Although current trends in global warming are of great concern, energy demand is still increasing, resulting in increasing pollutant emissions. To address this issue, we need reliable renewable energy sources, lowered pollutant emissions, and efficient and profitable processes for energy conversion. We also need to improve the use of the energy, produced by existing infrastructure. Consequently, the work presented in this thesis aims at investigating current scientific and technological challenges in energy conversion through biomass gasification and the alternative use of fossil fuels, such as diesel, in the generation of cleaner electricity through auxiliary power units in the transport sector. Production of chemicals, syngas, and renewable fuels is highly dependent on the development and innovation of catalytic processes within these applications. This thesis focuses on the development and optimization of catalytic technologies in these areas. One of the limitations in the commercialization of the biomass gasification technology is the effective catalytic conversion of tars, formed during gasification. Biomass contains high amounts of alkali impurities, which pass on to the producer gas. Therefore, a new material with alkali tolerance is needed. In the scope of this thesis, a new catalyst support, KxWO3 – ZrO2 with high alkali resistance was developed. The dynamic capability of KxWO3 – ZrO2 to store alkali metals in the crystal structure, enhances the capture of alkali metals "in situ". Alkali metals are also important electronic promoters for the active phase, which usually increases the catalysts activity and selectivity for certain products. Experimental results show that conversion of 1-methylnaphathalene over Ni/KxWO3 – ZrO2 increases in the presence of 2 ppm of gas-phase K (Paper I). This support is considered to contribute to the electronic equilibrium within the metal/support interface, when certain amounts of alkali metals are present. The potential use of this support can be extended to applications in which alkali "storage-release" properties are required, i.e. processes with high alkali content in the process flow, to enhance catalyst lifetime and regeneration. In addition, fundamental studies to understand the adsorption geometry of naphthalene with increasing temperature were performed in a single crystal of Ni(111) by STM analyses. Chapter 9 presents preliminary studies on the adsorption geometry of the molecule, as well as DFT calculations of the adsorption energy. In relation to the use of clean energy for transport applications, hydrogen generation through ATR for FC-APUs is presented in Papers II to V. Two promoted RhPt bimetallic catalysts were selected in a previous bench scale study, supported on La2O3:CeO2/d – Al2O3 and MgO : Y2O3/CeO2 – ZrO2. Catalyst evaluation was performed in a fullscale reformer under real operating conditions. Results showed increased catalyst activity after the second monolithic catalyst due to the effect of steam reforming, WGS reaction, and higher catalyst reducibility of the RhxOy species in the CeO2 – ZrO2 mixed oxide, as a result of the improved redox properties. The influence of sulfur and coke formation on diesel reforming was assessed after 40 h on stream. Sulfur poisoning was evaluated for the intrinsic activity related to the total Rh and Pt area observed after exposure to sulfur. Sulfur concentration in the aged catalyst washcoat was observed to decrease in the axial direction of the reformer. Estimations of the amount of sulfur adsorbed were found to be below the theoretical equilibrated coverage on Rh and Pt, thus showing a partial deactivation due to sulfur poisoning. / <p>QC 20150213</p>

La structure bronze quadratique de tungstène, grâce à sa flexibilité cristallochimique, est une candidate légitime pour le développement de matériaux fonctionnels. L’étude des propriétés diélectriques, pyroélectriques, et ferroélectriques de céramiques de formulation Ba2NdFeNb4-xTaxO15 montre un crossover relaxeur-ferroélectrique-paraélectrique, avec une hystérèse thermique de la transition ferroélectrique. L’étude structurale des monocristaux relaxeurs Ba2LnFeNb4O15 (Ln=La, Pr, Nd, Sm, Eu), obtenus par la méthode du flux, a mis en évidence une structure modulée. L’affinement de la structure de base prouve l’existence de moments dipolaires dans le plan ab (dus à une distorsion des octaèdres [NbO6]). Afin d’établir un lien entre la structure cristalline et les propriétés d’un composé dérivé du multiferroïque MnWO4, la croissance en four à image de Mn0,85Mg0,15WO4 a été entreprise avec succès. / Tetragonal Tungsten Bronze structure, thanks to its compositional flexibility, is a legitimate candidate for the development of functional materials. Study of dielectric, pyroelectric, and ferroelectric properties of Ba2NdFeNb4-xTaxO15 ceramics reveals a relaxor-ferroelectric-paraelectric crossover, with thermal hysteresis of the ferroelectric transition. Structural analysis of Ba2LnFeNb4O15 (Ln=La Pr, Nd, Sm, Eu), relaxor single crystals obtained by flux method, highlighted a modulated structure. Refinement of the basic structure proves the existence of a dipolar moment in the ab plane (induced by a distortion of [NbO6] octahedra). In order to establish a correlation between the crystal structure and the properties of a compound derived from multiferroic MnWO4, the crystal growth in image furnace of Mn0,85Mg0,15WO4 was successfully undertaken.

