Showing papers on "Oxidation state published in 2001"

Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study

Abstract: High-resolution Fe K-edge XANES spectra of a series of crystalline Fe 2+ - and Fe 3+ -bearing model compounds were measured in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms. The model compounds comprise 30 natural minerals and synthetic compounds, with Fe coordination environments ranging from 4 to 12 O atoms for Fe 2+ , including 5-coordinated trigonal bipyramidal Fe 2+ , and from 4 to 6 O atoms for Fe 3+ . Most pre-edge spectra show two components (due to crystal-field splitting) that are located just above the Fermi level. The most useful characteristics of the Fe-K pre-edge for determining Fe oxidation state and coordination number are the position of its centroid and its integrated intensity. The separation between the average pre-edge centroid positions for Fe 2+ and Fe 3+ is 1.4 ± 0.1 eV. Thus, the position of the pre-edge feature can be used as a measure of the average Fe-redox state, with the average pre-edge position for mixed Fe 2+ -Fe 3+ compounds occurring between positions for Fe 2+ and Fe 3+ . The lowest pre-edge normalized heights and integrated intensities are observed for the most centrosymmetric sites of Fe, in agreement with previous studies (see Waychunas et al. 1983). Examination of the pre-edge features of mechanical mixtures of phases containing different proportions of Fe 2+ and Fe 3+ suggests that the pre-edge position and intensity for these mixtures can vary quite non-linearly with the average redox state of Fe. However, distinctly different trends of pre-edge position vs. pre-edge intensity can be observed, depending on the coordination environment of Fe 2+ and Fe 3+ , with an accuracy in redox determination of ±10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe 3+ /Fe 2+ ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe 2+ /Fe 3+ .

Composition–activity effects of Mn–Ce–O composites on phenol catalytic wet oxidation

Abstract: Mn–Ce–O composite catalysts have been widely used in sub- and supercritical catalytic wet oxidation of toxic organics contained in aqueous streams. In order to investigate their composition–activity relationship, 11 samples with Ce/(Mn+Ce) atomic bulk ratios ranging from 0 to 100% were prepared by co-precipitation. Phenol was selected as a model pollutant and the catalytic oxidation was carried out in a batch slurry reactor using oxygen as the oxidizing agent under mild reaction conditions. The results showed that the catalytic activity was greatly influenced by the catalyst composition. The catalyst with Mn/Ce ratio=6/4 was found to be the most active in reducing both phenol concentration and total organic carbon (TOC). All catalysts were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), temperature programmed reduction (TPR) and nitrogen adsorption techniques. Systematic shifts in binding energy, diffraction angle, and reduction temperature were observed in the XPS, XRD and TPR spectra, respectively. XPS and XRD data revealed the occurrence of significant interactions between Mn and Ce oxides, resulting in the evolution of textural, structural and oxidation state with composition. TPR analysis showed that the interaction between Mn and Ce greatly improved the oxygen storage capacity of manganese and cerium oxides as well as oxygen mobility on the surface of catalyst. Catalytic active sites have been ascribed to manganese oxide species exhibiting higher oxidation state. Furthermore, XPS revealed that the most active catalyst, i.e. Mn/Ce 6/4, exhibits an electron-rich surface which may be very important in the activation of adsorbed oxygen.

Influence of catalyst pretreatments on volatile organic compounds oxidation over gold/iron oxide

Abstract: This paper reports a study on the influence of calcination pretreatments on the catalytic behaviour of the Au/iron oxide system towards the combustion of some representative volatile organic compounds (2-propanol, ethanol, methanol, acetone and toluene). The catalytic activity of Au/Fe 2 O 3 samples towards the total oxidation to CO 2 has been found to be strongly dependent on the catalyst pretreatment, decreasing on increasing the calcination temperature. On the basis of characterisation data (XPS, FT-IR, XRD, BET surface area) it has been proposed that the catalytic behaviour is related to the gold state and/or the iron oxide phase. It appears plausible to suggest that the gold oxidation state and/or the particle size play a key role in the catalytic combustion of volatile organic compounds.

Stability of cerium oxide on silicon studied by x-ray photoelectron spectroscopy

Abstract: The silicon-cerium oxide interface is studied using x-ray photoelectron spectroscopy. The oxidation and reduction of species at the interface are examined as a function of annealing temperature both in vacuum and oxygen ambient, in order to determine their relative stabilities. By depositing a very thin CeO2 film (similar to 30 Angstrom), the cerium and silicon core level peaks can be monitored simultaneously. The presence of characteristic chemical shifts of the Si 2p peak gives information about any SiOx, layer that may form at the interface. The oxidation state of the cerium can be probed from three different areas of the spectrum. From this information we can infer the oxidation state of both the silicon and the cerium. For the first time a complete picture of the interface is obtained. The implications of these findings on the utility of CeO2 in device applications are discussed.

