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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 1,351 records · Page 75
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Atomistic Insights into Nitrogen-Cycle Electrochemistry: A Combined DFT and Kinetic Monte Carlo Analysis of NO Electrochemical Reduction on Pt(100)

Electrocatalytic denitrification is a promising technology for the removal of NOx species in groundwater. However, a lack of understanding of the molecular pathways that control the overpotential and product distribution have limited the development of practical electrocatalysts, and additional atomic-level insights are needed to advance this field. Adsorbed NO has been identified as a key intermediate in the NOx electroreduction network, and the elementary steps by which it decomposes to NH4+, N2, NH3OH+, or N2O remain a subject of debate. Herein, we report a combined density functional theory (DFT) and kinetic Monte Carlo (kMC) study of this reaction on Pt(100), a catalytic surface that is known to be suitable for the activation of strong covalent bonds, in acidic electrolytes. This approach describes the effects of coverage-dependent adsorbate–adsorbate interactions, water-mediated protonation kinetics and thermodynamics, and transient potential sweeps, on reaction rates and selectivities. The results predict NO stripping curves in excellent agreement with experiments while, at the same time, providing a mechanistic interpretation of observed current peaks. Furthermore, production of NH4+ products is traced to the rapid kinetics of N–O bond breaking in reactive intermediates, whereas rapid hydrogenation of surface N* species prevent competing pathways from forming either N2 or N2O. The combined DFT-kMC methodology thus provides a unique tool to describe the mechanism and energetics of platinum-catalyzed electroreduction in the nitrogen cycle, and this approach should also find application to related electrocatalytic processes that are of technological and environmental interest.

Chun, Hee-Joon↗
DOE OSTI2022

Understanding Instability in Formamidinium Lead Halide Perovskites: Kinetics of Transformative Reactions at Grain and Subgrain Boundaries

Transformative and reconstructive reactions impart significant structural changes at particle boundaries of hybrid perovskites, which influence environmental stability and optoelectronic properties of these materials. Here, we investigate the moisture-induced transformative reactions in formamidinium based perovskites FAPbX 3 (X=I, Br) and show that the ambient stability of these materials can be adjusted from a few hours to several months. For FAPbI 3 , roles of water vapor, particle size, and light illumination on the kinetic pathways of the cubic (a) transformation to the hexagonal (d) phase are analyzed by X-ray diffraction, optical microscopy, photoluminescence and solid-state NMR spectroscopy techniques. The grain and sub-grain boundaries exhibit different α→δ-FAPbI 3 phase transformation kinetics. Our study suggests that the dynamic transformation involves the local water-induced dissolution of the cubic phase occurring at the crystal surfaces followed by precipitation of the hexagonal phase. Insights into structures and dynamics of a kinetically trapped α-|δ-FAPbI 3 are obtained by 1 H, 2 H, and 207 Pb ssNMR spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Mechanistic Study of 1,2-Dichloroethane Hydrodechlorination on Cu-Rich Pt–Cu Alloys: Combining Reaction Kinetics Experiments with DFT Calculations and Microkinetic Modeling