Les matériaux conducteurs de basse dimensionnalité électronique peuvent présenter des transitions vers des états électroniques complexes tels que la supraconduction, les Ondes de Densité de Spins (ODS) et les Ondes de Densité de Charges (ODC). La coexistence de plusieurs de ces instabilités au sein d’un même matériau a donné lieu à une recherche active de nouveaux systèmes conducteurs quasi-bidimensionnels. Dans cette thèse nous avons choisi de nous intéresser à la famille des Bronzes MonoPhosphate de Tungstène à tunnels pentagonaux (MPTBp), de formule chimique (PO2)4(WO3)2m ; l’un des principaux intérêts réside dans la possibilité de contrôler directement la dimensionnalité du matériau et sa densité de porteurs en faisant varier la valeur de m (2 ≤ m ≤ 14). Dans la littérature il a été montré que certains termes de cette famille (m=4, 5, 6) admettent des états ODC successifs alors que d’autres (m=10) présentent des ordres de type ferro électrique ; l’objectif de cette thèse est alors de montrer l’effet de la dimensionnalité du matériau sur l’apparition et la stabilité de ces états électroniques dans les MPTBp. Pour cela, des mesures de transport électronique, des études des structures modulées incommensurables et commensurables par diffraction des rayons X avant et après chaque transition et des mesures de diffusion inélastique ont été effectuées sur plusieurs termes à valeur de m pair. Dans ce travail nous avons pu montrer l’existence d’un état ODC pour les termes m=2, 4, 6, 8 et 10, caractérisé par la formation d’amas de tungstène dans certaines régions du matériau, accompagné, pour les m=8 et 10, de l’installation progressive d’une mise en ordre de type anti-ferroélectrique des déplacements des atomes de tungstène. Un phénomène de dépiégeage d’ODC a été observé pour le terme quasi-unidimensionnel m=2, ce qui n’avait encore jamais été reporté chez les MPTBp. Un couplage électron-phonon fort a pu être mis en évidence pour les hauts termes (m ≥ 8) à la fois via les analyses structurales, l’observation de transitions résistives du premier ordre mais également par des mesures de diffusion inélastique des rayons X. Pour ces hauts termes, une transition de type ordre-désordre est alors à envisager. / Conductive materials with low electronic dimensionality can present some transitions toward complex electronic states as superconductivity, Spin Density Waves (SDW) and Charge Density Waves (CDW). The coexistence of several of these instabilities in the same material leads to bustling investigations on new quasi-bidimensional conductors. In this thesis, we chose to study the MonoPhosphate Tungsten Bronzes with pentagonal channels family (MPTBp), of chemical formula (PO2)4(WO3)2m ; one of the main interests of this family is the possibility to directly control the compound’s dimensionality and its carriers density by varying m value (2 ≤ m ≤ 14). In the literature, it’s been shown that some MPTBp members (m=4, 5, 6) present successive CDW states whereas others (m=10) show ferroelectric-type orders. The aim of this thesis is thus to bring into light the effect of the material’s dimensionality on the appearance and the stability of these electronic states in the MPTBp family. In this way, transport measurements, X-Rays diffraction studies of the commensurate and incommensurate modulated structures below and above each transition and inelastic scattering measurements were done on several members with even value of m. In this work we reveal the existence of a CDW state for the m=2, 4, 6, 8 and 10 members, characterized by the formation of tungsten clusters in some areas of the crystal. For the m=8 and 10 members, the CDW is accompanied by a gradual installation of an anti-ferroelectric-like ordering of the tungsten atoms displacements. A CDW depinning phenomenon was observed for the quasi-unidimensional m=2 member, which has never been reported before in the MPTBp family. A strong electron-phonon coupling was evidenced for the high terms of the family (m ≥ 8) by structural analysis, first order resistive transitions observation and by X-Rays inelastic scattering measurements. For these high terms, an order-disorder transition must be considered.

