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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Influence of Adsorbates on the Electronic Structure, Bond Strain, and Thermal Properties of an Alumina-Supported Pt Catalyst

We describe the results of an X-ray absorption spectroscopy (XAS) study of adsorbate and temperature-dependent alterations of the atomic level structure of a prototypical, noble metal hydrogenation and reforming catalyst: ~1.0 nm Pt clusters supported on gamma alumina (Pt/γ-Al 2 O 3 ). This work demonstrates that the metal–metal (M–M) bonding in these small clusters is responsive to the presence of adsorbates, exhibiting pronounced coverage-dependent strains in the clusters’ M–M bonding, with concomitant modifications of their electronic structures. Hydrogen and CO adsorbates demonstrate coverage-dependent bonding that leads to relaxation of the M–M bond strains within the clusters. These influences are partially compensated, and variably mediated, by the temperature-dependent electronic perturbations that arise from cluster–support and adsorbate–support interactions. Taken together, the data reveal a strikingly fluxional system with implications for understanding the energetics of catalysis. Here, we estimate that a 9.1 ± 1.1 kJ/mol strain exists for these clusters under H 2 and that this strain increases to 12.8 ± 1.7 kJ/mol under CO. This change in the energy of the particle is in addition to the different heats of adsorption for each gas (64 ± 3 and 126 ± 2 kJ/mol for H 2 and CO, respectively).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Ensemble averaged structure–function relationship for nanocrystals: effective superparamagnetic Fe clusters with catalytically active Pt skin [Ensemble averaged structure-function relationship for composite nanocrystals: magnetic bcc Fe clusters with catalytically active fcc Pt skin]

Practical applications require the production and usage of metallic nanocrystals (NCs) in large ensembles. Besides, due to their cluster-bulk solid duality, metallic NCs exhibit a large degree of structural diversity. This poses the question as to what atomic-scale basis is to be used when the structure–function relationship for metallic NCs is to be quantified precisely. In this paper, we address the question by studying bi-functional Fe core-Pt skin type NCs optimized for practical applications. In particular, the cluster-like Fe core and skin-like Pt surface of the NCs exhibit superparamagnetic properties and a superb catalytic activity for the oxygen reduction reaction, respectively. We determine the atomic-scale structure of the NCs by non-traditional resonant high-energy X-ray diffraction coupled to atomic pair distribution function analysis. Using the experimental structure data we explain the observed magnetic and catalytic behavior of the NCs in a quantitative manner. Lastly, we demonstrate that NC ensemble-averaged 3D positions of atoms obtained by advanced X-ray scattering techniques are a very proper basis for not only establishing but also quantifying the structure–function relationship for the increasingly complex metallic NCs explored for practical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Dynamic and relativistic effects on Pt–Pt indirect spin–spin coupling in aqueous solution studied by ab initio molecular dynamics and two- vs four-component density functional NMR calculations

Treating 195Pt nuclear magnetic resonance parameters in solution remains a considerable challenge from a quantum chemistry point of view, requiring a high level of theory that simultaneously takes into account the relativistic effects, the dynamic treatment of the solvent–solute system, and the dynamic electron correlation. A combination of Car-Parrinello molecular dynamics (CPMD) and relativistic calculations based on two-component zeroth order regular approximation spin–orbit Kohn–Sham (2c-ZKS) and four-component Dirac–Kohn–Sham (4c-DKS) Hamiltonians is performed to address the solvent effect (water) on the conformational changes and JPtPt1 coupling. A series of bridged PtIII dinuclear complexes [L1–Pt2(NH3)4(Am)2–L2]n+ (Am = α–pyrrolidonate and pivalamidate; L = H2O, Cl−, and Br−) are studied. The computed Pt–Pt coupling is strongly dependent on the conformational dynamics of the complexes, which, in turn, is correlated with the trans influence among axial ligands and with the angle N–C–O from the bridging ligands. The J-coupling is decomposed in terms of dynamic contributions. The decomposition reveals that the vibrational and explicit solvation contributions reduce JPtPt1 of diaquo complexes (L1 = L2 = H2O) in comparison to the static gas-phase magnitude, whereas the implicit solvation and bulk contributions correspond to an increase in JPtPt1 in dihalo (L1 = L2 = X−) and aquahalo (L1 = H2O; L2 = X−) complexes. Relativistic treatment combined with CPMD shows that the 2c-ZKS Hamiltonian performs as well as 4c-DKS for the JPtPt1 coupling.

Chemistry↗
DOE OSTI2019

Unravelling the mechanism of the semiconducting-like behavior and its relation to superconductivity in (CaFe 1-x Pt x As) 10 Pt 3 As 8

The temperature-dependence of the in-plane optical properties of (CaFe 1–x Pt x As) 10 Pt 3 As 8 have been investigated for the undoped (x= 0) parent compound, and the optimally-doped (x = 0.1)superconducting material ( T c ≃12 K) over a wide frequency range. The optical conductivity has been described using two free-carrier (Drude) components, in combination with oscillators to describe interband transitions. At room temperature, the parent compound may be described by a strong, broad Drude term, as well as a narrow, weaker Drude component. Below the structural and magnetic transitions at ≃ 96 and 83 K, respectively, strength is transferred from the free-carrier components into a bound excitation at ≃1000 cm –1 , and the material exhibits semiconducting-like behavior. In the optimally-doped sample, at room temperature the optical properties are again described by narrow and broad Drude responses comparable to the parent compound; however, below T* ≃100 K, strength from the narrow Drude is transferred into a newly-emergent low-energy peak at 120 cm –1 , which arises from a localization process, resulting in semiconducting-like behavior. Interestingly, below T c , this peak also contributes to the superfluid weight, indicating that some localized electrons condense into Cooper pairs; this observation may provide insight into the pairing mechanism in iron-based superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Stability of PT and anti- PT -symmetric Hamiltonians with multiple harmonics

