Synchrotron Vacuum Ultraviolet Photoionization Mass Research Articles

Synergistic effects in toluene/C3H4 isomers co-pyrolysis: Formation of indene and naphthalene

A combination of experimental and kinetic modeling study is performed to explore synergistic effects between toluene and C3H4 isomers on the formation of polycyclic aromatic hydrocarbons (PAHs) and pyrolysis reactivity. Co-pyrolysis of toluene-allene and toluene-propyne is investigated in a flow reactor employing synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at 0.04 bar and 1 bar. Mole fraction profiles of fuels and intermediates up to two-ring PAHs are obtained. A kinetic model for co-pyrolysis of toluene-C3H4 isomers is established and examined against the present data. Sampled mass-specific photoionization efficiency (PIE) curves are employed to identify the presence of aliphatic aromatic species, favoring specific perception into interactions between phenyl/benzyl radicals and C3 species. The synergistic effects observed in this work are not sensitive to the molecular structure of allene and propyne but quite sensitive to the experimental pressures. The reason being that the interactions between phenyl/benzyl radicals and small molecules like CH3, C2H2 and C3H3 are pressure dependent. Both experimental and simulation results indicate the essential role of the aliphatically substituted aromatic in the growth reactions. Indene and naphthalene are identified as the predominant C9H8 and C10H8 products respectively, in all cases studied. Channels leading to the formation of indene and naphthalene vary with pressure, according to rate-of-production (ROP) analyses. The phenyl + C3H4/C3H3 channel and benzyl + C2H2 channel make comparable contributions to the formation of indene at 0.04 bar, while the latter channel dominates the formation of indene at 1 bar. Both C7H5 + C3H3 channel and benzyl + C3H3 channel can lead to the formation of naphthalene at 0.04 bar, while the latter channel is more competitive at 1 bar.

Experimental and kinetic modeling studies of phenyl acetate pyrolysis at atmospheric pressure

Phenyl acetate (CH3COOC6H5, PA) shares a similar aryl acetate group with vitamin E acetate, which is thought to be responsible for producing pulmonary toxic ketene in e-cigarettes. Hence, PA is reported to be a model compound of vitamin E acetate in producing ketene. To better understand the pyrolysis chemistry of vitamin E acetate, pyrolysis of PA in a jet-stirred reactor was investigated by using synchrotron vacuum ultraviolet photoionization mass spectrometry at atmospheric pressure and at temperature range of 700 – 1025 K. Several key products such as acetylene, ethylene, carbon monoxide, formaldehyde, carbon dioxide, vinyl acetylene, 1,3-butadiene, 1,3-cyclopentadiene, benzene, phenol, etc., and especially, ketene, were identified and measured. By extending the phenyl formate pyrolysis model, a detailed PA pyrolysis model containing 735 species and 3365 reactions was constructed and validated against the current experimental results of PA pyrolysis. Rate of production analysis and sensitivity analysis show that the main reaction pathways of PA pyrolysis are the unimolecular decomposition forming phenol and ketene, followed by the C–CH3 bond cleavage forming phenoxycarbonyl and methyl. The corresponding products of these two reactions and of the subsequent reactions, including phenol, ketene and carbon monoxide, etc., are demonstrated to be the key products in PA pyrolysis. Toxic aromatic compounds, such as benzene, toluene and ethylbenzene, etc., also have relatively high mole fractions in PA pyrolysis.