Des recherches très importantes ont été dédiées à l'étude des composés ferroélectriques de type bronze de tungstène quadratique (TTB). Cette thèse concerne la synthèse et la caractérisation de 7 matériaux TTB de formule générale A6M2M'8O30 (A = Sr2+, Ba2+ ou Pb2+, M = Ti4+, Zr4+ ou HF4+; M' = Nb5+ ou Ta5+). Ces composés cristallins de grande taille ∼100 nm ont été synthétisés sous forme de poudre. Les moyennes expérimentales : diffractions des rayons X, spectroscopie Raman, la microscopie électronique MEB et MET, l'optique non linéaire ONL, ont permis d'établir des relations étroites entre structure et propriétés physiques. Les susceptibilités optiques non linéaires de 2eme et 3eme ordres, χ(2), χ(3) de chaque composés ont été déterminées et les valeurs résultantes du χ(3) ne montrent pas les mêmes caractéristiques que χ(2), car le signal de THG est moins dépendant de structure que SHG qui requiert un milieu non-centrosymétrique. Le décalage fréquentiel (Δσ) des modes de vibrations entre les structures à base de strontium(Sr) et celles à base de Barium(Ba) a été constaté afin d'identifier les signatures propres de ces composés. L'interprétation de ces poudres n'était pas facile car les pics sont très délicats. Nos mesures expérimentales montrent clairement que les positions des pics peuvent être modifiées par le changement cationique au sein des matériaux, plus la maille cristalline est rigide, plus on a des pics plus discernables. La différence est nettement visible là ou l'émission est autour de 808 cm-1 pour la structure (Sr), alors que pour le (Br) elle est autour de 780cm-1. Nous attribuons ce Δσ (28cm-1) à la transition de groupe de symétrie spatiale de Pba2 vers P4bm. L'ensemble des résultats décrits dans ce travail ont montrés la grande variété des paramètres sur lesquels il est possible d'agir afin d'optimiser les compositions en vue d'une application spécifique. / Recently, very significant research has been devoted to the study of ferroelectric compounds of tetragonal tungsten bronze (TTB). This thesis deals with the synthesis and characterization of 7 TTB's material with general formula A6M2M'8O30 (A = Sr2+, Ba2+ or Pb2+, M = Ti4+, Zr4+ or HF4+, M '= Nb5+ or Ta5+). These crystalline compounds of large size ~ 100 nm were synthesized in powder form. The experimental Technics: X-ray diffraction, Raman spectroscopy, scanning electron microscopy SEM and TEM and nonlinear optics NLO, allowed us to establish close relationships between structure and physical properties. The nonlinear 2nd and 3rd order optical susceptibility χ(2), χ(3) of all compounds were determined and the resulting values of χ(3) did not show the same characteristics as measures of χ(2) because the THG signal is less dependent on the structure where SHG signal requires a non-centro-symmetric medium. The frequency shift between the vibration modes of structures containing strontium (Sr) and those based on Barium (Ba) has been detailed to identify signatures of these specific compounds. The interpretation of these powders wasn't that easy as these peaks are very delicate but our experimental results clearly show that the positions of the peaks can be modified by the change in cation materials, as long as the crystal lattice is more rigid, more we get discernible Raman modes. This difference is clearly visible where the emission is around 808 cm-1 for the structure Sr, whereas the Br it is centered around 780cm-1. We attribute these Δσ (28cm-1) to the transition of spatial symmetry group from Pba2 to P4bm. Results showed the wide variety of settings in which it is possible to act in order to optimize the compositions for a specific application.

