Fe Oxidation State Research Articles

Gd2O3–SrO–Fe2O3 system: The phase diagram and oxygen content in oxides

The phase diagram for the ½ Gd2O3–SrO–½ Fe2O3 system was constructed at 1100 °C in air. Six types of phases: Sr1-хGdхFeO3-δ (cubic, SG Pm3̅m, with 0.05≤х≤0.30 and orthorhombic, SG Pbnm, with 0.80≤х≤1.00), Sr2-yGdyFeO4-δ where y=0.75 and 0.80 with the K2NiF4-type structure, Sr3-uGduFe2O7-δ (two tetragonal oxides u=0 and 0.3 with SG I4/mmm and u=1.9 with SG P42/mnm), and Sr3.2Gd0.8Fe3O10-δ with the tetragonal structure (SG I4/mmm) were obtained. The structural parameters of single-phase samples were refined by the Rietveld analysis. A revised phase diagram for the GdFeO3-SrFeO3-δ system in the “T – composition” coordinates is proposed. Oxygen content in single-phase oxides is determined by TGA and iodometric titration. Oxygen content in the perovskite-type oxides gradually increases with Gd concentration. RP-type oxides with n=1 Sr2-yGdyFeO4-δ and with n=2Sr1.1Gd1.9Fe2O7-δ exhibit negligible oxygen losses with temperature while those with n=2 Sr3-uGduFe2O7-δ (0≤u≤0.3) and n=3 Sr3.2Gd0.8Fe3O10-δ demonstrate significant changes in oxygen content associated to higher Fe oxidation states.

Effect of Fe oxidation state (+2 versus +3) in precursor on the structure of Fe oxides/carbonates-based composites examined by XPS, FTIR and EXAFS

Here we investigate the influence of Fe oxidation state (either Fe(II) or Fe(III) sulfates) in precursor of the same chemical composition on the atomic scale structure, surface speciation and adsorptive anion removal of the purely inorganic composites produced under the urea supported hydrothermal synthesis. In case of utilization of Fe3+ precursor, the materials chemistry was solely dominated by the formation of highly crystalline Fe(III) oxides, however the particle surfaces were covered with small quantities of FeCO3 (not detectable by EXAFS/XANES/FTIR) precipitated after the autoclave was turned off. Within the reactive medium with Fe2+ precursor, due to high pressure in autoclave which facilitated reducing conditions and sedimentation of Fe divalent, two main processes took place one of which was the formation of Fe hydrous oxides; the second reaction resulted in generation of FeCO3, which become a predominant phase in volume averaged composition. Notably, despite the prevalence of Fe(II) carbonates in bulk structure, the narrow upper layers (XPS detectable) was enriched with Fe(III) oxides. At the chosen autoclave temperature of 150 °C, both samples had low hydration of physisorbed water, which confirmed our recent hypothesis about correlation between Fe (or another metal formed oxides) local structure in outer shells fitted with several (many) oxygen atoms and the material hydration with physisorbed water. None of the two composites demonstrated strong adsorptive removal of seven anions (F−, Br−, BrO3−, HAsO42−, H3AsO3, HPO42−, SeO42−), which verified another idea about the interdependence of distinguished (EXAFS simulated) outer shells of metals (here, Fe) fitted with two paths simultaneously ({Fe–Fe}+{Fe–O}) one peak and the anion exchange potential. Overall, the Fe(III)-precursor product was more crystalline, less heterogeneous (fewer phases), showed worse anion uptake than the other sample. The material prepared from Fe(II) precursor is considered to be a promising precursor for further phase transformations via thermal or hydrothermal treatments (due to generous presence of FeCO3).

Study the oxygen vacancies and Fe oxidation states in CaFeO3-δ perovskite nanomaterial

CaFeO3-δ perovskite nanomaterial has been synthesized by sol-gel method. Rietveld analysis of the X-Ray diffraction (XRD) data of the CaFeO3-δ sample revealed orthorhombic perovskite structure (space group pcmn). Field emission scanning electron microscope (FE-SEM) of the CaFeO3-δ sample showed the agglomerations of various particles. Mössbauer measurement of the CaFeO3-δ sample showed that the value of δ is 0.262 and Fe oxidation states are Fe3+ in octahedral and Fe4+ in tetrahedral coordination. X-ray photoelectron spectroscopy (XPS) revealed that the presence of Fe3+, Fe4+ ions, and oxygen vacancies at the surface of the CaFeO3-δ sample. Thermogravimetry analysis (TG) and differential scanning calorimetric (DSC) of the CaFeO3-δ sample revealed the presence of phase transitions. The magnetic properties of the CaFeO3-δ sample exhibit mainly antiferromagnetic state as confirmed by Mössbauer measurement. Based on the obtained results, the presence of oxygen vacancies and mixed oxidation states of iron (Fe3+ and Fe4+) makes the CaFeO3-δ sample useful for industrial applications.

