Compósitos mesoporosos de ZrO2-CeO2 estão sendo desenvolvidos devido às suas excelentes propriedades morfológicas e estruturais, necessárias para várias aplicações, que incluem sensores de gás, catálise automotiva e ânodos de células a combustível de óxido sólido. Nesse trabalho foi desenvolvido um novo método de síntese sol-gel com template cooperativo utilizando o polímero tribloco P-123 e os cloretos de Zr/Ce como precursores dos óxidos. Foram sintetizados ZrO2-x(mol)%CeO2 com x = 50, 70 e 90% de CeO2, uma vez que esses materiais apresentam melhores características para aplicações catalíticas. Dois processos de calcinação diferentes foram testados (até 540 e 400ºC). O NiO (60% m/m) foi impregnado para que o material obtenha a condutividade eletrônica necessária para aplicação em ânodos de SOFC. Os resultados de difração de raios X indicaram sistemas cuja fase cristalográfica predominante é a cúbica tipo fluorita (a fase tetragonal é minoritária). Fase única cúbica foi obtida para 90% de CeO2 após a calcinação até 400ºC. Dentre as características morfológicas dos materiais calcinados até 540ºC, os resultados de adsorção/dessorção de N2, imagens de microscopia eletrônica e espalhamento de raios X a baixos ângulos apresentaram aglomerados cristalinos de ZrO2-CeO2, formando um sistema mesoporoso bicontínuo, aleatéorio e sem forma definida, homogêneo em composição, com área superficial intermediária (30-40 m2/g), com alta dispersividade de poros/partículas. A calcinação a 400ºC apresentou menor dispersividade, menor tamanho de poros e maior área superficial (> 100 m2/g). O recobrimento da matriz de ZrO2-CeO2 pelas nanopartículas de NiO é superficial, sem obstrução ou preenchimento dos poros. A partir da redução à temperatura programada, observou-se que independentemente do conteúdo de CeO2, a porcentagem de redução do Ce4+ foi maior e ocorreu a menores temperaturas (início da redução em 300ºC) do que o padrão de CeO2 (750ºC). Esse comportamento se repete com as amostras após a incorporação com NiO, que se reduz a Ni também em baixas temperaturas (320ºC). A atividade catalítica para conversão do CH4 em oxidação total foi similar para ambas temperaturas de calcinação, para 90% de CeO2, atingindo 50% de conversão de CH4 para ~ 540ºC. Nos experimentos de absorção de raios X in-situ, na borda K do Ni e na borda LIII do Ce, foi possível observar que todos os conteúdos de CeO2 são ativos para oxidação parcial e total de CH4, assim como decomposição do CH4 e oxidação do CO, que ocorreu em torno de 600ºC. Os resultados de espectroscopia de impedância eletroquímica mostraram que, materiais com alto conteúdo de CeO2, apresentam baixa resistividade, de 0,97 cm2 a 750ºC em atmosfera de 5% CH4/3\\%H2O/N2. Portanto, o material desenvolvido neste trabalho apresenta as melhores propriedades morfológicas, estruturais, elétricas e catalíticas relatadas na literatura para aplicações como ânodo de SOFC e catalisador, comparado a materiais similares relatados na literatura. / Mesoporous ZrO2-CeO2 composites are being developed due to their excellent morphological and structural properties, which are necessary for their use in several applications, including gas sensors, three way catalysts (TWC) and solid oxide fuel cells (SOFCs). In this work a new synthesis method was developed based on a template cooperative sol-gel approach, using the tri-block polymer P-123 and Zr/Ce chlorides as the oxides\' precursors. Since high cerium oxide quantities lead to better catalytic performance, the ZrO2-x(mol)%CeO2 were synthesized with x = 50, 70 and 90. Two different calcination processes were tested (until 540 and 400 ºC). NiO was impregnated in order to obtain enough electronic conductivity for their application as SOFC anodes. X-ray diffraction results showed that these systems are biphasic and crystallized preferentially into cubic fluorite type structure together with smaller quantities of the tetragonal zirconia-ceria phase. A 100% cubic phase was retained for 90% of CeO2 after 400 ºC calcination. Textural and morphological characteristics for 540ºC calcination evaluated from N2 sorption, electronic microscopy images and small angle X-ray scattering revealed a two-density (pores/particles) random crystalline clusters of mesoporous ZrO2-CeO2, with homogeneous composition, average superficial area (30-40 m2/g), high dispersivity of pores/particle sizes. Calcination until 400 ºC presented a narrower pore size distribution and smaller pores, with higher superficial area (> 100 m2/g). It was observed that NiO particles formed an uniform layer over the ZrO2-CeO2 without filling or blocking the zirconia-ceria pores. Temperature programmed reduction experiments showed that for all ceria contents the reduction percentage of Ce4+ species in the samples was higher and at lower temperatures (beginning of reduction at 300 ºC) than standard CeO2 (750 ºC). After NiO impregnation this behavior was similar, with NiO reducing at lower temperatures (320 ºC) as well. Catalytic activity for methane total oxidation reaction was similar for both calcination temperatures, for 90% CeO2, showing 50% of CH4 conversion around 540 ºC. Absorption X-ray in-situ experiments at Ni K-edge and Ce LIII-edge showed that all ceria contents are active for total and partial methane oxidation, CH4 decomposition and CO oxidation at 600 ºC. Electrochemical impedance spectroscopy measurements showed low resistivity for higher ceria content, 0,97 Ocm2 at 750 ºC in 5% CH4/3% H2O/N2 atmosphere. Resuming, the material developed in this work presents the best morphological, structural, electrical and catalytical properties for applications as SOFC anode and catalyst, compared to similar materials reported in the literature.

O desenvolvimento de novas tecnologias para produção de combustíveis é assunto de vital importância, principalmente quando se trata de combustíveis de fontes limpas e renováveis. Nesse aspecto, surge o hidrogênio (H2), tecnologia limpa, vindo de fontes renováveis, como o etanol. Para a obtenção do H2 a partir do etanol, destaca-se a reforma a vapor. No entanto, a reforma a vapor apresenta alguns desafios como maximização da conversão e seletividade em H2, além da minimização na formação de carbono e dos seus precursores (acetaldeído e acetona), uma vez que a formação de carbono pode levar a perda da atividade do catalisador. Nesse contenxto, a ideia deste trabalho é desenvolver catalisadore a base de níquel (Ni) suportados em céria (CeO2) dopados com os lantanídeos gadolínio (Gd), samario (Sm) e neodímio (Nd) para aplicação na reforma a vapor do etanol de modo a aumentar a conversão do etanol e a seletividade em H2, bem como minimizar a formação de carbono e de seus percursores. Para isso, fixou-se o teor de Ni em 5% e variou-se os teores do dopantes em 1%, 5% e 10%, também sintetizou-se o catalisador sem presença de dopante para comparação. A partir desses catalisadores, fez-se a caracterização e os testes catalíticos a 400, 500 e 600 °C. Percebeu-se a formação de solução sólida para todos os dopantes (Gd, Sm e Nd), por meio da expansão dos parâmetros de rede, na qual a expansão de rede aumentou com o aumento do teor de dopante e com o aumento do raio iônico do dopante. A adição de 1% de dopante para os catalisadores dopados com Sm e Nd levaram a um aumento significativo da área superficial, bem como a diminuição do tamanho do cristalito do suporte. A temperatura de reação na qual se observa os melhores resultados catalíticos é a 600 °C de modo que houve uma conversão máxima de etanol, seletividades em H2 acima de 2,80 molprod.molEtOHconv-1 e minimização na formação de espécies líquidas como a acetona e acetaldeído, bem como na formação de coque. A conversão do etanol nessa temperatura foi similar para todos os catalisadores, mas os catalisadores 5Ni_5GdCeO2 e 5Ni_5NdCeO2 apresentaram a maior seletividade a H2. No entanto, os catalisadores com a presença de Nd apresentaram as menores taxas de formação de coque. / The development of new technologies for fuel production is a vital issue, especially clean and renewable fuels. In this apect, hydrogen (H2), a clean technology, is highlighted when it comes from renewable sources, such as ethanol. To obtain H2 from ethanol, the steam reform is pointed out. However, steam reforming presents some challenges, such as maximizing conversion and H2 selectivity, as well as minimizing the formation of carbon and its precursors (acetaldehyde and acetone), since carbon formation can lead to catalytic activity loss. In this context, the idea of this work is the development of nickel (Ni) based catalysts supported in ceria (CeO2), doped with the lanthanides gadolinium (Gd), samarium (Sm) and neodymium (Nd), for ethanol steam reforming application in order to increase ethanol conversion and H2 selectivity, as well as to minimize the formation of carbon and its precursors. For this, the Ni content was set at 5% and the dopant contents varied in 1%, 5% and 10%, the catalyst was also synthesized