The interaction of hydrogen and carbon monoxide with oxygen on Cu(111)-Fe Surfaces

The interaction of hydrogen and carbon monoxide with oxygen adsorbed on Cu(111)-Fe surfaces containing different amounts of iron has been studied with ellipsometry, Auger electron spectroscopy and low energy electron diffraction. With carbon monoxide copper can be reduced completely and if, at larger iron deposits, γ-Fe₂O₃ is present, γ-Fe₂O₃ can be reduced to Fe₃O₄. The maximum reaction rate is proportional to the square of the total copper surface. With hydrogen all oxygen can be removed. The reduction proceeds via a number of different stages. This is explained by the subsequent occurrence of γ-Fe₂O₃, Fe₃O₄ Fe_0.95O and Fe.     

Mössbauer spectroscopic properties of tin-doped iron oxides

The ⁵⁷Fe Mössbauer spectra recorded in situ from tin-doped Fe₃O₄ at elevated temperature in vacuo shows the Curie temperature to decrease with increasing concentrations of the dopant. Thermal treatment under oxidising conditions results in the initial formation of tin-doped γ-Fe₂O₃ which subsequently undergoes a phase transformation to tin-doped α-Fe₂O₃. ⁵⁷Fe Mössbauer spectroscopy at elevated temperatures shows the Néel temperature for tin-doped γ-Fe₂O₃ to be lower than that of pure γ-Fe₂O₃. The ¹¹⁹Sn Mössbauer spectra recorded from all the tin-doped iron oxides show the presence of a hyperfine magnetic field at the Sn⁴⁺ site which is more complex in the spectra recorded from tin-doped γ-Fe₂O₃ and α-Fe₂O₃.     

Thermal decomposition of almandine garnet: Mössbauer study

The thermal decomposition of almandine garnet from Zoltye Vody, Ukraine, has been studied using⁵⁷Fe Mössbauer spectroscopy. Room temperature Mössbauer spectrum of the initial powdered sample is characterised by one doublet corresponding to Fe²⁺ in dodecahedral position 24c. In the room temperature spectra of all heated almandine samples, a doublet corresponding to γ-Fe₂O₃ nanoparticles appeared. Depending on experimental conditions (heating temperature and time), the additional spectral lines of α-Fe₂O₃ and ε-Fe₂O₃ were observed in Mössbauer spectra. It is obvious that the thermal transformation of almandine garnet in air is related to the primary formation of γ-Fe₂O₃ superparamagnetic nanoparticles. γ-Fe₂O₃ nanoparticles are transformed into ε-Fe₂O₃ and consequently into α-Fe₂O₃ at higher temperatures. The mechanism and kinetics of the individual structural transformations depend on experimental conditions — mainly on the heating temperature and size of the particles.     

Mechanism study of selenium retention by iron minerals during coal combustion

The interactions between selenium vapors and coal accessory Ca/Fe-minerals favor selenium emission control by transferring selenium into fly ash during coal combustion. Considering the complicated effects of iron transformation on selenium retention, iron species in fly ashes from seven coal-fired power plants were distinguished and the associations between selenium and iron minerals were assessed. Iron oxides (including Fe₃O₄, γ-Fe₂O₃ and ɑ-Fe₂O₃) were determined as the main form of iron minerals in fly ash. The adsorption of selenium vapors by different iron oxides was conducted at temperatures ranging from 300 to 900 °C and the species of captured selenium were identified. Furthermore, reaction sites on the surfaces of fresh and reacted iron oxides were compared to investigate the mechanism regarding selenium adsorption over these iron oxides, which were further clarified through density functional theory study. The results showed that iron oxides were surely to play a significant role in selenium retention mainly through chemisorption and the reactions probably occurred at temperatures below 900 °C. At 300 °C, ɑ-Fe₂O₃ had better selenium adsorption performance than Fe₃O₄/γ-Fe₂O₃. Regardless of iron species, Fe atoms on iron oxides participated in the selenium adsorption by forming a Se–O–Fe Structure. With temperature increasing, selenium adsorption by Fe atoms was suppressed, which caused a drop off in selenium capture capacity of Fe₃O₄ and ɑ-Fe₂O₃. Differently, increasing temperature promoted selenium adsorption over γ-Fe₂O₃, which owned a high selenium adsorption capacity even at 700 °C. Further analysis confirmed that the presence of O₂/H₂O(g) in the flue gas contributed to the formation of oxygen vacancies on the surface of γ-Fe₂O₃ at high temperatures and facilitated selenium vapors to react with Fe atoms.     

