http://hdl.handle.net/10174/268 2026-04-05T17:38:48Z 2026-04-05T17:38:48Z Burke, Anthony http://hdl.handle.net/10174/24673 2019-02-14T17:35:50Z 1993-03-19T23:00:00Z

Title: Studies in the Synthesis and Chemistry of Flavonoid epoxides Authors: Burke, Anthony Abstract: Estudos nas sinteses e as reações de epóxidos

1993-03-19T23:00:00Z Ferreira, Teresa http://hdl.handle.net/10174/9062 2013-12-12T17:17:06Z 2007-06-18T23:00:00Z

Title: Estudos de Síntese e Caracterização de Óxidos Mistos do Tipo Espinela Authors: Ferreira, Teresa Abstract: A detailed study of the preparation and the characterization of polycrystalline FeCo2O4 and CoFe2O4 samples with the spinel structure, belonging to the Co1+xFe2-xO4 (x = 0; 1)system has been undertaken, in order to evaluate the ionic competition by site occupation and the relation between structure and properties, regarding to their potential use as electrode materials. Three distinct preparation methods were tested – the direct ceramic method, a wet chemistry route and a sol-gel type process – with different parameters: starting materials, stages in characterization corresponding to distinct annealing treatments and cooling process – quenched (q) and slowly cooled samples at 3 K min-1 (sc). After the optimization of these conditions, stoichiometric single-phase samples with spinel structure were obtained. All compounds were characterized by powder X-ray diffraction and scanning electron microscopy combined with X-ray microanalysis, energy dispersive spectroscopy for structural and morphological information. The total Fe and Co contents were estimated by inductively coupled plasma emission spectroscopy and atomic absorption spectroscopy respectively. Associated with the EDS results, these techniques supplied a useful tool for the evaluation of the chemical homogeneity and stoichiometry of the samples. Crystal data and site occupation factors were estimated from phase analysis and structure refinement by Rietveld analysis of powder diffractograms. The non-linear enthalpy of disordering model of O’Neill and Navrotsky was applied to FeCo2O4 and CoFe2O4 spinel phases and the degree of inversion, λ, could be successfully estimated, and afterwards it was compared with the one derived from the experimental techniques. The wet chemistry route has provided excellent reproducibility and a straightforward and rapid way for powder preparation. Moreover, the samples have shown good stability and chemical and morphological homogeneity. For this reason, these samples were chosen to be the subject of a more fundamental characterization and evaluation of their properties. 57Fe Mössbauer spectroscopy revealed that iron was present as Fe3+ and it allowed an estimation of the relative fractions of Fe3+ in tetrahedral and octahedral sites, which combined with powder XRD, providing a more accurate cation distribution. The surface of the materials was analysed by X-ray photoelectron spectroscopy, which showed cobalt segregation in samples prepared at low temperature for both FeCo2O4 and CoFe2O4 stoichiometries. The powders obtained at 1173 K presented that the overall Co/Fe ratios were in accordance with the theoretical values. The fast atom bombardment technique revealed that Fe+ ion was the preferential species being ejected (sputtered) from the sample, suggesting that Fe-O binding energy was smaller than the Co-O. The result was in agreement with the infrared spectra analyses, which showed a deviation of the absorption bands to lower frequency values when Co3+ ions were replaced by Fe3+ ions, with an increase of the Fe3+ fraction in tetrahedral coordination. Nitrogen adsorption studies at 77 K were carried out and revealed high specific surface areas in the samples prepared at 573 K (>100 m2 g-1). Thermogravimetric analysis supplied information about the samples behaviour in the 293 – 1273 K range and the possibility of Co2+ oxidation at ≈560K. Electronic properties were analysed by electric conductivity studies that pointed out a hopping semiconductor behaviour of carriers between localized states, and magnetization studies with temperature and applied field and hysteresis curves, showing a strong dependence of composition, cation distribution and morphology. The samples presented a ferrimagnetic order and the Néel collinear model successfully described the cation arrangement. As a conclusion, it was possible to synthesize the CoFe2O4 and FeCo2O4 samples using the defined preparing conditions. Furthermore, their characterization with the available techniques showed that these materials present potential interest for application, namely, as electrode materials.