Cu-rich Pt–Cu bimetallic catalysts are among the most promising candidates for actively catalyzing the hydrodechlorination of 1,2-dichloroethane (1,2-DCA) toward ethylene production. Combining reaction kinetics experiments with density functional theory (DFT) calculations and mean-field microkinetic modeling, we present a systematic mechanistic study for 1,2-DCA hydrodechlorination on Cu-rich Pt–Cu alloy catalysts. Our DFT (PBE+(TS+SCS)) results suggest that increasing Cu content in the Pt–Cu alloy destabilizes C 2 -species adsorption while stabilizing the binding of atomic chlorine. The reaction energetics of all the elementary steps in the 1,2-DCA reaction network were calculated on a Pt 1 Cu 3 (111) model surface. The DFT results were then used to construct a microkinetic model, and the model-predicted reaction rates were compared with our reaction kinetics experimental results on a Cu-rich SiO 2 -supported Pt–Cu alloy catalyst through a parameter estimation procedure. Both the reaction kinetics experiments and the microkinetic model after parameter adjustments yielded 100% selectivity to ethylene. The microkinetic model pointed to a reaction pathway involving two sequential chlorine-removal steps on the Pt–Cu alloy catalyst, a mechanism distinct from the one previously identified on pure Pt/SiO 2 catalysts, which involved an initial hydrogen-removal step. Adjustments to the DFT-derived parameters indicate the possible formation of chlorine-induced Cu-enriched surface sites during 1,2-DCA hydrodechlorination conditions, sites that are more active than those encountered in the bulk Pt 1 Cu 3 (111) alloy surface. Furthermore, our study offers valuable initial insights on the 1,2-DCA hydrodechlorination reaction mechanism and the nature of the active sites on PtCu bimetallic catalysts.

1,2-Dichloroethane↗
DOE OSTI2012

Kinetically Controlled Nanostructure Formation in Self-Assembled Globular Protein–Polymer Diblock Copolymers

Aqueous processing of globular protein–polymer diblock copolymers into solid-state materials and subsequent solvent annealing enables kinetic and thermodynamic control of nanostructure formation to produce block copolymer morphologies that maintain a high degree of protein fold and function. When model diblock copolymers composed of mCherry-b-poly(N-isopropylacrylamide) are used, orthogonal control over solubility of the protein block through changes in pH and the polymer block through changes in temperature is demonstrated during casting and solvent annealing. Hexagonal cylinders, perforated lamellae, lamellae, or hexagonal and disordered micellar phases are observed, depending on the coil fraction of the block copolymer and the kinetic pathway used for self-assembly. Here, good solvents for the polymer block produce ordered structures reminiscent of coil–coil diblock copolymers, while an unfavorable solvent results in kinetically trapped micellar structures. Decreasing solvent quality for the protein improves long-range ordering, suggesting that the strength of protein interactions influences nanostructure formation. Subsequent solvent annealing results in evolution of the nanostructures, with the best ordering and the highest protein function observed when annealing in a good solvent for both blocks. While protein secondary structure was found to be almost entirely preserved for all processing pathways, UV–vis spectroscopy of solid-state films indicates that using a good solvent for the protein block enables up to 70% of the protein to be retained in its functional form.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Evaporation Kinetics of Laboratory Generated Secondary Organic Aerosols at Elevated Relative Humidity

Secondary organic aerosols (SOA) dominate atmospheric organic aerosols that affect climate, air quality, and health. Recent studies indicate that, contrary to previously held assumptions, at low relative humidity (RH) these particles are semi-solid and evaporate orders of magnitude slower than expected. Elevated relative humidity has the potential to affect significantly formation, properties, and atmospheric evolution of SOA particles. Here we present a study of the effect of RH on the room-temperature evaporation kinetics of SOA particles formed by ozonolysis of α-pinene and limonene. Experiments were carried out on SOA particles generated, evaporated, and aged at 0%, 50% and 90% RH. We find that in all cases evaporation begins with a relatively fast phase, during which 30% to 70% of the particle mass evaporates in 2 hours, followed by a much slower evaporation rate. Evaporation kinetics at 0% and 50% RH are nearly the same, while at 90% RH a slightly larger fraction evaporates. In all cases, aging the particles prior to inducing evaporation reduces the evaporative losses, with aging at elevated RH leading to more significant effect. In all cases, SOA evaporation is nearly size-independent, providing direct evidence that oligomers play a crucial role in determining the evaporation kinetics.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2011

