Made available in DSpace on 2014-06-11T19:32:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-04-04Bitstream added on 2014-06-13T19:02:31Z : No. of bitstreams: 1 godoi_drm_dr_araiq_prot.pdf: 3455751 bytes, checksum: 7f41251a52df80f90667ad35cc7086b2 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Neste trabalho, as reações de oxidação de metanol e etanol foram estudadas respectivamente sobre nanocatalisadores de Pt-Ru e Pt-Sn suportados em carbono. Os estudos para cada sistema foram realizados com aproximadamente o mesmo tamanho de partícula e mesma composição total e diferentes quantidades das fases liga e óxido. Catalisadores Pt-Sn com diferentes composições também foram estudados. Os catalisadores foram preparados pelos métodos de microemulsão e poliol modificado e as quantidades das fases liga e óxido foram modificadas por tratamentos térmicos em diferentes atmosferas. O crescimento das partículas foi evitado utilizando-se condições brandas de temperatura, de modo que este estudo foi conduzido na ausência dos efeitos de tamanho de partícula. A caracterização das propriedades físicas foi realizada por difração de raios X (DRX), microscopia eletrônica de transmissão (TEM), calorimetria exploratória diferencial (DSC), espectroscopia de fotoelétrons excitados por raios X (XPS) e espectroscopia de absorção de raios X dispersivos (DXAS) in situ. O comportamento eletroquímico geral dos nanocatalisadores foi avaliado por voltametria cíclica em solução ácida e as atividades eletrocatalíticas frente às reações de oxidação de metanol e etanol foram estudadas por varredura linear de potencial e cronoamperometria. Cromatografia líquida de alta eficiência (HPLC) foi utilizada para analisar quantitativamente os produtos de oxidação do metanol sobre os catalisadores Pt-Ru. Os resultados obtidos com os catalisadores Pt-Ru mostram claramente que a presença de espécies oxidadas de Ru é necessária para melhorar a atividade eletrocatalítica frente à oxidação de metanol. A oxidação de CO adsorvido também foi estudada, sendo que ambas as reações mostraram-se bastante... / In this work, methanol and ethanol oxidation were studied on carbon-supported Pt-Ru and Pt- Sn nanocatalysts respectively. For each system, the studies were carried out with nearly the same particle size and identical overall composition, and different amounts of oxide and alloyed phases. For some Pt-Sn catalysts, overall composition was also varied. The catalysts were synthetized by microemulsion and modified poliol methods, and the amounts of alloyed and oxide phases were modified by heat treatments in different atmospheres. Because particle growth was avoided using mild temperature conditions, the study reported here was conducted in the absence of particle size effects. Characterization of physical properties was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and in situ dispersive X-ray absorption spectroscopy (DXAS). The general electrochemical behavior of the nanocatalysts was evaluated by cyclic voltammetry in acidic solution, and the electrocatalytic activities for the oxidation of methanol and ethanol were studied by linear potential sweeps and chronoamperometry. High performance liquid chromatography (HPLC) was used to assess the yields of soluble products of methanol oxidation on the Pt-Ru catalysts. The results obtained for the Pt-Ru catalysts clearly evidence that the presence of oxide species is necessary to enhance the electrocatalytic activity for methanol oxidation. Oxidation of adsorbed CO was also measured. Both reactions, methanol and adsorbed CO oxidation, were found to be very sensitive to the surface changes produced by the heat treatments. Interestingly, the best catalyst for methanol oxidation was not found to be the most efficient for the... (Complete abstract click electronic access below)

Nanohybrid materials have been prepared and examined for their electrocatalytic activity. The nanocomposites have been prepared from carbon nanomaterials (multiwalled carbon nanotubes (MWCNTs) and graphene nanosheets), cadmium selenide quantum dots and metallophthalocyanines (MPcs). The MPcs used in this work are cobalt tetraamino-phthalocyanine (CoTAPc) and tetra (4-(4,6-diaminopyrimidin-2-ylthio) phthalocyaninatocobalt (II)) (CoPyPc). Their activity has also been explored in different forms; polymeric MPcs, iodine doped MPcs and covalently linked MPcs. The premixed drop-dry, sequential drop-dry and electropolymerisation electrode modification techniques were used to prepare nanocomposite catalysts on the glassy carbon electrode (GCE) surface. The sequential drop dry technique for MPc and MWCNTs gave better catalytic responses in terms of limit of detection, catalytic and electron transfer rate constants relative to the premixed. MWCNTs and CdSe-QDs have been used as intercalating agents to reduce restacking of graphene nanosheets during nanocomposite preparation. Voltammetry, chronoamperometry, scanning electrochemical microscopy and electrochemical impedance spectroscopy methods are used for electrochemical characterization modified GCE. X-ray photoelectron spectroscopy, X-ray diffractometry, transmission electron microscopy, scanning electron microscopy, infra-red spectroscopy, Raman spectroscopy were used to explore surface functionalities, morphology and topography of the nanocomposites. Electrocatalytic activity and possible applications of the modified electrodes were tested using oxygen reduction reaction, l-cysteine oxidation and paraquat reduction. Activity of nanocomposites was found superior over individual nanomaterials in these applications.

M.Sc. (Chemistry) / Loading gold nanoparticles (Au NPs) on mesoporous materials via different methods has been reported in the literature. However, the immobilisation of the dendrimer-encapsulated Au NPs on materials is still considered amongst the hot topics in chemistry. This study describes the synthesis, characterisation as well as catalytic evaluation of unsupported and supported Au NPs....

Magister Scientiae - MSc / Characterization of nanophase electrocatalysts, which are an essential part in the direct methanol fuel cell (DMFC) and solid-polymer electrolyte (SPE) electrolyser, have been studied in this work. Their nanoparticulate size raises significant challenges in the analytical techniques used in their structural and chemical characterization. Hence, the applicability of analytical protocols for the qualitative and quantitative characterization of structural and chemical properties of nanophase platinum and platinum-ruthenium electrocatalysts was investigated. Also, fabricated carbon-supported platinum, platinum-ruthenium, iridium oxide, and mesoporous silica-templated platinum electrocatalysts were screened on the basis of their electrocatalytic activity. A set of structural and chemical parameters influencing the performance of nanophase electrocatalysts was identified. Parameters included crystallinity, particle size, particle size distribution, agglomeration, aggregation, surface area, thermal stability, chemical speciation, electrocatalytic activity, and electrochemically-active surface area. A large range of analytical tools were employed in characterizing the electrocatalysts of interest. High accuracy and precision in the quantitative and qualitative structural characterization of nanophase electrocatalysts, collected by x-ray diffractometry and transmission electron microscopy, was demonstrated. Selected-area electron diffraction was limited to a rapid qualitative evaluation of electrocatalyst polycrystallinity and crystal symmetry. Scanning electron microscopy was limited to the qualitative evaluation of the agglomeration state of supported electrocatalysts. High-performance particle sizing was unable to resolve the particle size of the electrocatalyst from that of the support and was therefore employed in the quantitative investigation of aggregate size and size distribution in supported electrocatalysts. The technique produced high precision data illustrating the reproducibility of the aggregate size data. N2-physisorption produced surface area and pore size distribution data of high quality, but was unable to determine surface areas specific to the metal phase in supported electrocatalysts. The technique was deemed inconsistent in the accurate determination of average pore size. The resolution of scanning electrochemical microscopy and proton-induced x-ray emission spectroscopy (SECM) did not allow for an investigation of characteristics at the nanoscale. Quantitative chemical information was difficult to extract from SECM maps and the technique was limited to the qualitative characterization of surface topography. Thermogravimetry was suitable for the qualitative investigation of the thermal stability of the nanophase electrocatalysts of interest. In this study, temperature-programmed reduction was able to qualitatively speciate the surface chemical state and investigate the strength of the metal-support interaction in supported nanophase electrocatalysts. Cyclic voltammetry and linear-sweep voltammetry were employed in the electrochemical characterization of nanophase electrocatalysts and both qualitative and quantitative information were obtained. The techniques were able to discriminate between various commercial and fabricated electrocatalysts and identify new highly-active materials. Preparation variables could be critically evaluated for the fabrication of cost-effective highly-active nanophase electrocatalysts. Certain techniques were deemed to be highly applicable in discriminating between high and low activity nanophase electrocatalysts based on their structural and chemical properties. The electrocatalyst characterization strategy and methodology was developed and will be implemented for future characterization of nanophase electrocatalysts. / South Africa

Magister Scientiae - MSc / The main objective of this work it to produce membrane electrode assemblies (MEAs) that have improved performance over MEAs produced by the conventional manner, by producing highly efficient, electroactive, uniform catalyst layers with lower quantities of platinum electrocatalyst. The catalyst coated membrane (CCM) method was used to prepare the MEAs for the PEM fuel cell as it has been reported that this method of MEA fabrication can improve the performance of PEM fuel cells. The MEAs performances were evaluated using polarisation studies on a single cell. A comparison of polarisation curves between CCM MEAs and MEAs produced in the conventional manner illustrated that CCM MEAs have improved performance at high current densities (>800 mA/cm2). / South Africa