Theoretical and experimental study of (Ga1-xFex)2O3 ternary alloys

Thin films of Ga2O3 and its alloys open the possibility of ultrawide bandgap heterostructure device designs that includes HEMT, quantum well UV optoelectronics, wide bandgap spintronics, high resistive buffer layers for FET and electrocatalytic applications. In this paper, (Ga1-xFex)2O3 (for x = 0.0–0.50) thin film alloys as a function of growth temperatures (450–750 °C) and partial oxygen pressures (10-1 −10-6 torr) grown using pulsed laser deposition are reported. The results suggest that, with the increase of Fe content at a fixed growth temperature and pressure, the bandgap decreases (4.93 to 3.43 eV and 4.39 to 2.25 eV for direct and indirect, respectively) due to a change in the Fe oxidation states as the Fe composition is increased from 0 to 50%. Density functional theory calculations also show a reduction in the bandgap of the alloy as the Fe content increases with the formation of a localized band of states close to the conduction band. Fe content, ≥ 40% in the alloy leads a phase change in the crystal structure with the formation of a gamma phase, also corroborated by theoretical calculations. Finally, room temperature ferromagnetic property was observed in thin films with the introduction of Fe into the Ga2O3 lattice.

Understanding electrochemical and structural properties of copper hexacyanoferrate: Application in hydrogen peroxide analysis

Copper hexacyanoferrate (CuHCF) exhibits great electrocatalytic activity towards H2O2 reduction, making it a suitable electrode material for H2O2 sensors and H2O2-based energy storage devices. In this work, the crystal structures, electronic structures, and electrochemical properties of CuHCF were investigated in detail by both experiments and simulation. Several different samples of CuHCF were synthesized and tested. The effects of Fe oxidation states and the Cu-to-Fe ratios on the electroactivities of CuHCF were evaluated. In-depth voltammetric and X-ray absorption near edge structure (XANES) studies revealed the underlying redox chemistry which contribute to the voltammetric responses of CuHCF. High electroactivity of CuHCF, high cycling stability, and high electrocatalytic activity towards H2O2 reduction were observed for the porous K0.09Cu[Fe(CN)6]0.6 •2.56 H2O particles synthesized at room temperature from chemical co-precipitation of CuSO4 and K3Fe(CN)6 at a 2:1 molar ratio. DFT calculation revealed favorable adsorption of H2O2 on Fe sites. The presence of Cu lowered the local charge on Fe, assisting the adsorption of electron-donating H2O2 molecule on the neighboring Fe atom. The excellent analytical performance of CuHCF towards H2O2 detection was further demonstrated, giving the linear range of 0.0 – 10.0 mM, the sensitivity of 10.6 µA mM‒1, and the limit of detection (3sB/m) of 0.033 mM.

Tracking the Nanoparticle Exsolution/Reoxidation Process in Ni-Doped SrTi0.3Fe0.7O3-δ Electrodes for Intermediate Temperature Symmetric SOFC

Mixed ionic and electronic conductor (MIEC) oxides have been proposed as candidates to replace Ni/YSZ composites as anodes for Solid Oxide Fuel Cells (SOFC) due to their good stability under C-based fuels. Some MIECs have also demonstrated a good electro-catalytic activity both for oxygen reduction and hydrogen oxidation, making them suitable for symmetric configurations (S-SOFC). This approach presents remarkable advantages for reducing manufacturing and operational costs, as well as for extending the cell’s lifetime by reversing gas flows and thus partially reversing the negative effects of sulphur poisoning and carbon deposition that may happen during operation. Also, the catalytic activity of MIEC electrodes can be improved by functionalizing the oxide surface with active nanoparticles. In this work, we study the formation of Ni-Fe alloy nanoparticles by exsolution from a Sr0.93(Ti0.3Fe0.63Ni0.07)O3-δ (STFN) perovskite in reducing atmospheres, and also the process of reoxidation when the exsolved material is exposed to an oxidizing atmosphere. The initial Sr-deficient composition was chosen to alleviate the segregation of Sr [1], which typically can occur in these materials.Exsolution has previously been reported to improve the electrochemical performance of STFN anodes [2], but the mechanisms underlying the exsolution process and the solid/gas interface are still not well understood. The possibility of using S-SOFC materials that undergo exsolution also raises the question of whether the material is regenerated during reoxidation. While oxidation-induced redissolution of exsolved nanoparticles has been observed for Fe-Co exsolution on La0.8Sr1.2Fe0.9Co0.1O4−δ perovskites [3], for the Ni-Fe exsolution in Sr2(Fe1.4Ni0.1Mo0.5)O6− δ, nanoparticles remained at the surface even after reoxidation [4]. The first case is a very interesting result to achieve larger cell lifetimes, and the latter case is interesting as it opens an additional route to increase the cathode performance. In fact, in ref. [5] Ni exsolution in SrTi0.1Fe0.85Ni0.05O3− δ is deliberately employed as design strategy, fully exploiting the non-reversibility of the exsolution of nanoparticles.However, it is not clear whether Ni-Fe nanoparticles oxidize to form a (Ni,Fe)Ox phase or if Fe is reincorporated into the lattice leaving only NiO particles at the surface. It is also not clear how Sr segregation is affected by the exsolution/reoxidation treatments, or how the reoxidized STFN perovskite is modified compared to the pristine sample.To address these questions directly, ambient pressure X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy (AP-XPS and NEXAFS) is used to study the chemical structure of STFN in a complete redox cycle in-situ. Based on the measurements, we can provide insights into the chemical states of Fe and Ni and can differentiate the surface and bulk species for Sr and O in each stage of the cycle. We observe that Ni exsolves readily, but we also note that the amount of surface Fe0 increases with increasing H2 content in the reducing atmosphere; Fe0 also increases with the reduction time following an exponential trend until a plateau value is reached within ~1h. Further, we find a significant Sr segregation in reducing atmospheres, which we presume occurs to compensate for the B-site cation exsolution. The amount of Sr segregation remains constant in the nearest surface after reoxidation, but is partially reversed for larger penetration depths; there is also a rapid reversibility in the Fe oxidation state during reoxidation. These observations were complemented with transmission (TEM) and scanning electron microscopy (SEM) studies, with simultaneous energy dispersive spectroscopy (EDS) analysis.In conclusion, we propose a reoxidation-induced reconstruction which forms a Fe- and Sr-rich STF perovskite in the near-surface region, leaving the Ni segregated from the perovskite. Finally, we link the results to the electrochemical impedance spectroscopy (EIS) response of the STFN electrode, observing that this STFN-reoxidized sample shows a significant improvement in its cathode performance compared to the pristine STFN.[1] Fagg, D. P. et al, J. Eur. Ceram. Soc. 21, 1831–1835 (2001).[2] Zhu, T., Troiani, H. E., Mogni, L. V, Han, M. & Barnett, S. A. Joule 2, 478–496 (2018).[3] Zhou, J. et al. Chem. Mater. 28, 2981–2993 (2016).[4] Liu, T. et al. J. Mater. Chem. A 8, 582–591 (2020).[5] Yang, G., Zhou, W., Liu, M. & Shao, Z. ACS Appl. Mater. Interfaces 8, 35308-35314 (2016). Figure 1

