Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry Research Articles

Carbon-free hydrogen production via plasma-catalytic ammonia decomposition over transition metal-based catalysts: In situ probing by DRIFTS and SVUV-PIMS

The lack of comprehensive understanding of the reaction mechanism hinders the development of plasma-catalytic ammonia (NH3) decomposition as a potential route for producing carbon-free hydrogen. Herein, we investigated the reaction process over transition metal-based catalysts in a dielectric barrier discharge (DBD) reactor. For the first time, a combination of an integrated in situ diffuse reflectance infrared Fourier transform spectroscopy-DBD (in situ DRIFTS-DBD) and Optical Emission Spectra (OES) were applied to reveal that as for Ni/Al2O3, less NH3* was observed in the gas phase and more NH3 adsorbed on the surface. Combined with online synchrotron vacuum ultraviolet-photoionization mass spectrometry (SVUV-PIMS) it was further illustrated that N2 desorbed more quickly from Ni/Al2O3 than other tested catalysts. Additionally, compared with thermal catalysis, plasma further enhanced the adsorption of NH3 on Lewis acid sites and facilitated the desorption of N2. This study provided a better understanding of the plasma-catalytic NH3 decomposition reaction.

A comparative pyrolysis study of tetraethoxysilane and tetraethoxymethane: Insight into congener substitution effects on pyrolysis chemistry of siloxane flame synthesis precursors

The pyrolysis of siloxane precursors, such as tetraethoxysilane (TEOS), is commonly employed in the flame synthesis and chemical vapor deposition of silica nanoparticles. In this work, the flow reactor pyrolysis of TEOS is studied using gas chromatography (GC) and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS), and the congener substitution effects from the central C atom to the central Si atom are investigated through comparison with its hydrocarbon counterpart tetraethoxymethane (TEOM). Pyrolysis models of TEOS and TEOM are constructed and validated against the measured results. Modeling analysis, including rate of production analysis and sensitivity analysis, provides insights into chemistry in fuel consumption and product formation. In contrast, the observations of silicon-containing products in SVUV-PIMS experiments provide evidence for crucial decomposition pathways of TEOS. It is observed that TEOS exhibits significantly higher stability than TEOM under pyrolysis conditions. The most abundant products in the pyrolysis of TEOS and TEOM are ethylene and ethanol, and TEOS produces more hydrocarbon products than TEOM. The lower pyrolysis reactivity of TEOS is attributed to the slower unimolecular decomposition reaction which dominates the decomposition of TEOS than TEOM. This can be explained by the hindrance of the extremely strong Si-O bond resulting from the significantly different electronegativity between Si and O. The higher initial decomposition temperature of TEOS enhances the contribution of other decomposition channels, such as C-C bond dissociation and H-abstraction reactions, in TEOS consumption. This leads to the abundant formation of hydrocarbon productions such as methane, ethane, and acetaldehyde. As the main pyrolysis product, the ethanol produced by TEOM pyrolysis is four times that of TEOS due to the congener substitution effects. The ethanol formation pathway in TEOS pyrolysis is different from that in TEOM pyrolysis, which is mainly formed via a newly proposed multi-step mechanism, resulting in a lower yield in TEOS pyrolysis.

Elucidating the low temperature chemistry of o-xylene enhanced by the reaction pool of dimethyl ether

The branched aromatic hydrocarbon with short side chains, such as o-xylene, has weak low temperature oxidation reactivity, while it could proceed low temperature chain recycling reactions by blending a fuel component with higher oxidation reactivity. However, the study of low temperature oxidation kinetics of o-xylene is very scarce. Therefore, this work investigated the low temperature oxidation process of o-xylene in an atmospheric jet stirred reactor by adding a biofuel with higher low temperature oxidation reactivity, i.e. dimethyl ether (DME). A series of characteristic low temperature intermediates, such as phenyl hydroperoxides and ketohydroperoxide from the oxidation of o-xylene, were detected by using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The chemical structures of representative low temperature intermediates were determined by comparing the ionization thresholds from measured photoionization efficiency curves with calculated ionization energies. A kinetic model for o-xylene and DME mixture was developed and verified by literature and present experimental data. Rate of production (ROP) and sensitivity analyses were performed, which indicate that the low temperature oxidation chemistry of o-xylene, such as the negative temperature coefficient behavior, is enhanced by the reaction pool of DME. According to the classical low temperature oxidation mechanism of n-alkanes, the low temperature reactions of o-xylene were proposed. In particular, phenyl hydroperoxides and ketohydroperoxides were revealed to be key indicators for the occurrence of first and second O2 addition reactions during the oxidation of o-xylene, respectively. In addition, the sensitivity analysis uncovers the interaction kinetics between the reaction pools of o-xylene and DME, as well as their effects on fuel consumption and products formation. In summary, present evidence elucidates the low temperature chemistry of a representative branched aromatic, which could contribute to the development of future engines and control of combustion pollutants.

Low-temperature oxidation of n-butanol in a jet-stirred reactor: Detailed species measurements and modeling studies

n-Butanol is a new generation of liquid biofuel. A detailed understanding of the low-temperature combustion chemistry of n-butanol is instructive for its practical application in advanced low-temperature engines. The high-temperature combustion chemistry of n-butanol has been extensively studied, but its low-temperature combustion chemistry remains underexplored, especially the detailed species distribution. In this work, ozone addition was used to enhance the low-temperature oxidation reaction activity to achieve the oxidation of n-butanol in a jet-stirred reactor (JSR) at atmospheric pressure and in the temperature range of 400–800 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for detailed measurements of species distribution. The identification of some intermediate species contributed to elucidating and verifying the low-temperature oxidation reaction pathways of n-butanol, such as butanal, C4 hydroxyalkyl hydroperoxides, C4 hydroxyl cyclic ether, and C4 keto-hydroperoxide. Furthermore, the formation of several elusive intermediates was also identified (e.g., C2H4O2, C3H6O2, C4H6On (n = 1∼3), and C4H8O2), highlighting the necessity of further kinetic studies on the low-temperature oxidation of n-butanol. Based on detailed experimental measurements, the n-butanol model was further developed to improve the model predictions by updating the rate constants for the H-atom abstraction reactions of n-butanol by the ȮH radical, adding the detailed butanal sub-mechanism, etc. Especially, the premature formation of CO2 in the present experiment was satisfactorily explained and predicted by the addition of a generally accepted reaction class (i.e., RȮ2 + RȮ2 = RȮ + RȮ + O2 of butanal). Overall, this updated model satisfactorily predicts both the ignition delay time reported in the literature and the species distribution obtained in this work.

A study on the pyrolysis of n-hexane initiated by 1-nitropropane: Molecular dynamics simulations and SVUV-PIMS experiments

To investigate the influence of 1-nitropropane (1-NP) initiator on the pyrolysis of n-hexane (n-C6H14), molecular dynamics (MD) simulations and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) experiments were used to analyze the pyrolysis processes of n-C6H14 and n-C6H14 with 10 % 1-NP added. ReaxFF MD simulations were used to analyze the pyrolysis rates and product distributions of n-C6H14 and 1-NP/n-C6H14 at different temperatures. The mechanism of action of n-C6H14 pyrolysis initiated by 1-NP was investigated. Additionally, the pyrolysis products of n-C6H14 and 1-NP/n-C6H14 were identified and quantitatively analyzed using the SVUV-PIMS technique. These experimental results were compared with the ReaxFF MD simulation results to validate the reliability of the simulations and to complement each other. The results indicate that the presence of 1-NP promotes the pyrolysis of n-C6H14 and significantly improves the chemical heat sink of the fuel. The chemical heat sink in the 1-NP/n-C6H14 system differs from that in the n-C6H14 system by 28.78 kJ/kg at 2103 K. A significant increase in C2H4 production in the pyrolysis products of n-C6H14 with the addition of 1-NP. The pyrolysis of 1-NP/n-C6H14 yielded approximately twice as much C2H4 as n-C6H14 at 2103 K. The pyrolysis reactions were described by first-order kinetics, and the activation energy for the pyrolysis of 1-NP/n-C6H14 was determined to be 140 kJ/mol, approximately 48 % lower than that of n-C6H14. The composition of the pyrolysis products was confirmed by SVUV-PIMS experiments, which were in good agreement with the ReaxFF MD simulations. The mechanism of pyrolysis of n-C6H14 initiated by 1-NP was elucidated: the NO2 radicals generated by 1-NP react with H radicals of the system to form OH radicals. The primary reaction of n-C6H14 involves the abstraction of H by OH, leading to the formation of C6H13. Subsequently, C6H13 is further decomposed to form 1-C3H7, 1-C4H8, 1-C5H10 and 2-C6H12.

