SEARCH · Engineering Papers

Results for “CEOS”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 199 records · Page 11

All sources NASA NTRS DOE OSTI

DOE OSTI2023

The Sluggish Diffusion of Cations in CeO 2 Probed through Molecular Dynamics and Metadynamics Simulations

Cation diffusion in fluorite‐structured CeO 2 , though far slower than anion diffusion, is an important, high‐temperature process because it governs diverse fabrication and degradation phenomena. Herein, cation diffusion is studied by means of classical molecular dynamics and metadynamics simulations. Three different mechanisms are examined: migration involving an isolated cerium vacancy, migration involving a cerium vacancy in a defect associate with an oxygen vacancy, and migration involving a cation divacancy. For each mechanism, defect diffusion coefficients are calculated as a function of temperature, from which the respective activation enthalpy of defect migration is obtained. Through comparisons with experimental cation diffusion data (specifically, of the absolute magnitude of the cation diffusivity as well as its activation enthalpy), it is concluded that cation diffusion takes place predominantly neither by isolated vacancies nor by cation vacancy–oxygen vacancy associates but by cation divacancies.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Engineering of Grain Boundaries in CeO 2 Enabling Tailorable Resistive Switching Properties

Abstract Defect engineering in valence change memories aimed at tuning the concentration and transport of oxygen vacancies are studied extensively, however mostly focusing on contribution from individual extended defects such as single dislocations and grain boundaries. In this work, the impact of engineering large numbers of grain boundaries on resistive switching mechanisms and performances is investigated. Three different grain morphologies, that is, “random network,” “columnar scaffold,” and “island‐like,” are realized in CeO 2 thin films. The devices with the three grain morphologies demonstrate vastly different resistive switching behaviors. The best overall resistive switching performance is shown in the devices with “columnar scaffold” morphology, where the vertical grain boundaries extending through the film facilitate the generation of oxygen vacancies as well as their migration under external bias. The observation of both interfacial and filamentary switching modes only in the devices with a “columnar scaffold” morphology further confirms the contribution from grain boundaries. In contrast, the “random network” or “island‐like” structures result in excessive or insufficient oxygen vacancy concentration migration paths. The research provides design guidelines for grain boundary engineering of oxide‐based resistive switching materials to tune the resistive switching performances for memory and neuromorphic computing applications.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Dry Reforming of Methane on a Highly‐Active Ni‐CeO 2 Catalyst: Effects of Metal‐Support Interactions on C−H Bond Breaking

Abstract Ni‐CeO 2 is a highly efficient, stable and non‐expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO 2 at temperatures as low as 300 K, generating CH x and CO x species on the surface of the catalyst. Strong metal–support interactions activate Ni for the dissociation of methane. The results of density‐functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO 2− x (111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CH x or C species are detected in the C 1s XPS region. The reforming of methane proceeds in a clean and efficient way.

Liu, Zongyuan↗

DOE OSTI2016

Dry reforming of methane on a highly-active Ni-CeO 2 catalyst: Effects of metal-support interactions on C–H bond breaking

Ni-CeO 2 is a highly efficient, stable and non-expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO 2 at temperatures as low as 300 K, generating CH x and CO x species on the surface of the catalyst. Strong metal–support interactions activate Ni for the dissociation of methane. The results of density-functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO 2–x (111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CH x or C species are detected in the C1s XPS region. As a result, the reforming of methane proceeds in a clean and efficient way.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Variations in the surface chemistry of methanol with CeO 2 (100) at low and elevated pressures

The reactions of methanol with and without O 2 were studied on a flat, highly crystalline CeO 2 (100) thin‐film surface with ambient pressure XPS. In the absence of O 2 , the ambient pressure XPS results indicate that in both low‐pressure (≤10 −5 Torr) and high‐pressure regimes (≥10 −1 Torr), the dominant surface species is methoxy. Methanol decomposition substantially reduces the ceria and C X deposit build‐up on the surface. When O 2 is present, C X does not accumulate on the surface and the dominant surface species is different in the low‐pressure and high‐pressure regimes. Methoxy dominates at low pressure, while formate dominates at the higher pressure. The type of surface species appears to be related to the ability of O 2 to fully oxidize the ceria surface during the methanol reaction.

