinum and pretreated at 200 • C.

		Temperature at 10% Temperature at 50%
		conversion ( • C)	conversion ( • C)
	1wt.%Pt/CaFeO 2.5 (cp) 1 st cycle	87	127
	1wt.%Pt/CaFeO 2.5 (cp) 3 rd cycle	129	161
	1wt.%Pt/CaFeO 2.5 (eaf) 1 st cycle	121	164
	1wt.%Pt/CaFeO 2.5 (eaf) 3 rd cycle	167	193
	1wt.%Pt/SrFeO x (cp) 1 st cycle	104	144
	1wt.%Pt/SrFeO x (cp) 3 rd cycle	135	165
	1wt.%Pt/SrFeO x (eaf) 1 st cycle	133	166
	1wt.%Pt/SrFeO x (eaf) 3 rd cycle	135	168
	ment. But with this sample again, the operating temperature range is lower (160-208

• C after 300 • C pretreatment/ 121-193 • C after 200

  • C) signal of C 16 O 2 starts to increase but signals of C 16 O 18 O and C 18 O 2 do not increase. Adding platinum allows C 16 O oxidation into C 16 O 2 at lower temperature (70

Table 4 .

 4 9: Elemental analysis of CaTi 1-x Fe x O 3-δ by X-Ray Fluorescence CaTiO 3 CaTi 0.9 Fe 0.1 O 3-δ CaTi 0.8 Fe 0.2 O 3-δ CaTi 0.7 Fe 0.3 O 3-δ Fe 0.4 O 3-δ CaTi 0.5 Fe 0.5 O 3-δ CaTi 0.4 Fe 0.6 O 3-δ CaTi 0.3 Fe 0.7 O 3-δ Fe 0.8 O 3-δ CaTi 0.1 Fe 0.9 O 3-δ Ca 2 Fe 2 O 5

	Ca (%)	100.0	100.0	100.0	100.0
	Fe (%)	0.0	10.1	19.6	29.7
	Ti (%)	100.5	89.8	73.5	69.2
	CaTi 0.6 Ca (%) 100.0	100.0	100.0	100.0
	Fe (%)	39.6	49.1	59.2	69.9
	Ti (%)	59.7	48.6	39.7	29.4
	CaTi 0.2 Ca (%) 100.0	100.0	100.0
	Fe (%)	79.4	89.3	100.8
	Ti (%)	20.2	9.9	0.0

Table 4 .

 4 10: Specific surface area measured by N 2 physisorption of some materials of CaTi 1-x Fe x O 3-δ system Compositions Specific Surface Area (m 2 • g -1 )

	CaTi 0.9 Fe 0.1 O 3-δ	7
	CaTi 0.8 Fe 0.2 O 3-δ	4
	CaTi 0.6 Fe 0.4 O 3-δ	7
	CaTi 0.4 Fe 0.6 O 3-δ	6
	CaTi 0.1 Fe 0.9 O 3-δ	8

Table 4 .

 4 when coordination number is 2 Adapting equation 4.14 to the substitution process performed in CaTi 1-x Fe x O 3-δ we obtain equation 4.15: 11: Goldschmidt tolerance factor in CaTi 1-x Fe x O 3-δ

	t =	√	r Ca + r O 2 * (x * r F e + (1 -x) * r T i + r O )	(4.15)
	Results are presented in Table 4.11:
	B site substitution (%)	0	10	20	30	40	50
	t		0.973 0.976 0.978 0.981 0.984 0.987
	B site substitution (%)	60	70	80	90	100
	t		0.990 0.992 0.995 0.998 1.001

  • / 38 • / 48 • / 55 • . CaTi 0.9 Fe 0.1 O 3-δ diffractogramm was already detailed at page 132. Regarding Mc-Cammon's phase diagram

  • C. 1wt%Pt/CaTi 0.6 Fe 0.4 O 3-δ has a behaviour similar to 1wt%Pt/CaTi 0.7 Fe 0.3 O 3-δ . Conversion starts at 85% at room temperature before decreasing to 65% at 45 • C. After this activity loss, conversion increases again, first quickly and then a bit slower from 80 • C. Finally, conversion reaches 100% around 125 • C. 1wt%Pt/CaTi 0.7 Fe 0.3 O 3-δ and 1wt%Pt/CaTi 0.6 Fe 0.4 O 3-δ

