Structure EXAFS

The Mo(IV) aqua ion was first reported by Souchay et al. as a monomeric species in 1966 (93), and the Mo3044+ core structure has been confirmed by single-crystal X-ray structure analyses, as described in this review, 95Mo NMR (94), EXAFS structure analysis (95), and 180-labeling experiments (96), after the appearance of many contradictory reports (97). Information on the reduction products of the Mo(IV) aqua ion is also available (98), Richens and Sykes have summarized the preparation of the different aqua ions of molybdenum in oxidation states II to V (99). 

In a subsequent report we shall present NEXAFS and EXAFS structural data for bimetallic supported catalysts prepared via the SMAD procedure and other techniques. 

Fe anchored amino-functionalized MCM-41 effectively adsorbs toxic oxyanions, arsenate, chromate, selenate and molybdate, in the aqueous solution. These adsorbents show ones of the largest adsorption capacities reported in the literature. The molar ratio, oxyanion / Fe, is 2.8, 1.8, 1.5 and 2.3, respectively. They are consistent with the coordination mmibers in the EXAFS structural analysis of adsorption complexes. Cr-Fe adsorption centre is uniform, where 2 1 complex is uniformly generated. On the contraiy, chloride ion remains in As- and Se-Fe complexes and two kinds of coordination with different bonding distances were found in As- and Mo-Fe complexes. 

AE McDermott, VK Yachandra, RD Guiles, K Sauer, MP Klein, KG Parrett and JH Golbeck (1989) EXAFS structural study ofFx, the low-potential FeS center in photosystem I. Biochemistry 28 8056-8059 V Petrouleas, JJ Brand, KG Parrett and JH Golbeck (1989) A M ssbuer a nalysis of the low-potential iron-sulfur center in photosystem I Spectroscopic evidence thatFx is a [4Fe-4S] cluster. Biochemistry 28 8980-8983 HV Scheller, I Svendsen and BL Mtller (1989) Subunit composition of photosystem I and identification of center X as a [4Fe-4S] iron-sulfur cluster. J Biol Chem 264 6929-6934... 

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.

Rossberg A, Baraniak L, Reich T, Hennig C, Bernhard G, Nitsche H (2000) EXAFS structural analysis of aqueous uranium(VI) complexes with lignin degradation products. Radiochim Acta 88 593-597 Rothe J, Kneedler EM, Pecher K, Tonner BP, Nealson KH, Grundi T, Meyer-Ilse W, Warwick T (1999) Spectromicroscopy of Mn distributions in micronodules produced by biomineralization. J Synchrotron Rad 6 359-361... 

The application of Co XAS techniques to organocobalamins and their models has been reported. Several of the results appear controversial, with a very recent (2001) methodological study presented that explains the origin of the controversies. " Proposals to improve EXAFS structure-parameter refinement by first-shell filtering, and the importance of mapping techniques in this regard, have been described. 

EXAFS structural study of, 10 RDF analysis of, 10 zirconia sol-gel layers, 27... 

Figure 6.28 Examples of the data collected to test the feasibility of the procedure of HTXAS procedure. Individual gene product samples from the Pyrococcus furiosus genome were loaded into a 5x5 spatial array of 3mL samples (middle left). A single raster scan was used to monitor both Ni (lower left) and Zn (upper left) K emission, identifying the presence of these metals in the indicated wells. Ni XANES speciation (lower right) clearly differentiates the t es of Ni binding sites and Zn EXAFS structural analysis (upper right) shows the sensitivity available for determining metal-site structures. 2005 International Union of Crystallography.

Hsiao YW et al (2006) EXAFS structure refinement supplemented by computational chemistry. Phys Rev B 74 214101... 

Table 10.1 Mn K-edge EXAFS structure parameter of the LiMn204 standards and the LiMn204/RG0 hybrid material...

While X-ray diffraction is well suited to the study of the structure of crystalline samples, it is less well suited to amorphous or disordered ones. In this area, the availability of synchrotron radiation has again led to the development of new techniques, in particular, extended X-ray absorption fine structure (EXAFS). In EXAFS, structural information is derived from an analysis of the oscillations that appear above an X-ray absorption edge, and this technique is now widely applied. The conventional EXAFS experiment is not surface sensitive but some variants are. Under HV conditions, the electron yield is a measure of the absorption, and since these electrons can only escape from a thin layer at the surface, this detection method is surface sensitive. This is the basis of SEXAFS. " Clearly, this cannot be used for in situ studies, but in these cases the fluorescent X-ray yield (which is also a measure of the absorption) can be measured and, particularly when this is coupled with a glancing incidence geometry, reasonable surface sensitivity is achieved. Some initial studies of this type on electrochemical systems have now been made, and these will be described in greater detail later. 

---