The surface of a dense Ni-YSZ anode was modified with a thin-film coating of niobium oxide (Nb2O5) in order to understand the mechanism of sulfur tolerance and the behavior of carbon deposition. Results suggest that the niobium oxide was reduced to NbO2 under operating conditions, which has high electrical conductivity. The NbOx coated dense Ni-YSZ showed sulfur tolerance when exposed to 50 ppm H2S at 700°C over 12 h. Raman spectroscopy and XRD analysis suggest that different phases of NbSx formed on the surface. Further, the DOS (density of state) analysis of NbO2, NbS, and NbS2 indicates that niobium sulfides can be considered as active surface phases in the H2S containing fuels. It was demonstrated that carbon formation was also suppressed with niobium oxide coating on dense Ni-YSZ in humidified CH4 (3% H2O) at 850ºC. In particular, under active operating conditions, there was no observable surface carbon as revealed using Raman spectroscopy due probably to electrochemical oxidation of carbon. Stable performances of functional cells consisting of Pt/YSZ/Nb2O5 coated dense Ni-YSZ in the fuel were achieved; there was no observable degradation in performance due to carbon formation. The results suggest that a niobium oxide coating has prevented carbon from formation on the surface probably by electrochemically oxidation of carbon on niobium oxide coated Ni-YSZ. On the other hand, computational results suggest that, among the metals studied, Mo seems to be a good candidate for Ni surface modification. Ni-based anodes were modified with Mo using wet-impregnation techniques, and tested in 50 ppm H2S-contaminated fuels. It was found that the Ni-Mo/CeO2 anodes have better sulfur tolerance than Ni, showing a current transient with slow recovery rather than slow degradation in 50 ppm H2S balanced with H2 at 700°C.

Oxide ultra-thin film surfaces have properties and structures that are significantly different from the terminations of the corresponding bulk crystals. For example, surface structures of epitaxial ultra-thin oxide films are highly influenced by the crystallinity and electronegativity of the metal substrates they grown on. Some enhanced properties of the novel reconstructions are related to catalysis, sensing and microelectronics, which has resulted in an increasing interest in this field. Ultra-thin TiO_x films were grown on Au(111) substrates in this work. Two well-ordered structures within monolayer coverage - honeycomb (HC) and pinwheel - were generated and investigated. Special attention has been paid to the uniform (2 x 2) Ti₂O₃ HC phase including its regular structure and imperfections such as domain boundaries (DBs) and point defects. Linear DBs with long-range repeating units have been observed; density functional theory (DFT) modelling has been used to simulate their atomic structures and calculate their formation energies. Rotational DBs/defects show up less frequently, however a six-fold symmetrical 'snowflake' DB loop stands out. Two types of point defects have been discovered and assigned to Ti vacancies and oxygen vacancies/hydroxyl groups. Their diffusion manners and pairing habits have been discussed within an experimental context. The results of growing NbO_x ultra-thin films on Au(111) are also presented in this thesis. An identical looking (2 x 2) HC structure to the Ti₂O₃ ultra-thin film has been formed; a stoichiometry of Nb2O3 is suggested. Another interesting reconstruction is a hollow triangle structure. Various sizes have been found, and sides of these equilateral triangles all show a double-line feature aligned along the { 1 ₁⁻ } directions of the Au(111) lattice. Chemical composition characterisations of NbO_x thin films are still required as is DFT modelling. Experimental techniques used in this thesis include scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). Ultra-thin oxide films were created by physical vapour deposition (PVD) in ultra-high vacuum (UHV) systems.

