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.

Mechanistic studies of the 'blue' Cu enzyme, bilirubin oxidase, as a highly efficient electrocatalyst for the oxygen reduction reaction

The 'blue copper' enzyme bilirubin oxidase from Myrothecium verrucaria shows significantly enhanced adsorption on a pyrolytic graphite 'edge' (PGE) electrode that has been covalently modified with naphthyl-2-carboxylate functionalities by diazonium coupling. Modified electrodes coated with bilirubin oxidase show electrocatalytic voltammograms for the direct, four-electron reduction of O(2) by bilirubin oxidase with up to four times the current density of an unmodified PGE electrode. Electrocatalytic voltammograms measured with a rapidly rotating electrode (to remove effects of O(2) diffusion limitation) have a complex shape (an almost linear dependence of current on potential below pH 6) that is similar regardless of how PGE is chemically modified. Importantly, the same waveform is observed if bilirubin oxidase is adsorbed on Au(111) or Pt(111) single-crystal electrodes (at which activity is short-lived). The electrocatalytic behavior of bilirubin oxidase, including its enhanced response on chemically-modified PGE, therefore reflects inherent properties that do not depend on the electrode material. The variation of voltammetric waveshapes and potential-dependent (O(2)) Michaelis constants with pH and analysis in terms of the dispersion model are consistent with a change in rate-determining step over the pH range 5-8: at pH 5, the high activity is limited by the rate of interfacial redox cycling of the Type 1 copper whereas at pH 8 activity is much lower and a sigmoidal shape is approached, showing that interfacial electron transfer is no longer a limiting factor. The electrocatalytic activity of bilirubin oxidase on Pt(111) appears as a prominent pre-wave to electrocatalysis by Pt surface atoms, thus substantiating in a single, direct experiment that the minimum overpotential required for O(2) reduction by the enzyme is substantially smaller than required at Pt. At pH 8, the onset of O(2) reduction lies within 0.14 V of the four-electron O(2)/2H(2)O potential.

Electrochemical oxidation of acid black 210 dye on the boron-doped diamond electrode in the presence of phosphate ions: Effect of current density, pH, and chloride ions

The electrochemical oxidation of acid black 210 dye (AB-210) on the boron-doped diamond (BDD) was investigated under different pH conditions. The best performance for the AB-210 oxidation occurred in alkaline phosphate solution. This is probably due to oxidizing agents such as phosphate radicals and peroxodiphosphate ions, which can be electrochemically produced with good yields on the BDD anode, mainly in alkaline solution. Under this condition, the COD (chemical oxygen demand) removal was higher than that obtained from the model proposed by Comninellis. Electrolyses performed in phosphate buffer and in the presence of chloride ions resulted in faster COD and color removals in acid and neutral solutions, but in alkaline phosphate solution, a better performance in terms of TOC removal was obtained in the absence of chloride. Moreover, organochloride compounds were detected in all electrolyses performed in the presence of chloride. The AB-210 electrooxidation on BDD using phosphate as supporting electrolyte proved to be interesting since oxidizing species generated from phosphate ions were able to completely degrade the dye without producing organochloride compounds. (C) 2009 Elsevier Ltd. All rights reserved.

The impact of water concentration on the catalytic oxidation of ethanol on platinum electrode in concentrated phosphoric acid

The electro-oxidation of ethanol on platinum in phosphoric acid opens the door to promote the oxidation reaction at higher temperatures. However, the effect of the presence of water is not well understood. In this work, the electro-oxidation of ethanol on platinum was studied in concentrated phosphoric acid containing different concentrations of water at room temperature. The results show that effect of bulk water on the rate electro-oxidation is highest at 0.60 V and decreases for increasing potentials. This was suggested as due to the increasing formation of oxygenated species on the electrode surface with potential, which in turn is more efficient than the increase of water content in the electrolyte. Altogether, these results were interpreted as an evidence of a Langmuir-Hinshelwood step involving oxygenated species as one of the adsorbed partners. (C) 2009 Elsevier B.V. All rights reserved.

The use of a graphite-silicone rubber composite electrode in the determination of rutin in pharmaceutical formulation

The possibility of using a graphite silicone-rubber composite electrode (GSR) in a differential pulse voltammetric(DPV) procedure for rutin (vitamin P) determination is described. Cyclic voltammograms of rutin presented a reversible pair of oxidation/reduction peaks respectively at 0.411 and 0.390 V (vs. SCE) at the GSR surface in Britton-Robinson(B-R) buffer solution pH 4.0. In DPV after optimization of conditions, an oxidation peak at 0.370 V (vs. SCE) was used to quantitative determination of rutin in B-R buffer solution pH 4.0. In this case a linear dynamic range of 5.0×10-8 to 50.0×10-8 mol L-1 was observed with a detection limit of 1.8×10-8 mol L-1 for the analyte. Recoveries from 94 to 113% were observed. The electrode surface was renewed by polishing after each determination, with a repeatability of 1.09 ± 0.06 µA (n = 10) peak current. Rutin was determined in a pharmaceutical formulation using the proposed electrode and the results agreed with those from an official method within 95% confidence level.

