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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 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Dynamical Change of Valence States and Structure in NiCu 3 Nanoparticles during Redox Cycling

Alloyed materials are promising candidates to improve catalytic processes. Ni–Cu nanoparticles are used for various reactions, including processes with biomass-derived components. However, dynamical restructuring effects alter the catalytic properties and can deactivate the sample. To understand these structural modifications, a multimodal investigation of NiCu 3 /C was performed to determine compositional and morphological changes during a redox cycle to simulate reduction and oxidation of the catalyst during reaction. We exploit a novel correlative, multimodal approach that combines in situ X-ray absorption spectroscopy (XAS) and in situ scanning transmission electron microscopy and electron energy-loss spectroscopy (STEM-EELS) to describe changes that occur in the sample in realistic conditions. In the fresh sample, there are two morphologies present: core–shell and hollow. Segregation of Cu was observed in both types of particles after synthesis, with Cu being more oxidized in the hollow structures. Upon reduction for 2 h under H 2 at 400 °C, the Cu was reduced, although segregation of Cu and Ni was still observed. Subsequent exposure to O 2 at 400 °C led to a strong reoxidation of Cu with the formation of hollow particles with compositional heterogeneities. The oxidation of metals and segregation phenomena can be related to known catalytic properties of NiCu 3 /C particles, especially regarding the hydrodeoxygenation of 5-hydroxymethylfurfural to 2,5-dimethylfuran.

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

Measuring Photoexcited Electron and Hole Dynamics in ZnTe and Modeling Excited State Core-Valence Effects in Transient Extreme Ultraviolet Reflection Spectroscopy

Transient extreme ultraviolet (XUV) spectroscopy is becoming a valuable tool for characterizing solar energy materials because it can separate photoexcited electron and hole dynamics with element specificity. We use surface-sensitive femtosecond XUV reflection spectroscopy to separately measure photoexcited electron, hole, and band gap dynamics of ZnTe, a promising photocathode for CO 2 reduction. We develop an ab initio theoretical framework based on density functional theory and the Bethe-Salpeter equation to robustly assign the complex transient XUV spectra to the material's electronic states. Applying this framework, we identify the relaxation pathways and quantify their time scales in photoexcited ZnTe, including subpicosecond hot electron and hole thermalization, surface carrier diffusion, ultrafast band gap renormalization, and evidence of acoustic phonon oscillations.

14 SOLAR ENERGY↗
DOE OSTI2025

Origin of Stabilization of Ligand-Centered Mixed Valence Ruthenium Azopyridine Complexes: DFT Insights for Neuromorphic Applications

Redox-driven conductance changes are critical processes in molecular- and coordination-complex-based memristive thin films and devices that are envisioned for neuromorphic technologies, but fundamental mechanisms of conductance switching are not fully understood. Here, we explore charge disproportionation (CD) processes in [Ru II L 2 ](PF 6 ) 2 molecular systems that intrinsically involve interfragment charge transfer (IFCT). Using a combination of ab initio molecular dynamics simulation (AIMD), time-dependent density functional theory (TD-DFT), and density functional theory (DFT) calculations, we investigate the electron transfer mechanisms and the roles of temperature and cell volumetric expansion in facilitating the counterion movements and electronic transitions required for low-cost IFCT and charge redistribution. A detailed analysis of the density of states and TD-DFT calculations highlights that unpaired electrons play a crucial role in low-energy transitions, with the azo (N=N) groups of the ligand serving as the primary sites for electronic transport between molecular fragments, further stabilizing the asymmetric state. Localization of added electrons on azo ligands occurs with negligible change at the Ru centers, supported by atomic volume expansions up to +4.74 bohr 3 , and goes along with a progressive reduction of the HOMO−LUMO gap across redox states, suggesting enhanced conductivity. The TD-DFT analysis reveals a dominant IFCT excitation at 2082.76 nm in the doubly reduced (22) state, while a stabilization energy of 1.20 eV of the asymmetric (13) state relative to the symmetric (22) state is predicted by constrained DFT. Periodic DFT and AIMD simulations emulating a molecular film show that the stabilization of the asymmetric state, relative to a symmetric one, translates in net charge separation values (order of ∼0.33 e) that are strongly linked to increased counterion mobility (average counterion displacements exceeding 0.7 Å per atom during CD events) and the involvement of azo groups in electron redistribution. These findings, which align with previously reported experimental and computational data, provide key insights into the IFCT mechanisms and electronic transport facilitated by azo groups, with important implications for redox-driven memristive and neuromorphic technologies.

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

Probing the Critical Dipole Moment To Support Excited Dipole-Bound States in Valence-Bound Anions

We report, in this paper, photodetachment spectroscopy and high-resolution photoelectron imaging of para -halogen substituted phenoxide anions, p -XC 6 H 4 O – (X = F, Cl, Br, I). The dipole moments of the p -XC 6 H 4 O neutral radicals increase from 2.56 to 3.19 D for X = F to I, providing a series of similar molecules to allow the examination of charge-dipole interactions by minimizing molecule-dependent effects. Excited DBSs ([XC 6 H 4 O]* – ) are observed for the four anions with binding energies of 8, 11, 24, and 53 cm –1 , respectively, for X = F to I, below their respective detachment thresholds. The binding energies exhibit a linear correlation with the dipole moments of the neutral radicals, extrapolating to a critical dipole moment of 2.5 D for zero binding energy. Because of the small binding energy of the excited DBS of [FC 6 H 4 O]* – , rotational autodetachment is observed to compete with vibrational autodetachment in the resonant photoelectron spectra, resulting in electrons with near zero kinetic energies.