  • C). Two different mechanisms can explain this. Either, the bi-atomic mechanism as described by equation 4.26: CHAPTER 4. BROWNMILLERITES AS CATALYTIC SUPPORTS C 16 O (ads) + 2 18 O (s) ↔ C 18 O 2(g) + 16 O (s) (4.26) Or the re-reaction of already formed C 16 O 18 O (ads) leading to the formation of C 18 O 2(g) detailed by equation 4.27: C 16 O 18 O (ads) + 18 O (s) ↔ C 18 O 2(g) (4.27) These two equation 4.26 and 4.27 are subsequently followed by reaction 4.25 in which the oxygen vacancies in the lattice are refilled. Signal of C 16 O 18 O (g) remains higher than C 16 O 2(g) until 13300 seconds (480 • C), but from this time C 16 O 18

Acknowledgements

This work has been conducted during a fruitful collaboration between the university of Montpellier and the research center of Saint-Gobain in Provence. First in would to thanks the director of the departrment in university of Montpellier Prof. Deborah Jones and the director of the research center of Saint-Gobain Dr. Robert Germar.

Isotopic studies 2.3.1 Isotopic enrichment

 CHAPTER 4. BROWNMILLERITES AS CATALYTIC SUPPORTS the impact of one parameter from the others. oxidation mechanism type over CaFeO 2.5 and SrFeO x brownmillerites to be Mars -van Krevelen. These supports were also tested for classic CO oxidation without 18 O. Here, complexation materials were also able to show slightly better performances than their respective EAF counterparts depending on the reductive pretreatment temperature.

The performances observed during CO oxidation over the supports were nor really competitive so we decided to add platinum over their surface as a reaction trigger. the four supports were then impregnated with one percent of their weight as platinum. The platinum dispersion was controlled by HAADF-STEM pictures revealing high platinum dispersion: 74% for CaFeO 2.5 (cp) , 80% for SrFeO x (cp) , 57% for CaFeO 2.5 (eaf) and 80% for SrFeO x (eaf) . ILPOR experiment was perfom over 1%Pt/CaFeO 2.5 (eaf) showing a mechanism shift from Langmuir-Hinshelwood (L-H) to Mars-van Krevelen (MvK) with temperature. Platinum impregnation is thus able to lower CO oxidation temperature following L-H shifting to MvK once the temperature is sufficient to allow oxygen mobility. Their catalytic properties for the classic CO oxidation were also tested. We observed a strong impact of the pretreatment temperature as the performances were better after a 200 • C reductive pretreatment than after a 300 • C one. After the 300 • C pretreatment we obtain the following classification (with the 50% conversion temperature during the third cycle): CaFeO 2.5 (eaf) (208

We managed to explain this classification using XRD after catalytic test: CaFeO 2.5 (cp) was partially polluted by calcium carbonates, SrFeO x (cp) was mostly turned into strontium carbonates, SrFeO x (eaf) was oxidised into the SrFeO 3 while CaFeO 2.5 (eaf) was not impacted by the catalytic process. After the 200 • C reductive pretreatment, the catalytic performance classification is: CaFeO 2.5 (cp) (161

We explain this classification by the difference in the micro-structure between complexation and EAF materials and because SrFeO x is supposed to be more active than CaFeO 2.5 . SrFeO x (cp) is thus supposed to show higher performances than CaFeO 2.5 (cp) but here is not the case. This is possibly due to the impact of the catalytic process over SrFeO x (cp) but is has not been evaluated yet. The differences in the performances observed when changing the pretreatment temperature could partially be explained by the differences we observed during temperature programmed reduction as SrFeO x compositions were reduced at lower temperature than the CaFeO 2.5 one. Nonetheless SrFeO x compositions are showing oxygen mobility at lower temperature than CaFeO 2.5 observed by TG-MS and explained by their different lattice dynamics.