2007-06-18T23:00:00Z Galacho, Cristina http://hdl.handle.net/10174/6218 2012-12-03T12:01:20Z 2006-05-11T23:00:00Z

Title: Materiais com estrutura mesoporosa ordenada contendo titânio: Preparação, Caracterização, Estudos de Adsorção e Propriedades Catalíticas Authors: Galacho, Cristina Abstract: his PhD thesis presents studies on the preparation, characterisation, thermal stability, stability during long term exposure to water vapour at 298K and evaluation of the catalytic properties of Ti-MCM-41 materials. Studies related to the synthesis and structural characterisation of Si-MCM-41 materials, prepared at room temperature, from structure directing agents with different chain lengths and by direct addition of the silica source or by its addition in alcoholic solution, allowed selection of tetra-, hexa- and octadecyltrimethylammonium bromides as the most suitable structure directing agents for the preparation of M41S type materials of high quality and exclusion of BuOH as the alcohol in the process of synthesis of the these materials. After developing and implementing a new method for the synthesis of Ti-MCM-41 materials, carried out at room temperature and based on the procedure for preparing Si-MCM-41 solids, a systematic studied of the effect of the principal synthesis parameters on the structural and chemical properties of the materials, namely, chain length of the structure directing agent, order of addition of the silicon and titanium precursors, direct or alcoholic solution addition of the titanium alcoxide, nature of the metal alcoxide used as titanium source and silicon/titanium molar synthesis ratio, was carried out. The results of structural characterisation, obtained by nitrogen adsorption at 77K and X-ray diffraction, show that it is possible to prepare Ti-MCM-41 materials with molar synthesis ratio Si/Ti ≥ 30 of high structural quality, and that the incorporation of titanium does not lead to a significant loss of quality of the substituted materials in comparison to the corresponding samples composed only of silica. Higher titanium contents imply a progressive decrease in the principal structural characterisation parameters without verifying the complete disappearance of the characteristic structure of MCM-41 materials. It was also shown that for low Si/Ti molar synthesis ratios the use of Ti(OEt)4 as titanium source leads to Ti-MCM-41 materials of slightly higher structural quality than the analogous samples synthesised from Ti(OPri)4. In the present work, the post-synthesis deposition of titanium in Si-MCM-41 precursor samples, in accordance with a nominal Si/Ti ratio of 10, did not prove to be an efficient synthesis method for obtaining substituted materials with the ideal structural characteristics of MCM-41. The results of chemical composition determination confirmed that the preparation of titanosilicates of the Ti-MCM-41 type by this new process leads to samples with a final Si/Ti molar ratio in agreement with the synthesis molar ratio, that is, it is possible to define, a priori, the final Si/Ti ratio intended for Ti-MCM-41 materials. The comparative analysis of the structural properties of the Ti-MCM-41 materials prepared in this work and those obtained by hydrothermal routes by different authors, allow us to conclude that in general the former present higher structural quality. In addition, the synthesis method presented allows Ti-MCM-41 materials to be obtained in a considerably short time, 60 to 90 minutes, without the need to apply external heat or pressure, which makes it very attractive for large scale production revealing itself to be an extremely convincing procedure for the preparation of Ti-MCM-41 materials. The results of the thermal stability studies show that the samples substituted with titanium have higher thermal stability than Si-MCM-41 samples, the collapse of the mesoporous structure being observed at 1373 and 1273K, respectively. Adsorption isotherms of nitrogen at 77K and n-pentane at 298K and X-ray diffraction patterns were determined on selected samples of Ti-MCM-41 before and after prolonged exposure to water vapour at room temperature (by carrying out two consecutive water vapour adsorption-desorption isotherms at 298K by the gravimetric method) and the results obtained also show that isomorphic substitution of silicon by titanium leads to a significant increase in stability. They also provided unequivocal confirmation that Ti-MCM-41 samples with a Si/Ti synthesis molar ratio ≤ 50 retain the initial structural uniformity associated to high pore volumes. It was also possible to show that the structural alterations occur principally during the determination of the first water vapour adsorption-desorption isotherm at 298K and that these alterations can be considered barely significant according to the criteria adopted by various authors. Studies of the evaluation of the oxidative catalytic properties of selected Ti-MCM-41 samples, using as model reaction system the epoxidation of cyclo-hexene, CH, with tert-butyl-hydroperoxide, TBHP, as oxidising agent, indicate that the presence of isolated and tetracoordinated titanium centres in the catalyst matrix is not a necessary or sufficient condition for obtaining high cyclohexene conversions coupled with selectivity in relation to cyclohexene oxide of 100%. The results show that the Ti-MCM-41 materials prepared with a Si/Ti synthesis molar ratio ≥ 10 exhibit good catalytic performance for the reaction tested, translated by high conversions and selectivities of 100%, coupled with practically residual acid catalytic reactivity and high structural stability in relation to the effect of leaching. We can therefore conclude that the tested materials possess structural properties, namely, high surface area and pore sizes in the meso range, suitable for an efficient dispersion of the active redox sites and for the diffusion of the substrate molecules, CH, oxidising agent, TBHP, and the principal reaction product, cyclohexene oxide, E.

