Post-edge Features Research Articles

Single-Atom Catalysts Supported by Crystalline Porous Materials: Views from the Inside.

Single-atom catalysts (SACs) have recently emerged as an exciting system in heterogeneous catalysis showing outstanding performance in many catalytic reactions. Single-atom catalytic sites alone are not stable and thus require stabilization from substrates. Crystalline porous materials such as zeolites and metal-organic frameworks (MOFs) are excellent substrates for SACs, offering high stability with the potential to further enhance their performance due to synergistic effects. This review features recent work on the structure, electronic, and catalytic properties of zeolite and MOF-protected SACs, offering atomic-scale views from the "inside" thanks to the subatomic resolution of synchrotron X-ray absorption spectroscopy (XAS). The extended X-ray absorption fine structure and associated methods will be shown to be powerful tools in identifying the single-atom site and can provide details into the coordination environment and bonding disorder of SACs. The X-ray absorption near-edge structure will be demonstrated as a valuable method in probing the electronic properties of SACs by analyzing the white line intensity, absorption edge shift, and pre-/postedge features. Emphasis is also placed on in situ/operando XAS using state-of-the-art equipment, which can unveil the changes in structure and properties of SACs during the dynamic catalytic processes in a highly sensitive and time-resolved manner.

Zinc K-edge XANES spectroscopy of mineral and organic standards.

Zinc K-edge X-ray absorption near-edge (XANES) spectroscopy was conducted on 40 zinc mineral samples and organic compounds. The K-edge position varied from 9660.5 to 9666.0 eV and a variety of distinctive peaks at higher post-edge energies were exhibited by the materials. Zinc is in the +2 oxidation state in all analyzed materials, thus the variations in edge position and post-edge features reflect changes in zinc coordination. For some minerals, multiple specimens from different localities as well as pure forms from chemical supply companies were examined. These specimens had nearly identical K-edge and post-edge peak positions with only minor variation in the intensity of the post-edge peaks. This suggests that typical compositional variations in natural materials do not strongly affect spectral characteristics. Organic zinc compounds also exhibited a range of edge positions and post-edge features; however, organic compounds with similar zinc coordination structures had nearly identical spectra. Zinc XANES spectral patterns will allow identification of unknown zinc-containing minerals and organic phases in future studies.

Observation of the flexoelectricity of a SrTiO3 single crystal by x-ray absorption and emission spectroscopies

Flexoelectricity, defined as the spontaneous electric polarization in a dielectric material induced by a strain gradient, is investigated from the microscopic viewpoint by x-ray spectroscopy. A single crystal SrTiO3 sample was used as a test system in order to reveal the appearance of the electric dipole moment by simple bending of the crystal. The spectral change characteristic of ferroelectric transition in SrTiO3 was not observed in the Ti K-edge absorption spectra. Instead, the gradual decrease (increase) of the post-edge feature (pre-edge structure) by bending was qualitatively explained using theoretical calculations that assumed the presence of oxygen vacancies and a slight crystal distortion. This assumption is also supported by the broadening of a tiny charge-transfer peak in the Ti Kβ resonant emission spectra. Therefore, it was revealed that the flexoelectric effect in SrTiO3 is easily drowned out through local imperfection induced by crystal deformations and cracks.

Near-surface Regions of Electrochemically Polarized Chalcopyrite (CuFeS2) as Studied Using XPS and XANES

Characteristics of the non-stoichiometric near-surface layers formed at reacted chalcopyrite are important for understanding its behavior as the main mineral of copper and for various materials applications. Here, we measured Cu L-, Fe L-, and S L-edge TEY XANES, and conventional XPS from chalcopyrite electrochemically polarized in 1M HCl solution. The spectra of copper, in contrast to those of Fe and S, remain almost unchanged on the first potential cycle despite strongly varying composition of the Fe-depleted near-surface regions. The Cu L-XANES spectra of chalcopyrite electrochemically pre-reduced and then anodized, and of that first anodized and then cathodically reduced, show the same leading peak but post-edge features indicative of covellite CuS and bornite Cu5FeS4, respectively. We suggest that the invariability of the Cu L-edge XANES and Cu 2p spectra is due to tetrahedral coordination CuS4 units stable even in the highly distorted non-stoichiometric structures.

XAS study of TiO 2 -based nanomaterials

Abstract X-Ray Absorption Spectroscopy studies of the W (0–1 at% W) and Mo-doped TiO 2 (0–1 at% Mo) nanoparticle specimens at the K edges of titanium and molybdenum as well as at the L 2 L 3 edges of tungsten are presented. The materials were prepared with Flame Spray Synthesis process by oxidation of metal-organic precursors. The Ti:K edge spectra in the XANES range show pre-edge and post-edge features characteristic for anatase. A decrease of the amplitude of the EXAFS function with doping is observed and attributed to a softening of the crystal lattice. The Mo EXAFS functions show a considerable decrease of the second-neighbour-shell peak with increasing Mo content, which is attributed to an increased number of cation vacancies. For tungsten a less pronounced effect is observed. The Mo and W XANES spectra do not show noticeable changes with doping level, which indicates their unchanged oxidation states.

Assembly, characterization, and electrochemical properties of immobilized metal bipyridyl complexes on silicon(111) surfaces.

Silicon(111) surfaces have been functionalized with mixed monolayers consisting of submonolayer coverages of immobilized 4-vinyl-2,2'-bipyridyl (1, vbpy) moieties, with the remaining atop sites of the silicon surface passivated by methyl groups. As the immobilized bipyridyl ligands bind transition metal ions, metal complexes can be assembled on the silicon surface. X-ray photoelectron spectroscopy (XPS) demonstrates that bipyridyl complexes of [Cp*Rh], [Cp*Ir], and [Ru(acac)2] were formed on the surface (Cp* is pentamethylcyclopentadienyl, acac is acetylacetonate). For the surface prepared with Ir, X-ray absorption spectroscopy at the Ir LIII edge showed an edge energy as well as post-edge features that were essentially identical with those observed on a powder sample of [Cp*Ir(bpy)Cl]Cl (bpy is 2,2'-bipyridyl). Charge-carrier lifetime measurements confirmed that the silicon surfaces retain their highly favorable photoelectronic properties upon assembly of the metal complexes. Electrochemical data for surfaces prepared on highly doped, n-type Si(111) electrodes showed that the assembled molecular complexes were redox active. However the stability of the molecular complexes on the surfaces was limited to several cycles of voltammetry.

Structural Characterization of Supported RhI(CO)2/γ-Al2O3Catalysts by Periodic DFT Calculations

Microscopic structures of monodispersed rhodium dicarbonyl species chemisorbed on a ceramic metal-oxide support (?-alumina) have been obtained by density functional theory (DFT) calculations with periodic boundary conditions applied. Several minimum energy structures of species were obtained and their relative energies indicate that, in the most energetically stable geometry, the rhodium atom is coordinated in a square-planar environment and forms a four-membered Rh–O–Al–O ring, with one Al atom octahedrally coordinated. Another docking geometry, close lying in energy, also has a square-planar coordination for the rhodium atom and involves a six-membered Rh–O–Al–O–Al–O ring with one Al octahedrally coordinated and one Al tetrahedrally coordinated. Computed bond lengths were found to be in reasonable agreement with experimental bond lengths as determined by EXAFS spectroscopy. Theoretical Rh K-edge XANES spectra suggest that the pre-edge region probes electronic states localized on the RhI(CO)2 unit, while postedge features probe the electronic states arising from the RhI(CO)2 interaction with the support, which partly depends on the docking geometry of the RhI(CO)2 units.

An X-ray absorption spectroscopic investigation of the geometry of Pt(IV) in H2PtCl6 ammonia solution

The geometrical structure of noble metal cations has an important impact in many areas. Therefore, the structure of some noble cations in solution has been investigated in order to understand the relationship between the structure and property of noble metal cations in solution. In this contribution, the coordination chemistry of H2PtCl6 in ammonia solution was investigated by X-ray absorption spectroscopy (XAS). We found that Cl ligands around Pt with a Pt–Cl bond length of 2.32Å are completely replaced by NH3 ligands with a Pt–N distance of 2.05Å. Theoretical calculations point out the occurrence of both a Pt–Cl and a Pt–N orbital hybridization. The observed ‘hybridization peak’ and the post-edge feature have been assigned to the contribution of Cl and N d-states.