Pyrolysis and kinetic study of dimethyl methylphosphonate (DMMP) by synchrotron photoionization mass spectrometry

Pyrolysis experiments of dimethyl methylphosphonate (DMMP) were carried out in a jet stirred reactor (JSR) coupled to synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at 1 atm, from 900 to 1200 K, to quantitatively measure the mole fraction profiles of DMMP and its pyrolysis products. Furthermore, theoretical calculations were carried out. Geometry optimization and vibrational frequency analysis were conducted for DMMP unimolecular and isomerization reactions at the MN15/6-311+G(2df,2p) level of theory, and kinetic calculations were performed using the kinetic code of MESS. The results show that DMMP mainly undergoes isomerization channels to form its isomers before decomposing into pyrolysis products. This is different from the DMMP decomposition channels adopted in the literature models. The kinetic model of DMMP in the literature was modified and improved by adding newly calculated reactions and their rate constants and updating the rate constants of unimolecular reactions for DMMP as well based on computations. Some rate tuning of reactions by 5 or 10 factors has been tried in model development to better describe the experimental measurements. Finally, the model developed by this work showed much better prediction of DMMP consumption and major species production than the prediction by literature models. In addition, the updated model was validated against various previous experimental data (CO mole fraction distributions in the shock tube, the flame speed and species mole fraction distributions in the premixed flame), and the satisfactory validation results indicate the reliability of the DMMP model that was improved in this work. This preliminary work laid a foundation to further study the combustion properties and to develop the kinetic model of organophosphorus compounds.

Chemical insight into the ozone-assisted low-temperature oxidation of propane

Ozone addition is not only a promising method for combustion control and enhancement but also provides a reliable platform to benchmark low-temperature oxidation chemistry. Studies of ozone-doped combustion provide additional insight into combustion chemistry at lower temperatures. This paper studies the ozone-assisted low-temperature oxidation of propane in an atmospheric jet-stirred reactor (JSR) from 350 to 770 K. More than twenty species are measured and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The experimental results show that even at temperatures as low as 450 K, the reaction of propane is initiated by Ö atoms that originate from the thermal decomposition of ozone. The reactivity of propane reaches a maximum at 650 K, decreases in the temperature range of 650 – 740 K, and then slightly increases from 740 to 770 K. The propane oxidation behavior observed in this study is effectively captured by NUIGMech1.3 combined with a Princeton ozone sub-mechanism. However, the mole fractions of certain species such as, CO, H2O, and H2O2 are over-estimated by the mechanism at temperatures above 700 K. Kinetics analyses indicate that fuel oxidation and intermediate species formation are sensitive to the reaction temperature which determines the competing reactions involving propyl-peroxy radicals and the formation of ȮH and HȮ2 radicals. These findings are significant in terms of improving the core mechanism and acquiring a deeper comprehension of the ozone-assisted low-temperature oxidation chemistry of alkanes.

