Direct methanol fuel cells (DMFC) are being investigated as a portable energy conversion device for military and commercial applications. DMFCs offer the potential to efficiently extract electricity from a dense liquid fuel. However, improvements in materials properties and lowering the cost of the electrocatalysts used in a DMFC are necessary for commercialization of the technology. The cathode electrocatalyst is a critical issue in DMFC because the state-of-the-art catalyst, platinum, is very expensive and rare, and its performance is diminished by methanol that crosses over from the anode to the cathode through the Nafion membrane. This thesis investigates the addition of platinum to a palladium-cobalt nanoalloy electrocatalyst supported on carbon black in order to improve catalyst activity for the oxygen reduction reaction (ORR) and catalyst stability against dissolution in acidic environment without significantly reducing the methanol-tolerance of the catalyst. Platinum was added to the palladium-cobalt nanoalloy catalyst using two synthesis methods. In the first method, platinum was directly alloyed with palladium and cobalt using a polyol reduction method, followed by heat treatment in a reducing atmosphere to form catalysts with 11 and 22 atom % platinum. In the second method, platinum was added to a palladium-cobalt alloy by galvanic displacement reaction to form catalysts with 10 and 22 atom % platinum. The palladium cobalt alloy was synthesized using a polyol method, followed by heat treatment in a reducing atmosphere to alloy the nanoparticles before the Pt displacement. It was found that both methods significantly improve catalyst activity and stability, with the displaced catalysts showing a higher activity than the corresponding alloy catalyst. However the alloy catalysts showed similar resistance to dissolution as the displaced catalysts, and the alloyed catalysts were more tolerant to methanol. The displaced catalyst with 22 atom % platinum (8 wt. % Pt overall) performed similar to a 20 wt. % commercial platinum catalyst in both RDE and single cell DMFC tests. The 10 and 22 atom % Pt displaced catalysts and 22 atom % Pt alloyed all showed higher Pt mass specific activities than a commercial Pt catalyst. / text

One of the most important challenges in low-temperature fuel cell technology is improving the catalytic efficiency at the electrode-catalyst where the oxygen reduction reaction (ORR) occurs. Platinum is the best pure catalyst for this reaction but its high cost and scarcity hinder the commercial implementation of fuel cells in automobiles. Pt-based alloys are promising alternatives to substitute platinum while maintaining the efficiency and life-time of the pure catalyst. However, the acid medium and the oxidation of the surface reduce the activity and durability of the alloy catalyst through changes in its local composition and structure. Molecular simulation techniques are applied to characterize the thermodynamics and dynamic evolution of the surface of platinum-based alloy catalysts under reaction conditions.1-10 A simulation scheme of the surface oxidation is proposed which combines classical molecular dynamics (MD) and density functional theory (DFT). This approach is able to reproduce the main features of the oxidation phenomena observed experimentally, it is concluded that the dissolution mechanism of metal atoms involves: 1) Surface segregation of alloy atoms, 2) oxygen absorption into the subsurface of the catalyst, and 3) metal detachment through the interaction with ions in the solvent. Therefore, to improve the durability of platinum-based alloy catalysts, the steps of the dissolution mechanism must be prevented. A versatile 3-D kinetic Monte Carlo (KMC) code is developed to study the degradation and dealloying in nanocatalysts. The results on the degradation of Pt nanoparticles under different potential regimes demonstrate that the dissolution depends on the potential path to which the nanocatalyst is exposed. Metal atoms detach from the boundaries of (111) facets expecting a reduction in the activity of the nanoparticle. Also, the formation of Pt hollow nanoparticles by the Kirkendall effect is addressed, the role of vacancies is crucial in the removal of the non-noble core that yields to hollow nanoparticles. To investigate the reasons for the experimentally found enhanced ORR activity in porous/hollow nanoparticles, the effect of subsurface vacancies on the main ORR activity descriptors is studied with DFT. It is found that an optimum amount of vacancies may enhance the ORR activity of Pt-monolayer catalysts over certain alloy cores by changing the binding energies of O and OH.

