69 research outputs found
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-ÎŽ (LSC), La0.6Sr0.4FeO3-ÎŽ (LSF), and SrTi0.7Fe0.3O3-ÎŽ (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 ()
for 73 sources in our sample. For those 73 sources, we fitted the
Comptonization model to estimate the temperature () of their
corona. 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 and
the photon index and (b) a significant negative correlation between
and the optical depth.Comment: 33 pages, 14 figures, Submitted to ApJ, comments are welcom
Origin of different deactivation of Pd/SnO<sub>2</sub> and Pd/GeO<sub>2</sub> 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 metalâsupport interaction after reduction at 573â673 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 metalâsupport interaction effect, but rather on a combination of missing active structural ensembles on Sn-enriched bimetallic phases and the formation of metallic ÎČ-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
Roadmap on exsolution for energy applications
Over the last decade, exsolution has emerged as a powerful new method for decorating oxide supports with uniformly dispersed nanoparticles for energy and catalytic applications. Due to their exceptional anchorage, resilience to various degradation mechanisms, as well as numerous ways in which they can be produced, transformed and applied, exsolved nanoparticles have set new standards for nanoparticles in terms of activity, durability and functionality. In conjunction with multifunctional supports such as perovskite oxides, exsolution becomes a powerful platform for the design of advanced energy materials. In the following sections, we review the current status of the exsolution approach, seeking to facilitate transfer of ideas between different fields of application. We also explore future directions of research, particularly noting the multi-scale development required to take the concept forward, from fundamentals through operando studies to pilot scale demonstrations
Production of a dual-species Bose-Einstein condensate of Rb and Cs atoms
We report the simultaneous production of Bose-Einstein condensates (BECs) of
Rb and Cs atoms in separate optical traps. The two samples are
mixed during laser cooling and loading but are separated by 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 La0.6Sr0.4CoO3âÎŽ 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
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