X-ray absorption edge studies

H2.5 X-ray absorption edge studies of the electronic structures of metal catalysts... 

Cole JL, Tan GO, Yang EK, Hodgson KO, Solomon El. Reactivity of the laccase trinuclear copper active site with dioxygen an X-ray absorption edge study. J Am Chem Soc 1990 112 2243-2249. 

The applications of polarized x-ray absorption spectroscopy (PXAS) for structure determination in inorganic and bioinorganic systems are discussed. PXAS studies of oriented samples add angular detail to the information obtained from x-ray absorption edges and from EXAFS. In some cases, PXAS can be used to determine molecular orientation. In other cases, PXAS can be used to infer the details of electronic structure or of chemical bonding. Some of the potential future applications of PXAS are discussed. 

XAS data comprises both absorption edge structure and extended x-ray absorption fine structure (EXAFS). The application of XAS to systems of chemical interest has been well reviewed (4 5). Briefly, the structure superimposed on the x-ray absorption edge results from the excitation of core-electrons into high-lying vacant orbitals (, ] ) and into continuum states (8 9). The shape and intensity of the edge structure can frequently be used to determine information about the symmetry of the absorbing site. For example, the ls+3d transition in first-row transition metals is dipole forbidden in a centrosymmetric environment. In a non-centrosymmetric environment the admixture of 3d and 4p orbitals can give intensity to this transition. This has been observed, for example, in a study of the iron-sulfur protein rubredoxin, where the iron is tetrahedrally coordinated to four sulfur atoms (6). 

Electronic Structure Determination, Cox and Beaumont have studied the polarized x-ray absorption edge of a single crystal of Zn 2 (23), in which the Zn has tetragonal (D ) site symmetry. The observed anisotropic K-absorption edges were explained in terms of a ls+4p and a ls-K4p, 4p ) transition. 

The problems associated with quantitative studies of structure based upon this viewpoint of X ray absorption-edge spectra may be similar to those encountered using electron beams of comparable energy, 3 to 100 ev., to carry out electron diffraction studies of crystal structure. Qualitatively, this analogy can be carried further, as both the X ray spectra and the electron beam diffraction in this energy range are influenced by only the first few atom layers. 

Traditionally, X-ray absorption edge measurements have been used to determine oxidation states of metals in complex materials. The extended X-ray absorption fine structure (EXAFS), on the other hand, provides structural information such as bond distances and coordination numbers even with powdered samples, crystalline or amorphous, the fine structure essentially resulting from short-range order around the absorbing atom. The technique is also useful for studying solid surfaces (SEXAFS). The observation of fine structure beyond the K-absorption edges of materials dates back to... 

Sarangi R. Aboelella N. Fujisawa K. Tolman W. B. Hedman B. Hodgson K. O. Solomon E. I. X-ray absorption edge spectroscopy and computational studies on LCu02 species superoxide-Cu11 versus peroxide-Cum bonding. J. Am. Chem. Soc. 2006, 128, 8286-8296. 

Kohn, S. C., Chamock, J. M., Henderson, C. M. B. Greaves, G. N. (1990) The structural environment of trace elements in dry and hydrous silicate glasses A manganese and strontium K-edge X-ray absorption spectroscopic study. Contrib. Mineral. Petrol., 105, 359-68. 

The analysis of the L2 and L3 X-ray absorption edges that was used in chapter 5 turned out to be more complex in later studies. The changes in the Pt L3 X-ray absorption edge induced by the adsorption of hydrogen are also sensitive to the mode of adsorption of hydrogen on the Pt... 

B. Near-Edge X-Ray Absorption Catalyst Studies Past and Present... 

The interaction of cupric ions with alumina supports has subsequently been studied more extensively as a function of the support surface area, metal loading, and calcination temperature (93,279) by means of EXAFS and X-ray absorption-edge shifts, in conjunction with XRD, EPR, XPS, and optical reflectance spectroscopy. These techniques, each sensitive to certain structural and electronic aspects, allow a unified picture of the phases present and the cation site location. Four Cu2 + ion sites are distinguished in the catalysts. In low concentrations (typically below about 4 wt. % Cu/100 m2/g support surface area) Cu2 + ions enter the defect spinel lattice of the A1203 support. The well-dispersed surface copper aluminate has Cu2+ ions predominantly occupying tetragonally (Jahn-Teller) distorted octahedral sites, although... 

Schwab s experiments on the effect of doped alumina carriers, the x-ray absorption edge experiments, and the study... 

Laccase contains four copper atoms and catalyzes the four-electron reduction of dioxygen to water. X-Ray absorption edge spectroscopy has been used to determine the oxidation states of copper in Rhus vernicifera laccase, following the reaction of the reduced enzyme with dioxygen (202). This study included the incorporation of mercury(II) in the Type 1 copper site (see Section IV,B). The results demonstrate that the Type 2/Type 3 trinuclear copper site, as found in ascorbate oxidase (103), represents the minimal active site required for the multielectron reduction of dioxygen. 

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