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The targeted design of new catalyst materials can only be accomplished once a fundamental understanding of the interactions between material surfaces and adsorbed molecules is developed. In situ infrared spectroscopy and mass spectrometry methods were employed to probe interactions at the gas-surface interface of oxide-supported metal nanoparticle materials. High vacuum conditions allowed for systematic investigations to describe detailed reaction mechanisms. Specifically, variable temperature infrared spectroscopy was utilized to uncover the binding energetics of CO to the oxide surface of TiO2-based materials. As binding energetics are related to the electronic structure of the adsorption site, differences in evaluated binding enthalpies are hypothesized to probe electronic metal-support interactions that describe charge transfer between the supported metal nanoparticles and TiO2. Cu/TiO2 aerogels were identified as a candidate for more in-depth studies. Flow reactor methods in combination with the surface-based infrared spectroscopy were utilized to elucidate the CO oxidation reaction mechanism over Cu/TiO2 aerogels. Bridging oxygen atoms on TiO2 regions of the material were identified as the active site for catalysis in a Cu-assisted Mars-van Krevelen lattice extraction mechanism. Methanol oxidation was then studied with similar methods to show the complete conversion to CO2 and H2O at high temperatures through the reduction of titania and formation of a formate intermediate. Higher-order carbonaceous alcohols were probed for adsorption and reactivity on Cu/TiO2 aerogels and were observed to follow a similar reaction pathway. The higher-order alcohols, however, were shown to undergo a partial oxidation pathway in the absence of gaseous O2 that is hypothesized to originate from enhanced binding to Cu sites. The decomposition of the chemical warfare agent simulant dimethyl chlorophosphate was also investigated. A hydrolysis pathway to form the significantly less toxic molecule CH3Cl was observed, highlighting the unique promotional effects and chemistry on Cu/TiO2 aerogels. The results presented exemplify both the influence of electronic metal-support interactions on catalysis and the versatile reactivity of Cu/TiO2 aerogels. / Doctor of Philosophy / Interactions between small gaseous molecules and material surfaces have very important implications for applications regarding the environment, industry, and military/public safety. The mechanisms in which gases interact with a solid surface can determine how the material can be functionally used as catalysts. Scientists and engineers start to build a fundamental understanding of what makes a catalyst successful for different applications by understanding the location and strength of interactions. A catalyst's surface acts to lower activation barriers and provide low-energy pathways for interacting molecules to chemically change, by breaking bonds for molecular decomposition and/or forming new bonds. The vibrations of chemical bonds that break and form on surfaces are probed with infrared spectroscopy at the gas-surface interface to study molecular adsorption and reactivity. In addition, a flow cell reactor is used to characterize reaction progress and identify products in real-time. A class of reactive nanoparticulate materials is utilized as a model system on which to study various chemical reactions for important applications including small molecule oxidation for industrial detoxification and clean energy applications, as well as the decomposition of chemical warfare agents. Reaction mechanisms for the oxidation of carbon monoxide and alcohols were elucidated through the utilization of the methods described above. In addition, the decomposition of a chemical warfare agent simulant is characterized. The discoveries and understanding of important chemical properties presented in this dissertation will aid in the synthesis of effective next-generation catalyst materials.

The polymer electrolyte fuel cell converts the chemical energy in a fuel, e.g. hydrogen or methanol, and oxygen into electrical energy. The high efficiency and the possibility to use fuel from renewable sources make them attractive as energy converters in future sustainable energy systems. Great progress has been made in the development of the PEFC during the last decade, but still improved lifetime as well as lowered cost is needed before a broad commercialization can be considered. The electrodes play an important role in this since the cost of platinum used as catalyst constitutes a large part of the total cost for the fuel cell. A large part of the degradation in performance can also be related to the degradation of the porous electrode and a decreased electrochemically active Pt surface. In this thesis, different fuel cell reactions, catalysts and support materials are investigated with the aim to investigate the possibility to improve the activity, stability and utilisation of platinum in the fuel cell electrodes. An exchange current density, i0, of 770 mA cm-2Pt was determined for the hydrogen oxidation reaction in the fuel cell with the model electrodes. This is higher than previously found in literature and implies that the kinetic losses on the anode are very small. The anode loading could therefore be reduced without imposing too high potential losses if good mass transport of hydrogen