Cover Feature: Spatially‐Resolved Insights Into Local Activity and Structure of Ni‐Based CO2 Methanation Catalysts in Fixed‐Bed Reactors (ChemCatChem 13/2021)

The Cover Feature illustrates the gradients in structure and activity along a catalytic fixed-bed reactor during CO2-methanation. In the focus of this study are Ni-based catalysts. In their Full Paper, by combining spatial activity profiling and complementary structural X-ray absorption spectroscopic studies, M.-A. Serrer, M. Stehle et al. uncovered relationships between the course of the reaction including gas phase concentration and the structure of the catalysts. In case of a bimetallic Ni-Fe-catalyst, a strong correlation between Fe oxidation state and the amount of water in the gas atmosphere was found. An oxidation of Fe under formation of FeOx species is able to protect the active Ni0 centers from oxidation and can offer an alternative CO2 activation pathway. Compared to a conventional Ni-based catalyst, this results in higher activity and selectivity. More information can be found in the Full Paper by M.-A. Serrer, M. Stehle et al.

Synthesis and characterization of Fe(III)-Fe(II)-Mg-Al smectite solid solutions and implications for planetary science

AbstractThis study demonstrates the synergies and limits of multiple measurement types for the detection of smectite chemistry and oxidation state on planetary surfaces to infer past geochemical conditions. Smectite clay minerals are common products of water-rock interactions throughout the solar system, and their detection and characterization provides important clues about geochemical conditions and past environments if sufficient information about their composition can be discerned. Here, we synthesize and report on the spectroscopic properties of a suite of smectite samples that span the intermediate compositional range between Fe(II), Fe(III), Mg, and Al end-member species using bulk chemical analyses, X-ray diffraction, Vis/IR reflectance spectroscopy, UV and green-laser Raman spectroscopy, and Mössbauer spectroscopy. Our data show that smectite composition and the oxidation state of octahedral Fe can be reliably identified in the near infrared on the basis of combination and fundamental metal-OH stretching modes between 2.1–2.9 μm, which vary systematically with chemistry. Smectites dominated by Mg or Fe(III) have spectrally distinct fundamental and combination stretches, whereas Al-rich and Fe(II)-rich smectites have similar fundamental minima near 2.76 μm, but have distinct combination M-OH features between 2.24 and 2.36 μm. We show that with expanded spectral libraries that include intermediate composition smectites and both Fe(III) and Fe(II) oxidation states, more refined characterization of smectites from MIR data is now possible, as the position of the 450 cm–1 absorption shifts systematically with octahedral Fe content, although detailed analysis is best accomplished in concert with other characterization methods. Our data also provide the first Raman spectral libraries of smectite clays as a function of chemistry, and we demonstrate that Raman spectroscopy at multiple excitation wavelengths can qualitatively distinguish smectite clays of different structures and can enhance interpretation by other types of analyses. Our sample set demonstrates how X-ray diffraction can distinguish between dioctahedral and trioctahedral smectites using either the (02,11) or (06,33) peaks, but auxiliary information about chemistry and oxidation state aids in specific identifications. Finally, the temperature-dependent isomer shift and quadrupole splitting in Mössbauer data are insensitive to changes in Fe content but reliability differentiates Fe within the smectite mineral structure.

