The marked physical changes in potassium boro-hydride such as decrepitation and the development of a deep blue coloration when the solid compound is exposed to ionizing radiation stimulated a study of the effects of 50 Kvp X-rays on the alkali metal borohydrides to determine the nature of the radiation induced changes such as color center formation and chemical decomposition. Methods were developed to prepare the alkali metal borohydrides in a form suitable for radiation studies. Solvents studies showed that anhydrous hydrazine was an exceptionally good solvent for potassium borohydride, the solubility being 28.3 grams KBH₄ per 100 grams at 18.5° C. The handling of hydrazine as a solvent required the construction of special apparatus. A study of the use of hydrazine as a solvent for other ionic borohydrides and/or the growth of crystals suitable for spectroscopic work is incomplete. Therefore, the spectroscopic studies on radiation induced absorption bands was done mainly with thin pressed pellets. The borohydrides of rubidium and cesium were prepared by metathesis reactions from potassium borohydride via a sulfonium borohydride. The preparation of the previously unreported trimethylsulfonium borohydride is described. Color center formation was studied by spectroscopic methods only and the F and U type centers have been tentatively identified. The thermal stability and optical bleachability of some of the radiation induced absorption bands were examined. Chemical studies of radiation damage in potassium borohydride failed to show the presence of free alkali metal. Gaseous boron hydrides were also undetectable. Mass spectrometric examination of gaseous material evolved during irradiation showed only hydrogen to be present. No gas was evolved when heavily irradiated samples of potassium borohydride were dissolved in liquid ammonia. A discussion of methods and apparatus characteristic to the radiation studies such as the X-ray generator, radiation vessels, vacuum system, and a section on radiation dosimetry is included in the thesis. The intensity of X-rays generated by the Machlett OEG-60 X-ray tube was determined by the application of the included dosimetry data together with a calorimetric measurement of the output flux of the tube. At 50 Kvp and 28 milliamperes the intensity output was found to be 0.220 cal.min.⁻¹ cm.⁻² at the tube port. Some suggestions for further work are outlined at the end of the study. / Science, Faculty of / Chemistry, Department of / Graduate

Includes bibliographical references (p. 46-47). / "Physics Chemsitry" -t.p.

In order to reduce our ties to fossil-based energy and mitigate the undeniable impacts of climate change on the environment, remarkable efforts have been directed over the last 4 decades toward developing renewable energy sources such as solar, wind, geothermal, etc. For transportation applications biofuels, electricity and hydrogen all offer potential solutions although current usage is still largely linked to fossil fuels (bio-based ethanol-gasoline mixtures, power generation for battery recharging, and steam reforming for hydrogen production). While hydrogen offers the greatest potential in terms of energy density, its poor volumetric density (0.01 MJ/L at RT) requires costly compression and pressurized storage. When future technology finally allows for efficient hydrogen release from water splitting, we need to have optimal solutions in place for hydrogen storage. One promising solution is chemical hydrogen storage in which thermolysis of a chemical precursor affords a controlled hydrogen release that can then be reversed in an off-board regeneration step. With a focus on maximum gravimetric hydrogen storage, various BNH compounds have been shown to be promising chemical hydrogen storage precursors. In this Thesis we summarize the state of the art in B-N-H hydrogen storage compounds (Chapter 1) and then investigate several new chemical hydrogen storage solutions with a focus on portable power generation. In the first project (Chapter 2) we sought to prepare a robust, base-metal borohydride hydrolysis catalyst for use in a custom hydrogen generator designed to use the reaction heat to help separate the borate spent fuel. Active ‘reverse opal’ layered double hydroxide (LDH) catalysts were prepared and tested. While the classical Ni-Mg-Al LDH released 3.4 equiv. of hydrogen at 50 °C in 150 minutes, the polystyrene templated Ni-Mg-Al catalyst released 4 equiv. of hydrogen with a higher initial rate under the same reaction conditions. The long-term objective of this project was to test these catalysts in fuel cells for underground mine forklifts with our industry collaborator (Kingston Process Metallurgy Inc.). In the next three chapters, the synthesis and hydrogen release properties of ammine metal borohydrides [M(BH4)m(NH3)n, AMBs] were investigated. As promising hydrogen storage materials with high hydrogen content (10-15 wt%), AMBs can access lower hydrogen release temperatures resulting from the combination of protic (N-Hδ+) and hydridic (B-Hδ-) hydrogens. While AMBs also do not suffer from diborane formation that plagues thermolysis of metal borohydrides, hydrogen release is often accompanied by small concentrations of ammonia that deactivate the fuel cell catalyst. Our objective for this work was to identify base metal catalysts that could suppress ammonia formation by further reducing the energy barrier to H2 release. In Chapter 3 our studies of the solution synthesis of AMB materials (Y, La, Zn, etc.) in coordinating solvents such as tetrahydrofuran (thf) and diethyl ether revealed the unexpected formation of ammonia-borane (H3NBH3, AB). It was shown that while the amounts of produced AB correlate with the Zhang electronegativity for the s- and p-block metals, ionic radius is a stronger determining factor for the transition metals. It was also observed that reducible metals such as Ti and V produce large amounts of AB while Zn produced the least. This knowledge was then used in Chapter 4 to prepare pure samples of the Y and La complexes, M(BH4)3(NH3)4 that were characterized by thermal analysis (TGA-MS), powder X-ray diffraction, FT-IR and 11B and 1H MAS NMR spectroscopy. Furthermore, a series of base-metal nanoparticle catalysts, prepared using a novel route from MCl2 and liquid hexylamine-borane, was shown to suppress ammonia formation from these Y and La AMBs. Immobilizing 5 wt.% of Co NPs on Y(BH4)3(NH3)4 and 5 wt.% of Fe NPs on La(BH4)3(NH3)4 resulted in reduction of ammonia release by three- and fourfold, respectively. In Chapter 5 the attempted solution synthesis of Zn(BH4)2(NH3)2 revealed complications due to preferred formation of MIZn(BH4)3 [instead of Zn(BH4)2] from the reaction of ZnCl2 and MIBH4 (MI= Li, Na, K). As a result, the mixed-metal AMB, KZn(BH4)3(NH3)n was prepared and characterized. Although the effects of both heterogeneous and homogeneous catalysts were not as pronounced as those for Y and La, using 5 wt.% FeNPs resulted in fourfold reduction in the amount of released ammonia which led to a purer hydrogen stream (98.9 mol%) compared to the uncatalyzed thermolysis (97.0 mol%). Finally, in Chapter 6 our results are considered vs. the current state of the art and suggestions are made for further investigations.

