Valence State Of Iron Research Articles

Supercritical water gasification of fuel gas production from waste lignin: The effect mechanism of different oxidized iron-based catalysts

Iron and iron oxides have been employed to catalyze supercritical water gasification (SCWG) of lignin, a typical component of pulp and paper mill wastewater. To investigate the effects of different oxidation sates of Fe-based catalysts during SCWG process, all simulations were carried out through ReaxFF molecular dynamics method. During the catalytic SCWG process, the degradation rate of guaiacyl dimer lignin (GDL) molecule was inversely proportional to the valence state of iron, the higher oxidation state of Fe in iron-based catalyst was, the lower the catalytic degradation ability would be, and then GDL molecule underwent a series of reactions, accompanying with the generation of small molecules, among most of them were fuel gas products. In terms of gas products, Fe catalyst had a unique advantage in catalytic hydrogen production. Moreover, it is found that iron with low oxidation state was beneficial to the formation of CO, while iron with high oxidation state was CO2. Our simulation results further revealed the formation mechanisms of CO, CO2 and CH4. Migration of lattice oxygen in iron oxides was also visualized through figure, and spent catalyst showed different sources in the final, demonstrating that SCW participates in the entire reaction providing not only H but also O free radicals.

Structural and optical properties of sol-gel synthesized LaFe1-xAgxO3

The band-gap energy of LaFeO3 is successfully tuned by substitution of Fe by monovalent Ag+. The physical properties of the as-synthesized nanopowders are evaluated using various structural and spectroscopic characterization techniques and supported with first principles calculation. All LaFe1-xAgxO3 nanopowders (x = 0, 0.02, 0.03, 0.04, 0.05) exhibit an orthorhombic crystal structure with increased a and c lattice parameters and that of the b parameter decreased when the silver content increases. The latter also contributed to the formation of highly porous crystallites with inter-pore spacings in the order of the nanometer. Moreover, the incorporation of silver shows a lattice distortion in particular the Fe-O bond which is responsible for the reduction of the band-gap energy value. Therefore, an additional iron valence state, Fe2+, is created along with that of the original one, Fe3+. Density functional theory calculation suggests that silver ions are more likely positioned at (a/2, 0, c/2) coordinates in the unit cell. The efficiency of such silver incorporation is demonstrated due to the band-gap energy red-shift (from 2.39 eV, for × = 0, to 1.9 eV, for × = 0.05) caused by the decrease in the minimum of the conduction band. This ability to control the energy value of the band-gap in multiferroic materials is very promising for photovoltaic applications.

High-capacity and high-rate Ni-Fe batteries based on mesostructured quaternary carbon/Fe/FeO/Fe3O4 hybrid material

SummaryThe Ni-Fe battery is a promising alternative to lithium ion batteries due to its long life, high reliability, and eco-friendly characteristics. However, passivation and self-discharge of the iron anode are the two main issues. Here, we demonstrate that controlling the valence state of the iron and coupling with carbon can solve these problems. We develop a mesostructured carbon/Fe/FeO/Fe3O4 hybrid by a one-step solid-state reaction. Experimental evidence reveals that the optimized system with three valence states of iron facilitates the redox kinetics, while the carbon layers can effectively enhance the charge transfer and suppress self-discharge. The hybrid anode exhibits high specific capacity of 604 mAh⋅g−1 at 1 A⋅g−1 and high cyclic stability. A Ni-Fe button battery is fabricated using the hybrid anode exhibits specific device energy of 127 Wh⋅kg−1 at a power density of 0.58 kW⋅kg−1 and maintains good capacity retention (90%) and coulombic efficiency (98.5%).

3D honeycomb monoliths with interconnected channels for the sustainable production of dihydroxybenzenes: towards the intensification of selective oxidation processes

Novel 3D Fe/SiC honeycomb monolithic reactors with different morphologies (i.e. cell geometry, cell density and interconnected channel pattern) have been conceptually designed, digitally prototyped and manufactured by robocasting. Square, tronco-conical and triangular cell geometries with parallel channels presenting staggered or faced interconnections have been tested in the phenol hydroxylation reaction with hydrogen peroxide to produce dihydroxybenzenes, such as catechol and hydroquinone. The analysis of the valence state of iron in the monoliths by Mössbauer spectroscopy identified iron silicides, viz. Fe3Si and α-FeSi2, as the iron catalytic species.The results demonstrate that an increased macro-channel tortuosity, favoured by a high density cell and a high number of not-facing inter-connected channels, facilitates the selectivity to the dihydroxybenzenes. In particular, 3D Fe/SiC monoliths with triangular cells provide an outstanding improvement with respect to the commercial process, not only because of their superior performance (SDHBZ=99.1% and YDHBZ=29.6% at 80 ºC) and stability (over 8 days on stream) but also in sustainability (i.e. operation in flow-reactor, no need of catalyst filtration, water as unique solvent). The additive manufacturing has allowed the smart integration of the catalytic phase into the monolithic structure, enabling, by this way, to architecture the reactor independently on its chemical composition.

