La distillation fractionnée des coupes pétrolières brutes se révèle insuffisante, pour répondre seule aux besoins en carburants et génère de fortes quantités de distillats à longue chaîne carbonée (C20-C50). Cette coupe d’hydrocarbures peu valorisable est ainsi transformée en molécules hydrocarbonées plus légères, par le biais de deux procédés : le craquage catalytique en lit fluidisé ou l’hydrocraquage. Ce dernier permet le craquage sélectif des hydrocarbures essentiellement en gazole et kérosène. Les catalyseurs acides utilisés sont « bifonctionnels » : une matrice zéolithique USY (zéolithe Y ultra-stabilisée) favorise le craquage des hydrocarbures, pendant qu’un sulfure mixte ou un métal noble greffé catalyse les réactions d’hydrogénation et de déshydrogénation. L’objectif de ce travail de thèse consiste à maximiser la sélectivité en distillats moyens, en diminuant le temps de séjour des molécules d’hydrocarbures dans la structure zéolithique, afin de réduire les réactions de surcraquage. Pour cela, il a été choisi de synthétiser directement des nanocristaux de zéolithe Y sans agent organique structurant, avec un rapport Si/Al le plus élevé possible. Cette propriété leur confère une meilleure stabilité lors des post-traitements, indispensables pour atteindre le rapport Si/Al des zéolithes USY actuellement utilisées. Grâce à l’élaboration d’une nouvelle stratégie de synthèse, des nanocristaux compris entre 20 et 90 nm avec un rapport Si/Al variant de 2,2 à 2,6 ont été obtenus. Puis, l’obtention de la forme protonée de ces nanocristaux (90 nm) a été étudiée et des mesures d’acidité ont également été effectuées. / The fractionated distillation of crude oil is not sufficient to cover all the fuel needs and produces large amounts of long chain carbon distillates (C20-C50). Thus, these hydrocarbons are transformed into lighter hydrocarbon molecules by two processes: fluid catalytic cracking or hydrocracking. The latter allows the selective cracking of hydrocarbons essentially in gas oil and kerosene. The acid catalysts used are “bifunctional”: a zeolite matrix USY (ultra-stabilized zeolite Y) performs the hydrocarbons cracking, at the same time a sulfide or a noble metal catalyzes hydrogenation and dehydrogenation reactions. The aim of this work is to maximize the selectivity to middle distillates by decreasing the residence time of hydrocarbon molecules in the zeolite framework. For this, it was chosen to directly synthesize zeolite Y nanocrystals with the highest possible Si/Al ratio, in a SDA-free medium. This property gives them a better stability during the post-treatments essential to reach the Si/Al ratio of the currently used USY zeolite. Thanks to a new synthesis strategy, nanocrystals between 20 and 90 nm with a Si/Al ratio varying from 2.2 to 2.6 were obtained. Then, the protonated form of these nanocrystals (90 nm) was studied and acidity measurements were also carried out.

