Acknowledgment

This work would not have been brought to life without the extensive endeavors of many people from so many parts who have been my support from the very beginning.

I would like first to thank the research council at the University of Balamand, the ecole doctorale at Sorbonne University and the European SOL-CARE project for financing my work when accomplishing my thesis.

An enormous thanks goes to my supervisors who have played the role of the engine that helped me propel through my thesis years in the smoothest possible fashion. In light of this, I would like to seize the opportunity to thank Dr. Nissrine El Hassan because she gracefully offered me the chance to apply for a PhD candidacy and paved the way for me to delve into the realm of research. She truly played the role of an authentic catalyst in accelerating my thesis work and she provided me with the full support, whether morally, emotionally, or educationally. She taught me how to cope with stress and overcome any impediment that comes along my way throughout my research years.

In parallel, I would like to thank my second supervisor Dr. Pascale Massiani for her continuous support when I was in France. She not only helped me with my research and thesis, but also made me feel at home. She also created an enriching environment which was conducive to an ever-rising drive for research and learning. Needless to say, her support encompassed all aspects: moral, emotional, and educational. She made me embrace positivity and made me strongly believe that nothing can abate me from reaching my goals. In addition, she constantly rectified my report and engaged in the exploitation of the work results.

Another series of thanks goes to both the Lebanese and French jury members who consented upon correcting my thesis in an objective and constructive ways. I would also like to specifically thank Dr. Julien Reboul for his help concerning the MOF part in this thesis. His expertise in this field provided me with a new interesting scientific knowledge. I am thankful for him also because he was always very kind and available whenever I needed him. I am grateful for Dr. Carlos Henriques who did not hesitate to host me in his lab in Portugal whereby I had the opportunity to further enrich my research work. His PhD student Carminha also imparted positivity and boosted my performance. I am indebted to Dr. Elie Chalhoub for his help and support in the kinetics measurements and modeling set-up. My appreciation goes also to Dr. Axel Lofberg, for he attended my "comité de suivi de thèse" that took place on a yearly basis. His evaluation of the work was very beneficial and he was ready to help in any possible way.

An enormous thanks goes to Ms. Sandra Casale for her help in microscopies experiments, to Dr. Jean Marc Krafft because he taught me how to use the Raman spectroscopy and Mr. Mohamed Selman for he helped me operate upon XRD. My warmest thanks goes to Ms. Annie Mettendorff who facilitated my stay in the laboratory, and arranged all my travels and accommodations in the smoothest possible way. It was always fun and beneficial to communicate with her because she allowed me to ameliorate my French language.

I want to express my deepest gratitude for two of my favorite professors at the university, Dr. Ghassan Akkary and Dr. Zeina Jundi, who have always encouraged and believed in me. As for Dr. Zeina, it is worth mentioning that she was not only a teacher, but also an excellent mentor who always guided me on the right path and will always be a great inspiration for me.

My colleagues and friends, especially Abdallah, Antoine, Asma, Aya, Claudia, David, Deurcy, Gabriela, Ghinwa, Hala, Hagop, Jackie, Katia, Kosty, Lena, Louay, Oscar, Riad, Ricardo, Samer, Vanessa, have played a pivotal role because they have provided me with a warm shoulder to lean on whenever I needed support. They made me embark on my thesis journey in the lightest possible way and converted all my stressors into laughers. I will also seize the great opportunity to explicitly thank my parents and my siblings for their unconditional love and support. Whenever an obstacle attempted to pull me down, my family used to push me up. Whenever an impediment put me in the deepest abyss, my family relocated me to the summit. They have truly made what is impossible possible. Therefore, I owe them my research, my work, and my thesis.

General introduction

Fossil fuels dependencies have been a serious addiction for most of the population since the middle of last century due to the need to ensure access to energy for quality of life and for economic development. Equally important, however, is the climate change problem encountered by the use of fossil fuels (non-renewable source) through the emissions of pollutants, CO2 specifically, contributing to global warming. As an alternative energy source, hydrogen (H2) and carbon monoxide (CO) mixture can be transformed following a specific reaction pathway to fuels, chemicals and electricity meeting everyone's need. There are several available processes to generate the required mixture of H2 and CO from methane, like steam reforming, dry reforming, and partial oxidation reforming. In this work, the focus will be targeted toward dry reforming of methane since this reaction uses CH4 and CO2 as reactants, both of which are greenhouse gases. Even if CH4 is a short-lived climate pollutant, its emission should not be underestimated. Indeed, CH4 is not only obtained from gas production, but it is also emitted from livestock and rice paddies, ruminant animals, side products of industrial activities, landfills (biomass), and waste water treatment. CO2 is mainly transferred to the atmosphere from combustion of fossil fuels. Thus, the conversion of both pollutants to energy will minimize fossil fuels dependency, and will contribute to reduce its disastrous climate consequences on the planet. Dry reforming of methane process is an endothermic reaction that is still under research studies striving to find an active and stable catalyst. Although noble-metals can be active and stable, they are scarcely available and expensive which led to the development of transition metals, nickel specifically. The drawback of using Ni-based catalyst is the sintering of Ni and the formation of carbon under the harsh reaction condition. The recent approaches are targeted towards enhancing the performance of Ni supported on different oxides, mainly Al2O3, by 1) employing different methods of Ni deposition on the support, 2) altering the shape of the support and 3) adding a second promoter or metal. However, the preparation methods were not yet efficient enough to stabilize Ni against thermal agglomeration and carbon deposition.

In this thesis, catalysts that are highly active and stable in dry reforming of methane are synthesized inspired by different positive parameters seen in literature. Needless to say, the Al2O3 oxide was selected as support for Ni. The direct one-pot method that involves adding Ni within the synthesis of Al2O3 was adopted along with alteration in the shape of alumina to ensure occlusion and confinement of nickel nanoparticles inside the pores or walls of structured oxide. MgO was also selected as basic dopant to prevent solid carbon accumulation. This research work is a joint PhD between the "Laboratoire de réactivité de Surface" at Sorbonne University and the Department of chemical engineering at University of Balamand.

It is also a part of a European ERANETMED project (SOL-CARE) that includes three European countries (Spain, Italy, Portugal) in addition to Lebanon.

The manuscript is divided into six main chapters. Chapter I provides a general overview on the non-renewable fossil fuels source of energy, their environmental issues, and the possible renewable alternative energy pathways involving C1-based molecules (CO2 and CH4). A detailed explanation is presented on dry reforming of methane reaction, then the state of the art catalysts already used in the reaction are elaborated and analyzed to understand the challenges faced.

Chapter II is dedicated to the technical part involving the principle of all the used characterization techniques, the applied procedures on the tested materials (calcined, reduced, and spent), and the derived results from each equipment. The chapter also embraces an evaluation of the thermodynamic equilibrium of the reactions, and the experimental catalytic tests set-ups. Note however that this chapter will not introduce the conditions of preparation of the samples that will be described in the respective chapters where they will be developed.

Chapter III tackles the effectiveness of using MgO as basic dopant in mesoporous Ni-Al2O3 materials by one-pot synthesis. The chapter begins with a brief bibliographic recall on the existing Mg-based alumina materials. Then, the preparation methodology used in this thesis is described in details before presenting the obtained physicochemical properties, the reducibility and the catalytic performances. The focus will be on the effect of magnesium loading with a clear explanation on why its use should be optimized.

Chapter IV introduces a novel technique to occlude Ni nanoparticles inside Al2O3 pores following an innovative metal organic framework (MOF) route. Firstly, this chapter gives concise recalls on MOF materials, their types, and the reason behind choosing MIL-53(Al) as MOF sacrificial precursor in our study. Then, the preparation of mesoporous Ni-Al2O3 from MOF is described and the material obtained is compared structurally, physically and catalytically to a second MOF-based sample prepared by impregnation and conventional Ni-Al2O3 material.

Chapter V applies the above-mentioned materials exhibiting good performances in dry reforming of methane to another reaction of interest in the energy domain, the CO2 methanation that also consumes CO2. A brief state of the art catalysts is given at the beginning of the chapter before explaining the preparation methods of the materials used and their physiochemical properties along with the catalytic tests results.

Chapter VI ends this report by treating some of the materials in a more realistic dry reforming gaseous conditions evolved from pyrolysis of biomass wastes as given by a Lebanese industry. The aim of this last pat is to investigate the efficiency of our catalysts under harsh conditions of dry reforming that contains CH4, CO2, H2 and CO in the reactants feed, making more difficult to obtain the expected H2/CO products ratios.

I. Chapter I: Literature review

I.1. Sources of energy from fossil resources

Mixtures of hydrogen (H2) and carbon monoxide (CO), known as syngas, are important in the energy domain since they can lead to the production of fuel. Such mixtures can be obtained from methane following different reforming reactions. In this chapter, the main resources of methane are presented along with the various pathways of converting it into H2 and CO. Through the description, an interest is highlighted on the dry reforming of methane (DRM) reaction that will be in the core of this work and where CO2 is used as a co-reactant. A state of the art is provided to summarize the advancements achieved in the existing catalysts for this reaction. The chapter ends with an outline of the objectives and methodology of the present thesis. I.1.1. Main resources a) Fossil fuels Fossil fuels were discovered over 150 years ago. They are formed from organic matter deposited over the course of million years and fabricated by natural processes deep in the earth or under the sea bed by anaerobic decomposition of buried dead organisms. The fossil fuels such as oil, coal and natural gas extracted by human operations can be further transformed and refined into petroleum products [START_REF] Shafiee | When will fossil fuel reserves be diminished?[END_REF][2][3]. In other words, fossil fuels exist in different forms (solid, liquid, gas) and they have a wide range of carbon content going from products with low carbon to hydrogen ratio like methane (volatile materials) to non-volatile pure carbon as anthracite coal. Oil, coal, and natural gas are known for being described as the energy of the world due to the versatile energy supplies they can offer [3,4] playing a crucial role in the 21 st century world energy market with an average rate of production of about several million barrels per day. For instance, oil is the world's primary fuel source for transportation capable of being transformed not only into fuel oil, diesel oil, kerosene, gasoline, and liquefied petroleum gas (propane, butane), but also into other products like fertilizers, plastics and pharmaceutical compounds [2,[START_REF] Opec | OPEC World Oil Outlook[END_REF]. Coal is mainly used to produce electricity, steel, manufacture cement and synthesize liquid fuel as well as for heating [2,6]. Nowadays, the use of natural gas has been growing in a fast pace into the sector of electric power generation (heat, electricity) with an interesting reliance in other areas (industries, residential, commercial, pipeline and distribution) [2,[START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF].

Chapter I Literature Review b) Methane (CH4)
As natural gas transport is expensive and problematically hard due to the relatively low energy content in relative to its volume, the global market of oil is much bigger and a basic requirement for energy needs. Nevertheless, and in close relationship towards the tendency of using greener energy, the natural gas consumption has increased in the last decade for it is considered as the cleanest burning petroleum based fuel, and this tendency is planned to continue with a net worldwide increase of about 2.8% annually from 2001 to 2025 [START_REF] Demirbas | The importance of natural gas as a World fuel, Energy Sources[END_REF]. It is important to mention that CH4 properties are utilized when speaking about natural gas because it accounts about 95% of its total volume. The other components with a total of 5%, contain traces of ethane, propane, butane, pentane, nitrogen, carbon dioxide and water vapor [START_REF] Demirbas | The importance of natural gas as a World fuel, Energy Sources[END_REF]. CH4 gas is a prominent raw material with high abundancy in remote areas distributed all around the world. According to BP statistical review and OPEC oil and gas data, Iran, Russia, Qatar and Turkmenistan have the highest natural gas reserves (Figure I-1) [START_REF] Esen | Natural gas reserve/production ratio in Russia, Iran, Qatar and Turkmenistan: A political and economic perspective[END_REF]. Methane has the ability to produce more heat and light energy by mass (H/C ratio: 4/1) than other hydrocarbons and fossil fuels (oil (H/C<2/1), coal (H/C<1/1)) resulting in significantly less carbon dioxide and other pollutants that create smog and pollutes the environment. In other words, one should rely more on CH4 and replace it for coal and oil to generate electricity and fuel transportation while contributing to environment protection.

I.1.2. Reserves, human consumption and potential depletion

There exists a dilemma about the persistence of this non-renewable source of energy (coal, oil and natural gas) and whether the existing reserves could withstand the intense consumption by the human being [START_REF] Shafiee | When will fossil fuel reserves be diminished?[END_REF][START_REF] Mathur | The carbon climax:End of hydrocarbon legacy-A decade of metamorphosis and rapid change[END_REF]. Estimations about fossil fuels reserves and availabilities differ and nobody can really have an accurate prediction on their exhaustion.

Global energy consumption has increased by 2.2% (2.1 million barrels per day) in 2017 compared to an average of 1.7% for the previous 10 years (BP) [2]. The energy information On the other hand, scientists and oil companies have a consensus that oil production for the long term forecast has reached a peak (Figure I-3) and we will at a certain moment run out of oil supplies [START_REF] Mathur | The carbon climax:End of hydrocarbon legacy-A decade of metamorphosis and rapid change[END_REF][START_REF] Peebles | Development of Hubbert's Peak Oil Theory and Analysis of its Continued Validity for U.S. Crude Oil Production[END_REF][START_REF] Deffeyes | Hubbert's peak: the impending world oil shortage[END_REF]. The size of oil reserves regularly discovered are getting smaller through years even if technologies (EOR: enhanced oil recovery) are highly advanced for finding new places to look for potential seams of oil. For instance, even if we take the most optimistic case scenario (4% depletion, green curve), in 2025 the world will be able to produce the same quantity of oil as in 1970.

Not to mention that the world population in 2025 will be 50% more than what we had in the 1970's [START_REF] Mathur | The carbon climax:End of hydrocarbon legacy-A decade of metamorphosis and rapid change[END_REF].

This means that the new discoveries and technologies have only delayed the fall and it is a must that we reduce our Figure I-3: Peak Oil Forecast, Source: BP statistical review of world energy 2009 [START_REF] Mathur | The carbon climax:End of hydrocarbon legacy-A decade of metamorphosis and rapid change[END_REF] Figure I-2: World liquid fuels production and consumption balance, Source: Energy information administration [START_REF] Iea | Short Term Energy Outlook. Organization for Economic cooperation and development, International energy agency[END_REF] dependence and adjust the falling oil supplies to avoid their depletion. In addition, shahriar Shafiee et al. calculated oil, coal and gas depletion following Klass and New model, and estimated that coal reserves will last until 2112 and it will be the single fossil fuel available after depletion of oil and gas in 2042 [START_REF] Shafiee | When will fossil fuel reserves be diminished?[END_REF] if people continue to use them as source of energy.

Concerning CH4, no matter what is the amount of reserve available, it should not be allowed to escape into the atmosphere due to environmental reasons that will be mentioned in the following paragraph.

I.1.3. Environmental issue I.1.3.1. CO2 emissions Another downstream of relying on this traditional method to produce energy, is the detrimental impact on the environment. Burning fossil fuels results in formation of carbon dioxide (CO2) which is one of the primary greenhouse gases (GHG) responsible for global warming [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Oemar | Catalytic Biomass Gasification to Syngas over Highly Dispersed Lanthanum-Doped Nickel on SBA-15[END_REF][START_REF] Riemer | Green house gas mitigation technologies, an overview of the CO2 capture, storage and future activities of the IEA Greenhouse Gas R&D programme[END_REF]. According to BP, the global emissions of CO2 from energy had increased on a basis of 10-year average (2006-2016) by 1.3%. However, in 2017 only, CO2 emitted faced a calamitous grown by 1.6% [2] which means that humans continue increasing dependencies on fossil fuels resources contributing to a calamitous effect on the environment. produced from any type of fossil fuels [START_REF]Carbon Dioxide Emissions Factor, Kg CO2 per MWh[END_REF]. We can see several points from this figure. For (CH4) also results in an inevitable release of CO2 but in much less quantity than other burning hydrocarbons (201.96 KgCO2/MWh). This is why it is considered as the cleanest type of fossil fuels. Moreover, its burning results in very low amount of NOx, particulates, and almost 0 SO2 emissions [2,[START_REF] Demirbas | The importance of natural gas as a World fuel, Energy Sources[END_REF]. Hence, CH4 presents important advantages compared to the other hydrocarbon feedstock, even if its utilization also contributes to an average of 2% increase per year of CO2 emissions which is planned to regularly continue as illustrated by predictions made until 2050 (Figure I-5) [START_REF] Scibioh | CO 2 Conversion-Relevance and Importance[END_REF].

I.1.3.2. Diversity of deleterious impacts

An additional environmental issue related to using fossil fuels comes from the fact that people nowadays are facing difficulties in extracting oil and gas sources. They rely on fracking and the use of energy intensive methods prone to escape of more CO2 gases and other air pollutants like sulfur dioxide from acid rain, nitrogen oxides (NOx), mercury and carcinogenic chemicals [START_REF]Carbon Dioxide Emissions Factor, Kg CO2 per MWh[END_REF]18,19].

This stark truth of CO2 greenhouse emission in addition to the chronic gases are further boosted by scientific facts of sea levels rise, hotter temperatures, and the occurrence of storm weather patterns like tsunamis and hurricanes. They can also contribute to serious health complications in the human being such as lung malfunctioning, asthma, cancer and cardiovascular diseases. They can lead as well to the death of aquatic animals and pollution of ground water if any oil spill is triggered [START_REF] Idso | Effects of atmospheric CO2 enrichment on plant constituents related to animal and human health[END_REF][START_REF] Ziska | Rising CO2, climate change, and public health: Exploring the links to plant biology[END_REF]. Not to mention that coal mines cause relocation and damage the vegetation, top-soil and contamination of rivers [START_REF] Elberling | Arctic vegetation damage by wintergenerated coal mining pollution released upon thawing[END_REF]. I.1.3.3. CH4 and CO2 as greenhouse gases

The negative consequences of burning fossil fuels and CH4, and producing CO2 is not the sole problem. It is significant to deem in mind that CH4 (oxidized carbon) is also by itself a pollutant if released into the atmosphere as CO2 gas (reduced carbon). CH4 and CO2 are considered as greenhouse gases because they both absorb and emit thermal infrared radiation at the range of our planet emitted wavelength, trapping the heat and making the planet warmer [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Scibioh | CO 2 Conversion-Relevance and Importance[END_REF]. CH4 is tougher than CO2 as greenhouse gas with around 120 times more radiative force but it has a shorter lifetime effect with only 12.4 years average. In contrary, CO2 is a complex molecule thermodynamically stable in the atmosphere with indefinite lifetime [START_REF] Independent | 4 The Importance of Methane[END_REF][START_REF] Dessus | Global Warming: the Significance of Methane[END_REF]. It is now well recognized that an effective way to control CH4 and CO2 emissions to air and reduce their negative effect in the atmosphere would be to use both as feedstock for energy production especially that there exist natural processes in which methane and CO2 are emitted.

Needless to say, Figure I-6 demonstrates that the concentrations of atmospheric greenhouse gases (CH4 and CO2) do not only come from burning of fossils fuels but they originate as well from different natural and human activities (biomass) sources [START_REF] Scibioh | CO 2 Conversion-Relevance and Importance[END_REF][START_REF] Independent | 4 The Importance of Methane[END_REF]. Based on a balance between GHG sources and sinks (conversion of the species to different chemical compounds), the amount of GHG in addition to its residence time (RT) in the atmosphere are estimated to be 73% (RT: 150 years) for CO2, 17% (RT: 12 years) for CH4, and 8% (RT: 100 years) for other greenhouse gases (N2O) [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Demirbas | The importance of natural gas as a World fuel, Energy Sources[END_REF][START_REF] Scibioh | CO 2 Conversion-Relevance and Importance[END_REF]25,[START_REF] Myers | Tropical Deforestation and Climatic Change[END_REF]. In brief (Figure I-6), CH4 is emitted Landfills and enteric fermentation are included in the biomass category that can be used as a source of energy limiting reliance on fossil fuels. This strategy is one of our interest and it will be tackled in more details in chapter VI of the thesis.

In order to mitigate the anthropogenic emissions (CH4 and CO2) a forward step has been taken by scientists considering few courses of action [START_REF] Scibioh | CO 2 Conversion-Relevance and Importance[END_REF][START_REF] Centi | Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries[END_REF] such as improving the efficiency of combustion engine, minimizing utilization of fossil fuels, constructing energy-efficient building, frequent usage of recycled materials, shifting to low carbon energy sources, capture and sequestration of greenhouse gases, reducing deforestation, and fertile land management.

Because these pathways are somehow daunting and require deeper investigations and studies to assure their efficiency and be pursued, chemists have been more concerned recently, as mentioned earlier, in utilizing CH4 and CO2 either together or separately as raw compounds for the production of useful fuels and chemicals. This is what will be elaborated in the next section.

I.3 Pathways for CH4 and CO2 conversions

There are several ways of converting CH4 and CO2 gases [START_REF] Scibioh | CO 2 Conversion-Relevance and Importance[END_REF][START_REF] Centi | Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries[END_REF][START_REF] Kumar | Catalytic bi-reforming of methane: From greenhouse gases to syngas[END_REF] through economic pathways. For example, CH4 and CO2 can be each used to produce chemicals such as food & beverages, solvent for pharma, fire extinguishers, solvents, and hydrocyanic acid. However, the market potential of fuel is 15 times higher than chemicals and speaking from the structural point of view, chemicals have higher molecular complexity than fuels which are made of simple [START_REF] Agency | Overview of greenhouse gases[END_REF] chains as in hydrocarbons [START_REF] Scibioh | CO 2 Conversion-Relevance and Importance[END_REF]. Thus, the focus of researchers is mainly towards energy production.

Methane can be used directly for the production of heat and power generation by combustion (severe conditions needed to break the stable C-H bond) or it can be converted (indirect pathway) to produce a synthesis gas (called syngas) that is a mixture of hydrogen (H2)

and carbon monoxide (CO) [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Centi | Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries[END_REF][START_REF] Kumar | Catalytic bi-reforming of methane: From greenhouse gases to syngas[END_REF][START_REF] Kawi | Progress in Synthesis of Highly Active and Stable Nickel-Based Catalysts for Carbon Dioxide Reforming of Methane[END_REF]. CO2 can be also converted (indirect pathway) to syngas.

Making syngas is of strong interest because it is a crucial intermediate for the production of fuels needed for transportation and energy. The ratio of the syngas produced varies and depending on this value the targeted application is altered. Figure I-8 summarizes the main existing pathways for syngas applications [START_REF] Kumar | Catalytic bi-reforming of methane: From greenhouse gases to syngas[END_REF][START_REF] Rauch | Biomass Gasification for Synthesis Gas Production and Applications of the Syngas[END_REF]. An important route (in red) is the use of syngas as reactant in Fischer-Tropsch synthesis for the subsequent production of fuel (gasoline, diesel, naphtha), another one (in grey) concerns power generation for fuel cells and gas turbines, a third one (in green) consists in using hydrogen as the source for transportation and refinery hydro treating and, finally, syngas can be converted to methanol to synthesize formaldehyde, acetic acid and ethylene propylene. In fact, in order to obtain the required syngas adapted to the targeted route, there are several reforming approaches in which methane reacts with variant oxidants (H2O, CO2, O2) to synthesize H2 and CO with different ratios. Table I-1 illustrates the six existing methane reforming processes along with their chemical reactions, their enthalpy and their resultant H2:CO ratio [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Kumar | Catalytic bi-reforming of methane: From greenhouse gases to syngas[END_REF][START_REF] Kawi | Progress in Synthesis of Highly Active and Stable Nickel-Based Catalysts for Carbon Dioxide Reforming of Methane[END_REF]. Reactions 1 and 2 are widely studied in literature and industrialized in some areas. These reactions in addition to reaction 3 rely on H2O, O2 or the combination of H2O and O2 to oxidize CH4 resulting in an H2:CO ratio between 2 and 3. In brief:

Table I-1: Chemical reactions of CH4 reforming processes along with their corresponding enthalpy and H2:CO ratios  In steam reforming (SRM), CH4 is oxidized by a steam generating a syngas with a H2:CO ratio of 3 suitable for H2 production. This reaction is very endothermic and its process is divided into four steps. The first reformer operates at very high temperatures (800-900 °C)

with pressure around 15-30 atm demanding high energy investment to produce the steam itself.

The second reformer works at a temperature of 1000 °C and uses a mixture of H2O and O2 in the feed. Then, H2/CO ratio is adapted using successive reactions of water gas shift at 400 °C and 200 °C, respectively [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Liu | Progresses in the preparation of coke resistant Ni-based catalyst for steam and CO 2 reforming of methane[END_REF][START_REF] Abdullah | Recent advances in dry reforming of methane over Ni-based catalysts[END_REF].

 Partial oxidation (POM) operates at lower temperature than SRM between 500 and 700 °C.

The syngas produced has a ratio of 2 adequate for methanol synthesis and purification of CH4 reactant is not always required. Despite these qualities, this reaction suffers from being highly exothermic. This problem can induce hot-spots in the reactor making the process more complicated and increase the risk of explosion. Pure oxygen supplies also come with a share of technical and economic problems inquiring industries to look for alternative solutions rather than cryogenic unit to separate oxygen from air [START_REF] Abdullah | Recent advances in dry reforming of methane over Ni-based catalysts[END_REF][START_REF] Lyubovsky | Catalytic partial "oxidation of methane to syngas" at elevated pressures[END_REF][START_REF] Supat | Combined Steam Reforming and Partial Oxidation of Methane to Synthesis Gas under Electrical Discharge[END_REF].  Auto thermal reforming (ATR) combines H2O to the process of partial oxidation to solve the economic issue raised. The bright side of adding H2O within the feed is the ability to impact and control the H2/CO ratio in the gas product allowing a simpler adaptation to several downstream operations such as ammonia and methanol. However, safety concerns associated with the presence of oxygen have limited their use. In addition, this reaction is more energy demanding and part of the methane fed is invested to generate heat in order to reach the appropriate operating temperature and achieve an optimal conversion of methane to syngas avoiding soot formation [START_REF] Kumar | Catalytic bi-reforming of methane: From greenhouse gases to syngas[END_REF].

Rxn

Since the trend nowadays is toward curtailing CO2 amount in the atmosphere which is also our main concern in this thesis, the focus becomes more towards the 3 next methane reforming reactions that consume such gas in addition to CH4 (Rxn 4,5,6 in Table I-1):

 Oxy-CO2 reforming resembles ATR in regards to the aim and outcome. The use of a sole O2 carrier favors the thermodynamic production of H2 and CO mixture. But, the economic barriers made it imperative to combine CO2 in the process. Adding CO2 within the feed leads to H2/CO ratio of around 6/4 suitable for further use in ammonia and methanol synthesis processes. It has also the appeal of auto thermal operations especially that the latter consumes CO2.

However, safety concerns associated with the presence of oxygen have limited their use.

 Dry reforming of methane (DRM) uses only carbon dioxide as an oxidizing agent reacting with CH4 to synthesize hydrogen and carbon monoxide. This reaction is recently attracting the attention of many scientists despite owing less favorable thermodynamics than the previous mentioned reactions. But, what makes this reaction more interesting is its ability to greenly consume the natural gas reducing the carbon impact by proportionally consuming 1 mole of carbon dioxide per one mole of methane invested. The product is eventually a syngas with an equimolar ratio of 1 used as a feedstock for the synthesis of large chain hydrocarbons by

Fischer-Tropch approach. The energy invested in the reaction may be curtailed by the use of appropriate catalytic systems which would downstream open the opportunity to dry reforming to become an economical process competing SRM that is already industrially mature. What prevents DRM from being scaled up to industrial level, is the difficulty in finding a catalyst that could withstand the high temperature required (due to thermodynamic reasons that will be explained in chapter II) and resist deactivation by sintering and carbon deposition [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Abdullah | Recent advances in dry reforming of methane over Ni-based catalysts[END_REF][START_REF] Dębek | A Short Review on the Catalytic Activity of Hydrotalcite-Derived Materials for Dry Reforming of Methane[END_REF][START_REF] Bian | A Review on Bimetallic Nickel-Based Catalysts for CO 2 Reforming of Methane[END_REF].

This reaction, and more specifically the question of improvement of catalysts, is our core interest in this thesis.

 Combined steam and dry reforming of methane (CSDRM) is a new emerging reforming reaction combining SRM and DRM. Compared to other CO2 consuming reactions, this chemical reaction produces a "metgas" with an H2:CO of 2 leading to formation of higher value products particularly methanol and its derivatives. This reaction has been rarely reported in literature due to its high endothermic feature and requirement of energy [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Kumar | Catalytic bi-reforming of methane: From greenhouse gases to syngas[END_REF][START_REF] Olah | Single Step Bireforming and Oxidative Bi-reforming of Methane (Natural Gas) with Steam and Carbon Dioxide to Metgas (CO-2H<inf>2</inf>) for Methanol Synthesis: Self-Sufficient Effective and Exclusive Oxygenation of Methane to Methanol with Oxygen[END_REF]. The main DRM process is a direct reaction between CH4 and CO2 and it is the most endothermic reaction in the series of equations because the CO2 molecule is very stable. Hence, in order for it to act as an oxidant and interact with CH4 a high energy is needed to activate it. Water gas shift (WGS) and reverse water gas shift (RWGS) are non-desirable side reactions that can thermodynamically occur in parallel to the main reaction. However, only the RWGS reaction is within the temperature range of DRM and thus it is more inclined to be triggered. This reaction consumes CO2 and H2 to produce CO and H2O and thus reduces the ratio of H2:CO product to less than the required value (1, section I.3.1). The presence of H2O in the product of the RWGS reaction is also a drawback. In fact, steam could oxidize the reduced active phase during the insitu run, decreasing catalytic activity and deactivating the catalyst. and CH4 decomposition reactions could strike simultaneously with the main reaction producing carbon that blocks the catalyst pore and covers the active sites [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Dębek | A Short Review on the Catalytic Activity of Hydrotalcite-Derived Materials for Dry Reforming of Methane[END_REF][START_REF] Nikoo | Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation[END_REF]. In this PhD, stability experiments were operated mainly at 650 °C coining the probable formation of carbon deposits to CH4 decomposition only. It is important to mention also that not all carbon deposits (Cα, Cβ) are detrimental leading to catalyst deactivation (Cγ), but their continuous accumulation inside the reactor could increase pressure, plug the reactor, and cease the reaction [START_REF] Castro Luna | Carbon dioxide reforming of methane over a metal modified Ni-Al2O3 catalyst[END_REF].

I.3.

