XAFS Spectroscopy Research Articles

Soluble soil Pb minimized by thermal transformation to Pb-bearing feldspar

Thermal transformation is an effective remediation measure to stabilize soil Pb and other heavy metals via transformation into less soluble compounds. This study aimed to determine the solubility of Pb in soils subjected to heating at a range of temperatures (100–900 °C) in relation to the changes in Pb speciation using XAFS spectroscopy. Lead solubility in the contaminated soils after thermal treatment corresponded well to the chemical species of Pb present. As the temperature was increased to 300 °C, cerussite and Pb associated with humus started to decompose in the soils. As the temperature was further increased to 900 °C, the amount of water and HCl extractable Pb decreased significantly from the soils, whereas Pb-bearing feldspar started to occur, accounting for nearly 70% of the soil Pb. During thermal treatment, Pb species in the soils were little affected by Fe oxides that showed a significant phase transformation into hematite. Our study proposes the following underlying mechanisms for Pb immobilization in thermally treated soils: i) thermally labile Pb species such as PbCO3 and Pb associated with humus start to decompose at temperatures around 300 °C, ii) aluminosilicates with crystalline and poorly ordered structures undergo thermal decomposition at temperatures around 400 °C, iii) liberating Pb in the soil is then associated with a Si and Al rich liquid derived from thermally decomposed aluminosilicates at higher temperatures, and iv) the formation of Pb-feldspar like minerals is enhanced at 900 °C.

Uranium oxides structural transformation in human body liquids

Uranium oxide microparticles ingestion is one of the potential sources of internal radiation doses to the humans at accidental or undesirable releases of radioactive materials. It is important to predict the obtained dose and possible biological effect of these microparticles by studying uranium oxides transformations in case of their ingestion or inhalation. Using a combination of methods, a complex examination of structural changes of uranium oxides in the range from UO2 to U4O9, U3O8 and UO3 as well as before and after exposure of uranium oxides in simulated biological fluids: gastro-intestinal and lung—was carried out. Oxides were thoroughly characterized by Raman and XAFS spectroscopy. It was determined that the duration of expose has more influence on all oxides transformations. The greatest changes occurred in U4O9, that transformed into U4O9-y. UO2.05 and U3O8 structures became more ordered and UO3 did not undergo significant transformation.

Off-centering of the Fe atom at the A site in SrTiO3: calculations and experiment

The influence of annealing temperature and deviation from stoichiometry on the incorporation of the Fe impurity into the A and B sites of the perovskite structure is studied using XAFS spectroscopy and X-ray diffraction. It is shown that at high annealing temperature about 30% of the iron atoms enter the A sites in SrTiO3. The obtained results agree with the results of first-principles calculations, according to which the Fe2+ ion at the A site exhibits strong off-centering (the displacement of ∼1 Å). The mechanism of magnetoelectric interaction for magnetic off-center ions is discussed.

Identification of Heavy Metal Compounds in Technogenically Transformed Soils Using Sequential Fractionation, XAFS Spectroscopy, and XRD Powder Diffraction

The transformation of technogenic Cu and Zn compounds in technogenically transformed soils (Spolic Technosols) with high and very high concentrations of metals formed at the site of a natural tailings pond in the floodplain of the Seversky Donets River, the main tributary of the Don River (Rostov oblast, Russia) has been studied. The Technosols are compared to an unpolluted meadow-chernozemic soil (Fluvisol) outside the impact zone. The state of Cu and Zn is assessed using three sequential extraction schemes—Miller’s, Tessier’s, and BCR, as well as synchrotron X-ray powder diffraction (XRD) and analysis of synchrotron X-ray absorption spectrometry (XAFS) spectra. It is shown that the distribution of metals in soil is largely related to their properties, such as electronegativity, hydrolyzability, and softness parameter. As is observed, Cu mainly concentrates in the residual fraction (to 42%) and in the fraction associated with organic matter (up to 27%). The mobility of Zn in the studied soils is higher than that of Cu. Its main part (up to 56%) is in the residual fraction and the fraction associated with Fe and Mn oxides (up to 48%), especially with Fe(III) crystalline compounds. The combination of a three-stage BCR scheme with XAFS and XRD techniques is used for the first time. Most of the peaks in diffraction patterns of soil samples after the first and second extraction stages correspond to the authigenic sulfur-containing minerals, namely, wurtzite (ZnS with a hexagonal structure), sphalerite (cubic ZnS), covellite (CuS), and bornite (Cu5FeS4). Wurtzite is present in the exchangeable and reducible fractions. These fractions also contain chalcocite (Cu2S). Sulfides are most abundant in soil sample after extraction of the oxidizable fraction, while phyllosilicates are prevalent in the sample after extraction of the reducible fraction. X-ray absorption spectroscopy demonstrates molecular structural changes in the Zn and Cu compounds in heavily polluted soils, suggesting the transformation of metals under different environmental conditions, which is important for assessment of the soil protective function.

