Non-equilibrium Reactions Research Articles

Parallel Numerical Simulation of Complex Unsteady Multi-Component Three-Dimensional Flow Field of Nonequilibrium Chemical Reaction

Parallel Numerical Simulation of Complex Unsteady Multi-Component Three-Dimensional Flow Field of Nonequilibrium Chemical Reaction

Enhanced mobility of cations and anions in the redox state: The polaronium mechanism

Hugely enhanced slow-ion diffusivity has been widely observed under extreme redox conditions and for unclear reasons. Aided by first-principles calculations on model systems of ZrO2, CeO2, BaTiO3 and Li4/3Mn2/3O2, here we successfully explained the intriguing phenomenon by a polaronium mechanism. We found a polaronium, defined as a transitory complex of a polaronic electron or hole and a migrating counterion, becomes highly mobile when the counterion comes from a d0 or f0 cation (e.g., Zr4+, Ti4+ and Ce4) or a p6 anion (e.g., O2−) in the host compound. Upon a redox reaction, the complex attains a d1/f1 or p5 configuration, which spontaneously forms because it is favored by an electron-phonon interaction (manifest as the Jahn-Teller effect in high symmetry systems) that enables local relaxation and lowers the system energy. Our calculations found such interaction reaching its peak at the saddle point where the local environment is softest, so soft that it allows a reorientation of the anisotropic d/f/p orbital to minimize the electron repulsion locally. Since the complex may dissolve after a successful ion-migration event, the redox electron/hole can be recycled to form another free-radical-like polaronium elsewhere, thereby enhancing ion migration repeatedly. The proposed polaronium mechanism, which also operates in ceramics doped with mixed-valence cations, is most relevant under dynamic and extreme thermal/field/irradiation conditions where extra electrons/holes are abundantly generated by non-equilibrium redox reactions. For such operations, some with emerging applications, our diffusion-enhancing mechanism may provide new theoretical insight to help understand their material/microstructure stability and performance.

Effects of near wall flow and non-equilibrium reaction coupling on heat flux prediction inside a 7-elements GOX/GCH4 combustion chamber

• The development and implementation of the coupled wall function are discussed in detail. • The chemistry effects are important for the accurate prediction of the methane combustion chamber. • The mechanism of the effect of the near wall chemistry on the heat transfer process is revealed. Methane/oxygen rocket engine is considered to be one of the most promising reusable rocket engines in future space activities. Adequate understanding and accurate prediction of heat transfer characteristics are considered key points for the development of reliable methane engines. In this paper, a methane combustion chamber with 7-elements is simulated using Reynolds averaged Navier-Stokes (RANS) method with Eddy Dissipation Concept (EDC) combustion model. The investigation reveals that the near-wall coupling effects of flow and chemistry have a significant influence on the wall heat load, and the coupled wall function developed by direct numerical simulation (DNS) is modified, validated, and incorporated in the RANS frame to consider the aforementioned coupling effects. The results show that the deviation of the wall heat load compared to experimental data is reduced from 25% to 5% for the wall of high temperature when chemistry effects are considered. The influence of the coupled wall function is limited near the wall and the properties of main flow are generally independent of the wall models adopted. The investigation also reveals that the turbulent flux of chemical enthalpy near the wall is comparable to the turbulent flux of sensible enthalpy in case of a methane combustion chamber. Finally, the effects of chemistry on the wall heat flux can be attributed to the coupling impacts of the chemical equilibrium shifting caused by the large temperature gradient near the wall and the non-uniform radial velocity brought by the powerful vortex system in the chamber.

Insights into the electrochemical performance of metal fluoride cathodes for lithium batteries

In recent years, energy storage and conversion have become key areas of research to address social and environmental issues, as well as practical applications, such as increasing the storage capacity of portable electronic storage devices. However, current commercial lithium-ion batteries suffer from low specific energy and high cost and toxicity. Conversion-type cathode materials are promising candidates for next-generation Li metal and Li-ion batteries (LIBs). Metal fluoride materials have shown tremendous chemical tailorability and exhibit excellent energy density in LIBs. Batteries based on such electrodes can compete with other envisaged alternatives, such as Li-air and Li-S systems. However, conversion reactions are typically multiphase redox reactions with mass transport phenomena and nucleation and growth processes of new phases along with interfacial reactions. Therefore, these reactions involve nonequilibrium reaction pathways and significant overpotentials during the charge-discharge process. In this review, we summarize the key challenges facing metal fluoride cathode materials and general strategies to overcome them in cells. Different synthesis methods of metal fluorides are also presented and discussed in the context of their application as cathode materials in Li and LIBs. Finally, the current challenges and future opportunities of metal fluorides as electrode materials are emphasized. With continuous rapid improvements in the electrochemical performance of metal fluorides, it is believed that these materials will be used extensively for energy storage in Li batteries in the future.

Kinetic and Continuum Modeling of High-Temperature Oxygen and Nitrogen Binary Mixtures

The present paper provides a comprehensive comparative analysis of thermochemistry models of various fidelity levels developed in leading research groups around the world. Fully kinetic, hybrid kinetic–continuum, and fully continuum approaches are applied to analyze parameters of hypersonic flows starting from the revision of single-temperature rate constants up to the application in 1-D postshock conditions. Comparison of state-specific and two-temperature approaches shows there are very significant and often qualitative differences in the time-dependent nonequilibrium reaction rates and their ratio to the corresponding single-temperature rates. A major impact of the vibration–dissociation coupling on the temporal relaxation of gas properties is shown. For instance, the legacy Park’s model has a strongly nonlinear behavior of nonequilibrium reaction rate with vibrational temperature, while a nearly linear shape exists for all state-specific approaches. Analysis of vibrational level populations in the nonequilibrium region shows a profound impact of the numerical approach and the model on the population ratios, and thus vibrational temperatures inferred from such ratios. The difference in the ultraviolet absorption coefficients, calculated by a temperature-based spectral code using vibrational populations from state-specific and kinetic approaches, is found to exceed an order of magnitude.

Self-Organized Spatiotemporal Mineralization of Hydrogel: A Simulant of Osteon.

Nature creates fascinating self-organized spatiotemporal patterns through the delicate control of reaction-diffusion dynamics. As the primary unit of cortical bone, osteon has concentric lamellar architecture, which plays a crucial role in the mechanical and physiological functions of bone. However, it remains a great challenge to fabricate the osteon-like structure in a natural self-organization way. Taking advantage of the nonequilibrium reaction in hydrogels, a simple mineralization strategy to closely mimic the formation of osteon in a mild physiological condition is developed. By constructing two reverse concentration gradients of ions from periphery to interior of cylindrical hydrogel, spatiotemporal self-organization of calcium phosphate in concentric rings is generated. It is noteworthy that minerals in different layers possess diverse contents and crystalline phases, which further guide the adhesion and spread of osteoblasts on these patterns, resembling the architecture and cytological behavior of osteon. Besides, theoretical data indicates the predominate role of ion concentrations and pH values of solution, in good accordance with experimental results. Independent of precise instruments, this lifelike method is easily obtained, cost-efficient, and effectively imitates the mineral deposition in osteon from a physiochemical view. The strategy may be expanded to develop other functional material patterns via spatiotemporal self-organization.

Constraints on error rate revealed by computational study of G•U tautomerization in translation.

In translation, G•U mismatch in codon-anticodon decoding is an error hotspot likely due to transition of G•U from wobble (wb) to Watson-Crick (WC) geometry, which is governed by keto/enol tautomerization (wb-WC reaction). Yet, effects of the ribosome on the wb-WC reaction and its implications for decoding mechanism remain unclear. Employing quantum-mechanical/molecular-mechanical umbrella sampling simulations using models of the ribosomal decoding site (A site) we determined that the wb-WC reaction is endoergic in the open, but weakly exoergic in the closed A-site state. We extended the classical ‘induced-fit’ model of initial selection by incorporating wb-WC reaction parameters in open and closed states. For predicted parameters, the non-equilibrium exoergic wb-WC reaction is kinetically limited by the decoding rates. The model explains early observations of the WC geometry of G•U from equilibrium structural studies and reveals discrimination capacity for the working ribosome operating at non-equilibrium conditions. The equilibration of the exoergic wb-WC reaction counteracts the equilibration of the open-closed transition of the A site, constraining the decoding accuracy and potentially explaining the persistence of the G•U as an error hotspot. Our results unify structural and mechanistic views of codon-anticodon decoding and generalize the ‘induced-fit’ model for flexible substrates.

Numerical simulation of hypersonic flow with non-equilibrium chemical reactions around sphere

High-temperature effects have a significant impact on the characteristics of aircraft moving at super- and hypersonic speeds. Due to the complexity of setting up a physical experiment, methods of mathematical modelling play an important role in finding the characteristics of hypersonic aircraft. Numerical modelling of super- and hypersonic air flow around the sphere is carried out taking into account high-temperature effects and non-equilibrium chemical reactions. The mathematical model includes equations of gas dynamics, equations of the turbulence model and equations of chemical kinetics. A critical review of the various models that are used to find the stand-off distance is provided. The results of numerical calculations on the distribution of flow quantities in the shock layer and the stand-off distance at different freestream Mach numbers are presented. The results of numerical calculations in a wide range of freestream Mach numbers are compared with the experimental data and computational results of other researchers. Standoff distance is computed at various Mach number and heights during a flight in the atmosphere at hypersonic speed.

The Distribution Behavior of Elements during the Top-Blowing Smelting Process of Electronic Waste

In this work, the local equilibrium modeling method of a non-equilibrium multi-phase reaction system in the top-blowing melting process of electronic waste was studied. The automatic judgment mechanism of phase transformation and the improvement of the trace component solving algorithm were explored to build the mathematical model of the element migration and transformation. Secondly, to determine the distribution mechanism of various elements in top-blowing smelting of electronic waste, the thermodynamic digital simulation system was developed according to the software platform of metallurgical process calculation. On this basis, combined with the industrial production practice, the coupling simulation experiment was carried out to investigate the influence of oxygen:feed ratio, oxygen concentration, amount of additive iron powder and CaO:SiO2 ratio of the slag on the smelting process. In addition, the direct yields of metals in the slag were Cu 90.69 wt%, Au 98.57 wt%, Ag 94.84 wt%, and Pd 97.87 wt% under the optimum conditions. Finally, the simulated values were consistent with industrial data, which can provide theoretical guidance for the industrial production practice of the top-blowing smelting of electronic waste.

Long-lasting accretion-powered chemical heating of millisecond pulsars

ABSTRACT We analyse the effect of magnetic field in superconducting neutron star (NS) cores on the chemical heating of millisecond pulsars (MSPs). We argue that the magnetic field destroys proton superconductivity in some volume fraction of the stellar core, thus allowing for unsuppressed non-equilibrium reactions of particle mutual transformations there. The reactions transform the chemical energy, accumulated by an NS core during the low-mass X-ray binary stage, into heat. This heating may keep an NS warm at the MSP stage (with the surface temperature $\sim 10^5\, \rm K$) for more than a billion of years after ceasing of accretion, without appealing to the rotochemical heating mechanism.

Nonequilibrium thermodynamics of accreted neutron-star crust

We show that, in order to determine the equation of state of the inner crust of an accreting neutron star, one should minimize not the Gibbs free energy, as it is generally assumed in the literature, but a different thermodynamic potential $\Psi$, which tends to the minimum at fixed pressure and neutron chemical potential. Once this potential is specified, one can calculate the heat-release distribution in the stellar crust due to nonequilibrium nuclear reactions induced by accretion of matter onto the neutron-star surface. The results are important for adequate modeling of the accreted crust and interpretation of the observations of accreting neutron stars in low-mass X-ray binaries.

The origins of kinetics hysteresis and irreversibility of monoclinic Li3V2(PO4)3

Monoclinic Li3V2(PO4)3 is a promising cathode material with complex charge–discharge behavior. Previous structural investigation of this compound mainly focuses on local environments; while the reaction kinetics and the driving force of irreversibility of this material remain unclear. To fully understand the above issues, both the equilibrium and the non-equilibrium reaction routes have been systematically investigated in this study. Multiple characterization techniques including X-ray diffraction, variable temperature (spinning rate) and ex/in situ7Li, 31P solid state NMR have been employed to provide comprehensive insights into kinetics, dynamics, framework structure evolution and charge ordering, which is essential to better design and application of lithium transition metal phosphate cathodes. Our results suggest that the kinetics process between the non-equilibrium and the quasi-equilibrium delithiation pathways from Li2V2(PO4)3 to V2(PO4)3 is related with a slow relaxation from two-site to one-site delithiation. More importantly, it has been demonstrated that the irreversibility in this system is not solely affected by cation and/or charge ordering/disordering, but mainly driven by framework structure distortion.