L’objectif de cette thèse est de développer une nouvelle génération de matériaux céramiques de confinement plasma pour les moteurs à effet Hall en mettant en oeuvre la voie PDCs pour polymer-derived ceramics ou voie des polymères précéramiques.Un état de l’art des différents matériaux de confinement et un bilan des travaux précédemment menés sur ce sujet ont permis de déterminer les paramètres essentiels de tels matériaux et de s’orienter vers l’utilisation des polymères précéramiques commeprécurseurs des matériaux envisagés. Après avoir détaillé les différents protocoles permettant de modifier chimiquement un polymère commercial avec le bore, de mettre en forme les composés obtenus puis de réaliser la pyrolyse pour générer la céramique, des pièces denses céramiques Si-B-(C)-N ont été réalisées avec une teneur variable en bore. Une étude complète allant de la structure chimique des polymères jusqu’aux propriétés des céramiques résultantes a permis de sélectionner la formulation optimale du polymère comme précurseur de céramique. Des composites c-BN/Si-B-(C)-N ont ensuite été préparés parajout de charges, puis mis en forme et caractérisés avant de procéder à un changement d’échelle visant à préparer des bagues céramiques de taille moteur. La dernière partie consiste en une ouverture sur l’utilisation des polymères précéramiques pour la réalisation de pièces denses Si-Al-(C)-N de composition contrôlée avec en particulier une étude sur l’impact de l’aluminium sur les propriétés des polymères et des céramiques. / The main objective is here to develop a new generation of ceramic materials used for plasma confinement in Hall-effect thrusters using the PDCs (polymer-derived ceramics) route. A state of the art of the different confinement materials and a review of the previous work done on this topic allowed to determine the key parameters of such materials and to move towards the use of preceramic polymers as precursors of the materials envisaged. After having detailed the various protocols used to chemically modify a commercial polymer with boron, to shape the compounds obtained and then to convert the polymers into ceramics bypyrolysis, Si-B-(C)-N dense ceramic pieces have been produced with various boron content. A complete study from the chemical structure of the polymers to the properties of the resulting ceramics allowed selecting the optimal formulation of the polymer as a ceramic precursor. c-BN/Si-B-(C)-N composites were then prepared by filler addition, then shaped and characterized before a scale-up to prepare engine-size ceramic rings. The last part consists of an opening on the use of the PDCs route for the realization of dense Si-Al-(C)-N pieces of controlled composition, with a study of the impact of aluminum on the polymers andceramics properties.

Many zeolites are deployed in various industrial processes owing to their robust catalytic performance and hydrothermal stability. Reactions in zeolites are catalyzed via framework aluminum. The Si/Al ratio is a metric that describes the relative aluminum content in zeolites. However, several researchers noted that the proximity of aluminum in the framework could impact the catalyst output [1–3]. In this work, the influence of paired acid sites is examined in ZSM-22. The 1-dimensional nature of ZSM-22 allows for direct assessment of aluminum proximity without the influence of channel intersection. Theoretical investigations via static density functional theory (DFT) optimization calculations on isolated and paired BAS in ZSM-22 revealed a potential increase in deprotonation potential energy (DPE), indicating a weaker acid with closer aluminum sites. One specific paired model, however, suggested stronger acid behavior, likely due to unfavorable proton-proton interactions influenced by proximity and orientation. Additionally, ammonia adsorption calculations inferred improved adsorption by isolated models, possibly due to unfavorable ammonium-proton interactions in the paired models. Reaction state calculations of ethylene and propylene oligomerization suggested enhanced stabilization of reactant molecules in paired sites. The synthesis of ZSM-22 showed sensitivity to precursor ratios and conditions, but pure samples were successfully achieved through iterative optimization. Catalytic testing of ethylene oligomerization with these samples, classified by their Si/Al ratios and unique fractions of paired acid sites, showed a correlation between higher fractions of paired BAS and increased catalytic activity and selectivity. Samples with higher fractions of paired BAS displayed a higher activity and selectivity for heavier hydrocarbons, explained by the enhanced adsorption capacity of paired BAS for larger reactant molecules, prompting further oligomerization and enhanced catalytic activity. Our findings demonstrate the impact of BAS proximity in dictating the activity and selectivity in ZSM-22 and provide valuable insights for designing more efficient industrial zeolite-based catalysts.

No description available.