FTIR study of CO and NO adsorption and coadsorption on a Cu/SiO2 catalyst: Probing the oxidation state of copper

Abstract: At room temperature CO is adsorbed strongly on Cu+ sites (band at 2131 cm−1) whereas NO is preferably adsorbed on Cu2+ sites (band at 1882 cm−1). Coadsorption of CO and NO allows simultaneous and selective detection of both kinds of cations. This observation is used to follow the changes on the sample surface occurring in the presence of oxygen. Addition of small amounts of O2 to the CO–NO–Cu/SiO2 system first leads to the oxidation of the Cu+ sites to Cu2+. This process is followed by formation of surface nitrates which block the Cu2+ sites for NO adsorption. Adsorption of CO at 85 K allows detection of Cu2+ cations (ca. 2200 cm−1) in addition to the Cu+, CO is replaced by NO from these sites. Cu0 sites form carbonyls which, when copper is highly dispersed, can absorb at the same frequency at which Cu+–CO carbonyls are detected. In this case both kinds of species could be distinguished by their stability: the Cu0–CO species are easily destroyed during evacuation.

NO–NH3 coadsorption on vanadia/titania catalysts: determination of the reduction degree of vanadium

Abstract: Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH3 and NO adsorption over a 15% w/w vanadia/titania catalyst. NH3 is adsorbed as coordinate NH3 and NH4+ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300°C, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the VO sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced VO groups, resulting in N2, according to a NO+V□→1/2N2+VO reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2ν(VO) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1.

Mn K-edge XANES and Kbeta XES studies of two Mn-oxo binuclear complexes: investigation of three different oxidation states relevant to the oxygen-evolving complex of photosystem II

Abstract: Two structurally homologous Mn compounds in different oxidation states were studied to investigate the relative influence of oxidation state and ligand environment on Mn K-edge X-ray absorption near-edge structure (XANES) and Mn Kβ X-ray emission spectroscopy (Kβ XES). The two manganese compounds are the di-μ-oxo compound [L‘2MnIIIO2MnIVL‘2](ClO4)3, where L‘ is 1,10-phenanthroline (Cooper, S. R.; Calvin, M. J. Am. Chem. Soc. 1977, 99, 6623−6630) and the linear mono-μ-oxo compound [LMnIIIOMnIIIL](ClO4)2, where L- is the monoanionic N,N-bis(2-pyridylmethyl)-N‘-salicylidene-1,2-diaminoethane ligand (Horner, O.; Anxolabehere-Mallart, E.; Charlot, M. F.; Tchertanov, L.; Guilhem, J.; Mattioli, T. A.; Boussac, A.; Girerd, J.-J. Inorg. Chem. 1999, 38, 1222−1232). Preparative bulk electrolysis in acetonitrile was used to obtain higher oxidation states of the compounds: the MnIVMnIV species for the di-μ-oxo compound and the MnIIIMnIV and MnIVMnIV species for the mono-μ-oxo compound. IR, UV/vis, EPR, and EXAFS spec...

Theoretical, thermodynamic, spectroscopic, and structural studies of the consequences of one-electron oxidation on the Fe-X bonds in 17- and 18-electron Cp*Fe(dppe)X complexes (X = F, Cl, Br, I, H, CH3).