without presence of dopant for comparison. From these catalysts, the characterization and the catalytic tests were carried out at 400, 500 and 600 °C. The formation of a solid solution for all dopants (Gd, Sm and Nd) was observed by means of the network parameters expansion , in which the network expansion increased with increasing dopant content and with increasing dopant ionic radius. The addition of 1% dopant, to the Sm and Nd doped catalysts, led to a significant increase in surface area as well as a decrease in the support crystallite size. The reaction temperature at which the best catalytic results are observed is at 600 °C, so that there was a maximum conversion of ethanol, H2 selectivity 2.80 molprod.molEtOHconv-1, and minimization of liquid species formation, such as acetone and acetaldehyde, as well as the coke formation. Ethanol conversion at this temperature was similar for all catalysts, but the catalysts 5Ni_5GdCeO2 and 5Ni_5NdCeO2 showed the highest selectivity to H2. However, the catalysts with the presence of Nd presented the lowest rates of coke formation.

Eletrólitos sólidos de céria com adição de íons de terras raras trivalentes apresentam elevada condutividade iônica quando comparados com a zircônia estabilizada com ítria, que é um eletrólito sólido padrão. Por isso, esses condutores de íons oxigênio à base de céria têm potencial de aplicação em células a combustível de óxido sólido que operam em temperaturas intermediárias (500-700ºC). Uma das abordagens mais investigadas para otimização da condutividade elétrica e de outras propriedades destes materiais é a incorporação de um segundo aditivo. Neste trabalho, composições de céria-20% mol gadolínia com SrO, TiO2 e SrTiO3 como co-aditivos, em teores de 1; 2,5 e 5% em mol foram preparadas por reação em estado sólido. O principal objetivo foi verificar o efeito dos aditivos na densificação, na microestrutura e na condutividade elétrica da céria-gadolínia. Os compactos sinterizados foram caracterizados por medidas de densidade aparente, difração de raios X e espectroscopia Raman para determinação das fases cristalinas, microscopia eletrônica de varredura para observar as diferentes microestruturas das composições, e medida da condutividade elétrica por espectroscopia de impedância. Os resultados mostraram que os aditivos exercem influência em todas as propriedades investigadas, mas de forma diferente dependendo do tipo e do teor. De forma geral, SrO exerce efeito benéfico na condutividade elétrica intergranular, mas prejudica a densificação. O TiO2 promove um aumento na densificação da céria-gadolínia, mas aumenta também o bloqueio aos portadores de carga nos contornos de grão, além de resultar na formação da fase pirocloro Gd2Ti2O7, quando adicionado em teores acima do limite de solubilidade. O SrTiO3 não produz alterações na densificação da céria-gadolínia. / Ceria containing trivalent rare-earths is a solid electrolyte with higher ionic conductivity than the standard yttria fully-stabilized zirconia ionic conductor. This property turns these ceria-based ionic conductors promising materials for application in solid oxide fuel cells operating at intermediate temperatures (500-700ºC). One of the most utilized approaches to optimize the electrical conductivity and other properties of these materials is the introduction of a second additive. In this work, ceria-20 mol% gadolinia with additions of 1, 2.5 and 5 mol% of SrO, TiO2 and SrTiO3 as co-additives were prepared by solid state reaction. The main purpose was to investigate the effects of the co-additives on densification, microstructure and electrical conductivity of the solid electrolyte. Sintered pellets were characterized by apparent density, X-ray diffraction, Raman spectroscopy, scanning electron microscopy and electrical conductivity by impedance spectroscopy. The additives were found to exert different influences in all studied properties. The way they influence the solid electrolyte properties depends on the type and content of the additive. SrO addition to doped ceria improves the intergranular conductivity, but decreases the apparent density of the pellets. Increase of densification was obtained with TiO2 addition. This additive promotes increase of the blocking of charge carriers at the grain boundaries due to solute exsolution and formation of the pyrochlore Gd2Ti2O7 phase at grain boundaries for contents in excess of the solubility limit. No influence on densification was found for SrTiO3 additions.