Revealing the Solution Structure of Pd(OAc)2 with Halide Additives

Comprehensive SummaryMechanistic studies of the palladium catalyst activation with halide additives are of great importance to palladium catalysis. In this work, XAFS spectroscopy and cyclic voltammetry were utilized to study real structures of Pd(OAc)2 in solution with different inorganic halogen additives. XAFS results demonstrate that Pd maintained +2 in the presence of excessive LiCl, LiBr, ZnCl2, ZnBr2, and NaI. Fitting results of EXAFS spectra revealed that the Pd's first shell is replaced by halogen partially or completely depending on the halogen species. DFT calculations were conducted to identify the most reliable solvated structures. The combined experimental and computational studies elucidate the critical role of inorganic halide additives in Pd chemistry.

On the possibility of incorporation of the iron impurity into A sites in SrTiO-=SUB=-3-=/SUB=-

The local environment and oxidation state of the Fe impurity in strontium titanate are studied using XAFS spectroscopy. The influence of annealing temperature and deviation from stoichiometry on the possibility of incorporation of the impurity into the A and B sites of the perovskite structure is studied. The results obtained from the X-ray diffraction, XANES spectra, and EXAFS spectra suggest that at high annealing temperature the iron atoms, at least partially (up to 30%), enter the A sites in SrTiO3. The obtained results agree with results of first-principles calculations, according to which the iron at the A site exhibits strong off-centering (the displacement of ~1 Angstrem), similar to that previously established in SrTiO3 samples doped with Mn and Co. Keywords: strontium titanate, iron, doping, XAFS, off-center atoms.

О возможности введения примеси железа в узлы A в SrTiO-=SUB=-3-=/SUB=-

The local environment and oxidation state of the Fe impurity in strontium titanate are studied using XAFS spectroscopy. The influence of annealing temperature and deviation from stoichiometry on the possibility of incorporation of the impurity into the A and B sites of the perovskite structure is studied. The results obtained from the X-ray diffraction, XANES spectra, and EXAFS spectra suggest that at high annealing temperature the iron atoms, at least partially (up to 30%), enter the A sites in SrTiO3. The obtained results agree with results of first-principles calculations, according to which the iron at the A site exhibits strong off-centering (the displacement of ~1 Angstrom), similar to that previously established in SrTiO3 samples doped with Mn and Co.

Speciation Study on Pb in Different Particle Size Fractions by Sequential Extraction and XAFS Spectroscopy

Speciation Study on Pb in Different Particle Size Fractions by Sequential Extraction and XAFS Spectroscopy

Formation of Active Centers of Nickel–Zinc Catalysts Deposited on the Nanodiamond for the Selective Hydrogenation of Phenylacetylene

A study is performed of bimetallic catalysts NiZn/ND with ratios Ni : Zn = 1 : 1 and 1 : 3 prepared by impregnation using detonation nanodiamond (ND) as a support. They were compared with monometallic Ni/ND and Zn/ND. It is shown by nitrogen adsorption/desorption, scanning and transmission electron microscopy that metal deposition does not affect the porous structure or morphology of a support. Coordination of metal precursors on a nanodiamond surface proceeds with the participation of functional groups, as is confirmed by a change in the electrokinetic charge of the surface. The reduction of metal precursors is studied by temperature-programmed reduction and in situ XAFS spectroscopy. In Ni-containing samples, two forms of Ni2+ are found that are bonded differently with the support. ZnO is not reduced in the samples upon treatment with hydrogen at temperatures up to 400°C. The fraction of reduced nickel is determined by analyzing XANES spectra. Virtually full reduction of nickel is observed in a catalyst with a Ni : Zn ratio of 1 : 1 after 4 h of in situ treatment with hydrogen inside a spectrometer cell at 400°C, but not at a Ni : Zn ratio of 1 : 3 under the same conditions. The highest selectivity of styrene formation in the reaction of phenylacetylene hydrogenation throughout the investigated range of temperatures (100–350°С) is ensured by NiZn/ND; NiZn3/ND is less active and selective, since ZnO closes the active nickel centers and prevents the adsorption of phenylacetylene.