Converting solar energy into useful chemical bonds via photocatalysis is a growing field aimed at addressing global challenges. The research disclosed describes heterogeneous photocatalysis as a nanophotoelectrochemical cell as photocatalysts enable both reduction and oxidation reactions using the local charge separation of photo-excited carriers. Herein, experimental and theoretical results of nanoscale electrolysis of water on the surface of CrOx/Pt/SrTiO3 showed that ohmic losses are negligible when the anode and cathode are within nanometer distances from each other. Additionally, increasing the photocatalytic rate of water splitting by increasing the light intensity demonstrated that pH gradients can still form at the nanoscale. These pH gradients can be minimized by the incorporation of buffers. Typically, photocatalysts decorated with noble-metal nanoparticles can be used for overall water splitting, but generally suffer from low yields due to the water-forming back reaction. The unwanted water-forming back reaction was successfully suppressed by coating Pt nanoparticles on the surface of SrTiO3 with a 2nm CrOx layer that block O2 gas from reaching the surface of the Pt nanoparticle. The back reaction can also be suppressed without the use of a protective layer material by changing the intrinsic nature of the Pt nanoparticle from a metallic state to an oxidized state. The Pt nanoparticles were able to maintain an oxidized state by reducing the particle size below 2 nm. Oxidized Pt particles are less likely to bind to H2, O2, and CO gas, unlike metallic Pt, thereby making it selective for hydrogen generation. Finally, CdS was found to be perform the direct trifluoromethylation of heteroarenes in a single step as opposed to the current multi-step synthetic procedures. The trifluoromethylation of organic compounds is relevant to the field of medicinal chemistry for the synthesis of pharmaceutical drugs. By improving overall water splitting via photocatalysis significantly, artificial photosynthesis may be achieved leading to a solution to the global energy security dilemma. By improving photoredox catalysis of organic compounds via photocatalysis, high value organic compounds (such as pharmaceuticals) can be synthesized more readily under milder conditions.

The utilization of porous materials for electrocatalytic applications has been of high interest due to their high surface area and increase in electrode-electrolyte interface. Metal-organic frameworks (MOFs) are an emerging class of 3-D porous materials consisting of inorganic nodes bound by multidentate organic linkers. MOFs have permanently large surface area, high stability, and the tunability of the structure. The metal source or the organic linker can be swapped to create a material with desirable features. MOFs have been explored for applications in electrocatalysis, conductivity and energy storage. The fundamental charge transfer methods of MOF thin films is discussed and the utilization of these conductive materials for the key reactions in artificial photosynthesis is explored to highlight methods to improve the efficiency of these materials for electrocatalysis. The Morris group has previously shown that charge transfer in MOFs can occur through a redox hopping mechanism in which the charge hops from redox center to redox center through space followed by the movement of a charge balancing ions. In Chapter 2, the charge transfer mechanism within the MOF is further investigated by utilization of spectroelectrochemistry. The incorporation of a redox center, Ru(bpy)2(dcbpy), "RuBPY" where bpy = 2,2′-bipyridine; bpy-(COOH)2 = 5,5′-dicarboxylic acid-2,2′-bipyridine into the UiO-67 framework creates a conductive MOF that is also electrochromic. RuBPY is a deep orange color in the standard RuII state and upon oxidation to RuIII it is pale green. The change in absorption profile of the redox center allows for the rate of oxidation to be determined through absorbance measurements. The material showed minimal change in absorbance upon applying an oxidative potential. The incorporation of a sulfonate group into the backbone of the RuBPY-UiO-67-SO3H MOF allowed for a much higher change in absorbance converting the entire MOF into the oxidized state. The change in level of absorbance indicates that the sulfonate groups improve the conductivity within the pores of the MOF allowing for oxidation of previously electrochemically inaccessible redox centers. The sulfonate groups are thought to break ion pairs of the electrolyte and increase effective electrolyte concentration within the pores. The sulfonate groups' ability to improve the conductivity within the MOF can be further investigated to improve charge transfer through porous materials. The sulfonate groups were again incorporated into the UiO-67 MOF framework for use in electrocatalytic applications by also incorporating the known water oxidation catalyst, RuTPY, Ru(tpy)(dcbpy)H2O. A RuTPY-UiO-67 film had previously shown reactivity as a water oxidation catalyst with improved activity over a monolayer of RuTPY on fluorine-doped tin oxide, FTO. The sulfonate groups were added to create a proton transfer chain that shuttled the generated protons away from the catalytic site to improve reactivity. The incorporation of sulfonate groups again showed improved charge transfer from the MOF materials with the RuTPY-UiO-67-SO3H being 100% electrochemically accessible. The water oxidation capabilities improved giving the material increased oxygen generation upon oxidation of water. The improvement of catalytic activity of RuTPY-UiO-67-SO3H was beyond the increased electrochemical accessibility means the proximal sulfonate groups were aiding in catalysis in some manor. This work highlights the use of multivariate approaches to MOFs to improved efficiency in various applications. The fourth chapter discusses the other half of artificial photosynthesis, CO2 reduction. The known CO2 reduction catalyst, Ni(cyclam), is incorporated into a zirconium-based MOF, VPI-100. The VPI-100 powder was electrochemically deposited onto a glassy carbon electrode and the film was used for electrochemical CO2 reduction into carbon dioxide. The film successfully generated CO as a major product with a faradaic efficiency of 56%. The film was stable under electroreduction conditions and was able to be recycled for continuous production of CO. The final chapter is a review that discusses the utilization of MOFs as photocatalysts for CO2 conversion using only abundant earth metals. While most CO2 catalysts are expensive noble metals, the development of cheap abundant catalytic materials is extremely relevant to a clean energy future. / Doctor of Philosophy / The climate crisis has led to a need to develop more efficient renewable energy sources and novel energy storage devices. Electrochemical systems have shown great promise in providing a clean energy future with developments in lithium-ion batteries, fuel cell technologies, and artificial photosynthesis to produce value added chemicals. The basis of all these electrochemical processes is efficient charge transfer to the necessary components. Metal-organic Frameworks (MOFs) which are crystalline, porous materials made up of inorganic metal nodes connected in a discrete pattern by organic linkers. The three-dimensional nature of the MOFs consisting of connected pore networks has shown great promise to be improved conductive materials and electrocatalysts compared to existing materials. Herein, we exploit the modular nature of the MOFs to incorporate functional groups that improve ion transport and allow catalytic activity for artificial photosynthesis. The work here shows that MOF structures can be further modified to provide even more efficient electrocatalytic behavior to aid in a clean energy future.

Orientador: Hebe de Las Mercedes Villullas / Banca: Estevam Vitorio Spinacé / Banca: Germano Tremiliosi Filho / Banca: Margarida Juri Saeki / Banca: Auro Atsushi Tanaka / Resumo: Neste trabalho, as reações de oxidação de metanol e etanol foram estudadas respectivamente sobre nanocatalisadores de Pt-Ru e Pt-Sn suportados em carbono. Os estudos para cada sistema foram realizados com aproximadamente o mesmo tamanho de partícula e mesma composição total e diferentes quantidades das fases liga e óxido. Catalisadores Pt-Sn com diferentes composições também foram estudados. Os catalisadores foram preparados pelos métodos de microemulsão e poliol modificado e as quantidades das fases liga e óxido foram modificadas por tratamentos térmicos em diferentes atmosferas. O crescimento das partículas foi evitado utilizando-se condições brandas de temperatura, de modo que este estudo foi conduzido na ausência dos efeitos de tamanho de partícula. A caracterização das propriedades físicas foi realizada por difração de raios X (DRX), microscopia eletrônica de transmissão (TEM), calorimetria exploratória diferencial (DSC), espectroscopia de fotoelétrons excitados por raios X (XPS) e espectroscopia de absorção de raios X dispersivos (DXAS) in situ. O comportamento eletroquímico geral dos nanocatalisadores foi avaliado por voltametria cíclica em solução ácida e as atividades eletrocatalíticas frente às reações de oxidação de metanol e etanol foram estudadas por varredura linear de potencial e cronoamperometria. Cromatografia líquida de alta eficiência (HPLC) foi utilizada para analisar quantitativamente os produtos de oxidação do metanol sobre os catalisadores Pt-Ru. Os resultados obtidos com os catalisadores Pt-Ru mostram claramente que a presença de espécies oxidadas de Ru é necessária para melhorar a atividade eletrocatalítica frente à oxidação de metanol. A oxidação de CO adsorvido também foi estudada, sendo que ambas as reações mostraram-se bastante... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In this work, methanol and ethanol oxidation were studied on carbon-supported Pt-Ru and Pt- Sn nanocatalysts respectively. For each system, the studies were carried out with nearly the same particle size and identical overall composition, and different amounts of oxide and alloyed phases. For some Pt-Sn catalysts, overall composition was also varied. The catalysts were synthetized by microemulsion and modified poliol methods, and the amounts of alloyed and oxide phases were modified by heat treatments in different atmospheres. Because particle growth was avoided using mild temperature conditions, the study reported here was conducted in the absence of particle size effects. Characterization of physical properties was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and in situ dispersive X-ray absorption spectroscopy (DXAS). The general electrochemical behavior of the nanocatalysts was evaluated by cyclic voltammetry in acidic solution, and the electrocatalytic activities for the oxidation of methanol and ethanol were studied by linear potential sweeps and chronoamperometry. High performance liquid chromatography (HPLC) was used to assess the yields of soluble products of methanol oxidation on the Pt-Ru catalysts. The results obtained for the Pt-Ru catalysts clearly evidence that the presence of oxide species is necessary to enhance the electrocatalytic activity for methanol oxidation. Oxidation of adsorbed CO was also measured. Both reactions, methanol and adsorbed CO oxidation, were found to be very sensitive to the surface changes produced by the heat treatments. Interestingly, the best catalyst for methanol oxidation was not found to be the most efficient for the... (Complete abstract click electronic access below) / Doutor

