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Daemen, Luke L.

Publications and source records attributed to Daemen, Luke L..

66 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Phonon Spectroscopy in Antimony and Tellurium Oxides

α-Sb 2 O 3 (senarmontite), β-Sb 2 O 3 (valentinite), and α-TeO 2 (paratellurite) are compounds with pronounced stereochemically active Sb and Te lone pairs. The vibrational and lattice properties of each have been previously studied but often lead to incomplete or unreliable results due to modes being inactive in infrared or Raman spectroscopy. Here in this paper, we present a study of the relationship between bonding and lattice dynamics of these compounds. Mössbauer spectroscopy is used to study the structure of Sb in α-Sb 2 O 3 and β-Sb 2 O 3 , whereas the vibrational modes of Sb and Te for each oxide are investigated using nuclear inelastic scattering, and further information on O vibrational modes is obtained using inelastic neutron scattering. Additionally, vibrational frequencies obtained by density functional theory (DFT) calculations are compared with experimental results in order to assess the validity of the utilized functional. Good agreement was found between DFT-calculated and experimental density of phonon states with a 7% scaling factor. The Sb–O–Sb wagging mode of α-Sb 2 O 3 whose frequency was not clear in most previous studies is experimentally observed for the first time at ~340 cm -1 . Softer lattice vibrational modes occur in orthorhombic β-Sb 2 O 3 compared to cubic α-Sb 2 O 3 , indicating that the antimony bonds are weakened upon transforming from the molecular α phase to the layer-chained β structure. The resulting vibrational entropy increase of 0.45 ± 0.1 kB/Sb 2 O 3 at 880 K accounts for about half of the α–β transition entropy. The comparison of experimental and theoretical approaches presented here provides a detailed picture of the lattice dynamics in these oxides beyond the zone center and shows that the accuracy of DFT is sufficient for future calculations of similar material structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Pressure-induced phase transition in barium hydride studied with neutron scattering

Barium hydride can undergo a structural phase transition from an orthorhombic phase to a hexagonal phase induced by high temperature or high pressure. This transition causes an immediate increase in the hydrogen diffusion rates by over an order of magnitude, and therefore, understanding the origin and details of such transition is of great interest not only for fundamental reasons but also for improving materials for future applications. In this work, the pressure evolution of the crystal structure was characterized using neutron powder diffraction up to a maximum pressure of 11.3 GPa. The pressure dependence of the unit cell volumes, lattice parameters, atomic sites, and compressibilities were determined for both phases. A structural phase transition occurred over a wide pressure range of P = 1.3 GPa–4.9 GPa. The transition to the higher density hexagonal phase reduced the volume per formula unit of BaD 2 by 13.6%, hence increasing the volumetric storage density. In addition, we investigated the hydrogen diffusion process using high pressure quasi-elastic neutron scattering up to 7.1 GPa. Our results show that the hydrogen mobility increases with pressure in the hexagonal phase. This work sheds light on the structural and dynamical aspects of barium hydride caused by the application of high pressure. Finally, the results may aid in the development of advanced metal hydride systems with increased hydrogen dynamics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Thermal Neutron Scattering Evaluation of Yttrium Hydride (FY2020 Progress)

This paper details the thermal neutron scattering measurements of yttrium hydride for various hydrogen concentrations and temperatures that were conducted at the Spallation Neutron Source at Oak Ridge National Laboratory. Measurements at a temperature range of 5 to 1,200 K were conducted to determine the change in inelastic scattering as a function of temperature and to probe for any anharmonic effects known to exist in other metallic hydrides. Additionally, hydrogen concentrations of YH x that range from x = 1.62 to 1.90 were measured to determine the effects of varying hydrogen concentration on the inelastic neutron spectra. Changes in temperature affected the inelastic spectra in unanticipated ways, indicating that there are anharmonic effects, whereas hydrogen concentration does not significantly affect the inelastic spectra. These measurements were compared against the ENDF/B-VIII.0 thermal scattering files of hydrogen in YH 2 and yttrium in YH 2 , as well as a new thermal scattering file created by using the stochastic temperature-dependent effective potential (s-TDEP). Both libraries were found to be in good agreement with the experimental data at lower temperatures. At higher temperatures, the s-TDEP method is better at predicting the experimentally observed softening of phonon modes in acoustic and optical regions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

On single-crystal neutron-diffraction in DACs: quantitative structure refinement of light elements on SNAP and TOPAZ