A study on the estimations of (n,t) reaction cross-sections at 14.5 MeV by using artificial neural network

In this paper, calculations of the [Formula: see text] reaction cross-sections at 14.5 MeV have been presented by utilizing artificial neural network algorithms (ANNs). The systematics are based on the account for the non-equilibrium reaction mechanism and the corresponding analytical formulas of the pre-equilibrium exciton model. Experimental results, obtained from the EXFOR database, have been used to train the ANN with the Levenberg–Marquardt (LM) algorithm which is a feed-forward algorithm and is considered one of the well-known and most effective methods in neural networks. The Regression [Formula: see text] values for the ANN estimation have been determined as 0.9998, 0.9927 and 0.9895 for training, testing and for all process. The [Formula: see text] reaction cross-sections have been reproduced with the TALYS 1.95 and the EMPIRE 3.2 codes. In summary, it has been demonstrated that the ANN algorithms can be used to calculate the [Formula: see text] reaction cross-section with the semi-empirical systematics.

Controlling biomolecular condensates via chemical reactions.

Biomolecular condensates are small droplets forming spontaneously in biological cells through phase separation. They play a role in many cellular processes, but it is unclear how cells control them. Cellular regulation often relies on post-translational modifications of proteins. For biomolecular condensates, such chemical modifications could alter the molecular interaction of key condensate components. Here, we test this idea using a theoretical model based on non-equilibrium thermodynamics. In particular, we describe the chemical reactions using transition-state theory, which accounts for the non-ideality of phase separation. We identify that fast control, as in cell signalling, is only possible when external energy input drives the reaction out of equilibrium. If this reaction differs inside and outside the droplet, it is even possible to control droplet sizes. Such an imbalance in the reaction could be created by enzymes localizing to the droplet. Since this situation is typical inside cells, we speculate that our proposed mechanism is used to stabilize multiple droplets with independently controlled size and count. Our model provides a novel and thermodynamically consistent framework for describing droplets subject to non-equilibrium chemical reactions.

(Invited) Leveraging Plasmas for Electrochemical Fuel Production: Synthesizing Novel Low-PGM Water Oxidation Catalysts and Enhancing the Rate of Electrochemical Ammonia Production

Plasmas have a number of unique properties which researchers may be able to leverage to enhance desired electrocatalyst material properties and steer electrochemical reaction performance. Two examples of the recent application of plasmas for advancements in electrochemical fuel production are presented. In the first, the nonequilibrium reaction environment of an air plasma enabled the synthesis of homogeneous phases of metastable compositions of a mixed W1-xIrxO3-δ alloy. For a low-platinum-group-metal (PGM) composition of only 1% Ir, plasma-synthesized W0.99Ir0.01O3-δ was observed to be an acid-stable oxygen evolution reaction (OER) catalyst of competitive overpotential. Moreover, the plasma-synthesized alloy was observed to have an OER overpotential ~ 570 mV less than that of a thermally oxidized alloy of the same composition. In the second example of a plasma enhancement, a nitrogen plasma was coupled into the cathode of a proton exchange membrane electrolyzer to provide energetically activated N2 species to the electrocatalyst surface and demonstrate a proof-of-concept synergistic enhancement in the rate of ammonia production. At an applied bias of ~3.5 V across the electrolyzer, plasma-assisted operation was observed to produce 47% more ammonia than the combination of plasma-without-bias and bias-without-plasma conditions.

Magnetohydrodynamic effect on first star formation: pre-stellar core collapse and protostar formation

ABSTRACT Recent theoretical studies have suggested that a magnetic field may play a crucial role in the first star formation in the universe. However, the influence of the magnetic field on the first star formation has yet to be understood well. In this study, we perform three-dimensional magnetohydrodynamic simulations taking into account all the relevant cooling processes and non-equilibrium chemical reactions up to the protostar density, in order to study the collapse of magnetized primordial gas cores with self-consistent thermal evolution. Our results show that the thermal evolution of the central core is hardly affected by a magnetic field because magnetic forces do not prevent the contraction along the field lines. We also find that the magnetic braking extracts the angular momentum from the core and suppresses fragmentation depending on the initial strength of the magnetic field. The angular momentum transport by the magnetic outflows is less effective than that by the magnetic braking because the outflows are launched only in a late phase of the collapse. Our results indicate that the magnetic effects become important for the field strength $B\gt 10^{-8}(n_{\rm H}/1\ \rm cm^{-3})^{2/3}\ \rm G$, where nH is the number density, during the collapse phase. Finally, we compare our results with simulations using a barotropic approximation and confirm that this approximation is reasonable at least for the collapse phase. Nevertheless, self-consistent treatment of the thermal and chemical processes is essential for extending simulations to the accretion phase, in which radiative feedback by protostars plays a crucial role.

