Kinetic study of plasma-assisted n-dodecane/O2/N2 pyrolysis and oxidation in a nanosecond-pulsed discharge
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Energy & Fuels↗
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Results for “formation kinetics”
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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Observationally Constrained Modeling of the Reactive Uptake of Isoprene-Derived Epoxydiols under Elevated Relative Humidity and Varying Acidity of Seed Aerosol Conditions
Isoprene is the non-methane volatile organic compound (VOC) emitted in largest amounts to the atmosphere, and it is a significant source of secondary organic aerosol (SOA) mass. The uptake of isoprene oxidation products followed by multiphase chemistry in fine particles is the key pathway to form isoprene epoxydiol-derived SOA (IEPOX-SOA). However, many parameters that relate to diffusion and reaction of IEPOX in the particle phase remain uncertain, since reaction kinetics previously measured in bulk aqueous phase solutions might be different from atmospheric aerosols. Here, we use simultaneous environmental chamber measurements of multiple parameters governing IEPOX-SOA formation at timescales of ~hours: particle size distribution, composition, and volatility of IEPOX-SOA to constrain the key parameters governing IE-POX-SOA formation under humid (i.e., 50% relative humidity, RH) and varying seed aerosol acidity conditions. Reducing the 2-methyltetrol (tetrol) reaction rate constants by a factor of 4 brings the model predictions in agreement with the IEPOX-SOA measurements with acidified ammonium bisulfate seed aerosols. For less acidic ammonium sulfate aerosols both the organo-sulfate (OS) and tetrol reaction rate constants need to be reduced to bring model predictions closer to chamber observations. Using the measured non-volatile content of IEPOX-SOA we constrain the oligomerization timescale of 2-methyltetrols. We find that the oligomerization timescale is 4 hours with acidified seed aerosols, but a much longer time scale of 24 hours is needed for non-acidified seed aerosols, indicating that the aerosol acidity greatly affects the oligomerization rate of tetrols. Here we show that the actual kinetics of IEPOX-SOA formation rate on aerosol seeds consisting of both ammonium bisulfate and ammonium sulfate are a factor of 4~5 slower under 50-60% RH conditions compared to their application in previous models, which were based on bulk aqueous solution measurements.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Geologic hydrogen as an emerging fuel: experimental insights, thermodynamics, kinetics, and reactive transport modeling
Geologic hydrogen (GeoH 2 ) is emerging as a viable clean energy source. It is largely produced through serpentinization, a geological process in which ultramafic rocks react with water under suitable temperature and pressure. Here, this review synthesizes the current understanding of H 2 generation by serpentinization, with an emphasis on reaction mechanisms, kinetics, and thermodynamics, as well as on modeling flow and transport of reacting fluids in geological formations. We describe the role of mineral assemblages, such as olivine and pyroxene, fluid-rock interactions, and catalytic surfaces, in influencing GeoH 2 yield and reaction rates. By integrating models of reaction kinetics, subsurface reactive flow and transport, and the serpentinization process, and by accounting for the thermodynamic state of the system, this review aims to guide future GeoH 2 research and to evaluate the potential of natural hydrogen as a sustainable clean energy source.
Moradi, Rasoul [Univ. of Southern California, Los ↗
Engineering Thermally Resilient and Kinetically Active Reversible Protonic Ceramic Cells via Interfacial Design
Achieving concurrent fast electrode kinetics and long-term thermo-mechanical durability remains a critical challenge for reversible protonic ceramic electrochemical cells (R-PCECs). Herein, we report an interfacial engineering strategy that integrates a perovs.kite-type PrBaRu0.1Co1.9O5+δ (PBRC) nanoparticle layer onto a PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) substrate (PBRC-PBSCF), together with a modified pellet-assisted sintering approach to fabricate dense BaZr0.4Ce0.4Y0.1Yb0.1O3-δ (BZCYYb4411) electrolytes. The in situ reconstructed heterointerface enhances oxygen reduction/evolution reaction (ORR/OER) kinetics, promotes H2O adsorption/dissociation, and improves steam tolerance, as verified by electrochemical measurements and interfacial microstructural analyses. Density functional theory reveals that Ru-induced electronic modulation at the PBRC-PBSCF interface lowers the energy of oxygen vacancy formation and optimizes the position of the O 2p band center, thereby accelerating oxygen redox kinetics and stabilizing the interface. The resulting R-PCECs deliver an excellent peak power density of 1.112 W cm−2 and an electrolysis current density of −1.257 A cm−2 at 1.3 V in 3% H2O wet air at 600°C, with a reasonable faradaic efficiency. Furthermore, the cells demonstrate excellent stability, sustaining 100 h of thermal cycling (400–600°C, 200°C h−1) in both fuel cell and electrolysis modes, with 600 h of stability in electrolysis mode (600°C, −0.5 to −2 A cm−2).