A introdução de cálcio e lítio como aditivos de sinterização na céria: 10% mol gadolínia foi investigada com o intuito de verificar sua influência na densificação e condutividade elétrica das cerâmicas sinterizadas. Pós contendo de 0 a 1,5% mol do metal foram preparados tanto por reação em estado sólido quanto pela co-precipitação dos oxalatos. As principais técnicas de caracterização utilizadas foram análise térmica, difração de raios X, microscopia eletrônica de varredura e medida da condutividade elétrica por espectroscopia de impedância. Os resultados obtidos mostraram que cerâmicas densas podem ser obtidas utilizando ambos os aditivos. O aumento no teor do aditivo resulta em aumento na densificação. A forma de adição, por reação em estado sólido ou por coprecipitação exerce influência na condutividade elétrica. A adição de cálcio promove maior crescimento dos grãos que o lítio. A condutividade elétrica das amostras contendo o segundo aditivo é inferior à da céria-gadolínia pura. Ambos os aditivos exercem influência na condutividade intergranular. Adição de cálcio resulta também em diminuição da condutividade intragranular. Os aditivos favorecem a exudação do gadolínio. / In this work, the use of calcium and lithium as sintering aid to gadolinia-doped ceria was systematically investigated. The main purpose was to verify the influence of these additives on the densification and electrical conductivity of sintered ceramics. Powder compositions containing up to 1.5 mol% (metal basis) of calcium or lithium were prepared by both solid state reaction and oxalate coprecipitation methods. The main characterization techniques were thermal analyses, X-ray diffraction, scanning electron microscopy and electrical conductivity by impedance spectroscopy. Both additives promoted densification of gadolinia-doped ceria. The densification increases with increasing the additive content. Different effects on microstructure and electrical conductivity result from the method of preparation, e.g., solid state reaction or coprecipitation. Calcium addition greatly enhances the grain growth compared to lithium addition. The electrical conductivity of specimens containing a second additive is lower than that of pure gadolinia-doped ceria. Both additives influence the intergranular conductivity and favor the exudation of gadolinium out of the solid solution.