Transparent glasses embedded with TTB structured ferroelectric nano/micro crystals (K3Li2Nb5O15, Ba5Li2Ti2Nb8O30) were fabricated in various tellurite and borate based glass matrices and characterized for their physical properties. Nanocrystals of K3Li2Nb5O15 were successfully grown inside tellurite glass matrix via conventional heat-treatment route. Eventhough, tellurite glasses preferentially crystallize only on the surface, bulk uniform crystallization was achieved in the (100-x) TeO2 - x(1.5K2O-Li2O-2.5Nb2O5) system. Heat capacity studies revealed them to be thermodynamically less fragile than any other tellurite glasses ever reported in the literature. Pyroelectric and ferroelectric effects as well as second harmonic generation were demonstrated for the heat treated (glass nanocrystal composites) samples in this system. The conventional method of melt-quenching of constituent oxides could not yield Ba5Li2Ti2Nb8O30 crystallites. So, Ba5Li2Ti2Nb8O30 microcrystals were successfully formed in tellurite glass matrix by mixing pre-reacted Ba5Li2Ti2Nb8O30 ceramic powders with TeO2. The glass transition temperature was found to be the highest ever reported and this system was kinetically strong based on the fragility parameter. Dielectric studies revealed a frequency and temperature independent nature of the dielectric constant and very low dielectric loss. The SHG measurement which was carried out as a function of temperature demonstrated the incidence of blue second harmonic generation in the microcrystals present in the glass matrix. Ba5Li2Ti2Nb8O30 nanocrystals were successfully crystallized in the transparent glass system (100-x)Li2B4O7 – x(Ba5Li2Ti2Nb8O30). Dielectric constant increased while the dielectric loss decreased with the increase in Ba5Li2Ti2Nb8O30 content. Nuclear magnetic resonance spectroscopic studies were carried out to have an insight into the structure of this system. Transmission studies and refractive index measurements were performed and various optical parameters were calculated. Dielectric and transport properties were studied for the glasses and glass nano/microcrystal composites of all the systems reported in this thesis. Li+ ion was found to be responsible for conduction in all these systems. Evolution of self-organized nanopatterns of K3Li2Nb5O15 crystals has been demonstrated in the glass system (100-x) TeO2 - x(1.5K2O-Li2O-2.5Nb2O5) by excimer laser irradiation. The second harmonic signal observed by the Maker fringe technique has been attributed to the presence of well-aligned nano-sized grating structures in the glass system. Glasses belonging to the systems TeO2-K3Li2Nb5O15, TeO2-Ba5Li2Ti2Nb8O30 and V2Te2O9 undergo spinodal decomposition on exposing to KrF pulsed excimer laser. The spinodally phase separated structures were observed on all the surfaces of the samples. Ring shaped patterns were observed on several locations of the samples at higher frequency of laser pulses probably owing to the shock waves produced by the high intense laser beam. Line shaped patterns were found to originate on the sample surfaces when irradiated for longer periods.

