Pre-edge Analysis Research Articles

Analysis of the Structure of Heavy Ion Irradiated LaFeO3 Using Grazing Angle X-ray Absorption Spectroscopy.

Crystalline ceramics are candidate materials for the immobilization of radionuclides, particularly transuranics (such as U, Pu, and Am), arising from the nuclear fuel cycle. Due to the α-decay of transuranics and the associated recoil of the parent nucleus, crystalline materials may eventually be rendered amorphous through changes to the crystal lattice caused by these recoil events. Previous work has shown irradiation of titanate-based ceramics to change the local cation environment significantly, particularly in the case of Ti which was shown to change from 6- to 5-fold coordination. Here, this work expands the Ti-based study to investigate the behavior in Fe-based materials, using LaFeO3 as an example material. Irradiation was simulated by heavy ion implantation of the bulk LaFeO3 ceramic, with the resulting amorphous layer characterized with grazing angle X-ray absorption spectroscopy (GA-XAS). Insights into the Fe speciation changes exhibited by the amorphized surface layer were provided through quantitative analysis, including pre-edge analysis, and modeling of the extended X-ray absorption fine structure (EXAFS), of the GA-XAS data.

X-ray Spectroscopic Study of the Electronic Structure of a Trigonal High-Spin Fe(IV)═O Complex Modeling Non-Heme Enzyme Intermediates and Their Reactivity.

Fe K-edge X-ray absorption spectroscopy (XAS) has long been used for the study of high-valent iron intermediates in biological and artificial catalysts. 4p-mixing into the 3d orbitals complicates the pre-edge analysis but when correctly understood via 1s2p resonant inelastic X-ray scattering and Fe L-edge XAS, it enables deeper insight into the geometric structure and correlates with the electronic structure and reactivity. This study shows that in addition to the 4p-mixing into the 3dz2 orbital due to the short iron-oxo bond, the loss of inversion in the equatorial plane leads to 4p mixing into the 3dx2-y2,xy, providing structural insight and allowing the distinction of 6- vs 5-coordinate active sites as shown through application to the Fe(IV)═O intermediate of taurine dioxygenase. Combined with O K-edge XAS, this study gives an unprecedented experimental insight into the electronic structure of Fe(IV)═O active sites and their selectivity for reactivity enabled by the π-pathway involving the 3dxz/yz orbitals. Finally, the large effect of spin polarization is experimentally assigned in the pre-edge (i.e., the α/β splitting) and found to be better modeled by multiplet simulations rather than by commonly used time-dependent density functional theory.

Mn K-edge X-ray absorption studies of mononuclear Mn(III)-hydroxo complexes.

Mn K-edge X-ray absorption spectroscopy experiments were performed on the solid- and solution-phase samples of [MnII(dpaqR)](OTf) (R=H, Me) and [MnIII(OH)(dpaqR)](OTf). The extended X-ray absorption fine structure (EXAFS) data show distinct differences between the MnII and MnIII-OH complexes, with fits providing metric parameters in excellent agreement with values from X-ray crystallography and density functional theory (DFT) computations. Evaluation of the EXAFS data for solid-phase [MnIII(OH)(dpaq)](OTf) resolved a short Mn-OH bond distance of 1.79Å; however, the short trans-amide nitrogen bond of the supporting ligand precluded the resolution of the Mn-OH bond distance in the corresponding solution-phase sample and for both [MnIII(OH)(dpaqMe)](OTf) samples. The edge energy also increases by approximately 2eV from the MnII to the MnIII-OH complexes. Experimental pre-edge analysis shows the MnII complexes to have pre-edge areas comparable to the MnIII-OH complexes, despite the presence of the relatively short Mn-OH distance. Time-dependent density functional theory (TD-DFT) computations illustrate that Mn 3d-4p mixing, a primary contributor to pre-edge intensities, decreases by~0.3% from the MnII to MnIII-OH complexes, which accounts for the very similar pre-edge areas. Collectively, this work shows that combined EXAFS and XANES analysis has great potential for identification of reactive MnIII-OH intermediates, such as those proposed in enzyme active sites.

Electronic State of Sodium trans-[Tetrachloridobis(1H-indazole)ruthenate(III)] (NKP-1339) in Tumor, Liver and Kidney Tissue of a SW480-bearing Mouse

Ruthenium complexes are promising candidates for anticancer agents, especially NKP-1339 (sodium trans-[tetrachloridobis(1H-indazole)ruthenate(III)]), which is on the edge to clinical applications. The anticancer mechanism seems to be tightly linked to the redox chemistry but despite progress in human clinical trials the in vivo Ru oxidation state and the coordination of Ru remains unclear. The Ru-based anticancer drug NKP-1339 was studied applying XANES (Cl K- and Ru L2,3-edges) in tumor, kidney and liver tissue of a SW480 bearing mouse. Based on coordination charge and 3D XANES plots containing a series of model compounds as well as pre-edge analysis of the ligand Cl K-edge it is suggested that NKP-1339 remains in its +III oxidation state after 24 hours and at least one of the four chlorido ligands remain covalently bound to the Ru ion showing a biotransformation from RuIIIN2Cl4 to RuIIIClx(N/O)6−x (X = 1 or 2).

Speciation using X-ray absorption fine structure (XAFS)

X-ray absorption fine structure (XAFS) spectroscopy is in use as a unique fingerprinting technique. By employing spectra of well-defined chemical species (standards), XAFS can be analyzed for speciation, i.e., identification and quantification of the different chemical species present in a sample. Several approaches to analyze the XAFS data for speciation have been developed. The principal component analysis and target transformation method are the two methods generally applied to check the number and identity of components in a sample. A widely used method for quantitatively determining the percentage of the species in the sample is linear combination fitting method, which can be applied to XANES spectra, derivative XANES spectra and EXAFS spectra. Other methods include residual phase analysis, normalized difference absorption edge analysis, pre-edge analysis method, derivative spectra methods and a method based on the relative position of the edge. The aim of the present article is to review the different (nearly all) methods of speciation using XAFS. The theory and principle of the different methods have been described and the analysis procedures for the methods have been illustrated with the help of examples. Some selected studies involving different elements have been reviewed and discussed.

Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis.

Analysis of the oxidation state and coordination geometry using pre-edge analysis is attractive for heterogeneous catalysis and materials science, especially for in situ and time-resolved studies or highly diluted systems. In the present study, focus is laid on iron-based catalysts. First a systematic investigation of the pre-edge region of the Fe K-edge using staurolite, FePO4, FeO and α-Fe2O3 as reference compounds for tetrahedral Fe(2+), tetrahedral Fe(3+), octahedral Fe(2+) and octahedral Fe(3+), respectively, is reported. In particular, high-resolution and conventional X-ray absorption spectra are compared, considering that in heterogeneous catalysis and material science a compromise between high-quality spectroscopic data acquisition and simultaneous analysis of functional properties is required. Results, which were obtained from reference spectra acquired with different resolution and quality, demonstrate that this analysis is also applicable to conventionally recorded pre-edge data. For this purpose, subtraction of the edge onset is preferentially carried out using an arctangent and a first-degree polynomial, independent of the resolution and quality of the data. For both standard and high-resolution data, multiplet analysis of pre-edge features has limitations due to weak transitions that cannot be identified. On the other hand, an arbitrary empirical peak fitting assists the analysis in that non-local transitions can be isolated. The analysis of the oxidation state and coordination geometry of the Fe sites using a variogram-based method is shown to be effective for standard-resolution data and leads to the same results as for high-resolution spectra. This method, validated by analysing spectra of reference compounds and their well defined mixtures, is finally applied to track structural changes in a 1% Fe/Al2O3 and a 0.5% Fe/BEA zeolite catalyst during reduction in 5% H2/He. The results, hardly accessible by other techniques, show that Fe(3+) is transformed into Fe(2+), while the local Fe-O coordination number of 4-5 is maintained, suggesting that the reduction involves a rearrangement of the oxygen neighbours rather than their removal. In conclusion, the variogram-based analysis of Fe K-edge spectra proves to be very useful in catalysis research.

Mitigation of Layered to Spinel Conversion of a Li-Rich Layered Metal Oxide Cathode Material for Li-Ion Batteries

We have prepared the Li-rich layered NMC composite cathode material of the composition 0.3Li2MnO30.7LiMn0.33Ni0.33Co0.33O2, (NMC) with 5 wt% Na doping. The latter material with composition of 0.3Li2MnO3.0.7Li0.97Na0.03Mn0.33Ni0.33Co0.33O2, synthesized as 200–300 nm size particles, was compared to its counterpart without Na. The discharge rate capability of the Li-rich NMC was greatly improved at both room temperature and 50°C with the Na doping. The Na doped material exhibited significantly higher conductivity than its un-doped analog as evidenced by dc electronic conductivity data and impedance of Li cells. Charge/discharge cycling results of Li cells at 50°C indicated that the voltage decay of Li-rich NMC accompanied by a layer to spinel structural conversion was mitigated with Na doping. XRD analysis revealed that ionic exchange of Na occurs upon contact of the cathode material with the electrolyte and produces a volume expansion of the crystal lattice which triggers a favorable metal (probably Ni) migration to Li depleted regions during oxidation of Li2MnO3 in the first cycle. XANES data showed that Na doped NMC has better Ni reduction efficiency to provide higher rate capability. EXAFS data supported this conclusion by showing that in the case of Na doped NMC, the structure has larger crystal cage allowing for better metal migration into the Li depleted regions located in the layered unit cell of C2/m space group. XANES of Mn K-edge supported by pre-edge analysis also revealed that during charging of the electrode, the Na doped NMC was oxidized to a higher Mn valence state compared to its undoped counterpart.

EXAFS, XANES, and DFT study of the mixed-valence compoundYMn2O5: Site-selective substitution of Fe for Mn

In ${\text{YMn}}_{2}{\text{O}}_{5}$, the Mn atoms occupy two nonequivalent Wyckoff sites within the unit cell exhibiting different oxygen coordinations, i.e., the system can be characterized as a mixed-valence compound. For the formation of the orthorhombic crystal structure, Jahn-Teller distortions are assumed to play an important role. In this study, we aimed at the investigation of the crystal structure changes upon the substitution of Mn by the non-Jahn-Teller cation ${\text{Fe}}^{3+}$. Therefore, we synthesized a series of ${\text{YMn}}_{2\ensuremath{-}x}{\text{Fe}}_{x}{\text{O}}_{5}$ powder samples with $x=0$, 0.5, and 1 by a citrate technique. We utilized extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) analysis as well as density-functional theory (DFT) to investigate the two nonequivalent Wyckoff sites within the orthorhombic crystal structure (confirmed for all compositions) occupied by transition-metal atoms. For quantitative determination of structural short-range order, all plausible options of substitution of Fe for Mn are discussed. On the basis of these evaluations, the EXAFS and XANES behavior is analyzed and appropriate crystallographic weights are assigned to the subset of structural models in accordance with the experimental data. From EXAFS analysis, using multiple-scattering theory, we conclude only the $4h$ Wyckoff site to be occupied by Fe [occupancy refined is $(100\ifmmode\pm\else\textpm\fi{}3)\mathrm{%}$ in case of $x=1$]. Furthermore, taking the XANES spectra into account, we are able to verify the EXAFS results and additionally explain the differences in the $\text{Mn}\text{ }K$ XANES spectra in dependence on $x$ to be caused by changes in the dipole transitions to $4p$ final states. From quantitative pre-edge analysis an oxidation number of $+4$ for the Mn atom for $x=1$ is determined whereas the Fe valence is shown to be unchanged. Since the substitution process only involves one Wyckoff site, the experimentally observed limit to a maximum amount of $x=1$ is explained. Additionally, a possible disorder, discussed in the literature, is not proven for our samples. With DFT calculations, the experimental findings are verified on the basis of the total energy of the different possible electronic configurations. Crystal-field effects are identified to be responsible for the site-selective substitution of Fe for Mn.

Oxidation state variation under β-irradiation in an iron-bearing soda lime glass system

The effect of ionizing radiation on glasses in the system SiO2-CaO-Na2O-Fe2O3 is investigated as a function of iron concentration or / and irradiation dose by a pre-edge analysis at the iron K-edge. While reduction phenomenon is clearly observed for large irradiation doses (5 C), the effect of irradiation for intermediate doses is more challenging to interpret. Comparison between X-ray absorption measurements, colorimetry results and Electron Paramagnetic Resonance measurements suggest that iron environment may be modified without the ions being reduced.

