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Title: Study of cerium oxide-metal intraction with gases - carbon monoxide oxidation and methanol decomposition Author: Tatiana Kolářová Department: Department of Surface and Plasma Science Supervisor: Doc. RNDr. Václav Nehasil, Dr., Department of Surface and Plasma Science Abstract: Dissertation thesis is focused on the study of two catalytic systems on the basis of cerium oxide-metal. The systems were characterized by various methods of surface analysis. The first part of the work concerns influence of the preparation parameters on the character of gold nanoparticles deposited onto the sputtered cerium oxide layers from a solution. Catalytic activity of prepared Au/CeOx/(C)/Si systems, containing particles with the mean sizes of 4- 10 nm, was tested toward CO oxidation at atmospheric pressure. The results show that with increasing amount of gold present on the samples, also the productivity of CO2 during the reactions grows. The work is further focused on the preparation of an inverse model CeOx/Pt(111) system. On this system, adsorption of methanol at low temeperatures and its decomposition during a heating was studied. It is apparent from presented results that the decomposition of methanol on the inverse CeOx/Pt(111) systems behaves in a different way than on a clean platinum sample or a thick...

It is accepted worldwide that the best final solution for spent nuclear fuel is to bury it in deep geological repositories. Despite the physical and chemical barriers that are supposed to isolate the nuclear waste for at least 100.000 years, some uncertainty factors may cause underground water to get in contact with the nuclear waste. Due to radioactivity and oxidation under air, dissolution experiments using UO2 pellets are difficult and frequently lead to incoherent results. Therefore, to enable a detailed study of the influence of microstructure and surface properties on the stability of spent nuclear fuel over time, it is necessary to produce analogues that closely resemble nuclear fuel in terms of crystallography and microstructure. At the same time, in-depth understanding of dissolution phenomena is crucial to geological processes such as dissolution precipitation creep and solvent mediated phase transformations. My thesis is based in two manuscripts. Paper I reports the microstructures obtained after sintering CaF2 powders at temperatures up to 1240°C. Pellets with microstructure, density and pore structure similar to that of UO2 spent nuclear fuel pellets were obtained in the temperature range between 900°C and 1000°C. Paper II reports how differences of surface chemistry and crystal symmetry, characteristics of each surface orientation, affect the topography of CaF2 pellets described in paper I during dissolution. I propose that every orientation of the fluorite structure can be decomposed in the three reference surfaces {100}, {110} and {111}. The {111} is the most stable surface with a dissolution rate of the top surface of 1,13x10-9 mol.m-2.s-1, and {112} the less stable surface with a dissolution rate 34 times faster that {111}. Surfaces that expose both Ca and F atoms in the same plan dissolve faster, possibly because the calcium is more susceptible to be solvated. The faster dissolving surfaces are replaced by the more stable {111} and {100} surfaces which causes the development of roughness on the top surface and stabilizes the surface on high energy sites; i.e. pores or grain boundaries. The main consequences of these observations are i) the increase of the total surface area; ii) the decrease of the overall surface energy. I present a dissolution model for surfaces of crystal with different surface energies. The main conclusions are: a) dissolution rates calculated from surface area are over estimated to the real dissolution rate; b) dissolution rates are faster at the beginning of dissolution and tend to diminish with time until a minimum value is reached.

Cerium oxide has become an outstanding material in catalytic applications over the last decades. In this thesis, the morphology and atomic structure of thick cerium oxide films and ceria single crystals is investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). The ceria films are prepared by annealing cycles from room temperature up to 1100K in ultra high vacuum (UHV) and in an oxygen atmosphere. The films exhibit large smooth terraces separated predominantly by O-Ce-O triple layer height step edges but in contrast to the ceria single crystals some inhomogeneities are observed on the terraces. By annealing the film at 1020K to 1070K in UHV several intermediate phases can be stabilized ranging from the fully oxidized phase CeO2 to the fully reduced phase Ce2O3. These phases have a unique stoichiometry with regular arranged vacancies in the surface and subsurface as revealed by density functional theory (DFT) calculations. The film can be reoxidized by annealing in an oxygen atmosphere as shown by X-ray spectroscopy (XPS). The annealing in oxygen atmosphere also results in a surface with less inhomogeneities. This makes the ceria films an excellent model system for catalytic applications. Further in this thesis a measurement series exhibiting absorbed water on the film surface is presented and discussed. Also line defects observed on the film and on the single crystal are analyzed.