2006-05-11T23:00:00Z Vicente, Henrique http://hdl.handle.net/10174/4865 2012-02-29T10:46:55Z 2004-01-01T00:00:00Z

Title: Especificação e Prototipação de Sistemas de Gestão e Controlo da Qualidade da Água de Albufeiras Authors: Vicente, Henrique Abstract: É possível re-equacionar os problemas subjacentes à temática da modelação da qualidade da água em albufeiras através de metodologias de resolução de problemas que emanam da Área Científica da Inteligência Artificial, assim como de ferramentas usadas na procura de soluções como as Árvores de Decisão, a Indução de Regras, as Redes Neuronais Artificiais e o Raciocínio Simbólico. O presente trabalho descreve modelos para a previsão da qualidade da água em albufeiras. Um destes modelos fundamenta-se em técnicas de Descoberta de Conhecimento em Bases de Dados, enquanto o outro se baseia no Raciocínio Simbólico. Os sistemas sub-simbólicos apresentam-se como complementares dos sistemas simbólicos permitindo, a partir destes, elaborar sobre sistemas de previsão e/ou simulação do estado futuro da qualidade das águas em albufeiras. Verificou-se que as fontes de informação na área do ambiente, particularmente no que concerne à qualidade da água em albufeiras, são variadas, heterogéneas e complexas, encontrando-se dispersas por diversas entidades, organismos e instituições. Esta dispersão não só torna difícil definir e caracterizar os problemas como impede a tomada das melhores decisões para os solucionar em tempo útil. Uma solução para este tipo de senãos passa pela integração, difusão e armazenamento desta informação diversificada num corpo único de forma a permitir a partilha de conhecimento com quem efectivamente dele necessita. Neste trabalho é apresentada uma Agência para a Integração, Difusão e Arquivo de Informação sobre a QUalidade da Água de Albufeiras – AQUA. A agência AQUA disponibiliza trabalhadores electrónicos inteligentes para tratar uma multitude de tarefas, tais como a comunicação entre os diferentes sub-sistemas, o enviar e receber informação (e.g. resultados das análises laboratoriais, resultados fornecidos pelos sistemas de aquisição automática de dados, informação relevante sobre as bacias, tais como cartas, levantamentos topográficos, modelos digitais do terreno, fontes de poluição pontual, utilização do solo, dados meteorológicos e climáticos, caracterização geomorfológica), o gerir, guardar e responder a pedidos de informação. A agência AQUA é, na sua essência, uma Data Warehousing associada a Sistemas MultiAgente. O objectivo principal passa por integrar, difundir, e arquivar grandes volumes de informação das mais diversas proveniências de forma a facilitar o rápido acesso à informação. A agência AQUA constitui um exemplo de aplicação de técnicas inovadoras da Área Científica da Inteligência Artificial a domínios do interesse público, neste caso à gestão e controlo dos recursos hídricos.

2004-01-01T00:00:00Z