Synchrotron Vacuum Ultraviolet Photoionization Mass 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.

Low temperature oxidation chemistry of n-butylbenzene in n-decane/n-butylbenzene mixture: Product characterization and kinetic modeling study

n-Alkanes and aromatics are two essential components in transportation fuels and have distinct oxidation reactivities. Due to products with different chemical functionalities generated in oxidation of n-alkanes and aromatics, the interaction kinetics may significantly affect the overall reactivity as well as product distributions. This work investigated the oxidation chemistry of the mixture of n-decane and n-butylbenzene, both of which are C10 hydrocarbons commonly used as jet fuel surrogate components. The experiments were conducted in a jet stirred reactor at pressure of 1 atm, the temperature range of 450–850 K, and the equivalence ratios of 0.5, 1.0 and 2.0. Key low temperature oxidation intermediates and radicals, including C10-cyclic ethers, C10-ketohydroperoxides, C10-ketodihydroperoxide, C10-diketohydroperoxide, benzyl hydroperoxide, methyl peroxy radical and methyl hydroperoxide, were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry. A detailed kinetic model was developed and validated against the speciation data in this work. Results reveal that the reactions of n-decane greatly enhance the low temperature oxidation reactivity of n-butylbenzene by efficiently producing reactive radicals, such as OH radicals. Besides, the reactions of n-butylbenzene play an inhibiting role in low temperature oxidation reactivity of n-decane. The measured intermediates, such as C10-cyclic ethers, C10-ketohydroperoxides and C10-ketodihydroperoxide, provide experimental evidence to validate the first, second and third O2-addition reactions, respectively. Due to the addition of n-decane, the evolution profiles of aromatic intermediates as a function of temperature are quite different from the cases of pure n-butylbenzene oxidation. For example, the formation of benzene, phenol and benzaldehyde at low temperatures is significantly enhanced, whose mole fractions present negative temperature coefficient behaviours. Finally, the reaction pathways of the binary fuel to produce C5C1 alkenes, alkynes, aldehydes, alcohols, acids and peroxides were analysed.

Exploration on the combustion chemistry of p-xylene: A comprehensive study over wide conditions and comparison among C8H10 isomers

Pyrolysis and oxidation of p-xylene over wide conditions were studied in this work in order to provide a comprehensive insight into the combustion chemistry of p-xylene. Concerning this goal, the pyrolysis of p-xylene was investigated in a flow reactor at pressures from 0.04 to 1 atm over the temperature range of 1000–1600 K. Key radicals, isomers and PAH products including three xylyl isomers, benzyl, xylylene isomers, styrene, benzene, fulvene, indene, naphthalene and phenanthrene, were identified and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry. The oxidation of p-xylene was studied in a jet-stirred reactor at 10 atm, 800–1300 K and equivalence ratios of 0.5–2.0 with a residence time of 0.5 s. Oxidation products were quantified by using gas chromatography combined with mass spectrometry and Fourier transform infrared spectrometry. A kinetic model for p-xylene combustion over wide conditions was developed based on our previously proposed p-xylene oxidation model and validated against the new experimental measurements in this work and literature reported experimental data, including flow reactor oxidation, laminar premixed flame, ignition delay time and laminar burning velocity. In the flow reactor pyrolysis, p-xylene mainly undergoes unimolecular decomposition to yield p-xylyl, which mainly suffers the H-loss reaction to form p-xylylene and isomerization reaction to form o-xylyl. The resonantly stabilized fulvenallenyl and benzyl radicals play an important role in the formation of bicyclic and tricyclic PAHs including indene, naphthalene, and phenanthrene. In the JSR oxidation, p-xylene is predominantly consumed via H-abstraction by oxygenated radicals such as OH, O, and HO2 forming p-xylyl, which can be oxidized by HO2 and eventually converted to p-methylbenzaldehyde. As the most abundantly produced oxygenated aromatics, further consumption of p-methylbenzaldehyde results in the formation of other observed oxygenated aromatics such as cresol, benzofuran, and benzaldehyde. The combustion chemistry of three C8H10 isomers, i.e., p-xylene, o-xylene, and ethylbenzene, were also compared in terms of pyrolysis and oxidation fuel reactivity, and PAHs formation and growth, showing remarkable fuel isomeric effects.