The aim of this work was to improve the heat capacity estimation of a material for usage within a CALPHAD-type assessment. An algorithm is derived that estimates the trend of heat capacity with temperature based on zero Kelvin properties and the thermal expansion coefficient at the Debye temperature. The algorithm predicts not only the trend of heat capacity but also the temperature trend of the volume and the bulk modulus, which can be also included in new thermodynamic databases. The algorithm is used to assess thermophysical properties of the intermetallic phases eta (Fe2Al5), epsilon~(Fe5Al8) and tau4 (FeAl3Si2). The heat capacity of the intermetallic phases zeta, eta, theta and epsilon of the Al-Fe system and of tau4 of the Al-Fe-Si system was measured using DSC. For the phases zeta, eta, and theta, a non-linearly increasing heat capacity approaching the melting temperature was observed. In addition, the heat capacity of three bcc-based Al-Fe samples including the B2-->A2 transition were determined. The Al-rich section of the Al-Fe phase diagram was studied using DTA and quenching experiments. The homogeneity ranges of the intermetallic phases were determined using SEM/WDS measurements. Based on own and literature values, a thermodynamic description of the Al-Fe system was assessed including the modelling of A2/B2 ordering and the homogeneity range of all intermetallic phases. In addition, thermodynamic parameters of the Al-Fe-Si, Al-Fe-Mg, and the Fe-Mg-Si system were assessed to obtain a thermodynamic description of the Al-rich side of the Al-Fe-Si-Mg system, which can be used to study phase transitions of typical A356-aluminium alloys.

The present thesis deals with Nb-Si alloy composites in both bulk and multilayer forms. The work has been divided into two parts. First part (chapter 4-6) deals with Nb based silicides binary and ternary alloys with alloying additions like Ga and Al. These alloys are synthesized by vacuum arc melting and suction casting (non-equilibrium processing techniques). The studies on intermetallic coatings of Nb-Si alloys and Nb/Si multilayer synthesized by pulsed laser deposition technique have been presented in the second part (chapter7-8). Nb-Si alloys are one of the candidate materials for the advanced structural and microelectronic applications. There are few issues with these materials like poor oxidation resistance, low fracture toughness and brittleness which need to be solved. Microstructure plays a crucial role to control these properties. The main focus of this work is to understand the process of phase transformation and thereby control the microstructure in both bulk alloys and thin films. We have also investigated in a limited manner mechanical and environmental properties of bulk alloys. This thesis is subdivided into nine chapters. After a brief introduction in the first chapter, a brief overview on Nb-Si phase diagram and literature reviews on Nb-Si based alloys are presented with emphasis on the current work in the second chapter. Literature reviews on the phase formations sequence and stability in Nb-Si alloys thin films and Nb/Si multilayers are also discussed in the same chapter. In the third chapter different experimental techniques, processing parameters and characterization tools like XRD, SEM, TEM etc. are briefly discussed. Special emphasis is given on two non-equilibrium techniques: laser deposition technique to deposit the thin film/multilayer and vacuum suction casting to produce the 3 mm diameter rods of different Nb-Si alloys. The fourth chapter discusses the microstructural aspects of Nb-Si alloys prepared by suction casting and its mechanical behavior. The samples have the compositions hypoeutectic (Nb-10at.%Si and Nb-14at.%Si), eutectic (Nb-18.7at.%Si) and hypereutectic (Nb-22at.% Si and Nb-25at.