Performance and Kinetics of a solid Amine Sorbent for Carbon Dioxide Removal

The kinetics of the reaction between CO{sub 2} and amine/bentonite particle were estimated over the range of 303–363 K from adsorption data obtained by TGA. The weight percent of amine, reaction temperature, and particle diameter were considered as experimental variables. The sorbent maximum or equilibrium CO{sub 2} uptake was found to be linearly dependent on temperature; decreasing with increasing temperature when tested in a 100% CO{sub 2} environment. Reactivity data for amine/bentonite particle with CO{sub 2} were presented and discussed. On the basis of the isothermal TGA results, reaction order and the value of activation energy have been obtained. These kinetic parameters are similar to those reported for MEA and DEA in aqueous solutions. The kinetic model was used to predict the fractional conversion at different temperature exhibiting good agreement with experimental data.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Thermogravimetric Analysis of Modified Hematite by Methane (CH{sub 4}) for Chemical-Looping Combustion: A Global Kinetics Mechanism

Iron oxide (Fe{sub 2}O{sub 3}) or in its natural form (hematite) is a potential material to capture CO{sub 2} through the chemical-looping combustion (CLC) process. It is known that magnesium (Mg) is an effective methyl cleaving catalyst and as such it has been combined with hematite to assess any possible enhancement to the kinetic rate for the reduction of Fe{sub 2}O{sub 3} with methane. Therefore, in order to evaluate its effectiveness as a hematite additive, the behaviors of Mg-modified hematite samples (hematite –5% Mg(OH){sub 2}) have been analyzed with regard to assessing any enhancement to the kinetic rate process. The Mg-modified hematite was prepared by hydrothermal synthesis. The reactivity experiments were conducted in a thermogravimetric analyzer (TGA) using continuous stream of CH{sub 4} (5, 10, and 20%) at temperatures ranging from 700 to 825 {degrees}C over ten reduction cycles. The mass spectroscopy analysis of product gas indicated the presence of CO{sub 2}, H{sub 2}O, H{sub 2} and CO in the gaseous product. The kinetic data at reduction step obtained by isothermal experiments could be well fitted by two parallel rate equations. The modified hematite samples showed higher reactivity as compared to unmodified hematite samples during reduction at all investigated temperatures.

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

Self-Assembly of Metallo-Supramolecules under Kinetic or Thermodynamic Control: Characterization of Positional Isomers Using Scanning Tunneling Spectroscopy

Coordination-driven self-assembly has been extensively employed to construct a variety of discrete structures as a bottom-up strategy. However, mechanistic understanding regarding whether self-assembly is under kinetic or thermodynamic control is less explored. To date, such mechanistic investigation has been limited to distinct, assembled structures. It still remains a formidable challenge to study the kinetic and thermodynamic behavior of self-assembly systems with multiple assembled isomers due to the lack of characterization methods. Herein, we use a stepwise strategy which combined self-recognition and self-assembly processes to construct giant metallo-supramolecules with 8 positional isomers in solution. With the help of ultrahigh-vacuum, low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy, we were able to unambiguously differentiate 14 isomers on the substrate which correspond to 8 isomers in solution. Through measurement of 162 structures, the experimental probability of each isomer was obtained and compared with the theoretical probability. Finally, such a comparison along with density functional theory (DFT) calculation suggested that although both kinetic and thermodynamic control existed in this self-assembly, the increased experimental probabilities of isomers compared to theoretical probabilities should be attributed to thermodynamic control.