High-temperature oxidation and a comprehensive kinetic mechanism of HFO-1234yf

To cope with the environmental problems caused by the greenhouse effect, the world has focused on the issue of refrigerant replacement. The potential alternative 2,3,3,3-tetrafluoropropene (CH2CFCF3, HFO-1234yf) has excellent thermal properties, system performances and environmental characteristics, while has the risk of combustion. In this study, the oxidation process of HFO-1234yf was first studied by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The important oxidation products of HFO-1234yf were detected, which included fluorine-containing compounds CH2F2, CHF3, CH2FCF3 and CF3CF3 except for CO, CO2, H2O. The molar fraction distributions of these specific fluorine-containing species were derived by the photoionization cross section measurements, which provided new test data to construct the combustion mechanism of HFO-1234yf. Furthermore, eleven newly oxidation paths of HFO-1234yf were proposed by the quantum chemistry calculation method. A modify kinetic model for HFO-1234yf combustion was established, which included 1,557 reactions and 193 species. In addition, rates of production (ROP) analysis of intermediates C2H2, CH2F2, CHF3, CF3CF3 were performed to probe the microscopic paths of HFO-1234yf oxidation. The results showed that most of HFO-1234yf were consumed by reacting with free radicals of O, H, OH, F and CF3. The sensitivity analysis indicated that HFO-1234yf was most sensitive to chemical reactions H + O2 = O + OH and CF3 + O2 = CF3O + O, and these reactions provided plenty of free radicals to promote the decomposition of HFO-1234yf.

Plasma-assisted low-temperature oxidation of n-butane: A synchrotron photoionization mass spectrometry and kinetic modeling study

The chemical kinetics of plasma-assisted low-temperature oxidation of n-butane (n-C4H10, 340 K, 30 Torr) is investigated by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and kinetic modeling. Species measurements are conducted in a flow reactor activated by a nanosecond discharge using SVUV-PIMS. Detailed species identification and quantification are reported in the present work. Fuel-specific oxygenated species including 2-butenal (2-C3H5CHO), methyl vinyl ketone (CH3COC2H3), butanone (CH3COC2H5), butanal (n-C3H7CHO), 2-methyloxetane (-CH(CH3)CH2CH2O-), 1,2-epoxybutane (-CH(C2H5)CH2O-), tetrahydrofuran (-CH2CH2CH2CH2O-), 1-/2-butanol (1-/2-C4H9OH), and butyl hydroperoxide (C4H9O2H) are detected. A kinetic model for n-C4H10 plasma is developed, which well predicts the mole fractions of most species observed in experiments. Kinetic modeling reveals that electron-impact reactions of n-C4H10 are the main sources for various C1-C4 alkenes, alkynes, and radicals. Therefore, these reactions have a significant effect on the fuel consumption pathways and species pool formation. Reactions involving alkyl peroxy (RO2), especially the self- and cross-reactions of RO2, are essential for RO2 depletion and the formation of alcohols, ketones, and aldehydes. On the other hand, the misprediction of the species detected in the system, such as C4 cyclic ethers, ethyl formate (HCOOC2H5), and acrolein (C2H3CHO), indicates that their formation pathways in the current mechanism are incompletely described. Further experimental and numerical research on these subjects is desired for the development of the kinetic models.

Experimental and kinetic model studies on the low- to moderate-temperature oxidation of N-methyl pyrrole in a jet-stirred reactor

N-methyl pyrrole (NMP) is an important nitrogenous heterocyclic compound in bio-oils produced from biomass conversion. NMP can be used as surrogate model compound to clarify the combustion chemistry of nitrogenous fuels and NOX formation mechanism. The low- to moderate-temperature oxidation experiments of NMP are carried out in atmospheric pressure jet-stirred reactor for 0.5% NMP/oxygen/Argon mixtures at different equivalence ratios (0.5, 1.0, and 2.0) over the temperature range of 760 – 1030 K. More than twenty species, including reactants, nitrogen-containing species, and oxygenated products, are measured using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A detailed and comprehensive kinetic model for NMP oxidation is developed for the first time based on the pyrrole oxidation model developed by Pelucchi et al. and Chen et al.. Simulated results show that the present model can reasonably reproduce the experimental results. Rate of production analysis indicates that the H-abstraction reactions on the methyl group dominate in NMP oxidation at all equivalence ratios. NMP oxidation produces C4 and C5 nitrogen-containing compounds including pyridine and pyrrole. These species are found to be products of the primary reactions of NMP, and their formation pathways can be explained satisfactorily by the present model. The present work provides a basis to further understand the oxidation characteristics of nitrogen-containing heterocyclic fuels.