is ensured. It was also shown that the electrochemically active surface area, activity and stability of the electrode can be affected by the support material. An increased activity was observed at higher potentials for Pt deposited on tungsten oxide, which was related to the postponed oxide formation for Pt on WOx. An improved stability was seen for Pt deposited on tungsten oxide and on iridium oxide. A better Pt stability was also observed for Pt on a low surface non-graphitised support compared to a high surface graphitised support. Pt deposited on titanium and tungsten oxide, displayed an enhanced electrochemically active surface area in the cyclic voltammograms, which was explained by the good proton conductivity of the metal oxides. CO-stripping was shown to provide the most reliable measure of the electrochemically active surface area of the electrode in the fuel cell. It was also shown to be a useful tool in characterization of the degradation of the electrodes. In the study of oxidation of small organic compounds, the reaction was shown to be affected by the off transport of reactants and by the addition of chloride impurities. Pt and PtRu were affected differently, which enabled extraction of information about the reaction mechanisms and rate determining steps. The polymer electrolyte fuel cell converts the chemical energy in a fuel, e.g. hydrogen or methanol, and oxygen into electrical energy. The high efficiency and the possibility to use fuel from renewable sources make them attractive as energy converters in future sustainable energy systems. Great progress has been made in the development of the PEFC during the last decade, but still improved lifetime as well as lowered cost is needed before a broad commercialization can be considered. The electrodes play an important role in this since the cost of platinum used as catalyst constitutes a large part of the total cost for the fuel cell. A large part of the degradation in performance can also be related to the degradation of the porous electrode and a decreased electrochemically active Pt surface. In this thesis, different fuel cell reactions, catalysts and support materials are investigated with the aim to investigate the possibility to improve the activity, stability and utilisation of platinum in the fuel cell electrodes. An exchange current density, i0, of 770 mA cm-2Pt was determined for the hydrogen oxidation reaction in the fuel cell with the model electrodes. This is higher than previously found in literature and implies that the kinetic losses on the anode are very small. The anode loading could therefore be reduced without imposing too high potential losses if good mass transport of hydrogen is ensured. It was also shown that the electrochemically active surface area, activity and stability of the electrode can be affected by the support material. An increased activity was observed at higher potentials for Pt deposited on tungsten oxide, which was related to the postponed oxide formation for Pt on WOx. An improved stability was seen for Pt deposited on tungsten oxide and on iridium oxide. A better Pt stability was also observed for Pt on a low surface non-graphitised support compared to a high surface graphitised support. Pt deposited on titanium and tungsten oxide, displayed an enhanced electrochemically active surface area in the cyclic voltammograms, which was explained by the good proton conductivity of the metal oxides. CO-stripping was shown to provide the most reliable measure of the electrochemically active surface area of the electrode in the fuel cell. It was also shown to be a useful tool in characterization of the degradation of the electrodes. In the study of oxidation of small organic compounds, the reaction was shown to be affected by the off transport of reactants and by the addition of chloride impurities. Pt and PtRu were affected differently, which enabled extraction of information about the reaction mechanisms and rate determining steps. / Polymerelektrolytbränslecellen omvandlar den kemiska energin i ett bränsle, exv. vätgas eller metanol, och syrgas  till elektrisk energi. Den höga verkningsgraden samt möjligheten att använda bränsle från förnyelsebara källor gör dem attraktiva som energiomvandlare i framtida hållbara energisystem. En enorm utveckling har skett under det senaste årtiondet men för att kunna introducera polymerelektrolytbränslecellen på marknaden i en större skala måste livstiden öka och kostnaden minska. Elektroderna har en central del i detta då den platina som används som katalysator står för en stor del av kostnaden för bränslecellen. En stor del av prestandaförsämringen med tiden hos bränslecellen kan också relateras till en degradering av den porösa elektroden och en minskad elektrokemiskt aktiv platinayta. I denna avhandling studeras olika bränslecellsreaktioner samt olika katalysatorer och supportmaterial med målet att undersöka möjligheten att förbättra platinakatalysatorns aktivitet, stabilitet och utnyttjandegrad i bränslecellselektroder. Utbytesströmtätheten, i0, för vätgasoxidationen i bränslecell bestämdes till 770 mA cm-2Pt genom försök med modellelektroderna. Denna var högre än vad som framkommit tidigare i litteratur, vilket visar att de kinetiska förlusterna på anoden är mycket små. Katalysatormängden på anoden borde därför kunna minskas utan några större potentialförluster så länge masstransporten av vätgas är tillräcklig. Den