Perovskite CaTi 1-x Fe x O 3-δ system as catalytic supports

The CaTi 1-x Fe x O 3-δ system was described in the literature by McCammon [START_REF] C A Mccammon | Shortrange ordering of oxygen vacancies in CaFe x Ti 1âx O 3âx/2 perovskites (0 < x < 0.4)[END_REF] to have a partial ordering in the range 0.1 < x < 0.4 and then few ordered phases: CaTi 0.5 Fe 0.5 O3 -δ, CaTi 0.44 Fe 0.66 O3 -δ and CaFeO 2.5 . We synthesised ten samples of this system: CaTi 1-x Fe x O 3-δ with 0.1 ≤ x ≤ 0.4 using a method derived from PE-CHINI's [START_REF] Pechini | PECHINI PATENT[END_REF]. The samples have low specific area between four and eight m 2 • g -1 . We observed crystallographic transition all along the phase diagram similarly to McCammon. We observed their red-ox potential to increase with iron content but were not able to determine the iron oxidation states and their respective relative amount in the materials using temperature programmed reduction.

To evaluated their catalytic performances the materials have been impregnated with one percent platinum the same as the brownmillerites. We also control the platinum dispersion over CaTi 0.9 Fe 0.1 O 3-δ and CaTi 0.6 Fe 0.4 O 3-δ reaching 79% and 77% respectively. The catalytic properties of the ten supports were evaluated for CO oxidation. The most performing materials were in the partially ordered range (CaTi 1-x Fe x O 3-δ with 0.1 ≤ x ≤ 0.4) showing catalytic activity even at room temperature. But they undergo a deactivation process lowering their catalytic activity until the temperature is sufficient enough to enable again high catalytic activity. Catalytic cycles revealed the instability of these materials as their performances were lowered when performing three catalytic cycles but CaTi 0.8 Fe 0.2 O 3-δ and CaTi 0.7 Fe 0.3 O 3-δ were still showing activity at room temperature. We performed isothermal CO oxidation at room temperature over CaTi 0.6 Fe 0.4 O 3-δ (one of the most efficient composition) and observe a deactivation that we could not easily reverse using helium or oxygen flush.

The study of the oxygen mobility was performed on 18 O enriched CaTi 0.7 Fe 0.3 O 3-δ and CaTi 0.6 Fe 0.4 O 3-δ without platinum. We observe the CO oxidation mechanism to occur through a MvK mechanism by the presence of C 18 O 16 and C 18 O 2 Both materials display very interesting oxygen mobility properties. Especially CaTi 0.7 Fe 0.3 O 3-δ which was producing higher quantity of C 18 O 16 than C 16 O 2 . So CaTi 0.7 Fe 0.3 O 3-δ is preferably using its lattice oxygen than 16 O 2 from the atmosphere.

We believe the materials studied in this work to be very promising for oxidation catalysis as they all display MvK mechanism. The most interesting compositions would be CaTi 0.7 Fe 0.3 O 3-δ and CaTi 0.6 Fe 0.4 O 3-δ because of their partial structural order. The EAF synthesis seems to be a good up-scaling strategy but up to now it only produces Example one is from [START_REF] Voorhoeve | Defect chemistry and catalysis in oxidation and reduction over perovskite-type oxides[END_REF]; example two is from [START_REF] Yamazoe | Oxygen permeation through pervoskite-type oxides[END_REF]; example three is from [START_REF] Gallagher | Studies of some supported perovskite oxidation catalysts[END_REF]; example four is from [START_REF] Wachowski | Influence of the method of preparation on the porous structure of perovskite oxides[END_REF]; example five is from [START_REF] Baythoun | Production of strontium-substituted lanthanum manganite perovskite powder by the amorphous citrate process[END_REF]; example six is from [START_REF] Taguchi | Surface characterization of LaCoO3synthesized using citric acid[END_REF]; example seven is from [START_REF] Gallagher | The activity of La0.7Sr0.3MnO3without Pt and La0.7Pb0.3MnO3with varying Pt contents for the catalytic oxidation of CO[END_REF][START_REF] Gallagher | Effects of the Pt content of La0.7Pb0.3MnO3on its catalytic activity for the oxidation of CO in the presence of SO2[END_REF]; example eight is from [START_REF] Wachowski | Influence of the method of preparation on the porous structure of perovskite oxides[END_REF] and example nine is from [START_REF] Ng Lee | Hydrogen peroxide decomposition over Ln1-xAxMnO3(Ln = La or Nd and A = K or Sr) perovskites[END_REF]; Taken from [START_REF] Royer | Perovskites as substitutes of noble metals for heterogeneous catalysis: Dream or reality[END_REF]

