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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 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Formation of Branched Ruthenium Nanoparticles for Improved Electrocatalysis of Oxygen Evolution Reaction

Branched nanoparticles are one of the most promising nanoparticle catalysts as their branch sizes and surfaces can be tuned to enable both high activity and stability. Understanding how the crystallinity and surface facets of branched nanoparticles affect their catalytic performance is vital for further catalyst development. In this work, a synthesis is developed to form highly branched ruthenium (Ru) nanoparticles with control of crystallinity. It is shown that faceted Ru branched nanoparticles have improved stability and activity in the oxygen evolution reaction (OER) compared with polycrystalline Ru nanoparticles. Here, this work achieves a low 180 mV overpotential at 10 mA cm –2 for hours, demonstrating that record–high stability for Ru nanocrystals can be achieved while retaining high activity for OER. The superior electrocatalytic performance of faceted Ru branched nanoparticles is ascribed to the lower Ru dissolution rate under OER conditions due to low–index facets on the branch surfaces.

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

Chasing the Achilles’ Heel in Hybrid Systems of Diruthenium Water Oxidation Catalysts Anchored on Indium Tin Oxide: The Stability of the Anchor

The development of hybrid devices for photo-driven water oxidation with dinuclear molecular ruthenium catalysts on solid supports aims both at high efficiencies of the catalyst and at high stability of the linking to the surface. We herein report a systematic study regarding the stability of catalysts anchored on an indium tin oxide (ITO) support via different acid functional groups at the ligand backbone: viz., a single carboxylate anchor, two carboxylate anchors, and a phosphonate anchor. The integrity of the hybrid electrodes ITO|mesoITO|catalyst was evaluated under acidic aqueous conditions as a function of the applied electric potential below and above the onset of the oxygen evolution reaction (OER). Rotating ring disk electrode (RRDE) experiments allowed us to distinguish between the water oxidation and the desorption processes. X-ray photoemission (XPS) and X-ray absorption spectroscopy (XAS) after electrochemical treatment showed high chemical stability of the catalyst core structure but pronounced dependence of the hybrid stability on the oxidation state of the ruthenium center. A combination of these different spectroscopic techniques shed light on the mechanisms underlying catalyst desorption. Specifically, catalysts equipped with carboxylate anchors are found to continuously desorb already at low applied potentials below the OER onset, while for the more rugged phosphonate-based hybrid oxidative P–C(aryl) bond cleavage is proposed to occur, but only after reaching the high-valent Ru V Ru IV state. These findings reveal specific challenges for anchoring strategies in molecular water oxidation catalysis.

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

Structure and stability of NaTaO 3 (001) and KTaO 3 (001) surfaces

In this work, we carried out first-principles slab calculations to investigate the structure, energetics, and electronic properties of the majority (001) surfaces of NaTaO 3 (NTO), a perovskite oxide with excellent photocatalytic properties, and KTaO 3 (KTO), a closely related but somewhat less active compound. Being polar, NTO(001) and KTO (001) require charge compensation to be stabilized.We examine a number of possible structural models for these surfaces by comparing their formation energies to those of the pure NaO/KO and TaO 2 terminations. Our results show that a “cation-exchange” reconstruction is energetically most favorable for NTO(001) under vacuum conditions, whereas for KTO(001) this reconstruction competes with a “striped” phase with equally exposed KO and TaO 2 terraces actually observed in recent experiments. NTO is found to exhibit enhanced structural flexibility and more effective charge compensation in comparison to KTO, which is attributed to the significantly smaller size of Na + relative to K + . Upon exposure to water, a (2 × 1) hydroxylated structure is by far most favorable for both NTO and KTO. This structure can thus provide a basis for the mechanistic understanding of photocatalytic processes on NTO and KTO surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Theoretical Investigation of Solvent Effects on the Hydrodeoxygenation of Propionic Acid over a Ni(111) Catalyst Model

The effect of two solvents, liquid water and 1,4-dioxane, has been studied from first principles on the hydrodeoxygenation of propionic acid over a Ni (111) catalyst surface model. A mean-field microkinetic model was developed to investigate these effects at a temperature of 473 K. Under all reaction conditions, a decarbonylation mechanism is favored significantly over a decarboxylation pathway. Although no significant solvent effects were observed on the decarbonylation rate, a substantial solvent stabilization of two key surface intermediates in the decarboxylation mechanism, CH 3 CCOO and CH 3 CHCOO, lead to a notable increase of the decarboxylation rate by two orders of magnitude in liquid water and by one order of magnitude in liquid 1,4-dioxane. Furthermore, a significant solvent stabilization of the transition state of C-H bond cleavage of the α-carbon of CH 3 CHCO, relative to the stabilization of the C-C bond cleavage of the α-carbon of CH 3 CHCO, leads to a change in dominant pathway in the liquid phase environments. Finally, a sensitivity analysis shows that the C-OH bond cleavage of propionic acid and C-C bond cleavage of the α-carbon of CH 3 CHCO are the most rate controlling states in the gas phase. In contrast, in solvents the dehydrogenation of CH 3 CHCO becomes the most influential step. This shift in rate controlling state is attributed to the solvent effect on the dehydrogenation of CH 3 CHCO, which is facilitated in aqueous phase. Altogether, it is likely that the investigated (111) facet of Ni is not active for the hydrodeoxygenation of propionic acid in neither the gas nor liquid phase and other Ni facets or phases must be responsible for the experimentally observed kinetics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Photocatalysis of Modified Transition Metal Oxide Surfaces

