Molecular Beam Mass Spectrometry Research Articles

Optical and mass spectrometric measurements of dissociation in low frequency, high density, remote source O2/Ar and NF3/Ar plasmas

Remote plasma sources are widely used in applications such as chamber cleaning and flowable chemical vapor deposition. In these processes, it is desirable that the dissociation rate of feed gases be as high as possible and stable. Here, the authors present results on radical densities and gas dissociation fractions for a 400 kHz toroidal transformer-coupled plasma source (MKS Instruments), operating at a power density of 5–50 W/cm3 with feed gas mixtures of O2 or NF3 in Ar and pressures of 0.4 or 2.0 Torr. Radical densities and feed gas dissociation percentages in the plasma were measured by optical emission spectroscopy combined with Ar actinometry. In the plasma, O2 was about 60% dissociated in dilute O2 mixtures (10%–20%). Dissociation decreased with the increasing addition of O2, dropping to 10% dissociation for 90% O2 in the feed gas. NF3 was >95% dissociated for all NF3/Ar mixtures. Little or no dependence on the flow rate was found. Plasma products flow into an anodized Al downstream chamber that is probed by vacuum ultraviolet (VUV) absorption spectroscopy and line-of-sight molecular beam mass spectrometry. In the downstream chamber, O recombined on the walls to form O2 (detected by VUV O2 absorption). The measured downstream O/O2 ratio was a strongly increasing function of an increasing flow rate reproduced by a downstream global model with O wall recombination probability of γO between 0.001 and 0.002. NF3 does not reform in the downstream chamber, as verified by VUV absorption and mass spectrometry. No NF or NF2 was detected, and F mostly recombined to form F2 at the back of the downstream chamber, along with N2. The F2, F, and N2 product absolute number densities were consistent with the 3:1 F:N mass balance of the NF3 feed gas.

Mass spectrometry of neutrals and positive ions in He/CO2 non-equilibrium atmospheric plasma jet

Neutral species and positive ions are measured by means of mass spectrometry in the effluent of the non-equilibrium atmospheric plasma jet operated in He/0%–1%CO2 gas mixture. The capacitively coupled plasma source is the predecessor of the known COST reference jet with identical performance and it is operated in a diffuse mode with gas temperature close to the room temperature. Threshold ionisation molecular beam mass spectrometry is used to measure absolute densities of CO, O2, O and O3 species. The CO molecules are generated with densities up to 2 × 1015 cm−3 at 1 W absorbed power. The O2 density is ∼6 times lower than the CO density and even lower than the O density at level of 4.5 × 1014 cm−3. The O3 density is negligibly small at 4 × 1012 cm−3. The measured O2, O3 and O absolute densities add up to ∼57% of the expected oxygen amount. The high O density could indicate that the vibrational excitation of CO2 is ineffective in this plasma and gas mixture, because O would otherwise react quickly with vibrationally excited CO2(v) to form CO and O2, the low energy efficiency is in agreement with the possibly low vibrational excitation. The highest energy efficiency is just 5% with 1% conversion efficiency. Ion mass spectrometry have been used to measure positive and negative ions, where only positive ions have been detected. The ion with highest signal is the ion, probably due to its low ionisation energy. , , , but no CO+, and protonated clusters or clusters containing water molecules have been detected as well. Neutrals and positive ions have been measured as a function of CO2 admixture, applied power and distance to the jet nozzle. The observed trends can be used for validation of plasma-chemistry models.

Absolute spatially and time‐resolved O, O3, and air densities in the effluent of a modulated RF‐driven atmospheric pressure plasma jet obtained by molecular beam mass spectrometry

AbstractIn this paper, we report a molecular beam mass spectrometer study of a time‐modulated radiofrequency (RF)‐driven atmospheric pressure plasma jet in Ar + 1% O2. Time‐resolved measurements of the absolute density of O3 during the RF modulation period revealed a temporal increase of O3 densities at the start and end of the power modulation. This increase correlates with the increase in O2 due to plasma‐induced transient vortices in the gas jet. Pseudo‐one‐dimensional plug flow modeling of the axial species densities as a function of distance match well with the experimentally recorded trends. The obtained results were used to assess the importance of the O flux in previously reported ClO− production in saline by the same plasma jet.

Effects of water addition on the combustion of iso-octane investigated in laminar flames, low-temperature reactors, and an HCCI engine

The effect of H2O injection on the combustion process of iso-octane was investigated with the aim to better understand the suitability of water addition as a potential engine control parameter for homogeneous-charge compression ignition (HCCI) combustion. Several experiments were combined including premixed low-pressure flames, a jet-stirred reactor (JSR) and a plug-flow reactor (PFR), both at atmospheric pressure, and a single-cylinder research engine (SCRE) operated with either iso-octane or RON 98 gasoline. The thermal effect of H2O addition was determined in laminar premixed iso-octane/O2/Ar flames (equivalence ratio Φ=1.4, 40 mbar) with H2O mole fractions of 0 to 0.22, where water addition reduced the temperature measured by laser-induced fluorescence (LIF) by up to 110 K. Speciation data were obtained from these flames as well as in the JSR (Φ=0.65, 933 mbar) and PFR experiments (Φ=0.65, 970 mbar) with and without H2O addition in the low- to intermediate temperature regime from 700–1100 K. The chemical analysis in these flame and reactor experiments was performed using molecular-beam mass spectrometry (MBMS) employing either electron ionization (EI) in the PFR and premixed flame or single-photon ionization (PI) by tunable vacuum-ultraviolet radiation in the JSR. The effects on species mole fractions were small which is supported by predictions from chemical-kinetic simulations. Quantitative speciation data of the exhaust gas of the SCRE were obtained by using Fourier-transform infrared (FTIR) spectroscopy. A very similar species pool was detected in the laboratory-scale experiments and for the engine operation. It is thus assumed that these results could assist in guiding both the improvement of fundamental chemical-kinetic as well as HCCI engine control models.

Laminar premixed and non-premixed flame investigation on the influence of dimethyl ether addition on n-heptane combustion

Several different ether components have been proposed as renewable replacements for diesel fuel in compression ignition engines. From a fundamental point of view, blends of n-heptane and dimethyl ether (DME) have been considered in this study, where n-heptane is a single-component diesel surrogate and DME acts as a representative for oxymethylene ethers. n-Heptane and n-heptane / DME blends have been investigated experimentally under laminar premixed low-pressure and non-premixed atmospheric-pressure counterflow flame conditions. For the premixed case, a series of three fuel-rich flames was studied with oxygen as the oxidizer, at a constant C/O ratio of 0.47, pressure of 4 kPa, with 50% argon dilution, and an equivalence ratio ϕ near 1.5, burning pure n-heptane and mixtures of n-heptane with DME in the ratios 1:1 and 1:4. In the non-premixed counterflow configuration, two fuel/oxygen/argon flames were studied with pure n-heptane and a 1:1 DME–n-heptane mixture as the fuel. Electron ionization molecular-beam mass spectrometry was used for both flame configurations to investigate the flame structure and determine quantitative mole fraction profiles of stable and reactive species formed in the combustion process. Particular attention was given to the changes caused by partial replacement of n-heptane by DME.Notwithstanding the experimental focus of this work, the experimental results were compared with predictions from a model suitable for both n-heptane and DME that was combined for this case from recent mechanisms available in the literature for these fuels. While the overall flame structure was not significantly altered upon DME addition, with some smaller differences mainly due to temperature effects, more prominent changes and interactive effects were found for a number of primary decomposition products, oxygenated species, and higher-molecular hydrocarbon compounds. In most cases, experimentally observed trends, but not always quantitative changes, could be satisfactorily reproduced and explained by the model.

Experimental and numerical study of the structure of premixed H2/CO/O2/Ar flames at atmospheric pressure

Elaboration of a predictive and well validated kinetic mechanism for combustion of H2/CO mixtures is the key to development of advanced technologies for effective syngas burning in power generation systems. Moreover, understanding the combustion chemistry of H2/CO mixtures is a prerequisite for a comprehensive description of hydrocarbons chemistry and development of predictive chemical kinetic models for combustion of transportation fuels. In this work we report new experimental data for chemical speciation in the atmospheric pressure burner-stabilized premixed flames diluted with Ar and fuelled with H2/CO (1:1) mixture at equivalence ratios of 1 and 2. Flame sampling molecular beam mass spectrometry with soft electron ionization was used to examine the flames. Recent detailed chemical kinetic mechanisms for syngas combustion available from literature were used to simulate the experimental profiles with the CHEMKIN code. A compact kinetic mechanism including 11 species and 16 reactions was elaborated to reproduce the experimental data.

Development and evaluation of a chemical kinetics reaction mechanism for tetramethylsilane-doped flames

Tetramethysilane (TMS) is a precursor for flame synthesis of silica (SiO2) nanoparticles. A chemical reaction mechanism was developed for the oxidation of TMS in a lean low-pressure (p ≈ 30 mbar) H2/O2/Ar flame using species mole fractions, obtained from molecular-beam mass spectrometry (MBMS) measurements in a matrix-supported flat flame doped with 600 ppm TMS. The thermodynamic data of Si-containing species were determined from quantum-chemical calculations at the G4 level of theory. The formation and subsequent consumption of Si(OH)4, one of the main products of TMS oxidation, and the formation of Si4O10H4 clusters are hypothesized to be the primary pathway in the synthesis of silica nanoparticles. The reaction rate coefficients are either estimated via an algorithmic optimization procedure or are assumed based on analogies to similar reactions in the literature. The mechanism was further validated based on MBMS measurements with the same base flame doped with 400 and 800 ppm TMS.

Probing the low-temperature chemistry of di-n-butyl ether: Detection of previously unobserved intermediates

Di-n-butyl ether (DBE, C8H18O) has been proposed as a promising biofuel for diesel engines, but details of its low-temperature (LT) oxidation chemistry are not well understood. This paper reports new speciation data obtained in the temperature range of 400–1100 K at ϕ=1 and nearly-atmospheric pressure, using a plug flow reactor combined with electron ionization molecular-beam mass spectrometry (MBMS) and two different jet-stirred reactors coupled with either online gas chromatography or tunable synchrotron vacuum ultraviolet photoionization-MBMS.The experimental results confirm that DBE is very reactive and exhibits two negative-temperature coefficient (NTC) zones around 500–550 K and 650–750 K. Speciation data with about 40 C0-C8 species are presented, including about 20 LT species not reported previously. Among those, fuel-specific C8H16O2 cyclic ethers were quantified. Also, butanoic acid, which is present in highest amounts among the detected LT intermediates, and C8H14O3 diones, were found to peak already near 500 K, suggesting their importance in the LT chemistry of DBE. Signals of several highly oxygenated peroxides (e.g., C8H14O5 and C8H16O6) were detected, indicating third O2 addition steps. Respective reaction pathways are suggested and discussed based on these experimental results.To better understand the LT chemistry of DBE, the present data were compared to two recent DBE models (Cai et al., 2014; Thion et al., 2017). Significant discrepancies between the experimental data and both models were found for important LT intermediates, of which many were not included in the respective mechanisms. The results reported in the present study thus provide new opportunities for refining DBE kinetic models.

Pyrolysis study of iso-propylbenzene with photoionization and molecular beam mass spectrometry

Pyrolysis study of iso-propylbenzene (IPB) at 30 and 760 torr was carried out by using synchrotron photoionization and molecular beam mass spectrometry techniques. 0.5%IPB was diluted by helium and heated to 798–1148 K in a flow tube. Styrene was the most abundant aromatic intermediate at both pressures. Several polycyclic aromatics were only observed at 760 torr, such as 2-ethenyl-1, 4-dimethyl-benzene, fluorene, anthracene, phenanthrene, 2-methyl-anthracene and pyrene, while fulvene, 1, 3-cyclohexadiene, and 5-ethenylidene-1, 3-cyclopentadiene were only detected at 30 torr. A detail mechanism involving 306 species and 1990 reactions was developed by updating ipso-addition and unimolecular dissociation reactions of IPB as well as reactions related to styrene, indene and naphthalene. The mechanism could predict the pyrolysis of IPB and production of other intermediates well. Rate-of-production analysis shows that the formations of 2-phenethyl, 1-iso-phenylpropyl and 2-iso-phenylpropyl radicals are the main consumption reactions of IPB independent of pressure. Sensitivity analysis indicates that the unimolecular dissociation of IPB forming 2-phenethyl radical is the most significant promoting reaction at both pressures. H-abstraction reaction forming 1-iso-phenylpropyl radical tends to play a significant inhibiting effect on IPB consumption at 30 torr, while the most inhibiting reaction is self-combination of methyl radicals at 760 torr. Compared to n-propylbenzene experiments reported by Liu et al. (Combustion and Flame, 191 (2018) 53–65) and Yuan et al. (Combustion and Flame,186 (2017) 178–192.), IPB tends to produce larger quantities of benzene, styrene and indene under pyrolysis and oxidation conditions. These results will improve the understanding of the combustion and soot formation of IPB as a potential aviation surrogate fuel.