In this thesis, the morphology and electric potential of pristine and gold covered surfaces with the fluorite structure are investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). The pristine (111) surface of calcium difluride is prepared by cleaving. Cleavege ledges are composed of short alternating type I and type II steps having different atomic structures and polarities. With respect to the stoichiometric terrace, the surface potential is slightly reduced at ledges predominately composed of type I steps, while the potential of ledges predominantly composed of type II steps is significantly higher. Pristine (111) surfaces of ceria single crystals and ceria thick films are prepared by repeated cycles of sputtering and annealing. Annealing bulk samples at 1100 K results in small terraces with rounded ledges and steps with predominantly one O-Ce-O triple layer height while annealing at 1200 K produces well-ordered straight step edges in a hexagonal motif and step bunching. The morphology and topographic details of films are similar, however, films are destroyed upon heating above 1100 K. NC-AFM and KPFM images exhibit uniform and atomically perfect terraces on a single crystal surface while films exhibit significant inhomogeneities even for best possible preparation conditions. Applying X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS), a significant contamination of the bulk ceria sample with fluorine within the first 5 nm below the surface is found while a possible fluorine contamination of films is below the detection limits. After deposition of gold, a surprisingly strong interaction of gold adatoms to the (111) surface of bulk ceria is found. The pinning of individual adatoms to one specific surface site and the stability against sintering for annealing temperatures up to 520 K is discussed considering the fluorine contamination of bulk ceria samples. Additionally nanoscale gold clusters supported on ceria are fully characterized by employing elaborate NC-AFM and KPFM experiments gaining detailed information on their shape, crystalinity, epitaxy and binding to the substrate.

In der vorliegenden Arbeit wird erstmals ein transparentes elektrochromes Display (ECD) höchster Auflösung (720 dpi) beschrieben. Die Entwicklung erforderte grundlegende Arbeiten an den einzelnen Komponenten, insbesondere (i) an der Arbeitselektrode und (ii) an der Gegenelektrode.Die Arbeitselektrode des ECD enthält ein digital festgelegtes elektrochromes Bild, welches mit Hilfe der Ink-Jet-Technik nach verschiedenen Methoden übertragen wurde. Der Druck eines Bildpositivs mit elektrochromer Tinte (Viologen) auf eine mit mesoporösem TiO2 beschichtete Glaselektrode lieferte dabei die besten Ergebnisse. Mit verschiedenen Elektrochromophoren konnten Mehrfarbendrucke erzeugt werden. Als Problem trat dabei auf, daß das Bild in einem ECD nach vier Monaten Lagerung unscharf wird. Die Lösung bestand darin, einen Viologenvorläufer zu drucken, der in einer anschließenden kaskadenartigen Synthese quervernetzt wurde. Zur weiteren Optimierung der Parameter (i) berflächenkonzentration der Viologenzentren, (ii) Stabilität gegenüber polaren Lösemitteln und (iii) Pimerisierungsgrad, wurden erstmals auch kombinatorische Experimente mit einem Ink-Jet Drucker durchgeführt. Ein weiteres Thema dieser Arbeit ist die Entwicklung transparenter Gegenelektroden, welche auf dem Prinzip der Lithiumioneninterkalation in CeO2 basieren. Dazu wurde ein bereits bekannter Typ von CeO2-TiO2-Mischoxidelektroden weiterentwickelt. Ein in dieser Arbeit neu entwickelter Typ einer Gegenelektrode mit verbesserter Kinetik besteht aus einem Zweiphasenoxidgemisch, mit CeO2 auf der inneren Oberfläche einer mesoporösen, antimondotierten SnO2-Schicht (ATO). Aus den Einzelkomponenten wurden verschiedene ECDs zusammengesetzt.

Esta Tesis Doctoral se ha enfocado en el diseño y síntesis de catalizadores de NiO-CeO2 para la metanación de CO2 y el reformado seco de metano, como alternativas para la revalorización de CO2. Concretamente, se ha estudiado el mecanismo de reacción de la metanación de CO2 sobre sistemas catalíticos NiO-CeO2 y se han optimizado los sitios activos implicados en esta reacción. Además, se ha estudiado la influencia de la morfología de los catalizadores NiO-CeO2 en su comportamiento catalítico para la reacción de metanación de CO2 utilizando diferentes tipos de materiales, como nanopartículas, nanopartículas soportadas, 3DOM, catalizadores macroporosos convencionales y xerogeles de carbón. Finalmente, se ha diseñado un catalizador basado en nanopartículas y se ha estudiado sus propiedades catalíticas para la reacción de reformado seco de metano.

