Controle de qualidade da vitamina C em produtos acabados

Vitamin C is vulnerable to oxidative degradation and then the aim of this study was conducting a literature review about quality control of vitamin C in finished products. The conclusion is determined multiple emulsions and products which contain encapsulant materials are able to protect it from oxidation ensuring its efficacy.

O efeito da ordenação e estrutura geométrica dos materiais PtSn/C e PdSn/C na eletrooxidação do glicerol em meio alcalino

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Influência do método de preparação de eletrocatalisadores PtRu/C sobre a atividade catalítica frente à reação de oxidação de etanol em meio ácido

In this work the influence of variations in the borohydrate reduction method on the properties of PtRu/C electrocatalysts was investigated. The electrocatalysts were prepared using 1:1 ; 2:1; 5:1; 50:1 and 250:1 molar ratios of NaBH₄ to metals. The reduction was also performed by dripping or by fast addition of the solution. The results showed that PtRu nanoparticles obtained by fast addition had the smallest crystallite sizes. It was also noted that the catalytic activity increased as the borohydrate:metal molar ratio increased. The PtRu/C electrocatalyst (50:1) obtained by fast addition presented the best catalytic activity for ethanol electro-oxidation.

The impact of the alkali cation on the mechanism of the electro-oxidation of ethylene glycol on Pt

By means of in situ IR spectroscopy we investigate the effect of dissolved alkali cations on the electro-oxidation of ethylene glycol on platinum in alkaline media. The results revealed that the increase in the oxidation currents (Li(+) < Na(+) < K(+)) is reflected in the increase in the ratio between carbonate and oxalate produced.

The influence of B4C and MgB2 additions on the behavior of MgO-C bricks

Carbon-containing refractory materials have received great attention over the last years due to their importance in the steelmaking process. The oxidation of carbon present in refractory materials at temperatures above 500 degrees C is usually accompanied by the decrease of their mechanical strength and chemical resistance. Aiming to improve the oxidation resistance of carbon-oxide refractories, the use of materials known as antioxidants has been extensively studied. In this work we evaluated the performance of MgB2 and B4C antioxidants when incorporated into MgO-C bricks. We observed that the co-addition of metallic antioxidants and B4C or MgB2 leads to refractory bricks with enhanced hot modulus of rupture and resistance against oxidation and slag corrosion. However, the excessive addition of these antioxidants could impair the performance of the obtained bricks. Thus, when determining the optimum concentration of MgB2 and B4C to be added into MgO-C refractories, one must take into consideration this behavior. (c) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Electrocatalytic Activity of Platinum-Niobium Nanoparticles for Ethanol Oxidation

Three nanostructured platinum-niobium supported on Vulcan XC-72R carbon black materials were prepared as catalysts for the ethanol electroxidation: (i) deposition of platinum and niobium on Vulcan XC-72R carbon black, (ii) platinum decorated on a mixture of commercial amorphous Nb2O5/carbon black, and (iii) the same than ii but using crystalline Nb2O5, by reduction of the precursors with sodium borohydride in ethanol. All the catalysts showed platinum crystal sizes in the range of 3-4 nm, with no or little modification of the lattice parameter. The analyses of the electronic structure from the XANES region of the XAS spectra displayed some interactions between platinum and niobium, despite the niobium was primarily in the form of pentoxide in all the catalysts. CO stripping exhibited a promising low onset potential and a large current density, especially in the case of the deposited catalyst. Ethanol electroxidation experiments revealed that the Pt-Nb(2)O(5)crystalline/C generated the largest current. However it was not effective to completely oxidize ethanol, leading to acetic acid as the main product. In this sense, the highest efficiency for the complete oxidation of ethanol was obtained for the deposited catalyst. These results were interpreted in terms of the physico-chemical characteristic displayed by the different catalysts. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.040210jes] All rights reserved.

On the stability of the extracellular hemoglobin of Glossoscolex paulistus, in two iron oxidation states, in the presence of urea

The stability of the Glossoscolex paulistus hemoglobin (HbGp), in two iron oxidation states (and three forms), as monitored by optical absorption, fluorescence emission and circular dichroism (CD) spectroscopies, in the presence of the chaotropic agent urea, is studied. HbGp oligomeric dissociation, denaturation and iron oxidation are observed. CD data show that the cyanomet-HbGp is more stable than the oxy-form. Oxy- and cyanomet-HbGp show good fits on the basis of a two state model with critical urea concentrations at 220-222 nm of 5.1 +/- 0.2 and 6.1 +/- 0.1 mol/L, respectively. The three-state model was able to reveal a subtle second transition at lower urea concentration (1.0-2.0 mol/L) associated to partial oligomeric dissociation. The intermediate state for oxy- and cyanomet-HbGp is very similar to the native state. For met-HbGp, a different equilibrium, in the presence of urea, is observed. A sharp transition at 1.95 +/- 0.05 mol/L of denaturant is observed, associated to oligomeric dissociation and hemichrome formation. In this case, analysis by a three-state model reveals the great similarity between the intermediate and the unfolded states. Analysis of spectroscopic data, by two-state and three-state models, reveals consistency of obtained thermodynamic parameters for HbGp urea denaturation. (C) 2012 Elsevier Inc. All rights reserved.