<p>Catalysts play an essential role in nearly every chemical production process. Platinum supported on high surface area carbon substrates (Pt/C) is one of the promising candidates as an electrocatalyst in low temperature polymer electrolyte fuel cells. Developing the activity of the Pt/C catalyst with narrow Pt particle size distribution and good dispersion has been a main concern in current research.</p> <p><br /> In this study, the main objective was the development and characterization of inexpensive and effective nanophase Pt/C electrocatalysts. A set of modified Pt/C electrocatalysts with high electrochemical activity and low loading of noble metal was prepared by the impregnation-reduction method in this research. The four home-made catalysts synthesized by different treatments conditions were characterized by several techniques such as EDS, TEM, XRD, AAS, TGA, BET and CV.</p> <p><br /> Pt electrocatalysts supported on acid treatment Vulcan XC-72 electrocatalysts were produced successfully. The results showed that Pt particle sizes of Pt/C (PrOH)x catalysts between 2.45 and 2.81nm were obtained with homogeneous dispersion, which were more uniform than the commercial Pt/C (JM) catalyst. In the electrochemical activity tests, ORR was confirmed as a structure-sensitive reaction. The Pt/C (PrOH/pH2.5) showed promising results during chemically-active surface area investigation, which compared well with that of the commercial standard Johnson Matthey Pt/C catalyst. The active surface area of Pt/C (PrOH/pH2.5) at 17.98m2/g, was higher than that of the commercial catalyst (17.22 m2/g ) under the conditions applied. In a CV electrochemical activity test of Pt/C catalysts using a Fe2+/Fe3+ mediator system study, Pt/C (PrOH/pH2.5) (67mA/cm2) also showed promise as a catalyst as the current density is comparable to that of the commercial Pt/C (JM) (62mA/cm2).</p> <p><br /> A remarkable achievement was attained in this study: the electrocatalyst Pt supported on CNTs was synthesized effectively. This method resulted in the smallest Pt particle size 2.15nm. In the electrochemically-active surface area study, the Pt/CNT exhibited a significantly greater active surface area (27.03 m2/g) and higher current density (100 mA/cm2) in the Fe2+/Fe3+ electrochemical mediator system than the other home-made Pt/C catalysts, as well as being significantly higher than the commercial Pt/C (JM) catalysts. Pt/CNT catalyst produced the best electrochemical activities in both H2SO4 and K4[Fe(CN)6] electrolytes. As a result of the characteristics of Pt/CNT，it can be deduced that the Pt/CNT is the best electrocatalyst prepared in this study and has great potential for use in fuel cell applications.</p>

Orientador: Hebe de las Mercedes Villullas / Banca: Rodrigo Fernando Costa Marques / Banca: Leandro Martins / Banca: Elisabete Inacio Santiago / Banca: Joelma Perez / Abstract: Carbon supported PdAu nanoparticles with different Au contents (20-50% in atoms) were synthesized using a procedure carried out in a liquid two-phase system. As-prepared materials presented similar average particle diameter (~3nm) with narrow distribution over the carbon support, as shown by Transmission Electronic Microscopy (TEM). The combined data from X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) suggest that nanoparticles had Pd-enriched surfaces and Au-rich interiors. Cyclic Voltammetry (CVs) studies in H2SO4 further reinforced these findings, confirming that the nanoparticle surfaces were enriched with Pd. Moreover, XPS results show that increasing the Au content of PdAu alloys leads to varying amounts of surface-like and bulk-like Pd oxide, with a significant increase of metallic Pd. This result is consistent with data of X-ray Absorption Spectroscopy (XAS) around Pd L3 edge, which revealed that Au promotes an increase in the electronic occupancy of the Pd 4d band. Therefore, this whole set of characterizations suggests that the presence of Au in PdAu nanoalloys decreases the Pd affinity for oxygen, giving Pd a more noble-like character. In addition, the influence of ligand and ensemble effects on electrochemical surface processes, such as oxide formation/reduction, CO oxidation and hydrogen adsorption were also investigated. This was also a necessary step in order to determine the best technique to measure the Electrochemical Active Area (EAA) of... (Complete abstract click electronic access below) / Resumo: Nanopartículas de PdAu suportadas em carbono com diferentes frações de Au (20-50% em átomos) foram sintetizadas em um sistema líquido de duas fases. As nanopartículas preparadas apresentaram diâmetro médio próximo a 3 nm, com uma distribuição homogênea sobre o suporte de carbono, o que foi demonstrado por microscopia eletrônica de transmissão (TEM). O conjunto dos dados coletados por difração de raios X (XRD) e por espetroscopia de fotoelétrons excitados por raios X (XPS) demonstrou que o interior