First Stage Of Oxidation Research Articles

Influence of the Neutralization Step on the Oxidative and Thermal Stability of Acid Olive Oil

This study elucidate the modification on oxidative and thermal stability of acid olive oils during the neutralization step. The neutralization of an acidified olive oil, chosen as model, was carried out by substituting lime (calcium hydroxide) for soda (sodium hydroxide) as the neutralizing agent. Resulting olive oil preserved almost 95% of their alpha-tocopherol content and had higher temperatures of thermal decomposition than oils neutralized with soda. Oils neutralized with lime had better oxidative stability since the losses of their natural antioxidants, particularly tocopherols, were very limited. The neutralization with soda was accompanied by a passage of epoxides and hydroperoxides (products of first stage of oxidation) to alpha, beta-unsaturated aldehydes and ketones (products of second stage of oxidation). These purposes were confirmed by thermal spectrophotometric analyses and the increase of the carbonyl value after neutralization.

Imaging by Photoelectrochemical Techniques of Laves-Phases γ Zr(Fe,Cr)<sub>2 </sub> Thermally Oxidized on Zircaloy-4

The oxidation of γ-Zr(Fe,Cr)2 intermetallic particles during the thermal exposition of Zircaloy-4 at 470°C in oxygen was investigated with PhotoElectroChemical techniques (PEC). Via the measurement of bandgap, haematite Fe2O3 (2.2 eV), rhomboedric solid solution (FexCr1-x)2O3 (2.6 eV) and chromia Cr2O3 (3.0 eV) phases were identified as components of oxidised particles. Evolution of size, lateral distribution and density of these particles was studied in function of zirconia scale thickness. During the first stage of oxidation, the density of oxidised particles increased with thickness but decreased during a second stage, highlighting in an innovative way the phenomenon of haematite and chromia dissolution in the zirconia matrix. It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in an oxide scale at the micron scale.

Modeling of oxidation of structural materials in LBE systems

In recent years, liquid metal alloys have been examined in the light of various applications in technical systems the most famous example is the sodium cooled Fast Breeder Reactor. One major problem in non-isothermal heavy liquid metal systems lies in the corrosion of their structural components. The formation of oxide scales on the structural components is considered as a viable measure in limiting the dissolution rates in the hot parts in lead and lead–bismuth loops. Models for oxide scale growth under the action of flowing liquid metals have been implemented in the newly developed code MATLIM, which allow calculating the evolution of the oxide scales on structural materials in multi-modular loops. There are thermo-hydraulic limitations on oxygen supply from the liquid metal to the structural materials, the oxygen mass transfer coefficient in the liquid metal, which depends on the flow conditions, being rate-determining. This seems to explain, for example, why in the first stage of oxidation of stainless steels slowly growing, dense single layer Fe/Cr spinel scales are formed.

Correlation between transport, dielectric, and optical properties of oxidized and nonoxidized porous silicon

Using the correlation between transport, dielectric relaxation phenomena, and the state of oxidation of porous silicon, we have found that there are two major transport routes in the porous silicon medium. The first route of conduction via the disordered tissue that surrounds the silicon nanocrystals dominates for nonoxidized porous silicon and gradually disappears during the first stage of oxidation, where the disordered tissue is oxidized. The second route of tunneling and hopping in between the nanocrystals persists up to the second stage of oxidation where the nanocrystals are oxidized. This dual transport route model is consistent with the presence of two Meyer-Neldel rules and two power-law ac conductivities at mid and high temperatures. In addition, we have found that dc conduction is limited by narrow geometrical constrictions along the transport path that give rise to a Coulomb blockade type activation process. At lower temperatures the transport is characterized by a Cole-Cole relaxation associated with thermally activated intercrystallites hopping due to the shorter distances needed to walk by the carriers. Finally, for oxidized porous silicon we have found a new relaxation mechanism of interfacial polarization that is characterized by long relaxation times at high temperatures.