Elucidating the photodissociation fingerprint and quantifying the determination of organic hydroperoxides in gas-phase autoxidation

Hydroperoxides are formed in the atmospheric oxidation of volatile organic compounds, in the combustion autoxidation of fuel, in the cold environment of the interstellar medium, and also in some catalytic reactions. They play crucial roles in the formation and aging of secondary organic aerosols and in fuel autoignition. However, the concentration of organic hydroperoxides is seldom measured, and typical estimates have large uncertainties. In this work, we developed a mild and environmental-friendly method for the synthesis of alkyl hydroperoxides (ROOH) with various structures, and we systematically measured the absolute photoionization cross-sections (PICSs) of the ROOHs using synchrotron vacuum ultraviolet-photoionization mass spectrometry (SVUV-PIMS). A chemical titration method was combined with an SVUV-PIMS measurement to obtain the PICS of 4-hydroperoxy-2-pentanone, a typical molecule for combustion and atmospheric autoxidation ketohydroperoxides (KHPs). We found that organic hydroperoxide cations are largely dissociated by loss of OOH. This fingerprint was used for the identification and accurate quantification of the organic peroxides, and it can therefore be used to improve models for autoxidation chemistry. The synthesis method and photoionization dataset for organic hydroperoxides are useful for studying the chemistry of hydroperoxides and the reaction kinetics of the hydroperoxy radicals and for developing and evaluating kinetic models for the atmospheric autoxidation and combustion autoxidation of the organic compounds.

Experimental investigation on the low-pressure pyrolysis of cyclohexane initiated by 1-nitropropane in a flow tube reactor

An experimental study of 5% 1-nitropropane (1-NP)-initiated cyclohexane pyrolysis was carried out in a flow reactor at 30 Torr (4.0 kPa) over a temperature range of 673–1273 K. The pyrolysis species were detected with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) technique. Pyrolysis species from m/z 15–106, including C1-C8 hydrocarbons, CO, NO, NO2, CH2O, CH3OH and C3H5NO, were identified with near-threshold photoionization efficiency (PIE) spectra and quantified at different temperatures. The experimental results indicated that the addition of 1-NP obviously reduced the initial pyrolysis temperature of cyclohexane from 1048 to 1098–723 K, and caused the decreases of the initial formation temperatures for the species from cyclohexane pyrolysis. Almost all the high-temperature pyrolysis species in the mechanisms proposed in the literature were identified in the experiments. Based on the experimental results in this work and rate coefficients proposed in the literature, the primary pyrolysis mechanisms of cyclohexane in the presence of 1-NP were discussed. The 1-NP pyrolysis occurs at a lower temperature than the cyclohexane does, and produces HONO, NO2, C3H7, etc. It was demonstrated that cyclohexane was mainly consumed via the H-atom abstractions by H and OH attacks in this work and the cyclohexyl radical formed via H-atom abstractions mainly isomerized into 1-C6H11-6 radical, which further combines with H to form 1-C6H12.

On the low-temperature chemistry of 1,3-butadiene

In this paper, species versus temperature profiles were measured during the oxidation of 1,3-butadiene in a jet-stirred reactor (JSR) at 1 atm, at different equivalence ratios (φ = 0.5, 1.0 and 2.0), in the temperature range 600 – 1020 K. Both synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC) methods were used to analyze the species. The experimental results show that a large proportion of the products are aldehydes (formaldehyde, acetaldehyde, acrolein, etc.) and ketenes (ketene, methyl-ketene), with acrolein being one of the major products. Moreover, furan, 1,3-cyclopentadiene and benzene are also present as intermediates in significant amounts. The reaction pathways leading to the formation of these species are discussed in detail. A new detailed mechanism, NUIGMech1.3, was developed to simulate these new data as well as other experimental data available in the literature. The validation results indicate that quantum calculations are also needed to explore the formation of some important species formed in the oxidation of 1,3-butadiene. Overall, the new 1,3-butadiene mechanism agrees well with various experimental data in the low- to high-temperature regimes and at different pressures. Flux and sensitivity analyses show that 1,3-butadiene shares some common reaction chemistry pathways with 1- and 2-butene via Ḣ atom and HȮ2 radical addition to the C = C double bond in 1,3-butadiene, reactions which are important for both systems. The low temperature chemistry of 1,3-butadiene is mainly controlled by the reaction pathways of ȮH radical addition to the C = C double bond of the fuel molecule. The 1-buten-4-ol-3-yl radicals so formed subsequently add to O2 and react via the Waddington mechanism, which is important in accurately simulating the oxidation and auto-ignition of 1,3-butadiene at engine relevant conditions.

