Reactive Molecular Dynamics of Fuel Oxidation and Catalytic Reactions

The present research employs the ReaxFF (a force field for reactive systems) molecular dynamics simulation method to investigate the detailed microscopic modelling for complex chemistry of fuel oxidation and catalytic reactions on graphenebased nanomaterials at the atomic level. Specifically, in total, four different systems are studied in detail. Firstly, the fundamental reaction mechanisms of hydrous ethanol oxidation in comparison with the ethanol oxidation under fuel-air condition is investigated. The results indicate that it is the addition of water that promotes the OH production due to the chemical effect of H2O leading to the enhancement of ethanol oxidation and reduction of CO production. Secondly, the fundamental study on mechanisms of thermal decomposition and oxidation of aluminium hydride is conducted. It is found that the thermal decomposition and oxidation of aluminium hydride proceed in three distinctive stages ((1) Pre-diffusion; (2) Core-shell integration; (3) Post-diffusion, and (I) Oxygen adsorption; (II) Fast dehydrogenation; (III) Al oxidation), respectively. Thirdly, the catalytic mechanisms and kinetics of methane oxidation assisted by Platinum/graphene-based catalysts are studied. Platinumdecorated functionalized graphene sheet is reported to be the most effective catalyst among all the involved nanoparticle candidates and it improves the catalytic activity by dramatically lowering the activation energy by approximately 73% compared with pure methane oxidation. Fourthly, the initiation mechanisms of JP-10 pyrolysis and oxidation with functionalized graphene sheets in comparison with normal JP-10 reactions are revealed. The results suggest that both pyrolysis and oxidation of JP-10 are advanced and enhanced in the presence of functionalized graphene sheets. Additionally, the functional groups also participate in various intermediate reactions to further enhance the pyrolysis and oxidation of JP-10. In summary, the new findings from the present research could contribute to the design and improvement of the future high-performance energy and propulsion systems, especially for the promising graphene-containing fuel/propellant formulations

Atomistic insight into enhanced thermal decomposition of energetic material on graphene oxide

Graphene oxide (GO)-based nanocomposites are promising additives for practical applications of cyclotrimethylenetrinitramine (RDX). GO is not only an excellent support for nanoparticles, but also has independent catalytic activities, which have not been well understood. In this study, the reactive molecular dynamics simulation method is employed to investigate the kinetics and fundamental catalytic mechanisms of the thermal decomposition of RDX on GO. The RDX decomposition reaction is found to be enhanced in the presence of GO and the catalytic effect is better at low than at high temperatures. Additionally, GO addition lowers the activation energy by 11.35% compared with the thermal decomposition of pure RDX. The study shows that the catalytic capabilities of GO primarily originate from its functional groups that promote both the initiation and intermediate reactions. Furthermore, the H exchange process between the functional groups on GO and RDX/RDX intermediates plays an important role in the reaction. GO is further oxidized with more functional groups during the reaction, which are also involved in the catalytic activities. Finally, the energy barrier of functional group-participated reactions is found to be lower than their corresponding unimolecular decomposition leading to enhanced thermal decomposition of RDX. The proposed catalytic mechanisms in the present research should also be applicable to other energetic materials of the same class with a similar structure as RDX

Classical and reactive molecular dynamics: Principles and applications in combustion and energy systems

