Neste trabalho foi estudado o efeito da composição e tratamento térmico em H2, de materiais de Pt-Rh/C com proporções atómicas Pt:Rh de 3:1, 1:1 e 1:3 e 40% em massa de metal com relação a carbono, para a oxidação de COads e a oxidação de etanol. Os catalisadores foram preparados utilizando redução química com ácido fórmico e caracterizados fisicamente pelas técnicas de energia dispersiva de raios-X (EDX), retroespalhamento de elétron (EBS) e microscopia eletrônica de transmissão (TEM), mostrando relações Pt:Rh muito próxima às previstas, tamanhos de partícula médios similares e uma apropriada distribuição do metal sobre o suporte de carbono na micro e nano escala. Experimentos de voltametria cíclica mostraram um enriquecimento da superfície em Pt, devido à instabilidade termodinámica do sistema na temperatura experimental. A normalização das correntes foi feitas utilizando a carga de dessorção oxidativa de CO permitindo observar as diferenças entre os níveis atuais gerados exclusivamente pelos efeitos eletrônicos do Rh na Pt. O tratamento térmico dos catalisadores de Pt-Rh em uma atmosfera de H2, mostrou grande estabilidade dos materiais e também um notório incremento nos níveis de corrente para as reações de eletro-oxidação de CO e etanol. Isto sugere a necessidade de melhor explorar os efeitos dos tratamentos térmicos na eletrocatálise da reação de oxidação do etanol.

In this work we studied the effect of the composition and thermal treatment in H2 of Pt-Rh/C materials with atomic ratios close to Pt:Rh 3:1, 1:1 and 1:3 and metal loading of 40 wt. %, for the COads and ethanol oxidation. Catalysts were prepared by chemical reduction with formic acid and physically characterized by energy dispersive X-rays spectroscopy (EDX), electron backscattering (EBS) and transmission electron microscopy (TEM), showing Pt:Rh ratios close to the nominals values, similar average particle sizes and an appropriated distributions of metal on carbon support at micro and nano scale. Cyclic voltammetry experiments showed a surface enriched in Pt due to the termodinamically unstable Pt-Rh system at the experimental temperature. The currents were normalized using the charge of oxidative desorption of CO allowing to observe differences among the current levels generated exclusively by the Rh electronic effects on the Pt. The thermal treatments of the Pt-Rh catalysts in a hydrogen atmosphere showed greater stability of the materials and notorious increases of the current levels for CO and ethanol electro-oxidation reactions. This suggests the necessity of better exploring the effects of thermal treatments in the electrocatalysis of the ethanol oxidation reaction.

Neste trabalho, a eletrooxidação oscilatória de ácido fórmico (FA) é investigada sobre paládio policristalino e comparada a resultados obtidos sobre platina policristalina; as principais diferenças entre ambas as superfícies são atribuídas a diferentes cinéticas da sub-rede química tanto quanto a rotas preferenciais admitidas sobre a superfície de paládio. Com o intuito de presumir a taxa cinética de acumulação de veneno sobre paládio, eletrooxidação de FA foi conduzida na presença e ausência de hidrogênio dissolvido na rede cristalina do paládio. As rotas preferenciais foram presumidas a partir do padrão temporal (série temporal). Notoriamente, oscilações de potencial durante a eletrooxidação de FA possuem mínimo de potencial em 0,2 V, o que é associado a uma rápida taxa de desidrogenação direta; além disso, períodos de indução (ca. 60 min) e oscilação (20 min) estão entre os mais compridos já observados em eletroquímica, pois o hidrogênio incluso na rede cristalina do paládio reduz a taxa de acumulação de CO sobre a superfície.

Herein, oscillatory formic acid (FA) electrooxidation on polycrystalline palladium is investigated and compared with the one on polycrystalline platinum; major differences between both are attributed to differences on the kinetics of sub-set chemical network as well as to preferential routes admitted on palladium surface. To presume the kinetic rate of poison accumulation on palladium, FA oxidation was accomplished in presence of occluded hydrogen and hydrogen-free electrodes. The preferential routes were presumed from the temporal pattern. Markedly, oscillations during FA electrooxidation have minimum potential at 0.2 V, which is linked to the fast rate of direct dehydrogenation; moreover, it has one of the largest induction period (ca. 60 min) and oscillatory period (20 min) observed in electrochemistry, since subsurface hydrogen slows down the rate of CO accumulation on the surface.