Oxidation study of n-propylamine with SVUV-photoionization molecular-beam mass spectrometry

This work reports the experimental results of n-propylamine (NPA) oxidation in a jet-stirred reactor at 1 atm within 625–875 K, equivalence ratios from 0.5 to 2.0. Oxidation products and intermediates were identified and quantified with synchrotron vacuum ultraviolet photoionization mass spectrometry. Apart from various hydrocarbons, oxygenated and nitrogenous species reported in previous studies of amines, several intermediates were newly detected, including formamide (H2NCHO), nitromethane (CH3NO2), nitrous acid (HNO2), 2-propen-1-ol (C3H5OH) and 2-propenal (C2H3CHO). A detailed kinetic model consisting of 277 species and 2314 reactions was developed with reasonable predictions against the measured data. The rate-of-production and sensitivity analyses results show that NPA oxidation at low temperatures is dominated by the reaction with HO2. Particular attention was paid to the main oxidation product HCN, because its formation is affected by both fuel structure and reaction temperature. The equivalence ratio changes have an opposite effect on HCN concentration in NPA oxidation compared with the pyrrole oxidation and ethylamine flame. In the current study, the peak mole fraction of HCN decreases with increasing equivalence ratio, because the formation of CN triple bond in HCN requires successive H-abstractions, dominantly controlled by the concentrations of OH/HO2 radicals and O2. In addition, a comparison between the experimental results of NPA oxidation and pyrolysis was performed to illustrate the effect of O2 concentration on reaction routes. Current results provide a preliminary insight into the combustion kinetics of more complicated aliphatic amines.

Evaluating the role of hydroxyl keto-hydroperoxide in the low temperature oxidation of alkenes

The low temperature oxidation of alkenes exhibits some distinct reaction pathways that may form different kinds of keto-hydroperoxides to start the chain-branching by further decomposition. This is mainly due to the existence of the double bond that the OH radical could add to it. This work explores that phenomenon by investigating the low temperature oxidation of three alkenes (1-hexene, 1-heptene and 1-decene) in a jet-stirred reactor (JSR) at atmospheric pressure. The synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) experiments combined with hydrogen-deuterium (H-D) exchange reactions were used to evaluate the chain branching pathways. Alkenyl keto-hydroperoxide (AnKHP) and hydroxyl keto-hydroperoxide (HyKHP) are identified. They represented two different chain branching reaction networks that are initiated from the H-abstraction of the primary and secondary CHs and via OH addition to the double bond. The experimental estimated and model predicted ratios of AnKHP to HyKHP are compared, and the result indicates that the literature model underpredicts the chain branching reactions via the formation of AnKHP, but overpredicts the chain branching reactions via HyKHP.

Quantification of n-heptane low temperature oxidation products by proton-transfer-reaction mass spectrometry

While studies on the low temperature oxidation of organic compounds have been in depth, fast and accurate quantitative analysis of oxidation products, especially for those without standard samples, remains challenging and requires analytical developments. This work studied n-heptane low temperature oxidation in a jet-stirred reactor (JSR) using proton-transfer-reaction mass spectrometry (PTR-MS). The oxidation products were also analyzed by a synchrotron vacuum ultraviolet radiation photoionization mass spectrometer (SVUV-PIMS) for comparison. The results showed that PTR-MS was able to measure most oxygenated intermediates compared to SVUV-PIMS. Furthermore, the method for the quantification of low temperature oxidation products was studied. By employing a series of calibration gases, the sensitivities of PTR-MS were confirmed to be linearly related to proton-transfer reaction rates, which were calculated using the trajectory parameterization theory. Mole fraction profiles of oxidation products measured by PTR-MS were compared to SVUV-PIMS and literature gas chromatography data, and in most cases, the results showed good consistency. Special attention is needed for some molecules, such as carboxylic acids, formaldehyde, and peroxides. The effect of the humidity of the reaction environment and the fragment on the quantification need to be considered. Overall, PTR-MS is a promising instrument for the quantitative analysis of combustion products and has the potential to measure the total mole fraction of complex isomers (could not distinguish isomers) and to predict the mole fraction of products without a standard sample.

An experimental and kinetic modeling investigation on low‐temperature oxidation chemistry of 1,3,5‐trimethylcyclohexane in a jet‐stirred reactor

Very scarce studies aiming at the low-temperature chemistry of multi-alkylated cycloalkanes were reported until now, which widely exist in real transportation fuels. Thus, this work presents the first study on the low-temperature oxidation characteristics of 1,3,5-trimethylcyclohexane (T135MCH) in an atmospheric jet-stirred reactor (JSR) combined with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at fuel-lean (ϕ = 0.5), stoichiometric (ϕ = 1.0) and fuel-rich (ϕ = 1.5) conditions. Abundant quantitative information of species is acquired including reactants, major products, reactive hydroperoxides, fuel-specific cycloalkenes and other C1–C5 oxygenated intermediates. Pronounced three-stage oxidation behavior was observed including two fuel decomposition processes and one negative-temperature-coefficient (NTC) window. A detailed model incorporating both high and low-temperature reaction scheme was proposed and validated against the measured data with reasonable agreement. Representative large oxygenated intermediates were detected in this work, such as cyclic ether (CE), cyclohexylhydroperoxide (ROOH), ketohydroperoxide (KHP), olefinic hydroperoxide (OFHP) and C9 acyclic/cyclic bi-carbonyl species whose signal profiles were further compared with the simulated mole fraction profiles justifying the occurrence of specific low-temperature chemistry. Based on the modeling analysis, the competition between the chain-branching processes forming OH and chain-propagating processes forming less reactive HO2 or CE was found to determine the distinct oxidation behavior at different temperature regions. As temperature rises, chain-branching processes are inhibited and the accumulation of HO2 is accelerated, which lead to the occurrence of self-combination forming H2O2, a chain-termination process, resulting in the NTC behavior. With the arrival of high-temperature region, H2O2 becomes thermally unstable and tends to dissociate to double OH via a chain-branching process, therefore triggering the second fuel decomposition via H-atom abstractions.

Ozone-assisted low temperature oxidation of methanol and ethanol

Ozone-assisted combustion is an advanced combustion technology that is considered a promising means for the active control of combustion. Ozone addition can enhance combustion processes and extend the experimental parameters to extreme conditions. In this work, ozone-assisted low temperature oxidation of methanol and ethanol in a jet-stirred reactor (JSR) was carried out at atmospheric pressure, with a fuel initial mole fraction of 0.005, an equivalence ratio of 0.28, and in the temperature range of 330–800 K. Significant conversions of methanol and ethanol were observed in the low temperature region when ozone was added. Numerous intermediate species were analyzed and quantified by synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography, which are crucially important for unraveling the low temperature oxidation reaction network. The kinetic model of NUIGMech1.1 is tentatively updated to better explain the measured results, although the improvements of the C1–C2 core mechanisms are difficult. Ozone addition enhances the action of the reaction channel of related radicals. Model analysis shows that the kinetics of Ö/ȮH/HȮ2 atoms/radicals are crucial for the overall reaction progress, as well as the reaction of HȮ2 radicals with ozone, which mainly contributes to the formation of ȮH radicals during the initial reaction stage. Inconsistency between measurements and model predictions highlights the considerable uncertainty in the C1-C2 alcohols subchemistry at lower temperatures. The experimental results obtained in this work and model development will motivate further study of the low temperature oxidation chemistry of methanol and ethanol.

Experimental and kinetic modeling studies of low- to moderate-temperature oxidation of 2-furfuryl alcohol in a jet-stirred reactor

2-Furfuryl alcohol, a promising platform chemical, and alternative fuels or additives, is produced from the hydrogenation of furfural. However, the low- to moderate-temperature oxidation study of 2-furfuryl alcohol is scarce up to now. The present study first performed the oxidation experiments in a jet-stirred reactor at the equivalence ratios of 0.5, 1.0, and 2.0. The oxidation species were identified and measured by the synchrotron vacuum ultraviolet photoionization mass spectrometry. No negative-temperature-coefficient behavior was observed. A detailed kinetic model was developed to better understand the consumption of 2-furfuryl alcohol and the formation of oxidation products. The present model can well reproduce the experimental results. Based on the rate of production analysis, H-abstraction reactions on hydroxymethyl group forming hydroxyl(2-furyl)methyl radical are the dominant pathways. Besides, the formation and consumption of main products, such as furfural, furan, acrolein, acetaldehyde, etc., are also discussed. Fuel radicals control the formation of furfural. Furan has multiple formation sources, such as the H-addition reactions on 2-furfuryl alcohol, furfural, and 2-hydroxyfuran. Acrolein is produced by the H-addition reaction on 2-hydroxyfuran and OH-addition reactions on furan. Also, the present work compares and discusses peak concentration for major oxidation products of 2-furfuryl alcohol and 2-methyl furan under the same simulated condition. 2-Furfuryl alcohol oxidation produces a higher concentration of furfural, furan, and acrolein than that of 2-methyl furan. OH on hydroxymethyl group promotes the H-abstraction reactions on the side chain and inhibits the OH-addition reaction on the furan ring.

