Submitted by Mery Piedad Zamudio Igami (mery@ipen.br) on 2017-03-10T16:23:34Z No. of bitstreams: 0 / Made available in DSpace on 2017-03-10T16:23:34Z (GMT). No. of bitstreams: 0 / Neste trabalho investigou-se a obtenção do composto TiFe a partir da moagem de alta energia de misturas de pós de TiH2 e Fe, seguida de aquecimento sob vácuo para a reação de síntese. No lugar do Ti, o TiH2 foi escolhido como precursor em razão de sua fragilidade, benéfica para a diminuição da aderência dos pós ao ferramental de moagem. Foram preparados dois lotes de misturas obedecendo-se a relação Ti:Fe de 50:50 e 56:44. Ambos foram processados em um moinho do tipo planetário por tempos que variaram de 5 até 40 horas, sob atmosfera de argônio de elevada pureza. Em todos os experimentos foram mantidos constantes a velocidade de rotação do prato do moinho, a quantidade de amostra, o diâmetro e o número de bolas. As amostras moídas foram caracterizadas por calorimetria exploratória diferencial (DSC), termogravimetria (TG), microscopia eletrônica de varredura (MEV), difração de raios X (DRX) e fluorescência de raios X por dispersão de energia (EDXRF). Apenas TiH2 e Fe foram observados nas amostras moídas, com um grau crescente de mistura em função do tempo de moagem. O composto TiFe nanoestruturado (12,5 a 21,4nm) foi obtido de forma majoritária em todas as amostras após a reação de síntese promovida pelo tratamento térmico a 600ºC (873K). As amostras reagidas foram caracterizadas por microscopia eletrônica de transmissão (MET) e DRX. Um equipamento do tipo Sievert, operando sob um fluxo constante (modo dinâmico), foi utilizado para levantar as curvas termodinâmicas de absorção e dessorção de hidrogênio. Todas as amostras absorveram hidrogênio à temperatura ambiente (~298K) sem a necessidade de ciclos térmicos de ativação. Os melhores resultados foram obtidos com as amostras moídas por 25 e 40 horas, de composição não estequiométrica 56:44. Tais amostras absorveram e dessorveram hidrogênio à temperatura ambiente, sob os platôs de aproximadamente 6,4 e 2,2bar (~0,6 e 0,2MPa), respectivamente. A capacidade máxima de armazenamento foi de 1,06% em massa de hidrogênio (H:M~0,546), sob pressão de até 11bar (1,1MPa), com reversão de até 1,085% em massa de hidrogênio (H:M~0,559), sob pressão de até 1bar (0,1MPa). Estas amostras também apresentaram maior cinética de absorção e dessorção de hidrogênio com fluxos de 1,23 (25h) e 2,86cm3/g.min. (40h). Tais resultados são atribuídos à variação composicional da fase TiFe e à maior quantidade de TiH2 livre. / Tese (Doutorado em Tecnologia Nuclear ) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

No description available.

Solid state chemical storage of hydrogen in metals offers promising advantages over compressed hydrogen gas and condensed liquid hydrogen, especially for mobile applications with respect to safety and energy efficiency. However, no single metal hydride simultaneously satisfies the essential performance criteria for onboard hydrogen storage namely, high gravimetric and/or volumetric energy density, fast kinetics and favorable thermodynamics. Recently, a breakthrough achievement was made by the development of reactive hydride composites in which two metal hydride systems (e.g. NaBH₄ and MgH₂) are mixed together resulting in better sorption properties than the individual pure systems. In this approach, the formation of MgB₂ by exothermic reaction destabilizes the composite and consequently reduces the overall enthalpy and sorption temperature of the endothermic desorption reaction. In this work the thermodynamic and kinetic properties of reactions in 2NaH + MgB₂ + 4H₂ ↔ 2NaBH₄ + MgH₂ and 3NaH + MgB₂ + 4H₂ ↔ 2NaBH₄ + NaMgH₃ were established using multiple experimental techniques like volumetric measurements, ex-situ and in-situ X-ray diffraction, calorimetry, and especially electron microscopy. Under the applied experimental conditions of 50 bar hydrogen and 400 °C during the hydrogenation of 2NaH + MgB2 and 0.1 bar hydrogen and 450 °C during the dehydrogenation of 2NaBH₄ + MgH₂, both reactions were kinetically limited and proceeded in multisteps. The absorption reaction was partial, being restricted by the unexpected formation of NaMgH₃ which limits the formation of NaBH₄ while the desorption reaction was complete and limited by the growth of MgB₂ through some intermediate complexes at the Mg/NaBH₄ interface where the intermediate phase forms a barrier to diffusion. Conversely, in the 3NaH + MgB₂ system, absorption in 100 bar hydrogen and 300 °C was complete but slow, while in the 2NaBH₄ + NaMgH₃ system, complete desorption was achieved in multisteps under 0.1 bar hydrogen and 450 °C. The formation of intermediate and stable complexes during these reactions poses a significant restraint to hydrogen sorption reactions. However, lower onset sorption temperatures have been established in these systems than in the pure compounds due to their simultaneous destabilization in the composite state. This study have demonstrated the complexity of desorption and absorption mechanisms in these composite systems and the difficulty of obtaining such reactions at low temperatures required for mobile applications. This understanding of the rate limiting reaction steps in reactive hydride composites provides the basis for further optimization of these materials for efficient hydrogen storage applications.

