Formation Of Higher Oxidation State Research Articles

Phase evolution in the UO2–CeO2 system under oxidizing and reducing conditions: X-ray diffraction and spectroscopic studies

The solid-solution formation mechanism and defect dynamics in U1-xCexO2±δ solid solutions have been investigated by means of X-ray diffraction (XRD) analysis, Raman spectroscopy, and X-ray absorption near-edge structure (XANES) spectroscopy. Diffraction studies have revealed the formation of single-phase solid solutions with a fluorite-type structure at x ≤ 0.2 and a single-phase fluorite-type structure with broadened diffraction peaks, attributed to inhomogeneity in the O/M ratio, at 0.3 ≤ x ≤ 0.8 under reducing conditions. Oxidizing conditions result in the formation of single-phase solid solutions with fluorite-type structure at x ≥ 0.4. Upon the formation of Ce3+ in solid solutions at x ≤ 0.2 under reducing conditions, charge neutrality is achieved through oxygen vacancy creation and formation of higher oxidation states of uranium through a charge compensation mechanism. Lattice parameter–composition relationships for fluorite-type phases synthesized under oxidizing conditions show a decrease in lattice parameters as compared to their stoichiometric counterparts due to the oxidation of U4+ to U5+/U6+. XANES studies provide direct experimental evidence of the presence of Ce3+ in solid solutions, despite synthesis under oxidizing conditions.

Electrochemical Study of Metallic Rhenium in Methanol Acidic Aqueous Solutions

Cyclic voltammograms were performed in a window potential from -0.4 to 1.6 V, in solutions of a) H2SO4 0.5M, b) CH3OH 2 M + 0.5 M, c) HCl 0.145 M, d) CH3OH 2 M + HCl 0.145 M. In the sweeps towards anodic direction a soluble oxide is formed at potential superior to 1 V, which is electrodeposited in the sweeps towards cathodic direction. The electrodeposited surface is formed by Re(I) and Re(0) and anodic peaks (0.47 V and 0.71 V vs SHE) correspond to the formation of higher oxidation states (Re(I)→Re(II)→Re(VI)). Methanol affects the structure of anodic peaks separately of the presence of ions chlorides or sulfates. In both acids the soluble oxide is formed and also this specie is produced in sulfuric acid solutions with methanol. Contrariwise, in hydrochloric acid solutions with methanol the perrhenate is the predominant specie. We suggest new hydrogen bonding structures in this solution. Furthermore, we propose a new bonding structure when methanol is dissolve in sulfuric acid or hydrochloric acid solutions. This specie produces bands at 317 nm or 321 nm in the UV-visible spectra of soluble oxide. The high corrosion of metallic rhenium impedes its use as an anode for methanol electrooxidation

XFEL Crystal Structures of Peroxidase Compound II.

Oxygen activation in all heme enzymes requires the formation of high oxidation states of iron, usually referred to as ferryl heme. There are two known intermediates: Compound I and Compound II. The nature of the ferryl heme-and whether it is an FeIV=O or FeIV-OH species-is important for controlling reactivity across groups of heme enzymes. The most recent evidence for Compound I indicates that the ferryl heme is an unprotonated FeIV=O species. For Compound II, the nature of the ferryl heme is not unambiguously established. Here, we report 1.06 Å and 1.50 Å crystal structures for Compound II intermediates in cytochrome c peroxidase (CcP) and ascorbate peroxidase (APX), collected using the X-ray free electron laser at SACLA. The structures reveal differences between the two peroxidases. The iron-oxygen bond length in CcP (1.76 Å) is notably shorter than in APX (1.87 Å). The results indicate that the ferryl species is finely tuned across Compound I and Compound II species in closely related peroxidase enzymes. We propose that this fine-tuning is linked to the functional need for proton delivery to the heme.

XFEL Crystal Structures of Peroxidase Compound II.

