Efeito dos dióxidos de enxofre e de nitrogênio no desempenho de uma célula a combustível de membrana de intercâmbio de prótons

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.

Photo-fenton degradation of poly(Ethyleneglycol)

Polyethyleneglycol (PEG) was photooxidized in a photo-Fenton system and results compared with the dark reaction. The products were analysed using GPC and HPLC. In the absence of light, PEG samples needed 490 min to reduce their w by 50%, whereas under UV irradiation, only 10 min were necessary. The exponential decay of w with a concomitant increase in polydispersity and number of average chain scission, characterized a random chain scission mechanism. The degradation products of PEG in both systems showed the presence of lower molecular weight products, including smaller ethyleneglycols and formic acid. The mechanism involves consecutive processes, were the larger ethyleneglycols give rise, successively, to smaller ones. This suggests that the mechanism involves successive scissions of the polymer chain. Irradiated samples decomposed faster than those kept in the dark This study proves that the foto-Fenton method associated with UV-light is a good reactant for PEG photodegradation.

Electrochemical Evidence of Strong Electronic Interaction of PtRu on Carbon Nanotubes with High Density of Defects

We show that carbon nanotubes (CNTs) with high density of defects can present a strong electronic interaction with nanoparticles of Pt-Ru with average particle size of 3.5 +/- 0.8 nm. Depending on the Pt-Ru loading on the CNTs, CO and methanol oxidation reactions suggest there is a charge transfer between Pt-Ru that in turn provokes a decrease in the electronic interaction taking place between Ru and Pt in the PtRu alloy. The CO stripping potentials were observed at about 0.65 and 0.5 V for Pt-Ru/CNT electrodes with Pt-Ru loadings of 10 and 20, and 30 wt %, respectively. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.2990222] All rights reserved.

Gold is not a Faradaic-Efficient Borohydride Oxidation Electrocatalyst: An Online Electrochemical Mass Spectrometry Study

Direct borohydride fuel cells are promising high energy density portable generators. However, their development remains limited by the complexity of the anodic reaction: The borohydride oxidation reaction (BOR) kinetics is slow and occurs at high overvoltages, while it may compete with the heterogeneous hydrolysis of BH(4)(-). Nevertheless, one usually admits that gold is rather inactive toward the heterogeneous hydrolysis of BH(4)(-) and presents some activity regarding the BOR, therefore yielding to the complete eight-electron BOR. In the present paper, by coupling online mass spectrometry to electrochemistry, we in situ monitored the H(2) yield during BOR experiments on sputtered gold electrodes. Our results show non-negligible H(2) generation on Au on the whole BOR potential range (0-0.8 V vs reversible hydrogen electrode), thus revealing that gold cannot be considered as a faradaic-efficient BOR electrocatalyst. We further propose a relevant reaction pathway for the BOR on gold that accounts for these findings.

In situ infrared (FTIR) study of the mechanism of the borohydride oxidation reaction

Early reports stated that Au was a catalyst of choice for the BOR because it would yield a near complete faradaic efficiency. However, it has recently been suggested that gold could yield to some extent the heterogeneous hydrolysis of BH(4)(-),therefore lowering the electron count per BH(4)(-), especially at low potential. Actually, the blur will exist regarding the BOR mechanism on Au as long as no physical proof regarding the reaction intermediates is not put forward. In that frame, in situ physical techniques like FTIR exhibit some interest to study the BOR. Consequently, in situ infrared reflectance spectroscopy measurements (SPAIRS technique) have been performed in 1 M NaOH/1 M NaBH(4) on a gold electrode with the aim to detect the intermediate species. We monitored several bands in B-H ((nu) over bar similar to 1180,1080 and 972 cm(-1)) and B-O bond regions ((nu) over bar =1325 and similar to 1425cm(-1)), which appear sequentially as a function of the electrode polarization. These absorption bands are assigned to BH(3), BH(2) and BO(2)(-) species. At the light of the experimental results, possible initial elementary steps of the BOR on gold electrode have been proposed and discussed according to the relevant literature data.

