Orientador: Miguel Angel Ramírez Gil / Coorientadora: Maria Olívia Berengue / Banca: José Vitor Candido de Souza / Banca: Miguel Adolfo Ponce / Resumo: Este trabalho apresenta e discute o uso do óxido de cério na fabricação de filmes espessos por "screen printing" para aplicações em sensores de gás. Nesse estudo o CeO2 puro foi obtido pelo método dos precursores poliméricos utilizando como resina precursora o citrato de céria. O "puff" - espuma resultante da primeira fase do tratamento térmico da resina- foi calcinado a 550, 600, 700 e 750°C. O pó foi caracterizado por termogravimetria (TG) e as propriedades estruturais, morfológicas foram avaliadas por difratometria de raios X (DRX), espectroscopia Raman, área de superfície por isotermas Brunauer, Emmett e Taller (BET) e microscopia eletrônica de varredura (MEV). A resposta sensora foi estudada em uma câmara de teste construída no Laboratório de Catálise e Superfícies do Instituto de Ciência de Tecnologia de Materiais (INTEMA) da Universidade de Mar del Plata. A TG mostrou a formação de óxido de cério a 550°C, temperatura relativamente baixa quando comparada com outros métodos. Mediante DRX todas as amostras mostraram picos correspondentes à fase pura de CeO2 o qual cristaliza em uma estrutura cúbica do tipo fluorita, entretanto, maiores temperaturas de calcinação mostraram aumento da cristalinidade e tamanho do cristalito. No espectro Raman, um forte pico em torno do 461 cm-1 foi detectado, atribuído às vibrações simétricas do Ce-O. A área de superfície BET dos pós foi de 301, 77 m2/g o que evidencia a formação de partículas muito pequenas e altamente reativas. As microgra... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This paper presents and discusses the use of cerium oxide in the production of thick films for "screen printing" for applications in gas sensors. In this study the pure CeO2 was obtained by the polymeric precursor method using as a precursor resin citrate ceria. The "puff" - resulting foam from the first stage of thermal treatment of the resin-calcined at 550, 600, 700 and 750 °C. The powder was characterized by thermogravimetry (TG) and structural, morphological were evaluated by X-ray diffraction (XRD), Raman spectroscopy, isothermal Brunauer surface area, Emmett and Taller (BET) and scanning electron microscopy (SEM) . The sensor response was studied in a test chamber built in the Laboratory of Catalysis and Surface Materials Technology Institute of Science (INTEMA), University of Mar del Plata. The thermogravimetry showed the formation of cerium oxide at 550° C, relatively low temperature compared with other methods. Upon XRD all samples showed peaks corresponding to pure CeO2 phase which crystallizes in a cubic fluorite type structure, however, higher calcination temperatures showed increased crystallinity and crystallite size. In the Raman spectrum, a strong peak around 461 cm-1 was detected, assigned to symmetric vibrations of the Ce-O. The BET surface area of the powders was 301, 77 m2 /g which shows the formation of very small and highly reactive particles. The SEM micrographs show the presence of different sizes in the form of agglomerates. The characterization of the sensor response showed that the sensors manufactured from pure CeO2 powder has a good response time, achieving better performance at 400 °C working temperature in both reducing and oxidizing atmospheres. The main feature observed was that the results are dependent on previous cycles, the system reproducibility is guaranteed when it deletes the "memory" of the system, exposing it to vacuum / Mestre

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A comparative study of the electro-oxidation of ethanol and methanol was carried out on a Pt/ceria composite electrode prepared by electro-deposition. Modification of the Pt electrode was realized by co-deposition from a 1.0 mM K₂PtCl₆ solution that also contained a 20 mM suspension of ceria. The electro-catalytic activities and stabilities of the Pt/ceria catalyst towards ethanol electro-oxidation reactions (EOR) and methanol electro-oxidation reactions (MOR) were investigated by potentiodynamic and potentiostatic methods in 0.5 M sulfuric acid and 1.0 M sodium hydroxide solutions at various concentrations of ethanol and methanol. The kinetics of ethanol and methanol on a Pt/ceria composite electrode were measured in 0.5 M sulfuric acid and 1.0 M sodium hydroxide solutions using a rotating disk electrode (RDE). Cyclic voltammetry was employed in temperatures ranging from 15 to 55°C to provide quantitative and qualitative information on the kinetics of alcohol oxidation. The temperature dependence of the electro-catalytic activities afforded the determination of apparent activation energies for ethanol and methanol oxidation.

