Determination of the iron oxidation state in earth materials using XANES pre-edge information

Role of Fe decoration on the oxygen evolving state of Co3O4 nanocatalysts

The production of green hydrogen through alkaline water electrolysis is the key technology for the future carbon-neutral industry. Nanocrystalline Co3O4 catalysts are highly promising electrocatalysts for the oxygen evolution reaction and their activity strongly benefits from Fe surface decoration. However, limited knowledge of decisive catalyst motifs at the atomic level during oxygen evolution prevents their knowledge-driven optimization. Here, we employ a variety of operando spectroscopic methods to unveil how Fe decoration increases the catalytic activity of Co3O4 nanocatalysts as well as steer the (near-surface) active state formation. Our study shows a link of the termination-dependent Fe decoration to the activity enhancement and a significantly stronger Co3O4 near-surface (structural) adaptation under the reaction conditions. The near-surface Fe- and Co-O species accumulate an oxidative charge and undergo a reversible bond contraction during the catalytic process. Moreover, our work demonstrates the importance of low coordination surface sites on the Co3O4 host to ensure an efficient Fe-induced activity enhancement, providing another puzzle piece to facilitate optimized catalyst design.

Angular orientation between the cores of iron oxide nanoclusters controls their magneto-optical properties and magnetic heating functions

Oriented attachment of nanobricks into hierarchical multi-scale structures such as inorganic nanoclusters is one of the crystallization mechanisms that has revolutionized the field of nano and materials science. Herein, we show that the mosaicity, which measures the misalignment of crystal plane orientation between the nanobricks, governs their magneto-optical properties as well as the magnetic heating functions of iron oxide nanoclusters. Thanks to high-temperature and time-resolved millifluidic, we were able to isolate and characterize (structure, properties, function) the different intermediates involved in the diverse steps of the nanocluster's formation, to propose a detailed dynamical mechanism of their formation and establish a clear correlation between changes in mosaicity at the nanoscale and their resulting physical properties. Finally, we demonstrate that their magneto-optical properties can be described using simple molecular theories.

Operando QEXAFS Study of Pt–Fe Ammonia Slip Catalysts During Realistic Driving Cycles

Bifunctional Fe–Pt ammonia slip catalysts were studied by operando quick-scanning extended X-ray absorption fine structure spectroscopy (QEXAFS) under conditions mimicking rapid temperature variations that occur in an automotive exhaust gas aftertreatment system during real driving. Two catalysts, Pt/Al2O3 and Fe-ZSM-5, were tested individually, as mixtures and in dual bed arrangements. Applying QEXAFS allowed to track changes of active metal state with high time resolution. It uncovered a strong dependence of the active metal state on reaction conditions and catalyst bed layout. For example, proximity to platinum stabilized iron species in their more active oxidized state and led to higher Fe-ZSM-5 activity. On the contrary, isolated iron species were more susceptible to overreduction by ammonia which led to deactivation and low selectivity. The use of transient conditions uncovered the influence of non-equilibrium phenomena on catalytic performance under industrially relevant conditions. Specifically, the effect of ammonia storage on the increase of activity was shown. This was also accompanied by elevated N2O production not observed during tests with gradual heating. Additionally, unusually high NOx selectivity was detected for Fe-ZSM-5 under these conditions. Lastly, tracking catalyst state under dynamic reaction conditions disclosed that Fe-ZSM-5 activity did not grow directly with temperature increase but rather depended on the oxidation state of Fe and surface concentration of ammonia.

