Electrochemical reduction of aromatic sulfinic acids in dimethylsulfoxide

The electrochemical reduction of benzenesulfinic, p-toluenesulfinic, and p-nitrobenzenesulfinic acids was studied in dimethylsulfoxide solutions. From cyclic voltammetry experiments, a chemical reaction following the first electron transfer was detected during the reduction process. A cyclic voltammetry technique using ultramicroelectrodes has provided kinetic parameters for the electron-transfer steps, from which it was possible to observe the influence of the ring substituent on the electrochemical reduction. The mechanism of the electroreduction of aromatic sulfinic acids in dimethylsulfoxide depends upon the nucleophilic attack of the radical anion produced on the starting compound during the reduction processes.

Electrochemical study of perchlorate reduction at tin electrodes

The electrochemical behaviour of tin in de-aerated sodium perchlorate was studied using potentiodynamic and potentiostatic techniques. Tin behaviour in sodium perchlorate has been complicated unexpectedly by the reduction of the perchlorate anion. It is shown that the reduction process takes place within a potential region comprising the negative side of the double layer region and the positive side of the hydrogen region (-0.7 less than or equal to E less than or equal to -1.3 V). The presence of oxide on the electrode surface favours the reduction reaction, which may occur in two steps: the formation of basic tin(II) chloride followed by its reduction, producing chloride.

Electrochemical reduction of azo-based chlorotriazine reactive dyes

The reduction process of the azo dyes reactive red 120 and reactive green 19 was investigated in B-R buffer pH 2-12 by differential pulse polarography, cyclic voltammetry and controlled potential electrolyse. The reactive red 120 presents two azo groups reducible in a single step of 8 electrons followed by simultaneous reduction of the two clorotriazine groups. The reduction of reactive green 19 is complicated by the presence of azo groups and chlorotriazine moyeties in a non symmetrical molecule. The peaks can be monitored for dyes determination in concentration level up to 1x10(-7) mol/L and 1x10(-9) mol/L using differential pulse polarography or cathodic stripping voltammetry.

Decolourization of anthraquinone reactive dye by electrochemical reduction on reticulated glassy carbon electrode

The electrochemistry reduction for the removal of Reactive Blue 4 (RB4) dye from aqueous solution using reticulated glassy carbon electrode is investigated. At pH < 8.0 the anthraquinone group of the RB4 dye are reduced in one cathodic step to hidroquinone after a reversible two-electron process involving a precedent two protons reaction. A stable semiquinone is detected by spectrophotometric technique. At pH > 8.0 the reduction process involves two reversible 2-electron steps, whose species are generated by a protonation equilibrium of anthraquinone group. The results shows that 60% of color removal was obtained after 3 hours of RB4 dye electrolysis at acidic and neutral conditions and only 37% at alkaline conditions. Simultaneously 64% of total organic carbon was removed after electrolysis at pH 2.0.

Caracterização de estruturas de óxido de estanho modificado como sensores de gases

Pós-graduação em Química - IQ

Oxygen reduction reaction catalyzed by epsilon-MnO2: Influence of the crystalline structure on the reaction mechanism

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on carbon supported epsilon-manganese dioxide (epsilon-MnO2/C). The epsilon-MnO2/C catalyst was prepared via thermal decomposition of manganese nitrate and carbon powder (Vulcan XC-72) mixtures. X-ray powder diffraction (XRD) measurements were performed in order to determine the crystalline structure of the resulting composite, while energy dispersive X-ray analysis (EDX) was used to evaluate the chemical composition of the synthesized material. The electrochemical studies were conducted using cyclic voltammetry (CV) and quasi-steady state polarization measurements carried out with an ultra thin layer rotating ring/disk electrode (RRDE) configuration. The electrocatalytic results obtained for 20% (w/w) Pt/C (E-TEK Inc., USA) and alpha-MnO2/C for the ORR, considered as one of the most active manganese oxide based catalyst for the ORR in alkaline media, were included for comparison. The RRDE results revealed that the ORR on the MnO2 catalysts proceeds preferentially through the complete 4e(-) reduction pathway via a 2 plus 2e(-) reduction process involving hydrogen peroxide as an intermediate. A benchmark close to the performance of 20% (w/w) Pt/C (E-TEK Inc., USA) was observed for the epsilon-MnO2/C material in the kinetic control region, superior to the performance of alpha-MnO2/C, but a higher amount of HO2- was obtained when epsilon-MnO2/C was used as catalyst. The higher production of hydrogen peroxide on epsilon-MnO2/C was related to the presence of structural defects, typical of this oxide, while the better catalytic performance in the kinetic control region compared to alpha-MnO2/C was related with the higher electrochemical activity for the proton insertion kinetics, which is a structure sensitive process. (C) 2012 Elsevier Ltd. All rights reserved.