elektrokemiskt aktiva ytan, aktiviteten och stabiliteten hos elektroden visade sig kunna påverkas av supportmaterialet. Platina deponerad på volfram oxid hade en högre aktivitet vid höga potentialer vilket relaterades till den förskjutna oxidbildningen på ytan. Elektroder med platina på volframoxid och iridiumoxid var mer stabila än elektroder med platina på kol. Det var även platina på ett icke grafitiserat kol med låg yta jämfört med platina på grafitiserade kol med en hög yta. Platina på metalloxidskikt av volfram och titan visade en högre elektrokemiskt aktiv yta i de cykliska voltamogrammen än platina på kol, vilket förklarades med att båda metalloxiderna har en bra protonledningsförmåga. CO-stripping gav det säkraste måttet på den elektrokemiskt aktiva ytan i en elektrod i bränslecell. CO-stripping visade sig även vara användbart för karaktärisering av degraderingen av en elektrod. Oxidationen av små organiska föreningar påverkades av borttransporten av intermediärer samt av kloridföroreningar. Pt aoch PtRu påverkades olika vilket gjorde det möjligt att få fram information om reaktionsmekanismer och hastighetsbestämmande steg. / QC 20101014

To limit the environmental impact from fuel based transport of goods and mobility, the knowledge-based improvement of the hydrothermal long-term stability of Pd exhaust catalysts is demanded. Thus, the present study evaluates the performance of hydrothermally aged Pd/gamma-Al2O3 powder catalysts on lean CO oxidation (60-200°C) in the presence of H2O, and discusses structure-activity correlations. A decline in the number of Pd surface sites is quantified with growing aging temperature (650-950°C), and correlates with the decrease in the CO oxidation activity in transient state. However, unexpected shifts in the activation energy of the steady state CO oxidation are observed for the different aging temperatures and total time on stream. The characterization of the active Pd sites of the aged and non-aged catalysts before and after the CO oxidation indicate that the oxidation state of Pd changes during the lean and wet CO oxidation. The observed trends of the oxidation state of Pd correlate with the determined variations of the activation energy, successfully elucidating structure sensitivity of the CO oxidation at temperatures of 750°C or higher, and with changing total time on stream.:1 Introduction and aims 2 Fundamentals 2.1 Emissions from combustion engines 2.2 Diesel exhaust after-treatment systems 2.3 Pd catalysts for CO oxidation 2.3.1 Characterization and CO oxidation activity of Pd catalysts 2.3.2 Mechanism of the CO oxidation on Pd catalysts 2.3.3 Influence of H2O on the CO oxidation on Pd catalysts 2.3.4 Structure sensitivity of the CO oxidation on Pd catalysts 2.4 Thermal and hydrothermal aging of supported Pd catalysts 2.4.1 Sintering and phase transformations of γ-Al2O3 2.4.2 Sintering of Pd in supported catalysts 2.4.3 Kinetic modelling of the CO oxidation on aged Pd catalysts 3 Materials and methods 37 3.1 Preparation of the Pd/γ-Al2O3 catalyst 3.2 Hydrothermal aging of the catalyst 3.3 Performance of the catalytic CO oxidation 3.3.1 Temperature-programmed CO oxidation 3.3.2 Steady state CO oxidation 3.4 Characterization of the catalysts 3.4.1 N2 physisorption 3.4.2 Temperature-programmed desorption of H2 3.4.3 Scanning transmission electron microscopy 3.4.4 X-ray diffraction 3.4.5 X-ray photoelectron spectroscopy 4 Results and discussion 4.1 Transient CO oxidation activity of the non-aged and aged Pd/γ-Al2O3 catalysts 12 Contents 4.2 Quantification of the active sites of the non-aged and aged Pd/γ-Al2O3 catalysts 4.2.1 Number of Pd surface sites 4.2.2 Pd particle sizes 4.2.3 Pd crystallite sizes 4.2.4 Content of surface Pd 4.2.5 Comparative discussion of the quantification of the active sites 4.3 Investigation of the aging mechanism of the Pd/γ-Al2O3 catalysts 4.4 Steady state CO oxidation kinetics of the non-aged and aged Pd/γ-Al2O3 catalysts 4.4.1 Activation energy of the steady state CO oxidation and discussion of structure sensitivity 4.4.2 Dispersion of Pd after the steady state CO oxidation 4.4.3 Oxidation state of Pd after the steady state CO oxidation 4.4.4 Comparative discussion of the transient and steady state CO oxidation activity 4.5 Summarizing discussion of the structure-activity correlations of the nonaged and aged Pd/γ-Al2O3 catalysts for CO oxidation 5 Conclusions and outlook Bibliography List of Figures List of Tables A Annexes

Esse tese experimental versa sobre reações eletroquímicas que ocorrem sobre estado termodinâmico afastado do equilíbrio. O sistema químico escolhido é o mais fundamental em eletrocatálise, a oxidação de monóxipdo de carbono (CO), cujo mecanismo fundamental envolve a remoção do CO por uma etapa Langmuir-Hinshelwood. A cinética complexa da oxidação do CO é um sistema biestável e corresponde a assinatura característica da oxidação do CO-bulk em um voltamograma cíclico. Se contanto, uma reação paralela é adicionada, por exemplo a oxidação de hidrogênio ou equivalentemente a adsorção/dessorção de íons cloreto, tal reação paralela quebra o vínculo conservativo entre os sítios livres e as respectivas coberturas de CO e espécie oxigenadas; o que permite o surgimento de oscilações autossustentáveis no tempo como resultado da passagem pela bifurcação de Hopf. Nesse sentido, o sistema H2/CO torna-se de crucial interessante pois mimetiza o oscilador mais corriqueiro em eletroquímica, o