Oxygen activation in all heme enzymes requires the formation of high oxidation states of iron, usually referred to as ferryl heme. There are two known intermediates: Compound I and Compound II. The nature of the ferryl heme—and whether it is an FeIV=O or FeIV‐OH species—is important for controlling reactivity across groups of heme enzymes. The most recent evidence for Compound I indicates that the ferryl heme is an unprotonated FeIV=O species. For Compound II, the nature of the ferryl heme is not unambiguously established. Here, we report 1.06 Å and 1.50 Å crystal structures for Compound II intermediates in cytochrome c peroxidase (CcP) and ascorbate peroxidase (APX), collected using the X‐ray free electron laser at SACLA. The structures reveal differences between the two peroxidases. The iron‐oxygen bond length in CcP (1.76 Å) is notably shorter than in APX (1.87 Å). The results indicate that the ferryl species is finely tuned across Compound I and Compound II species in closely related peroxidase enzymes. We propose that this fine‐tuning is linked to the functional need for proton delivery to the heme.

Hierarchically connected electrospun WO3 nanowires – An acetaldehyde sensor

Hierarchically connected low-dimensional metal oxide nanostructures have been preferred to fabricate gas-sensing elements owing to their enhanced sensing response. In this juncture, hierarchical architectures of WO3 nanowires were synthesized using electrospinning technique with polyvinyl alcohol and ammonium metatungstate hydrate as precursor. X-ray diffraction analysis revealed the polymer evaporation and metal oxide formation with phase transformation of WO3 nanowires from monoclinic to orthorhombic and back to monoclinic as an effect of calcination temperatures (673–973 K). The variation in the interconnected grain features of WO3 nanowires as a function of calcination temperature was observed using field emission scanning electron microscope and transmission electron microscope. The formation of higher oxidation states of tungsten (W6+) and increased oxygen vacancies were observed for the samples calcined at higher temperatures through X-ray photoelectron spectrometer. Electrical studies for 873 K sample showed the maximum mobility of electrons due to the highly interconnected nanowires with minimum grain boundary resistance. The gas sensing responses of WO3 nanostructures were investigated and WO3 sample calcined at 873 K with maximum mobility showed a selective response of 5537 towards 75 ppm of acetaldehyde at room temperature.

Electronic Structure of Pt and Pt-Co Nano-Particles with O2 and O2/H2o Adsorption Revealed By in Situ xafs and Hard X-Ray Photoelectron Spectroscopy

In order to clarify the effect of water adsorption on fuel cell cathode catalysis surface, we have investigated electronic structure of Pt and Pt-Co nano-particles with O2 adsorption and O2/H2O co-adsorption by in situ XAFS and in situ hard X-ray photoelectron spectroscopy (HAXPES). First, the electronic structure change under dry and humidified oxygen conditions at room temperature was measured by in situ HAXPES equipped with an ambient cell. The valence band (mainly Pt 5d) and Pt 4f spectra were successfully obtained under up to 1 mbar for the first time. Both valence band and Pt 4f spectra show that O2/H2O co-adsorption hindered oxygen adsorption. Based on our first principles calculation of valence band density-of-states (DOS) we have found that DOS just below Fermi level strongly decreases upon oxygen adsorption, while valence band spectra remain almost unchanged upon H2O adsorption. These results suggest that H2O molecules may occupy the oxygen adsorption sites on Pt surface more easily than oxygen, resulting in hindering the successive oxygen adsorption. However, under the more realistic condition at atmospheric pressure (1 bar) quite different results that the formation of higher oxidation states of Pt in Pt Pt L 3-edge absorption spectra was enhanced by water adsorption were obtained by high resolution (FWHM of about 2.5 eV) in situ XAFS, taking advantage of life-time free measurements1). It is difficult to observe these changes by conventional XAFS spectra with poor energy resolution (FWHM of about 5.2 eV). At the atmospheric pressure more frequent attack by oxygen molecules onto water-adsorbed Pt surface may replace water molecules by oxygen, resulting in the formation of hydrated hydroxyl intermediates and higher oxidation states which hinder oxygen reduction reaction. The molecular dynamics simulation2) also confirms the oxygen replacement on water-adsorbed Pt surfaces. This enhanced oxygen adsorption is more clearly observed for Pt nano-particles, probably because Pt nano-particles with stronger Pt-O bonding than Pt-Co nano-particles may further stabilize Pt-O bonding by additional adsorbed water. On the other hand, oxygen molecules on Pt-Co nano-particles may be difficult to form hydrated hydroxyl intermediates with adsorbed water, leading to less water effect on oxygen adsorption on Pt-Co. These results would be much helpful to understand why Pt-Co nano particles show higher ORR activity than Pt nano particles.