Mullins, David R.↗

DOE OSTI2020

Microkinetic simulation and fitting of the temperature programmed reaction of methanol on CeO 2 (111): H 2 and H 2 O + V production

The kinetics and mechanism for temperature programmed reaction following adsorption of an adsorbate can be better understood by simulation and fitting with comparison to experiment. A case study is presented here for the chemistry of adsorption of methanol on a CeO 2 (111) surface followed by heating. The gas products observed are CH 3 OH, CH 2 O, H 2 , H 2 O, CO, CO 2 . At low temperatures (< 500 K), there is formation of H 2 and H 2 O, where the H 2 O formation is accompanied by lattice oxygen vacancy (V) formation and is thus important in determining the selectivity towards different products. Microkinetic modeling was performed using a recently published method for fitting to gain mechanistic knowledge of the H 2 and H 2 O + V formation at < 500 K. In the kinetic models used here, most of the H 2 and H 2 O + V V formation can be explained by a mechanism in which a metastable state of hydrogen on the surface (H*) acts as an intermediate. Two possibilities were investigated for the source of the metastable H* intermediate: H* from CH bond breaking of methoxies, or promotion of H+ to H* via electron transfer from ionic methoxies absorbed in oxygen vacancies (CH 3 O - /V). From this study, we consider the latter to be more likely at < 500 K. For the H 2 O formation, it was found to be critical that H 2 O cannot dissociate directly on oxygen vacancies. Finally, catalytic chemistry was observed in simulations, including catalytic formation of oxygen vacancies. Various features of the experimental results were reproduced, including methoxies being the major carbon containing species on the surface at < 500 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Elevated Temperature Solid Particle Erosion Performance of Plasma-Sprayed Co-based Composite Coatings with Additions of Al{sub 2}O{sub 3} and CeO{sub 2}

In this paper, investigation into solid particle erosion behavior of atmospheric plasma-sprayed composite coating of CoCrAlY reinforced with Al{sub 2}O{sub 3} and CeO{sub 2} oxides on Superni 76 at elevated temperature of 600 °C is presented. Alumina particles are used as erodent at two impact angles of 30° and 90°. The microstructure, porosity, hardness, toughness and adhesion properties of the as-sprayed coatings are studied. The effects of temperature and phase transformation in the coatings during erosion process are analyzed using XRD and EDS techniques. Optical profilometer is used for accurate elucidation of erosion volume loss. CoCrAlY/CeO{sub 2} coating showed better erosion resistance with a volume loss of about 50% of what was observed in case of CoCrAlY/Al{sub 2}O{sub 3}/YSZ coating. Lower erosion loss is observed at 90° as compared to 30° impact angle. The erosion mechanism evaluated using SEM micrograph revealed that the coatings experienced ductile fracture exhibiting severe deformation with unusual oxide cracks. Reinforced metal oxides provide shielding effect for erodent impact, enabling better erosion resistance. The oxidation of the coating due to high-temperature exposure reforms erosion process into oxidation-modified erosion process.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Comparison of Pt-BaO/Al 2 O 3 and Pt-CeO 2 /Al 2 O 3 for NOx storage and reduction: Impact of cycling frequency

NOx reduction under net lean and near-stoichiometric conditions was carried out on Pt/Al 2 O 3 , Pt/CeO 2 /Al 2 O 3 and Pt/BaO/Al 2 O 3 washcoated monoliths to compare performance features and identify reaction pathways. The impact of the storage components (BaO, CeO 2 ) on the NOx conversion and byproduct (NH 3 and N 2 O) yields was quantified for a range of feed temperatures, reductant types (H 2 , and C 3 H 6 ), O 2 feed concentrations, and cycle times. The NOx storage functionality is essential for NOx reduction under net lean conditions while the oxygen storage functionality promotes NOx reduction for near-stoichiometric conditions. NOx conversion by H 2 under lean conditions is dependent on the NOx storage capacity of the catalyst, with Pt/CeO 2 /Al 2 O 3 and Pt/BaO/Al 2 O 3 exhibiting the highest NOx conversion below and above 300 °C, respectively. High NOx conversion is achieved over Pt/CeO 2 /Al 2 O 3 for anaerobic rich feeds at temperatures above 400 °C. Increasing the O 2 feed concentration enhances NOx conversion over Pt/CeO 2 /Al 2 O 3 below 400 °C but inhibits NOx conversion above 400 °C. The former is attributed to promotion of NO oxidation leading to NOx storage while the latter is attributed to O 2 inhibition of NO decomposition/reduction. Shorter cycle times increase the NOx conversion with C 3 H 6 as reductant over Pt/BaO/Al 2 O 3 under lean conditions and over PCA for the near-stoichiometric feed. The findings confirm that improved NOx storage utilization is mainly responsible for NOx conversion enhancement. A ceria redox pathway has only a secondary effect on NOx conversion under excess O 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