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Os materiais nanoestruturados vêm sendo extensivamente estudados, não somente pelas novas propriedades e suas possíveis aplicações tecnológicas, mas também pela busca de uma melhor compreensão dos aspectos físicos e químicos causados por suas reduzidas dimensões. A céria (CeO2) tem despertado grande interesse nas últimas décadas, pois tem papel vital em tecnologias emergentes para aplicações em setores que vão desde a área ambiental através do desenvolvimento de novos catalisadores, passando pela área energética com o desenvolvimento de células de combustível em estado sólido e em novas tecnologias como a spintrônica por meio do desenvolvimento de novos materiais, os óxidos magnéticos diluídos. As propriedades físico-químicas da céria são diretamente dependentes do método de síntese e da morfologia das partículas. Nesse contexto, esse trabalho teve como objetivo realizar a síntese através dos métodos dos precursores poliméricos e hidrotérmico assistido por micro-ondas e a caracterização físico-química de nanopartículas do sistema Ce(1-x)MxO2 (M = Cu, Co). Ensaios catalíticos destes pós foram realizados para verificar sua eficácia como catalisadores na reação de oxidação preferencial de monóxido de carbono, e na produção de hidrogênio através da reforma a vapor de etanol. As amostras foram caracterizadas por difração de raios X, espectroscopia UV-Visível, EPR, Raman e fotoluminescência, BET, microscopia eletrônica de varredura e transmissão e TPR. As amostras sintetizadas pelo método dos precursores poliméricos são constituídas por nanopartículas com tamanho da ordem de 30 nm, enquanto que as amostras sintetizadas pelo método hidrotérmico assistido por micro-ondas são constituídas por nanobastões com tamanho da ordem de 10 nm de diâmetro por 70 nm de comprimento. Dentre as amostras sintetizadas pelo método dos precursores poliméricos, o catalisador Ce0,97Cu0,03O2 foi o que apresentou o melhor resultado na conversão do CO (100%)(CO-PROX), e o catalisador Ce0,80Co0,20O2 apresentou 100% de conversão de etanol. Para as amostras sintetizadas pelo método hidrotérmico assistido por micro-ondas o catalisador Ce0,97Cu0,03O2 foi o que apresentou o melhor resultado na conversão do CO (85%)(CO-PROX), e todos os catalisadores com cobalto apresentaram 100% de conversão de etanol. As caracterizações físico-químicas revelaram que diferentes espécies de cobre/cobalto presentes nos catalisadores, o teor de cobre/cobalto presente e as interações entre o cobre/cobalto e o suporte de CeO2 são os fatores que mais contribuem na eficiência dos catalisadores. / Nanostructured materials have been extensively studied, not only by new properties and their possible technological applications, but also by the search for a better understanding of the physical and chemical aspects caused by its small size. Ceria (CeO2) have attracted great interest in recent decades, because it plays a vital role in emerging technologies for applications in sectors ranging from environmental area through the development of new catalysts, passing by the energy area with the development of solid state fuel cells and in new technologies such as spintronics through the development of new materials, the diluted magnetic oxides. The physico-chemical properties of ceria are directly dependent on the synthesis method and the morphology of the particles. In this context, this study aimed the synthesis via polymeric precursors and microwave-assisted hydrothermal methods and the physico-chemical characterization of nanoparticles from the Ce(1-x)MxO2 (M = Cu, Co) system. Catalytic tests of the powders were carried out to verify its efficiency as catalysts on carbon monoxide preferential oxidation reaction, and hydrogen production by steam reforming of ethanol. The samples were characterized by X-ray diffraction, UV-visible, EPR, Raman and photoluminescence spectroscopies, BET, scanning and transmission electron microscopy and TPR. Samples synthesized by polymeric precursors method consist of nanoparticles with sizes of about 30 nm, while the samples synthesized by microwave-assisted hydrothermal method consist of nanorods with 10 nm average diameter and 70 nm length. Among the samples synthesized by the polymeric precursors method, Ce0,97Cu0,03O2 was the one that presented the best result in the CO conversion (100%)(CO-PROX), and the Ce0,80Co0,20O2 catalyst presented 100% of ethanol conversion. For samples synthesized by microwave-assisted hydrothermal method the Ce0,97Cu0,03O2 catalyst was the one that presented the best result in the CO conversion (85%) (CO-PROX), and all catalysts with cobalt presented 100% of ethanol conversion. The physical-chemical characterizations revealed that different species of copper/cobalt present in the catalysts, the copper/cobalt content and interactions between copper/cobalt and the CeO2 support are the main factors that contribute on the efficiency of the catalysts.

