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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Post-Mortem Characterization of Cerium Speciation in Molten Calcium Chloride

Abstract Advancing radiochemistry and nuclear materials science requires understanding actinide interactions with molten salts, which are used in next-generation nuclear reactors and for processing of actinides to recover useful fissile material. Understanding chemical interactions of actinides with molten salts has been limited by challenges in developing spectroscopic and X-ray techniques that are compatible with the high temperatures necessary to study molten salts. In this work, interactions of CeO2 (serving as a nonradioactive surrogate for uranium and plutonium) with CaCl2 are characterized following heating. Scanning electron microscopy indicates CeO2 morphological changes from small (<1 μm) particles to 3–5 μm sheets. Powder X-ray diffraction and infrared and Raman spectroscopies show the formation of CeOCl at higher (850–1050 °C) temperatures. In the absence of a chemical reducing agent, it was found that a high-temperature, low-oxygen environment is the key to the formation of oxychloride and that oxychloride formation is inhibited by annealing the CeO2 starting material. Lastly, thermal analysis revealed lowering of the melting point of CaCl2 after heating with CeO2. In all, this work identifies applicable spectroscopic techniques to target studies of heavy elements in molten salt environments and highlights the relationship between chemical speciation and melting behavior, a key thermophysical property.

Kelly, Sheridon N. [Lawrence Livermore National La↗
DOE OSTI2021

Unraveling the Origin of Photocatalytic Deactivation in CeO 2 /Nb 2 O 5 Heterostructure Systems during Methanol Oxidation: Insight into the Role of Cerium Species

The study provides deep insight into the origin of photocatalytic deactivation of Nb 2 O 5 after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO 2 /Nb 2 O 5 heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV-visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb 2 O 5 and CeO 2 ) and as-prepared CeO 2 / Nb 2 O 5 heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by operando-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO 2 /Nb 2 O 5 heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb 2 O 5 to CeO 2 . However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

High-Pressure Structural and Thermodynamic Properties of Cerium Orthosilicates (CeSiO 4 )

Pressure-induced phase transitions from the zircon structure-type (I4 1 /amd) to the scheelite structure type (I4 1 /a) are known for many ternary oxides systems (ABO 4 ). In this work, we present the first high-pressure study on synthetic stetindite (CeSiO 4 ) by a combination of in situ high-pressure synchrotron powder X-ray diffraction up to 36 GPa, implemented with and without dual sided laser heating, and in situ high-pressure Raman spectroscopy up to 43 GPa. Two phase transitions were identified: zircon to a high-pressure low-symmetry (HPLS) phase at 15 GPa and then to a scheelite at 18 GPa. The latter from HPLS scheelite phase was found irreversible; i.e., scheelite is fully quenchable at ambient conditions, as in other zircon-type phases. The bulk moduli (K 0 ) of stetindite, HPLS, and high-pressure scheelite phases were determined, respectively, as 171(5), 105(4), and 221(40) GPa by fitting to a second-order Birch-Murnaghan equation of state. The pressure derivatives of vibrational modes and Gru''neisen parameters of the zircon-structured polymorph are similar to those of other orthosilicate minerals. In conclusion, due to the larger ionic radii of Ce 4+ , with respect to Zr 4+ , stetindite was found to possess a softer bulk modulus and undergo the phase transitions at a lower pressure than zircon (ZrSiO 4 ), such observations are consistent with what were found in coffinite (USiO 4 ).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Oxidation and Reduction of Polycrystalline Cerium Oxide Thin Films in Hydrogen

This study investigates the oxidation state of ceria thin films' surface and subsurface under 100 mTorr hydrogen using ambient pressure X-ray photoelectron spectroscopy. We examine the influence of the initial oxidation state and sample temperature (25-450 °C) on the interaction with hydrogen. Our findings reveal that the oxidation state during hydrogen interaction involves a complex interplay between oxidizing hydride formation, reducing thermal reduction, and reducing formation of hydroxyls followed by water desorption. In all studied conditions, the subsurface exhibits a higher degree of oxidation compared to the surface, with a more subtle difference for the reduced sample. The reduced samples are significantly hydroxylated and covered with molecular water at 25 °C. We also investigate the impact of water vapor impurities in hydrogen. We find that although 1 × 10 -6 Torr water vapor oxidizes ceria, it is probably not the primary driver behind the oxidation of reduced ceria in the presence of hydrogen.