CO2 methanation (Sabatier reaction), seen as possible undesirable reaction in

Chapter I

I.4. State of the art catalysts for dry reforming of methane

Dry reforming of methane reaction needs a robust catalyst capable of withstanding the harsh operating conditions imposed by the reaction, i.e. the high temperature levels and the occurrence of possible side reactions mentioned above. Thus, researchers are trying to design a convenient catalyst using modern catalysis techniques combining several features like low cost, recyclability and environmentally benign ensuring 100% selectivity towards the desired products and good stability [START_REF] Aramouni | Catalyst design for dry reforming of methane: Analysis review[END_REF][START_REF] Jang | A review on dry reforming of methane in aspect of catalytic properties[END_REF][START_REF] Zhang | A review of CH4-CO2 reforming to synthesis gas over Nibased catalysts in recent years (2010-2017[END_REF][START_REF] Singh | Advanced synthesis strategies of mesoporous SBA-15 supported catalysts for catalytic reforming applications: A state-of-the-art review[END_REF]. The catalytic activity, selectivity and stability per surface atom rely mainly on the type of metal adopted (active component), particle size, structure and location, reduction extent, accessibility of active metal sites, and choice of oxide support. Thus, the current section of this report tackles the type of catalysts employed in dry reforming covering the active phases, the promoters, and the supports examined in this reaction along with their corresponding catalytic activity and stability and the reason behind their deactivation if any. Note that in heterogeneous DRM catalysis, people use oxides as support for the metals and it has no direct catalytic role. In our case, we will focus on Al2O3-based support because it is cheap, has a high attrition resistance and high thermal stability [START_REF] Seo | Recent Scientific Progress on Developing Supported Ni Catalysts for Dry (CO2) Reforming of Methane[END_REF] that make it industrially enticing. Co. A decent number of studies have concealed experimentally and theoretically the excellent activity and stability of precious noble metals in dry reforming of methane due to their high capability of hindering carbon deposition as well as tolerance towards corrosion and oxidation [START_REF] Bian | A Review on Bimetallic Nickel-Based Catalysts for CO 2 Reforming of Methane[END_REF][START_REF] Pakhare | A review of dry (CO2) reforming of methane over noble metal catalysts[END_REF]. On the other side, these metals are not economically convenient for large-scale industrial service due to their high price and scarce availability [START_REF] Lavoie | Review on dry reforming of methane , a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF][START_REF] Zhang | A review of CH4-CO2 reforming to synthesis gas over Nibased catalysts in recent years (2010-2017[END_REF]. Thus, the number of papers embracing noble metals as a primary active phase on Al2O3 support has diminished through years especially in the period of 2010-2019 inspecting around 6 papers of different noble metals (Pd, Pt, Ir, Rh) as main active phase with small to moderate amount (0.5-1 wt%) [START_REF] Yentekakis | Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO2 reforming of methane[END_REF][START_REF] Maina | Study of the performance and stability in the dry reforming of methane of doped alumina supported iridium catalysts[END_REF][START_REF] Fernández | Insights into dynamic surface processes occurring in Rh supported on Zr-grafted γ-Al2O3 during dry reforming of methane[END_REF][START_REF] Ocsachoque | Role of CeO2 in Rh/α-Al2O3 Catalysts for CO2 Reforming of Methane[END_REF][START_REF] Ballarini | Platinum supported on alkaline and alkaline earth metaldoped alumina as catalysts for dry reforming and partial oxidation of methane[END_REF][START_REF] Ghelamallah | Impact of barium and lanthanum incorporation to supported Pt and Rh on α-Al 2O 3 in the dry reforming of methane[END_REF][START_REF] Shi | Role of MgO over γ-Al2O3-supported Pd catalysts for carbon dioxide reforming of methane[END_REF]. The presence of such small amount of noble metal is not enough to cast carbon formation and increasing further the amount of noble metals added enhance the activity and stability but increase a lot the price of the catalyst. As a consequence these samples were adjusted with alkaline or alkaline earth metals. The CO2 activation is boosted to form O species that reacts with C(s) to form CO enhancing basicity of the catalyst and resistance to deactivation [START_REF] Maina | Study of the performance and stability in the dry reforming of methane of doped alumina supported iridium catalysts[END_REF][START_REF] Ballarini | Platinum supported on alkaline and alkaline earth metaldoped alumina as catalysts for dry reforming and partial oxidation of methane[END_REF][START_REF] Ghelamallah | Impact of barium and lanthanum incorporation to supported Pt and Rh on α-Al 2O 3 in the dry reforming of methane[END_REF][START_REF] Shi | Role of MgO over γ-Al2O3-supported Pd catalysts for carbon dioxide reforming of methane[END_REF]. Other non-alkaline oxygen carriers (PrO2, ZrO2) were added to low amount noble metals to benefit from the transfer of oxygen to the reactants abating carbon filaments [START_REF] Yentekakis | Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO2 reforming of methane[END_REF][START_REF] Ocsachoque | Role of CeO2 in Rh/α-Al2O3 Catalysts for CO2 Reforming of Methane[END_REF].

All the advances in drawing attention towards low-amount noble based-materials by altering them with promoters did not totally eliminate the presence of carbon, and higher loadings are still needed to avoid deactivation and ensure excellent activity and stability.

Due to the restrictions of noble metals (high cost and scarce availability), transition metals have invaded dry reforming of methane studies with nickel receiving much attention due to its vast availability, cheapness and agnate noble metals catalytic activity in DRM [START_REF] Álvarez | CO2 reforming of methane over Ni-Ru supported catalysts: On the nature of active sites by operando DRIFTS study[END_REF].

Nevertheless, using nickel promotes sintering and coke formation that are two major concerns in dry reforming decreasing catalytic stability [START_REF] Abdullah | Recent advances in dry reforming of methane over Ni-based catalysts[END_REF][START_REF] Aramouni | Catalyst design for dry reforming of methane: Analysis review[END_REF][START_REF] Jang | A review on dry reforming of methane in aspect of catalytic properties[END_REF]. Fewer attention has been given to Co-Al2O3 based catalysts since they are less performing in DRM [START_REF] Park | Enhanced stability of Co catalysts supported on phosphorus-modified Al2O3 for dry reforming of CH4[END_REF][START_REF] Horváth | Dry reforming of CH4on Co/Al2O3catalysts reduced at different temperatures[END_REF][START_REF] Li | Effect of granulation on the activity and stability of a Co-Al 2O3 aerogel catalyst in a fluidized-bed reactor for CH4-CO2 reforming[END_REF][START_REF] Zeng | Modification effect of natural mixed rare earths on Co/γ-Al 2O 3 catalysts for CH 4/CO 2 reforming to synthesis gas[END_REF][START_REF] Park | Effect of supports on the performance of Co-based catalysts in methane dry reforming[END_REF]. They are sensitive to deactivation from oxidation by CO2 perturbing its stability [START_REF] Park | Enhanced stability of Co catalysts supported on phosphorus-modified Al2O3 for dry reforming of CH4[END_REF]. Thus, the key in dry reforming studies is to design high carbon and sintering-resistance nickel-based catalysts. In order to reach this goal, a design of highly active coke-resistant Ni-Al2O3 based catalysts can be achieved by utilizing promoters, alloying Ni with other metals, changing support characteristics and catalysts synthesis techniques.

b) Al2O3 as support for metal Ni:

The supports are in direct physical contact with the supported active sites and affect their behavior and structure, not to mention that in some cases it could interfere in the chemical reactions. The used materials are expected to have a large surface area to efficiently disperse the metal nanoparticles, uniform size and shape morphology, strong mechanical strength, high thermal stability, high surface to volume ratio and a number of defect sites, corners, edges and kinks [START_REF] Zhang | A review of CH4-CO2 reforming to synthesis gas over Nibased catalysts in recent years (2010-2017[END_REF][START_REF] Singh | Advanced synthesis strategies of mesoporous SBA-15 supported catalysts for catalytic reforming applications: A state-of-the-art review[END_REF].

To date, there are many oxides that have been studied and investigated to support the active phase for methane dry reforming such as SiO2, Al2O3, La2O3, MgO, ZrO2, and CeO2 [START_REF] Pakhare | A review of dry (CO2) reforming of methane over noble metal catalysts[END_REF][START_REF] Park | Effect of supports on the performance of Co-based catalysts in methane dry reforming[END_REF]. Back in 2000 and 2002, studies showed that metal dispersion (Rh) increases in the order of TiO2 < La2O3 < CeO2 < ZrO2 < MgO < SiO2 < MCM-41 < Al2O3 [START_REF] Wang | Carbon dioxide reforming of methane to synthesis gas over supported rhodium catalysts: The effect of support[END_REF][START_REF] Yokota | Support effect of metal oxide on Rh catalysts in the CH4-CO2reforming reaction[END_REF] . Since nickel is our main interest as active phase we checked its performance over different supports in several literatures. Ruckenstein and Hu were the first to highlight the importance of the support nature for nickel. In their study, they proved that the activity and selectivity of the catalyst were highly dependent on the nature of the support following the order of TiO2< SiO2< Al2O3. They related such behavior to the metal-support interaction since it can activate and modify the Chapter I Literature Review electronic and chemical properties of the active phase [START_REF] Ruckenstein | Role of support in CO2 reforming of CH4 to syngas over Ni catalysts[END_REF]. For instance, TiOx molecules covered the active sites during reduction by migrating over the surface of Ni particles curtailing the surface free energy and decreasing the catalyst activity. Several other researchers asserted such assumption in their publications [START_REF] Bian | A Review on Bimetallic Nickel-Based Catalysts for CO 2 Reforming of Methane[END_REF][START_REF] Pakhare | A review of dry (CO2) reforming of methane over noble metal catalysts[END_REF]. Zhang R.J. et al. [START_REF] Rong-Jun | Effect of support on the performance of Ni-based catalyst in dry reforming[END_REF] conducted similar studies to show the significant influence of the support types on the state of active phase (NiO species)

and their dry reforming catalytic performance. This group related again the poor performances of SiO2, TiO2, and ZrO2 based catalysts to the low reduction temperature commonly associated to weak interaction with the active Ni species. On the other side, Al2O3 based catalyst had very high stability for more than 100 h even at high gas hourly space velocity.

Despite the fact that both silica and alumina have received much attention compared to other supports type due to the high metal dispersion they provide [START_REF] Zhang | A review of CH4-CO2 reforming to synthesis gas over Nibased catalysts in recent years (2010-2017[END_REF], Al2O3-based catalysts are still taking the most important space for DRM with almost 109 papers published between the years 2010-early 2019 compared to 55 papers for its silica counterpart (Data taken from Web of Science). In brief, these 2 supports differ chemically. Kinetics and mechanistic studies in literature stated that there is a dual function for DRM depending on the support nature. Thus, it is proved that on acidic or basic supports the CH4 is activated on the metal and CO2 on the carrier forming formates or oxy-carbonates. However, for inert materials like SiO2, CH4 and CO2 are activated on the metal only, following a mono functional pathway so when carbon forms by dehydrogenation of methane, the activation of CO2 and reaction with carbon will become limited resulting in catalyst deactivation [START_REF] Pakhare | A review of dry (CO2) reforming of methane over noble metal catalysts[END_REF]. All this leads to inert SiO2 weakly interacting with the metal, less stable and active compared to mildly acidic Al2O3. This chemical Another important chemical feature that can be addressed is the hydrophobicity and hydrophilicity in both supports. Al2O3 is more hydrophilic so its surface is more prone to be hydrated when in contact with H2O and therefore less available for oxidizing metallic sites and more stable [START_REF] Pakhare | A review of dry (CO2) reforming of methane over noble metal catalysts[END_REF]. A study was done by our group on the effect of Ni-based silica and Ni-based alumina catalysts on their performance in reforming. Results indicated a re-oxidation of the Ni 0 active phase in silica-based materials while alumina-based samples preserved the metallic nickel state [START_REF] Jabbour | Factors affecting the long-term stability of mesoporous nickel-based catalysts in combined steam and dry reforming of methane[END_REF].

However, even if alumina appears to be a better support than silica, Ni-supported on Al2O3 still faces deficiencies due to the acidic feature of the support in addition to the common problem encountered on any support that is the high reaction temperature triggering agglomeration of Ni particle that in turn develops carbon. At the present moment, efforts are being made to adjust the Ni-Al2O3 stability and synthesize a material that could persist the high reaction temperatures with desirable small, active and stable metal nanoparticles.

I.4.2. Recent developments in stabilization of Ni-Al2O3 catalysts

The unstable status of Ni is related to its low Tammann temperature (0.5Tmelting point:590 °C) increasing the mobility of the bulk atoms and making the catalyst more susceptible to sintering [START_REF] Bian | A Review on Bimetallic Nickel-Based Catalysts for CO 2 Reforming of Methane[END_REF][START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF]. Thus, deactivation of Ni-Al2O3 catalysts very often arises from nickel sintering or thermal agglomeration. Sintering induces a loss of active surface by structurally modifying the catalyst with crystalline growth of the active phase. This phenomenon is strongly favored in DRM by the elevated temperature applied (the reasons will be detailed in section II.2.1.1) and the atmosphere (H2, O2) in contact with the catalyst. Carbon residues developed from CH4 decomposition side reaction, as mentioned earlier, deactivate also the catalyst by either covering the active sites and/or by blocking the pores [START_REF] Rostrup-Nielsen | Concepts in syngas manufacture[END_REF]. Many researchers attributed a correlation between nickel sintering and carbon formation suggesting that large nickel particles have higher carbon deposition rate during methane decomposition [START_REF] Chen | Carbon deposited on Ni/Ce{single bond}Zr{single bond}O isooctane autothermal reforming catalysts[END_REF]. Taking a look into the Table, several researchers tested different methods of Ni deposition on commercial Al2O3 to enhance its performance. For instance, some used the wet impregnation (W.I.) method to add nickel. However, this technique usually results in a weak interaction between the Ni active phase and the Al2O3 support forming large nickel particles that triggered filamentous carbon during DRM reaction [START_REF] Mcfarlane | The application of inelastic neutron scattering to investigate the "dry" reforming of methane over an alumina-supported nickel catalyst operating under conditions where filamentous carbon formation is prevalent[END_REF][START_REF] Goula | Nickel on alumina catalysts for the production of hydrogen rich mixtures via the biogas dry reforming reaction: Influence of the synthesis method[END_REF][START_REF] Fu | Hydrogen production from bio-oil model compounds dry (CO 2 ) reforming over Ni/Al 2 O 3 catalyst[END_REF][START_REF] Schwengber | Methane dry reforming using Ni/Al2O3 catalysts: Evaluation of the effects of temperature, space velocity and reaction time[END_REF]. Incipient wetness impregnation (I.W.I) was developed to further improve the dispersion and is quite frequently used in many reforming application. It is based on adding a metal-containing solution with volume equivalent to the catalyst support volume [START_REF] Xu | A comparison of Al2O3 and SiO2 supported Ni-based catalysts in their performance for the dry reforming of methane[END_REF][START_REF] Silverwood | Persistent species formed during the carbon dioxide reforming of methane over a nickel-alumina catalyst[END_REF]. The nickel particle size could be as small as 4 nm using this method [START_REF] Xu | A comparison of Al2O3 and SiO2 supported Ni-based catalysts in their performance for the dry reforming of methane[END_REF], however Ni generally sinters during the high reaction temperature making it vulnerable to be encapsulated and detached from the Al2O3 matrix by carbon, decreasing drastically CH4 conversion from 80 to 50% as seen in the table. Other techniques applied like the equilibrium deposition filtration technique (EDF) [START_REF] Goula | Nickel on alumina catalysts for the production of hydrogen rich mixtures via the biogas dry reforming reaction: Influence of the synthesis method[END_REF], and electrospinning [START_REF] Liu | Synthesis of a highly dispersed Ni/Al 2 O 3 catalyst with enhanced catalytic performance for CO 2 reforming of methane by an electrospinning method[END_REF], achieved nickel particle sizes of 12 and 6.5 nm respectively. Despite being innovative routes, they do not fit the industry requirements, like the W.I. and I.W.I. that are very simple to apply, and control of many parameters are needed and expensive. Not to mention Chapter I that for the case of electrospinning synthesized catalyst, a high amount of carbon was formed after 300 minutes of DRM reaction only [START_REF] Liu | Synthesis of a highly dispersed Ni/Al 2 O 3 catalyst with enhanced catalytic performance for CO 2 reforming of methane by an electrospinning method[END_REF].

I.4.2.1.2. Simultaneous Ni addition to Al2O3

In order to further minimize the particle size and enhance its distribution, few researchers have suggested the direct incorporation of Ni metal into the Al2O3 framework (

I.4.2.2. Alterations in Al2O3 nature

Trials were also made to increase the surface area of Al2O3 (Table in the Appendix) and further enhance the nickel dispersion and catalytic properties. Thus, the type of Al2O3 support was changed from being commercial, to nanoflakes hollow hierarchical spheres [START_REF] Zhang | Facile synthesis of hollow hierarchical Ni/γ-Al2O3 nanocomposites for methane dry reforming catalysis[END_REF], nanosheets/nanoplates [START_REF] Sun | Carbon dioxide reforming of methane over nanostructured Ni/Al2O3 catalysts[END_REF], over coated oxide by ALD [START_REF] Littlewood | Ni-Alumina Dry Reforming Catalysts: Atomic Layer Deposition and the Issue of Ni Aluminate[END_REF], and mesoporous oxide [START_REF] Wang | Methane Dry Reforming over Coke-Resistant Mesoporous Ni-Al2O3 Catalysts Prepared by Methane Dry Reforming over Coke-Resistant Mesoporous Ni-Al 2 O 3 Catalysts Prepared by Evaporation-Induced Self-Assembly Method[END_REF]. The Ni deposited on hollow hierarchical spheres Al2O3 were in strong interaction with good dispersion, but the formation of coke during DRM was not eliminated [START_REF] Zhang | Facile synthesis of hollow hierarchical Ni/γ-Al2O3 nanocomposites for methane dry reforming catalysis[END_REF]. A comparison was done between nanosheets and nanoplates alumina-based samples. The study showed that the nanosheets sample exhibited higher activity due to possessing a higher surface area (245 m 2 .g - 1 compared to 48 m 2 .g -1 for the nanoplates) enhancing therefore Ni dispersion and metal-support interaction contributing to a better stability over 300 minutes time on stream [START_REF] Sun | Carbon dioxide reforming of methane over nanostructured Ni/Al2O3 catalysts[END_REF]. Kim W. et al. [START_REF] Kim | Coke tolerance of Ni/Al2O3 nanosheet catalyst for dry reforming of methane[END_REF] [START_REF] Wang | Methane Dry Reforming over Coke-Resistant Mesoporous Ni-Al2O3 Catalysts Prepared by Methane Dry Reforming over Coke-Resistant Mesoporous Ni-Al 2 O 3 Catalysts Prepared by Evaporation-Induced Self-Assembly Method[END_REF] process with the presence of a soft template (CTAB or P123) as structuring agent (Figure I-13) [START_REF] Mo | Preparation of porous Al 2 O 3 by template method and its application in Ni-based catalyst for CH 4 /CO 2 reforming to produce syngas[END_REF][START_REF] Xu | Ordered mesoporous alumina supported nickel based catalysts for carbon dioxide reforming of methane[END_REF]. Compared with the first pathway requiring more than one impregnation and calcination steps, the evaporation-induced self-assembly (EISA) method was the most successful one with Al2O3 support carrying ordered hexagonal pores and high surface area (225 m 2 .g -1 ) available for Ni species during impregnation. This structure formed rigid, stable and high dispersion of Ni metals inside the mesoporous cage of alumina limiting the nickel sintering to reach a cluster size of 15 nm and resulting in highly active amorphous carbon.

I.4.2.3. Doped Ni-Al2O3 catalyst

Besides developing monometallic catalysts with different support modifications and Ni deposition methods, studies were done also on alloying nickel with a second metal to alter the catalyst surface properties (not presented in the Table Appendix). This second metal forms bimetallic materials where Ni is alloyed to a noble [START_REF] Bian | A Review on Bimetallic Nickel-Based Catalysts for CO 2 Reforming of Methane[END_REF][START_REF] Garcia-Dieguez | Nanostructured Ptand Ni-based catalysts for CO 2 -reforming of methane[END_REF][START_REF] Mu | Applied Surface Science Reversible surface structural changes in Pt-based bimetallic nanoparticles during oxidation and reduction cycles[END_REF][START_REF] Menning | Regenerating Pt -3d -Pt model electrocatalysts through oxidationreduction cycles monitored at atmospheric pressure[END_REF][START_REF] Menning | Thermodynamics and kinetics of oxygen-induced segregation of metals in and bimetallic structures[END_REF][START_REF] Menning | General trend for adsorbate-induced segregation of subsurface metal atoms in bimetallic surfaces[END_REF][START_REF] Deckers | Initial growth and surface alloy formation of Pt on Ni ( 111 )[END_REF][START_REF] Li | Controlled Surface Segregation Leads to Efficient Coke-Resistant Nickel / Platinum Bimetallic Catalysts for the Dry Reforming of Methane[END_REF][START_REF] Ozkara-Aydinoglu | CO2 reforming of methane over PtNi/Al2O3 catalysts : Effects of catalyst composition , and water and oxygen addition to the feed[END_REF][START_REF] Mahoney | The effects of Pt addition to supported Ni catalysts on dry (CO2) reforming of methane to syngas[END_REF][START_REF] Ocsachoque | Rh-Ni/CeO2-Al2O3 catalysts for methane dry reforming[END_REF][START_REF] Al-Fatesh | ScienceDirect Rh promoted and ZrO2/Al2O3 supported Ni/Co based catalysts : High activity for CO2 reforming , steam CO2 reforming and oxy CO2 reforming of CH4[END_REF][START_REF] Luisetto | Ni supported on γ -Al 2 O 3 promoted by Ru for the dry reforming of methane in packed and monolithic reactors[END_REF][START_REF] Foppa | Intrinsic reactivity of Ni , Pd and Pt surfaces in dry reforming and competitive reactions : Insights from first principles calculations and microkinetic modeling simulations[END_REF][START_REF] Ma | Catalysis Science & Technology bimetallic Pd -Ni catalysts for methane dry[END_REF][START_REF] Gould | General Enhanced dry reforming of methane on Ni and Ni-Pt catalysts synthesized by atomic layer deposition[END_REF] or a transition metal [START_REF] Foo | Evaluation of lanthanide-group promoters on Co -Ni/Al2O3 catalysts for CH4 dry reforming[END_REF][START_REF] Mehdi | Hydrogen production via CO2 -reforming of methane over Cu and Co doped Ni/Al2O3 nanocatalyst : impregnation versus solgel method and effect of process conditions and promoter[END_REF][START_REF] Illa | Low metal content Co and Ni alumina supported catalysts for the CO 2 reforming of methane[END_REF][START_REF] Liao | An indepth understanding of the bimetallic effects and coked carbon species on an active bimetallic Ni(Co)/Al2O3 dry reforming catalyst[END_REF][START_REF] Huang | Ordered mesoporous CoO-NiO-Al2O3 bimetallic catalysts with dual confinement effects for CO2 reforming of CH4[END_REF][START_REF] Horlyck | Elucidating the impact of Ni and Co loading on the selectivity of bimetallic NiCo catalysts for dry reforming of methane[END_REF][START_REF] Ji | Hydrogen production from CH 4 dry reforming over bimetallic NiCo/Al2O3 catalyst[END_REF][START_REF] Gonzalez-Delacruz | In Situ XAS Study of Synergic Effects on NiCo/ZrO 2 Methane Reforming Catalysts[END_REF][START_REF] Tu | Consequences of Surface Oxophilicity of Ni , Ni-Co , and Co Clusters on Methane Activation[END_REF] having distinctive bi-functional assets when in contact with Al2O3 support. However, the bimetallic systems are outside the scope of this thesis, where Ni is the focus since it has better performance than any other transition metal even when used alone and it is way cheaper than noble metals that is being avoided. Same reasoning goes for tri-metallic Ni-alumina catalysts [START_REF] Al-Fatesh | Suppression of carbon formation in CH4-CO2reforming by addition of Sr into bimetallic Ni-Co/γ-Al2O3catalyst[END_REF][START_REF] Son | Study on coke formation over Ni/γ-Al2O3, Co-Ni/γ-Al2O3, and Mg-Co-Ni/γ-Al 2O3 catalysts for carbon dioxide reforming of methane[END_REF][START_REF] Zhang | Effect of Gd Promoter on the Structure and Catalytic Performance of Mesoporous Ni/Al 2 O 3 -CeO 2 in Dry Reforming of Methane[END_REF][START_REF] Charisiou | Syngas production via the biogas dry reforming reaction over nickel supported on modified with CeO2 and/or La2O3 alumina catalysts[END_REF][START_REF] Yang | Promotional effect of La2O3 and CeO2 on Ni/γ-Al2O3 catalysts for CO2 reforming of CH4[END_REF][START_REF] Khajeh Talkhoncheh | Hydrogen production over ternary supported Ni/Al2O3-clinoptilolite-CeO2 nanocatalyst via CH4/CO2 reforming: Influence of support composition[END_REF][START_REF] Chen | High carbon-resistance Ni/CeAlO3-Al2O3 catalyst for CH4/CO2 reforming[END_REF][START_REF] Ramezani | Promotional effect of Mg in trimetallic nickelmanganese-magnesium nanocrystalline catalysts in CO2 reforming of methane[END_REF][START_REF] Yahyavi | Ultrasound-assisted synthesis and physicochemical characterization of Ni-Co/Al2O3-MgO nanocatalysts enhanced by different amounts of MgO used for CH4/CO2 reforming[END_REF][START_REF] Abdollahifar | Sono-synthesis and characterization of bimetallic Ni-Co/Al2O3-MgO nanocatalyst: Effects of metal content on catalytic properties and activity for hydrogen production via CO2 reforming of CH4[END_REF][START_REF] Ghani | Dry reforming of methane for syngas production over Ni-Co-supported Al2O3-MgO catalysts[END_REF][START_REF] Mahboob | Sonochemically preparation and characterization of bimetallic Ni-Co/Al2O3-ZrO2 nanocatalyst: Effects of ultrasound irradiation time and power on catalytic properties and activity in dry reforming of CH4[END_REF][START_REF] Sharifi | Syngas production via dry reforming of CH4 over Co-and Cu-promoted Ni/Al2O3-ZrO2 nanocatalysts synthesized via sequential impregnation and sol-gel methods[END_REF][START_REF] Rahemi | The effect of the calcination temperature on the physicochemical properties and catalytic activity in the dry reforming of methane over a Ni-Co/Al2O3-ZrO2 nanocatalyst prepared by a hybrid impregnation-plasma method[END_REF][START_REF] Akbari | CeO2 Promoted Ni-MgO-Al2O3 nanocatalysts for carbon dioxide reforming of methane[END_REF][START_REF] Faria | Hydrogen production through CO2 reforming of methane over Ni/CeZrO2/Al2O3 catalysts[END_REF] that make the system more complex, expensive and far from the purpose required.

Adding dopants to reduce and remove carbon deposition on Ni-Al2O3 is an interesting approach. Thus, we focused on basified dopants and magnesium specifically since it is less expensive than other dopants seen in literature like Ce [START_REF] Wang | One-pot Synthesis of Ordered Mesoporous NiCeAl Oxide Catalysts and a Study of Their Performance in Methane Dry Reforming[END_REF][START_REF] Charisiou | Studying the stability of Ni supported on modified with CeO 2 alumina catalysts for the biogas dry reforming reaction[END_REF][START_REF] Chein | ScienceDirect Syngas production via dry reforming of methane over CeO 2 modified Ni / Al 2 O 3 catalysts[END_REF][START_REF] Aghamohammadi | Sequential impregnation vs . sol-gel synthesized Ni / Al 2 O 3 -CeO 2 nanocatalyst for dry reforming of methane : Effect of synthesis method and support promotion[END_REF][START_REF] Jamal | Sol-gel vs . impregnation preparation of MgO and CeO 2 doped Ni / Al 2 O 3 nanocatalysts used in dry reforming of methane : Effect of process conditions , synthesis method and support composition[END_REF][START_REF] Laosiripojana | Catalytic dry reforming of methane over high surface area ceria[END_REF][START_REF] Wang | Facile Route for Synthesizing Ordered Mesoporous Ni -Ce -Al Oxide Materials and Their Catalytic Performance for Methane Dry Reforming to Hydrogen and Syngas[END_REF][START_REF] Rahemi | Synthesis and physicochemical characterizations of Ni/Al 2O3-ZrO2 nanocatalyst prepared via impregnation method and treated with non-thermal plasma for CO 2 reforming of CH4[END_REF][START_REF] Shin | Dry reforming of methane over Ni / ZrO2-Al2O3 catalysts : Effect of preparation methods[END_REF], La [START_REF] Soloviev | Carbon dioxide reforming of methane on monolithic Ni/Al 2O3-based catalysts[END_REF][START_REF] Hassan | Understanding the role of lanthanide promoters on the structureactivity of nanosized Ni/␥ -Al2O3 catalysts in carbon dioxide reforming of methane[END_REF][START_REF] Al-Fatesh | Role of La2O3 as Promoter and Support in Ni/Ȗ-Al2O3 Catalysts for Dry Reforming of Methane[END_REF][START_REF] Pegios | Comparative study on Lapromoted methane dry reformingspray drying for enhanced nickel dispersion and strong metalsupport interactions[END_REF][START_REF] Charisiou | Investigating the correlation between deactivation and the carbon deposited on the surface of Ni/Al2O3 and Ni/La2O3 -Al 2O3 catalysts during the biogas reforming reaction[END_REF][START_REF] Shamskar | Preparation and characterization of ultrasoundassisted co-precipitated nanocrystalline La-, Ce-, Zrpromoted Ni-Al2O3 catalysts for dry reforming reaction[END_REF][START_REF] Jiménez-González | Structural characterisation of Ni/alumina reforming catalysts activated at high temperatures[END_REF], and Mn [START_REF] Guo | CO 2 reforming of methane over Mn promoted Ni / Al 2 O 3 catalyst treated by N 2 glow discharge plasma[END_REF][START_REF] Chem | Reforming of Methane by CO2 over Bimetallic Ni-Mn / γ-Al2O3 Catalyst[END_REF].

Basic metal oxides have received a great attention in DRM because of their high Lewis basicity that enhances chemisorption of CO2 (acidic gas) and improves coke resistance. Barium, calcium and magnesium were applied in dry reforming [START_REF] Tan | Ni-Mg-Al solid basic layered double oxide catalysts prepared using surfactant-assisted coprecipitation method for CO<inf>2</inf> reforming of CH4[END_REF][START_REF] Alipour | Effect of Ni loadings on the activity and coke formation of MgO-modified Ni/Al2O3 nanocatalyst in dry reforming of methane[END_REF][START_REF] Min | Carbon dioxide reforming of methane on Ni-MgO-Al2O3 catalysts prepared by sol-gel method: Effects of Mg/Al ratios[END_REF][START_REF] Herrera | NiBa catalysts for CO2-reforming of methane[END_REF][START_REF] Ranjbar | Dry reforming reaction over nickel catalysts supported on nanocrystalline calcium aluminates with different CaO /Al2O3 ratios[END_REF]. However, Ba has been barely used because Ni tends to sinter in presence of Barium forming big aggregates and barium carbonates. It may also lead to nickel re-oxidation during handling of catalyst and carbon deposition with large particle size of 20 nm [START_REF] Herrera | NiBa catalysts for CO2-reforming of methane[END_REF]. Ni-doped calcium alumina catalyst revealed in a study an important role in inducing high basicity but did not eliminate the occurrence of carbon and the Ni nanoparticle size obtained was quite large (21 nm) resulting in catalyst deactivation [START_REF] Ranjbar | Dry reforming reaction over nickel catalysts supported on nanocrystalline calcium aluminates with different CaO /Al2O3 ratios[END_REF]. Ni alloyed with Mg on commercial alumina sample provided the highest CH4 and CO2 conversions compared to Ba and Ca supported samples [START_REF] Alipour | Effect of alkaline earth promoters (MgO, CaO, and BaO) on the activity and coke formation of Ni catalysts supported on nanocrystalline Al2O3 in dry reforming of methane[END_REF][START_REF] Alipour | Effects of support modifiers on the catalytic performance of Ni/Al 2O3 catalyst in CO2 reforming of methane[END_REF].

The author explained the difference in performances by a study done on Ni supported on MgO, CaO, and BaO [START_REF] Ruckenstein | Carbon dioxide reforming of methane over nickel/alkaline earth metal oxide catalysts[END_REF]. CO temperature programed desorption results (TPD) showed in this study that Mg promoted sample produces less amount of CO2 from CO compared to Ba and Ca samples. This means that the Mg-containing sample is more resistant to the disproportionation side reaction 2CO → C + CO2 over Ni sites implying higher stability. In addition, MgO is the only face-centered cubic type promoter with lattice parameter (a= 4.21 Å, A-B= 2.11 Å) and bond distance close to NiO (a= 4.19 Å, A-B= 2.10 Å) [START_REF] Ruckenstein | Carbon dioxide reforming of methane over nickel/alkaline earth metal oxide catalysts[END_REF]154]. Due to this similarity, a solid solution can be formed retarding reduction of NiO to Ni strengthening the interaction with the Al2O3 support and reducing Ni segregation. It is also important to deem in mind that NiO amount should not be too low compared to Mg since only a very small amount of Ni will segregate at the solid-gas interface during reduction curtailing the catalyst activity.

Ni-Mg-Al2O3 used so far in DRM still faces certain deficiencies that are elaborated in Chapter III.

I.5. Thesis objectives and methodology

Based on this review on dry reforming of methane and on recent advances of Nialumina, obtaining highly active and stable catalyst has not been reached. Studies have demonstrated that different parameters during the material preparation can greatly influence its behavior during catalytic test. Needless to say, the one-pot method seems to be the best solution for Ni addition within Al2O3 support. The shape of alumina has to be constructed in a way that ensures occlusion and confinement of nickel particles within the support structure to avoid sintering as much as possible. MgO is the most suitable basic dopant to Ni-Al2O3 suppressing carbon formation. In this context, the objectives of this thesis is to take advantage of the different positive routes described above to go even further in the elaboration of highly active and stable catalyst for DRM. The proposed approaches are the following:

1. In chapter 3: To combine Mg-dopant with Ni precursors by one-pot within the synthesis of mesoporous alumina catalyst. Altering the EISA method, Ni, Mg and Al precursors are all mixed together to synthesize highly uniform dispersed Ni and Mg among the mesoporous framework of alumina. This technique takes advantages of two features: first, Ni is trapped within the parallel ordered mesopores that cease particle sintering, and second, the presence of MgO suppresses carbon formation by enhancing the chemisorption and activation of CO2. A target will be also to find the Chapter I optimum Mg loading that seems from bibliography (as will be introduced in the chapter) to play a determining role.

In chapter 4:

To adopt a novel technique derived from one-pot and different type of alumina support as a new way of occluding Ni inside the pores. Thus, we chose an Al-containing Metal-organic framework (MOF) as sacrificial precursor to prepare the catalysts. This route combines the advantages of high porosity, high surface area and intimate mixing between the nickel and support components to synthesize highly stable and active Ni/alumina catalysts for DRM.