das nanopartículas é enriquecido por Au, enquanto a superfície é mais rica em Pd. A análise por XPS também demonstrou que o aumento da fração de Au nas ligas de PdAu leva a uma variação na fração de diferentes espécies de óxidos de Pd e um aumento na quantidade total de Pd metálico. Este resultado é consistente com aquele obtido por espectroscopia de absorção de raios-X (XAS), realizada na borda L3 do Pd, a qual revelou que o Au promove um preenchimento eletrônico na banda 4d do Pd. Ou seja, a presença do Au parece diminuir a afinidade do Pd pelo oxigênio. Ademais, foram estudados a influência de efeitos eletrônicos e do arranjo superficial de átomos sobre os processos eletroquímicos de formação/redução de óxidos, oxidação de CO adsorvido e adsorção de hidrogênio. Estes estudos também permitiram a determinação da área eletroquímica ativa de Pd. Por meio de todas estas caracterizações foi possível traçar correlações entre a composição no cerne das nanopartículas de PdAu e suas propri... (Resumo completo, clicar acesso eletrônico abaixo) / Doutor

La diminution du coût des catalyseurs est l'une des conditions nécessaires pour rendre la technologie PEMFC économiquement viable au grand public. Ces catalyseurs, habituellement composés de nanoparticules de platine, sont limités par leur coût, leur performance et leur durabilité. La nanostructuration est une des solutions envisageables pour ces catalyseurs car elle permet d'augmenter considérablement la surface spécifique, de diminuer le chargement en platine et d'augmenter les performances pour la réaction de réduction de l'oxygène, la plus limitante dans la technologie PEMFC.Les travaux présentés dans ce mémoire, ont été réalisés autour de deux types de particules nanostructurées (particules coeur-coquille et particules creuses) à base de platine ou d'alliage de platine. Ces particules ont été étudiées, caractérisées en électrochimie à 3 électrodes (électrode tournante disque-anneau et montage demi-pile) puis intégrées dans des systèmes pile à combustible. Nous avons observé les améliorations de performances offertes par ce type de particules électro-catalytiques vis-à-vis de la réaction de réduction de l'oxygène. Puis nous avons commencé à étudier et à optimiser leur intégration dans les piles à combustible en conditions réelles de fonctionnement. / The decrease in cost of catalysts is one of the conditions necessary to make economically viable PEMFC technology to the general public. These catalysts, usually composed of platinum nanoparticles, are limited by cost, performance and durability. Nanostructuring is one of the possible solutions for these catalysts because it greatly increases the surface area, reducing the platinum loading and increase performance for the reaction of oxygen reduction, the most limiting in PEMFC technology.The works presented in this thesis were performed on two types of nanostructured particles (core-shell particles, hollow particles) based on platinum or platinum alloy. These particles have been studied, characterize in electrochemistry to 3 electrodes (rotating ring-disk electrode and half-cell assembly) and then integrated in fuel cell systems. We observed performance improvements offered by this type of electro-catalytic particles towards the reduction reaction of oxygen and then we started studying and optimize their integration into fuel cells and actual conditions of operation.

Orientador: Prof. Dr. Mauro Coelho dos Santos / Tese (doutorado) - Universidade Federal do ABC. Programa de Pós-Graduação em Ciência e Tecnologia/Química, 2016. / Esta tese descreve um estudo comparativo entre diferentes processos oxidativos avançados na degradação do antibiótico ciprofloxacino. Inicialmente, foi feita a preparação e a caracterização de eletrocatalisadores nanoestruturados baseados em estanho e níquel, preparados pelo método dos precursores poliméricos e suportados em Vulcan XC72R. O objetivo foi o de avaliar a atividade catalítica destes materiais na reação de redução de oxigênio (RRO) pelo mecanismo de 2 elétrons, no sentido da formação de H2O2, para posteriormente empregá-los na degradação do fármaco. Foram estudadas quatro proporções em massa de metais em carbono: 3%, 6%, 9% e 13%, sendo que os materiais binários foram preparados variando-se as proporções atômicas de Sn e Ni em 6:1, 3:1, 1:1, 1:3 e 1:6, respectivamente, para cada percentual testado. Dos resultados, concluiu-se que o material de SnNi/C 9% 6:1 foi o mais promissor, transferindo 2,2 elétrons na RRO e produzindo 88% de H2O2, enquanto que o carbono Vulcan eletrogerava apenas 63%. Foi então preparado um eletrodo de difusão gasosa (EDG) baseado no melhor material, a fim de se quantificar a sua produção de H2O2 in situ e verificou-se que o mesmo era capaz de gerar 275 mg L-1 de H2O2, uma quantidade três vezes maior à gerada pelo carbono (83 mg L-1). O EDG de material nanoparticulado foi empregado na degradação de 350 mL de