Flame structure and a compact reaction mechanism for combustion of dimethyl ether at atmospheric pressure

To extend the available experimental database on DME flame speciation, this paper reports new measurement data on the chemical structure of premixed stoichiometric and fuel-rich (ϕ = 2.2) DME/O2/Ar flames stabilized on a Botha-Spalding burner at atmospheric pressure. Flame sampling molecular beam mass spectrometry with soft electron ionization was used to detect many DME combustion intermediates, including labile species such as H, CH3, and HO2. The new experimental data were used to develop and validate a compact mechanism of DME combustion which is able to reproduce not only laminar burning velocity and ignition delay times, but also peak mole fractions of important species in DME combustion at a minimum number of species and reactions included. The compact chemical kinetic mechanism consisting of 41 species and 110 elementary reactions is proposed based on the literature kinetic mechanism of DME combustion. Comparison of predictions made using the compact mechanism and detailed literature models with three sets of measurement data (chemical speciation profiles reported in this paper, laminar burning velocity and ignition delay time data obtained by other authors) shows that they are in satisfactory agreement, which generally proves the good predictive capability of the mechanism. Further investigations, however, are needed to better understand the kinetics of H2O2 and C2H2 included in the mechanism, because under fuel-rich conditions, the compact mechanism as well as other detailed models do not adequately predict the peak mole fractions of these important intermediates.

Chemical insights into the larger sooting tendency of 2-methyl-2-butene compared to n-pentane

A comprehensive, chemically detailed mechanism for the combustion of 2-methyl-2-butene and n-pentane is presented to provide insights into the different sooting tendencies of these two structurally different C5 hydrocarbons. A hierarchically assembled mechanism has been developed to specifically target speciation data from low-pressure premixed flames of 2-methyl-2-butene [Ruwe et al., Combust. Flame, 175, 34-46, 2017] and newly measured mole fraction data for a fuel-rich (ɸ=1.8) n-pentane flame, in which species profiles up to phenol were quantified. The partially isomer-resolved chemical composition of this flame was determined using flame-sampling molecular-beam mass spectrometry with single-photon ionization by tunable, synchrotron-generated vacuum-ultraviolet radiation. The presented model, which includes a newly determined, consistent set of the thermochemistry data for the C5 species, presents overall satisfactory capabilities to predict the mole fraction profiles of common combustion intermediates. The analysis of the model predictions revealed the fuel-structure dependencies (i.e. saturated vs. unsaturated and linear vs. branched) of the formation of small aromatic species that are considered as soot precursors. The propensity of the 2-methyl-2-butene flame to form larger concentrations of aromatic species was traced back to the readily available formation routes of several small precursor molecules and the efficient formation of “first aromatic rings” beyond benzene.

Flame structure of dimethyl ether/O2/Ar mixture at 1 atm

For diesel engines, dimethyl ether (DME) is widely used in the world, it is a perspective candidate to replace traditional diesel in the future. The consumption of DME as a motor fuel is constantly increasing. DME has a very low propensity to soot formation during combustion, which allows not only to reduce harmful emissions into the atmosphere, but also to improve the completeness of combustion. To validate available chemical kinetic models for DME combustion, in this work we report our experimental data on the chemical structure of a laminar premixed stoichiometric DME/O2/Ar flame stabilized at 1 atm on a flat burner. Flame sampling molecular beam-mass spectrometry is used to obtain spatial distribution of mole fractions of many species in the flame. The experimental data are compared with the calculation results obtained using PREMIX code and two detailed chemical kinetic mechanisms available from literature for DME combustion.

