<p>The development and improvement of the functionality of metal oxides in heterogeneous catalysis and other surface chemical processes can greatly benefit from an atomic-level understanding of the surface chemistry. Atomistic calculations such as quantum mechanical (QM) calculations and molecular dynamics (MD) simulations can provide highly detailed information about the atomic and electronic structure, and constitute valuable complements to experimental surface science techniques. </p><p>In this thesis, an embedded-cluster approach for quantum mechanical calculations has been developed to model the surface chemistry of metal oxides. In particular, CO adsorption on the MgO(001) and CeO₂(110) surfaces as well as O vacancy formation at the CeO₂(110) surface have been investigated. The cluster model has been thoroughly tested by comparison with electronic structure calculations for the periodic slab model.</p><p>The chemical implications of distorted surface structures arising from the surface dynamics have been investigated by combining the QM embedded-cluster calculations with force-field based MD simulations. Here QM embedded-cluster calculations were performed using surface structures sampled from the MD simulations.</p><p>This combined MD+QM embedded-cluster procedure was applied to the CO adsorption on MgO(001) at 50 K and the O vacancy formation on CeO₂(110) at 300 K. Significant thermal variations of the CO adsorption energy and the O vacancy formation energy were observed. It was found that these variations could be estimated using the force field of the MD simulation as an interaction model. With this approach, the QM results were extrapolated to higher temperature and doped systems.</p>

The solid oxide fuel cell (SOFC) technology has been under research since thelate 1950s, and most of the research has been on designs utilizing yttria stabilized zirconia (YSZ) as the electrolyte of choice. However, the SOFC technology has the major drawback of requiring high operation temperatures (up to 1000 degrees Celcius), so research of alternative materials have come into interest that would possibly require a lower working temperature without any significant loss of conductivity.One such material of interest for the electrolyte is compounds of ceriumdioxide (ceria). Ceria is well known for its ability to release oxygen by formingoxygen vacancies under oxygen-poor conditions, which increases its oxygen ionconductivity, and works at a lower temperature than the YSZ compounds whenproperly doped. Conversely, ceria is also able to absorb oxygen under oxygen-rich conditions, and those two abilities make it a very good material to use in catalytic converters for reduction of carbon monoxide and nitrogen oxide emission. The ability for the oxygen ions to easily relocate inbetween the different lattice sites is likely the key property of oxygen ion transportation in ceria. Also, in oxygen-rich conditions, the absorbed oxygen atom is assumed to join the structure at either the roomy octrahedral sites, or the vacant tetrahedral sites. Following that, the oxygen atom may relocate to other vacant locations, given it can overcome a possible potential barrier. This thesis studies how those interstitial oxygen vacancies (defects) affect theenergy profile of ceria-based supercells by first principles calculations. The system is modeled within the density functional theory (DFT) with aid of (extended) local density approximation (LDA+U) using the software VASP. Furthermore, it is studied how those vacancies affect neighbouring oxygen atoms, and wether or not it is energetically benificial for the neighbouring atoms to readjust their positions closer or further away from the vacancy. The purpose of this thesis is to analyze wether or not it is theoretically possible that interstitial oxygen vacancies may cause neighbouring oxygen atoms to naturally relocate to the octahedral site in ceria, and how this affects the overall energy profile of the material.

The development and improvement of the functionality of metal oxides in heterogeneous catalysis and other surface chemical processes can greatly benefit from an atomic-level understanding of the surface chemistry. Atomistic calculations such as quantum mechanical (QM) calculations and molecular dynamics (MD) simulations can provide highly detailed information about the atomic and electronic structure, and constitute valuable complements to experimental surface science techniques. In this thesis, an embedded-cluster approach for quantum mechanical calculations has been developed to model the surface chemistry of metal oxides. In particular, CO adsorption on the MgO(001) and CeO2(110) surfaces as well as O vacancy formation at the CeO2(110) surface have been investigated. The cluster model has been thoroughly tested by comparison with electronic structure calculations for the periodic slab model. The chemical implications of distorted surface structures arising from the surface dynamics have been investigated by combining the QM embedded-cluster calculations with force-field based MD simulations. Here QM embedded-cluster calculations were performed using surface structures sampled from the MD simulations. This combined MD+QM embedded-cluster procedure was applied to the CO adsorption on MgO(001) at 50 K and the O vacancy formation on CeO2(110) at 300 K. Significant thermal variations of the CO adsorption energy and the O vacancy formation energy were observed. It was found that these variations could be estimated using the force field of the MD simulation as an interaction model. With this approach, the QM results were extrapolated to higher temperature and doped systems.