We present in this work an experimental investigation of the effect of temperature (from 25 to 180 ºC) in the electro-oxidation of ethanol on platinum in two different phosphoric acid concentrations. We observed that the onset potential for ethanol electro-oxidation shifts to lower values and the reaction rates increase as temperature is increased for both electrolytes. The results were rationalized in terms of the effect of temperature on the adsorption of reaction intermediates, poisons, and anions. The formation of oxygenated species at high potentials, mainly in the more diluted electrolyte, also contributes to increase the electro-oxidation reaction rate.

Neste trabalho, foram preparados diferentes eletrocatalisadores de Pt-Cu e de Pt-Cu modificados com Pd, suportados em carbono por meio da redução química de precursores de Pt, Cu e Pd com borohidreto de sódio. A atividade eletrocatalítica frente à reação de redução de oxigênio foi estudada. A análise dos resultados de difração de raios X mostrou que há formação de liga. Com microscopia eletrônica de transmissão (TEM) foi observado que as nanopartículas metálicas estão bem distribuídas no suporte de carbono. Espectroscopia de absorção de raios X dispersiva (DXAS) foi empregada para compreender melhor as propriedades intrínsecas dos materiais. Os eletrocatalisadores foram caracterizados eletroquimicamente por voltametria cíclica em H2SO4 0.5 mol L-1 e a atividade eletrocatalítica destes foi estudada por meio de experimentos em estado estacionário em células a combustível de metanol direto (DMFC). A introdução de Pd ocasionou um aumento da atividade dos catalisadores de Pt-Cu. O melhor desempenho da DMFC foi obtido usando como cátodo o catalisador da liga de Pt50Pd40Cu10/C tratado termicamente a 500 °C em atmosfera de H2/Ar.

In this work, different carbon supported Pt-Cu catalysts and Pd modified Pt-Cu catalysts were prepared by chemical reduction of Pt, Cu and Pd precursors with sodium borohydride, and their electrocatalytic activity for the oxygen reduction reaction was studied. X-ray diffraction analysis indicated alloy formation. Transmission electron microscopy (TEM) showed that the metal nanoparticles are well-distributed on the carbon support. Dispersive X-ray absorption spectroscopy (DXAS) was used to achieve a better understanding of the intrinsic properties of the materials. The electrocatalysts were characterized electrochemically by cyclic voltammetry in 0.5 mol L-1 H2SO4 and their electrochemical activity was studied with steady-state experiments in a direct methanol fuel cell (DMFC). The introduction of Pd led to an improvement of the catalytic activity of the Pt-Cu catalysts. The best performance in a DMFC was obtained using, as cathode, a Pt50Pd40Cu10/C alloy catalyst thermally treated at 500 ºC in an H2/Ar atmosphere.

There is presently much interest in the clean and efficient generation of energy by proton exchange membrane fuel cells (PEMFC), using hydrogen as fuel. The generation of hydrogen by the reforming of other fuels, anaerobic fermentation of residual waters and other methods, often produce contaminants that affect the performance of the cell. In this work, the effect of gaseous SO2 and NO2 on the performance of a H2/O2 single PEMFC is studied. The results show that SO2 decreases irreversibly the performance of the cell under operating conditions, while NO2 has a milder effect that allows the recovery of the system.

It is investigated in the present contribution the oscillatory co-electrodeposition of CuSn on a polycrystalline gold surface in the presence of Triton X-100 surfactant and citric acid as additive, in acidic media. The experiments were conducted under potentiostatic control and the system dynamics characterized in terms of the morphology and stability of the current oscillations. Besides modulations in the frequency and amplitude of the current oscillations, several patterned states were observed, including relaxation-like and mixed mode oscillations. The oscillations were found to be very robust and some time series presented regular motions up to about two hours.

Fuel cells are attracting much interest as efficient and clean energy conversion devices. The main components of low temperature fuel cells are the electrocatalysts used to promote the anodic and cathodic reactions, which are based on platinum and platinum alloys. These electrocatalysts are normally prepared in the form of metal nanoparticles supported on a conductive material, usually high surface area carbon, to improve catalyst utilization and reduce cost. This work presents and comments some methods used presently to produce these electrocatalysts. The performances of the produced electrocatalysts are compared to that of state-of-the-art commercial E-TEK electrocatalysts.