X-ray Absorption and Emission Spectroscopy of CrIII (Hydr)Oxides: Analysis of the K-Pre-Edge Region

Pre-edge spectral features below the main X-ray absorption K-edge of transition metals show a pronounced chemical sensitivity and are promising sources of structural information. Nevertheless, the use of pre-edge analysis in applied research is limited because of the lack of definite theoretical peak-assignments. The aim of this study was to determine the factors affecting the chromium K-pre-edge features in trivalent chromium-bearing oxides and oxyhydroxides. The selected phases varied in the degree of octahedral polymerization and the degree of iron-for-chromium substitution in the crystal structure. We investigated the pre-edge fine structure by means of high-energy-resolution fluorescence detected X-ray absorption spectroscopy and by 1s2p resonant X-ray emission spectroscopy. Multiplet theory and full multiple-scattering calculations were used to analyze the experimental data. We show that the chromium K-pre-edge contains localized and nonlocalized transitions. Contributions arising from nonlocalized metal-metal transitions are sensitive to the nearest metal type and to the linkage mode between neighboring metal octahedra. Analyzing these transitions opens up new opportunities for investigating the local coordination environment of chromium in poorly ordered solids of environmental relevance.

A pre-edge analysis of Mn K-edge XANES spectra to help determine the speciation of manganese in minerals and glasses

High-resolution manganese K-edge X-ray absorption near edge structure spectra were collected on a set of 40 Mn-bearing minerals. The pre-edge feature information (position, area) was investigated to extract as much as possible quantitative valence and symmetry information for manganese in various “test” and “unknown” minerals and glasses. The samples present a range of manganese symmetry environments (tetrahedral, square planar, octahedral, and cubic) and valences (II to VII). The extraction of the pre-edge information is based on a previous multiple scattering and multiplet calculations for model compounds. Using the method described in this study, a robust estimation of the manganese valence could be obtained from the pre-edge region at 5% accuracy level. This method applied to 20 “test” compounds (such as hausmannite and rancieite) and to 15 “unknown” compounds (such as axinite and birnessite) provides a quantitative estimate of the average valence of manganese in complex minerals and silicate glasses.

Titanium reduction processes in oxide glasses under electronic irradiation

Borosilicate and disilicate glasses doped with 0–2mol% of TiO2 have been irradiated with 2MeV electrons. Reduction of Ti4+ is attested by the presence of an intense electronic paramagnetic resonance (EPR) signal around g=1.92 corresponding to Ti3+, not observed in samples that were not irradiated. Moreover, two distinct sites of Ti3+ have been detected in each glass composition. Ti–K-edge X-ray absorption near edge structure (XANES) experiment and more precisely the pre-edge analysis of non-irradiated samples indicates the presence of four, five and six coordinated Ti(IV) in borosilicate glass compositions. In contrast, the average Ti(IV) coordination in DS composition is a mixture between fivefold and sixfold. In irradiated glasses, the Ti–K-edge energy (as well as the pre-edge position) shows a shift to smaller energies, which we suggest is caused by Ti3+. Comparison between EPR, Raman and XANES results in a coherent description of Ti4+ reduction process in the glass samples.

Unusual Coordination Behavior of Cr3+ in Microporous Aluminophosphates

A CrAPO-5 molecular sieve has been investigated with X-ray absorption spectroscopy (EXAFS-XANES) as dehydrated material and after loading with water and ammonia to unravel the coordination geometries of Cr3+ in the framework of a microporous crystalline aluminophosphate, more particularly of the AFI-type. A comparison of the XANES data, a preedge analysis with crystal field multiplet calculations and EXAFS data, pointed toward the presence of framework Cr3+ which, on dehydration, takes on a distorted tetrahedral coordination state. Due to the 3d3 configuration of Cr3+, this unusual tetrahedral coordination environment strongly tends to transform into the more stable 6-fold coordination geometry by binding two extraframework water molecules during hydration. In the presence of ammonia, tetrahedral Cr3+ readily transforms into a 5-fold coordination geometry by binding one ammonia molecule. Therefore, depending on the environmental conditions, the Cr3+ ions can occur in a 4-, 5-, or 6-fold coordination. This observation underlines the flexibility of transition metal ions, such as Cr3+, to cope with unusual coordination geometries in inorganic hosts, making them interesting as potential active sites in heterogeneous catalysis.

