Reaction Of CH Research Articles

Automated Skeleton Network Generation for ReaxFF Molecular Dynamics Simulations of Hydrocarbon Fuel Pyrolysis and Oxidation via a Rate-Based Algorithm.

In this study, we present an automated approach of rate-based skeleton network generation for ReaxFF MD simulation (RxMD-SN) for deriving the reaction kinetic mechanism of large hydrocarbon fuels in pyrolysis and oxidation from large-scale ReaxFF MD simulations. The approach contains the statistical calculation of reaction rate constants and the generation of skeleton reaction networks using a rate-based algorithm. The RxMD-SN method takes advantage of reaction flux ranking at a small time interval in terms of temporal reaction rate to extract the core reaction networks, which allows for keeping the rare reaction events that may be dominant in a certain period of the reaction network. The kinetic models derived from ReaxFF MD simulation in CH4 oxidation can reproduce what was obtained in the ReaxFF MD simulation, which demonstrates the capability of RxMD-SN in capturing the global reaction kinetics. An evaluation of reaction rate constants indicates that close kinetic parameters are shared for n-octane oxidation of similar reaction classes, shared oxidation reactions of CH4 against n-heptane, and shared pyrolysis reactions of the RP-3 surrogate fuel against n-heptane. This capability of RxMD-SN is particularly beneficial in meeting the challenges in characterizing the oxidation reaction kinetics of large hydrocarbon molecules. RxMD-SN approach is potentially a general approach in chemical kinetics modeling on the basis of ReaxFF MD simulations.

Optimization of activity and selectivity for syngas-to-ethanol conversion on metal-based catalysts via the first-principles microkinetic simulations

Achieving high selectivity in converting syngas to ethanol has long been a challenging task, requiring identification of key factors favoring ethanol production over other by-products such as methane, methanol, and acetaldehyde. Herein, we perform systematic density functional theory (DFT) calculations and microkinetic simulations to shed light on the activity and selectivity trends of ethanol relative to three competing by-products in syngas conversion, which quantitatively unveils the key determining factors on the transition metal-based catalysts. The scaling relations involved are revealed as functions of the adsorption energies of C and O species (EC and EO), and the three-dimensional activity and selectivity surfaces are constructed quantitatively, indicating that the optimal condition (peak position) correspond to EC = -6.10 eV and EO = -5.70 eV. Furthermore, we find that ethanol activity at the peak is primarily constrained by the kinetic barriers of CH3O dissociation and the coupling reaction of CH3 and CO, while its selectivity can be enhanced by increasing the energy barrier of CH3 hydrogenation and strengthening the adsorption of CH3CHO. More significantly, following these rules, the single-atom metal alloy catalysts (i.e., Cu1Co, Cu1Ni) are identified that could serve as promising candidates for ethanol synthesis, which are superior to the common alloy-type catalysts. We believe that these insights further deepen the understanding of theoretical design of metal alloy catalysts in ethanol production.

Gas-phase reaction of CH4 and H2S – Evidence from pyrolysis experiments and case study from the Sichuan Basin

The application of routine geochemical analyses for identification of dry natural gas sources and post generation processes poses a challenge as its hydrocarbon composition is typically dominated by CH4 (∼≥99 %) while the presence of non-hydrocarbons may be limited. In particular, the possibility of dry gas interaction with H2S at reservoir conditions (low temperatures, high pressures, millions of years residence time) is not established as previous studies of this reaction focused on conditions typical for industrial reactors (high temperature, ambient pressure, residence time of seconds). To address these issues, we studied the molecular and isotopic (δ34S) composition of volatile organic sulfur compounds (VOSC) formed during pyrolysis experiments between CH4 and H2S at 360 °C (4–96 h), reaching %Ro equivalent of 0.80–1.25 which represents thermally mature oil and gas reservoirs. The results demonstrated that methanethiol (MeSH) is the main product of the reaction, while its δ34S value suggest equilibrium isotopic effect with its parent H2S. Subsequent analysis of the molecular and isotopic (δ13C, δ2H, δ34S) composition of thermogenic, H2S containing, dry natural gases from the Jiannan gas field in China revealed the presence of short thiols and sulfides dominated by MeSH. The δ34S of the VOSC identified suggests they all formed by gas-phase reaction of alkanes (mainly CH4) with the associated H2S.This study demonstrates the applicability of VOSC as a proxy for identification of interaction between H2S and dry gas and identification of H2S sources within a gas reservoir or a basin.

The use of δ 13C in CO to determine removal of CH4 by Cl radicals in the atmosphere *

The reaction of CH4 with chlorine (Cl) radicals in the atmosphere is associated with an extraordinarily strong isotopic fractionation, where 12CH4 reacts about 70 ‰ faster with Cl than 13CH4. Therefore, although the Cl-based sink of CH4 constitutes only a small contribution to its total removal rate, the uncertainty in this small sink has been identified as one of the two largest uncertainties of isotope-based CH4 source apportionment at the global scale. The uncertainty arises from the fact that Cl levels in the atmosphere are so low that they cannot be detected directly. One very sensitive indirect method to identify and quantify the CH4 + Cl reaction in the atmosphere is the detection of the extremely 13C-depleted reaction product carbon monoxide (CO) from this reaction. This article reviews the concept of this approach, its successful application in the atmosphere, its challenges and opportunities for identifying and quantifying Cl-based removal of CH4 at the regional and global scale and its potential to detect and evaluate possible attempts to enhance CH4 removal from the atmosphere.

Controllable decomposition of CH4/CO using Fe3C catalyst and carbon structural properties: Combine experimental study with a ReaxFF simulation

This study utilizes a two-stage method to prepare high-performance Fe3C catalysts through the continuous reduction of Fe2O3 and carburization of metallic iron. Firstly, metal iron with well-developed pores and high specific surface area is obtained through reduction, and then the target product Fe3C is synthesized through the carburization effect of CH4. The performance analysis of the catalyst shows that it has a strong gas adsorption capacity and well-developed pores and its structural and surface chemical properties have been analyzed using XRD, TEM, and XPS methods. The carbon deposition reaction of CH4 and CO on Fe3C surface is characterized by combining Reactive force field (ReaxFF) and experimental methods, and the degree of ordering of the products has been measured which indicates that CH4 is more likely to generate carbon black with a lower degree of ordering on the surface, while CO tends to generate graphite with a higher degree of ordering.

The coupling influence of chemical/physical effects of NH3 on the NO formation in the premixed CH4–NH3 flames

The individual and coupling chemical/physical effects of NH3 addition on the NO formation of the CH4 premixed combustion process are investigated using the numerical simulation and the detailed chemical mechanism. The roles of chemical and physical effects of NH3 component in NO formation routes and major NO-related reactions of CH4–NH3 premixed flames are identified at different equivalence ratios (φ) and blending ratios (ηNH3). The results show that, with the increased NH3 addition, the continuously increased peak NO concentration at the fuel-lean condition is predominated by the chemical effects of NH3, while the variation of peak NO concentration at the fuel-rich condition can be ascribed to both chemical and physical effects of NH3. Additionally, for the pure CH4 fuel or the CH4/NH3 blends with the low NH3 addition, the NO/NO2 conversion process is always significant to the NO concentration variations at φ = 0.8. Furthermore, with the NH3 addition, the enhanced importance of the NO/NO2 conversion and the HNO route to the NO formation are attributed to the physical and chemical effects respectively at φ = 0.8. By contrast, with the NH3 addition at φ = 1.2, the NO formation is predominated by the HNO route, the thermal route and the NNH route, while their contribution variations are all dominated by chemical effects of NH3. Specifically, the HNO route becomes increasingly significant to the NO production, while the thermal route and the NNH route play the more paramount roles in the NO consumption in the CH4–NH3 flame at φ = 1.2. Besides, with the increased NH3 addition up to the medium level, the considerably decreased contribution of the thermal NO pathway is mainly attributed to physical effects of NH3, while the improved HNO pathway is undoubtedly dominated by chemical effects. Furthermore, the NO/NO2 conversion process always plays a significant role in the NO formation with the increased NH3 addition, and its contribution is increased initially due to physical effects but decreased subsequently thanks to chemical effects. In addition, it is also confirmed that the NO destruction can be accelerated by the increased NHi radicals with the NH3 addition, while the C–N kinetics could be important to the NO formations in the CH4–NH3 flame considering the certain contribution of CH3+HNONO + CH4 to the NO production.

Thermal Reactions of NiAl3O6+ and Al4O6+ with Methane: Reactivity Enhancement by Doping.

Investigation of the reactivity of heteronuclear metal oxide clusters is an important way to uncover the molecular-level mechanisms of the doping effect. Herein, we performed a comparative study on the reactions of CH4 with NiAl3O6+ and Al4O6+ cluster cations at room temperature to understand the role of Ni during the activation and transformation of methane. Mass spectrometric experiments identify that both NiAl3O6+ and Al4O6+ could bring about hydrogen atom abstraction reaction to generate CH3• radical; however, only NiAl3O6+ has the potential to stabilize [CH3] moiety and then transform [CH3] to CH2O. Density functional theory calculations demonstrate that the terminal oxygen radicals (Ot-•) bound to Al act as the reactive sites for the two clusters to activate the first C-H bond. Although the Ni atom cannot directly participate in methane activation, it can manipulate the electronic environment of the surrounding bridging oxygen atoms (Ob) and enable such Ob to function as an electron reservoir to help Ot-• oxidize CH4 to [H-O-CH3]. The facile reduction of Ni3+ to Ni+ also facilitates the subsequent step of activating the second C-H bond by the bridging "lattice oxygen" (Ob2-), finally enabling the oxidation of methane into formaldehyde. The important role of the dopant Ni played in improving the product selectivity of CH2O for methane conversion discovered in this study allows us to have a possible molecule-level understanding of the excellent performance of the catalysts doping with nickel.

An experimental and modeling study on homogeneous oxidative coupling of methane utilizing N2O as oxidant

Experimental and kinetic modeling studies were performed to investigate the homogeneous oxidative coupling of methane utilizing nitrous oxide as the oxidant (N2O-OCM). The experiments were performed in a jet stirred reactor at atmospheric pressure and temperatures of 773–1173 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to achieve comprehensive and isomer-resolved identification of major products and nitrogenous, carbonyl and hydrocarbon intermediates. To interpret the experiments, a detailed kinetic model was developed by extending the widely used hydrocarbon growth mechanisms. Rate of production and sensitivity analyses were performed to provide a deep insight into the complex reaction network. The results demonstrate that N2O dissociation is the rate-controlling step, and the mild oxidation of N2O effectively reduces the production of CO2 as byproduct and meanwhile improves the selectivity of C2 species. In the presence of N2O, CH3 can actively participates in H-abstraction reactions of C2H6 and C2H4, besides the self-coupling reactions. Due to chain-propagation competition reactions (N2O+H=N2+OH and N2O+H=NH+NO), NH3 and NO exhibit low concentrations. The detailed formation pathways for NH3 and NO were elaborated, and the evolution routes of carbonyl intermediates in the presence of N2O were analyzed. Benzene formation is found mainly through C3 self-recombination and C5 ring reforming reactions. Finally, comparative analysis with O2-OCM reveals that the presence of oxygen promotes the conversion of CH3 to CH2O and deep oxidation of C2H3, explaining the lower C2 yield of O2-OCM. The inherent limitation of the C2 yield of N2O-OCM primarily arises from the reaction of C2H4 and O, as well as the subsequent reaction CH2(S)+N2O=CH2O+N2. The experiment observations confirmed the existence of C2 carbonyl intermediates such as ketene and acetaldehyde. The study highlights the notable role of O2 molecular and O atoms in the homogeneous OCM and offers kinetic support for the OCM reaction process with catalysts.

Theoretical dynamics studies of the CH3 + HBr → CH4 + Br reaction: effects of isotope substitution and vibrational excitation of CH3.

The rate coefficient for two deuterium substituted isotopologues of reaction CH3 + HBr → CH4 + Br has been determined using the quasiclassical trajectory (QCT) method. We used the analytical potential energy surface (PES) fitted to high-level ab initio points in earlier work. The PES exhibits a pre-reaction van der Waals complex and a submerged potential barrier. The rate coefficients of the deuterated isotopologue reactions, similarly to the pure-protium isotopologue, show significant deviation from the Arrhenius law, namely, the activation energy is negative below about 600 K and positive above it: k[CH3 + DBr] = 1.35 × 10-11 exp(- 2472/T) + 5.85 × 10-13 exp(335/T) and k[CD3 + HBr] = 2.73 × 10-11 exp(- 2739/T) + 1.46 × 10-12 exp(363/T). The CH3 + DBr reaction is slower by a factor of 1.8, whereas CD3 + HBr isotopologue is faster by a factor of 1.4 compared to the HBr + CH3 system across a wide temperature range. The isotope effects are interpreted in terms of the properties of various regions of the PES. Quantum state-resolved simulations revealed that the reaction of CH3 with HBr becomes slower when any of the vibrational modes of the methyl radical is excited. This contradicts the assumption that vibrational excitation of methyl radicals enhances its reactivity, which is of historical importance: this assumption was used as an argument against the existence of negative activation energy in a decade-long controversy in the 1980s and 1990s.

Detection and prebiotic chemistry of possible glycine precursor molecule methylenimine towards the hot molecular core G10.47+0.03

Abstract Amino acids are essential for the synthesis of protein. Amino acids contain both amine (R–NH2) and carboxylic acid (R–COOH) functional groups, which help to understand the possible formation mechanism of life in the universe. Among the 20 types of amino acids, glycine (NH2CH2COOH) is known as the simplest non-essential amino acid. In the last 40 years, all surveys of NH2CH2COOH in the interstellar medium, especially in the star-formation regions, have failed at the millimetre and sub-millimetre wavelengths. We aimed to identify the possible precursors of NH2CH2COOH, because it is highly challenging to identify NH2CH2COOH in the interstellar medium. Many laboratory experiments have suggested that methylenimine (CH2NH) plays a key role as a possible precursor of NH2CH2COOH in the star-formation regions via the Strecker synthesis reaction. After spectral analysis using the local thermodynamic equilibrium (LTE) model, we successfully identified the rotational emission lines of CH2NH towards the hot molecular core G10.47+0.03 using the Atacama Compact Array (ACA). The estimated column density of CH2NH towards G10.47+0.03 is (3.40 ± 0.2) × 1015 cm−2 with a rotational temperature of 218.70 ± 20 K, which is estimated from the rotational diagram. The fractional abundance of CH2NH with respect to H2 towards G10.47+0.03 is 2.61 × 10−8. We found that the derived abundance of CH2NH agree fairly well with the existing two-phase warm-up chemical modelling abundance value of CH2NH. We discuss the possible formation pathways of CH2NH within the context of hot molecular cores, and we find that CH2NH is likely mainly formed via neutral–neutral gas-phase reactions of CH3 and NH radicals towards G10.47+0.03.

Enhanced C2-CN sub-mechanism: Impact on NO/N2O and soot precursor yields during C2H2/HCN oxidation

The combustion of hydrocarbon-ammonia fuels poses challenges related to the formation of soot and nitrogen oxides (NOx) emissions. Various kinetic mechanisms have been developed to describe the pathways of soot and NOx formation, but limited attention has been paid to the role of the C2-CN species, such as HC3N and CH2CHCN, in influencing the yields of soot and NOx. To address this gap, we present a detailed C2-CN sub-mechanism integrated into a NOx and polycyclic aromatic hydrocarbons (PAHs) kinetic model. The rate constants of the barrierless association reactions of C2H+CN, C2H3+CN, C2H5+CN, and CH3+CH2CN were updated using the variable reaction coordinate transition state theory (VRC-TST). We numerically investigated the oxidation of C2H2 and HCN at equivalence ratios of 2.0 and 3.0, which are major precursors for NOx and PAHs, in a perfectly stirred reactor (PSR) employing two kinetic models (with/without the developed C2-CN sub-mechanism). By comparing the rates of production (ROPs) obtained via both models, we analyzed the formation pathways of NO/N2O and Benzo(ghi)fluoranthene (BGHIF). Our findings reveal temperature-dependent trends in the calculated rate constants of C2H5+CN and CH3+CH2CN associations (positive dependence) as well as C2H+CN and C2H3+CN associations (negative dependence). The peak mole fractions of BGHIF at equivalence ratio (φ) of 2.0 and 3.0 predicted by the updated mechanism were 18.4 % and 13.6 % lower than the base mechanism. This reduction can be attributed to an additional consumption channel of C2H2 (C2H2+CN=HC3N+H) predicted by the C2-CN sub-mechanism. Furthermore, the C2-CN sub-mechanism exhibited a stronger suppression effect on N2O formation compared with NO formation at φ=2.0 and 3.0. This limitation can be explained by an additional consumption channel of CN via CN+C2H2=HC3N+H, resulting in decreased NO/N2O through the reaction sequence CN→NO→N2O and CN→NCO→HNCO→NH2→HNO→NO→N2O.

