XANES

About: XANES is a research topic. Over the lifetime, 7737 publications have been published within this topic receiving 188032 citations.

Simultaneous Presence of Cationic and Reduced Gold in Functioning MgO-Supported CO Oxidation Catalysts: Evidence from X-ray Absorption Spectroscopy

Abstract: Gold clusters on the surface of MgO powder (calcined at 673 K) were prepared from adsorbed [Au(CH3)2(acac), where acac is C5H7O2] and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES). One sample initially contained gold predominantly in the form of clusters approximated as Au6 on the basis of the EXAFS data showing first- and second-shell Au−Au coordination numbers of 4.0 ± 0.4 and 1.0 ± 0.1, respectively. The other sample initially contained larger clusters, with an average diameter of about 30 A (containing about 100 atoms each, on average), as shown by the EXAFS first- and second-shell Au−Au coordination numbers of 9.4 ± 0.9 and 3.5 ± 0.4, respectively. The samples, in each of the three gases CO, O2, and He and in the presence of CO + O2 during CO oxidation catalysis, were investigated by EXAFS spectroscopy and XANES in a cell that was also a flow reactor. Data obtained during steady-state CO oxidation indicate the presen...

Soft X-ray radiation-damage studies in PMMA using a cryo-STXM.

Abstract: Radiation damage sets a fundamental limit for studies with ionizing radiation; cryo-methods are known to ease these limits. Here, measurements on mass loss and the decrease in the C=O bond density as measured by oxygen-edge XANES (NEXAFS) spectroscopy in thin films of poly(methylmethacrylate) (PMMA), studied in a vacuum, are reported. While cryo-methods allow more than 95% of the mass to remain at doses up to 10^7 Gy, there is little difference in C=O bond density versus dose between 298 K and 113 K sample temperatures. At both temperatures the critical dose for bond breaking is ∼15 × 106 Gy.

Xanes spectroscopy of sulfur in earth materials

Abstract: X-ray absorption near-edge structure (XANES) spectroscopy is the ideal non-destructive technique for characterizing and quantifying S species in compositionally complex natural materials such as silicate glasses and minerals, coals, asphalts and asphaltenes, kerogens and humic substances. Sulfur absorption edges represent the transition of S 1 s and 2 p core electrons to unoccupied antibonding orbitals at the bottom of the conduction band. Shifts in the position of the absorption-edge feature of S K - and L -edge XANES spectra constitute a chemical ruler for oxidation state of both inorganic and organic species of S; the S K edge shifts from 2469.5 eV for chalcopyrite (2− oxidation state) to 2482 eV for gypsum (6+). However, chemical state of S in Earth materials is most readily assigned by comparing the overall XANES profile with spectra for reference compounds. Sulfur XANES spectra are reviewed for pyrite, troilite, pyrrhotite and NiAs-type Co 0.923 S and Ni 0.923 S, niningerite (MgS), oldhamite (CaS), alabandite (MnS) and cubic FeS, and sphalerite and related phases, as well as for selected solid-solutions of the monosulfides. Sulfur XANES spectra for FeS, CoS, NiS, MgS, CaS, MnS and ZnS have been simulated by multiple scattering calculations. The S K -edge XANES of transition-metal-bearing monosulfides generally show anomalous absorption consistent with hybridization of the final S 3 p σ* antibonding states with empty 3 d orbitals on the metal atoms. Various applications of S K - and L -edge XANES fingerprinting are discussed, including speciation of inorganic S in basaltic glasses, lazurite and hauyne, identification of organic functional groups of S in coals, kerogens and humic substances extracted from subtropical soils and marine sediments, and the association of sulfated sugars with calcification of coral aragonite skeletons.

Quantitative organic and light-element analysis of comet 81P/Wild 2 particles using C-, N-, and O-μ-XANES

Abstract: Synchrotron-based soft X-ray micro-analysis was performed on particles extracted from the Stardust aerogel collector in order to obtain detailed organic functional group information on any organic solids captured as part of the Principal Examination suite of analyses for samples from comet 81P/Wild 2. It is observed that cometary organic carbon captured in aerogel is present in a number of different manifestations and often intimately associated with silicates. Carbon X-ray absorption near edge structure (XANES) spectra reveal considerable chemical complexity in all of the organic particles studied so far. Universally, the comet 81P/Wild 2 organic particles contain low concentrations of aromatic and/or olefinic carbon relative to aliphatic and heteroatom-containing functional groups, e.g., amide, carboxyl, and alcohol/ethers. N-XANES confirms the presence and assignments of these functional groups. In general, the XANES data record considerable chemical complexity across the range of organic samples currently analyzed. The atomic ratios, N/C and O/C, derived from XANES data reveal a wide range in heteroatom content; in all cases these elemental ratios are higher than that of primitive meteoritic organic matter. The wide range in chemistry, both in elemental abundances and specific organic functional groups, suggests that the comet 81P/Wild 2 organic solids may have multiple origins.

A XANES determination of the oxidation state of chromium in silicate glasses

Abstract: Cr K -edge X-ray absorption near-edge structure (XANES) spectra were recorded for silicate glasses of various compositions in the system CaO + MgO + Al2O3 + SiO2 ± TiO2, quenched from melts equilibrated as a function of oxygen fugacity ( f O2) at 1400°C. The spectra vary systematically with f O2 (log f O2 ~0 to −16) indicating changes in the Cr oxidation state. The intensity of a shoulder on the absorption edge (attributed to the 1 s → 4 s transition) was quantified using the corresponding peak in the XANES derivative spectrum and used to determine Cr2+/∑Cr. The resulting Cr2+/∑Cr values are in agreement with the theoretical dependence on log f O2 suggesting that the 1s → 4s feature is diagnostic of Cr2+ in a silicate glass and σ(Cr2+/∑Cr) = ± 0.015. The Cr2+/∑Cr ratio for a given log f O2 changes with the glass composition which may reflect the ability of the melt to accommodate the Jahn-Teller distorted coordination geometry which stabilizes Cr2+. Cr2+/∑Cr varies between ~0.3 and 0.8 over the log f O2 range bounded by the Ni/NiO and Fe/FeO f O2 buffers, suggesting that Cr2+ may be important in natural melts even though this oxidation state has never been identified in a terrestrial material. The development of a methodology for determining Cr2+/∑Cr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature.