Ambient Pressure XPS Study of Mixed Conducting Perovskite-type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization

The oxygen exchange activity of mixed conducting oxide surfaces has been widely investigated, but a detailed understanding of the corresponding reaction mechanisms and the rate-limiting steps is largely still missing. Combined in situ investigation of electrochemically polarized model electrode surfaces under realistic temperature and pressure conditions by near-ambient pressure (NAP) XPS and impedance spectroscopy enables very surface-sensitive chemical analysis and may detect species that are involved in the rate-limiting step. In the present study, acceptor-doped perovskite-type La0.6Sr0.4CoO3-δ (LSC), La0.6Sr0.4FeO3-δ (LSF), and SrTi0.7Fe0.3O3-δ (STF) thin film model electrodes were investigated under well-defined electrochemical polarization as cathodes in oxidizing (O2) and as anodes in reducing (H2/H2O) atmospheres. In oxidizing atmosphere all materials exhibit additional surface species of strontium and oxygen. The polaron-type electronic conduction mechanism of LSF and STF and the metal-like mechanism of LSC are reflected by distinct differences in the valence band spectra. Switching between oxidizing and reducing atmosphere as well as electrochemical polarization cause reversible shifts in the measured binding energy. This can be correlated to a Fermi level shift due to variations in the chemical potential of oxygen. Changes of oxidation states were detected on Fe, which appears as FeIII in oxidizing atmosphere and as mixed FeII/III in H2/H2O. Cathodic polarization in reducing atmosphere leads to the reversible formation of a catalytically active Fe0 phase

On the properties of X-ray corona in Seyfert 1 galaxies

We carried out a uniform and systematic analysis of a sample of 112 nearby bright Seyfert 1 type AGN, the observations of which were carried out by the {\it Nuclear Spectroscopic Telescope Array (NuSTAR)} between August 2013 and May 2022. The main goal of this analysis is to investigate the nature of the X-ray corona in Seyfert 1 galaxies. From the physical model that fits the {\it NuSTAR} spectra, we could constrain the high energy cut-off ($\rm{E_{cut}}$) for 73 sources in our sample. For those 73 sources, we fitted the Comptonization model to estimate the temperature ($\rm{kT_{e}}$) of their corona. $\rm{kT_{e}}$ could be constrained in 42 sources. We investigated for possible correlations between various properties of the corona obtained from physical model fits to the observed spectra and between various coronal parameters and physical properties of the sources such as Eddington ratio and black hole mass. We found (a) a strong correlation between $\rm{E_{cut}}$ and the photon index and (b) a significant negative correlation between $\rm{kT_{e}}$ and the optical depth.Comment: 33 pages, 14 figures, Submitted to ApJ, comments are welcom

Origin of different deactivation of Pd/SnO₂ and Pd/GeO₂ catalysts in methanol dehydrogenation and reforming: A comparative study

Pd particles supported on SnO2 and GeO2 have been structurally investigated by X-ray diffraction, (High-Resolution) transmission and scanning electron microscopy after different reductive treatments to monitor the eventual formation of bimetallic phases and catalytically tested in methanol dehydrogenation/reforming. For both oxides this included a thin film sample with well-defined Pd particles and a powder catalyst prepared by incipient wetness impregnation. The hexagonal and the tetragonal polymorph were studied for powder GeO2. Pd2Ge formation was observed on all GeO2-supported catalysts, strongly depending on the specific sample used. Reduction of the thin film at 573 K resulted in full transformation into the bimetallic state. The partial solubility of hexagonal GeO2 in water and its thermal structural instability yielded Pd2Ge formation at 473 K, at the cost of a structurally inhomogeneous support and Ge metal formation at higher reduction temperatures. Pd on tetragonal GeO2 entered a state of strong metal–support interaction after reduction at 573–673 K, resulting in coalescing Pd2Ge particles on a sintered and re-crystallized support, apparently partially covering the bimetallic particles and decreasing the catalytic activity. Pd2Ge on amorphous thin film and hexagonal GeO2 converted methanol primarily via dehydrogenation to CO and H2. At 573 K, formation of Pd2Sn and also PdSn occurred on the Pd/SnO2 thin film. Pd3Sn2 (and to some extent Pd2Sn) were predominantly obtained on the respective powder catalyst. Strong deactivation with increasing reduction temperature was observed, likely not based on the classical strong metal–support interaction effect, but rather on a combination of missing active structural ensembles on Sn-enriched bimetallic phases and the formation of metallic β-Sn. Correlations to Pd and its bimetallics supported on ZnO, Ga2O3 and In2O3 were also discussed