Este trabalho descreve a preparação e caracterização de ligas dispersas de Pt-Ru sobre carbono de alta área superficial, as quais foram avaliadas para a oxidação de CO em eletrodos de disco rotatório/camada fina porosa e para a oxidação de hidrogênio em células a combustível de eletrólito polimérico alimentadas com hidrogênio contendo 100 ppm de CO. Tratamentos térmicos (H2, 300 °C) aplicados aos catalisadores melhoram a tolerância a pequenas quantidades de CO e, em alguns casos, reduzem o potencial necessário para promover a oxidação de CO durante a varredura do potencial. Sob condições operacionais em uma célula a combustível na presença de CO, foi observado que os melhores resultados foram obtidos quando a liga Pt-Ru/C foi preparada por redução simultânea dos íons Pt (IV) e Ru (III), diferentemente da redução seqüencial.

This work describes the preparation and characterization of Pt-Ru alloys dispersed on high surface area carbon, which were evaluated for CO oxidation on thin porous coating rotating disk electrodes and for hydrogen oxidation on polymer electrolyte fuel cells fed with hydrogen containing 100 ppm CO. A thermal treatment (H2, 300 ºC) applied to the catalysts improves the tolerance to small quantities of CO and, in some cases, reduces the potential necessary to promote the CO oxidation during a linear potential scan. Under operational conditions in a fuel cell in the presence of CO it was observed that the best results were obtained when the Pt-Ru/C alloy was prepared by simultaneous reduction of the ions Pt (IV) and Ru (III), as opposed to a sequential reduction.

The concepts of dissipation and feedback are contained in the behavior of many natural dynamical systems. They have been used to predict the evolution of populations leading to the formulation of the quadratic logistic equation (QLE). More recently, the QLE has been used to provide a better understanding of physicochemical systems with promising results. Many physical, chemical and biological dynamic phenomena can be understood on the basis of the QLE and this work describes the main aspects of this equation and some recent applications, with emphasis on electrochemical systems. Also, it is illustrated the concept of potential energy as a convenient way of describing the stability of the fixed points of the QLE.

A study of the kinetics of oxygen evolution in alkaline conditions from ceramic films of Mn2O3 supported on stainless steel was carried out. This study has been done through the determination of transfer coefficients, Tafel slopes and exchange currents using potentiodynamic and quasi-potentiostatic measurements. The activation energy was determined as a function of the overpotential and, additionally, the electrode active surface was estimated. The results are consistent with data already published for other electrodes, implying that the methods used in this work were reliable and precise.

This work considers mainly the problem of environmental pollution due to the production of energy by burning fossil fuels, particularly in urban vehicles. Electrochemical energy conversion is proposed as a partial solution to this problem, through the use of hydrogen in fuel cells. In both the production of hydrogen by electrolysis and in the electrochemical processes in fuel cells, the key factor is electrocatalysis. The concept of electrocatalysis, the production of catalysts and supported catalysts and their use in practical systems for the conversion of energy is discussed.

A oxidação eletroquímica do metanol foi estudada sobre ligas de Pt:Mo dispersa em carbono, preparadas utilizando um método recentemente desenvolvido neste laboratório. Análises de EDX confirmaram que a redução simultânea dos sais precursores de Pt e Mo conduzem a presença destes metais nas composições nominais inicialmente calculadas. A adição de Mo sobre Pt causa um aumento da corrente de oxidação, mas não produz um efeito catalítico para oxidação do metanol. A partir dos dados de polarização em várias concentrações de metanol foram obtidos diagramas de Tafel que apresentaram duas inclinações. A técnica de espectroscopia de massa diferencial aplicada "on line" a um experimento voltamétrico foi utilizada para investigar a distribuição de produtos e intermediários formados na oxidação de metanol.

The electrocatalytic oxidation of methanol on Pt:Mo dispersed on carbon prepared using an alternative method recently developed in this laboratory was investigated. The EDX analysis confirmed that the simultaneous reduction of the precursor salts of Pt and Mo leads to the presence of these materials at the nominal composition initially calculated. The addition of Mo to Pt causes an increase of the oxidation currents, but does not improve the catalytic effect for methanol oxidation. Tafel plots for various methanol concentrations showed the presence of two slopes. On line differential electrochemical mass spectrometry (DEMS) was used to investigate the distribution of products and intermediates in methanol oxidation.