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron

The core and valence level XPS spectra of Fe_xO (x ~ 0.90–0.95); Fe₂O₃ (α and γ); Fe₃O₄; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). Fe^II and Fe^III states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below E_F, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for Fe_xO and Fe₃O₄. Fe surfaces (films, foils, (100) face) yield predominantly Fe^III species when exposed to high exposures of oxygen or air, though there is evidence for some Fe^II also. At low exposures the Fe^II/Fe^III ratio increases.     

57Fe Mössbauer study under high pressure; ε-Fe and Fe2O3

Using diamond anvil cell, the⁵⁷Fe Mössbauer spectra of pure iron foil and α-Fe₂O₃ powder under high pressure have been measured at room temperature.⁵⁷Fe Mössbauer spectra of α-Fe were measured from 15 GPa to 45 GPa. Isomer shift value decreased and the quadrupole splitting slightly increased as the pressure increased.⁵⁷Fe Mössbauer spectra of Fe₂O₃ under high pressure up to 72 GPa were observed. Above 52 GPa, the new lines appeared at the center portion of the spectrum corresponding to the new high pressure phase. The spectrum of new high pressure phase consisted of 6-line splitting and doublet, suggesting the existence of the two different kinds of iron states in it.     

A study of nanostructures formed in the hydrogen reduction of Fe(OH)<Subscript>3</Subscript>

A complex study of the hydrogen reduction of nanosized iron hydroxide Fe(OH)₃ at 400°C was performed. It was shown that, during the reduction of Fe(OH)₃ to iron metal α-Fe, intermediate compounds such as Fe(OH)₂, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH, and FeO are formed along with stable iron oxides α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄. A scheme of chemical and structural transformations that occur in the reduction of nanosized Fe(OH)₃ is presented. The scheme takes into account the possibility of the bifurcation mechanism of reaction development.     

Mossbauer investigations of corrosion environment influence on Fe valence states in oxide films of zirconium alloys

Mössbauer investigations about iron atom redistribution in oxide films of zirconium alloys subjected to corrosion at 500°C in pure oxygen and water pair have been analysed. The alloys were also subjected to autoclave conditions at a pressure of 10.0 MPa and autoclave conditions at 350°C and at a pressure of 16.8 MPa, using distilled water and water with additives of lithium and fluorine. It is shown that, depending on the corrosion environment, various compounds of iron, such as α-Fe₂O₃, Fe₃O₄, and FeO, as solid solutions of iron in ZrO₂ are formed in oxide films.     

Facile synthesis of flower-like α-Fe2O3/ZnFe2O4 architectures with self-assembled core-shell nanorods for superior TEA detection

Uniform flower-like α-Fe₂O₃ architectures with self-assembled core-shell nanorods are constructed and successfully prepared via the facile process. The concentration of Fe salt plays a great significance for morphological evolution from nanorods to self-assembled microflowers. Flower-like α-Fe₂O₃/ZnFe₂O₄ consisting of α-Fe₂O₃ core and ZnFe₂O₄ shell nanorods are derived from FeOOH/ZIF-8 precursors. The detailed studies reveal that the tunable growth of ZIF-8 nanoparticles on three-dimensional FeOOH microflowers at room temperature and the availble calcination regulation are responsible for the formation of core-shell Fe₂O₃/ZnFe₂O₄ composites. The highest response value of flower-like α-Fe₂O₃/ZnFe₂O₄ architectures to 100 ppm triethylamine (TEA) has been improved to 141 at 280 °C, which is calculated to be 6.2 times compared with flower-like α-Fe₂O₃ architectures (22.7). The enhanced gas-sensing mechanism of α-Fe₂O₃/ZnFe₂O₄ composites can be attributed to the typical microflowers structures, the large specific surface area, the effective heterojunctions between α-Fe₂O₃ core and ZnFe₂O₄ shell, and the improved electron transfer process.     

The study of novel Fe3O4@γ-Fe2O3 core/shell nanomaterials with improved properties

The surface structure of the iron oxide nanoparticles obtained by the co-precipitation method has been investigated, and a thin layer of α-FeOOH absorbed on surface of the nanoparticle is confirmed by analyses of Fourier transform infrared (FTIR), X-ray photoelectron spectra (XPS) and surface photovoltage spectroscopy (SPS). After annealed at 400 °C, the α-FeOOH can be converted to γ-Fe₂O₃. The simple-annealed procedure resulted in the formation of Fe₃O₄@γ-Fe₂O₃ core/shell structure with improved stability and a higher magnetic saturation value, and also the simple method can be used to obtain core/shell structure in other similar system.     

A Mössbauer effect study of the electronic and structural properties of Fe9(PO4)O8

Fe₉(PO₄)O₈ is a mixed valence compound with both layers of (FeO)₆, which are similar to those in stoichiometric wüstite, FeO, and layers of Fe₃PO₆, which are similar to those found in anhydrous iron(III) phosphate, FePO₄. A detailed Mössbauer effect study between 232 and 850 K of the electronic and structural properties of Fe₉(PO₄)O₈ has been undertaken for comparison purposes and to study any valence averaging electron delocalization or exchange that may be present. An earlier single crystal X-ray study has revealed that Fe₉(PO₄)O₈ crystallizes with five distinct iron sites in the ratio of 1∶1∶2∶4∶1. The differently distorted octahedral Fe(1), Fe(3), and Fe(4) sites contain divalent iron, the tetrahedral Fe(5) site contains divalent iron, and the octahedral Fe(2) site contains trivalent iron. Because of the variety of iron sites, the paramagnetic Mössbauer spectra of Fe₉(PO₄)O₈ are complex and exhibit many partially resolved lines. The logarithm of the Mössbauer spectral absorption area and the median isomer shift vary linearly with temperature and yield an effective Mössbauer temperature of 300 K for Fe₉(PO₄)O₈. The temperature dependence of the median isomer shift indicates electron delocalization into an unspecified conduction band above 630 K. The differing site degeneracies, site symmetries, and site valencies make it possible to fit the Mössbauer spectra of Fe₉(PO₄)O₈ with two different models, both of which yield a realistic temperature dependence of the hyperfine parameters, but which lead to different conclusions about the presence of valence averaging electron exchange. Hence, the Mössbauer spectra can not, unequivocally, demonstrate the presence of valence averaging in Fe₉(PO₄)O₈. However, the spectra do indicate the presence of structural changes, both above and below 295 K, which are consistent with a monoclinic space group as suggested by the presence of the weak superlattice reflections reported earlier. The relative component spectral areas indicate, in agreement with the relative Wigner-Seitz cell volumes, that the iron(III) on the Fe(2) site has a relatively low recoil-free fraction, whereas the six-coordinate iron(II) on the Fe(1) site has a relatively high recoil-free fraction.     

Characterisation of soil-oxide analogs by applied-field 57Fe Mössbauer spectroscopy

In this contribution, the effect of high external magnetic fields upon the Mössbauer spectra of aluminous α- and γ-Fe₂O₃, of aluminous α- and γ-FeOOH and of aluminous Fe₃O₄ is reviewed. It is shown that the shapes of these spectra are characteristic of these materials and also to some extent of their crystallinities and Al-for-Fe substitutions. Based upon this evaluation, the potential for application of external-field Mössbauer spectroscopy to soil-related analytical purposes is demonstrated for two soil samples. Limitations of the technique are discussed. Finally, some suggestions for further research in this field are indicated.     

Pressure induced phase transition of nanocrystalline and bulk maghemite (γ-Fe2O3) to hematite (α-Fe2O3)

Phase transition and bulk moduli of bulk and nanocrystalline γ-Fe₂O₃ were studied using synchrotron X-ray diffraction under high pressure. Contrary to most other nanomaterials, nanocrystalline γ-Fe₂O₃ begins to transform into α-Fe₂O₃ at the same pressure as bulk γ-Fe₂O₃, which is caused by a special structure of γ-Fe₂O₃, in which there exist vacancies of crystal. It is believed that phase transition starts from a certain site of vacancy because of the stress concentration at vacancy sites. Compared to bulk material, nanocrystalline γ-Fe₂O₃ has a larger bulk modulus, which is ascribed to the large ratio of surface to volume.     

The nature of the passivating oxide layer on iron powder

Passivated iron powder is studied by Mössbauer spectroscopy, x-ray diffraction and magnetization measurements in order to elucidate the nature of the oxide layer. A gradual transition from FeO at the metal/oxide interface via Fe₃O₄ to γ-Fe₂O₃ at the outside agrees with the experimental results and with the available data on chemical composition, calorimetry, in fared spectroscopy and chemical stability.     

Effect of the Calcination Atmosphere on the Structural Properties of the Reduced Fe/SiO2 System