30 DIRECT ENERGY CONVERSION↗
Hydrogen-silicon carbide interactions
A study of the thermochemistry and kinetics of hydrogen environmental attack of silicon carbide was conducted for temperatures in the range from 1100 C to 1400 C. Thermodynamic maps based on the parameters of pressure and oxygen/moisture content were constructed. With increasing moisture levels, four distinct regions of attack were identified. Each region is defined by the thermodynamically stable solid phases. The theoretically stable solid phases of Region 1 are silicon carbide and silicon. Experimental evidence is provided to support this thermodynamic prediction. Silicon carbide is the single stable solid phase in Region 2. Active attack of the silicon carbide in this region occurs by the formation of gases of SiO, CO, CH4, SiH4, and SiH. Analysis of the kinetics of reaction for Region 2 at 1300 C show the attack of the silicon carbide to be controlled by gas phase diffusion of H2O to the sample. Silicon carbide and silica are the stable phases common to Regions 3 and 4. These two regions are characterized by the passive oxidation of silicon carbide and formation of a protective silica layer.
Eckel, Andrew J.↗
Hydrogen-silicon carbide interactions
A study of the thermochemistry and kinetics of hydrogen environmental attack of silicon carbide was conducted for temperatures in the range from 1100 C to 1400 C. Thermodynamics maps based on the parameters of pressure and oxygen/moisture content were constructed. With increasing moisture levels, four distinct regions of attack were identified. Each region is defined by the thermodynamically stable solid phases. The theoretically stable solid phases of region 1 are silicon carbide and silicon. Experimental evidence is provided to support this thermodynamic prediction. Silicone carbide is the single stable solid phase in region 2. Active attack of the silicon carbide in this region occurs by the formation of gases of SiO, CO, CH4, SiH4 and SiH. Analyses of the kinetics of reaction for region 2 at 1300 C show the attack of the silicon carbide to be controlled by gas phase diffusion of H2O to the sample. Silicon carbide and silica are the stable phases common to regions 3 and 4. These two regions are characterized by the passive oxidation of silicon carbide and formation of a protective silica layer.
Eckel, Andrew J.↗
Kinetics and Thermochemistry of the Cl((sup 2)P(sub J)) + C2Cl4 Association Reaction
A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the Cl(sup 2)P(sub j) + C2Cl4 association reaction as a function of temperature (231-390 K) and pressure (3-700 Torr) in nitrogen buffer gas. The reaction is found to be in the falloff regime between third and second order over the range of conditions investigated, although the second-order limit is approached at the highest pressures and lowest temperatures. At temperatures below 300 K, the association reaction is found to be irreversible on the experimental time scale of approximately 20 m-s. The kinetic data at T is less than 300 K have been employed to obtain falloff parameters in a convenient format for atmospheric modeling. At temperatures above 330 K, reversible addition is observed, thus allowing equilibrium constants for C2Cl5 formation and dissociation to be determined. Second- and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the association reaction: Delta-H(298) = -18.1 +/- 1.3 kcal/mol, Delta-H(0) = -17.6 +/- 1.3 kcal/mol, and Delta-S(298) = -27.7 +/- 3.0 cal/mol.K. In conjunction with the well-known heats of formation of Cl((sup 2)P(sub j)) and C2Cl4 the above Delta-H values lead to the following heats of formation for C2Cl5, at 298 and 0 K: Delta-H(f,298) = 8.0 +/- 1.3 kcal/mol and Delta-H(f,0) = 8.1 +/- 1.5 kcal/mol. The kinetic and thermochemical parameters reported above are compared with other reported values, and the significance of reported association rate coefficients for understanding tropospheric chlorine chemistry is discussed.