Nanoscale cerium oxide (nanoceria) have shown to possess redox active property , and has been widely studied for potential use in catalysis, chemical-mechanical planarization, biomedical and solid oxide fuel cell (SOFC), etc. The redox state of nanoceria can be tuned by controlling the defects within the lattice and thus its physical and chemical properties. Perfect ceria lattice has fluorite structure and the research in last decade has shown that oxide and mixed oxide systems with pyrochlore and fluorite have better structural stability under high energy radiation. However, the current literature shows a limited number of studies on the effect of high energy radiation on nanoceria. This dissertation aims at understanding the phenomena occurring on irradiation of nanoceria lattice through experiments and atomistic simulation. At first, research was conducted to show the ability to control the defects in nanoceria lattice and understand the effect in tailoring its properties. The defect state of nanoceria was controlled by lower valence state rare earth dopant europium. Extensive materials characterization was done using high resolution transmission electron microscopy (HRTEM), UV-Visible spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to understand the effect of dopant chemistry in modifying the chemical state of nanoceria. The defects originating in the lattice and redox state was quantified with increasing dopant concentration. The photoluminescence property of the control and doped nanoceria were evaluated with respect to its defect state. It was observed that defect plays an important role in modifying the photoluminescence property and that it can be tailored in a wide range to control the optical properties of nanoceria. iv Having seen the importance of defects in controlling the properties of nanoceria, further experiments were conducted to understand the effect of radiation in cerium oxide thin films of different crystallinity. The cerium oxide thin films were synthesized using oxygen plasma assisted molecular beam epitaxy (OPA-MBE) growth. The thin films were exposed to high energy radiation over a wide range of fluence (1013 to 1017 He+ ions/cm3 ). The current literature does not report the radiation effect in nanoceria in this wide range and upto this high fluence. The chemical state of the thin film was studied using in-situ XPS for each dose of radiation. It was found that radiation induced defects within both the ceria thin films and the valence state deviated further towards non-stoichiometry with radiation. The experimental results from cerium oxide thin film irradiation were studied in the light of simulation. Classical molecular dynamics and Monte Carlo simulation were used for designing the model ceria nanoparticle and studying the interaction of the lattice model with radiation. Electronic and nuclear stopping at the end of the range were modeled in ceria lattice using classical molecular dynamics to simulate the effect of radiation. It was seen that displacement damage was the controlling factor in defect production in ceria lattice. The simulation results suggested that nanosized cerium oxide has structural stability under radiation and encounters radiation damage due to the mixed valence states. A portion of the study will focus on observing the lattice stability of cerium with increasing concentration of the lower valence (Ce3+) within the lattice. With this current theoretical understanding of the role of redox state and defects during irradiation, the surfaces and bulk of nanoceria can be tailored for radiation stable structural applications

Devido a estabilidade e capacidade de armazenar oxigênio (oxygen storage capacity) óxidos de cério (CeO2, CeO2 - x, 0 < x < 1/2, Ce2O3) possuem muitas aplicações, tais como catalisadores e suporte para metais de transição (Ni, Pt, Pd, Rh, etc.). A capacidade do átomo de cério alterar reversivelmente seu estado de oxidação entre Ce4+ e Ce3+ é crucial em suas aplicações e pode ser induzida por metais de transição ou outra espécie químicam (átomos, moléculas) depositados sobre suas superfícies. Porém, não se tem conhecimento de estudos da adsorção de metais transição sobre clusters neutros de óxido de cério, nem mesmo estudos de clusters de composição fixa e carga variável. Portanto, neste projeto são estudadas as propriedades físico-químicas dos clusters de (CeO2)6- , (CeO2)6 e (CeO2)6+ e da adsorção dos ametais F e Na e dos metais de transição Ni, Pd e Pt sobre (CeO2)6. Os resultados foram obtidos mediante cálculos ab initio da teoria do funcional da densidade (do inglês - Density Functional Theory, DFT), onde os funcionais PBE e HSE06 foram utilizados para descrição do funcional de troca-correlação. Os resultados apontam divergências dos funcionais PBE e HSE06 na sequência de energia total relativa das 22 configurações representativas de (CeO2)6. As energias de ionização e afinidade eletrônica indicam que as configurações (CeO2)6 de maior energia perdem um elétron mais facilmente que as de menor energia, que acomodam melhor o elétron. O pGMC (do inglês -putavive Global Minimum Configuration) aniônico é derivado do pGMC neutro pela localização do elétron no orbital Ce fz3 e consequnete mudança do estado de oxidação Ce4+ para Ce3+. O pGMC catiônico não é derivado do pGMC neutro, esse cluster não possui ligações Ce-O terminais e o elétron é removido de um orbital com maior contribuição do estado O p. Nas configurações de adsorção apenas ligações iônicas Ce-X ou O-X com maior diferença de eletronegatividade são formadas, nas quais ocorre transferência de carga para o elemento mais eletronegativo. Análises das configurações de menor energia mostram que, ao contrário da adsorção de flúor, os átomos de Na, Ni, Pd e Pt transferem carga para o cluster. Contudo, as configurações Pd/(CeO2)6 e Pt/(CeO2)6 não possuem momento magnético resultante. Apenas nas configurações Na/(CeO2)6 e Ni/(CeO2)6 o elétron se localiza no orbital Ce fz3 com a concomitante mudança do estado de oxidação Ce4+ para Ce3+. Nas configurações de menor energia a adsorção de sódio é favorecida em 0,05 eV em relação a adsorção de flúor. Por sua vez, o níquel é o metal de transição que se adsorve mais fortemente ao cluster, ao passo que o paládio apresenta a menor energia de adsorção. / Due to the stability and oxygen storage capacity, cerium oxides (CeO2, CeO2 - x, 0 < x < 1/2 , Ce2O3) have many applications, such as catalysts and support for transition metals (Ni, Pt, Pd, Rh, etc.). The ability of the cerium atom to reversibly change its oxidation state between Ce4+ and Ce3+ is crucial in its applications and can be induced by transition metals or another chemistry species (atoms, molecules) deposited on their surfaces. However, no studies of the transition metals adsorption on cerium oxide neutral clusters, nor studies of clusters of fixed composition and variable charge are known. In this project, the physicochemical properties of the (CeO2)6- , (CeO2)6 and CeO2)6+ clusters and the adsorption of the F and Na ametals and Ni, Pd and Pt transition metals on (CeO2)6.The results were obtained by ab initio calculations of the density functional theory (DFT), where the PBE and HSE06 functionals were employed for the exchange-correlation functional. The results point out divergences of the PBE and HSE06 functional in the 22 representative (CeO2)6 configurations relative total energy sequence. Ionization and electron affinity energies indicate that the higher energy (CeO2)6 configurations lose an electron more easily than the lower energy ones, which better accommodate the electron. The anionic pGMC (putavive Global Minimum Configuration) is derived from the neutral pGMC by the location of the electron in the Ce fz3 orbital and consequent oxidation state change from Ce4+ to Ce3+. The cationic pGMC is not derived from the neutral pGMC, this cluster has no terminal Ce-O bonds and the electron is removed from an orbital with the highest contribution of the Op state. In the adsorption configurations only ionic bonds Ce-X or O-X with the greatest difference in electronegativity are formed, in which charge transfer occurs to the most electronegative element. Unlike fluorine adsorption, lower energy configurations analyses show that Na, Ni, Pd e Pt atoms transfer charge to the cluster. However, the Pd/(CeO2)6 and Pt/(CeO2)6 configurations have no resulting magnetic moment. Only in the Na/(CeO2)6 and Ni/(CeO2)6 configurations the electron is located in the Ce fz3 orbital with the concomitant oxidation state change from Ce4+ to Ce3+. In the lower energy configurations the sodium adsorption is favored in 0,05 eV in relation to the fluorine adsorption. In turn, nickel is the transition metal that most strongly adsorbs on cluster, while palladium has the lowest adsorption energy.

