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

Carbon encapsulation of magnetite nanoparticles enhances magnetism at room-temperature due to spin-polarized charge transfer

This study fabricated amorphous-carbon-encapsulated magnetite (Fe3O4@a-C) nanoparticles (NPs) and observed that the encapsulation significantly enhanced the room-temperature magnetization of the NPs by approximately 22.5%. Generalized gradient approximation with the Hubbard-U correction involved the ionic picture hypothesis of electron acceptors for Fe3O4, namely, e2↓ t23↓ → e2↓ t23↓ e↑, which link the spin-polarized charge transfer to Fe ions at the tetrahedral site FeA of Fe3O4. Fe K-edge X-ray absorption near-edge structure (XANES) spectra suggested a formal charge reduction in the FeA of the Fe3O4@a-C NPs. Moreover, Fe L3-edge x-ray magnetic circular dichroism yielded the same conclusion as that obtained using the XANES spectra and suggested the restoration of an orbital moment in the Fe3O4@a-C NPs. Synchrotron radiation spectroscopy confirmed the charge reduction of FeA, which accounted for the magnetic enhancement.

A combined Fourier transform infrared and Cr K-edge X-ray absorption near-edge structure spectroscopy study of the substitution and diffusion of H in Cr-doped forsterite

Abstract. Single crystals of synthetic Cr-doped forsterite (Cr:Mg2SiO4) containing both Cr3+ and Cr4+ were partially hydroxylated in piston-cylinder apparatuses at 750–1300 ∘C and pressures from 0.5 to 2.5 GPa, with p(H2O) ≈Ptotal. The oxygen fugacity (fO2) was buffered by graphite-water, Ni–NiO, Re–ReO2, Fe2O3–Fe3O4 or Ag–Ag2O, and the silica activity (aSiO2) was buffered by powdered forsterite plus either enstatite (Mg2Si2O6), periclase (MgO) or zircon–baddeleyite (ZrSiO4–ZrO2). Profiles of OH content versus distance from the crystal edge were determined using Fourier transform infrared (FTIR) spectroscopy, and profiles of the oxidation state and coordination geometry of Cr were obtained, at the same positions, using K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The techniques are complementary – FTIR spectroscopy images the concentration and nature of O–H bonds, where Cr K-edge XANES spectroscopy shows the effect of the added H on the speciation of Cr already present in the lattice. Profiles of defect-specific absorbance derived from FTIR spectra were fitted to solutions of Fick's second law to derive diffusion coefficients, which yield the Arrhenius relationship for H diffusion in forsterite: log⁡10D̃[001]=-2.5±0.6+-(224±12+4.0±2.0P)2.303RT, where D̃ is the measured diffusion coefficient in m2 s−1, valid for diffusion parallel to [001] and calibrated between 1000 and 750 ∘C, P and T are in GPa and K, and R is 0.008314 kJK−1 mol−1. Diffusivity parallel to [100] is around 1 order of magnitude lower. This is consistent with previous determinations of H diffusion associated with M-site vacancies. The FTIR spectra represent a variety of Cr-bearing hydrous defects, along with defects associated with the pure Mg–Si–O–H system. It is proposed that all of the defects can form by interaction between the dry lattice, including Cr3+ and Cr4+, and fully hydroxylated M-site vacancies. The initial diffusive wave of hydroxylation is associated with neither reduction nor oxidation of Cr but with Cr4+ changing from tetrahedral to octahedral coordination. Superimposed on the H diffusion and concomitant change in Cr4+ site occupancy, but at a slower rate, producing shorter profiles, is reduction of Cr4+ to Cr3+ and potentially of Cr4+ and Cr3+ to Cr2+. In addition, by comparing FTIR data to trace element contents measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), constraints can be placed on absorption coefficients used for converting absorbance to H2O contents – our data support either wavenumber- or defect-dependent values of absorption coefficients. We estimate absorption coefficients of between 60 200 and 68 200 L mol−1 cm−1 for OH− associated with octahedral Cr3+ and an M-site vacancy and 18 700 to 24 900 L mol−1 cm−1 for two OH− associated with octahedrally coordinated Cr4+ and a Si vacancy (i.e. a “clinohumite-type” point defect).