Neste trabalho a oxidação eletroquímica do formaldeído é estudada sobre eletrodos dimensionalmente estáveis preparados por decomposição térmica de percursores (correspondentes cloretos). Foram utilizados como eletrodos de trabalho: Ti/Ir0.3Ti0.7O2, Ti/Ru0.3Ti0.7O2 e Ti/Ir0.2Ru0.2Ti0.6O2. As eletrólises foram realizadas galvanostaticamente em uma célula do tipo filtro prensa em soluções 0,5 mol L-1 H2SO4 com concentração inicial de formaldeído de 100 mmol L-1. A concentração de formaldeído decresce rapidamente com o tempo de eletrólise sendo que o eletrodo ternário (Ir + Ru + Ti) é o que apresenta maior atividade catalítica. O ânodo contendo somente Ir, apesar de maior carga superficial é o de menor atividade eletrocatalítica. Para a oxidação de ácido fórmico, formado pela oxidação de formaldeído, a presença de Ir na composição do ânodo não favorece o processo, sendo o ânodo contendo somente Ru o mais efetivo para este processo.

The electrochemical oxidation of formaldehyde is studied on dimensionally stable anodes prepared by thermal decomposition of precursors (the corresponding chlorides). The working electrodes used were: Ti/Ir0.3Ti0.7O2, Ti/Ru0.3Ti0.7O2 and Ti/Ir0.2Ru0.2Ti0.6O2. The electrolyses were performed galvanostatically in a filter press cell with 0.5 mol L-1 H2SO4 solutions with initial formaldehyde concentration equal to 100 mmol L-1. The concentration of formaldehyde decreases fast with the electrolysis time, with the ternary anode (Ir + Ru + Ti) presenting the best performance for this step. The anode containing only Ir, despite presenting the higher superficial charge, is the one with the lowest electrocatalytic activity. For the formic acid oxidation step, the presence of iridium in the anode composition does not promote the process, the anode containing only ruthenium being the most effective for this step.

This paper describes in detail a technique employed to grow quasi-spherical single crystals of noble metals for electrochemical applications, using platinum as an example. The metal beads were formed by melting the extremity of a wire in an oxygen / butane flame. X-ray techniques were used to check the crystallization and to determine the orientation of the crystals. Treatment with a pure hydrogen flame followed by a cooling procedure in a hydrogen / argon atmosphere were used for conditioning the well-defined platinum single crystal surfaces. Finally, electrochemical characterization of the Pt(111), Pt(110) and Pt(100) surfaces was done in diluted sulfuric acid solution in the hydrogen adsorption / desorption potential region.

A deposição, em regime de sub-tensão, de chumbo sobre ouro, foi estudada por voltametria cíclica e espectroscopia de reflectância diferencial no domínio UV-visível. Os experimentos foram realizados em forma comparativa empregando-se eletrodos de ouro policristalino e monocristalinos de orientações (111) e (210). Para as superfícies monocristalinas, os resultados de reflectância confirmam o significativo efeito da orientação cristalográfica durante a deposição e redissolução do chumbo sobre as diferentes superfícies de ouro estudadas. O trabalho mostra que a existência de diferentes tipos de estruturas para o chumbo adsorvido depende não só da distribuição atômica do substrato mas acima de tudo do potencial aplicado. Adicionalmente, é feita uma tentativa para explicar o comportamento do ouro policristalino com base nos resultados obtidos para os dois monocristais pesquisados.

The underpotential deposition of lead on gold has been studied by cyclic voltammetry and UV-visible differential reflectance spectroscopy. Comparative experiments were carried out with a polycrystalline gold electrode and two single-crystal gold electrodes with the orientations (111) and (210). For the single crystal surfaces, the reflectance results confirm the large effect of the crystallographic orientation on the deposition and redissolution of lead on gold surfaces. The existence of different types of structures for adsorbed lead, depending not only on the atomic distribution of the substrate but above all on the electrode potential, is demonstrated. An attempt is made to explain the behavior of polycrystalline gold on the basis of the results obtained for the two investigated gold single crystal faces.