<p>Katalytisk förbränning är en lovande teknik för användning vid kraftgenerering, särskilt för</p><p>gasturbiner. Genom att använda katalytisk förbränning kan man nå mycket låga emissioner av kväveoxider</p><p>(NOX), kolmonoxid (CO) och oförbrända kolväten (UHC) samtidigt, vilket är svårt vid</p><p>konventionell förbränning. Förutom att man erhåller låga emissioner, kan katalytisk förbränning stabilisera</p><p>förbränningen och kan därmed användas för att uppnå stabil förbränning för gaser med låga</p><p>värmevärden. Denna avhandling behandlar huvudsakligen högtemperaturdelen av den katalytiska</p><p>förbränningskammaren. Kraven på denna del har visat sig svåra att nå. För att den katalytiska förbränningskammaren</p><p>ska kunna göras till ett alternativ till den konventionella, måste katalysatorer</p><p>med bättre stabilitet och aktivitet utvecklas.</p><p>Målet med denna avhandling har varit att utveckla katalysatorer med högre aktivitet och stabilitet,</p><p>lämpliga för högtemperaturdelen av en katalytisk förbränningskammare för förbränning av naturgas.</p><p>En mikroemulsionsbaserad framställningsmetod utvecklades för att undersöka om den kunde ge</p><p>katalysatorer med bättre stabilitet och aktivitet. Bärarmaterial som är kända för sin stabilitet, magnesia</p><p>och hexaaluminat, framställdes med den nya metoden. Mikroemulsionsmetoden användes också</p><p>för att impregnera de framställda materialen med de mer aktiva materialen perovskit (LaMnO3) och</p><p>ceriumdioxid (CeO2). Det visade sig att mikroemulsionsmetoden kan användas för att framställa katalysatorer</p><p>med bättre aktivitet jämfört med de konventionella framställningsmetoderna. Genom att</p><p>använda mikroemulsionen för att lägga på aktiva material på bäraren erhölls också en högre aktivitet</p><p>jämfört med konventionella beläggningsstekniker.</p><p>Eftersom katalysatorerna ska användas under lång tid i förbräningskammaren utfördes också en</p><p>åldringsstudie. Som jämförelse användes en av de mest stabila materialen som rapporterats i litteraturen:</p><p>LMHA (mangan-substituerad lantan-hexaaluminat). Resultaten visade att LMHA deaktiverade</p><p>mycket mer jämfört med flera av katalysatorerna innehållande ceriumdioxid på hexaaluminat som</p><p>framställts med den utvecklade mikroemulsionstekniken.</p> / <p>Catalytic combustion is a promising technology for power applications, especially gas turbines.</p><p>By using catalytic combustion ultra low emissions of nitrogen oxides (NO_X), carbon monoxide (CO)</p><p>and unburned hydrocarbons (UHC) can be reached simultaneously, which is very difficult with conventional</p><p>combustion technologies. Besides achieving low emission levels, catalytic combustion can</p><p>stabilize the combustion and thereby be used to obtain stable combustion with low heating-value</p><p>gases. This thesis is focused on the high temperature part of the catalytic combustor. The level of</p><p>performance demanded on this part has been proven hard to achieve. In order to make the catalytic</p><p>combustor an alternative to the conventional flame combustor, more stable catalysts with higher activity</p><p>have to be developed.</p><p>The objective of this work was to develop catalysts with higher activity and stability, suitable</p><p>for the high-temperature part of a catalytic combustor fueled by natural gas. A microemulsion-based</p><p>preparation method was developed for this purpose in an attempt to increase the stability and activity</p><p>of the catalysts. Supports known for their stability, magnesia and hexaaluminate, were prepared using</p><p>the new method. The microemulsion method was also used to impregnate the prepared material with</p><p>the more active materials perovskite (LaMnO₃) and ceria (CeO₂). It was shown that the microemulsion</p><p>method could be used to prepare catalysts with better activity compared to the conventional</p><p>methods. Furthermore, by using the microemulsion to apply active materials onto the support a</p><p>significantly higher activity was obtained than when using conventional impregnation techniques.</p><p>Since the catalysts will operate in the catalytic combustor for extended periods of time under</p><p>harsh conditions, an aging study was performed. One of the most stable catalysts reported in the</p><p>literature, LMHA (manganese-substituted lanthanum hexaaluminate), was included in the study for</p><p>comparison purposes. The results show that LMHA deactivated much more strongly compared to</p><p>several of the catalysts consisting of ceria supported on lanthanum hexaaluminate prepared by the</p><p>developed microemulsion method.</p>

Although current trends in global warming are of great concern, energy demand is still increasing, resulting in increasing pollutant emissions. To address this issue, we need reliable renewable energy sources, lowered pollutant emissions, and efficient and profitable processes for energy conversion. We also need to improve the use of the energy, produced by existing infrastructure. Consequently, the work presented in this thesis aims at investigating current scientific and technological challenges in energy conversion through biomass gasification and the alternative use of fossil fuels, such as diesel, in the generation of cleaner electricity through auxiliary power units in the transport sector. Production of chemicals, syngas, and renewable fuels is highly dependent on the development and innovation of catalytic processes within these applications. This thesis focuses on the development and optimization of catalytic technologies in these areas. One of the limitations in the commercialization of the biomass gasification technology is the effective catalytic conversion of tars, formed during gasification. Biomass contains high amounts of alkali impurities, which pass on to the producer gas. Therefore, a new material with alkali tolerance is needed. In the scope of this thesis, a new catalyst support, KxWO3 – ZrO2 with high alkali resistance was developed. The dynamic capability of KxWO3 – ZrO2 to store alkali metals in the crystal structure, enhances the capture of alkali metals "in situ". Alkali metals are also important electronic promoters for the active phase, which usually increases the catalysts activity and selectivity for certain products. Experimental results show that conversion of 1-methylnaphathalene over Ni/KxWO3 – ZrO2 increases in the presence of 2 ppm of gas-phase K (Paper I). This support is considered to contribute to the electronic equilibrium within the metal/support interface, when certain amounts of alkali metals are present. The potential use of this support can be extended to applications in which alkali "storage-release" properties are required, i.e. processes with high alkali content in the process flow, to enhance catalyst lifetime and regeneration. In addition, fundamental studies to understand the adsorption geometry of naphthalene with increasing temperature were performed in a single crystal of Ni(111) by STM analyses. Chapter 9 presents preliminary studies on the adsorption geometry of the molecule, as well as DFT calculations of the adsorption energy. In relation to the use of clean energy for transport applications, hydrogen generation through ATR for FC-APUs is presented in Papers II to V. Two promoted RhPt bimetallic catalysts were selected in a previous bench scale study, supported on La2O3:CeO2/d – Al2O3 and MgO : Y2O3/CeO2 – ZrO2. Catalyst evaluation was performed in a fullscale reformer under real operating conditions. Results showed increased catalyst activity after the second monolithic catalyst due to the effect of steam reforming, WGS reaction, and higher catalyst reducibility of the RhxOy species in the CeO2 – ZrO2 mixed oxide, as a result of the improved redox properties. The influence of sulfur and coke formation on diesel reforming was assessed after 40 h on stream. Sulfur poisoning was evaluated for the intrinsic activity related to the total Rh and Pt area observed after exposure to sulfur. Sulfur concentration in the aged catalyst washcoat was observed to decrease in the axial direction of the reformer. Estimations of the amount of sulfur adsorbed were found to be below the theoretical equilibrated coverage on Rh and Pt, thus showing a partial deactivation due to sulfur poisoning. / <p>QC 20150213</p>

The solid oxide fuel cell (SOFC) is a potential high efficient electrochemical device for vehicles, auxiliary power units and large-scale stationary power plants combined heat and power application. The main challenges of this technology for market acceptance are associated with cost and lifetime due to the high temperature (700-1000 oC) operation and complex cell structure, i.e. the conventional membrane electrode assemblies. Therefore, it has become a top R&amp;D goal to develop SOFCs for lower temperatures, preferably below 600 oC. To address those above problems, within the framework of this thesis, two kinds of innovative approaches are adopted. One is developing functional composite materials with desirable electrical properties at the reduced temperature, which results of the research on ceria-based composite based