HNDR (acrônimo que traduzido para português lê \"Resistência Diferencial Negativa Escondida\"). Essa questão mecanística foi esclarecida nos experimentos com o sistema fundamental de três eletrodos onde foi testado diferentes geometrias e as variáveis mecanísticas essenciais reveladas. Nessa tese, há um segundo aspecto da dinâmica oscilatória da oxidação do H2/CO que consiste em tratar o mecanismo oscilatório sobre uma superfície espacialmente estendida, e nesse intuito foi escolhido usar uma célula a combustível (CaC), que é, em essência, um reator eletroquímico com grande área superficial. A cinética complexa da oxidação do H2/CO em uma CaC, apresentou transições dinâmicas de p1 → p2 → aperiodics. Suspeita-se de duas rotas para o caos: rota Feigenbaum (dobramento de período) e sobreposição de diferentes regiões de MMOs. O espaço de fase da dinâmica obtidas na CaC apresentou uma ordem quanto a distribuição dos períodos e amplitudes de uma séries caótica que permite predizer o comportamento a um curto alcance, e é uma prova indubitável de caos determinístico. A presença de caos foi diretamente atribuído à presença de pelo menos uma variável espacial. Apesar de nenhuma medida espacial ter sido realizada, inferências sobre acoplamentos espaciais são discutidos com base em argumentos lógicos, e sugere-se que exista acoplamento pela fase gasosa porém é improvável haver acoplamento elétrico exceto entre o anodo e o cátodo. A energia de ativação da CaC oscilante revelou que a condução protônica é a etapa determinante do período de oscilação. Diretamente indica que a variação espacial da condutividade da membrana pode ser considerado como uma variável espacial. Por fim, o terceiro aspecto dessa tese refere-se ao cálculo de eficiência para o estado oscilatório, experimental e teórico. Experimentalmente, o balanco energético de um sistema oscilante indicou uma maior eficiência que o respectivo estado estacionário. Teoricamente, a produção de entropia no ponto de bifurcação de Hopf deve ser maior que o respectivo estado estacionário devido a uma defasagem entre a força e potencial termodinâmico. Uma boa prova de que um estado oscilatório é um sistema dissipativo por promover uma conversão mais eficiente entre dois estados energéticos. / This thesis deals with an experimental electrochemical reactions that occur on thermodynamic state far from equilibrium. The chemical system selected is the most fundamental in electrocatalysis, the carbon monoxide (CO) oxidation, whose fundamental mechanism involves the removal of CO by a Langmuir-Hinshelwood step. The kinetics of the complex oxidation of CO is a bistable system and corresponds to the characteristic signature of CO-bulk oxidation in a cyclic voltammogram. If a parallel reaction is added, for instance the hydrogen oxidation or the chloride ion adsorption / desorption, this parallel reaction breaks the conservative link between the free sites and the coverage of both CO and oxygen species; which allows the appearance of self-sustaining oscillations in time as a result of passage through the Hopf bifurcation. In this sense, the system H2/CO becomes crucially interesting because it mimics the oscillator more commonplace in electrochemistry, the HNDR (\"Hidden Negative Differential Resistance\"). This mechanistic issue was cleared mainly in the experiments with the fundamental system in which different geometries were tested thus reavealing the essential mechanistic variables. In this thesis, there is a second aspect of the oscillatory oxidation of H2/CO which consists in treating the oscillating mechanism of a surface spatially extended, and to this end a fuel cell (FC), which is essentially an electrochemical reactor with a large surface area, was chosen to be used. The kinetics of the complex oxidation of H2/CO in a FC, showed dynamic transitions p1 → p2 → aperiodics. Two routes to chaos are suspected: Feigenbaum route (folding time) and different overlapping regions of MMOs. The FC phase space presented an order regarding the distribution of periods and amplitudes of a chaotic series that allows predicting the behavior in a short range, and it is an undoubtable proof of deterministic chaos. The presence of chaos was directly attributed to the presence of at least one spatial variable. Although no spatial measurement has been performed, inferences about spatial couplings are discussed based on logical arguments and suggests that there is coupling through the gas phase and whereas a electrical coupling between the anode and cathode is the unique with relevance with respect to the coupling by electrical field. The activation energy of the oscillating FC revealed that the proton conduction is a decisive step of the oscillation period. This directly indicates that the spatial variation of the conductivity of the membrane can be considered as a spacial variable. Finally, the third aspect of this thesis refers to the efficiency calculation from both experimental and theoretical points of view. Experimentally, the energy balance of an oscillating system indicated a higher efficiency than the repective steady state. Theoretically, the entropy production at the point of Hopf bifurcation must be greater than the steady state due to a lag between the thermodynamic force and potential. This is a good proof that an oscillatory state is a dissipative system becouse it promote a more efficient conversion between two energetic states.