Defect luminescence and lattice strain in Mn2+ doped ZnGa2O4

Undoped and Mn2+ doped ZnGa2O4 phosphors were prepared by solution combustion method and characterized by XRD, SEM, luminescence and electron paramagnetic resonance (EPR) techniques. Based on XRD results, it is inferred that, strain in ZnGa2O4 host lattice increases with incorporation of Mn2+ ions in the lattice. Mn2+ doping at concentration levels investigated, lead to significant reduction in the defect emission and this has been attributed to the formation of higher oxidation states of Mn ions in the lattice. Electron Paramagnetic Resonance studies confirmed that majority of Mn ions exist as Mn2+ species and they occupy tetrahedral Zn2+ site in ZnGa2O4 lattice with an average hyperfine coupling constant, Aiso∼82G.

Mössbauer study of matrix effects in formation of high oxidation states of iron and cobalt

To fix unstable states of pentavalent iron and cobalt, a matrix containing (according to the preparation conditions) potassium manganates (V) was simultaneously doped with iron and radioactive cobalt. The absorption and emission spectra of this matrix, with allowance for the reversal of the source and absorber functions, are similar, which is indicative of the identity of the oxidation states (IV, V, and VI) of iron and cobalt in it. Stabilization of 57Fe and 57Co in the β-MnO2 matrix is also investigated by Mossbauer absorption and emission spectroscopy.

The nature of the active site in the Fe-ZSM-5/N 2O system studied by (resonant) inelastic X-ray scattering

Resonant inelastic X-ray scattering (RIXS) was applied to investigate whether the iron–oxygen species created upon reaction of Fe-ZSM-5 with N 2O have a (partial) 3d 4 configuration, i.e. oxidation state Fe(IV). Fe-ZSM-5 samples with low iron loading were pre-treated at high temperatures before reaction with N 2O. Then Fe K-edge XANES spectra were recorded either by selecting the fluorescence of the main Kβ 1,3 line or the satellite Kβ′ line in the Kβ emission spectrum. The experiments were performed in situ and imitated the experiments performed in the laboratory reactor. The absence of a pre-edge in the Kβ′ line spectrum proves that the configuration of the active iron–oxygen species is purely 3d 5. The result was confirmed for several differently treated Fe-ZSM-5 catalysts. The high reactivity of Fe-ZSM-5 is therefore not due to the formation of high oxidation states of iron, but must be ascribed to the particular oxygen species that is created by the reaction of N 2O with the catalyst.

ELECTROCHEMICAL DETECTION AND CATALYTIC OXIDATION OF PHENOLIC COMPOUNDS OVER NICKEL COMPLEX MODIFIED GRAPHITE ELECTRODE

A Novel Ni(II) mixed ligand complex modified graphite electrode is developed for detection and oxidation of the phenolic compounds based on simple cyclic voltammetric technique. The Ni(II) mixed ligand complex with thiocyanate and macro cyclic ligand 1, 4, 8, 11 tetraaza cyclotetradecane (cyclam) was synthesised and used for catalytic oxidation of phenolic compounds viz. phenol, nitro-phenol and chloro-phenol etc. The Ni(II)-cyclam-thiocyanate mixed ligand complex was synthesised in typical molar ratio of 1:2:5:: Ni(II):cyclam:thiocyanate and crystallized followed by characterization and catalytic applications. The electrochemical study of the complex mediated graphite electrode revealed its fast electron-transfer property, as formation of higher oxidation state of central Ni(II) metal ion as Ni(III) on applying a positive potential, which catalyses the oxidation of the target species. The Ni(II) complex modified graphite paste electrode enhanced the oxidation current of phenolic compounds many times relatively at lower oxidation potential in comparison to simple graphite paste electrode. It is also found to be sensitive for the detection of various phenolic compounds in the range of 20 mg/L to 0.5mg/L in aqueous solution. The Ni(II)-complex stability at higher potential and ambient condition proved its potential for sensing and detoxification of phenolic compounds released in the ecosystem from polluting industries.