Nicovich, J. M.↗
Laboratory studies of low temperature rate coefficients: The atmospheric chemistry of the outer planets
The objectives are to measure laboratory rate coefficients for key reactions of hydrocarbon molecules and radicals at low temperatures, which are relevant to the atmospheric photochemistry of Saturn, Jupiter, and Titan. Upcoming NASA planetary missions, such as Cassini, will probe the atmosphere of Titan in more detail, offering an excellent opportunity to test kinetic models and to establish fiducial standards for using kinetic models to interpret various parameters of the outer planets. Accurate low temperature kinetic data, which are presently lacking, may require crucial revisions to the rates of formation and destruction and are of utmost importance to the success of these efforts. In this program, several key reactions of ethynyl radicals (C2H) with acetylene (C2H2), methane (CH4), and oxygen (O2), down to temperatures of 170 K were successfully investigated. The experimental apparatus developed in our laboratory for measuring reaction kinetics at low temperatures consists of a laser photolysis/infrared probe laser setup. The rate measurements are carried out as a function of (low) temperature with a transverse flow cell designed specifically for these studies. A 193 nm argon fluoride pulsed excimer laser is used to photolyze a suitable precursor molecule, such as acetylene to produce C2H, and a high resolution, tunable infrared F-center laser (2.3-3.35 mu m) probes the transient concentrations of the radical species directly in absorption to extract the kinetic rate coefficients.
Leone, Stephen R.↗
Fuel-rich oxidation of gasoline surrogate components in an atmospheric flow reactor
Fuel-rich oxidation of three typical gasoline surrogate components, toluene, isooctane, and n-heptane, was investigated in an atmospheric-pressure flow reactor at mean gas temperatures from 1050 to 1350 K, equivalence ratio of 9.0, and residence times of 0.45 and 1.2 s. Not only polycyclic aromatic hydrocarbons (PAHs) up to 3 ring structure but also small intermediate products from C 1 to C 5 were quantified by a gas chromatograph mass spectrometry coupled with photon ionization and gas chromatograph with flame ionization detector, respectively. The kinetic model recently developed by Lawrence Livermore National Laboratory was revised to reflect the results of many recent investigations. Basically, the updated model could satisfactorily reproduce the experimental mole fractions of many species. The experimental and simulated results showed that PAH mole fractions produced were in the order of toluene, isooctane, and n-heptane. The kinetic analysis using the model was carried out to explore PAH formation pathways, especially focusing on naphthalene, acenaphthylene, and phenanthrene. Through rate of production analysis, it was found that the main formation pathways of many PAHs were affected by the fuels. Although resonantly stabilized radicals, such as benzyl and fulvenallenyl radicals, played a crucial role in the formation pathways of many PAHs in every fuel, they were produced through hydrogen elimination of toluene in toluene fuel, while they were formed from small intermediate products in isooctane and n-heptane fuels. Sensitivity analysis revealed the difference and similarity of the reactions with large positive coefficients according to the fuels studied here. In conclusion, the molecular growth reactions of aromatic species were influential in PAH production in every fuel, whereas the ring formation reactions from small species and the reactions involving toluene had large positive sensitivity coefficients in isooctane/n-heptane and toluene fuels, respectively.
32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
Direct Deoxygenation of Phenol over Fe-Based Bimetallic Surfaces Using On-the-Fly Surrogate Models
We present an accelerated nudged elastic band (NEB) study of phenol direct deoxygenation (DDO) on Fe-based bimetallic surfaces using a recently developed Gaussian process regression (GPR) calculator. Our test calculations demonstrate that the GPR calculator achieves up to 3 times speedup compared to conventional density functional theory calculations while maintaining high accuracy, with energy barrier errors below 0.015 eV. Using GPR-NEB, we systematically examine the DDO mechanism on pure Fe(110) and surfaces modified with Co and Ni in both top and subsurface layers. Our results show that subsurface Co and Ni substitutions preserve favorable thermodynamics and kinetics for both C–O bond cleavage and C–H bond formation, comparable to those on the pure Fe(110) surface. In contrast, top-layer substitutions generally increase the C–O bond cleavage barrier, render the step endothermic, and result in significantly higher reverse reaction rates, making DDO unfavorable on these surfaces. This work demonstrates the effectiveness of GRR-accelerated transition state searches for complex surface reactions and provides insights into rational design of bimetallic catalysts for selective deoxygenation.
Aromatic compounds↗
Interplay of Mechanochemistry and Material Processes in the Graphite to Diamond Phase Transformation
The manifestation of intramolecular strains in covalent systems is widely known to accelerate chemical reactions and open alternative reaction paths. This process is moderately well understood for isolated molecules and unimolecular processes. However, in condensed matter processes such as phase transformations, material properties and structure may influence typical mechanochemical effects. Therefore, we utilize steered molecular dynamics to induce out of plane strains in graphite and compress the system under a constant strain rate to induce phase transformation. We show that the out of plane strain allows phase transformations to initiate at small amounts of compressive strain. Yet, in contrast to typical mechanochemical results, the sum of compressive and out of plane work needed to form a diamond has a local minimum due to altered defect formation processes during phase transformation. Additionally, these altered processes slow the kinetics of the phase transformation, taking longer from initiation to total material transformation.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Toward Efficient Entropic Recycling by Mastering Ring–Chain Kinetics
Traditional chemical recycling approaches for condensation polymers suffer compounding energy losses and CO 2 emissions across multiple polymerization and depolymerization cycles. Entropic recycling can address these energy losses by entrapping free energy within the deconstruction products. Entropic recycling involves depolymerization to macrocyclic monomers, but such processes have not been feasible due to the high dilutions typically required to generate macrocyclic compounds. Here, we leverage selective catalysis to allow entropic recycling at concentrations 20–2000× higher than typical for macrocyclization reactions. We find that Ru-based olefin metathesis catalysts containing bulky iodine ligands significantly bias the ring–chain kinetic product distribution during ring-closing metathesis (RCM) toward the formation of oligomeric cycloalkenes. Further improvements in reaction concentration and macrocycle yield are obtained by using high catalyst loadings and by predisposing the alkene substrates to undergo favorable macrocyclization. These RCM optimizations translate effectively to cyclodepolymerization (CDP) of an olefin-containing polymer, with RCM and CDP affording similar macrocycle product distributions under identical reaction conditions. Macrocycle polymerization by entropy-driven ring-opening metathesis provides much higher molecular weight polymers than condensation polymerization of linear analogues, reducing the time to achieve high molecular weight from hours to minutes and enabling polymerization at room temperature. Finally, our findings re-emphasize the importance of energy consumption during a polymer’s lifecycle and provide a framework for the design of efficient entropic recycling systems.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Laser-driven, ion-scale magnetospheres in laboratory plasmas. II. Particle-in-cell simulations
Ion-scale magnetospheres have been observed around comets, weakly magnetized asteroids, and localized regions on the Moon and provide a unique environment to study kinetic-scale plasma physics, in particular in the collision-less regime. In this work, we present the results of particle-in-cell simulations that replicate recent experiments on the large plasma device at the University of California, Los Angeles. Using high-repetition rate lasers, ion-scale magnetospheres were created to drive a plasma flow into a dipolar magnetic field embedded in a uniform background magnetic field. The simulations are employed to evolve idealized 2D configurations of the experiments, study highly resolved, volumetric datasets, and determine the magnetospheric structure, magnetopause location, and kinetic-scale structures of the plasma current distribution. We show the formation of a magnetic cavity and a magnetic compression in the magnetospheric region, and two main current structures in the dayside of the magnetic obstacle: the diamagnetic current, supported by the driver plasma flow, and the current associated with the magnetopause, supported by both the background and driver plasmas with some time-dependence. From multiple parameter scans, we show a reflection of the magnetic compression, bounded by the length of the driver plasma, and a higher separation of the main current structures for lower dipolar magnetic moments.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes
Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η 5 -pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η 4 -pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2′-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)] + , an elusive hydride complex that has been spectroscopically and kinetically characterized here. Tautomerization of the hydride leads to the clean formation of [(Cp*H)Rh(bpy)] + . Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic monitoring of the second proton transfer event reveals that both the hydride and related Cp*H complex can be involved in further reactivity, showing that [(Cp*H)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands.