Geochemistry of reduction haloes in ODP Holes 135-834A and 135-837A

The sediment column overlying basement in the Lau Basin consists of a sequence of volcaniclastic turbidites interbedded with hemipelagic clayey nannofossil mixed sediments, overlain in turn by a sequence of hemipelagic clayey nannofossil oozes containing sporadic calcareous turbidites. The clayey nannofossil oozes and mixed sediments are pervasively stained by hydrothermally derived iron and manganese oxyhydroxides. Sharply defined, lighter colored bands occur in the hemipelagic sediments, immediately beneath some (but by no means all) volcaniclastic and calcareous turbidites. These are identified as reduction haloes, of a type previously identified in quite different turbidite/pelagic sequences. The haloes are attributed to the burial of labile surficial Corg by turbidites, followed by the remineralization of this Corg with Mn and Fe oxyhydroxides as electron acceptors. The resultant characteristic Mn and Fe concentration/depth profiles are described, and a model is proposed for their development. The color alteration of the halo is ascribed to the removal of Mn oxyhydroxides, because, although the Fe content fluctuates through the haloes, this does not appear to affect their color. Other elements (Co, Cu, and Ni) are also at low concentration levels in the haloes like Mn, consistent with remobilization and migration out of the halo section, although the profile shapes are not identical with those of Mn. The behavior of V is distinctive in that it appears to have migrated into the haloes to be enriched there. Haloes are unlikely to form if turbidite emplacement is erosive and removes the near-surface layer, which generally is the most fluid part of the sediment and contains the highest levels of reactive Corg to drive the reduction process. Conversely, the presence of a halo implies that emplacement of the overlying turbidite did not significantly erode the pre-existing sediment/water interface.

Nitrate reduction at Pt(1 0 0) single crystals and preferentially oriented nanoparticles in neutral media

The electroreduction of nitrate on Pt(1 0 0) electrodes in phosphate buffer neutral solution, pH 7.2, is reported. The sensitivity of the reaction to the crystallographic order of the surface is studied through the controlled introduction of defects by using stepped surfaces with (1 0 0) terraces of different length separated by monoatomic steps, either with (1 1 1) or (1 1 0) symmetry. The results of this study show that nitrate reduction occurs mainly on the well defined (1 0 0) terraces in the potential region where H adsorption starts to decrease, allowing the nitrate anion to access the surface. Adsorbed NO has been detected as a stable intermediate in this media. An oxidation process observed at 0.8 V has been identified as leading to the formation of adsorbed NO and being responsible for a secondary reduction process observed in the subsequent negative scan. Using in situ FTIRS, ammonium was found to be the main product of nitrate reduction. This species can be oxidized at high potentials resulting in adsorbed NO and nitrate (probably with nitrite as intermediate).

The Importance of Dissimilatory Nitrate Reduction to Ammonium (DNRA) in the Nitrogen Cycle of Coastal Ecosystems

Until recently, it was believed that biological assimilation and gaseous nitrogen (N) loss through denitrification were the two major fates of nitrate entering or produced within most coastal ecosystems. Denitrification is often viewed as an important ecosystem service that removes reactive N from the ecosystem. However, there is a competing nitrate reduction process, dissimilatory nitrate reduction to ammonium (DNRA), that conserves N within the ecosystem. The recent application of nitrogen stable isotopes as tracers has generated growing evidence that DNRA is a major nitrogen pathway that cannot be ignored. Measurements comparing the importance of denitrification vs. DNRA in 55 coastal sites found that DNRA accounted for more than 30% of the nitrate reduction at 26 sites. DNRA was the dominant pathway at more than one-third of the sites. Understanding what controls the relative importance of denitrification and DNRA, and how the balance changes with increased nitrogen loading, is of critical importance for predicting eutrophication trajectories. Recent improvements in methods for assessing rates of DNRA have helped refine our understanding of the rates and controls of this process, but accurate measurements in vegetated sediment still remain a challenge.