Orientador: Hebe de las Mercedes Villullas / Banca: Antonio Carlos Dias Angelo / Banca: Elisabete Inacio Santiago / Resumo: Este trabalho visou contribuir ao desenvolvimento de materiais com bom desempenho para a catálise da reação de redução de oxigênio, que ocorre nos cátodos de células a combustível, através do estudo da influência da composição e da nanoestrutura de nanopartículas de PtNi suportadas em carbono na atividade eletrocatalítica. Os estudos envolveram a preparação de catalisadores contendo nanopartículas de PtNi com diferentes composições pelo método do poliol modificado. Partes destes materiais foram submetidas a tratamento térmico em hidrogênio a 300ºC com o intuito de promover um aumento da incorporação de Ni na rede da Pt sem provocar severas modificações no tamanho e distribuição das nanopartículas sobre o suporte. As propriedades físicas dos materiais foram estudadas pelas técnicas de difração de raios X e microscopia eletrônica de transmissão. Os resultados mostraram que os catalisadores de PtNi têm um elevado grau de liga e boa homogeneidade de distribuição das partículas sobre o carbono suporte. As propriedades eletrônicas foram avaliadas por espectroscopia de absorção de raios X in situ. A atividade eletrocatalítica para a redução do oxigênio foi estudada utilizando-se a técnica de eletrodo de disco-anel rotatório. Em meio ácido, a presença de Ni nos catalisadores promove um aumento da vacância da banda 5d da Pt, ao tempo que o tratamento térmico provoca uma diminuição da vacância em relação aos materiais como preparados. Foi verificada uma maior atividade catalítica dos materiais de PtNi em relação à Pt. Em meio alcalino, os catalisadores que contêm Ni mostraram a banda 5d mais preenchida, provavelmente resultado da formação de óxidos de Ni. De modo geral, os catalisadores PtNi são menos ativos que a Pt em meio alcalino. A tolerância à presença de metanol foi avaliada através de comparação entre as curvas de polarização obtidas em soluções ácida e alcalina com as... / Abstract: This study aimed to contribute to the development of materials with good performance for the catalysis of the oxygen reduction reaction, which occurs at the cathodes of fuel cells, by studying the influence of composition and nanostructure of carbon-supported PtNi nanoparticles on the electrocatalytic activity. The studies involved the preparation of catalysts containing PtNi nanoparticles of different compositions by the modified polyol method. Parts of these materials were submitted to a heat treatment in hydrogen at 300 °C aiming to promote an increase of the incorporation of Ni into the Pt lattice without causing severe changes on the size and distribution of particles on the support. The physical properties of the materials were evaluated by X-ray diffraction and transmission electron microscopy. The results showed that PtNi catalysts have a high degree of alloying and that particle distribution on the carbon support has good homogeneity. The electronic properties were evaluated in situ by X-ray absorption spectroscopy. The electrocatalytic activity for the oxygen reduction reaction was studied by the rotating ring disk technique. In acid medium, the presence of Ni in the catalysts causes an increase in the Pt 5d band, while the heat treatment resulted in a Pt 5d band vacancy decrease. A catalytic activity for PtNi materials higher than that of Pt was verified. In alkaline medium, the catalysts containing Ni showed a Pt 5d band more filled, probably resulting from the formation of Ni oxides. In general, Pt Ni catalysts are less active than Pt in alkaline medium. The tolerance to the presence of methanol was evaluated by comparing the polarization curves obtained in acid and alkaline solutions containing alcohol. A decrease of catalytic activity in the presence of methanol in acid medium and good tolerance in alkaline medium were observed. / Mestre

Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.