The effect of inert gases (Xe, Ar, Ne, He) on decomposition reactions of N2O, CH4 and CO2 at high pressures

In a pulsed compression reactor (PCR) experiments were done with N2O (4%mol), CH4 (1%mol) and CO2 (2%mol) diluted in the inert gases Xe, Ar, Ne and He. The mixtures were compressed up to 250 bar, reaching temperatures of up to 4000 K. At equal temperature, pressure and volume, significant differences (up to 20%) were measured in the conversion of the three species in different noble gases. The measurements with N2O decomposition showed that the reaction is the fastest in the most heavy noble gas tested. The conversion decreased as the molar mass of the noble gas decreased. Likewise, methane pyrolysis was measured to be the fastest in xenon and slowed down in accordance with the mass of the inert molecule. A reverse trend was measured for the decomposition of CO2 to CO and O⋅, which is explained by the dominant role of the reverse reaction. As a result, the CO2 data is also explained by conversion rates that are higher in heavier gases. This paper provides a first attempt to understand the observed influence of the molar mass of the inert bathing gas on the reaction rate in the high pressure domain. A theory is proposed based on a Newtonian description of reactant activation by the inert bathing gas.

Partial Methane Oxidation to Methanol on Ru-Porphyrins - on the Role of Non-Innocent Ligands and Spin Crossover.

The partial oxidation reaction of CH4 led to the formation of CH3 OH in the presence of Ru-porphyrin oxo complexes (denoted as POR, POR-O and POR-OH where in the case of the last two, oxygen atom and the OH group were attached to the active site, respectively), in which Ru was present on different oxidation states. The simulations were performed based on Density Functional Theory (DFT) with extended geometric and electronic structure analyses of each reaction step. Moreover, the reaction pathways were investigated in different spin states. The Spin Crossover (SCO) phenomenon was found to play an important role in the kinetics of the reaction in the presence of POR and POR-O. Harmonic Oscillator Model of Aromaticity (HOMA) index was applied for different spin states to estimate the aromaticity changes of the pyrrole rings in the Ru-porphyrin complexes. In order to characterize the nature of bonding, the Natural Bond Orbitals (NBO) analysis including the Wiberg Bond Index (WBI) and Natural Population Analysis (NPA) was carried out. Finally, the Non-Covalent Interactions (NCI) index was employed to gain insight into interactions between the components of the reaction. It was found that the non-covalent interactions cannot be neglected in the studied reaction paths.

IR spectroscopic characterization of products of methane and cyclopropane activation by Ru cations

Methane and cyclopropane (c-C3H6) were reacted with Ru+ ions in a room temperature ion trap and the resulting products were identified using a combination of mass spectrometry, IR action spectroscopy, and density functional theory calculations. In the reaction with methane, no products with odd numbers of carbon atoms were located, whereas significant amounts of products with even numbers of carbon atoms were observed. We identified [Ru,2C,4H]+ as the Ru+ ion with an ethene ligand attached, and [Ru,4C,6H]+ as a Ru(η4-cis-1,3-butadiene)+ complex. The barrier toward formation of Ru(C2H4)+ + 2H2 was calculated at the B3LYP/def2-TZVPPD level to be 0.80 eV above the energy of the ground state Ru+ (4F) + 2 CH4 reactants. In the reaction of c-C3H6 with Ru+, we identified the dehydrogenation product [Ru,3C,4H]+ as Ru(η2-propyne)+, [Ru,2C,2H]+ as Ru+ with an ethyne ligand, and [Ru,5C,5H]+ as Ru(η5-c-C5H5)+ having a cyclopentadienyl ligand.

Spin current in the early stage of radical reactions and its mechanisms.