An x-ray absorption spectroscopy study of Ni–Mn–Ga shape memory alloys

The austenite to martensite phase transition in Ni–Mn–Ga ferromagnetic shape memory alloys was studied by extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectroscopy. The spectra at all the three elements’, namely, Mn, Ga and Ni, K-edges in several Ni–Mn–Ga samples (with both Ni and Mn excess) were analyzed at room temperature and low temperatures. The EXAFS analysis suggested a displacement of Mn and Ga atoms in opposite direction with respect to the Ni atoms when the compound transforms from the austenite phase to the martensite phase. The first coordination distances around the Mn and Ga atoms remained undisturbed on transition, while the second and subsequent shells showed dramatic changes indicating the presence of a modulated structure. The Mn rich compounds showed the presence of antisite disorder of Mn and Ga. The XANES results showed remarkable changes in the unoccupied partial density of states corresponding to Mn and Ni, while the electronic structure of Ga remained unperturbed across the martensite transition. The post-edge features in the Mn K-edge XANES spectra changed from a double peak like structure to a flat peak like structure upon phase transition. The study establishes strong correlation between the crystal structure and the unoccupied electronic structure in these shape memory alloys.

Probing the Structure of Salt Water under Confinement with First-Principles Molecular Dynamics and Theoretical X-ray Absorption Spectroscopy.

We investigated the structure of liquid water around cations (Na(+)) and anions (Cl(-)) confined inside of a (19,0) carbon nanotube with first-principles molecular dynamics and theoretical X-ray absorption spectroscopy (XAS). We found that the ions preferentially reside near the interface between the nanotube and the liquid. Upon confinement, the XAS signal of water molecules surrounding Na(+) exhibits enhanced pre-edge and reduced post-edge features with respect to that of pure water, at variance with the solvation shell of Na(+) in bulk water. Conversely, the first solvation shell of confined Cl(-) has a main-edge intensity comparable to that of bulk solvated Cl(-), likely as a result of a high number of acceptor hydrogen bonds in the first solvation shell. Confined nonsolvating water molecules exhibit bulk-like or water-monomer-like properties, depending on whether they belong to core or interfacial layers, respectively.

Cations Strongly Reduce Electron-Hopping Rates in Aqueous Solutions

We study how the ultrafast intermolecular hopping of electrons excited from the water O1s core level into unoccupied orbitals depends on the local molecular environment in liquid water. Our probe is the resonant Auger decay of the water O1s core hole (lifetime ∼3.6 fs), by which we show that the electron-hopping rate can be significantly reduced when a first-shell water molecule is replaced by an atomic ion. Decays resulting from excitations at the O1s post-edge feature (∼540 eV) of 6 m LiBr and 3 m MgBr(2) aqueous solutions reveal electron-hopping times of ∼1.5 and 1.9 fs, respectively; the latter represents a 4-fold increase compared to the corresponding value in neat water. The slower electron-hopping in electrolytes, which shows a strong dependence on the charge of the cations, can be explained by ion-induced reduction of water-water orbital mixing. Density functional theory electronic structure calculations of solvation geometries obtained from molecular dynamics simulations reveal that this phenomenon largely arises from electrostatic perturbations of the solvating water molecules by the solvated ions. Our results demonstrate that it is possible to deliberately manipulate the rate of charge transfer via electron-hopping in aqueous media.

Phosphorus K-edge XANES spectroscopy of mineral standards.

