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

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

Development of a ReaxFF reactive force field for silicon dioxide/hydrogen fluoride etching systems and first-principle calculation of adsorption energies in lithium-sulfur batteries

학위논문(박사) -- 서울대학교대학원 : 공과대학 화학생물공학부, 2021.8. 이원보.본 박사과정 졸업 논문에서는, 다양한 계산화학 방법론을 통해 규소 산화막의 불화 수소를 통한 식각 공정과 리튬-황 배터리의 구성 요소인 양극재 바인더, 그리고 분리막 코팅 재료와 반응 부산물인 황화 리튬 간의 흡착 에너지에 대한 모델링 및 계산을 수행하였다. 첫 번째로, 규소(Si)-산소(O)-수소(H)-불소(F)를 포함한 식각 공정을 모델링 하기 위해 분자동역학 기법 중 하나인 ReaxFF 힘장을 개발하였다. ReaxFF 힘장 매개 변수는 양자역학 계산을 통해 생성된 물질의 구조, 결합 길이에 따른 에너지, 결합 각 차이에 따른 에너지, 그리고 규소 산화막과 불화 수소 간 반응에 대한 에너지 등의 학습 자료들을 기반으로 다시 구성되었다. 반응 에너지를 위한 계산 자료들은 작은 단위의 분자 모델과 표면 모델로 구성하여 계산하였다. 새로 학습된 ReaxFF 힘장은 양자역학 계산을 통한 분자 모델 및 표면 모델에서 발생하는 식각 공정 메커니즘의 에너지를 잘 모사하였다. 이렇게 새로 개발된 힘장을 통해 불화 수소를 통한 규소 산화막의 식각 공정을 분자동역학을 통해 모사하였다. 이를 통해 주입되는 식각 기체인 불화 수소에 주입되는 초기 에너지에 따른 식각 정도를 식각 수율 및 생성물의 양적 차이를 통해 비교하였다. 두 번째로, 리튬-황 배터리의 구성 요소 중 양극재 바인더와 분리막에 코팅된 금속 산화막의 개선을 통한 배터리 내구성 및 효율 증대를 위해 양자역학 계산 방법론 중 밀도 범함수 이론을 통해 황화 리튬과 구성 요소 간의 흡착 메커니즘 및 에너지를 확인하였다. 실제 실험 및 측정에 앞서, 이론적인 계산화학 방법론을 통해 양극재 바인더인 키토산과 XNBR 로 구성된 분자를 모사하고 황화 리튬과의 흡착 메커니즘을 확인하고 흡착 에너지를 계산하였다. 또한, 분리막에 코팅하는 산화막의 종류에 따른 황화 리튬과의 흡착 메커니즘 및 흡착 에너지를 계산하여 최적의 양극재 바인더 및 금속 산화막에 대한 정보를 계산 화학을 통해 규명하고 이를 실험을 통해 확인하였다.In this thesis, etching processes with silicon dioxide/hydrogen fluoride gas systems and adsorption mechanisms of lithium polysulfides (LPS) and battery components such as functional binder in sulfur cathode and separator coated with functional metal oxide shields are modelled and calculated with various computational methods. First, a new ReaxFF reactive force field has been developed to describe reactions in the Si-O-H-F system. The ReaxFF force field parameters have been fitted to a quantum mechanical (QM) training set containing structures and energies related to bond dissociation energies, angle and dihedral distortions, and reactions between silicon dioxide and hydrogen fluoride as well as experimental crystal structures, heats of formation and various reaction mechanisms. Model configurations for the training set were based on density functional theory (DFT) calculations on molecular clusters and periodic bulk and surface systems. ReaxFF reproduces accurately the QM training data for structures and energetics of small clusters and surfaces. The results of ReaxFF match reasonably well with those of QM for energies of initial etching process, transition state, and final production process. In addition to this, this force field was applied to etching simulations for silicon dioxide and hydrogen fluoride gas. In etching simulations, silicon dioxide slab models with hydrogen fluoride gas were used in molecular dynamics simulations. The etching yield and number of reaction products with different incident energies of hydrogen fluoride etchant are investigated. Second, the adsorption energies of LPS with functional binder and functional shield in lithium-sulfur batteries were calculated with DFT method. Before various actual evaluations, the chemical adsorption capacity of the prepared polymer binders composed with chitosan and carboxylated nitrile butadiene rubber (XNBR) for LPS (Li2Sx, x = 4, 6, 8) based on DFT calculations. In addition, the adsorption capability of metal oxides to LPS was investigated by predicting the interaction of the as-prepared metal oxides with LPS with DFT calculations. Calculation included well-known metal oxides for comparison. As a result, with computational method, functional binder and functional shield for enhanced lithium-sulfur batteries were investigated.Chapter 1. Introduction 1 1.1. Overall Introduction 1 1.2. Outline 1 Chapter 2. Theoretical Background for Computational Chemistry 4 2.1. DFT calculations 4 2.1.1. Introduction 4 2.1.2. Kohn-Sham method 5 2.2. The ReaxFF reactive force field 8 2.2.1. Introduction 8 2.2.2. The ReaxFF reactive force field method 10 2.2.3. Energy descriptions in ReaxFF 12 Chapter 3. Molecular Dynamics Simulation of Silicon Dioxide Etching by Hydrogen Fluoride Using ReaxFF Reactive Force Field 16 3.1. Introduction 16 3.2. Simulation model and details 19 3.3. Results 21 3.4. Summary and discussion 26 3.5. Acknowledgement 27 Chapter 4. Adsorption energy calculations in Lithium–Sulfur Batteries 56 4.1. Elastic chitosan based lean content binder 56 4.1.1. Introduction 56 4.1.2. Model and computational method 58 4.1.3. Results 60 4.1.4. Conclusions 64 4.1.5. Acknowledgement 64 4.2. Multifunctional Ga2O3 shield for Li-S batteries 75 4.2.1. Introduction 75 4.2.2. Model and computational method 75 4.2.3. Results 77 4.2.4. Conclusions 83 4.2.5. Acknowledgement 84 Chapter 5. Conclusions 101 Bibliography 103 국문 초록 115박