Enhancement in the Magnetic Properties of Cr3+ and Ce3+ Substitution Thulium Iron Garnet Synthesized via Sol-Gel Route

The structure and magnetic properties of Ce0.04Tm2.96CrxFe5-xO12 (x = 0–0.30) prepared via sol-gel route were studied in this paper. X-ray diffraction (XRD) data and Raman spectra (RS) data revealed that Cr and Ce strongly enter the Tm3Fe5O12 (TmIG) structure. RS results showed the characteristic peaks of TmIG garnets ferrites. The X-ray photoelectron spectroscopy (XPS) results in Ce0.04Tm2.96Cr0.25Fe4.75O12 revealed that single valence state of iron was “+3.” Moreover, by comparing Ce0.04Tm2.96Fe5O12 with Cr-doped sample, saturation magnetization (Ms) was increased, which may be related to the fact that Cr3+ ions occupied octahedral position (a) to increase the effective moments formed by Fe3+ cations. The Ms of the sample x = 0.15 reaches a maximum of 18.24 emu/g. Furthermore, these nanoferrites with different magnetic properties and structures may be used for longitudinal recording and sensor applications.

Mössbauer spectroscopy, XRPD, and SEM study of iron‐containing Na2O‐B2O3‐SiO2 glasses

AbstractGlasses in the Na2O‐B2O3‐SiO2‐Fe2O3 system are suitable for producing magnetic porous glasses—host matrices for multiferroic composite materials for spintronics applications. Successful synthesis of such materials is dependent on the knowledge of crystallization and immiscibility areas in this system. Here, we report new findings for such glasses with a constant SiO2 concentration of 70 mol %, containing 7‐12 mol % Fe2O3, whose compositions lie in the low (2 mol % Na2O) and higher (8‐12 mol % Na2O) alkali regions, heat‐treated at 550°C. Glasses were studied using analytical chemistry methods and investigated by means of Mössbauer spectroscopy, XRPD and SEM. We outlined the immiscibility area boundary in the chosen silica cross‐section for 550°C, indicating the region of interconnected morphology and the crystallization fields of magnetite and FeSiO3, and correlated them with data on the valence and coordination state of iron in glasses. We find that both Fe3+ and Fe2+ ions in the low‐alkali region are octahedrally coordinated, in higher alkali area Fe3+ and Fe2+ are tetrahedrally and octahedrally coordinated, respectively. A significant amount of Fe2+ in the low‐alkali region can be the cause of the FeSiO3 formation. The usual crystalline phase in non‐X‐Ray amorphous glasses is magnetite, precipitating at the annealing stage.

Effect of the Valence State of Iron in the Precursors on the Fischer–Tropsch Synthesis Performance of an Fe/Fe Foam Catalyst

The relationship between iron precursors and Fischer–Tropsch synthesis (FTS) performance is not well understood due to the lack of evidence for the role of the iron valence state. Here, the iron pr...

High-Capacity and High-Rate Ni-Fe Batteries Based on Mesostructured Quaternary Carbon/Fe/Feo/Fe <sub>3</sub>O <sub>4</sub> Hybrid Material

There is now an increasing demand for green, long-lasting and sustainable battery technology, due to the rapid development of photovoltaic and other renewable energy power systems. The Ni-Fe battery is a promising alternative to lithium ion batteries due to its long life, high reliability and eco-friendly characteristics. However, passivation and self-discharge of the iron anode are currently two of the main issues with Ni-Fe batteries, which lead to low specific energy (20-30 Wh·kg-1) and poor cycle efficiency (~65%). Here, we demonstrate that controlling the valence state of the iron and coupling with carbon can solve these problems. We develop a novel mesostructured quaternary carbon/Fe/FeO/Fe3O4 hybrid, containing iron oxides wrapped with a few atomic layers of carbon synthesized by a one-step solid state reaction. Experimental evidence reveals that the optimized system with three valence states of iron facilitates the redox kinetics, while the carbon layers can effectively enhance the charge transfer and suppress self-discharge. The hybrid anode exhibits high specific capacity of 604 and 528 mAh · g-1 at 1 and 10 A·g-1, respectively and high cyclic stability. A Ni-Fe button battery is fabricated using the carbon/Fe/FeO/Fe3O4 hybrid, delivering specific device energy of 127 and 110 Wh·kg-1 at a power density of 0.58 and 5.07 kW·kg-1, respectively. ~98.5 % Coulombic efficiency and ~90 % capacity retention have been achieved after 2500 cycles at 300 mA ·g-1. These features suggest that the Ni-Fe battery holds promise to become an important alternative in stationary energy storage.