Screening process for activity determination of conductive oxide electrodes for organic oxidation

A modified method for the calculation of the normalized faradaic charge (q fN) is proposed. The method involves the simulation of an oxidation process, by cyclic voltammetry, by employing potentials in the oxygen evolution reaction region. The method is applicable to organic species whose oxidation is not manifested by a defined oxidation peak at conductive oxide electrodes. The variation of q fN for electrodes of nominal composition Ti/RuX Sn1-X O2 (x = 0.3, 0.2 and 0.1), Ti/Ir0.3Ti0.7O2 and Ti/Ru0.3Ti0.7O2 in the presence of various concentrations of formaldehyde was analyzed. It was observed that electrodes containing SnO2 are the most active for formaldehyde oxidation. Subsequently, in order to test the validity of the proposed model, galvanostatic electrolyses (40 mA cm-2) of two different formaldehyde concentrations (0.10 and 0.01 mol dm-3) were performed. The results are in agreement with the proposed model and indicate that this new method can be used to determine the relative activity of conductive oxide electrodes. In agreement with previous studies, it can be concluded that not only the nature of the electrode material, but also the organic species in solution and its concentration are important factors to be considered in the oxidation of organic compounds.

Characterization of thiol-functionalised silica films deposited on electrode surfaces

Thiol-functionalised silica films were deposited on various electrode surfaces (gold, platinum, glassy carbon) by spin-coating sol-gel mixtures in the presence of a surfactant template. Film formation occurred by evaporation induced self-assembly (EISA) involving the hydrolysis and (co)condensation of silane and organosilane precursors on the electrode surface. The characterization of such material was performed by IR spectroscopy, thermogravimetry (TG), elemental analysis (EA), atomic force microscopy (AFM), scanning electron microscopy (SEM) and cyclic voltammetry (CV).

Determination of paracetamol at a graphite-polyurethane composite electrode as an amperometric flow detector

A bare graphite-polyurethane composite was evaluated as an amperometric flow injection detector in the determination of paracetamol (APAP) in pharmaceutical formulations. A linear analytical curve was observed in the 5.00 x 10-5 to 5.00 x 10-3 mol L-1 range with a minimum detectable net concentration of 18.9 µmol L-1 and 180 determinations h-1, after optimization of parameters such as the detection potential, sample loop volume, and carrier solution flow rate. Interference of ascorbic acid was observed, however, it was possible overcome the interference, reaching results that agreed with HPLC within 95% confidence level. These results showed that the graphite-polyurethane composite can be used as an amperometric detector for flow analysis in the determination of APAP.

Electrochemical Reduction Using Glassy Carbon Electrode in Aqueous Medium of a Potential Anti-Chagas Drug: NFOH

Nitrofurazone (NF) presents activity against Chagas' disease, yet it has a high toxicity. Its analog, hydroxymethylnitrofurazone (NFOH), is more potent against Trypanosoma cruzi and much less toxic than the parent drug, NF. The electrochemical reduction of NFOH in an aqueous medium using a glassy carbon electrode (GCE) is presented. By cyclic voltammetry in anacidic medium, one irreversible reduction peak related to hydroxylamine derivative formation was registered, being linearly pH dependent. However, from pH > 7, a reversible reduction peak at a more positive potential appears and corresponds to the formation of a nitro radical anion. The radical-anion kinetic stability was evaluated by Ip(a)/Ip(c) the current ratio of the R-NO(2)/R-NO(2)-redox couple. The nitro radical anion decays with a second-order rate constant (k(2)) of 6.07, 2.06, and 1.44(X 10(3)) L mol(-1) s(-1) corresponding to pH 8.29, 9.29, and 10.2, respectively, with a corresponding half-time life (t(1/2)) of 0.33, 0.97, and 1.4 s for each pH value. By polishing the GCE surface with diamond powder and comparing with the GCE surface polished with alumina, it is shown that the presence of alumina affects the lifetime of the nitro radical anion. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3130082] All rights reserved.

Signatures of oxygen on Cu(3)Au(100): From isolated impurity to oxide regimes

We analyze the scanning tunneling microscopy (STM) signatures for the O/Cu(3)Au(100) surface from the low-coverage (isolated impurity) to high-coverage (oxide) regimes. First-principles calculations show that oxygen signatures switch from dark to bright spots as the oxygen coverage increases. This behavior is nicely traced back to a change in the oxygen orbital character of the Fermi-level electronic states. Our results allow for the chemical identification by STM of oxygen and copper atoms in the fully ordered O/Cu(3)Au(100)-c(2x2) surface.