In chapter 5:

To extrapolate the study of the catalytic performances of the most interesting synthesized mesoporous Ni-alumina based materials (tested in chapters 3 and 4 in DRM) to another reaction of potential interest as well, namely CO2 methanation reaction.

In chapter 6:

To move to extended and more realistic reactant mixtures for methane reforming reaction after finishing from the conventional model mixtures. Treatment of biomass wastes do not only result in CH4 and CO2 formation but includes H2 and CO. As a result, a product gasification mixture composition from a Lebanese industry containing CH4, CO2, H2 and CO was treated under Ni-Mg-Al2O3 based catalysts in reforming reaction to investigate the efficiency of our catalysts under harsh conditions.

To achieve our purpose, many characterization techniques were applied and used in order to analyze the materials prepared. They will be presented first, in chapter 2 where the equipment and experimental conditions for characterizations in addition to the set-up of both DRM and CO2 methanation catalytic tests and considerations on thermodynamic equilibrium are elaborated. Then, above-mentioned chapters (3 to 6) will systematically recall some Chapter I Literature Review 32 bibliographic elements specific to the treated topic, then introduce the conditions of preparation of the studied samples, analyze (if relevant) their physicochemical properties (both in calcined and reduced states) and finally discuss catalytic features.

II. Chapter 2: Technical Part

Chapter II

Technical Part

The methods adopted for synthesizing the materials employed in this thesis will be presented in the respective chapters where these materials will be studied. The aim of the present chapter is to give a rigorous explanation about the characterization techniques used to determine the physico-chemical properties of the samples prepared embracing the structure, the texture, the reducibility and the state of the active phase in the catalyst system. After explaining each technique used, the catalytic schemes of both dry reforming and CO2 methanation will be elaborated in details along with the appropriate conditions used in each reaction. This last part will also include a discussion on thermodynamic equilibrium which has to be considered to estimate catalysts performances.

II.1. Characterization techniques

II.1.1. Physicochemical techniques II.1.1.1. N2-Sorption

Principle of the technique:

This technique provides information about surface area, pore volume and pore diameter. It is based on a gas-solid adsorption phenomenon involving intermolecular forces (Van der Waals), analysed on a volumetric basis at constant temperature.

The volume of adsorbed gas (nitrogen in our case) is measured as a function of partial pressure (P/P0). The adsorption process is followed by a desorption that coexists to represent the opposite operation in which the amount of adsorbed species decreases. The recorded values at increasing and decreasing P/P0 allow for the plotting of adsorption-desorption isotherms. If the adsorption and desorption curves do no coincide, a hysteresis loop appears. The shape of the isotherms informs on the types of pores present in the materials that can include macropores (pore width Figure II-1: Classification of physisorption isotherms (left graphs) and hysteresis loops (right graphs) [START_REF] Kaufman | X-ray diffraction[END_REF] > 50 nm), mesopores (2 < pore width < 50 nm) and micropores (pore width < 2nm) [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF][START_REF] Storck | Characterization of micro-and mesoporous solids by physisorption methods and pore-size analysis[END_REF].

There exist 6 distinct types of isotherms identified by IUPAC (International Union of Pure and Applied Chemistry), among which three of them only will be of interest in our study (Figure II-1): type I is dedicated to microporous materials with (a) narrow micropores (< 1 nm in diameter) and (b) wide micropores (with diameter between 1 and 2.5 nm) giving capillary condensation at very low pressure, type II is typical of nonporous/macroporous adsorbents characterized by monolayer then multilayer adsorption on surfaces, and type IV is related to mesoporous samples with the presence of capillary condensation in the mesopores giving a hysteresis. For this latter type, many different kinds of hysteresis loops have been reported

where the main ones are illustrated in Figure II-1. To elaborate, H1 loop is given by materials with a narrow range of uniform mesopores, H2 loop is found in more complex pore structure where network effect is crucial associated to pore blocking, and the unusual type H5 hysteresis could be found in plugged hexagonal template silica having open and partially blocked mesopores. H3 resembles a type II isotherm that belongs to plate-like particles such as clays or to macropore networks not fully filled with pore condensate. H4 loop possesses an adsorption branch that is a composite of types I and II and is found in zeolites and micro-mesoporous carbons.

Procedure: Nitrogen adsorption-desorption experiments were conducted on an ASAP 2020 Micromeritics apparatus that contains degas and analysis ports. 70-80 mg of the sample (support, materials calcined and reduced) to be analysed was enclosed in a glass tube and degassed under vacuum for 2 hours at 250 °C with a rate of 10 °C.min -1 in order to remove all impurities (H2O, CO2) from the surface of the sample prior to N2-sorption analysis. After degassing, the sample was placed in a liquid nitrogen bath (-196 °C). The adsorption isotherms were obtained by introducing stepwise known volumes of N2 into the cell (adsorption) then evacuating the cell progressively (desorption), and plotting the volumes as a function of the equilibrium pressure P/P0 at each step in the range of 0 to 1.

Derived results:

The values of surface area, pore volume and pore diameter were obtained directly from the machine software that did automatically the calculations: a) Surface area (Brunauer-Emmet-Teller method): The conventional linear equation applied is the following:

P P0 n×(1- P P0 ) = 1 nm×C + C-1 nm×C × P P0
where,

 n is the volume of gas adsorbed at relative P/P0 at standard temperature and pressure (0 °C, 760 mmHg) and is deduced from the isotherm curve (y-axis)

 P/P0 is the total pressure over the vapor pressure of the adsorbate at experimental temperature conditions and is deduced from the isotherm curve (x-axis)

 nm is the specific monolayer capacity, calculated as specified below  C is the BET constant related to heat of adsorption and this parameter is exponentially related to the energy of monolayer adsorption.

A convenient way to derive nm is to plot P/P0/(n × (

Principle of the technique:

Derived results:

The nature of the nano-crystalline phases in the samples are determined by a direct comparison of the obtained diffraction patterns to standard powder XRD files found in the International Center for Diffraction Data (ICDD). The particle size of the crystal domains is calculated based on the full width at half maximum using Scherrer equation: D= K.λ/β.cosθ where, D is the average nanocrystals size (A), K is the shape factor (0.9 for spherical particles), λ is the X-ray wavelength (1.5405 A), β is the full width at half maximum (FWHM) calculated from the Warren equation: β=√(β 2 measured -β 2 reference) with βreference=0.18°, and θ is the peak position (radians).

II.1.1.3 X-ray fluorescence (XRF)

Principle of the technique: X-ray fluorescence (XRF) is a technique of non-destructive chemical analysis (Al and Ni in our case). This is made possible by the respond of atoms to the interacted X-radiation. When the sample is illuminated by an intense X-ray beam (incident beam), the sample is excited by the primary X-radiation and emits X-rays along a spectrum of wavelengths which characteristics depend on the type of atoms present in the material. To Procedure: XRF was performed on an XEPOS spectrometer (Spectro Ametek).

Quantitative data were obtained using the MicroPowder method combining the well-established fundamental parameters approach to XRF spectrometer calibration (calibration curve previously established from NiO and alumina mixtures with known chemical compositions) with automatic correction for matrix effects by using the Compton backscatter information from the sample to calculate the matrix interference. This technique can be used also in diffraction mode opening the feature of determining reticular distances and crystalline zones that facilitates identifying the nature of the particle present in correlation with the International Center for Diffraction Data (ICDD) sheets. The chemical composition can be also determined when enough energy is present in the electron beam ejecting a core-level electron energy from an atom that is further filled by a higher-level electron energy. The difference between these two energy levels emits an X-ray energy that is a Derived results: For the mesoporous materials, when the organized alumina grains are parallel to the electron beam, organized parallel channels are detected, whereas, the perpendicular orientation of the grain to the beam shows hexagonal pore apertures at high magnification. Average sizes of nickel particles (calcined, reduced and spent materials), when visible, were estimated using the ImageJ software and taking at least 500 particles into consideration. This technique also allows for visualization of carbon deposits if present.

II.1.2.2. Scanning electron microscopy (SEM)

Principle of the technique: SEM provides information on the external surface topology of the grains. A specimen is scanned with a focused beam of electron that interacts with the electrons of the material observed generating pictures to study the surface topography (secondary electrons imaging) but also to check existence of external nano-metal particles (backscattered electrons imaging).

Procedure: Scanning Electron Microscopy (SEM) images were taken on a Hitachi SU-70 SEM-FEG microscope (LaB6 Gun) with an electron acceleration tension of 7 kV. The samples were deposited on the analysis Al support and observed in mixed mode (70% of secondary electrons and 30% of retro-diffused signals). given incident energy of the X-ray beam (hν): hν = Ek + EB [START_REF] Knop-Gericke | X-ray Photoelectron Spectroscopy. An Introduction to Principles and Practices. By Paul van der Heide[END_REF]. The XPS curve is plotted using the emitted photoelectron intensity as a function of the binding energy.

Procedure: An Omicron (ESCA+) X-ray photoelectron spectrometer with Al Kα (hν =1486.6 eV) X-ray source having a 300 W electron beam power was used to visualize the XPS spectra. The sample was first evacuated under a vacuum of less than 10 -10 mbar, then a monochromatic X-ray (Al Kα) irradiated the sample and excited the electrons.

Derived results:

The surface composition of the material is determined by attributing the observed energy peaks to a specific element and quantifying their area using the Casa XPS software.

II.1.3.2. Raman spectroscopy

Principle of the technique:

The sample is illuminated with a monochromatic laser beam that creates a scattered light upon interaction with the sample molecules. The strike monochromatic radiation scatters in all direction after the collision with the sample. Most of the scattered radiation acquire similar frequency to that of the incident one (Rayleigh scattering)

A very small fraction of the scattered light has a distinct frequency than that of the incident light which allows the construction of the Raman spectrum. With higher incident frequency than that of the scattered one, Stokes lines are seen in the Raman spectrum whereas anti-stokes lines appear when the frequency of the scattered radiation is higher. Conventional Raman spectroscopy measures Stokes bands because they are more intense than anti-stokes bands at normal temperature. The latter are usually used to measure fluorescing samples that interfere with Stoke bands. The Raman spectrum appears in the form of intensity as a function of wavenumber shift that is a characteristic of a chemical component. The Raman spectrum provides further information about the quantity of the component present inferred from the intensity of the spectrum, stress and strain states identified from the peak shift, the degree of crystallinity revealed by the width of the peak and crystal symmetry and orientation detected from the polarization state [START_REF] Bumbrah | Raman spectroscopy -Basic principle, instrumentation and selected applications for the characterization of drugs of abuse, Egypt[END_REF].

Procedure: Raman measurements were conducted on a Kaiser microscope optical system (RXN1) containing a charge-coupled detector. The laser beam ( = 785 nm) was focused by adjusting the microscope to an objective of 50X long working distance (8 mm) lens.

The spectra obtained were collected over a range of 150-3200 cm -1 with a 10 mW laser power, 10 seconds acquisition time and 30 accumulation per each spectrum.

Derived results:

In our case, Raman was adopted to identify the presence of carbon and its properties on the spent catalysts after DRM reaction. Graphene and graphite in particular have stacks of sp 2 bonded planar graphene sheets that give a Raman spectrum with a main G band at around 1585 cm -1 . Carbon nanotubes are composed of rolled up graphene sheets in the form of hollow tubes that induce an additional D band that is located at 1350 cm -1 . The presence of this latter peak depicts a defect in the structure of carbon formed and lessen its degree of crystallinity. This assumption is more estimated by the calculation of the intensity ratio between these 2 bands. For instance the lower the ID/IG ratio, the higher is the degree of crystallinity of the carbon and the more difficult becomes its removal. Procedure: Measurements were performed on an Autochem 2920 equipment (Micromeritics) (Figure II-4). The calcined samples (80 mg) were deposited on a quartz wool inside a U-shaped quartz reactor and the H2 consumption profiles were registered during heating from 25 °C till 900 °C with a rate of 10 °C.min -1 and a 5 vol% H2/Ar flow of 30 ml.min - 1 at 1 atm.

Derived results:

The amount of hydrogen consumed by each material was evaluated by integrating the area under the TCD curve over the reduction temperature range. Different reduction peak temperatures could give an idea about the interaction between the active phase (Ni) and the alumina support used with higher reduction temperature triggering stronger metalsupport interaction. II.1.4.2. Thermo-gravimetric analysis (TGA) coupled with differential thermal analysis (DTA) Principle of the technique: Thermogravimetric analysis (TGA) coupled with differential thermal analysis (DTA), depicts weight changes (TGA) and exothermic or endothermic heat flow (DTA) as a function of both time and temperature during heating. The increase in temperature decomposes components that their corresponding weight loss/gain can be calculated and the components nature can be as well identified by mass spectrometry (MS).

Procedure: Experiments were performed on a TA SDT Q600 instrument working in horizontal mode. One of the two alumina crucibles was filled with about 10 mg of sample while the other (reference) crucible was kept empty. Both were located in the heating chamber and equipped with thermocouples to follow simultaneously weight and heat flow changes during heating, carried out in air flow (50 ml.min -1 ) from room temperature to 900 °C with a ramping rate of 5 °C.min -1 . A capillary placed at the exit of the chamber and connected to a Fisons VG Thermolab Mass Spectrometer was also used to perform continuous mass spectrometry (MS) analysis of the exhaust gases.

Derived results:

We used this technique for the detection in our samples of carbon and nitrate species able to be oxidatively decomposed upon heating in air accompanied by a weight loss. When present, the associated exothermic behavior was detected by the positive increase of the heat flow and MS was used to identify the nature of the evolved gaseous products.

II.1.4.3. Hydrogen chemisorption

II.2. Experimental catalytic tests set-ups

This part describes the equipment and conditions used to perform the methane reforming reaction (both dry reforming of methane alone, and reforming of waste pyrolysis mixture) and the CO2 methanation reaction. Before doing that, thermodynamic equilibrium is calculated for each type of reaction and the employed procedure is explained. This is important to understand the choice of experimental conditions (temperature) and to properly evaluate latter catalysts performances. In the second step, carbon is allowed to take place in DRM and therefore was added as an additional product component to H2, CO and H2O. A Gilson high performance liquid chromatography (HPLC) pump is present on the top of the unit to generate water if needed (not used in our case). A computer equipped with remote control system can be seen next to the MAR including communications through Ethernet capable of following continuously all variable parameters including pressure, temperature, hot box temperature, and gas flow rates. This MAR is connected online to an Inficon3000 gas chromatograph Micro-GC unit to analyze the reactor outlet (reactant and product gases). This Micro-GC is composed of thermal conductivity detectors (TCD) and two columns for gas composition detection: molecular sieve column (column A) adsorbs CH4 (retention time (RT): The products obtained go to a water trap placed next to the quartz reactor to condensate the H2O produced (CO2(g) + 4H2(g) → CH4(g) + 2H2O(g)) and the remaining gas passes through a 3ways valve either to an in-line flow meter to measure the outlet flow or to several gas detectors to analyze the product gas (dry base results). To this end, four gas IR detectors are available:

II.2.1. Conditions for

the Ultramat 23 Siemens one is used to detect low amount of COx gases (<1 vol% CO2, CO)

and three Guardian NG Edinburg sensors are available for higher ranges of CO2, CH4, and CO (<10 vol%). CO usually has a low selectivity, thus the low concentrations detector is needed.

Concerning the CO2 component, its concentration will be always higher than 5 vol% and therefore the reading is sufficient from the Guardian sensor only. The reactant gases (H2 and CO2) are regulated with mass flow controllers (Brooks) with 99.99% purity before reaching the reactor inlet. N2 gas (purity: 99.99%) is used as a diluent at the exit of the reactor to dilute the product gases before reaching the detectors in order to reach a concentration and a flow (500 ml.min -1 ) suitable for the apparatus sensitivity. and CO are calculated relying on the inlet and outlet molar flow rates following the formulas:

CO2 conversion (%)= FCO2,inlet-FCO2,outlet FCO2,inlet ×100 CH4 selectivity= FCH4,outlet FCO2,inlet-FCO2,outlet ×100 CO selectivity= FCO,outlet FCO2,inlet-FCO2,outlet ×100

III. Chapter 3: Effect of magnesium loading over ordered mesoporous Ni 5 -Mg y -Al 2 O 3 DRM catalysts

This chapter is an elaboration of one of the thesis aim that is to improve the stability of ordered Ni-alumina mesoporous catalyst for dry reforming of methane by addition of an appropriate amount of Mg to increase the basicity. Mg can act as a promoter by increasing the number of basic sites in the catalyst that strengthen chemisorption of CO2 and boost catalytic performances. The state of the art specific to mesoporous Ni-Mg-Al2O3 materials used for reforming reactions will be recalled and compared with the present work. Our methodology was to fix the nickel content and vary the MgO one in order to understand the origin of the existence of an optimum magnesium content, so it has been varied from 0 to 26 wt%. The method of preparation will be explained in details, followed by the physicochemical characterizations of the obtained materials. The resulting catalytic performances will be presented and explained in accordance with the physico-chemical properties.

III.1. Literature review

Several preparation methods have been used in the literature for the preparation of alumina-based catalysts containing both nickel and magnesium and were applied in DRM reaction with a CH4:CO2 ratio of 1: sol-gel [START_REF] Min | Carbon dioxide reforming of methane on Ni-MgO-Al2O3 catalysts prepared by sol-gel method: Effects of Mg/Al ratios[END_REF], two-step hydrothermal [START_REF] Zhang | Dry reforming of methane over Ni/MgO-Al 2 O 3 catalysts prepared by two-step hydrothermal method[END_REF], impregnation BET SA (m 2 .g -1 ) PD (nm) PV (cm 3 .g -1 )

T(°C), P(atm) Cw(mg), GHSV,CH4:CO2:N2 TOS (hr)

Conversions (%) C(s) (%) XCH4 XCO2
Ni-MgO-Al2O3 [148] Sol-gel 15 Ni-Mg-Al [172] Coprecipitation Ni-Mg-Al [173] Coprecipitation 10 SA : surface area, PD : pore diameter, PV : pore volume, Cw : catalyst weight, GHSV : gas hourly space velocity in L.g -1 .h -1 , TOS : time in stream, XCH4/XCO2 : conversions of CH4 and CO2 respectively, r,s : reduced and spent materials by the formation of a NiO-MgO solid solution. However, it still resulted in the formation of carbon filaments even if in less amount compared to the other impregnation sequences.

A
The authors suggest that the formation of carbon is due to the occurrence of a partial phase transformation of alumina from γ-Al2O3 to α-Al2O3 and MgAl2O4 spinel phase during calcination and reduction which alters the porous structure of the catalyst leading to a decrease in the surface area (56 m 2 .g -1 ) and altering the Ni-support interaction.

 The co-precipitation method led to catalysts with acceptable surface areas and Ni particle sizes ranging between 6.5 and 17 nm. Again, the method did not prevent the formation of

III.2.2. Synthesis procedure

A series of alumina based samples with 5 wt% nickel and different magnesium loading was prepared. The method used is the well-known one-pot evaporation induced self-assembly (EISA) method [START_REF] Morris | Ordered mesoporous alumina-supported metal oxides[END_REF][START_REF] Yuan | Facile Synthesis for Ordered Mesoporous γ-Aluminas with High Thermal Stability[END_REF] applied for synthesis of mesoporous Ni-Al2O3 materials. In our case, we modified this method to include magnesium within the synthesis. The procedure of the catalysts preparation steps is described in Ni5-Mg50-Al2O3 26 Ni5Mg50Al45 [START_REF] Meynen | Verified syntheses of mesoporous materials[END_REF][START_REF] Grosman | Capillary condensation in porous materials. Hysteresis and interaction mechanism without pore blocking/percolation process[END_REF]. Similar behavior has been seen in literature, where scientists proposed that the addition of nickel nitrate could ameliorate the ordered mesoporosity of the alumina support [START_REF] Morris | Ordered mesoporous alumina-supported metal oxides[END_REF] and contribute to larger mesopores by acting as sustained-released agents to maintain an acidic equilibrium environment. According to these authors, the nickel nitrate precursor contains crystallographic water that hydrolyzes and produces in-situ H + by being transformed into free water in nitrate.

The hydrolysis process of the nitrates can be also affected by the trace water in ethanol and the relative humidity in the air above the solution. The influence of the metal loading was also investigated using small angle XRD as seen in Figure III-6 A. Small angle X-Ray Diffraction has been widely applied on ordered mesoporous materials such as SBA-15 [START_REF] Karam | Tuning the properties of nickel nanoparticles inside SBA-15 mesopores for enhanced stability in methane reforming[END_REF], MCM-41 [START_REF] Kresge | Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism[END_REF] and Al2O3 [START_REF] Morris | Ordered mesoporous alumina-supported metal oxides[END_REF] to prove the presence of two-dimensional structure. Pure Al2O3 exhibits a strong diffraction peak around 0.7-0.9° and a weak diffraction peak around 1.5-1.7° characteristic of [START_REF] Luisetto | Ni supported on γ -Al 2 O 3 promoted by Ru for the dry reforming of methane in packed and monolithic reactors[END_REF] and [START_REF] Ji | Hydrogen production from CH 4 dry reforming over bimetallic NiCo/Al2O3 catalyst[END_REF] plane reflections, respectively. This behavior is an evidence of the presence of hexagonally ordered mesopores with p6mm symmetry [START_REF] Morris | Ordered mesoporous alumina-supported metal oxides[END_REF][START_REF] Yuan | Facile Synthesis for Ordered Mesoporous γ-Aluminas with High Thermal Stability[END_REF]. These reflection peaks are also detectable in Ni5-Mgy-Al2O3 samples having an Mg wt% range from 0 to 7 wt% and are more intense in the case of Ni5-Mg10-Al2O3, and Ni5-Mg15-Al2O3 which means that the hexagonal arrangement is highly uniform in the latter samples [START_REF] Erdem | ??ks??zo??lu, A. ????tak, Effect of calcination temperature on the structural and magnetic properties of Ni/SBA-15 nanocomposite[END_REF]. Magnesium (dissolution of magnesium nitrate into Mg 2+ ) triggers certain interaction between non-ionic surfactant P123 and metal ions (Ni 2+ ), leading to a better ordered structure than Ni effect [START_REF] Pan | Structural and surface properties of highly ordered mesoporous magnesium-aluminium composite oxides derived from facile synthesis[END_REF][START_REF] Wang | Synthesis, characterization and catalytic performance of MgO-coated Ni/SBA-15 catalysts for methane dry reforming to syngas and hydrogen[END_REF][START_REF] Wei | In-Situ Coating of SBA-15 with MgO: Direct Synthesis of Mesoporous Solid Bases from Strong Acidic Systems[END_REF]. An appropriate amount Mg 2+ Note that Ni5-Mg5-Al2O3 sample is always out of the trend followed by the other Mgbased materials. This could be due to the fact that this sample was firstly prepared and the preparation procedure was not fully optimized by that time, and it will be excluded from the rest of the comparison. The absence of crystallite domains suggests homogeneous elements distribution and high metal dispersion. Compared to other nickel-magnesium mesoporous alumina prepared using different methods [START_REF] Huang | Ordered mesoporous CoO-NiO-Al2O3 bimetallic catalysts with dual confinement effects for CO2 reforming of CH4[END_REF][START_REF] Min | Carbon dioxide reforming of methane on Ni-MgO-Al2O3 catalysts prepared by sol-gel method: Effects of Mg/Al ratios[END_REF][START_REF] Zhang | Dry reforming of methane over Ni/MgO-Al 2 O 3 catalysts prepared by two-step hydrothermal method[END_REF][START_REF] Roh | Carbon dioxide reforming of methane over Ni catalysts supported on Al 2O3 modified with La2O3, MgO, and CaO[END_REF]171,[START_REF] Hou | Meso-porous Ni/Mg/Al catalysts for methane reforming with CO2[END_REF] (Table III-1), the nickel particle size is here much smaller which assures that the one-pot method plays a crucial role in ensuring high nickel dispersion.

-Mg30-Al2O3 - - - - - - Ni5-Mg50-Al2O3 - - - - - -
Adding higher amount of Mg (15-26 wt%) contributes to the appearance of MgO (JPCDS: 01-079-9866) diffraction peaks. The size of the crystal domains is around <3 and 5 nm for Ni5-Mg30-Al2O3 and Ni5-Mg50-Al2O3, respectively (Table III-5) at 2θ= 63°. This reveals a disturbance in the Mg dispersion in the samples with high Mg loadings. NiO species with mean sizes high enough to become detectable might be also present as deduced from the peaks at 2θ equal to 37, 43 and 63° but they however superpose those of MgO having similar positions, making their identification questionable. dark field (HAADF) observations were performed on selected samples using ultra-thin microtomic sections for higher quality and better identification of porosity. NiO species [START_REF] Xu | Ordered mesoporous alumina supported nickel based catalysts for carbon dioxide reforming of methane[END_REF][START_REF] Charisiou | Synthesis gas production via the biogas reforming reaction over Ni/MgO-Al2O3 and Ni/CaO-Al2O3 catalysts[END_REF][START_REF] Li | Methane reforming to synthesis gas over Ni catalysts modified with noble metals[END_REF] and that spinel and boost the appearance of MgAl2O4 species, which facilitates the reduction of nickel oxide particles, lowers the interaction towards alumina, and shifts the reduction peak to a lower temperature [START_REF] Charisiou | Synthesis gas production via the biogas reforming reaction over Ni/MgO-Al2O3 and Ni/CaO-Al2O3 catalysts[END_REF][START_REF] Xu | Ordered mesoporous MgO-Al2O3 composite oxides supported Ni based catalysts for CO2 reforming of CH4 : Effects of basic modifier and mesopore structure[END_REF][START_REF] Dias | Influence of calcium content in Ni/CaO/Al2O3 catalysts for CO2reforming of methane[END_REF].

III.4. Reducibility of mesoporous Ni5-Mgy-Al2O3 catalysts

Increasing further the amount of Mg (Ni5-Mg30-Al2O3) induces a small deviation of the reduction peak towards a higher temperature with an additional peak shoulder noticeable at around 800 °C. This phenomenon is more pronounced in the case of Ni5-Mg50-Al2O3 with 26

wt% Mg where the reduction temperature for the whole single peak position is shifted to about

III.7. Dry reforming of methane reaction

The above characterization properties of the calcined and reduced samples, clearly indicate that there exists an optimal magnesium content in our series of samples with regard to their structural and porous properties. Indeed, the incorporation of Mg up to a certain extent (7 wt%) preserves the mesoporous alumina framework and leads to samples with high repartition of both Ni and Mg species. In contrary, a higher amount of magnesium leads to nonhomogeneous and less ordered samples having lower surface area and pore volume, and in which the oxide Al2O3 and MgO phases are segregated. This seen trend resembles the "volcanoshape" curve seen at the beginning of this chapter for catalytic performances taken from bibliography and that will be also seen below for our series of catalyst tested in DRM. III.7.1. Activity in DRM of Ni5-Al2O3 and Ni5-Mg15-Al2O3 at a GHSV of 36 L.g -1 .h -1

Catalytic tests in DRM were first carried out on two of the prepared catalysts, Ni5-Al2O3 and Ni5-Mg15-Al2O3, using 100 mg of sample each time, giving a GHSV of 36 L.g -1 .h -1 as often applied in the literature (see Table III Looking in more details, Ni5-Mg15-Al2O3 leads to slightly higher conversions (by 4 wt% in CO2 conversion) as well as a slightly higher stability than Ni5-Al2O3 that faces around 5% loss of its activity after 50 hours, attributable to higher coke deposition since the H2:CO value directing the reverse water gas shift reaction retarding and hindering the activity of the main reaction [START_REF] Özkara-Aydınoğlu | A comparative study on the kinetics of carbon dioxide reforming of methane over Pt-Ni/Al2O3 catalyst: Effect of Pt/Ni Ratio[END_REF].

The XRD for the spent samples (Table III-5) agree well with the catalytic test results.

Ni5-Mg15Al2O3 has a 4.2 nm Ni 0 particle size that is quite similar to its reduced value and therefore no sintering has occurred under the harsh reaction conditions imposed. On the other hand, the nanoparticle size of Ni5-Mg50-Al2O3 increased from 8.8 nm in its reduced form to 9.7 nm after reaction implying some sort of sintering and growth of metallic Ni 0 that could also As estimated from literature, the methane apparent activation energies in DRM for all Ni based catalysts is in the range of 20-360 kJ.mol -1 . This wide range of values is dependent on the type of support, the nature of promoters, and the catalytic tests conditions applied. It can be mentioned that our results of Ea are in the same order of magnitude compared to literature [START_REF] Zhang | Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/Γ-alumina nanocomposites by one-pot synthesis[END_REF][START_REF] Lemonidou | Carbon dioxide reforming of methane over 5 wt.% Ni/CaO-Al2O3catalyst[END_REF]. As shown in Table III-7, regardless of the amount of Mg added, the Ea of CH4 consumption for all catalysts studied is higher than that of CO2. This means that the Ea of CH4 is more sensitive to temperature change than CO2 [START_REF] Özkara-Aydınoğlu | A comparative study on the kinetics of carbon dioxide reforming of methane over Pt-Ni/Al2O3 catalyst: Effect of Pt/Ni Ratio[END_REF][START_REF] Wang | A Comprehensive Study on Carbon Dioxide Reforming of Methane over Ni/γ-Al2O3 Catalysts[END_REF][START_REF] Nandini | Kinetic study of the catalytic carbon dioxide reforming of methane to synthesis gas over Ni-K/CeO2-Al2O3 catalyst[END_REF][START_REF] Oemar | CO2 reforming of methane over highly active La-promoted Ni supported on SBA-15 catalysts: mechanism and kinetic modelling[END_REF][START_REF] Kathiraser | Kinetic and mechanistic aspects for CO2 reforming of methane over Ni based catalysts[END_REF], consequently higher CO2 who demonstrated that when the partial pressure of CO2 is below 8.4 kPa, the reaction order of CO2 is almost 1 [START_REF] Wang | A comprehensive study on carbon dioxide reforming of methane over Ni/γ-Al2O3 catalysts[END_REF]. On the other hand, the power constant for CO2 is higher than the order of CH4 suggesting that the reaction is more sensitive to CO2 partial pressure than CH4 partial pressure. This dependence of Ni-Al2O3 on CO2 partial pressure might be probably due to the utilization of oxygen coming from CO2 to clean deposited carbon in agreement with the deactivation seen in stability results. As for the Ni5-Mg15-Al2O3, the amount of basic sites present could enhance the chemisorption of CO2 that provides the oxygen needed in removing carbon, hence limiting CO2 dependency [START_REF] Özkara-Aydinoĝlu | A comparative study on the kinetics of carbon dioxide reforming of methane over Pt-Ni/Al2O3 catalyst: Effect of Pt/Ni Ratio[END_REF]. However, this is just a hypothesis that needs to be justified by similar kinetics experiments on the Mg-based material. The kinetics of DRM as Chapter III Effect of Mg loading over ordered mesoporous Ni5-Mgy-Al2O3 DRM catalysts the simple power-law model is a simple model that provides rough estimation of the parameters, but is insufficient under a large extent of partial pressure data. Description of the mechanistic steps is needed by using Langmuir-Hinshelwood model to describe DRM kinetics. This process was also tackled in the core of the thesis but is not presented because it has not finished yet.

III.8. Conclusion

Ni5-Mgy-Al2O3 ordered mesoporous catalysts containing a wide range of Mg wt% (0-26 wt%) were synthesized using the evaporation induced self-assembly procedure associated to a "one-pot" methodology. Based on extensive physicochemical properties seen through the chapter, the inclusion of a maximum of 7 wt% Mg loading preserves high and homogenous Ni dispersion with simultaneous excellent Mg repartition. Mesoporous Ni5-Mg15-Al2O3 acquired also organized porosity that is even better than Al2O3 and Ni5-Al2O3. For instance, Mg 2+ ions facilitate the rate of hydrolysis of P123 and aluminum precursor especially that some AlOx sites in the mesoporous alumina are preferable doping sites for Mg atoms. These good structural and textural characteristics, in addition to the enhanced basicity seen by infra-red (creation of basic sites) that accelerates CO2 chemisorption, contributed to superior catalytic performances (lowest activation energy) with the best activity and stability among the rest of the samples.

On the contrary, if we continue to increase the amount of Mg (15-26 wt%), Ni species lose their excellent repartition over the alumina support and MgO is segregated. The organized support with high activity and stability in DRM. A short literature review about MOF materials will be elaborated specifying the selected type in this thesis. The preparation steps will be then described in details along with the proper textural and structural properties of the novel obtained Ni-Al2O3 materials that were then treated in DRM reaction to compare their performance to those of a commercial-based catalyst.

IV.1. Literature review

MOFs (Metal Organic Framework) are highly porous hybrid materials made by a regular assembly of metal clusters connected to one another's by multifunctional organic linkers. For example, MOF structures can be synthesized using common organic ligands like 1,4-benezenedicarboxylic acid, its functionalized forms and 1,3,5-benzenedicarboxylic acid in conjunction with metal nodes of Zn 2+ , Cu 2+ , Cr 3+ , Al 3+ , Fe 3+ and Zr 4+ [START_REF] Lee | Synthesis of metal-organic frameworks: A mini review[END_REF]. MOF abbreviation is usually used as a general name of a class of compounds, however there are many MOFs [START_REF] Stock | Synthesis of Metal-Organic Frameworks ( MOFs ): Routes to Various MOF topologies, morphologies, and composites[END_REF]. In the field of catalysis, the high accessibility of their metal nodes and their reactivity have started to be exploited as well, even if stability issues in the conditions of catalysis still often remain to be solved.

Besides, the scope of MOF application was recently broadened to another level. Indeed, these hybrid materials are used as sacrificial templates to synthesize either carbonaceous or purely inorganic materials with unique textural properties depending on the thermal treatment used for their transformation [START_REF] Zhao | Isobutane dehydrogenation over the mesoporous Cr2O 3/Al2O3 catalysts synthesized from a metal-organic framework MIL-101[END_REF][START_REF] Yuan | Worm-like mesoporous carbon synthesized from metal-organic coordination polymers for supercapacitors[END_REF][START_REF] Yan | Synthesis of mesoporous and tetragonal zirconia with inherited morphology from metal-organic frameworks[END_REF][START_REF] Lü | MOF-templated synthesis of porous Co3O4 concave nanocubes with high specific surface area and their gas sensing properties[END_REF]. Starting with the carbon-based supports, there is quite abundant literature about it. The first 8 lines in Table IV-1 summarize the reported Ni/C-based catalysts doped with nickel derived from MOF and details the treatment conditions applied as well as surface areas obtained and types of applications. In all cases, to obtain a carbon support from the MOF, the sample was thermally treated in an inert gas (N2, Ar). The Ni particle size ranged from 5 nm when coupled with Pt to 50 nm when present alone. The surface area could be as small as 7 m 2 .g -1 and as large as 750 m 2 .g -1 .

As our main interest in the present work concerned metal oxides (more specifically alumina) derived from MOFs, Table IV-1 also embraces the different existing bibliographic examples on such systems. To obtain metal oxide, the MOF structure has then to be thermally treated under air or pure O2. As seen in the table, in the case of alumina, no active metal phase was added and the prepared materials, with surface areas varying from 50 to 270 m 2 .g -1 , were not tested in any application. For these samples, Table IV-2 details the differences between the type of the parents MOF used, the ligand nature, the Al source and the thermal treatment applied. Despite the fact that these alumina were already reported, they were not chosen in our study because they have expensive ligand or Al source in addition to the high temperature and long time required for the preparation of the MOF that is later calcined to obtain the Al2O3 oxide.

In this thesis, the selected MOF is instead an aluminum-based MIL-53(Al) composed of aluminum hydroxyl chains connected by terephthalate organic linkers and running along one-dimensional channels. The benefit of this MIL-53(Al) is that it is synthesized using a microwave within 30 minutes reaction only, the linker and Al source are cheaper than the ones mentioned in Table IV Pt-Ni/PC [220] MOF-5 Ar 5 234 Fuel cells Mn3O4 [221] Mn-MOF Air -108-160 No Application

Co3O4 [210] ZIF-67 Air 8-28 22-120 Gas sensing ZnO/Co3O4 [222] ZIF-8@ZIF67 Air n.m.

2-25 CO2 photo reduction

Cr2O3/Al2O3 [207] MIL-101 O2 30 102 Isobutane dehydrogenation ZrO2 [209] UiO-66 Air

174 No Application

Cu/ZnO [223] HKUST-1 Air 24 5.5 Hydrogenolysis of glycerol Au/TiO2 [224] MIL-125 O2 n.m.

CO2 reduction

Al2O3 [225] MOF A520 Air -205 No Application

Al2O3 [226] MIL-110 Air -138 No Application

Al2O3 [226] MIL-100 Air -123 No Application

Al2O3 [227] MIL-96 Air -56 No Application

Al2O3 [228] MIL-53 O2 -88 No Application

Al2O3 [228] DUT- This MIL-53(Al) will be impregnated for the first time with nickel precursor and used as a source of porous Ni-Al2O3 catalyst that will be applied in dry reforming of methane reaction. The MIL-53(Al) host gathers the attributes required to obtain a porous nickel-alumina solid solution after its calcination: on one hand, it possesses a high porosity and metal-cluster accessibility expected to favor the homogeneous dispersion of nickel species within the oxide structure; on the other hand, it contains organics whose decomposition enables the formation of porosity in the obtained alumina [START_REF] Liu | Mesoporous alumina nanosheets and nanorolls derived from topologically identical Al-based MOFs[END_REF]. An additional benefit is the occurrence in this structure of aluminum bridging hydroxyl groups lying along the inorganic chains that can interact with the nickel precursors and hence act as anchorage sites.

IV.2. Synthesis of mesoporous Ni-Al2O3 derived from MOF (MIL-53(Al))

The parent Al-containing MIL-53(Al) material was synthesized according to a known microwave-based procedure reported to provide nanocrystallites of homogeneous distribution in shape and size [START_REF] Isaeva | Control of morphology and size of microporous framework MIL-53(Al) crystals by synthesis procedure[END_REF]. . Such new reflections were to our knowledge never reported for MIL-53, strongly suggesting that nickel cations are inserted within the pores where they provoke structural deformation by interacting with the framework as in the case of adsorbed organic guest molecules [START_REF] Horcajada | Flexible Porous Metal-Organic Frameworks for a Controlled Drug Delivery[END_REF].

In order to confirm the origin of these new reflections, a series of experiments was done with the aim to demonstrate that they appear only in the presence of Ni and do not simply come from different hydration states generating cell volumes and symmetry changes in response to the above-mentioned "breathing behavior". We prepared to this end a nickel free hydrated MIL- pressures above P/P0 = 0.9 characteristic of a substantial intercrystalline meso-macro porosity occurring between aggregated nanocrystals.

IV.3.4. Organic linkers removal followed by thermogravimetric analysis

Another evidence of the intimate mixing between the nickel precursor species and the porous network in Ni/MIL-53 is given by thermogravimetric analyses as discussed in this paragraph. This technique was applied to study the decomposition in air of the carbonaceous (terephthalate) linkers constitutive of the parent MIL-53, the aim being to form the oxide alumina matrix. (2) this is followed between 200 and 400 °C by nitrates decomposition into NOx (NO, NO2, etc.) [START_REF] Yuvaraj | Thermal decomposition of metal nitrates in air and hydrogen environments[END_REF] with simultaneous formation of a NiO-based phase (for the sake of neutrality preservation); (3) then, strongly adsorbed NOx oxidizers remaining within the pores in close proximity (or in contact) with the organic linkers promote their decomposition, decreasing the temperature at which it occurs [START_REF] Pourmortazavi | Effect of nitrate content on thermal decomposition of nitrocellulose[END_REF].

In view of these TG profiles, we decided to set the calcination temperature of Ni/MIL-53 during sample preparation (to form Ni5-Al2O3 MIL-53) at 500 °C, a temperature high enough to degrade the linkers but guaranteeing as well the development of the porosity and the large surface area of the pure mineral final phase. Indeed, it was recently reported that a temperature close to the onset of the parent MOF calcination temperature is optimal for the preparation of inorganic structures with preserved morphology and dimensions [START_REF] Lü | MOF-templated synthesis of porous Co3O4 concave nanocubes with high specific surface area and their gas sensing properties[END_REF], which is important for preserving an intercrystalline porosity and large surface area in the obtained catalyst. The BET specific surface of 239 m 2 .g -1 is still high, even if much smaller than in the parent 1.4 Å that can be estimated above from XRD for NiAl2O4 and also not far from those typical of γ-Al2O3 (d= 1.4 Å and 1.98 Å attributed to ( 440) and (400) planes, respectively). On the contrary, the lattice spacing expected around 2.4 Å and 2.1 Å for ( 111) and ( 200) crystal planes of NiO [START_REF] Long | Growth of Hierarchal Mesoporous NiO Nanosheets on Carbon Cloth as Binder-free Anodes for High-performance Flexible Lithiumion Batteries[END_REF] are not distinguished confirming the absence of such phase as discussed above.

IV.4.2. Comparative properties of calcined Ni5@Al2O3 MIL-53 prepared by impregnation of Al2O3 MIL-53

In order to evaluate the impact on the material properties of the adopted preparation procedure (i.e. impregnating first the activated MIL-53 with nickel then calcining it), a different order was applied by first calcining the activated MIL-53 (to obtain Al2O3 MIL-53) then impregnating and calcining it at 500°C (to remove adsorbed nitrates and water molecules). This second sequence order, in which the impregnation is done on a preformed alumina support (here No evidence of spinel phase can be observed on the diffractogram of Ni5@Al2O3 COM, although diffraction peaks corresponding to a small fraction of this phase might be masked by those of the γ-Al2O3 phase. The TPR trace of this material displays two peaks, as for Ni5@Al2O3 MIL-53, but centered at lower temperatures than previously (630 °C and 800 °C, COM catalysts in addition to the expected thermodynamic values at different temperatures (light off measurements). A slightly higher CO2 than CH4 conversion is also depicted over the entire temperature range accounting for the RWGS reactions that consumes CO2 and increases its conversion level. This trend is seen over both catalysts. However, the conversions of Ni5 0 @Al2O3 COM catalyst starts at higher temperature (450 °C) compared to those of Ni5 0 -Al2O3 MIL-53 (390 °C) with 25% less conversion values. This difference in conversions and the superior . A rather good selectivity to DRM is also observed on Ni5 0 @Al2O3 MIL-53 but with a H2:CO ratio higher than 1 that suggests the occurrence of some CH4 decomposition (CH4(g) → C(s) + 2H2(g)). Noteworthy Ni5 0 -Al2O3 MIL-53 is not only the most active catalyst but it is also very stable as its performance after the prolonged 100 confinement effect may also explain the absence of carbon production on this sample. This explanation is in line with previous data showing that the occlusion of nickel nanoparticles within the mesopores of alumina supports prevent the catalysis of carbon nanotube formation, which occurs only from the nickel nanoparticles located on the external surface of the support [START_REF] Jabbour | Ordered mesoporous "one-pot" synthesized Ni-Mg (Ca)-Al 2 O 3 as effective and remarkably stable catalysts for combined steam and dry reforming of methane (CSDRM)[END_REF].

IV.7. Comparison of Ni5 0 -Al2O3 MIL-53 with other materials in literature

It is worth adding that although the nickel nanoparticles reported here are not the smallest reported in the literature so far (the application of atomic layer deposition recently afforded nanoparticle size below 3 nm [START_REF] Li | Steam reforming of n-dodecane over mesoporous alumina supported nickel catalysts: Effects of metal-support interaction on nickel catalysts[END_REF]), this study demonstrates the potential of MOFs as sacrificial hosts for preparing highly dispersed nickel nanoparticles whose size is below than those prepared by employing other type of oxides as stable nickel cation hosts, such as alkaline earth metal substituted MZr1-xNixO3-δ perovskites [START_REF] Dama | Active and durable alkaline earth metal substituted perovskite catalysts for dry reforming of methane[END_REF] or the lanthanum zirconate pyrochlore La2Zr2-xNixO7-δ [START_REF] Saché | Ni stabilised on inorganic complex structures: superior catalysts for chemical CO 2 recycling via dry reforming of methane[END_REF]. Noteworthy, the use of MOF results in materials with a relatively high surface area (S. A.= 240 m 2 .g -1 ) compared to that obtained with other types of purely inorganic sacrificial nickel host cited above (S.A. < 20 m 2 .g -1 ), which can be seen as an additional benefit of this strategy in view of catalysis applications.

To end, and as a comparison Ni5-Al2O3 MIL-53 showed superior performance than an organized mesoporous based alumina catalyst. Organized mesoporous Al2O3 was prepared following the EISA route and impregnated with 5wt% Ni (Ni5-Al2O3 Meso). The latter exhibited lower surface area than that of the MOF-based catalyst by 25% accompanied with a lower reduction temperature emphasizing on the less Ni-support interaction. This leads to larger particle size with retarded activity and appearance of carbon at the end of the reaction (Table IV-6).

IV.8. Conclusion

In this work we highlighted on the benefit of using aluminum carboxylate MIL-53 as a sacrificial nickel precursor host to prepare a dry methane reforming catalyst composed of nickel nanoparticles dispersed within a porous γ-Al2O3 lamellar phase. The higher surface area of this metal-organic framework compared to preformed porous alumina supports allows for a more efficient dispersion of the nickel precursors and, in turn, for a superior dispersion and uniformity of the nickel nanoparticles obtained after the MOF calcination and subsequent reduction steps.

Consequently, the performances in terms of activity, selectivity and stability of this new catalyst for dry reforming of methane within the reaction conditions applied in this study are higher than those of comparative nickel@alumina catalysts prepared by conventional nickel impregnation of preformed alumina supports. The results displayed in this report demonstrate that MOF can be considered as a promising hybrid alternative to the sacrificial inorganic and non-porous hosts (perovskite, hydrotalcites) used so far to stabilize nickel species towards the synthesis of catalysts based on nickel metal nanoparticles. (chemical and energy production) from CO2 (valorization of greenhouse gas) and H2 that can be generated by electrolysis of water using excess of renewable electric energy (from solar and wind) produced in low consumption periods [START_REF] Centi | Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries[END_REF][START_REF] Rönsch | Review on methanation -From fundamentals to current projects[END_REF][START_REF] Stangeland | CO2Methanation: The Effect of Catalysts and Reaction Conditions[END_REF][START_REF] Specht | Storing bioenergy and renewable electricity in the natural gas grid[END_REF]. This wind to natural gas (CH4) concept frequently called power-to-gas (P2G) relies on the existing gas grid infrastructures for storing and converting the generated fuel to electricity as seen in Figure V-1 [START_REF] Centi | Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries[END_REF].

CO2 methanation projects are dominant in

Germany due to the transformation of its energy system to a 100% renewable one, triggered by the huge demand to chemically store the electric energy and the need to compensate the intermittency of wind and solar energy provision. The produced methane can be applied in the chemical industry, lowering As already presented in chapter II, thermodynamic analysis demonstrates that, with a stoichiometric feed gas H2/CO2 of 4 and a temperature range of 200-350 °C, the main products will be only CH4 and H2O. As a result, the assumed methanation reaction is the following: CO2 + 4H2 ↔ CH4 + 2H2O. Since this reaction is an eight-electron process to fully reduce carbon dioxide to methane with significant kinetic limitations, a catalyst is mandatory to achieve acceptable rates and selectivity. Despite the difference in operating temperatures between DRM and CO2 methanation, the catalysts used for both reactions have many similarities and the good catalysts found for DRM could be also suitable for methanation.

V.1.2. Catalysts applied in literature

In the state of the art regarding CO2 methanation catalysts, many studies are based on nickel due to its high activity, selectivity and comparatively low price [START_REF] Rönsch | Review on methanation -From fundamentals to current projects[END_REF][START_REF] Gao | Recent advances in methanation catalysts for the production of synthetic natural gas[END_REF][START_REF] Guo | Carbon Dioxide Methanation over Nickel-Based Catalysts Supported on Various Mesoporous Material[END_REF][START_REF] Riani | Unsupported versus aluminasupported Ni nanoparticles as catalysts for steam/ethanol conversion and CO 2 methanation[END_REF]. However, the operating temperatures should be high enough to avoid the potential formation of highly toxic nickel carbonyls from carbon monoxide, which defines the possible temperatures range between 200 and 550 °C [START_REF] Schaaf | Methanation of CO2 -storage of renewable energy in a gas distribution system[END_REF]. Table V-1 illustrates some of the newest research studies that applied Ni-Al2O3 based catalysts in CO2 methanation, summarizing the preparation method, nickel amount, some of the physicochemical properties of the reported materials and their performances towards CO2 methanation. This table shows that, as observed in dry reforming of methane, Ni nanoparticles were also deposited on Al2O3 support using different insertion methods. However, a direct comparison between the reported catalysts is difficult due to the differences in the operating conditions used. Needless to say, Ni/Al2O3 materials prepared by standard impregnation methods [START_REF] Guo | Carbon Dioxide Methanation over Nickel-Based Catalysts Supported on Various Mesoporous Material[END_REF][START_REF] Riani | Unsupported versus aluminasupported Ni nanoparticles as catalysts for steam/ethanol conversion and CO 2 methanation[END_REF] lead to relatively large particle sizes (10-30 nm), weakening the interaction with the support and favoring agglomeration processes during the course of the reaction. Ni/Al2O3 materials prepared by deposition-precipitation resulted in Chapter V Mesoporous nickel-alumina catalysts for CO2 methanation 121 smaller particle sizes (e.g. 3.2 nm) [START_REF] Mutz | Surface Oxidation of Supported Ni Particles and Its Impact on the Catalytic Performance during Dynamically Operated Methanation of CO2[END_REF] but, the Ni interaction with Al2O3 was poor, leading to low activity (CO2 conversion of 22%). The occurrence of deactivation during the test was attributed to the oxidation of the most active Ni sites in the presence of O2 impurities. Moreover, as observed for DRM in chapter III, some reported studies on methanation also revealed that the addition of a basic additives such as Mn, CaO, Mg, Sr, Ba, or La ameliorate performances [START_REF] Yang | Promotion Effect of CaO Modification on Mesoporous Al2O3-Supported Ni Catalysts for CO2 Methanation[END_REF][START_REF] Liang | Methanation of CO 2 over Ni/Al 2 O 3 modified with alkaline earth metals: Impacts of oxygen vacancies on catalytic activity[END_REF][START_REF] Garbarino | A study of Ni/La-Al 2 O 3 catalysts: A competitive system for CO 2 methanation[END_REF] due to an enhancement of the chemical adsorption of CO2 that decreases in turn its activation energy [START_REF] Liang | Methanation of CO 2 over Ni/Al 2 O 3 modified with alkaline earth metals: Impacts of oxygen vacancies on catalytic activity[END_REF]. Ni/Al2O3 [259] In-situ growth 20 a: deposition-precipitation, b: incipient wetness impregnation, c: H2:CO2 = 5:1 * : unit in h -1 , n.m.: not mentioned, S.A.: surface area, XCO2: CO2 conversion, SCH4: CH4 selectivity Some reports on preparation methods, similar to ours, involving in-situ growth (one-pot Ni addition), microwave or EISA techniques can also be found in literature for methanation, but not numerous. They resulted in better catalytic performance with improved CO2 conversion and CH4 selectivity, but still suffered from deactivation due to sintering (from 5 to 8.5 nm) [START_REF] He | A surface defect-promoted Ni nanocatalyst with simultaneously enhanced activity and stability[END_REF], oxidation [START_REF] Aljishi | The effect of synthesis parameters on ordered mesoporous nickel alumina catalyst for CO2 methanation[END_REF], or little carbon formation [START_REF] Song | Obtaining well-dispersed Ni/Al2O3 catalyst for CO2 methanation with a microwave-assisted method[END_REF].

The use of MOF-based catalysts is also not null in CO2 methanation as it was in DRM, but it is very new and introduced, to our knowledge, to few research studies involving expensive noble metals [START_REF] Lippi | Highly active catalyst for CO2 methanation derived from a metal organic framework template[END_REF][START_REF] Jiang | The synergistic effect of Pd NPs and UiO-66 for enhanced activity of carbon dioxide methanation[END_REF][START_REF] Mihet | Pt/UiO-66 Nanocomposites as Catalysts for CO₂ Methanation Process[END_REF], cobalt [START_REF] Li | Low Temperature CO2 Methanation: ZIF-67-Derived Co-Based Porous Carbon Catalysts with Controlled Crystal Morphology and Size[END_REF], and nickel [START_REF] Zhen | Enhancing catalytic activity and stability for CO2 methanation on Ni@MOF-5 via control of active species dispersion[END_REF][START_REF] Zhen | Enhancing activity for carbon dioxide methanation by encapsulating (111) facet Ni particle in metal-organic frameworks at low temperature[END_REF] nanoparticles with different size.

However, even for Ni-MOF, the catalysts were very distinct from ours since they were mostly Chapter V Mesoporous nickel-alumina catalysts for CO2 methanation 122 used in non-calcined form. In the example given by Zhen et al. [START_REF] Zhen | Enhancing catalytic activity and stability for CO2 methanation on Ni@MOF-5 via control of active species dispersion[END_REF] MOF-5 was used to disperse 5 to 12.5 wt% Ni. The best catalyst, with 10 wt% Ni, gave 75% CH4 yield at 320 °C (GHSV=125 ml.min -1 .g -1 , P= 1atm) and had a metal Ni average size of 9 nm. The same authors showed that, under similar catalytic test conditions, the activity of 20 wt% Ni deposited on MIL-101 [START_REF] Zhen | Enhancing activity for carbon dioxide methanation by encapsulating (111) facet Ni particle in metal-organic frameworks at low temperature[END_REF] was better than other nickel-supported materials (γ-Al2O3, SiO2, TiO2). Even though these MOFs displayed good catalytic performances in CO2 methanation, they are unstable at high temperature and thermally decompose.

For this reason, MOFs can be used instead as a sacrificial template resulting in carbonaceous or purely inorganic materials depending on the thermal treatment adopted. This route has not been applied so far, to our knowledge, for Ni-based MOF and the catalyst prepared following this method in CO2 methanation is a Co metal [START_REF] Li | Low Temperature CO2 Methanation: ZIF-67-Derived Co-Based Porous Carbon Catalysts with Controlled Crystal Morphology and Size[END_REF]. In this study the MOF used is ZIF-67 pyrolyzed under N2 atmosphere, and later reduced with H2 to obtain Co-based porous carbon (PC) with Co nanoparticles between 7-20 nm in size. This catalyst had a high surface area of 346 m 2 .g -1 with a CH4 yield of 52 % at 270 °C (GHSV=1200 ml.min -1 .g -1 , P= 30 atm)

and no sintering of Co nanoparticles which make them enticing compared to Co supported on commercial activated carbon catalyst.

From this aspect, and attracted by the high surface area and metal dispersion that our mesoporous Ni-Mg-Al2O3 synthesized by one-pot method (chapter III) and Ni-Al2O3 derived from MIL-53(Al) (chapter IV) have, and their encouraging results in DRM, we thought that these new materials can also bring new outcomes to the field of CO2 methanation.

V.2. Synthesis of mesoporous one-pot Nix-Mgy-Al2O3 and Nix-Al2O3 derived from MIL-53(Al) materials

The preparation methods followed for the two sets of materials studied in this chapter are similar to the elaborated explanation in section III. a determined from TGA-DSC experiments; b total surface area (S.A.) and total pore volume (Vtot) calculated from the N2 sorption isotherms, in brackets: reduced materials; c value estimated from the histograms obtained from representative TEM images of each reduced sample; d nanoparticle dispersion calculated from TEM of reduced samples

V.3.1.2. Surface adsorption properties

The interaction with water was evaluated since H2O is a reaction product that could have an eventual relevant inhibitory role in the methanation reaction [166,[START_REF] Bacariza | Enhanced activity of CO2 hydrogenation to CH4 over Ni based zeolites through the optimization of the Si/Al ratio[END_REF][START_REF] Delmelle | Development of improved nickel catalysts for sorption enhanced CO2 methanation[END_REF][START_REF] Borgschulte | Sorption enhanced CO2 methanation[END_REF][START_REF] Walspurger | Sorption enhanced methanation for substitute natural gas production: Experimental results and thermodynamic considerations[END_REF][START_REF] Aziz | CO2 methanation over Ni-promoted mesostructured silica nanoparticles: Influence of Ni loading and water vapor on activity and response surface methodology studies[END_REF][START_REF] Ocampo | Développement de catalyseurs pour la réaction de méthanation du dioxyde de carbone[END_REF][START_REF] Yang | Preparation and characterization of LiNiO2 synthesized from Ni(OH)2 and LiOH•H2O[END_REF][START_REF] Borgschulte | Manipulating the reaction path of the CO2 hydrogenation reaction in molecular sieves[END_REF]. In addition, and in the specific case of zeolites (crystalline alumina-silicate materials) water could be chemically adsorbed on CO2 activation sites, playing another inhibitory role in the reaction [START_REF] Bacariza | Enhanced activity of CO2 hydrogenation to CH4 over Ni based zeolites through the optimization of the Si/Al ratio[END_REF][START_REF] Mfoumou | The preferential adsorption sites of H2O on adsorption sites of CO2 at low temperature onto NaX and BaX zeolites[END_REF][START_REF] Bacariza | DBD plasma-assisted CO2 methanation using zeolite-based catalysts: Structure composition-reactivity approach and effect of Ce as promoter[END_REF]. The strength of the water interaction with the catalysts was estimated by Chapter V Mesoporous nickel-alumina catalysts for CO2 methanation 126 intensifies CO2 chemisorption and, subsequently, activates CO2 during the course of the reaction [START_REF] Xu | One-Pot Synthesis of Ordered Mesoporous NiO-CaO-Al2O3 Composite Oxides for Catalyzing CO2 Reforming of CH4[END_REF][START_REF] Kim | Bifunctional Mechanism of CO2 Methanation on Pd-MgO/SiO2 Catalyst: Independent Roles of MgO and Pd on CO2 Methanation[END_REF][START_REF] Liu | Cobalt doped Ni based ordered mesoporous catalysts for CO2 methanation with enhanced catalytic performance[END_REF][START_REF] Chen | Size-Tunable Ni Nanoparticles Supported on Surface-Modified, Cage-Type Mesoporous Silica as Highly Active Catalysts for CO2 Hydrogenation[END_REF][START_REF] Xu | CO2 methanation over Ca doped ordered mesoporous Ni-Al composite oxide catalysts: The promoting effect of basic modifier[END_REF][START_REF] Xu | CO2 methanation over a Ni based ordered mesoporous catalyst for the production of synthetic natural gas[END_REF][START_REF] Xu | Alkaline-promoted Ni based ordered mesoporous catalysts with enhanced low-temperature catalytic activity toward CO 2 methanation[END_REF][START_REF] Bacariza | Magnesium as Promoter of CO2 Methanation on Ni-Based USY Zeolites[END_REF]. A suggested mechanism can explain the difference: on Ni5-Al2O3 catalyst, CO2 methanation is expected to be indicated by dissociation of CO2 into carbon species (COads) and an oxygen species (Oads) over the catalyst surface. COads then reacts with H atoms to generate CH4 through several reaction mechanism steps on Ni active sites, responsible for dissociating H2 into H atoms. When Mg is added to the sample, the formation of COads is increased suppressing its desorption process into CO leading to higher CH4 selectivity.

Reaching 26 wt% of Mg (Ni5-Mg50-Al2O3), CO2 conversion is not improved and its behavior looks similar to the 7 wt% Mg sample. This unimproved activity could be related to the lower Ni dispersion (11%), higher interaction with water (h index: 0. with around 800 μmol.g -1 H2 consumed for Ni5-Mg15-Al2O3, 1600 μmol.g -1 for Ni10-Mg15-Al2O3, 2400 μmol.g -1 for Ni15-Mg15-Al2O3, and 3300 μmol.g -1 consumed for Ni20-Mg15-Al2O3.

Reduced Nix-Mg15-Al2O3 samples exhibit an increment of metallic nickel particle size To conclude this part, the results of the most promising sample (Ni15-Mg15-Al2O3) were compared to other Ni-based prepared by conventional or by one-pot method in literature (Table V -5). All the presented studies used much more favorable contact times (almost six times higher than the one used in this work) and milder conditions than ours. Therefore, a direct comparison is complicated, but, with the harsh conditions used, in our case, the results obtained for mesoporous Ni15-Mg15-Al2O3 are comparable and even better (first 2 rows) than those shown in the table, indicating promising future for these materials in the CO2 methanation reaction. 111), ( 200) and ( 220) with face centered cubic cell (JCPDS: 01-071-4750) suggesting lower interaction with the support and larger particle size. indicating that this method is adequate for a wide range of nickel loadings (5-20 wt%).

Regarding Ni15@Al2O3 MIL-53 sample that is calcined then impregnated, its surface area (156 m 2 .g -1 ) and pore volume (0.42 cm 3 .g -1 ) are lower by 30 and 50% respectively than the samples contains Ni species that are present mainly as NiO and whose reduction processes occur at ~320 ºC (NiO in the zeolite external surface) and 400 ºC (NiO in the zeolite supercages) as demonstrated elsewhere [START_REF] Bacariza | Enhanced activity of CO2 hydrogenation to CH4 over Ni based zeolites through the optimization of the Si/Al ratio[END_REF][START_REF] Bacariza | Magnesium as Promoter of CO2 Methanation on Ni-Based USY Zeolites[END_REF][START_REF] Bacariza | The effect of the compensating cation on the catalytic performances of Ni/USY zeolites towards CO2 methanation[END_REF][START_REF] Bacariza | CO2 Hydrogenation Over Ni-Based Zeolites: Effect of Catalysts Preparation and Pre-reduction Conditions on Methanation Performance[END_REF][START_REF] Graça | CO2 hydrogenation into CH4 on NiHNaUSY zeolites[END_REF]. A smaller additional reduction peak occurred at a temperature above 600 ºC attributed to the reduction of NiO species present in the mesoporous cavities of this specific commercial zeolite [START_REF] Bacariza | Magnesium as Promoter of CO2 Methanation on Ni-Based USY Zeolites[END_REF][START_REF] Bacariza | Micro-and mesoporous supports for CO2 methanation catalysts: A comparison between SBA-15, MCM-41 and USY zeolite[END_REF][START_REF] Bacariza | CO2 Hydrogenation Over Ni-Based Zeolites: Effect of Catalysts Preparation and Pre-reduction Conditions on Methanation Performance[END_REF][START_REF] Graça | CO2 hydrogenation into CH4 on NiHNaUSY zeolites[END_REF]. Ni15@SBA-15 (Figure V-12 B f) presents also two reduction peaks, one at ~400 ºC and the second one is at ~600 ºC, attributed to NiO species weakly (external surface) and strongly (mesoporous channels) interacting with the silica support, respectively [START_REF] Karam | Tuning the properties of nickel nanoparticles inside SBA-15 mesopores for enhanced stability in methane reforming[END_REF][START_REF] Bacariza | Micro-and mesoporous supports for CO2 methanation catalysts: A comparison between SBA-15, MCM-41 and USY zeolite[END_REF][START_REF] Liu | Preparation, characterization and activities of the nano-sized Ni/SBA-15 catalyst for producing COx-free hydrogen from ammonia[END_REF]. Ni15-Al2O3 One-pot (Figure V-12 B g) presents a main reduction peak at ~700 ºC, as in the case of the MOF-derived material, due to reduction of Ni strongly interacting with Al2O3 [START_REF] Drobná | Analysis of Ni species formed on zeolites, mesoporous silica and alumina supports and their catalytic behavior in the dry reforming of methane[END_REF][START_REF] Espinosa-Alonso | Effect of the Nickel Precursor on the Impregnation and Drying of γ-Al2O3 Catalyst Bodies: A UV-vis and IR Microspectroscopic Study[END_REF]. However, this sample presents also a less Ni15@Al2O3 MIL-53 respectively (Table V-6). These particle sizes are systematically smaller than the ones between 9 and 20 nm reported in literature on other MOF materials [START_REF] Li | Low Temperature CO2 Methanation: ZIF-67-Derived Co-Based Porous Carbon Catalysts with Controlled Crystal Morphology and Size[END_REF][START_REF] Zhen | Enhancing catalytic activity and stability for CO2 methanation on Ni@MOF-5 via control of active species dispersion[END_REF].Thus, these data highlight again that this new strategy of occluding nickel inside the pores of alumina following MIL-53 route is very efficient and capable of producing small nickel particles even for high nickel loading. Chapter V Mesoporous nickel-alumina catalysts for CO2 methanation 139 V-8). Needless to say, the contact times used in the present work are less favorable than the ones reported in other studies. However, results indicate that using an Al-based MOF as precursor results in better outcomes in terms of both CO2 conversion and CH4 selectivity.

Consequently, the presented work constitutes a great advance in the use of active, selective and stable Ni-MOF-derived materials for CO2 methanation, especially when taking into account that few studies have been reported till the moment in this area. 1%Ru-ZrO2 [263] UiO-66 This chapter ends the thesis report by tackling dry reforming over a more complex mixture containing not only CH4 and CO2 in the feed, but also H2 and CO. The addition of the product within the feed mixture makes the reaction more challenging, but more realistic. The non-conventional model mixture was taken from a pyrolysis product unit after treatment of biomass wastes in a Lebanese industry partner of the SOL-CARE project. A thermodynamic study was done on the obtained mixture in dry reforming to check the expected behavior. Then, the reaction was applied on mesoporous Ni5-Mgy-Al2O3 materials to investigate the efficiency of these catalysts, with good performance in standard DRM, under harsh conditions.