Study on Remediation of Hexachlorobenzene Contaminated Soil by Mechanochemical Method

The mechanochemical method is a potential way to destroy pollutants such as heavy metals and organic compounds due to its advantages such as complete reaction, adaptation of various pollutants and low energy consumption, etc. Research work was conducted to investigate the feasibility of remediating the persistent organic pollutants (POPs) contaminated soil and how the parameters influence the destruction of the pollutants. In the study, hexachlorobenzene (HCB) was used as a presentative of the POPs in soil. Natural minerals such as albite and magnetite were selected as additives to treat HCB contaminated soil with the application of mechanochemical method. The reasonable operation parameters as well as the soil properties on the destruction of HCB were determined. Analysis such as Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffractometer (XRD), X-ray Photoelectron Spectroscopy (XPS) and Raman Spectrometer were conducted for the supplement of mechanism study. A degradation rate of 92.5% for HCB was achieved under the optimal reaction condition. According to the XPS analysis results, the transformed valence state of iron, provided electrons for the destruction of HCB, on the basis of specific structure of albite. The amorphous carbon and graphite carbon were the final products of the destruction of HCB in the process of ball milling. The selected reagents with albite and magnetite would be viable for the damage of other POPs by mechanochemical method.

The importance of the iron valence state in NiCoFe nanosheet array catalysts for the oxygen evolution reaction

The catalytic performances of NiCoFe nanosheet array catalysts for the OER are strongly dependent on the valence state of iron cations in the precursors.

A deep insight on the coal ash-to-slag transformation behavior during the entrained flow gasification process

Recent research provided deep insight on the coal ash-to-slag transformation characterization during the entrained flow gasification process, with experimentation on a 40 kg/hr (dry basis) coal-fed opposed multi-burner (OMB) entrained flow gasifier and simulation via FactSage™ software. A general mechanism is presented to relate the gasifier design temperature, ash fluid temperature, and operating temperature with the degree of the slag polymerization. The change of the high temperature zone, the corresponding particle residence time in the high temperature zone, and syngas composition have obvious effects on the slag mineral transformation behavior. Mineral types formed on the wall of the gasifier chamber were mainly anorthite (CaAl2Si2O8), aluminum oxide (Al2O3), and calcium sulfide (CaS). These minerals transformed to anorthite and diopside (CaMgSi2O6) at the slag hole zone, while the minerals at the lock hopper were anorthite, orthoclase (KAlSi3O8), quartz (SiO2), gypsum (CaSO4), calcite (CaCO3), and halite (NaCl). FactSage™ predicted minerals as anorthite, diopside, orthoclase, and albite (NaAlSi3O8), etc., where the slag temperature was below the ash fluid temperature and when the ratios of CO/CO2 and (CO + H2)/CO2 were lower than 1.0 and 2.0, respectively. By simulation, residual carbon was found to be the dominant factor over syngas composition to cause mineral transformation, and this was verified experimentally. The Ca-based crystals, typically anorthite, was shifted to diopside, near the slag hole zone, and a linear relationship was found between the content ratios of diopside/(anorthite + diopside), CaO/SiO2, and (CaO + MgO)/SiO2. A dimensionless number, θ, was defined to characterize the changing chemical composition and the degree of slag polymerization, with temperature deviation from the design condition. Three zones of θ were identified and related to the deviation between the actual gasification condition from the design condition. A low slag polymerization degree corresponded with a higher temperature deviation between the actual condition and design condition, and this proved that increased residual carbon content and changing iron valence state increased the mineral types when the slag temperature was below the ash fluid temperature.