A search for temperature induced time-dependent structural transitions in 10mol%Sc 2 O 3 –1mol%CeO 2 –ZrO 2 and 8mol%Y 2 O 3 –ZrO 2 electrolyte ceramics

Both Sc 2 O 3 –CeO 2 -stabilized-ZrO 2 (SCSZ) and Y 2 O 3 -stabilized-ZrO 2 (YSZ) show similar Young's modulus damping at 175–400 °C and 200–400 °C, respectively, by impulse excitation acoustic technique. The phase transition in SCSZ is considered responsible for the damping; however for YSZ, such a phase transition has never been reported. To clarify the relation of damping and structural transition in these two materials, the time-dependent high temperature stabilities are studied by in situ X-ray diffraction, neutron diffraction and Raman scattering during long-term annealing of SCSZ at 350 °C and YSZ at 275 °C. The cubic-to-rhombohedral transition in SCSZ is detected. However, the existence of t' phase in YSZ raw powders is confirmed without significant changes of the cubic structure during isothermal annealing. It is concluded that the phase transition contributes to the Young's modulus damping in SCSZ, while the structural transition is excluded from the reason for damping in YSZ.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Pd-CeO 2 catalyst facilely derived from one-pot generated Pd@Ce-BTC for low temperature CO oxidation

Due to the capacity to offer abundant catalytic sites within porous solids featuring high surface areas, metal-organic frameworks (MOFs) and their derivatives have garnered considerable attention as prospective catalysts in environmental catalysis. Here, to promote the industrial application of MOFs, there is an urgent need for an effective and environmental-friendly preparation approach. Breaking through the limitation of the traditional two-step preparation method that Pd was introduced to the already prepared Ce-BTC (Pd/Ce-BTC, BTC = 1, 3, 5 benzenetricarboxylate), in this work, we present a novel one-pot solvothermal method for synthesizing the Pd material supported by Ce-BTC (Pd@Ce-BTC). After pyrolysis in N 2 flow or air flow, Pd-CeO 2 catalysts derived from Pd@Ce-BTC exhibited much higher CO oxidation activity than those from Pd/Ce-BTC. Moreover, Pd/Ce-BTC and Pd@Ce-BTC pyrolyzed in N 2 flow (Pd/Ce-BTC-N and Pd@Ce-BTC-N) could better catalyze the oxidation of CO than Pd/Ce-BTC and Pd@Ce-BTC pyrolyzed in air flow (Pd/Ce-BTC-A and Pd@Ce-BTC-A). Further characterizations revealed that the abundant surface Ce 3+ species, rich surface adsorbed oxygen species and superior redox properties were the main reasons for the superior CO oxidation activity of Pd@Ce-BTC-N.

36 MATERIALS SCIENCE↗

DOE OSTI2021

High temperature mechanical properties of fluorite crystal structured materials (CeO 2 , ThO 2 , and UO 2 ) and advanced accident tolerant fuels (U 3 Si 2 , UN, and UB 2 )

The mechanical interaction between the fuel and cladding that occurs during operation of a nuclear reactor is important to understand as it can lead to cladding failures and release of radioactive material into the coolant. Additionally, in order to develop better models of the pellet-clad mechanical interactions, the mechanical properties of the fuel at relevant operating temperatures, like the elastic moduli, are needed for current and advanced accident tolerant fuels (ATFs). In this work, elevated temperature nanoindentation and resonant ultrasound spectroscopy were used to measure the moduli and hardness of several fluorite materials (CeO 2 , ThO 2 , UO 2 ) and several ATF candidates (ATF) (U 3 Si 2 , UN, UB 2 ). In addition, a comparison of the two techniques was performed in this study to independently validate the mechanical properties.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2014