Free radical biology and medicine

The cysteine (Cys) proteome is a major component of the adaptive interface between the genome and the exposome. The thiol moiety of Cys undergoes a range of biologic modifications enabling biological switching of structure and reactivity. These biological modifications include sulfenylation and disulfide formation, formation of higher oxidation states, S-nitrosylation, persulfidation, metalation, and other modifications. Extensive knowledge about these systems and their compartmentalization now provides a foundation to develop advanced integrative models of Cys proteome regulation. In particular, detailed understanding of redox signaling pathways and sensing networks is becoming available to allow the discrimination of network structures. This research focuses attention on the need for atlases of Cys modifications to develop systems biology models. Such atlases will be especially useful for integrative studies linking the Cys proteome to imaging and other omics platforms, providing a basis for improved redox-based therapeutics. Thus, a framework is emerging to place the Cys proteome as a complement to the quantitative proteome in the omics continuum connecting the genome to the exposome.

Oxidation of heated diamond C(100):H surfaces

This paper extends a previous study (Pehrsson and Mercer, submitted to Surf. Sci.) on unheated, hydrogenated, natural diamond (100) surfaces oxidized with thermally activated oxygen (O ∗ 2). In this paper, the oxidation is performed at substrate temperatures from T sub=24 to 670°C. The diamond surface composition and structure were then investigated with high resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), electron loss spectroscopy (ELS) and low energy electron diffraction (LEED). The oxygen coverage ( θ) increased in two stages, as it did during oxidation at T<80°C. However, there are fundamental differences between the oxidation of nominally unheated and heated diamond surfaces. This difference is attributed to simultaneous adsorption and rapid desorption of oxygen species at higher temperatures; the desorption step is much slower without heating. The initial oxidation rates were similar regardless of the substrate temperatures, but the peak coverage ( θ) was lower at higher temperatures. For example, θ plateaued at 0.4±0.1 ML at 600°C. The lower saturation coverage is again attributed to oxygen desorption during oxidation. Consistent results were obtained on fully oxidized surfaces, which when heated in vacuum to T sub=600°C, lost ∼60% of their adsorbed oxygen. ELS revealed few CC dimers on the oxidized surfaces, and more graphitization than on unheated surfaces. Oxidation at elevated temperatures also increased the carbonyl to ether ratio, reflecting etching-induced changes in the types of surface sites. The carbonyl and C–H stretch frequencies increased with oxygen dose due to formation of higher oxidation states and/or hydrogen bonding between adjacent groups. The oxygen types did not interconvert when the oxidized surfaces were heated in vacuum. Oxygen desorption generated a much more reactive surface than heating-induced dehydrogenation of the smooth, hydrogenated surface.

Oxidation of the hydrogenated diamond (100) surface

The surface composition and structure of natural diamond (100) surfaces subsequently oxidized with activated oxygen at T sub≤35°C were investigated with high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy, electron loss spectroscopy (ELS) and low-energy electron diffraction (LEED). Complete surface oxidation (oxygen coverage θ=1 ML) required doses of hundreds of kilolangmuirs of O 2. HREELS vibrational spectra permitted identification of the specific surface oxygen species, and also provided information about the diamond surface states. Most surface sites lost their hydrogen at least once before becoming oxidized. The oxygen coverage θ increased quickly at first, and then more slowly as saturation was approached; different mechanisms or sites may have accounted for the decreased rate. The relative distribution of oxygen species varied with the oxidation conditions. Ether, carbonyl and hydroxyl groups appeared during the initial stages of oxidation, but the hydroxyl groups disappeared at higher coverages. Bridge-bonded ether groups dominated at saturation coverage, although smaller amounts of carbonyl and hydroxyl were still observed. The carbonyl and CH stretch frequencies increased with oxygen dose due to formation of higher oxidation states and/or hydrogen bonding between adjacent groups. ELS revealed only a low concentration of CC dimers on the oxidized surfaces, and no evidence of graphitization. Surfaces generated by oxygen addition and then desorption were more reactive than surfaces generated by hydrogen desorption. Oxidized surfaces that were heated in vacuum and then rehydrogenated did not recover the sharp LEED patterns and HREELS spectra of the original plasma-smoothed surface. This effect was presumably due to surface roughening caused by oxygen desorption as CO and CO 2, and creation of reactive high-energy sites that quickly bonded to available background gases and prevented large areas of organized surface reconstruction.