No description available.

This Thesis considers the use of solid-state metal ammines as ammonia storage materials and endeavours to understand these materials on a fundamental chemical level. The ammines of LiBH₄, MgCl₂, MgBr₂, MgI₂ and Mg(BH₄)₂, are investigated. The structures of lithium borohydride ammines, Li(NH₃)_nBH₄ with n = 1, 2, 3 and 4 are solved using X-ray and neutron diffraction, vibrational spectroscopy, nuclear magnetic resonance, and first-principles calculations. The reversibility, bonding and ammonia storage properties of this system are discussed, and investigated using gravimetric analysis and vibrational spectroscopy. The ammines of magnesium halides are investigated using X-ray and neutron powder diffraction, gravimetric techniques, nuclear magnetic resonance, first-principles calculations and vibrational spectroscopy. Their disordered structures, bonding, and decomposition are discussed, and the trends in their properties are used to interpret the properties of other ammines. The ammines of magnesium borohydride are investigated using X-ray and neutron powder diffraction, gravimetric techniques, first-principles calculations and vibrational spectroscopy. The structure, decomposition and reversibility of Mg(NH₃)₆(BH₄)₂ as an ammonia store are presented. Throughout the Thesis and at the end of each Chapter the possibility of using these ammines as solid-state ammonia stores is discussed.

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.

Made available in DSpace on 2014-10-09T12:35:43Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:54Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:35:43Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:54Z (GMT). No. of bitstreams: 0 / Foram preparados eletrocatalisadores PtSnCu/C (com diferentes razões atômicas Pt:Sn:Cu) e PtSn/C (50:50) com 20 % em massa de metais pelos métodos da redução por borohidreto (IRB) e redução por álcool (RA). Utilizou-se H2PtCl6.6H2O, SnCl2.2H2O e CuCl2.2H2O como fonte de metais, NaBH4 e etilenoglicol como agentes redutores, 2-propanol e etilenoglicol/água como solventes e carbono como suporte. Numa segunda etapa, estes eletrocatalisadores foram ativados pelos processos de dealloying químico (DQ), por tratamento com HNO3 e dealloying eletroquímico (DE), utilizando a técnica de eletrodo de camada fina porosa. Os materiais obtidos foram caracterizados por energia dispersiva de raios-X (EDX), difração de raios-X (DRX), microscopia eletrônica de transmissão (MET), energia dispersiva de raios-X de varredura linear (EDX-VL) e voltametria cíclica (VC). Estudos eletroquímicos para a oxidação eletroquímica do etanol foram realizados por voltametria cíclica, cronoamperometria e células unitárias (conjunto eletrodos/membrana). Os efluentes anódicos provenientes dos testes em células unitárias foram analisados por cromatografia a gás de alta eficiência (CG). Os difratogramas de raios-X dos eletrocatalisadores sintetizados mostraram a típica estrutura cúbica de face centrada (CFC) de liga de platina e após tratamento por dealloying, observou-se que a estrutura (CFC) foi preservada. O tamanho de cristalito dos eletrocatalisadores como preparados variou na ordem de 2 nm a 3 nm e, após processos de dealloying, não foram observadas variações de tamanho significativas. Análises por EDX dos eletrocatalisadores como preparados mostraram similaridade entra a razão atômica Pt:Sn e Pt:Sn:Cu obtida e a nominal. Após dealloying químico e eletroquímico, observou-se variação nas razões atômicas Pt:Sn e Pt:Sn:Cu, indicando a remoção parcial de Cu e Sn. Contudo, o processo de dealloying químico mostrou-se mais eficiente para a remoção de Cu e o dealloying eletroquímico para a remoção de Sn. As análises por EDX-VL mostraram que os processos de dealloying foram efetivos na remoção dos átomos mais superficiais de Cu e/ou Sn da estrutura CFC da Pt. Os resultados obtidos por cronoamperometria e voltametria cíclica mostraram que os eletrocatalisadores com teores de Pt maiores ou iguais a 30 %, após dealloying químico e eletroquímico apresentaram melhora significativa na atividade eletrocatalítica para a oxidação eletroquímica do etanol no potencial de interesse (0,5 V). Os eletrocatalisadores que apresentaram maior eficiência para oxidação eletroquímica do etanol foram PtSn/C (50:50) IRB/DE > PtSnCu/C (50:40:10) RA/DE > PtSnCu/C (50:10:40) IRB/DQ. Foram testados em células unitárias alimentadas diretamente com etanol os eletrocatalisadores PtSn/C (50:50) IRB/DQ, PtSnCu/C (50:10:40) IRB/DQ, PtSnCu/C (50:40:10) RA/DQ e os eletrocatalisadores comerciais Pt/C BASF e PtSn/C (75:25) BASF. Os eletrocatalisadores apresentaram a seguinte ordem de desempenho: PtSn/C (50:50) IRB/DQ > PtSnCu/C (50:40:10) RA/DQ > PtSn/C (75:25) BASF > PtSnCu/C (50:10:40) IRB/DQ > Pt/C BASF. Análises por cromatografia gasosa dos efluentes anódicos desses eletrocatalisadores mostraram formação de ácido acético e acetaldeído como produtos principais. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

