Fe K-edge X-ray Absorption Near Edge Structure Research Articles

Saturation magnetisation as an indicator of the disintegration of barium hexaferrite nanoplatelets during the surface functionalisation

Barium hexaferrite nanoplatelets (BHF NPLs) are permanent nanomagnets with the magnetic easy axis aligned perpendicular to their basal plane. By combining this specific property with optimised surface chemistry, novel functional materials were developed, e.g., ferromagnetic ferrofluids and porous nanomagnets. We compared the interaction of chemically different phosphonic acids, hydrophobic and hydrophilic with 1–4 phosphonic groups, with BHF NPLs. A decrease in the saturation magnetisation after functionalising the BHF NPLs was correlated with the mass fraction of the nonmagnetic coating, whereas the saturation magnetisation of the NPLs coated with a tetraphosphonic acid at 80 °C was significantly lower than expected. We showed that such a substantial decrease in the saturation magnetisation originates from the disintegration of BHF NPLs, which was observed with atomic-resolution scanning transmission electron microscopy and confirmed by a computational study based on state-of-the-art first-principles calculations. Fe K-edge XANES (X-ray absorption near-edge structure) and EXAFS (Extended X-ray absorption fine structure) combined with Fourier-transformed infrared (FTIR) spectroscopy confirmed the formation of an Fe–phosphonate complex on the partly decomposed NPLs. Comparing our results with other functionalised magnetic nanoparticles confirmed that saturation magnetisation can be exploited to identify the disintegration of magnetic nanoparticles when insoluble disintegration products are formed.

Local structure examination of mineral-derived Fe2O3 powder by Fe K-edge EXAFS and XANES

Fe K-edge XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) spectra have been used to investigate the local structures of hematite (Fe2O3) powders which were synthesized from local iron stone. The formation of hematite with various calcination temperature (500, 650, 800) °C was observed from XRD data of each powder, and Fe K-edge EXAFS and XANES were performed to sample with calcination temperature 800 °C. The XANES spectra confirmed the oxidation state of the powders. The energy position of the first pre-edge peak at (7113.5 ± 0.5) eV confirm Fe3+. The absorption edges of Fe2O3 powders was 7123.41 eV. The first EXAFS data analysis showed that Fe2O3 powder exhibited nearest-neighbor distances RFe-O = 1.56449 Å and RFe-Fe = 3.01449 Å. The XRD lattice parameter values for the sample is in a fair agreement with the nearest-neighbor distances according to the EXAFS data.

Geochemical characteristics of oil sands fluid petroleum coke

The geochemical characteristics of fluid petroleum coke from the Athabasca Oil Sands Region (AOSR) of northern Alberta, Canada were investigated. Continuous core samples were collected to 8 m below surface at several locations (n = 12) from three coke deposits at an active oil sands mine. Bulk elemental analyses revealed the coke composition was dominated by C (84.2 ± 2.3 wt%) and S (6.99 ± 0.26 wt%). Silicon (9210 ± 3000 mg kg−1), Al (5980 ± 1200 mg kg−1), Fe (4760 ± 1200 mg kg−1), and Ti (1380 ± 430 mg kg−1) were present in lesser amounts. Vanadium (1280 ± 120 mg kg−1) and Ni (230 ± 80 mg kg−1) exhibited the highest concentrations among potentially-hazardous minor and trace elements. Sequential extractions revealed potential for release of these metals under field-relevant conditions. Synchrotron powder X-ray diffraction revealed the presence of Si and Ti oxides, organically-complexed V and hydrated Ni sulfate, and provided information about the asphaltenic carbon matrix. X-ray absorption near edge structure (XANES) spectroscopy at the V and Ni K-edges revealed that these metals were largely hosted in porphyrins and similar organic complexes throughout coke grains. Minor differences among measured V and Ni K-edge spectra were largely attributed to slight variations in local coordination of V(IV) and Ni(II) within these organic compounds. However, linear combination fits were improved by including reference spectra for inorganic phases with octahedrally-coordinated V(III) and Ni(II). Sulfur and Fe K-edge XANES confirmed that thiophenic coordination and pyritic-ilmenitic coordination are predominant, respectively. These results provide new information on the geochemical and mineralogical composition of oil sands fluid petroleum coke and improve understanding of potential controls on associated water chemistry.

Determination of transition metal ion distribution in cubic spinel Co1.5Fe1.5O4using anomalous x-ray diffraction

We report anomalous x-ray diffraction studies on Co ferrite with composition Co1.5Fe1.5O4 to obtain the distribution of transition metal ions in tetrahedral and octahedral sites. We synthesize spinel oxide (Co1.5Fe1.5O4) through co-precipitation and subsequent annealing route. The imaginary part (absorption) of the energy dependent anomalous form factor is measured and the real part is calculated theoretically through Kramers–Kronig transformation to analyze anomalous x-ray diffraction peak intensities. Fe and Co K-edge x-ray absorption near edge structure (XANES) spectra are used to estimate charge states of transition metals. Our analysis, within experimental errors, suggests 44% of the tetrahedral sites contain Co in +2 oxidation state and the rest 56% sites contain Fe in +2 and +3 oxidation states. Similarly, 47% of the octahedral sites contain Fe in +3 oxidation states, whereas, the rest of the sites contain Co in +2 and +3 oxidation states. While a distinct pre-edge feature in the Fe K-edge XANES is observed, Co pre-edge remains featureless. Implications of these results to magnetism are briefly discussed.

A copper and iron K-edge XANES study on chalcopyrite leached by mesophiles and moderate thermophiles

To reveal the mechanism of chalcopyrite biooxidation at different temperatures, pure chalcopyrite leached by mesophiles (30°C) and moderate thermophiles (48°C) was characterized by synchrotron X-ray absorption near-edge structure (XANES). Solution studies show that elevated temperatures promoted chalcopyrite dissolution. A final copper recovery of 41% was achieved at 48°C, while only 15% at 30°C. Cu K-edge XANES analyses indicated more iron than copper dissolved, leaving on the particle surface CuSn-like species in the early stages of leaching. In the late stages of leaching, copper in CuSn-like species was leached, leaving behind Sn-like species. The slowness of leaching of Sn-like species may be the reason why no CuSn-like species were formed in the late stages, and thus why chalcopyrite passivated. In addition, Fe K-edge XANES indicates no significant amount of jarosite formed at 30°C; however, at 48°C, jarosite precipitation was profoundly accelerated, which accounted for 51% of the iron species in the residue at day 35.

Local structure of magnetite and maghemite and chemical shift in Fe K-edge XANES

Local structures around Fe and chemical shifts in X-ray absorption fine structure (XAFS) spectra were investigated for synthetic Fe1-dO, Fe3O4 (magnetite), γ-Fe2O3(maghemite), and SrFeO3-e, as well as natural Fe3O4 and α-Fe2O3 (hematite) specimens. XAFS spectra near the Fe K-edge were measured at BL-9C and BL-12C of the Photon Factory, KEK, Japan. Similar measurements for Fe-N pairs in Fe nitrides (FeN, Fe2N, Fe4N) were obtained for comparison. The X-ray absorption near edge structure (XANES) spectra for various iron compounds, in particular Fe3O4 and γ-Fe2O3, and FexN specimens, showed clear chemical shifts of half-maximum positions with the changing oxidation states of the Fe ions in their structures. The energy at half maximum position should be used for the quantitative discussion of the oxidation and valence states of the Fe ion in Fe compounds rather than the threshold energy E0 found using differentiating XAFS spectra. The δ values in Fe3-δO4 (δ=0.333 for end-member γ-Fe2O3) for magnetite and maghemite were estimated by the extent of the chemical shifts. Decreases in the average Fe-O distances and extended X-ray absorption fine structure (EXAFS) Debye-Waller factor σ2 values for spinel-type Fe3-δO4 solid-solutions were consistent with the estimated δ values. The error in proving the Fe3+/Fetotal ratio in the system by the XAFS method would appear to be less than ±0.10, and higher reliability than this value would be acquired in the comparison of relative values at half-maximum positions.

Electronic Structure and Local Atomic Arrangement of Transition Metal Ions in Nanoporous Iron-Substituted Nickel Phosphates, VSB-1 and VSB-5

The electronic structure and local atomic arrangement of transition metal ions in nanoporous iron-substituted nickel phosphates VSB-1 and VSB-5 have been investigated using X-ray absorption near-edge structure (XANES) spectroscopy at Fe K- and Ni K-edges. The Fe K-edge XANES study clearly demonstrated that substituted iron ions were stabilized in octahedral nickel sites of nanoporous nickel phosphate lattice. A comparison with several Fe-references revealed that the substituted irons have mixed Fe2+/Fe3+ oxidation state with the average valence of +2.8-3.0. According to the Ni K-edge XANES analysis, the aliovalent substitution of Ni2+ with Fe2+/Fe3+ induced a slight reduction of divalent nickel ions in VSB-5 to meet a charge balance. On the contrary, Fe substitution for the VSB-1 phase did not cause notable decrease in the oxidation state of nickel ions, which would be related either to the accompanying decrease of pentavalent phosphorus cations or to the increase of oxygen anions. In conclusion, the present findings clearly demonstrated that the nanoporous lattice of nickel phosphate can accommodate effectively iron ions in its octahedral nickel sites.

Classification of Fe-bearing species fromK-edge XANES data using two-parameter correlation plots

Solid iron compounds are extremely common in the environment as well as in meteorites and comets. Fe K-edge XANES (X-ray absorption near-edge structure) measurements can be carried out quickly, theoretically allowing one to categorize many areas within a sample or set of samples in a short time. However, interpretation of such data is not straightforward unless one has the appropriate reference spectra, hence a way of classifying an unknown spectrum to a family group (trivalent, divalent, oxide, silicate etc.) is required. Methods of abstracting Fe XANES spectra to produce pairs of variables which, when plotted, cluster in distinct regions depending on the family are presented. For instance, divalent minerals fall in a different region than trivalent minerals, and sulfides in a different region than oxides.

