Kinetic oscillations in the oxidation of CO occur because of local changes in the catalyst structure inside the reactor. Activity loss within an oscillation originates from partial reduction of the active surface, which occurs at distinct positions within the catalyst bed. The original activity is regained partly by re‐oxidation of the catalyst, during which a short‐lived phase is formed.

Quick scanning extended X-ray absorption fine structure (QEXAFS) studies in the subsecond time scale have been performed to gain insight into the reaction mechanism of Heck-type C-C coupling reactions in the presence of supported Pd-based catalysts. Using a specially designed in situ EXAFS cell, both the solid catalyst and the liquid reaction mixture during the reaction of phenyl bromide (PhBr) with styrene were monitored. Soluble Pd species were only, but rapidly, detected in the liquid reaction phase once the reaction temperature of 150 °C was reached. At the same time, the conversion of PhBr started, and during the following "active phase" of the catalyst hardly any changes in the corresponding EXAFS and XANES spectra were observed. The present species could be identified as colloidal Pd(0) clusters with a size of ∼2 nm estimated from the corresponding EXAFS spectra. The QEXAFS mode not only allowed monitoring rapid changes in the second time scale but also permitted minimization of effects caused by the heterogeneity of the systems. When the reaction rate started to decrease, pronounced changes in the EXAFS spectra were observed, which were attributed to an increased formation of bromo-palladates ([PdBr(4)](2-), [Pd(2)Br(6)](2-)). In addition to the liquid-phase species, significant changes were observed for the solid catalyst that was also probed in situ during the reaction. The originally oxidized Pd catalyst was efficiently reduced upon heating. Additionally, growth of the supported Pd particles was observed by both EXAFS and STEM. The above results confirm the role of the soluble molecular Pd species as the catalytically active species and clarify their conjunction with the in situ formed Pd colloids. Furthermore, the investigation demonstrates the potential of the QEXAFS not only for monitoring rapid changes during catalysis but also for gaining deeper insight into the mechanism of such complex industrially important systems under relevant reaction conditions.

The dynamics of the ignition and extinction of the catalytic partial oxidation (CPO) of methane to hydrogen and carbon monoxide over Pt-Rh/Al(2)O(3) and Pt/Al(2)O(3) were studied in the subsecond timescale using quick-EXAFS with a novel cam-driven X-ray monochromator employing Si(111) and Si(311) crystals. The experiments were performed under reaction conditions in a small fixed-bed capillary reactor. For the first time XAS data were taken with this QEXAFS technique with a Si(311) crystal that opens the energy range up to 35 keV. In addition, both XANES and EXAFS data are shown at the Pt L(3)-edge, allowing to discuss the potential and limitation of this technique in catalysis and related areas. With respect to the noble metal catalysed partial oxidation of methane, several interesting observations were made: structural changes during ignition were-independent of the chosen reaction conditions-significantly faster than during the extinction of the reaction. The dynamic behavior of the catalysts was dependent on the flow conditions and the respective noble metal component(s). Higher reaction gas flow led to a faster ignition process. While the ignition over Pt-Rh/Al(2)O(3) occurred at lower temperature than over Pt/Al(2)O(3), the structural changes during ignition were significantly faster in the latter case. The rate of reduction of the catalyst during ignition was also dependent on the axial position in the fixed-bed. The spectroscopic results provide important insight into the ignition and extinction behavior of the CPO of methane and are complementing results from time-resolved infrared thermography and full field X-ray microscopy studies.

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New concepts for time-resolved x-ray absorption spectroscopy using the quick-extended x-ray absorption fine structure (QEXAFS) method are presented. QEXAFS is a powerful tool to gain structural information about, e.g., fast chemical reactions or phase transitions on a subsecond scale. This can be achieved with a monochromator design that employs a channel-cut crystal on a cam driven tilt table for rapid angular oscillations of the Bragg angle. A new angular encoder system and a new data acquisition were described and characterized that were applied to a QEXAFS monochromator to get spectra with a directly measured accurate energy scale. New electronics were designed to allow a fast acquisition of the Bragg angle values and the absorption data during the measurements simultaneously.

Insight into the complex chemical mechanism for the formation of tungstite nanoparticles obtained by the reaction of tungsten hexachloride with benzyl alcohol is presented herein. The organic and inorganic species involved in the formation of the nanoparticles were studied by time-dependent gas chromatography and X-ray diffraction as well as by time-resolved in situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy. Principal component analysis revealed two intermediates, which were identified as WCl(4) and WOCl(4) by using linear combination analysis. Quick-scanning extended X-ray absorption fine structure spectroscopy enabled the time-dependent evolution of the starting compound, the intermediates and the product to be monitored over the full reaction period. The reaction starts with fast chlorine substitution and partial reduction during the dissolution of the tungsten hexachloride in benzyl alcohol followed by the generation of intermediates with W=O double bonds and finally the construction of the W-O-W network of the tungstite structure.

The hydrolysis of titanium(IV) tetraisopropoxide and the following condensation and aggregation to titania nanoparticles under slow water solution addition were concomitantly investigated by in situ QEXAFS and UV/vis spectroscopies. The amount of dominant titanate species in the solution was determined as a function of time, temperature, and water addition rate. Resulting from fast hydrolysis and condensation of isopropoxide precursors an intermediate oligomeric species is initially formed at the early stages of the reaction. This oligomeric species has been identified as a dodecatitanate species with Ti11O13 or Ti12O16 molecular framework. At the subsequent stage of the reaction corresponding to slower processes compared to the fast hydrolysis, titania nanoparticles are formed by a cluster−cluster growth mechanism involving the consumption of the dodecatitanate species following a pseudo first-order reaction. Finally, at the advanced stage, sudden aggregation of titania nanoparticles occurs at a given threshold of amount of nanoparticles.

The SuperXAS beamline at the Swiss Light Source (SLS) features a permanently installed monochromator for quick scanning EXAFS (QEXAFS) spectroscopy in series with a conventional double crystal monochromator (DCM). All installed optical components like collimating and focussing mirrors can be used by both devices. The remote exchange of the monochromators is possible in less than five minutes while maintaining the beam geometry on the sample. The QEXAFS system allows fast absorption scans down to the millisecond range for the investigation of time dependent processes. Using a Si(111) channel cut crystal the energy range from 5-16 keV can be covered, with a Si(311) cut the range 9.5-30 keV. Usually a quick scanning interval of 0.1° -2° in Bragg angle is selected, thus covering XANES, full EXAFS or multiple edge scans of e.g. all L-edges of a heavy element. Up to about 80 spectra per second can be collected, corresponding to a time resolution of 12.5 ms. The high intensity of the beamline even facilitates fluorescence measurements on dilute samples.