Defect Chemistry of Mixed Conducting Pcfc Cathode Materials with Protons, Oxygen Vacancies and Holes

In recent years significant progress was achieved for protonic ceramic fuel cells (PCFC) concerning fundamental understanding, materials processing, and device performance. PCFC cathode materials require sufficient proton conductivity to extend the reaction zone beyond the three phase boundary. The hydration thermodynamics of BaFeO3- d-related perovskites was studied using thermogravimetry.[1] Despite a high oxygen vacancy concentration, the degree of hydration is lower for cathode materials compared to Ba(Zr,Y)O3-x electrolytes. A partial substitution of iron by redox-inactive, oversized Zn2+ or Y3+ drastically increases the proton uptake. Measurements of oxygen nonstoichiometry and proton uptake indicate pronounced deviations from ideally dilute defect chemistry (assigned to hole-hole and hole-proton defect interactions [1]). Based on DFT calculations [2] and EXAFS/XRS measurements [3], these interactions are related to the partial transfer of electron holes from iron to adjacent oxygen ions, which in turn disfavors protonation. The obtained detailed defect-chemical understanding serves as the basis for further PCFC cathode optimization, in particular since proton uptake, catalytic activity, electronic conductivity, and stability show conflicting dependencies on cation composition.[1] R. Zohourian, R. Merkle, G. Raimondi, J. Maier, Adv. Funct. Mater. 28 (2018) 1801241.[2] M.F. Hoedl, D. Gryaznov, R. Merkle, E. A. Kotomin, J. Maier, J. Phys. Chem. C 124 (2020) 11780.[3] G. Raimondi, F. Giannici, A. Longo, R. Merkle, A. Chiara, M. F. Hoedl, A. Martorana, J. Maier, Chem. Mater. 32 (2020) 8502.We thank GIF (I-1342-302.5/2016) for financial support.Figure 1: (a) Increase of proton uptake by Y or Zn doping [1]. (b) Dependence of hydration enthalpy on formal Fe oxidation state [2]. (c) Increased local deformations by Y doping [3]. (d) Conflicting trends; e.g. Ba is beneficial for proton uptake and catalytic activity but detrimental for electronic conductivity and chemical stability. Figure 1

Correlating Crystallization-Induced Structural and Electrical Evolutions in SrTi0.65Fe0.35O3-X Thin Films

Rapid oxygen exchange kinetics and fast ionic and electronic transport are central to the performance of materials in solid-state electrochemical devices, such as metal-air batteries, solid oxide fuel/electrolysis cells, and gas sensors [1]. When fabricated at high temperatures, these materials are plagued by surface segregation of large cations, which leads to sluggish oxygen exchange [2]. When further operated at high temperatures, ongoing surface degradation can occur, while operation at low temperatures yields higher electrical resistances and even slower oxygen surface exchange kinetics due to their thermally-activated nature. By contrast, recent work in our group has demonstrated a low-thermal-budget route to fabricate SrTi0.65Fe0.35O3-x (STF35) thin films with enhanced low-temperature oxygen exchange kinetics by crystallizing amorphous-grown STF35 thin films, thereby maintaining a low Sr surface concentration [3,4]. While this result is promising, the crystallization-induced structural evolution and its influence on the charge transport behavior have yet to be studied.In this work, amorphous STF35 thin films grown by room temperature pulsed laser deposition (PLD) were used to study the relationship between structure and conductivity during crystallization of a mixed conductor. The degree of crystallinity and local ion environment were evaluated by X-ray diffraction (XRD) and synchrotron X-ray absorption spectroscopy (XAS). In situ AC impedance spectroscopy was used to monitor the conductivity of the film during crystallization over a 400 °C isothermal hold in a controlled gas environment. The dependence of the conductivity on the volume percent crystalline showed two distinct behaviors: a small, fast increase in conductivity after a short time at temperature and a larger, percolation-type increase later in the anneal. The percolation behavior suggests a microstructural influence on the transport behavior. From the XAS results, we observed an increase in the oxidation number of Fe within the first 10 minutes of the anneal, indicating rapid oxidation. Additionally, there was an increase in symmetry and coordination number in the Fe coordination unit over the first 30 minutes of the anneal, which is correlated to an increase in the orbital overlap of the O 2p and Fe 3d orbitals and thus the hole mobility. Combined with the conductivity and XRD results, the difference in annealing-time dependence of the Fe oxidation state and Fe coordination unit symmetry enable us to separate the relative contributions of changing orbital overlap, hole concentration, and microstructure to the evolving charge transport behavior.[1] Hong, W.T., Risch, M., Stoerzinger, K.A., Grimaud, A., Suntivich, J. and Shao-Horn, Y., 2015. Energy & Environmental Science, 8(5), pp.1404-1427.[2] Perry, N.H. and Ishihara, T., 2016. Materials, 9(10), p.858.[3] Chen, T., Harrington, G.F., Sasaki, K. and Perry, N.H., 2017. Journal of Materials Chemistry A, 5(44), pp.23006-23019.[4] Chen, T., Harrington, G., Masood, J., Sasaki, K. and Perry, N.H., 2019. ACS applied materials & interfaces.

