Seuils d'absorption des rayons X : un outil de caractérisation indispensable en chimie de coordination.

National audienceSynchrotron radiation allows to perform spectroscopic experiments in an energy range from infra-red to hard X-rays. The paper shows, through particular examples (role of the geometry, of the oxidation and spin state of the absorber ...) how it is possible to extract unique informations from X-ray absorption edges to characterize the electronic structure of the absorbing elements whatever the state of the sample (crystalline, amorphous, liquid, solution, gaseous). The interpretation of the data is proposed at a double level : qualitative, thanks to symmetry theory and more quantitative, thanks to quantum calculations

Multivariate Analysis of Coupled Operando EPR/XANES/EXAFS/UV–Vis/ATR-IR Spectroscopy: A New Dimension for Mechanistic Studies of Catalytic Gas-Liquid Phase Reactions

Operando EPR, XANES/EXAFS, UV-Vis and ATR-IR spectroscopic methods have been coupled for the first time in the same experimental setup for investigation of unclear mechanistic aspects of selective aerobic oxidation of benzyl alcohol by a Cu/TEMPO catalytic system (TEMPO=2,2,6,6-tetramethylpiperidinyloxyl). By multivariate curve resolution with alternating least-squares fitting (MCR-ALS) of simultaneously recorded XAS and UV-Vis data sets, it was found that an initially formed (bpy)(NMI)CuI- complex (bpy=2,2′-bipyridine, NMI=N-methylimidazole) is converted to two different CuII species, a mononuclear (bpy)(NMI)(CH3CN)CuII-OOH species detectable by EPR and ESI-MS, and an EPR-silent dinuclear (CH3CN)(bpy)(NMI)CuII(μ-OH)2⋅CuII (bpy)(NMI) complex. The latter is cleaved in the further course of reaction into (bpy)(NMI)(HOO)CuII-TEMPO monomers that are also EPR-silent due to dipolar interaction with bound TEMPO. Both Cu monomers and the Cu dimer are catalytically active in the initial phase of the reaction, yet the dimer is definitely not a major active species nor a resting state since it is irreversibly cleaved in the course of the reaction while catalytic activity is maintained. Gradual formation of non-reducible CuII leads to slight deactivation at extended reaction times. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

XANES study of rhenium oxide compounds at the L1 and L3 absorption edges

8 pagesInternational audienceWe report on the study of a set of rhenium oxide reference compounds (NH4ReO4, NaReO4, ReO3, ReO2, and Re2O7) using x-ray-absorption near-edge structure. The parallel use of Re L1 and L3 absorption edges permits a concomitant understanding of both the oxidation state and the local symmetry for these compounds. Experiments are compared with ab initio simulations. A good agreement is reached in most cases. The choice of the cluster size and the calculation method (full potential or not), which are mandatory ingredients allowing a satisfactory reproduction of the recorded spectra, is discussed in detail. In the meantime, these parameters give important pieces of information on the studied materials

X-ray-Absorption Spectral Study of the R₂Fe₁₇₋ₓMₓ Solid Solutions (R=Ce, Nd and M=Al, Si)

The x-ray-absorption near-edge structure (XANES) spectra obtained at the cerium LIII edge of the Ce2Fe17-xAlx solid solutions and Ce2Fe14Si3, show two absorption peaks characteristic of the 4f1 and 4f0 configurations of cerium, peaks which indicate that cerium is in a mixed valent state in these compounds. All the XANES spectra have been consistently and excellently fit with one sigmoidal function and two Gaussianbroadened Lorentzian functions. The cerium spectroscopic valence obtained from the relative areas of the two peaks decreases from 3.64 to 3.43 between x=0 and 9 in Ce2Fe17-x,Alx, and correlates linearly with the cerium site volume This correlation confirms that the cerium valence is strongly dependent upon steric effects. In contrast the cerium valence obtained from the XANES spectrum of Ce2Fe14Si3 is not determined by stenc effects and indicates, in agreement with other measurements and calculations, that silicon is more covalently bonded with its near-neighbor cerium atoms than is aluminum. The neodymium LIII,-edge XANES spectra of the Nd2Fe17-x Alx solid solutions, where x is 0, 3, and 8, reveal the presence of only trivalent neodymium and an increase of the empty 5d state density when aluminum is substituted in place of iron. XANES measurements at the iron K edee of the Ce2Fe17-xAlx, and Nd2Fe17-1Alx, solid solutions show changes m the relative intensity of the multiple scattering peaks, changes which are related to the changing composition of the first three neighbor shells with increasing aluminum content

Evolution of iron speciation during hydration of C4 AF

International audienceIt is now well accepted and demonstrated that calcium silicate, calcium aluminate and calcium sulfo aluminate (ettringite, AFm) phases exhibit a good capability to fix metals and metalloids. Unfortunately the role of minor phases and especially calcium-ferric aluminate phase, shorthand C(4)AF is not well defined. In other systems like in soils or sediments iron phases play a key role in the fixation of pollutant. In cement sorption isotherms, indicated that various metals can be retained by the C(4)AF hydrated products. Therefore the capabilities of those phase to retain heavy metal should not be neglected. Previous investigations have shown that the minerals formed during the hydration of C(4)AF are similar to those formed from C3A (pure tri-calcium aluminate) under comparable conditions. Nevertheless no investigation was conducted at the molecular level and there is still a controversy whether Fe substitutes for Al in the hydrated minerals in whole or in part, or if it forms FeOOH clusters scattered throughout the matrix. In this context we have conducted XAS experiments using synchrotron radiation. It was found that the hydration of C(4)AF forms C(3)AH(6) (hydrogarnet) in which Fe randomly substitutes for Al as well as an amorphous FeOOH phase. Intermediate products like AFm (i.e., an ill organized lamellar phase) are also formed but rapidly evolve to C(3)AH(6); iron does not seem to be incorporated in the AFm structure

Local order around rare earth ions during the devitrification of oxyfluoride glasses.

International audienceErbium L(3)-edge extended x-ray absorption fine structure (EXAFS) measurements were performed on rare earth doped fluorosilicate and fluoroborate glasses and glass ceramics. The well known nucleating effects of erbium ions for the crystallization of cubic lead fluoride (based on x-ray diffraction measurements) and the fact that the rare earth ions are present in the crystalline phase (as indicated by Er(3+) emission spectra) seem in contradiction with the present EXAFS analysis, which indicates a lack of medium range structural ordering around the Er(3+) ions and suggests that the lead fluoride crystallization does not occur in the nearest neighbor distance of the rare earth ion. Molecular dynamics simulations of the devitrification process of a lead fluoride glass doped with Er(3+) ions were performed, and results indicate that Er(3+) ions lower the devitrification temperature of PbF(2), in good agreement with the experimental results. The genuine role of Er(3+) ions in the devitrification process of PbF(2) has been investigated. Although Er(3+) ions could indeed act as seeds for crystallization, as experiments suggest, molecular dynamics simulation results corroborate the experimental EXAFS observation that the devitrification does not occur at its nearest neighbor distance