Jan Stötzel

Dynamic Structure Changes of a Heterogeneous Catalyst within a Reactor: Oscillations in CO Oxidation over a Supported Platinum Catalyst

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.

Novel angular encoder for a quick-extended x-ray absorption fine structure monochromator.

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.

Hydrothermal Synthesis of Bi6S2O15 Nanowires: Structural, in situ EXAFS, and Humidity‐Sensing Studies

J. Stötzel, Prof. R. Frahm Department of Physics, University of Wuppertal 42097 Wuppertal (Germany) [ ] We gratefully acknowledge the Swiss Light Source (SLS, Villigen, Switzerland) for providing beam time at the superXAS beamline for QEXAFS experiments. We are grateful to M. J. Beier for help and J.D.G. to DANSCATT for financial support. The support of the Electron Microscopy ETH Zurich, EMEZ, and Center for Microscopy and Image Analysis, University of Zurich, is acknowledged. We are grateful to Prof. W. Bensch and Dr. N. Pienack (University of Kiel, Germany) for in situ EDXRD experiments and to HASYLAB (DESY, Hamburg, Germany) for providing beam time at beamline F3. Financial support from the Sino Swiss Science and Technology Cooperation (SSSTC, project no. EG05-092008) is gratefully acknowledged. We thank the Swiss National Science Foundation (SNF Professorship PP002 114711/1) and the University of Zurich for financial support. : Supporting Information is available on the WWW under http:// www.small-journal.com or from the author.

The dedicated QEXAFS facility at the SLS: Performance and Scientific Opportunities

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 flu...

Study of the Chemical Mechanism Involved in the Formation of Tungstite in Benzyl Alcohol by the Advanced QEXAFS Technique

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.

A new flexible monochromator setup for quick scanning x-ray absorption spectroscopy

A new monochromator setup for quick scanning x-ray absorption spectroscopy in the subsecond time regime is presented. Novel driving mechanics allow changing the energy range of the acquired spectra by remote control during data acquisition for the first time, thus dramatically increasing the flexibility and convenience of this method. Completely new experiments are feasible due to the fact that time resolution, edge energy, and energy range of the acquired spectra can be changed continuously within seconds without breaking the vacuum of the monochromator vessel and even without interrupting the measurements. The advanced mechanics are explained in detail and the performance is characterized with x-ray absorption spectra of pure metal foils. The energy scale was determined by a fast and accurate angular encoder system measuring the Bragg angle of the monochromator crystal with subarcsecond resolution. The Bragg angle range covered by the oscillating crystal can currently be changed from 0 degrees to 3.0 degrees within 20 s, while the mechanics are capable to move with frequencies of up to ca. 35 Hz, leading to ca. 14 ms/spectrum time resolution. A new software package allows performing programmed scan sequences, which enable the user to measure stepwise with alternating parameters in predefined time segments. Thus, e.g., switching between edges scanned with the same energy range is possible within one in situ experiment, while also the time resolution can be varied simultaneously. This progress makes the new system extremely user friendly and efficient to use for time resolved x-ray absorption spectroscopy at synchrotron radiation beamlines.

A new stand-alone QEXAFS data acquisition system for in situ studies.

To meet the demands of the QEXAFS (quick-scanning extended X-ray absorption fine structure) technique for a fast, user-friendly and flexible data acquisition a new stand-alone system with new software exploiting a multi-functional USB board was designed. The chosen approach allows the scanning of several analogue and digital data sources with up to 500000 samples each second over hours storable in binary or ASCII format without any dead-time. At the same time it is possible to visualize the acquired data instantaneously which provides a maximum of interactivity during the running experiment and also optimal conditions to select the best suited beamline and detector settings prior to each measurement. Furthermore, the QEXAFS monochromator and typically three current amplifiers are entirely controlled by the new software so that all monochromator settings can be synchronized with the data acquisition enabling programmed scans with alternating parameter sets. This versatile concept also enables the user to react immediately to changes in the sample during in situ studies. An interface to a three-axis stepper motor control unit is additionally included to control a sample stage which can again be synchronized with the data acquisition. Thus, spatially resolved scans and the usage of scan tools for sample alignment are feasible with the new system. Typical examples to demonstrate the features of the new data acquisition system are presented, the designed graphical user interface is described in detail and, furthermore, the crucial design parameters of a typical QEXAFS set-up are discussed.

Advancing Time-resolved Methods in Monitoring and Characterization of Catalysts

Time dependent X-ray studies allow the investigation of the activation of a catalyst and its activity during operation. Because of their ability to perform in situ studies under real environmental conditions, both X-ray absorption spectroscopy (XAFS) and X-ray diffraction (XRD) have been widely used [1]. Typical processes take place in the time range from subseconds to few seconds. Thus it is essential to collect data sets within fractions of a second to uncover, e.g., possible intermediates, which can occur during reduction/oxidation processes involving several species with different oxidation states. Such measurements allow the quantitative modelling of the reaction kinetics – knowledge that is indispensible for specific optimizations of catalysts.

Reduction and re-oxidation of Cu/Al2O3 catalysts investigated with quick-scanning XANES and EXAFS

In the present study the structure of copper catalysts on alumina support were investigated in situ and time resolved during reduction and re-oxidation at different temperatures with the quick-scanning EXAFS (QEXAFS) technique. Different impregnation times (2 min and 90 min) were chosen for the preparation which resulted in different copper species that show a strong variation in the reduction/re-oxidation behaviour. These dynamic changes as well as possible intermediate phases during the gas atmospheres changes were followed with up to 20 EXAFS spectra per second at the copper K-edge covering an energy range of 450 eV. The high time resolution provided new insights into the dynamics of the catalysts e.g. revealing Cu(I) as intermediate state during re-oxidation. Latest advances in the data acquisition hardware are leading to an improved data quality of spectra collected at the SuperXAS beamline. Thus, not only accurate analysis of the catalysts via XANES but also by EXAFS was possible. This is also due to the recent upgrade to monitor the Bragg angle directly with an encoder during the experiments.

Investigation of the ignition behaviour of the noble metal catalyzed catalytic partial oxidation of methane

Catalytic partial oxidation (CPO) of methane to hydrogen and carbon monoxide over Pt-Rh/Al2O3 and Pt/Al2O3 was studied in-situ with a new QEXAFS setup. The structural changes of the catalysts were investigated on the subsecond timescale during two reaction steps by recording both XANES and full EXAFS spectra: (1) heating and ignition in 6%CH4/3%O2/He, (2) periodic changes between the reaction gas mixture and H2 atmosphere. The results showed that the ignition occurred at lower temperatures for Pt-Rh/Al2O3 while it was completed in a significant shorter time interval for Pt/Al2O3. Some structural changes during the heating phase were detectable before the reaction ignited, especially for Pt/Al2O3, as reflected by the performed principle component analysis. However, a closer analysis of the FT-QEXAFS data did not evidence a defined intermediate. In addition, the composition of the gas atmosphere was altered between hydrogen and the reaction mixture, enabling modulation excitation spectroscopy. This technique was for the first time applied to QEXAFS data and resulted in significantly enhanced data quality.