Reflectance spectroscopy of ilmenites and related Ti and Ti[sbnd]Fe oxides (200 to 2500 nm): Spectral–compositional–structural relationships

Ilmenite is an important mineral for understanding lunar evolution. Ilmenite is also a primary ore of titanium on the Earth. Here we present a comprehensive examination of the spectral-compositional-structural relationships of ilmenites and related FeTi oxides. Ilmenite spectral features of interest include maxima near ~250 nm (due Ti4+-O and Fe2+-O charge transfers), ~335 nm (due to Fe2+-Ti4+ charge transfer), and 950 nm (interband maximum), and absorption features near ~540 nm (due to Fe2+-Ti4+ charge transfers), ~630 nm (due to Ti3+-Ti4+ charge transfers), and a ~ 1300/1600 nm absorption doublet (due to Fe2+ crystal field transitions). Absorption features transition from Fresnel peaks to valley around 400 nm, as absorption coefficients decrease toward longer wavelengths. Ilmenite powders are generally darkest and show the greatest spectral contrast and reflectance rise beyond ~1300 nm for the smallest grain sizes. Other Ti and FeTi oxides share some spectral properties with ilmenites but also exhibit many differences. The most common feature they share is a rise in reflectance toward shorter wavelengths below ~500 nm (i.e., blue spectral slope). Ti ± Fe oxide spectra can also exhibit absorption features attributable to Ti, Fe, and Ti ± Fe, and these features vary in intensity, shape and wavelength position due to factors such as Ti and Fe oxidation states, coordination environment, and nearest neighbor cation types. Ilmenite differs most from silicates in the region below ~500 nm: it shows a reflectance increase versus decrease toward shorter wavelengths for silicates, as well as diagnostic Ti/Ti-Fe maxima or minima. Thus, detection of ilmenite in mixtures is best accomplished by including the UV region in spectral analysis. With increasing ilmenite in mixtures, its diagnostic spectral features become increasingly apparent.

Combined multiplet theory and experiment for the Fe 2p and 3p XPS of FeO and Fe2O3.

The Al K alpha, 1486.6 eV, based x-ray photoelectron spectroscopy (XPS) of Fe 2p and Fe 3p for Fe(III) in Fe2O3 and Fe(II) in FeO is compared with theoretical predictions based on ab initio wavefunctions that accurately treat the final, core-hole, multiplets. The principal objectives of this comparison are to understand the multiplet structure and to evaluate the use of both the 2p and 3p spectra in determining oxidation states. In order to properly interpret the features of these spectra and to use the XPS to provide atomistic insights as well as atomic composition, it is necessary to understand the origin of the multiplet energies and intensities. The theoretical treatment takes into account the ligand field and spin-orbit splittings, the covalent mixing of ligand and Fe 3d orbitals, and the angular momentum coupling of the open shell electrons. These effects lead to the distribution of XPS intensity into a large number of final, ionic, states that are only partly resolved with energies spread over a wide range of binding energies. For this reason, it is necessary to record the Fe 2p and 3p XPS spectra over a wide energy range, which includes all the multiplets in the theoretical treatment as well as additional shake satellites. We also evaluate the effects of differing assumptions concerning the extrinsic background subtraction, to make sure our experimental spectrum may be fairly compared to the theory. We conclude that the Fe 3p XPS provides an additional means for distinguishing Fe(III) and Fe(II) oxidation states beyond just using the Fe 2p spectrum. In particular, with the use of the Fe 3p XPS, the depth of the material probed is about 1.5 times greater than for the Fe 2p XPS. In addition, a new type of atomic many-body effect that involves excitations into orbitals that have Fe f,ℓ = 3, symmetry has been shown to be important for the Fe 3p XPS.

Timing of formation and cause of coloration of brown nephrite from the Tiantai Deposit, South Altyn Tagh, northwestern China