Molecular dynamics (MD) has evolved into a ubiquitous, versatile and powerful computational method for fundamental research in science branches such as biology, chemistry, biomedicine and physics over the past 60 years. Powered by rapidly advanced supercomputing technologies in recent decades, MD has entered the engineering domain as a first-principle predictive method for material properties, physicochemical processes, and even as a design tool. Such developments have far-reaching consequences, and are covered for the first time in the present paper, with a focus on MD for combustion and energy systems encompassing topics like gas/liquid/solid fuel oxidation, pyrolysis, catalytic combustion, heterogeneous combustion, electrochemistry, nanoparticle synthesis, heat transfer, phase change, and fluid mechanics. First, the theoretical framework of the MD methodology is described systemically, covering both classical and reactive MD. The emphasis is on the development of the reactive force field (ReaxFF) MD, which enables chemical reactions to be simulated within the MD framework, utilizing quantum chemistry calculations and/or experimental data for the force field training. Second, details of the numerical methods, boundary conditions, post-processing and computational costs of MD simulations are provided. This is followed by a critical review of selected applications of classical and reactive MD methods in combustion and energy systems. It is demonstrated that the ReaxFF MD has been successfully deployed to gain fundamental insights into pyrolysis and/or oxidation of gas/liquid/solid fuels, revealing detailed energy changes and chemical pathways. Moreover, the complex physico-chemical dynamic processes in catalytic reactions, soot formation, and flame synthesis of nanoparticles are made plainly visible from an atomistic perspective. Flow, heat transfer and phase change phenomena are also scrutinized by MD simulations. Unprecedented details of nanoscale processes such as droplet collision, fuel droplet evaporation, and CO2 capture and storage under subcritical and supercritical conditions are examined at the atomic level. Finally, the outlook for atomistic simulations of combustion and energy systems is discussed in the context of emerging computing platforms, machine learning and multiscale modelling

Double-layer distribution of hydronium and hydroxide ions in the air-water interface

The acid-base nature of the aqueous interface has long been controversial. Most macroscopic experiments suggest that the air-water interface is basic based on the detection of negative charges at the interface that indicates the enrichment of hydroxides (OH–), whereas microscopic studies mostly support the acidic air-water interface with the observation of the hydronium (H3O+) accumulation in the top layer of the interface. It is crucial to clarify the interfacial preference of OH– and H3O+ ions for rationalizing the debate. In this work, we perform deep potential molecular dynamics simulations to investigate the preferential distribution of OH– and H3O+ ions at aqueous interfaces. The neural network potential energy surface is trained based on density functional theory calculations with the SCAN functional, which can accurately describe the diffusion of these two ions both in the interface and in the bulk water. In contrast to the previously reported single ion enrichment, we show that both OH– and H3O+ surprisingly prefer to accumulate in interfaces, but at different interfacial depths, rendering a double-layer ionic distribution within ~1 nm below the Gibbs dividing surface. The H3O+ is preferentially adsorbed in the topmost layer of the interface, but the OH–, which is enriched in the deeper interfacial layer, has a higher equilibrium concentration due to the more negative free energy of interfacial stabilization (–0.90 (OH–) vs. –0.56 (H3O+) kcal/mol). The air-water interface is therefore negatively charged, in agreement with the macroscopic charge detection and not in contradiction with the microscopic studies. The present finding of the ionic double-layer distribution qualitatively offers a self-consistent explanation for the long-term controversy about the acid-base nature of the air-water interface

Fundamental Study on Mechanisms of Thermal Decomposition and Oxidation of Aluminum Hydride

Aluminum hydride (AlH3) has great potential for a variety of propulsion and energy-storage applications. In this study, the ReaxFF reactive force field molecular dynamics simulation is employed to investigate the fundamental reaction mechanisms of thermal decomposition and oxidation of AlH3. The effects of an oxide layer and/or defect are examined, and the detailed process and mechanism of H2 and H2O formation are illustrated. With the presence of an oxide layer, H2 production of core–shell AlH3 during the thermal decomposition is slower than that of bare AlH3. As far as oxidation is concerned, any defect enhances the initiation of core–shell AlH3 oxidation and accelerates the oxidation at the early stage of the reaction. Additionally, the presence of O2 promotes the production of OH. Both thermal decomposition and oxidation of core–shell AlH3 show significant H2O production, and H2O is preferentially formed compared with H2 at the beginning of the reaction. The results reveal that the structural evolution of core–shell AlH3 during the thermal decomposition and oxidation proceeds in three distinctive stages, respectively. It is found that during the oxidation, dehydrogenation and oxidation proceed simultaneously although the oxidation rate is limited during the dehydrogenation period

Preparation and characterization of β-cyclodextrin grafted N-maleoyl chitosan nanoparticles for drug delivery

