U. Hatje

Curved Crystal Transmission Optics for Energy-Dispersive X-ray Absorption Spectroscopy.

The development of a curved crystal monochromator of the Laue type for energy-dispersive X-ray absorption spectroscopy is presented. The quality of the X-ray absorption spectra at high photon energies is compared with spectra measured with silicon crystals in the more frequently used Bragg geometry. In the Bragg case, an asymmetric broadening of the reflectivity profile leads to strong distortions of the near-edge fine structure and to a reduction in spectral resolution. The reflectivity profiles of fiat and curved crystals for Laue and Bragg geometry have been calculated using dynamical theory and are compared with experimental data. The new optics have been used for in situ time-resolved X-ray absorption spectroscopy. An example of the application of the technique for the characterization of a Pd catalyst is given. The X-ray absorption fine structure at the Pd K-edge has been measured during the activation and during the heterogeneous catalytic oxidation of carbon monoxide.

Zeolite-catalyzed low-temperature combustion of ecologically harmful chlorobenzene

The low-temperature combustion of chlorobenzene over Cu-and Pd-containing faujasites has been investigated applying reactor experiments and X-ray absorption spectroscopy. Although being active catalysts, the samples revealed deactivation, which could be protracted by increasing the Pd content and protonation. Formation of an intermediate Pd-Cl complex can be proven for the unprotonated catalysts but does not seem to be responsible for deactivation. Protonation also reduces the chlorine incorporation into the catalyst and stabilizes the oxidation state of Pd. Chlorine can be removed by hydrogen treatment which simultaneously gives rise to the formation of small palladium clusters.

Application of in situ dispersive XAFS studies in catalysis

Abstract Useful application of in situ dispersive XAFS spectroscopy to time-resolved processes, e.g. activation, solid-state ion exchange of and catalytic reaction over noble metal containing zeolites are described.

XAS Studies on the Interaction of with PtY and PdY Zeolites

Summary The primary step of chlorobenzene combustion in air over PtY and PdY zeolites turns out to be the reduction of the noble metal component to zero oxidation number followed by clustering. During reaction the formation of an intermediate Pt-Cl complex can be proven but chlorine seems not to be responsible for deactivation by irreversible blocking of the active centres. Comparative studies with bromobenzene reveal different behaviour of PdY and PtY concerning the uptake of bromine during reaction, providing an explanation for the far longer reaching activity of the PtY catalyst.

DEXAFS studies on the diffusion of ammonia into zeolite CuNaY

Summary The diffusion process of ammonia into the host channels of zeolite CuNaY has been followed using synchrotron radiation and time-resolved Dispersive Extended X-ray Absorption Fine Structure technique (DEXAFS). Adsorption of NH 3 guest molecules on copper ions bound to the framework of the zeolite leads to strong changes in the x-ray absorption fine stucture at the Cu K-edge. Difusion coefficients, activation energy of diffusion as well as distances and corrdination numbers of the nearest Cu neighbors have been determined.

XAFS with an energy-dispersive Laue monochromator

Abstract A Laue-type curved monochromator has been used for the first time for energy-dispersive XAFS spectroscopy on a synchrotron beam line. Experiments at the Pd K-edge at 24.35 keV photon energy have been performed at the DEXAFS/HASYLAB station. The quality of the XAFS spectra proved to be superior to spectra measured with silicon crystals in the dispersive Bragg geometry. In this case an asymmetric broadening of the reflectivity profile leads to strong distortions of the near edge fine structure and a reduction in the spectral resolution. XAFS spectra measured in the Laue and Bragg geometry are compared. The reflectivity profiles have been calculated using the dynamical theory.

Selected applications of time-resolved in situ investigations using dispersive X-ray absorption spectroscopy

Abstract In this report the capability of dispersive x-ray absorption spectroscopy is demonstrated for four examples selected from a variety of chemical applications.

DEXAFS studies on the reactivity of exfoliated graphite intercalation compounds

Abstract The exfoliation (rapid thermal decomposition) of a H2PtCl6 GIC was investigated using time resolved X-ray absorption spectroscopy in an energy dispersive mode. The experiments were carried out at the DEXAFS beamline at HASYLAB (Hamburg). The process results under nitrogen atmosphere as well as in air in a reduction of the platinum to graphite fixed metal clusters. During the exfoliation under nitrogen atmosphere the occurrence of PtCl4−x GIC, which cannot be prepared by application of the standard synthesis conditions, can be observed.