Temperature effects on the oscillatory electro-oxidation of methanol on platinum

We report in this paper the effect of temperature on the oscillatory electro-oxidation of methanol on polycrystalline platinum in aqueous sulfuric acid media. Potential oscillations were studied under galvanostatic control and at four temperatures ranging from 5 to 35 degrees C. For a given temperature, the departure from thermodynamic equilibrium does not affect the oscillation period and results in a slight increase of the oscillation amplitude. Apparent activation energies were also evaluated in voltammetric and chronoamperometric experiments and were compared to those obtained under oscillatory conditions. In any case, the apparent activation energies values fell into the region between 50 and 70 kJ mol(-1). Specifically under oscillatory conditions an apparent activation energy of 60 +/- 3 kJ mol(-1) and a temperature coefficient q(10) of about 2.3 were observed. The present findings extend our recently published report (J. Phys. Chem. A, 2008, 112, 4617) on the temperature effect on the oscillatory electro-oxidation of formic acid. We found that, despite the fact that both studies were carried out under similar conditions, unlike the case of formic acid, only conventional, Arrhenius, dynamics was observed for methanol.

Catalytic oxidation of ethanol on gold electrode in alkaline media

This work presents a study of the catalytic oxidation of ethanol on polycrystalline gold electrode in alkaline media. The investigation was carried out by means of chronoamperometry, cyclic voltammetry, and in situ FTIR spectroscopy. The main goal was to investigate the early stages of ethanol electrooxidation, namely at fairly low potentials (E = 600 mV vs. RHE) and for moderate reaction times (t < 300 s). Chronoamperometric experiments show a current increase accompanying the increasing in the ethanol concentration up to about 2 M and then a slight decrease at 3 M. Adsorbed CO has been observed as early as about 200 mV vs. RHE and indicates that the cleavage of the C-C bond might occur, probably to a small extent, at very low overpotentials during ethanol adsorption on gold surface. The amount of dissolved acetate ions produced during the chronoamperomentry was followed by the asymmetric stretching band at 1558 cm(-1) as a function of time, and found to increase linearly with time up to 300 s. This allowed estimating the reaction order of acetate formation with respect to ethanol concentration.

Surface structure effects on the electrochemical oxidation of ethanol on platinum single crystal electrodes

Ethanol oxidation has been studied on Pt(111), Pt(100) and Pt(110) electrodes in order to investigate the effect of the surface structure and adsorbing anions using electrochemical and FTIR techniques. The results indicate that the surface structure and anion adsorption affect significantly the reactivity of the electrode. Thus, the main product of the oxidation of ethanol on the Pt(111) electrode is acetic acid, and acetaldehyde is formed as secondary product. Moreover, the amount of CO formed is very small, and probably associated with the defects present on the electrode surface. For that reason, the amount of CO(2) is also small. This electrode has the highest catalytic activity for the formation of acetic acid in perchloric acid. However, the formation of acetic acid is inhibited by the presence of specifically adsorbed anions, such as (bi) sulfate or acetate, which is the result of the formation of acetic acid. On the other hand, CO is readily formed at low potentials on the Pt(100) electrode, blocking completely the surface. Between 0.65 and 0.80 V, the CO layer is oxidized and the production of acetaldehyde and acetic acid is detected. The Pt(110) electrode displays the highest catalytic activity for the splitting of the C-C bond. Reactions giving rise to CO formation, from either ethanol or acetaldehyde, occur at high rate at any potential. On the other hand, the oxidation of acetaldehyde to acetic acid has probably the lower reaction rate of the three basal planes.