Ozone-assisted low-temperature oxidation of methane and ethane

Ozone addition is a promising way to control and monitor the combustion process, e.g., enhancing the low-temperature oxidation reaction. This paper studied the ozone-initiated low-temperature oxidation of methane and ethane in a jet-stirred reactor (JSR) at atmospheric pressure. For the first time, we observed that the low-temperature oxidation of methane and ethane started ca. 450 K. Dozens of low-temperature oxidation products, which are dominantly oxygenated species, were identified and quantified by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). These data are valuable for evaluating the C1–C2 chemistry of base models. NUIGMech1.1 was used to simulate the experimental data in this work. The results reveal that the model can predict methane conversion, but a large deviation is observed for the intermediate species. In the case of ethane, NUIGMech1.1 overpredicts the low-temperature reactivity of ethane. We found that the reaction mechanism of the alkylperoxy radicals (CH3Ȯ2 and C2H5Ȯ2) is crucial in accurately predicting the low-temperature oxidation chemistry of methane and ethane. The bimolecular reactions of the alkylperoxy radicals with Ö atoms and ȮH and HȮ2 radicals, as well as the self and cross reactions of the alkylperoxy radicals, determine the balance of the radical pool of Ö, ȮH, and HȮ2, and subsequently the fuel reactivity and the formation of the oxidation intermediates. This work provides new insights into the reaction mechanism of the key intermediates in the base model for chemically sensitized low-temperature fuel oxidation that need to be thoroughly studied in the future.

Exploring fuel molecular structure effects on the pyrolysis chemistry of branched hexenes

3,3-Dimethyl-1-butene (NEC6D3) and 2,3-dimethyl-2-butene (XC6D2) are representative branched alkene components in gasoline. This work experimentally investigated the pyrolysis of NEC6D3 and XC6D2 in a flow reactor (T = 950–1350 K, P = 0.04 atm) and a jet-stirred reactor (T = 730–1000 K, P = 1 atm) using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC). A pyrolysis model of branched hexenes was proposed and validated against the new experimental data. The combined experimental observations and modeling analyses provide insights into the predominant fuel decomposition pathways and specific formation pathways of products under pyrolysis conditions. NEC6D3 exhibits a much higher reactivity than XC6D2 due to the existence of allylic CC bonds. Unimolecular decomposition reactions play the most crucial role in NEC6D3 decomposition, while in XC6D2 pyrolysis, fuel consumption is dominated by H-abstraction reactions and the H-assisted isomerization reaction. Fuel-specific pathways can remarkably influence the formation of pyrolysis products, especially the key C1C2 products, isomer pairs and dialkenes. Furthermore, the reactions involving propargyl radical dominate the formation of fulvene and aromatic products in the pyrolysis of both fuels, leading to more abundant production of C6 and larger cyclic products in XC6D2 pyrolysis.

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.

Measurement and summary of photoionization data for biomass-derived compounds.

Biomass is a potential feedstock for making liquid fuels and valuable chemicals. Quantitative analysis of biomass conversion in real time by photoionization mass spectrometry (PIMS) is an important way to understand the reaction process. However, the lack of photoionization data for biomass-derived compounds limits the research using PIMS. Measurements of photoionization data were performed with synchrotron vacuum ultraviolet PIMS. Toluene and methanol were used as calibrated references and solvents in this experiment since their photoionization cross-sections (PICS) are well documented in the literature. The ionization energies (IEs) of 23 biomass-derived compounds were measured. Among them, the PICSs of 14 compounds were calibrated and presented. Besides, the IEs of 95 other biomass-derived compounds and their typical fragment ions were also summarized. A photoionization database related to IEs and PICSs of biomass-derived compounds (m/z < 200) is established. PICSs of most biomass-derived compounds have low values at the most frequently used photoionization energy of 10.5eV. Lignin-derived compounds have lower IEs than carbohydrate-derived compounds.

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.

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.

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.