Origins of low lattice thermal conductivity of Pb1-xSnxTe alloys for thermoelectric applications.

Lead telluride is a well-established material for direct conversion of heat into electricity. However, the aspects of the heat transport phenomena for PbTe-alloys remain not fully understood. Here, for the first time, origins of the phonon scattering in Pb1-xSnxTe compounds were studied through changes in the effective anharmonic pair potential obtained from X-ray Absorption Fine Structure (XAFS) spectroscopy. Results indicate that the interatomic pair potential of Pb-Te and Sn-Te bonds changes with the level of substitution x and that the anharmonicity of bonds in the solid solution is increased leading to the lower values of the lattice thermal conductivity. Furthermore, due to the existence of a soft TO mode in Pb1-xSnxTe, the Grüneisen parameter γE determined using XAFS much more precisely corresponds with the changes of lattice thermal conductivity κlat compared to γS obtained from the speed of sound measurements. This study explains the observed drastic reduction in κL in Pb1-xSnxTe solid solution (2.3 W m-1 K-1 for PbTe vs. 1.0 W m-1 K-1 for Pb0.75Sn0.25Te), due to changes in the interatomic pair potential of Pb-Te and Sn-Te and provides guidelines into its effective modification related to thermal transport in alloys based on PbTe. The estimated range of low thermal conductivity for the PbTe-SnTe solid solutions (<1 W m-1 K-1) reveals opportunities for further enhancement of energy conversion for this promising family of compounds. Moreover, this work provides a new concept for the estimation of the Grüneisen parameter through the EXAFS spectra analysis.

Counterion Binding in Adsorbed Surfactant Films Studied by Surface Tensiometry and Total Reflection XAFS Spectroscopy

Counterion Binding in Adsorbed Surfactant Films Studied by Surface Tensiometry and Total Reflection XAFS Spectroscopy

The Local Structure and Exchange Bias Effect of (Ho, Co)-codoped BiFeO3 investigated by XAFS spectroscopy

Two elements (Ho, Co) co-doped Bi1-xHoxFe0.95Co0.05O3 (x = 0.05, 0.10, 0.15) samples were prepared by sol-gel method. The X-ray diffraction patterns demonstrate that Ho ions doping plays a major role in a systematic structural phase transition. The details of the magnetic properties of the samples are investigated. The magnetic properties of (Ho, Co) co-doped BiFeO3 have been improved greatly, compared with the pure phase BiFeO3. An exchange bias effect is observed in the samples. The local structure of the samples measured by X-ray absorption fine structure is studied to analyze the origin of the enhancement of the magnetic properties and the change of exchange bias effect of the samples. The occurrence of the exchange bias effect is attributed to the changes of the Fe-O bond length and of the local symmetry around Fe atom.

Study of the Interaction between Components at Different Stages of Preparing Ru–Rb/Sibunit Catalysts for the Decomposition of Ammonia

The interaction between the components of a Ru–Rb/Sibunit catalyst for the decomposition of ammonia is studied via XAFS spectroscopy, X-ray diffraction with synchrotron radiation, and high-resolution transmission electron microscopy. The porous structure of the samples is examined at the stages of preparation. A set of measures reveals the presence of ruthenium in the catalyst (mainly in the metallic state) and the absence of joint phases of ruthenium and rubidium. It is found that rubidium is in an oxidized state in the catalyst in the form of compounds in direct contact with particles of active component and comparable in size to the latter, and as small clusters found mainly in the structure of carbon support.

Синтез наночастиц палладия на поверхности оксида церия(IV) под воздействием ультрафиолетового излучения и их диагностика

Проведен синтез наночастиц палладия с использованием комплексного оксалата палладия в качестве прекурсора в водной среде методом УФ-облучения. Образцы исследованы различными лабораторными методиками, такими как просвечивающая электронная микроскопия, рентгенофлуоресцентный анализ, рентгеновская порошковая дифракция, XAFS-спектроскопия и ИКспектроскопия диффузного отражения с использованием зондирующих молекул CO. Микроизображения, полученные с помощью просвечивающей электронной микроскопии, не позволили отличить наночастицы Pd от материала подложки, но показали отсутствие влияния УФ-излучения на размеры наночастиц CeO2. Данные рентгенофлуоресцентного анализа показали наличие в материале атомов церия и палладия. Дифрактограммы свидетельствуют о присутствии фаз диоксида церия и металлического палладия, также анализ спектров XAFS за K-краем палладия показал наличие в системе фазы PdO. Приблизительный размер наночастиц палладия оценен по ИК-спектрам после адсорбции CO, он составил не более 2 нм, что существенно меньше среднего размера наночастиц Pd, полученных аналогичным способом без подложки CeO2(1.5–9.5 нм).