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

Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides

The kinetics of hydride transfer from Re( R bpy)(CO) 3 H (bpy = 4,4'-R-2,2'-bipyridine; R = OMe, t Bu, Me, H, Br, COOMe, CF 3 ) to CO 2 and seven different cationic N-heterocycles were determined. Additionally, the thermodynamic hydricities of complexes of the type Re( R bpy)(CO) 3 H were established primarily using computational methods. Linear free-energy relationships (LFERs) derived by correlating thermodynamic and kinetic hydricities indicate that, in general, the rate of hydride transfer increases as the thermodynamic driving force for the reaction increases. Kinetic isotope effects range from inverse for hydride transfer reactions with a small driving force to normal for reactions with a large driving force. Hammett analysis indicates that hydride transfer reactions with greater thermodynamic driving force are less sensitive to changes in the electronic properties of the metal hydride, presumably because there is less buildup of charge in the increasingly early transition state. Bronsted α values were obtained for a range of hydride transfer reactions and along with DFT calculations suggest the reactions are concerted, which enables the use of Marcus theory to analyze hydride transfer reactions involving transition metal hydrides. It is notable, however, that even slight perturbations in the steric properties of the Re hydride or the hydride acceptor result in large deviations in the predicted rate of hydride transfer based on thermodynamic driving forces. This indicates that thermodynamic considerations alone cannot be used to predict the rate of hydride transfer, which has implications for catalyst design.

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

Thermodynamic, Kinetic, and Transport Contributions to Hydrogen Evolution Activity and Electrolyte-Stability Windows for Water-in-Salt Electrolytes

Concentrated water-in-salt electrolytes (WiSEs) are used in aqueous batteries and to control electrochemical reactions for fuel production. The hydrogen evolution reaction is a parasitic reaction at the negative electrode that limits cell voltage in WiSE batteries and leads to self-discharge, and affects selectivity for electrosynthesis. Mitigating and modulating these processes is hampered by a limited fundamental understanding of HER kinetics in WiSEs. Here, in this study, we quantitatively assess how thermodynamics, kinetics, and interface layers control the apparent HER activities in 20 m LiTFSI. When the LiTFSI concentration is increased from 1 to 20 m, an increase in proton activity causes a positive shift in the HER equilibrium potential of 71 mV. The exchange current density, i o , derived from the HER branch for 20 m LiTFSI in 98% purity (0.56 ± 0.05 μA/cm Pt 2 ), however, is 8 times lower than for 20 m LiTFSI in 99.95% (4.7 ± 0.2 μA/cm Pt 2 ) and 32 times lower than for 1 m LiTFSI in 98% purity (18 ± 1 μA/cm Pt 2 ), demonstrating that the WiSE's impurities and concentration are both central in significantly suppressing HER kinetics. The ability and applicability of the reported methods are extended by examining additional WiSEs formulations made of acetates and nitrates.

25 ENERGY STORAGE↗
DOE OSTI2017

Lithiation Thermodynamics and Kinetics of the TiO 2 (B) Nanoparticles

TiO2 (B) has attracted a lot of attention in recent years because it exhibits the largest capacity among all studied titania polymorphs with high rate performance for Li intercalation achieved when this material is nanostructured. However, due to the complex nature of its lithiation mechanism and practical challenges in probing Li local environments in nanostructured materials, a definitive understanding of the lithiation thermodynamics has yet to be established. A comprehensive mechanistic investigation of the TiO2 (B) nanoparticles is therefore presented using a combination of in situ / operando X-ray pair distribution function (PDF) and electrochemical techniques. The discharge begins with surface reactions involving surface hydroxyl groups. Such reactions contribute to the capacity loss and take place in parallel with Li insertion into the near-surface region of the nanoparticles. The Li bulk insertion starts with a single-phase reaction into the A2 site, a position adjacent to the b channel. A change of the Li diffusion pathway from that along this open channel to that along the c-direction is likely to occur at the composition of Li0.25TiO2 until Li0.5TiO2 is attained, leading to a two-step A2-site incorporation with one step kinetically distinct from the other. Subsequent Li insertion involves C’ site, a position situated inside the channel, and follows a rapid two-phase reaction to form Li0.75TiO2. Due to the high diffusion barrier associated with the further lithiation, Li insertion into the A1 site, another position adjacent to the channel neighboring the A2 sites, is kinetically restricted. It can be promoted by either nanostructuring or raising the operating temperature, the latter however triggering concurrent electrolyte decomposition giving rise to additional capacity loss. This study not only provides compelling experimental evidence for the unresolved reaction thermodynamics of nanoparticulate TiO2 (B), but also serves as a strong reference for future studies of the bulk phase, and for future calculations to study the kinetic properties of TiO2 (B).