The goal of this project has been to establish a cause-effect relationship for photocatalytic activity variations of different structures of the same material; and furthermore gain fundamental understanding on modification of photocatalysts by compositional or surface modifications. The reasoning is that gaining atomic scale understanding of how surface and bulk modifications alter the photo reactivity will lead to design principles for next generation photocatalysts. As a prototypical photocatalyst the research focused on TiO 2 synthesized in well-defined single crystalline form to enable fundamental characterizations.We have obtained results in the following areas: (a) Preparation of epitaxial anatase TiO 2 samples by pulsed laser deposition. (b) Comparison of hydrogen diffusion on different crystallographic surface. (c) Determining the stability of the TiO 2 (011)-2x1 reconstruction upon interactions with adsorbates. (d) Characterization of adsorption and (thermal and photo) reaction of molecules with nitro-endgroups, (e) Exploring the possibility of modifying planar model photocatalyst surfaces with graphene to enable fundamental studies on reported enhanced photocatalytic activities of graphene modified transition metal oxides, (f) gained fundamental understanding on the role of crystallographic polymorphs of the same material for their photocatalytic activities.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effects of electron cyclotron current drive on magnetic islands in tokamak plasmas

The effects of the electron cyclotron current drive on magnetic islands in tokamak plasmas are studied using gyrokinetic simulations. By investigating the effects of different characteristics of the driven current, such as current density distribution and deposition location, the factors which can determine the suppression effect on the resistive tearing modes have been explored. It is found that an electron cyclotron wave (ECW) driven current with a larger peak value and more focused deposition region has a better stabilization effect. When the ECW-driven current is closer to the rational surface, it has a better stabilizing effect. These gyrokinetic toroidal code (GTC) linear simulations in the electron fluid limit of the tearing modes in the cylindrical geometry agree well with the magnetohydrodynamic codes. In conclusion, the optimal timing control of the current deposition on resistive tearing modes is demonstrated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Hydrophobic Inorganic Oxide Pigments via Polymethylhydrosiloxane Grafting: Dispersion in Aqueous Solution at Extraordinarily High Solids Concentrations

Building on the recent demonstration of aqueous-dispersible hydrophobic pigments that retain their surface hydrophobicity even after drying, we demonstrate the synthesis of surface-modified Ti-Pure R-706 (denoted R706) titanium dioxide-based pigments, consisting of a thin (one to three monolayers) grafted polymethylhydrosiloxane (PMHS) coating, which (i) are hydrophobic in the dry state according to capillary rise and dynamic vapor sorption measurements and (ii) form stable aqueous dispersions at solid contents exceeding 75 wt % (43 vol %), without added dispersant, displaying similar rheology to R706 native oxide pigments at 70 wt % (37 vol %) consisting of an optimal amount of conventional polyanionic dispersant (0.3 wt % on pigment basis). Additionally, the surface-modified pigments have been characterized via 29 Si and 13 C cross-polarization/magic angle spinning solid-state NMR spectroscopy; infrared spectroscopy; thermogravimetric and elemental analyses; and ζ potential measurements. On the basis of these data, the stability of the surface-modified PMHS-R706 aqueous dispersions is attributed to steric effects, as a result of grafted PMHS strands on the R706 surface, and depends on the chaotropic nature of the base used during PMHS condensation to the pigment/polysiloxane interface. The lack of water wettability of the surface-modified oxide particles in their dry state translates to improved water-barrier properties in coatings produced with these surface-modified pigment particles. Finally, the synthetic approach appears general as demonstrated by its application to various inorganic-oxide pigment particles.

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

Orientation-dependent stabilization of MgCr 2 O 4 spinel thin films

AB 2 O 4 normal spinels with a magnetic B site can host a variety of magnetic and orbital frustrations leading to spin-liquid phases and field-induced phase transitions. Here, we report the epitaxial growth of (111)-oriented MgCr 2 O 4 thin films. By characterizing the structural and electronic properties of films grown along the (001) and (111) directions, the influence of growth orientation has been studied. Despite distinctly different growth modes observed during deposition, the comprehensive characterization reveals no measurable disorder in the cation distribution nor multivalency issue for Cr ions in either orientation. Contrary to a naive expectation, the (111) stabilized films exhibit a smoother surface and a higher degree of crystallinity than (001)-oriented films. The preference in growth orientation is explained within the framework of heteroepitaxial stabilization in connection to a significantly lower (111) surface energy. Furthermore, these findings open broad opportunities in the fabrication of two-dimensional kagome-triangular heterostructures with emergent magnetic behavior inaccessible in bulk crystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