Made available in DSpace on 2014-12-17T15:01:35Z (GMT). No. of bitstreams: 1 SamuelAO_DISSERT.pdf: 6693517 bytes, checksum: 0be8bd399faaff3fe4e2da515c357889 (MD5) Previous issue date: 2014-01-30 / Among the heterogeneous catalysts materials made from niobium show up as an alternative to meet the demand of catalysts for biodiesel production. This study aims to evaluate the potential of a heterogeneous catalyst derived from a complex of niobium in the reaction of methyl esterification of oleic acid. The catalyst was synthesized after calcination at different temperatures of a niobium complex ((NH4)3[NbO(C2O4)3].H2O) generating a niobium oxide nanostructure with a different commercial niobium oxide used to synthesize the complex. The commercial niobium oxide, the complex niobium and niobium catalyst were characterized by thermogravimetry (TG and DTA), surface area analysis (BET), scanning electron microscopy (SEM) and X-ray diffraction (XRD), showing the catalyst has researched morphological and crystallographic indicating a catalytic potential higher than that of commercial niobium oxide characteristics. Factorial with central composite design point, with three factors (calcination temperature, molar ratio of alcohol/oleic acid and mass percentage of catalyst) was performed. Noting that the optimal experimental point was given by the complex calcination temperature of 600?C, a molar ratio alcohol/oleic acid of 3.007/1 and the catalyst mass percentage of 7.998%, with a conversion of 22.44% oleic acid in methyl oleate to 60 min of reaction. We performed a composite linear and quadratic regression to determine an optimal statistical point of the reaction, the temperature of calcination of the complex at 450?C, the molar ratio of alcohol/oleic acid 3.3408/1 and mass percentage of catalyst of 7.6833% . Kinetic modeling to estimate parameters for heterogeneous catalysis it set well the experimental results with a final conversion of 85.01% with 42.38% of catalyst and without catalyst at 240 min reaction was performed. Allowing to evaluate the catalyst catalytic studied has the potential to be used in biodiesel production / Dentre os catalisadores heterog?neos os materiais a base de ni?bio mostram-se como uma alternativa para suprir a demanda de catalisadores para a produ??o de biodiesel. O presente trabalho visa avaliar o potencial de um catalisador heterog?neo derivado de um complexo de ni?bio na rea??o de esterifica??o met?lica do ?cido oleico. O catalisador foi sintetizado ap?s a calcina??o em diferentes temperaturas de um complexo de ni?bio ((NH4)3[NbO(C2O4)3].H2O) gerando um ?xido de ni?bio com uma nanoestrutura diferente do ?xido de ni?bio comercial, usado para sintetizar o complexo. O ?xido de ni?bio comercial, o complexo de ni?bio e o catalisador de ni?bio foram caracterizados por termogravimetria (TG e DTA), an?lise de ?rea superficial (BET), microscopia eletr?nica de varredura (MEV) e difra??o de raios-X (DRX), demostrando que o catalisador pesquisado possui caracter?sticas morfol?gicas e cristalogr?ficas que indicam um potencial catal?tico superior ao ?xido de ni?bio comercial. Foi realizado um planejamento fatorial composto com ponto central, com tr?s fatores (temperatura de calcina??o, raz?o molar de ?lcool/?cido oleico e percentual m?ssico de catalisador). Verificando que o ponto ?timo experimental se deu mediante a temperatura de calcina??o do complexo em 600?C, uma raz?o molar de ?lcool/?cido oleico de 3,007/1 e percentual m?ssico de catalisador de 7,998%, apresentando uma convers?o de 22,44% do ?cido oleico em oleato de metila em 60 min de rea??o. Realizou-se uma regress?o linear e quadr?tica composta para se determinar de um ponto ?timo estat?stico da rea??o, sendo a temperatura de calcina??o do complexo em 450?C, raz?o molar de ?lcool/?cido oleico de 3,3408/1 e percentual m?ssico de catalisador de 7,6833%. Foi realizada uma modelagem cin?tica para estima??o de par?metros para a cat?lise heterog?nea que se ajustou satisfatoriamente aos resultados experimentais com uma convers?o final de 85,01% com catalisador e de 42,38% sem o catalisador em 240 min de rea??o. Permitindo avaliar que o catalisador estudado possui potencial catal?tico para ser utilizado na produ??o de biodiesel

La déshydratation du glycérol en acroléine a été étudiée sur de nouveaux catalyseurs à base d'oxydes en vue de mettre au point un nouveau procédé de production de la méthionine. Ces catalyseurs performants sont des zircones niobiées et des zircones tungstées dopées à la silice. Si les catalyseurs proposés dans la littérature jusqu'à présent pouvaient être sélectifs, ils se désactivaient rapidement par cokage. Les nouveaux catalyseurs mis au point sont aussi actifs et sélectifs mais beaucoup plus stables sous flux réactionnel. Les performances des zircones niobiées sont très sensibles à la méthode de préparation; les catalyseurs ont été caractérisés par DRX, spectroscopie Raman, MET, EDX, XPS. Ces analyses ont montré qu'une partie du niobium est incorporée dans la zircone qui est recouverte d'espèces polymères de niobium. Les zircones tungstées performantes sont constituées d'une zircone recouverte de silice et de polytunsgtates. L'acido-basicité des catalyseurs a été étudiée par différentes techniques: réactions modèles de transformation du méthylbutynol et du 2-propanol, TPD-NH3, adsorption de NH3 et SO2étudiée par microcalorimétrie et adsorption de pyridine suivie par IR. Ces études ont permis de montrer que les sites basiques de la zircone sont néfastes pour les performances et que les sites acides de Brönsted avec une large gamme de force acide sont actifs et sélectifs en acroléine. Nous avons montré par microcalorimétrie que même les sites acides très faibles participent à la réaction / Glycerol dehydration to acrolein has been studied over innovative oxide catalysts in order to develop a new methionine production process. These performing catalysts are niobiate zirconia mixed oxides and tungstated zirconia doped with silicium. The catalysts proposed so far in the literature are active and selective but they show fast deactivation under reaction flow due to coke deposition. The new catalysts appear at least as active and selective but much more stable. The niobiate zirconia mixed oxides, which catalytic performances are very sensitive to the preparation method, have been characterized by XRD, Raman spectroscopy, TEM with EDX analyses and XPS. Theses analyses show that niobium cations are partially incorporated into zirconia and covered its surface as polymeric niobium oxide species. The efficient tungstated zirconia catalysts consist of zirconia cover with silicium and polymeric tungstate species. Acid-base properties of the catalysts have been studied by model reaction of methylbutynol and 2-propanol, NH3-TPD, SO2 and NH3adsorption followed by microcalorimetry and IR spectroscopy of pyridine adsorption. These studies show that the zirconia uncovered by silica or containing no niobium are unselective sites and that Brönsted acid sites of the polymeric tungstate or niobium oxide species are the selective sites in acrolein. The strength of these sites does not appear as a key parameter to acrolein selectivity and even weak sites participate to the reaction

Dielektrická relaxační spektroskopie je jednou z užitečných metod pro studium molekulární dynamiky materiálů. Díky nedávnému pokroku v přístrojové a měřicí technice je dnes možné získat dielektrické spektrum v širokém frekvenčním intervalu a pro velice rozdílné materiály. Cílem mé práce bylo studium dielektrických relaxačních spekter a vodivosti oxidů titanu, niobu, tantalu, lanthanu a hafnia pro katody pracující na principu studené emise. Cílem výzkumu bylo analyzovat frekvenční a teplotní chování těchto oxidů, včetně jejich vodivosti, v širokém frekvenčním a teplotním rozsahu, a pokusit se stanovit původ relaxačního mechanismu. Vzhledem k tomu, že původně zadaný rozsah oxidů byl dosti široký, soustředila se pozornost pouze na oxidy tantalu a niobu, rovněž s ohledem na jejich aplikace v elektrolytických kondenzátorech. Elektrické, tepelné a mechanické (při zpracování) vlastnosti oxidů tantalu a niobu jsou dnes již dobře prozkoumány. K dispozici je však jen málo poznatků o jejich dielektrických relaxačních mechanismech. Výsledky získané pro Ta2O5 ukazují existence relaxačního maxima, nacházejícího se v experimentálně dostupném teplotním a frekvenčním intervalu 187 K – 385 K a 1 Hz – 10 MHz. Frekvence ztrátového maxima se řídí Arrheniovým zákonem s aktivační energií 0.048 eV. Ve vodivostních spektrech vykazují tenké vrstvy Ta2O5 na nízkých frekvencích ustálenou hodnotu a při vysokých frekvencích monotónní nárůst, který závisí na teplotě. Pozorovanou vodivost lze popsat mocninnou funkcí s exponentem nepatrně větším než jedna (tzv. superlineární závislost). Výsledky získané pro Nb2O5 v podobné teplotní a frekvenční oblasti, 218 K – 373 K, 1 Hz – 1 MHz rovněž ukazují jedno relaxační maximum. Frekvence ztrátového maxima se opět řídí Arrheniovým zákonem s poněkud vyšší aktivační energií 0.055 eV. Niobové kondenzátory vykazují vodivostní mechanismus shodný s kondenzátory tantalovými.