Ligand migration from cluster to support: a crucial factor for catalysis by Thiolate-protected gold clusters

Thiolate protected metal clusters are valuable precursors for the design of tailored nanosized catalysts. Their performance can be tuned precisely at atomic level, e.g. by the configuration/ type of ligands or by partial/complete removal of the ligand shell through controlled pre-treatment steps. However, the interaction between the ligand shell and the oxide support, as well as ligand removal by oxidative pre-treatment, are still poorly understood. Typically, it was assumed that the thiolate ligands are simply converted into SO 2 , CO 2 and H 2 O. Herein, we report the first detailed observation of sulfur ligand migration from Au to the oxide support upon deposition and oxidative pre-treatment, employing mainly S K-edge XANES. Conse- quently, thiolate ligand migration not only produces clean Au cluster surfaces but also the surrounding oxide support is modified by sulfur-containing species, with pronounced effects on catalytic propertiesPeer ReviewedPostprint (published version

Production of a dual-species Bose-Einstein condensate of Rb and Cs atoms

We report the simultaneous production of Bose-Einstein condensates (BECs) of $^{87}$Rb and $^{133}$Cs atoms in separate optical traps. The two samples are mixed during laser cooling and loading but are separated by $400 \mu$m for the final stage of evaporative cooling. This is done to avoid considerable interspecies three-body recombination, which causes heating and evaporative loss. We characterize the BEC production process, discuss limitations, and outline the use of the dual-species BEC in future experiments to produce rovibronic ground state molecules, including a scheme facilitated by the superfluid-to-Mott-insulator (SF-MI) phase transition

The Chemical Evolution of the La0.6Sr0.4CoO3−δ Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity

© The Author(s) 2018Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO3 (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O2 reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.Fonds zur Förderung der wissenschaftlichen Forschung (FWF)212921411

CuZn-, PdGa- und PdZn- Modellkatalysatoren: Herstellung, In-situ-Charakterisierung und selektive Katalyse in der Methanol-Dampfreformierung