VI.1. Literature Review

As mentioned earlier in Chapter I of this report, biomass is one of the alternative sources of energy that can be used to limit the dependency on fossil fuels and their impact on the global environment. The process of conversion of biomass to energy encompasses a wide range of different renewable sources. The organic biomass wastes are derived from garbage, animal manure and biodegradable materials. This technology consists a thermochemical route to convert biomass. This method divided into 2 options which choice depends on the desired form of energy and on the type and quantity of biomass feedstock available [START_REF] Yilmaz | A review on the methods for biomass to energy conversion systems design[END_REF][START_REF] Mckendry | Energy production from biomass (part 2): conversion technologies[END_REF][START_REF] Sharma | Biomass Conversion Technologies for Renewable Energy and Fuels: A Review Note[END_REF]:

1. Combustion: It is the direct burning of biomass into air where the chemical energy stored inside it will be converted to heat, mechanical power and electricity that can be used for example in furnaces, boilers (heating water, cooking), and steam turbines. This method is wasteful of resources (low conversion efficiency, less than 25% chemical energy of the solids converted to thermal and electrical energy).

2. Pyrolysis and gasification: It is the partial oxidation of biomass in absence of O2 or presence of air at around 500-800 °C [START_REF] Bridgwater | Fast pyrolysis processes for biomass 被引1124次[END_REF]. Gaseous products mostly contain a mixture of CH4, CO2, H2, and CO gases, some light hydrocarbons (chars), tars, ash and minor contaminants (NH3, H2S, SO2) [START_REF] Chen | Utilization of municipal solid and liquid wastes for bioenergy and bioproducts production[END_REF]. The produced gas can be burnt directly or used as a fuel for gas turbines and engines if it has low calorific value. Otherwise the gaseous mixture (CH4, CO2, H2, CO) can be transformed under suitable reactions to syngas (H2 and CO) and used as feedstock in the production of liquid fuels (Fischer-Tropsch synthesis) and methanol, each of which may be served as fuels for transportation. It is recommended, for time saving and environmental considerations, to carry out gasification/pyrolysis in a unique one-stage reactor [START_REF] Hossain | Prospect of hydrogen production from oil palm biomass by thermochemical process -A review[END_REF][START_REF] Li | Absorption-enhanced steam gasification of biomass for hydrogen production: Effects of calcium-based absorbents and NiO-based catalysts on corn stalk pyrolysis-gasification[END_REF][START_REF] Sobek | Solar pyrolysis of waste biomass: Part 1 reactor design[END_REF][START_REF] Couto | Municipal solid waste gasification in semi-industrial conditions using air-CO2 mixtures[END_REF]. Nevertheless, no satisfactory production of clean syngas (H2, CO) was obtained and formation of tar can block the reactor in such conditions. A two-stage reactor [START_REF] Wu | Thermal degradation of waste plastics in a two-stage pyrolysiscatalysis reactor over core-shell type catalyst[END_REF][START_REF] Zhang | Fe-Ni-MCM-41 Catalysts for Hydrogen-Rich Syngas Production from Waste Plastics by Pyrolysis-Catalytic Steam Reforming[END_REF][START_REF] Wang | Hot char-catalytic reforming of volatiles from MSW pyrolysis[END_REF] was developed where the waste is first pyrolyzed to produce a range of pyrolysis gases that pass directly to a second-stage catalytic reactor where catalytic reforming takes place to generate higher yield of hydrogen or syngas.

Even if very attractive, such type of combined reactor will not be considered in this work because efforts are still needed to improve each individual step, and the focus will remain targeted towards the reforming stage to find a good catalyst that can handle the complex product mixture of the pyrolysis reaction.

As seen in chapter I, a great effort has been made in literature on developing reforming catalysts conducted using model compounds (CH4, and CO2 only), whereas those involving real raw material are still limited. Few studies worked on reforming of gasification/pyrolysis products using different type of catalysts. The final reforming product also had distinct syngas ratio (probably due to the type of waste used). For example, Juniza MD Saad et al. [START_REF] Saad | Pyrolysis-Catalytic-Dry Reforming of Waste Plastics and Mixed Waste Plastics for Syngas Production[END_REF] performed catalytic dry reforming on evolved gas from pyrolysis of plastics. The composition of the pyrolysis product gas was not specified. Ni-Co-Al catalyst used increased syngas yields with an H2:CO=0.5. However, carbon formation occurred during reaction deactivating the catalyst. Katsuya Kawamoto et al. [START_REF] Kawamoto | Gasification and reforming of biomass and waste samples by means of a novel catalyst[END_REF] performed reforming after gasification on a wood feedstock sample. The complex mixture used was composed of 9% CH4, 32% CO2, 23% H2 and 32% CO. In this study, mesoporous Ni/SBA-15 was applied playing on the mode of Ni insertion on the SBA-15 support which was either by impregnation or direct synthesis.

Maximum concentrations were achieved when 20 wt% of NiO were added over SBA-15 with an H2:CO ratio of 2.2. Yeshui Zhang [START_REF] Zhang | Pyrolysis-catalytic reforming/gasification of waste tires for production of carbon nanotubes and hydrogen[END_REF] investigated four different Al2O3-based catalysts (Co/Al2O3, Cu/Al2O3, Fe/Al2O3 and Ni/Al2O3) prepared by incipient wetness impregnation and their effect in dry reforming of waste tires pyrolysis product. The composition of the reforming feed was not detailed. The highest gas yield and hydrogen production was achieved in Ni/Al2O3 with an H2:CO ratio of 3.6 and the least amount of carbon nanotubes deposition.

Few other researchers relied on modeling and theoretical paths to evaluate the influence of H2 or CO addition to CH4 and CO2 in the feed mixture [START_REF] Hiblot | Steam reforming of methane in a synthesis gas from biomass gasification[END_REF][START_REF] Liu | Dry reforming of model biomass pyrolysis products to syngas by dielectric barrier discharge plasma[END_REF]. Liu L. 328 et al. suggests that the addition of H2 (CH4:CO2:H2: 1:2:1) or CO (CH4:CO2:CO: 1:2:1) to the initial feed blocks the conversions of CH4 and CO2 directing different reactions. H. Hublot stated that CO2 addition promotes DR reaction while CO had no kinetic effect 327. These studies did not rely on any experiment, and therefore no catalyst was synthesized and tested.

Chapter VI Promotion of mesoporous nickel-alumina based catalysts by magnesium addition for reforming of waste gasification products In our case, the realistic mixture to be applied in reforming was taken from a pilot plant pyrolysis product. The model gas composition provided from a Lebanese industry (INDEVCO) containing CH4:CO2:H2:CO = 0.75:1:1.57:0.85 will be tested theoretically (HSC software) and experimentally (DRM on Ni5-Mgy-Al2O3 catalysts seen in chapter III). Before applying such mixture directly in reforming, the standard feed ratio was changed progressively from a standard reforming with a CH4:CO2=1 to CH4:CO2=0.75, till the addition of H2 (CH4:CO2:H2=0.75:1:1.57) and CO (CH4:CO2:CO=0.75:1:0.85) in the initial feed mixture. The results are then compared to the overall composition with the inclusion of CH4, CO2, H2 and CO initially as reactants. Altering the initial feed composition to meet or be closer to realistic conditions make the reaction itself more complicated and more prone to side reactions (production of water and carbon deposition) which diminish the conversions compared to classical DRM.

VI.2. Thermodynamic analysis

Before doing the catalytic tests, we performed thermodynamic simulations to check the changes in the feed mixture going from the case of CH4:CO2 of 0.75:1 to CH4:CO2:H2:CO = 0.75:1:1.57:0.85. Moreover, the regions of side reactions are specified in order to select the conditions where these reactions are limited. Table VI-1 summarizes the cases of inlet feed composition used and the obtained H2:CO ratios by the software considering or not the presence of carbon C(s). thermodynamic curves. This is explained by the fact that carbon forming reactions do not occur at elevated temperature above 600 °C which avoid the occurrence of side reactions seen in section 1 and 2. The only possible side reaction producing carbon at such elevated temperature is the CH4 decomposition. However, it is seen from all the profiles that this reaction does not have an important influence under the reactant conditions applied, since C(s) formation is very little with H2:CO values (Table VI-1) slightly higher than those of the free-carbon case.

The change in the reactant feed affects the selectivity with different values seen in each case for H2:CO ratio. The presence of the complex mixture does not cast water production with a possibility of carbon formation making such realistic conditions more challenging than standard ones. The next paragraphs focus on the evaluation of the performances of mesoporous Ni5-Mgy-Al2O3 catalysts under similar reaction conditions.

VI.3. Catalytic performance of Ni5-Mgy-Al2O3 mesoporous catalysts

To measure the catalytic test in each reactant mixture case, the total gas hourly space velocity (GHSV) was set at 180 L.g -1 .h -1 . The reactant feed ratio was changed four times to thermodynamic curve in absence of C(s). This means that the dry reforming operation is done in a free-carbon formation or that the amount of carbon formed is very little. Moreover, the conversions for all catalysts (highest activity for Ni5-Mg15-Al2O3) were enhanced compared to standard dry reforming results (CH4:CO2 = 1:1) due to the presence of more CO2 amount that shifts the reaction towards production of H2 and CO. The excess of CO2 also contributed to lower CO2 conversions than CH4 because some unreacted CO2 will be left at the end of reaction.

However, this difference between both conversions was not big since CO2 plays the role of oxidant and could react with C deposits to form CO, but can also be further consumed by favoring the occurrence of RWGS producing more H2O than the standard case (CH4:CO2 = 1:1) [START_REF] Karemore | On the dry and mixed reforming of methane over Ni/Al2O3 ??? Influence of reaction variables on syngas production[END_REF][START_REF] Serrano-Lotina | Influence of the operating parameters over dry reforming of methane to syngas[END_REF][START_REF] Jafarbegloo | Thermodynamic analysis of carbon dioxide reforming of methane and its practical relevance[END_REF].

VI.3.2. Effect of H2 addition to CH4 and CO2 feed stream

In process of adapting the mixture to the industrial case, hydrogen was added to the feed mixture with a stoichiometric ratio of CH4:CO2:H2=0.75:1:1.57. Al2O3<Ni5-Mg15-Al2O3. CO2 conversion was higher for all catalysts than CH4 conversion unlike thermodynamics, because the addition of H2 promotes the RWGS [START_REF] Liu | Dry reforming of model biomass pyrolysis products to syngas by dielectric barrier discharge plasma[END_REF], however the H2:CO ratio did not decrease a lot. This could be due to a competitive side reactions effect between RWGS and CH4 decomposition [START_REF] Behroozsarand | Modeling of microreactor for methane dry reforming: Comparison of Langmuir-Hinshelwood kinetic and microkinetic models[END_REF]. Moreover, it has been mentioned in other studies [START_REF] Gac | Methane decomposition over Ni-MgO-Al2O3 catalysts[END_REF][START_REF] Villacampa | Catalytic decomposition of methane over Ni-Al2O3 coprecipitated catalysts: Reaction and regeneration studies[END_REF] that the presence of additional amount of H2 to the reaction can prevent the surface of metal Ni crystallite from deactivation by reacting with carbon deposits formed by CH4 dissociation. This is true in the case of Ni5-Mg15-Al2O3. The presence of both H2 and moderate amount of Mg leads to an equilibrium between methane adsorption, dissociation, diffusion and desorption of hydrogen, with CO2 chemisorption and activation. This equilibrium induces higher CH4 and CO2 conversions in Ni5-Mg15-Al2O3 with sustained stability because the carbon formed are removed from two sides reaching a syngas mixture of H2:CO ratio around 2 close to the expected value.

On the other hand, when higher amount of Mg (Ni5-Mg30-Al2O3) is present, the number of basic sites becomes excessive that the adsorbed CO2 could cover some of the Ni active sites of the catalyst and desorption from the active sites become difficult due to the strong alkaline sites among the framework of the catalysts. As a result, CH4 and CO2 conversions are restrained from further increase, decreasing stability [START_REF] Zhan | Biogas reforming of carbon dioxide to syngas production over Ni-Mg-Al catalysts[END_REF]. In Ni5-Al2O3, the presence of an excess of H2

is not sufficient to cast the carbon formed, and thus it was the least stable acquiring the highest H2:CO ratio of around 2.8.

VI.4. Conclusion

As a conclusion of this chapter, it is evident that alteration of the initial DR feed mixture influences the products behavior and the selectivity of the reaction. Briefly, a CH4:CO2 ratio of 0.75:1 has a positive effect on the reaction due to CH4 present as limiting reactant, and thus the more CO2 available helps in oxidizing carbon formed during reaction. Adding H2 to the feed blocks CH4 conversions at low temperatures to direct CO2 methanation instead. In such condition, DR is dominated at elevated temperatures, but the presence of high H2 amount shows occurrence of RWGS. The H2:CO ratio achieved in this case was around 2. When CO is initially added to CO2 and CH4 no side reaction takes place at low reaction temperatures (<400 °C), but its presence retards conversions of CH4 and CO2 for DRM. The selectivity achieved in this case is as low as H2:CO of 0.5. The complex model mixture (CH4, CO2, H2, CO) is a combination of the above mentioned mixtures with a selectivity of H2:CO of 1.3.

Results also revealed that Ni5-Mg15-Al2O3, similar to the case of standard dry reforming of methane (CH4:CO2 = 1:1), had the best catalytic performance among all the other tested materials. This confirms the importance of high order mesoporous structure and the presence of an optimum amount of basic modifier to highly disperse nickel particles restricting sintering and carbon formation. Thus, Ni5-Mg15-Al2O3 proved to be a good candidate in reforming of complex and realistic mixtures with a great stability for a long period of time (50 hrs).

General conclusion and perspectives

Despite the enormous studies tackled on dry reforming of methane reaction, this reaction still faces obstructions regarding the utilized catalysts. The problems encountered are mainly related to the sintering of nickel active sites or/and to the formation of carbon species deactivating the catalyst and hindering DRM reaction.

In Second, and inspired by approaches one and two, a deep explanation was given about the benefit of using metal-organic framework (MIL-53(Al)) as a sacrificial nickel precursor host to highly disperse nickel nanoparticles within a porous γ-Al2O3 lamellar phase. Indeed, Ni5-Al2O3 MIL-53 synthesized by impregnation of Ni on the MOF prior to calcination resulted in high surface area, with superior dispersion and uniformity of nickel nanoparticles leading to high activity, selectivity and stability in DRM. Under similar test conditions, this new material was even better than Ni5@Al2O3 MIL-53 and traditional Ni5@Al2O3 COM prepared by conventional nickel impregnation over preformed alumina supports that acquired larger nickel nanoparticles with eventually lower dispersion.

Third, the application of Ni5-Mgy-Al2O3 and Nix-Al2O3 MIL-53 catalysts was extended to CO2 methanation reaction where their behavior was as outstanding as in dry reforming of methane. This proves the efficiency of these materials to be applied in versatile energy applications ranging from low to high reaction temperatures. Ni5-Mgy-Al2O3 catalyst (Ni5-Mg15-Al2O3 specifically) showed also good resistance in dry reforming of pyrolysis gaseous products towards sintering and carbon formation. This further highlights on the capability of the synthesized materials to withstand and operate in realistic conditions.

As for the perspectives, starting first with the materials synthesis, it could be a good idea to work on following same strategies but with cheaper and more "eco-friendly" templates. In the case of one-pot mesoporous Nix-Mgy-Al2O3 P123 commercial organic surfactant was used and is applied commonly to many mesoporous materials of Al2O3 and silica. But it was recently revealed to have poor biodegradability and toxic to several biological organisms at high concentrations in addition to its high relative cost. The trend should be towards using more environmentally friendly surfactants derived from renewable sources. For instance, chitosan polymers, derived from the shell of crustaceans such as shrimps and crabs, can be used. A basic trials on using such template in the following thesis was tried but it yielded to poorly ordered mesoporouss material. So the parameters of synthesis are suggested to be optimized. In the MOF case for example, the terephtalic acid used in the synthesis produce can be obtained from PET (polyethylene terephthalate) that is an industrially important waste stream and consists of more than 85% of terephtalic acid. Moreover, PET hydrolysis can be achieved at 200 °C in water, which is close to the synthesis conditions of MIL-53(Al). This idea took a part in my PhD work (training of an Italian masters student as internship in the frame of SOL-CARE project) where MIL-53(Al) was successfully synthesized by in-situ/ex-situ generation of linkers from PET hydrolysis. The obtained MIL-53(Al) followed later the same strategy as the seen samples and was impregnated with Ni and calcined.

For industrial application, even though the catalysts used in this work showed very good activity and stability, they need to be further tested at high pressures and for longer period of time (more than two weeks). It is also important to obtain kinetic and mechanistic information to design any reactor and predict products flow rate of the scaled-up operation. A fair part of the kinetics and mechanistic aspects are exploited on mesoporous Ni5-Al2O3 catalyst but they still need to be optimized and applied on the other materials for comparison.

List of figures

Figure I- 1 :

 1 Figure I-1: Natural gas reserves distribution across the world [9]

  The intergovernmental panel on climate change (IPCC) has summarized in the form of a graph (Figure I-4) the amount in Kg of CO2 emitted per megawatt-hour (MWh) of energy

Figure I- 4 :

 4 Figure I-4: Carbon dioxide (CO₂) emissions factor, measured in kilograms of CO₂ produced per megawatt-hour (MWh) of energy produced from a given fossil fuel source. Source: intergovernmental panel on climate change (IPCC)[START_REF]Carbon Dioxide Emissions Factor, Kg CO2 per MWh[END_REF]

Figure I- 5 :

 5 Figure I-5: Global carbon dioxide levels since the beginning of industrial revolution[START_REF] Scibioh | CO 2 Conversion-Relevance and Importance[END_REF]

Figure I- 6 :

 6 Figure I-6: Summary of human sources of greenhouse gas emissions[START_REF] Scibioh | CO 2 Conversion-Relevance and Importance[END_REF]

Figure I- 7 :

 7 Figure I-7: Percentage of greenhouse gases sources[START_REF] Agency | Overview of greenhouse gases[END_REF]

Figure I- 8 : 17 I. 3 . 1 .

 81731 Figure I-8: Schematic with a brief summary about synthetic gas applications[START_REF] Kumar | Catalytic bi-reforming of methane: From greenhouse gases to syngas[END_REF]

2 .

 2 Dry reforming of methane regarding literature The "web of science" research engine was used as a reliable basic source to estimate the amount of papers published on DRM, SRM, and POX through the years. Statistics were performed on two bases: topic and title search.

I. 3 . 3 .

 33 Figure I-9: Statistical histograms displaying the evolution of the publications number as a function of year for SRM, POX, DRM and CO2 methanation reported studies. The search was evaluated between 2000 and 2018 using either the topic or title in the search engine

I. 4 . 1 .

 41 Type of metals and supports in dry reforming of methane a) Noble and transition metals: The active metal elements are often divided into two categories: noble metals such as Pt, Ru, Pd, Ir and Rh, and transition metals like Ni, Fe, Cu and

  difference was recently tackled by Xu Yan et al. [64] in a study where Ni/SiO2 and Ni/Al2O3 were prepared by a same technique to elucidate the effect of the support properties on the catalytic performances. Results have indicated that Ni/SiO2 depicts initially higher conversion, but poorer stability due to the reduction of Ni at lower temperature than that of Al2O3 forming very large Ni particles of 31 nm and promoting the formation of coke precursors. Ni/Al2O3 kept stable for 50 h time on stream due to the formation of NiAlxOy species reducible at very high temperature stabilizing Ni nanoparticles (Figure I-10).

Figure I- 10 :

 10 Figure I-10: Scheme representing effect of support on nickel particles [64]

  describes a new method to include Ni precursors within the synthesis of the support [77-79,82]. A comparison between impregnated Ni and one-pot (with Ni put in the synthesis mixture) Ni-Al2O3 shows that Ni can be trivially separated from the support when added after Al2O3 synthesis due to the weak interaction that forms destructive carbon in DRM (Figure I-11 b) encapsulating Ni particles by layers and increasing rate of catalyst deactivation. The "onepot" route managed to form Ni particles in the range of 5-15 nm, with good stability but did not cast the development of carbon, for 28 % of carbon was formed on the spent catalyst compared to 64 % for its impregnated counterpart [77].

Figure I- 11 :

 11 Figure I-11: Schematic representing the influence of Ni insertion in the catalyst on coke formation during DRM[START_REF] Wang | One-pot Synthesis of Ordered Mesoporous NiCeAl Oxide Catalysts and a Study of Their Performance in Methane Dry Reforming[END_REF]

  compared a unique nanosheet morphology to a random Al2O3 morphology prepared by co-precipitation method. The results demonstrated that the Al2O3 nanosheet catalyst exhibited better coke tolerance in DRM due to randomconnected large intersheet pore channels that facilitate transport of reactant product molecules and heavy coke precursors. Whereas, the precipitation catalyst possesses randomly small distributed pores, trapping coke precursors to form coke particles that deactivate the catalyst (Figure I-12). Coating Ni/Al2O3 catalysts with alumina ALD overcoats have proved resistance to sintering and coking with higher stability compared to the uncoated material attributed to the formation of NiAl2O4 species that preserves the small nickel particle size [85]. However, most of the initial active sites were encapsulated by the ALD alumina resulting in lower initial activity in DRM. Recently, attention was directed towards synthesizing mesoporous Al2O3 supports that can boost activity and stability. Several methods were applied such as the nanocasting method (SBA-15 or Carbon as hard templates) or the modified sol-gel self-assembly

Figure I- 12 :

 12 Figure I-12: Schematic illustrating the catalyst morphology effect on coke catalyst deactivation[START_REF] Kim | Coke tolerance of Ni/Al2O3 nanosheet catalyst for dry reforming of methane[END_REF]

  This technique determines the crystalline structure and size of crystalline domains in the catalyst. An incident X-ray beam is directed towards the solid material used in which the intensity of the diffracted Xray is collected (Figure II-2). Diffraction occurs when the angle between the incident rays and a crystallographic plan is identical to the angle between this plan and the diffracted rays. The resulting constructive interference of the diffracted beam enhances the diffracted intensity following what is called the Bragg's law 2dsinθ=nλ [157] where, θ is half the diffraction angle, n is the order of diffraction (equal to 1), λ is the wavelength of the radiation, d is the distance between two planes of the same crystallographic family (Å) with indices hkl. Procedure: The small angle XRD patterns were measured in the 2 Theta range between 0.5 and 5°on a Brüker D8 diffractometer (available at LRS, Paris) operating at 30 kV and 10 mA, using the CuKα wavelength (1.5418 Å) and a step size of 0.01° (1 s per step). These patterns were recorded for the calcined and reduced Ni-based materials. The wide angle XRD measurements were conducted on a PANalytical XPert 3 diffractometer in Lebanon (Cu Kα, λ=1.5405 nm) in a 2 Teta range between 5 and 90°. A voltage of 30 kV was used with a current of 10 mA and a step size of 0.02° at 2 seconds time per step.

Figure II- 2 :

 2 Figure II-2: Mode of X-ray diffraction

Derived results :

 results The content in elements obtained by the analyses were used to calculate the chemical compositions of the material studied (Al, Ni, Si). II.1.2. Electron microscopies II.1.2.1. Transmission electron microscopy (TEM) Principle of the technique: Information about the support morphology and nickel nanoparticles dispersion will be trivially identified in this technique. The electron beam passes through the sample where the electrons are absorbed as a function of sample density, thickness, and chemical nature. The electrons interaction forms an image on Ni-Al2O3 material morphology with Ni particles appearing as black dots since it is thicker than Al2O3 support.

Figure II- 3 :

 3 Figure II-3: Principle of X-ray fluorescence

Derived results :

 results The grain morphology and external surface of the sample are visualized with depiction of external Ni particles if present and visualization of carbon species over spent catalysts. II.1.3. Spectroscopies II.1.3.1. X-ray photoelectron spectroscopy (XPS) Principle of the technique: It allows for a clear understanding of the nature and chemical states of the elements present on a material surface. Photoelectrons are ejected from core energy levels of different atoms in the sample after X-ray bombardment. They are then collected and their kinetic energy (Ek) measured is transformed to binding energy (EB) by a

II. 1 . 4 .Figure

 14 Figure II-4: TPR schematic

  Principle of the technique: H2chemisorption measurements allow for the estimation of the number of metal surface sites which is used in the calculation of turn over frequency (TOF) for any metal-containing catalyst for a specific reaction. This method is of peculiar interest when the optimization of a catalytic formulation is targeted as it estimates the available metal surface area of samples resulting from various preparation procedures and conditions (temperature, type of support), and thus provides an indirect information about the catalytic reactivity of the catalyst. Metal adsorption sites must be accessible in large amount to the probe molecule. Weakly adsorbed species "R" (reversible adsorption) are most often present on the surface of the support, whereas more strongly chemisorbed species "I" (irreversible adsorption) are usually bound to metals [163] (Figure II-5). To distinguish between reversible and irreversible adsorption, a second chemisorption isotherm is run after the sample is evacuated following the first chemisorption isotherm to remove the weakly adsorbed molecules. In this 2 nd chemisorption isotherm measurement, the surface metal sites are already covered by a monolayer of chemisorbed species and thus the recorded H2 uptake is only related to the reversibly adsorbed species. The isotherm due to irreversible adsorption of the probe (Figure II-6) is obtained by subtracting the uptake of the reversibly adsorbed species (2 nd isotherm) from the total uptake (reversible + irreversible, 1 st isotherm). The dissociative adsorption of H2 occurs onto 2 surface metal sites (H2 + 2Ms → 2H-Ms, Ms is a surface metal atom (Ni)).Procedure: H2-chemisorption measurements were conducted on BELsorp-max (MicrotracBEL) apparatus. The samples (~ 200 mg) were firstly reduced under hydrogen for 2 hours at 650/800 °C with a heating rate of 10 °C.min -1 before evacuation at 620/770 °C for 1h.Then the H2-chemisorption experiments were performed at 298 K in the 0-50 kPa domain with intermediate evacuation of the samples for 2 h at 298 K between the first and the second isotherm.

Figure II- 5 :

 5 Figure II-5: Steps required to determine chemisorption isotherm

7 Figure II- 7 :

 77 Figure II-7: Thermodynamic equilibrium curve for DRM (CH4:CO2 = 1:1) reactants and products at P= 1 atm (A) without and (B) with carbon as side product and Argon as diluent

  Figure II-8: MAR equipment and Micro-GC

2. 2 3 .

 23 min), CO (RT: 3.2 min) and H2 (RT:0.8 min) gases and plot U column (column B) adsorbs CO2 (RT: 0.65 min) gas. CH4 is detected as well on Plot U column but the Mol Sieve column has been used for CH4.The PID (Process Integral Development) diagram (FigureII-9) provides a detailed description of the interior operation, divided into 3 main parts. The first one includes the gas delivery system including in our case CH4, CO2, H2 and Ar gases connected each to a mass flow controller to obtain the required flow rate before being mixed together in a gas mixer. The 2 nd part of the scheme involves the hot box inside which the main reactor is settled. Thus, the mixed gas reactants are entered into the hot box that is equipped with an electric forced convention heater to control the temperature (between 160 and 180 °C) to restrict any possible condensation. The preheated streams then merge into a 6-port valve controlled to direct the flow path either towards the reactor or rerouting it towards the system bypassing the reactor.Reaching the fixed bed tubular reactor (Hastelloy X), the gas flow inside is up-down where the reactants are fed from the upper part and the products are collected from the lower part of the reactor. The reactor is equipped with a thermocouple in contact with a catalyst bed placed inside a porous plate deposited between 2 layers of quartz wool, it has a length of 305 mm, an external diameter of 14.5 mm, an internal diameter of 9 mm, a maximum recommended temperature of 1200 °C and a maximum pressure of 100 bar. The product gases at the reactor outlet pass through the hot box to the 6-port valve reaching the liquid-gas separator (Peltier) to condense the liquid at low temperature. The remained gases are again directed to the hot box under controlled pressure (micrometric regulating valve) before being analyzed online by micro gas chromatography. Explanation of procedure adopted Any calcined sample is first introduced inside the Hastelloy-X reactor and a leak test is performed prior to the reaction. It is done by using argon as an inert gas with a flow rate of 50 ml.min -1 that contributes to a continuous increase of pressure until reaching a set value of 5 bar (otherwise a leak is present in the line connections). After attaining 5 bar, the gas inlet (Ar) is closed, and the maintaining of the pressure for a sufficient period of time (5 minutes) is

Figure II- 9 : 1 .

 91 Figure II-9: General scheme of dry reforming system

Figure II- 10 :

 10 Figure II-10: Temperature profile during insitu reduction (grey area), activity (orange area), and stability (green area) tests in DRM

Figure II- 12 :

 12 Figure II-12: General scheme of the CO2 methanation system

II. 2 . 2 . 3 .

 223 Explanation of procedure adopted 200 mg of catalyst is used for the CO2 methanation reaction. As for DRM, the catalyst is first pre-treated in-situ at 650/800 °C for 1 h with a heating rate of 5 °C.min -1 and an 80 vol% H2/N2 flow of 250 ml.min -1 at atmospheric pressure to reduce nickel oxide species to metallic Ni 0 which is also the active phase for methanation (Figure II-13). After reduction, the reactor is cooled down to 250 °C and the feed gas is switched to CO2 methanation reactants containing H2, CO2 and N2 with a molar ratio of 36:9:10 respectively. The activity is then followed while increasing the temperature gradually from 250 °C till 450 °C (50 °C step size) with a heating rate of 5 °C.min -1 and a total flow of H2 and CO2 equal to 288 ml.min -1 . At each temperature, CO2 conversion and CH4 selectivity are measured for 3 successive times after stabilization. The stability test was conducted at 300 °C

Figure II- 13 :

 13 Figure II-13: Temperature profile during in-situ reduction (grey area), activity (orange area), and stability (green area) tests in CO2 methanation

[ 168 -

 168 Figure III-1: SEM images of spent Ni/Mg/Al2O3catalysts[START_REF] Arbag | Effect of impregnation sequence of Mg on performance of mesoporous alumina supported Ni catalyst in dry reforming of methane[END_REF]

  contents. Nevertheless, all examples in Figure III-2 show an optimum in Mg content giving the best conversion level. An explanation of such specific behavior has to our knowledge not been detailed yet and will be targeted in this chapter. Thus, the current work aims in producing a defined catalyst porosity, using fewer preparation steps than the previous mentioned ones.Moreover, an inspection on Mg content influence regarding the physicochemical properties and the catalytic performance of Ni-Mg-Al2O3 material that resembles a parabola (volcano-shape curve) is constructively demonstrated.

Figure III- 2 :III. 2 .Figure

 22 Figure III-2: Variation of CH4 conversion as a function of Mg content in different methods of Ni-Mg-Al2O3 preparation present in literature: Sol-gel[START_REF] Min | Carbon dioxide reforming of methane on Ni-MgO-Al2O3 catalysts prepared by sol-gel method: Effects of Mg/Al ratios[END_REF], 2-step hydrothermal[START_REF] Zhang | Dry reforming of methane over Ni/MgO-Al 2 O 3 catalysts prepared by two-step hydrothermal method[END_REF], co-precipitation[START_REF] Zhan | Biogas reforming of carbon dioxide to syngas production over Ni-Mg-Al catalysts[END_REF], and co-precipitation[START_REF] Hou | Meso-porous Ni/Mg/Al catalysts for methane reforming with CO2[END_REF]

  Figure III-4: Chronological steps summary of preparing mesoporous one-pot Nix-Mgy-Al2O3 following EISA route

*

  Atomic formula does not take into account oxygen III.3. Textural and structural properties of the calcined Ni5-Mgy-Al2O3 materials III.3.1. Porosity by N2-sorption technique The nitrogen physisorption isotherms of the calcined Ni5-Mgy-Al2O3 samples as well as their corresponding pore size distributions are displayed in Figure III-5. All the materials exhibit a type IV isotherm with H1 hysteresis loop attributable to mesoporous solids [155]. All samples with a magnesium weight percent between 0 and 7 wt% (Figure III-5 A) exhibit capillary steps of very steep hysteresis loop, which suggests the presence of uniform mesopores carrying cylindrical shaped patterns [155] [180,182]. Further increase in magnesium weight (Ni5-Mg30-Al2O3 and Ni5-Mg50-Al2O3 samples) leads to a decrease in the steepness of capillary condensation step and alteration in the hysteresis loop (Figure III-5 B) indicative of some loss of mesopores ordering. Simultaneously, a shift of the condensation step to a higher relative pressure is seen, revealing the formation of larger mesopores [180]. This is indeed confirmed by the BJH curves derived from the desorption part of the N2-sorption presented as insets in Figure III-5. For instance, Ni5-Mg30-Al2O3 and Ni5-Mg50-Al2O3 exhibit broader pore size distribution leading to larger mean pore diameter compared to the other samples with narrow pore size distribution and eventually a smaller pore diameter.

Figure III- 5 :

 5 Figure III-5: Nitrogen adsorption/desorption isotherms and pore size distribution profiles (insets) of Ni5-Mgx-Al2O3 catalysts calcined at 600 °C: (a) Al2O3, (b) Ni5-Al2O3, (c) Ni5-Mg5-Al2O3, (d) Ni5-Mg10-Al2O3, (e) Ni5-Mg15-Al2O3, (f) Ni5-Mg30-Al2O3, (g) Ni5-Mg50-Al2O3

  Figure III-6: (A) low and (B) wide angles XRD patterns of Ni5-Mgx-Aly catalysts calcined at 600 °C: (a) Al2O3, (b) Ni5-Al2O3, (c) Ni5-Mg5-Al2O3, (d) Ni5-Mg10-Al2O3 , (e) Ni5-Mg15-Al2O3, (f) Ni5-Mg30-Al2O3, (g) Ni5-Mg50-Al2O3

  d100 * : host Al2O3 periodicity (interplane distance) derived from low-angle XRD a0 * : lattice parameter, a0= 2d100/√3 t * : pore wall thickness, t= a0 -DBjH n.d. not determined The wide angle XRD curves with different magnesium content are presented in Figure III-6 B. Mesoporous alumina support (curve a) has no visible diffraction peak indicating an amorphous Al2O3 walls oxide. Ni5-Mgy-Al2O3 samples having Mg content of 7 wt% or less,exhibit no apparent NiO, MgO, and even Al2O3 diffraction peaks as well. This observance means that NiO, MgO, and Al2O3 crystallites are either lower than the detection limit in XRD (smaller than 3 nm) or that the nickel and Al2O3 present are not crystalline (amorphous phase).

  Figure III-7 shows typical micrographs obtained for the calcined mesoporous Ni5-Al2O3 sample. Sticking to the HRTEM mode when the alumina grains are parallel to the electronic beam, arranged and parallel channels are intelligible and clear (Figure III-7 A). Moreover, Figure III-7 A' is the result of a perpendicular orientation of the grain to the beam and depicts very regular hexagonal pore apertures at high magnification (2D p6mm hexagonally structured mesoporous channels) confirming N2-sorption results. Both orientations show absence of Ni particles to the naked eye, in line with the absence in section III.3.2 of any XRD detection of NiO crystals for this sample. In order to have more precise and clearer observations, STEM-HAADF images (Figure III-7 B-B') were performed on the same sample since it plays more on the atomic contrast and could help differentiate between Ni and Al species. However, again, the pictures did not contribute to detection of any Ni-based particles distinct from the alumina. These results, in coherence with XRD ones, insist on the excellent dispersion of Ni with a size of nano-clusters undetectable or inserted inside the wall of the mesoporous alumina framework. In agreement with textural properties discussed above (section III.3.1), the successive increase of MgO content till 7 wt% does not alter the well-organized parallel channels and the honeycomb orientation of the alumina pores (Figure III-8 A-A'). Again, no Ni-based particles are detected by HRTEM and Ni remains homogenously spread on the alumina support. On the contrary, further magnesium increment (Figure III-8 B-B') confirms the deterioration of the structured channels and pores (suspected in N2-sorption), and formation of ill-defined pore morphology. Few nickel oxide particles start to appear at this stage, indicating a less homogeneous distribution of Ni, and Ni-based (NiO) particles become clearly distinct when 26 weight percent of magnesium is present in the sample. In addition, Figure III-8 B' depicts a darker zone over alumina support that could be due to magnesium segregation justifying the MgO peaks observed in XRD experiments that are superposing those of NiO peaks. This hypothesis cannot be confirmed without making mapping and HAADF on the same area of the

Figure III- 7 :

 7 Figure III-7: Representative HRTEM (A-A') and STEM-HAADF (B-B') micrographs of calcined mesoporous Ni5-Al2O3 showing the intimate mixing between Ni and its alumina support after synthesis; (B-B') are pictures for preliminary experiments done on MPQ-Paris Diderot microscopy (JEOL ARM 200F, coll. J. Nelayah)

Figure III- 8 :

 8 Figure III-8: Representative STEM micrographs of calcined mesoporous (A-A') Ni5-Mg15-Al2O3 and (B-B') Ni5-Mg50-Al2O3 catalysts

Figure III- 10

 10 Figure III-10 displays the TPR (temperature-programmed reduction) profiles of calcined Ni5-Mgy-Al2O3 materials with different Mg contents. Beginning with the Al2O3 support, no reduction peak is observed confirming that the support does not play a role in H2consumption. Ni5-Mgy-Al2O3 materials with Mg content up to 7 wt% show a single hydrogen reduction peak at a temperature range between 640 and 710 °C. For all these samples, the H2-uptake calculated from the area under the curve (Figure III-10) is close to the theoretical value (860 μmol.g -1 ) confirming the sole Ni reduction and the expected 5 wt% Ni content (TableIII-6). The absence of a reduction peak in the range of 300-400 °C reveals that there is no free

Figure III- 10 :

 10 Figure III-10: H2-TPR profiles of mesoporous Ni5-Mgx-Al2O3 catalysts calcined at 600 °C: (a) Al2O3, (b) Ni5-Al2O3, (c) Ni5-Mg5-Al2O3, (d) Ni5-Mg10-Al2O3, (e) Ni5-Mg15-Al2O3, (f) Ni5-Mg30-Al2O3, (g) Ni5-Mg50-Al2O3, (h) Mg50-Al2O3

Table III- 6 :Figure III- 11 :III. 5 .

 6115 Figure III-11: XPS spectra of the Ni2p in calcined (a) Ni5-Al2O3 and (b) Ni5-Mg15-Al2O3 materials

Figure III- 14 :

 14 Figure III-14: SEM micrographs of in-situ reduced (650 °C/2h, H2/Ar) (A) Ni5-Al2O3, (B) Ni5-Mg15-Al2O3, (C) Ni5-Mg50Al2O3

Figure III- 16 :

 16 Figure III-16: Representative standard HRTEM micrographs of reduced (650 °C/2h) mesoporous (A) Ni5-Mg15-Al2O3, (B) Ni5-Mg30-Al2O3 and (C) Ni5-Mg50-Al2O3 and STEM-HAADF (A'-A'') of Ni5-Mg15-Al2O3 showing presence of Ni nano-species after reduction

Figure III- 18 :

 18 Figure III-18: H2-TPD profiles of alumina based-materials: (a) Ni5-Al2O3, (b) Ni5-Mg10-Al2O3, (c) Ni5-Mg15-Al2O3, (d) Ni5-Mg30-Al2O3, and (e) Ni5-Mg50-Al2O3

  the conversions obtained are almost half of those of the current results with very high percentage of coke produced (37%). This reveals a very high activity for both of our catalysts, in line with the excellent Ni dispersions deduced above from physicochemical characterizations on both samples.

Figure III- 19 :

 19 Figure III-19: Variation of (A) CH4 (filled circle) and CO2 conversion (empty circle), and (B) H2:CO ratio as a function of time on stream during DRM (T= 650 °C, P=1 atm, CH4:CO2=1:1, GHSV= 36 L.g -1 .h -1 ) over (a) Ni5-Al2O3 and (b) Ni5-Mg15-Al2O3

is higher than 1 (

 1 . This agrees well with the proposal discussed in chapter I that adding a moderate amount of Mg boosts the creation of basic sites that favor CO2 chemisorption on the catalyst, and hence its reaction with formed carbon species, thus reducing carbon deposition. Nevertheless, the promoting effect of Mg is not clearly intelligible, and cannot be analyzed with great accuracy, because of the operating conditions (limited GHSV) that lead to conversions close to equilibrium. III.7.3. Performances of the Ni5-Mgy-Al2O3 catalysts at a GHSV of 180 L.g -1 .h -1 III.7.3.1. Light off curves Higher GHSV (180 L.g -1 .h -1 ) was applied to lower the conversions and allow for a correct comparison of performances between the prepared Ni5-Mgy-Al2O3 catalysts. Figure III-20 shows results of light off curves (conversions followed while increasing the temperature) for four chosen catalysts (Ni5-Al2O3, Ni5-Mg15-Al2O3, Ni5-Mg30-Al2O3, and Ni5-Mg50-Al2O3). Both CH4, and CO2 conversions in graphs A and B increase with temperature no matter what is the amount of MgO indicative of an endothermic DRM as demonstrated in the thermodynamics graphs in chapter II. Conversions on mesoporous Ni5-Mg15-Al2O3 are always the highest compared to the other three samples, and by more than 20% for instance at the temperature of 750 °C. Increasing further the amount of Mg (Ni5-Mg30-Al2O3) is no longer beneficial and leads to a catalyst with similar activity to that of the Mg-free Al2O3 based sample.

Figure III- 20 :

 20 Figure III-20: Variation of (A) CH4 and (B) CO2 conversion levels as a function of temperature (400-800 °C) during DRM (P=1 atm, CH4:CO2=1:1, GHSV=180 L.g -1 .h -1 ) over (green) Ni5-Al2O3, (blue) Ni5-Mg15-Al2O3, (pink) Ni5-Mg30-Al2O3, (purple) Ni5-Mg50-Al2O3

4 .

 4 Turn over frequency and activation energy of Ni5-Mgy-Al2O3 catalysts Turn over frequency (TOF) are used to make a direct comparison between the samples since they consist of normalized values taking into account the dispersion and number of metallic active sites. The TOF of each sample was calculated by taking an average value of the conversion at 650 °C after 2 hours reaction and presented in TableIII-7. The TOF value increased from 5.77 s -1 in Ni5-Al2O3 to 6.64 s -1 in Ni5-Mg15-Al2O3 that is the maximum value achieved, then decreased to reach 3.03 s -1 in Ni5-Mg50-Al2O3. Table III-7: Apparent activation energies (Ea) for the prepared Ni5-Mgy-Al2O3 catalysts To further understand the promotion effect of MgO for DRM, the activation energy (Ea) of all the catalysts was obtained based on Arrhenius formula over a temperature range of 550-700 °C with much higher space velocity of 666 L.g -1 .h -1 guarantying both low conversion levels (<15%) and absence of transport effect necessary for obtaining kinetics information. The relationship between natural logarithm of CH4 consumption rate and reciprocal of temperature is depicted in Figure III-23 and the values of Ea are presented in Table III-7. The intrinsic experimental data of each catalyst are taken after 60 minutes time on stream to ensure stable performance.

  Figure IV-1: Types of several MOFs and their applications

Figure IV- 2 Figure IV- 2 :

 22 Figure IV-2: Chronological steps summary of preparing mesoporous Ni-Al2O3 with 5wt% nickel following the new metal organic framework route

Figure IV- 3 : 95 Table IV- 3 :Figure IV- 4 :

 39534 Figure IV-3: Schematic representing two types of Ni-Al2O3 catalyst derived from MIL-53 framework based on the order of Ni impregnation with respect to calcination

53 sample to be

  Figure IV-5: N2 physisorption isotherms after degassing at 120 °C of: (a) freshly activated MIL-53, (b) rehydrated MIL-53, (c) Ni/MIL-53, (d) Ni5-Al2O3 MIL-53 and (e) Ni5 0 -Al2O3 MIL-53. Filled and empty symbols correspond to adsorption and desorption isotherms, respectively.

Figure IV- 6 A

 6 Figure IV-6 A shows the TGA profiles obtained on freshly activated MIL-53, rehydrated MIL-53 and impregnated Ni/MIL-53 (curves a to c, respectively). A weight loss of about 15 % between 200 °C and 420 °C is distinctly seen for impregnated Ni/MIL-53 (Figure IV-6 A c) but not for the Ni-free materials. As confirmed by the simultaneous MS detection of NO (mass 30) in the exhaust gas (Figure IV-6 B, bottom, blue curve), this weight loss

Figure IV- 6 :

 6 Figure IV-6: (A) TGA (top curves), DTG (middle curves) and DTA (bottom curves) profiles for samples: (a) freshly activated MIL-53, (b) rehydrated MIL-53 and (c) impregnated Ni/MIL-53. (B) Mass spectrometry profiles corresponding to the detection of H2O (mass 18), NO (mass 30) and CO2 (mass 44) for hydrated MIL-53 (red dotted lines) and impregnated Ni/MIL-53 (blue lines). The temperatures of peak maxima in B are slightly higher than in A due to the delay for the exhaust gases to reach the MS analysis chamber.

100 IV. 4 .

 1004 Figure IV-7 and Figure IV-8 show typical SEM and TEM micrographs of the parent (MIL-53) and impregnated then calcined (Ni5-Al2O3 MIL-53) materials. In both scanning (Figure

Figure IV- 7 :

 7 Figure IV-7: Representative SEM images of (a) MIL-53, (b) Ni5-Al2O3 MIL-53, (c) Al2O3 MIL-53 and (d) Ni5@Al2O3 MIL-53

a

  Figure IV-8: Representative HR-TEM images of (a) MIL-53, (b) Ni5-Al2O3 MIL-53, (c) Al2O3 MIL-53 and (d) Ni5@Al2O3 MIL-53

Figure IV- 9 :

 9 Figure IV-9: Diffractograms of the Ni containing samples: (a) Ni5-Al2O3 MIL-53, (a') Ni5 0 -Al2O3 MIL-53, (b) Ni5@Al2O3 MIL-53, (b') Ni5 0 @Al2O3 MIL-53, (c) Ni5@Al2O3 COM, (c') Ni5 0 @Al2O3 COM. The diffractograms of the preformed alumina supports (light grey lines) are shown for comparison: (b*) Al2O3 MIL-53 and (c*) Al2O3 COM. Intensities are divided by 3 in patterns c*, c and c'.

Al2O3 MIL- 53 )

 53 , resembles a conventional preparation route. From XRD, the structure of Al2O3 MIL-53 is typical of an amorphous alumina (Figure IV-9 b*). It changes after Ni impregnation and calcination (sample Ni5@Al2O3 MIL-53, Figure IV-9 b) to a crystalline phase very similar to that in Ni5-Al2O3 MIL-53 except for two reflections

Figure IV- 11 :

 11 Figure IV-11: STEM-HAADF image of Ni5-Al2O3 MIL-53 (left) and EDX elemental mappings of the same area for Al (in green) and Ni (in yellow).

Figure IV- 12 :

 12 Figure IV-12: Representative TEM micrographs with its corresponding local SAED image over selected grain zones of calcined Ni5-Al2O3 MIL-53

5 .

 5 Figure IV-10 c), indicating a weaker Ni-support interaction than in both MOF-based samples, especially Ni5-Al2O3MIL-53. Similarly to Ni5@Al2O3 MIL-53, a fraction of the nickel in Ni5@Al2O3 COM is probably existing in the material as amorphous NiO, as revealed by the TPR signal observed at relatively low temperature (T < 700°C). The significantly higher dispersion and stabilization of Ni in Ni5-Al2O3 MIL-53 sample can be explained as follows: the impregnation of nickel nitrate on the parent MIL-53 with especially high surface area enables the initial occlusion of the nickel precursors along the organic linkers and [Al(OH)O]n clusters decorating the pores, leading after the MOF calcination step (organic removal) to the formation of an Al2O3-NiAl2O4 phase in which nickel and aluminum cations are intimately mixed in a homogenous oxide phase. In contrast, impregnation of a preformed alumina (obtained by calcination of either MIL-53 or boehmite) then impregnation and calcination leads to a mixture of alumina, NiAl2O3 and NiO with an overall weaker interaction between nickel and the alumina matrix. Interestingly, the presence of nickel at the very first step of the preparation seems to direct the formation of the crystalline spinel NiAl2O4 phase during calcination (giving Ni5-Al2O3 MIL-53), while no spinel phase is formed when starting from pure MIL-53 (giving amorphous Al2O3 MIL-53). Chapter IV Porous nickel-alumina derived from metal-organic framework (MIL-53) in DRM IV.Physicochemical properties of reduced Ni5-Al2O3 MIL-53, Ni5@Al2O3 MIL- 53, and Ni5@Al2O3 COM Previous to catalytic tests, all the prepared nickel-alumina materials were treated under a flowing H2 atmosphere to obtain reduced Ni 0 , the active phase for DRM. The H2 consumption during TPR was used to estimate the amount of reducible Ni 2+ in all nickel containing samples, considering the consumption of one H2 molecule per Ni 2+ cation. For all samples, the Ni content is in accordance with that measured by X-fluorescence and close to the expected 5 wt% Ni, even if slightly lower, by less than 10%, for sample Ni5 0 @Al2O3 MIL-53 (Table IV-4). The XRD patterns of the samples reduced at 800 °C are shown in Figure IV-9 and compared to those before reduction. The reduction of Ni5-Al2O3 MIL-53 to Ni5 0 -Al2O3 MIL-53 leads to the formation of new diffraction peaks assignable to the (111), (200) and (220) planes of crystalline Ni 0 (at 2θ = 44.5°, 51.9° and 76°, respectively, Figure IV-9 a'). Bands attributable to an alumina-based phase are still present but they are slightly shifted (compared to those before reduction) towards positions characteristic of pure γ-Al2O3 which attests of Ni extraction from the oxide matrix towards the pore surface. The reduction of the sample was followed by H2 in-situ XRD techniques (done at University of Lille in collaboration with "unité de catalyse et chimie du solide" (UCCS)) in the temperature range of 25-850 °C (Figure IV-14). In good agreement with XRD and TPR measurements, the reduction does not start before 700 °C and it is at this temperature also that we start to see disappearance of NiAl2O4 spinel and presence of metallic Ni 0 species and γ-Al2O3 phase. Application of the Scherrer equation (Figure IV-9) to the Ni 0 reflection at 2θ = 51.9° leads to a mean Ni 0 nanoparticle size of 5.3 nm, slightly smaller than the value estimated by HR-TEM after measuring the sizes of at least 500 nanoparticles (Table IV-4). Considering the TEM image of Ni5 0 -Al2O3 MIL-53 shown in Figure IV-15 a and the non-symmetrical shape of the histogram of particles sizes for this sample (Figure IV-16), it can be assumed that this average size discrepancy comes from contrast issues making uneasy the detection of the smallest

Figure IV- 14 :

 14 Figure IV-14: In-situ XRD patterns of Ni5-Al2O3 MIL-53 during reduction by hydrogen (5% H2 in Ar) up to 850 °C

Figure IV- 16 :IV. 6 .

 166 Figure IV-16: Histograms established from at least 500 Ni 0 nanoparticles in typical HR-TEM images of (A) Ni5 0 -Al2O3MIL-53, (B) Ni5 0 @Al2O3 MIL-53 and (C) Ni5 0 @Al2O3 COM before and after 13h of catalytic test (black and white bares, respectively). Related mean sizes are shown in (D) for the catalysts before (black circles) and after (grey circles) test

Figure IV- 17

 17 Figure IV-17: (A) CH4, (B) CO2 conversions, and (C) H2:CO product ratio during DRM upon continuous increase of temperature from 400 °C till 750 °C over in-situ reduced (800 °C/2h): (blue) Ni5 0 -Al2O3 MIL-53, and (orange) Ni5 0 @Al2O3 COM. Grey columns are the calculated values at the thermodynamic equilibrium for the CO2, CH4 conversions and H2:CO ratio

Figure IV- 18 :

 18 Figure IV-18: Comparison of the catalytic performances in DRM of (a) Ni5 0 -Al2O3 MIL-53, (b) Ni5 0 @Al2O3 MIL-53 and (c) Ni5 0 @Al2O3 COM as a function of (A) CH4 and (B) CO2 conversions measured during 13h of reaction. Black continued and dashed lines are the calculated values at the thermodynamic equilibrium for the CH4 and CO2 conversions, respectively

h

  on stream remains nearly equal to that measured at the early stage of the reaction (Figure IV-20).

Figure IV- 19 : 111 VI. 6 . 3 .

 1911163 Figure IV-19: H2:CO ratios in DRM (a) Ni5 0 -Al2O3 MIL-53, (b) Ni5 0 @Al2O3 MIL-53 and (c) Ni5 0 @Al2O3 COM

Figure IV- 21 :

 21 Figure IV-21: Raman spectroscopy profiles of the spent catalysts: a) Ni5 0 -Al2O3 MIL-53, (b-) Ni5 0 @Al2O3 MIL-53, and (c) Ni5 0 @Al2O3 COM

  chapter V is to check if the good materials seen in the previous chapters are efficient in another reaction of strong interest in the energy domain, CO2 methanation. The principle of this application is to produce methane by converting CO2 (greenhouse valorization) together with H2 (generated by electrolysis from renewable energy surplus (wind/sun)). A brief summary about CO2 methanation process and the Ni-Al2O3 based catalysts state-of-art are first reported. Then the preparation method and the composition of the Ni-Al2O3 materials used are described including different Ni loading and other classical Ni-based support (for comparison), before presenting their physicochemical properties and catalytic performances in this specific reaction. Note that this work was done in collaboration with the Portuguese team (University of Lisbon) in the core of the ERANETMED project providing me with the opportunity to do a 1 month internship in their laboratory and using the available facilities (i.e. CO2 methanation equipment).V.1. Literature ReviewV.1.1. General definition CO2 methanation is the hydrogenation of CO2 to methane, also known as Sabatier reaction. This process is receiving great attention because it is based on the production of CH4

Figure V- 1 :

 1 Figure V-1: Schematic describing wind to natural gas concept for bidirectional coupling of the electricity andgas grids[START_REF] Centi | Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries[END_REF]

Figure V- 2 :

 2 Figure V-2: Chemical energy storage usingheat from renewables and DRM and methanation as energy carriers[START_REF] Wang | Carbon dioxide reforming of methane to produce synthesis gas over metal-supported catalysts: State of the art[END_REF]

  2 for mesoporous Nix-Mgy-Al2O3 and section IV.2 for mesoporous Nix-Al2O3 derived from MIL-53(Al). The only difference that might reside between the samples from this chapter and the previous ones is the amount of Ni added. Table V-2 and Table V-3 summarize all the mesoporous magnesium-based alumina and MOF derived samples, respectively. Some of the obtained Ni-Al2O3 samples (MOF-based materials specifically) were also compared to other more classical materials used in the Portuguese partner team as listed at the bottom of Table V-3. They all contain 15 wt% Ni Chapter V Mesoporous nickel-alumina catalysts for CO2 methanation 123 deposited by incipient wetness impregnation on Cs-USY zeolite (Si/Alglobal of 38) [269,270], mesoporous commercial SBA-15 [271], and one-pot EISA alumina supports. The following results are divided into two parts. The first part tackles the magnesiumbased mesoporous alumina materials and the effect of nickel and magnesium amount on CO2 methanation. The second part includes the MOF based alumina samples with different nickel contents that are in turn compared to other Ni-based samples and evaluated also in CO2 methanation reaction.

124 V. 3 .

 1243 Physicochemical properties followed by CO2 methanation over mesoporous Nix-Mgy-Al2O3 materials V.3.1. Variation of Mg content and its influence on CO2 methanation V.3.1.1. Physicochemical properties In this part, three samples were chosen for comparison: Ni5-Al2O3, Ni5-Mg15-Al2O3 and Ni5-Mg50-Al2O3. To avoid repetition, their physicochemical properties results are not presented here since they are detailed in Chapter III, and Table V-4 summarizes certain values. Note that the values in brackets and italic font for the surface area and pore volume as well as the italic font for the nickel nanoparticles size correspond to the reduced form of the materials.

  determining the h index by TGA-DSC experiments that were done in Portugal. Briefly, an h index value close to 1 indicates that the samples are not strongly interacting with water (most of the water molecules will be easily desorb below 150 ºC) while indexes close to 0 indicate stronger interaction between water and the catalysts (only a small fraction of water molecules desorb below 150 ºC). As seen in Table V-4, increasing the Mg loading strengthens the interaction between water and the samples with h index decreasing from 0.55 in Ni5-Al2O3 to Chapter V Mesoporous nickel-alumina catalysts for CO2 methanation 125 0.38 in Ni5-Mg50-Al2O3. This decrease can lead to a higher water concentration over the catalysts during the reaction and higher inhibitory effect. The interaction of the samples with CO2 was also characterized by collecting CO2 isotherms (experiments performed in Portugal by degassing the calcined samples then adsorbing CO2 at 0 °C). This technique, was adopted in the present study for comparing the affinity of the samples towards CO2. Results, illustrated in Figure V-3, present the same CO2 uptake for all the three materials at high pressures but, in terms of affinity (low relative pressure points), Ni5-Mg50-Al2O3 presents significantly enhanced CO2 uptakes. This agrees with the higher content of CO2 adsorbed on this sample compared to those containing 0 and 7 wt% Mg as seen by Infra-red data in chapter III (Figure III-9). V.3.1.3. CO2 methanation on Ni5-Mgy-Al2O3 mesoporous materials Catalytic test results of these materials are depicted in Figure V-4. CO2 conversions increase with temperatures, and do not follow the trend of the theoretical CO2 conversion curve that starts at very high value (97 %) and decrease with temperatures. This difference is due to the presence of a barrier in the kinetics that limits CO2 activation. As for the variation in Mg content, mesoporous Ni5-Mg15-Al2O3 catalyst has higher CO2 conversions than Ni5-Al2O3 over all temperatures range. This behavior resembles what was seen for DRM reaction and solidifies the efficiency of Mg in enhancing performances of the catalyst by increasing the surface basicity that

Figure V- 3 :

 3 Figure V-3: CO2 isotherms obtained for (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3 and (c) Ni5-Mg50-Al2O3 samples

  Figure V-5: XRD patterns of calcined (a) Ni5-Mg15-Al2O3, (b) Ni10-Mg15-Al2O3, (c) Ni15-Mg15-Al2O3, and (d) Ni20-Mg15-Al2O3 samples

Figure V- 6 :

 6 Figure V-6:N2-sorption curves of (A) calcined and (B) reduced (a-a') Ni5-Mg15-Al2O3, (b-b') Ni10-Mg15-Al2O3, (cc') Ni15-Mg15-Al2O3, and (d-d') Ni20-Mg15-Al2O3 samples

from 4 .

 4 5 to 9 nm in the Ni content range of 5-20 wt% as demonstrated by TEM (Table V-4). V.3.2.3. CO2 methanation on mesoporous Nix-Mg15-Al2O3 catalysts All the reduced samples were tested in CO2 methanation (Figure V-8). When going from the Ni5-Mg15-Al2O3 to Ni10-Mg15-Al2O3 then Ni15-Mg15-Al2O3 catalysts the activity (CO2 conversion) progressively increases in line with the increment in Ni content, each metal site being active for the dissociation of H2 into H atoms [290-294]. This increase is however not totally proportional to Ni content due to slight changes in the metal dispersion (Table V-4). When moving from Ni15-Mg15-Al2O3 to Ni20-Mg15-Al2O3, the CO2 conversion is no longer

Figure V- 7 :

 7 Figure V-7: (A) CO2 isotherms and (B) H2-TPR profiles of calcined (a-a') Ni5-Mg15-Al2O3, (b-b') Ni10-Mg15-Al2O3, (c-c') Ni15-Mg15-Al2O3, and (d-d') Ni20-Mg15-Al2O3 samples

Figure V- 9 : 130 V. 4 .

 91304 Figure V-9: Stability test in terms of (a-b) CO2 conversion and (a'-b') CH4 selectivity of (a-a') Ni5-Mg15-Al2O3 and (b-b') Ni15-Mg15-Al2O3

  that are firstly impregnated by Ni on MIL-53(Al) framework and later calcined. The textural properties obtained by physisorption are well confirmed by HRTEM images. For instance, both Ni5-Al2O3 MIL-53, and Ni15-Al2O3 MIL-53 present a fused ill-defined and defective alumina flakes composed of several randomly interwoven nanosheets with approximately 20 nm in length (Figure V-11 A, A' and B, B'). Ni15-Al2O3 MIL-53 (Figure V-11 B, B') has no detection of nickel particles confirming the results of XRD that reveal the presence of NiAl2O4 instead of NiO. This accentuates on the efficiency of MIL-53(Al) derived method to highly and homogeneously distribute Ni throughout the samples even if higher Ni amount is used.

  determined for the different Nix-Al2O3 MIL-53 samples are somehow quite similar (~0.6 in all cases, Table V-7) neglecting any impact of this parameter on the catalytic performances. V.4.2. Reducibility of Nix-Al2O3 MIL-53 materials The reducibility of all calcined Nix-Al2O3 MIL-53 materials (Figure V-12 A) with different Ni wt% (5-20 wt%) present similar reduction profiles with one reduction curve occurring at temperatures above 500 ºC. The absence of a second reduction peak at lower temperature indicates that the Ni species present in these sample are all strongly interacting with the support [271], as in spinel mixed phases with no weakly bounded Ni species [287-289]. These results agree well with what has been seen in XRD and TEM. Thus, the impregnation of nickel nitrate on the parent MIL-53 allows the formation of an Al2O3-NiAl2O4 phase in which nickel and aluminium cations are intimately mixed in a homogenous oxide phase over a wide range of Ni loading (5-20 wt%). Note that a small displacement in the maximum reduction temperature towards lower temperatures was observed for Ni20-Al2O3 MIL-53 (Figure V-12 A, c) insisting on the formation of small fraction of easier reducible Ni species that is in good agreement with XRD results showing a shift of the peak from 2θ of 65° to 63°.

Figure V- 11 :

 11 Figure V-11: HR-TEM Micrographs of fresh Ni-Al2O3 MIL-53 catalysts: (A-A') Ni5-Al2O3 MIL-53, (B-B') Ni15-Al2O3 MIL-53.

Figure V- 12 :

 12 Figure V-12: H2-TPR profiles for calcined (A) MOF-derived samples: (a) Ni5-Al2O3 MIL-53, b) Ni15-Al2O3 MIL-53, c) Ni20-Al2O3 MIL-53, and d) Ni15@Al2O3 MIL-53 and (B) other Ni-based materials: (e) Ni15@USY, f) Ni15@SBA-15, and g) Ni15-Al2O3

Figure V- 13 :CO2

 13 Figure V-13: HR-TEM Micrographs and Ni 0 particle size distribution of reduced catalysts: (A-A'') Ni5-Al2O3 MIL-53, (B-B'') Ni15-Al2O3 MIL-53, and (C-C'') Ni20-Al2O3 MIL-53

Figure V- 14 :

 14 Figure V-14: (A) CO2 conversion and (B) CH4 selectivity of a) Ni5-Al2O3 MIL-53, b) Ni15-Al2O3 MIL-53, c-) Ni20-Al2O3 MIL-53, and d) Ni15@Al2O3 MIL-53

Figure V- 16 : 1 Figure V- 15 :

 16115 Figure V-16: TEM micrograph of spent Ni15-Al2O3 MIL-53 after CO2 methanation reaction. Conditions: T=350 °C, GHSV=1440 ml.g -1 .min -1 , H2:CO2= 4:1

Figure V- 17 :

 17 Figure V-17: (A) CO2 conversion and (B) CH4 selectivity of (a) Ni15-Al2O3 MIL-53, (b) Ni15@USY, c) Ni15@SBA-15, d) Ni15-Al2O3

  Porous carbon; b MOF-5, UiO-66, MIL-101 structures not collapsed during the preparation nor the preactivation, * This work V.5. Conclusion In summary, mesoporous Ni5-Mgy-Al2O3 and Ni5-Al2O3 derived from MIL-53 framework, good in DRM (chapters III-IV), also exhibit good performances in CO2 methanation reaction. As for DRM, in the Ni5-Mgy-Al2O3 series of catalysts, an optimum amount of 7 wt% Mg favors both CO2 conversions and CH4 selectivity avoiding MgO segregation and keeping sufficient basic sites to activate CO2 species and enhance the reaction mechanism. This chapter also shows that Ni loading can be increased up to 15 wt% conserving to a certain extent very good dispersions in both the proposed one-pot EISA and MOF routes. This leads to enhanced conversion of carbon dioxide and high selectivity to methane provided by the high metallic surface area available for the dissociation of hydrogen. The obtained Ni15-Al2O3 MIL-53 catalyst has superior performances than USY zeolite, SBA-15 material and Ni-Al2O3 commercial or synthesized by a different method. Ni-Al2O3 derived from MIL-53 framework led to superior results compared to the few existing MOF-based materials already used in CO2 methanation. Needless to say, the promising activity, selectivity and stability of the synthesized materials turn them into potential catalysts for a large-scale application of power-to-gas as in DRM. Chapter V Mesoporous nickel-alumina catalysts for CO2 methanation Chapter VI Promotion of mesoporous nickel-alumina based catalysts by magnesium addition for reforming of waste gasification products VI. Chapter VI: Promotion of mesoporous nickel-nickel-alumina based catalysts by magnesium addition for reforming of waste gasification products 143

Table VI- 1 : 15 (

 115 Figure VI-1 A simulates case 1 with CH4:CO2 ratio of 0.75:1. No reforming reaction takes place below 400 °C in region 1 (absence of products (H2, CO) formation) stimulating the high temperature needed for the DR reaction to occur no matter what the feed composition is.Formation of the syngas is further promoted at higher temperatures. However, in this case

Figure VI- 1 :

 1 Figure VI-1: Thermodynamic equilibrium curves for DR reactants and products at P= 1 atm excluding carbon from the calculations

Figure VI- 2 D

 2 Figure VI-2 D, contains all the feed mixture combining the side reactions seen in the cases of B and C. The inhibition effect of carbon on the main reaction is noticed from the H2:CO ratio achieved in Table VI-1 for all reactant conditions that are very high. Section 3, the favorable temperature range for reforming reaction, shows similar trend in the 4 cases as the carbon-free Figure VI-2: Thermodynamic equilibrium curves for DR reactants and products at P= 1 atm including carbon as side product

  Figure VI-3: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of temperature of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, and (c) Ni5-Mg30-Al2O3; black line and black dashed line correspond to thermodynamic values without and with C(s); Initial feed mixture: CH4:CO2=0.75:1

  Figure VI-4 shows the light off curves of the three tested catalysts. Both CH4 and CO2 conversions increase with temperature (Figure VI-4 A, B), however the CO2 conversions start at higher values than CH4 that was always negligible between 400-500 °C. This is due to the presence of H2 initially in the feed that blocks CH4 conversion by favoring the CO2 methanation reaction consuming CO2 to produce CH4 and H2O (hydrogen consumption exceeds the production from hydrocarbon reforming). The main dry reforming reaction dominates then in the high temperature region (550-750 °C) showing consumption of both CH4 and CO2 (hydrogen formation from methane balances the oxidation of H2 by CO2).