This thesis investigates the structure and thermal behaviour of LiBH4, NaBH4, KBH4, LiK(BH4)2, Ca(BH4)2 and Sr(BH4)2. LiK(BH4)2 is the first mixed alkali metal borohydride and was synthesised and characterised during this work. The crystal structures of these borohydrides were studied using variable temperature neutron and synchrotron X-ray diffraction. The synthesis of isotopically enriched samples of 7Li11BD4, Li11BD4, Na11BD4 and K11BD4 allowed high quality neutron diffraction data to be collected. Particular attention was paid to the exact geometry of the borohydride ions which were generally found to be perfect tetrahedra but with orientational disorder. New structures of Ca(BH4)2 were identified and the first crystal structure of Sr(BH4)2 was determined from synchrotron X-ray diffraction data. Solid state 11B NMR and Raman spectroscopy provided further information about the structure of these borohydrides. The thermal behaviour of the borohydrides was investigated using thermogravimetric analysis with mass spectrometry of the decomposition gas products. Hydrogen is the main decomposition gas product from all of these compounds but small amounts of B2H6 and BH3 were also detected during decomposition. Thermogravimetic analyses of Na11BD4 and K11BD4 were completed whilst collecting in-situ neutron diffraction data allowing information about structural changes and mass losses to be combined in order to better understand the decomposition process.

Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2016-11-11T09:51:24Z No. of bitstreams: 0 / Made available in DSpace on 2016-11-11T09:51:24Z (GMT). No. of bitstreams: 0 / PtBi/C e PdBi/C foram preparados em diferentes razões atômicas (100:0, 90:10, 80:20, 70:30, 60:40 e 50:50) pelo método de redução via borohidreto de sódio (com adição total da solução de borohidreto em uma única etapa) utilizando H2PtCl6.6H2O, Pd(NO3)2, (BiNO3)3.5H2O como fonte de metais, Vulcan® (XC72-Cabot) como suporte de carbono e com uma carga metálica correspondente a 20% em massa. Os eletrocatalisadores obtidos foram caracterizados por difração de raios-X (DRX), microscopia eletrônica de transmissão (MET) e voltametria cíclica (VC). A atividade dos diferentes materiais preparados para a oxidação eletroquímica do ácido fórmico foi realizada em eletrólito ácido e alcalino utilizando-se as técnicas de voltametria cíclica, e cronoamperometria. Para estes estudos foi utilizado a técnica do eletrodo de camada fina porosa. A caracterização eletroquímica permitiu comparar o desempenho eletroquímico da platina e paládio, além de avaliar o benefício da presença do bismuto nas razões atômicas propostas. Os difratogramas de raio-X (DRX) confirmaram para todos os compostos de PtBi/C e PdBi/C a formação da estrutura cúbica de face centrada (cfc) característicos da rede cristalina da platina e do Paládio respectivamente. Outros picos encontrados foram associados a presença de fases de óxido de bismuto em ambos os compostos, PtBi/C e PdBi/C. A microscopia eletrônica de transmissão (MET) indicou que a presença de maiores teores de bismuto não acarretaram em aumento do tamanho médio da partícula. Os resultados eletroquímicos em meio alcalino indicaram que ainda é necessário uma otimização da concentração de ácido fórmico para que possamos observar melhores resultados quanto à adição de bismuto na platina ou paládio, no entanto os estudos em meio ácido mostraram o efeito benéfico da adição de bismuto tanto para platina quanto para o paládio. / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP