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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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10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Performance testing of a prototype Pd-Ag diffuser

The fusion fuel cycle has gained significant attention over the last decade as interest in fusion programs has increased. One of the critical components of the fusion process is the tritium fuel cycle. The tritium fuel cycle is designed to supply and recycle process tritium at a specific throughput rate. One of the most important processes within the tritium fuel cycle is the clean-up of the of the process tritium. This step will initially separate the hydrogen isotopes (H2, D2, and T2) from the rest of the process gas using Pd-Ag diffusers or permeators. The Pd-Ag diffuser is an integral component for any tritium purification system; whether part of the United States’ defense mission or fusion programs. Domestic manufacturers of Pd-Ag diffusers are extremely limited and only a few manufacturers exist. Johnson-Matthey (JM) Pd-Ag diffusers (permeators) have previously been evaluated for the separation of hydrogen isotopes from non-hydrogen gas species in the process. JM is no longer manufacturing Pd-Ag diffusers and a replacement vendor needs to be identified to support future needs. A prototype Pd-Ag diffuser has been manufactured by Power and Energy, and is considered a potential replacement for the JM diffuser for tritium service. New diffuser designs for a tritium facility for any fusion energy applications must be characterized by evaluating their operating envelope prior to installation in a tritium processing facility. The prototype Pd-Ag diffuser was characterized to determine the overall performance as a function of the permeation of hydrogen through the membrane. The tests described in this report consider the effects of feed gas compositions, feed flow rates, pump configuration and internal tube pressure on the permeation of H2 through the Pd-Ag tubes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Pd-Ag chronometry of IVA iron meteorites and the crystallization and cooling of a protoplanetary core

To constrain the timescales and processes involved in the crystallization and cooling of protoplanetary cores, we examined the Pd-Ag isotope systematics of the IVA iron meteorites Muonionalusta and Gibeon. A Pd-Ag isochron for Muonionalusta provides an initial 107 Pd/ 108 Pd = (2.57 ± 0.07) × 10 -5 . The three metal samples analyzed from Gibeon plot below the Muonionalusta isochron, but these samples also show significant effects of cosmic ray-induced neutron capture reactions, as is evident from 196 Pt excesses in the Gibeon samples. After correction for neutron capture effects on Ag isotopes, the Gibeon samples plot on the Muonionalusta isochron, indicating that these two IVA irons have indistinguishable initial 107 Pd/ 108 Pd. Collectively, the Pd-Ag data indicate cooling of the IVA core below Pd-Ag closure between 2.9 ± 0.4 Ma and 8.9 ± 0.6 Ma after CAI formation, where this age range reflects uncertainties in the initial 107 Pd/ 108 Pd ratios of the solar system, which in turn result from uncertainties in the Pb-Pb age of Muonionalusta. The Ag isotopic data indicate that the IVA core initially evolved with a modestly elevated Pd/Ag, but the low Ag concentrations measured for some metal samples indicate derivation from a source with much lower Ag contents and, hence, higher Pd/Ag. These contrasting observations can be reconciled if the IVA irons crystallized from an initially more Ag-rich core, followed by extraction of Fe-S melts during compaction of the nearly solidified core. Owing to its strong tendency to partition into Fe-S melts, Ag was removed from the IVA core during compaction, leading to the very low Ag concentration observed in metal samples of IVA irons. Alternatively, Ag was lost by evaporation from a still molten metallic body just prior to the onset of crystallization. The Pd-Ag isotopic data indicate that Muonionalusta cooled at >500 K/Ma through the Pd-Ag closure temperature of ~900 K, consistent with the rapid cooling inferred from metallographic cooling rates for IVA irons. Finally, combined, these observations are consistent with cooling of IVA irons in a metallic body with little or no silicate mantle.

58 GEOSCIENCES↗
DOE OSTI2023

Achieving Ultra-High Selectivity to Hydrogen Production from Formic Acid on Pd–Ag Alloys

Palladium-silver based alloy catalysts have a great potential for CO-free hydrogen production from formic acid for fuel cell applications. However, the structural factors affecting the selectivity of formic acid decomposition is still debated. Herein, the decomposition pathways of formic acid on Pd-Ag alloys with different atomic configurations have been investigated to identify the alloy structures yielding high H 2 selectively. Several Pd x Ag 1-x surface alloys with various compositions were generated on a Pd(111) single crystal; their atomic distribution and electronic structure were determined by a combination of infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). It was established that the Ag atoms with Pd neighbors are electronically altered, and the degree of alteration correlates with the number of nearest Pd. Temperature programmed reaction spectroscopy (TPRS) and DFT demonstrated that the electronically altered Ag domains create a new reaction pathway that selectively dehydrogenates formic acid. In contrast, Pd monomers surrounded by Ag are demonstrated to have a similar reactivity compared to pristine Pd(111), yielding CO and H 2 O in addition to the dehydrogenation products. Furthermore, they bind to the produced CO weaker than pristine Pd, demonstrating an enhancement in resistance to CO poisoning. This work therefore shows that surface Ag domains modified by interaction with subsurface Pd are the key active sites for selective decomposition of formic acid, while surface Pd atoms are detrimental for selectivity. Hence the decomposition pathways can be tailored for CO-free H 2 production on Pd-Ag alloy systems.