Phosphorus K-edge X-ray absorption near-edge structure (XANES) spectroscopy was performed on phosphate mineral specimens including (a) twelve specimens from the apatite group covering a range of compositional variation and crystallinity; (b) six non-apatite calcium-rich phosphate minerals; (c) 15 aluminium-rich phosphate minerals; (d) ten phosphate minerals rich in either reduced iron or manganese; (e) four phosphate minerals rich in either oxidized iron or manganese; (f) eight phosphate minerals rich in either magnesium, copper, lead, zinc or rare-earth elements; and (g) four uranium phosphate minerals. The identity of all minerals examined in this study was independently confirmed using X-ray powder diffraction. Minerals were distinguished using XANES spectra with a combination of pre-edge features, edge position, peak shapes and post-edge features. Shared spectral features were observed in minerals with compositions dominated by the same specific cation. Analyses of apatite-group minerals indicate that XANES spectral patterns are not strongly affected by variations in composition and crystallinity typical of natural mineral specimens.

A XANES calibration for determining the oxidation state of iron in mantle garnet

Iron K-edge X-ray absorption near edge structure (XANES) spectra were recorded for synthetic garnets of the almandine–skiagite (Fe 3Al 2Si 3O 12–Fe 3Fe 2Si 3O 12), andradite–skiagite (Ca 3Fe 2Si 3O 12–Fe 3Fe 2Si 3O 12), Fe–knorringite–skiagite (Fe 3Cr 2Si 3O 12–Fe 3Fe 2Si 3O 12), and andradite–grossular (Ca 3Fe 2Si 3O 12–Ca 3Al 2Si 3O 12) solid solutions. Fe 3+/ΣFe varied systematically in these samples from 0 to 1. The variation in the energy and intensity of spectral features as a function of Fe 3+/ΣFe was investigated to identify correlations that could be used to determine Fe 3+/ΣFe of unknowns. The pre-edge energy, which is commonly used for quantification, was found to be relatively insensitive for garnet, particularly at low values of Fe 3+/ΣFe. The best correlation was for the absorption edge energy, which may provide an accurate and precise method for determining Fe 3+/ΣFe of garnets in metapelitic rocks. The resulting calibration curve, however, significantly and non-systematically overestimates the Fe 3+/ΣFe value of mantle garnets (from xenoliths and megacrysts) for which Fe 3+/ΣFe had been determined previously by Mössbauer spectroscopy. This is probably a result of differences in composition between the synthetic and natural garnets. For the mantle garnets, Fe 3+/ΣFe is strongly correlated with the intensity ratio of post-edge features at 7138.4 and 7161.7 eV. The deviation of the Mössbauer results from a linear fit is less than 0.01 (1 σ). This suggests a new method for determining Fe 3+/ΣFe from XANES spectra that does not require precise energy calibration or spectral fitting. The accuracy and precision for mantle garnets (within the compositional range studied: 0.75 ≤ Mg/(Mg + Fe) ≤ 0.86, 3.7–6.3 wt.% CaO, and 0.3–7.4 wt.% Cr 2O 3) is comparable to those of Mössbauer spectrosopy, however, XANES spectra can be acquired in ~ 15 min, allowing large numbers of samples to be analysed or the distribution of Fe 3+/ΣFe to be mapped with micron spatial resolution.

Local Effects in the X-ray Absorption Spectrum of Salt Water

Both first-principles molecular dynamics and theoretical X-ray absorption spectroscopy have been used to investigate the aqueous solvation of cations in 0.5 M MgCl(2), CaCl(2), and NaCl solutions. We focus here on the species-specific effects that Mg(2+), Ca(2+), and Na(+) have on the X-ray absorption spectrum of the respective solutions. For the divalent cations, we find that the hydrogen-bonding characteristics of the more rigid magnesium first-shell water molecules differ from those in the more flexible solvation shell surrounding calcium. In particular, the first solvation shell water molecules of calcium are able to form acceptor hydrogen bonds, and this results in an enhancement of a post-edge peak near 540 eV. The absence of acceptor hydrogen bonds for magnesium first shell water molecules provides an explanation for the experimental and theoretical observation of a lack of enhancement at the post-main-edge peak. For the sodium monovalent cation we find that the broad tilt angle distribution results in a broadening of postedge features, despite populations in donor-and-acceptor configurations consistent with calcium. We also present the reaveraged spectra of the MgCl(2), CaCl(2), and NaCl solutions and show that trends apparent with increasing concentration (0.5, 2.0, 4.0 M) are consistent with experiment. Finally, we examine more closely both the effect that cation coordination number has on the hydrogen-bonding network and the relative perturbation strength of the cations on lone pair oxygen orbitals.

Geometry of Pt(IV) in H 2PtCl 6 aqueous solution: An X-ray absorption spectroscopic investigation

The noble metal ions play an important role in many chemical reactions, but at the present time they represent also potentially new environmental contaminants. There is relatively little information available to adequately assess the potential health hazards, so that to evaluate the potential hazards and identify the necessary actions to reduce the risks associated with exposure to these metals and their compounds it is important to understand the local structure around noble metal ions. In this contribution, the local coordination around platinum (IV) ions e.g., Pt 4+ in aqueous solution, has been investigated by using X-ray absorption spectroscopy (XAS). X-ray absorption near-edge spectra (XANES) of both [PtCl 6] 2− and [PtCl 4(OH) 2] 2− in an aqueous solution have been calculated using FEFF8.2 and both are characterized by an octahedral geometry. From these calculations, we may also assign a characteristic post-edge feature to a contribution of Cl d-states. From the EXAFS analysis we also determined the corresponding Pt bond distances, e.g., 2.33 Å for the Pt–Cl distance and 2.03 Å for the Pt–O distance in these aqueous solutions. The same analysis provides evidence that the peaks in the Fourier transform at about 4.0 Å are due to multiple scattering collinear Cl–Pt–Cl contributions.

The blue colouring of beryls from Licungo, Mozambique: an X-ray absorption spectroscopy study at the iron K-edge

AbstractThe colours of natural and synthetic beryl (ideally Be3Al2Si6O18) have been attributed either tothe presence of chromophore ions partially replacing Al and/or Be (Cr3+, Fe2+/3+, Mn2+) or to the occurrence of colour centres related to the presence of multi-atomic groups. However, no full explanation has been proposed for the blue colour in natural beryl gemstones (aquamarine) despite it being well established that bluish-green synthetic aquamarines containing Fe turn blue when heated for 1 h at 400°C.A X-ray absorption near-edge spectroscopy (XANES) study at the Fe K-edge was performed on blue beryl crystals from Licungo pegmatite (Mozambique) to ascertain the speciation state of this colouring element, reported in blue sapphire (ideally Al2O3) as a mixture of Fe2+ and Fe3+ in octahedral coordination. The general trend of the spectra is the same when synchrotron radiation (SR) impinges along the c axis of the hexagonal crystal, as well as along the a axis with c vertical. Conversely, a shoulder in the absorption edge is noticed when the incident beam is perpendicular to c, with this axis horizontal in the polarization plane of the SR beam, a reverse in the relative intensities of the two post-edge features is simultaneously observed. Pre-edge features accounting for Fe speciation (valence plus coordination) are identical in all three situations. Considering previous approaches which account for polarization, an explanation is suggested for the special effects observed on the Fe K-edge XANES spectra of analysed blue beryl crystals. Ideas on the origin of the blue colour of this mineral are also presented.

Effect of epitaxial strain on the exchange-bias properties of[La0.67Ca0.33MnO3∕La0.33Ca0.67MnO3]15multilayers: Resonant x-ray scattering measurements

Complementary x-ray synchrotron radiation diffraction (XRD) and resonant scattering (RXS) measurements were performed at the $\mathrm{Mn}\phantom{\rule{0.3em}{0ex}}K$-edge between 10 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in order to analyze the effect of epitaxial strain and tetragonal lattice distortions on the exchange bias (EB) mechanism observed in $[{\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}{\mathrm{MnO}}_{3}(\mathrm{FM})∕{\mathrm{La}}_{1∕3}{\mathrm{Ca}}_{2∕3}{\mathrm{MnO}}_{3}(\mathrm{AF}){]}_{15}$ multilayers below a blocking temperature, ${T}_{B}$, of $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. XRD measurements showed that the $c∕a$ axial ratio, an indication of the tetragonal lattice distortion in pseudocubic lattice settings, reaches its maximum at the onset of the EB effect and the corresponding structural correlation length varies substantially at the onset of ${T}_{B}$. The in-plane lattice parameter $a$ at ${T}_{B}$ is close to the bulk lattice parameters of the antiferromagnetic (AF) layers, thus indicating that the EB effect is related with the accommodation of strain inside the ferromagnetic (FM) and AF layers. RXS measurements revealed a ferrodistortive (FD) state. The RXS intensity difference signal exhibits a main-edge feature and a postedge feature at $\ensuremath{\sim}6.57\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ that scales linearly with temperature and the $c∕a$ ratio up to $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The disappearance of the postedge feature above $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}(={T}_{B})$ may signify a rearrangement of $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Mn}$ bonding angles due to strain-driven effects at the FM/AF interfaces, inducing disorder in FD octahedral tilt ordering which may pin the local distortions below the ${T}_{B}$.

