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[en] The structure of supported platinum catalyst (2 wt% Pt/Al2O3) has been determined during the sudden change in the activity of the catalyst, referred as 'ignition', during oxidation of carbon monoxide. The catalyst was prepared by incipient wetness impregnation method. The active phase of the catalyst is shown to be a highly disordered platinum oxide. This surface oxide increases with the increase of carbon monoxide conversion during the ignition process, which further increased the conversion, making the process autocatalytic. The QEXAFS spectra collected at Pt L3 edge with a time resolution of 0.5 sec are compared with the standard EXAFS that are generally collected in transmission mode and are shown to be useful to extract valuable information about the changing structure of the catalyst during the ignition.
[en] An algorithm and code for the determination of the factorized atomic part of the absorption cross-section from experimental spectra in the XANES region were generated and tested on Al and Si K-edge XANES in reference compounds and applied to zeolite Beta with various Si/Al ratios. However, replacement of the theoretically determined background with the experimental absorption cross-section significantly improved the agreement with experiment. The spectral analysis showed that the XANES region can be used to extract detailed structural information when the Muffin-tin approximation can be avoided. Asymmetry in the silicon oxide tetrahedron was observed when increasing the aluminum content of the zeolite
[en] In situ X-ray absorption spectroscopy at the Pd L3 edge was applied to determine the particle size effect on the formation of palladium hydride and on surface hydrogen adsorption at room temperature. Pd L3 edge XANES spectra allow direct detection of hydride formation via a characteristic spectral feature caused by the formation of a Pd-H anti-bonding state. This feature showed strong particle size dependence. The L3 edge spectra were reproduced using full multiple scattering analysis and density of states calculations and the contributions of bulk-dissolved and surface hydrogen to the XANES spectra could be distinguished. The ratio of hydrogen on the surface versus that in the bulk increased with decreasing particle size, and smaller particles dissolved less hydrogen.
[en] The effects of hydrogen in Pd nanoparticles on the features of Pd K-edge X-ray absorption near edge structure (XANES) are studied. Simulations indicate a linear dependence of the intensities of XANES maxima upon the concentration of hydrogen in the bulk of Pd clusters at low H-concentrations and stepwise increase of the first near edge peak at high concentrations. The effects of the diffusion of H atoms over the interstitial sites are considered. The application of the obtained results to the experimental spectra measured in PdH nanoparticles of sizes from 1.3nm to 10.5nm indicates low H-concentrations and high mobility of H-atoms in the bulk of the nanoparticles.
[en] In situ x-ray absorption spectroscopy identified partially oxidized platinum as the active phase for generating high activity during carbon monoxide oxidation over Pt/Al2O3. CO covers and poisons metallic platinum, which results in a low catalytic activity. The conversion in CO abruptly increased during ignition, switching from low activity to high activity, accompanied by oxidation as observed in the increased intensity of the XANES. EXAFS analysis indicated breaking of Pt-Pt bonds and the appearance of a Pt-O scatterer. The partially oxidized catalyst had a lower oxygen coordination number and shorter Pt-O bond length than bulk PtO2, suggesting that a strongly defected platinum oxide was formed with a coordination possibly lower than octahedral. From infrared data, ignition was accompanied by the abrupt disappearance of CO adsorbed on metallic particles. The reverse changes happen with the sudden decrease in CO conversion during extinction: partially oxidized catalyst becomes reduced and covered with CO.
[en] 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 deg. - 2 deg. 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.
[en] Ce L1-edge high-energy-resolution fluorescence detected (HERFD) X-ray absorption spectroscopy (XAS) was applied to unravel the electronic structure of Pt-promoted CeO2 nanoparticles under different redox conditions. The Ce Lγ3 emission line was chosen for partial fluorescence yield detection to improve the XAS spectral resolution. Oxygen vacancies were formed on the surface of ceria nanoparticles upon reducing in 5% CO in He. Theoretical Ce L1-edge simulations showed good agreement with the experimental data.
[en] The first high-spin transition in C2 (1 5Πu − 1 5Πg) is observed by perturbation-facilitated optical-optical double resonance spectroscopy. The experiment is performed by applying unfolded two-color resonant four-wave mixing. C2 radicals in the initial a 3Πu, v = 5 state are produced by using a discharge source in a molecular beam environment. The final quintet state is excited via intermediate “gateway” states exhibiting both substantial triplet and quintet character due to a perturbation between the 1 5Πg, v = 0 and the d 3Πg, v = 6 states. Fifty seven rotational transitions in the P, Q, and R branches of all spin sub-states are measured and yield accurate molecular constants of the newly found upper level 1 5Πu. In addition, satellite transitions (ΔJ ≠ ΔN) are observed and allow an accurate determination of the spin-orbit constant. The results are compared with high-level ab initio computations at the multi-reference configuration interaction level of theory. The high-lying quintet state is found to be predissociative and displays a shallow potential that accommodates three vibrational levels only
[en] A new apparatus for attosecond time-resolved photoelectron spectroscopy of liquids and gases is described. It combines a liquid microjet source with a magnetic-bottle photoelectron spectrometer and an actively stabilized attosecond beamline. The photoelectron spectrometer permits venting and pumping of the interaction chamber without affecting the low pressure in the flight tube. This pressure separation has been realized through a sliding skimmer plate, which effectively seals the flight tube in its closed position and functions as a differential pumping stage in its open position. A high-harmonic photon spectrometer, attached to the photoelectron spectrometer, exit port is used to acquire photon spectra for calibration purposes. Attosecond pulse trains have been used to record photoelectron spectra of noble gases, water in the gas and liquid states as well as solvated species. RABBIT scans demonstrate the attosecond resolution of this setup
[en] Single and double layer phase change memory structures based on GeTe and GaSb thin films were deposited by pulsed laser deposition (PLD). Their crystallization behavior was studied using in-situ synchrotron techniques. Electrical resistance vs. temperature investigations, using the four points probe method, showed transition temperatures of 138 °C and 198 °C for GeTe and GaSb single films, respectively. It was found that after GeTe crystallization in the stacked films, Ga atoms from the GaSb layer diffused in the vacancies of the GeTe crystalline structure. Therefore, the crystallization temperature of the Sb-rich GaSb layer is decreased by more than 30 °C. Furthermore, at 210 °C, the antimony excess from GaSb films crystallizes as a secondary phase. At higher annealing temperatures, the crystalline Sb phase increased on the expense of GaSb crystalline phase which was reduced. Extended X-ray absorption fine structure (EXAFS) measurements at the Ga and Ge K-edges revealed changes in their local atomic environments as a function of the annealing temperature. Simulations unveil a tetrahedral configuration in the amorphous state and octahedral configuration in the crystalline state for Ge atoms, while Ga is four-fold coordinated in both as-deposited and annealed samples