low temperature ceramic fuel cell (LTCFC). The other one is discovering novel energy conversion technology - Single-component/ electrolyte-free fuel cell (EFFC), in which the electrolyte layer of conventional SOFC is physically removed while this device still exhibits the fuel cell function. Thus, the focus of this thesis is then put on the characterization of materials physical and electrochemical properties for those advanced energy conversion applications. The major scientific content and contribution to this challenging field are divided into four aspects except the Introduction, Experiments and Conclusions parts. They are: Continuous developments and optimizations of advanced electrolyte materials, ceria-carbonate composite, for LTCFC. An electrolysis study has been carried out on ceria-carbonate composite based LTCFC with cheap Ni-based electrodes. Both oxygen ion and proton conductance in electrolysis mode are observed. High current outputs have been achieved at the given electrolysis voltage below 600 oC. This study also provides alternative manner for high efficient hydrogen production.  Compatible and high active electrode development for ceria-carbonate composite electrolyte based LTCFC. A symmetrical fuel cell configuration is intentionally employed. The electro-catalytic activities of novel symmetrical transition metal oxide composite electrode toward hydrogen oxidation reaction and oxygen reduction reaction have been experimentally investigated. In addition, the origin of high activity of transition metal oxide composite electrode is studied, which is believed to relate to the hydration effect of the composite oxide. A novel all-nanocomposite fuel cell (ANFC) concept proposal and feasibility demonstration. The ANFC is successfully constructed by Ni/Fe-SDC anode, SDC-carbonate electrolyte and lithiated NiO/ZnO cathode at an extremely low in-situ sintering temperature, 600 oC. The ANFC manifests excellent fuel cell performance (over 550 mWcm-2 at 600 oC) and a good short-term operation as well as thermo-cycling stability. All results demonstrated its feasibility and potential for energy conversion. Fundamental study results on breakthrough research Single-Component/Electrolyte-Free Fuel Cell (EFFC) based on above nanocomposite materials (ion and semi-conductive composite) research activities. This is also the key innovation point of this thesis. Compared with classic three-layer fuel cells, EFFC with an electrolyte layer shows a much simpler but more efficient way for energy conversion. The physical-electrical properties of composite, the effects of cell configuration and parameters on cell performance, materials composition and cell fabrication process optimization, micro electrochemical reaction process and possible working principle were systematically investigated and discussed. Besides, the EFFC, joining solar cell and fuel cell working principle, is suggested to provide a research platform for integrating multi-energy-related device and technology application, such as fuel cell, electrolysis, solar cell and micro-reactor etc. This thesis provides a new methodology for materials and system innovation for the fuel cell community, which is expected to accelerate the wide implementation of this high efficient and green fuel cell technology and open new horizons for other related research fields. / <p>QC 20131122</p>

Made available in DSpace on 2014-12-17T14:06:52Z (GMT). No. of bitstreams: 1 BeatrizC.pdf: 4078508 bytes, checksum: 01a5dec5db97a60acc69b635fdbd40bc (MD5) Previous issue date: 2009-01-04 / The direct use of natural gas makes the Solid Oxide Fuel Cell (SOFC) potentially more competitive with the current energy conversions technologies. The Intermediate Temperature SOFC (IT-SOFC) offer several advantages over the High Temperature SOFC (HT-SOFC), which includes better thermal compatibility among components, fast start with lower energy consumption, manufacture and operation cost reduction. The CeO2 based materials are alternatives to the Yttria Stabilized Zirconia (YSZ) to application in SOFC, as they have higher ionic conductivity and less ohmic losses comparing to YSZ, and they can operate at lower temperatures (500-800?C). Ceria has been doped with a variety of cations, although, the Gd3+ has the ionic radius closest to the ideal one to form solid solution. These electrolytes based in ceria require special electrodes with a higher performance and chemical and termomechanical compatibility. In this work compounds of gadolinia-doped ceria, Ce1-xGdxO2-&#948; (x = 0,1; 0,2 and 0,3), used as electrolytes, were