Les oxydes de métaux de transition lamellaires peuvent servir de catalyseurs pour la conversion de la biomasse, mais leur développement nécessite une meilleure compréhension de leurs propriétés. En conséquence, plusieurs matériaux lamellaires, tels que HNbMoO6, HNbWO6, H2W2O7 et H2WO4, ainsi qu'un nouveau type d'oxydes en couches basés sur Nb et W et caractérisés par des phases d'Aurivillius “en escalier” ont été largement caractérisés notamment par spectroscopie DRX et spectroscopie Raman. La possibilité de convertir les solides précurseurs au lithium ou bismuth en phases protonées a été étudiée. Ces oxydes ont la spécificité d'intercaler des molécules entre les couches, ce qui peut contribuer à l’activité catalytique en phase liquide. Pour identifier les rôles respectifs de l’intercalation et des propriétés de surface comme l’acidité, les matériaux ont été caractérisés en phase liquide par spectroscopie Raman en utilisant des bases organiques telles que les n-alkylamines (butylamine et octylamine) et la pyridine. L'intercalation avec des réactifs possibles, les n-alcools et le 2,5-hexanediol, a également été étudiée. L'activité catalytique de ces oxydes lamellaires a été déterminée grâce à une nouvelle réaction: la cyclo-déshdrataion du 2,5-hexanediol en 2,5-diméthyltétrahydrofurane. HNbMoO6 s’est avéré le catalyseur le plus actif, comparé à des catalyseurs acides conventionnels ou aux autres matériaux lamellaires. L’acidité et la capacité d’intercalation de ces divers matériaux lamellaires ont été comparées pour comprendre les différences observées pour l’activité catalytique. / Layered transition metal oxides have a potential as catalysts for biomass conversions, but their development necessitates a better understanding of their properties. Consequently, several layered materials such as HNbMoO6, HNbWO6, H2W2O7 and H2WO4 as well as new types of layered oxides based on Nb and W and characterized by a “stair-like” Aurivillius phases were extensively characterized notably by XRD and Raman spectroscopy. The possibility to convert the as-synthesised lithium or bismuth precursors to protonated phases was also thoroughly evaluated. Layered oxides have the specificity to intercalate molecules within their interlayer regions, which may be a key feature to catalytic activity for reactions in the liquid phase. In order to evaluate the respective roles of intercalation and surface properties such as acidity, the materials were characterized in the liquid phase by Raman spectroscopy using organic bases such as n-alkylamines (butylamine and octylamine) and pyridine. Intercalation with possible reagents, n-alcohols and 2,5-hexanediol, was also studied. The catalytic activity of these layered oxides was evaluated using a novel test reaction: the cyclo-dehydration of 2,5-hexanediol into 2,5-dimethyltetrahydrofuran. HNbMoO6 proved to be the most active catalyst, compared to conventional acidic catalysts or other layered materials. The acidity and intercalation ability of the various layered materials were compared to understand the differences observed for the catalytic activity.

