X-Ray Absorption Spectroscopy XAS

XAS has been applied to the study of electrolyte side reactions much less often than XPS. Nonetheless, a few examples exist in the hterature of the kind of insight that can be gathered with this tool [189,192,193]. The possibility of collecting data with two different detectors with significantly different, built-in depth sensitivities affords an opportunity to compare the electrode surface with its bulk in a single experiment. Such measurements on layered transition metal oxides revealed a difference in the formal oxidation state of Ni with sample depth that pointed at the active participation of the transition metals in the decomposition of the electrolyte [194]. 

Many conventional techniques based on X-rays can be applied in grazing inci-dence/exit mode. While they have been developed for many years and their potential is obviously large, only a handful of reports exist of their application to the study of EEIs. They provided a unique insight into the reorganization of the electrode surface and EEI structure in numerous cathode materials [195-198], as well as evaluate the crystallinity of any reaction deposits [199]. This structural reorganization is of critical importance, as it can lead to increased activation barriers for the transfer of ions from the electrolyte to the electrode. Further, because hard X-rays, which penetrate deep into matter, were employed, setups have been proposed and demonstrated to carry out experiments in situ to probe live, electrified interfaces [196]. 

XRR has been applied to the study of EEIs on several systems [201-205]. The technique was found to be sensitive not only to the formation of reaction layers but also to mass loss at the electrode surface due to processes of corrosion (dissolution) [201]. Of particular interest is the application of high energy synchrotron beams as sources, as their deep penetration capabilities enables the design of operando cells (Fig. 7.10a) [203], Therefore, uncertainty due to equilibration in the absence of an electrochemical potential is eliminated. The structural and chemical stability of EEIs during the lithium insertion/extraction processes have thus been evaluated (Fig. 7.10b) [201-204]. The dependence of these irreversible reactions on the crystal facet of the electrode material forming the EEI was established. It was found that electrolyte decomposition processes were coupled with the redox process occurring in the bulk of the electrode, which is a critical piece of information when designing materials that bypass such layer formation. 

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EXAFS is part of the field of X-ray absorption spectroscopy (XAS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternatively called the near-edge X-ray absorption fine structure, NEXAFS). The combination of XANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article following this one for a discussion of SEXAFS and NEXAFS. 

The prepared catalysts were characterized by x-ray diffraction (XRD), N2 adsorption and CO chemisorption. Also, X-ray absorption spectroscopy (XAS) at the Ni K edge (8.333 keV) of reference and catalyst samples was carried out in the energy range 8.233 to 9.283 keV at beamline X18B of the... 

Principles and Characteristics In X-ray absorption spectroscopy (XAS) the absorption coefficient (p.)... 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

X-ray spectroscopic techniques are powerful tools for the analysis of electronic structures of materials. There are two principal techniques, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), both of which have... 

The most prevalent technique exploiting synchrotron radiation is X-ray absorption spectroscopy (XAS, also called X-ray absorption fine structure, XAFS). Two related types of experiments are conducted X-ray absorption near-edge spectroscopy (XANES), which probes the initial absorption edge and related nearby structure, and... 

Bacteria and their composites with soil minerals or organic matter are capable of taking up a wide range and variety of toxic metals in soil environments. Research done over the last decade or so has greatly improved our understanding of the mechanisms on biosorption of metals and bacte-ria-metal-soil component interactions. However, more studies from molecular level are needed in order to enhance the ability of bacteria and their association with soil components to remediate toxic metals-contaminated soils. The focus of future investigations should be on the mechanisms by which metals are sorbed and bound by bacterial cell surfaces and bacteria-soil/mineral composites. In this connection, X-ray absorption spectroscopy (XAS) is a promising technique because it can provide information about... 

X-RAY ABSORPTION SPECTROSCOPY (XAS) AND EXTENDED X-RAY ABSORPTION FINE STRUCTURE (EXAFS)... 

The use of X-ray diffraction from crystalline samples can result in a complete three-dimensional crystal structure of a molecule, but requires a single crystal suitable for proper diffraction (see Section 3.3). X-ray absorption spectroscopy (XAS) can yield limited molecular structural information on noncrystalline (amorphous) solid... 

X-Ray Absorption Spectroscopy (XAS) and Extended X-Ray Absorption Fine Structure (EXAFS), 68... 

Synchotron based techniques, such as surface X-ray scattering (SXS) and X-ray absorption spectroscopy (XAS), have found increased use in characterization of electrocatalysts during electrochemical reactions.37 These techniques, which can be used for characterization of surface structures, require intricate cell designs that can provide realistic electrochemical conditions while acquiring spectra. Several examples of the use of XAS and EXAFS in non-precious metal cathode catalysts can be found in the literature.38 2... 

Most research on the structure of skeletal catalysts has focused on nickel and involved methods such as x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), electron diffraction, Auger spectroscopy, and x-ray photoelectron spectroscopy (XPS), in addition to pore size and surface area measurements. Direct imaging of skeletal catalyst structures was not possible for a long while, and so was inferred from indirect methods such as carbon replicas of surfaces [54], The problem is that the materials are often pyrophoric and require storage under water. On drying, they oxidize rapidly and can generate sufficient heat to cause ignition. 

X-Ray Absorption Spectroscopy (XAS). The XAS measurements were similar to those described elsewhere.Grazing incidence (GI)-XAS measurements were performed at beamline 11-2 at Stanford Synchrotron Radiation Laboratory (SSRL). A double Si(220) crystal spectrometer was used to select the energy of the synchrotron X-rays, and the beam size was set to 400 pm x 2 mm. The bandwidth of the spectrometer was about 1 eV. Routine procedures were used to optimize the positions of the samples so that the angle of incidence was about 0.17°, with the X-ray... 

Recently, however, considerable doubt has been cast on these conclusions. In the case of the putative Mg/Al/Sn - CO3, Mg/Al/Zr - CO3 and Co/Al/Sn - CO3 materials it has been unambiguously shown by X-ray absorption spectroscopy (XAS) and Mossbauer spectroscopy that the tetravalent cations are segregated from the LDH structure and form amorphous oxide-hke particles [69]. It was further demonstrated that the increased values of Uo previously attributed to the introduction of the large cations... 

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