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.

AbstrPct. Self-sustained temperature as well as rate oscillations during the oxidation of CO at atmospheric pressure on sup ported Pd catalysts were studied in-sifu at the Pd K edge by employing energvdispersive XAS. From the absorption speca phase transition from palladium oxide to palladium clusters can be excluded as driving force of the observed oscillations. Nevertheless a constant phase correlation between edge position and the observed temperature oscillations could be revealed. This points toward a periodical surface oxidation 1 reduction process in phase with deactivation I activation cycles of the catalyst. Furthermore, an oscillatory behaviour in height and position of the first Pd peak could be seen in the radial distribution fun* tion. From theoretical cluster calculations using FEFF6 it can be deduced, that the evaluated oscillations in Pd wordination number and Pd-Pd distance indicate an oscillatory change of surface coverage h m CO to oxygen and vice-versa. Taking these swfacewverage oscillations and the periodical oxidation 1 reduction into account, a surface activation 1 deactivation process is proposed as kinetic mechanism for the observed chemical oscillations.

The combustion of carbon monoxide on transition metals is known to exhibit an intriguing oscillatory behavior in reaction rate as well as reaction temperature under certain conditions. Chemical oscillations during the oxidation of CO at atmospheric pressure on supported Pd catalysts were studied in situ by employing X-ray absorption spectroscopy (XAS) in an energy-dispersive mode. Self-sustained thermokinetic oscillations with a period of about 12 min were found in both reaction temperature and carbon dioxide partial pressure. Energy-dispersive XAS experiments were carried out in situ at the Pd K-edge (24.35 keV) during the occurrence of chemical oscillations with a time resolution of several seconds. An elliptically bent, rectangular Si(400) crystal in transmission geometry was used. From the Pd absorption spectra, a constant phase correlation of edge position (threshold energy) with respect to the observed temperature oscillations could be revealed. This points toward a periodic oxidation/reduction process accompanying the deactivation/activation cycles of the catalyst. Furthermore, an oscillatory behavior in height and position of the first Pd peak was found in the radial distribution function FT(χ(k)). From a comparison with theoretical cluster calculations, it can be deduced that the evaluated oscillations in Pd coordination number and Pd−Pd distance indicate an oscillatory change of the oxygen surface coverage as well as the ratio of linearly and bridged bonded carbon monoxide. Taking these surface-coverage oscillations and the periodic oxidation/reduction process into account, a surface activation/deactivation mechanism for chemical oscillations during CO oxidation on supported Pd catalyst can be confirmed. The activity of the catalyst appears to be decreased by a surface oxidation leading to an increasing concentration of linearly bonded CO molecules. Thereafter, an increasing amount of bridging CO and a decreasing amount of linearly bonded CO accompany the reduction of the catalyst and subsequently the return to the high activity branch of the reaction.

No abstract

No abstract