Abstract: The compounds Cp*Fe(dppe)X ([Fe]X) and the corresponding cation radicals [Fe*]X*+ are available for the series X = F, Cl, Br, I, H, CH3. This has allowed for a detailed investigation of the dependence of the nature of Fe-X bonding on the identity of X and the oxidation state (charge) of the complex. Cyclic voltammetry demonstrates that the electrode potentials for the [Fe]X0/+ couples decrease in the order I > Br > Cl > H > F > CH3. An "inverse halide order" is seen, in which the most electronegative X leads to the most easily oxidized complex. This suggests that F is the best donor among the halides. The halide trend is also reflected in NMR spectroscopic data. Mossbauer spectroscopy data also suggest that the F ligand is a strong donor (relative to H and CH3) in [Fe*]X*+. DFT calculations on CpFe(dpe)X ([Fe]X) model complexes nicely reproduce the trend in the electrode potentials for the [Fe*]X0/+ couples. Analysis of the theoretical data within the halogen series indicates that the energy of the [Fe]X HOMO does not correlate with the extent of its Fe(d(pi))-X(p(pi)) antibonding character, which varies in the order I > Br > Cl > F, but rather depends on the destabilizing electrostatic effect caused by X. This effect varies in the order F > Cl > Br > I. A thermochemical cycle that incorporates the [Fe*]X0/+ and [Fe*]0/+ electrode potentials was used to investigate the effect of the oxidation state of the complex on the homolytic bond dissociation energy (BDEhom), defined for the processes Fe-X --> Fe* + X* and Fe-X*+ --> Fe*+ + X*. For all X, it was found that a one-electron oxidation leads to a weakening of the Fe-X bond. This trend was reproduced by the DFT calculations. On the other hand, IR nu(Fe-X) spectroscopy data showed an increase in the stretching frequencies for X = H and Cl upon oxidation. X-ray crystallographic data showed a shortening of the Fe-Cl bond upon oxidation. The trends in IR and Fe-Cl bond distances were reproduced in the DFT calculations. The combined data therefore suggest that oxidation leads to weaker, but shorter, Fe-X bonds. A second thermochemical cycle was applied to investigate the effect of the one-electron oxidation on the heterolytic bond dissociation energies (BDEhet), defined for the processes Fe-X --> Fe+ + X- and Fe-X*+ --> Fe2+ + X-. In this case, the oxidation led to bond strengthening in all cases. The computed BDE values have been analyzed within Ziegler's transition state methodology and decomposed into two components, one electrostatic and one covalent, describing the interaction between the unrelaxed fragments. In all the computed BDEhom and BDEhet values of the [Fe]X models the electrostatic component is important. This helps to understand their respective variations upon oxidation.

Investigation of reduction of Cu(II) complexes in positive-ion mode electrospray mass spectrometry

Abstract: The electrospray ionization mass spectrometry (ESI-MS) behavior of seven Cu(II) complexes with tetradentate ligands has been studied. An unexpected reduction process, in positive ion mode, of the Cu oxidation state was observed, and shown to be due to charge transfer between the metal complex and the solvent molecules in the gas phase. Ion trap collision-induced dissociation experiments and deuterated solvents were used to support the proposed mechanism that is not a common electrochemical redox reaction at the ESI tip, but a gas-phase process. A series of solvents (acetonitrile, methanol, ethanol, propanol and iso-butanol) were tested, and a correlation between ionization energy (IE) and the amount of Cu(I) produced in ESI has been demonstrated for the alcohols, although some other solvent properties should also be taken into account. The electrochemical reduction potential of the complexes in solution is also an important parameter, since complexes more easily reduced in solution are also easier to reduce in the gas phase. Copyright © 2001 John Wiley & Sons, Ltd.

From the Metal to the Molecule-Ternary Bismuth Subhalides.

Abstract: Subvalent compounds, that is, metalrich substances in which the average oxidation state of the cation is smaller than would be expected from the (8ˇ N) rule, have proved to be a rich source of unexpected structural and physical features. The extraordinary structural chemistry generally observed in subvalent compounds is a consequence of the low and often non-integer oxidation states of the metal atoms coupled with the low concentration of valence electrons. Both factors can lead to a wide-range of bonding types within the same compound. A characteristic of these compounds is the interplay between “metallic” regions, with delocalized electrons and mainly nonpolar bonds between the metal atoms, and “saltlike” regions, which are characterized by strong localization of the electrons and heteropolar exchange between the metal and nonmetal atoms. The volumes of the different structural regions as well as the extent to which they interpenetrate can vary from compound to compound. The ternary subhalides of bismuth belong to a new class of substances which cover the whole spectrum from partially oxidized “porous” metals, through one- and two-dimensional metals, up to semiconducting ionic or molecular cluster compounds. These subvalent compounds with their unusually high chemical stabilities provide excellent vehicles for further research and their potential is described in the following article.

Crystal Chemistry of Dichalcogenides MX2

Abstract: Crystal-chemical analysis of the structures of metal (other than REE) dichalcogenides of composition X:M = 2:1 has been carried out. It is shown that there are two types of structure depending on the formal degree of metal oxidation. The first group are compounds M(X2) containing metals in the oxidation state 2+ and covalently bonded pairs of chalcogen elements; the main structural types here are cubic (pyrite) and orthorhombic (marcasite) FeS2. The second group involves the compounds MX2 with metals in the formal oxidation state 4+ and chalcogenide ions X 2 - ; the main structural types in this group are CdI2, CdCl2, MoS2. The bond lengths M–X, X–X, and X...X are analyzed; it is shown that the high polarizability of the chalcogenide ions is reflected in shortened X...X distances. In the covalently bonded pairs X–X, the bond lengths change within the following limits: 2.03-2.30, 2.35-2.55, and 2.70-2.83 A for X...X ion contacts, the minimal values are 3.07, 3.10, and 3.20 A (X = S, Se, and Te, respectively).