O uso do peróxido de hidrogênio eletrogerado in situ em Processos Oxidativos Avançados (POAs) é um promissor método para o tratamento de efluentes orgânicos. Neste contexto, o desenvolvimento de materiais mais eficientes para viabilizar a reação de redução do oxigênio (RRO) pelo mecanismo envolvendo a transferência de dois elétrons ainda é de grande importância. O presente estudo objetiva a obtenção e a avaliação da atividade catalítica de nanopartículas de óxidos metálicos, Ta2O5, MoO3, Nb2O5 ou ZrO2, incorporadas em carbono Printex 6L e em grafeno no estudo da redução do oxigênio visando a eletrogeração de peróxido de hidrogênio. A caracterização morfológica e microestrutural desses materiais foi investigada por difração de raios X, microscopia eletrônica de transmissão e espectroscopia de fotoelétrons excitados por raios X. O mecanismo da RRO foi analisado pela técnica do eletrodo de disco-anel rotatório. O carbono Printex 6L apresentou uma eficiência de corrente para a eletrogeração de H2O2 (I(H2O2)%) igual a 65,3% em K2SO4 0,1 mol L-1 (pH =2). Após o tratamento térmico desse substrato pelo método dos precursores poliméricos, a eficiência de corrente aumentou 17,1%, ou seja, I(H2O2)% igual a 76,5%. A modificação do carbono Printex 6L com nanopartículas cristalinas de Ta2O5, MoO3, Nb2O5 ou ZrO2 deslocou o potencial de meia-onda para a RRO para valores menos negativos e aumentou a I(H2O2)%. O Nb2O5/C apresentou a maior I(H2O2)%, 87,1%, enquanto o ZrO2/C deslocou o potencial em 137 mV. O estudo comparativo entre três substratos de carbono para a redução do oxigênio, revelou que o óxido de grafeno reduzido apresentou maior atividade para a RRO comparado ao óxido de grafeno e ao carbono Printex 6L, tanto em eletrólito ácido quanto em eletrólito alcalino. A modificação do rGO com Nb2O5 ou ZrO2 pelo método hidrotermal aumentou a atividade catalítica desse substrato para a RRO. A maior eletrogeração de H2O2 foi observada na presença do ZrO2. De fato, um aumento de 73,7% para 88,5% em K2SO4 0,1 mol L-1 (pH =2), e de 72,9% para 83,1% em NaOH 0,1 mol L-1, foi observado em rGO e ZrO2-rGO, respectivamente. Além disso, o ZrO2-rGO apresentou menor sobrepotencial para a RRO comparado ao rGO sem modificador. Portanto, a presença das nanopartículas de óxidos metálicos na matriz condutora de carbono amorfo intensificou a atividade catalítica para a eletrogeração do H2O2, tanto em carbono Printex 6L quanto em óxido de grafeno reduzido, indicando o efeito sinérgico entre as nanopartículas e o subtrato de carbono. Consequentemente, os catalisadores avaliados neste trabalho são promissores para a eletrogeração de espécies oxidantes in situ e sua aplicaçao em POAs. / In situ electrogeneration of hydrogen peroxide has been greatly application in Advanced Oxidation Processes (AOPs) as an effective water treatment technology. In this context, the development of electrode materials that enable the oxygen reduction reaction (ORR) mechanism through a two-electron pathway with high selectivity at low overpotential has a pronounced importance. In the present study, we investigate the properties of Ta2O5, MoO3, Nb2O5 or ZrO2 nanoparticles supported on Printex 6L carbon and reduced graphene oxide for the electrocatalysis of oxygen reduction to H2O2. The structures and morphologies of these materials were characterized by X-ray diffraction analysis, transmission electron microscopy and X-ray photoelectron spectroscopy. Electrochemical activities toward the ORR were evaluated using a rotating ring-disk electrode system. Printex 6L carbon showed a current efficiency for H2O2 production (I(H2O2)%) of 65.3% in K2SO4 0.1 mol L-1 (pH =2) whereas the carbon heat-treated by the polymeric precursor method displayed 76.5% yield of H2O2 electrogeneration. Carbon Printex 6L modified with Ta2O5, MoO3, Nb2O5 or ZrO2 nanoparticles shifted the half-wave potential for the ORR to less negative potential and increased the I(H2O2)%. The Nb2O5/C showed the highest I(H2O2)%, 87,1%, and the ZrO2/C shifted the potential 137 mV. The comparative study of ORR in acid and alkaline media for the three different conductive carbon pigment showed that the reduced graphene oxide displayed higher activity to oxygen reduction than graphene oxide and Printex 6L carbon. Reduced graphene oxide modified with Nb2O5 or ZrO2 nanoparticles by hydrothermal method increased the catalytic activity of this substrate for ORR which the highest H2O2 electrogeneration was observed for ZrO2-rGO. Indeed, an increase from 73.7% to 88.5% in K2SO4 0.1 mol L-1 (pH =2) and from 72.9% to 83.1% in NaOH 0.1 mol L-1 was obtained for rGO and ZrO2-rGO, respectively. Additionally, the ZrO2-rGO electrocatalyst exhibited overpotential lower than that of rGO unmodified. Therefore, the metallic oxides nanoparticles in both carbon Printex 6L and reduced graphene oxide enhanced the catalytic activity for H2O2 electrogeneration indicating the synergistic effect between the nanoparticles and the amorphous carbon. The catalysts evaluated in this study are promising for in situ electrogeneration of oxidizing agents to be used in the degradation of organic pollutants.