The world’s largest and economically valuable nephrite deposits are distributed along the Western Kunlun Orogen in northwestern China. During the past 20 years, extensive white and brown nephrite deposits have been discovered in both China and Russia, and several of these deposits have been found in South Altyn Tagh in the eastern part of the Western Kunlun Orogen. However, the timing of formation, genesis, petrographic characteristics, mineralogical composition, and reasons for the white–brown color of the nephrite remain unclear. We analyzed 50 samples of white, brown, and white–brown nephrite from the Tiantai nephrite deposit in Qiemo County with the aim of understanding the processes of formation, age, and diversity of these nephrites. Three main types of nephrite can be distinguished: white, brown, and dark brown. X-ray diffraction and petrographic studies indicate that the brown and dark-brown nephrite samples have fewer mineral inclusions compared with the white nephrites. The nephrite formed during early retrogression through the sequence of diopside skarn → coarse-grained tremolite skarn and coarse-grained tremolite → fine-grained tremolite. The higher bulk Fe content of the white nephrite (0.76 wt% FeO) relative to the brown nephrite (0.39 wt%), and the uniform, very low Fe2O3 contents of both of these color varieties (<0.15 wt%), suggest that the brown coloration is unrelated to Fe content and oxidation state. The white nephrite is characterized by a wide range of SiO2 contents (49.6–56.7 wt%), in contrast to the brown nephrites, which are characterized by nearly constant SiO2 contents (~58 wt%), close to the ideal content for tremolite. On average, the white nephrite is enriched in Al2O3 (3.22 wt%) and water (3.96 wt% loss on ignition) and depleted in CaO (12.35 wt%) relative to the brown nephrite (0.80 wt% Al2O3, 2.23 wt% loss on ignition, 12.95 wt% CaO). These characteristics, combined with greater transparency and higher Al and water contents in the chlorite formula relative to that of tremolite, as well as a near-absence of Ca in the chlorite formula, can explain the lower transparency and associated more intense coloration (i.e., yellow to brown) of the brown nephrite in terms of the lower modal abundance of chlorite. The spatial distribution of selected elements agrees with this hypothesis, whereby the brown-colored zones are dominated by Ca- and Sr-rich (substituting Ca) domains reflecting the highest modal volume of the tremolite. In contrast, the white zones are characterized by a substantially higher volumetric ratio of Al-rich domains, reflecting the higher chlorite content. Thus, the different coloration is not caused by specific elements, but by modal proportions of tremolite and chlorite. The slightly higher average Cr (10.1 ppm) and Ni (6.4 ppm) contents of brown nephrite, relative to white nephrite (8.7 ppm Cr and 5.5 ppm Ni), might also contribute to the color intensity and brown coloration. Ore-forming fluids involved in the formation of the studied nephrite have isotopic compositions of δ18O = 1.5‰ to 9.4‰ (330–430 °C) and δD = −87‰ to −50‰ (330–450 °C), suggesting that the formation of the Mg-skarn deposit was related to metasomatism of dolomite by fluids derived from local granite/granodiorite intrusions. Zircons from three brown–white samples yield concordant SHRIMP ages of 438 ± 14 Ma (2σ, MSWD=10.4), 916 ± 10 (2σ, MSWD = 1.5), 438 ± 9 Ma (2σ, MSWD = 5.3), and 431.1 ± 2.5 Ma (2σ, MSWD = 2.4). The younger ages (ca. 430 Ma) are close to those for placer nephrite from the Yurungkash and Karakash rivers, as well as those for Yecheng, Alamas, and Buya granodiorites, and probably represent the primary formation ages of nephrite within the Hetian Nephrite Belt. Overall, these ages indicate that the primary deposits formed during pre- or post-orogenic stages in the Western Kunlun Orogen. 40Ar/39Ar dating of hydrothermal muscovite and phlogopite intergrown with tremolite in the nephrite yielded ages of 418.8 ± 3.4 Ma (2σ, MSWD = 0.65), 379.5 ± 3.0 Ma (2σ, MSWD = 0.81), and 354.5 ± 3.6 Ma (2σ, MSWD = 0.71), which suggest multiple metasomatic activity. The similar formation ages of nephrites from Altyn Tagh (433 Ma) and from the previously studied areas of West Kunlun (441–378 Ma) and East Kunlun (416 Ma) indicate that these nephrites formed during the closure of the Proto-Tethys and in the accompanying post-collisional extensional environment.

Characteristic of Fe in tektite observed from XANES and UV-Vis spectroscopy

Tektite is a geological sample formed as a result of a collision of the meteorite on the surface. An energy from the collision results in an excavation of the melted material into the Earth’s atmosphere and landed back on the surface further away from the impact site having glassy or crystalline textures. Tektite was deposited in Thailand and was identified as part of the Australasian strewn field. Chemical compositions of tektites have previously been studied. However, there is no study has established a relationship between composition and color of tektites. Thus, this study aims to relate the characteristic of color-center element, Fe, to the color of tektites using X-ray Absorption Near Edge Structure (XANES) and UV-Vis spectroscopy. Nine tektite samples were collected from Korat plateau and purchased from Thailand and Vietnam gem markets. XANES spectra of all (nine) samples show similar Fe K-edge pattern with pre-edge and E0 at 7080-7089 and 7118 eV, respectively. The spectrum is 100 percent matched with XANES structure of FeO standard suggesting Fe component of Fe2+ oxidation state in the sample. Seven, out of nine, samples establish similar UV-Vis broad absorption peak at 492-539 nm, which corresponds to absorption peak of Fe2+. The UV-Vis absorption spectrum cannot be obtained from two samples possibly due to high Fe-content in the samples. The calculated energy band gaps (E g ) of the samples are in between 1.35 eV to 1.76 eV with maximum absorption in between 1.88 eV to 1.95 eV. The result is in consistent with a black color of the tektite. It indicates that cause of color in tektites is related to Fe-content and can be explained by the energy band. This study can be furthered compare with moldavite (green variety of tektite) in order to identify cause of color in impact-related materials like tektites, as well as to provide information for identification of synthetic and genuine moldavites and also for characterization of tektites sources using the component of various Fe oxidation states.