Protection of Lipids from Oxidation by Epicatechin, trans-Resveratrol, and Gallic and Caffeic Acids in Intestinal Model Systems

Consumption of polyphenols is associated with health promotion through diet, although many are poorly absorbed in animals and humans alike. Lipid peroxides may reach the intestine and initiate deleterious oxidation. Here we measured inhibition of the oxidation of linoleic acid (LA) in authentic fluid from rat small intestine (RIF) by two dietary polyphenols, a flavonoid, epicatechin (EC), and a stilbene, resveratrol (RV), and by gallic (GA) and caffeic (CA) acids, and their partition coefficients. Both polyphenols inhibited 80%, and CA inhibited 65%, of the production of hexanal. GA was the weakest antioxidant in this assay. Interestingly, measuring peroxides production in RIF showed that only epicatechin inhibited the first stage of oxidation. The oxidizing agent, the antioxidant comound, the solution pH and lipophilicity are known to affect the total antioxidative activity. We suggest that the mechanism of this activity changes in accord with the environment: i.e., RV may act as a free radial scavenger, but here, in protecting lipids in intestinal fluid from oxidation, it acts as a hydrogen atom donor. Since the concentration of phenolics is much higher in the intestinal fluid than is ever achieved in plasma or other body tissues, it is suggested that their antioxidant activity could be exerted in the gastrointestinal tract (GIT), breaking the propagation of lipid peroxides oxidation and production of toxic compounds.

The preparation, crystal structure and electrochemistry of (5 Z,5′ Z)-2,2′-(alkane-α,ω-diylsulfanyldiyl)bis(5-(3-pyridylmethylene)-3,5-dihydro-4 H-imidazol-4-ones) and their complexes with cobalt(II) chloride

A series of S-alkylated derivatives of 2-thiohydantoins, containing two 5-(3-pyridylmethylene)-3,5-dihydro-4 H-imiazole-4-one fragments joined by polymethylene bridges with different numbers of carbon atoms, was synthesized. The title (5 Z,5′ Z)-2,2′-(alkane-α,ω-diylsulfanyldiyl)bis(5-(3-pyridylmethylene)-3,5-dihydro-4 H-imidazol-4-ones) ( L) were obtained by alkylation of 5-(3-pyridylmethylene)-3-phenyl-2-thiohydantoine by 1,2-dibromoethane, 1,6-dibromohexane or 1,10-dibromodecane in DMF in the presence of potassium carbonate. The complexes of ligands L with CoCl 2 · 6H 2O have been synthesized. It is shown that, regardless of the L:CoCl 2 ratio, complexes with LCoCl 2 composition are obtained in all cases. The structure of the cobalt(II) chloride complex with (5 Z,5′ Z)-2,2′-(ethane-1,2-diyldisulfanyldiyl)bis(5-(3-pyridylmethylene)-3-phenyl-3,5-dihydro-4 H-imidazol-4-one) was determined by means of the RSA method. The cobalt atom in this complex has a tetrahedral ligand environment and it is coordinated by two chloride anions and two nitrogen atoms of pyridine fragments, forming a 19-membered metallocycle. Electrochemical investigations of the synthesized ligands and complexes have been made by CVA and RDE methods. It is established that the first stage of reduction for the complexes takes place on the metal, whereas the first stage of oxidation takes place on the coordinated chloride-anions.

Magnetization and magnetic anisotropy energy of ultrathin Fe films on GaAs(001) exposedto oxygen

The remanent magnetization and the magnetic anisotropy energy of 5–16 monolayer (ML)epitaxial Fe films grown on GaAs(001) were quantitatively monitored in situ bysuperconducting quantum interference device magnetometry and ferromagnetic resonanceas a function of oxygen exposure at 300 K. We find that at an oxygen dose of 10 L theperpendicular uniaxial anisotropy decreases by almost 50%, whereas the magnetization is decreased by8% only in the case of a 5 ML Fe film. The fourfold anisotropy constantK4 and in-plane uniaxial anisotropy constant , however, remain unchanged. Low-energy electron diffraction spots of the Fe surface disappearafter 6 L of oxygen, indicating a disordered Fe oxide growth. For exposures below 70 L we observea reduction of the remanent magnetization irrespective of the film thickness. At exposures>70 L we find a faster decrease of the remanent magnetization for the thinner films which isattributed to the finite size effect and a thickness dependent surface roughness of the ironfilms, respectively. As evidenced by Auger electron spectroscopy and x-ray diffraction, theiron oxide is found to be FeO in the first stage of oxidation, transforming intoγ-Fe2O3 at higherdosages (>1000 L).

Influence of Oxygen Dissociation on the Oxidation of Iron

Kinetics of the oxidation of iron in fluxes of atomic and molecular oxygen have been studied over a wide temperature range from 573 to 1273 K. The influence of oxygen pre-dissociation on the oxidation rate was found negligible at 573–1073 K. At temperatures above 1073 K, when only FeO is formed, the rate of iron oxidation in atomic oxygen is substantially higher than that in molecular oxygen. Decarburization occurs during the first stage of oxidation of iron containing carbon. The rate of carbon elimination in atomic oxygen exceeds that in molecular oxygen due to the higher chemical potential of atomic gas.