Disorder-Induced Broadband Near-Infrared Persistent and Photostimulated Luminescence in Mg2SnO4:Cr3.

Materials with near-infrared (NIR) persistent luminescence (PersL) and NIR-to-NIR photostimulated luminescence (PSL) properties are attractive platforms for photonic energy harvesting and release. In this work, we develop Mg2SnO4:Cr as a broadband NIR PersL and NIR-to-NIR PSL material (luminescence maxima at ∼800 nm) and reveal the origin of the PersL and PSL properties. The material has an inverse spinel structure with the Mg2+ and Sn4+ disorder at the Wyckoff 16d site based on the Rietveld refinement. Cr K-edge X-ray absorption near-edge structure (XANES) spectra uncover that the doped Cr ions have a +3 valence state and occupy the disordered (Mg,Sn) site with octahedral coordination. The disorder results in multiple Cr3+ centers, and the broadband luminescence originates from the 4T2(4F) → 4A2 transition of Cr3+ at sites with intermediate crystal field strength. The distribution of trap depths is continuous according to the analysis of thermoluminescence (TL) spectra using the initial rising method, which relates to the random distribution of Mg2+ and Sn4+ at the second coordination sphere of the Cr3+ centers rather than the oxygen-related defects. Stimulating the material with a NIR laser, the NIR PersL gets significantly enhanced due to a PSL process. The broadband PersL and PSL are detectable beyond 100 h and have good tissue penetrability and therefore the developed Mg2SnO4:Cr3+ has potential in applications of optical information storage/reading and autofluorescence-free bioimaging. Finally, three crystal and electronic structure factors are proposed for screening new Cr3+-activated PersL and PSL materials.

Revealing differences in the chemical form of zinc in brain tissue using K-edge X-ray absorption near-edge structure spectroscopy.

Zinc is a prominent trace metal required for normal memory function. Memory loss and cognitive decline during natural ageing and neurodegenerative disease have been associated with altered brain-Zn homeostasis. Yet, the exact chemical pathways through which Zn influences memory function during health, natural ageing, or neurodegenerative disease remain unknown. The gap in the literature may in part be due to the difficulty to simultaneously image, and therefore, study the different chemical forms of Zn within the brain (or biological samples in general). To this extent, we have begun developing and optimising protocols that incorporate X-ray absorption near-edge structure (XANES) spectroscopic analysis of tissue at the Zn K-edge as an analytical tool to study Zn speciation in the brain. XANES is ideally suited for this task as all chemical forms of Zn are detected, the technique requires minimal sample preparation that may otherwise redistribute or alter the chemical form of Zn, and the Zn K-edge has known sensitivity to coordination geometry and ligand type. Herein, we report our initial results where we fit K-edge spectra collected from micro-dissected flash-frozen brain tissue, to a spectral library prepared from standard solutions, to demonstrate differences in the chemical form of Zn that exist between two brain regions, the hippocampus and cerebellum. Lastly, we have used an X-ray microprobe to demonstrate differences in Zn speciation within sub-regions of thin air-dried sections of the murine hippocampus; but, the corresponding results highlight that the chemical form of Zn is easily perturbed by sample preparation such as tissue sectioning or air-drying, which must be a critical consideration for future work.

Coordination Number Regulation of Molybdenum Single-Atom Nanozyme Peroxidase-like Specificity

Summary Nanozymes are promising alternatives to natural enzymes, but their use remains limited owing to poor specificity. Overcoming this and controlling the targeted enzyme-like performance of traditional nanozymes is extremely challenging due to the intrinsic structural complexity of these systems. We report theoretical design and experimental realization of a series of heterogeneous molybdenum single-atom nanozymes (named MoSA–Nx–C), wherein we find that the peroxidase-like specificity is well regulated by the coordination numbers of single Mo sites. The resulting MoSA–N3–C catalyst shows exclusive peroxidase-like behavior. It achieves this behavior via a homolytic pathway, whereas MoSA–N2–C and MoSA–N4–C catalysts have a different heterolytic pathway. The mechanism of this coordination-number-dependent enzymatic specificity is attributed to geometrical structure differences and orientation relationships of the frontier molecular orbitals toward these MoSA–Nx–C peroxidase mimics. This study demonstrates the rational design of peroxidase-specific nanozymes and precise regulation of their enzymatic properties.

Composting with biochar or woody peat addition reduces phosphorus bioavailability

Biochar and woody peat have been recognized as an additive to reduce carbon and nitrogen loss during composting. Yet little is known about their influences on the transformation of phosphorus (P) fractions in composting. This study investigated the quantitative and qualitative changes in different P forms during composting with adding biochar or woody peat using sequential extraction and P K-edge X-ray absorption near-edge structure (XANES). The results showed that compost products from the treatment with adding woody peat had a higher HA/FA (the ratio of humic acid to fulvic acid) compared to biochar treatment and the control, suggesting that the addition of woody peat might benefit the humification process of composting. Sequential extraction and XANES illustrated that adding biochar or woody peat limited the P availability. Biochar increased the proportion of Pi and woody peat decreased the conversion from Po to Pi compared to the control. Structural equation modeling and redundancy analysis suggested that biochar improved the refractory P based on the indirect effects of NH4+-N by regulating microbial community, while woody peat was beneficial for Po accumulation by affecting humic acid. Taken together, this research provides basis for regulating the nutrient level of carbon, nitrogen, and phosphorus in composts and reducing environmental risks.

Damages Induced by Synchrotron Radiation-Based X-ray Microanalysis in Chrome Yellow Paints and Related Cr-Compounds: Assessment, Quantification, and Mitigation Strategies.

Synchrotron radiation (SR)-based X-ray methods are powerful analytical tools for several purposes. They are widely used to probe the degradation mechanisms of inorganic artists' pigments in paintings, including chrome yellows (PbCr1-xSxO4; 0 ≤ x ≤ 0.8), a class of compounds often found in Van Gogh masterpieces. However, the high intensity and brightness of SR beams raise important issues regarding the potential damage inflicted on the analyzed samples. A thorough knowledge of the SR X-ray sensitivity of each class of pigment in the painting matrix is therefore required to find analytical strategies that seek to minimize the damage for preserving the integrity of the analyzed samples and to avoid data misinterpretation. Here, we employ a combination of Cr K-edge X-ray absorption near-edge structure spectroscopy, Cr-Kβ X-ray emission spectroscopy, and X-ray diffraction to monitor and quantify the effects of SR X-rays on the stability of chrome yellows and related Cr compounds and to define mitigation strategies. We found that the SR X-ray beam exposure induces changes in the oxidation state and local coordination environment of Cr ions and leads to a loss of the compound's crystalline structure. The extent of X-ray damage depends on some intrinsic properties of the samples (chemical composition of the pigment and the presence/absence and nature of the binder). It can be minimized by optimizing the overall fluence/dose released to the samples and by working in vacuum and under cryogenic conditions.

Spin cascade and doming in ferric hemes: Femtosecond X-ray absorption and X-ray emission studies

The structure-function relationship is at the heart of biology, and major protein deformations are correlated to specific functions. For ferrous heme proteins, doming is associated with the respiratory function in hemoglobin and myoglobins. Cytochrome c (Cyt c) has evolved to become an important electron-transfer protein in humans. In its ferrous form, it undergoes ligand release and doming upon photoexcitation, but its ferric form does not release the distal ligand, while the return to the ground state has been attributed to thermal relaxation. Here, by combining femtosecond Fe Kα and Kβ X-ray emission spectroscopy (XES) with Fe K-edge X-ray absorption near-edge structure (XANES), we demonstrate that the photocycle of ferric Cyt c is entirely due to a cascade among excited spin states of the iron ion, causing the ferric heme to undergo doming, which we identify. We also argue that this pattern is common to a wide diversity of ferric heme proteins, raising the question of the biological relevance of doming in such proteins.

