Papers by Keyword: Oxidation State

Abstract: We investigated the stability of the high-spin state of the iron β-NaFeO₂ based on the structural refinement. The oxidation of the Fe²⁺ ion in the as-synthesized sample is evidenced by its green color. Due to its sensitivity in air and CO₂, this compound will decompose into a reddish Fe³⁺ state. The smaller crystal volume of the decomposed compound is mainly related to the shorter ionic radius of the high-spin state Fe³⁺ and this result will be compared to the single crystal sample. In contrast to the polycrystalline sample, the decomposition single crystal sample only taking place on the surface of the as-grown crystal.

Abstract: It is shown that the value of the equilibrium oxygen partial pressure, Po₂ as a value available for measurements is possible to be taken as a measure of slag redox potential of, taking into account its electronic system performance. Application of the electromotive force method (EMF) allowed establishing the character of a change in the average oxidation state of iron ν_Fe depending on Po₂, the temperature and slag composition. The study of Mössbauer absorption spectra of quenched slag samples confirmed the possibility of simultaneous presence of iron in the flux in oxidation states from 0 to +3.

Abstract: This research is aimed to examine oxidation state of Copper (Cu) in both as-synthesized and reduced T’-Pr_2-xCe_xCuO₄ (T’-PCCO) with x = 0, 0.10, and 0.15 using Cu K-edge x-ray absorption near edge structure (XANES). The T‘-PCCO nanopowders were successfully synthesized by the chemically dissolved method with HNO₃ as a dissolving agent continued by calcination at 1000°C for 15 h. The reduced T’-PCCO nanopowders were obtained by reduction annealing process at 700°C for 5 h under Ar gas atmosphere. The analyses of XANES spectra show that oxidation states of the Cu ions in all of the T'-PCCO nanopowders have values between +1 and +2. This indicates the existence of electron doping in the CuO₂ planes, even in the undoped T’-structure. It is found that the oxidation states of the Cu ions change after reduction annealing depending on the existence of apical oxygen in the T'-structure. Based on the XANES analyses, it is revealed that the change of oxidation state is influenced by the presence of both electron and hole carriers in the two-carrier model of T’-structure.

Abstract: Three ways were used to prepare sulfide in soda-lime-silicate glass samples, i.e., glass heat-treated in N₂/H₂ (90/10 volume ratio), doped with different Carbon/Sulfur in mol ratio, as well as different sulfur source respectively. The oxidation state of sulfur on the air side surface of float glass heat-treated in N₂/H₂ and the bulk samples of the other two were determined by X-ray absorption near edge structure spectra (XANES). It was observed on the air side that: 1) A weak pre-edge exhibits at 2466.8eV which appears only with sulfite at high temperature and related to sulfur source; 2) A typical peak of sulfate at 2482.4eV decreases with increasing the temperature and heating duration. 3) Two peaks associated with sulfide, a broader one at about 2476.3eV and a sharper one with an accurate position at 2473.7eV. The S^2- content at air side of glass heat-treated at 750 is higher than that of the glass treated at other temperatures, and increases with the heating duration, which is just the reverse at 1050, because the S^2- volatilizes to the atmosphere easily at higher temperature. In addition, the content of S^2- increases with increasing C/S ratio. The local environment of S^2- in the glass with Na₂S is different from that of with FeS, as indicated by the broader resonance. The spectra indicate that the glass heat-treated in N₂/H₂ (90/10), doped with different C/S ratio and with Na₂S have the same sulfites or sulfites.

Abstract: Bond valence sum (BVS) analysis is used by many researchers to determine the oxidation state of metal ions in solids based on crystallographically determined metal-ligand bond distances. In this paper, bond valence sums (BVS) have been calculated for a series of mixed ligand complexes involving Nickel(II)dithiocarbamates and phosphorous donor ligands. The BVS values are calculated first time for the complexes whose crystal structures have been reported from our laboratory and a few others reported from other laboratories. The calculated values show that the BVS values are higher than the expected formal oxidation state of +2. The higher values observed in the complexes support the fact that the Ni-S, Ni-P bonds are more covalent and the back bonding effects are very highly pronounced.