Oxygen Vacancies and Valence States of Iron in SrFeO3 – δ Compounds

The brownmillerite phase has been studied by X-ray diffraction and Mossbauer methods. The available data on single-phase compounds with the perovskite structure in strontium ferrite SrFeO3 – δ are analyzed. All the valence states of iron for any phase state of strontium ferrite are found to be determined by its local oxygen environment. This fact enables us to understand the regularity of transitions of iron from one valence state to another state when adding oxygen vacancies and to explain the structural states of iron in strontium ferrite, such as the single-phase and two-phase compositions. This approach is a more general case for description of all known compounds and phase combinations synthesized in SrFeO3 – δ than the formula for single-phase structures that is considered in the literature and fit well into the scheme proposed in this work.

Surface metal valence state regulating on hematite to weaken dependence of charge transport to catalyst loading

Generally, the loading of oxygen evolution catalyst (OEC) must be carefully controlled to an extremely thin level or the photo-generated holes will mainly charge the catalyst rather than oxidizing water. To overcome the extreme sensitivity of charge transport to catalyst thickness, we proposed the iron valence state regulating on hematite to negatively shift the surface defect state, thereby suppressing the severe OEC charging. In detail, a fluorine layer (FeOxFy) was inserted between Fe2O3 and OEC (ZIF-67) to reduce the oxidation state of iron atoms, forming a case where only part of Co(II) can be oxidized (build a rapid Co(II)/Co(III) cycle). Even in the case of thicker OEC (~10 nm), Fe2O3@[email protected] exhibits increased photocurrent density of 2.56 mA cm−2 at 1.23VRHE (~170% higher than Fe2O3@ZIF-67), accompanied by a rapid decrease in sensitivity to OEC loading. Above pretreatment not only relaxed the catalyst process requirements, but also provide new insights into interfacial catalysis process.

Effects of composition and pressure on electronic states of iron in bridgmanite

Abstract Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10–50% Fe/total cations, 0–25% Al/total cations, 12–100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4–3.1 and 3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.

Structural characterizations and dielectric, magnetic, and optical properties of Ba2FeVO6 double perovskite ceramics

Here, we present the structural characterization and physical properties of Ba2FeVO6 (BFVO) ceramics prepared by the solid-state ceramic method, which is regarded as potential Mott multiferroics. Structural studies demonstrate that the BFVO ceramic samples crystallize in a hexagonal structure with R3c symmetry. SEM images show a spherical morphology of the ceramic grains with an average composition of Bi:Fe:V close to 2.0:1.0:1.0 determined by x-ray energy dispersive spectroscopy. Infrared spectra revealed the vibrational modes of [FeO6] and [VO6] octahedrons in the ceramics, and XPS spectra confirmed the dual chemical valence states of iron (Fe2+ and Fe3+) and vanadium (V3+ and V4+) and two states of oxygen species in the BFVO ceramics, while barium was present as Ba2+. The BFVO ceramics exhibit relaxor-like dielectric behavior with an activation energy of 0.62 eV. The antiferromagnetic (AFM) nature was confirmed in the BFVO ceramics, whereas a weak magnetism was observed at 2 K. The temperature dependence of magnetizations measured with (and without) field cooling across 2 K–300 K under external magnetic fields indicates the spin glass characteristics in the present system. A strong variation of the dielectric behavior observed close to the AFM transition temperature was due to the induced dielectric polarization via the magnetostrictive effect driven by AFM transition. The bandgaps (Eg) of the investigated ceramic samples were deduced from their UV–visible spectra. The Eg values were about 1.15 eV and 1.50 eV, which were ascribed to the d–d electronic transitions of Fe 3d t2g → Fe 3d eg and 6A1g → 4T1g, respectively.

Synthesis optimization and investigation on electrical properties of Fe+2-doped Mg2TiO4 ceramics for energy storage

Polycrystalline MgFeTiO4 ceramic is prepared using a solid-state reaction route. The structural, vibrational, morphology, and electrical characteristics of the compound are studied. Crystal structure and phase analysis results suggest that MgFeTiO4 has a cubic spinel structure corresponding to the Fd3m space group. Fourier transform infrared (FTIR) and Raman spectroscopy indicate that Mg‒O, Fe‒O, and Ti‒O bonds are present in the sample. X-ray photoelectron spectroscopy study suggests the valence state of iron (Fe); thus, the formation of MgFeTiO4 is confirmed. The morphology and compositional analyses are done by field emission scanning electron microscopy and high-resolution transmission electron microscopy. The electrical and dielectric behaviors of the sample are studied at various temperature ranges. The dc conductivity of 5.68 × 10−4 S cm−1 is obtained at 300 °C, which is higher compared to LiFeTiO4. The activation energy is estimated and observed two different temperature-dependent conductivity regions.