We study the electronic spin flux (atomic-scale flow of the spin density in molecules) by a perturbation analysis and abinitio nonadiabatic calculations. We derive a general perturbative expression of the charge and spin fluxes and identify the driving perturbation of the fluxes to be the time derivative of the electron-nucleus interaction term in the Hamiltonian. We then expand the expression in molecular orbitals so as to identify relevant components of the fluxes. Our perturbation theory describes the electronic fluxes in the early stage of reactions in an intuitively clear manner. The perturbation theory is then applied to an analysis of the spin flux obtained in abinitio calculations of the radical reaction of O2 and CH3· starting from three distinct spin configurations; (a) CH3· and triplet O2 with total spin of the system set Stot=1/2 (b) CH3· and singlet O2, Stot=1/2, and (c) CH3· and triplet O2, Stot=3/2. Further analysis of the time-dependent behaviors of the spin flux in these numerical simulations reveals (i) the spin flux induces rearrangement of the local spin structure, such as reduction of the spin polarization arising from the triplet O2 and (ii) the spin flux flows from O2 to CH3· in the reaction starting from spin configuration (a) and from CH3· to O2 in that starting from configuration (b), whereas no major intermolecular spin flux was observed in that starting from configuration (c). Our study thus establishes the mechanism of the spin flux that rearranges the local spin structures associated with chemical bonds.

Adsorption and surface reactions of C2H2 and C2H4 on Co(0001)

In this paper we have studied adsorption and surface reactions of acetylene and ethylene on Co(0001) in detail by temperature desorption spectrum (TDS), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). C2H4 adsorption at 130 and 300 K followed by subsequent heating mainly forms C2 clusters and graphitic carbon, respectively, while C2H4 decomposes at 400 and 500 K to form dominant graphitic carbon and carbon adatoms, respectively. C2H2 molecularly adsorbs at 130 K but exclusively dehydrogenates upon heating. The resulting C2H2(a) species at low coverages remains stable up to 400 K and then exclusively dehydrogenates into C2 clusters, while the resulting C2H2(a) species at high coverages remains stable up to 310 K and then majorly dehydrocyclizates into (C2H)n intermediates with ring structures at 340 K which further dehydrogenates into graphitic carbon, and minorly dehydrogenates into C2 clusters. Exposed at 370 K, C2H2 dehydrocyclizates into (C2H)n intermediates with ring structures. These temperature and coverage dependent surface reactions of C2H2 and C2H4 on Co(0001) greatly enrich our fundamental understanding of Co-catalyzed F-T synthesis reaction.

Investigation of oxy-fuel combustion for methane and acid gas in a diffusion flame

Co-combustion of methane (CH4) and acid gas (AG) is required to sustain the temperature in Claus reaction furnace. In this study, oxy-fuel combustion of methane and acid gas has been experimentally studied in a diffusion flame. Three equivalence ratios (ER = 1.0, 1.5, 2.0) and CH4-addition ratios (CH4/AG = 0.3, 0.5, 0.7) were examined and the flame was interpreted by analyzing the distributions of the temperature and species concentration along central axial. CH4-AG diffusion flame could be classified into three sections namely initial reaction, oxidation and complex reaction sections. Competitive oxidation of CH4 and H2S was noted in the first section wherein H2S was preferred and both were mainly proceeding decomposition and partial oxidation. SO2 was formed at oxidation section together with obvious presence of H2 and CO. However, H2 and CO were inclined to be sustained under fuel rich condition in the complex reaction section. Reducing ER and increasing CH4/AG contributed to higher temperature, H2S and CH4 oxidation and CO2 reactivity. Hence a growing trend for CH4 and AG to convert into H2, CO and SO2 could be witnessed. And this factor enhanced the generation of CS2 and COS in the flame inner core by interactions of CH4 and CO2 with sulfur species. COS was formed through the interactions of CO and CO2 with sulfur species. The CS2 production directly relied on reaction of CH4 with sulfur species. The concentration of COS was greater than CS2 since CS2 was probably inhibited due to the presence of H2. COS and CS2 could be consumed by further oxidation or other complex reactions.

Experimental and Theoretical Study of the Kinetics of the CH3 + HBr → CH4 + Br Reaction and the Temperature Dependence of the Activation Energy of CH4 + Br → CH3 + HBr.