La thèse porte sur la conception et la simulation d'un réservoir de stockage solide de l'hydrogène sous forme d'hydrure de magnésium (MgH2). La particularité du réservoir conçu réside dans sa capacité à stocker l'énergie d'absorption grâce à un matériau de changement de phase (MCP). Afin de pouvoir prouver la viabilité du système, une étude portant sur le comportement de l'hydrure de magnésium compacté lors du cyclage à été effectuée. Celle-ci montre qu'après 100 cycles, les cinétiques de réaction et les taux massiques de stockage d'hydrogène ne sont pas affectés. En revanche, un changement de morphologie important a été observé puisqu'une dilatation ainsi qu'une augmentation importante de la conductivité des matériaux composites ont été relevées. L'étude du MCP révéla l'importance de certains paramètres, en particulier la conductivité thermique et l'enthalpie de fusion. Le MCP sélectionné est un alliage métallique en composition eutectique. Celui ci est bon conducteur de chaleur, présente une enthalpie de fusion élevée et une stabilité de comportement thermique au cyclage. Le réservoir construit contient 10 kg d'hydrure de magnésium co-broyé + 5 % de Graphite Naturel Expansé. Il est capable de stocker 7000 NL d'hydrogène (625 g) en 3h. L'avantage principal du réservoir est son efficacité énergétique, puisque la chaleur stockée par le MCP à l'absorption est refournie lors de la désorption. Afin de pouvoir prédire les comportements thermiques et cinétiques des prochains réservoirs basés sur cette technologie, 2 modèles numériques utilisant Matlab et Fluent ont été développés et validés. / The thesis's subject is about creation and modeling of a solid state hydrogen tank using magnesium hydride (MgH2). The main characteristic of this tank is the ability to store the heat of absorption due to the use of a Phase Change Material (PCM). In order to prove the sustainability of this system, a study, on the magnesium hydride's behavior, has been carried out. On one hand, kinetic properties and the amount of the stored hydrogen do not decrease after 100 cycles. On the other hand, a significant change on material morphology has been noticed. Indeed, a swelling and an increasing of thermal conductivity have been measured. Investigations about the MCP showed the importance of the thermal conductivity and the heat of fusion. That's why a metallic eutectic alloy has been selected. His atomic composition is Mg69Zn28Al3, he is a good thermal conductor, having a high heat of fusion, and presenting a good chemical stability during cycling. The designed tank contains 10 kg of magnesium hydride ball-milled added with Expanded Natural Graphite. It can absorb 7000 NL (625 g) of hydrogen in 3 hours and a half. On one total cycle, the energetic efficiency can be estimated to more than 70 %. At the same time, two numerical modeling have been achieved with Fluent and Matlab softwares, in order to make the design of next generation of tanks easier.

