Showing papers by "Markus Janousch published in 2009"

Sulphur poisoning of Ni catalysts in the SNG production from biomass: A TPO/XPS/XAS study

Abstract: Ni-based catalysts are prone to deactivation (poisoning) of their active surface sites by sulphur and carbon species contained in the gas fed to the reactor. This study focuses on Ni/Al2O3-based catalyst samples which had allegedly been deactivated by sulphur poisoning. The samples had been collected from a 10 kW methanation reactor fed with producer gas from the industrial biomass gasifier in Gussing (Austria). The samples allowed intensive investigation using several analytical tools to identify the chemical nature (inorganic, organic) of the S-poisoning species. Temperature-programmed oxidation (TPO) allowed quantification of the sulphur content, but not the identification of the S species responsible. S 2p X-ray photoelectron spectroscopy (XPS) pointed at the presence of sulphide and sulphate, but the data were too noisy to reach more specific conclusions. Ni K-edge X-ray absorption spectroscopy (XAS) in the fine structure (EXAFS) region suggested the presence of elemental or thiophenic sulphur, but the contribution was masked heavily by other backscattering paths. Only S Kedge analysis in the near edge (XANES) region showed unambiguously that the catalyst could not have been deactivated by inorganic H2S only. This conclusion is supported by S K-edge XANES results with model catalysts which had either been poisoned by H2S or thiophene (C4H4S), representing a cyclic, aromatic S compound. Short-term H2S poisoning in the absence of air led to a white-line position characteristic for sulphide (2470 eV), whereas with thiophene the white-line position started at 3 eV higher energy. The XANES signatures changed with the catalyst samples after contacting air, but remained unique for each of the two S-poison types studied here. (C) 2009 Elsevier B.V. All rights reserved.

Spatial distribution of oxygen vacancies in Cr-doped SrTiO3 during an electric-field-driven insulator-to-metal transition

Abstract: Spatially resolved x-ray fluorescence maps are presented that show the introduction and the evolution of oxygen vacancies in chromium-doped strontium titanate during an electric-field-driven insulator-to-metal transition. The vacancies are introduced at the anode and diffuse through the crystal toward the cathode. The spatial distribution of vacancies is explained by a model describing the electrical breakdown as a percolation process. Strong differences in the vacancy distribution were found when the transition took place in air and in a hydrogen-enriched atmosphere. In air, the vacancies disappeared from the surface, whereas in the reducing hydrogen atmosphere, they remained at the surface.

Speciation of Mg in biogenic calcium carbonates

Abstract: A selection of marine biominerals, mostly aragonitic coral skeletons were probed at the Mg K-edge by XANES spectroscopy coupled to μXRF methods and compared to an extensive set of relevant model compounds (silicates, carbonates, oxides and organic). Extensive methodologies are required to better describe the speciation of Mg in those minerals. A combination of ab-initio XANES calculations for defective clusters around Mg in aragonite together with wavelets analyzes of the XANES region are required to robustly interpret the spectra. When using those methodologies, the speciation of Mg ranges from a magnesite-type environment in some scleractinian corals to an organic-type environment. In all environments, the Mg-domains probed appear to be less than 1 nm in size.

Origin of oxygen vacancies in resistive switching memory devices

Abstract: The resistive switching state in Cr-doped SrTiO3 was induced by applying an electric field. This was done in ambient air and in an atmosphere of H2/Ar. The distribution of the thereby introduced oxygen vacancies was studied by spatially resolved X-ray fluorescence images. It was concluded that the oxygen vacancies were introduced in the interface between the SrTiO3 and the positively biased electrode.

High-resolution hard-X-ray fluorescence spectrometer

Abstract: A Johann-type X-ray fluorescence spectrometer for XES, RXES/RIXS, HERFD XAS and RXS experiments was designed, constructed and commissioned at the X10DA-SuperXAS beamline of the Swiss Light Source. The spectrometer consists of three key elements: a sample manipulator, an X-ray dispersive element (spherically bent silicon or germanium crystal), and an one-dimensional-array X-ray detector. The detected X-ray fluorescence energy is scanned by changing the angle between the sample, crystal and detector. The energy resolution of the spectrometer ranges from sub-eV to several eV. Thanks to the use of a one-dimensional array detector the spectrometer is easy to align and operate.

Detecting oxygen vacancies in SrTiO3 by 3d transition-metal tracer ions

Abstract: X-ray absorption experiments on 3d transition-metal tracer ions in SrTiO3 are presented. The absorption spectra of the tracer-ion changed upon reduction in the SrTiO3. This change is due to an oxyg ...