Evaluation of Several Carbon-Supported Nanostructured Ni-Doped Manganese Oxide Materials for the Electrochemical Reduction of Oxygen

Physical and electrochemical properties of nanostructured Ni-doped manganese oxides (MnO(x)) catalysts supported on different carbon powder substrates were investigated so as to characterize any carbon substrate effect toward the oxygen reduction reaction (ORR) kinetics in alkaline medium. These NiMnO(x)/C materials were characterized using physicochemical analyses. Small insertion of Ni atoms in the MnO(x) lattice was observed, which consists of a true doping of the manganese oxide phase. The corresponding NiMnO(x) phase is present in the form of needles or agglomerates, with crystallite sizes in the order of 1.5-6.7 nm (from x-ray diffraction analyses). Layered manganite (MnOOH) phase has been detected for the Monarch 1000-supported NiMnO(x) material, while different species of MnO(x) phases are present at the E350G and MM225 carbons. Electrochemical studies in thin porous coating active layers in the rotating ring-disk electrode setup revealed that the MnO(x) catalysts present better ORR kinetics and electrochemical stability upon Ni doping. The ORR follows the so-called peroxide mechanism on MnO(x)/C catalysts, with the occurrence of minority HO(2)(-) disproportionation reaction. The HO(2)(-) disproportionation reaction progressively increases with the Ni content in NiMnO(x) materials. The catalysts supported on the MM225 and E350G carbons promote faster disproportionation reaction, thus leading to an overall four-electron ORR pathway. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3528439] All rights reserved.

Self-organized distribution of periodicity and chaos in an electrochemical oscillator

We report a detailed numerical investigation of a prototype electrochemical oscillator, in terms of high-resolution phase diagrams for an experimentally relevant section of the control (parameter) space. The prototype model consists of a set of three autonomous ordinary differential equations which captures the general features of electrochemical oscillators characterized by a partially hidden negative differential resistance in an N-shaped current-voltage stationary curve. By computing Lyapunov exponents, we provide a detailed discrimination between chaotic and periodic phases of the electrochemical oscillator. Such phases reveal the existence of an intricate structure of domains of periodicity self-organized into a chaotic background. Shrimp-like periodic regions previously observed in other discrete and continuous systems were also observed here, which corroborate the universal nature of the occurrence of such structures. In addition, we have also found a structured period distribution within the order region. Finally we discuss the possible experimental realization of comparable phase diagrams.

Complex kinetics, high frequency oscillations and temperature compensation in the electro-oxidation of ethylene glycol on platinum

Despite the fact that the majority of the catalytic electro-oxidation of small organic molecules presents oscillatory kinetics under certain conditions, there are few systematic studies concerning the influence of experimental parameters on the oscillatory dynamics. Of the studies available, most are devoted to C1 molecules and just some scattered data are available for C2 molecules. We present in this work a comprehensive study of the electro-oxidation of ethylene glycol on polycrystalline platinum surfaces and in alkaline media. The system was studied by means of electrochemical impedance spectroscopy, cyclic voltammetry, and chronoamperometry, and the impact of parameters such as applied current, ethylene glycol concentration, and temperature were investigated. As in the case of other parent systems, the instabilities in this system were associated with a hidden negative differential resistance, as identified by impedance data. Very rich and robust dynamics were observed, including the presence of harmonic and mixed mode oscillations and chaotic states, in some parameter region. Oscillation frequencies of about 16 Hz characterized the fastest oscillations ever reported for the electro-oxidation of small organic molecules. Those high frequencies were strongly influenced by the electrolyte pH and far less affected by the EG concentration. The system was regularly dependent on temperature under voltammetric conditions but rather independent within the oscillatory regime.