La thèse s’inscrit dans le contexte de sûreté nucléaire relatif à l'hydruration des gainages combustibles en Zircaloy, en modélisant, à l’échelle atomique, les phénomènes d’ordre chimique hydrogène - lacune atomique, sur le sous-réseau interstitiel tétraédrique des hydrures de zirconium CFC. Une telle démarche s’est déclinée en deux étapes : en premier lieu, le développement d’un modèle énergétique atomistique à la fois précis et peu coûteux numériquement, puis l’implémentation d’approches thermostatistiques de type Monte-Carlo à l’aide de ce modèle. En prenant pour point de départ un Hamiltonien de Liaisons Fortes (TB), la construction du modèle énergétique a reposé sur la dérivation d’interactions multiatomiques entre atomes d’hydrogène, à l’aide de la méthode des perturbations généralisée (GPM) basée sur une représentation de l’état de désordre interstitiel dans l’Approximation du Potentiel Cohérent (CPA). La démarche a permis de réduire l'énergie d'ordre à un modèle d'Ising effectif dérivé des liaisons fortes (TBIM), basé sur les interactions de paires effectives entre atomes d’hydrogène. Le TBIM a ensuite été validé, en comparant les énergies de structures ordonnées d’une part reconstruites en TBIM, et d’autre part obtenues par des calculs directs d’énergie totale effectués soit en Liaisons Fortes, soit par des méthodes ab initio (DFT). L'implémentation d'une approche Monte-Carlo canonique par le TBIM a permis de caractériser les différentes transitions ordre-désordre, et d'établir un diagramme de phase de l’ordre chimique hydrogène - lacune atomique, sur le sous-réseau interstitiel tétraédrique des hydrures de zirconium CFC. / The thesis addresses the nuclear safety issue of Zircaloy fuel cladding hydruration, by modelling, at atomistic scale, chemical ordering processes between hydrogen and atomic vacancies on tetrahedral interstitial sublattice of CFC zirconium hydrides. This has been achieved into two steps : first the development of an atomistic energetic model sufficiently precise and not too much CPU time consuming, and secondly its implementation in Monte-Carlo thermostatistical simulations. Starting from a Tight-Binding (TB) Hamiltonian, the energetic model has been derived from the calculation of multiatomic interactions between hydrogen atoms, using the Generalized Perturbation Method (GPM) applied to an interstitial disorder described within the Coherent Potential Approximation (CPA). The path allows us to reduce the ordering energy to a Tight-Binding Ising Model (TBIM), based on effective pairwise interactions between hydrogen atoms. The TBIM has been validated by comparing ordering energies of ordered structures either reconstructed using TBIM, or directly obtained from total energy calculations perfor- med both within TB and ab initio (DFT) methods.By implementing a canonical Monte-Carlo with TBIM, we obtain different order-disorder phase transitions, and a phase diagram of H-vacancy chemical ordering, on the tetrahedral interstitial sublattice of CFC zirconium hydrides.

Demands for zero greenhouse-gas emission vehicles have sharpened with today’s increased focus on global warming. Hydrogen storage is a key technology for the implementation of hydrogen powered vehicles. Metal hydrides can claim higher energy densities than alternative hydrogen storage materials, but a remaining challenge is to find a metal hydride which satisfies all current demands on practical usability. Several metals store large amounts of hydrogen by forming a metal hydride, e.g., Mg, Ti and Al. The main problems are the weight of the material and the reaction energy between the metal and hydrogen. Magnesium has a high storage capacity (7.6 wt.% hydrogen) in forming MgH2; this is a slow reaction, but can be accelerated either by minimizing the diffusion length within the hydride or by changing the diffusion properties. Light-metal hydrides have been studied in this thesis with the goal of finding new hydrogen storage compounds and of gaining a better understanding of the parameters which determine their storage properties. Various magnesium-containing compounds have been investigated. These systems represent different ways to address the problems which arise in exploiting magnesium based materials. The compounds were synthesized in sealed tantalum tubes, and investigated by in situ synchrotron radiation X-ray powder diffraction, neutron powder diffraction, isothermal measurements, thermal desorption spectroscopy and electron microscopy. It is demonstrated that hydrogen storage properties can be improved by alloying magnesium with yttrium or scandium. Mg-Y-compounds decompose in hydrogen to form MgH2 nano-structures. Hydrogen desorption kinetics are improved compared to pure MgH2. The influence of adding a third element, gallium or zinc has also been studied; it is shown that gallium improves hydrogen desorption from YH2. ScAl1-xMgx is presented here for the first time as a hydrogen storage material. It absorbs hydrogen by forming ScH2 and Al(Mg) in a fully reversible reaction. It is shown that the hydrogen desorption temperature of ScH2 is reduced by more than 400 °C by alloying with aluminium and magnesium.

