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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Molecular Hydroxo-Bridged Dimers of Uranium(VI), Neptunium(VI), and Plutonium(VI): [Me 4 N] 2 [(AnO 2 ) 2 (OH) 2 (NO 3 ) 4 ]

The synthesis of a series of molecular actinyl(VI), namely, uranium(VI), neptunium(VI), and plutonium(VI), hydroxo-bridged dimers is reported here. These complexes were isolated from an aqueous nitrate solution by titration with tetramethylammonium hydroxide. The solid-state structures were determined using single-crystal X-ray diffraction, revealing molecular complexes with the formula [Me 4 N] 2 [(AnO 2 ) 2 (μ 2 -OH) 2 (NO 3 ) 4 ], where An = U VI , Np VI , and Pu VI . Spectroscopic data—UV–vis–near-IR absorption, IR, and Raman—were collected on the solutions and solid-state complexes where available and compared to those of the aqueous solutions from which the crystals formed. These data provide structural evidence for higher-order polynuclear complexes of actinyl(VI) complexes upon a pH increase in the aqueous solution, confirming earlier thermodynamic models.

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

Water Solubility at Saturation for CO 2 –CH 4 Mixtures at 323.2 K and 9.000 MPa

The concentration of H 2 O dissolved in CO 2 –CH 4 supercritical fluids is an important parameter that can control shale permeability, affect CH 4 transmissivity, and ultimately impact the efficiency of CO 2 enhanced gas recovery (EGR) operations. Here, we use in situ high-pressure infrared (IR) spectroscopic titrations to quantify the solubility of H 2 O in six CO 2 –CH 4 mixtures, ranging from pure CO 2 to pure CH 4 , at shallow shale reservoir conditions of 323.2 K and 9.000 MPa. Measured concentrations of H 2 O at saturation increase with increasing mole percent CO 2 , and our results are in agreement with limited data available in the literature. We use these experimental results to benchmark three current thermodynamic multiphase routines: the mixed-solvent electrolyte (MSE), STOMP-COMP, and the Statistical Associating Fluid Theory for variable range Mie potentials (SAFT γ-Mie) equations of state. We find that, of these models, MSE and STOMP-COMP accurately predict maximum H 2 O solubilities of the binary CO 2 –H 2 O and CH 4 –H 2 O systems, and they also reproduce the shape of the water solubility curve as a function of mole percent CO 2 . Hence, these routines should work well to predict H 2 O content in reservoir simulations and help to make informed decisions concerning injection strategies for CO 2 EGR in shale plays at shallow depths.

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

Constant-pH Molecular Dynamics Simulations for Large Biomolecular Systems

We report that an increasingly important endeavor is to develop computational strategies that enable molecular dynamics (MD) simulations of biomolecular systems with spontaneous changes in protonation states under conditions of constant pH. The present work describes our efforts to implement the powerful constant-pH MD simulation method, based on a hybrid nonequilibrium MD/Monte Carlo (neMD/MC) technique within the highly scalable program NAMD. The constant-pH hybrid neMD/MC method has several appealing features; it samples the correct semigrand canonical ensemble rigorously, the computational cost increases linearly with the number of titratable sites, and it is applicable to explicit solvent simulations. The present implementation of the constant-pH hybrid neMD/MC in NAMD is designed to handle a wide range of biomolecular systems with no constraints on the choice of force field. Furthermore, the sampling efficiency can be adaptively improved on-the-fly by adjusting algorithmic parameters during the simulation. Finally, illustrative examples emphasizing medium- and large-scale applications on next-generation supercomputing architectures are provided.

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

CpHMD-Then-QM/MM Identification of the Amino Acids Responsible for the Anabaena Sensory Rhodopsin pH-Dependent Electronic Absorption Spectrum

Anabaena Sensory Rhodopsin (ASR), a microbial photoactive protein featuring the retinal chromophore in two different conformations, exhibits a pH-dependent electronic absorption spectrum. Using the recently developed CpHMD-then-QM/MM multiscale protocol applied to ASR embedded in a membrane model, the pH-induced changes in its maximum absorption wavelength have been reproduced and analyzed. While the acidic tiny red-shift is essentially correlated with the deprotonation of an aspartic acid located on the ASR extracellular side, the larger blue-shift experimentally reported at pH values larger than 5 involves a cluster of titrating residues sitting on the cytoplasmic side. The ASR pH-dependent spectrum is the consequence of the competitive stabilization of retinal ground and excited states by the protein electrostatic potential.