Neste trabalho são apresentados resultados dos estudos da reação de oxidação de hidrogênio puro realizados em eletrodos rotatório em camada ultrafina porosa e na presença de CO em células a combustível utilizando catalisadores formados por Pt dispersas em substratos de óxidos, tais como, Pt/RuO 2 -C, Pt/RhO 2 -C, Pt/WO 3 -C e em Pt/WC-C. As reações foram também estudadas em suporte puro, como, RhO 2 /C e WC/C em diferentes proporções atômicas. Este estudo teve como finalidade estudar as propriedades catalíticas destes materiais visando elucidar os mecanismos da reação de oxidação de hidrogênio (ROH) sobre estes catalisadores dispersos, possibilitando a obtenção de parâmetros cinéticos das reações. Em conjunto com as medidas eletroquímicas, foram realizados estudos sobre as propriedades eletrônicas e estruturais destes catalisadores, o que possibilita relacionar suas propriedades eletrônicas e estruturais com a cinética da HOR. Observou-se que a presença dos óxidos de Ru, Rh e WO 3 favorecem a diminuição do grau de recobrimento da Pt por CO, deixando mais sítios disponíveis a ROH, em consequência do esvaziamento da banda 5d da Pt, o que diminui a retro-doação de elétrons da Pt ao CO, diminuindo a força da ligação Pt-CO. Como observado, essas modificação eletrônicas observadas nos espectros de XANES não induziram a nenhuma mudança perceptível na cinética ou no mecanismo reacional. Para quase todos os catalisadores, a tolerância ao CO pode ser explicada tanto em termos do mecanismo eletrônico como do bifuncional evidenciado pela formação de CO 2 nas medidas de EMS. / This work shows results of studies of the hydrogen oxidation reaction (HOR) pure in rotation in ultra porous layer and in the presence of CO in the fuel cell to the electrocatalysts of Pt alloy dispersed on oxide substrates such Pt/RuO 2 -C, Pt/RhO 2 -C, Pt/WO 3 -C, Pt/WC-C and pure materials, as RhO 2 /C and WC. The study of the catalytic properties of these materials to elucidate the mechanisms of hydrogen oxidation reaction on these dispersed catalysts, allowing the collection of kinetic parameters of reactions. Together with the electrochemical measurements were carried out studies on the structural and electronic properties of these catalysts, which allow relating their structural and electronic properties with the kinetics of the HOR. It was observed that the presence of oxides of Ru, Rh and WO 3 encourage the reduction of the degree of coating of Pt by CO, leaving more sites available to ROH, as a consequence of emptying of the Pt 5d band, which reduces the backdonation of electrons from Pt to CO by reducing the strength of Pt-CO binding. As noted, these changes observed in electronic spectra of XANES not led to any perceptible change in the kinetics or the reaction mechanism. For almost all catalysts, the CO tolerance can be explained in terms of the electronic effect and the bifunctional mechanism evidenced by the formation of CO 2 in the EMS.

Submitted by Aelson Maciera (aelsoncm@terra.com.br) on 2017-05-19T19:35:37Z No. of bitstreams: 1 DissARLM.pdf: 2999284 bytes, checksum: cb32a2cd7ed71b40fd67f2b5604ce434 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-23T19:48:47Z (GMT) No. of bitstreams: 1 DissARLM.pdf: 2999284 bytes, checksum: cb32a2cd7ed71b40fd67f2b5604ce434 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-23T19:48:57Z (GMT) No. of bitstreams: 1 DissARLM.pdf: 2999284 bytes, checksum: cb32a2cd7ed71b40fd67f2b5604ce434 (MD5) / Made available in DSpace on 2017-05-23T19:58:53Z (GMT). No. of bitstreams: 1 DissARLM.pdf: 2999284 bytes, checksum: cb32a2cd7ed71b40fd67f2b5604ce434 (MD5) Previous issue date: 2016-03-29 / Outra / The CO preferential oxidation reaction (PROX) is used in the purification of hydrogen streams for various applications. Since the discovery of improved catalytic properties of gold particles, at nanometer range, in the CO oxidation reaction at low temperature, Au catalysts have been the subject of numerous investigations of CO oxidation and PROX. Concerning the heterogeneous catalysis, the catalyst activity is determined by several factors: the nanoparticles size, the metal loading and dispersion, the nature and porosity of the support, and the metal/support interaction. To understand the dependence of the support and the influence of the synthesizing method on the material properties and catalytic performances, gold nanoparticles supported on three different oxides (CeO2, MnO2 and TiO2) were successfully synthesized by the deposition precipitation (DP) method, using urea, and modified polyol (MP) method, using PVP as surfactant and sodium borohydride as reducing agent. Additionally, Au/SiO2 was obtained by the DP method. Characterizations were performed to assess the crystallinity, the metal mass percentage, the reduction temperatures and the respective reducing gas consumptions of pure oxides and supported materials synthesized by both methods. In addition, microscopic pictures were obtained to evaluate the materials morphology and the gold nanoparticles diameter. The Au catalysts synthesized by the DP method presented smaller particle sizes as compared to catalysts obtained by the MP method. Catalytic evaluation of the CO oxidation and PROX reactions were performed using temperature ramp starting at room temperature up to 200 °C. As a comparative parameter, the ratio between the catalyst mass and the gas feed flow remained constant at 1 mg / 1 mL / min for all reactions. The results showed that Au supported on TiO2 prepared by the DP method presented complete CO conversion at room temperature, but with the simultaneous supply of H2, this catalyst tends to oxidize H2, decreasing the CO conversion activity. On the other hand, the Au/CeO2 catalyst prepared by the DP method presented activity up to about 200 °C for PROX and better selectivity towards the CO2 formation. / A reação de oxidação preferencial de CO (PROX) é