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

Tale of Three Molecular Nitrides: Mononuclear Vanadium (V) and (IV) Nitrides As Well As a Mixed-Valence Trivanadium Nitride Having a V 3 N 4 Double-Diamond Core

Here, transmetallation of [VCl 3 (THF) 3 ] and [TlTp tBu,Me ] afforded [(Tp tBu,Me )VCl 2 ] (1, Tp tBu,Me = hydro-tris(3-tert-butyl-5-methylpyrazol-1-yl)borate), which was reduced with KC 8 to form a $C_{3v}$ symmetric V II complex, [(Tp tBu,Me )VCl] (2). Complex 1 has a high-spin ($\textit{S}$ = 1) ground state and displays rhombic high-frequency and -field electron paramagnetic resonance (HFEPR) spectra, while complex 2 has an $\textit{S}$ = 3/2 4 A 2 ground state observable by conventional EPR spectroscopy. Complex 1 reacts with NaN 3 to form the V V nitride-azide complex [(Tp tBu,Me )V≡N(N 3 )] (3). A likely V III azide intermediate en route to 3, [(Tp tBu,Me )VCl(N 3 )] (4), was isolated by reacting 1 with N 3 SiMe 3 . Complex 4 is thermally stable but reacts with NaN3 to form 3, implying a bis-azide intermediate, [(Tp tBu,Me )V(N 3 ) 2 ] (A), leading to 3. Reduction of 3 with KC 8 furnishes a trinuclear and mixed-valent nitride, [{(Tp tBu,Me )V} 2 ($μ_{4-}$VN 4 )] (5), conforming to a Robin–Day class I description. Complex 5 features a central vanadium ion supported only by bridging nitride ligands. Contrary to 1, complex 2 reacts with NaN 3 to produce an azide-bridged dimer, [{(Tp tBu,Me )V} 2 (1,3-$μ_2$-N 3 ) 2 ] (6), with two antiferromagnetically coupled high-spin V II ions. Complex 5 could be independently produced along with [($κ_2$-Tp tBu,Me ) 2 V] upon photolysis of 6 in arene solvents. The putative {V IV ≡N} intermediate, [(Tp tBu,Me )V≡N] (B), was intercepted by photolyzing 6 in a coordinating solvent, such as tetrahydrofuran (THF), yielding [(Tp tBu,Me )V≡N(THF)] (B-THF). In arene solvents, B-THF expels THF to afford 5 and [($κ_2$-Tp tBu,Me ) 2 V]. A more stable adduct (B-OPPh 3 ) was prepared by reacting B-THF with OPPh 3 . These adducts of B are the first neutral and mononuclear V IV nitride complexes to be isolated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Mixed Valence {Ni 2+ Ni 1+ } Clusters as Models of Acetyl Coenzyme A Synthase Intermediates

Acetyl coenzyme A synthase (ACS) catalyzes the formation and deconstruction of the key biological metabolite, acetyl coenzyme A (acetyl-CoA). The active site of ACS features a {NiNi} cluster bridged to a [Fe4S4] n+ cubane known as the A-cluster. The mechanism by which the A-cluster functions is debated, with few model complexes able to replicate the oxidation states, coordination features, or reactivity proposed in the catalytic cycle. In this work, we isolate the first bimetallic models of two hypothesized intermediates on the paramagnetic pathway of the ACS function. The heteroligated {Ni 2+ Ni 1+ } cluster, [K(12-crown-4) 2 ][1], effectively replicates the coordination number and oxidation state of the proposed “A red ” state of the A-cluster. Addition of carbon monoxide to [1] - allows for isolation of a dinuclear {Ni 2+ Ni 1+ (CO)} complex, [K(12-crown-2) n ][2] (n = 1–2), which bears similarity to the “A NiFeC ” enzyme intermediate. Structural and electronic properties of each cluster are elucidated by X-ray diffraction, nuclear magnetic resonance, cyclic voltammetry, and UV/vis and electron paramagnetic resonance spectroscopies, which are supplemented by density functional theory (DFT) calculations. Calculations indicate that the pseudo-T-shaped geometry of the three-coordinate nickel in [1] – is more stable than the Y-conformation by 22 kcal mol –1 , and that binding of CO to Ni 1+ is barrierless and exergonic by 6 kcal mol –1 . UV/vis absorption spectroscopy on [2] - in conjunction with time-dependent DFT calculations indicates that the square-planar nickel site is involved in electron transfer to the CO π*-orbital. Further, we demonstrate that [2] - promotes thioester synthesis in a reaction analogous to the production of acetyl coenzyme A by ACS.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