synthesized by polymeric precursors method, Pechini, as well as the composite material NiO - Ce0,9Gd0,1O1,95, used as anode, also attained by oxide mixture method, mixturing the powders of the both phases calcinated already. The materials were characterized by X ray diffraction, dilatometry and scanning electronic microscopy. The refinement of the diffraction data indicated that all the Ce1-xGdxO2-&#948; powders were crystallized in a unique cubic phase with fluorite structure, and the composite synthesized by Pechini method produced smaller crystallite size in comparison with the same material attained by oxide mixture method. All the produced powders had nanometric characteristics. The composite produced by Pechini method has microstructural characteristics that can increase the triple phase boundaries (TPB) in the anode, improving the cell efficiency, as well as reducing the mass transport mechanism effect that provokes anode degradation / A utiliza??o direta do g?s natural torna a c?lula a combust?vel de ?xido s?lido (SOFC) potencialmente mais competitiva com as atuais tecnologias para convers?o de energia. A SOFC de temperatura intermedi?ria (IT-SOFC) oferece muitas vantagens sobre a SOFC de alta temperatura (HT-SOFC), que incluem melhor compatibilidade t?rmica entre os componentes, partida r?pida com menos consumo energ?tico, redu??o de custos de obten??o e opera??o. Os materiais baseados em CeO2 s?o alternativas aos eletr?litos de zirc?nia estabilizada com ?tria (YSZ) para aplica??es em SOFC, pois t?m condutividade i?nica maior e menores perdas ?hmicas em compara??o a YSZ, e podem operar a temperaturas mais baixas (500-800?C). C?ria tem sido dopada com uma variedade de c?tions, entretanto, o Gd3+ possui o raio i?nico mais pr?ximo do ideal para forma??o da solu??o s?lida. Esses eletr?litos baseados em c?rio requerem eletrodos especiais com um alto desempenho e compatibilidade termomec?nica e qu?mica. Neste trabalho compostos c?ria dopada com gadol?nia, Ce1-xGdxO2-&#948; (x = 0,1; 0,2 e 0,3), utilizadas como eletr?litos, foram sintetizados a partir do m?todo dos precursores polim?ricos, Pechini, assim como o material comp?sito NiO - Ce0,9Gd0,1O1,95, usado para anodo, obtido tamb?m pelo m?todo de mistura dos ?xidos, p?s das duas fases j? calcinadas. Os materiais foram caracterizados atrav?s das t?cnicas de difra??o de raios X, dilatometria e microscopia eletr?nica de varredura. O refinamento dos dados obtidos pela difra??o de raios X indicou que todos os p?s de Ce1-xGdxO2-&#948; cristalizaram em uma ?nica fase c?bica com estrutura fluorita, e que o comp?sito obtido por Pechini produziu menores tamanhos de cristalitos das fases em compara??o com o p? sintetizado por mistura de ?xidos em uma mesma temperatura de calcina??o. Todos os p?s obtidos t?m caracter?sticas nanom?tricas. O comp?sito obtido por Pechini possui caracter?sticas microestruturais que podem aumentar a fronteira de fase tripla (TPB) dentro do anodo, melhorando a efici?ncia da c?lula, assim como reduzir o efeito do mecanismo de transporte de massa que provoca degrada??o do anodo

Made available in DSpace on 2014-12-17T14:07:09Z (GMT). No. of bitstreams: 1 AmandaLM_DISSERT.pdf: 3676559 bytes, checksum: 256cb3ce22bbf3b5f114a2ff3021de96 (MD5) Previous issue date: 2013-02-06 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / Fuel cells are electrochemical devices that convert chemical energy into electricity. Due to the development of new materials, fuel cells are emerging as generating clean energy generator. Among the types of fuel cells, categorized according to the electrode type, the solid oxide fuel cells (SOFC) stand out due to be the only device entirely made of solid particles. Beyond that, their operation temperature is relatively high (between 500 and 1000 ?C), allowing them to operate with high efficiency. Another aspect that promotes the use of SOFC over other cells is their ability to operate with different fuels. The CeO2 based materials doped with rare earth (TR+3) may be used as alternatives to traditional NiO-YSZ anodes as they have higher ionic conductivity and smaller ohmic losses compared to YSZ, and can operate at lower temperatures (500-800?C). In the composition of the anode, the concentration of NiO, acting as a catalyst in YSZ provides high electrical conductivity and high electrochemical activity of reactions, providing internal reform in the cell. In this work compounds of NiO - Ce1-xEuxO2-&#948; (x = 0.1, 0.2 and 0.3) were synthesized from polymeric precursor, Pechini, method of combustion and also by microwave-assisted