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

Stable and Catalytically Active Shape-Engineered Cerium Oxide Nanorods by Controlled Doping of Aluminum Cations

Shape-engineered nanocrystals (SENs) promise a better selectivity and a higher activity in catalytic reactions than the corresponding non-shape-engineered ones because of their larger specific surface areas and desirable crystal facets. However, often, it is challenging to apply SENs in practical catalytic applications at high reaction temperatures, where SENs deforms into more stable, less active nanoparticles. In this paper, we show that atomic layer deposition (ALD) of Al 2 O 3 at 200 °C can controllably dope Al cations into the shape-engineered CeO 2 nanorods (NRs) to not only increase their shape transition temperature from 400 °C to beyond 700 °C but also greatly increase their specific reversible oxygen storage capacity (srOSC). Furthermore, the substituted Al3+ ions impede the surface diffusion of Ce ions and therefore improve the thermal stability of CeO 2 NRs. These Al3+ dopants form -Al–O–Ce–O– clusters, which are new Ce species and can be reversibly reduced and oxidized at 500–700 °C. This low-temperature chemical doping method decouples the synthesis process of SENs from the doping process and maintains the shape of the SENs during the activation of dopants. This concept could be adopted to enable the applications of other SENs in challenging high-temperature environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Creation of an unexpected plane of enhanced covalency in cerium(III) and berkelium(III) terpyridyl complexes

Abstract Controlling the properties of heavy element complexes, such as those containing berkelium, is challenging because relativistic effects, spin-orbit and ligand-field splitting, and complex metal-ligand bonding, all dictate the final electronic states of the molecules. While the first two of these are currently beyond experimental control, covalent M‒L interactions could theoretically be boosted through the employment of chelators with large polarizabilities that substantially shift the electron density in the molecules. This theory is tested by ligating Bk III with 4’-(4-nitrophenyl)-2,2’:6’,2”-terpyridine (terpy*), a ligand with a large dipole. The resultant complex, Bk(terpy*)(NO 3 ) 3 (H 2 O)·THF, is benchmarked with its closest electrochemical analog, Ce(terpy*)(NO 3 ) 3 (H 2 O)·THF. Here, we show that enhanced Bk‒N interactions with terpy* are observed as predicted. Unexpectedly, induced polarization by terpy* also creates a plane in the molecules wherein the M‒L bonds trans to terpy* are shorter than anticipated. Moreover, these molecules are highly anisotropic and rhombic EPR spectra for the Ce III complex are reported.

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

Reforming of methane with carbon dioxide over cerium oxide promoted nickel nanoparticles deposited on 4-channel hollow fibers by atomic layer deposition

Ni nanoparticles were deposited on four-channel structured α-Al2O3 hollow fibers by atomic layer deposition (ALD). CeO 2 was loaded by a liquid phase incipient wetness method to promote Ni catalysts for dry reforming of methane. For Ni/Al 2 O 3 prepared by ALD, inactive NiAl2O4 that originated from the Ni ALD process was incompletely reduced. The introduced CeO 2 was found to weaken the NiO–Al 2 O 3 interaction, free NiO from NiO–Al 2 O 3 or NiAl 2 O 4 , and improve the reducibility of NiO. The higher reducibility of NiAl 2 O 4 , tuned by CeO 2 , further activated the catalyst during DRM, because a larger proportion of NiAl 2 O 4 was gradually reduced to metallic nickel by the reaction products. The optimal catalytic performance reached a methane reforming rate of 2410 L h -1 gNi -1 at 850 °C. The CeO 2 promoted catalyst also exhibited an excellent performance after regeneration. In addition, the inhibition effect of CeO 2 on coke formation was observed, due to the enhanced CO 2 dissociative adsorption by CeO 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