On the Role Played by Dimers of Radical Cations in the Process of Electrochemical Oxidation–Reduction of Polyaniline: The Data that Were Obtained Using the Method of Cyclic Voltabsorptometry in the Presence of Counteranions of a Diverse Nature

The electrochemical oxidation–reduction of films of polyaniline is studied in the presence of various counteranions. The differential cyclic voltabsorptometric (DCVA) curves (dA/dt relative to potential, where A is the optical absorption) are recorded at characteristic wavelengths corresponding to individual spectrum components previously isolated with the Alentsev–Fok method. The DCVA curves for aqueous solutions of HCl, HClO4, and H2SO4 at different potential scan rates are compared to traditional cyclic voltammograms. For the DCVA curves obtained at 665 nm discovered is one broad peak of dA/dt in the region of potentials between the first and second stages of polyaniline oxidation ( 0.6 V vs. Ag/AgCl). An assumption is made about a chemical nature of the process of generation of absorption in this spectral region, which, more likely than not, is connected with the emergence of dimers of radical cations of polyaniline. It is discovered that the electrochemical processes in the region of potentials that correspond to the first stage of oxidation give rise to variations in the absorption inside several regions of spectrum: 435 nm (radical cations) and 755 nm (localized polarons). An assumption is made that the first peak of the current corresponds to several successive processes that occur in a polyaniline molecule in the course of oxidation as well as to the existence of heterogeneous regions where the generation of localized polarons proceeds at different rates.

Effects of hydrogen on the oxidation of coarse decrepitated powder of Nd–Dy–Fe–B alloy

Four kinds of coarse Nd–Dy–Fe–B powder were oxidized at room temperature in air and in a dryer, which were dehydrogenated at different temperatures in vacuum after hydrogen decrepitation (HD). The oxidation and water absorption of these coarse powders were investigated by a weight change method. It was found that the undegassed powder had a higher oxidation rate, as well as higher water absorption, than the dehydrogenated powder, especially in the first stage of oxidation. The higher oxidation resistance of degassed powder results from the lower hydrogen concentration and smaller amounts of hydrides. So, the mechanisms of hydrogen accelerating the oxidation probably focus on that hydrogen bonds between hydrides in powder and water molecules in air accelerate the absorption of water from ambient air, which speeds the electrochemical reaction in the powder; and that defects caused by hydrogen in powder induce oxygen diffusion more quickly.

Electrochemical conversions of pentaphenyl(methoxycarbonyl)cyclopentadiene derivatives

Redox processes of fluctuating pentaphenyl(methoxycarbonyl)cyclopentadiene derivatives on platinum electrode in acetonitrile are studied by cyclic voltammetry. The reduction of derivatives of pentaphenylcyclopentadiene C5Ph5M yields a radical anion which decomposes to anion M- and stable radical C5Ph 5 • ; the latter reduces to C5Ph5. In the first stage of oxidation of these compounds, a stable radical cation forms, with substituent M intact. Reduction of derivatives of pentamethoxycarbonylcyclopentadiene [C5(CO2Me)5]M yields a dianion. Such electrochemical behavior of fluctuating cyclopentadiene compounds reveals additional information on the mechanism of displacement of the element-centered substituents M in them

Protection of γ-based TiAl against high temperature oxidation using ion implantation of chlorine

The effect of ion beam implantation and plasma immersion ion implantation of chlorine on the high temperature oxidation of titanium aluminides above 800°C in air was investigated. Thermogravimetric oxidation tests (TGA) were performed to examine the long term protection. Depth profiling with Auger electron spectroscopy (AES) was used to investigate Cl diffusion and oxide formation during the first stage of oxidation. A microscopic model of the ‘Cl-effect’ will be discussed. A systematic variation of the implantation energy and fluence shows that there is a narrow regime of Cl concentration for optimum protective effect. The time to form a protective Al2O3 layer depends on the local Cl concentration. The oxidation rate after this incubation time is reduced by about two orders of magnitude compared to untreated Ti50Al and is nearly independent of the fluence. The implantation energy is not a sensitive parameter because the implanted chlorine profile changes very quickly during high temperature oxidation.