hydrothermal method. The materials were characterized by the techniques of TG, TPR, XRD and FEG-SEM. The refinement of data obtained by X-ray diffraction showed that all powders of NiO - Cex-1EuxO2-&#948; crystallized in a cubic phase with fluorite structure, and also the presence of Ni. Through the characterizations can be proved that all routes of preparation used were effective for producing ceramics with characteristics suitable for application as SOFC anodes, but the microwave-assisted hydrothermal method showed a significant reduction in the average grain size and improved control of the compositions of the phases / C?lulas a combust?vel s?o dispositivos eletroqu?micos que convertem a energia qu?mica em el?trica. Em virtude do desenvolvimento de novos materiais, as c?lulas a combust?vel v?m se destacando como promissores na gera??o de energia de forma limpa. Dentre os tipos de c?lulas a combust?vel, classificadas de acordo com o tipo de eletr?lito, destacam-se as de ?xido s?lido (SOFC), por serem as ?nicas inteiramente constitu?das por s?lidos. Al?m disso, pela sua temperatura de opera??o ser relativamente elevada (entre 500 e 1000 ?C), estas c?lulas operam com alta efici?ncia. Outro aspecto que favorece o emprego de SOFC ? a sua habilidade de operar com diferentes combust?veis, como fontes de hidrog?nio.Os materiais a base de CeO2 dopados com terras raras (TR+3) podem ser utilizados como alternativas aos tradicionais anodos de NiO-YSZ. Al?m de maior condutividade i?nica maior e menores perdas ?hmicas, elas podem operar a temperaturas mais baixas (500- 800?C). Na composi??o do anodo, a concentra??o de NiO, atuando como catalisador confere alta condutividade el?trica e alta atividade eletroqu?mica das rea??es, proporcionando a reforma interna do combust?vel na c?lula. Neste trabalho compostos de NiO - Ce1-xEuxO2-&#948; (x = 0,1; 0,2 e 0,3), foram sintetizados a partir do m?todo dos precursores polim?ricos, Pechini, do m?todo de combust?o e, tamb?m, pelo m?todo hidrotermal assistido por micro-ondas. Os materiais obtidos foram caracterizados atrav?s das t?cnicas de TG, DRX, TPR e MEV-FEG. O refinamento dos dados obtidos pela difra??o de raios X indicou que todos os p?s de NiO - Ce1- xEuxO2-&#948; cristalizaram-se em uma fase c?bica com estrutura fluorita, e tamb?m a presen?a de NiO. Todas as rotas de prepara??o utilizadas mostraram-se eficientes para a produ??o de p?s com caracter?sticas adequadas para aplica??o como anodos de SOFC, por?m o m?todo hidrotermal assistido por micro-ondas apresentou significativa redu??o do tamanho m?dio de gr?os e melhor controle das composi??es das fases

Cerias, appropriately doped with trivalent rare earth ions in particular, can have high oxide ion conductivity and are attractive as both SOFC (solid oxide fuel cell) electrolytes and anodes. Here, four groups of candidate electrolyte materials were synthesised using a low temperature method in order to determine the effect of multiple doping on their microstructure and ionic conductivity. In an initial study, seven compositions of Ce0.8SmxGd[sub]yNd[sub]zO1.9 (where x, y and z = 0.2, 0.1, 0.0667 or 0 and x + y + z = 0.2) were synthesised and the properties of multiply-doped materials were compared with the corresponding singly-doped parent materials. The effect of co-doping with Gd and Sm was investigated in more detail by preparing and studying five compositions of Ce1−2xSmxGdxO2−x (where x = 0.125, 0.1, 0.0875, 0.075 or 0.05) and seven compositions of Ce0.825SmxGd0.175−xO1.9125 (where x = 0.175, 0.14, 0.105, 0.0875, 0.07, 0.035 or 0). The effect of additional doping with a divalent ion- Ca2+- was studied in six compositions of Ce[sub](0.825+y)Sm[sub](0.0875-y)Gd[sub](0.0875-y)Ca[sub]yO1.9125 (where y = 0, 0.00875, 0.0175, 0.02625, 0.035 or 0.04375). The materials were characterised using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry and X-ray diffraction. Crystallite sizes were determined in the powders and relative densities and grain size distributions were obtained in sintered pellets. Total, bulk and grain boundary conductivities were obtained using impedance spectroscopy and corresponding activation energies and enthalpies of ion migration and defect association were calculated. The most promising material for SOFCs operating at intermediate temperatures was found to be Ce0.825Sm0.0875Gd0.0875O1.9125 which had a total conductivity at 600 °C of 2.23 S m−1. Lastly, doped ceria materials, primarily Ce0.8Sm0.2O1.9, were employed as catalytic supports for Pd and PdO nanoparticles and these were investigated as SOFC anode materials.