Ho Chun Man. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Abstracts in English and Chinese. / ABSTRACT --- p.iv / DECLARATION --- p.vi / ACKNOWLEDGEMENT --- p.vii / TABLE OF CONTENTS --- p.viii / LIST OF TABLES --- p.xi / LIST OF FIGURES --- p.xii / Chapter Chapter One: --- Introduction --- p.1 / Chapter 1.1 --- Overview --- p.1 / Chapter 1.2 --- Fundamental of CeO2 --- p.2 / Chapter 1.3.1 --- Synthesis and Modification of Ceria-based Materials --- p.5 / Chapter 1.3.1 --- Synthetic Method --- p.5 / Chapter 1.3.2 --- "Mesoporous Structure of Ce02, CexZr1-x02" --- p.6 / Chapter 1.3.3 --- Doped Ce02 Materials --- p.6 / Chapter 1.3.4 --- Fabrication of Ceria and Cerium-based Nanoparticles --- p.7 / Chapter 1.4 --- Scope of work --- p.8 / Chapter 1.5 --- References --- p.11 / Chapter Chapter Two: --- Meso- and Macro-porous Pd/CexZr1-x02 as Carbon Monoxide Oxidation Catalysts --- p.16 / Chapter 2.1 --- Introduction --- p.16 / Chapter 2.2 --- Experimental Section --- p.18 / Chapter 2.2.1 --- Sample Preparation - Synthesis of the Catalyst Support --- p.18 / Chapter 2.2.2 --- Addition of Pd to the Catalyst Support --- p.19 / Chapter 2.2.3 --- Characterization --- p.20 / Chapter 2.2.4 --- Carbon monoxide oxidation measurement --- p.21 / Chapter 2.3 --- Results and Discussion --- p.22 / Chapter 2.3.1 --- XRD analysis --- p.22 / Chapter 2.3.2 --- SEM and TEM --- p.25 / Chapter 2.3.3 --- N2-Soprtion --- p.32 / Chapter 2.3.4 --- X-ray Photoelectron Spectroscopy --- p.40 / Chapter 2.3.5 --- Thermal Catalysis Study --- p.45 / Chapter 2.4 --- Conclusion --- p.52 / Chapter 2.5 --- References --- p.54 / Chapter Chapter Three: --- Morphology-Controllable Synthesis of Ce02 Nano and Meso-structures --- p.60 / Chapter 3.1 --- Introduction --- p.60 / Chapter 3.2 --- Experimental Section --- p.62 / Chapter 3.2.1 --- Materials and Experimental Conditions --- p.62 / Chapter 3.2.2 --- Characterization --- p.64 / Chapter 3.3 --- Results and Discussion --- p.67 / Chapter 3.3.1 --- SEM and TEM Analysis --- p.67 / Chapter 3.3.2 --- XRD Analysis --- p.75 / Chapter 3.3.3 --- N2-Soprtion --- p.78 / Chapter 3.3.4 --- X-ray Photoelectron Spectroscopy --- p.84 / Chapter 3.3.5 --- FT-IR Analysis --- p.87 / Chapter 3.3.6 --- GC-MS Analysis --- p.89 / Chapter 3.3.7 --- Proposed Formation of Ce02 nanospheres and their transformation to microrods --- p.95 / Chapter 3.3.8 --- UV absorption spectra and band gap energies --- p.97 / Chapter 3.3.9 --- Thermal Catalysis Study --- p.100 / Chapter 3.4 --- Conclusion --- p.103 / Chapter 3.5 --- References --- p.105 / Chapter Chapter Four: --- Conclusion --- p.110 / LIST OF PUBLICATIONS --- p.112

A new reactor and a novel in-situ sampling technique were developed for the study of the synthesis of Ce02 powders produced from dissolved cerium nitrate salts. The reactor minimized particle recirculation and provided a highly symmetric and undisturbed plasma flow suitable for the analysis of the phenomena affecting the formation of Ce02 powders. The sampling probe provided in-situ sampling of in-flight CeCb particles and allowed continuous sampling without cross contamination. The sampled particles were collected using a wet collection system composed of a mist atomizer acting as a scrubber and a custom-made spray chamber. The entire collection system is portable and it was tested in the particle range of 20 nm to 100 jam. This information provided a picture of how Ce02 particles were formed. A numerical simulation of different plasma operating parameters using Fluent was presented. A comprehensive droplet-to-particle formation mechanism was deduced based on calorimetry. thermodynamics of Ce02 formation, numerical simulations and collected particles. The effect of adding water soluble fuels (alanine and glycine) to the original cerium nitrate solutions was investigated. Fuel addition decreased the temperature of CeC>2 formation by acting as a local heat source as a result of fuel auto-ignition. The addition of fuel caused “particle size discrimination’*, and a single mode particle size distribution centered between 50-140 nm was achieved along the centerline of the reactor. [...] / Un nouveau reacteur et une technique innovatrice d’echantillonnage in situ furent developpes pour etudier la synthese de poudres de Ce02 produites a partir de sels de nitrate de cerium dissous. Le reacteur minimise la recirculation des particules et fournit un plasma non perturbe et grandement symetrique approprie