Pb-doped Bi-2212: Homogeneity Area, Oxidation and Effect on jc

ABSTRACTThe most promising way to improve jc in superconductors is likely to be a creation of small precipitates in the material during decomposition of supersaturated solid solution. It was recognised recently that Pb-doping of 2212 could have beneficial effect on jc. Bi2.1−xPbxSr2CaCu2O8+d (x=0.1…0.7) was prepared at different temperatures and oxygen partial pressure (pO2=10−3 – 0.21 atm) via nitrate route. The effect of pO2 and Pb-doping on Bi-2212 phase formation was explored with XRD, SEM/EPMA and TEM/SAED. The limit of Pb solubility in Bi-2212 was found to increase with a decrease in pO2 and corresponded to a decrease in x=0.3 at pO2 = 0.21 atm (850 °C) and x=0.6 at pO2 ∼ 10−3 atm (750 °C). TEM study of the as-synthesized solid solutions revealed significant nanoscale inhomogeneity attributed to Pb-segregation. For the samples with x > 0.3 the inhomogeneity quite often looked like a well- ordered wave with the period of about 10 nm. The samples of (Bi,Pb)-2212 processed under pO2 conditions were oxidized at 600–750°C in air for 0.5–60 h in order to cause precipitation of secondary phases from the supersaturated solid solution. Thermogravimetrical study (TG) of the oxidation gave a strong evidence of two stages composing the process. It was found that jc was enhanced by a factor of 2 after annealing at 650°C (i.e. during the first stage of oxidation).

Electrooxidation of primary nitramine anions on platinum in MeCN

Oxidation of primary nitramine anions RNNO2 −Bu4N+ (R=Me, Et, Pr1, or methoxyfurazanyl) in 0.1N Bu4NClO4 in MeCN on a Pt anode was studied by voltammetry and controlled potential electrolysis. It was found that the first stage of oxidation affords the corresponding radicals, which are further stabilized due to hydrogen abstraction from the medium. These radicals are also involved in other reactions, including those, which yield azo derivatives. The possibility of generation of nitrene species in these reactions is discussed.

XPS and transmission electron microscopy of themulticomponent hydride-forming alloys

The photoelectron spectrograms of the Ni2p, Ni3p, Co3p, Mn2p, Al2p,Ols and Cls in LaNi 4.6Mn 0.4, LaNi 4.5Al 0.5, MmNi 3.5Co 0.7Al 0.8, and Zr3d, Ti2p, V2p, Ni2p, Co2p, Ols, and Cls inZr 0.9VCo 0.55Ni 0.55 and Zr 0.59Ti 0.41V 0.53Cr 0.22 Fe 0.20Co 0.27Ni 0.78 have been obtained.In a surface layer about 2–2.5 nm of both alloys, the main part of nickel atoms are in metallic state(Ni 0) and only a third of them in oxidized state (Ni +2, Ni +3). At the same time the concentration of nickel in surface layer of Zr-containing alloy is muchlower as compared with Mm-containing alloy. The chemical state of Co and V atoms is similar toNi atoms state, but their concentrations, especially of V, in surface layer is lower than that ofnickel. The results of XPS have been compared with those of transmission electron microscopy ofthe thin foils of La- and Zr- based alloys. Dark field microphotographs of the oxidized alloysindicate presence of two different phases, which, according to electron microdiffraction, arenickel-based alloy and oxide. On the first stage of oxidation on the surface of foil the fine nickelparticles (about 10 nm) and amorphous oxide phase appear. Complete oxidation of the foil leadsto formation of eutectic-like structure of mixture of metallic and oxide phase. Some difference ofthe oxidized layer structure of investigated alloys have been observed.

The Detailed Mechanism of Oxidation of Ni-P Alloys

DTA in conjuction with X-ray diffraction analysis with a high-temperature camera and infrared spectroscopy was employed to determine the mechanism of oxidation of Ni-P alloys. Amorphous Ni-P powders were obtained from a nickel(II) sulphate bath as a nickel source and sodium dihydrophosphate(I) as a reducing agent. The crystallization product is composed of two phases: (f.c.c.) Ni and (b.c.t.) Ni3P. The amorphous to crystalline transformation takes place in the temperature range 280–330°C. Ni3P samples were heated from room temperature to 1050°C in air atmosphere at 5°C min−1. It was found that the first stage of oxidation of Ni3P goes through the intermediate phase of Ni12P5 formation to Ni2P. Some exothermic reactions were observed. Heating runs were interrupted after each reaction for crystal structure determination by IR spectrometry. Infrared spectra are reported and it is shown that the structure units present in the amorphous products at about 700°C were the oxoanions PO3− and P2O7−. The final products of the oxidation process are NiO and Ni3(PO4)2.