Made available in DSpace on 2016-06-02T19:56:50Z (GMT). No. of bitstreams: 1 5035.pdf: 2696735 bytes, checksum: 1e1df725e495abb44c7ceab896e17284 (MD5) Previous issue date: 2013-03-12 / Universidade Federal de Sao Carlos / Particle size effect and Ce addition on the catalytic properties of Cu/Al2O3catalysts were investigated for the water gas shift reaction (WGS). The catalysts were prepared by dry impregnation of an aqueous solution of nitrates of the respective metals on alumina, synthesized by sol-gel method. Samples were prepared with 5, 10 and 15% w/w of metallic copper and 12% w/w of CeO2. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR) spectroscopy, X-ray absorption (XAS). The WGS reaction was performed with reagents ratio of H2O:CO = 1:3 with temperature range from 200 to 350° C. The crystallites CuO were not detected by XRD. As the Cu content increased, the crystallite size of CeO2 decreased with a fluorite type structure from 7.4 to 3.4 nm. The results of TPR showed that the interaction Cu-O-Al was crucial to reduce temperature and ceria addition on the catalysts did not affect the temperature reduction of the CuO. The XANES in situ results along the WGS reaction showed that metallic Cu predominated and ceria was partially reduced. EXAFS results showed that the Cu particle size increased from 0.65 to 0.91 nm with an increased load of copper from 5 to 15%, respectively. After the reduction, step prior to reaction, the catalysts were not completely reduced. The degree of reduction increased with the Cu particle size and it was also dependent on the temperature and the oxidation potential of mixing of the reactants. The addition of ceria did not change the degree of reduction of samples Cu/Al2O3. The results suggest that the Cu particles have a reduced Cu core covered with an oxide layer. The catalytic activity increased as the Cu particle size decreased, which can be associated with the presence of the redox couple Cu+/Cu0. This provides a possibility of CO oxidation and its reoxidation due to water activation. The ceria addition also increased catalytic activity and it is probably attributed to activation of the water on the surface of ceria, followed by transfer of oxygen from its structure to the oxidation of CO in an interface Cu-CeO2. / O efeito do tamanho da partícula de Cu e a adição de céria nas propriedades catalíticas dos catalisadores de Cu/Al2O3 foram investigados para a reação de deslocamento gás água (WGS). Os catalisadores foram preparados por impregnação da solução alcoólica dos respectivos nitratos dos metais em alumina, sintetizada pelo método sol-gel. As amostras foram preparadas com teores de Cu de 5, 10 e 15% m/m e 12% m/m de CeO2. Os catalisadores foram caracterizados por difração de raios X (DRX), redução a temperatura programada (TPR) e espectroscopia de absorção de raios X (XAS). A reação de WGS foi realizada com a razão de reagentes H2O:CO = 3:1 em temperaturas entre 200 e 350ºC . Os cristalitos de CuO não foram detectados por DRX. Com o aumento do teor de Cu de 5 para 15% m/m verificou-se um decréscimo no tamanho de cristalitos de CeO2 com uma estrutura do tipo fluorita de 7,4 para 3,4 nm. A interação Cu-O-Al foi determinante na temperatura de redução dos catalisadores e a adição da céria não afetou a temperatura da redução do CuO. Os resultados de XANES in situ mostraram que ao longo da reação de WGS o Cu na forma metálica foi predominante e a céria encontrava-se parcialmente reduzida. Os resultados de EXAFS mostraram que o tamanho das partículas de Cu aumentou de 0,65 para 0,91nm com o aumento do teor do cobre de 5 para 15%, respectivamente. Após a etapa de redução que antecede a reação, os catalisadores não se encontraram completamente reduzidos. O grau de redução aumentou com o tamanho da partícula de Cu e mostrou-se dependente também da temperatura e do potencial de oxidação da mistura dos reagentes. A adição da céria não modificou o grau de redução das amostras de Cu/Al2O3. Tais resultados sugerem que as partículas de cobre apresentam um núcleo reduzido com óxido de cobre na superfície. A atividade catalítica aumentou com a diminuição do tamanho de partícula de Cu, o que pode estar associado à maior presença do par redox Cu+/Cu0 nas menores partículas. Este possivelmente proporciona a oxidação do CO, reduzindo o Cu+ ao Cu0 e a reoxidação ocorre devido à ativação da água. A adição da céria também aumentou a atividade catalítica, a qual foi atribuída provavelmente à ativação da água nas vacâncias de oxigênio da céria, seguida da transferência de oxigênio de sua estrutura para a oxidação do CO em uma interface Cu-CeO2.