utilizada na purificação de correntes de hidrogênio para diversas aplicações. Desde a descoberta da melhoria das propriedades catalíticas de partículas de ouro, em escala nanométrica, na reação de oxidação de CO a baixa temperatura, catalisadores de Au têm sido objeto de numerosas investigações em oxidação de CO e de PROX. No que diz respeito a catálise heterogênea, a atividade do catalisador é determinada por vários fatores: o tamanho das nanopartículas, a carga e dispersão metálica, a natureza e a porosidade do suporte, e a interação metal/suporte. Para entender a dependência do suporte e a influência do método de síntese nas propriedades dos materiais e nos desempenhos catalíticos, foram sintetizados com sucesso nanopartículas de ouro suportadas em 3 diferentes óxidos (CeO2, MnO2 e TiO2) pelos métodos de deposição precipitação (DP) com ureia e poliol modificado (PM) utilizando PVP como agente surfactante e borohidreto de sódio como agente redutor. Adionalmente, Au/SiO2 foi obtido pelo método DP. Foram realizadas caracterizações para avaliar a cristalinidade, porcentual mássico de metal, temperaturas de redução e respectivos consumos de gás redutor dos óxidos puros e dos materiais suportados sintetizados por ambos os métodos. Além disso, foram obtidas imagens de microscopia para avaliar a morfologia dos materiais e o diâmetro das nanopartículas de ouro. Os catalisadores de Au sintetizados pelo método DP apresentaram menores tamanhos de partícula, comparados aos catalisadores obtidos pelo método PM. A avaliação catalítica nas reações de oxidação de CO e PROX foram realizadas através de rampa de temperatura, começando pela temperatura ambiente e aquecendo até cerca de 200 ºC. Como parâmetro comparativo, a razão entre a massa de catalisador e a vazão dos gases alimentados manteve-se constante em 1 mg / 1 mL/min em todas as reações. Os resultados mostraram que o catalisador de Au suportado em TiO2 obtido pelo método DP apresentou conversão total de CO em temperatura ambiente, porém com a alimentação simultânea de H2, esse catalisador tende a oxidar o H2, diminuindo a atividade à conversão de CO. Por outro lado, o catalisador de Au/CeO2 preparado pelo método DP apresentou atividade até cerca de 200 ºC na PROX e melhor seletividade à formação de CO2. / PRH-ANP/MCT N° 44

Dans ce travail, les nanoparticles d’or supportées ont été étudiées en raison de leur fort potentiel dans les domaines de la catalyse. Des sphères de silice obtenues par la voie Stöber ont été synthétisées et utilisées comme supports des nanoparticules d’or. Les précurseurs d’or sont des solutions colloïdales possédant des nanoparticules d’or homogènes et de petite taille (environ 3 nm). Les matériaux préparés avec ou sans addition d’un nouvel élément ont été utilisés dans les réactions de l’oxydation du CO et une réaction inédite pour l’or: l’oxydation sélective de l’H2S.Les résultats indiquent que l’utilisation de colloïdes d’or comme précurseurs permet de contrôler la taille et la dispersion avant le dépôt et ceci est facilité par les propriétés optiques des nanoparticules de ce métal. Pour les propriétés catalytiques, outre la dispersion, on confirme très clairement par cette méthode de synthèse que l’interaction Au/Support joue un rôle majeur. L’addition d’un second métal augmente significativement l’activité des matériaux à base d’or. Les particules d’or et de Cu se rassemblent sous les conditions réactionnelles testées. Dans le cas de l’oxydation du CO, de petites quantités d’espèce d’or partiellement oxydée sont générées et forment une glue avec les espèces Cu2+ également formées au cours de la réaction et ceci permet de fixer les nanoparticules sur le support. Dans le cas de l’oxydation sélective de H2S, il a été montré que le système est résistant au soufre, y compris sur une longue durée de test. Les silices Stöber se sont avérées comme des supports catalytiques très intéressants car non poreux et formés de particules de petites tailles, ils sont aptes à exposer les phases actives de l’or aux réactifs, diminuant la pression de surface et facilitant le transfert thermique. Il a également été prouvé que ce matériau peut être mis en compétition avec les matériaux mésoporeux, ces derniers étant jugés indispensables pour obtenir de hautes activités en catalyse hétérogènes jusque là. / Supported gold nanoparticles have been focused on due to their high efficiency in the catalysis field. In this work, Stöber silica consisted by globules were synthesized and used as support for gold nanoparticles. The gold colloids with homogeneous and small nanoparticles (about 3 nm) were applied as gold precursor. The prepared materials with further metal addition were used to active the CO oxidation and another very special reaction- selective oxidation of H2S.The results suggested that by using the gold colloid as precursor the sizes of nanoparticles could be controlled before loading onto silica under the help of its optical properties. The results for catalytic reactions indicated that the interaction between Au-support is another essential factor impacting the activity beyond the size effect. The additions of metal largely enhance the activity of gold materials. The gold and copper (oxide) particles go through reassembling during the reactions. In the CO oxidation, small amounts of partially oxidized gold species were generated and “glued” strongly by the new formed Cu2+ species to the support. While in the selective H2S oxidation, it was found that the material is sulfur-resistant during long time reaction. The Stöber silica is proved to be a very promising support, which is non-porous but beneficial to expose active sites of gold nanoparticles to the reactants, lower the pressure drop, and facilitate the heat transformation. It was proved even can be matched with the well-know mesoporous materials that is claimed to be indispensable for the high activity.