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

In Situ Tracking Kinetic Pathways of Li + /Na + Substitution during Ion-Exchange Synthesis of Li x Na 1.5– x VOPO 4 F 0.5

Ion exchange is a ubiquitous phenomenon central to wide industrial applications, ranging from traditional (bio)chemical separation to the emerging chimie douce synthesis of materials for batteries and other energy applications. The exchange process is complex, involving substitution and transport of different ions under non-equilibrium conditions, and thus difficult to probe, leaving a gap in mechanistic understanding of kinetic exchange pathways toward final products. Herein, we report in situ tracking kinetic pathways of Li + /Na + substitution during solvothermal ion-exchange synthesis of Li x Na 1.5-x VOPO 4 F 0.5 (0 ≤ x ≤ 1.5), a promising multi-Li polyanionic cathode for batteries. The real-time observation, corroborated by first-principles calculations, reveals a selective replacement of Na + by Li + , leading to peculiar Na + /Li + /vacancy orderings in the intermediates. Contradicting the traditional belief of facile topotactic substitution via solid solution reaction, an abrupt two-phase transformation occurs and predominantly governs the kinetics of ion exchange and transport in the 1D polyanionic framework, consequently leading to significant difference of Li stoichiometry and electrochemical properties in the exchanged products. The findings may help to pave the way for rational design of ion exchange synthesis for making new materials.

25 ENERGY STORAGE↗
DOE OSTI2014

Adsorption, Desorption, and Displacement Kinetics of H2O and CO2 on Forsterite, Mg2SiO4(011)

We have examined the adsorbate-substrate interaction kinetics of CO2 and H2O on a natural forsterite crystal surface, Mg2SiO4(011), with 10-15% of substitutional Fe2+. We use temperature programmed desorption (TPD) and molecular beam techniques to determine the adsorption, desorption, and displacement kinetics for H2O and CO2. Neither CO2 nor H2O has distinct sub-monolayer desorption peaks but instead both have a broad continuous desorption feature that evolve smoothly into multilayer desorption. Inversion of the monolayer coverage spectra for both molecules reveals that the corresponding binding energies for H2O are greater than that for CO2 on all sites. The relative strength of these interactions is the dominant factor in the competitive adsorption/displacement kinetics. In experiments where the two adsorbates are co-dosed, H2O always binds to the highest energy binding sites available and displaces CO2. The onset of CO2 displacement by H2O occurs between 65 and 75 K.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Kinetic gating mechanism of DNA damage recognition by Rad4/XPC

The xeroderma pigmentosum C (XPC) complex initiates nucleotide excision repair by recognizing DNA lesions before recruiting downstream factors. How XPC detects structurally diverse lesions embedded within normal DNA is unknown. Here we present a crystal structure that captures the yeast XPC orthologue (Rad4) on a single register of undamaged DNA. The structure shows that a disulphide-tethered Rad4 flips out normal nucleotides and adopts a conformations similar to that seen with damaged DNA. Contrary to many DNA repair enzymes that can directly reject non-target sites as structural misfits, our results suggest that Rad4/XPC uses a kinetic gating mechanism whereby lesion selectivity arises from the kinetic competition between DNA opening and the residence time of Rad4/XPC per site. This mechanism is further supported by measurements of Rad4-induced lesion-opening times using temperature-jump pertubation spectroscopy. Kinetic gating may be a general mechanism used by site-specific DNA-binding proteins to minimize time-consuming interrogations of non-target sites.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

In situ microbeam surface X-ray scattering reveals alternating step kinetics during crystal growth