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

Pd3Ag(111) as a Model System for Hydrogen Separation Membranes: Combined Effects of CO Adsorption and Surface Termination on the Activation of Molecular Hydrogen

Abstract The co-adsorption of hydrogen and carbon monoxide on Pd 3 Ag(111) alloy surfaces has been studied as a model system for Pd-Ag alloys in membrane and catalysis applications using periodic density functional theory calculations (PW91-GGA). We explored the effects of Pd–Ag surface composition, since segregation of silver towards and away from the surface has been suggested to explain the experimentally observed changes in H 2 activation, CO inhibition and reactivity. We found that CO pre-adsorbed on the surface weakens the adsorption of H on Pd 3 Ag(111) alloy surfaces irrespective of whether the surface termination corresponds to the bulk Pd 3 Ag composition, or is purely Pd-terminated. A higher coverage of H with CO present is obtained for the Pd-terminated surface; this surface also exhibits a larger range of chemical potentials for co-adsorbed hydrogen and CO. The barrier for H 2 activation increases with increasing CO coverage, but the surface composition has the largest impact on H 2 activation at intermediate CO coverage. The results imply that Pd-based membranes with typically ~ 23 wt% Ag are less prone to CO poisoning if the surface becomes Pd-terminated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Tungsten and molybdenum isotopic evidence for an impact origin of pallasites

The origin of pallasites—stony-iron meteorites mainly composed of olivine and Fe-Ni metal—is debated and proposed formation scenarios broadly range from models that explain pallasite formation by internal processes in the mantle of a differentiated planetesimal to those that involve impact–induced mixing of core and mantle materials. Here, the origin of pallasites is examined by studying the nebular source regions of their precursor material using Mo isotopes and their history of metal-silicate segregation using Hf-W chronometry. We report new Mo and W isotopic data for a large suite of pallasite metal samples, alongside Pt isotope data to quantify superimposed cosmic ray exposure effects. Most main-group pallasites exhibit uniform pre-exposure 182 W and Mo isotopic compositions that bear an excellent similarity to those of IIIAB iron meteorites. Four main-group pallasites and the IIIAB iron Thunda have more radiogenic pre-exposure 182 W compositions, but display the same Mo isotopic composition as other main-group pallasites and IIIAB irons. This strong chronological and genetic link strongly suggests that main-group pallasite metal originated in the IIIAB parent body core. This, combined with prior Pd-Ag chronometric evidence for an early collisional disruption of the IIIAB parent body, implies that main-group pallasites formed by impact–induced mixing of metal and silicates rather than by an internal process on the IIIAB parent body. This mixing led to elevated 182 W compositions in some pallasites, which are best accounted for by partial re-equilibration of IIIAB metal with radiogenic 182 W from the colliding body. Altogether, our results support models that explain main-group pallasite formation by injection of pallasite metal into the mantle of another differentiated body, implying that pallasite silicates did not primarily derive from the IIIAB mantle, but instead from that of the colliding body.

58 GEOSCIENCES↗
DOE OSTI2020

Evolution of Metastable Structures at Bimetallic Surfaces from Microscopy and Machine-Learning Molecular Dynamics

The restructuring of interfaces plays a crucial role in materials science and heterogeneous catalysis. Bimetallic systems, in particular, often adopt very different compositions and morphologies at surfaces compared to the bulk. For the first time, we reveal a detailed atomistic picture of long-time scale restructuring of Pd deposited on Ag using microscopy, spectroscopy, and novel simulation methods. By developing and performing accelerated machine-learning molecular dynamics followed by an automated analysis method, we discover and characterize previously unidentified surface restructuring mechanisms in an unbiased fashion, including Pd-Ag place exchange and Ag pop-out as well as step ascent and descent. Remarkably, layer-by-layer dissolution of Pd into Ag is always preceded by an encapsulation of Pd islands by Ag, resulting in a significant migration of Ag out of the surface and a formation of extensive vacancy pits within a period of microseconds. These metastable structures are of vital catalytic importance, as Ag-encapsulated Pd remains much more accessible to reactants than bulk-dissolved Pd. We report our approach is broadly applicable to complex multimetallic systems and enables the previously intractable mechanistic investigation of restructuring dynamics at atomic resolution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Tuning the activity and selectivity of electroreduction of CO 2 to synthesis gas using bimetallic catalysts

The electrochemical carbon dioxide reduction reaction to syngas with controlled CO/H 2 ratios has been studied on Pd-based bimetallic hydrides using a combination of in situ characterization and density functional theory calculations. When compared with pure Pd hydride, the bimetallic Pd hydride formation occurs at more negative potentials for Pd-Ag, Pd-Cu, and Pd-Ni. Theoretical calculations show that the choice of the second metal has a more significant effect on the adsorption strength of *H than *HOCO, with the free energies between these two key intermediates (i.e., ΔG(*H)–ΔG(*HOCO)) correlating well with the carbon dioxide reduction reaction activity and selectivity observed in the experiments, and thus can be used as a descriptor to search for other bimetallic catalysts. The results also demonstrate the possibility of alloying Pd with non-precious transition metals to promote the electrochemical conversion of CO 2 to syngas.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Separation of Hydrogen Using Pd/Ag Membranes: Experimental and Modeling Results with Potential Application to Direct Internal Recycle