Structure-Property-Function Relationships of Iron Oxide Multicore Nanoflowers in Magnetic Hyperthermia and Photothermia

Magnetite and maghemite multicore nanoflowers (NFs) synthesized using the modified polyol-mediated routes are to date among the most effective nanoheaters in magnetic hyperthermia (MHT). Recently, magnetite NFs have also shown high photothermal (PT) performances in the most desired second near-infrared (NIR-II) biological window, making them attractive in the field of nanoparticle-activated thermal therapies. However, what makes magnetic NFs efficient heating agents in both modalities still remains an open question. In this work, we investigate the role of many parameters of the polyol synthesis on the final NFs' size, shape, chemical composition, number of cores, and crystallinity. These nanofeatures are later correlated to the magnetic, optical, and electronic properties of the NFs as well as their collective macroscopic thermal properties in MHT and PT to find relationships between their structure, properties, and function. We evidence the critical role of iron(III) and heating ramps on the elaboration of well-defined NFs with a high number of multicores. While MHT efficiency is found to be proportional to the average number of magnetic cores within the assemblies, the optical responses of the NFs and their collective photothermal properties depend directly on the mean volume of the NFs (as supported by optical cross sections numerical simulations) and strongly on the structural disorder in the NFs, rather than the stoichiometry. The concentration of defects in the nanostructures, evaluated by photoluminescence and Urbach energy (E_U), evidence a switch in the optical behavior for a limit value of E_U = 0.4 eV where a discontinuous transition from high to poor PT efficiency is also observed.

Unveiling the Genesis and Effectiveness of Negative Fading in Nanostructured Iron Oxide Anode Materials for Lithium-Ion Batteries

Iron oxide anode materials for rechargeable lithium-ion batteries have garnered extensive attention because of their inexpensiveness, safety, and high theoretical capacity. Nanostructured iron oxide anodes often undergo negative fading, that is, unconventional capacity increase, which results in a capacity increasing upon cycling. However, the detailed mechanism of negative fading still remains unclear, and there is no consensus on the provenance. Herein, we comprehensively investigate the negative fading of iron oxide anodes with a highly ordered mesoporous structure by utilizing advanced synchrotron-based analysis. Electrochemical and structural analyses identified that the negative fading originates from an optimization of the electrolyte-derived surface layer, and the thus formed layer significantly contributes to the structural stability of the nanostructured electrode materials, as well as their cycle stability. This work provides an insight into understanding the origin of negative fading and its influence on nanostructured anode materials.

High Temperature Continuous Flow Syntheses of Iron Oxide Nanoflowers Using the Polyol Route in a Multi-Parametric Millifluidic Device

One of the most versatile routes for the elaboration of nanomaterials in materials science, including the synthesis of magnetic iron oxide nanoclusters, is the high-temperature polyol process. However, despite its versatility, this process still lacks reproducibility and scale-up, in addition to the low yield obtained in final materials. In this work, we demonstrate a home-made multiparametric continuous flow millifluidic system that can operate at high temperatures (up to 400 °C). After optimization, we validate its potential for the production of nanomaterials using the polyol route at 220 °C by elaborating ferrite iron oxide nanoclusters called nanoflowers (CoFe₂O₄, Fe₃O₄, MnFe₂O₄) with well-controlled nanostructure and composition, which are highly demanded due to their physical properties. Moreover, we demonstrate that by using such a continuous process, the chemical yield and reproducibility of the nanoflower synthesis are strongly improved as well as the possibility to produce these nanomaterials on a large scale with quantities up to 45 g per day.

Fe K-edge x-ray absorption spectroscopy of corrosion phases of archaeological iron: results, limitations, and the need for complementary techniques

Data analysis methods for iron x-ray absorption spectroscopy can provide extensive information about the oxidation state and co-ordination of an Fe-species. However, the extent to which techniques developed using a single-phase iron sample may be applied to complex, mixed-phase samples formed under real-world conditions is not clear. This work uses a combination of pre-edge fitting and linear combination analysis to characterise the near edge region of the x-ray absorption spectrum (XANES) for a set of archaeological iron corrosion samples from a collection of cast iron cannon shot excavated from theMary Roseshipwreck and compares the data with phase compositions determined by synchrotron x-ray powder diffraction (SXPD). Archaeological powder and cross-section samples were compared to a library of iron standards and diffraction data. The XANES are consistent with previous observations that generation of the chlorinated phase akaganeite, β-FeO(OH,Cl), occurs in those samples which have been removed form passive storage and subjected to active conservation. However, the results show that if any metallic species is present in the sample, the contribution from Fe(0) to the spectral region containing a pre-edge for oxidised iron-Fe(II) and Fe(III)-causes the analysis to be less effective and the conclusions unreliable. Consequently, while the pre-edge fitting methodology may be applied to a mixture of iron oxides or oxyhydroxides, the procedure is inappropriate for a mixed metal-oxide sample without the application of a complimentary technique, such as SXPD.