uma solução de ciprofloxacino (0,245 mmol L-1) em meio ácido, apresentando uma mineralização de 80% do composto após 360 minutos de tratamento pelo processo fotoeletro-Fenton. A segunda parte envolveu um sistema que contava também com a atuação de um anodo; assim, promoveu-se a oxidação direta e a indireta simultaneamente em uma célula eletroquímica com 100 mL de solução do antibiótico (0,245 mmol L-1), em 0,05 mol L-1 de Na2SO4, pH 3,0, por oxidação eletroquímica. As eletrólises foram realizadas sob agitação constante, utilizando um anodo de diamante dopado com boro (DDB) ou de platina e um cátodo de difusão gasosa de carbono. Nos processos Fenton, o ciprofloxacino foi rapidamente degradado devido à reação com os radicais hidroxila formados no meio reacional. A maior taxa de mineralização do antibiótico foi alcançada pelo processo fotoeletro-Fenton solar com o anodo de DDB, com 95% de incineração após 360 minutos de tratamento. A identificação dos intermediários primários e derivados hidroxilados por LC-MS permitiu a proposta de um mecanismo de reação para a mineralização do fármaco. Um comportamento diferente foi observado quando a mesma concentração de ciprofloxacino foi oxidada em uma matriz de urina sintética com elevado teor de ureia e uma mistura de íons PO43-, SO42- e Cl-. A reação de Fenton foi inibida neste meio e apenas os processos de oxidação eletroquímica e oxidação eletroquímica com H2O2 eletrogerado/com cátodo de aço inoxidável foram eficientes na mineralização, sendo que as fases orgânicas foram degradadas principalmente pelo HClO, formado a partir do cloreto. O processo utilizando o anodo de DDB e o cátodo de aço inoxidável foi o tratamento que apresentou maior eficácia na matriz de urina, removendo 96% do ciprocloxacino e mineralizando 98% de compostos orgânicos após 360 minutos de eletrólise, em densidade de corrente de 66,6 mA cm-2. / This thesis describes a comparative study of different methods in the degradation of the antibiotic ciprofloxacin. Initially, were carried out the preparation and characterization of nanostructured electrocatalysts based on tin and nickel, prepared by the polymeric precursor method and supported on Vulcan XC72R. The objective was to evaluate the catalytic activity of these materials in the ORR mechanism 2 electrons towards H2O2 formation, to later use them in drug degradation. Were studied four proportions by mass of metals on carbon: 3%, 6%, 9% and 13%, and that the binary materials were prepared by varying the atomic proportions of Sn and Ni in 6:1, 3:1, 1:1, 1:3 and 1:6, respectively for each percentage tested. The results showed that the material SnNi/C 9% 6:1 was the most promising transferring 2.2 electrons in RRO and producing 88% of H2O2, whereas Vulcan carbon electrogenerated only 63%. It was prepared a gas diffusion electrode (GDE) of the best material in order to quantify its production of H2O2 in situ and found that it was able to generate 275 mg L-1 of H2O2, an amount three times that generated by the carbon (83 mg L-1). The GDE based on the nanoparticulated material was used in the degradation of 350 mL of a ciprofloxacin solution (0.245 mM) in an acidic medium, having a mineralization of 80% after 360 minutes of the treatment with fotoeletro-Fenton process. The second part involved a system that also had the action of an anode, thereby direct and indirect oxidation were promoted simultaneously in a stirred tank reactor with 100 mL of antibiotic solution (0.245 mM) in 0.05 M Na2SO4 at pH 3.0 has also been studied by electrochemical oxidation. The electrolyses were carried out under constant agitation using a boron doped diamond (BDD) or a platinum anode and a carbon air-diffusion cathode. In Fenton processes, ciprofloxacin was rapidly removed due to its oxidation with hydroxyl radicals formed in the reaction medium. The larger electrochemical incineration of the antibiotic was achieved by solar photoeletro-Fenton with the anode BDD with 95% mineralization after 360 minutes of treatment. Up to 10 primary intermediates and 11 hydroxylated derivatives were identified by LC-MS allowing the proposal of a reaction equence for ciprofloxacin mineralization. A different behavior was found when the same antibiotic concentration was oxidized in a synthetic urine matrix with a high urea content and a mixture of PO43-, SO42- and Cl- ions. Since Fenton's reaction was inhibited in this medium, only the electrochemical oxidation processes and electrochemical oxidation with H2O2 electrogenerated with a stainless steel cathode were useful for mineralization, being the organic mainly degraded by HClO formed from Cl- oxidation. The electrochemical oxidation process with a BDD/stainless steel cell was found to be the most powerful treatment for the urine solution, yielding 96% ciprofloxacin removal and 98% mineralization after 360 minutes of electrolysis at optimum values of pH 3.0 and current density of 66.6 mA cm-2.