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.

Revealing the low to moderate temperature oxidation kinetics of 1,2,4-trimethylbenzene sensitized by dimethyl ether

Low to moderate temperature oxidation has been a vital topic in the combustion chemistry due to its critical role in controlling the combustion characteristics such as autoignition and pollutant emissions. Low temperature oxidation kinetics of alkanes and oxygenated biofuels have been widely explored, while that of branched aromatics is poorly understood. This work aims to investigate the low to moderate temperature oxidation kinetics of 1,2,4-trimethylbenzene (TMB, C9H12), which is a multi-branched alkylbenzene with weak low temperature oxidation reactivity. To enhance the low temperature oxidation reactivity of TMB, dimethyl ether (DME) was added to efficiently provide chain initiators. The oxidation experiment of TMB/DME mixture was performed with an atmospheric jet stirred reactor coupled with synchrotron vacuum ultraviolet radiation photoionization mass spectrometry. The negative temperature coefficient (NTC) behavior, as well as featured aromatic intermediates, especially phenyl hydroperoxides and unexpected highly oxygenated molecules (HOMs), were observed in the experiment. The detection of these fingerprint aromatic intermediates provide crucial evidence for revealing the low to moderate oxidation kinetics of TMB. In addition, a kinetic model was developed to provide in-depth kinetic analysis. The results demonstrate that the active chain initiators produced from DME oxidation trigger the low temperature chain-branching reactions of TMB. The chain recycling process is further sustained due to the regeneration of active intermediates such as phenyl hydroperoxides and •OH radicals. Aromatic intermediates with chemical compositions of C9H12Ox and C9H10Ox were measured and identified to be key indicators for the low temperature chemistry of TMB. In particular, the aromatic HOMs (C9H12O6 and C9H10O5) are fingerprint intermediates produced from the third O2 addition and subsequent reactions of TMB. Besides, the sensitivity analysis indicates that the interaction kinetics between TMB and DME by sharing the highly sensitive reactions of hydroperoxides and •OH radicals. The present work extends the conceptual reaction schemes proposed in alkanes towards branched aromatics, especially sequential O2 addition reactions that contributing to the formation of phenyl hydroperoxides and HOMs.

Experimental and theoretical study of cyclopropanated fuel exo,exo-Tetracyclo[3.3.1.02,4.06,8]nonane pyrolysis in a jet-stirred reactor

Exo,exo-Tetracyclo[3.3.1.02,4.06,8]nonane (exo-TCN), a kind of cyclopropanated fuel with high tension three-membered ring, which has the characteristics of high density, high net heat of combustion and good low-temperature performance, is a potential high-energy-density liquid fuel. To deeply understand its combustion behavior, and lay a foundation for practical application, detailed experimental and theoretical study is necessary. The pyrolysis of exo-TCN was investigated in a jet-stirred reactor at a temperature range of 500–1010 K under atmospheric pressure with a residence time of 2 s. 54 products and intermediates ranging from m/z = 2–204 were identified and quantified by synchrotron vacuum ultraviolet photoionization mass spectrometry and online gas chromatography, the major products were hydrogen, ethylene, benzene, methane, propene, toluene and styrene. The potential energy surfaces of exo-TCN unimolecular decomposition reactions were calculated by quantum chemistry at CBS-QB3 level. It is found that the pyrolysis of exo-TCN experiences three stages. In the first stage, exo-TCN isomerizes into Tricyclo[3.3.1.02,4]non-6-ene (TCN-6) by ring expansion of the three-membered ring. In the second stage, TCN-6 generates two main isomers through ring expansion reactions, and decomposes into pyrolysis products. The third stage is the secondary reaction of the intermediate products. Finally, combined with the experimental results and theoretical calculation, the possible reaction mechanism of exo-TCN pyrolysis was proposed, and the related reaction rate constants were calculated.