Mutations of noncatalytic sulfhydryl groups influence the stability, folding, and oxidative susceptibility of rhodanese.

Mutants of rhodanese (EC 2.8.1.1) which substitute serine residues for each of the 4 cysteine residues have been studied to determine the roles of cysteines in the structure and function of the enzyme. The proteins compared in these studies were: the wild-type, C63S, C247S, C254S, C263S, C254S/C263S, and C63S/C254S/C263S. These current studies show that cysteine 247 is the only cysteine required for the activity of the enzyme. Although the other sulfhydryl groups do not participate in sulfur transfer, mutations of the noncatalytic cysteines result in the destabilization of the native structure of the enzyme. All the active proteins had similar kinetic parameters. Mutants substituting cysteine 254, compared with the other species, were: (a) more resistant than wild-type to inactivation by dithiothreitol, (b) more readily reactivated following oxidative inactivation, and (c) found to adopt conformations that show increased exposure of hydrophobic surfaces following removal of the transferable sulfur. On the other hand, cysteine to serine substitutions had very little effect on: (a) the rates of oxidative inactivation, (b) the increased fluorescence following the removal of transferable sulfur, or (c) the effectiveness of spontaneous refolding after urea denaturation. Forms of rhodanese that were formerly considered to be irreversibly oxidized can be reactivated if the protein is denatured in urea before reductants are used. It is proposed that these forms differ from reversibly oxidized states due to the inaccessibility of intramolecular disulfides to reductants and not to the formation of higher oxidation states of the protein.

Cytochrome c-catalyzed membrane lipid peroxidation by hydrogen peroxide

Cytochrome c 3+ -catalyzed peroxidation of phosphatidylcholine liposomes by hydrogen peroxide (H 2O 2) was indicated by the production of thiobarbituric acid reactive substances, oxygen consumption, and emission of spontaneous chemiluminescence. The iron chelator diethylenetriaminepentaacetic acid (DTPA) only partially inhibited peroxidation when H 2O 2 concentrations were 200 μ m or greater. In contrast, iron compounds such as ferric chloride, potassium ferricyanide, and hemin induced H 2O 2-dependent lipid peroxidation which was totally inhibitable by DTPA. Cyanide and urate, which react at or near the cytochrome—heme, completely prevented lipid peroxidation, while hydroxyl radical scavengers and superoxide dismutase had very little or no inhibitory effect. Changes in liposome surface charge did not influence cytochrome c 3+ plus H 2O 2-dependent peroxidation, but a net negative charge was critical in favoring cytochrome c 3+ -dependent, H 2O 2-independent lipid auto-oxidative processes. These results show that reaction of cytochrome c with H 2O 2 promotes membrane oxidation by more than one chemical mechanism, including formation of high oxidation states of iron at the cytochrome—heme and also by heme iron release at higher H 2O 2 concentrations. Cytochrome c 3+ could react with mitochondrial H 2O 2 to yield “site-specific” mitochondrial membrane lipid peroxidation during tissue oxidant stress.

SiO2-Si interface formation by catalytic oxidation using alkali metals and removal of the catalyst species

We studied the catalytic action of adsorbed cesium or sodium on the oxidation of Si(100)2×1 by core-level photoemission using synchrotron radiation. At room temperature, a large enhancement of the silicon oxidation rate was observed with formation of high oxidation states. By moderate thermal annealing, we were able to remove the catalyst species from the surface and to form a clean SiO2-Si interface at lower temperatures than with other processes.