Os objetivos deste trabalho foram: i) estudar a degradação de catalisadores do tipo Pt-M (onde M = Co e Cr) suportados em carbono após tratamento ácido e a atividade catalítica dos mesmos para a reação de redução de oxigênio; ii) avaliar o desempenho e a estabilidade de eletrocatalisadores de platina suportada em monocarbeto e dióxido de tungstênio (WC/C e WO2/C, respectivamente) para a reação de redução de oxigênio em eletrólitos ácidos. O tratamento ácido foi realizado mantendo-se uma certa quantidade do eletrocatisador em uma solução ácida (H2SO4 0,5 mol L-1) a 90ºC por 24 h. Os catalisadores Pt3Co/C e Pt3Cr/C foram caracterizados antes e após o tratamento pelas seguintes técnicas: Espectroscopia por Dispersão de Energia de Raios X (EDX), Difratometria de Raios X (DRX), Microscopia Eletrônica de Transmissão (MET), Espectroscopia de Absorção de Raios X (XAS), como também submetidos a testes eletroquímicos. Os resultados mostraram que os metais menos nobres foram lixiviados devido ao tratamento ácido e que houve variações da região do potencial de redução de óxidos. Em relação aos carbertos de tungstênio, este foram sintetizados pelo método sonoquímico e caracterizados por EDX, DRX e MET. Os resultados eletroquímicos obtidos a partir desses materiais não apresentaram atividade catalítica significante para a RRO, porém, quando se tem Pt ancorada nesse material, a reação de redução de oxigênio é catalisada, similarmente ao desempenho de Pt/C, e segue uma reação via 4 elétrons. Já os materiais formados por dióxido de tungstênio foram sintetizados pelo método de impregnação e também foram caracterizados por EDX, DRX e MET. Os dados obtidos nos testes eletroquímicos destes materiais demonstraram uma certa atividade catalítica para RRO, e que esta reação se ocorre via 2 elétrons. Já para os materiais com Pt suportada em WO2/C, a reação segue a conhecida via 4 elétrons, como no caso de Pt/C. Ao serem submetidos ao tratamento ácido, tanto os catalisadores suportados em WC/C quanto os suportados em WO2/C apresentaram maior estabilidade quando comparados ao Pt/C comercial. / The objectives of this work were: i) to study the degradation of Pt-M catalysts (where M = Co and Cr) supported on carbon after acid treatment and their catalytic activities for the oxygen reduction reaction; ii) to evaluate the performance and stability of platinum electrocatalysts based in tungsten monocarbide (WC/C) and tungsten dioxide (WO2/C) for the oxygen reduction reaction in acidic electrolytes. The acid treatment was carried out by keeping a certain amount of eletrocatalyst in an acidic solution (H2SO4 0.5 mol L-1) at 90°C for 24 h. The Pt3Co/C and Pt3Cr/C catalysts were characterized before and after treatment by the following techniques: Energy Dispersive Spectroscopy (EDS), X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), X-Ray Absorption Spectroscopy (XAS), followed by electrochemical tests. The results showed that the non-noble metals have been leached due to the acid treatment and that there are variations in the potential region of oxide reduction. Tungsten carbides, were synthesized by a chemistry and characterized by EDS, XRD and TEM. The electrochemical results obtained for the WC/C materials showed no significant catalytic activity for the ORR, but when Pt is anchored, the oxygen reduction reaction is catalyzed, and the performance is similar to that of Pt/C, and with reaction following the 4 electrons pathways. The materials formed by tungsten dioxide were synthesized by an impregnation method, and also characterized by EDS, XRD and TEM. The data obtained in the electrochemical tests of the bare materials showed some catalytic activity for ORR, but the reaction is promoted via the 2 electrons pathways. As for the previous cases the materials formed by Pt supported on WO2/C the reaction follows the 4 electrons steps. When subjected to the acid treatment, as in the case of the catalysts supported on WC/C, the WO2/C-supported materials had higher stability when compared to the commercial Pt/C catalyst.

