314 research outputs found
Quantitative powder diffraction using a (2+3) surface diffractometer and an area detector
X-ray diffractometers primarily designed for surface x-ray diffraction are
often used to measure the diffraction from powders, textured materials, and
fiber-texture samples in so-called scans. Unlike high-energy powder
diffraction only a fraction of the powder rings is typically measured and the
data consists of many detector images across the range. Such
diffractometers typically scan in directions not possible on a conventional
lab-diffractometer, which gives enhanced control of the scattering vector
relative to the sample orientation. There are, however, very few examples where
the measured intensity is directly used, such as for profile/Rietveld
refinement, as is common with other powder diffraction data. Although the
underlying physics is known, converting the data is time-consuming and the
appropriate corrections are dispersed across several publications, often not
with powder diffraction in mind. In this paper we present the angle
calculations and correction factors required to calculate meaningful
intensities for scans with a (2+3)-type diffractometer and an area
detector. We also discuss some of the limitations with respect to texture,
refraction, and instrumental resolution, and what kind of information one can
hope to obtain.Comment: Research paper, 15 pages, 12 figures, 3 table
CO Oxidation on a Pd(100)
A stagnation flow reactor has been designed and characterized for both experimental and modeling studies of single-crystal model catalysts in heterogeneous catalysis. Using CO oxidation over a Pd(100) single crystal as a showcase, we have employed planar laser-induced fluorescence (PLIF) to visualize the CO2 distribution over the catalyst under reaction conditions and subsequently used the 2D spatially resolved gas phase data to characterize the stagnation flow reactor. From a comparison of the experimental data and the stagnation flow model, it was found that characteristic stagnation flow can be achieved with the reactor. Furthermore, the combined stagnation flow/PLIF/modeling approach makes it possible to estimate the turnover frequency (TOF) of the catalytic surface from the measured CO2 concentration profiles above the surface and to predict the CO2, CO and O2 concentrations at the surface under reaction conditions
Infrared Spectroscopy as Molecular Probe of the Macroscopic Metal-Liquid Interface
Peer reviewedPublisher PD
An in situ set up for the detection of CO(2) from catalytic CO oxidation by using planar laser-induced fluorescence.
We report the first experiment carried out on an in situ setup, which allows for detection of CO(2) from catalytic CO oxidation close to a model catalyst under realistic reaction conditions by the means of planar laser-induced fluorescence (PLIF) in the mid-infrared spectral range. The onset of the catalytic reaction as a function of temperature was followed by PLIF in a steady state flow reactor. After taking into account the self-absorption of CO(2), a good agreement between the detected CO(2) fluorescence signal and the CO(2) mass spectrometry signal was shown. The observed difference to previously measured onset temperatures for the catalytic ignition is discussed and the potential impact of IR-PLIF as a detection technique in catalysis is outlined
Local Density of States and Interface Effects in Semimetallic ErAs Nanoparticles Embedded in GaAs
The atomic and electronic structures of ErAs nanoparticles embedded within a
GaAs matrix are examined via cross-sectional scanning tunneling microscopy and
spectroscopy (XSTM/XSTS). The local density of states (LDOS) exhibits a finite
minimum at the Fermi level demonstrating that the nanoparticles remain
semimetallic despite the predictions of previous models of quantum confinement
in ErAs. We also use XSTS to measure changes in the LDOS across the ErAs/GaAs
interface and propose that the interface atomic structure results in electronic
states that prevent the opening of a band gap.Comment: 9 pages, 3 figur
Anisotropic strain variations during the confined growth of Au nanowires
The electrochemical growth of Au nanowires in a template of nano-porous
anodic aluminum oxide was investigated in situ by means of grazing-incidence
transmission small- and wide-angle x-ray scattering (GTSAXS and GTWAXS), x-ray
fluorescence (XRF) and 2-dimensional surface optical reflectance (2D-SOR). The
XRF and the overall intensity of the GTWAXS patterns as a function of time were
used to monitor the progress of the electrodeposition. Furthermore, we
extracted powder diffraction patterns in the direction of growth and in the
direction of confinement to follow the evolution of the direction-dependent
strain. Quite rapidly after the beginning of the electrodeposition, the strain
became tensile in the vertical direction and compressive in the horizontal
direction, which showed that the lattice deformation of the nanostructures can
be artificially varied by an appropriate choice of the deposition time. By
alternating sequences of electrodeposition to sequences of rest, we observed
fluctuations of the lattice parameter in the direction of growth, attributed to
stress caused by electromigration.. Furthermore, the porous domain size
calculated from the GTSAXS patterns was used to monitor how homogeneously the
pores were filled.Comment: Short communication manuscript. Four figure
Metastable Precursor Structures in Hydrogen-infused Super Duplex Stainless Steel Microstructure – An Operando Diffraction Experiment
We report the evolution of metastable precursor structures during hydrogen infusion in the near-surface region of a super duplex stainless steel. Grazing-incidence x-ray diffraction was employed to monitor, operando, the lattice degradation of the austenite and ferrite phases. Electrochemical hydrogen charging resulted in the splitting of the diffraction peaks of the austenite phase, suggesting the evolution of a metastable precursor structure. This may be explained by the formation of quasi-hydrides, which convert back into the austenite parent structure during hydrogen effusion. The ferrite showed less lattice deformation than the austenite and no phase transformation
Formation and Structure of Graphene Waves on Fe(110)
A very rich Fe-C phase diagram makes the formation of graphene on iron surfaces a challenging task. Here we demonstrate that the growth of graphene on epitaxial iron films can be realized by chemical vapor deposition at relatively low temperatures, and that the formation of carbides can be avoided in excess of the carbon-containing precursors. The resulting graphene monolayer creates a novel periodically corrugated pattern on Fe(110). Using low-energy electron microscopy and scanning tunneling microscopy, we show that it is modulated in one dimension forming long waves with a period of similar to 4 nm parallel to the [001] direction of the substrate, with an additional height modulation along the wave crests. The observed topography of the graphene/Fe superstructure is well reproduced by density functional theory calculations, and found to result from a unique combination of the lattice mismatch and strong interfacial interaction, as probed by core-level photoemission and x-ray absorption spectroscopy
hydride breathing during the template-assisted electrodeposition of Pd nanowires
We investigated the structural evolution of electrochemically fabricated Pd
nanowires by means of grazing-incidence transmission small-
and wide-angle x-ray scattering (GTSAXS and GTWAXS), x-ray fluorescence (XRF)
and 2-dimensional surface optical reflectance (2D-SOR). This shows how
electrodeposition and the hydrogen evolution reaction (HER) compete and
interact during Pd electrodepositon. During the bottom-up growth of the
nanowires, we show that -phase Pd hydride is formed. Suspending the
electrodeposition then leads to a phase transition from - to
-phase Pd hydride. Additionally, we find that grain coalescence later
hinders the incorporation of hydrogen in the Pd unit cell. GTSAXS and 2D-SOR
provide complementary information on the volume fraction of the pores occupied
by Pd, while XRF was used to monitor the amount of Pd electrodeposited.Comment: 17 pages, 11 figures, 4 appendice
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