% Si). SEM microstructural analyses of all the samples clearly show the enhancement in the volume fraction of eutectic and decease in the eutectic spacings in microstructure due to large undercooling. Rod eutectic is observed in most of places with irregular eutectic a few places in all samples. First check of phases has been done by XRD in all samples. Phase confirmation using TEM showed the eutectic between Nbss and Nb3Si phases in all samples. The primary phase for hypoeutectic alloys is Nbss (dendritic structure), Nb3Si phase for eutectic composition and β-Nb5Si3 phase for hypereutectic alloys. Compositional analysis using EDS and EPMA also supported the above results. No signature of eutectoid reaction (Nb3Si→Nb+α-Nb5Si3) is observed. Mechanical properties like hardness, strength, ductility and indentation fracture toughness have been determined for above mention alloy compositions. SEM micrographs showed that silicides fractured by cleavage and Nb phase in a ductile manner during the compression tests carried out at room temperature. We attempt to explain how the above mention mechanical properties change with alloy compositions and processing. Chapter five deals with the effect of Ga addition on the microstructure and mechanical properties of the Nb-Si alloy. The composition selected for this study is Nb-20.2at.%Si-2.7at.%Ga. The results of ternary alloy have been compared with the binary alloy composition Nb-18.7at.%Si. Phase analysis has been carried out using TEM and XRD. Ga addition has suppressed the formation of Nb3Si phase and promoted the formation of β-Nb5Si3 phase. Ga addition also established the eutectic between Nbss and β-Nb5Si3, which is a metastable eutectic. Ga added ternary alloy, on suction casting, yields ultrafine eutectic with nanometer length scale (50-100nm). From the compression tests, it is concluded that the combination of ultrafine eutectic (Nbss-β-Nb5Si3) and primary β-Nb5Si3 in ternary alloy results in a high compressive strength ~2.8±0.1 GPa with 4.3% plasticity. In contrast binary alloy under identical conditions shows the compressive strength ~1.35±0.1 GPa and 0.2% plasticity. Ga addition also enhances the indentation fracture toughness from 9.2±0.05 MPa√m (binary) to 24.11±0.5 MPa√m (ternary). Composite hardness values of the ternary and binary alloys are 1064±20 Hv and 1031±20 Hv respectively. Chapter six deals with Al added Nb-Si ternary alloy. Here we have discussed microstructural and mechanical properties like in chapter 5 along with oxidation behavior for the alloy composition Nb-12.7at.%Si-9at.%Al. SEM micrograph shows the presence of primary dendrites structure with ultra fine lamellar eutectic (50-100nm). Detailed TEM studies confirm the Nbss as primary phase present in form of dendrites. These dendrites contain the plate shape precipitates of δ-Nb11Si4 (body centered orthorhombic structure) phase in Nb matrix (primary dendrites). Eutectic phases are Nbss and β-Nb5Si3. The analysis of the results indicates that Al addition promote the formation of β-Nb5Si3 phase in the eutectic. The results of this ternary composition were also compared with the binary alloy composition Nb-18.7at.%Si. Compression tests have been carried out at room and elevated temperatures to measure the strength of the material. Al added ternary alloy yields the compressive strength value 1.6±0.01 GPa whereas binary alloy yields the compressive strength value 1.1±0.01 GPa. Enhancement in indentation fractured toughness is observed in Al added ternary alloy (20.4±0.5MPa√m) compare to binary alloy (9.2±0.05 MPa√m). Thermal analysis by TGA and DTA were used to see the oxidation behavior of Al added ternary alloy. Chapter seven deals with the deposition characteristics and the TEM studies on the laser deposited Nb-Si thin films. Films were deposited on the NaCl crystals and Si single crystal substrates. The compositions chosen in this case are Nb-25at.%Si, Nb-37.5at.%Si and Nb-66.7at.%Si. These compositions correspond to the equilibrium intermetallic compounds Nb3Si, Nb5Si3 and NbSi2 respectively. In this chapter we have briefly discussed the microstructural and phase evolutions in the intermetallic coatings. The smooth films quenched from the vapor and/or plasma state show amorphous structure. The sequence of crystallization was studied by hot stage TEM experiments as well as by cross sectional TEM in the films deposited at the elevated temperatures (600oC and 700oC) on Si substrates. During the hot stage experiment, crystallization is observed in Nb-25at.%Si film around 850oC with nucleation of metastable cubic Nb3Si phase. Occasionally metastable hexagonal Nb3Si3 phase has also been observed (close to Si substrate) along with cubic Nb3Si phase in the films at elevated temperatures. For Nb-37.5at.%Si film, crystallization is observed at 800oC with the nucleation of grains of metastable hexagonal Nb5Si3 phase. Cross-sectional TEM shows the presence of hexagonal Nb5Si3 phase along with few grains of NbSi3 (equilibrium) phase in the films deposited at elevated temperatures. Hot stage experiment of Nb-66.3at.