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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 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Energetics of boron near tungsten surfaces: A first-principles study

Interest exists in utilizing boron (B) wall conditioning of fusion tokamaks containing tungsten (W) plasma facing components, in order to improve plasma confinement. To understand the interactions of B with W surfaces, first-principles density functional theory calculations have been performed to model the adsorption, diffusion, and solution of B near the W(100), W(110), and W(111) surfaces. The results show that B within a distance of 0.6 nm above the surfaces is adsorbed to the surfaces without activation barriers. B atoms are strongly adsorbed on the W(100) surface with an adsorption energy of 7.80 eV, which is 1.22 and 1.35 eV larger than on the W(110) and W(111) surfaces. B diffusion on the W(100), W(110), and W(111) surfaces has an activation energies of 2.08, 1.12, and 1.47 eV, respectively; while, diffusion from the adsorption sites into the bulk requires 2.2–2.3 eV. The B solution energy below a clean W(100) surface is the lowest, followed by the W(111) and W(110) surfaces. B clustering and B-induced surface deformation as a function of B coverage has been investigated. B on the W(100) surface occupy epitaxial sites at coverages of 0–1.25 ML, but form clusters at higher coverages. B clustering on the W(110) and W(111) surfaces is expected throughout the adsorption process. Compared to a clean surface, B atoms on the W(100) reduce the surface effect on the B solution energy below the surface, while the presence of B on the W(110) and W(111) surfaces generally decreases or increases the B solution energy below surfaces, respectively.

Yang, L. (ORCID:0000000322166071)↗
DOE OSTI2021

Quasiparticle electronic structure of phthalocyanine:TMD interfaces from first-principles GW

Interfaces formed between monolayer transition metal dichalcogenides and (metallo)phthalocyanine molecules are promising in energy applications and provide a platform for studying mixed-dimensional molecule-semiconductor heterostructures in general. An accurate characterization of the frontier energy level alignment at these interfaces is key in the fundamental understanding of the charge transfer dynamics between the two photon absorbers. Here, we employ the first-principles substrate screening GW approach to quantitatively characterize the quasiparticle electronic structure of a series of interfaces: metal-free phthalocyanine (H 2 Pc) adsorbed on monolayer MX 2 (M = Mo, W; X = S, Se) and zinc phthalocyanine (ZnPc) adsorbed on MoX 2 (X = S, Se). Furthermore, we reveal the dielectric screening effect of the commonly used α-quartz (SiO 2 ) substrate on the H 2 Pc:MoS 2 interface using the dielectric embedding GW approach. Our calculations furnish a systematic set of GW results for these interfaces, providing the structure–property relationship across a series of similar systems and benchmarks for future experimental and theoretical studies.

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

Role of ripples in altering the electronic and chemical properties of graphene

Ripples of graphene are known to manipulate electronic and hydrogenation properties of graphitic materials. More detailed work is needed to elucidate the structure–property relationship of these systems. In this work, the density functional theory is used to compute the energy and electronic structure of the graphene models with respect to variable curvatures and hydrogen adsorption sites. The magnitude of finite bandgap opening depends on the orientation of ripples, and the hydrogen adsorption energy depends on the local curvature of graphene. An adsorbed hydrogen alters the local curvature, resulting in relatively weakened adsorption on the neighboring three sites, which gives a rationale to experimentally observed dynamic equilibrium stoichiometry (H:C = 1:4) of hydrogenated graphene. The surface diffusion transition state energy of adsorbed hydrogen is computed, which indicates that the Eley–Rideal surface recombination mechanism may be important to establish the dynamic equilibrium, instead of the commonly assumed Langmuir–Hinshelwood mechanism.