Experimental and modeling study of the N, N-dimethylformamide pyrolysis at atmospheric pressure

As a polar solvent, N, N-Dimethylformamide (DMF) is a potential source of volatile organic compounds during industrial processing. The potential energy surface and pressure-dependent rate coefficients of DMF primary and secondary thermal decomposition were theoretically investigated by automatic ab initio calculations. A singlet aminohydroxycarbene route generating CO is found to be the most energetically favored channel among the unimolecular initiation reactions with a tight transition state. A kinetic model was proposed for DMF pyrolysis prediction and validated against the speciation data newly obtained from synchrotron vacuum ultraviolet photoionization mass spectrometry in a flow reactor within 773 K–1048 K and literature data in a jet stirred reactor within 450 K–900 K at atmospheric pressure. Satisfactory agreements were obtained for the major intermediates and products, but the model failed to capture the formation of some oxygenated compounds, possibly due to the missing reaction pathway for bimolecular addition. During pyrolysis, DMF is mainly consumed by the hydrogen abstraction reaction as the relatively high CN bond energy hinders the barrierless homolytic cleavage. Unlike aldehydes where the carbonyl site is favored, H-abstraction from DMF methyl sites is prevalent, although the acylamino group readily undergoes decarbonylation and β-scission forming oxygenated and nitrogenous parts. Consequently, CO and HCN are the major pyrolysis products. Besides, by incorporating essential high temperature kinetics, extended validation was conducted against the available intermediate temperature autoignition delay times (1000–1350 K, 2 bar) and laminar burning velocity data, and the reason for the discrepancy between the model prediction and experiments was inferred according to the sensitivity analyses.

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.

Methyl radical chemistry in non-oxidative methane activation over metal single sites

Molybdenum supported on zeolites has been extensively studied as a catalyst for methane dehydroaromatization. Despite significant progress, the actual intermediates and particularly the first C-C bond formation have not yet been elucidated. Herein we report evolution of methyl radicals during non-oxidative methane activation over molybdenum single sites, which leads selectively to value-added chemicals. Operando X-ray absorption spectroscopy and online synchrotron vacuum ultraviolet photoionization mass spectroscopy in combination with electron microscopy and density functional theory calculations reveal the essential role of molybdenum single sites in the generation of methyl radicals and that the formation rate of methyl radicals is linearly correlated with the number of molybdenum single sites. Methyl radicals transform to ethane in the gas phase, which readily dehydrogenates to ethylene in the absence of zeolites. This is essentially similar to the reaction pathway over the previously reported SiO2 lattice-confined single site iron catalyst. However, the availability of a zeolite, either in a physical mixture or as a support, directs the subsequent reaction pathway towards aromatization within the zeolite confined pores, resulting in benzene as the dominant hydrocarbon product. The findings reveal that methyl radical chemistry could be a general feature for metal single site catalysis regardless of the support (either zeolites MCM-22 and ZSM-5 or SiO2) whereas the reaction over aggregated molybdenum carbide nanoparticles likely facilitates carbon deposition through surface C-C coupling. These findings allow furthering the fundamental insights into non-oxidative methane conversion to value-added chemicals.

Ozone-initiated low-temperature oxidation of two propanol isomers: revealing the effects of Ö/ȮH atoms/radicals

The low-temperature oxidation of propanol isomers (n-propanol and iso-propanol) initiated by ozone addition was carried out in a jet-stirred reactor at an equivalence ratio of 0.5, in a temperature range of 400–800 K and at atmospheric pressure. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to follow the temperature dependence of key intermediates, including aldehydes, ketones, hydroperoxides, alcohols, alkanes, and alkenes, to explore their low-temperature oxidation reaction pathways. n-Propanol and iso-propanol presented different intermediate distributions during low-temperature oxidation, due to the different position of the hydroxyl group in the molecular structure. The elucidation of the distribution of these intermediate species contributed to the understanding of the low-temperature oxidation reaction network of n-propanol and iso-propanol. Furthermore, a detailed kinetic model of propanol isomers was tentatively developed by systematically updating the sub-mechanism of β-hydroxypropyl radical and by adding the four reaction channels for CO2 formation. These four reactions involve the reactions of the Ö/ȮH atom/radical with formaldehyde/acetaldehyde. The rate of production analysis reveals the crucial roles of Ö/ȮH atoms/radicals on the whole reaction process, highlighting the controlling effects of reactions directly related to Ö/ȮH atoms/radicals. Especially, the reaction between HȮ2 radicals and ozone dominates ȮH radical formation at lower temperatures of ca. 400–475 K, while Waddington reactions, the cyclization reactions of hydroperoxyl hydroxyalkyl radicals and the H-atom abstraction reactions of fuels/intermediates are dominantly responsible for the formation of ȮH radicals at elevated temperatures of ca. > 500 K.

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.

Experimental and theorical study on the pyrolysis behaviors of 2,3,3,3-tetrafluroropropene

The pyrolysis behaviors of 2,3,3,3-tetrafluroropropene (R1234yf) had been first studied in a flow reactor pyrolysis apparatus coupled with the synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography. The pyrolysis temperature, reaction products and intermediates were measured at atmospheric pressure. The experimental results revealed that the initial pyrolysis temperature of R1234yf was 1040K and the thermal decomposition was accelerated with the increase of temperature. It was found that the pyrolysis reaction of R1234yf will be replaced by a self-polymerization reaction at 1160 K. In order to explore the chemical process of R1234yf pyrolysis, a detailed kinetic simulation was performed and compared with experimental results. The calculated results indicated that the unimolecular decomposition paths took a dominant role in the whole pyrolysis processes. The intermediates of C2H2, C2HF, CF2, CHF3, CF4 and CF3CF3 were detected in the pyrolysis experiments. Moreover, the distribution characteristics and sensitivity analysis of the vital fluorinated species of CF2, C2H2, CF4, CF3CF3 and HF were discussed. All of these founding could provide reliable data for the development of pyrolysis models and guidance for the use of R1234yf in the refrigeration system.

Experimental study on n-hexane pyrolysis initiated by 1-nitropropane in a low-pressure flow reactor

In order to investigate the species pool and primary channels of initiated pyrolysis of n-hexane, the low-pressure (30 Torr, 4.0 kPa) flow reactor pyrolysis of n-hexane initiated by 5% 1-nitropropane (1-NP) was studied experimentally in a wide range of temperature from 623 K to 1223 K. Combined with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectra scanned at photon energies ranging from 9.0 to 14.7 eV and near-threshold photoionization efficiency (PIE) spectra, 29 pyrolysis species, covering C1–C6 hydrocarbons and several pyrolysis species containing O or N atoms, were identified. Based on the ion signal intensities of the visible peaks in the mass spectra, the mole fraction profiles of all the experimental species were evaluated. The experimental results suggested that the initial pyrolysis temperature of n-hexane was reduced by approximately 275–325 K. On the basis of the mole fraction profiles and the initial formation temperatures reported in this work and rate coefficients proposed in previous studies, the 1-NP-initiated pyrolysis mechanisms of n-hexane were discussed. It was suggested that n-hexane was dominantly consumed by the H-abstraction reactions by the attack of OH radical in this work. The 1-NP-initiated pyrolysis of n-hexane mainly proceeded via reactions to produce C3H6 + C3H7, or 1-C4H8 + C2H5, or 1-C5H10 + CH3.

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.

Experimental and kinetic modeling study of low-temperature oxidation of n-pentane

Attractiveness in advanced low-temperature combustion engines drives a constantly updated understanding of low-temperature oxidation chemistry. In this work, the low-temperature oxidation chemistry of n-pentane in two jet-stirred reactors at atmospheric pressure and in the temperature range of 500–825 K was investigated using combined analysis methods of synchrotron vacuum ultraviolet photoionization mass spectrometry, gas chromatography, and Fourier transform infrared spectroscopy. Furthermore, the gaseous mixture from JSR was collected in acetonitrile for subsequent product characterization using flow injection analysis, high-pressure and ultra-high-pressure liquid chromatography coupled to a Thermo Scientific™ Orbitrap® Q-Exactive high-resolution mass spectrometry. Numerous intermediate species were identified by these analytical methods, which contributed to unraveling the low-temperature oxidation reaction network of n-pentane. A detailed n-pentane model was tentatively developed to reduce deviations between experimental measurements and model predictions by updating the rate constants of C5 keto-hydroperoxide decomposition, C5 hydroperoxy cyclic ether decomposition, and Korcek reactions of C5 keto-hydroperoxide, and by introducing pressure-dependent rate constants for the reaction classes of Q˙OOH + O2, Q˙OOH decompositions, concerted HȮ2-elimination of RȮ2, C5 keto-hydroperoxide decomposition, C5 hydroperoxy cyclic ether decomposition, and Korcek reactions of C5 keto-hydroperoxide, and by adding more detailed sub-mechanisms for C5 cyclic ethers and C5 keto-hydroperoxides. This updated model was validated against a set of available experimental data, including jet-stirred reactor species data and ignition delay times. These exploratory updates of the kinetic model reveal the considerable influence of the rate constants of hydroperoxide decomposition and the pressure-dependent rate constants of key reaction classes on the kinetic model predictions, highlighting the future demands for high-precision quantum chemistry calculations of the pressure-dependent rate constants of the aforementioned reaction classes to reduce mechanism uncertainties and to develop accurate and robust chemical kinetic models.