The C5 chemistry preceding the formation of polycyclic aromatic hydrocarbons in a premixed 1-pentene flame

The formation of small polycyclic aromatic hydrocarbons (PAHs) and their precursors can be strongly affected by reactions of C5 species. For improving existing combustion mechanisms for small PAH formation, it is therefore valuable to understand the fuel-specific chemistry of C5 fuels. To this end, we provide quantitative isomer-resolved species profiles measured in a laminar premixed (ϕ = 1.8) low-pressure (4 kPa) flame of 1-pentene with photoionization molecular-beam mass spectrometry (PI-MBMS) using tunable synchrotron vacuum-ultraviolet (VUV) radiation. These experimental results are accompanied with numerical simulations, starting from models from the literature by Wang et al. [JetSurF version 2.0 (2010)] and Healy et al. [Energy Fuels 24 (2010) 1521–1528] that were developed for different fuels, but which include 1-pentene as an intermediate, and by Narayanaswamy et al. [Combust. Flame 157 (2010) 1879–1898] focusing on the small PAH chemistry. Taking observed discrepancies between experimental results and simulations into consideration, a mechanism for C5 chemistry was newly developed including PAH formation pathways, and its performance analyzed in detail. Special emphasis was placed on the initial fuel consumption of 1-pentene as well as on formation pathways of small aromatics. The mechanisms show differences regarding fuel decomposition and hydrocarbon growth reactions. These contribute to noticeable differences between the simulations with different models on one hand, and deviations between model predictions and experimental results on the other. While the new model presents overall satisfactory capabilities to predict the mole fraction profiles of common combustion intermediates, the predictive capability of the literature models was not fully satisfying for some intermediate species, including C4H6, C7H8, and C10H8. The results indicate that the fuel-specific C5 reaction routes as well as the mechanism for small PAH formation need further investigation.

Providing effective constraints for developing ketene combustion mechanisms: A detailed kinetic investigation of diacetyl flames

Ketene is an important combustion intermediate, but due to its high reactivity, it has been difficult to validate the corresponding kinetic sub-mechanisms with direct experimental measurements. It is shown here the possibility to refine the ketene combustion model through relevant experimental and modeling analyses of diacetyl [2,3-butadione, (CH3CO)2] combustion. The consumption of diacetyl leads to abundant ketene under high-temperature oxidation conditions. From the aspect of the hierarchical structure of a kinetic mechanism, the ketene sub-mechanism acts as a part of a secondary mechanism in the kinetic scheme exploring diacetyl combustion. Therefore, this strategy to understand the kinetics of ketene through studying the combustion chemistry of diacetyl should be based on a good understanding of the diacetyl combustion kinetics. To this end, we investigated the flame chemistry of diacetyl through experimental, modeling, and theoretical approaches. Flame-sampling molecular-beam mass spectrometry with single-photon ionization was employed to measure the detailed chemical structure of a premixed diacetyl flame stabilized at the pressure of 18.0 Torr and the equivalence ratio of 1.2. The quantitative speciation measurements were essential in identifying important reactions in the fuel sub-mechanism, and the corresponding rate coefficients were obtained through high-level theoretical calculations. A kinetic model was proposed, which showed satisfactory performances in predicting the measured flame structure. With the diacetyl flame chemistry better understood, global sensitivity analyses were carried out to suggest conditions for future experiments potentially facilitating the development of ketene sub-mechanisms.

Experimental Investigation of Soot Oxidation under Well-Controlled Conditions in a High-Temperature Flow Reactor

ABSTRACTOxidation by molecular oxygen of freshly produced soot from a flat ethylene-air-flame is investigated under well-controlled conditions in a flow reactor at 773–1273 K and atmospheric pressure. Soot particles are characterized before and after oxidation by electrical mobility technique. In addition to the size-classified soot number density, the molecular gas-phase species are obtained simultaneously by molecular-beam mass spectrometry. Oxidation behavior of soot particles as well as some hydrocarbon species sampled from the soot source was determined quantitatively. Oxygenated species are formed as molecular intermediates during the oxidation process inside the reactor. Different particle size distributions have been investigated by varying the sampling position and equivalence ratio of the flame. Oxidation of soot starts at different temperatures, but is clearly separated from oxidation of flame species starting earlier at lower temperatures. Soot oxidation rates were calculated and comparison with the Nagle-Strickland-Constable model indicates that the investigated flame-sampled soot is more reactive than graphite under the investigated conditions. The presented dataset may help to validate existing soot models.

Experimental and kinetic investigation of pyrolysis and oxidation of nitromethane

The pyrolysis and oxidation of nitromethane (NM) were studied in a plug flow reactor over the temperature range of 735–1476 K at 5 Torr and in a jet-stirred reactor over the temperature range of 600–875 K at atmospheric pressure, respectively. Mole fraction profiles of major products and intermediates were identified with tunable synchrotron vacuum ultraviolet photoionization and molecular-beam mass spectrometry. The current study represents the first comprehensive characterization of NM conversion under these conditions. The experimental results were compared to predictions with a detailed chemical kinetic model involving 364 species and 2389 reactions. The rate constants for significant dissociation reactions CH3NO2 = CH3 + NO2 and CH3NO2 = HCNO + H2O were calculated by the RRKM/Master Equation method using Variflex program in this work. The model provided an overall reasonable agreement with the measured data, but for pyrolysis conditions future work is required to improve predictions of some intermediates. Rate-of-production analysis indicates that the primary decomposition pathways of NM are different for pyrolysis and oxidation due to the effects of CN bond fission and roaming mediated isomerization. From the sensitivity analysis, the two pathways mentioned above have promoting effects on NM consumption for all the studied conditions, while the reaction of CH3+NO2CH3O+NO with inhibiting effect at Φ=2.0 shows promoting effects for Φ=0.4 and pyrolysis. This work extends the experimental database and helps to improve the understanding of low temperature chemistry of NM.

Plug-Flow Reactor Study of the Partial Oxidation of Methane and Natural Gas at Ultra-Rich Conditions

ABSTRACTThe homogeneous partial oxidation of fuel-rich CH4/O2, CH4/C2H6/C3H8/O2 as well as CH4/C2H6/C3H8/H2/O2 mixtures is investigated in a plug-flow reactor at intermediate temperatures (473 ≤ T ≤ 973 K) and a pressure of 6 bar. Experiments are carried out at equivalence ratios (Φ) of 2, 10, and 20. Product species are analyzed using time-of-flight molecular-beam mass spectrometry. The experimental results are further compared with kinetic simulations. It was found that under the investigated conditions, the onset temperature for CH4 oxidation is above 773 K. The highest methane conversion at equivalence ratios of 10 and 20 was between 0–3% for neat methane as fuel and 10–13% for natural gas as fuel. The conversions yield useful chemicals like synthesis gas (H2/CO), C2H4, C2H6, or C3H6. Higher CH4 conversion in the natural gas mixtures results in much higher yields of all products. The natural gas components ethane and propane do not influence the reaction onset temperature.

Experimental and kinetic study on the low-temperature oxidation of pyridine as a representative of fuel-N compounds

The low-temperature oxidation (LTO) of pyridine was studied in a jet-stirred reactor over the temperature range of 700–1000 K at atmospheric pressure and equivalence ratio of 2.0. Mole fraction profiles of the reaction products were obtained based on molecular beam mass spectrometry and tunable vacuum ultraviolet synchrotron photoionization techniques. Hydrogen peroxide, methanamine, acetylenamine, ethenamine, acetaldimine, ethylamine, allyamine, and methylformamide were newly identified compared with previous studies of pyridine flame and pyrolysis. HCN was found to be the dominant N-containing species of pyridine LTO. Pyrrole, acrylonitrile, acetonitrile, and ammonia were also found at the same level of N2O and NO. Based on the new measurements and updated rate constants of several reactions including the H-abstractions of pyridine as well as the oxidation of ortho-pyridyl using density functional theory calculations, a new pyridine LTO kinetic model consisting of 588 species and 3516 reactions was developed with a reasonable agreement with the experimental results. In general, the predictions of the predominant species have been improved compared with the existing model. Rate-of-production analysis indicates that pyridine mainly consumes via C5H5N→C5H4N→C5H4NO2→HCN+CO+CH2CHC˙O, and C5H5N→C5H5NO→C2H2+HCN+CH2CO. Sensitivity analysis shows that C5H4N+O2=>C5H4NO2, and C5H5N+OHC5H4N+H2O have significant promoting effect on pyridine consumption, while the reverse of C5H4N+HO2C5H4NO+OH has strong inhibiting effect. The results will enrich the understanding of pyridine low-temperature oxidation mechanism, which can be applied to the fields of coal pre-treatment, staged combustion and mild combustion.

Ion chemistry in premixed rich methane flames

External electric field and plasma assisted combustion show great potential for combustion enhancement, e.g., emission and ignition control. To understand soot suppression by external electric fields and flame ignition in spark ignition engines, flame ion chemistry needs to be investigated and developed. In this work, comprehensive and systematic investigations of neutral and ion chemistry are conducted in premixed rich methane flames. Cations are measured by quadrupole molecular beam mass spectrometry (MBMS), and neutrals are measured by synchrotron vacuum ultra violet photoionization time of flight MBMS (SVUV-PI-TOF-MBMS). The molecular formula and dominant isomers of various measured cations are identified based on literature survey and quantum chemistry calculations. Experimentally, we found that H3O+ is the dominant cation in slightly rich flame (ϕ=1.5), but C3H3+ is the most significant in very rich flames (ϕ=1.8 and 2.0). An updated ion chemistry model is proposed and used to explain the effects of changing equivalence ratio. To further verify key ion-neutral reaction pathways, measured neutral profiles are compared with cation profiles experimentally. Detailed cation and neutral measurements and numerical simulations by this work help to understand and develop ion chemistry models. Deficiencies in our current understanding of ion chemistry are also highlighted to motivate further research.

From benzene to naphthalene: direct measurement of reactions and intermediates of phenyl radicals and acetylene.

Hydrogen-abstraction-C2H2-addition (HACA) is one of the most important pathways leading to the formation of naphthalene, the simplest two-ring polycyclic aromatic hydrocarbon (PAH). The major reaction channels for naphthalene formation have previously been calculated by Mebel et al., but few experiments exist to validate the theoretical predictions. In this work, time-resolved molecular beam mass spectrometry (MBMS) was used to investigate the time-dependent product formation in the reaction of a phenyl radical with C2H2 for the first time, at temperatures of 600 and 700 K and pressures of 10 and 50 Torr. A pressure-dependent model was developed with rate parameters derived from Mebel et al.'s calculations and from newly calculated pathways on the C8H7 PES at the G3(MP2,CC)//B3LYP/6-311G** level of theory. The model prediction is consistent with the MBMS product profiles at a mass-to-charge ratio (m/z) of 102 (corresponding to the H-loss product from C8H7, phenylacetylene), 103 (the initial C8H7 adduct and its isomers plus the 13C isotopologue of phenylacetylene), 128 (naphthalene), and 129 (C10H9 isomers plus the 13C isotopologue of naphthalene). An additional C8H7 isomer, bicyclo[4.2.0]octa-1,3,5-trien-7-yl, not considered by Mebel et al.'s calculations, contributes significantly to the signal at m/z 103 due to its stable energy and low reactivity. At high C2H2 concentrations, bimolecular reactions dominated the observed chemistry, and the m/z 128 and m/z 102 MBMS signal ratio was measured to directly determine the product branching ratio. At 600 K and 10 Torr, branching to the H-loss product (phenylacetylene) on the C8H7 PES accounted for 7.9% of phenyl radical consumption, increasing to 15.9% at 700 K and 10 Torr. At 50 Torr, the branching was measured to be 2.8% at 600 K and 6.2% at 700 K. Adduct stabilization is favored at higher pressure and lower temperature, which hinders the formation of the H-loss product. The pressure-dependent model predicted the observed branching ratios within the experimental uncertainty, indicating that the rate parameters reported here can be used in combustion mechanisms to provide insights into phenyl HACA reactions and PAH formation.