Amostras de dióxido de cério dopadas com La e Fe foram confeccionadas e caracterizadas por uma técnica nuclear baseada em interações hiperfinas conhecida como Correlação Angular Gama-Gama Perturbada (CAP). Como o composto em questão não é radioativo, foram utilizados núcleos radioativos como ponta de prova 111In 111Cd, que decaem através de uma cascata gama-gama 171-245 keV, com nível intermediário possui uma meia-vida de 84ns, spin 5/2- e um momento de quadrupolo elétrico Q= 0,83. Inicialmente uma metodologia para a produção das amostras precisou ser elaborada. As amostras de dióxido de cério e seus dopantes foram produzidas através do processo Sol-Gel, passando por uma calcinação e sinterização até a finalização da amostra. Sendo caracterizada por diversos tipos de técnicas (Difração de Raios-X e Microscopia Eletrônica de Varredura) culminando no estudo através da técnica CAP, para uma compreensão das interações quadrupolares elétricas das amostras e também a possibilidade da existência de comportamento magnético (assunto intensamente investigado dado o interesse na área da spintrônica). Os resultados encontrados foram analisados frente aos conhecimentos encontrados na literatura e as discussões foram feitas em função da variação do elemento dopante, tipos de tratamentos térmicos usados na sinterização e as diferentes temperaturas de medidas. Permitindo uma discussão e interpretação física dos resultados encontrados. / Samples of cerium dioxide doped with La and Fe were fabricated and characterized by a technique based on nuclear hyperfine interactions known as Perturbed Angular Gamma-Gamma Correlation (PAC). As the used compound is not radioactive and the PAC technique requires a radioactive decaying through a cascade gamma-gamma, a radioactive probe nuclei were used (111In 111Cd - 171-245 keV), with intermediate has a half-life of 84ns, spin 5/2- and a moment of electric quadrupole Q= 0,83. Initially a methodology for the production of the samples had to be prepared. The cerium dioxide and its dopants were produced by the Sol-Gel process, undergoing a calcining and sintering. The sample are characterized by various types of techniques (X-ray Diffraction and Scanning Electron Microscopy) succeeding study through hyperfine Interactions, with the PAC technique, to an understanding of the electric quadrupole inteirações samples and also the possibility of the existence of magnetic behavior (subject intensively investigated given the interest in the field of spintronics). The results were analyzed taking the knowledge found in the literature and discussions were made on the basis of the variation of the doping element, thermal treatments used in the sintering temperatures and the different measures. Allowing a discussion and physical interpretation of results.

CuNi nanoparticles were synthesized using a new polyol synthesis method. Three different CuxNi1-x catalysts were synthesized where x = 20, 50 and 80. The nanoparticles were deposited on carbon, C, gamma-alumina, γ-Al2O3, yttria-stabilized zirconia, YSZ, and samariumdoped ceria, SDC. Each set of catalysts was tested using the Reverse Water Gas Shift, RWGS, reaction under atmospheric pressure and at temperatures ranging from 400°C-700°C. The experiments were repeated 3 times to ensure stability and reproducibility. Platinum nanoparticles were also deposited on the same supports and tested for the RWGS reaction at the same conditions. The CuNi nanoparticles were characterized using a variety of different techniques. Xray diffraction, XRD, measurements demonstrate the resence of two CuNi solid solutions: one Cu rich solid solution, and the other a Ni rich solid solution. X-ray photo electron spectroscopy, XPS, measurements show Cu enrichment on all catalytic surfaces. Scanning electron microscopy, SEM, measurements show CuNi nanoparticles ranging in size from 4 nm to 100 nm. Some agglomeration was observed. SDC showed the best yield with all catalysts. Furthermore, high oxygen vacancy content was shown to increase yield of CO for the RWGS reaction. Cu50Ni50/SDC shows the combination of highest yield of CO and the best stability among CuNi catalysts. It also has similar yields (39.8%) as Pt/SDC at 700°C, which achieved the equilibrium yield at that temperature (43.9%). The catalyst was stable for 48 hours when exposed to high temperatures (600-700°C). There was no CH4 observed during any of the experiments when the partial pressure of the reactant gases was fed stoichiometrically. Partial pressure variation experiments demonstrated the presence of CH4 when the partial pressure of hydrogen was increased to twice the value of the partial pressure of CO2.