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

Pyrrole- and Naphthobipyrrole-Strapped Calix[4]pyrroles as Azide Anion Receptors

Here, the binding interactions between the azide anion (N 3 – ) and the strapped calix[4]pyrroles 2 and 3 bearing auxiliary hydrogen bonding donors on the bridging moieties, as well as of normal calix[4]pyrrole 1, were investigated via 1 H NMR spectroscopic and isothermal titration calorimetry analyses. The resulting data revealed that receptors 2 and 3 have significantly higher affinities for the azide anion in organic media as compared with the unfunctionalized calix[4]pyrrole 1 and other azide receptors reported to date. Single crystal X-ray diffraction analyses and calculations using density functional theory revealed that receptor 2 binds CsN 3 in two distinct structural forms. As judged from the metric parameters, in the resulting complexes one limiting azide anion resonance contributor is favored over the other, with the specifics depending on the binding mode. In contrast to what is seen for 2, receptor 3 forms a CsN 3 complex in 20% CD 3 OD in CDCl 3 , wherein the azide anion is bound only vertically to the NH protons of the calix[4]pyrrole and the cesium cation is complexed within the cone shaped-calix[4]pyrrole bowl. The bound cesium cation is also in close proximity to a naphthobipyrrole subunit present in a different molecule, forming an apparent cation-π complex.

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

Realistic Ion Dynamics through Charge Renormalization in Nonaqueous Electrolytes

While many practically important electrolytes contain lithium ions, interactions of these ions are particularly difficult to probe experimentally because of their small X-ray and neutron scattering cross sections and large neutron absorption cross sections. Molecular dynamics (MD) is a powerful tool for understanding the properties of nonaqueous electrolyte solutions from the atomic level, but the accuracy of this computational method crucially depends on the physics built into the classical force field. In this study, we demonstrate that several force fields for lithium bistriflimide (LiTFSI) in acetonitrile yield a solution structure that is consistent with the neutron scattering experiments, yet these models produce dramatically different ion dynamics in solution. Such glaring discrepancies indicate that inadequate representation of long-range interactions leads to excessive ionic association and ion-pair clustering. We show that reasonable agreement with the experimental observations can be achieved by renormalization of the ion charges using a “titration” method suggested herewith. This simple modification produces realistic concentration dependencies for ionic diffusion and conductivity in <2 M solutions, without loss in quality for simulation of the structure.

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

Electrostatic Control of Polymorphism in Charged Amphiphile Assemblies

Stimuli-induced structural transformations of molecular assemblies in aqueous solutions are integral to nanotechnological applications and biological processes. In particular, pH responsive amphiphiles as well as proteins with various degrees of ionization can reconfigure in response to pH variations. Here, we use in situ small and wide-angle X-ray scattering (SAXS/WAXS), transmission electron microscopy (TEM), and Monte Carlo simulations to show how charge regulation via pH induces morphological changes in the assembly of a positively charged peptide amphiphile (PA). Monte Carlo simulations and pH titration measurements reveal that ionic correlations in the PA assemblies shift the ionizable amine pK ~ 8 from pK ~ 10 in the lysine headgroup. SAXS and TEM show that with increasing pH, the assembly undergoes spherical micelle to cylindrical nanofiber to planar bilayer transitions. SAXS/WAXS reveal that the bilayer leaflets are interdigitated with the tilted PA lipid tails crystallized on a rectangular lattice. The details of the molecular packing in the membrane result from interplay between steric and van der Waals interactions. We speculate that this packing motif is a general feature of bilayers comprised of amphiphilic lipids with large ionic headgroups. Altogether, our studies correlate the molecular charge and the morphology for a pH-responsive PA system and provide insights into the Å-scale molecular packing in such assemblies.

36 MATERIALS SCIENCE↗
DOE OSTI2026

In Situ UV–vis Quantification of Chlorobasicity in Binary Molten Chlorides

Optical basicity measurements provide a basis for determining the relative chlorobasicity of a given molten salt composition. This study uses Bi3+ as a spectroscopic probe to investigate optical basicity across three binary molten chloride systems (LiCl-KCl, MgCl2–KCl, and ZnCl2–KCl) through in situ UV–vis spectroscopy measurements of the 1S0→3P1 transition. LiCl-KCl systems exhibit the highest optical basicity with minimal compositional dependence, while both divalent systems show stronger, yet nonlinear, dependence that is reminiscent of acid–base titration curves. The influence of cation electronegativity and polarizing power (Li+ < Mg2+ < Zn2+) directly reflects the basicity curves, with ZnCl2–KCl displaying the steepest gradients and lowest basicity values. Nonlinear dependencies in divalent systems indicate formation of chloride-bridged network structures with coordination states dictated by the amount of KCl in a salt composition, which was further investigated and validated by molecular dynamics simulations of polymeric structures formed in ZnCl2 and MgCl2-based salts using machine learning force fields. The temperature dependence of chlorobasicity in terms of optical basicity was investigated, and a loose, positive correlation between the two was revealed. These findings establish Bi3+ as a sensitive probe for understanding electronic environments in molten chloride systems and provide insights for predicting the chemical behavior of molten salts for use in high-temperature environments.