Fuel-Driven Dynamic Combinatorial Libraries.

In dynamic combinatorial libraries, molecules react with each other reversibly to form intricate networks under thermodynamic control. In biological systems, chemical reaction networks operate under kinetic control by the transduction of chemical energy. We thus introduced the notion of energy transduction, via chemical reaction cycles, to a dynamic combinatorial library. In the library, monomers can be oligomerized, oligomers can be deoligomerized, and oligomers can recombine. Interestingly, we found that the dynamics of the library's components were dominated by transacylation, which is an equilibrium reaction. In contrast, the library's dynamics were dictated by fuel-driven activation, which is a nonequilibrium reaction. Finally, we found that self-assembly can play a large role in affecting the reaction's kinetics via feedback mechanisms. The interplay of the simultaneously operating reactions and feedback mechanisms can result in hysteresis effects in which the outcome of the competition for fuel depends on events that occurred in the past. In future work, we envision diversifying the library by modifying building blocks with catalytically active motifs and information-containing monomers.

Heat release in accreting neutron stars

Observed thermal emission from accreting neutron stars (NSs) in a quiescent state is believed to be powered by nonequilibrium nuclear reactions that heat the stellar crust (deep crustal heating paradigm). We derive a simple universal formula for the heating efficiency, assuming that an NS has a fully accreted crust. We further show that, within the recently proposed thermodynamically consistent approach to the accreted crust, the heat release can be parametrized by the only one parameter -- the pressure $P_{\rm oi}$ at the outer-inner crust interface (as we argue, this pressure should not necessarily coincide with the neutron-drip pressure). We discuss possible values of $P_{\rm oi}$ for a selection of nuclear models that account for shell effects, and determine the net heat release and its distribution in the crust as a function of $P_{\rm oi}$. We conclude that the heat release should be reduced by a factor of few in comparison to previous works.

Probing photoinduced proton coupled electron transfer process by means of two-dimensional resonant electronic-vibrational spectroscopy.

We develop a detailed theoretical model of photo-induced proton-coupled electron transfer (PPCET) processes, which are at the basis of solar energy harvesting in biological systems and photovoltaic materials. Our model enables us to analyze the dynamics and the efficiency of a PPCET reaction under the influence of a thermal environment by disentangling the contribution of the fundamental electron transfer and proton transfer steps. In order to study quantum dynamics of the PPCET process under an interaction with the non-Markovian environment, we employ the hierarchical equations of motion. We calculate transient absorption spectroscopy (TAS) and a newly defined two-dimensional resonant electronic-vibrational spectroscopy (2DREVS) signals in order to study the nonequilibrium reaction dynamics. Our results show that different transition pathways can be separated by TAS and 2DREVS.

Molecular Pumps and Motors.

Pumps and motors are essential components of the world as we know it. From the complex proteins that sustain our cells, to the mechanical marvels that power industries, much we take for granted is only possible because of pumps and motors. Although molecular pumps and motors have supported life for eons, it is only recently that chemists have made progress toward designing and building artificial forms of the microscopic machinery present in nature. The advent of artificial molecular machines has granted scientists an unprecedented level of control over the relative motion of components of molecules through the development of kinetically controlled, away-from-thermodynamic equilibrium chemistry. We outline the history of pumps and motors, focusing specifically on the innovations that enable the design and synthesis of the artificial molecular machines central to this Perspective. A key insight connecting biomolecular and artificial molecular machines is that the physical motions by which these machines carry out their function are unambiguously in mechanical equilibrium at every instant. The operation of molecular motors and pumps can be described by trajectory thermodynamics, a theory based on the work of Onsager, which is grounded on the firm foundation of the principle of microscopic reversibility. Free energy derived from thermodynamically non-equilibrium reactions kinetically favors some reaction pathways over others. By designing molecules with kinetic asymmetry, one can engineer potential landscapes to harness external energy to drive the formation and maintenance of geometries of component parts of molecules away-from-equilibrium, that would be impossible to achieve by standard synthetic approaches.