Product Identification in the Low-Temperature Oxidation of Cyclohexane Using a Jet-Stirred Reactor in Combination with SVUV-PEPICO Analysis and Theoretical Quantum Calculations.

Cyclohexane oxidation chemistry was investigated using a near-atmospheric pressure jet-stirred reactor at T = 570 K and equivalence ratio ϕ = 0.8. Numerous intermediates including hydroperoxides and highly oxygenated molecules were detected using synchrotron vacuum ultraviolet photoelectron photoion coincidence spectroscopy. Supported by high-level quantum calculations, the analysis of photoelectron spectra allowed the firm identification of molecular species formed during the oxidation of cyclohexane. Besides, this work validates recently published gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry data. Unambiguous detection of characteristic hydroperoxides (e.g., γ-ketohydroperoxides) and their respective decomposition products provides support for the conventional O2 addition channels up to the third addition and their relative contribution to the cyclohexane oxidation. The results were also compared with the predictions of a recently proposed new detailed kinetic model of cyclohexane oxidation. Most of the predictions are in line with the current experimental findings, highlighting the robustness of the kinetic model. However, the analysis of the recorded slow photoelectron spectra indicating the possible presence of C5 species in the kinetic model provides hints that the substituted cyclopentyl radicals from cyclohexyl ring opening might play a minor role in cyclohexane oxidation. Potentially important missing reactions are also discussed.

Characterization of the low-temperature oxidation chemistry of an unsaturated aldehyde 2-butenal in a Jet-stirred reactor

Unsaturated aldehydes such as butenal are essential intermediates in the combustion of various alkenes and oxygenated biofuels. 2-Butenal is a typical intermediate included in the core mechanism, containing a C=C double bond adjacent to an aldehyde group. In the present work, the oxidation of 2-butenal is studied in a jet-stirred reactor (JSR) at atmospheric pressure under temperature ranging from 500 to 850 K. The synchrotron vacuum ultraviolet photoionization mass spectrometry is employed to identify the key intermediates. A kinetic model for 2-butenal oxidation is developed and validated against the experimental datasets. Fuel flux and sensitivity analyses are performed to clarify reactions governing the reactivity of 2-butenal. OH addition to the C=C double bond is essential for fuel reactivity at the initial stage. A combination of experimental observations and kinetic simulations is used to illuminate the Waddington mechanism initiated by OH addition. The resonance-stabilized feature of fuel radicals facilitates their interactions with HO2 radicals, which replenishes a large amount of OH radicals and contributes to the formation of CO2.

Experimental and kinetic model studies on the pyrolysis of 2-furfuryl alcohol at two reactors: Flow reactor and jet-stirred reactor

2-Furfuryl alcohol, derived from non-edible biomass, is the significant component of bio-oil in the pyrolysis of lignocellulose biomass and a potential alternative fuel or fuel additive. To better optimize the pyrolysis model of lignocellulose to improve the properties of pyrolysis bio-oil and further understand the combustion characteristics of 2-furfuryl alcohol, the pyrolysis experiments were performed in a flow reactor at 30 Torr for T = 1006 – 1339 K and 760 Torr for T = 850 – 1131 K, and in a jet-stirred reactor at 760 Torr for T = 750 – 1050 K. The pyrolysis products, including radicals (methyl, propargyl, allyl, and cyclopentadienyl), isomers (furfural/2-ethyl furan, furan/vinyl ketene, and allene/propyne), and aromatics (benzene, toluene, indene, etc.) were identified and measured using synchrotron vacuum ultraviolet photoionization mass spectrometry. A comprehensive kinetic model for 2-furfuryl alcohol pyrolysis was developed and was validated against the experimental data in the present and previous work. Rate of production analysis indicated that the C–O bond dissociation reaction, H-abstraction reactions on the hydroxymethyl group, and the H-addition reactions on the furan-ring controlled the consumption of 2-furfuryl alcohol and the formation of primary pyrolysis products like 2-methyl furan, 2-ethyl furan, furfural, and furan. In addition, the OH-addition reaction on the furan-ring had a certain contribution to the consumption of 2-furfuryl alcohol, while the contribution of H-shift reactions was negligible. The present work systematically compared the effects of methyl, ethyl, and hydroxymethyl groups on the decomposition of furanic fuels and the formation of major products. The results showed that the substituent groups can reduce the decomposition temperature of the fuels, especially the ethyl and hydroxymethyl groups. The alkyl substituent group can promote the formation of small hydrocarbons, of which the mole fraction increased with the length of the carbon chain increasing. The hydroxymethyl group facilitated the formation of oxygenated products.

Pyrolysis study of iso-propylamine with SVUV-photoionization molecular-beam mass spectrometry

The pyrolysis of isopropylamine (IPA) was studied in a jet-stirred reactor at 1 atm within 670–1100 K. Pyrolysis products and intermediates were identified and quantified with both synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography analysis. Propiolonitrile, propanenitrile, 1-methylethenylamine, 3-butenenitrile, pyridine and benzonitrile were newly identified compared with previous studies of amines. HCN and 1-methylethenylamine were found to be the dominant N-containing species in IPA pyrolysis. The peak mole fractions and temperatures of C1-C2 hydrocarbons are generally higher than C3-C4 ones. The peak temperatures tend to rise with the increase of degree of unsaturation for hydrocarbons with the same C numbers. Based on the observations, a detailed kinetic model consisting of 429 species and 2407 reactions was developed with reasonable predictions against the measured data. Rate-of-production analysis indicates that IPA is mainly consumed by the H-abstraction at the Cα atom: (CH3)2CHNH2+H = (CH3)2CNH2+H2. Sensitivity analysis shows that the unimolecular decomposition via CC bond cleavage and H-abstraction at the Cα atom play promoting roles. The H-abstraction of propene by H-atom yielding allyl radical and self-combination of methyl radicals forming ethane exhibit inhibiting effects. Compared to the pyrolysis of n-propylamine (NPA), more amine- or imine-like species were generated during NPA pyrolysis while more nitriles were detected in IPA pyrolysis. Moreover, the peak temperatures of hydrocarbons in both NPA and IPA pyrolysis shift to higher values with the increase of degree of unsaturation. The results will enrich the understanding of IPA for further combustion kinetic studies of low sooting fuels.

Improving quantification of hydrogen peroxide by synchrotron vacuum ultraviolet photoionization mass spectrometry

Hydrogen peroxide (H2O2) plays a key role in the autoxidation of fuels related to ignition in compression engines, and in the degradation of volatile organic compounds (VOC) in the atmospheric autoxidation. Advanced synchrotron vacuum ultraviolet radiation photoionization mass spectrometry (SVUV-PIMS) is widely used for in-situ analysis of H2O2 in the gas phase autoxidation process. However, considerable uncertainty has been observed in the quantification of H2O2 by SVUV-PIMS. This work improves the quantification of H2O2 by re-evaluating its photoionization cross section (PICS). Spray vaporization, combined with a low pressure flow reactor, was used to minimize decomposition and wall loss of H2O2 in the PICS measurement. It was found that the PICS of H2O2 obtained here was lower than that reported in the literature by a factor of two. Adopting this newly obtained PICS significantly improved quantification of H2O2, and will prove valuable in evaluating the reaction kinetics of H2O2 and the development of combustion and atmospheric autoxidation models.

Reaction mechanism of toluene decomposition in non-thermal plasma: How does it compare with benzene?

Non-thermal plasma (NTP) catalysis is considered one of the most promising technologies to address a wide range of energy and environmental needs, such as carbon dioxide(CO2) conversion, NH3 synthesis, and volatile organic compounds (VOCs) removal. A systematic approach to optimizing NTP systems benefits from understanding VOCs' fundamental NTP destruction behavior and analyzing the correlations between molecular structures and conversion and selectivity. Herein, the mechanical performance of the toluene destruction in NTP is examined and compared with benzene bearing a similar molecular structure. Different experimental and theoretical techniques are applied, including synchrotron vacuum ultraviolet photoionization mass spectrometry(SVUV-PIMS), thermochemistry, and quantum chemistry. Comparatively, toluene is more readily destroyed under the same NTP conditions than benzene. More intriguingly, the distribution of the decomposition species is significantly different. The theoretical calculations reveal that the abundant methyl radicals generated in toluene decomposition mainly lead to the various species distribution. These radicals promote some reactions, such as the decomposition of o-benzoquinone, one of the key intermediates, thus leading to new reaction pathways and products different from benzene. Finally, the critical mechanism steps of toluene decomposition in the current NTP condition are established, including the reactions between toluene and electrons or active radicals, aromatic ring breakup reactions, and reactions of methyl radicals. This study provides a powerful approach to revealing the different fundamental reaction mechanisms resulting from a slight structural difference and brings new insights for probing the underlying plasma chemistry, which is crucial for its various promising environmental and energy applications.

Exploring low temperature oxidation of iso-octane under atmospheric pressure

Iso-octane is the dominant component of primary reference fuel (PRF), with high knock resistance during the combustion process. Understanding the low temperature oxidation chemistry of iso-octane aids in clarifying the auto-ignition properties of gasoline surrogates and developing their kinetic models. However, its low temperature oxidation performance has only been observed under high pressure conditions; no report in the literature includes detailed speciation, hindering a comprehensive examination of the iso-octane low temperature oxidation mechanism. In this work, low temperature oxidation behavior of iso-octane was clearly observed in jet-stirred reactors (JSRs) under an initial fuel mole fraction of 0.01, residence time of 2 s, equivalence ratio of 0.25, and pressure of 1 bar. The low temperature oxidation reaction networks of iso-octane were explored by measuring low temperature oxidation intermediates in three experimental approaches: synchrotron vacuum-ultraviolet photoionization mass spectrometry (SVUV-PIMS), gas chromatography (GC), and cw-cavity ring-down spectroscopy (cw-CRDS). More than 80 intermediates were qualitatively analyzed in combination with the photoionization efficiency (PIE) spectrum, quantum chemistry calculation of the ionization energies, and GC analysis. Hydroperoxides derived from iso-octane low temperature oxidation were reported experimentally for the first time, including keto-hydroperoxides (KHP) and/or hydroperoxyl cyclic ethers (HPCE). Approximately 40 products were quantified, including aldehydes, ketones, olefins, and carboxylic acids, etc. Kinetic models in the literature have been examined using the datasets obtained from this work and reaction pathways for the important intermediates discussed. Comparison indicated that the iso-octane models in the literature must be improved to accurately capture the performance of iso-octane low temperature oxidation.