Buttice, Kailee [Department of Nuclear Engineering↗
DOE OSTI2021

Probing Acid–Base Properties of Anatase TiO 2 Nanoparticles with Dominant {001} and {101} Facets Using Methanol Chemisorption and Surface Reactions

In the present study, we investigate the surface acid-base properties of anatase TiO 2 nanomaterials with dominant {101} and {001} facets via methanol titrations. Two anatase nanoparticles, TiO 2 (101) and TiO 2 (001), with well-defined morphology are prepared. TiO 2 (101) is predominantly enclosed by the {101} facets (> 90%), and TiO 2 (001) contains ~46% {001} facets and ~54% {101} facets. Upon adsorption of methanol 423 K, DRIFTS measurements show that both molecular and dissociative adsorption occurs on TiO 2 (101), while dissociative adsorption dominates on TiO 2 (001). During methanol TPD, TiO 2 (001) mainly generates acid-base product dimethyl ether and thermal cracking products CO and H 2 , as anticipated. In contrast, substantial amounts of formaldehyde and methane also desorb from TiO 2 (101), suggesting strong participation of surface defects (e.g., oxygen vacancies).

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

Preparation and Characterization of Model Homotopic Catalysts: Rh Adatoms, Nanoparticles, and Mixed Oxide Surfaces on Fe 3 O 4 (001)

The atomic-level characterization of active sites is essential to understanding the mechanisms behind catalytic reactions. In this study, using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we follow the morphological changes of a model Rh catalysts supported on Fe 3 O 4 (001) as a function of temperature and Rh coverage. Here, we identify the preparation conditions leading to model catalysts containing homotopic or nearly homotopic Rh species bound as adatoms, small clusters, substitutional within the Fe 3 O 4 (001), and nanoparticles. Adsorbates such as CO and CO 2 are further subsequently used to characterize the properties of different Rh sites. Using temperature programmed desorption (TPD), we demonstrate that adatoms, clusters, and nanoparticles exhibit high-temperature CO desorption (250-600 K). Strong binding on such sites further allows for CO oxidation to CO 2 via the Mars-van-Krevelen mechanism. In contrast, CO 2 was found to interact weakly with all Rh sites. Differences in desorption temperature enable the use of CO and CO 2 as titration methods for nanoparticles and Fe 3 O 4 (001), respectively. A small quantity of CO 2 was found to be reduced to CO on Rh adatoms and clusters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Charge Transfer and Recombination Pathways through Fullerene Guests in Porphyrin-Based MOFs

Porphyrin-based metal–organic frameworks (MOFs) offer a unique platform for building porous donor–acceptor networks that exhibit long-lived charge separation and transport upon incorporation of electron acceptor guest species. Here, in this work, porphyrin-based MOFs, PCN-222(H 2 ) and PCN-222(Zn), synthesized as nanoparticle suspensions, are successfully infiltrated with fullerene acceptor molecules, C 60 or PC 61 BM, in both polar and nonpolar solvent environments. The location and relative binding strength of these guest species are evaluated through a combination of N 2 physisorption measurements, photoluminescence quenching, and UV–vis absorption titration experiments. Semiempirical tight binding calculations are used to screen potential locations of the fullerene guest within the MOF pores, and hybrid density functional theory (DFT)-computed interaction energies confirm the energetically favorable positions. The fundamental photophysics of these donor–acceptor host–guest combinations are probed using ultrafast transient absorption spectroscopy. Sub-picosecond electron transfer involving initial exciplex population is observed, with slow charge recombination lifetimes on the order of τ ~1 ns for all systems in both dimethylformamide and 1,4-dioxane. Charge recombination occurs through population of fullerene and/or framework porphyrin triplet states depending on the porphyrin metalation status. The photophysics of the fullerene-loaded MOFs are discussed in the context of relevant porphyrin–fullerene donor–acceptor molecules to highlight the unique role of the framework environment in dictating photoinduced electron transfer and decay pathways.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Revealing the Influence of Binding Motifs on Electron Transfer and Recombination Kinetics for CdSe Quantum Dots Functionalized with a Modified Viologen