Neste trabalho são apresentados resultados dos estudos da reação de oxidação de hidrogênio puro realizados em eletrodos rotatório em camada ultrafina porosa e na presença de CO em células a combustível utilizando catalisadores formados por Pt dispersas em substratos de óxidos, tais como, Pt/RuO 2 -C, Pt/RhO 2 -C, Pt/WO 3 -C e em Pt/WC-C. As reações foram também estudadas em suporte puro, como, RhO 2 /C e WC/C em diferentes proporções atômicas. Este estudo teve como finalidade estudar as propriedades catalíticas destes materiais visando elucidar os mecanismos da reação de oxidação de hidrogênio (ROH) sobre estes catalisadores dispersos, possibilitando a obtenção de parâmetros cinéticos das reações. Em conjunto com as medidas eletroquímicas, foram realizados estudos sobre as propriedades eletrônicas e estruturais destes catalisadores, o que possibilita relacionar suas propriedades eletrônicas e estruturais com a cinética da HOR. Observou-se que a presença dos óxidos de Ru, Rh e WO 3 favorecem a diminuição do grau de recobrimento da Pt por CO, deixando mais sítios disponíveis a ROH, em consequência do esvaziamento da banda 5d da Pt, o que diminui a retro-doação de elétrons da Pt ao CO, diminuindo a força da ligação Pt-CO. Como observado, essas modificação eletrônicas observadas nos espectros de XANES não induziram a nenhuma mudança perceptível na cinética ou no mecanismo reacional. Para quase todos os catalisadores, a tolerância ao CO pode ser explicada tanto em termos do mecanismo eletrônico como do bifuncional evidenciado pela formação de CO 2 nas medidas de EMS. / This work shows results of studies of the hydrogen oxidation reaction (HOR) pure in rotation in ultra porous layer and in the presence of CO in the fuel cell to the electrocatalysts of Pt alloy dispersed on oxide substrates such Pt/RuO 2 -C, Pt/RhO 2 -C, Pt/WO 3 -C, Pt/WC-C and pure materials, as RhO 2 /C and WC. The study of the catalytic properties of these materials to elucidate the mechanisms of hydrogen oxidation reaction on these dispersed catalysts, allowing the collection of kinetic parameters of reactions. Together with the electrochemical measurements were carried out studies on the structural and electronic properties of these catalysts, which allow relating their structural and electronic properties with the kinetics of the HOR. It was observed that the presence of oxides of Ru, Rh and WO 3 encourage the reduction of the degree of coating of Pt by CO, leaving more sites available to ROH, as a consequence of emptying of the Pt 5d band, which reduces the backdonation of electrons from Pt to CO by reducing the strength of Pt-CO binding. As noted, these changes observed in electronic spectra of XANES not led to any perceptible change in the kinetics or the reaction mechanism. For almost all catalysts, the CO tolerance can be explained in terms of the electronic effect and the bifunctional mechanism evidenced by the formation of CO 2 in the EMS.