08 HYDROGEN↗
Cryo-$\mathrm{EM}$ reveals an entangled kinetic trap in the folding of a catalytic $\mathrm{RNA}$
Functional RNAs fold through complex pathways that can contain misfolded “kinetic traps.” A complete model of RNA folding requires understanding the formation of these misfolded states, but they are difficult to characterize because of their transient and potentially conformationally dynamic nature. We used cryo–electron microscopy (cryo-EM) to visualize a long-lived misfolded state in the folding pathway of the Tetrahymena thermophilagroup I intron, a paradigmatic RNA structure-function model system. The structure revealed how this state forms native-like secondary structure and tertiary contacts but contains two incorrectly crossed strands, consistent with a previous model. This incorrect topology mispositions a critical catalytic domain and cannot be resolved locally as extensive refolding is required. This work provides a structural framework for interpreting decades of biochemical and functional studies and demonstrates the power of cryo-EM for the exploration of RNA folding pathways.
59 BASIC BIOLOGICAL SCIENCES↗
Theoretical description of spectral line profiles of parent molecules in cometary comae
The present overview of cometary spectral-line profiles obtainable through advancements in high-resolution spectroscopic studies, which allow the retrieval of coma kinematic properties from velocity-resolved spectral-line profiles, incorporates the most important gas dynamic processes into an outflow model which is tailored to the interpretation of spectroscopic observations of parent molecules. The model is then used to study the influence on parent-molecule spectral line profile formation of the field-of-view, the expansion velocity, the kinetic temperature, and the anisotropic outflow distributions.
Hu, Hong-Yao↗
Kinetic conversion of CO to CH4 in the Solar System
Some of the most interesting chemistry in the Solar System involves changes in the oxidation state of the simple carbon species. The chemical pathways for the conversion of CH4 to CO and CO2 are for the most part known. The reverse process, the reduction of CO to CH4, is, however, poorly understood. This is surprising in view of the importance of the reduction process in the chemistry of the Solar System. Recently we investigated the chemical kinetics of a hitherto unsuspected reaction. It is argued that the formation of the methoxy radical (CH3O) from H+H2CO may play an essential role in the reduction of CO to CH4. The rate coefficient for this reaction has been estimated using the approximate theory of J. Troe and transition state theory. We will discuss the implications of this reaction for the chemistry of CO on Jupiter, in the solar nebula, for interpreting the laboratory experiments of A. Bar-Nun and A. Shaviv and A. Bar-Nun and S. Chang, and for organic synthesis in the prebiotic terrestrial atmosphere. The possible relation of CO reduction in the solar nebula and polyoxymethylene observed in comet Halley will be discussed.
Yung, Y. L.↗
Modeling High Current Pulsed Discharge in AA Battery Cathodes: The Effect of Localized Charging during Rest
During high current operation, substantial heterogeneity develops within battery cathodes, particularly when their thickness is large. Heterogeneity relaxation during subsequent rest is important for understanding battery performance under pulsed conditions. Localized charge balancing phenomena within batteries at zero net current are not well understood and merit investigation. In this work, the heterogeneity within cathodes of commercial alkaline Zn–MnO 2 batteries is measured during discharge and monitored during rest using energy dispersive X-ray diffraction (EDXRD). Significant gradients in protonation form during discharge and partially relax under rest. It is demonstrated that the proton gradient relaxation is through local redox activity at zero net current, where local (de)protonation works to redistribute charge across the cathode thickness. To support this redox-based relaxation, a fundamental kinetic study on prismatic MnO 2 cathodes is conducted to determine an appropriate model to describe both discharge and charge kinetics of MnO 2 . These kinetics are incorporated into a computational model to simulate the proton gradient formation and partial relaxation under identical discharge conditions as the operando EDXRD experiments. Model and experimental data are found to be in excellent agreement, correctly predicting localized charge balancing at rest.
Batteries↗