Figure VI- 4 :

 4 Figure VI-4: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of time of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, and (c) Ni5-Mg30-Al2O3; black line and black dashed line correspond to thermodynamic values without and with C(s) respectively; Initial feed mixture: CH4:CO2:H2=0.75:1:1.57

Figure VI- 5 :

 5 Figure VI-5: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of time of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, and (c) Ni5-Mg30-Al2O3; black line and black dashed line correspond to thermodynamic values of CH4 and CO2 conversions, respectively; Initial feed mixture: CH4:CO2:H2=0.75:1:1.57 (T= 650 °C)

Figure VI- 6 :

 6 Figure VI-6: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of time of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, (c) Ni5-Mg30-Al2O3, and Ni5-Mg50-Al2O3; black line and black dashed line correspond to thermodynamic values without and with C(s) respectively; Initial feed mixture: CH4:CO2:CO=0.75:1:0.85

Figure VI- 10 :

 10 Figure VI-10: TEM images of spent (A, A') Ni5-Al2O3, (B, B') Ni5-Mg15-Al2O3, and (C, C') Ni5-Mg50-Al2O3.

Figure VI- 11 :

 11 Figure VI-11: (A) TGA profiles (at the top) associated to heat flow signals (at the bottom) and (B) Raman spectroscopy signals of the spent (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, and (c) Ni5-Mg50-Al2O3.

  this work, stable and selective Ni-based alumina material were designed by taking into consideration different key factors: 1) Simultaneous addition of Ni precursors within the synthesis of Al2O3 support, 2) mutation in the shape of the alumina support for better trapping and confinement of nickel nanoparticles inside the support structure (pores/walls) preventing sintering, and 3) modification of the material basicity by adding MgO as a basic dopant to enhance reactivity by annealing coke deposition First, Ni5-Mgy-Al2O3 ordered mesoporous catalysts were synthesized combining the three aforementioned approaches. Mg loading was varied between 0 and 26 wt%. Ni5-Mg15-Al2O3 with intermediate Mg content (7 wt%) has the best catalytic performances with as superior activity and stability among all the other Mg amounts. This was explained by several combined features: i) the high, homogenous and simultaneous dispersion of both Ni and Mg species, ii) the high surface area and ordered channels for Al2O3 support and iii) the enhancement of material basicity that accelerates CO2 chemisorption reducing the activation energy. Further increment of Mg loading (15-26 wt%) has negative catalytic response due to a strong MgO segregation in the material that disturbs the structure and pores ordering, and strongly affects Ni dispersion.

Figure I- 1 :

 1 Figure I-1: Natural gas reserves distribution across the world [9] ...........................................

Figure I- 2 :

 2 Figure I-2: World liquid fuels production and consumption balance, Source: Energy information administration [11] ............................................................................

Figure I- 3 :

 3 Figure I-3: Peak Oil Forecast, Source: BP statistical review of world energy 2009 [10] ........

Figure I- 4 :

 4 Figure I-4: Carbon dioxide (CO₂) emissions factor, measured in kilograms of CO₂ produced per megawatt-hour (MWh) of energy produced from a given fossil fuel source. Source: intergovernmental panel on climate change (IPCC) [16] ........................

Figure I- 5 :

 5 Figure I-5: Global carbon dioxide levels since the beginning of industrial revolution [17] ....

Figure I- 6 :

 6 Figure I-6: Summary of human sources of greenhouse gas emissions [17].............................

Figure I- 7 :

 7 Figure I-7: Percentage of greenhouse gases sources [27] ........................................................

Figure I- 8 :

 8 Figure I-8: Schematic with a brief summary about synthetic gas applications [29] ................

Figure I- 9 :

 9 Figure I-9: Statistical histograms displaying the evolution of the publications number as a function of year for SRM, POX, DRM and CO2 methanation reported studies. The search was evaluated between 2000 and 2018 using either the topic or title in the search engine ...................................................................................................

Figure I- 10 :

 10 Figure I-10: Scheme representing effect of support on nickel particles [64] ...........................

Figure I- 11 :

 11 Figure I-11: Schematic representing the influence of Ni insertion in the catalyst on coke formation during DRM [82] .................................................................................

Figure I- 12 :

 12 Figure I-12: Schematic illustrating the catalyst morphology effect on coke catalyst deactivation [86] ...................................................................................................

Figure I- 13 :

 13 Figure I-13: Porous alumina structure [77] ..............................................................................

Figure II- 1 :

 1 Figure II-1: Classification of physisorption isotherms (left graphs) and hysteresis loops (right graphs) [157] .........................................................................................................

Figure II- 2 :

 2 Figure II-2: Mode of X-ray diffraction ....................................................................................

Figure II- 3 :

 3 Figure II-3: Principle of X-ray fluorescence ............................................................................

Figure II- 4 :

 4 Figure II-4: TPR schematic ......................................................................................................

Figure II- 5 :

 5 Figure II-5: Steps required to determine chemisorption isotherm ...........................................

Figure II- 6 :

 6 Figure II-6: Chemisorption isotherm ........................................................................................

Figure II- 7 :

 7 Figure II-7: Thermodynamic equilibrium curve for DRM (CH4:CO2 = 1:1) reactants and products at P= 1 atm (A) without and (B) with carbon as side product and Argon as diluent ...............................................................................................................

Figure II- 8 :

 8 Figure II-8: MAR equipment and Micro-GC ...........................................................................

Figure II- 9 :

 9 Figure II-9: General scheme of dry reforming system .............................................................

Figure II- 10 :

 10 Figure II-10: Temperature profile during in-situ reduction (grey area), activity (orange area), and stability (green area) tests in DRM ................................................................

Figure II- 11 :

 11 Figure II-11: Thermodynamic CO2 conversion as a function of temperature and pressure obtained from HSC chemistry software. ..............................................................

Figure II- 12 :

 12 Figure II-12: General scheme of the CO2 methanation system ................................................

Figure II- 13 :

 13 Figure II-13: Temperature profile during in-situ reduction (grey area), activity (orange area), and stability (green area) tests in CO2 methanation ..............................................

Figure III- 1 :

 1 Figure III-1: SEM images of spent Ni/Mg/Al2O3 catalysts [170] ............................................

Figure III- 2 :

 2 Figure III-2: Variation of CH4 conversion as a function of Mg content in different methods of Ni-Mg-Al2O3 preparation present in literature: Sol-gel [148], 2-step hydrothermal [167], co-precipitation [172], and co-precipitation [173] .....................................

Figure III- 3 :

 3 Figure III-3: A representative schematic of the mechanism involved in the synthesis of mesoporous Ni-Mg-Al2O3 catalyst .......................................................................

Figure III- 4 :

 4 Figure III-4: Chronological steps summary of preparing mesoporous one-pot Nix-Mgy-Al2O3 following EISA route ............................................................................................

Figure III- 5 :

 5 Figure III-5: Nitrogen adsorption/desorption isotherms and pore size distribution profiles (insets) of Ni5-Mgx-Al2O3 catalysts calcined at 600 °C: (a) Al2O3, (b) Ni5-Al2O3, (c) Ni5-Mg5-Al2O3, (d) Ni5-Mg10-Al2O3, (e) Ni5-Mg15-Al2O3, (f) Ni5-Mg30-Al2O3, (g) Ni5-Mg50-Al2O3 ...............................................................................................

Figure III- 6 :

 6 Figure III-6: (A) low and (B) wide angles XRD patterns of Ni5-Mgx-Aly catalysts calcined at 600 °C: (a) Al2O3, (b) Ni5-Al2O3, (c) Ni5-Mg5-Al2O3, (d) Ni5-Mg10-Al2O3 , (e) Ni5-Mg15-Al2O3, (f) Ni5-Mg30-Al2O3, (g) Ni5-Mg50-Al2O3 ..................................

Figure III- 7 :

 7 Figure III-7: Representative HRTEM (A-A') and STEM-HAADF (B-B') micrographs of calcined mesoporous Ni5-Al2O3 showing the intimate mixing between Ni and its alumina support after synthesis; (B-B') are pictures for preliminary experiments done on MPQ-Paris Diderot microscopy (JEOL ARM 200F, coll. J. Nelayah) ..

Figure III- 8 :

 8 Figure III-8: Representative STEM micrographs of calcined mesoporous (A-A') Ni5-Mg15-Al2O3 and (B-B') Ni5-Mg50-Al2O3 catalysts .........................................................

Figure III- 9 :

 9 Figure III-9: CO2 desorption by Infra-red measurements on mesoporous: Al2O3 (black curve), Ni5-Mg5-Al2O3 (pink curve), Ni5-Mg15-Al2O3 (red curve), Ni5-Mg30-Al2O3 (green curve) and Ni5-Mg50-Al2O3 (blue curve) ..............................................................

Figure III- 10 :

 10 Figure III-10: H2-TPR profiles of mesoporous Ni5-Mgx-Al2O3 catalysts calcined at 600 °C: (a) Al2O3, (b) Ni5-Al2O3, (c) Ni5-Mg5-Al2O3, (d) Ni5-Mg10-Al2O3, (e) Ni5-Mg15-Al2O3, (f) Ni5-Mg30-Al2O3, (g) Ni5-Mg50-Al2O3, (h) Mg50-Al2O3 ........................

Figure III- 11 :

 11 Figure III-11: XPS spectra of the Ni2p in calcined (a) Ni5-Al2O3 and (b) Ni5-Mg15-Al2O3 materials ................................................................................................................

Figure III- 12 :

 12 Figure III-12: Nitrogen adsorption/desorption isotherms and pore size distribution profiles (insets) of Ni5-Mgx-Al2O3 catalysts reduced at 650 °C for 2 hours: (a) Al2O3, (b) Ni5-Al2O3, (c) Ni5-Mg10-Al2O3, (d) Ni5-Mg15-Al2O3, (e) Ni5-Mg30-Al2O3, (f) Ni5-Mg50-Al2O3 ...........................................................................................................

Figure III- 13 :

 13 Figure III-13: (A) small angle and (B) wide angle XRD patterns of in-situ H2-reduced Ni5-Mgx-Al2O3 catalysts: (a) Al2O3, (b) Ni5-Al2O3, (c) Ni5-Mg5-Al2O3, (d) Ni5-Mg10-Al2O3, (e) Ni5-Mg15-Al2O3, (f) Ni5-Mg30-Al2O3, (g) Ni5-Mg50-Al2O3 .................

Figure III- 14 :

 14 Figure III-14: SEM micrographs of in-situ reduced (650 °C/2h, H2/Ar) (A) Ni5-Al2O3, (B) Ni5-Mg15-Al2O3, (C) Ni5-Mg50Al2O3 ....................................................................

Figure III- 15 :

 15 Figure III-15: Representative (A-A'), (B-B') STEM-HAADF micrographs of reduced mesoporous Ni5-Al2O3 showing presence of Ni nano-species after reduction .....

Figure III- 16 :

 16 Figure III-16: Representative standard HRTEM micrographs of reduced (650 °C/2h) mesoporous (A) Ni5-Mg15-Al2O3, (B) Ni5-Mg30-Al2O3 and (C) Ni5-Mg50-Al2O3 and STEM-HAADF (A'-A'') of Ni5-Mg15-Al2O3 showing presence of Ni nanospecies after reduction ..........................................................................................

Figure III- 17 :

 17 Figure III-17: HAADF with EDS mapping images of (A) Ni5-Al2O3, (B) Ni5-Mg15-Al2O3, and (C) Ni5-Mg50-Al2O3. Atoms of Al (red), Ni (yellow), and Mg (blue) were identified within EDS maps ..................................................................................

Figure III- 18 :

 18 Figure III-18: H2-TPD profiles of alumina based-materials: (a) Ni5-Al2O3, (b) Ni5-Mg10-Al2O3, (c) Ni5-Mg15-Al2O3, (d) Ni5-Mg30-Al2O3, and (e) Ni5-Mg50-Al2O3 ..........

Figure III- 19 :

 19 Figure III-19: Variation of (A) CH4 and CO2 conversion, and (B) H2:CO ratio as a function of time on stream during DRM (P=1 atm, CH4:CO2=1:1, GHSV= 36 L.g -1 .h -1 ) over (a) Ni5-Al2O3 and (b) Ni5-Mg15-Al2O3 .................................................................

Figure III- 20 :

 20 Figure III-20: Variation of (A) CH4 and (B) CO2 conversion levels as a function of temperature (400-800 °C) during DRM (P=1 atm, CH4:CO2=1:1, GHSV=180 L.g - 1 .h -1 ) over (green) Ni5-Al2O3, (blue) Ni5-Mg15-Al2O3, (pink) Ni5-Mg30-Al2O3, (purple) Ni5-Mg50-Al2O3 .......................................................................................

Figure III- 21 :

 21 Figure III-21: Variation of H2:CO ratio as a function of temperature during DRM (P=1 atm, CH4:CO2=1:1, GHSV=180 L.g -1 .h -1 ) over (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, (c) Ni5-Mg30-Al2O3, (d) Ni5-Mg50-Al2O3 ...................................................................

Figure III- 22 :

 22 Figure III-22: Variation of (A) CH4 conversion, (B) CO2 conversion, and (C) H2:CO ratio as a function of time on stream during DRM (P=1 atm, CH4:CO2=1:1, GHSV=180 L.g -1 .h -1 ) over (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, (c) Ni5-Mg30-Al2O3, (d) Ni5-Mg50-Al2O3. Thermodynamic values of CH4 and H2:CO (straight black line) and CO2 (dashed black line) are calculated using the HSC 7.1 software ....................

Figure III- 23 :

 23 Figure III-23: Example of Arrhenius plot in DRM of Ni5-Al2O3 catalyst ...............................

Figure IV- 1 :

 1 Figure IV-1: Types of several MOFs and their applications ....................................................

Figure IV- 2 :

 2 Figure IV-2: Chronological steps summary of preparing mesoporous Ni-Al2O3 with 5wt% nickel following the new metal organic framework route ....................................

Figure IV- 3 :

 3 Figure IV-3: Schematic representing two types of Ni-Al2O3 catalyst derived from MIL-53 framework based on the order of Ni impregnation with respect to calcination ....

Figure IV- 4 :

 4 Figure IV-4: X-ray diffraction patterns of: (a) as synthesized MIL-53(Al), (b) activated MIL-53, (c) rehydrated MIL-53 dried in room atmosphere or (c') at 200 °C, (d) impregnated Ni/MIL-53 dried in room atmosphere or (d') at 200 °C. Triangles and spheres indicate the peak positions corresponding to the open and closed forms of the MIL-53 structure, respectively (as reported in [230]). Diamonds show unattributed reflections that may correspond to an intermediate state. Stars indicate peaks appearing after nickel impregnation. The dot vertical line shows the position of the 011 reflection characteristic of the MIL-53 large-pore phase.

Figure IV- 5 :

 5 Figure IV-5: N2 physisorption isotherms after degassing at 120 °C of: (a) freshly activated MIL-53, (b) rehydrated MIL-53, (c) Ni/MIL-53, (d) Ni5-Al2O3 MIL-53 and (e) Ni5 0 -Al2O3 MIL-53. Filled and empty symbols correspond to adsorption and desorption isotherms, respectively. ........................................................................................

Figure IV- 6 :

 6 Figure IV-6: (A) TGA (top curves), DTG (middle curves) and DTA (bottom curves) profiles for samples: (a) freshly activated MIL-53, (b) rehydrated MIL-53 and (c) impregnated Ni/MIL-53. (B) Mass spectrometry profiles corresponding to the detection of H2O (mass 18), NO (mass 30) and CO2 (mass 44) for hydrated MIL-53 (red dotted lines) and impregnated Ni/MIL-53 (blue lines). The temperatures of peak maxima in B are slightly higher than in A due to the delay for the exhaust gases to reach the MS analysis chamber. ..............................................................

Figure IV- 7 :

 7 Figure IV-7: Representative SEM images of (a) MIL-53, (b) Ni5-Al2O3 MIL-53, (c) Al2O3 MIL-53 and (d) Ni5@Al2O3 MIL-53 ....................................................................................

Figure IV- 8 :

 8 Figure IV-8: Representative HR-TEM images of (a) MIL-53, (b) Ni5-Al2O3 MIL-53, (c) Al2O3 MIL-53 and (d) Ni5@Al2O3 MIL-53 ........................................................................... Figure IV-9: Diffractograms of the Ni containing samples: (a) Ni5-Al2O3 MIL-53, (a') Ni5 0 -Al2O3 MIL-53, (b) Ni5@Al2O3 MIL-53, (b') Ni5 0 @Al2O3 MIL-53, (c) Ni5@Al2O3 COM, (c') Ni5 0 @Al2O3 COM. The diffractograms of the preformed alumina supports (light grey lines) are shown for comparison: (b*) Al2O3 MIL-53 and (c*) Al2O3 COM. Intensities are divided by 3 in patterns c*, c and c'. ...........................................

Figure IV- 10 :

 10 Figure IV-10: Temperature program Reduction analyses of the samples (a) Ni5-Al2O3 MIL-53, (b) Ni5@Al2O3 MIL-53, and (c) Ni5@Al2O3 COM. ..................................................

Figure IV- 11 :

 11 Figure IV-11: STEM-HAADF image of Ni5-Al2O3 MIL-53 (left) and EDX elemental mappings of the same area for Al (in green) and Ni (in yellow). .......................................

Figure IV- 12 :

 12 Figure IV-12: Representative TEM micrographs with its corresponding local SAED image over selected grain zones of calcined Ni5-Al2O3 MIL-53 .......................................

Figure IV- 13 :

 13 Figure IV-13: N2 physisorption isotherms of the samples: (a) Ni5@Al2O3 MIL-53 and (b) Ni5@Al2O3 COM ...................................................................................................

Figure IV- 14 :

 14 Figure IV-14: In-situ XRD patterns of Ni5-Al2O3 MIL-53 during reduction by hydrogen (5% H2 in Ar) up to 850 °C .............................................................................................

Figure IV- 15 :

 15 Figure IV-15: HR-TEM images of the reduced samples: (a and a') Ni5 0 -Al2O3 MIL-53, (b and b') Ni5 0 @Al2O3 MIL-53, (c and c') Ni5 0 @Al2O3 COM. ............................................

Figure IV- 16 :

 16 Figure IV-16: Histograms established from at least 500 Ni 0 nanoparticles in typical HR-TEM images of (A) Ni5 0 -Al2O3MIL-53, (B) Ni5 0 @Al2O3 MIL-53 and (C) Ni5 0 @Al2O3 COM before and after 13h of catalytic test (black and white bares, respectively). Related mean sizes are shown in (D) for the catalysts before (black circles) and after (grey circles) test ........................................................................................

Figure IV- 17

 17 Figure IV-17: (A) CH4, (B) CO2 conversions, and (C) H2:CO product ratio during DRM upon continuous increase of temperature from 400 °C till 750 °C over in-situ reduced (800 °C/2h): (blue) Ni5 0 -Al2O3 MIL-53, and (orange) Ni5 0 @Al2O3 COM. Grey columns are the calculated values at the thermodynamic equilibrium for the CO2, CH4 conversions and H2:CO ratio ......................................................................

Figure IV- 18 :

 18 Figure IV-18: Comparison of the catalytic performances in DRM of (a) Ni5 0 -Al2O3 MIL-53, (b) Ni5 0 @Al2O3 MIL-53 and (c) Ni5 0 @Al2O3 COM as a function of (A) CH4 and (B) CO2 conversions measured during 13h of reaction. Black continued and dashed lines

Figure IV- 19 :

 19 Figure IV-19: H2:CO ratios in DRM (a) Ni5 0 -Al2O3 MIL-53, (b) Ni5 0 @Al2O3 MIL-53 and (c) Ni5 0 @Al2O3 COM ..................................................................................................

Figure IV- 20 :

 20 Figure IV-20: Catalytic performances in DRM of Ni5 0 -Al2O3 MIL-53, as a function of (a) CH4 and (a') CO2 conversions measured during 100h of reaction. Black continued and dashed lines are the calculated values at the thermodynamic equilibrium for the CH4 and CO2 conversions, respectively .............................................................

Figure IV- 21 :

 21 Figure IV-21: Raman spectroscopy profiles of the spent catalysts: a) Ni5 0 -Al2O3 MIL-53, (b-) Ni5 0 @Al2O3 MIL-53, and (c) Ni5 0 @Al2O3 COM ......................................................

Figure IV- 22 :

 22 Figure IV-22: TGA profiles (at the top) associated to MS signals corresponding to CO2 (at the bottom) of the spent (a-a') Ni5 0 -Al2O3 MIL-53, (b-b') Ni5 0 @Al2O3 MIL-53, and (c-c') Ni5 0 @Al2O3 COM ..................................................................................................

Figure IV- 23 :

 23 Figure IV-23: TEM (a,b) and SEM (a',b') images of spent (a,a') Ni5-Al2O3 MIL-53 and (b,b') Ni5@Al2O3 MIL-53. The dashed oval and white arrows in (b) highlight the presence of carbon nanotubes ............................................................................................

Figure V- 1 :

 1 Figure V-1: Schematic describing wind to natural gas concept for bidirectional coupling of the electricity and gas grids [28] .........................................................................

Figure V- 2 :

 2 Figure V-2: Chemical energy storage using heat from renewables and DRM and methanation as energy carriers [253] ......................................................................................

Figure V- 3 :

 3 Figure V-3: CO2 isotherms obtained for (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3 and (c) Ni5-Mg50-Al2O3 samples .....................................................................................................

Figure V- 4 :

 4 Figure V-4: (A) CO2 conversion and (B) CH4 selectivity of (a-a') Ni5-Al2O3, (b-b') Ni5-Mg15-Al2O3, and (c-c') Ni5-Mg50-Al2O3 ......................................................................

Figure V- 5 :

 5 Figure V-5: XRD patterns of calcined (a) Ni5-Mg15-Al2O3, (b) Ni10-Mg15-Al2O3, (c) Ni15-Mg15-Al2O3, and (d) Ni20-Mg15-Al2O3 samples ..................................................

Figure V- 6 :

 6 Figure V-6:N2-sorption curves of (A) calcined and (B) reduced (a-a') Ni5-Mg15-Al2O3, (b-b') Ni10-Mg15-Al2O3, (c-c') Ni15-Mg15-Al2O3, and (d-d') Ni20-Mg15-Al2O3 samples ............................................................................................................................

Figure V- 7 :

 7 Figure V-7: (A) CO2 isotherms and (B) H2-TPR profiles of calcined (a-a') Ni5-Mg15-Al2O3, (b-b') Ni10-Mg15-Al2O3, (c-c') Ni15-Mg15-Al2O3, and (d-d') Ni20-Mg15-Al2O3 samples ...............................................................................................................

Figure V- 8 :

 8 Figure V-8: (A) CO2 conversion and (B) CH4 selectivity of (a-a') Ni5-Mg15-Al2O3, (b-b') Ni10-Mg15-Al2O3, (c-c') Ni15-Mg15-Al2O3, and (d-d') Ni20-Mg15-Al2O3 samples ............................................................................................................................

Figure V- 9 :

 9 Figure V-9: Stability test of Ni15-Mg15-Al2O3 ........................................................................

Figure V- 10 :

 10 Figure V-10: XRD patterns of calcined (a-g) and reduced (a'-g') samples respectively: a-a') Ni5-Al2O3 MIL-53, b-b') Ni15-Al2O3 MIL-53, c-c') Ni20-Al2O3 MIL-53, d-d') Ni15@Al2O3 MIL-53, e-e') Ni15@USY, f-f') Ni15@SBA-15, and g-g') Ni15-Al2O3 ............................................................................................................................

Figure V- 11 :

 11 Figure V-11: HR-TEM Micrographs of fresh Ni-Al2O3 MIL-53 catalysts: (A-A') Ni5-Al2O3 MIL- 53, (B-B') Ni15-Al2O3 MIL-53. ................................................................................ Figure V-12: H2-TPR profiles for calcined (A) MOF-derived samples: (a) Ni5-Al2O3 MIL-53, b) Ni15-Al2O3 MIL-53, c) Ni20-Al2O3 MIL-53, and d) Ni15@Al2O3 MIL-53 and (B) other Nibased materials: (e) Ni15@USY, f) Ni15@SBA-15, and g) Ni15-Al2O3 ..............

Figure V- 13 :

 13 Figure V-13: HR-TEM Micrographs and Ni 0 particle size distribution of reduced catalysts: (A-A'') Ni5-Al2O3 MIL-53, (B-B'') Ni15-Al2O3 MIL-53, and (C-C'') Ni20-Al2O3 MIL-53 ............................................................................................................................

Figure V- 14

 14 Figure V-14: (A) CO2 conversion and (B) CH4 selectivity of a) Ni5-Al2O3 MIL-53, b) Ni15-Al2O3 MIL-53, c-) Ni20-Al2O3 MIL-53, and d) Ni15@Al2O3 MIL-53 ....................................... Figure V-15: Stability test of Ni15-Al2O3 MIL-53 over 20 h time on stream .............................

Figure V- 16 :

 16 Figure V-16: TEM micrograph of spent Ni15-Al2O3 MIL-53 after CO2 methanation reaction. Conditions: T=350 °C, GHSV=1440 ml.g -1 .min -1 , H2:CO2= 4:1 .......................

Figure V- 17

 17 Figure V-17: (A) CO2 conversion and (B) CH4 selectivity of (a) Ni15-Al2O3 MIL-53, (b) Ni15@USY, c) Ni15@SBA-15, d) Ni15-Al2O3.....................................................

Figure V- 18

 18 Figure V-18: (A) CO2 conversion and CH4 selectivity and (B) stability at 350 °C of Ni20-Al2O3 MIL-53 and Ni@γ-Al2O3 ..............................................................................

Figure VI- 1 :

 1 Figure VI-1: Thermodynamic equilibrium curves for DR reactants and products at P= 1 atm excluding carbon from the calculations ..............................................................

Figure VI- 2 :

 2 Figure VI-2: Thermodynamic equilibrium curves for DR reactants and products at P= 1 atm including carbon as side product ........................................................................

Figure VI- 3 :

 3 Figure VI-3: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of temperature of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, and (c) Ni5-Mg30-Al2O3; black line and black dashed line correspond to thermodynamic values without and with C(s); Initial feed mixture: CH4:CO2=0.75:1 .......................................................

Figure VI- 4 :

 4 Figure VI-4: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of time of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, and (c) Ni5-Mg30-Al2O3; black line and black dashed line correspond to thermodynamic values without and with C(s) respectively; Initial feed mixture: CH4:CO2:H2=0.75:1:1.57 ............................

Figure VI- 5 :

 5 Figure VI-5: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of time of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, and (c) Ni5-Mg30-Al2O3; black line and black dashed line correspond to thermodynamic values of CH4 and CO2 conversions, respectively; Initial feed mixture: CH4:CO2:H2=0.75:1:1.57 ............................

Figure VI- 6 :

 6 Figure VI-6: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of time of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, (c) Ni5-Mg30-Al2O3, and Ni5-Mg50-Al2O3; black line and black dashed line correspond to thermodynamic values without and with C(s) respectively; Initial feed mixture: CH4:CO2:CO=0.75:1:0.85 ............

Figure VI- 7 :

 7 Figure VI-7: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of time of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, (c) Ni5-Mg30-Al2O3, and Ni5-Mg50-Al2O3; black line corresponds to thermodynamic values without C(s); Initial feed mixture: CH4:CO2:CO=0.75:1:0.85 ....................................................................

Figure VI- 8 :

 8 Figure VI-8: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of time of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, (c) Ni5-Mg30-Al2O3, and Ni5-Mg50-Al2O3; black line and black dashed line correspond to thermodynamic values without and with C(s) respectively; Initial feed mixture: CH4:CO2:H2:CO=0.75:1:1.57:0.85152 Figure VI-9: (A) CH4 and (B) CO2 conversions with (C) H2:CO ratio as a function of time of: (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, (c) Ni5-Mg30-Al2O3, and (d) Ni5-Mg50-Al2O3; black line corresponds to thermodynamic values without C(s); Initial feed mixture: CH4:CO2:H2:CO=0.75:1:1.57:0.85 ......................................................

Figure VI- 10 :

 10 Figure VI-10: TEM images of spent (A, A') Ni5-Al2O3, (B, B') Ni5-Mg15-Al2O3, and (C, C') Ni5-Mg50-Al2O3. ..................................................................................................

Figure VI- 11

 11 Figure VI-11: (A) TGA profiles (at the top) associated to heat flow signals (at the bottom) and (B) Raman spectroscopy signals of the spent (a) Ni5-Al2O3, (b) Ni5-Mg15-Al2O3, and (c) Ni5-Mg50-Al2O3. .....................................................................................

Table I -

 I

2

[START_REF] Abdullah | Recent advances in dry reforming of methane over Ni-based catalysts[END_REF][START_REF] Nikoo | Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation[END_REF]

:

Table I -

 I

	Reactions	Equations	∆H˚298 (kJ/mol)	Temperature range (°C)
		Main reaction
	Dry reforming of methane (DRM) CH4 + CO2 → 2H2 + 2CO	247	600-1000
	Side reactions with no C(s) production
	Water gas shift (WGS)	CO + H2O→ CO2 + H2	-42.1	200-500
	Reverse water gas shift (RWGS)	CO2 + H2→ CO + H2O	42.1	500-800
	CO2 methanation	CO2 + 4H2→ CH4+ 2H2O	-165	100-500
	Side reactions inducing C(s)
	CO2 hydrogenation	CO2 + 2H2→ 2H2O + C(s)	-90	100-500
	CO dehydrogenation	CO + H2→ H2O + C(s)	-131	100-500
	CO disproportionation	2CO → CO2 + C(s)	-172	100-600
	CH4 decomposition	CH4 → 2H2 + C(s)	75	600-900

2: List of possible involved reactions in dry reforming of methane

Table I

 I

-2, is not in the thermodynamic range of DRM, however, it has also gained our attention in the present work because it is a particular technique that further curtails CO2 emissions and produces CH4 as an energy carrier. It has also been newly developed in literature since it is seen in Figure

I-9

D that the number of research studies related to this application has been recently continuously increasing through time in a manner similar to that for DRM. Details about this reaction and a literature review will be explained in details in Chapter V.

The formation of carbon should be also taken into account because it is the most limiting feature deactivating DRM catalysts in most published studies. Table

I

-2 encloses the chemical reactions related to solid carbon deposition in methane reforming processes. In fact, there are four reactions that produce carbon as a product. CO2 hydrogenation and CO dehydrogenation are exothermic reactions feasible at low temperatures and therefore they are exempted from being considered in DRM side reactions. On the other hand, CO disproportionation

(Boudouard)

  Cγ is the most deleterious type of carbon The table in the Appendix summarizes the advances done in literature during the past 10 years on Ni-Al2O3 preparation methods (17 publications). The table mainly comprises

	demanding very high temperatures to be removed (600-850 °C) destroying catalyst stability
	[69,70].
	Around 102 papers were published in the recent years (2010-early 2019) on the
	improvements made in synthesizing Ni-alumina catalyst that can resist sintering and carbon
	deposition.