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

Calculating adsorption isotherms using the two-phase thermodynamic method and molecular dynamics simulations

We describe the calculation of adsorption isotherms from molecular dynamics simulations based on the two-phase thermodynamic (2PT) model. The 2PT model developed for bulk fluid phases treats the gas-like components as hard spheres (HSs), which correctly recovers the limiting behaviors of unconfined fluids. We showed that this treatment, however, does not always lead to the correct zero-loading behavior in strongly confining systems. For methane adsorption into zeolite MFI, the HS reference state underestimates entropy by up to 20% at low loadings and leads to an order-of-magnitude increase in the adsorption onset pressure. To fix these issues, we propose the use of ideal adsorbed gas (IAG) as the gas reference model, the properties of which can be computed using the Widom insertion method on an empty adsorbent. We further describe three routes to compute adsorption isotherms from the Helmholtz free energy at different loadings. Comparing against established Monte Carlo (MC) methods, we found that the adsorption isotherms obtained using the IAG reference state agrees to within 40%, which corresponds to deviations of <5% in adsorption free energy. The isotherms calculated using the HS reference state underestimate the adsorption uptake at low to medium loadings in strongly confining systems, but its accuracy improves at higher loadings and as the pore size increases relative to the sorbate diameter. The methods described here provide an alternative approach for computing adsorption isotherms when MC simulations in an open ensemble are undesirable and enable a direct comparison of computed adsorption thermodynamics with experiments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS

Interactions of N2 at oxide surfaces are important for understanding electrocatalytic nitrogen reduction reaction (NRR) mechanisms. Interactions of N2 at the polycrystalline vanadium oxide/vapor interface were monitored at room temperature and total pressures up to 10−1 Torr using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). The oxide film was predominantly V(IV), with V(III) and V(V) components. XPS spectra were acquired in environments of both pure N2 and equal pressures of N2 and H2O vapor. In pure N2, broad, partially resolved N1s features were observed at binding energies of 401.0 and 398.7 eV, with a relative intensity of ∼3:1, respectively. These features remained upon subsequent pumpdown to 10−9 Torr. The observed maximum N surface coverage was ∼1.5 × 1013 cm−2—a fraction of a monolayer. In the presence of equal pressures of H2O, the adsorbed N intensity at 10−1 Torr is ∼25% of that observed in the absence of H2O. The formation of molecularly adsorbed H2O was also observed. Density functional theory-based calculations suggest favorable absorption energies for N2 bonding to both V(IV) and V(III) cation sites but less so for V(V) sites. Hartree–Fock-based cluster calculations for N2–V end-on adsorption show that experimental XPS doublet features are consistent with the calculated shake-up and normal, final ionic configurations for N2 end-on bonding to V(III) sites but not V(IV) sites. The XPS spectra of vanadium oxide transferred in situ between electrochemical and UHV environments indicate that the oxide surfaces studied here are stable upon exposure to the electrolyte under NRR-relevant conditions.

Balogun, K.↗
DOE OSTI2022

Symmetry-resolved CO desorption and oxidation dynamics on O/Ru(0001) probed at the C K-edge by ultrafast X-ray spectroscopy

In this work, we report on carbon monoxide desorption and oxidation induced by 400 nm femtosecond laser excitation on the O/Ru(0001) surface probed by time-resolved X-ray absorption spectroscopy (TR-XAS) at the carbon K-edge. The experiments were performed under constant background pressures of CO (6x10 -8 Torr) and O 2 (3x10 -8 Torr). Under these conditions, we detect two transient CO species with narrow 2π* peaks, suggesting little 2π* interaction with the surface. Based on polarization measurements, we find that these two species have opposing orientations: (1) CO favoring a more perpendicular orientation and (2) CO favoring a more parallel orientation with respect to the surface. We also directly detect gas-phase CO 2 using a mass spectrometer and observe weak signatures of bent adsorbed CO 2 at slightly higher X-ray energies than the 2π* region. These results are compared to previously reported TR-XAS results at the O K-edge where the CO background pressure was three times lower (2x10 -8 Torr) while maintaining the same O 2 pressure. At the lower CO pressure, in the CO 2π* region, we observed adsorbed CO and a distribution of OC-O bond lengths close to the CO oxidation transition state, with little indication of gas-like CO. The shift towards 'gas-like' CO species may be explained by the higher CO exposure, which blocks O adsorption, decreasing O coverage and increasing CO coverage. These effects decrease the CO desorption barrier through dipole-dipole interaction, while simultaneously increasing the CO oxidation barrier.