Mechanistic insights into defect generation and tuning of optical properties in Zn1-xFexAl2O4(0.01 ≤ x ≤ 0.40) nanocrystals.

The correlation of several defects and optical and magnetic properties with Fe content in Zn1-xFexAl2O4 (0.01 ≤ x ≤ 0.40) nanocrystals has been scrutinized through X-ray diffraction, O K-edge X-ray absorption near-edge structure, FT-IR, diffuse reflectance, photoluminescence and electron spin-resonance spectroscopies, and vibrating sample magnetometry. Increasing Fe content causes elongation in the octahedral units of the lattice, accompanied by distortion in the octahedral coordination. Fe introduces non-radiative centres in the forbidden gap, thereby tuning the band gap from 4.37 to 3.88 eV and eliminating emission in the visible region. Zn vacancies are found to tail off, while {\rm Fe}_i^{\bullet \bullet \bullet}, {\rm Al}_{\rm Zn}^\bullet and FeAl× antisite defects increase in concentration with increasing Fe content. Inhomogeneous broadening of spin-resonance signals infers strong spin-lattice interactions of Fe3+ ions at distorted octahedral and non-symmetric tetrahedral sites. A transition is observed from paramagnetism to superparamagnetism at higher Fe concentrations. A visual colour change from pearly white to orange-brown is observed in Zn1-xFexAl2O4 nanocrystals with increasing Fe content, revealing its potential candidature for pigments in the paint and dye industries.

A contrast in the electronic structures of B ion implanted ZnO thin films grown on glass and silicon substrates by using x-ray absorption spectroscopy

Fabrication of suitable c-axis oriented boron doped zinc oxide (BZO) thin films is vital for many applications. The properties of films critically depend on film preparation methods and the type of substrate. Furthermore, ion implantation at a specific position or depth of a sample substantially affects the properties of films. In this study, a contrast between the electronic structure of ZnO thin films grown on glass and silicon (Si) substrates and then implanted with B ions at 40 keV with a concentration of 5 × 1013 ions/cm2 was investigated. The B ions were implanted using a 3 MV tandem accelerator. Secondary ion mass spectrometry, atomic force microscopy, and x-ray diffraction were used to study the distribution and depth of B ions, surface morphology, and the crystallinity of ZnO and BZO thin films. The preferential orientation, texture, surface roughness, and grain size of the ZnO thin films were affected by B ion implantation. O K-, Zn L3-, and K-edge x-ray absorption near-edge structure (XANES), and Zn K-edge extended x-ray absorption fine structure techniques were used to investigate the electronic structure of the BZO thin films and to determine the substrate influence. O K- and Zn L3-edge XANES spectra of the BZO/glass thin films revealed an enhanced electron transfer from Zn to O as compared to the BZO/Si thin film, implying that B implantation increased the negative effective charge on the O ions. Interestingly, in contrast to the definition of electronegativity, an increase (decrease) in the number of O 2p (Zn 3d/4sp) unoccupied states due to an enhanced O 2p–Zn 3d/4sp hybridization is observed in the BZO/Si thin film as compared to the BZO/glass thin film, suggesting an increase in the number of O 2p-dangling bonds. This would affect the conductivity and luminescent behavior of the ZnO/Si thin films after B doping, which will be useful in optoelectronic applications.

Phosphate-Suppressed Selenite Biotransformation by Escherichia coli.