Electrocatalytic performance of environmentally friendly synthesized gold nanoparticles towards the borohydride electro-oxidation reaction

In this paper we report a simple and environmentally friendly synthesis of gold nanoparticles (AuNps) and their electrocatalytic activity for borohydride oxidation reaction (BOR). Ultraviolet spectroscopy (UV- vis) and transmission electron microscopy (TEM) confirmed the formation of poly(vinyl pyrrolidone)protected colloidal AuNps through direct reduction of Au3+ by glycerol in alkaline medium at room temperature. For the BOR tests the AuNps were directly produced onto carbon to yield the Au/C catalyst. Levich plots revealed that the process occured via 7.2 electrons, therefore near the theoretical value of 8 electrons. When compared to bulk Au, the gold nanoparticles presented enhanced catalytic properties since the onset potential for BOR was shifted 200 mV towards negative potentials. (C) 2012 Elsevier B.V. All rights reserved.

Partial oxidation of methane on NiO-MgO-ZrO2 catalysts

Catalysts containing mixtures of NiO, MgO and ZrO2 were synthesized by the polymerization method. They were characterized by X-ray diffraction (XRD), physisorption of N-2 (BET), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES), and then tested in the partial oxidation of methane (POM) in the presence of air (2CH(4):1O(2)) at 750 degrees C for 6 h. Among the ternary oxides, the catalyst with 40 mol% MgO showed the highest conversion rates in the catalytic processes, but also the highest carbon deposition values (48 mmol h (1)). The greater the amount of NiO-MgO solid solution formed, the higher was the conversion rate of reactants (CH4), peaking at 40 mol% of MgO. Catalysts with lower Ni content on the surface achieved a high rate of CH4 conversion into synthesis gas (H-2 + CO). The formation of more NiO-MgO solid solution seemed to inhibit the deactivation of Ni degrees during reaction. The values of the H-2/CO product ratio were generally found to be slightly lower than stoichiometric. (C) 2012 Elsevier Ltd. All rights reserved.

An optimization study of PtSn/C catalysts applied to direct ethanol fuel cell: Effect of the preparation method on the electrocatalytic activity of the catalysts

The aim of this work was to perform a systematic study of the parameters that can influence the composition, morphology, and catalytic activity of PtSn/C nanoparticles and compare two different methods of nanocatalyst preparation, namely microwave-assisted heating (MW) and thermal decomposition of polymeric precursors (DPP). An investigation of the effects of the reducing and stabilizing agents on the catalytic activity and morphology of Pt75Sn25/C catalysts prepared by microwave-assisted heating was undertaken for optimization purposes. The effect of short-chain alcohols such as ethanol, ethylene glycol, and propylene glycol as reducing agents was evaluated, and the use of sodium acetate and citric acid as stabilizing agents for the MW procedure was examined. Catalysts obtained from propylene glycol displayed higher catalytic activity compared with catalysts prepared in ethylene glycol. Introduction of sodium acetate enhanced the catalytic activity, but this beneficial effect was observed until a critical acetate concentration was reached. Optimization of the MW synthesis allowed for the preparation of highly dispersed catalysts with average sizes lying between 2.0 and 5.0 nm. Comparison of the best catalyst prepared by MW with a catalyst of similar composition prepared by the polymeric precursors method showed that the catalytic activity of the material can be improved when a proper condition for catalyst preparation is achieved. (C) 2012 Elsevier B.V. All rights reserved.

Photo-Fenton oxidation of phenol and organochlorides (2,4-DCP and 2,4-D) in aqueous alkaline medium with high chloride concentration

A highly concentrated aqueous saline-containing solution of phenol, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4-dichlorophenol (2.4-DCP) was treated by the photo-Fenton process in a system composed of an annular reactor with a quartz immersion well and a medium-pressure mercury lamp (450 W). The study was conducted under special conditions to minimize the costs of acidification and neutralization, which are usual steps in this type of process. Photochemical reactions were carried out to investigate the influence of some process variables such as the initial concentration of Fe2+ ([Fe2+](0)) from 1.0 up to 2.5 mM, the rate in mmol of H2O2 fed into the system (F-H2O2,F-in) from 3.67 up to 7.33 mmol of H2O2/min during 120 min of reaction time, and the initial pH (pH(0)) from 3.0 up to 9.0 in the presence and absence of NaCl (60.0 g/L). Although the optimum pH for the photo-Fenton process is about 3.0, this particular system performed well in experimental conditions starting at alkaline and neutral pH. The results obtained here are promising for industrial applications, particularly in view of the high concentration of chloride, a known hydroxyl radical scavenger and the main oxidant present in photo-Fenton processes. (C) 2012 Elsevier Ltd. All rights reserved.