Quantitative single crystal neutron-diffraction in diamond anvil cells has so far been limited by the neutron flux available at the various neutron sources. As a result, highly precise measurements of the exact position of light elements have not been possible preventing, for example, structural studies of hydrogen and hydrogen bonds under pressure. Here we report experiments carried out on SNAP at the Spallation Neutron Source (ORNL, TN, USA) to explore the possibility and current limits of such studies. Furthermore, we benchmarked the obtained data quality with reference experiments carried out on TOPAZ, a dedicated single-crystal instrument.We show that measuring single-crystal diffraction intensities on SNAP is possible to such a precision that we are able to resolve the hydrogen bonds in potassium dideuterium phosphate (DKDP) as well as in ice VI.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Activation and surface reactions of CO and H 2 on ZnO powders and nanoplates under CO hydrogenation reaction conditions

Here, activation and surface reactions of CO and H 2 on ZnO powders and nanoplates under CO hydrogenation reaction conditions were (quasi) in situ studied using temperature programmed surface reaction spectra, diffuse reflectance Fourier transform infrared spectroscopy, inelastic neutron scattering spectroscopy and electron paramagnetic resonance. CO undergoes disproportion reaction to produce gaseous CO 2 and surface carbon adatoms, and adsorbs to form surface formate species. H 2 adsorption forms dominant irreversibly-adsorbed surface hydroxyl groups and interstitial H species and very minor surface Zn-H species. Surface formate species and hydroxyl groups react to produce CO 2 and H 2 , while surface carbon adatoms are hydrogenated by surface Zn-H species sequentially to produce CH(a), CH 2 (a), CH 3 (a) and eventually gaseous CH 4 . The ZnO nanoplates, exposing a higher fraction of Zn-ZnO(0001) and O-ZnO(000–1) polar facets, are more active than the ZnO powders to catalyze CO hydrogenation to CH 4 . These results provide fundamental understanding of the reaction mechanisms and structural effects of CO hydrogenation reaction catalyzed by ZnO-based catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Discriminating the Role of Surface Hydride and Hydroxyl for Acetylene Semihydrogenation over Ceria through In Situ Neutron and Infrared Spectroscopy

Ceria has been used as a hydrogenation catalyst especially in selective alkyne hydrogenation, but the reaction mechanism regarding the role of different surface hydrogen species remains unclear. Here, we utilized in situ neutron and infrared vibration spectroscopy to show the catalytic role of cerium hydride (Ce–H) and hydroxyl (OH) groups in acetylene hydrogenation over ceria surfaces with different degree of reduction. In situ inelastic neutron scattering spectroscopy (INS) proved that not only Ce–H but also surface atomic hydrogen species on the reduced ceria surface can participate in acetylene semihydrogenation. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results implied that bridging OH groups both on the oxidized and reduced ceria are active in the selective hydrogenation of acetylene. It appears that surface Ce–H is more reactive than the coexisting OH species on the reduced ceria surface, but over-reduction of ceria also results in strongly bound species that may lead to catalyst deactivation. These spectroscopic results clearly explain the reaction mechanism including not only the surface chemistry but also the nature of the active hydrogen species for selective hydrogenation over ceria, providing insights into the design of more active and stable ceria-based catalysts for hydrogenation reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Nature of Reactive Hydrogen for Ammonia Synthesis over a Ru/C12A7 Electride Catalyst

Recently, there have been renewed interests in exploring new catalysts for ammonia synthesis under mild conditions. Electride-based catalysts are among the emerging ones. Ruthenium particles supported on an electride composed of a mixture of calcium and aluminum oxides (C12A7) have attracted great attention for ammonia synthesis due to their facile ability in activating N2 under ambient pressure. However, the exact nature of the reactive hydrogen species and the role of electride support still remain elusive for this catalytic system. Here, we report for the first time that the surface-adsorbed hydrogen, rather than the hydride encaged in the C12A7 electride, plays a major role in ammonia synthesis over the Ru/C12A7 electride catalyst with the aid of in situ neutron scattering techniques. Combining in situ neutron diffraction, inelastic neutron spectroscopy, density functional theory (DFT) calculation, and temperature-programmed reactions, the results provide direct evidence for not only the presence of encaged hydrides during ammonia synthesis but also the strong thermal and chemical stability of the hydride species in the Ru/C12A7 electride. Steady state isotopic transient kinetic analysis (SSITKA) of ammonia synthesis showed that the coverage of reactive intermediates increased significantly when the Ru particles were promoted by the electride form (coverage up to 84%) of the C12A7 support rather than the oxide form (coverage up to 15%). Such a drastic change in the intermediate coverage on the Ru surface is attributed to the positive role of electride support where the H2 poisoning effect is absent during ammonia synthesis over Ru. The finding of this work has significant implications for understanding catalysis by electride-based materials for ammonia synthesis and hydrogenation reactions in general.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