A pesquisa e desenvolvimento de novos materiais catalisadores para as reações eletródicas em células a combustível de membrana trocadora de prótons são de grande importância para a viabilização destes sistemas como geradores de energia. Neste trabalho foram preparados catalisadores de Pt-Co suportados em carbono (Pt-Co/C) por diferentes métodos e são apresentados os resultados da caracterização física por diferentes técnicas. Os resultados da avaliação da atividade eletrocatalítica para a reação de redução de oxigênio (RRO) em ácido sulfúrico, na ausência e presença de metanol e a avaliação do desempenho destes materiais em células a combustível unitárias alimentadas com H2 e/ou metanol (no ânodo) e O2 (no cátodo) são também apresentados. Em linhas gerais, observou-se que os catalisadores de Pt-Co/C apresentaram maior atividade catalítica para a RRO quando comparados ao catalisador Pt/C sendo o método de impregnação o melhor método de preparação dentre os investigados. O catalisador Pt75Co25/C, preparado pela deposição do Co sobre Pt/C seguido pelo tratamento à alta temperatura, apresentou o melhor desempenho para a RRO devido à provável formação de uma liga que apresenta menor tamanho de partícula e menor distância interatômica Pt-Pt que o Pt/C. Adicionalmente, este material apresentou boa estabilidade nos testes em célula a combustível alimentada com H2/O2. Catalisadores Pt-Co/C mostraram boa tolerância ao metanol quando usados como cátodos com maior atividade para a RRO que o Pt/C, tanto em ácido sulfúrico na ausência e presença de metanol como na célula a combustível de metanol direto. / Research and development of new catalyst materials for the electrodic reactions in polymer electrolyte fuel cells are of great importance for the development of these systems as sources of energy. In this work electrocatalyts Pt-Co supported on carbon (Pt-Co/C) were prepared by different methods. Additionally to conventional electrochemical characterization, the materials were physically characterized by means of several different techniques. The evaluation of the electrocatalytic activity for the oxygen reduction reaction (ORR) in acid media, in the absence and presence of methanol, as well as the evaluation of the performance in fuel cells fed with H2 or methanol (in the anode) and O2 (in the cathode) are investigated. Pt-Co/C electrocatalyts presented better catalytic activity for the ORR when compared to Pt/C. Additionally, the impregnation method was found as being the best preparation method investigated. Pt75Co25/C electrocatalyts prepared by deposition of Co on Pt/C followed by thermal treatment at high temperatures presented the best performance for the ORR due the probable formation of an alloy with small particle size and shorter Pt-Pt bond distance compared to Pt/C. This material presented good stability in fuel cells. Pt-Co/C electrocatalyts showed good tolerance to methanol when used as cathode materials, showing better activity for the ORR compared to Pt/C, in acid medium in the absence and presence of methanol and in direct methanol fuel cells.