L'objectif de ce travail était l'élaboration par ablation laser pulsé (PLD) et la caractérisation de couches minces d'oxydes dans le système K-Nb-O, et plus précisément d'une phase de structure bronze de tungstène quadratique (TTB) sous forme de nanorods, potentiellement intéressante dans le contexte de la recherche de nouveaux piézoélectriques sans plomb. Malgré une forte compétition de croissance entre les différentes phases, l'étude approfondie des conditions de dépôt a montré la possibilité d'obtenir les phases KNb3O8, K4Nb6O17, K6Nb10,88O30 (TTB) et KNbO3, en films minces après une phase d’optimisation essentielle. Nous avons déterminé l'influence des conditions de dépôt sur la formation et la nanostructuration de ces composés en couches minces. En particulier, il a été démontré que la température et la composition de la cible PLD avaient une forte influence sur la croissance de la phase de structure TTB. Une étude plus approfondie de ces phases a révélé que toutes avaient une morphologie spécifique liée à leur structure anisotrope, que nous avons pu contrôler par la croissance épitaxiale sur les substrats SrTiO3 orienté (100) et (110). L'existence d'une activité piézoélectrique dans des couches minces de la phase TTB, mise en évidence par PFM, lui confère un intérêt certain. Cette phase TTB a également été obtenue dans le système Na-K-Nb-O, très connu pour ses propriétés piézoélectriques et ferroélectriques, ouvrant la voie sur de nouvelles recherches. / The purpose of this work was the elaboration by pulsed laser deposition (PLD) and the characterization of thin films of oxides in the K-Nb-O system, and more precisely that of a tetragonal tungsten bronze phase (TTB) as nanorods, of potential interest as a new lead free piezoelectric. In spite of a strong growth competition between the different phases, the detailed study of the deposition conditions showed that it is possible to obtain KNb3O8, K4Nb6O17, K6Nb10,88O30 (TTB ) and KNbO3 in thin films form after an important optimization step. We have determined the influence of these deposition conditions on the formation and the nanostructuration of these compounds as thin films. In particular, it was shown that the temperature and the PLD target’s composition has a strong influence on the growth of the TTB structure. A further study of these phases revealed that all have a specific morphology related to their anisotropic structure, that we have controlled by the epitaxial growth on the (100) and (110) SrTiO3 substrates. The existence of a piezoelectric activity in the TTB thin films, evidenced by PFM, gives a great interest to this phase. This TTB phase was also obtained in the Na-K-Nb-O system, well known for its piezoelectric and ferroelectric properties, opening the way to new research.

Übergangsmetalloxidbronzen (TMOB) – Verbindungen mit der allgemeinen chemischen Summenformel AxByOz (A = Element der Gruppen 1, 2, 11, 12, 13, NH4+ oder Kombination; B = Übergangsmetall) – sind seit längerem als Material für ionensensitive Festkörperelektroden bekannt. Sie reagieren als Elektroden in elektrochemischen Messungen auf Ionen in wässrigen Elektrolyten in höchst unterschiedlicher Weise. Bis heute gibt es dazu nur wenige systematische vergleichende Untersuchungen, die von der Präparation der Bronzen, der Charakterisierung ihrer Eigenschaften bis zur Untersuchung ihrer Elektrodeneigenschaften reichen. Gegenstand der Arbeit war deshalb die Synthese von in der Literatur beschriebenen TMOB auf der Basis von Wolfram und Molybdän, die Bestimmung ihrer Kristallstruktur und die Untersuchung ihrer chemischen und physikalischen Eigenschaften. Aus den synthetisierten Verbindungen waren anhand einer elektrochemischen Vorcharakterisierung einige auszuwählen, um diese systematisch als Elektroden zu untersuchen. Insbesondere HxMoO3, Ba0,12WO3 und Li0,4Mo0,95W0,05O3 zeichneten sich durch gegenüber den anderen Oxidbronzen abweichende potentiometrische Charakteristika aus. Hinsichtlich der Einsetzbarkeit dieser Materialien für potentiometrische Elektroden wurden diese Verbin¬dungen noch nicht untersucht. Daher wurden diese Verbindungen eingehender erforscht. Die bezüglich ihrer potentiometrischen Eigen¬schaften bereits ausreichend untersuchte Verbindung Na0,9Mo6O17 diente dem Vergleich und wurde deshalb den selben Tests ausgesetzt. Dazu waren im Einzelnen potentiometrische Messungen in Abhängigkeit von der Konzentration der Alkaliionen und der Wasserstoffionen sowie von der Konzentration verschiedener Redoxspezies in wässrigen Medien durchzuführen. Unklar war bisher wie sich die Wechselwirkungen der gelösten Ionen mit den Molybdän- und Wolframionen im Festkörper vollziehen und warum die Elektroden Sensitivitäten für verschiedene Ionen aufweisen, die sich nicht aus der chemischen Zusammensetzung oder mit der Kristallstruktur der jeweiligen Oxidbronze erklären lassen. Zur Aufhellung dieses Problems waren unter anderem XPS-Untersuchungen an verschiedenen Oxidbronzen durchzuführen, aus denen man auf den Oxidationszustand des Wolframs und Molybdäns im Festkörper schließen kann.