List of Figures

Perovskite and Brownmillerite as catalyst support materials

In this work, we are studying unconventional support materials such as Perovskites ABO 3 and Brownmillerites ABO 2.5 . We are interested in those materials because of their oxygen conductivity at low temperature. The two structures follow inherently different oxygen mobility mechanisms: perovskites show oxygen conducting properties due to oxygen vacancies created by partial substitution on the perovskite's B site. Brownmillerites show oxygen mobility due to changes in the orientation of the FeO 4 tetrahedra. Particular attention is paid on the link between oxygen mobility, crystallographic defects and catalytic performances for CO oxidation.

The first part of this manuscript is dedicated to the optimization of a technique named ILPOR. Using 18 O-enriched support materials, this method allows the observation of oxygen mobility from the solid phase (catalyst support) and its potential involvement in the catalytic reaction. Inspired by pulsed chemisorption methods, it may provide catalytic mechanisms identification. Evaluation of different parameters enables the differentiation between the instrumental impact and materials properties.

The second part of this manuscript is centred on evaluating CaFeO 2.5 and SrFeO x Brownmillerites as support materials. While CaFeO 2.5 is a stable line phase, SrFeO x exists in several modifications, e.g. SrFeO 2.5 , SrFeO 2.75 and SrFeO 2.875 . SrFeO 2.5 is known to show higher oxygen mobility than CaFeO 2.5 which might have an impact of the catalytic performance. The two compositions are synthesized by two different methods, a wet chemical route and electric arc fusion (EAF), revealing a very low specific surface area for EAF materials and a low one for wet chemical route. These materials are showing oxygen mobility determined by TG-MS at moderate temperature (300 °C for wet chemistry materials 400 °C for EAF). The catalytic performances of the four impregnated supports with 1wt.% platinum is studied for CO oxidation. The catalytic performance of the different materials depends strongly on the H 2pretreatment temperature. In all cases wet chemistry materials are showing a lower stability. 18 O was used to study oxygen mobility in ILPOR configuration on the bare materials. The results show oxygen mobility at lower temperature for EAF-synthesized SrFeO x composition (onset temperature 200 °C) than EAF-synthesized CaFeO 2.5 (onset temperature 300 °C). By ILPOR we identify that CO oxidation over the bare support is following a Mars -Van Krevelen (MvK) mechanism. Upon impregnation with Pt, the mechanism is changing from Langmuir-Hinshelwood (L-H) to MvK with increasing temperature.

A third part focuses on the impact of iron substitution in CaTiO 3 on the B site and a possible relation between catalytic performances and oxygen mobility. XRD analysis is used to describe the partial ordering of oxygen vacancies. Temperature-programmed reduction (TPR) shows that reducibility evolves with increasing iron content, which correlates to the iron content and its oxidation states. Catalytic performances of Pt-impregnated samples were evaluated on CO oxidation. Room temperature activity was observed for compositions with partial oxygen vacancy ordering (CaTi 1-x Fe x O 3-δ where 0.1<x<0.4). These compositions without platinum were tested using ILPOR showing oxygen mobility from 200 °C on also occurring through a MvK mechanism.

To sum up, we observe that contrary to the two studied brownmillerites, Fe doped CaTiO 3 shows room temperature catalytic activity. The catalytic performances depend strongly on the iron content. The CO oxidation mechanism is following MvK mechanism for Fe-doped CTF and bare brownmillerites. But in the case of Pt-impregnated EAF CFO we observe a shift from L-H to MvK. The difference in catalytic performance is thus reflected by different reaction mechanisms. In additions, we observe a trade-off between stability and performances: wet chemical materials degrade during the catalytic process leading to carbonate formation. Structural changes were also observed in SrFeO x made by EAF: SrFeO x converts to a perovskite (SrFeO 3 ) during CO oxidation at elevated temperatures.