Biotransformation of selenite to valuable elemental selenium nanoparticles (Se0) is a promising avenue to remediate seleniferous environments and simultaneously recover selenium (Se). However, the underlying oxyanion competition and selenite transformation mechanism in prokaryotes are poorly understood. In this work, the impacts of phosphate on selenite uptake and transformation were elucidated with Escherichia coli and its mutant deficient in phosphate transport as model microbial strains. Selenite uptake was inhibited by phosphate in E. coli. Moreover, the transformation of internalized Se was shifted from Se0 to toxic organo-Se with elevated phosphate levels, as evidenced by the linear combination fit analysis of the Se K-edge X-ray absorption near-edge structure. Such a phosphate-regulated selenite biotransformation process was mainly assigned to the competitive uptake of phosphate and selenite, which was primarily mediated by a low affinity phosphate transporter (PitA). Under phosphate-deficient conditions, the cells not only produced abundant Se0 nanoparticles but also maintained good cell viability. These findings provide new insights into the phosphate-regulated selenite biotransformation by prokaryotes and contribute to the development of new processes for bioremediating Se-contaminated environments, as well as bioassembly of Se0.

Charge–discharge mechanism and capacity degradation of Co-substituted Li5AlO4 during cycling

High-performance advanced and futuristic products require batteries with higher energy densities than those of conventional batteries. Anion-redox active materials have high specific capacities, but they exhibit low life-cycle performance. In this study, the charge–discharge mechanism and the primary factors contributing to the capacity degradation of Co-substituted Li5AlO4 (CSLA) during charge–discharge cycling were investigated. X-ray absorption spectroscopy (XAS) was performed to determine the change in the electronic state of CSLA during the charge–discharge process. The changes in the XAS peak intensities were attributed to the formation and consumption of peroxide and superoxide, as observed in the O K-edge X-ray absorption near-edge structure (XANES) spectra of CSLA obtained over the first ten charge–discharge cycles. The results indicated that the redox reaction involving the formation and consumption of peroxide and superoxide allows for the charge compensation of CSLA. Furthermore, the intensity of the O K-edge XANES peak attributed to the formation and consumption of peroxide decreased with the increasing number of charge–discharge cycles of CSLA, which correlated with the capacity degradation of CSLA. This study provides new insights into the charge–discharge and capacity degradation mechanisms of anion-redox cathode materials.

Evaluation of Salt-Induced Damage to Aged Wood of Historical Wooden Buildings.

Salt is a common cause of damage to building materials used in cultural and historical buildings. The damage to aged wood in historical wooden buildings has not been extensively studied, resulting in the need for a more detailed analysis. In this work, Yingxian Wooden Pagoda, a typical historical wooden structure, was taken as the research object. Multichemical analyses were conducted to evaluate and understand the salt-induced damage to the aged wood using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer, sulphur K-edge X-ray absorption near-edge structure spectroscopy, X-ray fluorescence spectroscopy, X-ray powder diffraction, and attenuated total reflectance fourier transformed infrared spectroscopy. The results showed the presence of invasive salt crystallisations and ions in the aged samples. The source of these invasive elements was deduced by identifying the type, amount, and valency of the elements; they were found to be derived from environmental factors such as acid rain and atmospheric pollutant. The unique damage mechanism and route induced by salt in historical buildings made of wood were summarised; the damage was attributed to the accumulation of sulphate salt causing hydrolysis of the carbohydrates and salt crystallisation resulting in mechanical damage. This interdisciplinary study is significant for decision making in studies related to the preservation and evaluation of historical wooden buildings.

Correlation among photoluminescence and the electronic and atomic structures of Sr2SiO4:xEu3+ phosphors: X-ray absorption and emission studies

A series of Eu3+-activated strontium silicate phosphors, Sr2SiO4:xEu3+ (SSO:xEu3+, x = 1.0, 2.0 and 5.0%), were synthesized by a sol–gel method, and their crystalline structures, photoluminescence (PL) behaviors, electronic/atomic structures and bandgap properties were studied. The correlation among these characteristics was further established. X-ray powder diffraction analysis revealed the formation of mixed orthorhombic α'-SSO and monoclinic β-SSO phases of the SSO:xEu3+ phosphors. When SSO:xEu3+ phosphors are excited under ultraviolet (UV) light (λ = 250 nm, ~ 4.96 eV), they emit yellow (~ 590 nm), orange (~ 613 nm) and red (~ 652 and 703 nm) PL bands. These PL emissions typically correspond to 4f–4f electronic transitions that involve the multiple excited 5D0 → 7FJ levels (J = 1, 2, 3 and 4) of Eu3+ activators in the host matrix. This mechanism of PL in the SSO:xEu3+ phosphors is strongly related to the local electronic/atomic structures of the Eu3+–O2− associations and the bandgap of the host lattice, as verified by Sr K-edge and Eu L3-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure, O K-edge XANES and Kα X-ray emission spectroscopy. In the synthesis of SSO:xEu3+ phosphors, interstitial Eu2O3-like structures are observed in the host matrix that act as donors, providing electrons that are nonradiatively transferred from the Eu 5d and/or O 2p–Eu 4f/5d states (mostly the O 2p–Eu 5d states) to the 5D0 levels, facilitating the recombination of electrons that have transitioned from the 5D0 level to the 7FJ level in the bandgap. This mechanism is primarily responsible for the enhancement of PL emissions in the SSO:xEu3+ phosphors. This PL-related behavior indicates that SSO:xEu3+ phosphors are good light-conversion phosphor candidates for use in near-UV chips and can be very effective in UV-based light-emitting diodes.

Chlorine K-Edge X-ray Absorption Near-Edge Structure Discrimination of Crystalline Solvates and Salts in Organic Molecules

The crystal forms of active pharmaceutical ingredients must be precisely determined to properly regulate their physical properties. In this study, chlorine K-edge X-ray absorption near-edge structu...

On the coordination of Mg2+ in aragonite: Ab-initio absorption spectroscopy and isotope fractionation study

Magnesium in biogenic aragonite skeletons is widely used to infer the geochemical conditions under which the minerals precipitated. However, because molecular speciation of Mg2+ in the aragonite crystal structure remains uncertain, questions are often posed about the reliability and utility of Mg as a paleoenvironment proxy. In this study, we investigated the Mg2+ coordination in abiogenic aragonite by applying first-principles density functional theory (DFT). We report for the first time a Mg2+-site disordered structure model of aragonite: substituted Mg2+ is five-fold coordinated and induces substantial disorder in the bond distances that depends on the relative positions of Mg2+ sites. Using the site disorder model, DFT qualitatively reproduces the key features of experimental Mg K-edge X-ray absorption near-edge structure (XANES) spectra of aragonite. DFT also resolves a long-standing conflict between experimental and theoretical studies pertaining to the enrichment of 26Mg in aragonite relative to calcite. The Boltzmann-averaged 26Mg/24Mg reduced partition function ratios were calculated in the order of aqueous Mg2+ > Mg-doped aragonite > Mg-doped calcite. The disorder structure model is likely applicable to the substitution of cations with radii smaller than the radius of Ca2+ ions in aragonite. Our study provides atomistic insight into metal substitution effects on the metal content and isotope fractionation in biogenic aragonite, which are critical for developing a reliable paleoenvironment proxy.

Chemical state of Fe3+ in a Fe3+-type cation exchange resin for the removal and recovery of phosphate ions and the adsorption mechanism of phosphate ion to the resin