The stacking sequence of hexagonal close-packed and related crystals typically results in steps on vicinal {0001} surfaces that have alternating A and B structures with different growth kinetics. However, because it is difficult to experimentally identify which step has the A or B structure, it has not been possible to determine which has faster adatom attachment kinetics. Here we show that in situ microbeam surface X-ray scattering can determine whether A or B steps have faster kinetics under specific growth conditions. We demonstrate this for organo-metallic vapor phase epitaxy of (0001) GaN. X-ray measurements performed during growth find that the average width of terraces above A steps increases with growth rate, indicating that attachment rate constants are higher for A steps, in contrast to most predictions. Overall, our results have direct implications for understanding the atomic-scale mechanisms of GaN growth and can be applied to a wide variety of related crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2023

UniKP: a unified framework for the prediction of enzyme kinetic parameters

Prediction of enzyme kinetic parameters is essential for designing and optimizing enzymes for various biotechnological and industrial applications, but the limited performance of current prediction tools on diverse tasks hinders their practical applications. Here, we introduce UniKP, a unified framework based on pretrained language models for the prediction of enzyme kinetic parameters, including enzyme turnover number (k cat ), Michaelis constant (K m ), and catalytic efficiency (k cat / K m ), from protein sequences and substrate structures. A two-layer framework derived from UniKP (EF-UniKP) has also been proposed to allow robust k cat prediction in considering environmental factors, including pH and temperature. In addition, four representative re-weighting methods are systematically explored to successfully reduce the prediction error in high-value prediction tasks. We have demonstrated the application of UniKP and EF-UniKP in several enzyme discovery and directed evolution tasks, leading to the identification of new enzymes and enzyme mutants with higher activity. UniKP is a valuable tool for deciphering the mechanisms of enzyme kinetics and enables novel insights into enzyme engineering and their industrial applications.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Design principles for energy transfer in the photosystem II supercomplex from kinetic transition networks

Photosystem II (PSII) has the unique ability to perform water-splitting. With light-harvesting complexes, it forms the PSII supercomplex (PSII-SC) which is a functional unit that can perform efficient energy conversion, as well as photoprotection, allowing photosynthetic organisms to adapt to the naturally fluctuating sunlight intensity. Achieving these functions requires a collaborative energy transfer network between all subunits of the PSII-SC. In this work, we perform kinetic analyses and characterise the energy landscape of the PSII-SC with a structure-based energy transfer model. With first passage time analyses and kinetic Monte Carlo simulations, we are able to map out the overall energy transfer network. We also investigate how energy transfer pathways are affected when individual protein complexes are removed from the network, revealing the functional roles of the subunits of the PSII-SC. In addition, we provide a quantitative description of the flat energy landscape of the PSII-SC. We show that it is a unique landscape that produces multiple kinetically relevant pathways, corresponding to a high pathway entropy. These design principles are crucial for balancing efficient energy conversion and photoprotection.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2018

Phase-field model of pitting corrosion kinetics in metallic materials

This study presents a thermodynamically consistent phase field model for the quantitative prediction of the pitting corrosion kinetics in metallic materials. An order parameter is introduced to represent the metal’s physical state at each point in the entire domain. The free energy of the metal-electrolyte system is defined in terms of its metal ion concentration and the order parameter. The mass transport in the electrolyte and the electrochemical reactions in the electrolyte/metal interface are taken into consideration in the model to simulate metal corrosion in a corrosive environment. The governing equations for mass transport and the order parameter are solved in such a manner that the system’s free energy is reduced as a result of diffusion and migration processes, and the distribution of electrostatic potential is governed by Poisson’s equation. A calibration study is performed to couple the kinetic interface parameter with the corrosion current density, which results in a direct relationship between overpotential and the kinetic interface parameter. A comparative study is performed to validate the phase field model against the experimental results. Several case studies are also examined to understand the corrosion behavior of closely located pits, stressed material, ceramic particles-reinforced steel, and crystallographic orientation dependence.

36 MATERIALS SCIENCE↗