The possibility of detecting some organic compounds directly in a petroleum/water interface was described. Aqueous phase was composed by 1 x 10-3 mol L-1 NaCl solution and the oil phase was carried out using crude oil forming an oil/water interface. Carbon based screen-printed electrodes was used in conjunction with square-wave voltammetry technique, constituting the main tools of this technical note. Petroleum was contaminated with phenol, naphthalene and cyclohexane. The oxidation potentials for these three molecules were 0,72, 0,81 and 0,87 V vs. Ag, respectively. Phenol was chosen to study the stability of currents during the process. After 15 minutes, the currents of this process were stable. Although the formation of a passivation film, the qualitative detection of the molecules was possible. Finally, the main advantage of this methodology is the possibility of detecting contaminants in oil without the necessity of complicated pre-treatment steps.

The oxygen reduction reaction was studied in alkaline media using manganese oxides obtained from spent batteries as electrocatalysts. Three processes were used to recover manganese oxides from spent batteries. The particles obtained were in the range from 8 to 11 nm. The electrochemical experiments indicated a good electrocatalytic activity toward oxygen reduction using the different samples and showing approximately a direct transference of 4 electrons during the process. Even though all the processes were efficient, the best result was observed for the prepared sample using reactants of low cost.

Este trabalho propõe estudar ânodos baseados em Pt e PbOx frente à reação de oxidação do ácido fórmico. Os materiais estudados foram sintetizados por uma metodologia sol-gel adaptada. Estudos voltamétricos mostraram que o processo de oxidação ocorre em potenciais extremamente baixos. Este bom desempenho foi comprovado por meio de curvas de polarização e cronoamperometrias.

The performance of Pt-PbOx-based catalysts to promote the formic acid oxidation is described here. All materials were synthesized by a modified sol-gel method. Voltammetric studies showed that the Pt-PbOx/C anode starts the oxidation process at extremely low potentials. Quasi-stationary polarization experiments and current vs. time measurements confirmed this affirmation.

RESUMO Com o objetivo de aprofundar investigações anteriores, foram realizadas modificações superficiais diretas em eletrodos de diamante dopados com boro (DDB), com óxidos metálicos (PtOx, RuO2, IrO2 e PbO2) e com alguns compósitos mistos, utilizando a técnica Sol-Gel. Estes materiais foram estudados por microscopia de força atômica (MFA) e os resultados indicaram a formação de sítios de deposição heterogênea. Técnicas eletroquímicas foram utilizadas para estabelecer a atividade catalítica para a reação de desprendimento de oxigênio (RDO) e para reação de oxidação de etanol em meio ácido utilizando os compósitos de PtOx e PtOx-RuO2. Testou-se ainda a estabilidade de um depósito de PtOx, recoberto por um filme de Nafion®, por meio de operação contínua de longa duração. Estudos de voltametria cíclica comparativa mostraram que a área superficial ativa dos novos materiais mudou consideravelmente com o método de modificação proposto. O eletrodo de IrO2/BDD mostrou o melhor desempenho para a RDO, iniciando o processo de oxidação em ~ 1.4 V vs. HESS, valor este 200 mV menor do que o observado sobre um eletrodo de PtOx/BDD. Foi observada uma pequena perda da atividade do eletrodo de PtOx/BDD (6%) depois de realizada eletrólise de 4 horas, enquanto que 1000 ciclos voltamétricos deixaram a superfície praticamente inalterada. Os estudos realizados mostram que as atividades satisfatórias dos eletrodos preparados pelo método Sol-Gel podem constituir ferramentas interessantes de pesquisa, visando principalmente a utilização destes materiais em aplicações práticas.

Continuing previous investigations, direct surface modifications of boron-doped diamond (BDD) electrodes with metal oxides (PtOx, RuO2, IrO2 and PbO2) and with some mixed composites were carried out by the Sol-Gel technique. The materials were studied by atomic force microscopy (AFM) to determine their surface topologies and by electrochemical techniques to establish the catalytic activity towards the oxygen evolution reaction (OER) and also, for the PtOx and PtOx-RuO2 composites, the ethanol oxidation reactions in acid media. The stability of PtOx coating covered by a Nafion® film was also tested by long-term operation. The AFM results indicated sites of heterogeneous deposition and the electrochemical studies demonstrated that the active surface area changed considerably with the proposed method of modification. The IrO2/BDD electrode showed the best performance to the OER with the onset of the oxidation current at ~1.4 V, a value 200 mV lower than for the PtOx/BDD electrode. The enhanced stability of PtOx/BDD electrodes achieved by the application of a Nafion® film and already reported in acid media was further proved using the ethanol oxidation reaction. Only a small loss of activity (6%) was observed after 4-hours electrolysis while one-thousand voltammetric cycles left the surface practically unchanged. In addition, preliminary studies for the same reaction on PtOx/BDD and PtOx-RuO2/BDD electrodes demonstrated the excellent activity of these mixed Sol-Gel coatings on the BDD surface and the possibility of further investigations for practical applications.

As reações eletródicas que ocorrem em ambos extremos da extensa janela eletroquímica do eletrodo de diamante dopado com boro foram analisadas por curvas de polarização visando determinar as respectivas energias de ativação aparentes para a etapa determinante da velocidade em cada caso. Encontrou-se que a reação de desprendimento de hidrogênio ocorre por um mecanismo Volmer-Heyrovsky, com a primeira etapa sendo a determinante da velocidade. A energia de ativação aparente foi calculada como sendo 150 kJ mol-1, indicando a formação de um intermediário do tipo M-H, típico da etapa de Volmer. Por outro lado, a energia de ativação aparente para a reação de desprendimento de oxigênio foi encontrada como sendo 106 kJ mol-1, o que também indica a etapa de adsorção do mecanismo como a determinante da velocidade. Desta forma, demonstrou-se que a interação entre as moléculas de água e a superfície do eletrodo de diamante é fortemente inibida, o que explica a extensa janela de potenciais observada para este eletrodo.

The electrochemical processes occurring at both edges of the wide electrochemical window of the boron doped diamond (BDD) electrode were studied by polarization curves experiments to evaluate the apparent energy of activation for the rate determining step in each reaction. It was found that the hydrogen evolution reaction occurs by a Volmer-Heyrovsky mechanism with the first step being the RDS. Moreover, the apparent energy of activation calculated from the Tafel plots presented a value as high as 150 kJ mol-1, indicating the formation of the M-H intermediate that is characteristic for the Volmer step. On the other hand, the apparent energy of activation for the oxygen evolution reaction was found to be 106 kJ mol-1 suggesting that the RDS in this mechanism is the initial adsorption step. In this way, it was demonstrated that the interaction between water molecules and the electrode surface is strongly inhibited on BDD thus justifying the extended potential window observed for this material.