Elucidation of Active Site and Mechanism of Metal Catalysts Supported in NU-1000

Advances in extraction of shale oil and gas has increased the production of geographically stranded natural gas (primarily consisting of methane (C1) and ethane (C2)) that is burned on site. A potential utilization strategy for shale gas is to convert it into fuel range hydrocarbons by catalytic dehydrogenation followed by oligomerization by direct efficient catalysts. This work focuses on understanding metal cation catalysts supported on metal-organic framework (MOF) NU-1000 that will actively and selectively do this transformation under mild reaction conditions, while remaining stable to deactivation (via metal agglomeration or sintering). I built computational models validated by experimental methods to elucidate the structure-function relationship of catalysts for reactions of small molecules (ethane in this work) in natural gas. Computational techniques and characterization data from experimental collaborations at Argonne National Lab and Northwestern University were used to build kinetic models to learn about mechanism of ethene hydrogenation on M-NU-1000 catalysts (M = Ni, Cu, Zn, Co, Mn, Fe). Hydrogen adsorption and dissociation barrier is identified as the reason for discrepancy between experimental and computational data. Quantum density functional theory (DFT) simulations and microkinetic modeling on an expanded mechanism with multiple hydrogen adsorption and dissociation steps is performed. The model predicted spin state of metal as an important design variable with high spin and low spin metals following different mechanistic pathways due to different hydrogen adsorption and dissociation energies. This resolved the discrepancies between the model and experiments. The impact of different modeling choices on microkinetic modeling is analyzed by expanding the method to include Molecular Dynamics (MD) simulations and comparing different catalyst models in ethene dimerization reaction on Ni@NU-1000. Adsorption and desorption steps are identified as being more significant for determining rates than the activated steps. In collaboration with Northwestern University and Stonybrook University, polyoxometalate and polysulfidometalate catalysts supported on NU-1000 active for CO oxidation and electrochemical hydrogen evolution reaction are studied. Computationally elucidated structure of these catalysts is validated by experimental methods (XAS, XRD and DRIFTS) and provided the insight that the clusters need to be reduced further to remove the peripheral sulfur atoms to tailor them for more challenging reductive chemistry. Using this information our collaborators synthesized a a catalyst with lower sulfur content that was found to be active for acetylene hydrogenation. Overall, this work furthered our understanding of catalyst structure and mechanisms for reductive chemical transformation for shale gas to liquid conversion with insights that are applicable generally to MOF catalysts

Étude de l’adsorption de l’oxyde nitreux sur le Zeolitic Imidazolate Framework-8 (ZIF-8) : modélisation moléculaire et chimie computationnelle

Nitrous oxide (N2O) is a greenhouse gas (GHG) mainly issued from agriculture and waste management activities, such as the use of nitrogen-based fertilizers and livestock manure storage. This gas has a global warming potential 298 times higher than carbon dioxide (CO2) and an atmospheric lifetime of 116 years. The N2O capture, in order to remove it partially or completely from gas emissions, becomes necessary to act against global warming and climate change. Even if N2O is mainly emitted under ambient average conditions of temperature and pressure, most of the available technologies to control N2O emissions operate at medium/high pressure, being high energy-consuming technologies. Gas clean processes based on regenerative adsorption are a feasible techno-economical solution to control N2O emissions. However, the challenge to develop N2O adsorption processes is the production of highly specific adsorbents that can capture N2O under atmospheric pressure. The molecular modeling of adsorption phenomena is a helpful tool to develop engineered adsorbents and to save time and material resources. The development and validation of analytical informatics procedures to obtain thermodynamic and kinetics data about gas adsorption by molecular modeling can be done using representative and well-known adsorbent materials. Due to its easy and reproducible synthesis, its thermal stability, its flexible structure, its hydrophobic behavior, and its high adsorption capacity for a wide range of gases, including some GHGs, ZIF 8 (Zeolitic Imidazolate Framework-8) has been selected in this study as adsorbent. In this research, the adsorption phenomenon refers to the gas capture (adsorbate, N2O) on a solid surface (adsorbent, ZIF-8) as a result of the molecular interactions between both. The thermodynamic parameters of these interactions have been predicted by using molecular modeling and computational chemistry which include quantum mechanics simulations and molecular dynamics simulations. Mathematical methods such as Semi-Empirical Method and Density Functional Theory have been used to simulate the most probable gas-adsorbent interaction sites. The thermodynamic stability of these interactions trough the adsorption energy has been analyzed to select the most stable adsorption site. The Molecular Mechanics 3 force field, adapted for porous materials like ZIF-8, has been used to simulate the gas-adsorbent molecular interactions over time. The adsorption capacity of the adsorbent material has been studied through adsorption isotherms curves to analyze the performance of the material under certain conditions. Finally, a thermogravimetric adsorption-desorption technique has been used to validate the molecular simulation study performed for CO2. A method for calculating experimentally the adsorption capacity of ZIF-8 with CO2 has been developed based on this study and data issued from scientific literature. The use of molecular modeling and computational chemistry has helped to have a better understanding of gas adsorption process under different operating conditions and different adsorbent materials. This has allowed to optimize resources and to establish the basis to study other physicochemical processes in which the adsorption technique is part of them.L’oxyde nitreux (N2O) est un gaz à effet de serre (GES) issu, majoritairement, des activités agricoles telles que l’utilisation d’engrais à base d’azote et l’élevage intensif, ainsi que des activités de gestion des déchets. Ce gaz a un potentiel de réchauffement global 298 fois plus élevé que celui du dioxyde de carbone (CO2) et une durée de vie atmosphérique de 116 années. Par conséquent, la capture ou l’élimination du N2O des émissions gazeuses est fondamentale pour agir contre le réchauffement planétaire et les changements climatiques. Même si le N2O est émis principalement, sous des conditions de température et de pression ambiantes, les technologies disponibles pour contrôler les émissions de N2O fonctionnent à moyenne ou à haute pression parce qu’elles sont énergivores. Les procédés d’épuration des gaz basés sur l’adsorption régénérative sont des technologies économiques et faisables pour contrôler les émissions de N2O. Cependant, le défi consiste à produire des adsorbants spécifiques capables de capturer le N2O sous une pression atmosphérique. La modélisation moléculaire pour le procédé d’adsorption est un outil pratique pour développer des adsorbants, en économisant, à la fois, des ressources et du temps. Le développement et la validation de procédures d’analyses informatiques pour obtenir des données thermodynamiques et cinétiques sur l’adsorption de gaz par modélisation moléculaire peuvent être réalisés à l’aide de matériaux adsorbants représentatifs et bien connus. En raison de sa synthèse facile et reproductible, sa stabilité thermique, sa structure flexible, son comportement hydrophobe et ses capacités d’adsorption élevées pour une grande gamme de gaz, incluant certains GES, le ZIF-8 (Zeolitic Imidazolate Framework-8) a été choisi comme adsorbant pour étudier l’adsorption du N2O. Dans cette étude, le phénomène d’adsorption fait référence à la capture du gaz (adsorbat, N2O) présent à la surface du solide (adsorbant, ZIF-8) grâce à des interactions moléculaires entre l’adsorbat et l’adsorbant. Les paramètres thermodynamiques des interactions ont été prédits par la modélisation moléculaire et la chimie computationnelle qui incluent des simulations de mécanique quantique et des simulations de dynamique moléculaire. Des méthodes mathématiques telles que le Semi-Empirical Method et le Density Functional Theory ont été utilisées pour simuler les sites d'interaction gaz-adsorbant. La stabilité thermodynamique de ces interactions, obtenue par l'énergie d'adsorption, a été analysée pour sélectionner le site d'adsorption le plus stable. Le champ de force Molecular Mechanics 3, adapté aux matériaux poreux comme le ZIF-8, a été utilisé pour simuler les interactions moléculaires gaz-adsorbant au fil du temps. La capacité d'adsorption du matériau adsorbant a été étudiée en construisant des isothermes d'adsorption pour analyser la performance du matériau sous certaines conditions. Finalement, une technique d'adsorption-désorption thermogravimétrique a été utilisée pour valider l'étude de simulation moléculaire réalisée pour le CO2. Une méthode de calcul expérimentale de la capacité d’adsorption du CO2 sur le ZIF-8 a été développée en se basant sur ces données et sur des données issues de la littérature scientifique. L’utilisation de la modélisation moléculaire et la chimie computationnelle a permis de mieux comprendre le procédé d’adsorption de gaz sous des conditions de température et pression ambiante et en utilisant différents matériaux adsorbants. Cette méthode a également permis d’optimiser les ressources et d’établir les bases pour étudier d'autres procédés physico-chimiques dans lesquels la technique d'adsorption fait partie d'eux