Composite electrode of carbon nanotubes and vitreous carbon for electron field emission

In this work, the electron field emission behaviour of electrodes formed by carbon nanotubes (CNTs) grown onto monolithic vitreous carbon (VCarbon) substrates with microcavities is presented. Scanning electron microscopy was used to characterize the microstructure of the films. Tungsten probes, stainless steel sphere, and phosphor electrodes were employed in the electron field emission study. The CNT/VCarbon composite represents a route to inexpensive excellent large area electron emission cathodes with fields as low as 2.1 V mu m(-1). In preliminary lifetime tests for a period of about 24 h at an emission current of about 4 mA cm(-2), there is an onset degradation of the emission current of about 28%, which then stabilizes. Electron emission images of the composites show the cavity of the samples act as separate emission sites and predominantly control the emission process. The emission of CNTs/VCarbon was found to be stable for several hours. (c) 2008 American Institute of Physics.

Hyperbaric Oxygen Prevents Early Death Caused by Experimental Cerebral Malaria

Background: Cerebral malaria (CM) is a syndrome characterized by neurological signs, seizures and coma. Despite the fact that CM presents similarities with cerebral stroke, few studies have focused on new supportive therapies for the disease. Hyperbaric oxygen (HBO) therapy has been successfully used in patients with numerous brain disorders such as stroke, migraine and atherosclerosis. Methodology/Principal Findings: C57BL/6 mice infected with Plasmodium berghei ANKA (PbA) were exposed to daily doses of HBO (100% O(2), 3.0 ATA, 1-2 h per day) in conditions well-tolerated by humans and animals, before or after parasite establishment. Cumulative survival analyses demonstrated that HBO therapy protected 50% of PbA-infected mice and delayed CM-specific neurological signs when administrated after patent parasitemia. Pressurized oxygen therapy reduced peripheral parasitemia, expression of TNF-alpha, IFN-gamma and IL-10 mRNA levels and percentage of gamma delta and alpha beta CD4(+) and CD8(+) T lymphocytes sequestered in mice brains, thus resulting in a reduction of blood-brain barrier (BBB)dysfunction and hypothermia. Conclusions/Significance: The data presented here is the first indication that HBO treatment could be used as supportive therapy, perhaps in association with neuroprotective drugs, to prevent CM clinical outcomes, including death.

The role of oxygen vacancy in the photoluminescence property at room temperature of the CaTiO(3)

In this paper, electron paramagnetic resonance, photoluminescence (PL) emission, and quantum mechanical calculations were used to observe and understand the structural order-disorder of CaTiO(3), paying special attention to the role of oxygen vacancy. The PL phenomenon at room temperature of CaTiO(3) is directly influenced by the presence of oxygen vacancies that yield structural order-disorder. These oxygen vacancies bonded at Ti and/or Ca induce new electronic states inside the band gap. Ordered and disordered CaTiO(3) was obtained by the polymeric precursor method. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3190524]

Direct evidence of singlet molecular oxygen generation from peroxynitrate, a decomposition product of peroxynitrite

The decomposition of peroxynitrite to nitrite and dioxygen at neutral pH follows complex kinetics, compared to its isomerization to nitrate at low pH. Decomposition may involve radicals or proceed by way of the classical peracid decomposition mechanism. Peroxynitrite (ONOOH/ONOO(-)) decomposition has been proposed to involve formation of peroxynitrate (O(2)NOOH/O(2)NOO(-)) at neutral pH (D. Gupta, B. Harish, R. Kissner and W. H. Koppenol, Dalton Trans., 2009, DOI: 10.1039/b905535e, see accompanying paper in this issue). Peroxynitrate is unstable and decomposes to nitrite and dioxygen. This study aimed to investigate whether O(2)NOO(-) formed upon ONOOH/ONOO(-) decomposition generates singlet molecular oxygen [O(2) ((1)Delta(g))]. As unequivocally revealed by the measurement of monomol light emission in the near infrared region at 1270 nm and by chemical trapping experiments, the decomposition of ONOO(-) or O(2)NOOH at neutral to alkaline pH generates O(2) ((1)Delta(g)) at a yield of ca. 1% and 2-10%, respectively. Characteristic light emission, corresponding to O(2) ((1)Delta(g)) monomolecular decay was observed for ONOO(-) and for O(2)NOOH prepared by reaction of H(2)O(2) with NO(2)BF(4) and of H(2)O(2) with NO(2)(-) in HClO(4). The generation of O(2) ((1)Delta(g)) from ONOO(-) increased in a concentration-dependent manner in the range of 0.1-2.5 mM and was dependent on pH, giving a sigmoid pro. le with an apparent pK(a) around pD 8.1 (pH 7.7). Taken together, our results clearly identify the generation of O(2) ((1)Delta(g)) from peroxynitrate [O(2)NOO(-) -> NO(2)(-) + O(2) ((1)Delta(g))] generated from peroxynitrite and also from the reactions of H(2)O(2) with either NO(2)BF(4) or NO(2)(-) in acidic media.