Impacts of NO on low-temperature oxidation of n-heptane in a jet-stirred reactor

Low-temperature (low-T) oxidation experiments of n-heptane –with and without NO addition– were experimentally and numerically investigated at stoichiometric conditions in a jet-stirred reactor. Experiments were performed at atmospheric pressure over a temperature range of 500–800 K. Reactants, intermediates, and products, were measured using synchrotron vacuum ultraviolet photoionization mass spectrometry. A detailed kinetic model was developed to gain insight into the chemical effect of NO on low-T oxidation chemistry of n-heptane. Taking 650 K as the transition temperature, the results revealed that NO addition exhibited an inhibiting effect on fuel reactivity below 650 K and a promoting effect above 650 K. The reactions of ROO + NO = RO + NO2 and HO2 + NO = OH + NO2 at different temperature regions were responsible for the inhibition and promotion effects, respectively. Evidence gathered from both experimental measurements and kinetic model predictions indicated that NO addition had a significant inhibitory effect on the formation of cool flame species during the low-T oxidation process. NO suppressed low-T oxidation via the reaction of ROO + NO = RO + NO2, which impeded the subsequent isomerization, O2 addition, OH-, and HO2-elimination reaction, and influenced product distribution of the cool flame species. The experimental observations provided detailed information about these reactive intermediates, which offered new insights into low-T oxidation phenomena and clarified the importance of NO reactions which prevent the formation of cool flame products during low-T oxidation.

Mechanistic Insights into Radical-Induced Selective Oxidation of Methane over Nonmetallic Boron Nitride Catalysts

Boron-based nonmetallic materials (such as B2O3 and BN) emerge as promising catalysts for selective oxidation of light alkanes by O2 to form value-added products, resulting from their unique advantage in suppressing CO2 formation. However, the site requirements and reaction mechanism of these boron-based catalysts are still in vigorous debate, especially for methane (the most stable and abundant alkane). Here, we show that hexagonal BN (h-BN) exhibits high selectivities to formaldehyde and CO in catalyzing aerobic oxidation of methane, similar to Al2O3-supported B2O3 catalysts, while h-BN requires an extra induction period to reach a steady state. According to various structural characterizations, we find that active boron oxide species are gradually formed in situ on the surface of h-BN, which accounts for the observed induction period. Unexpectedly, kinetic studies on the effects of void space, catalyst loading, and methane conversion all indicate that h-BN merely acts as a radical generator to induce gas-phase radical reactions of methane oxidation, in contrast to the predominant surface reactions on B2O3/Al2O3 catalysts. Consequently, a revised kinetic model is developed to accurately describe the gas-phase radical feature of methane oxidation over h-BN. With the aid of in situ synchrotron vacuum ultraviolet photoionization mass spectroscopy, the methyl radical (CH3•) is further verified as the primary reactive species that triggers the gas-phase methane oxidation network. Theoretical calculations elucidate that the moderate H-abstraction ability of predominant CH3• and CH3OO• radicals renders an easier control of the methane oxidation selectivity compared to other oxygen-containing radicals generally proposed for such processes, bringing deeper understanding of the excellent anti-overoxidation ability of boron-based catalysts.

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.