Thermal decarboxylation for the generation of hierarchical porosity in isostructural metal-organic frameworks containing open metal sites.

The effect of metal-cluster redox identity on the thermal decarboxylation of a series of isostructural metal–organic frameworks (MOFs) with tetracarboxylate-based ligands and trinuclear μ3-oxo clusters was investigated. The PCN-250 series of MOFs can consist of various metal combinations (Fe3, Fe/Ni, Fe/Mn, Fe/Co, Fe/Zn, Al3, In3, and Sc3). The Fe-based system can undergo a thermally induced reductive decarboxylation, producing a mixed valence cluster with decarboxylated ligand fragments subsequently eliminated to form uniform mesopores. We have extended the analysis to alternative monometallic and bimetallic PCN-250 systems to observe the cluster's effect on the decarboxylation process. Our results suggest that the propensity to undergo decarboxylation is directly related to the cluster redox accessibility, with poorly reducible metals, such as Al, In, and Sc, unable to thermally reduce at the readily accessible temperatures of the Fe-containing system. In contrast, the mixed-metal variants are all reducible. We report improvements in gas adsorption behavior, significantly the uniform increase in the heat of adsorption going from the microporous to hierarchically induced decarboxylated samples. This, along with Fe oxidation state changes from 57Fe Mössbauer spectroscopy, suggests that reduction occurs at the clusters and is essential for mesopore formation. These results provide insight into the thermal behavior of redox-active MOFs and suggest a potential future avenue for generating mesoporosity using controlled cluster redox chemistry.

Causes of color in purple- and yellow- quartz

Quartz is a variety of gemstone consisting of SiO2. The various colors of quartz are caused by Fe as a trace element. This study focused on the investigation of Fe oxidation states affecting colorations of purple- and yellow- quartz by X-ray absorption spectroscopy (XAS) and UV-Vis-NIR spectroscopy. The amethyst (purple- quartz) and citrine (yellow- quartz) were collected as the samples. As the XAS results, there was only Fe3+ oxidation state on both purple-and yellow- quartz considering with the absorption energy position at the Fe K-edge compared to the Fe-oxide chemical standards i.e., FeO and Fe2O3. However, the UV-Vis-NIR absorption spectra of the purple- and yellow- quartz were different. Therefore, we propose that the coloration of purple- and yellow- quartz was caused by Fe3+ oxidation states as the donor states at 3.59 eV and 2.28 eV with the different energy band gaps of the purple- and yellow- quartz that were 4.13 eV and 4.88 eV respectively derived from Tauc Plot method.

Quantitative microscale Fe redox imaging by multiple energy X-ray fluorescence mapping at the FeKpre-edge peak

AbstractFe oxidation/reduction reactions play a fundamental role in a wide variety of geological processes. In natural materials, Fe redox state commonly varies across small spatial scales at reaction interfaces, yet the approaches available for quantitatively mapping the Fe redox state at the microscale are limited. We have designed an optimized synchrotron-based X-ray spectroscopic approach that allows microscale quantitative mapping of Fe valence state by extending the Fe XANES pre-edge technique. An area of interest is mapped at nine energies between 7109–7118 eV and at 7200 eV, allowing reconstruction, baseline subtraction, and integration of the pre-edge feature to determine Fe(III)/ΣFe with 2 μm spatial resolution. By combining the Fe redox mapping approach with hyperspectral Raman mineralogy mapping, the Fe oxidation state distributions of the major mineral phases are revealed. In this work, the method is applied to a partially serpentinized peridotite with various Fe-bearing secondary mineral phases to trace the Fe transformations and redox changes that occurred during its alteration. Analysis with the Fe redox mapping technique revealed that the peridotite contained relict olivine with abundant Fe(II), while serpentine, pyroaurite, and another hydroxide phase are secondary mineral reservoirs of Fe(III). Although serpentine is not Fe-rich, it contained approximately 74% ± 14% Fe(III)/ΣFe. These analytical results are integral to interpreting the sequence of alteration reactions; serpentinization of primary olivine formed Fe(II)-rich brucite and oxidized serpentine, which could have contributed to H2 production during serpentinization. Subsequent weathering by oxidizing, CO2-bearing fluids led to the partial carbonation and oxidation of brucite, forming pyroaurite and a hydroxide phase containing dominantly Fe(III). This Fe redox imaging approach is applicable to standard petrographic thin sections or grain mounts and can be applied to various geologic and biogeochemical systems.

Metal-Ligand Cooperativity via Exchange Coupling Promotes Iron- Catalyzed Electrochemical CO2 Reduction at Low Overpotentials.