β-cyclodextrin (CD) grafted N-maleoyl chitosan (CD-g-NMCS) with two different degrees of substitution (DS) of N-maleoyl (DS = 21.2% and 30.5%) were synthesized from maleic anhydride and chitosan bearing pendant cyclodextrin (CD-g-CS). CD-g-NMCS based nanoparticles were prepared via an ionic gelation method together with chitosan and CD-g-CS nanoparticles. The size and zeta potential of prepared CD-g-NMCS nanoparticles were 179.2~274.0 nm and 36.2~42.4 mV, respectively. In vitro stability test indicated that CD-g-NMCS nanoparticles were more stable in phosphate-buffered saline compared with chitosan nanoparticles. Moreover, a poorly water-soluble drug, ketoprofen (KTP), was selected as a model drug to study the obtained nanoparticle's potentials as drug delivery carriers. The drug loading efficiency of CD-g-NMCS20 nanoparticles were 14.8% for KTP. MTT assay showed that KTP loaded CD-g-NMCS nanoparticles were safe drug carriers. Notably, in vitro drug release studies showed that KTP was released in a sustained-release manner for the nanoparticles. The pharmacokinetic of drug loaded CD-g-NMCS20 nanoparticles were evaluated in rats after intravenous administration. The results of studies revealed that, compared with free KTP, KTP loaded CD-g-NMCS20 nanoparticles exhibited a significant increase in AUC0→24h and mean residence time by 6.6-fold and 2.9-fold, respectively. Therefore, CD-g-NMCS nanoparticles could be used as a novel promising nanoparticle-based drug delivery system for sustained release of poorly water-soluble drugs. The carboxylic acid groups of the CD-g-NMCS molecule provide convenient sites for further structural modifications including introduction of tissue- or disease- specific targeting groups

How sodium chloride extends lifetime of bulk nanobubbles in water

We present a molecular dynamics simulation study on the effects of sodium chloride addition on stability of a nitrogen bulk nanobubble in water. We find that the lifetime of the bulk nanobubble is extended in the presence of NaCl and reveal the underlying mechanisms. We do not observe spontaneous accumulation or specific arrangement of ions/charges around the nanobubble. Importantly, we quantitatively show that the N2 molecule selectively diffuses through water molecules rather than pass by any ions after it leaves the nanobubble due to the much weaker water-water interactions than ion-water interactions. The strong ion-water interactions cause hydration effects and disrupt hydrogen bond networks in water, which leave fewer favorable paths for the diffusion of N2 molecules, and by that reduce the degree of freedom in the dissolution of the nanobubble and prolong its lifetime. These results demonstrate that the hydration of ions plays an important role in stability of the bulk nanobubble by affecting the dynamics of hydrogen bonds and the diffusion properties of the system, which further confirm and interpret the selective diffusion path of N2 molecules and the extension of lifetime of the nanobubble. The new atomistic insights obtained from the present research could potentially benefit the practical application of bulk nanobubbles

A reactive force field molecular dynamics study on the inception mechanism of titanium tetraisopropoxide (TTIP) conversion to titanium clusters

We performed ReaxFF reactive molecular dynamics simulations to investigate the inception mechanism of TTIP precursor droplet conversion to Ti-containing clusters in 1000 K–2500 K with or without gaseous O2 molecules. A new Ti/C/H/O ReaxFF force field has been developed. Key intermediate titanium species and the initial decomposition pathways of TTIP are identified. The effects of temperature, O2 concentration and high-temperature residence time on the conversion of TTIP to incipient titanium clusters are investigated. Results suggest that high pyrolysis temperature does not necessarily promote the formation of incipient Ti-containing clusters, due to less stable Tisingle bondO bonds at high temperatures. Ti2Ox Cy Hz species appear earlier than TiO2 during TTIP pyrolysis, while TiO2 forms earlier than Ti2OxCyHz species and has much higher concentration with ambient O2. Decreasing high-temperature residence time boosts the formation of Ti-containing clusters by facilitating the condensation of TiO2 vapors. The growth pattern of the incipient titanium clusters is elucidated as formation of Tisingle bondO bond with Ti2OxCyHz species or titanium clusters followed by continuous breakage of Tisingle bondO or Csingle bondO bonds to release hydrocarbon moieties