A comparative study of the electrochemical oxidation of the herbicide tebuthiuron using boron-doped diamond electrode

The thiadiazolylurea derivative tebuthiuron (TBH) is commonly used as an herbicide even though it is highly toxic to humans. While various processes have been proposed for the removal of organic contaminants of this type from wastewater, electrochemical degradation has shown particular promise. The aim of the present study was to investigate the electrochemical degradation of TBH using anodes comprising boron-doped (5000 and 30000 ppm) diamond (BDD) films deposited onto Ti substrates operated at current densities in the range 10-200 mA cm(-2). Both anodes removed TBH following a similar pseudo first-order reaction kinetics with k(ap)p close to 3.2 x 10(-2) min(-1). The maximum mineralization efficiency obtained was 80%. High-pressure liquid chromatography with UV-VIS detection established that both anodes degraded TBH via similar intermediates. Ion chromatography revealed that increasing concentrations of nitrate ions (up to 0.9 ppm) were formed with increasing current density, while the formation of nitrite ions was observed with both anodes at current densities >= 150 mA cm(-2). The BDD film prepared at the lower doping level (5000 ppm) was more efficient in degrading TBH than its more highly doped counterpart. This unexpected finding may be explained in terms of the quantity of impurities incorporated into the diamond lattice during chemical vapor deposition. (C) 2012 Elsevier Ltd. All rights reserved.

Investigation of a Pt3Sn/C Electro-Catalyst in a Direct Ethanol Fuel Cell Operating at Low Temperatures for Portable Applications

A 20% Pt3Sn/C catalyst was prepared by reduction with formic acid and used in a direct ethanol fuel cell at low temperatures. The electro-catalytic activity of this bimetallic catalyst was compared to that of a commercial 20% Pt/C catalyst. The PtSn catalyst showed better results in the investigated temperature range (30 degrees-70 degrees C). Generally, Sn promotes ethanol oxidation by adsorption of OH species at considerably lower potentials compared to Pt, allowing the occurrence of a bifunctional mechanism. The bimetallic catalyst was physico-chemically characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. The presence of SnO2 in the bulk and surface of the catalyst was observed. It appears that SnO2 can enhance the ethanol electro-oxidation activity at low potentials due to the supply of oxygen-containing species for the oxidative removal of CO and CH3CO species adsorbed on adjacent Pt active sites.

Effects of catalyst load in Pt and Pb-based catalysts using formic acid oxidation as a model

In this paper, we discuss the effects of catalyst load with respect to carbon powder for several Pt and Pb-based catalysts, using formic acid as a model molecule. The discussion is based on electrochemical tests, a complete morphological investigation and theoretical calculations. We show that the Pt and Pb-based catalysts presented activity in formic acid oxidation at very low catalyst loads (e.g., 0.5% in respect to the carbon content). Physical characterisations demonstrate that the electrodes are composed of separated phases of Pt and lead distributed in Pt nanometric-sized islands that are heterogeneously dispersed on the carbon support and Pb ultra-small particles homogeneously distributed throughout the entire carbon surface, as demonstrated by the microscopy studies. At high catalyst loads, very large clusters of Pb(x)O(y) could be observed. Electrochemical tests indicated an increase in the apparent resistance of the system (by a factor of 19.7 Omega) when the catalyst load was increased. The effect of lead in the materials was also studied by theoretical calculations (OFT). The main conclusion is that the presence of Pb atoms in the catalyst can improve the adsorption of formic acid in the catalytic system compared with a pure Pt-based catalyst. (C) 2011 Elsevier B.V. All rights reserved.

Structural features of lignin obtained at different alkaline oxidation conditions from sugarcane bagasse

Lignin is a macromolecule frequently obtained as residue during technological processing of biomass. Modifications in chemical structure of lignin generate valuable products, some with particular and unique characteristics. One of the available methods for modification of industrial lignin is oxidation by hydrogen peroxide. In this work, we conducted systematic studies of the oxidation process that were carried out at various pHs and oxidizing agent concentrations. Biophysical, biochemical, structural properties of the oxidized lignin were analyzed by UV spectrophotometry, Fourier transform infrared spectroscopy, scanning electron microscopy and small angle X-ray scattering. Our results reveal that lignin oxidized with 9.1% H(2)O(2) (m/v) at pH 13.3 has the highest fragmentation, oxidation degree and stability. Although this processing condition might be considered quite severe, we have concluded that the stability of the obtained oxidized lignin was greatly increased. Therefore, the identified processing conditions of oxidation may be of practical interest for industrial applications. (C) 2011 Elsevier B.V. All rights reserved.