The valence and speciation of sulfur in glasses by x-ray absorption spectroscopy

Abstract: The geochemical behavior of sulfur in magmas depends strongly on the oxidation state of sulfur, but this is not easily determined by standard analytical methods. We have measured XANES absorption spectra at the sulfur K-edge and have found that such measurements are useful to characterize the oxidation state and speciation of sulfur in silicate glasses of geological rel evance. Measured spectra of a set of reference minerals show the effects of different oxidation states and coordination numbers of sulfur; there is a large shift in energy (~10‐12 eV) of the sulfur K-edge between S 2‐ and S 6+ . This large and easily detectable difference makes possible the measurement of the valence of sulfur in unknown samples by measuring the shift in energy of the absorption edge. This approach is applicable to both crystalline and glassy materials, and useful results have been obtained on samples with as little as 450 ppm S. We have used XANES measurements to characterize oxidation state and speciation of sulfur in a set of natural and synthetic sulfur-bearing glasses. The samples cover a range of composition from basaltic to almost rhyo litic, and some were synthesized over a range of pressure, temperature and oxygen fugacity; glass S content varies between 450 and 3000 ppm. XANES analyses, carried out in fluorescence mode at LURE, allowed determination of the sulfur oxidation state in all of the samples and clearly show that some samples contain a mixture of S 2‐ and S 6+ ; no other sulfur species were observed. Quantitative determination of the abundance of sulfide and sulfate shows good agreement with independent measurements based on electron-microprobe determination of the wavelength shift of sulfur K X-rays.

Nature and distribution of chemical species on oxidized pyrite surface: Complementarity of XPS and nuclear microprobe analysis

Abstract: The coupling of X-ray photoelectron spectroscopy (XPS) and nuclear microprobe analysis (NMA) using resonant reactions 16 O ( α , α ) 16 O and 12 C ( p , p ) 12 C at 3.05 and 1.725 MeV, respectively, is particularly adapted to the characterization of thin oxidation layers onto pyrites. XPS permits to determine both oxidation state and chemical environment of S and Fe. NMA gives an information about spatial distribution and chemical composition heterogeneity of oxidation products. Pyrites oxidized in acidic medium produce few solid components. Only FeII sulfate is detected on the oxidized pyrite surface. In carbonate medium, oxidation layer is more complex. Iron is mainly with a (+II) oxidation state under siderite or FeII sulfate form. As illustrated by the comparison of Fe3p and Fe2p3/2 peaks, iron has an (+III) oxidation state to a minor extent under α-FeOOH and FeIII sulfate forms from the first oxidized pyrite layers. Sulfur oxidation induces intermediate species (polysulfides and sulfoxyanions as S2O32− also evidenced in solution) indicating that oxidation occurs at solid state. NMA has shown that oxidation occurs only on localized points of pyrite surface, with oxidation layers showing spatial distribution and thickness heterogeneities.

The effect of the nature of vanadium species on benzene total oxidation

Abstract: The nature of the vanadium species present on V2O5/Al2O3 catalysts was investigated by using solid state 51V NMR, diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD) and temperature programmed reduction (TPR). 51V NMR and DRS analyses indicated the presence of V5+ in tetrahedral symmetry at low vanadium loading. A surface polymeric vanadium species and/or the bulk crystalline V2O5 were mainly observed at high vanadium loading as also detected by XRD. The positions of the absorption edges determined through the UV–VIS spectra allowed distinguishing between various tetrahedral symmetries. After TPR, the average oxidation state of vanadium depended on the vanadium content. The nature of vanadium species was related to the catalyst behavior on the benzene oxidation reaction. The catalysts containing high vanadium content were more active suggesting that a high amount of V4+ is responsible for the higher activity.

System and method for removal of arsenic from aqueous solutions

Abstract: A method and system of removing arsenic from aqueous solutions is provided. Specifically, the aqueous solution includes arsenic, said arsenic being present in the +3 oxidation state. Arsenic can also be present in the +5 oxidation state. The pH of the aqueous solution is first adjusted to a pH in the range of about 3 to 5. Iron salts , such as ferric or an ferrous salts, are introduced into the aqueous solution. Hydrogen peroxide is added to the aqueous solution wherein the arsenic present in the +3 oxidation state is oxidized to the +5 oxidation state. Next, the pH of the aqueous solution is adjusted to a value in the range of about 5 to 8 to form an insoluble ferric hydroxide compound including arsenic in the +5 oxidation state adsorbed onto the compound which is then removed from the aqueous solution.