Effect of Ni on Pt/C and PtSn/C prepared by the Pechini method

Different compositions of Pt, PtNi, PtSn, and PtSnNi electrocatalysts supported on carbon Vulcan XC-72 were prepared through thermal decomposition of polymeric precursors. The nanoparticles were characterized by morphological and structural analyses (XRD, TEM, and EDX). XRD results revealed a face-centered cubic structure for platinum, and there was evidence that Ni and Sn atoms are incorporated into the Pt structure. The electrochemical investigation was carried out in slightly acidic medium (H(2)SO(4) 0.05 mol L(-1)), in the absence and in the presence of ethanol. Addition of Ni to Pt/C and PtSn/C catalysts significantly shifted the onset of ethanol and CO oxidations toward lower potentials, thus enhancing the catalytic activity, especially in the case of the ternary PtSnNi/C composition. Electrolysis of ethanol solutions at 0.4 V us. RHE allowed for determination of acetaldehyde and acetic acid as the reaction products, as detected by HPLC analysis. Due to the high concentration of ethanol employed in the electrolysis experiments (1.0 mol L(-1)), no formation of CO(2) was observed. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Eliminação do Cr(VI) na fase de neutralização de um efluente industrial proveniente de processos de galvanoplastia

A presente tese teve por base a identificação e resolução de um problema existente no tratamento de efluentes provenientes dos processos de tratamento de superfícies por galvanoplastia, na OGMA – Indústria Aeronáutica de Portugal S.A.. Observou-se a ocorrência, esporádica, de crómio hexavalente, (Cr (VI)), em valor superior ao valor limite de emissão (VLE). Os resultados foram monitorizados e os dados recolhidos no decorrer da actividade de tratamento de efluentes, durante o período de, aproximadamente, 5 anos (2006 a 2011). A recolha de resultados decorreu no âmbito da actividade profissional da mestranda, que, para além da responsabilidade técnica dos processos de galvanoplastia na empresa, é também responsável pelo suporte técnico ao processo de tratamento de efluentes resultantes da actividade de tratamento de superficies por processos de galvanoplastia. A empresa OGMA – Indústria Aeronáutica de Portugal S.A., é uma empresa de actividade aeronáutica dedicada à Fabricação e Manutenção de aeronaves, nomeadamente a prestação de serviços de Manutenção, Revisão e Modernização de, Aeronaves, Motores e Componentes, bem como Fabricação e Montagem de Aeroestruturas. Integrada na OGMA, S.A. encontra-se a área de tratamentos electroquímicos, onde são realizados processos de tratamento de materiais metálicos por electrodeposição, deposição química e conversão química. Desta actividade resulta uma quantidade considerável de efluentes líquidos que necessitam de tratamento adequado previamente à sua descarga em cursos de água. Devido ao tipo de contaminantes que estes efluentes possuem, o tratamento dos mesmos é realizado em várias etapas, passando pela oxidação de cianetos, a redução de cromatos e a neutralização. Posteriormente segue-se uma sedimentação e a remoção de lamas. De modo a garantir um controlo dos parâmetros de descarga dos efluentes tratados, de acordo com a legislação ambiental em vigor, o efluente obtido é analisado periodicamente em laboratório acreditado. Na perspectiva de solucionar o problema em questão, procedeu-se à realização de ensaios experimentais utilizando os efluentes provenientes dos tanques de reacção da redução de cromatos e da oxidação de cianetos da linha com cádmio, com especial incidência na variação dos intervalos de pH recomendados para cada uma das fases do tratamento de efluentes, e observação do comportamento das misturas em termos de presença de Cr (VI), quando sujeitos a variações de pH. Após análise dos dados disponíveis e realização de todos os ensaios, conclui-se que, o processo de oxidação de cianeto da linha com cádmio e o processo de redução de cromatos na mesma linha estão a funcionar adequadamente. Concluiu-se que o reaparecimento de Cr (VI) ocorre devido à existência de hipoclorito de sódio, em excesso, no tanque de oxidação de cianeto que, quando passa para o tanque de neutralização e entra em contacto com o efluente proveniente do tanque de redução de cromatos, oxida parte do crómio trivalente, (Cr (III)), existente, a Cr (VI). Para impedir a ocorrência deste fenómeno separou-se todo o efluente contendo crómio que passou a ser tratado na linha de tratamento de efluentes isenta de cádmio, não entrando assim em contacto com o efluente que contém hipoclorito não reagido, evitando a oxidação do Cr (III) a Cr (VI).

δ 15N measurement of organic and inorganic substances by EA-IRMS: a speciation-dependent procedure