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

Charting C–C coupling pathways in electrochemical CO 2 reduction on Cu(111) using embedded correlated wavefunction theory

The electrochemical CO 2 reduction reaction (CO 2 RR) powered by excess zero-carbon-emission electricity to produce especially multicarbon (C 2+ ) products could contribute to a carbon-neutral to carbon-negative economy. Foundational to the rational design of efficient, selective CO 2 RR electrocatalysts is mechanistic analysis of the best metal catalyst thus far identified, namely, copper (Cu), via quantum mechanical computations to complement experiments. Here, we apply embedded correlated wavefunction (ECW) theory, which regionally corrects the electron exchange-correlation error in density functional theory (DFT) approximations, to examine multiple C–C coupling steps involving adsorbed CO (*CO) and its hydrogenated derivatives on the most ubiquitous facet, Cu(111). We predict that two adsorbed hydrogenated CO species, either *COH or *CHO, are necessary precursors for C–C bond formation. The three kinetically feasible pathways involving these species yield all three possible products: *COH–CHO, *COH–*COH, and *OCH–*OCH. The most kinetically favorable path forms *COH–CHO. In contrast, standard DFT approximations arrive at qualitatively different conclusions, namely, that only *CO and *COH will prevail on the surface and their C–C coupling paths produce only *COH–*COH and *CO–*CO, with a preference for the first product. This work demonstrates the importance of applying qualitatively and quantitatively accurate quantum mechanical method to simulate electrochemistry in order ultimately to shed light on ways to enhance selectivity toward C 2+ product formation via CO 2 RR electrocatalysts.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2023

Tuning the surface reactivity of oxides by peroxide species

The Mars–van Krevelen mechanism is the foundation for oxide-catalyzed oxidation reactions and relies on spatiotemporally separated redox steps. Herein, we demonstrate the tunability of this separation with peroxide species formed by excessively adsorbed oxygen, thereby modifying the catalytic activity and selectivity of the oxide. Using CuO as an example, we show that a surface layer of peroxide species acts as a promotor to significantly enhance CuO reducibility in favor of H 2 oxidation but conversely as an inhibitor to suppress CuO reduction against CO oxidation. Together with atomistic modeling, we identify that this opposite effect of the peroxide on the two oxidation reactions stems from its modification on coordinately unsaturated sites of the oxide surface. By differentiating the chemical functionality between lattice oxygen and peroxide, these results are closely relevant to a wide range of catalytic oxidation reactions using excessively adsorbed oxygen to activate lattice oxygen and tune the activity and selectivity of redox sites.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Elemental characterization of crystalline silicotitanate following Hanford tank waste processing

To expedite vitrification of low activity waste at the Hanford Site, a Tank-Side Cesium Removal system is being evaluated. This method utilizes an inorganic ion exchange (IX) media, crystalline silicotitanate (CST), to remove 99.9% of the 137Cs from tank waste supernate. Laboratory scale testing of the ion exchange process has been conducted by Pacific Northwest National Laboratory to evaluate Cs removal with CST in supernates collected from Hanford tanks 241-AP-107 and 241-AW-102 under prototypic plant operating conditions. In an effort to understand the selectivity and partitioning of other analytes toward CST, specifically Resource Conservation and Recovery Act (RCRA) hazardous metals, selected +2 cations, and transuranic elements, a method for the complete dissolution of CST and Cs removal from the digestate was applied to the spent CST post-processing Hanford tank waste. The CST was shown to adsorb a relevant fraction of Ca, Cd, Pb, Sr, and potentially Fe by IX and/or other mechanisms. The spent CST also adsorbed actinides, including U, Pu, Np, and Am. This work discusses digestion of CST and removal of Cs from the digestate allowing effective analysis and provides insight as to what analytes may be competing for IX sites on the CST media.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2021