29Si solid state NMR and Ti K-edge XAFS pre-edge spectroscopy reveal complex behavior of Ti in silicate melts

An understanding of the mechanisms of Ti is incorporation into silicate glasses and melts is critical for the field of petrology. Trace-element thermobarometry, high-field-strength element partitioning, and the physical properties of magmas are all be influenced by Ti incorporation into glasses and changes therein in response to changes in composition and temperature. In this study, we combine 29Si solid state NMR and Ti K-edge XAFS spectroscopy to investigate how Ti is incorporated into quenched Na-silicate glasses, and the influence of Ti on the structure of silicate species in these glasses. 29Si NMR shows that in both Ti-bearing Na2O•4SiO2 (NS4) and Na2O•8SiO2 (NS8) glasses, increasing the amount of Ti in the melt results in a shift of Si Q4 peak in the 29Si NMR spectra reflecting Ti nearest neighbors for Si in Q4 speciation. The Ti XAFS results from NS8 glass indicate that Ti is primarily incorporated in [5]-fold coordination. At higher Ti content, there is a shift of the XAFS pre-edge feature suggesting mixing of [4]-fold Ti into the spectra. Combined, the 29Si NMR and XAFS pre-edge data are consistent with Ti incorporation as isolated [5]Ti atoms and the formation of [5]Ti clusters at relatively low Ti concentrations, with no evidence for Ti–Na interactions as suggested by previous studies. As the Ti content increases, the Ti atoms begin to occupy 4-fold coordinated sites interacting primarily with Si in Q4 speciation (no significant Na–[4] Ti bonding). The internal consistency of these two techniques provides a uniquely complete snapshot of the complexity of Ti incorporation in silicate melts and underlies the importance of understanding Ti incorporation mechanisms in natural magmatic systems.

Detection of key transient Cu intermediates in SSZ-13 during NH3-SCR deNO x by modulation excitation IR spectroscopy.

The small pore zeolite Cu-SSZ-13 is an efficient material for the standard selective catalytic reduction of nitrogen oxides (NO x ) by ammonia (NH3). In this work, Cu-SSZ-13 has been studied at 250 °C under high conversion using a modulation excitation approach and analysed with phase sensitive detection (PSD). While the complementary X-ray absorption near edge structure (XANES) spectroscopy measurements showed that the experiments were performed under cyclic Cu+/Cu2+ redox, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) experiments provide spectroscopic evidence for previously postulated intermediates Cu-N([double bond, length as m-dash]O)-NH2 and Cu-NO3 in the NH3-SCR deNO x mechanism and for the role of [Cu2+(OH-)]+. These results therefore help in building towards a more comprehensive understanding of the reaction mechanism which to date has only been postulated in silico.

Ba-vacancy induces semiconductor-like photocatalysis on insulator BaSO4

Semiconductor-based photocatalysis has attracted considerable interdisciplinary attention for its diverse applications in environmental remediation and solar energy conversion. However, pure earth-abundant insulators have been seldom considered as photocatalysts because of the unfeasible electronic excitation. In this work, we make the earth-abundant insulator BaSO4 as conceptually new photocatalyst via Ba-vacancy engineering for the first time. The BaSO4 with Ba-vacancy is synthesized by a facile precipitation method and applied for photocatalytic NO removal in air. XAFS spectroscopy and DFT calculations demonstrate the formation of Ba-vacancy. Also, defect level induced by Ba-vacancy between the wide band gap is demonstrated, which endows insulator BaSO4 with semiconductor-like photocatalytic performance. Besides, according to the TPD analysis and theoretical simulation, Ba species is functioned as the NOx storage center for the primary accumulation of NO molecule on substrate, and simultaneously Ba-vacancy is equipped with the capability to redistribute charge carriers and thus accelerate the activation of NO molecule by the donation of electrons to electron-deficient areas, facilitating the conversion of NO into a higher valance state for further favorable photocatalytic oxidation. In situ DRIFTS spectra are applied to dynamically monitor intermediates and products on photocatalyst surface, revealing the reaction process and the enhancement role of Ba-vacancy. This work opens a new research doorway on earth-abundant insulators for the development of a new family of photocatalyst.