The 1998 Center for Simulation of Dynamic Response in Materials Annual Technical Report

Introduction: This annual report describes research accomplishments for FY 98 of the Center for Simulation of Dynamic Response of Materials. The Center is constructing a virtual shock physics facility in which the full three dimensional response of a variety of target materials can be computed for a wide range of compressive, tensional, and shear loadings, including those produced by detonation of energetic materials. The goals are to facilitate computation of a variety of experiments in which strong shock and detonation waves are made to impinge on targets consisting of various combinations of materials, compute the subsequent dynamic response of the target materials, and validate these computations against experimental data

Reducing CO2 and Corrosion: Insights from Thermodynamic Descriptors Calculated with Density Functional Theory

Catalytic reaction mechanisms can be extremely complex, and it is difficult to determine all the factors that control reaction rates. Fortunately, complex chemical phenomena can frequently be described by thermodynamic properties (such as molecular pKas and reaction overpotentials) that correlate with catalytic reaction rates. While these properties can be difficult or time intensive to measure experimentally, they can be easily computed using Kohn-Sham density functional theory (KS-DFT). We have developed a thermodynamic descriptor-based model that uses molecular pKas and redox potentials calculated with KS-DFT to predict the electrochemical conditions at which aromatic N-heterocycle (ANH) molecules could facilitate multi-proton and multi-electron reduction reactions. By automating this procedure using the ADF modeling suite, we can rapidly screen through potential catalysts with minimal user input. To establish a baseline procedure for studying the chemical reduction of CO2 via hydride transfers from ANH molecules, we characterized the chemical reduction of CO2 by hydride transfers from sodium borohydride. We located hydride transfer pathways with nudged elastic band calculations and obtained free energy barriers from potentials of mean force derived from constrained molecular dynamics simulations along the reaction pathways. These simulations provided reaction energetics at realistic operating conditions and highlighted the potential pitfalls of only studying reaction pathways at 0 K. Cathodic reduction reactions can limit galvanic corrosion rates in atmospheric environments. To help guide the design of titanium alloys that resist galvanic corrosion, we used density functional theory to predict dopants that inhibit cathodic reduction reaction kinetics on oxide surfaces. We calculated overpotentials for the oxygen reduction reaction (ORR) occurring on metal dopants in an amorphous TiO2 surface. These overpotential trends successfully predicted six dopants that have been experimentally verified to inhibit ORR activity by up to 77% (Sn, Cr, Co, Al, Mn, and V). Next, we used this approach to study the native oxides of Ti-6Al-4V, a Ti alloy with improved corrosion resistance. We used Behler-Parrinello neural networks to create defective and amorphous surface models for TiAl2O5 (the oxide that forms on Ti-6Al-4V surfaces in addition to TiO2) and predicted how ORR activity was altered by different complex oxide surface morphologies

Nanoporous Materials and Their Applications

This book is a special collection of articles dedicated to the preparation and characterization of nanoporous materials, such as zeolitic-type materials, mesoporous silica (SBA-15, MCM-41, and KIT-6), mesoporous metallic oxides, metal–organic framework structures (MOFs), and pillared clays, and their applications in adsorption, catalysis, and separation processes. This book presents a global vision of researchers from international universities, research centers, and industries working with nanoporous materials and shares the latest results on the synthesis and characterization of such materials, which have given rise to the special interest in their applications in basic and industrial processes