Im Rahmen dieser Arbeit wurden neuartige, gemischte Tetrahydridoborate (Borhydride), die für die Wasserstoffspeicherung im Festkörper für die mobile Anwendung geeignet sein könnten, synthetisiert und vollständig charakterisiert. Entscheidende Materialanforderungen für die Kombination mit einer Tieftemperaturbrennstoffzelle sind die hohe Wasserstoffspeicherkapazität von min. 6 m% bei einer Wasserstoffdesorption unterhalb von 100°C. Um beide dieser Hauptkriterien zu erfüllen, wurden Li-Al- und Na-Al-Borhydrid entsprechend dem Konzept von Nakamori u.a. ausgewählt. Beide Borhydride desorbieren unterhalb von 100°C, wobei das synthetisierte Li-Al-Borhydrid aufgrund des hohen Wasserstoffgehalts (17,2 m% H2) die vielversprechendsten Eigenschaften zeigte. Beide Systeme wurden mittels Pulverdiffraktometrie am Synchrotron hinsichtlich Ihrer Struktur aufgeklärt, wobei die Struktur der einzelnen komplexen Ionen anhand von Schwingungsspektroskopie (Infrarot-, Ramanspektroskopie) ebenfalls bestätigt werden konnte. Mit Hilfe verschiedener kombinierter Desorptionsanalysen war es möglich den Zersetzungspfad, insbesondere die Bildung instabiler Desorptionsprodukte, aufzuklären. So erfolgt die Zersetzung des Li-Al-Borhydrids über die Bildung von Lithiumborhydrid in der Festphase, das mittels in-situ Ramanspektroskopie in einer speziellen Ramanzelle beobachtet werden konnte. Die Infrarotspektroskopie des Desorptionsgases zeigte zunächst die Freisetzung von Aluminiumborhydrid, dass wiederrum Diboran und Wasserstoff desorbiert. Weiterhin wurden verschiedene Möglichkeiten verfolgt, wie der Zusatz von Kohlenstoff oder das Nanoconfinement von Lithiumalanat, um den Zersetzungsweg hinsichtlich ausschließlicher Wasserstofffreisetzung zu modifizieren und somit Reversibilität zu ermöglichen. Es konnte jedoch kein reversibles System mit hoher gravimetrischer Wasserstoffspeicherdichte und Desorption unterhalb von 100°C erzeugt werden. / Aim of the work was the synthesis and characterisation of novel mixed tetrahydroborates (borohydrides) for solid state hydrogen storage suitable for mobile applications. The combination with a PEM fuel cell requires a material with at least 6 wt% hydrogen combined with hydrogen desorption below 100°C. To fulfill both criteria, Li-Al- und Na-Al-borohydride were selected according to Nakamori’s concept. Both mixed borohydrides desorb well below 100°C whereas the mixed Li-Al-borohydride showed the most promising properties due to its high gravimetric hydrogen content (17.2 wt% H2). The crystal structures were examined by powder diffraction with a synchrotron source. The symmetry of the containing complex cations and anions was confirmed with vibrational spectroscopy (infrared, raman spectroscopy). The desorption pathway was clarified using a variety of combined thermal analysis techniques. Especially the desorption of unstable products of the most promising Li-Al-borohydride was possible via spectroscopy. Hence the desorption of Li-Al-borohydride leads to the formation of lithium borohydride in the solid state which was monitored via in-situ raman spectroscopy in a special raman cell. Infrared spectroscopy of the desorbed gas showed the initial desorption of aluminium borohydride which desorbs diborane and hydrogen in the following. Different options were examined in order to modify this desorption pathway by carbon addition or nanoconfinement of lithium alanate. However, none of the materials showed high hydrogen content combined with exclusive hydrogen desorption below 100°C and reversibility.

Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2015-12-10T18:03:27Z No. of bitstreams: 0 / Made available in DSpace on 2015-12-10T18:03:27Z (GMT). No. of bitstreams: 0 / Tese (Doutorado em Tecnologia Nuclear) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2015-12-10T18:03:27Z No. of bitstreams: 0 / Made available in DSpace on 2015-12-10T18:03:27Z (GMT). No. of bitstreams: 0 / Foram preparados eletrocatalisadores Pt-Sn/C a partir da deposição de Pt sobre Sn/C por diferentes metodologias. Os suportes Sn/C foram preparados pela redução com boroidreto de sódio (BH) e pelo método da redução por álcool (MRA). A deposição da Pt foi efetuada pelo processo de troca galvânica e utilizando agentes redutores pelos métodos BH e MRA. Os materiais obtidos foram caracterizados por energia dispersiva de raios X (EDX), difração de raios X (DRX), microscopia eletrônica de transmissão (TEM), energia dispersiva de raios X de varredura linear (EDX- line scan), voltametria cíclica (VC) e stripping de CO e testados na oxidação eletroquímica do etanol. O difratograma de raios X do Sn/C preparado pelo método BH mostrou a presença da fase Sn metálico, enquanto que, o Sn/C preparado pelo método MRA levou a formação da fase SnO2. Apesar de o material obtido por deposição espontânea apresentar-se mais ativo que o suporte Sn(BH)/C, sua atividade foi inferior ao do catalisador comercial PtSn/C BASF. Os materiais obtidos apresentaram-se mais ativos que o catalisador comercial. A deposição da Pt pelo método BH sobre o suporte Sn(BH)/C levaram a catalisadores com desempenhos inferiores ao catalisador comercial, isto devido a deposição da Pt ocorrer preferencialmente sobre as nanopartículas de Sn metálico, tornando a superfície do catalisador rica em Pt. Os estudos realizados por microscopia eletrônica de transmissão EDX scan-line mostraram que este material apresentou a distribuição mais homogênea dos sítios de Pt e Sn na superfície do catalisador. Assim, nas condições estudadas, a deposição de Pt na superfície do Sn/C, apesar de alguns casos, os materiais obtidos apresentarem melhor atividade que o catalisador PtSn/C comercial, estes apresentaram-se menos ativos que os materiais preparados em uma única etapa por co-redução. / Tese (Doutorado em Tecnologia Nuclear) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

La préparation de nouveaux catalyseurs à base de tungstène par la voie Chimie Organométallique de Surface a été abordée dans cette thèse pour la production du propylène à partir de l'éthylène et/ou de butènes. Deux types de systèmes catalytiques ont été développés. Des hydrures de tungstène supportés, obtenus par réaction de surface entre un complexe de tungstène [W(CtBu)(CH2 tBu)3] et la -alumine suivi d'un traitement sous H2 à 150°C ont été préparés. Les caractérisations par différentes techniques spectroscopiques (IR, RMN solide, Raman et EXAFS) et réactivité stoechiométrique ainsi que les modélisations par calculs théoriques (DFT) ont montré la présence à la surface de deux espèces trishydrures : neutre et cationique. Ce catalyseur s'est révélé particulièrement actif pour la conversion directe de l'éthylène en propylène à 150°C selon un mécanisme trifonctionnel (dimérisation, isomérisation et métathèse croisée). L'accent a été mis sur la détermination du mode de désactivation par oligomérisation de l'éthylène, due principalement à la présence en surface d'espèce cationique. Pour pallier ce problème, d'autres réactions permettant la production de propylène avec de meilleures activités ont été développées (conversion des butènes, métathèse croisée éthylène/2-butène, 2-butène/isobutène). La conversion directe de 2-butène en propylène, inconnue jusqu'à lors, a notamment été étudiée. Enfin, un second type de systèmes catalytiques, modèles du site actif de l'hydrure de tungstène supporté et du catalyseur industriel WO3/SiO2, a été préparé et caractérisé. Ces nouveaux catalyseurs, portant un ligand oxo, se sont montrés bien plus actif en métathèse des oléfines que leurs homologues portant un ligand imido. Ces derniers se désactivent rapidement par décomposition de métallacyclobutane entraînant une réduction du tungstène non observée dans le cas des systèmes oxo / The preparation of new tungsten based catalyst using Surface Organometallic Chemistry is described in this thesis. These catalysts have been prepared for the production of propylene from ethylene and/or butenes. Two types of catalyst have been developed. Supported tungsten hydrides, resulting from the surface reaction of [W(CtBu)(CH2 tBu)3] with -alumina followed by a treatment under H2 at 150°C have been prepared. The characterizations by several spectroscopic techniques (IR, SSNMR, Raman and EXAFS) and stoichiometric reactivity combined by with theoretical calculations (DFT) have demonstrated the presence of two tungsten hydride surface species: a neutral and a cationic. This catalyst have shown outstanding reactivity for the direct conversion of ethylene to propylene 150°C following a tri-functional mechanism (dimerization, isomerization and crossmetathesis). The deactivation pathway has been attributes to ethylene oligomerization mainly due to the presence of cationic surface species. To circumvent this deactivation, other reactions affording propylene have been developed (butenes conversion, ethylene/2-butene and isobutene/2-butene cross metathesis). The conversion of 2-butene to propylene, until then unreported, has been notably studied. Finally, a second type of catalyst, models of the active site of supported tungsten hydrides and of WO3/SiO2 catalyst, has been prepared and characterized. These new catalysts, bearing an oxide ligand, have been shown to be more active than their imido counterparts. These later deactivates quikely by metallacyclobutane decomposition leading to the reduction of the tungsten non observed in the case of oxo systems