Esse tese experimental versa sobre reações eletroquímicas que ocorrem sobre estado termodinâmico afastado do equilíbrio. O sistema químico escolhido é o mais fundamental em eletrocatálise, a oxidação de monóxipdo de carbono (CO), cujo mecanismo fundamental envolve a remoção do CO por uma etapa Langmuir-Hinshelwood. A cinética complexa da oxidação do CO é um sistema biestável e corresponde a assinatura característica da oxidação do CO-bulk em um voltamograma cíclico. Se contanto, uma reação paralela é adicionada, por exemplo a oxidação de hidrogênio ou equivalentemente a adsorção/dessorção de íons cloreto, tal reação paralela quebra o vínculo conservativo entre os sítios livres e as respectivas coberturas de CO e espécie oxigenadas; o que permite o surgimento de oscilações autossustentáveis no tempo como resultado da passagem pela bifurcação de Hopf. Nesse sentido, o sistema H2/CO torna-se de crucial interessante pois mimetiza o oscilador mais corriqueiro em eletroquímica, o HNDR (acrônimo que traduzido para português lê \"Resistência Diferencial Negativa Escondida\"). Essa questão mecanística foi esclarecida nos experimentos com o sistema fundamental de três eletrodos onde foi testado diferentes geometrias e as variáveis mecanísticas essenciais reveladas. Nessa tese, há um segundo aspecto da dinâmica oscilatória da oxidação do H2/CO que consiste em tratar o mecanismo oscilatório sobre uma superfície espacialmente estendida, e nesse intuito foi escolhido usar uma célula a combustível (CaC), que é, em essência, um reator eletroquímico com grande área superficial. A cinética complexa da oxidação do H2/CO em uma CaC, apresentou transições dinâmicas de p1 → p2 → aperiodics. Suspeita-se de duas rotas para o caos: rota Feigenbaum (dobramento de período) e sobreposição de diferentes regiões de MMOs. O espaço de fase da dinâmica obtidas na CaC apresentou uma ordem quanto a distribuição dos períodos e amplitudes de uma séries caótica que permite predizer o comportamento a um curto alcance, e é uma prova indubitável de caos determinístico. A presença de caos foi diretamente atribuído à presença de pelo menos uma variável espacial. Apesar de nenhuma medida espacial ter sido realizada, inferências sobre acoplamentos espaciais são discutidos com base em argumentos lógicos, e sugere-se que exista acoplamento pela fase gasosa porém é improvável haver acoplamento elétrico exceto entre o anodo e o cátodo. A energia de ativação da CaC oscilante revelou que a condução protônica é a etapa determinante do período de oscilação. Diretamente indica que a variação espacial da condutividade da membrana pode ser considerado como uma variável espacial. Por fim, o terceiro aspecto dessa tese refere-se ao cálculo de eficiência para o estado oscilatório, experimental e teórico. Experimentalmente, o balanco energético de um sistema oscilante indicou uma maior eficiência que o respectivo estado estacionário. Teoricamente, a produção de entropia no ponto de bifurcação de Hopf deve ser maior que o respectivo estado estacionário devido a uma defasagem entre a força e potencial termodinâmico. Uma boa prova de que um estado oscilatório é um sistema dissipativo por promover uma conversão mais eficiente entre dois estados energéticos. / This thesis deals with an experimental electrochemical reactions that occur on thermodynamic state far from equilibrium. The chemical system selected is the most fundamental in electrocatalysis, the carbon monoxide (CO) oxidation, whose fundamental mechanism involves the removal of CO by a Langmuir-Hinshelwood step. The kinetics of the complex oxidation of CO is a bistable system and corresponds to the characteristic signature of CO-bulk oxidation in a cyclic voltammogram. If a parallel reaction is added, for instance the hydrogen oxidation or the chloride ion adsorption / desorption, this parallel reaction breaks the conservative link between the free sites and the coverage of both CO and oxygen species; which allows the appearance of self-sustaining oscillations in time as a result of passage through the Hopf bifurcation. In this sense, the system H2/CO becomes crucially interesting because it mimics the oscillator more commonplace in electrochemistry, the HNDR (\"Hidden Negative Differential Resistance\"). This mechanistic issue was cleared mainly in the experiments with the fundamental system in which different geometries were tested thus reavealing the essential mechanistic variables. In this thesis, there is a second aspect of the oscillatory oxidation of H2/CO which consists in treating the oscillating mechanism of a surface spatially extended, and to this end a fuel cell (FC), which is essentially an electrochemical reactor with a large surface area, was chosen to be used. The kinetics of the complex oxidation of H2/CO in a FC, showed dynamic transitions p1 → p2 → aperiodics. Two routes to chaos are suspected: Feigenbaum route (folding time) and different overlapping regions of MMOs. The FC phase space presented an order regarding the distribution of periods and amplitudes of a chaotic series that allows predicting the behavior in a short range, and it is an undoubtable proof of deterministic chaos. The presence of chaos was directly attributed to the presence of at least one spatial variable. Although no spatial measurement has been performed, inferences about spatial couplings are discussed based on logical arguments and suggests that there is coupling through the gas phase and whereas a electrical coupling between the anode and cathode is the unique with relevance with respect to the coupling by electrical field. The activation energy of the oscillating FC revealed that the proton conduction is a decisive step of the oscillation period. This directly indicates that the spatial variation of the conductivity of the membrane can be considered as a spacial variable. Finally, the third aspect of this thesis refers to the efficiency calculation from both experimental and theoretical points of view. Experimentally, the energy balance of an oscillating system indicated a higher efficiency than the repective steady state. Theoretically, the entropy production at the point of Hopf bifurcation must be greater than the steady state due to a lag between the thermodynamic force and potential. This is a good proof that an oscillatory state is a dissipative system becouse it promote a more efficient conversion between two energetic states.