Effect of 8 MeV electron beam irradiation on the structural and optical properties of CeO{sub 2} nanoparticles

The effect of 8 MeV electron beam irradiation on the structural and optical properties of cerium oxide nanoparticles was investigated. Ceria nanoparticles were synthesized by chemical precipitation method, and characterized by X-ray diffraction, transmission electron microscopy, ultraviolet–visible, photoluminescence and Raman spectroscopy. Ultraviolet–visible absorption spectra, photoluminescence and Raman spectra of beam irradiated samples were modified, and shifted to blue region, which were attributed to quantum size effect. Systematic observations found that nonstoichiometry, defects and size reduction caused by beam irradiation have great influence on optical band gap, blue shift, photoluminescence and Raman band modifications. Moreover, electron beam irradiation is a suitable technique to enhance the structural and optical properties of nanoceria by controlling the particle size, which may lead to potentially useful technological applications. - Highlights: • Investigated effect of beam irradiation on CeO{sub 2} nanoparticles • Beam irradiation caused size reduction and surface modification. • It increases microstrain, decreases d-spacing and broadens XRD peaks. • It also modifies optical band gap, absorption, PL and Raman bands.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Water-gas shift reaction on alumina-supported Pt-CeO x catalysts prepared by supercritical fluid deposition

Alumina-supported platinum catalysts, both with and without ceria, were prepared by supercritical fluid deposition and evaluated for activity for water-gas shift reaction. The organometallic precursor, platinum(II) acetylacetonate, was deposited from solution in supercritical carbon dioxide. Analysis of the catalysts by high resolution scanning transmission electron microscopy indicated that platinum was present in the form of highly dispersed metal nanoparticles. Pretreatment of the alumina-supported ceria in hydrogen prior to the deposition of the platinum precursor resulted in more platinum nucleated on ceria than non-pretreated alumina-supported ceria but varied in both particle size and structure. The ceria-containing catalyst that was not pretreated exhibited a more uniform particle size, and the Pt particles were encapsulated in crystalline ceria. Reaction rate measurements showed that the catalyst was more active for water-gas shift, with reaction rates per mass of platinum that exceeded most literature values for water-gas shift reaction on Pt-CeO x catalysts. The high activity was attributed to the significant fraction of platinum/ceria interfacial contact. We found that these results show the promise of supercritical fluid deposition as a scalable means of synthesizing highly active supported metal catalysts that offer efficient utilization of precious metals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Water–gas shift reaction over gold nanoparticles dispersed on nanostructured CeO x –TiO 2 (110) surfaces: Effects of high ceria coverage

We used scanning tunnelling microscopy to study the morphology of an overlayer of ceria in contact with a TiO 2 (110) substrate. Two types of domains were observed after ceria deposition. An ordered ceria film covered half of the surface and high-resolution imaging suggested a near-c(6 × 2) relationship to the underlying TiO 2 (110)-(1 × 1). For the other half of the surface, it comprised CeO x nanoparticles and reconstructed TiOx supported on TiO 2 (110)-(1 × 1). Exposure to a small amount of gold resulted in the formation of isolated gold atoms and small clusters on the ordered ceria film and TiO 2 (110)-(1 × 1) areas, which exhibited significant sintering at 500 K and showed strong interaction between the sintered gold clusters and the domain boundaries of the ceria film. The Au/CeO x /TiO 2 (110) model system proved to be a good catalyst for the water–gas shift (WGS) exhibiting much higher turnover frequencies (TOFs) than Cu(111) and Pt(111) benchmarks, or the individual Au/TiO 2 (110) and Au/CeO 2 (111) systems. Finally, for Au/CeO x /TiO 2 (110) catalysts, there was a decrease in catalytic activity with increasing ceria coverage that correlates with a reduction in the concentration of Ce 3 + formed during WGS reaction conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Synthesis of cerium oxide (CeO 2 ) by co-precipitation for application as a reference material for X-ray powder diffraction peak widths