%Si film showed the onset of crystallization much earlier at 400oC and complete crystallization at 600oC. This crystallization leads to the nucleation of grains of NbSi2 phase. Films of this composition deposited at elevated temperatures showed the presence of NbSi2 and metastable hexagonal Nb5Si3 phases (occasionally). The laser ablated films, besides the film matrix also contain the micron and submicron sized spherical droplets of different sizes. These droplets travel at very high velocities and impinge on the substrate resulting in a very high rate of heat transfer during solidification from liquid state. Therefore in this work we have also studied the microstructural evolution in the droplets for each composition. The phases observed in the droplets embedded in the matrix of Nb-25 at% Si alloy film are the bcc Nb and the cubic Nb3Si (metastable phase). The droplets in the matrix of Nb-37.5 at% Si alloy showed the bcc Nb and tetragonal β-Nb5Si3 phases. The phases observed in the droplets of in the Nb-66.3at.%Si alloy are the bcc Nb, tetragonal β-Nb5Si3 and the hexagonal NbSi2 (metastable phase). Chapter eight describes the synthesis and microstructural characterization using TEM of Nb/Si multilayers. The aim of this work is to check the stability and phase formation sequence in Nb/Si multilayer. Nb/Si multilayers were first annealed at different time intervals at 600oC and at different temperatures (for 2 hours) and then characterized by the cross-sectional transmission electron microscopy. As-deposited Nb layer is crystalline while Si layer is amorphous. Microstructural and compositional evidences suggest the intermixing between the Nb and Si layers at the interfaces. Nb/Si multilayer annealed at 600oC for 1 hour, NbSi2 was identified as the first crystalline nucleating phase. However amorphous silicide layers were also observed between Nb and NbSi2 layers. Metastable hexagonal Nb5Si3 was identified as the next crystalline phase that nucleated from the amorphous silicide layers at the interfaces of Nb and NbSi2 layers. Occasionally few grains of cubic Nb3Si phase were also observed after 8 hours of annealing at 600oC. In the chapter we have compared the results to the other reported works in Nb-Si bulk diffusion couples and also thin film couples. The final chapter summarizes the major conclusions of the present work and scope of future work.

<p>Selective catalytic reduction (SCR) of NO_xusing NH₃as a reductant (4NH₃+ 4NO + O₂ 6H₂O + 4N₂) over Cu-SSZ-13 zeolites is a commercial technology used to meet emissions targets in lean-burn and diesel engine exhaust. Optimization of catalyst design parameters to improve catalyst reactivity and stability against deactivation (hydrothermal and sulfur poisoning) necessitates detailed molecular level understanding of structurally different active Cu sites and the reaction mechanism. With the help of synthetic, titrimetric, spectroscopic, kinetic and computational techniques, we established new molecular level details regarding 1) active Cu site speciation in monomeric and dimeric complexes in Cu-SSZ-13, 2) elementary steps in the catalytic reaction mechanism, 3) and deactivation mechanisms upon hydrothermal treatment and sulfur poisoning.</p><p>We have demonstrated that Cu in Cu-SSZ-13 speciates as two distinct isolated sites, nominally divalent Cu^IIand monovalent [Cu^II(OH)]⁺complexes exchanged at paired Al and isolated Al sites, respectively. This Cu site model accurately described a wide range of zeolite chemical composition, as evidenced by spectroscopic (Infrared and X-ray absorption) and titrimetric characterization of Cu sites under <i>ex situ </i>conditions and <i>in situ </i>and <i>operando </i>SCR reaction conditions. Monovalent [Cu^II(OH)]⁺complexes have been further found to condense to form multinuclear Cu-oxo complexes upon high temperature oxidative treatment, which have been characterized using UV-visible spectroscopy, CO-temperature programmed reduction and dry NO oxidation as a probe reaction. Structurally different isolated Cu sites have different susceptibilities to H₂and He reductions, but are similarly susceptible to NO+NH₃reduction and have been found to catalyze NO_xSCR reaction at similar turnover rates (per Cu^II; 