Kβ-Detected XANES of Framework-Substituted FeZSM-5 Zeolites

The valence and local symmetry of iron in framework-substituted FeZSM-5 with a high Fe dilution (Si/Fe = 360) was studied by means of Kβ-detected X-ray absorption spectroscopy. This technique combines high-resolution (ΔE ∼1 eV) fluorescence detection of the 3p to 1s (Kβ) transition with the X-ray absorption near-edge structure (XANES) at the Fe K-edge. An absorption-like spectrum is recorded by detecting the Kβ fluorescence intensity as a function of the incident energy that is scanned through the K absorption edge. Kβ-detected XANES spectra allow for a more precise separation of the weak K pre-edge structure from the main edge as compared to conventional absorption spectroscopy. Subsequent analysis and interpretation of the pre-edge spectral features therefore is more accurate. The pre-edge is sensitive to changes in the local coordination and oxidation state of Fe. Using this technique we were able to quantitatively determine the degree of iron extraction out of a zeolite framework upon steaming. With the use of appropriate reference compounds, the pre-edge analysis was used to monitor the activation of low-loaded, framework-substituted FeZSM-5 (0.3 wt % Fe). Template removal and calcination distort the zeolite framework and induce a deviation from Td symmetry for incorporated iron. The (deliberate) presence of water at high temperature (T > 500 °C) facilitates the hydrolysis of the Si−O−Fe bonds and increases the formation of extraframework iron species. The amount of FeIII occupying tetrahedral sites in the MFI-type zeolite decreases to 32% and 19%, respectively, for mild- and hard-steamed samples.

Spectroscopic studies of the effect of ligand donor strength on the Fe-NO bond intradiol dioxygenases.

The geometric and electronic structure of NO bound to reduced protocatechuate 3,4-dioxygenase and its substrate (3,4-dihydroxybenzoate, PCA) complex have been examined by X-ray absorption (XAS), UV-vis absorption (Abs), magnetic circular dichroism (MCD), and variable temperature variable field (VTVH) MCD spectroscopies. The results are compared to those previously published on model complexes described as [FeNO]7 systems in which an S = 5/2 ferric center is antiferromagnetically coupled to an S = 1 NO-. XAS pre-edge analysis indicates that the Fe-NO units in FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] lack the greatly increased pre-edge intensity representative of most [FeNO]7 model sites. Furthermore, from extended X-ray absorption fine structure (EXAFS) analysis, the FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] active sites are shown to have an Fe-NO distance of at least 1.91 A, approximately 0.2 A greater than those found in the model complexes. The weakened Fe-NO bond is consistent with the overall lengthening of the bond lengths and the fact that VTVH MCD data show that NO(-)-->FeIII CT transitions are no longer polarized along the z-axis of the zero-field splitting tensor. The weaker Fe-NO bond derives from the strong donor interaction of the endogenous phenolate and substrate catecholate ligands, which is observed from the increased intensity in the CT region relative to that of [FeNO]7 model complexes, and from the shift in XAS edge position to lower energy. As NO is an analogue of O2, the effect of endogenous ligand donor strength on the Fe-NO bond has important implications with respect to O2 activation by non-heme iron enzymes.

Spectroscopic Investigation of Reduced Protocatechuate 3,4-Dioxygenase: Charge-Induced Alterations in the Active Site Iron Coordination Environment.

Chemical reduction of the mononuclear ferric active site in the bacterial intradiol cleaving catecholic dioxygenase protocatechuate 3,4-dioxygenase (3,4-PCD, Brevibacterium fuscum) produces a high-spin ferrous center. We have applied circular dichroism (CD), magnetic circular dichroism (MCD), variable-temperature-variable-field (VTVH) MCD, X-ray absorption (XAS) pre-edge, and extended X-ray absorption fine structure (EXAFS) spectroscopies to investigate the geometric and electronic structure of the reduced iron center. Excited-state ligand field CD and MCD data indicate that the site is six-coordinate where the (5)E(g) excited-state splitting is 2033 cm(-)(1). VTVH MCD analysis of the ground state indicates that the site has negative zero-field splitting with a small rhombic splitting of the lowest doublet (delta = 1.6 +/- 0.3 cm(-)(1)). XAS pre-edge analysis also indicates a six-coordinate site while EXAFS analysis provides accurate bond lengths. Since previous spectroscopic analysis and the crystal structure of oxidized 3,4-PCD indicate a five-coordinate ferric active site, the results presented here show that the coordination number increases upon reduction. This is attributed to the coordination of a second solvent ligand. The coordination number increase relative to the oxidized site also appears to be associated with a large decrease in the ligand donor strength in the reduced enzyme due to protonation of the original hydroxide ligand.