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.

Respective Role of Fe and Mn Oxide Contents for Arsenic Sorption in Iron and Manganese Binary Oxide: An X-ray Absorption Spectroscopy Investigation

Abstract: In our previous studies, a synthesized Fe-Mn binary oxide was found to be very effective for both As(V) and As(III) removal in aqueous phase, because As(III) could be easily oxidized to As(V). As(III) oxidation and As(V) sorption by the Fe-Mn binary oxide may also play an important role in the natural cycling of As, because of its common occurrence in the environment. In the present study, the respective role of Fe and Mn contents present in the Fe-Mn binary oxide on As(III) removal was investigated via a direct in situ determination of arsenic speciation using X-ray absorption spectroscopy. X-ray absorption near edge structure results indicate that Mn atoms exist in a mixed valence state of +3 and +4 and further confirm that MnOx (1.5 < x < 2) content is mainly responsible for oxidizing As(III) to As(V) through a two-step pathway [reduction of Mn(IV) to Mn(III) and subsequent Mn(III) to Mn(II)] and FeOOH content is dominant for adsorbing the formed As(V). No significant As(III) oxidation by pure FeOOH had been observed during its sorption, when the system was exposed to air. The extended X-ray absorption fine structure results reveal that the As surface complex on both the As(V)- and As(III)-treated sample surfaces is an inner-sphere bidentate binuclear corner-sharing complex with an As-M (M = Fe or Mn) interatomic distance of 3.22-3.24 angstrom. In addition, the MnOx and FeOOH contents exist only as a mixture, and no solid solution is formed. Because of its high effectiveness, low cost, and environmental friendliness, the Fe-Mn binary oxide would play a beneficial role as both an efficient oxidant of As(III) and a sorbent for As(V) in drinking water treatment and environmental remediation.

Fischer–Tropsch synthesis: study of the promotion of Re on the reduction property of Co/Al2O3 catalysts by in situ EXAFS/XANES of Co K and Re LIII edges and XPS

Abstract: A direct relationship between the Fischer–Tropsch synthesis (FTS) rate and the number of surface cobalt atoms available for reaction is usually obtained. For Co/Al2O3 catalysts in particular, the site density depends on two primary factors: (1) the average size of the cobalt clusters on the support; and (2) the fraction of cobalt reduced to the metallic state. The addition of small amounts of noble metal promoters, such as Pt and Ru to cobalt alumina, may catalyze the reduction of cobalt oxide shifting the temperature of reduction of both steps (Co3O4→CoO and CoO→Co0) to lower temperatures. However, Re affects only the second step. Re is reduced at a higher temperature than Pt or Ru, and at approximately the same temperature as the first step of cobalt reduction (Co3O4→CoO). Thus, Re metal is present to catalyze only the second step. In situ extended X-ray absorption fine structure (EXAFS) at the LIII edge of Re has been used to show that there is direct contact of Re with cobalt atoms, while evidence for ReRe bonds is not observed. Even though direct atom-to-atom contact is found, temperature-programmed reduction (TPR) suggests that hydrogen spillover from the promoter to cobalt oxide clusters is important for the catalysis of cobalt oxide reduction. In situ EXAFS at the K edge of Co shows that the average cobalt cluster size decreases with increasing Re loading. Re promotes reduction of smaller species which interact with the support, and therefore, for a given reduction temperature, the average cobalt metal cluster size decreases as a function of increasing Re content. After reduction at a temperature slightly above the first peak in the TPR (Co3O4→CoO), the species remaining on the surface displayed XANES spectra identical to that of CoO. After reduction at a temperature above the second broad TPR peak, XPS showed that a residual oxide species was present, with a binding energy equivalent to cobalt aluminate.

Extended X-ray absorption fine structure determination of the structure of cobalt in carbon-supported Co and Co-Mo sulfide hydrodesulfurization catalysts

Abstract: The structure of the cobalt present in carbon-supported Co and Co-Mo sulfide catalysts was studied by means of X-ray absorption spectroscopy at the Co K-edge and by X-ray photoelectron spectroscopy (XPS). Thiophene hydrodesulfurization activities were used to measure the catalytic properties of these catalysts. By comparison of the EXAFS and XANES spectra of the catalysts with those of c09sS and Cos2 model compounds, it was concluded that all Co atoms in a catalyst prepared with nitrilotriacetic acid as complexing agent were in the "Co-Mo-S" state, while the Co atoms in a conventionally prepared catalyst were partly present in a CO$8-like structure and partly in a "Co-Mo-S" structure. The Co atoms in the To-Mc-S" state have a distorted 5- to 6-fold sulfur coordination, and on the average, every Co atom is in contact with two Mo atoms at a distance of 2.80 A. On the basis of these data, the most likely position for the Co atoms is in front of the square sulfur faces of the MoS6 trigonal prisms along the edges of the MoS, crystallites with two additional sulfur atoms or H2S molecules attached. The Co atoms in the sulfided Co/C catalyst have Co-S and Co-Co coordinations as in c09sg, although the sulfur coordination number is higher.

Extended-x-ray-absorption-fine-structure study of small Fe molecules isolated in solid neon

Abstract: We have used rare gas matrix isolation techniques in combination with extended x-ray absorption fine structure (EXAFS) to study the variation in interatomic distances for small Fe molecules in solid neon. A considerable contraction in the interatomic distances was observed for the metal molecules. An Fe-Fe distance of 2.02 \ifmmode\pm\else\textpm\fi{} 0.02 \AA{} for the lowest concentration of metal was observed. This is in good agreement with early EXAFS measurements in ${\mathrm{Fe}}_{2}$-Ar. We also carried out a careful study of the x-ray-absorption near-edge structure (XANES), and observed the appearance of considerable structure for a 1.5-at.% Fe sample. The XANES spectra were analyzed in terms of $1s$-to-($3d,4s$) and $1s$-to-$4p$ transitions.