To apply the Fe3+-type cation exchange resin, Fe3+-type UBK 10, which is an effective adsorbent of selenite ions, for the removal and recovery of phosphate ions (PO43−), the adsorption mechanism of PO43− to the Fe3+-type UBK 10 resin was investigated. To elucidate the aforementioned mechanism, the chemical state of Fe3+ in the Fe3+-type UBK 10 resin and the adsorption mode of PO43− into the adsorbent were examined via 57Fe Mössbauer spectroscopy and Fe and P K-edge X-ray absorption spectroscopy. The optimum pH for the adsorption of PO43− was found to be approximately 6 and the adsorbed PO43− was rapidly desorbed using 6 mol dm-3 hydrochloric acid (HCl) through the conversion of Fe3+ to FeCl4-, which is practical for the removal and recovery of PO43−. The Fe K-edge X-ray absorption near-edge structure (XANES) spectra showed, through the appearance of three isobestic points, that Fe3+ in the Fe3+-type UBK 10 resin exists as the following four species in equilibrium, depending on the pH: two monomeric hydrolytic species of ferric ions, (-S)2FeOH and (-S)Fe(OH)2, a μ-oxo dimer with corner sharing structure, (-S)2Fe-O-Fe(S-)2, and a dihydroxo dimer with edge sharing structure, (-S)2Fe-(OH)2-Fe(S-)2 (S: SO3 group in UBK 10). It should be emphasized that a PO43− can combine with a (-S)2Fe-(OH)2-Fe(S-)2 site at pH of 6, which is an optimum pH for adsorption of PO43−, forming a bidentate-binuclear configuration, (-S)2Fe-(OH)2(OPO2O)-Fe(S-)2, through the formation of Fe-O-P bonds.

Promoting effect of 2000 ppm H2S on the dry reforming reaction of CH4 over pure CeO2, and in situ observation of the behavior of sulfur during the reaction

Biomass-derived gas and unused industrial byproducts are resources for H2 production with low CO2 emission; however, they contain > 1000 ppm H2S, which necessitates the use of H2S-tolerant catalysts. Herein, a pure CeO2 catalyst demonstrated stable and efficient H2 production via the dry reforming reaction in the presence of 2000 ppm H2S. No impurity phases and carbon deposition appeared on the catalyst after the reaction. Moreover, CeO2 catalytic activity improved with H2S concentration. The effect of H2S on CeO2 was determined by in situ infrared spectroscopy and S K-edge X-ray absorption near-edge structure studies. The re-oxidation of CeO2 by CO2, a key step of the dry reforming reaction, was accelerated by the rapid redox cycle of sulfur, which promoted CH4 activation and decreased the apparent activation energy from 125 to 101 kJ/mol. The redox cycle of S made the pure CeO2 catalyst resistant to high concentration of H2S.

In Situ XANES Characterization of V2O5/TiO2–SiO2–MoO3 Catalyst for Selective Catalytic Reduction of NO by NH3

The structures of V2O5 catalysts for the selective catalytic reduction (SCR) of NO by NH3 at low temperatures are investigated using in situ V K-edge X-ray absorption near-edge structure measuremen...

First-principles modeling of X-ray absorption spectra enlightens the processes of scandium sequestration by iron oxides

Abstract Scandium is often associated with iron oxides in the environment. Despite the use of scandium as a geochemical tracer and the existence of world-class supergene deposits, uncertainties on speciation obscure the processes governing its sequestration and concentration. Here, we use first-principles approaches to interpret experimental K-edge X-ray absorption near-edge structure spectra of scandium either incorporated in or adsorbed on goethite and hematite, at concentrations relevant for the environment. This modeling helps to interpret the characteristic spectral features, providing key information to determine scandium speciation when associated with iron oxides. We show that scandium is substituted into iron oxides at low concentrations without modifying the crystal structure. When scandium is adsorbed onto iron oxide surfaces, the process occurs through outer-sphere complexation with a reduction in the coordination number of the hydration shell. Considering available X-ray absorption spectra from laterites, the present results confirm that scandium adsorption onto iron oxides is the dominant mechanism of sequestration in these geochemical conditions. This speciation explains efficient scandium recovery through mild metal-lurgical treatments of supergene lateritic ores. The specificities of scandium sorption mechanisms are related to the preservation of adsorbed scandium in million-years old laterites. These results demonstrate the emerging ability to precisely model fine X-ray absorption spectral features of trace metals associated with mineral phases relevant to the environment. It opens new perspectives to accurately determine trace metals speciation from high-resolution spatially resolved X-ray absorption near-edge structure spectroscopy in order to constrain the molecular mechanisms controlling their dynamics.