Insights into the CO2 Stability-Performance Trade-Off of Antimony-Doped SrFeO3-δ Perovskite Cathode for Solid Oxide Fuel Cells

One major challenge for the further development of solid oxide fuel cells is obtaining high-performance cathode materials with sufficient stability against reactions with CO₂ present in the ambient atmosphere. However, the enhanced stability is often achieved by using material systems exhibiting decreased performance metrics. The phenomena underlying the performance and stability trade-off has not been well understood. This paper uses antimony-doped SrFeO_3-δ as a model material to shed light on the relationship between the structure, stability, and performance of perovskite-structured oxides which are commonly used as cathode materials. X-ray absorption revealed that partial substitution of Fe by Sb leads to a series of changes in the local environment of the iron atom, such as a decrease in the iron oxidation state and increase in the oxygen coordination number. Theoretical calculations show that the structural changes are associated with an increase in both the oxygen vacancy formation energy and metal-oxygen bond energy. The area-specific resistance (ASR) of the perovskite oxide increases with Sb doping, indicating a deterioration of the oxygen reduction activity. Exposure of the materials to CO₂ leads to depressed oxygen desorption and an increased ASR, which becomes less pronounced at higher Sb doping levels. Origin of the stability-performance trade-off is discussed based on the structural parameters.

Temperature-Induced Formation of Lubricous Oxides in Vanadium Containing Iron-Based Arc Sprayed Coatings

In the field of surface engineering, the use of self-lubricous coatings with the incorporation of vanadium represent a promising approach to reduce friction, thus contributing to the wear behavior. For vanadium containing hard coatings produced by means of thin film technology, the reduction in friction at elevated temperatures was repeatedly attributed to temperature-induced and tribo-oxidatively formed oxides which act as solid lubricant. Only very few studies focused on the tribological characteristics of vanadium containing arc sprayed coatings. In this study, the tribological characteristics of a vanadium containing iron-based arc sprayed deposit were investigated in dry sliding experiments under ambient conditions and different temperatures. Types of wear at the worn surfaces and counterparts were examined by means of electron microscopy and energy dispersive X-ray (EDX) spectroscopy. The speciation of vanadium in the superficial layer was determined using X-ray absorption near edge structure (XANES) spectroscopy. It was found that the vanadium-containing coating exhibited a distinctly reduction of the coefficient of friction above 450 °C which further decreased with increasing temperature. XANES spectroscopy indicated an increased oxidation state for the V component on the coating surface, suggesting the prevalence of specific vanadium oxides which promote a self-lubricating ability of the coating.

Iron oxidation dynamics vs. temperature of synthetic potassic-ferro-richterite: a XANES investigation

We investigated the oxidation behaviour of a synthetic potassic-ferro-richterite up to 750 °C by using simultaneous X-ray absorption spectroscopy and X-ray diffraction experiments with synchrotron radiation. From the X-ray diffraction results, we observed an abrupt decrease of cell dimensions at ∼335 °C accompanied by an anomalous increase in the monoclinic cell angle β. From the analysis of the XANES spectra at the iron K-edge, we observed that the structural shrinkage is due to the iron oxidation process, coupled to hydrogen loss, occurring at ∼315 °C, slightly before the cell contraction. Combining these results with previous studies performed on similar samples by single-crystal structure refinement, Mössbauer, high temperature-Fourier transform IR and Raman spectroscopies, we show that the temperature evolution in Fe-amphiboles is a multi-step process. Following the iron oxidation driven by temperature, the structural dynamics in this double-chain silicate is ruled by local strains in the ribbon of iron-filled octahedra, mainly due to the contraction of the M(1) site.

Iron chelators target both proliferating and quiescent cancer cells

Poorly vascularized areas of solid tumors contain quiescent cell populations that are resistant to cell cycle-active cancer drugs. The compound VLX600 was recently identified to target quiescent tumor cells and to inhibit mitochondrial respiration. We here performed gene expression analysis in order to characterize the cellular response to VLX600. The compound-specific signature of VLX600 revealed a striking similarity to signatures generated by compounds known to chelate iron. Validation experiments including addition of ferrous and ferric iron in excess, EXAFS measurements, and structure activity relationship analyses showed that VLX600 chelates iron and supported the hypothesis that the biological effects of this compound is due to iron chelation. Compounds that chelate iron possess anti-cancer activity, an effect largely attributed to inhibition of ribonucleotide reductase in proliferating cells. Here we show that iron chelators decrease mitochondrial energy production, an effect poorly tolerated by metabolically stressed tumor cells. These pleiotropic features make iron chelators an attractive option for the treatment of solid tumors containing heterogeneous populations of proliferating and quiescent cells.

Formation of environmentally persistent free radical (EPFR) in iron(III) cation-exchanged smectite clay

Environmentally persistent free radicals (EPFRs) have been found at a number of Superfund sites, with EPFRs being formed via a proposed redox process at ambient environmental conditions. The possibility of such a redox process taking place at ambient environmental conditions is studied utilizing a surrogate soil system of phenol and iron(III)-exchanged calcium montmorillonite clay, Fe(III)CaM. Sorption of phenol by the Fe(III)CaM is demonstrated by Fourier-transformed infra-red (FT-IR) spectroscopy, as evidenced by the peaks between 1345 cm(-1) and 1595 cm(-1), and at lower frequencies between 694 cm(-1) and 806 cm(-1), as well as X-ray diffraction (XRD) spectroscopy, as shown by an increase in interlayer spacing within Fe(III)CaM. The formation and characterization of the EPFRs is determined by electron paramagnetic resonance (EPR) spectroscopy, showing phenoxyl-type radical with a g-factor of 2.0034 and ΔHP-P of 6.1 G at an average concentration of 7.5 × 10(17) spins per g. EPFRs lifetime data are indicative of oxygen and water molecules being responsible for EPFR decay. The change in the oxidation state of the iron redox center is studied by X-ray absorption near-edge structure (XANES) spectroscopy, showing that 23% of the Fe(III) is reduced to Fe(II). X-ray photoemission spectroscopy (XPS) results confirm the XANES results. These findings, when combined with the EPFR concentration data, demonstrate that the stoichiometry of the EPFR formation under the conditions of this study is 1.5 × 10(-2) spins per Fe(II) atom.

Effect of post-synthesis hydrogen-treatment on the nature of iron species in Fe-BEA as NH3-SCR catalyst

The relative amount of monomeric iron species increases after post-synthesis treatment with hydrogen, which improves the low-temperature activity during NH₃-SCR.

Microstructure determination of IQ-WB clays: a direct procedure by small-angle X-ray scattering

An accurate characterization of the microstructure of raw and pure homoionic clays from Wadi Bashira in Iraq (IQ-WB) has been carried out experimentally, using small-angle X-ray scattering (SAXS) and dynamic light scattering. The structures of lamellar IQ-WB dispersions were identified by these techniques and by complementary free swelling and dialysis experiments. SAXS measurements were used to resolve the characteristic distance,hs, between individual platelets in Na IQ-WB and between platelets inside tactoids formed in Ca clays. The tactoids in raw and Ca IQ-WB have well formed lamellar structures wherehs= 1.9 nm. The average lateral size, 〈D〉, of a platelet is determined to be a few hundred nanometres. Analysis of the SAXS peaks, based on the Scherrer relation, revealed a small difference in the average number of platelets per tactoid, 〈N〉 = 6–9, depending on platelet size and the concentrations of divalent counter-ions. It was also found that the average tactoid size can be estimated from an empirical relation as follows: 〈N〉 ≃ δ + α〈D〉, where δ is a constant and α the slope.

Acidification and buffering mechanisms in acid sulfate soil wetlands of the Murray-Darling Basin, Australia

The acid generation mechanisms and neutralizing capacities of sulfidic sediments from two inland wetlands have been studied in order to understand the response of these types of systems to drying events. The two systems show vastly different responses to oxidation, with one (Bottle Bend (BB) lagoon) having virtually no acid neutralizing capacity (ANC) and the other (Psyche Bend (PB) lagoon) an ANC that is an order of magnitude greater than the acid generation potential. While BB strongly acidifies during oxidation the free acid generation is less than that expected from the measured proton production and consumption processes, with additional proton consumption attributed to the formation of an acid-anion (chloride) FeIII (oxyhydr)oxide product, similar to akaganéite (Fe(OH)2.7Cl0.3). While such products can partially attenuate the acidification of these systems, resilience to acidification is primarily imparted by sediment ANC.

New Methodological Approach for the Vanadium K-Edge X-ray Absorption Near-Edge Structure Interpretation: Application to the Speciation of Vanadium in Oxide Phases from Steel Slag

This paper presents a comparison between several methods dedicated to the interpretation of V K-edge X-ray absorption near-edge structure (XANES) features. V K-edge XANES spectra of several V-bearing standard compounds were measured in an effort to evaluate advantages and limits of each method. The standard compounds include natural minerals and synthetic compounds containing vanadium at various oxidation state (from +3 to +5) and in different symmetry (octahedral, tetrahedral, and square pyramidal). Correlations between normalized pre-edge peak area and its centroid position have been identified as the most reliable method for determining quantitative and accurate redox and symmetry information for vanadium. This methodology has been previously developed for the Fe K edge. It is also well adapted for the V K edge and is less influenced by the standard choice than other methods. This methodology was applied on an "environmental sample," i.e., a well-crystallized leached steel slag containing vanadium as traces. Micro-XANES measurements allowed elucidating the microdistribution of vanadium speciation in leached steel slag. The vanadium exhibits an important evolution from the unaltered to the altered phases. Its oxidation state increases from +3 to +5 together with the decrease of its symmetry (from octahedral to tetrahedral).

Integration of EXAFS, Spectroscopic, and DFT Techniques for Elucidation of the Structure of Reactive Diiron Compounds

Strategies for modelling the EXAFS of a range of compounds with structural features common to the diiron subsite of the [FeFe] hydrogenase H-centre are compared, and this has allowed identification of highly constrained models that still permit expression of the main structural characteristics of the compounds. Despite giving self-consistent values of the iron–scatterer distances, the EXAFS analysis fails to give unambiguous identification of the stereochemistry and composition of the compounds, and this necessitates the integration of results obtained using other spectroscopic and computational approaches. The combination of infrared spectroscopy, EXAFS, and ab initio DFT calculations are shown to provide a particularly potent approach for the study of metal carbonyl compounds of this class. In this case the EXAFS-derived iron–scatterer distances provide the basis of the starting point for DFT geometry optimization calculations, and the final distances together with the calculated infrared spectrum provides a means of validating the computed geometry. The approach is applied both to compounds of known structure and to the examination of the unstable products of chemical or electrochemical reduction.

XAS study of iron and arsenic speciation during Fe(II) oxidation in the presence of As(III)

The speciation of As and Fe was studied during the oxidation of Fe(II)-As(III) solutions by combining XAS analysis at both the Fe and As K-edges. Fe(II) and As(III) were first hydrolyzed to pH 7 under anoxic conditions; the precipitate was then allowed to oxidize in ambient air for 33 h under vigorous stirring. EXAFS analysis at the As K-edge shows clear evidence of formation of inner-sphere complexes between As(III) and Fe(II), i.e., before any oxidation. Inner-sphere complexes were also observed when Fe became sufficiently oxidized, in the form of edge-sharing and double-corner linkages between AsIIIO3 pyramids and FeIIIO6 octahedra. XAS analyses at the Fe K-edge reveal that the presence of As(III) in the solution limits the polymerization of Fe(II) and the formation of green rust and inhibits the formation of goethite and lepidocrocite. Indeed, As(III) accelerates the Fe(II) oxidation kinetics and leads to the formation of nanosized Fe-As subunits of amorphous aggregates. These observations, rather than a presumed weaker affinity of As(III) for iron oxyhydroxides, might explain why As(III) is more difficult to remove than As(V) by aerating reducing groundwater.

Behavior of Extraframework Fe Sites in MFI and MCM-22 Zeolites upon Interaction with N2O and NO

We report on the characterization of an isomorphously substituted Fe-MCM-22 sample containing both Fe and Al in framework positions (Si/Fe = 44, Si/Al = 25). XANES spectroscopy was used to study the evolution of Fe sites as a consequence of thermal activation at high temperature (1073 K) and subsequent oxidation with N2O. The results were compared to those obtained in the same conditions on a well-known Fe-silicalite sample (Si/Fe = 68, Si/Al = infinity). In both samples, thermal activation causes migration of a fraction of Fe ions from framework to extraframework positions, this migration being accompanied by a reduction of Fe3+ to Fe2+. Upon oxidation with N2O at 523 K, the two samples show a different behavior. While in Fe-silicalite practically all of the Fe2+ sites formed by thermal activation are reoxidized to Fe3+, in Fe-MCM-22 only a fraction of the extraframework iron sites is involved in the reoxidation process. The accessibility of the extraframework Fe sites was also investigated by using the NO molecule as a surface probe. Upon NO dosage on the sample, the modification of the pre-edge peak and of the edge position suggests an important charge release from the extraframework Fe2+ ions to the adsorbed molecules. This could be formalized with the formation of Fe3+(NO-) complexes, compatible (on the basis of the simple molecular orbital theory) with a bent NO geometry. The formation of a complex family of Fe2+ mono-, di-, and trinitrosyl complexes was also confirmed by FTIR spectroscopy. Similarly to what was observed in the oxidation experiments, the fraction of extraframework Fe sites able to interact with NO in Fe-MCM-22 sample is smaller than that in Fe-silicalite treated in the same conditions. This trend is explained with a major clustering of extraframework Fe sites in Fe-MCM-22 sample, as was also suggested by FTIR experiments. These results suggest that the dispersion of iron in zeolitic matrixes prepared by isomorphous substitution could also depend on the zeolitic structure.

Raman spectroscopy of the mineral rhodonite

The mineral rhodonite an orthosilicate has been characterised by Raman spectroscopy. The Raman spectra of three rhodonites from Broken Hill, Pachapaqui and Franklin were compared and found to be similar. The spectra are characterised by an intense band at around 1000 cm(-1) assigned to the nu(1) symmetric stretching mode and three bands at 989, 974 and 936 cm(-1) assigned to the nu(3) antisymmetric stretching modes of the SiO(4) units. An intense band at around 667 cm(-1) was assigned to the nu(4) bending mode and showed additional bands exhibiting loss of degeneracy of the SiO(4) units. The low wave number region of rhodonite is complex. A strong band at 421.9 cm(-1) is attributed to the nu(2) bending mode. The spectra of the three rhodonite mineral samples are similar but subtle differences are observed. It is proposed that these differences depend upon the cationic substitution of Mn by Ca and/or Fe(2+) and Mg.

A high-pressure iron K-edge x-ray absorption spectral study of the spin-state crossover in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) and (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2)

The room temperature iron K-edge X-ray absorption near edge structure spectra of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) and (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) have been measured between ambient and 88 and 94 kbar, respectively, in an opposed diamond anvil cell. The iron(II) in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2)undergoes the expected gradual spin-state crossover from the high-spin state to the low-spin state with increasing pressure. In contrast, the iron(II) in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) remains high-spin between ambient and 78 kbar and is only transformed to the low-spin state at an applied pressure of between 78 and 94 kbar. No visible change is observed in the preedge peak in the spectra of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) with increasing pressure, whereas the preedge peak in the spectra of ((e[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) changes as expected for a high-spin to low-spin crossover with increasing pressure. The difference in the spin-state crossover behavior of these two complexes is likely related to the unusual behavior of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) upon cooling.