Chemical Kinetic Model Research Articles

Simulations of Modified Shallow Water Equation and Reaction Kinetics of Poly Etching on Single Wafer Process

With the downscaling of semiconductor feature size, single wafer wet etching has shown better within-wafer and wafer-to-wafer uniformity. According to previous research and analysis, liquid film thickness has a close relationship with etch loss, and numerical simulations have been applied to calculate liquid film thickness with fixed nozzle dispensing. This study investigated liquid film thickness with scanning nozzle processing and a modeled aqueous tetramethylammonium hydroxide (TMAH) poly etching method. A modified shallow water equation was used to calculate the liquid film thickness under nozzle scanning conditions. Additionally, the liquid film thickness and liquid velocity were coupled with a chemical reaction kinetic model to calculate the TMAH poly etching reaction rate on the wafer surface. Scan dispensing etch loss was calculated by integrating the reaction rate. Both the experimental and simulation results showed that the scan speed can affect within-wafer etch loss profile and that a fast scan speed could increase etch loss in wafer donut area. The simulation model could be a predictive tool for improving within-wafer etching profile.

Simulations of Heat and Drag Reduction of Opposing Jet in Hypersonic Flow

To reduce aerodynamic heat and drag, and thus improve the speed of hypersonic vehicles, opposing jet technology has become a research focus because of its excellent heat and drag reduction effect. In this paper, Navier–Stokes equations, the shear stress transport turbulent models, the Gupta chemical kinetics model, and the five-species air model are considered; and a numerical simulation program of hypersonic aerodynamic heat and drag with an opposing jet is developed and verified by existing experimental data. On this basis, the flight conditions of , 14, and 15 at a 30 km flight altitude are simulated numerically; and the complex flowfield structure of the leading edge of a blunt body with or without the opposing jet is analyzed. It is found that the jet can push away the shock wave, playing a good role in reducing heat and drag. The influence of the pressure ratio, Mach number, and jet temperature on reducing heat and drag is investigated. The results indicate that, under the same conditions, increasing the pressure ratio, increasing the jet Mach number, and decreasing the inlet Mach number will be more helpful for reducing heat and drag; whereas increasing the jet temperature is not conducive to heat reduction and has little impact on aerodynamic drag.

Fast uncertainty reduction of chemical kinetic models with complex spaces using hybrid response-surface networks

Response-surface (RS) surrogate approaches permit efficient inverse uncertainty quantification (UQ) of combustion kinetic models, wherein the uncertainty of reaction rates is reduced from observed targets. For kinetic models with parameters characterized by large uncertainty factors, strong nonlinearities, and reaction couplings (e.g., reduced mechanisms of real fuels; such models are referred to be “complex” in this work), the global RS is difficult to approximate, precluding conventional surrogate approaches. This paper proposes a framework that is extendable to such systems, termed Hybrid Response Surface Networks followed by a Stochastic Gradient Descent Ensemble (HRSN-SGDE). This technique focuses on mapping the local RS of just the uncertain spaces in the vicinity of the observed target, referred to as the rate target subspace. Two neural network surrogates are considered: a classifier that predicts the probability of data residing in the rate target subspace and a local RS surrogate which maps the RS of this subspace. A hybrid surrogate loss function is then defined using these surrogates to optimize uncertain rates repeatedly to get an ensemble of solutions representing the constrained rate space. HRSN-SGDE is demonstrated on a complex jet fuel model developed using the hybrid chemistry (HyChem) approach with a low temperature chemistry sub-model using a series of ignition delay times as targets. Results show that the method's local RS objective enables efficient and accurate construction of the surrogates through active learning-based sampling. Also, the unique formulation of the surrogate loss function enables optimization that is robust to suboptimal local minima and faster than evolutionary algorithms by several orders of magnitude. It is shown that HRSN-SGDE method is highly efficacious and suitable to conducting inverse UQ on such complex kinetic models.

Machine learned compact kinetic models for methane combustion

Chemical kinetic models are an essential component in the development and optimisation of combustion devices through their coupling to multi-dimensional simulations such as computational fluid dynamics (CFD). Due to the significant level of detail contained within, detailed chemical kinetic models are computationally prohibitive for use in CFD. Therefore, low-dimensional kinetic models which retain good fidelity to the reality are needed, the production of which requires considerable human-time cost and expert knowledge.Here, we present a novel automated compute intensification methodology to produce overly-reduced and optimised (“compact”) chemical kinetic models. The Machine Learned Optimisation of Chemical Kinetics (MLOCK) coded algorithm systematically perturbs each of the four chemical kinetic model components to discover what combinations of terms results in a model with high fidelity calculations. A virtual reaction network comprised of n species is first obtained using conventional mechanism reduction procedures. Once n is lower than a threshold value, the model performance is typically of low fidelity.To adjust for this, the weights (virtual reaction rate constants) of important connections (virtual reactions) between each node (species) of the virtual reaction network are numerically optimised across four sequential phases to replicate select calculations.The first version of MLOCK (MLOCK1.0), simultaneously perturbs all three virtual Arrhenius reaction rate constant parameters for important connections and assesses the suitability of the new parameters through objective error functions, which quantify the error in the calculations of each model candidate to a set of optimisation targets, comprised of detailed model calculations.In this study, the MLOCK algorithm is demonstrated by automatic creation of compact models for the archetypal case of methane/air combustion. It is shown that the NUGMECH1.0 detailed model comprised of 2789 species is reliably compacted to 15 species (nodes), whilst retaining an overall fidelity of 79–90% to the detailed model at Industry-defined performance target calculations, outperforming the prior state-of-art.

Study of the interaction between NH3 and NO in the reduction zone of air-staged ammonia combustion under high moisture atmosphere

NH3 is used as a hydrogen-rich fuel, and the water vapor content produced by combustion can reach 30%. However, high NOx emission is one of the main problems restricting its application. The air-staged combustion is an effective low-NOx combustion strategy, subdividing the combustion process into three typical sections: the main combustion zone, the reduction zone, and the burnout zone. Fully utilizing the reaction between NH3 and NO in the reduction zone is expected to achieve an effective control of NOx emission. This research aims to explore the reduction ability of the residual NH3 on NO reduction in the reduction zone under high moisture atmosphere formed by ammonia combustion. An isothermal flow reactor was used to study the kinetic characteristics of the NH3/NO reaction under no-oxygen or residual low-oxygen condition in a wide temperature range (1073–1773K) and a high water vapor content range (0–50%). Additionally, three chemical kinetic models were used in simulations. The results showed that oxygen significantly changes the effect of water vapor on the NH3/NO reaction. In the absence of oxygen, water vapor has less effect on NH3/NO reaction at lower temperatures (<1500K). In the presence of oxygen, the increase of water vapor content leads to a significant shift of the reaction temperature window to higher temperatures and a decrease of the maximum NO reduction efficiency. Finally, the remarks about using residual NH3 to complete NOx removal in ammonia combustion were discussed.

Hierarchical nano-reactor with core-shell structure for efficient removal of atrazine via heterogeneous catalytic ozonation: Experimental parameters, kinetics and performance analysis

The engineering application of the heterogeneous catalytic ozonation process (HCOP) is severely hampered by inefficient heterogeneous catalysts. Herein, a novel hierarchical core-shell MnO2@mSiO2 nano-reactor was prepared using hydrothermal and sol-gel methods. Its catalytic performance was systematically evaluated through the degradation and mineralization of atrazine (ATZ) in HCOP under various reaction parameters. At optimal parameters (O3/7.5 mg/L, ATZ/15 uM, catalyst/30 mg/L, pH = 7), 96.5 % and 36.1 % of ATZ and total organic carbon (TOC) removal efficiencies could be obtained after 30 min, respectively. After five cycles, the ATZ removal efficiency of MnO2@mSiO2 only decreased by approximately 10 %. The synergy effects between the core and mesoporous shell played a vital role in its efficient catalytic activity. The large specific surface area and confined environment favored ozone adsorption, mass transfer and enhanced the enrichment of ATZ, intermediate products, and oxidants. Quenching experiments and the chemical kinetic model indicated that hydroxyl radical (·OH) was the primary oxidant. The Mn3+/ Mn4+ and oxygen vacancies (OVs) acted as active sites to promote ozone decomposition via electron transfer facilitated by the shell. This work provides new advancements for the development of high-efficiency catalysts for the engineering application of HCOP.

Accelerating models for multiphase chemical kinetics through machine learning with polynomial chaos expansion and neural networks

Abstract. The heterogeneous chemistry of atmospheric aerosols involves multiphase chemical kinetics that can be described by kinetic multi-layer models (KMs) that explicitly resolve mass transport and chemical reactions. However, KMs are computationally too expensive to be used as sub-modules in large-scale atmospheric models, and the computational costs also limit their utility in inverse-modeling approaches commonly used to infer aerosol kinetic parameters from laboratory studies. In this study, we show how machine learning methods can generate inexpensive surrogate models for the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB) to predict reaction times in multiphase chemical systems. We apply and compare two common and openly available methods for the generation of surrogate models, polynomial chaos expansion (PCE) with UQLab and neural networks (NNs) through the Python package Keras. We show that the PCE method is well suited to determining global sensitivity indices of the KMs, and we demonstrate how inverse-modeling applications can be enabled or accelerated with NN-suggested sampling. These qualities make them suitable supporting tools for laboratory work in the interpretation of data and the design of future experiments. Overall, the KM surrogate models investigated in this study are fast, accurate, and robust, which suggests their applicability as sub-modules in large-scale atmospheric models.

Impact of Hydrothermal and Chemical Aging on SCR Storage Characteristics and NO &lt;italic&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/italic&gt; Reduction Performance in an Ultra-Low NO &lt;italic&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/italic&gt; System

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;This work is a part of medium-duty Low NO&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; technology development project with a focus on evaluating a combination of engine and advanced aftertreatment for 0.02 g/bhp-hr NO&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; regulation proposed by CARB (California air resource board). In this project, a control oriented chemical kinetics model of SCR (Selective catalytic reduction) was used in the aftertreatment controller that is susceptible to performance degradation due to hydrothermal and chemical aging. This paper focuses on modeling the NO&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; conversion and NH3 storage characteristics using a controls oriented SCR plant model which is further used for a model-based urea dosing scheme. A set of steady state reactor tests were used to calibrate the SCR performance at degreened, hydrothermal only and hydrothermal + chemical aging conditions and also to determine inhibition factors related to aging. The resultant model is capable of simulating SCR performance deterioration such as a reduction in NO&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; conversion and NH3 storage. A non-linear aging profile was observed for Lo-SCR and downstream SCR showing a change in the NO&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; conversion in the aged system when compared to a degreened system. Upon chemical aging further deterioration of low temperature performance was observed. This aging phenomenon impacts the dosing control strategy of the system. The results on controller performance for a set of Heavy Duty Federal Test Protocol(FTP), Ramp Modal Cycle (RMC), and Low Load Cycle (LLC) are presented.&lt;/div&gt;&lt;/div&gt;

Numerical modeling of gas‐phase reactions of tetraethoxysilane/O2/Ar atmospheric dielectric barrier discharge for deposition

AbstractA chemical kinetic model is developed to study the decomposition of tetraethoxysilane (TEOS) in O2/Ar atmospheric pressure dielectric barrier discharge and compared with gas chromatography and optical emission spectroscopy results. The calculations indicate that the excited Ar dominates the fragmentation of TEOS in the absence of oxygen and mainly breaks the carbon–carbon bonds in TEOS. However, in the presence of oxygen, the primary decomposition process of TEOS is the substitution of ethoxy (–OC2H5) by hydroxyl (–OH). The variation of these two reactions with oxygen composition could explain the transition of the deposition layer from organic to norganic. The model and its results provide a theoretical basis for further modeling and regulating the quality of the deposition layer.

Quantum Chemical and Kinetic Study of the Gas-Phase Reactions of Methane Sulfonamide with Cl Atom and the Fate of •CH2S(═O)2NH2 with 3O2 in the Atmosphere

The gas phase mechanism of the chlorine atom (•Cl)-initiated oxidation of methane sulfonamide (CH3S(═O)2NH2; MSAM) has been elucidated using ab initio/DFT electronic structure methods and chemical kinetic modeling. A reaction that commences via abstraction of an H-atom from the methyl group of MSAM to form a transition state with a barrier height of ∼4.8 kcal mol–1 above that of the MSAM + •Cl reactants, yielding •CH2S(═O)2NH2 + HCl was found to be a major path in comparison with the other possibilities. Rate coefficients for all possible H-atom abstraction reactions were calculated using the canonical variational transition state theory (CVT) with the small curvature tunneling (SCT) method in the temperature range of 200–400 K. The rate coefficient for the major reaction was found to be 1.6 × 10–14 cm3 molecule–1 s–1 at 300 K, while the overall rate coefficient for the MSAM + •Cl reaction is found to be 1.7 × 10–14 cm3 molecule–1 s–1 at 300 K. In addition, SCT contributions, branching ratios for each reaction path, and the atmospheric implications are provided and discussed. Based on the results, the MSAM + •Cl reaction proceeds to form •CH2S(═O)2NH2, which then further reacts with 3O2 under oxygen-rich conditions to form the corresponding RO2 adduct (•OOCH2S(═O)2NH2). Subsequent reactions of this radical result in the formation of greenhouse gases such as sulfur dioxide (SO2), carbon dioxide (CO2), carbon monoxide (CO), nitric acid (HNO3), nitrous oxide (N2O), and formic acid (HC(O)OH), which may contribute to climate change and formation of secondary organic aerosols and acid rain.

Synergy among Pausing, Intrinsic Proofreading, and Accessory Proteins Results in Optimal Transcription Speed and Tolerable Accuracy.

Cleavage of dinucleotides after the misincorporational pauses serves as a proofreading mechanism that increases transcriptional elongation accuracy. The accuracy is further improved by accessory proteins such as GreA and TFIIS. However, it is not clear why RNAP pauses and why cleavage-factor-assisted proofreading is necessary despite transcriptional errors in vitro being of the same order as those in downstream translation. Here, we developed a chemical-kinetic model that incorporates most relevant features of transcriptional proofreading and uncovers how the balance between speed and accuracy is achieved. We found that long pauses are essential for high accuracy, whereas cleavage-factor-stimulated proofreading optimizes speed. Moreover, in comparison to the cleavage of a single nucleotide or three nucleotides, RNAP backtracking and dinucleotide cleavage improve both speed and accuracy. Our results thereby show how the molecular mechanism and the kinetic parameters of the transcriptional process were evolutionarily optimized to achieve maximal speed and tolerable accuracy.

Dissipation and energy propagation across scales in an active cytoskeletal material

Living systems are intrinsically nonequilibrium: They use metabolically derived chemical energy to power their emergent dynamics and self-organization. A crucial driver of these dynamics is the cellular cytoskeleton, a defining example of an active material where the energy injected by molecular motors cascades across length scales, allowing the material to break the constraints of thermodynamic equilibrium and display emergent nonequilibrium dynamics only possible due to the constant influx of energy. Notwithstanding recent experimental advances in the use of local probes to quantify entropy production and the breaking of detailed balance, little is known about the energetics of active materials or how energy propagates from the molecular to emergent length scales. Here, we use a recently developed picowatt calorimeter to experimentally measure the energetics of an active microtubule gel that displays emergent large-scale flows. We find that only approximately one-billionth of the system's total energy consumption contributes to these emergent flows. We develop a chemical kinetics model that quantitatively captures how the system's total thermal dissipation varies with ATP and microtubule concentrations but that breaks down at high motor concentration, signaling an interference between motors. Finally, we estimate how energy losses accumulate across scales. Taken together, these results highlight energetic efficiency as a key consideration for the engineering of active materials and are a powerful step toward developing a nonequilibrium thermodynamics of living systems.

Chemical Kinetic Modeling of Air-Steam Gasification of Eucalyptus Wood Sawdust for H2-Rich Syngas Production.

The aim of this research paper is to develop a chemical kinetic model, based on the mechanism of surface reactions, for air-steam gasification of eucalyptus wood sawdust (CH1.63O1.02) and analyze the hydrogen-rich syngas production. Experiments are performed on a bubbling fluidized-bed gasifier using air-steam as a gasifying agent. For validation of the developed kinetic model, the outcome of the model is compared with that of experimental data, which shows a root-mean-square error of less than 4. Different parameters such as equivalence ratios (0 ≤ ER ≤ 0.4), particle size (100 ≤ Dp ≤ 1000 μm), gasification temperature (900 ≤ T ≤ 1200 K), pressure (1 ≤ P ≤ 20 atm), and steam-to-biomass ratio (0 ≤ SBR ≤ 2) are considered for the analysis. The one-parameter-at-a-time concept is employed to maximize the production of H2-rich syngas. Results indicate that the maximum concentration of hydrogen is 55.04 vol % (experimental) and 51.81 vol % (predicted) at optimum conditions: ER = 0, Dp = 100 μm, T = 1100 K, P = 1 atm, and SBR = 0.75. Gasification performance parameters such as hydrogen gas yield, heating values, cold gas efficiency, etc., are evaluated.

Experimental and numerical study of curvature effects and NO formation in ammonia Bunsen flames

Analysis of ammonia Bunsen flame is meaningful for understanding ammonia combustion details and solving the two major problems, the poor combustion characteristics and the severe NO emission. In the present work, ammonia Bunsen flames with equivalence ratio from 0.9 to 1.2 were successfully achieved at an elevated preheat temperature, 873 K, using an elevated-preheat-temperature jet flame facility. Detailed simulations were conducted using the CFD method combined with a chemical kinetic model. A good agreement was achieved between the calculated flames and the experimental flame images. The strong effects of curvature at the flame tip were revealed both experimentally and numerically, which were consistent with the literature data. In the lean Bunsen flame, curvature inhibits combustion, while, in the stoichiometric or rich flames, curvature enhances combustion. Based on the calculated flames, comparison of the detailed flame structures between the tip and the planar portion was conducted and the mechanism of the curvature effects was uncovered, which were closely related to the H2 transport characteristics. Besides, NO formation was investigated and a strong dependence on the local combustion intensity and atmosphere was observed. In stoichiometric and rich flames, two specific NO peaks were uncovered at the flame tip and on the base, respectively.

In-depth mechanistic and kinetic investigation of sonochemically mediated atom transfer radical polymerization using modeling approach

Mechano/sonochemically mediated reversible deactivation radical polymerization has attracted great attention because of the novel electron transfer regulation process and high-efficient process intensification on mass transfer. Herein, a multi-scale modeling strategy combining quantum chemical calculation and kinetic modeling was developed for sonochemically induced electron transfer controlled atom transfer radical polymerization (SET-ATRP) to study the complicated SET process and its effects on polymerization. At the molecular scale, density functional theory (DFT) calculations were used to acquire the electron characteristic and structure–reactivity of the reactants, intermediates and products during SET process, which provides guideline to figure out the underlying mechanism. Results showed that DMSO•• generated under ultrasonication could react with free Me6TREN to induce electron transfer for the reduction of CuIIBr/L+ to produce CuI/L+, which is the key step for a successful polymerization. The corresponding elementary reaction rate coefficients were calculated by thermodynamic formulae combining transition state theory and Marcus theory. At the micro-scale, kinetic modeling based on method of moment was established to account for the role of SET process in regulating polymerization. Good agreements between the simulation results and experimental data confirmed the reliability of the as-developed multi-scale modeling strategy. More Me6TREN and CuIIBr/L+ resulted in more electron transfer and faster polymerization rate. This work provides an in-depth mechanism insights and kinetic understanding for SET-ATRP, which has a promising potential to promote its progress of application.

A wide range experimental and kinetic modeling study of the oxidation of 2,3-dimethyl-2-butene: Part 1

2,3-Dimethyl-2-butene (TME) is a potential fuel additive with high research octane number (RON) and octane sensitivity (S), which can improve internal combustion engine performance and efficiency. However, the combustion characteristics of TME have not been comprehensively investigated. Thus, it is essential to study the combustion characteristics of TME and construct a detailed chemical kinetic model to describe its combustion. In this paper, two high-pressure shock tubes and a constant-volume reactor are used to measure ignition delay times and laminar flame speeds of TME oxidation. The ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in “air”, at pressures of 5 and 10 bar, in the temperature range of 950 – 1500 K. Flame speeds of the TME/ “air” mixtures were measured at atmospheric pressure, at a temperature of 325 K, for equivalence ratios ranging from 0.78 to 1.31. Two detailed kinetic mechanisms were constructed independently using different methodologies; the KAUST TME mechanism was constructed based on NUIGMech1.1, and the MIT TME mechanism was built using the Reaction Mechanism Generator (RMG). Both mechanisms were used to simulate the experimental results using Chemkin Pro. In the present work, reaction flux and sensitivity analyses were performed using the KAUST mechanism to determine the critical reactions controlling TME oxidation at the conditions studied.

A new approach for mathematical modeling of chemical kinetics

A new approach for describing chemical kinetics for reactions in gas phase is proposed, which removes some observed inconsistencies. This new approach is referring mainly to the law of mass actions in both forms, chemical equilibrium and reaction rate. A new interpretation of the activation energy is also given, so that the Arrhenius law becomes consistent with both Boltzmann statistic distribution and the definition of equilibrium constant. It will be shown that the activation energy is not a simple constant, it depends on the temperature at which the reaction begins.

Extraction of hops pelletized (Humulus lupulus) with subcritical CO2 and hydrodistillation: Chemical composition identification, kinetic model, and evaluation of antioxidant and antimicrobial activity

Hop essential oil and hop extract using carbon dioxide (CO2) are products with high added value because they have bioactive and sensory properties. In this context, the objective of this study was to obtain and characterize essential oil and extracts from pelleted hops of El Dorado, Polaris, Hallertau Blanc and Callista varieties using hydrodistillation and subcritical CO2 extraction methods. Extraction yield ranged from 0.38 % to 1.97 % (m/m) for essential oils and from 8.76 % to 15.35 % (m/m) for extracts using subcritical CO2. The chemical compositions of the essential oils were mainly monoterpene (18.14 % to 29.91 %) and sesquiterpene (46.01 % to 59.03 %) hydrocarbons and for the extracts were sesquiterpene hydrocarbons (33.05 % to 71.90 %) and oxygenated sesquiterpenes (14.80 % to 34.89 %). The extracts showed better antioxidant activity than essential oils due to the presence of phenolic compounds and flavonoids. Hop extracts showed some antimicrobial activity, but essential oils did not demonstrate antimicrobial potential. Hop extracts obtained with subCO2 have the potential to be used in the brewing industry as a flavoring and as natural antioxidants.

Oxidation study of small hydrocarbons at elevated pressure part II: A two-fuel compound mixture of acetylene and 1,3-butadiene