In situ ambient pressure XPS study of Pt/Cu(111) single-atom alloy in catalytically relevant reaction conditions

The presence of multiple reactant gases as well as reaction intermediates in a heterogeneous catalytic reaction results in a complex interaction between different components of the catalyst with each gas, which can alter the surface and chemical state of the catalyst differently than in the presence of an individual gas alone. In this study, we used in situ ambient pressure x-ray photoelectron spectroscopy to study the surface state of Pt/Cu(111) single-atom alloy model system in two catalytically relevant reaction conditions: CO 2 hydrogenation and CO oxidation. We found that the activation of CO 2 results in the formation of CO, which adsorbs on Pt sites at up to 400 K. In the presence of CO 2 and H 2 , Pt catalyzes the reverse water–gas shift reaction, which produces more CO and further stabilizes surface Pt atoms at 450 K. On the other hand, in CO oxidation condition, the presence of O 2 results in the formation of a thick Cu 2 O layer at higher temperatures, and Pt atoms are no longer detected in the surface and subsurface layers. When O 2 is introduced to the sample before CO, the formation of a complete Cu 2 O layer that covers all Pt atoms occurs immediately at room temperature. Finally, however, when CO is introduced at room temperature before O 2 , the presence of adsorbed CO on Pt sites stabilizes the surface Pt atoms and prevents the formation of a complete Cu 2 O layer, thus exposing the Pt atoms in 'holes' in the Cu 2 O layer.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Insight the process of hydrazine gas adsorption on layered WS 2 : a first principle study

The process of hydrazine gas adsorption on layered WS 2 has been systematically studied from first principle calculations. Our results demonstrate that this adsorption process is exothermic, and hydrazine molecules are physically adsorbed. The layer-dependent adsorption energy and interlayer separation induced by van der Waals interaction exerted by hydrazine molecules lead to the difficulty in desorbing hydrazine molecules from layered WS 2 as the number of layers increases. The most interesting finding is the emergence of localized impurity states below the Fermi level upon the hydrazine adsorption, irrespective of the number of WS 2 layers, resulting in a significant effect on the band structures and subsequently changing its electrical conductivity. Furthermore, a layer-dependent small charge transfer occurs between hydrazine and layered WS 2 , leading to a charge redistribution and considerable polarization in the adsorbed systems. The existence of defects and the humidity, on the other hand, influences the sensitivity of layered WS 2 to the hydrazine adsorption. Here, obtained results show that a perfectly layered WS 2 might be a promising candidate as an efficient nanosensor to detect such toxic gas in dry environment.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ab Initio modeling of Near-Edge EELS spectra for chemisorbed molecules

Abstract Electron energy loss spectroscopy (EELS) has recently been applied to probe chemisorbed molecules on metal nanostructures, but a fundamental understanding of the correlation between these spectra and the electronic structures of the adsorbates has been limited. We report here on the insights afforded by time-dependent density functional theory to decipher the energy loss near edge structure (ELNES) of EELS spectra associated with chemisorption. These first-principles calculations simulate the ELNES-EELS spectra for chemisorbed CO on various facets of Au and Pt. Computational predictions of key signatures such as the ‘red shift’ and reductions in the peak intensity of the 2 π * and 6 σ * peaks, as compared to free CO in the gas phase, are validated in comparison to experimentally collected EELS spectra. These signatures are revealed to arise from changes in the electronic structure in terms of unoccupied density of states associated with the chemisorption process. They are consistent with a Blyholder model that incorporates donation and back-donation of electrons. They are also characteristic of the chemisorption process, such as the choice of metal, site of adsorption and the coverage and distribution of adsorbates. Our simulations thus provide guidelines for the use of ELNES-EELS to characterize the atomic structure and adsorption property of nanostructured surfaces and facilitate the development of advanced nanomaterials for catalytic applications.