The rate coefficient of the reaction of CH3 with HBr was measured and calculated in the temperature range 225-960 K. The results of the measurements performed in a flow apparatus with mass spectrometric detection agree very well with the quasiclassical trajectory calculations performed on a previously developed potential energy surface. The experimental rate coefficients are described well with a double-exponential fit, k1(exp) = [1.44 × 10-12 exp(219/T) + 6.18 × 10-11 exp(-3730/T)] cm3 molecule-1 s-1. The individual rate coefficients below 500 K accord with the available experimental data as does the slightly negative activation energy in this temperature range, -1.82 kJ/mol. At higher temperatures, the activation energy was found to switch sign and it rises up to about an order of magnitude larger positive value than that below 500 K, and the rate coefficient is about 50% larger at 960 K than that around room temperature. The rate coefficients calculated with the quasiclassical trajectory method display the same tendencies and are within about 8% of the experimental data between 960 and 300 K and within 25% below that temperature. The significant variation of the magnitude of the activation energy can be reconciled with the tabulated heats of formation only if the activation energy of the reverse CH4 + Br reaction also significantly increases with the temperature.

Methanol Formation through Reaction of Low-energy CH3 + Ions with an Amorphous Solid Water Surface at Low Temperature

We have performed experimental investigations of methanol formation via the reactions of low-energy CH3 + ions with an amorphous solid water (ASW) surface at ∼10 K. A newly developed experimental apparatus enabled irradiation of the ASW surface by several eV ions and detection of trace amounts of reaction products on the surface. It was found that methanol molecules were produced by low-energy CH3 + irradiation of the ASW surface and that hydroxy groups in the produced methanol originated from water molecules in the ASW, as predicted in a previous theoretical study. Little temperature dependence of the observed methanol intensity is apparent in the temperature range 12–60 K. Ab initio molecular dynamics simulations under constant-temperature conditions of 10 K suggested that this reaction spontaneously produced a methanol molecule and an H3O+ ion, regardless of the contact point of CH3 + on the ASW surface. We have performed a simulation with an astrochemical model under molecular-cloud conditions, where the reaction between CH3 + and H2O ice, leading to methanol formation, was included. We found that the impact of the reaction on methanol abundance was limited only at the edge of the molecular cloud (<1 mag) because of the low abundance of CH3 + in the gas phase, whereas the reaction between the abundant molecular ion (HCO+) and H2O ice, which has not yet been confirmed experimentally, can considerably affect the abundance of a complex organic molecule. This work sheds light on a new type of reaction between molecular ions and ice surfaces that should be included in astrochemical models.

Reactive molecular dynamics simulation and chemical kinetic modeling of ammonia/methane co-combustion

Under the vision of “low carbonization”, ammonia (NH3), as a carbon-free hydrogen-rich fuel, blended with reactive additives, is an effective means to achieve low carbon emissions and reduce the consumption of traditional fossil fuels. In this work, the combustion behaviors of NH3 and CH4, the distribution of main products and the formation and reduction characteristics of NO at different temperatures, CH4 mixing ratios, and O2 equivalence ratios are explored by means of reactive molecular dynamics simulation to reveal the microscopic mechanism of NH3/CH4 co-combustion. The calculated results show that CH4 takes the lead in producing H radicals which react with O2 to generate OH radicals to promote the oxidation of NH3, and it is found that the co-combustion process of NH3/CH4 in the radical chain transfer stage is similar to the respective pyrolysis and oxidation reactions of CH4 and NH3. Accordingly, an optimized kinetic mechanism of NH3/CH4 co-combustion is constructed based on the mechanisms proposed by Glarborg et al. and Okafor et al. by incorporating the NH3 pyrolysis sub-mechanisms related to NH2, HNO, and N2H2 and the skeletal mechanism of CH4 oxidation. The optimized mechanisms of NH3/CH4 co-combustion are found to improve the prediction of the ignition delay time (IDT) and laminar burning velocity (LBV) of NH3/CH4 mixture under wide ranges of conditions compared to the original mechanisms of Glarborg et al. and Okafor et al. To decrease the computational cost of three-dimensional combustion numerical simulations, the skeletal mechanism models are established by simplification of the optimized mechanisms of NH3/CH4 co-combustion using direct relation graph method with error propagation and sensitivity analysis, which can provide nearly the same estimates of IDTs and LBVs as the optimized mechanisms mentioned above.