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-08-23T19:44:24Z No. of bitstreams: 1 THIAGO RODRIGO BARBOSA BARROS – DISSERTAÇÃO (PPGEQ) 2017.pdf: 2174476 bytes, checksum: 3268fd475d20f6eff1be072f2e7fa4e3 (MD5) / Made available in DSpace on 2018-08-23T19:44:24Z (GMT). No. of bitstreams: 1 THIAGO RODRIGO BARBOSA BARROS – DISSERTAÇÃO (PPGEQ) 2017.pdf: 2174476 bytes, checksum: 3268fd475d20f6eff1be072f2e7fa4e3 (MD5) Previous issue date: 2017-02-20 / Capes / Biodiesel, que é produzido a partir de óleos vegetais ou gorduras animais através da reação de transesterificação com álcool é um excelente substituto para o diesel de petróleo por ser um combustível biodegradável derivado de fontes renováveis. A peneira molecular MCM-41 apresenta potencial para processamento de ácidos graxos visando a produção de biodiesel, por controle da composição química e por tratamentos químicos posterior a sua síntese. A inserção de óxidos metálicos pode proporcionar um aumento no desempenho catalítico da peneira molecular, possibilitando a preparação de catalisadores com maior atividade, seletividade e estabilidade térmica. O presente trabalho tem como objetivo principal a obtenção de biodiesel a partir da reação de transesterificação do óleo de soja utilizando o catalisador MoO3/25CeO2/MCM-41. Para tal afinidade, a peneira molecular foi preparada partindo-se de um gel com a seguinte composição molar da mistura reacional: 1SiO2 : 0,30 CTABr : 11 NH3 : 144 H2O. A incorporação do cério e molibdênio (em peso) no suporte MCM-41 foi por via seca. Através dos difratogramas foi possível identificar os picos característicos da peneira molecular MCM-41, como também as fases dos óxidos de cério e molibdênio na estrutura da peneira molecular, confirmando os processos de impregnação. Por meio da isoterma de adsorção de nitrogênio a peneira molecular apresentou uma área superficial SBET de 1072 m2/g e isoterma do tipo IV característica de materiais mesoporosos. O catalisador contendo 15% em peso de molibdênio e 25% em peso de cério atingiu a maior conversão em éster metílico em torno de 93% como também densidade de 0.8832 g/cm3, viscosidade cinemática 4,044 mm2/s e acidez de 0,49 mg KOH/g. / Biodiesel, which is produced from vegetable oils or animal fats by transesterification reaction with alcohol, is an excellent substitute for diesel oil being a biodegradable fuel derived from renewable. The molecular sieve MCM-41 has the potential to process fatty acids aiming biodiesel production, by controlling the chemical composition and subsequent chemical treatments to their synthesis. The inclusion of metallic oxides can provide an increase in catalytic performance of the molecular sieve, allowing th e preparation of catalysts with higher activity, selectivity and thermal stability. This study aims to obtain biodiesel from the transesterification reaction of soybean oil using the catalyst MoO3/25CeO2/MCM-41. To this affinity, the molecular sieve was prepared starting with a gel with the following molar composition of the reaction mixture: 1SiO2 : 0,30 CTABr : 11 NH3 : 144 H2O. The incorporation of cerium and molybdenum (by weight) in the MCM-41 was supported by dry. Through the XRD patterns it was possible to identify the peaks characteristic of MCM-41 molecular sieve, as well as phases of cerium and molybdenum oxides in the molecular sieve structure, confirming the impregnation process. By means of the nitrogen adsorption isotherm molecular sieve had a surface area SBET de 1072 m2/g and isotherm type IV characteristic of mesoporous materials. The catalyst containing 15% by weight molybdenum and 25% by weight cerium reached the highest methyl ester conversion around 93% as well as density of 0.8832 g/cm3, kinematic viscosity 4,044 mm2/s and acidity of 0,49 mg KOH/g