Materials Science↗
DOE OSTI2021

Performance study of charcoal-based radon reduction systems for ultraclean rare event detectors

The continuous emanation of radon due to trace amounts of uranium and thorium in detector materials introduces radon to the active detection volume of low-background rare event search detectors. 222 Rn produces a particularly problematic background in the physics region of interest by the "naked" beta decay of its 214 Pb daughter nucleus. While charcoal-based adsorption traps are expected to be effective for radon reduction in auxiliary circulation loops that service the warm components of current ton-scale detectors at slow flow rates (0.5-2 SLPM), radon reduction in the entire circulation loop at high flow rates $\mathcal{O}$(100s SLPM) is necessary to reach high sensitivity in future generation experiments. In this article we explore radon dynamics with a charcoal-based radon reduction system in the main circulation loop of time projection chamber detectors. Here, we find that even for perfect radon traps, circulation speeds of 2,000 SLPM are needed to reduce radon concentration in a 10 ton detector by 90%. This is faster by a factor of four than the highest circulation speeds currently achieved in dark matter detectors. We further find that the effectiveness of vacuum swing adsorption systems, which have been employed very successfully at reducing atmospheric radon levels in clean-rooms, is limited by the intrinsic radon activity of the charcoal adsorbent in ultra-low radon environments. Adsorbents with significantly lower intrinsic radon activity than in currently available activated charcoals would be necessary to build effective vacuum swing adsorption systems operated at room temperature for rare event search experiments. If such VSA systems are cooled to about 190 K, this requirement relaxes drastically.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Extensive Attention Mechanisms in Graph Neural Networks for Materials Discovery

We present our research where attention mechanism is extensively applied to various aspects of graph neural net- works for predicting materials properties. As a result, surrogate models can not only replace costly simulations for materials screening but also formulate hypotheses and insights to guide further design exploration. We predict formation energy of the Materials Project and gas adsorption of crystalline adsorbents, and demonstrate the superior performance of our graph neural networks. Moreover, attention reveals important substructures that the machine learning models deem important for a material to achieve desired target properties. Our model is based solely on standard structural input files containing atomistic descriptions of the adsorbent material candidates. We construct novel methodological extensions to match the prediction accuracy of state-of-the-art models some of which were built with hundreds of features at much higher computational cost. We show that sophisticated neural networks can obviate the need for elaborate feature engineering. Our approach can be more broadly applied to optimize gas capture processes at industrial scale.

Cong, Guojing↗
DOE OSTI2021

Experimental Study of Thermal Management and Enhancement of Adsorption-Based Onboard Hydrogen Storage System

Abstract Although hydrogen has one of the highest specific energies, its energy density in terms of volume is very poor compared to liquid fuels. Thus, to achieve attractive energy density for hydrogen, either high-pressure compression or a storage method is needed. For onboard (vehicles) hydrogen storage, up to 700 bars are needed for commercial fuel cell vehicles. This creates extreme requirements for material strength and thus safety concerns. A new metal-organic framework 5 (MOF-5) was selected as the adsorbent for H2 storage, as it provides promising storage capacity and is commercially available. Under the same H2 storage capacity and tank volume, the adsorption system is expected several folds reduction in pressure. Under the current study, a unique thermal management design using Modular Adsorbing Tank Insert (MATI) is paired with conduction-enhanced compressed MOF-5 beds. Compared to bare beds without conduction enhancement, all beds with conduction enhancement using either aluminum pins or expanded natural graphite (ENG) have shown various levels of improvement on bed thermal response, which can potentially help expedite system charge and discharge cycle times for real applications.

