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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

MOF-Transformed In 2 O 3-x @C Nanocorn Electrocatalyst for Efficient CO 2 Reduction to HCOOH

For electrochemical CO 2 reduction to HCOOH, an ongoing challenge is to design energy efficient electrocatalysts that can deliver a high HCOOH current density (J HCOOH ) at a low overpotential. Indium oxide is good HCOOH production catalyst but with low conductivity. In this work, we report a unique corn design of In 2 O 3-x @C nanocatalyst, wherein In 2 O 3-x nanocube as the fine grains dispersed uniformly on the carbon nanorod cob, resulting in the enhanced conductivity. Excellent performance is achieved with 84% Faradaic efficiency (FE) and 11 mA cm –2 J HCOOH at a low potential of – 0.4 V versus RHE. At the current density of 100 mA cm –2 , the applied potential remained stable for more than 120 h with the FE above 90%. Density functional theory calculations reveal that the abundant oxygen vacancy in In 2 O 3-x has exposed more In 3+ sites with activated electroactivity, which facilitates the formation of HCOO* intermediate. Operando X-ray absorption spectroscopy also confirms In 3+ as the active site and the key intermediate of HCOO* during the process of CO 2 reduction to HCOOH.

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

Laminar burning velocity predictions for C 1 and C 2 hydrofluorocarbon refrigerants with air

Due to their high global warming potentials, many existing working fluids are being phased out. Their replacements will often be slightly flammable, and the burning velocity of refrigerant-air mixtures is a metric for ranking their flammability. To allow estimates of the flammability of new blends of agents, predictive tools for the burning velocity are being developed and require a kinetic mechanism. A hydrofluorocarbon (HFC) mechanism was developed 20 years ago to describe hydrocarbon-air flames with added trace amounts of hydrofluorocarbon fire retardants. In the present work, the mechanism has been updated slightly to include new HFC compounds, new rate data. The modified mechanism is used to predict steady, planar, 1D, unstretched burning velocities for mixtures of air with one- and two-carbon saturated HFC compounds R41 (CH 3 F), R32 (CH 2 F 2 ), R161 (C 2 F 5 H), R152 (CH 2 F-CH 2 F), R152a (CH 3 -CHF 2 ), R143 (CH 2 F-CHF 2 ), R143a (CH 3 -CF 3 ), R134 (CHF 2 -CHF 2 ), and R134a (CH 2 F-CF 3 ), for which existing experimental data were available.

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

Crystal structure of nicarbazin, (C 13 H 10 N 4 O 5 )(C 6 H 8 N 2 O)

The crystal structure of nicarbazin has been solved and refined using synchrotron X-ray powder diffraction data and optimized using density functional theory techniques. Nicarbazin is a co-crystal of 4,4′-dinitrocarbanilide (DNC) and 2-hydroxy-4,6-dimethylpyrimidine (HDP) molecules. Nicarbazin crystallizes in space groupP-1(#2) witha= 6.90659(8),b= 12.0794(4),c= 13.5040(7) Å,α= 115.5709(11),β= 102.3658(6),γ= 91.9270(4)°,V= 982.466(5) Å 3 , andZ= 2. The DNC and HDP molecules are linked by two strong N–H⋯O and N–H⋯N hydrogen bonds, and the HDP molecules are linked into centrosymmetric dimers by another N–H⋯O hydrogen bond. These strong hydrogen bonds link the molecules into layers parallel to theab-plane and parallel stacking of both DNC and HDP molecules is prominent in the structure. The powder pattern has been submitted to ICDD for inclusion in the Powder Diffraction File™ (PDF®).

Materials Science↗
DOE OSTI2021

Elucidating Electron Storage and Distribution within the Pentaheme Scaffold of Cytochrome c Nitrite Reductase (NrfA)

