Energy-dispersive EXAFS

Newton MA, Jyoti B, Dent AJ, Fiddy SG, Evans J. Synchronous, time resolved, diffuse reflectance FT-IR, energy dispersive EXAFS (EDE) and mass spectrometric investigation of the behaviour of Rh catalysts during NO reduction by CO. Chem Commun. 2004 2382. 

Dent, A. I. (2002). Development of time-resolved XAFS insti umentation for quick EXAFS and energy-dispersive EXAFS measurements on catalyst systems. Top. Catal. 18, 27-35. 

FIGURE 12. A schematic diagram of the energy-dispersive EXAFS configuration. 

Linear absorption measurements can therefore give the first indication of possible alloy formation. Nevertheless, in systems containing transition metals (Pd-Ag, Co-Ni,. ..) such a simple technique is no longer effective as interband transitions completely mask the SPR peak, resulting in a structurless absorption, which hinders any unambiguous identification of the alloy. In such cases, one has to rely on structural techniques like TEM (selected-area electron diffraction, SAED and energy-dispersive X-ray spectroscopy, EDS) or EXAFS (extended X-ray absorption fine structure) to establish alloy formation. 

Table 5.2 Summary of selected analytical methods for molecular environmental geochemistry. AAS Atomic absorption spectroscopy AFM Atomic force microscopy (also known as SFM) CT Computerized tomography EDS Energy dispersive spectrometry. EELS Electron energy loss spectroscopy EM Electron microscopy EPR Electron paramagnetic resonance (also known as ESR) ESR Electron spin resonance (also known as EPR) EXAFS Extended X-ray absorption fine structure FUR Fourier transform infrared FIR-TEM Fligh-resolution transmission electron microscopy ICP-AES Inductively-coupled plasma atomic emission spectrometry ICP-MS Inductively-coupled plasma mass spectrometry. Reproduced by permission of American Geophysical Union. O Day PA (1999) Molecular environmental geochemistry. Rev Geophysics 37 249-274. Copyright 1999 American Geophysical Union...

Several years ago, it was demonstrated that time-resolved EXAFS measurements could be performed by use of the energy-dispersive method (41),... 

Most of the routine work in structural analysis is performed with D5000 Siemens diffractometer equipped with a Gobel mirror and an energy-dispersive detector. Raman micro-spectrometry has been recently introduced with a Labram infinity spectrometer with two laser sources, fitted with a horizontal output adapted to the investigation of vertical items like paintings or statues. For the most fine structural investigations, experiments are conducted with EXAFS, XANES or diffraction lines from various synchrotron facilities (ESRF at Grenoble, BESSY at Berlin, LURE at Paris). 

Keegan et al. studied the calcination and reduction of the same Pt(NH3)4 on HZSM-5 system with energy dispersive Extended X-ray Absorption Fine Structure (EXAFS). They showed that during calcination the Pt-Pt coordination rises, indicating agglomeration. The final metal particle size obtained after direct reduction (no calcination prior to the reduction) was smaller than the particle size obtained after calcination prior to the reduction. The authors did not clarify the chemistry of the pretreatment process. 

On one hand, TR-XAFS investigations that require the best time resolution available (such as isothermal reactions or rapid decompositions with half lives of the order of one minute) may be performed at an energy-dispersive XAFS station, with full advantage taken of the time resolution in the sub-second range. On the other hand, TR-XAFS investigations of processes with half lives of the order of several minutes may be performed in the QEXAFS mode, with advantage taken of the increased EXAFS data quality for a detailed structural analysis. 

Fig. 3.35 An XAS spectrum of ZnSe under a pressure of 10 GPa at the Zn K-edge obtained with an energy-dispersive set-up (X) the XANES structure, and (E) the EXAFS structure.

Fig. 17. The krypton lattice parameter as measured by EXAFS and energy dispersive diffraction...

Warburton WK (1986) Filtered energy dispersive detector (EDD) arrays superior detectors of EXAFS from very diluted solutions. Nucl Inst Methods A246 541-546 Waychunas GA, Brown GE (1994) Fluoresence yield XANES and EXAFS experiments Applications to highly dilute and surface samples. Adv X-ray Anal 37 607-617 Waychunas GA, Rea BA, Fuller CC, Davis JA (1993) Surface chemistry of ferrihydrite. Part I. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate. Geochim Cosmocliim Acta 57 2251-2269... 

Extended X-ray absorption fine structure (EXAFS) measurements for Pd-Pt systems prepared either by direct redox or refilling method indicated the presence of platinum in the vicinity of palladium for both preparations [13]. Transmission electron microscopy (TEM) coupled with energy dispersive X-ray (EDX) microanalysis was carried out on Pd or Pt modified by Cu, Ag and Au [35], Pd-Sn [38], Pd-Cu [36], Rh-Ge [41], Pt-Ru-Sn and Pt-Ru-Mo (Fig. 9.5) [76], Pd-Sn-Au and Pt-Sn-Au [46] to collect more information on metallic particles in terms of particle size and composition. The catalysts prepared by redox reactions showed that the parent metal is always associated to the additive, but can be found isolated for supports which are able to adsorb the modifier. 

Further significant developments of the EXAFS technique are envisaged. Energy-dispersive X-ray absorption tine structure (DXAFS) allows collection of a spectrum in much less than 1 s, and its extension to the fj,s or even the ns time scale is not impossible. Polarisation-dependent total-reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) permits high spatial resolution and has been applied to the copper trimer Cus on a Ti(llO) surface. The use of time-and space-resolved XAFS observation for studies of dynamic aspects of the local structure at catalyst surfaces under working conditions is forseen . Note that the acronym XAFS is used, rather than EXAFS. 

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