O uso do peróxido de hidrogênio eletrogerado in situ em Processos Oxidativos Avançados (POAs) é um promissor método para o tratamento de efluentes orgânicos. Neste contexto, o desenvolvimento de materiais mais eficientes para viabilizar a reação de redução do oxigênio (RRO) pelo mecanismo envolvendo a transferência de dois elétrons ainda é de grande importância. O presente estudo objetiva a obtenção e a avaliação da atividade catalítica de nanopartículas de óxidos metálicos, Ta2O5, MoO3, Nb2O5 ou ZrO2, incorporadas em carbono Printex 6L e em grafeno no estudo da redução do oxigênio visando a eletrogeração de peróxido de hidrogênio. A caracterização morfológica e microestrutural desses materiais foi investigada por difração de raios X, microscopia eletrônica de transmissão e espectroscopia de fotoelétrons excitados por raios X. O mecanismo da RRO foi analisado pela técnica do eletrodo de disco-anel rotatório. O carbono Printex 6L apresentou uma eficiência de corrente para a eletrogeração de H2O2 (I(H2O2)%) igual a 65,3% em K2SO4 0,1 mol L-1 (pH =2). Após o tratamento térmico desse substrato pelo método dos precursores poliméricos, a eficiência de corrente aumentou 17,1%, ou seja, I(H2O2)% igual a 76,5%. A modificação do carbono Printex 6L com nanopartículas cristalinas de Ta2O5, MoO3, Nb2O5 ou ZrO2 deslocou o potencial de meia-onda para a RRO para valores menos negativos e aumentou a I(H2O2)%. O Nb2O5/C apresentou a maior I(H2O2)%, 87,1%, enquanto o ZrO2/C deslocou o potencial em 137 mV. O estudo comparativo entre três substratos de carbono para a redução do oxigênio, revelou que o óxido de grafeno reduzido apresentou maior atividade para a RRO comparado ao óxido de grafeno e ao carbono Printex 6L, tanto em eletrólito ácido quanto em eletrólito alcalino. A modificação do rGO com Nb2O5 ou ZrO2 pelo método hidrotermal aumentou a atividade catalítica desse substrato para a RRO. A maior eletrogeração de H2O2 foi observada na presença do ZrO2. De fato, um aumento de 73,7% para 88,5% em K2SO4 0,1 mol L-1 (pH =2), e de 72,9% para 83,1% em NaOH 0,1 mol L-1, foi observado em rGO e ZrO2-rGO, respectivamente. Além disso, o ZrO2-rGO apresentou menor sobrepotencial para a RRO comparado ao rGO sem modificador. Portanto, a presença das nanopartículas de óxidos metálicos na matriz condutora de carbono amorfo intensificou a atividade catalítica para a eletrogeração do H2O2, tanto em carbono Printex 6L quanto em óxido de grafeno reduzido, indicando o efeito sinérgico entre as nanopartículas e o subtrato de carbono. Consequentemente, os catalisadores avaliados neste trabalho são promissores para a eletrogeração de espécies oxidantes in situ e sua aplicaçao em POAs. / In situ electrogeneration of hydrogen peroxide has been greatly application in Advanced Oxidation Processes (AOPs) as an effective water treatment technology. In this context, the development of electrode materials that enable the oxygen reduction reaction (ORR) mechanism through a two-electron pathway with high selectivity at low overpotential has a pronounced importance. In the present study, we investigate the properties of Ta2O5, MoO3, Nb2O5 or ZrO2 nanoparticles supported on Printex 6L carbon and reduced graphene oxide for the electrocatalysis of oxygen reduction to H2O2. The structures and morphologies of these materials were characterized by X-ray diffraction analysis, transmission electron microscopy and X-ray photoelectron spectroscopy. Electrochemical activities toward the ORR were evaluated using a rotating ring-disk electrode system. Printex 6L carbon showed a current efficiency for H2O2 production (I(H2O2)%) of 65.3% in K2SO4 0.1 mol L-1 (pH =2) whereas the carbon heat-treated by the polymeric precursor method displayed 76.5% yield of H2O2 electrogeneration. Carbon Printex 6L modified with Ta2O5, MoO3, Nb2O5 or ZrO2 nanoparticles shifted the half-wave potential for the ORR to less negative potential and increased the I(H2O2)%. The Nb2O5/C showed the highest I(H2O2)%, 87,1%, and the ZrO2/C shifted the potential 137 mV. The comparative study of ORR in acid and alkaline media for the three different conductive carbon pigment showed that the reduced graphene oxide displayed higher activity to oxygen reduction than graphene oxide and Printex 6L carbon. Reduced graphene oxide modified with Nb2O5 or ZrO2 nanoparticles by hydrothermal method increased the catalytic activity of this substrate for ORR which the highest H2O2 electrogeneration was observed for ZrO2-rGO. Indeed, an increase from 73.7% to 88.5% in K2SO4 0.1 mol L-1 (pH =2) and from 72.9% to 83.1% in NaOH 0.1 mol L-1 was obtained for rGO and ZrO2-rGO, respectively. Additionally, the ZrO2-rGO electrocatalyst exhibited overpotential lower than that of rGO unmodified. Therefore, the metallic oxides nanoparticles in both carbon Printex 6L and reduced graphene oxide enhanced the catalytic activity for H2O2 electrogeneration indicating the synergistic effect between the nanoparticles and the amorphous carbon. The catalysts evaluated in this study are promising for in situ electrogeneration of oxidizing agents to be used in the degradation of organic pollutants.

Made available in DSpace on 2014-12-17T14:07:15Z (GMT). No. of bitstreams: 1 MariaVMS_Parcial.pdf: 533267 bytes, checksum: dfba6ad9f2b0c2ad0d26dac46721fabf (MD5) Previous issue date: 2013-10-31 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The refractory metal carbides have proven important in the development of engineering materials due to their properties such as high hardness, high melting point, high thermal conductivity and high chemical stability. The niobium carbide presents these characteristics. The compounds of niobium impregnated with copper also have excellent dielectric and magnetic properties, and furthermore, the Cu doping increases the catalytic activity in the oxidation processes of hydrogen. This study aimed to the synthesis of nanostructured materials CuNbC and niobium and copper oxide from precursor tris(oxalate) oxiniobate ammonium hydrate through gas-solid and solid-solid reaction, respectively. Both reactions were carried out at low temperature (1000?C) and short reaction time (2 hours). The niobium carbide was produced with 5 % and 11% of copper, and the niobium oxide with 5% of copper. The materials were characterized by X-Ray Diffraction (XRD), Rietveld refinement, Scanning Electron Microscopy (SEM), X-Ray Fluorescence Spectroscopy (XRF), infrared spectroscopy (IR), thermogravimetric (TG) and differential thermal analysis (DTA , BET and particle size Laser. From the XRD analysis and Rietveld refinement of CuNbC with S = 1.23, we observed the formation of niobium carbide and metallic copper with cubic structure. For the synthesis of mixed oxide made of niobium and copper, the formation of two distinct phases was observed: CuNb2O6 and Nb2O5, although the latter was present in small amounts / Os carbetos de metais refrat?rios t?m se revelado importantes no desenvolvimento de materiais de engenharia devido as suas propriedades, tais como: alta dureza, alto ponto de fus?o, alta condutividade t?rmica e alta estabilidade qu?mica. O carbeto de ni?bio apresenta essas caracter?sticas. Os compostos de ni?bio impregnados com cobre tamb?m possuem excelentes propriedades diel?tricas e magn?ticas e, al?m disso, a dopagem com Cu aumenta a atividade catal?tica em processos de oxida??o de hidrog?nio. Este trabalho teve como objetivo a s?ntese dos materiais CuNbC e ?xido de ni?bio e cobre nanoestruturados a partir do precursor tris(oxalato)oxiniobato de am?nio hidratado, atrav?s de rea??o g?s-s?lido e s?lido-s?lido, respectivamente. Para ambos, as rea??es foram realizadas a baixa temperatura (1000?C) e curto tempo de rea??o (2 horas). O carbeto de ni?bio foi produzido com 5% e 11% de cobre e o ?xido de ni?bio e cobre com 5% de cobre. Os materiais obtidos foram caracterizados atrav?s dos ensaios de Difra??o de Raios X (DRX), Refinamento Rietveld, Microscopia Eletr?nica de Varredura (MEV), Espectroscopia por Fluoresc?ncia de Raios-X (FRX), Espectroscopia de Infravermelho (IV), Termogravim?trica (TG), An?lise Termodiferencial (DTA), BET e granulometria a Laser. A partir das an?lises de DRX e do refinamento Reitiveld para o CuNbC com S= 1,23, observou-se a forma??o do carbeto de ni?bio e cobre puro com estrutura c?bica. Na s?ntese realizada do ?xido misto de ni?bio e cobre correu a forma??o de duas fases distintas: CuNb2O6 e Nb2O5, embora a ?ltima tenha sido formada em pequena quantidade / 2020-01-01

Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro / É crescente o interesse nos materiais cerâmicos, devido as suas características como baixa massa específica e maior resistência a ambientes agressivos do que a maioria das ligas metálicas. Este trabalho tem o objetivo de produzir a partir de material descartado, cerâmicas em diferentes temperaturas de sinterização e avaliar a sua tensão de ruptura em ensaio de flexão de três pontos e a confiança desta medida. Devido ao custo de produzir pó de vidro, tanto pelo alto gasto de energia para fundir a matéria-prima como pelo consumo de minerais industriais, foi proposto utilizar vidro de para-brisa obsoleto reduzindo despesas operacionais e definindo um destino econômico e ambiental viável para estes rejeitos. A metodologia consistiu-se na obtenção do pó de vidro com características adequadas para ser conformado e sinterizado. Foram usadas duas composições e quatro tratamentos térmicos para obter oito materiais. Uma composição com apenas o pó oriundo da moagem de para-brisa e outra com este pó mais 4% de óxido de nióbio. A resistência à flexão dos produtos obtidos foi avaliada. Utilizou-se a estatística de Weibull para caracterizar estes resultados. Os resultados obtidos indicam que o material de composição pó de vidro e temperatura final de sinterização de 650C obteve a maior resistência mecânica entre os materiais sintetizados. A adição do óxido de nióbio provoca um decréscimo na resistência mecânica se comparada com o material sem a adição deste óxido. Entretanto, comparando as duas composições na mesma temperatura final de sinterização, a adição de óxido de nióbio provocou um aumento no módulo de Weibull, excetuando-se dois de oito materiais obtidos. As diferentes composições e temperaturas de sinterização afetaram as propriedades mecânicas dos materiais obtidos. / The growing interest in the materials in ceramics, is due to its characteristics such as low density and greater resistance to aggressive environments more than most metal alloys. This work aims to produce from discarded material, ceramics at different sintering temperatures and evaluate its resistance of three points bending and a confidence measure of this. Due to the cost of producing powdered glass, both on the high expenditure of energy to melt the raw material as the consumption of industrial minerals, it was proposed to use windshield obsolete for reducing operating costs and defining a viable economic and environmental fate for these tailings. The methodology consisted on getting the glass powder with adequate characteristics to be shaped and sintered. Two compositions and four thermal treatments were used to obtain eight materials. One with just the milling powdered of the windshield and another with this powder and 4 wt% niobium oxide powder. The flexural strength of the obtained products were evaluated. It was used the statistic of Weibull to characterize these results. The results indicate that the powder material glass composition and final sintering temperature of 650C got greater strength among the materials synthesized. The addition of niobium oxide causes a decrease in the mechanical strength compared with the material without addition of the oxide. However, comparing the two compositions in the same final sintering temperature, the addition of niobium oxide caused an increase in Weibull modulus, except for two of the eight materials obtained. The different compositions and the sintering temperatures modified the mechanical properties of the obtained materials.