Biological and heterogeneous catalysts for the electrochemical CO2 reduction reaction (CO2RR) often exhibit a high degree of electronic delocalization that serves to minimize overpotential and maximize selectivity over the hydrogen evolution reaction (HER). Here, we report a molecular iron(II) system that captures this design concept in a homogeneous setting through the use of a redox non-innocent terpyridine-based pentapyridine ligand (tpyPY2Me). As a result of strong metal-ligand exchange coupling between the Fe(II) center and ligand, [Fe(tpyPY2Me)]2+ exhibits redox behavior at potentials 640 mV more positive than the isostructural [Zn(tpyPY2Me)]2+ analog containing the redox-inactive Zn(II) ion. This shift in redox potential is attributed to the requirement for both an open-shell metal ion and a redox non-innocent ligand. The metal-ligand cooperativity in [Fe(tpyPY2Me)]2+ drives the electrochemical reduction of CO2 to CO at low overpotentials with high selectivity for CO2RR (>90%) and turnover frequencies of 100 000 s-1 with no degradation over 20 h. The decrease in the thermodynamic barrier engendered by this coupling also enables homogeneous CO2 reduction catalysis in water without compromising selectivity or rates. Synthesis of the two-electron reduction product, [Fe(tpyPY2Me)]0, and characterization by X-ray crystallography, Mössbauer spectroscopy, X-ray absorption spectroscopy (XAS), variable temperature NMR, and density functional theory (DFT) calculations, support assignment of an open-shell singlet electronic structure that maintains a formal Fe(II) oxidation state with a doubly reduced ligand system. This work provides a starting point for the design of systems that exploit metal-ligand cooperativity for electrocatalysis where the electrochemical potential of redox non-innocent ligands can be tuned through secondary metal-dependent interactions.

Reduction of iron (hydr)oxide-bound arsenate: Evidence from high depth resolution sampling of a reducing aquifer in Yinchuan Plain, China

Sediment in fluvial-deltaic plains with high-As groundwater is heterogenous but its characterization of As and Fe oxidation states lacks resolution, and is rarely attempted for aqueous and solid phases simultaneously. Here, we pair high-resolution (> 1 sample/meter) Fe extended fine-structure spectroscopy (EXAFS, n = 40) and As X-ray absorption near-edge spectroscopy (XANES, n = 49) with groundwater composition and metagenomics measurements for two sediment cores and their associated wells (n = 8) from the Yinchuan Plain in northwest China. At shallower depths, nitrate and Mn/Fe reducing sediment zones are fine textured, contain 9.6 ± 5.6 mg kg−1 of As(V) and 2.3 ± 2.7 mg kg−1 of As(III) with 9.1 ± 8.1 g kg−1 of Fe(III) (hydr)oxides, with bacterial genera capable of As and Fe reduction identified. In four deeper 10-m sections, sulfate-reducing sediments are coarser and contain 2.6 ± 1.3 mg kg−1 of As(V) and 1.1 ± 1.0 mg kg−1 of As(III) with 3.2 ± 2.6 g kg−1 of Fe(III) (hydr)oxides, even though groundwater As concentrations can exceed 200 μg/L, mostly as As(III). Super-enrichment of sediment As (42–133 mg kg−1, n = 7) at shallower depth is due to redox trapping during past groundwater discharge. Active As and Fe reduction is supported by the contrast between the As(III)-dominated groundwater and the As(V)-dominated sediment, and by the decreasing sediment As(V) and Fe(III) (hydr)oxides concentrations with depth.

Ca-doped rare earth perovskite materials for tailored exsolution of metal nanoparticles.

Perovskite-type oxide materials (nominal composition ABO3) are a very versatile class of materials, and their properties are tuneable by varying and doping A- and B-site cations. When the B-site contains easily reducible cations (e.g. Fe, Co or Ni), these can exsolve under reducing conditions and form metallic nanoparticles on the surface. This process is very interesting as a novel route for the preparation of catalysts, since oxide surfaces decorated with finely dispersed catalytically active (often metallic) nanoparticles are a key requirement for excellent catalyst performance. Five doped perovskites, namely, La0.9Ca0.1FeO3-δ, La0.6Ca0.4FeO3-δ, Nd0.9Ca0.1FeO3-δ, Nd0.6Ca0.4FeO3-δ and Nd0.6Ca0.4Fe0.9Co0.1O3-δ, have been synthesized and characterized by experimental and theoretical methods with respect to their crystal structures, electronic properties, morphology and exsolution behaviour. All are capable of exsolving Fe and/or Co. Special emphasis has been placed on the influence of the A-site elemental composition on structure and exsolution capability. Using Nd instead of La increased structural distortions and, at the same time, hindered exsolution. Increasing the amount of Ca doping also increased distortions and additionally changed the Fe oxidation states, resulting in exsolution being shifted to higher temperatures as well. Using the easily reducible element Co as the B-site dopant significantly facilitated the exsolution process and led to much smaller and homogeneously distributed exsolved particles. Therefore, the Co-doped perovskite is a promising material for applications in catalysis, even more so as Co is catalytically a highly active element. The results show that fine-tuning of the perovskite composition will allow tailored exsolution of nanoparticles, which can be used for highly sophisticated catalyst design.