Extended X-ray absorption fine structure

About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.

Reaction-Relevant Gold Structures in the Low Temperature Water-Gas Shift Reaction on Au-CeO2

Abstract: Combined in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies have been conducted to follow gold structural changes of low-content (<1%Au) gold-ceria catalysts in water-gas shift (WGS) reaction tests at 100 and 200 °C; and after heating the used catalysts in oxygen gas at 150 °C. Gold in the fresh (400 °C-calcined) material was atomically dispersed in cerium oxide. Under WGS reaction conditions, reduction of the oxidized gold species was observed, accompanied by gradual gold aggregation. The Au−Au coordination number is zero for the fresh material, but increases with the reaction temperature, to 6.5 ± 2.4 (after use at 100 °C) and to 8.7 ± 1.5 (after 200 °C) in a gas mixture containing 5% CO- 3% H2O in helium. The second important parameter is the reaction gas composition which determines the extent of Au−O reduction. The lower the reduction potential of the reaction gas mixture, the more oxidized the gold is in the used catalyst, and the higher...

EXAFS Study of the Inner Shell Structure in Copper(II) Complexes with Humic Substances

Abstract: The structure of humic substances (HS) and their complexes with metal cations is a matter of ongoing debate. In this paper, the structure of Cu2+−HS complexes was studied by extended X-ray absorption fine structure spectroscopy (EXAFS). This method is highly sensitive to the local structure around the target (e.g., Cu2+ or other cations). The CuO6 octahedron was used to model the inner complexation shell of the Cu2+−HS complexes. The quality of the fit of the EXAFS data was tested using model systems (copper(II)−aqua and −salicylate complexes). On the basis of the dissimilarity of the EXAFS spectra for the Cu2+−tetrahydrofurantetracarboxylic acid complex from those of the Cu2+−HS complexes, it is concluded that complexation sites of HS are not likely to contain carboxylic groups attached to a furan ring. The Cu−O distances in all Cu2+−HS complexes were shorter than those in aqua or salicylate complexes. The contraction of the Cu−O distance was especially prominent for the axial direction, and the distorti...

Possibility of adsorbate position determination using final-state interference effects

Abstract: Novel methods for the determination of adsorbate position on surfaces based on final-state interference effects are discussed. The first is the measurement of extended x-ray absorption fine structure (EXAFS) from excitation of the adsorbed atom by monitoring the intensity of the Auger emission line as a function of photon energy. Such an experiment should be feasible with synchrotron radiation. The sensitivity of surface EXAFS to adatom position is demonstrated by calculations appropriate for adsorption of Se on several crystal faces of Ni. We also examine the possibility of measuring EXAFS by monitoring the photoelectron intensity and conclude that an average over a hemisphere outside the crystal is not sufficient to reduce the photoemission data to EXAFS. Relations between EXAFS and angular-resolved photoemission are elucidated and it is shown that the single scattering EXAFS theory includes second-order scattering from the excited atom in photoemission theory.

Where Water is Oxidized to Dioxygen: Structure of the Photosynthetic Mn4Ca Cluster from X-ray Spectroscopy

Abstract: Light-driven oxidation of water to dioxygen in plants, algae and cyanobacteria is catalyzed within photosystem II (PS II) by a Mn4Ca cluster. Although the cluster has been studied by many different methods, the structure and the mechanism have remained elusive. X-ray absorption and emission spectroscopy and EXAFS studies have been particularly useful in probing the electronic and geometric structure, and the mechanism of the water oxidation reaction. Recent progress, reviewed here, includes polarized X-ray absorption spectroscopy measurements of PS II single crystals. Analysis of those results has constrained the Mn4Ca cluster geometry to a set of three similar high-resolution structures. The structure of the cluster from the present study is unlike either the 3.0 or 3.5 Angstrom-resolution X-ray structures or other previously proposed models. The differences between the models derived from X-ray spectroscopy and crystallography are predominantly because of damage to the Mn4Ca cluster by X-rays under the conditions used for structure determination by X-ray crystallography. X-ray spectroscopy studies are also used for studying the changes in the structure of the Mn4Ca catalytic center as it cycles through the five intermediate states known as the Si-states (i=0-4). The electronic structure of the Mn4Ca cluster has been studied more recently using resonant inelastic X-ray scattering spectroscopy (RIXS), in addition to the earlier X-ray absorption and emission spectroscopy methods. These studies are revealing that the assignment of formal oxidation states is overly simplistic. A more accurate description should consider the charge density on the Mn atoms that includes the covalency of the bonds and delocalization of the charge over the cluster. The geometric and electronic structure of the Mn4Ca cluster in the S-states derived from X-ray spectroscopy are leading to a detailed understanding of the mechanism of the O-O bond formation during the photosynthetic water splitting process.