Diese vorliegende Dissertation beschreibt meine Arbeit zum Thema der Methanol Dampfreformierung auf den bimetallischen "inversen" Modellsystemen PdZn, PdGa und CuZn. Hierbei wurde besonders das katalytische Verhalten dieser Systeme in Zusammenhang mit ihrer geometrischen und elektronischen Oberflächenstruktur und ihrer Zusammensetzung untersucht. Mit Hilfe der am HZB/BESSY II unter Reaktionsbedingungen durchgeführten In-situ-XPS-Spektroskopie, konnte so der aktive und selektive Zustand der jeweiligen Modellsysteme identifiziert und genau beschrieben werden. Der vergleichende Überblick über alle drei Modellkatalysator-Systeme zeigt, dass das katalytische Verhalten aller drei Systeme einem klaren Grundschema bzw. Mechanismus folgt. Als wichtigste Grundeigenschaft für einen aktiven und selektiven Methanoldampfreformier-Katalysator kann die Fähigkeit der Wasseraktivierung hervorgehoben werden. Ist diese Fähigkeit nicht gegeben, kommt es je nach System zu vermehrter Formaldehydbildung (bei Kupfer/Zink) oder CO-Bildung (bei Palladium/Zink bzw. Palladium/Gallium). Auf reinem Kupfer bleibt die Methanolzersetzung auf Stufe des Formaldehyds stehen, wohingegen dieses auf Palladium bzw. palladiumartigen Oberflächen weiter zu CO dehydrogeniert wird. Für nicht kupferbasierte Katalysatoren ist daher auch eine elektronische Modifikation unumgänglich um den Weg zu CO zu unterdrücken. Dies wurde durch die Modifikation des Palladiums durch Zink erreicht. Eine dickere Pd:Zn-1:1-intermetallische Oberflächenphase zeigt eine Reduktion der DOS an der Fermikante (ähnlich dem Kupfer). Es hat sich jedoch gezeigt, dass nicht nur die elektronische Modifikation bei PdZn wichtig ist, sondern auch eine geometrische Modifikation. Erst durch die Korrugation der Oberfläche (Zink außen, Palladium innen) wird eine Wasseraktivierung auf der Modellkatalysatoroberfläche möglich. Nur eine ausreichend dicke Pd:Zn-1:1-intermetallische Oberflächenphase bildet diese Korrugation aus. Beim PdGa-System wird zwar durch das Gallium die elektronische Struktur des Palladiums verändert, aber die von mir präparierte 1:1-intermetallische Oberfläche ist nicht in der Lage unter den vorgegebenen MSR-Bedingungen Wasser zu aktivieren. Dass theoretisch eine CO2-selektive MSR-Reaktion möglich wäre, zeigen die Versuche zur OSR auf PdGa. Dort ist bereits bei niedrigen Reaktionstemperaturen eine hohe Aktivität und Selektivität zu beobachten. Die 1:1-PdGa-intermetallische Phase kann also - im Gegensatz zu Wasser - Sauerstoff sehr gut aktivieren. Beim Vorhandensein von Coadsorbiertem Sauerstoff auf der Oberfläche läuft die Reaktion dann bereits bei niederen Temperaturen bis zum CO2 ab. Kupfer weist bereits die richtige elektronische Struktur auf, ist jedoch nicht in der Lage Wasser zu aktivieren. Erst durch die Zinkmodifikation wird dies ermöglicht. Dabei ist der richtige Gehalt an Zink extrem wichtig. Ist die CuZn-Legierung zu zinkreich, überzieht sie sich unter MSR-Bedingungen mit einer Zinkoxidschicht und wird dadurch katalytisch inaktiv. Ist der Zinkgehalt zu niedrig, wird ebenfalls keine selektive MSR-Reaktion beobachtet. Nur beim richtigen Zinkgehalt (ca. 6 - 8%) bildet sich unter Reaktionsbedingungen die maximale Anzahl an CuZn-ZnO(H) Grenzflächenplätzen an der Oberfläche aus, welche für die Wasseraktivierung benötigt werden. Mit den Erkenntnissen aus der Dissertation kann aktiv zu einem besseren Verständnis zwischen der Struktur eines Katalysators, und der damit in Relation stehenden MSR-Selektivität eines Katalysators, beigetragen werden.This thesis describes my work on the topic of methanol steam reforming on the bimetallic "inverse" model systems PdZn, PdGa and CuZn. In particularly the catalytic behavior of these systems in relation to their geometric and electronic surface structure and composition was studied. With the support of the HZB/BESSY II synchrotron in-situ-XPS-spectroscopy was carried out under MSR reaction conditions. As result of these measurements the active and selective state of the model systems could be identified and described in detail. The comparative overview over all three model catalyst systems shows that the catalytic behaviors of all three systems follow a clear basic pattern or mechanism. As the most important property of an active and selective MSR-catalyst, the ability of water activation can be highlighted. If this capability is not given, increased formaldehyde formation is visible (for copper/zinc) or CO-formation (for palladium/zinc or palladium/gallium). On pure copper the methanol decomposition remains at the level of formaldehyde. On palladium and palladium-like surfaces it is further dehydrogenated to CO. For non-copper-based catalysts, therefore, an electronic modification is necessary to suppress the way to CO. This was achieved through the modification of palladium by zinc. A thicker Pd:Zn-1:1-intermetallic surface-phase shows a reduced DOS at the Fermi level (similar to copper). It has been shown that not only the electronic modification of PdZn is important, but also a geometric modification is crucial. Only by the corrugation of the surface (zinc outside, palladium inside) water activation is possible on the model catalyst surface. Only a sufficiently thick Pd:Zn-1:1-intermetallic phase forms the needed surface corrugation. For the PdGa-system the addition of gallium to palladium alters the electronic structure of palladium but the prepared 1:1-intermetallic surface is not able to activate the water under the given MSR-conditions. The measurements for OSR on PdGa show that theoretically a CO2-selectiv reaction is possible. For OSR a high activity and selectivity is already observed at low reaction temperatures. The 1:1-PdGa-intermetallic phase can activate oxygen (in contrast to water). In the presence of coadsorbed oxygen on the surface the reaction runs at low temperatures until CO2. Copper has already the right electronic structure but it is not able to activate water. Only through the modification with zinc the water activation is possible. The correct amount of zinc is extremely important. If the CuZn-alloy is to rich in Zink, it will be covered very fast by a zinc oxide layer under MSR-conditions and is then catalytically inactive. Is the zinc content too low also no selective and active MSR-reaction is visible. Only with a proper content of zinc (about 6-8%) the maximum amount of CuZn-ZnO(H) interface places, which are required for water activation, are formed on the surface under reaction conditions. With the findings in the dissertation a better understanding between the structure of a catalyst and the related selectivity/activity can be provided. The key role of the water activation is also demonstrated