Abstract: Visible light absorption of TiO2 can be induced by the addition of transition metal impurities. However, many dissimilar results have been reported about this subject and there are various interpretations about the origin of these absorption features. In this work, samples of chromium-doped titania (TiO2-Cr) with different dopant contents (0.1, 0.5, 1.0 and 5.0 wt. %) were prepared by a sol-gel method. Their particle size was determined by dynamic light scattering and it was on the nanometer scale (18 nm). X-ray powder diffraction and Raman spectroscopy showed only the presence of anatase phase in all samples. X-ray photoelectron spectroscopy reveals that the oxidation state of chromium in the prepared materials is different than in the dopant precursor. This change can be associated to the oxidative gelling conditions used in the materials preparation. UV VIS diffuse reflectance spectroscopy showed that the chromium doping, until 1.0 wt. %, did not effectively narrow the TiO2 band-gap but it induces the visible light absorption probably through the formation of color centers.

Abstract: Mineral wool products can be used for thermal and acoustic insulation as well as for fire protection. The high temperature properties and the crystallization behaviour (devitrification) of the amorphous fibres during heating have been examined. Commercial stone wool and commercial hybrid wool (stone wool produced by a glass wool process) have been compared, as well as specially produced stone wool fibres. The fibres differed in chemical compositions and degree of oxidation given by Fe3+/Fetotal ratios. The materials were studied by thermal stability tests, X-ray diffraction, Mössbauer spectroscopy, secondary neutral mass spectroscopy, differential scanning calorimetry and thermal gravimetric analysis. When stone wool fibres were heated at 800 °C in air, oxidation of Fe2+ to Fe3+ occurred simultaneously with migration of divalent cations (especially Mg2+) to the surface. Decreasing Fe3+/Fetotal ratios resulted in increasing migration and improved thermal stability. The cations formed a surface layer mainly consisting of MgO. When heated to above 800 °C, bulk crystallization of the fibres took place with diopside and nepheline as the main crystalline phases. Commercial stone wool and the specially made fibres were considerably more temperature stable than the commercial hybrid wool. Commercial hybrid wool has a high Fe3+/Fetotal ratio of 65% resulting in less migration of cations during heat treatment.

Abstract: Al doped Li(Ni1/3Co1/3Mn1/3-xAlx)O2 (x=0.005, 0.01, 0.05) and Li(Ni1/3-x/2Co1/3Mn1/3-x/2Alx)O2 (x=0.01, 0.05) cathode materials for lithium ion batteries were synthesized using an ultrasonic spray pyrolysis and heat treatment. The substitution with Al reduced the content of Mn3+, promoted grain growth, and broadened the particle size distribution of synthesized powders. The initial discharge capacity of cells made with 0.5 mol% Al doped Li(Ni1/3Co1/3Mn1/3-0.005Al0.005)O2 powder was as high as that of the undoped (~180 mAhg-1, 3.0
4.5 V), and showed an excellent cycle stability. The improvement of the cycle stability was considered to be due to the decrease of Mn3+ in Li(Co1/3Ni1/3Mn1/3-xAlx)O2 by Al doping.

Abstract: Lithium manganese oxide (LiMn2O4) powders for lithium ion batteries were synthesized from two separate raw material pairs, LiOH/MnO and LiOH/MnO2. The prepared powders and their electrochemical properties were investigated. Powders calcined at 780°C were composed of a single-phase spinel structure but those treated at 850°C showed a higher intensity ratio of I400 to I311, a slightly larger lattice parameter, and an increased discharge capacity by 10% under 3.0~4.3V voltage range. The XPS study on the oxidation states of manganese repealed that powders made from LiOH/MnO had less Mn3+ ion and gave better battery performances than those from LiOH/MnO2.