473 K) via a Cu^II/Cu^Iredox cycle, as their structurally different identities are masked by NH₃solvation during reaction. </p><p><br></p><p>Molecular level insights on the low temperature Cu^II/Cu^Iredox mechanism have been obtained using experiments performed <i>in situ</i>and <i>in operando </i>coupled with<i></i>theory. Evidence has been provided to show that the Cu^II to Cu^Ireduction half-cycle involves single-site Cu reduction of isolated Cu^IIsites with NO+NH₃, which is independent of Cu spatial density. In contrast, the Cu^I to Cu^IIoxidation half-cycle involves dual-site Cu oxidation with O₂to form dimeric Cu-oxo complexes, which is dependent on Cu spatial density. Such dual-site oxidation during the SCR Cu^II/Cu^Iredox cycle requires two Cu^I(NH₃)₂sites, which is enabled by NH₃solvation that confers mobility to isolated Cu^Isites and allows reactions between two Cu^I(NH₃)₂species and O₂. As a result, standard SCR rates depend on Cu proximity in Cu-SSZ-13 zeolites when Cu^Ioxidation steps are kinetically relevant. Additional unresolved pieces of mechanism have been investigated, such as the reactivity of Cu dimers, the types of reaction intermediates involved, and the debated role of Brønsted acid sites in the SCR cycle, to postulate a detailed reaction mechanism. A strategy has been discussed to operate either in oxidation or reduction-limited kinetic regimes, to extract oxidation and reduction rate constants, and better interpret the kinetic differences among Cu-SSZ-13 catalysts.</p><p><br></p><p>The stability of active Cu sites upon sulfur oxide poisoning has been assessed by exposing model Cu-zeolite samples to dry SO₂and O₂streams at 473 and 673 K, and then analyzing the surface intermediates formed via spectroscopic and kinetic assessments. Model Cu-SSZ-13 zeolites were synthesized to contain distinct Cu active site types, predominantly either divalent Cu^IIions exchanged at proximal framework Al (Z₂Cu), or monovalent [Cu^IIOH]⁺complexes exchanged at isolated framework Al (ZCuOH). SCR turnover rates (473 K, per Cu) decreased linearly with increasing S content to undetectable values at equimolar S:Cu ratios, consistent with poisoning of each Cu site with one SO₂-derived intermediate. Cu and S K-edge X-ray absorption spectroscopy and density functional theory calculations were used to identify the structures and binding energies of different SO₂-derived intermediates at Z₂Cu and ZCuOH sites, revealing that bisulfates are particularly low in energy, and residual Brønsted protons are liberated at Z₂Cu sites as bisulfates are formed. Molecular dynamics simulations also show that Cu sites bound to one HSO₄^-are immobile, but become liberated from the framework and more mobile when bound to two HSO₄^-. These findings indicate that Z₂Cu sites are more resistant to SO₂poisoning than ZCuOH sites, and are easier to regenerate once poisoned.</p><p><br></p><p>The stability of active Cu sites on various small-pore Cu-zeolites during hydrothermal deactivation (high temperature steaming conditions) has also been assessed by probing the structural and kinetic changes to active Cu sites. Three small-pore, eight-membered ring (8-MR) zeolites of different cage-based topology (CHA, AEI, RTH) have been investigated. With the help of UV-visible spectroscopy to probe the Cu structure, in conjunction with measuring differential reaction kinetics before and after subsequent treatments, it has been suggested that the RTH framework imposes internal transport restrictions, effectively functioning as a 1-D framework during SCR catalysis. Hydrothermal aging of Cu-RTH results in complete deactivation and undetectable SCR rates, despite no changes in long-range structure or micropore volume after hydrothermal aging treatments and subsequent SCR exposure, highlighting beneficial properties conferred by double six-membered ring (D6R) composite building units. Exposure aging conditions and SCR reactants resulted in deleterious structural changes to Cu sites, likely reflecting the formation of inactive copper-aluminate domains. Therefore, the viability of Cu-zeolites for practical low temperature NO_xSCR catalysis cannot be inferred solely from assessments of framework structural integrity after aging treatments, but also require Cu active site and kinetic characterization after aged zeolites are exposed to low temperature SCR conditions.</p>