Indoors lead corrosion: Reassessing the role of formaldehyde

The present work is focused on the role of formaldehyde in indoors Pb corrosion, that is still a controversial issue. Pb coupons were exposed to the atmosphere produced by formaldehyde aqueous solutions (1% and 4% in volume) and corrosion was followed by Raman Microscopy. The compounds formed in both experiments were the same, but were not in agreement with previously reported results in the literature, that identified plumbonacrite, hidrocerussite and Pb oxide. The experiments here reported have clearly shown that formates are produced on Pb surfaces exposed to formaldehyde and that oxidants, such as H(2)O(2), are not necessary. Formaldehyde oxidation also occurs with powdered PbO in a controlled environment. The Raman spectra of the Pb formates are much more complex than the Pb(HCO(2))(2) spectrum and change when exposed to room conditions, by a slow reaction with CO(2), forming Pb carbonates (hidrocerussite and plumbonacrite mostly) and Pb(HCO(2))(2). Such spectral change may be responsible for the differences in terms of chemical composition of the corrosion layer when the data here reported is compared with the literature. Other factors that must be considered are the storage conditions (particularly relative humidity and CO(2) concentration) and time; the effect of metal composition cannot be discarded as it is well known that the presence of other metals can change significantly the Pb resistance to oxidation. (C) 2010 Elsevier B.V. All rights reserved.

Pentachlorophenol (PCP) dechlorination in horizontal-flow anaerobic immobilized biomass (HAIB) reactors

This study verifies the potential applicability of horizontal-flow anaerobic immobilized biomass (HAIB) reactors to pentachlorophenol (PCP) dechlorination. Two bench-scale HAIB reactors (R1 and R2) were filled with cubic polyurethane foam matrices containing immobilized anaerobic sludge. The reactors were then continuously fed with synthetic wastewater consisting of PCP, glucose, acetic acid, and formic acid as co-substrates for PCP anaerobic degradation. Before being immobilized in polyurethane foam matrices, the biomass was exposed to wastewater containing PCP in reactors fed at a semi-continuous rate of 2.0 mu g PCP g(-1) VS. The applied PCP loading rate was increased from 0.05 to 2.59 mg PCP l(-1) day(-1) for RI, and from 0.06 to 4.15 mg PCP l(-1) day(-1) for R2. The organic loading rates (OLR) were 1.1 and 1.7 kg COD m(-3) day(-1) at hydraulic retention times (HRT) of 24 h for R1 and 18 In for R2. Under such conditions, chemical oxygen demand (COD) removal efficiencies of up to 98% were achieved in the HAIB reactors. Both reactors exhibited the ability to remove 97% of the loaded PCP. Dichlorophenol (DCP) was the primary chlorophenol detected in the effluent. The adsorption of PCP and metabolites formed during PCP degradation in the packed bed was negligible for PCP removal efficiency. (C) 2009 Elsevier Ltd. All rights reserved.

Development and validation of high performance liquid chromatographic and UV-derivative spectrophotometric methods for the determination of sotalol hydrochloride in tablets

High performance liquid chromatographic (HPLC) and UV derivative spectrophotometric (UVDS) methods were developed and validated for the quantitative determination of sotalol hydrochloride in tablets. The HPLC method was performed on a C18 column with fluorescence detection. The excitation and emission wavelengths were 235 and 310nm, respectively. The mobile phase was composed of acetonitrile-water containing 0.1% trietylamine (7:93v/v) and pH adjusted to 4.6 with formic acid. The UVDS method was performed taking a signal at 239.1nm in the first derivative. The correlation coefficients (r) obtained were 0.9998 and 0.9997 for HPLC and UVDS methods, respectively. The proposed methods are simple and adaptable to routine analysis.

The effect of methanol on the stability of Pt/C and Pt-RuO(x)/C catalysts

The behavior of Pt/C and Pt-RuO(x)/C electrodes subjected to a larger number of potential scans and constant potential for prolonged time periods was investigated in the absence and presence of methanol. The structural changes were analyzed on the basis of the modifications observed in the X-ray diffraction pattern of the catalysts. Carbon monoxide stripping experiments were performed before and after the potential scans, thus enabling analysis of the behavior of the electrochemically active surface area. The resulting solutions were examined by inductively coupled plasma mass spectrometry (ICP-MS). There was reduction in the electrochemically active surface area, as well as increase in crystallite size and dissolution of catalyst components after the potential scan tests. Catalyst degradation was more pronounced in the presence of methanol, and cyclic potential conditions accelerate the degradation mechanisms. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.