Electronic states of LaCoO3: Co K-edge and La L-edge x-ray absorption studies

Room-temperature Co K-edge and La L-edge x-ray absorption studies have been carried out onLaCoO3. Experimental near-edge structures have been analysed by theoreticalLDA+U density of states (DOS) and multiple scattering (MS) calculations. Use of both MSand DOS calculations yields additional information about hybridization of thestates of the central atom with neighbouring atoms responsible for producingthe near-edge structures. Absorption processes at the Co K-edge, and the LaL1-,L2-,and L3-edges have been attributed to electronic transitions from Co 4p, La 6p, La 5d, and La 5d, respectively. All the pre-edge and post-edge features including the shape of the mainabsorption edge have been generated by taking the convolution of the calculatedDOS, indicating that single particle approximation is sufficient to express allexperimentally observed major structures. Two pre-edge structures observed in theCo K-edge spectrum are attributed to Co and quadrupole transitions in contrast to earlier identification of the same to Co and eg transitions. The influence of La 6p states on the Co 4p states is such that the inclusion of Laatoms in the MS calculations is necessary to generate post-edge structures in the Co K-edgespectrum. The importance of the hybridization of the O 2p state with La 6p and 5d in theL-edge absorption processes is also discussed. A 10% contribution of the quadrupolechannel has been estimated in the La L-edges.

Redox reaction of iodine in paddy soil investigated by field observation and the I K-Edge XANES fingerprinting method

In order to elucidate the cause for the leaching of iodine in a flooded paddy field, we investigated the transformation of an iodine species affected by the water management of the paddy field. The increased concentration of iodide (I −) in soil solution of a flooded paddy field suggested that I − was leached from the soil under anaerobic conditions. The post-edge feature of X-ray absorption near-edge structure (XANES) for iodate (IO 3 −) spiked to soil totally disappeared after anaerobic incubation of the soils, and I − was dissolved in the solution. On the other hand, I − in contact with the soil was not likely to be oxidized to IO 3 − under aerobic incubation. Iodine was leached out in soil solution as I − under anaerobic conditions, whereas part of the iodine species was retained by soil as I 2 or organoiodine both under anaerobic and aerobic conditions.

Soft X-ray absorption at the O K-edge in rutile-type natural oxides

An X-ray absorption study on O 1s edge was carried out on natural oxides with rutile-type crystal structure – the minerals rutile (TiO2), pyrolusite (MnO2), cassiterite (SnO2) and plattnerite (β-PbO2). Despite the chemical bonding being essentially ionic, the differences in the outer electron shell of cations clearly influence post-edge features of XAS spectra. The implications of site symmetry on coordination environment are emphasized for [MO6] octahedra (with quite regular geometry in cassiterite) and for the isosceles triangle [OM3] that displays either two long plus one shorter (pyrolusite and plattnerite) or two short plus one longer (rutile and cassiterite) M–O distances, with M–O–M bond angles always close to 100° plus twice 130°.Theoretical modelling of XAS spectra was carried out. Details of O 1s absorption spectra like the presence of a single post-edge feature for plattnerite and cassiterite may be interpreted by the separation or mixing between O 2p and M nd plus mixing with M ns and np states. Spectra features are commented in relation to crystal structure details but the energy shift assigned for the O K-edge in cassiterite still lacks a plausible explanation.