Durante o desenvolvimento deste projeto, foram obtidos diversos resultados relativos à construção de distintos sensores eletroquímicos para a determinação de oxigênio em meio aquoso em amostras biológicas e ambientais. Para a construção destes sensores foram usadas diferentes superfícies eletródicas, preparadas pela incorporação de porfirinas de cobalto sintetizadas e caracterizadas no laboratório do Prof. Dr. Koiti Araki, tais como a 5,10,15,20-meso-tetrafenilporfirina de cobalto (CoTPP) e a [tetrakis-bisdimetil-bipiridina cloro rutênio(II)]-5,10,15,20-Tetrapiridinaporfirina de cobalto (II) (CoTRP). Eletrodos de ouro modificados com nanoestruturas de ouro também foram empregados para a detecção de oxigênio. Em alguns casos, óxido de grafeno (GO) foi imobilizado nas superfícies eletródicas para melhorar o transporte de elétrons. Porfirinas de CoTPP foram imobilizadas em eletrodo de carbono vítreo (GCE) e o comportamento eletroquímico desse eletrodo modificado para a redução de oxigênio em solução aquosa de KNO3 0,1 mol L-1 foi comparado com o do GCE, observando-se diminuição do sobrepotencial. A utilidade do sensor foi demonstrada pelo monitoramento amperométrico contínuo do consumo de oxigênio mitocondrial e os resultados foram concordantes com aqueles obtidos em medições paralelas realizadas usando um eletrodo comercial (\"eletrodo de Clark\"). GO foi misturado com a porfirina de CoTRP para a preparação de compósitos, os quais foram posteriormente imobilizados em superfícies eletródicas. Verificou-se que as moléculas de CoTRP ficavam em forma horizontal na superfície do GO, gerando nanoestruturas mais dispersas em meios aquosos dependendo da quantidade relativa de CoTRP e GO. Esta característica foi usada para modular a atividade eletrocatalítica dos compostos na reação de redução de oxigênio (ORR) em meio neutro. O material foi caracterizado por microscopia eletrônica de transmissão, Raman, espectroscopia UV-vis e microscopia de força atômica (AFM), e os resultados confirmaram que a porfirina CoTRP é fortemente ancorada no GO por meio de interações eletrostáticas. A ORR no eletrodo modificado CoTRP/GO foi avaliada por técnicas eletroquímicas e baixo sobrepotencial (0,05 V) foi notado no processo de quatro elétrons envolvendo a redução do oxigênio. Esse valor é dramaticamente deslocado para potenciais menos negativos (0,88V em relação ao GCE), e o excelente desempenho do eletrodo permite sua utilização como sensor efetivo para o monitoramento contínuo de oxigênio dissolvido em meio aquoso. Filmes nanoporosos de ouro (NPGF) foram preparados em superfície de ouro pela aplicação de potencial de 2,0 V por 60 minutos em solução de H2SO4 0,5 mol L-1. A presença de nanoestruturas de ouro foi confirmada por microscopia eletrônica de varredura (MEV) e o eletrodo NPGF mostrou atividade eletrocatalítica superior para a ORR em comparação com a superfície polida de ouro Superfícies de ouro mais ativas eletrocataliticamente também foram preparadas por redução eletroquímica de Au3+ em eletrodos de carbono vítreo. Com um tempo de deposição ótimo de 90 s, superfícies rugosas foram obtidas e caracterizadas por MEV e difração de raios X (XRD). Esses eletrodos modificados foram empregados com sucesso para o monitoramento de oxigênio dissolvido em amostras de água e o desempenho analítico foi avaliado com base em parâmetros como sensibilidade, seletividade, reprodutibilidade, estabilidade e limite de detecção (LOD). / During the development of this project, several results were obtained concerning the construction of different electrochemical sensors for the determination of oxygen in aqueous media in biological and environmental samples. Different electrode surfaces were used for the construction of these sensors, which were prepared by the incorporation of cobalt porphyrins synthesized and characterized in the laboratory of Prof. Koiti Araki, such as cobalt 5,10,15,20-meso- tetraphenylporphyrin (CoTPP) and [tetrakis-bisdimethyl-bipyridine chlorine (II)] - 5,10,15,20-Cobalt tetrahydrin (II) (CoTRP). Gold electrodes modified with gold nanostructures were also employed for the detection of oxygen. In some cases, graphene oxide (GO) was immobilized on the electrode surfaces to improve the electron transport. CoTPP porphyrins were immobilized on a vitreous carbon electrode (GCE) and the electrochemical behavior of this modified electrode for the reduction of oxygen in 0.1 mol L-1 KNO3 aqueous solution was compared with that of a GCE, a reduction of the overpotential being observed. The utility of the sensor was demonstrated by the continuous amperometric monitoring of the mitochondrial oxygen consumption and the results were in agreement with those obtained in parallel measurements performed by using a commercial electrode (\"Clark electrode\"). GO was mixed with the CoTRP porphyrin for the preparation of composites, which were subsequently immobilized on electrode surfaces. The CoTRP molecules were found to be horizontal in the GO surface, generating more dispersed nanostructures in aqueous media depending on the relative amount of CoTRP and GO. This characteristic was used to modulate the electrocatalytic activity of the compounds in the oxygen reduction reaction (ORR) in neutral medium. The material was characterized by transmission electron microscopy, Raman, UV-vis spectroscopy and atomic force microscopy (AFM), and the results confirmed that CoTRP porphyrin is strongly anchored in the GO by means of electrostatic interactions. The ORR in the modified CoTRP / GO electrode was evaluated by electrochemical techniques and a low overpotential (0.05 V) was observed in the four-electrons process involving the reduction of oxygen. This value is dramatically shifted to less negative potentials (0.88V compared to GCE), and the excellent electrode performance allows its use as an effective sensor for the continuous monitoring of oxygen dissolved in aqueous medium. Gold nanoporous films (NPGF) were prepared on a gold surface by application of 2.0 V for 60 minutes in 0.5 mol L-1 H2SO4 solution. The presence of gold nanostructures was confirmed by scanning electron microscopy (SEM) and the NPGF electrode showed superior electrocatalytic activity for the ORR compared to the gold polished surface. More electrocatalytically active gold surfaces were also prepared by electrochemical reduction of Au3+ on glassy carbon electrodes. With an optimum deposition time of 90 s, rough surfaces were obtained and characterized by SEM and XRD. These modified electrodes were successfully used for the monitoring of dissolved oxygen in water samples and the analytical performance was evaluated based on parameters such as sensitivity, selectivity, reproducibility, stability and limit of detection (LOD).