There are three types of carbon classified as Cα, Cβ, and Cγ. Cα is known as the most reactive carbon species trivially eliminated by oxidation or reduction at low temperatures (300-450 °C). This type of carbon does not deactivate the catalyst and can play an intermediate role in DRM reaction to in-situ form CO product. Cβ derives from carbon nanotubes can be removed also at temperatures not exceeding 600 °C. This type of carbon does not lead to catalyst deactivation unless Ni particles become totally encapsulated within the nanotubes preventing access to the gaseous reactants. The formation of carbon nucleus detach and drive Ni nanoparticles away Chapter I Literature Review from the Al2O3 support. If carbon nanotubes are formed in large amount they can contribute to reactor plugging and cease of reaction. Cγ is a graphitic crystalline carbon created from the interaction between Cα and Cβ or/and between carbons of the same type (Cα-Cα/ Cβ-Cβ) that gradually transform into Cγ with time on stream. the type of Al2O3 used, the method of Ni insertion and its particle size with the test conditions applied, the CH4/CO2 conversions obtained, and the reasons of deactivation if any. Thus, the following paragraphs highlight on the main developments made concerning Ni-Al2O3 materials and their main results. I.4.2.1. Alterations in Ni addition to Al2O3 support I.4.2.1.1. Post-Ni addition to Al2O3

  Table in the Appendix)[START_REF] Wang | Methane Dry Reforming over Coke-Resistant Mesoporous Ni-Al2O3 Catalysts Prepared by Methane Dry Reforming over Coke-Resistant Mesoporous Ni-Al 2 O 3 Catalysts Prepared by Evaporation-Induced Self-Assembly Method[END_REF][START_REF] Jiang | Comparative study of the dry reforming of methane on fluidised aerogel and xerogel Ni / Al 2 O 3 catalysts[END_REF][START_REF] Noh | Nickel Supported on Mesoporous Alumina for Dry Reforming of Methane: Combustion Method[END_REF]. Newnham J. et al. compared the synthesis of Ni/Al2O3 using W.I. and I.W.I. to the one synthesized directly in a polyethylene bottle in presence of both Ni and Al precursor[START_REF] Newnham | Highly stable and active Nimesoporous alumina catalysts for dry reforming of methane[END_REF]. The latter catalyst showed a decrease of 10% in conversion in 10 h then it was stabilized for 200 h with only 2-3% conversion loss. However, the size of the Ni crystallite was sintered after test from 11 to 15 nm contributing eventually to the formation of a lot of carbon nanotubes. Zhan L. et al.[START_REF] Zhang | An active and stable Ni/Al2O3 nanosheet catalyst for dry reforming of CH4[END_REF] developed also a 2-step hydrothermal technique where Ni(OH)2 nanoparticles were synthesized by a 1 st -step hydrothermal then anchored into a 2 nd -step hydrothermal in presence of alumina precursor to adjust the environment and form a thermally stable catalyst. The unique leaf-like Ni/Al2O3 obtained offers more active sites contributing to higher catalytic activity and stability as well as carbon resistance compared to

	one-step	hydrothermal	and
	conventional impregnation method.
	In addition, the Table Appendix

Table III -

 III 1: Bibliographic listing of Ni-MgO-Al2O3 catalysts synthesized by different techniques and their performances in DRM

	Sample	Method of Preparation	Ni(wt%)	Order of Ni/Mg addition	MgO(wt%)	Al2O3 (wt%)	Particle Size (nm)

Mg/Al:0.67 11.6 r /15 s 223 4.6 0.42 84-85 (84.7) 86-86 -

					Mg/Al:0.25		16.8 r /19 s	145	8	0.45				78-80 (78.5)	82-84	13.4
				solution of	Mg/Al:0.43		15.3 r /18 s	179	6.9	0.43				85-86 (85)	86-87	5
				Ni, Mg, Al added to		-					800,1	50,60,1:1	4
				Na2CO3	Mg/Al:1		11.9 r /13 s	218	4.2	0.46				83-84 (83.6)	85-86	-
					Mg/Al:4		9.4 r /11 s	190	3.9	0.36				82-83 (82.7)	85-85	-
					1	99		208	6.6	0.31
				Ni	2	98		184	7.8	0.37
	Ni/MgO-Al2O3 [174]	Impregnation	5	impregnated on one-pot	3 5	97 95	n.m.	188 163	4.9 7.8	0.27 0.28	700,1	100,15,1:1	100	77-73	77-73	8
				Mg Al	8	92		159	5.6	0.27
					10	90		163	7.8	0.33

  In addition, all the aforementioned ways of doping Ni with Mg suffer from certain deficiencies like the need of a long preparation time (i.e. several impregnations/calcinations)[START_REF] Huang | Ordered mesoporous CoO-NiO-Al2O3 bimetallic catalysts with dual confinement effects for CO2 reforming of CH4[END_REF][START_REF] Zhang | Dry reforming of methane over Ni/MgO-Al 2 O 3 catalysts prepared by two-step hydrothermal method[END_REF][START_REF] Roh | Carbon dioxide reforming of methane over Ni catalysts supported on Al 2O3 modified with La2O3, MgO, and CaO[END_REF]171,[START_REF] Hou | Meso-porous Ni/Mg/Al catalysts for methane reforming with CO2[END_REF], tough conditions as high temperature[START_REF] Min | Carbon dioxide reforming of methane on Ni-MgO-Al2O3 catalysts prepared by sol-gel method: Effects of Mg/Al ratios[END_REF][START_REF] Zhang | Dry reforming of methane over Ni/MgO-Al 2 O 3 catalysts prepared by two-step hydrothermal method[END_REF], hard operation steps[START_REF] Zhang | Dry reforming of methane over Ni/MgO-Al 2 O 3 catalysts prepared by two-step hydrothermal method[END_REF]171,[START_REF] Hou | Meso-porous Ni/Mg/Al catalysts for methane reforming with CO2[END_REF], and expensive substrates.More interestingly, it can be noted from the data in TableIII-1 that, regardless of the preparation protocol used, there is always an intermediate optimum amount of magnesium that leads the highest activity and stability of CH4 and CO2 conversions. To illustrate this, Figure III-2 reports the conversion of CH4 as a function of Mg amount (expressed as Mg/Al molar ratio) for the representative work taken from the table. The scale applied for these studies is not always the same because of the distinct preparation methods used, shifting the trends in Mg

	whiskers carbon, oxidation of metallic Ni and sintering thus affecting the stability in long
	term tests [171-173].
	example, in a recent study done by Bruna Rego de Vasconcelos et al., a hydrotalcite catalyst
	needed 70 wt% of Mg to enhance its stability. However, even with this high amount of Mg,
	core-shell carbon and carbon nanotubes were inevitably present [178].

 The 2-step hydrothermal technique giving non-porous materials was the least efficient (lower catalytic activity) with sintering and large amounts of carbon deposition were observed on all the catalysts after only 10 h reaction independently from the Mg amount added

[START_REF] Zhang | Dry reforming of methane over Ni/MgO-Al 2 O 3 catalysts prepared by two-step hydrothermal method[END_REF]

.

 The sol-gel pathway came up with porous materials having non ordered mesoporosity. The catalysts were somehow stable through time with little deactivation, but, even the best catalyst with 52 wt% MgO encountered carbon nanotubes formation with sintered Ni crystallites from 8 to 15 nm.  Leilei Xu et al. compared the deposition of Ni by incipient wetness impregnation on mesoporous and non-porous Mg-Al2O3 support. As mentioned earlier, using Al2O3 support with ordered mesoporosity serves as a good way to confine nickel inside the pores resisting sintering and subsequent coke deposition. Thus, non-porous Ni/Mg-Al2O3 was less stable with 28.3% carbon formation compared to 8 wt% for the mesoporous Ni/Mg-Al2O3 sample

[START_REF] Xu | Ordered mesoporous MgO-Al2O3 composite oxides supported Ni based catalysts for CO2 reforming of CH4 : Effects of basic modifier and mesopore structure[END_REF]

.

Studies have been tackled also on hydrotalcite catalysts since they are cheap with basic properties that showed good activity in DRM. However, these catalysts suffer from fast sintering and coke formation due to their poor texture disturbing Ni dispersion

[START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF][START_REF] Vallezi Paladino | Hydrotalcites derived catalysts for syngas production from biogas reforming: Effect of nickel and cerium load[END_REF][START_REF] Abdelsadek | In-situ hydrogasification/regeneration of NiAl-hydrotalcite derived catalyst in the reaction of CO2 reforming of methane: A versatile approach to catalyst recycling[END_REF]

. For Based on the above-mentioned literature, a lack is still present in synthesizing a stable porous Ni-Al2O3 catalyst taking advantage of the Mg basic properties in casting carbon.

  TableIII-2). All mentioned chemicals were employed in the catalysts preparations as received and without additional purifications. The obtained mixture was then covered with a polyethylene film (PE)and stirring was kept overnight to reach a complete dissolution of the chemicals added. Then,

	the solution was dried at 60 °C for 48 h to slowly evaporate ethanol and HNO3 with the aid of
	a Julabo that circulates water to regulate and control the temperature (Figure III-4 B). The
	resulting Xerogel (Figure III-4 C) was calcined in a muffle furnace under air at 600 °C for 5 h
	with a heating rate of 0.5 °C.min -1 (thin bed conditions). The samples obtained were denoted as
	Nix-Mgy-Al2O3 where x and y correspond to the molar percentage of Ni and Mg, respectively
	calculated as follows: x= nNi / (nNi + nAl + nMg) *100 and y= nMg / (nNi + nAl + nMg) *100; n
	represents the number of atoms of each element.

Note that the "y" numbers of Mg 0, 5, 10, 15, 30 and 50 are mole contents. They correspond to Mg weight percentage detailed in Table

III

-2 and that are 0, 2.5, 5, 7, 15, and 26 wt%, respectively.

Table III -

 III

		Metal
	Sample code	contents (wt%) Ni Mg	NixMgyAlz atomic formula *	Weight of chemical added (g) Ni(NO3)2 Mg(NO3)2 P123 Al[OCH(CH3)2]3 .6H2O •6H2O
	Al2O3	-	-	Ni0Mg0Al100		14.3	0	0
	Ni5-Al2O3		0	Ni5Mg0Al95		13.6	1.02	0
	Ni5-Mg5-Al2O3		2.5 Ni5Mg5Al90		12.9	1.02	0.9
	Ni5-Mg10-Al2O3 Ni5-Mg15-Al2O3	5	5 7	Ni5Mg10Al85 Ni5Mg15Al80	7	12.2 11.5	1.02 1.02	1.8 2.7
	Ni5-Mg30-Al2O3		15 Ni5Mg30Al65		9.3	1.02	5.4

2: Summary of the synthesis conditions of the Ni5-Mgy-Al2O3 mesoporous materials prepared by one-pot EISA route

Table III -

 III 3 reports the values of surface areas (SA) and pore volumes (PV) of all the prepared materials. For the sake of easy comparison, the Table reports the data obtained for calcined samples (this section) but it also includes those measured in a next section for reduced (section III.5) materials. An increase in Mg content up to 7 wt% did not have a significant effect or change on the BET surface area and pore volume of the solids in comparison with pure

	alumina. They all demonstrate high SA between 230 and 262 m 2 .g -1 with comparable PV (0.47-
	0.52 cm 3 .g -1 ). Whereas, Ni5-Mg30-Al2O3 and Ni5-Mg50-Al2O3 have lower surface area of about
	25% less than Al2O3 support confirming the presence of irregular mesopores with probably
	blocking parts (plugs).
	It can be also noted on Figure III-5 that the physisorption curve of pure Al2O3 shows a

second small desorption step at lower partial pressures (P/P0=0.5) unlike the nickel-magnesium based catalysts. This could be attributed to the presence of plugs in the mesopores creating inkbottle sections within the channels which is absent in Ni and Mg enriched samples having only one desorption step at P/P0=0.8 corresponding to open pores

Table III

 III Structural properties and crystallite type/size by small and wide angle XRD

	III.3.2.
	-3: Porous characteristics of calcined and reduced Ni5-Mgy-Al2O3 materials
	Sample S.A. Al2O3 Ni content a (wt%) -258 (127)	0.52 (0.28)	4.8 (5.3)
	Ni5-Al2O3	5.0	229 (246)	0.49 (0.54)	6.2 (6.6)
	Ni5-Mg5-Al2O3	5.0	235 (207)	0.40 (0.62)	5.8 (8.5)
	Ni5-Mg10-Al2O3	4.52	258 (207)	0.47 (0.40)	5.8 (5.1)
	Ni5-Mg15-Al2O3	4.91	262 (229)	0.49 (0.38)	5.5 (5.7)
	Ni5-Mg30-Al2O3	4.93	196 (144)	0.48 (0.36)	6.9 (7.6)
	Ni5-Mg50-Al2O3	4.96	200 (172)	0.52 (0.42)	7.4 (8.0)
	a: Ni amount determined by XRF, b: total surface area (S.A.), total pore volume (Vtot) and mean
	pore diameter (mean) calculated from the N2 sorption isotherms for calcined (normal font) or
	reduced (in brackets) materials properties

b (m 2 .g -1 ) Vtot b (cm 3 .g -1 ) Mean b (nm)

Table III

 III

	-4: Interplanar spacing and unit cell parameter of calcined and reduced Ni5-Mgy-Al2O3
			samples
	Samples	Calcined d100 * (nm) a0 * (nm) t * (nm) d100 * (nm) a0 * (nm) t * (nm) Reduced
	Al2O3	10.5	12.1	7.3	10.3	11.9	6.6
	Ni5-Al2O3	9.8	11.3	5.1	9.7	11.2	4.7
	Ni5-Mg5-Al2O3	10.9	12.5	6.7	n.d.	n.d.	n.d.
	Ni5-Mg10-Al2O3	9.4	10.8	5.0	8.4	9.7	4.6
	Ni5-Mg15-Al2O3	9.5	10.7	5.2	8.7	10.1	4.4
	Ni5

Table III -

 III 5: Ni average particle size for calcined, reduced and spent materials

		Particle size (nm) a
	Sample
		Calc. ФNiO Red. ФNi 0 Spent ФNi 0
	Al2O3	n.d. (n.d.)	-	-
	Ni5-Al2O3	n.d. (n.d.)	4 (4.6)	n.p.
	Ni5-Mg5-Al2O3	n.d. (n.d.)	4.3 (n.p.)	n.p.
	Ni5-Mg10-Al2O3 n.d. (n.d.)	5.6 (5.3)	n.p.
	Ni5-Mg15-Al2O3 n.d. (n.d.)	4.4 (4.5)	4.2 (5)
	Ni5-Mg30-Al2O3	<3 * (n.p.)	4.7 (7.4)	n.p.
	Ni5-Mg50-Al2O3 5.0 * (n.p.)	7.7 (8.8)	9.7
	a: mean size values estimated from XRD (normal font) by applying the Scherrer equation considering the Ni 0
	reflection at 2θ =51.9°, and from the histograms obtained from representative TEM images (in brackets), n.d.: not
	detectable, * MgO crystalline domains, n.p. not performed

III.3.3. Visualization of porosities by TEM

High resolution transmission electron microscopy (HRTEM) and high angular annular

  Example of Arrhenius plot in DRM of conversions compared to CH4 are obtained. In addition, Ni5-Mg15-Al2O3 has the lowest activation energy for CH4 and CO2 consumption (28.14 and 22.30 kJ.mol -1 , respectively) and the trend is in good agreement with what was obtained in the activity of those catalysts. Thus, the addition of a proper amount of magnesium promoter can remarkably reduce the energy consumption of the system by lowering adsorbed CH4 and CO2 species Ea in DRM. However, with a very high amount of Mg (15-26 wt%), the activation barrier becomes higher and the Ea value increases. Not to mention, that the values of Ea obtained in this study are lower than several Ni-Al2O3 based catalysts studied in literature (Table III-7) independent of the amount of promoter present. III.7.5. Power law rate expression for Ni5-Al2O3 catalystThe influence of the partial pressures of CH4 and CO2 on the DRM rate was studied over mesoporous Ni5-Al2O3 catalyst. A constant CH4 partial pressure of 0.05 atm was used as the CO2 partial pressure was varied between 0.01 and 0.05 atm and vice versa. The CH4 consumption rates in the kinetics measurements

	Sample	TOF (s -1 )	EaCH4 (kJ.mol -1 )	EaCO2 (kJ.mol -1 )	Reference
	Ni5-Al2O3	5.77	35.92	26.32	This work
	Ni5-Mg15-Al2O3	6.64	28.14	22.30	This work
	Ni5-Mg30-Al2O3	6.23	34.30	26.22	This work
	Ni5-Mg50-Al2O3	3.03	36.59	28	This work
	Ni7-Cey-Al2O3	-	41.40	-	[134]
	Ni7-Cey-Al2O3	-	38.93	-	[82]
	Ni13-Lay-Al2O3	3.45-5.77	44.38	-	[196]
	Ni-Pt-Al2O3	-	104.67	-	[195]
	Ni5-CaO21-Al2O3	-	108.85	98.74	[197]
	Ni-Al2O3	0.6	70.60	69.00	[198]
	Ni-γAl2O3	-	51.9	56.1	[199]
	Ni-K/CeO2-Al2O3	-	46.1	46.2	[200]

Figure III-23: XCH4×FCH4 Wcat ; XCH4 is CH4 conversion, FCH4 is the initial flow rate of CH4 (mmol.s -1 ) and Wcat is the catalyst weight (g)) are calculated from the slopes of the conversions versus residence time plots. The emperical power low -rCH4=K×P a (CH4)×P b (CO2) was used to estimate the reaction orders (a, b) and the apparent rate constant (K) by applying the non-linear regression analysis in POLYMATH 6.1 software choosing Levenberg-Marquardt algorithm. The obtained rates in Table

III

-8 and their corresponding partial pressure values obtained at 650 °C were utilized in order to calculate and provide the values of the reactants orders presented. Results of the regression analysis give the following values: a= 0.75, b= 0.99 and K= 13.48 (mmol.g -1 .s -1 .atm -(a+b) ) with a variance (σ 2 ) of 6.242E -06 . Both CH4 and CO2 orders were close to 1, similar to a study done by Wang S. et al.

Table III

 III

	Chapter III	Effect of Mg loading over ordered mesoporous Ni5-Mgy-Al2O3 DRM catalysts
	mesoporosity also disappears forming Mg-O bonds with longer length than those of Al-O bonds
	expanding the Al2O3 structure. As a result, the catalytic performance faces a calamitous
	decrease (increase in activation energy) especially that the excessive number of basic sites can
	lead to CO2 coverage of active Ni sites. Hence, this chapter has justified the reason behind
	observing a "volcano-shape" curve in most of the Ni-Mg-Al2O3 studies.
		-8: Initial rates of DRM at 650 °C over mesoporous Ni5-Al2O3 catalyst
	Partial pressures (atm) CH4 CO2	CH4/CO2	Reaction rate (mmol.g -1 .s -1 )
	0.01	0.05	0.2	0.0188
	0.02	0.05	0.4	0.0363
	0.03	0.05	0.6	0.0548
	0.04	0.05	0.8	0.0645
	0.05	0.05	1	0.0701
	0.05	0.04	1.2	0.0576
	0.05	0.03	1.6	0.0441
	0.05	0.02	2.4	0.0284
	0.05	0.01	4.8	0.0159
				85

IV.

Chapter IV: Porous nickel-alumina derived from metal-organic framework (MIL-53): a new approach to achieve active and stable catalysts in methane dry reforming

	Chapter IV	Porous nickel-alumina derived from metal-organic
		framework (MIL-53) in DRM
	This chapter tackles a second strategy of synthesizing a material that combines the
	features of high porosity, high surface area and intimate mixing between Ni metal and Al2O3
		89

Table IV -

 IV 1: Summary about Ni/C and oxides-MOF-derived materials and their applications

	Sample	Derived MOF Treatment	Particle size (nm)	Surface area (m 2 .g -1 )	Application
	Ni@C-N [213]	MOF	Ar	17.4	100-200	Alkanes oxidation
	Ni/C [214]	MOF	N2	19-40	7, 51, 389	4-nitrophenol reduction
	Ni/C [215]	NiOF	N2	11	130	Super capacitors
	Ni-Zn/C [216]	ZIF-8	N2	n.m.	750	CO2 capture and separation
	Ni/C [217]	MOF	Ar	18-50	18-165	H2 evolution reaction
						Furfural
	CuNi/C [218]	MOF	N2	15	91	Conversion to
						cyclopentanone
	Ni/C [219]	MOF	N2	n.m.	460-600	Super capacitors

Table IV -

 IV

	Sample names	Textural properties of calcined samples a S.A. (m 2 .g -1 ) Φ Vtot (nm) (cm 3 .g -1 ) mean	Reduction temperature (°C)	Ni content b (wt%)	Ni 0 nanoparticle size after reduction (nm) c reduced spent	Ni 0 nanoparticle dispersion after reduction (%) d
	MIL-53	1130	0.80	n.d
	Ni5-Al2O3 MIL-53	239	0.82	10	830	5.1 (5.2)
	Ni5 0 -Al2O3 MIL-53	200	1.02	16			6.8 (5.3)	7.5	19 (18.3)
	Al2O3 MIL-53	318	1.19	6.2
	Ni5@Al2O3 MIL-53	244	0.94	5.1	700/820	4.7 (4.6)
	Ni5 0 @Al2O3 MIL-53	130	0.55	5.6			10.2 (8.0)	11.0	12 (11.8)
	Al2O3 COM	185	0.51	9.2
	Ni5@Al2O3 COM	168	0.47	9.9	630/800	5 (5)
	Ni5 0 @Al2O3 COM	125	0.43	10.3			22 (18)	29	5.6 (9.6)

4: Textural characteristics, reduction temperature and nickel content

Table IV -

 IV 5: Catalytic test results for all the prepared catalysts

				Conversion (%)			Molar composition of main products (H2/CO)	Coke content (%)
	Catalyst	t=0.5h	t=13h		t=100h
								t=0.5h t=13h t=100h t=13h t=100h
		XCH4 XCO2 XCH4 XCO2 XCH4 XCO2
	Ni5-Al2O3MIL-53	75	79	76	79	74	80	1.02	1.01	1.03	0	0
	Ni5@Al2O3MIL-53	71	79	70	77 n.p.	n.p.	1.12	1.09	n.p.	5	n.p.
	Ni5@Al2O3COM	22	27	20	26 n.p.	n.p.	0.93	0.89	n.p.	7	n.p.

Table IV

 IV

	Chapter IV		Porous nickel-alumina derived from metal-organic
					framework (MIL-53) in DRM
		-6: Comparison between Ni5-Al2O3 MIL-53 and Ni5-Al2O3 Meso
	Catalyst	Maximum reduction temperature	Surface Area (m 2 .g -1 )	Particle size (nm)	Activity at 650 °C	C(s)
		(°C)		reduced spent t=0.5h	t=13h
	Ni5-Al2O3MIL-53	830	239	6.8	7	75-79	76-79	No
	Ni5-Al2O3 Meso	708	182	8.5	10	41-46	41-47	Yes

Table V -

 V 1: Bibliography of some of the current Ni-Al2O3 based catalysts applied in CO2 methanation

	Catalyst	Preparation method	Ni amount (wt%)	S.A. (m 2 .g -1 )	Particle size (nm)	GHSV (L.g -1 .h -1 )/ T(°C)	XCO2 (%)	SCH4 (%)	TOF (CO2, s -1 )
	Ni/Al2O3 [255]	DP a	10	200	3.7	26700 * /300	22	85	8.10 -2

Table V -

 V 2: Summary of the prepared one-pot mesoporous Nix-Mgy-Al2O3 samples

			Metal
			contents		Weight of chemical added (g)
	Sample code	NixMgyAlz atomic formula	(wt%) Ni Mg	P123	Al[OCH (CH3)2]3	Ni(NO3)2 .6H2O	Mg(NO3)2 •6H2O
	Al2O3	Ni0Mg0Al100	-	-		14.3	0	0
	Ni5-Al2O3	Ni5Mg0Al95		0		13.6	1.02	0
	Ni5-Mg15-Al2O3	Ni5Mg15Al80	5	7		11.5	1.02	2.7
	Ni5-Mg50-Al2O3	Ni5Mg50Al45		26	7	6.5	1.02	9
	Ni10-Mg15-Al2O3	Ni10Mg15Al75	10			10.7	2.03	2.7
	Ni15-Mg15-Al2O3	Ni15Mg15Al70	15	7		10.0	3.05	2.7
	Ni20-Mg15-Al2O3	Ni20Mg15Al65	20			9.3	4.07	2.7

Table V-3: Summary of mesoporous Nix-Al2O3 materials derived from MIL-53 framework

Table V -

 V 4: Physicochemical properties of calcined and reduced (in italic) Nix-Mgy-Al2O3 materials

	Catalyst code	h index a	Vtot b (cm 3 /g)	S.A. b (m 2 /g)	ФNi 0 c (nm)	Dispersion d (%)
	Ni5-Al2O3	0.55	0.49 (0.32) 234 (173)	4.6	21
	Ni5-Mg15-Al2O3	0.45	0.56 (n.d.) 240 (n.d.)	4.5	21
	Ni5-Mg50-Al2O3	0.38	0.54 (n.d.) 205 (n.d.)	8.8	11
	Ni10-Mg15-Al2O3	0.41	0.57 (0.52) 247 (219)	5.85	17
	Ni15-Mg15-Al2O3	0.46	0.54 (0.45) 203 (160)	6.37	15
	Ni20-Mg15-Al2O3	0.45	0.47 (0.48) 197 (198)	9	11

Table V -

 V 5: Comparison of the performances revealed by the most promising Ni-Mg-Al2O3 material from the present work with other traditional (first two rows) and one-pot Al2O3 samples reported in the literature

			Catalytic tests	Best catalytic performances
	Catalyst	P (bar)	conditions H2:CO2 GHSV (ml.min -1 .g -1 )	T (ºC)	achieved CO2 conversion (%)	CH4 selectivity (%)	Ref.
	15Ni-20Ca/Al2O3	1	4:1	-	450	66	92	[256]
	10Ni-8Mg/Al2O3	1	4:1	16000 h -1	500	67	35	[257]
	10Ni-8Ca-Al2O3	1	4:1	250	350	76	98	[284]
	10Ni-3Pr-Al2O3	1	4:1	250	350	74	98	[295]
	10Ni-1Ru-2Ca-Al2O3	1	4:1	500	380	84	>99	[194]
	Ni15-Mg15-Al2O3	1	4:1	1440	350	65	95	This work

Table V

 V

	-6 indicates NiO average particle size of 17.2 nm, 8.6 nm, and 8 nm for Ni15@USY,
	Ni15@SBA-15, and Ni15-Al2O3 respectively.

Table V -

 V 6: Ni-nanoparticle sizes of the calcined and reduced materials, and metallic Ni dispersion on reduced samples a: value of NiO particle size estimated from XRD diffractograms of calcined materials; b: value of metallic Ni 0 nanoparticle estimated from XRD of reduced materials, in brackets: nanoparticle size from the histograms obtained from representative TEM images of each samples; c: nanoparticle dispersion calculated from TEM Increasing the Ni amount from 5 to 20 wt% does not alter the shape of the isotherm in all Ni-Al2O3 materials derived from MIL-53 and all of them resembles the shape already seen and discussed in chapter III. Ni5-Al2O3 MIL-53, Ni15-Al2O3 MIL-53, and Ni20-Al2O3 MIL-53 have close values of surface area (230-250 m 2 .g -1 , Table V-7) and pore volume (0.97-1.09 cm 3 .g -1 )

	Chapter V	Mesoporous nickel-alumina catalysts for CO2 methanation
	Catalyst code	ФNiO a (nm)	ФNi 0 b (nm)	Ni 0 dispersion c (%)
	Ni5-Al2O3MIL-53	-	-(7.0)	14
	Ni15-Al2O3MIL-53	-	5.6 (8.6)	12
	Ni20-Al2O3 MIL-53	-	7.1 (9.4)	11
	Ni15@Al2O3MIL-53	-	6.2 (9.2)	11
	Ni15@USY	17.2	18.9 (25)	4
	Ni15@SBA-15	8	9.7 (17.1)	6
	Ni15-Al2O3 one-pot	8.6	8.8 (9.0)	11
				132

Table V -

 V 7: Main textural properties of the calcined and reduced samples prepared in the present work

	Catalyst code	h index a	Vmicro b (cm 3 .g -1 )	Vmeso b (cm 3 .g -1 )	S.A. b (m 2 .g -1 )
	MIL-53	-	0.4	0.4	1130
	Al2O3 MIL-53	-	-	1.19	318
	Ni5-Al2O3 MIL-53	0.64	-	0.97 (0.85)	230 (225)
	Ni15-Al2O3 MIL-53	0.60	-	1.09 (1.02)	251 (150)
	Ni20-Al2O3 MIL-53	0.58	-	1.05 (0.71)	245 (175)
	Ni15@Al2O3MIL-53	0.54	-	0.42 (0.29)	156 (81)
	USY	-	0.208	0.271	298
	Ni15@USY	0.87	0.16 (0.01) 0.20 (0.25) 259* (261)*
	SBA-15	-	-	1.02	701
	Ni15@SBA-15	0.98	0.11 (0.07) 0.66 (0.62)	487 (422)
	Al2O3 One-Pot	-	-	0.52	258
	Ni15-Al2O3 One-Pot	0.38	-	0.13 (0.11)	60 (44)
	a determined from TGA-DSC experiments; b total surface area (S.A.), microporous (Vmicro) and
	mesoporous (Vmeso) pore volume calculated from the N2 sorption isotherms, in brackets: reduced
	materials *Sext determined using t-plot method for the zeolite sample after calcination and reduction

Table V -

 V 8: Comparison of the performances revealed by the best MOF-derived material from the present work with similar reported catalysts in the literature.

			Metal	Catalytic tests conditions	Best catalytic performances achieved
	Catalyst	MOF	amount (wt%)	P (bar)	H2:CO2	GHSV (ml.min -1 .g -1 )	T (ºC)	CO2 conversion (%)	CH4 selectivity (%)	TOF (s -1 )

  bed (d:8 mm), 100 mg, 700 °C, 300 min, 48 L.g -1 .h -1 bed, 50 mg, , 800 °C, 80 h, 72 L.g -1 .h-1

									n.m.	25	n.m.	n.m.	C(s)
	[74]		Commercial	W.I.	n.m.	n.m.	Tubular U-shaped, 300 mg, 650 °C, 10 h, 45 L.g -1 .h -1	40	50	30	45	n.m.	C(s)
	[75]		Commercial α	I.W.I.	n.m.	n.m.	Tubular U-shaped	n.m.	n.m.	n.m.	n.m.	n.m.	n.m.
	[64]		Commercial	I.W.I.	4	n.m.	Fixed bed (d:9 mm), 200 mg, 800°C, 50h, 48 L.g -1 .h -1	80	85	50	65	0.8	C(s)
	[72]	8	Commercial γ	EDF	12	n.m.	Fixed bed, 750 °C, 120 L.g -1 .h -1	14.5	32	n.m.	n.m.	0.4	C(s)
	[76]		Commercial	electrospinning	6.5	n.m.		55	82	50	80	0.8	C(s)
	[77]		Al precursor	EISA	15	n.m.		90	90	88	88	n.m.	C(s)
	[78]		Al precursor	aerogel	9	20	Quartz, (d:18 mm), 200 mg, 800 °C, 30 h	95	95	90	90	n.m.	Amorphous C(s) and S
	[79]		Al precursor	combustion	4.6	16.8	Fixed bed, 100 mg, 700 °C, 24 h	40	52	35	47	n.m.	Whiskers C(s) and S
	[80]		Meso-Al2O3	One-pot	11	15	Fixed bed, 100 mg, , 800 °C, 500 min, 52 L.g -1 .h -1	62	75	52	62	0.78	Nanotubes C(s)
	[81]		Al precursor	2-step hydrothermal	7	n.m.	100 mg, , 800 °C, 50 h, 21 L.g -1 .h -1	95	90	95	90	0.92	Amorphous C(s)
	[83]		γ nanoflakes	Hydrothermal	5.5	8.5	Fixed bed quartz, 100 mg, , 800 °C, 50 h	82	n.m.	80	n.m.	0.90	Nanotubes C(s)
	[84]		Nanosheets γ	I.W.I.	6.7	7	Quartz (d=8mm), 100 mg, 700 °C, 300 min, 48 L.g -1 .h -1	55	n.m.	55	n.m.	n.m.	-
	[86]		Nanosheets	Hydrothermal	12	14	Quartz (d=8mm), 100 mg, 800 °C, 100 h, 108 L.g -1 .h -1	95	94	90	92	0.95	35% C(s)
	[87]	5	Meso-Al2O3	Hydrolysis deposition	5.5	20	Fixed bed s.s.(d= 6 mm), 500 mg, , 800 °C, 10 h, 14.4 L.g -1 .h -1	92	97	90	92	0.85	C(s) and S
	[88]		Al precursor	I.W.I.	14	15	Fixed bed Quartz (d=8mm), 100 mg, 700 °C, 100 h, 15L.g -1 .h -1	80	80	78	78	0.8	Graphitic C(s)

Fixed Fixed

List of abbreviations

List of tables