Cytochrome c nitrite reductases (CcNIR or NrfA) play important roles in the global nitrogen cycle by conserving the usable nitrogen in the soil. Here, the electron storage and distribution properties within the pentaheme scaffold of Geobacter lovleyi NrfA were investigated via EPR spectroscopy coupled with chemical titration experiments. Initially, a chemical reduction method was established to sequentially add electrons to the fully oxidized protein, one equivalent at a time. The step–by–step reduction of the hemes was then followed using UV–Vis and EPR spectroscopy. EPR–spectral simulations elucidate the sequence of heme reduction within the pentaheme scaffold, and identify the signals of all five hemes in the EPR spectra. Electrochemical experiments ascertain the reduction potentials for each heme, observed in a narrow +11 (Heme 5) to –212 mV (Heme 3) range (vs SHE). Based on quantitative analysis/simulation of the EPR data, we demonstrate that Hemes 4 and 5 are reduced first (before the active site Heme 1) and serve the purpose of an electron storage unit within the protein. To probe the role of the central Heme 3, a H108M NrfA variant was generated where the reduction potential of Heme 3 is shifted positively (from –212 mV to +29 mV). Here, the H108M mutation significantly impacts the distribution of electrons within the pentaheme scaffold and the reduction potentials of the hemes, reducing the catalytic activity of the enzyme to 3% compared to wild–type. We propose that this is due to Heme 3’s important role as an electron gateway in the wild–type enzyme.

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

A Copper Relay System Involving Two Periplasmic Chaperones Drives cbb 3 -Type Cytochrome c Oxidase Biogenesis in Rhodobacter capsulatus

PccA and SenC are periplasmic copper chaperones required for the biogenesis of cbb 3 -type cytochrome c oxidase (cbb 3 -Cox) in Rhodobacter capsulatus at physiological Cu concentrations. However, both proteins are dispensable for cbb 3 -Cox assembly when the external Cu concentration is high. PccA and SenC bind Cu using Met and His residues and Cys and His residues as ligands, respectively, and both proteins form a complex during cbb 3 -Cox biogenesis. SenC also interacts directly with cbb 3 -Cox, as shown by chemical cross-linking. Here we determined the periplasmic concentrations of both proteins in vivo and analyzed their Cu binding stoichiometries and their Cu(I) and Cu(II) binding affinity constants (KD) in vitro. Our data show that both proteins bind a single Cu atom with high affinity. In vitro Cu transfer assays demonstrate Cu transfer both from PccA to SenC and from SenC to PccA at similar levels. We conclude that PccA and SenC constitute a Cu relay system that facilitates Cu delivery to cbb 3 -Cox.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2016

Intrinsic selectivity and structure sensitivity of Rhodium catalysts for C 2+ oxygenate production [On the intrinsic selectivity and structure sensitivity of Rhodium catalysts for C 2+ oxygenate production]

Synthesis gas (CO + H 2 ) conversion is a promising route to converting coal, natural gas, or biomass into synthetic liquid fuels. Rhodium has long been studied as it is the only elemental catalyst that has demonstrated selectivity to ethanol and other C 2+ oxygenates. However, the fundamentals of syngas conversion over rhodium are still debated. In this work a microkinetic model is developed for conversion of CO and H 2 into methane, ethanol, and acetaldehyde on the Rh (211) and (111) surfaces, chosen to describe steps and close-packed facets on catalyst particles. The model is based on DFT calculations using the BEEF-vdW functional. The mean-field kinetic model includes lateral adsorbate–adsorbate interactions, and the BEEF-vdW error estimation ensemble is used to propagate error from the DFT calculations to the predicted rates. The model shows the Rh(211) surface to be ~6 orders of magnitude more active than the Rh(111) surface, but highly selective toward methane, while the Rh(111) surface is intrinsically selective toward acetaldehyde. A variety of Rh/SiO 2 catalysts are synthesized, tested for catalytic oxygenate production, and characterized using TEM. The experimental results indicate that the Rh(111) surface is intrinsically selective toward acetaldehyde, and a strong inverse correlation between catalytic activity and oxygenate selectivity is observed. Furthermore, iron impurities are shown to play a key role in modulating the selectivity of Rh/SiO 2 catalysts toward ethanol. The experimental observations are consistent with the structure-sensitivity predicted from theory. As a result, this work provides an improved atomic-scale understanding and new insight into the mechanism, active site, and intrinsic selectivity of syngas conversion over rhodium catalysts and may also guide rational design of alloy catalysts made from more abundant elements.