In situ x-ray absorption spectroscopic study of oxide film on Fe alloy

The nature of passive film grown on the bulk surface of Fe–20Cr alloy in a pH 8.4 boric-borate buffer was investigated with in situ synchrotron x-ray absorption spectroscopy. The main electrochemical test method was potentiostatic polarization and various potentials were applied from −1.0 V to 0.6 V vs saturated calomel electrode (SCE). X-ray absorption data from the passive film formed in the boric-borate buffer were collected from an in situ reflection x-ray absorption spectroscopic experiment in a grazing-incident geometry that is suitable for investigating the nanometer-thick passive film. It was observed in the Fe K-edge XANES (X-ray Absorption Near-Edge Structure) region that the Fe absorption edge moved from the absorption edge of metal Fe to that of Fe oxide as the applied potential increased. It was observed from an analysis of the Radial Distribution Function (RDF) from Fe K-edge EXAFS (Extended X-ray Absorption Fine Structure) data that the Fe–Fe bond intensity of the first shell decreased and Fe–O bond intensity increased as the applied potential increased. The RDF of the Fe oxide is consistent with γ-Fe2O3.

AmorphousAl90FexCe10−xalloys: X-ray absorption analysis of the Al, Fe and Ce local atomic and electronic structures

X-ray-absorption fine structure (XAFS) above the Fe K edge, the Ce ${L}_{3}$ edge, and the Al K edge in amorphous ${\mathrm{Al}}_{90}{\mathrm{Fe}}_{x}{\mathrm{Ce}}_{10\ensuremath{-}x}$ $(x=3,$ 5, and 7) alloys have been measured and analyzed. Quantitative analyses of the Fe K-edge and Ce ${L}_{3}$-edge extended XAFS spectra are limited to local structure parameters of the first coordination sphere. Comparison of experimental x-ray-absorption near-edge structure (XANES) spectra with theoretical multiple-scattering (MS) XANES spectra for model compounds, such as crystalline ${\mathrm{FeAl}}_{6}$ and ${\mathrm{CeAl}}_{4},$ allows one to determine the local structure around the aluminum, iron, and Ce sites in ternary amorphous alloys. Using the theoretical MS approach, we show that the Fe and Al K-edge XANES are sensitive to the structure of coordination spheres, which extend up to nearly 4.4 and 3.3 \AA{}, respectively. The Ce ${L}_{3}$-edge XANES, on the other hand, is sensitive to the structure that extends up to 3.15 \AA{}.

Preparation and EXAFS Studies of Highly Dispersed Iron Catalyst Derived from Fe3(CO)12 Precipitated on Alumina

Supported tri-iron dodecacarbonyl has been prepared by a novel method, precipitating the cluster on alumina without any pretreatment. The precipitated Fe3(CO)12/Al2O3 retains a molecular structure similar to that of its parent compound as demonstrated by Fe K-edge XANES (X-ray absorption near-edge structure) and infrared results. After decarbonylation under very mild conditions (310 K in vacuum), the Fe/Al2O3 obtained exhibits a good catalytic selectivity for lower olefins in the hydrogenation of carbon monoxide. K-edge EXAFS (extended X-ray absorption fine structure) analyses of the fresh catalyst and the used catalyst suggest that the active site on the surface is an ensemble of three iron atoms with one short FeFe bond of 2.52 ± 0.05 Å and two longer FeFe bonds of 2.74 ± 0.04 Å. However, the aggregation of these Fe3 groups to particles over 10 Å in size brings about a short lifetime and poor selectivity for the catalyst. The aggregated particles are still two-dimensional, consisting of at least three iron trimers in which the interatomic distance for each trimer of iron on the average is 2.48 ± 0.08 Å, and the nearest interatomic distance between two trimers is 2.83 ± 0.03 Å.

Influence of allosteric effectors on the heme conformation of dromedary ferrous nitrosylhemoglobin detected by XANES spectroscopy

The changes of the Fe heme-active site conformation of dromedary (Camelus dromedarius) nitrosylhemoglobin (HbNO) induced by inositol hexakisphosphate (IHP) and chlofibric acid (CFA) have been studied by using X-ray absorption near-edge structure (XANES) spectroscopy. Structural information has been determined by multiple scattering analysis of the Fe K-edge XANES spectra. The proximal histidine is found to move away from iron centers by about 0.4 Angstrom on the average over the four hemes upon binding of CFA or stoichiometric amount of IHP. In molar excess of polyanion or in the simultaneous presence of IHP, CFA and chloride, the proximal histidine moves back to a position very close to that observed in pure buffer; yet, the structure modulation induced by the allosteric effectors is not completely reversible. Such findings parallel with the functional properties and the spectroscopic (e.g., EPR and absorbance) characteristics of HbNO.