The high-pressure oxidation experiments of a two-fuel component mixture of 0.2%1,3-butadiene (1,3-C4H6), 0.2% acetylene (C2H2), nitrogen (N2) and oxygen (O2) (see Table 3, No.1 & 2) were carried out in a jet-stirred reactor (JSR) at 12 atm within 575–1025 K under fuel-lean (φ = 0.5) and fuel-rich (φ = 3.0) conditions. 15 species profiles of products and intermediates were identified and quantified by gas chromatography (GC) and gas chromatography-mass spectrometer (GC–MS). This work is enlarging the experimental data base obtained from the study of C2H2/1,3-C4H6 mixtures reported earlier. The updated comprehensive chemical reaction kinetic model based on the experimental data of the current work and the AramcoMech 3.0 model is containing 635 species and 3291 reactions. The comparison between the new experimental data base and the data predicted by the updated model was done for major products and intermediates, such as oxygenated species and aromatics. In general, a good agreement was found. It was shown that the fuel-rich oxidation of C2H2/1,3-C4H6 mixtures (see Table 3, No.2) contributes largely to benzene (C6H6) formation compared to neat oxidation of C2H2 (see Table 3, No.4) as well as of 1,3-C4H6 (see Table 3, No.6). At temperatures below 900 K, C6H6 is mainly formed through the C2 + C5 channel: C2H2 + C5H5 → C6H5CH2 → C6H6. The C2 + C4 pathway: C4H5-I/N + C2H2 → FULVENE → C6H6 tends to be the dominant route for C6H6 formation at temperatures above 900 K. A rate-of-production (ROP) analysis indicates that both C2H2 and 1,3-C4H6 are consumed mainly by addition reactions with mainly OH radicals involved. The consumption of C2H2 decreases with increasing values of the fuel-equivalence (φ), while the consumption of 1,3-C4H6 is much less effected by the specific φ values. Compared with the results obtained at fuel-lean condition (φ=0.5), the C3H5-A and C5H5 radicals consume more C2H2 with increasing temperatures at fuel-rich condition (φ=3.0). The oxidation process of 1,3-C4H6 is occurring mainly by H-abstraction with OH radicals, H-addition, and subsequent reactions. According to sensitivity analysis, the reaction H2O2 (+M) ⇌ OH + OH (+M) can promote the consumption of C2H2 and of 1,3-C4H6; however, the promoting effect gradually decreases with increasing of φ values. On the other hand, the consumption of C2H2 can also be promoted by the reactions C3H5-A + C2H2 ⇌ CVCCVCCJ and 1,3-C4H6 + OH ⇌ C3H5-A + CH2O, in addition, the promotion effect can gradually enhance with increasing φ values. For 1,3-C4H6 consumption, the promoting effect of the reaction 1,3-C4H6 + CH3 ⇌ C4H5-I + CH4 also strengthens. The results of the present work contribute to a better understanding of benzene formation at high-pressures in particular and provide valuable information for the study of the formation pathways of PAHs and soot as well.

Unimolecular Reactions of 2,4-Dimethyloxetanyl Radicals.

Alkyl-substituted oxetanes are cyclic ethers formed via unimolecular reactions of QOOH radicals produced via a six-membered transition state in the preceding isomerization step of organic peroxy radicals, ROO. Owing to radical isomer-specific formation pathways, cyclic ethers are unambiguous proxies for inferring QOOH reaction rates. Therefore, accounting for subsequent oxidation of cyclic ethers is important in order to accurately determine rates for QOOH → products. Cyclic ethers can react via unimolecular reaction (ring-opening) or via bimolecular reaction with O2 to form cyclic ether-peroxy adducts. The computations herein provide reaction mechanisms and theoretical rate coefficients for the former type in order to determine competing pathways for the cyclic ether radicals. Rate coefficients of unimolecular reactions of 2,4-dimethyloxetanyl radicals were computed using master equation modeling from 0.01 to 100 atm and from 300 to 1000 K. Coupled-cluster methods were utilized for stationary-point energy calculations, and uncertainties in the computed rate coefficients were accounted for using variation in barrier heights and in well depths. The potential energy surfaces reveal accessible channels to several species via crossover reactions, such as 2-methyltetrahydrofuran-5-yl and pentanonyl isomers. For the range of temperature over which 2,4-dimethyloxetane forms during n-pentane oxidation, the following are the major channels: 2,4-dimethyloxetan-1-yl → acetaldehyde + allyl, 2,4-dimethyloxetan-2-yl → propene + acetyl, and 2,4-dimethyloxetan-3-yl → 3-butenal + methyl, or, 1-penten-3-yl-4-ol. Well-skipping reactions were significant in a number of channels and also exhibited a markedly different pressure dependence. The calculations show that rate coefficients for ring-opening are approximately an order of magnitude lower for the tertiary 2,4-dimethyloxetanyl radicals than for the primary and secondary 2,4-dimethyloxetanyl radicals. Unlike for reactions of the corresponding ROO radicals, however, unimolecular rate coefficients are independent of the stereochemistry. Moreover, rate coefficients of cyclic ether radical ring-opening are of the same order of magnitude as O2 addition, underscoring the point that a competing network of reactions is necessary to include for accurate chemical kinetics modeling of species profiles for cyclic ethers.