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

Formation of 2- and 1-methyl-1,4-dihydronaphthalene isomers via the crossed beam reactions of phenyl radicals (C 6 H 5 ) with isoprene (CH 2 C(CH 3 )CHCH 2 ) and 1,3-pentadiene (CH 2 CHCHCHCH 3 )

Crossed molecular beam reactions were exploited to elucidate the chemical dynamics of the reactions of phenyl radicals with isoprene and with 1,3-pentadiene at a collision energy of 55 ± 4 kJ mol -1 . Both reactions were found to proceed via indirect scattering dynamics and involve the formation of a van-der-Waals complex in the entrance channel. The latter isomerized via the addition of the phenyl radical to the terminal C1/C4 carbon atoms through submerged barriers forming resonantly stabilized free radicals C 11 H 13 , which then underwent cis–trans isomerization followed by ring closure. The resulting bicyclic intermediates fragmented via unimolecular decomposition though the atomic hydrogen loss via tight exit transition states located 30 kJ mol -1 above the separated reactants in overall exoergic reactions forming 2- and 1-methyl-1,4-dihydronaphthalene isomers. The hydrogen atoms are emitted almost perpendicularly to the plane of the decomposing complex and almost parallel to the total angular momentum vector (‘sideways scattering’) which is in strong analogy to the phenyl–1,3-butadiene system studied earlier. RRKM calculations confirm that 2- and 1-methyl-1,4-dihydronaphthalene are the dominating reaction products formed at levels of 97% and 80% in the reactions of the phenyl radical with isoprene and 1,3-pentadiene, respectively. This barrier-less formation of methyl-substituted, hydrogenated PAH molecules further supports our understanding of the formation of aromatic molecules in extreme environments holding temperatures as low as 10 K.

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

Very high commutation quality factor and dielectric tunability in nanocomposite SrTiO 3 thin films with T c enhanced to >300 °C

We report on nanoengineered SrTiO 3 –Sm2O 3 nanocomposite thin films with the highest reported values of commutation quality factor (CQF or K-factor) of >2800 in SrTiO 3 at room temperature. The films also had a large tunability of dielectric constant (49%), low tangent loss (tan δ = 0.01) and a Curie temperature for SrTiO 3 > 300 °C, making them very attractive for tunable RF applications. The enhanced properties originate from the unique nanostructure in the films, with <20 nm diameter strain-controlling Sm 2 O 3 nanocolumns embedded in a SrTiO 3 matrix. Very large out-of-plane strains (up to 2.6%) and high tetragonality (c/a) (up to 1.013) were induced in the SrTiO 3 . Finally, the K-factor was further enhanced by adding 1 at% Sc 3+ (acceptor) dopant in SrTiO 3 to a value of 3300 with the tangent loss being ≤0.01 up to 1000 kV cm -1 .

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH{sub 2}BrI

It is well known that ultraviolet photoexcitation of halomethanes results in halogen-carbon bond cleavage. Each halogen-carbon bond has a dominant ultraviolet (UV) absorption that promotes an electron from a nonbonding halogen orbital (n{sub X}) to a carbon-halogen antibonding orbital (σ*{sub C-X}). UV absorption into specific transitions in the gas phase results primarily in selective cleavage of the corresponding carbon-halogen bond. In the present work, broadband ultrafast UV-visible transient absorption studies of CH{sub 2}BrI reveal a more complex photochemistry in solution. Transient absorption spectra are reported spanning the range from 275 nm to 750 nm and 300 fs to 3 ns following excitation of CH{sub 2}BrI at 266 nm in acetonitrile, 2-butanol, and cyclohexane. Channels involving formation of CH{sub 2}Br + I radical pairs, iso-CH{sub 2}Br-I, and iso-CH{sub 2}I-Br are identified. The solvent environment has a significant influence on the branching ratios, and on the formation and stability of iso-CH{sub 2}Br-I. Both iso-CH{sub 2}Br-I and iso-CH{sub 2}I-Br are observed in cyclohexane with a ratio of ∼2.8:1. In acetonitrile this ratio is 7:1 or larger. The observation of formation of iso-CH{sub 2}I-Br photoproduct as well as iso-CH{sub 2}Br-I following 266 nm excitation is a novel result that suggests complexity in the dissociation mechanism. We also report a solvent and concentration dependent lifetime of iso-CH{sub 2}Br-I. At low concentrations the lifetime is >4 ns in acetonitrile, 1.9 ns in 2-butanol and ∼1.4 ns in cyclohexane. These lifetimes decrease with higher initial concentrations of CH{sub 2}BrI. The concentration dependence highlights the role that intermolecular interactions can play in the quenching of unstable isomers of dihalomethanes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