Electrochemistry↗
DOE OSTI2022

Sulfur-enhanced dynamics of coinage metal(111) surfaces: Step edges versus terraces as locations for metal-sulfur complex formation

The propensity of trace amounts of sulfur adsorbed on coinage metal(111) surfaces to dramatically enhance surface dynamics has been demonstrated by STM observations of accelerated 2D island decay for Cu and Ag. It is generally accepted that this enhancement is due to the formation of adsorbed metal-sulfur complexes, which facilitate surface mass transport of the metal. These complexes were originally proposed to form on terraces following the extraction of metal atoms from step edges and subsequent combination with sulfur on the terraces. However, even when thermodynamically feasible, this mechanism may not be kinetically viable for some complexes due to limited coupling of the complex concentration to the surface diffusion flux of metal atoms. Focusing on the case of Cu, we assess various scenarios where complexes are formed either on terraces or instead directly at step edges, the latter being a new paradigm. In this work, a new pathway is proposed for the formation on terraces. A rich variety of structures incorporating S at step edges exist, which could provide a viable source for complexes, at least from a thermodynamic perspective. However, it is necessary to also assess the activation barrier for complex formation and detachment from step edges. This is facilitated by the nudged-elastic-band analysis of the minimum energy path for this process utilizing machine-learning derived potentials based on density functional theory energetics for the metal-sulfur system.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Bias-Modulated ALD of ZnO: Insights into Precursor-Surface Interactions for ZnO Films

Atomic layer deposition (ALD) is widely used to deposit conformal thin films but is often limited in the tunability of the resulting material’s properties. Substrate bias and electric fields alter precursor-surface interactions and provide means to tune material properties. To explore this, we performed zinc oxide (ZnO) ALD using diethylzinc (DEZ) and water on silicon native oxide substrates at 150 °C in a sample holder designed to create a static electrical field by biasing one plate of a parallel plate capacitor-style sample holder during deposition. ZnO films prepared in an electric field/on a biased sample holder were thinner, changed relative crystalline composition, and contained more carbon compared to samples grown in identical sample holders without bias. The thickness was independent of the magnitude of the eletric field between plates, indicating that the primary driver for the change was substrate biasing not the electric field between plates of the parallel plate capacitor-style sample holder. Density functional theory calculations showed enhanced electron migration between dissociatively adsorbed DEZ molecules and the ZnO (002) facet with increasing force from an electric field at the substrate surface, which strengthens the electronic interactions between the surface and the adsorbate. These models offer a compelling explanation for the inhibited growth, changes in the crystallinity, and increase in carbon content of films grown in an electric field/on biased plates.

Jones, Jessica C. (ORCID:0000000174754620)↗
DOE OSTI2020

Asphaltene-induced spontaneous emulsification: Effects of interfacial co-adsorption and viscoelasticity

Asphaltenes are a class of high molecular weight aromatic compounds found in crude oil. They adsorb onto toluene-water interfaces and induce a spontaneous emulsification phenomenon, whereby stable water-in-oil emulsions form without the need of an external energy input. This work aims to control and understand the factors affecting spontaneous droplet formation in the presence of asphaltene adsorption. This is particularly useful for crude oil refining, where the presence of a stable emulsion hampers the efficiency of downstream processing operations. We explore the effect of the addition of copolymers designed as crude oil flow improvers as a means to control the extent of emulsion formation. We find that the polymers competitively adsorb onto the toluene-water interface and diminish spontaneous emulsification. Additionally, we conduct fluorescence microscopy experiments and measurements of the interfacial energy to determine the mechanism of spontaneous emulsification in asphaltene systems. We conclude that an emulsion forms via the diffusion of molecular water into the oil phase and subsequent binding with asphaltene aggregates, leading to the nucleation of micrometer-sized water droplets. We find that the polymer forms complexes with the dissolved asphaltenes, possibly hampering the ability of diffused water to bind to the asphaltenes and reducing the extent of spontaneous emulsification. Finally, we investigate the role of interfacial shear and dilatational viscoelasticity to better understand which fundamental interfacial properties are important in the emulsification of asphaltene-laden systems. We find that the rate of formation of an interfacial microstructural network is inversely correlated with the extent and rate of spontaneous emulsification.

42 ENGINEERING↗