Experimental and kinetic modeling investigations on low-temperature oxidation of 2-ethylfuran in a jet-stirred reactor

Biomass fuel, as a kind of alternative fuel, offers the possibility of a higher sustainability of the energy system. Furanic fuels that are proposed as next-generation biomass-derived fuels have become one of the targets of recent researches. Here the studies on the low-temperature oxidation of 2-ethylfuran (EF2) in a jet-stirred reactor were performed at atmospheric pressure, at temperature range of 600–900 K and at three equivalence ratios of 0.5, 1.0 and 2.0, respectively. Over 20 oxidation products were detected and measured by using synchrotron vacuum ultraviolet photoionization mass spectrometry, including hydrocarbons, such as methane, ethylene, acetylene, et al., and oxygenated species, such as formaldehyde, ketene, acetaldehyde, 2-methylfuran and 2-vinylfuran, especially the isomers (furan/vinyl ketene) et al. A detailed low-temperature oxidation kinetic model with 723 species and 3300 reactions based on the reported models and theoretical calculations was developed to demonstrate the experimental results in this work. Rate of production analysis shows that, in the low-temperature oxidation of EF2, the consumption of EF2 is dominated by hydrogen atom-abstraction to produce 1-(2-furyl)ethyl radical and by hydroxyl radical-addition onto C(2) of EF2 to produce 2,3-dihydro-2-hyroxyl-2-ethyl-3-furanyl radical at 50% EF2 consumption. The subsequent dominant consumption pathway of 1-(2-furyl)ethyl radical reacting with hydroperoxyl radical will generate 1-(2-furyl)ethyloxyl radical or 2,5-dihydro-5-ethyliden-2-furanyloxyl radical, while 2,3-dihydro-2-hyroxyl-2-ethyl-3-furanyl radical will subsequently go through β-scission to produce carbon monoxide + acetylene + formyl radical eventually. Sensitivity analysis indicates that reactions related to these four radicals, i.e., hydroxyl, 1-(2-furyl)ethyl, 2,5-dihydro-5-ethyliden-2-furanyloxyl and 2,3-dihydro-2-hyroxyl-2-ethyl-3-furanyl, are critical in the consumption of EF2. Hydroxyl radical is mainly produced by the decomposition of hydroperoxide at the equivalence ratio of 0.5 and 1.0, while at the equivalence ratio of 2.0, reactions between 1-(2-furyl)ethyl radical and hydroperoxyl radical, as well as that between methyl radical and hydroperoxyl radical, become more important.

An experimental and modeling study of ammonia oxidation in a jet stirred reactor

Ammonia is a promising carbon-free fuel and has attracted massive attention nowadays. However, there are still large uncertainties in the mechanism of ammonia oxidation that motivate further fundamental investigations. In this work, oxidation experiments of ammonia were carried out in a jet-stirred reactor (JSR) equipped with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry and gas chromatography. Ammonia/O2/Ar mixtures were tested under wide-range operating conditions covering an oxidation temperature of 700–1200 K and an equivalence ratio of ϕ = 0.1–1.0. Important intermediates and products (e.g., NO, NO2, N2O, N2, H2, and H2O) were detected and quantified. A chemical kinetic model for ammonia oxidation was proposed by integrating the rate constants of critical reactions from different models. The results show that ammonia oxidation reactivity is quite strong at ultra-lean conditions, manifesting lower oxidization temperatures and greater oxidization rates. However, this behavior becomes weakened rapidly with the elevation of equivalence ratio, such that ammonia oxidation characteristics are highly similar between ϕ = 0.5 and 1.0. Meanwhile, important intermediates like NO2 and N2O are only detected under ultra-lean conditions. Reaction pathway analysis for equivalence ratios shows that high oxygen concentration is beneficial to the formation of HO2, which leads to a huge amount of OH and promotes the dehydrogenation of ammonia. With the elevation of equivalence ratio, the dehydrogenation of ammonia becomes difficult and reactions NO+HO2=NO2+OH and NH2+NO2=N2O+H2O are suppressed, which answers for the vanishing of NO2 and N2O at ϕ = 0.5 and 1.0. Reaction pathway analysis for oxidation temperatures also emphasizes the significance of HO2 and its conversion pathway to OH, but they are also strongly influenced by equivalence ratios. The current work provides useful insights into ammonia oxidation and a reliable experimental database for modeling.