Electrocatalytic reduction of CO2 to CO over iron phthalocyanine-modified graphene nanocomposites

The non-precious materials with iron-pyrrolic nitrogen-carbon (Fe–N–C) moiety have attracted an increasing attention on the CO2 reduction reaction (CO2RR). However, the influence of iron valence state and environmental synergy is not still clear. Here, iron phthalocyanine (FePc) molecule with an intrinsic Fe–N4–C moiety is enough dispersed on graphene as the single-atom iron catalyst through a facile chemical method. The FePc-graphene composites with different FePc content and iron valence state are synthesized to investigate their catalysis for the CO2RR to CO. The onset overpotential of 190 mV and Faradaic efficiency of >90% may be achieved in the optimal composites. Experimental and calculational results prove the positive role and synergy of Fe(II)Pc with Fe(III)Pc and graphene. The formation of ∗COOH intermediate is confirmed to be the rate-limiting step. The theoretical calculation reveals that Fe(II)Pc should have higher activity than Fe(III)Pc, and Fe(II)Pc/FePc(III) dimer may be better than individual one. This work clearly exhibits the effect of Fe2+/3+- pyrrolic N4–C on the CO2RR.

A comparative study of the R-P phase Srn+1FenO3n+1 (n= 1, 2 and 3) cathodes for intermediate temperature solid oxide fuel cells

A comparative investigation of the Ruddlesden-Popper (R-P) phase Srn+1FenO3n+1 (SFO, n = 1, 2 and 3) series of materials has been explored to determine their applicability as cathodes for proton-conducting solid oxide fuel cells (H-SOFCs). With the traditional solid-state reaction method, pure SFO powders were obtained. As n increases, the composition of perovskite in the material increases gradually, and the proportion of the rock salt layer decreases accordingly. The valence state of iron in the SFOs was analyzed using X-ray photoelectron spectroscopy (XPS), and the results were consistent with those estimated by an X-ray absorption fine structure (XAFS) analysis. The results show that the average valence state of iron is the lowest in SFO2 and the highest in SFO3. Notably, Sr3Fe2O7 (SFO2, n = 2) and Sr4Fe3O10 (SFO3, n = 3) showed better electronic conductivity compared with Sr2FeO4 (SFO1, n = 1), in which SFO2 was the best. Paralleling the conductivity test, the oxygen incorporation kinetics of SFO2 were better than the other two components, as measured by the electricity conductivity relaxation (ECR) method. Single cells with the structure of NiO-BaZr0.1Ce0.7Y0.2O3-δ (NiO-BZCY) anode|-BZCY electrolyte|-SFO-5wt.% BZCY cathode were also fabricated and measured from 550 to 700 °C. At 700 °C, the maximum power density of 703.8 mWcm-2 was achieved with the SFO2-5wt.%BZCY cathode. In conclusion, SFO2 with two BO6 layers connected with a perovskite-related R-P structure is a promising cathode candidate for H-SOFCs, providing new directions for other R-P phase electrode materials for H-SOFCs.

Valence state of iron and molybdenum cations under conditions of anionic deficiency in Sr2FeMoO6–δ

Valence state of iron and molybdenum cations under conditions of anionic deficiency in Sr2FeMoO6–δ

Solvent directed discrimination of metal ions using a coumarin-pyridine fluorescence receptor

The development of reliable methods for large scale discrimination of versatile metal ions is meaningful to avoid metal poisoning accidents and investigate their biological roles. Herein, a solvent directed strategy is reported for readily construction of a robust and powerful fluorescent sensor array R1SA for effective metal ion discrimination. The four sensors of R1SA were obtained by simply dissolving a unique coumarin-pyridine compound R1 in four different solvents, i.e., methanol, ethanol, acetonitrile and DMSO respectively. The sensing mechanism was based on the fact that R1 showed fluorescence responses towards various metal ions in a solvent directed manner. R1SA was able to generate distinct fluorescence response patterns for 12 kinds of metal ions including Cd2+, Co2+, V3+, Sn2+, Fe3+, Cr3+, Hg2+, Mn2+, Cu2+, Bi3+, Pb2+, and Ag+. It was able to discriminate these 12 metal ion species reliably in both aqueous solution and serum media, and identify the unknown metal ion in serum samples with high accuracy. Moreover, R1SA was also successfully applied to discriminate metal ion mixtures of Sn2+/Pb2+, Cd2+/Hg2+ and the valence states of iron in water and 5 % serum media, indicating that it can be considered as a practical tool for investigating the biological roles of iron.