Several methods can be used to obtain, from powder diffraction patterns, crystallite size and lattice strain of polycrystalline samples. Some examples are the Scherrer equation, Williamson–Hall plots, Warren/Averbach Fourier decomposition, Whole Powder Pattern Modeling, and Debye function analysis. To apply some of these methods, it is necessary to remove the contribution of the instrument to the widths of the diffraction peaks. Nowadays, one of the main samples used for this purpose is the LaB6 SRM660b commercialized by the National Institute of Standard Technology; the width of the diffraction peak of this sample is caused only by the instrumental apparatus. However, this sample can be expensive for researchers in developing countries. In this work, the authors present a simple route to obtain micron-sized polycrystalline CeO 2 that have a full width at half maximum comparable with the SRM660b and therefore it can be used to remove instrumental broadening.

de Lima Batista, Anderson Márcio↗

DOE OSTI2023

Oxygen K-Edge X-ray Absorption Spectra of ThO 2 and CeO 2 : Experiment, Interpretation, and Structural Effects

Experimental oxygen K-edge spectra of ThO 2 and CeO 2 are presented and interpreted based on density functional theory (DFT). The contribution of d and f orbitals to the O K-edge spectrum is identified as well-distinguished peaks, the presence of which evidences the strong hybridization of Th and Ce metal centers with O orbitals. The sensitivity of the O K-edge to both f- and d-states in the absence of a core–hole on the metal ion results in an insightful overview of the electronic structure involved in the chemical bond. In particular, the large bandwidth of the Th 5f band as compared to the Ce 4f band is observed as a set of wider and more substantial set of peaks in the O K-edge, confirming the stronger hybridization of the former with O orbitals. The peak ascribed to the 5f band of ThO 2 is found at higher energy than the 6d band, as predicted from DFT calculations on actinide dioxides. Here, to highlight the sensitivity and the potential use of the O K-edge for the characterization of ThO 2 -based systems, the sensitivity of the spectrum to structural changes such as lattice expansion and size reduction are calculated and discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Coadsorbed Species Explain the Mechanism of Methanol Temperature-Programmed Desorption on CeO 2 (111)

Here, we have used density functional theory calculations to investigate the temperature-programmed desorption (TPD) of methanol from CeO 2 (111). For the first time, low-temperature water formation and high-temperature methanol desorption are explained by our calculations. High coverages of methanol, which correspond to experimental conditions, are required to properly describe these features of the TPD spectrum. We identify a mechanism for the low-temperature formation of water involving the dissociation of two methanol molecules on the same surface O atom and filling of the resulting surface vacancy with one of the methoxy products. After water desorption, methoxy groups are stabilized on the surface and react at higher temperatures to form methanol and formaldehyde by a disproportionation mechanism. Alternatively, the stabilized methoxy groups undergo sequential C–H scission reactions to produce formaldehyde. Calculated energy requirements and methanol/formaldehyde selectivity agree with the experimental data.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Kinetic Understanding of Field-Induced Phase Transition from Tetragonal to Ferroelectric Orthorhombic Phase in Ferroelectric CeO 2 –HfO 2 –ZrO 2 Films

The ferroelectric properties and structural phase transition behaviors of fluorite-type CeO 2 −HfO 2 −ZrO 2 films were investigated. The epitaxial films on indium tin oxide (ITO) (111)/yttria-stabilized zirconia (YSZ) (111) substrates were grown through pulsed laser deposition at room temperature and subsequently heat-treated at 1000 °C under a N 2 gas flow. The crystalline phases and Curie temperatures of the films were investigated by X-ray diffraction. An increase in the Ce or Zr content in the films led to a higher crystallographic symmetry, such as orthorhombic or tetragonal. In addition, electrical characterization revealed that the orthorhombic films and some of the tetragonal films displayed ferroelectricity. This was due to the field-induced phase transition from the tetragonal to ferroelectric orthorhombic phase in the films, where the Curie temperatures were relatively low. The tetragonal metastable phase was kinetically frozen and could not change into the stable orthorhombic phase at such a low temperature. The critical electric field where the field-induced phase transition occurred was below 0.8 MV/cm, which was sufficiently small compared to the coercive field. These results evidence the kinetic driving force that causes a field-induced phase transition from the paraelectric tetragonal phase to the ferroelectric orthorhombic phase in HfO 2 -based ferroelectrics. They also enhance our understanding of the thermodynamic phase stabilities of HfO 2 -based material polymorphs.

36 MATERIALS SCIENCE↗