Iron supported systems are frequently used as catalysts in the Fischer–Tropsch synthesis being the Fe⁰ the active phase for the reaction. We have studied the influence of the calcination atmosphere (air or nitrogen) on the iron oxide reducibility and the metallic iron particle size obtained in Fe/SiO₂ system. We have impregnated a silicagel with Fe(NO₃)₃·9H₂O aqueous solution and the solid obtained was calcinated in air or N₂ stream. These precursors, with 5% (wt/wt) of Fe, were characterized by Mössbauer Spectroscopy at 298 and 15 K. Amorphous Fe₂O₃ species with 3 nm diameter in the former, and α-Fe₂O₃ crystals of 48 nm diameter were detected in the last one. Both precursors were reduced in H₂ stream. Two catalysts were obtained and characterized by Mössbauer spectroscopy in controlled atmosphere at 298 and 15 K, CO chemisorption and volumetric oxidation. α-Fe⁰, Fe₃O₄ and Fe²⁺ were identified in the catalyst calcined in air. Instead, only α-Fe⁰ was detected in the catalyst calcined in N₂. The iron metallic crystal sizes were estimated as ≈2 nm for the former and ≈29 nm for the last one. The different oxide crystal sizes, obtained from the diverse calcination atmospheres, have led to different structural properties of the reduced solids. It has been possible to reduce totally the existing iron in an Fe/SiO₂ system with iron loading lower than 10% (wt/wt).     

Analysis of atmospheric corrosion products of steel and coated steel by means of scattering Mössbauer spectrometry

Wearthering steels treated with and without zinc phosphate solution were exposed to atmosphere for 15 years and rust layers produced on the steels were analysed by scattering Mössbauer spectrometry (CEMS and XMS). γ-FeOOH, fine α-FeOOH, 5Fe₂O₃·9H₂O, γ-Fe₂O₃ and Fe₃O₄ were identified to be present in the rust formed on the steel without phosphate coating. Large particles of γ-Fe₂O₃ and Fe₃O₄ formed on the uncoated steel exposed to atmosphere in a position facing north on vertical plane. The layer structure of rust was affected by the position. The thin rust layer formed on the phosphate + carylite resin coated steel was considered to consist of γ-FeOOH, fine α-FeOOH, and fine γ-Fe₂O₃.     

Mössbauer spectroscopy on57Fe in highT c superconductor Bi2Sr2Ca1Cu2O y

Mössbauer measurements were performed on polycrystalline⁵⁷Fe: Bi₂Sr₂Ca₁Cu₂O _y , super-conductor in the temperature range of 77–296 K. The samples were obtained in a solid phase synthesis using 0.01, 0.03, 0.1 and 0.5 mol fractions of α-Fe₂O₃ (96% enriched in⁵⁷Fe). A prevailing quadrupole doublet practically independent of temperature and iron concentration characterizes the obtained Mössbauer spectra. The corresponding hyperfine parameters suggest the presence of high spin Fe¹¹¹ ions in a strongly distorted octahedral symmetry which indicates a probable copper substitution by iron in the system.     

溅射氧化铁薄膜矫顽力的研究

本文报道了有关溅射氧化铁薄膜磁性能的系统研究,特别着重于矫顽力的温度依赖关系和矫顽力随不同氧化铁相的变化。从理论分析与实验测量结果的对比中,给出了形状各向异性、磁晶各向异性及应力各向异性各自对Fe₃O₄薄膜、γ-Fe₂O₃薄膜及二者混合相薄膜的贡献,并且得到了Fe₃O₄薄膜和γ-Fe₂O₃薄膜的磁晶各向异性常数K₁的温度依赖关系曲线。 关键词：     

Low Temperature CEMS of Corrosion Products by Sulfuric Acid Water

Conversion electron Mössbauer spectroscopy (CEMS) was used to study the corrosion products by sulfuric acid water. The acid water was put on the iron foil and evaporated under the atmosphere of several oxygen ratios in nitrogen-oxygen gas mixture in order to investigate the influence of oxygen contents on the corrosion products. The hyperfine fields of the observed sextets at 15 K suggest that the corrosion products consist of α-FeOOH, γ-FeOOH and γ-Fe₂O₃. The amount of γ-FeOOH becomes larger as the oxygen concentration increases. It is also found that the amount of γ-Fe₂O₃ becomes the largest at 1.4 % of the oxygen concentration.     

Iron-containing atmospheric aerosols in the Chernobyl fallout

Mössbauer spectroscopy was applied to determine the composition and the iron concentration in the atmospheric aerosols contaminated in Sofia, Bulgaria after the Chernobyl accident. The results confirm the major conclusion of the Kopcewiczs for Poland, i.e. that in the initial filters, collected during the contaminating fallout (30.04–05.05.1986), the iron concentration was highest, 3.69 μg/m³ and that magnetite Fe₃O₄ was present. For the following days a change in the chemical composition including the presence of α-Fe₂O₃, α-FeOOH and γ-FeOOH as well as the absence of magnetite, was detected. Input of industrial iron contamination was negligible since the nearby steel plant had worked at minimum power due to official holidays. Unfortunately, Mössbauer spectroscopy studies only, do not allow a definite conclusion about an increase of the isotope abundance of ⁵⁷Fe in the Chernobyl fallout.