pour l’analyse des phenomenes affectant la formation de poudres de Ce02. De plus, une sonde permet un echantillonnage in situ et en vol des particules de Ce02 et ce, en continu et sans contamination croisee. Les particules ainsi captees sont recueillies grace a un systeme de collecte par voie humide qui est compose d’un atomiseur de brume (mist atomizer) qui joue le role d’un recureur (scrubber) et d’une chambre d’atomisation (spray chamber) maison. Le systeme d’echantillonnage est mobile et fut teste sur des particules ayant des tailles de 20 nm a 100 jam. C’est grace a l’analyse des particules ainsi recueillies que nous sommes en mesure de comprendre comment les particules de Ce02 sont formees. Nous presentons aussi une simulation numerique, effectuee avec le logiciel Fluent, qui utilise les differents parametres d’experimentation. Le mecanisme detaille de la formation des particules a partir de gouttelettes fut deduit grace a des etudes calorimetriques, a une etude thermodynamique de la formation du Ce02, a des simulations numeriques et a l’analyse des particules recueillies. Nous avons aussi etudie l’effet d’un ajout de combustibles hydrosolubles (l’alanine et la glycine) a la solution originale de nitrate de cerium. Cette addition de combustibles diminue la temperature de formation du Ce02 en agissant comme une source locale de chaleur resultant de 1’auto-ignition du combustible. Aussi, le combustible cause une « segregation des particules » selon leur taille.

A new reactor and a novel in-situ sampling technique were developed for the study of the synthesis of CeO2 powders produced from dissolved cerium nitrate salts. The reactor minimized particle recirculation and provided a highly symmetric and undisturbed plasma flow suitable for the analysis of the phenomena affecting the formation of CeO2 powders. The sampling probe provided in-situ sampling of in-flight CeO2 particles and allowed continuous sampling without cross contamination. The sampled particles were collected using a wet collection system composed of a mist atomizer acting as a scrubber and a custom-made spray chamber. The entire collection system is portable and it was tested in the particle range of 20 nm to 100 mum. This information provided a picture of how CeO2 particles were formed. A numerical simulation of different plasma operating parameters using Fluent was presented. A comprehensive droplet-to-particle formation mechanism was deduced based on calorimetry, thermodynamics of CeO2 formation, numerical simulations and collected particles. The effect of adding water soluble fuels (alanine and glycine) to the original cerium nitrate solutions was investigated. Fuel addition decreased the temperature of CeO2 formation by acting as a local heat source as a result of fuel auto-ignition. The addition of fuel caused "particle size discrimination", and a single mode particle size distribution centered between 50-140 nm was achieved along the centerline of the reactor. / Also, heat and mass transfer effects were numerically investigated in evaporating solution droplets (20-40 mum in diameter) containing dissolved hexahydrated cerium nitrate in a stationary rf Ar-O2 thermal plasma. This model was developed to study the evaporation of a solution droplet surrounded by a porous crust in a stagnant rf Ar-O2 thermal plasma under reduced pressure. It considered a three phase system: a liquid core of dissolved Ce(NO 3)3.6H2O in water, a dry porous crust of homogeneously precipitated spherical crystals of equal size, and an Ar-O2 plasma under reduced pressure. The impact of different plasma operating parameters on the temperature and dissolved solid content profiles in the droplet was studied, i.e. surrounding plasma temperature, initial salt content and droplet size, plasma gas composition, and system pressure. Temperature and composition dependant thermophysical properties were used. The model was solved in a moving boundary frame using an ALE approach and considering Stefan flow. It provided the necessary information to understand the droplet to particle transformation steps in regions where in-flight probing was unfeasible, i.e. torch zone.

Thesis (Ph.D.)--Tufts University, 2004. / Submitted to the Dept. of Chemical and Biological Engineering. Includes bibliographical references.

Thesis (M.S.)--Mississippi State University. Dave C. Swalm School of Chemical Engineering. / Title from title screen. Includes bibliographical references.