This work deals with the preparation of thin cerium oxide films on the Cu(110) single crystal. Physico-chemical properties of this system were studied by surface science techniques (XPS, UPS, ARUPS, LEED, LEEM and STM). The first part of the work concerns interaction of Cu(110) single crystal with oxygen. Condi- tions for formation of O(2x1) and Oc(6x2) oxygen reconstructions were found. Various methods of preparation of CeOx films were discussed. A novel method for continuous control of ceria stoichiometry from CeO2 to Ce2O3 through variation of oxygen vacancy concentration has been developed. Ceria facilitated oxygen spill-over was observed on copper substrate. It was found that a restructuring of copper substrate occurs at the copper-ceria interface with subsequent formation of Cu(13 13 1) facets, which support a Carpet-like ceria overlayer. Interaction of this system with platinum was studied. Finally, high temperature growth of CeOx films was studied and creation of ceria islands exposing the (110) plane was observed. 1

The aim of this thesis is preparation of inverse model catalyst CeOx/Pt(111) and its investigation using combination of surface physics methods and electrochemistry. New electrochemical cell was designed and built for electrochemical experiments. CeOx/Pt(111) samples were prepared and studied in UHV using STM and XPS methods. After that, samples were transferred to the electrolyte environment and studied by means of cyclic voltammetry and AFM. For high surface coverage of CeOx, new reaction was observed. We have identified this reaction as a combination of the reduction of cerium(IV) oxide by interaction with hydrogen adsorbed on the Pt(111) surface and oxidation of cerium(III) oxide by dissociative adsorption of water molecules. Powered by TCPDF (www.tcpdf.org)

The aim of this work was to manufacture and test non noble metal catalysts for CO oxidation and soot combustion. The feeding gases consist in the products of the combustion diesel in a Reformtech heater. These gases contain CO, CO2, H2O as well as small amounts of NOx and hydrocarbons. Two different catalysts were prepared for CO oxidation, based on cobalt oxide supported on ceria. 12Co/CeO2 with 12% weight of cobalt and 15CoOx/CeO2 with 15%. The first one was prepared by impregnation of cobalt nitrates in cerium oxide support; the second one was prepared by co-precipitation of cobalt and cerium nitrates. Another catalyst called 12Co4.5K/CeO2, with 12% cobalt and 4.5% potassium, was made for the simultaneous combustion of soot and oxidation of CO. The base also consisted in cobalt oxide supported on ceria, but with the addition of potassium which could stabilize the cobalt oxide particles. Both co-precipitation and impregnation methods gave the desired catalyst structure in the CO oxidation catalysts and both catalysts (12Co/CeO2 and 15CoOx/CeO2) showed activity. Nevertheless, the activity was lower than desired due to low surface area and mass transfer limitations. The catalysts also deactivated in less than three hours on stream, probably due to poisoning. The co-precipitation method for the 12Co4.5K/CeO2 catalyst gave the desired cobalt and cerium oxides, but no conclusion can be drawn regarding potassium since it was not shown in the XRD tests. The catalyst for both CO oxidation and soot combustion (12Co4.5K/CeO2) showed no activity for any of the reactions. Nevertheless, the tests performed to test the soot combustion ability were not conclusive and should be improved in future studies.

Due to continuously stricter regulations regarding emissions from power generation processes, development of existing gas turbine combustors is essential. A promising alternative to conventional flame combustion in gas turbines is catalytic combustion, which can result in ultra low emission levels of NOx, CO and unburned hydrocarbons. The work presented in this thesis concerns the development of methane oxidation catalysts for gas turbine combustors. The application of catalytic combustion to different combustor concepts is addressed in particular. The first part of the thesis (Paper I) reports on catalyst development for fuel-lean methane combustion. The effect on catalytic activity of diluting the reaction mixture with water and carbon dioxide was studied in order to simulate a combustion process with exhaust gas recirculation. Palladium-based catalysts were found to exhibit the highest activity for methane oxidation under fuel-lean conditions. However, the catalytic activity was significantly decreased by adding water and CO2, resulting in unacceptably high ignition temperatures of the fuel. In the second part of this thesis (Paper II), the development of rhodium catalysts for fuel-rich methane combustion is addressed. The effect of water addition on the methane conversion and the product gas composition was studied. A significant influence of the support material and Rh loading on the catalytic behavior was found. The addition of water influenced both the low-temperature activity and the product gas composition. / QC 20101126