Sorption mechanisms of diphenylarsinic acid on ferrihydrite, goethite and hematite using sequential extraction, FTIR measurement and XAFS spectroscopy

Diphenylarsinic acid (DPAA) is an organoarsenic compound derived from abandoned chemical weapons. DPAA sorption by iron (hydr)oxides is of considerable importance but remains largely unexplored. The current study aimed at investigating the sorption mechanisms of DPAA on ferrihydrite, goethite and hematite using both macroscopic sorption kinetics and sequential extraction procedure (SEP) as well as microscopic Fourier transformed infrared (FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques. Sorption kinetics studies show that >93% of added DPAA (4–100 mg L−1) was sorbed on ferrihydrite and hematite within 5 min, while only 84% of added DPAA (100 mg L−1) was sorbed on goethite after 24 h. The sequential extraction results and FTIR measurements reveal that DPAA formed simultaneously inner- and outer-sphere complexes on goethite and hematite, but predominantly inner-sphere complexes on ferrihydrite with limited formation of outer-sphere complexes (<15%). A combination of SEP, FTIR and EXAFS techniques further enables identification of the interfacial reactions between DPAA and solid surfaces of iron (hydr)oxides and the mechanisms involved. Results indicate that DPAA interacted with these iron (hydr)oxides via (1) electrostatic attraction or hydrogen bonding, (2) surface complexation and (3) complexation embedded inside the mineral particles. EXAFS studies further demonstrate that DPAA formed mainly bidentate binuclear corner-sharing (2C) complexes regardless of the iron substrate, with As-Fe distances at 3.19–3.32 Å. Comparison of these results with available data in the literature on inorganic, methyl and phenyl arsenics (As) suggests that it is the phenyl group substitution that finally determines the predominance of 2C complexes. Results from the present study will improve our knowledge of DPAA interaction with solid surfaces and may help in the prediction of the environmental fate and environmental risk management of DPAA in the soil-water system.

Insights into Pb(II) binding by Fe/Al hydroxide–microbe composite: XAFS spectroscopy and isothermal titration calorimetry study

Soil is a complex assemble of colloidal and particulate constituents, including minerals, organic matter and microbes. Minerals and organics are not separate entities; rather, they are constantly interacting with each other. Such mineral-organic interaction can greatly affect the sequestration of metallic elements at the soil-water interfaces. This study aims to explore the binding of Pb(II) in a model Fe/Al hydroxide-organo composite (Fe/Al-OM) made with Pseudomonas putida cells which, for the purposes of this study, can be considered to be an analogue of some natural soil microaggregates. We measure an enhanced Pb adsorption to the Fe/Al hydroxide-bacteria composite (Fe/Al-B) over pure Fe/Al hydroxide at pH < 5.5. We find that Fe/Al-B sorbs an intermediate amount of Pb between the end-member Fe/Al hydroxide and bacteria. We identify a bidentate edge-sharing Pb complex on the Fe/Al oxide surfaces with a Pb–Fe/Al bond distance of ~3.33 Å, as well as a monodentate carboxyl binding on the bacterial surfaces. We report that Pb binding is an exothermic, energetically favorable process accompanied by positive entropy. Moreover, Pb distribution shifts from the bacterial fraction to the mineral fraction in Fe/Al-B when pH increases from 4 to 6.5. Results from batch to spectroscopic and thermodynamic experiments will facilitate our understanding of Pb speciation, mobility and fate in soils and sediments.

Study of mixed-valence Mn2BO4 using XRD, XPS and XAFS spectroscopies

The valence states and local structure around Mn atoms in mixed-valence Mn2BO4 have been studied by temperature dependent X-ray powder diffraction (XRPD), X-ray photoelectron (XPS) and Mn K-edge X-ray absorption (XAFS) spectroscopies measurements. X-ray absorption near-edge structure (XANES) and XPS have been used to measure the average oxidation state of Mn in bulk and near-surface of the material. The edge position, peak shapes and pre-edge features of Mn K-edge XANES spectra have been discussed. The pronounced temperature dependence of the Debye-Waller (DW) factor corresponding to the MnO coordination shell has been found from the extended x-ray absorption fine structure (EXAFS) analysis and has been associated with variations in the local distortions in MnO6 octahedra and emergence of short-range magnetic correlations at low temperatures. The XRPD measurements have been carried out at 298, 523 and 773 K. The monoclinic symmetry (P21/n) was found to persist up to highest temperature measured. The BVS calculations have revealed large valence difference between two manganese sites that strongly supports the presence of charge ordering up to high temperatures.