Improvement of the Thermal Insulation Performance of Silica Aerogel by Proper Heat Treatment: Microporous Structures Changes and Pyrolysis Mechanism

A simple heat treatment method was used to optimize the three-dimensional network structure of the hydrophobic aerogel, and during the heat treatment process at 200–1000 °C, the thermal conductivity of the aerogel reached the lowest to 0.02240 W/m·K between 250 °C and 300 °C, which was mainly due to the optimization of microstructure and pyrolysis of surface groups. Further Fluent heat-transfer simulation also confirmed the above results. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used to finely measure the pyrolysis process of aerogels, and the pyrolysis process of aerogel was divided into four stages. (I) Until 419 °C, as the temperature continued to rise, surface methyl groups were oxidized to form hydroxyl. (II) As the temperature reached to 232 °C, the oxidation proceeded. In addition, inside the aerogel, because of lacking oxygen, the reaction produced CH4 and C–Si bonds would form. (III) After 283 °C, Si–OH groups began to condense to form Si–O–Si, which optimized the three-dimensional network structures to be beneficial to improve the thermal insulation performance of silica aerogel. (IV) When it reached 547 °C, the chemical reaction was terminated, and all the primary particles gradually fused into secondary particles and sintered to form clusters.

Experimental and kinetic modeling study of the homogeneous chemistry of NH3 and NOx with CH4 at the diluted conditions

Homogeneous oxidation of CH4/NH3/NO/NO2 mixtures in the temperature range from 600 to 1200 K was investigated in this work. Seven gas mixtures were designed to reveal the influence of CH4 and NO/NO2 ratio during the reaction process. The experiments were carried out in a laminar flow reactor at atmospheric pressure. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to achieve comprehensive detection of major products and critical hydrocarbon, carbonyl and nitrogenous intermediates. An improved kinetic model, which incorporates a detailed C1-C2/NOx/NH3 sub-mechanism was developed to complement the experiments. In general, the experimental results are in good agreement with prediction of the kinetic model. Rate of production analysis and sensitivity analysis were performed to interpret the experimental observations. The results show that in the oxidation of NH3/NOx mixtures, the NO/NO2 ratio can affect the conversion of NO through the reaction NH2 + NO2 = H2NO + NO. In the presence of CH4, the oxidation reactivity of NH3/NOx mixture is significantly promoted. As the added amount of CH4 increases, the production of H2O is enhanced while N2 formation is inhibited. Beside, the addition of CH4 also promotes the production of carbonyl and nitrogenous species. In the oxidation of CH4/NH3/NOx mixtures, the change of NO2/NO ratios only has little effect on the formation of major products and intermediates.

Variable pressure JSR study of low temperature oxidation chemistry of n-heptane by synchrotron photoionization mass spectrometry

Low temperature oxidation chemistry is crucial in the auto-ignition process of internal combustion engines. The development of laboratory based reactors and diagnostic systems promotes understanding of the low temperature oxidation mechanism. This work develops a variable pressure jet-stirred reactor (VP-JSR) platform working from one to ten bar. Compared to previous high pressure studies which routinely use gas chromatography to analyze low temperature oxidation products, the difference in this work is the possibility of probing intermediates by synchrotron vacuum ultraviolet photoionization mass spectrometry via molecular beam sampling. The setup was validated by repeating n-heptane low temperature oxidation experiments at one and ten bar, respectively. Good agreement was observed between the data in this work and the data in the literature. A much more detailed species pool was probed compared to the literature study, including H2O2, carbonyl acids, alkylhydroperoxides, and keto-hydroperoxides. As a first demonstration of the usefulness of this design, the low temperature oxidation of n-heptane with initial fuel mole fraction of 0.005, residence time of 2 s, and equivalence ratio of 1.0 was studied at one, five, and ten bar. The preliminary results, including reactants, final products, and the initial low temperature oxidation intermediates, are discussed. The VP-JSR system developed in this work is valuable for study of the fuel chemistry from one to ten bar, to develop and examine chemical kinetic models, and to guide the development of the reaction system at even higher pressures, such as those in engine function and supercritical conditions.