Oxidation of Saturated Fatty Acids Esters DSC investigations

Oxidation of saturated fatty acids ethyl esters: laurate, myristate, palmitate and stearate was investigated by means of DSC techniques under isothermal and non-isothermal conditions. The activation energies of isothermal oxidation were similar to each other (112–123 kJ mol−1) and no influence of carbon length on the rate of oxidative decomposition was observed. Results obtained from non-isothermal experiments were similar only for the first stage of oxidation.

Mechanisms of oxidation of Ni(II)-Fe(II) hydroxides in chloride-containing aqueous media: role of the pyroaurite-type Ni-Fe hydroxychlorides

AbstractNi-Fe pyroaurite-type hydroxychlorides were prepared by aerial oxidation of Ni(II)-Fe(II) hydroxides precipitated in aqueous solution with various P = Fe/Ni ratios. When P≥ 1/3, Ni(II)-Fe(II)-Fe(III) hydroxychlorides characterized by a specific Fe(IIl)/[Fe(II)+Ni(II)] ratio of 1/3, corresponding to the idealized formula of NiII3-xFeIIxFeIII(OH)8Cl.nH2O (with 0 ≤ x ≤ 3), were obtained at the end of the first stage of oxidation, In a second reaction stage, these hydroxychlorides oxidize with deprotonation of hydroxyl ions into O2- ions, i.e. the remaining FeII(OH)2 groups are transformed into FeIIIOOH groups. Along with the Ni(II)-Fe(III) hydroxychloride which contains a part of the FeIIIOOH groups a second phase is obtained. It is an amorphous Fe(III) or Ni(II)-Fe(III) oxyhydroxide when 1/3<P≤3/2, and a ferric oxyhydroxide identified as γ-FeOOH (lepidocrocite) when P>3/2. On the other side of the domain, when P<1/3, the Fe(III)/[Fe(II)+Ni(II)] ratio cannot reach the specific value of 1/3; this gives rise to a pyroaurite-type Ni(II)-Fe(III) hydroxychloride with a lower chloride content, that is with an average composition of NiII3+yFeIII1-y(OH)8Cl1-y.nH2O where y = {[4/(1+P)] - 3}, down to minimum Fe(III) and Cl contents corresponding to y = 1/3 (P = 1/5). The in situ mechanisms of oxidation of Ni(II)-Fe(II) hydroxides into Ni(H)-Fe(II)-Fe(III) hydroxychlorides are discussed.

XPS and transmission electron microscopy of the multicomponent hydride-forming alloys for electrochemical applications

Photoelectron spectra of the Ce4d, Ni2p, Ni3p, Co3p, Al2p, O1s, C1s in Mm(Ni,Co,Al) 5 and Zr3d, Ti2p, V2p, Ni2p, O1s, C1s in Zr 1− X Ti X (Ni,V,Co,Cr) 2 alloys have been obtained. In the surface layer, about 2–2.5 nm of both alloys, the main part of the nickel atoms are in the metallic state (Ni 0) and only a third of them in oxidized states (Ni +2, Ni +3). At the same time the concentration of nickel in surface layer of Zr-content alloy is much smaller when compared with Mm-content alloy. The chemical state of the Co and V atoms is similar to the Ni atoms state, but their concentrations, especially of V, in the surface layer is smaller than that of nickel. The results of XPS have been compared with the data of transmission electron microscopy of the thin foils of La- and Zr- based alloys. Dark field microphotographs of the oxidized alloys indicate the presence of two different phases, which, according to electron microdiffraction, are nickel-based alloy and oxide. In the first stage of oxidation on the surface of the foil, fine nickel particles (about 10 nm) and an amorphous oxide phase appear. Complete oxidation of the foil leads to the formation of a eutectic like structure consisting of a mixture of metallic and oxide phases. Some difference in the oxidized layer structure of the investigated alloys has been observed.

FTIR Study of the Oxidation of Porous Silicon

The oxidation of hydrogen-terminated porous silicon surfaces produced by electrochemical etching has been studied using transmission FTIR spectroscopy. The surface is passivated to oxidation by surface hydrogen below about 523 K. Above this temperature as hydrogen depletion occurs by H2 evolution, Si surface dangling bond sites, capable of O2 dissociation, are involved in initiating the first stage of oxidation. Two reactions are observed. The first, O insertion into Si−Si back-bonds, leads to −OySiHx surface species which exhibit frequency shifts to the blue compared to parent SiHx stretching modes. In addition, Si−O−Si modes are also observed to form. The second reaction involves oxygen atom insertion into Si−H bonds to produce isolated Si−OH surface species.