Hermitian Hamiltonians with time-periodic coefficients can be analyzed via Floquet theory, and have been extensively used for engineering Floquet Hamiltonians in standard quantum simulators. Generalized to non-Hermitian Hamiltonians, time periodicity offers avenues to engineer the landscape of Floquet quasienergies across the complex plane. We investigate two-level non-Hermitian PT and anti- PT -symmetric Hamiltonians with coefficients that have multiple harmonics using Floquet theory. By analytical and numerical calculations, we obtain their regions of stability, defined by real Floquet quasienergies, and contours of exceptional point (EP) degeneracies. We extend our analysis to study the phases that accompany these cyclic changes with the biorthogonality approach. Our results demonstrate that these time-periodic Hamiltonians generate a rich landscape of stable (real) and unstable (complex) regions. Published by the American Physical Society 2024

Cen, Julia (ORCID:0000000299353943)↗
DOE OSTI2016

Electronic structure of Pt-substituted clathrate silicides Ba{sub 8}Pt{sub x}Si{sub 46–x}(x = 4–6)

The results of calculation of the electronic structure of Si-based Pt-substituted clathrates are reported. Calculation is carried out by the linearized-augmented-plane-wave method. The effect of the number of substitutions and their crystallographic position in the unit cell on the electron-energy spectrum and the electronic properties of Pt-substituted clathrates is analyzed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

The {sup 136}Xe + {sup 198}Pt Reaction: a Detailed Re-examination

We extend previous measurements of the transfer product yields in the reaction of E-c.m. = 450 MeV Xe-136 with Pt-198 by measurements of the product yields using Gammasphere. By recording events occurring in beam bursts and in between beam bursts, we are able to extend the number of measured product yields from 78 to 171 nuclides. Our new data span a much wider range of Z and A than observed in previous work and when compared to theoretical predictions, these new measurements provide a more stringent and thorough test of models of multi-nucleon transfer (MNT) reactions.

Desai, V. V.↗
DOE OSTI2015

Morphology Tailoring of Pt Nanocatalysts for the Oxygen Reduction Reaction: The Paradigm of Pt 13

Understanding the relationship between the morphology of Pt nanocatalysts and their catalytic performance towards the oxygen reduction reaction (ORR) is important for their applications as electrocatalysts. In this work, using density functional theory calculations, we investigate structural properties, ORR activity, and CO tolerance of five morphological forms of the Pt13 cluster. Four of these are lowenergy, lower symmetry conformations (Pt13-1 and Pt13-4 have CS symmetry, Pt13-2 and Pt13-3 are of C2v symmetry) and the fifth (Pt13-5) is a higher energy, icosahedral structure. Our results indicate that of the five considered, Pt13-1 and Pt13-2 possesses the best, overall comparable, combination of catalytic characteristics—activity and CO tolerance—relevant to the ORR. Computational characterization of morphology-dependent catalytic properties of sub-nano and nanosized particles herein can inform the design and synthesis of nanocatalysts with superior targeted functionalities

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Relativistic MR-MP calculations of the energy levels and transition probabilities in Ni- to Kr-like Pt ions

Ni- to Kr-like Pt ions have been studied by relativistic multi-reference Møller–Plesset many-body perturbation theory calculations. Energy levels and lifetimes of low-lying excited states within the n=4 complex are reported for each ion. Wavelengths and transition probabilities for the strongest electric-dipole transitions are compared with available experimental data. Synthetic radiative spectra are shown for various wavelength regions.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Synthesis of Structurally Ordered Pt 3 Ti and Pt 3 V Nanoparticles as Methanol Oxidation Catalysts

Structurally ordered Pt3Ti or Pt3V intermetallic nanoparticle catalysts with ultrasmall particle sizes have never been successfully synthesized. Herein, we present a KCl-nanoparticle method to successfully provide such compounds. These two catalysts show enhanced catalytic activity and stability for methanol oxidation compared to pure Pt.

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

LixNiO/Ni Heterostructure with Strong Basic Lattice Oxygen Enables Electrocatalytic Hydrogen Evolution with Pt-like Activity

The low-cost hydrogen production from water electrolysis is crucial for practical deployment of sustainable hydrogen economy, but is often constrained by the lack of active and robust electrocatalysts from Earth-abundant materials. We describe here an unconventional heterostructure composed of strongly coupled Ni-deficient LixNiO nanoclusters and polycrystalline Ni nanocrystals, and its exceptional activities toward hydrogen evolution reaction (HER) in aqueous electrolytes. The presence of unique lattice oxygen species with strong Brønsted basicity is a significant feature, which spontaneously split water molecules and effectively accelerate the sluggish Volmer H-OH dissociation step in neutral and alkaline HER. In combination with the unpreceded level of intimate LixNiO and Ni interfacial junctions that produces abundant “hotspots” for promoted hydride coupling, the catalyst exhibited kinetic activities almost identical as Pt/C and decent long term stability in universal pH.

Lu, Ke↗