Anchoring of molecules to the surfaces of semiconductor nanocrystals (NCs) presents an opportunity to leverage the precise synthetic tunability of molecular function and the remarkable light harvesting properties of NCs to drive photochemical reactions. However, charge transfer between the two species depends not only on the energy level alignments but also on the details of their binding interactions, which are difficult to probe. Here, in this work, we characterize the binding between CdSe quantum dots (QDs) and a new phosphonated derivative of the electron acceptor methyl viologen, designed to attach to the QD surface via the phosphonate group. We use isothermal titration calorimetry to probe the thermodynamics of the QD–molecule interaction and use the parameters determined therein to analyze transient absorption spectroscopy measurements of forward and back electron transfer from QDs to the viologen. We find that the ligand-like phosphonate binding leads to an electron-transfer rate constant that is 3 orders of magnitude smaller than that for the face-on binding of the bipyridine ring of methyl viologen. Back electron transfer is also significantly slower in the derivative. Interestingly, a minor fraction of the phosphonated derivative also binds in the face-on configuration, with similar forward and back electron transfer kinetics as methyl viologen. Numerical simulations show that the ligand-like binding will lead to significantly improved quantum yields of photocatalysis over a wide range of reaction rates. By independently characterizing binding thermodynamics and charge transfer kinetics, this work reveals how the complexities underlying electron transfer at the NC–molecule interface determine photocatalytic outcomes. This work also represents a step toward controlling forward and back electron transfer kinetics via rational molecular design.

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

Lithium Superoxide Hydrolysis and Relevance to Li–O 2 Batteries

Fundamental understanding of reactions of lithium peroxides and superoxides is essential for the development of Li–O 2 batteries. In this context, an investigation is reported of the hydrolysis of lithium superoxide, which has recently been synthesized in a Li–O 2 battery. Surprisingly, the hydrolysis of solid LiO 2 is significantly different from that of NaO 2 and KO 2 . Unlike KO 2 and NaO 2 , the hydrolysis of LiO 2 does not produce H 2 O 2 . Similarly, the reactivity of Li 2 O 2 toward water differs from LiO 2 , in that Li 2 O 2 results in H 2 O 2 as a product. The difference in the LiO 2 reactivity with water is due to the more exothermic nature of the formation of LiOH and O 2 compared with the corresponding reactions of NaO 2 and KO 2 . Here, we also show that a titration method used in this study, based on reaction of the discharge product with a Ti(IV)OSO 4 solution, provides a useful diagnostic technique to provide information on the composition of a discharge product in a Li–O 2 battery.

25 ENERGY STORAGE↗
DOE OSTI2017

Low-Temperature Pd/Zeolite Passive NO x Adsorbers: Structure, Performance, and Adsorption Chemistry

Pd/zeolite passive NOx adsorber (PNA) materials were prepared with solution ion-exchange between NH4/zeolites (Beta, ZSM-5 and SSZ-13) and PdCl2 solutions. The nature of Pd (dispersion, distribution and oxidation states) in these materials was characterized with Na+ ion-exchange, TEM imaging, CO titration with FTIR and in situ XPS. The NOx trapping and release properties were tested using feeds with different compositions. It is concluded that multiple Pd species coexist in these materials: atomically dispersed Pd in the cationic sites of zeolites, and PdO2 and PdO particles on the external surfaces. While Pd is largely atomically dispersed in ZSM-5, the small pore opening for SSZ-13 inhibits Pd diffusion such that the majority of Pd stays as external surface PdO2 clusters. NOx trapping and release are not simple chemisorption and desorption events, but involve rather complex chemical reactions. In the absence of CO in the feed, cationic Pd(II) sites with oxygen ligands and PdO2 clusters are reduced by NO to Pd(I) and PdO clusters. These reduced sites are the primary NO adsorption sites. In the presence of H2O, the as-formed NO2 desorb immediately. In the presence of CO in the feed, metallic Pd, “naked” Pd2+, and Pd+ sites are responsible for NO adsorption. For Pd adsorption sites with the same oxidation states but in different zeolite frameworks, NO binding energies are not expected to vary greatly. However, NO release temperatures do vary substantially with different zeolite structures. This indicates that NO transport within these materials play an important role in determining release temperatures. Finally, some rational design principles on efficient PNA materials are suggested. The authors gratefully acknowledge the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Office for the support of this work. The research described in this paper was performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle. Discussions with Drs. A. Yezerets, J.Y. Luo, K. Kamasamudram and N. Currier Cummins, Inc., and H.Y. Chen from Johnson-Matthey are greatly appreciated.