Made available in DSpace on 2016-08-19T12:56:38Z (GMT). No. of bitstreams: 1 Jaldyr de Jesus.pdf: 960775 bytes, checksum: 15b1d54e03bab9da8818d65bfa95a1e8 (MD5) Previous issue date: 2006-07-14 / This work presents oxygem reduction reaction (ORR) studies on platinum (Pt/C) and platinum-chromium alloys containing 50% (PtCr/C) or 70% in chromium (Pt3Cr) dispersed on high surface area, in acid aqueous solutions with and without metanol, with cyclic voltammetry, chronoamperometry and rotating disk electrode techniques. Electrochemical measurements have been also performed in a direct methanol unit fuel cell. The electrocatalysts were characterized by energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analyses. The ORR studies in absence of methanol in solution showed that chromium adittion does not change the reaction mechanism observed on the Pt/C and the electrocatalytic activity increases in the order PtCr/C < Pt/C < Pt3Cr/C. Such increase was attributed to a favourable interatomic distance for O2 dissociative adsorption. The alloys showed higher tolerance to methanol presence in solution than Pt, mainly the PtCr electrode due to smaller amount of active sites to methanol adsorption with respect to Pt. In solutions containing low concentration of methanol, the ORR mechanism was not affected by methanol presence. The polarization curves recorded with the unit cell showed that na increase in temperature results in an increase in the electrocatalitic activity of the anode, as well as na increase in methanol diffusion through the Nafion® membrane with consequent cathode depolarization. Furthermore, the system containing Pt3Cr/C as cathodic electrode material presented the best electrochemical performance, with power densities of ~ 55 mW cm-2, in contrast to ~ 40 mW cm-2 for the unit cell with Pt/C cathode and ~ 20 mW cm-2 with PtCr/C. / This work presents oxygem reduction reaction (ORR) studies on platinum (Pt/C) and platinum-chromium alloys containing 50% (PtCr/C) or 70% in chromium (Pt3Cr) dispersed on high surface area, in acid aqueous solutions with and without metanol, with cyclic voltammetry, chronoamperometry and rotating disk electrode techniques. Electrochemical measurements have been also performed in a direct methanol unit fuel cell. The electrocatalysts were characterized by energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analyses. The ORR studies in absence of methanol in solution showed that chromium adittion does not change the reaction mechanism observed on the Pt/C and the electrocatalytic activity increases in the order PtCr/C < Pt/C < Pt3Cr/C. Such increase was attributed to a favourable interatomic distance for O2 dissociative adsorption. The alloys showed higher tolerance to methanol presence in solution than Pt, mainly the PtCr electrode due to smaller amount of active sites to methanol adsorption with respect to Pt. In solutions containing low concentration of methanol, the ORR mechanism was not affected by methanol presence. The polarization curves recorded with the unit cell showed that na increase in temperature results in an increase in the electrocatalitic activity of the anode, as well as na increase in methanol diffusion through the Nafion® membrane with consequent cathode depolarization. Furthermore, the system containing Pt3Cr/C as cathodic electrode material presented the best electrochemical performance, with power densities of ~ 55 mW cm-2, in contrast to ~ 40 mW cm-2 for the unit cell with Pt/C cathode and ~ 20 mW cm-2 with PtCr/C.