Implementation of fusion energy requires processing the deuterium-tritium (D-T) mixture used to fuel the reaction, and separation of hydrogen isotopes from other gases is imperative. Specifically, the separation of hydrogen isotopes from helium is a matter of importance to the fusion fuel cycle community. Initial testing with a palladium-silver (Pd-Ag) membrane indicates that even moderate vacuum (~100 torr permeate pressure) can provide a high degree of separation (>90%) at a high ratio of H 2 to He. Given the presence of He in many fusion systems, a high technology readiness level (TRL) for Q 2 /He (where Q represents any isotope of hydrogen) separations is needed. This study demonstrates the efficacy of H 2 removal from He via permeation and potential applications for direct internal recycle. Modeling will accompany the experimental campaign to generate a predictive capability and quantify the separation performance. Modeling from previous hydrogen permeation studies has demonstrated that the typical Sieverts’ law fails to predict the measured permeation rates at high hydrogen fluxes. Existing models are being refined to integrate the effects of surface phenomena into permeation predictions, which have been expanded to account for mixtures with large ranges of Q 2 concentrations. These data will improve the TRL of permeators as a separation technology for the fusion fuel cycle.

08 HYDROGEN↗
DOE OSTI2020

Electron Microscopic Examination of an Irradiated TRISO Coated Particle of AGR-2 Experiment: AGR2-222-RS019

The transmission electron microscopic (TEM) examination of an irradiated tristructural isotropic (TRISO) coated particle from advanced gas reactor (AGR) 2 fuel Compact 2-2-2 is presented in this report. Compact 2-2-2 refers to the compact in Capsule 2 at Level 2 of Stack 2. The examination on AGR2 222 RS019 is focused on the interface between the silicon carbide (SiC) to inner pyrolytic carbon (IPyC) layer, central part of SiC, and the interface between SiC and outer pyrolytic carbon layer. The SiC grain boundary distribution and associated fission product precipitates were characterized by scanning transmission electron microscopy (STEM) energy dispersive spectroscopy (EDS), and precession electron diffraction (PED) in the transmission electron microscope. Two separate locations on SiC layer were selected for on the basis of the intact of the buffer layer in the irradiated TRISO particle. The key observation in this particle is that the compositional nature of grain boundary precipitates is more complex near IPyC/SiC layer where Pd, Pd-Ag, Pd-U containing precipitates can be observed. However, the compositions of precipitate towards outer most region of SiC mainly contain Pd. Ag has been observed in the IPyC/SiC layer in one of the two locations. Comparison of the results to previously irradiated and analyzed, Variant 3, safety tested AGR-1 TRISO particles (AGR1-433-001 and AGR1 222-RS004) showed the grain boundary distributions have some noticeable differences. The differences in coincident site lattice grain boundary and misorientation angles are possibly inherent to the fabrication process. Neutron irradiation induced void distribution has been found to be nonuniform. A high concentration of smaller voids is found at stacking faults compared to those in the SiC matrix. Larger void sizes are observed in the SiC location that is near to the broken buffer layer. This void size variation with integrity of buffer layer can potentially affect the fission product retention. However, the average void size continuously decreases along the radius of SiC in both locations.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2023

Silver-decorated palladium on carbon catalyst for enhanced ammonium formate dehydrogenation

Palladium (Pd)-based catalysts efficiently convert ammonium formate solution to hydrogen at low temperatures (<100 °C), but they tend to deactivate quickly during stability testing. This manuscript presents a systematic investigation into the catalytic properties of Pd–Ag bimetallic catalysts, focusing on their surface compositions and exploring the mechanisms behind the deactivation of Pd/Ag-based catalysts. Here this study reports a carbon-supported Pd–Ag bimetallic nanoparticle (NP) catalyst obtained through a galvanic replacement method, which showed enhanced formate dehydrogenation performance. The best catalyst, Pd 3 Ag 10 /ACA-G (Pd–Ag bimetallic NPs with a 3 : 10 mass ratio loaded on acid-washed activated carbon, prepared by the galvanic replacement method), presents the highest activity with a TOF of 5202 h -1 (~2.6-fold of commercial Pd/C). The enhanced electron density of Pd–Ag bimetallic nanoparticles, coupled with the advantages of a smaller nanoparticle size, and the modulation of hydrogen adsorption energy through the Ag/Pd surface alloy on the Ag/Pd(111) facet, collectively resulted in experimentally higher turnover rates of hydrogen production. The changes on the catalyst surface, including surface Ag fraction decrease, NP size growth, and O-containing species (carboxylate, etc.) adsorption, gradually resulted in catalyst deactivation.

08 HYDROGEN↗