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

A Single Layer of MoS 2 Activates Gold for Room Temperature CO Oxidation on an Inert Silica Substrate

Here, we demonstrate the facile deposition of catalytically active stable Au nanoparticles on single-layer MoS 2 , on an inert oxide substrate (SiO 2 ). Unlike a gold surface, adsorption of CO on these gold particles proceeds even at room temperature. Subsequent exposure to oxygen leads to CO 2 formation and desorption along a reaction pathway described, in detail, by density functional theory. The barrier to CO oxidation, along this pathway, is less than 300 meV, corroborating the facile nature of the CO oxidation reactions in this system. X-ray photoelectron spectroscopy directly reveals that the carbon monoxide, adsorbed on Au nanoparticles on single-layer MoS 2 , can be repeatedly titrated off the substrate by exposure to oxygen. Comparison of computational results assuming a defect-laden and pristine MoS 2 substrate suggest that the pristine single-layer material may be superior in supporting this reaction.

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

Activation Mechanism and Surface Intermediates during Olefin Metathesis by Supported MoO x /Al 2 O 3 Catalysts

The activation of supported MoO x /Al 2 O 3 catalysts and the resulting surface intermediates were examined with in situ DRIFTS during olefin metathesis reaction conditions, aided by C 3 D 6 -C 3 H 6 isotopically labeled switching, C 2 H 4 -C 4 H 8 titration, and temperature programmed reaction experiments. The studies shows that activation by propylene of the surface MoO x sites on Al 2 O 3 initiates by forming surface isopropoxide species that subsequently dehydrogenate to acetone and surface Mo-OH. Desorption of acetone from the catalyst surface reduces the Mo +6 sites to Mo +4 sites, creating a vacancy for coordination of the next CH 2 =CHCH 3 molecule. Oxidative addition from subsequent adsorption of propylene on the surface Mo +4 sites results in formation of surface Mo=CH 2 and Mo=CHCH3 reactive intermediates, oxidizing the reduced surface molybdena sites back to Mo +6 . This study demonstrates the activation mechanism and surface intermediates during olefin metathesis by supported MoO x /Al 2 O 3 catalysts under reaction conditions.

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

Distinctive Trend of Metal Binding Affinity via Hydration Shell Breakage in Nanoconfined Cavity

Here, we report a distinctive trend in the binding affinity of metal cations when they interact with lacunary Keggin-type polyoxometalate K 7 [α-PW 11 O 39 ] clusters in aqueous media. The binding affinity of metal cations follows the special trends of Li + > Na + > K + for monovalent cations and Mg 2+ > Ca 2+ > Sr 2+ > Ba 2+ for divalent cations, completely reversed from the common trends of Li + < Na + < K + and Mg 2+ < Ca 2+ < Sr 2+ < Ba 2+ for macroion–counterion ion-pair formation in dilution solution. Isothermal titration calorimetry and NMR measurements confirm that the hydration shells of metal cations are significantly broken during the interaction in the nanoscale cavity, making the binding affinity stronger for the cations with smaller ionic sizes instead of hydrated sizes. Furthermore, the same phenomenon is observed for the lacunary Dawson-type clusters, which contain well-defined cavity in their structures as well, demonstrating the universality of this reverse trend and its connection to nanoconfinement effect in small submolecular cavities.

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

Monovalent versus Divalent Cation Diffusion in Thiospinel Ti2S4

Diffusion coefficients (D) for both Li+ and Mg2+ in Ti2S4 were measured using the galvanic intermittent titration technique (GITT) as a function of both ion concentration (x) and temperature. During discharge at 60 degrees C, D-Li descends gradually from 2 X 10(-8) cm(2)/s at x(Li) approximate to 0 to 2 X 10(-9) cm(2)/s at x(Li) approximate to 1.9. In contrast, D-Mg decreases sharply from 2 X 10(-8) to 1 x 10(-12) cm(2)/s by x(Mg) approximate to 0.8. This kinetic factor limits the maximum practical discharge capacity of MgxTi2S4. The difference in behavior vis a vis Li+ implies that either increasing Mg2+ occupation of the tetrahedral site at x(Mg) > 0.6 and/or interactions between diffusing cations play a larger role in mediating the diffusion of divalent compared to monovalent cations. Diffusion activation energies (E-a) extracted from the temperature-dependent data revealed that E-a,E-Mg (540 +/- 80 meV) is about twice that of E-a,E-Li (260 +/- 50 meV), explaining the poorer electrochemical performance of MgxTi2S4 at room temperature.

Bonnick, Patrick J.↗