Little attention has been paid so far to the influence of the chemical nature of the substance when measuring δ 15N by elemental analysis (EA)-isotope ratio mass spectrometry (IRMS). Although the bulk nitrogen isotope analysis of organic material is not to be questioned, literature from different disciplines using IRMS provides hints that the quantitative conversion of nitrate into nitrogen presents difficulties. We observed abnormal series of δ 15N values of laboratory standards and nitrates. These unexpected results were shown to be related to the tailing of the nitrogen peak of nitrate-containing compounds. A series of experiments were set up to investigate the cause of this phenomenon, using ammonium nitrate (NH4NO3) and potassium nitrate (KNO3) samples, two organic laboratory standards as well as the international secondary reference materials IAEA-N1, IAEA-N2-two ammonium sulphates [(NH4)2SO4]-and IAEA-NO-3, a potassium nitrate. In experiment 1, we used graphite and vanadium pentoxide (V2O5) as additives to observe if they could enhance the decomposition (combustion) of nitrates. In experiment 2, we tested another elemental analyser configuration including an additional section of reduced copper in order to see whether or not the tailing could originate from an incomplete reduction process. Finally, we modified several parameters of the method and observed their influence on the peak shape, δ 15N value and nitrogen content in weight percent of nitrogen of the target substances. We found the best results using mere thermal decomposition in helium, under exclusion of any oxygen. We show that the analytical procedure used for organic samples should not be used for nitrates because of their different chemical nature. We present the best performance given one set of sample introduction parameters for the analysis of nitrates, as well as for the ammonium sulphate IAEA-N1 and IAEA-N2 reference materials. We discuss these results considering the thermochemistry of the substances and the analytical technique itself. The results emphasise the difference in chemical nature of inorganic and organic samples, which necessarily involves distinct thermochemistry when analysed by EA-IRMS. Therefore, they should not be processed using the same analytical procedure. This clearly impacts on the way international secondary reference materials should be used for the calibration of organic laboratory standards.

Iron oxidation on the surface of adventitious roots and its relation to aerenchyma formation in rice genotypes

Establishment of the water layer in an irrigated rice crop leads to consumption of free oxygen in the soil which enters in a chemical reduction process mediated by anaerobic microorganisms, changing the crop environment. To maintain optimal growth in an environment without O2, rice plants develop pore spaces (aerenchyma) that allow O2 transport from air to the roots. Carrying capacity is determined by the rice genome and it may vary among cultivars. Plants that have higher capacity for formation of aerenchyma should theoretically carry more O2 to the roots. However, part of the O2 that reaches the roots is lost due to permeability of the roots and the O2 gradient created between the soil and roots. The O2 that is lost to the outside medium can react with chemically reduced elements present in the soil; one of them is iron, which reacts with oxygen and forms an iron plaque on the outer root surface. Therefore, evaluation of the iron plaque and of the formation of pore spaces on the root can serve as a parameter to differentiate rice cultivars in regard to the volume of O2 transported via aerenchyma. An experiment was thus carried out in a greenhouse with the aim of comparing aerenchyma and iron plaque formation in 13 rice cultivars grown in flooded soils to their formation under growing conditions similar to a normal field, without free oxygen. The results indicated significant differences in the volume of pore spaces in the roots among cultivars and along the root segment in each cultivar, indicating that under flooded conditions the genetic potential of the plant is crucial in induction of cell death and formation of aerenchyma in response to lack of O2. In addition, the amount of Fe accumulated on the root surface was different among genotypes and along the roots. Thus, we concluded that the rice genotypes exhibit different responses for aerenchyma formation, oxygen release by the roots and iron plaque formation, and that there is a direct relationship between porosity and the amount of iron oxidized on the root surface.

Considerações sobre a eletrogeração de peróxido de hidrogênio

This paper presents some results that may be used as previous considerations to a hydrogen peroxide electrogeneration process design. A kinetic study of oxygen dissolution in aqueous solution is carried out and rate constants for oxygen dissolution are calculated. Voltammetric experiments on vitreous carbon cathode shown that the low saturation concentration drives the oxygen reduction process to a mass transfer controlled process which exhibits low values of limiting currents. Results have shown that the hydrogen peroxide formation and its decomposition to water are separated by 400 mV on the vitreous carbon surface. Diffusion coefficients for oxygen and hydrogen peroxide are calculated using data taken from Levich and Tafel plots. In a series of bulk electrolysis experiments hydrogen peroxide was electrogenerated at several potential values, and concentration profiles as a function of the electrical charged passed were obtained. Data shown that, since limiting current plateaus are poorly defined onto reticulated vitreous carbon, cathodic efficiency may be a good criterion for choosing the potential value in which hydrogen peroxide electrogeneration should be carried out.

Copper electrodeposition on glassy carbon and highly oriented pyrolytic graphite substrates from perchlorate solutions

In present work, we analyzed the copper electrodeposition onto GCE (System I) and HOPGE (System II) from perchlorate solutions. The current density transients obtained from system I and II were well described through a kinetic mechanism that involves four different contributions: (a) a Langmuir type adsorption process, b) an electron transfer from Cu2+→Cu+, (c) a 3D nucleation limited by a mass transfer reaction and (d) a proton reduction process. It was observed that the values of the nucleation rate, the number of active nucleation sites were increased with the overpotential and they are bigger onto GCE in comparison with HOPGE.