Weiye Chen

Qiang Xu

Hao Lou

Qimei Di

Cheng Xie

Bingzhi Liu

Jiuzhong Yang
email: jzhyang@ustc.edu.cn

Hervé Le Gall

L S Tran

Xudi Wang

Herve Le Gall

Zongyu Xia

Olivier Herbinet

Fre De Rique Battin-Leclerc
email: frederique.battin-leclerc@univ-lorraine.fr

Zhandong Wang
email: zhdwang@ustc.edu.cn

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

Keywords: autoxidation, peroxides, kinetic modeling, synchrotron radiation photoionization mass spectrometry

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Investigating low temperature oxidation chemistry of hydrocarbons and alternative fuels is prerequisite to improving the efficiency of internal combustion engines, and developing new concepts for combustion engines and their fuels [START_REF] Yao | Progress and recent trends in homogeneous charge compression ignition (HCCI) engines[END_REF][START_REF] Saxena | Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits[END_REF]. Laboratory based reaction systems, such as flow reactors (FR) [START_REF] Kurimoto | Quantitative measurements of HO2/H2O2 and intermediate species in low and intermediate temperature oxidation of dimethyl ether[END_REF][START_REF] Guo | Measurements of H2O2 in low temperature dimethyl ether oxidation[END_REF], shock tubes (ST) [START_REF] Hanson | Recent advances in laser absorption and shock tube methods for studies of combustion chemistry[END_REF], rapid compression machines (RCM) [START_REF] Sung | Using rapid compression machines for chemical kinetics studies[END_REF], and jetstirred reactors (JSR) [START_REF] Herbinet | Progress in Understanding Low-Temperature Organic Compound Oxidation Using a Jet-Stirred Reactor[END_REF][START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF] are commonly used to examine the low temperature oxidation chemistry of fuels. Because of the well-defined physical properties and 0-D structure for simulation, the JSRs were coupled to various analytic tools that contributed significantly to exploring the low temperature oxidation mechanism and providing speciation data for kinetic model development [START_REF] Herbinet | Progress in Understanding Low-Temperature Organic Compound Oxidation Using a Jet-Stirred Reactor[END_REF][START_REF] Dagaut | A jet-stirred reactor for kinetic studies of homogeneous gas-phase reactions at pressures up to ten atmospheres (1 MPa)[END_REF][START_REF] Qi | Combustion chemistry probed by synchrotron VUV photoionization mass spectrometry[END_REF].

The JSR systems that are used by several labs in the combustion community are presented in Table 1. The JSR system currently being used functions at the pressure range of one to 40 bar.

Recent work by Zhao et al. [START_REF] Zhao | Studies of high-pressure n-butane oxidation with CO2 dilution up to 100 atm using a supercritical-pressure jet-stirred reactor[END_REF] developed a supercritical pressure JSR that can work at 100 bar.

The analytic methods include gas chromatography (GC) with a flame ionization detector (FID), a thermal conductivity detector (TCD) and a mass spectrometer detector, Fourier transform Infrared spectroscopy (FTIR), cavity ring down spectroscopy (CRDS), and mass spectrometry (MS). The GC and FTIR can analyze stable products, while the CRDS method detects H2O2 and HO2 radical [START_REF] Bahrini | Quantification of Hydrogen Peroxide during the Low-Temperature Oxidation of Alkanes[END_REF][START_REF] Djehiche | Quantitative Measurements of HO2 and Other Products of n-Butane Oxidation (H2O2, H2O, CH2O, and C2H4) at Elevated Temperatures by Direct Coupling of a Jet-Stirred Reactor with Sampling Nozzle and Cavity Ring-Down Spectroscopy (cw-CRDS)[END_REF]. The mass spectrometry system used can be categorized into two groups, according to the ionization source. The first group adopts photoionization like the synchrotron vacuum ultraviolet photoionization (SVUV-PI) with tunable photon energy from 7 -20 eV [START_REF] Qi | Combustion chemistry probed by synchrotron VUV photoionization mass spectrometry[END_REF], and single photoionization (SPI) with fixed photon energy of 10.5 eV [START_REF] Rodriguez | Measuring hydroperoxide chain-branching agents during n-pentane low-temperature oxidation[END_REF]. The time-of-flight mass spectrometer (TOF-MS) with a mass resolution from 1000 to 5000 is commonly used at the Hefei Light Source [START_REF] Zhou | The vacuum ultraviolet beamline/endstations at NSRL dedicated to combustion research[END_REF], Advanced Light Source [START_REF] Moshammer | Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether[END_REF], and LRGP-CNRS [START_REF] Rodriguez | Measuring hydroperoxide chain-branching agents during n-pentane low-temperature oxidation[END_REF]. Another group used chemical ionization coupled with a high mass resolution mass spectrometer (HRMS) such as the orbitrap mass spectrometer with mass resolution higher than 100,000, like the systems with atmospheric pressure chemical ionization (APCI) used at KAUST [START_REF] Wang | n-Heptane cool flame chemistry: Unraveling intermediate species measured in a stirred reactor and motored engine[END_REF][START_REF] Wang | Unraveling the structure and chemical mechanisms of highly oxygenated intermediates in oxidation of organic compounds[END_REF] and ICARE-CNRS [START_REF] Belhadj | Oxidation of di-n-butyl ether: Experimental characterization of low-temperature products in JSR and RCM[END_REF][START_REF] Belhadj | Experimental characterization of n-heptane low-temperature oxidation products including keto-hydroperoxides and highly oxygenated organic molecules (HOMs)[END_REF]. Moreover, recent work by Battin-Leclerc et al. [START_REF] Battin-Leclerc | Chemistry deriving from OOQOOH radicals in alkane low-temperature oxidation: A first combined theoretical and electron-ion coincidence mass spectrometry study[END_REF][START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF] coupled a JSR to a double imaging photoelectron photoion COincidence (i 2 PEPICO) scheme at the DESIRS beamline of SOLEIL. The coincident mass-tagged threshold photoelectron spectra (TPES) produced better identification of low temperature oxidation intermediates. Previous studies have shown that the JSR, coupled with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS), is sufficiently powerful to reveal species distribution and reaction mechanisms of low temperature oxidation chemistry, especially complex oxygenated intermediates with carbonyl, carboxy, and peroxy groups [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF][START_REF] Qi | Combustion chemistry probed by synchrotron VUV photoionization mass spectrometry[END_REF]. Currently, only the JSR system at one bar is coupled to the SVUV-PIMS system. Recent studies by Dagaut et al. [START_REF] Belhadj | Oxidation of di-n-butyl ether: Experimental characterization of low-temperature products in JSR and RCM[END_REF][START_REF] Belhadj | Experimental characterization of n-heptane low-temperature oxidation products including keto-hydroperoxides and highly oxygenated organic molecules (HOMs)[END_REF][START_REF] Belhadj | Experimental Characterization of Tetrahydrofuran Low-Temperature Oxidation Products Including Ketohydroperoxides and Highly Oxygenated Molecules[END_REF] analyzed low temperature oxidation intermediates from the high pressure JSR at ten bar. Products were collected in a trap, separated by high performance liquid chromatography (HPLC) and analyzed by an atmospheric pressure chemical ionization orbitrap mass spectrometer. Their study showed that low temperature oxidation at high pressure formed a highly complex species pool (for example hydroperoxides (C7H16O2), keto-hydroperoxides (C7H14O3), cyclic ethers (C7H14O), carboxylic acids (C2H4O2, C3H6O2, C4H8O2), ketones (C3-5H6-10O), diones (C7H12O2)), and highly oxygenated molecules (C7H14O5, C7H14O7, C7H14O9, C7H14O11) in n-heptane low temperature oxidation [START_REF] Belhadj | Experimental characterization of n-heptane low-temperature oxidation products including keto-hydroperoxides and highly oxygenated organic molecules (HOMs)[END_REF], indicating that current understanding of the low temperature oxidation mechanism and kinetic models require improvement. However, Dagaut's work is not online analysis, and chemical ionization makes quantification of the intermediates a challenge. An online measurement, comprehensive species analysis, and species quantification system is needed to study low temperature oxidation mechanism at high pressures.

This work develops a variable pressure jet-stirred reactor (VP-JSR) system working from one to ten bar, and couples the VP-JSR to the SVUV-PIMS at the Hefei Light Source. Molecular beam sampling was used to probe the reaction intermediates, which was analyzed by the SVUV-PIMS. This paper focuses on development of the sampling system, validation of the experimental setup, and a preliminary study of n-heptane low temperature oxidation from one to ten bar. The kinetic model of n-heptane low temperature oxidation by Zhang et al. [START_REF] Zhang | An updated experimental and kinetic modeling study of n-heptane oxidation[END_REF] was used for comparison with the experimental data obtained in the present study.

Experimental method and kinetic modeling

The VP-JSR used in this work was similar to the high pressure JSR used in LRGP-CNRS [START_REF] Herbinet | An experimental and modeling study of the oxidation of n-heptane, ethylbenzene, and n-butylbenzene in a jet-stirred reactor at pressures up to 10 bar[END_REF].

The JSR is composed of fused silica, with a diameter of 52 mm and a volume of 65.4 cm 3 .

Thermocoax resistances were used to heat the preheating zone and the reactor. Temperature was monitored by K-type thermocouples fixed between the Thermocoax resistances and the reactor wall. The temperature of the reaction was measured by another K-type thermocouple.

The extremity of the thermocouple was located in the center of the reactor. From the outlet of the reactor, a thermocouple was inserted to measure the temperature distribution inside the JSR.

Good temperature homogeneity was observed, except for a slight decrease of the temperature at the edge of the reactor outlet. The uncertainty of the reaction temperature measurement was estimated to be ±10 K.

In the experiment, the flow rate of the liquid fuel (e.g., n-heptane) was controlled by a HPLC plunger pump (model FL2200, Fuli Analytical Instruments Inc., Zhejiang, China); the flow rate of the gasses (O2 and argon) was measured using MKS Ⓡ mass flow controllers. The liquid was injected into a handmade vaporizer and vaporized at 373 K with argon as the dilution. Argon and O2 were mixed and flowed into the preheating zone, and then to the reactor. The VP-JSR was set in a stainless-steel vessel; its pressure was adjustable from one to ten bar. Pressure was controlled by a needle valve. There were two holes with a diameter of 4 mm at the reactor outlet, to obtain the pressure balance between the inside and the outside of the VP-JSR. An auxiliary argon was made to flow into the steel vessel to flush the stainless-steel chamber.

The outlet of the JSR used in Hefei was shorter than the VP-JSR used in LRGP-CNRS; this modification was made to shorten the distance between the sampling nozzle and the edge of the reactor. The quartz nozzle could not resist the pressure difference of ca. ten bar in the ten bar experiment. Here, a stainless-steel nozzle was used for the sampling, as shown in Fig. 1a. The nozzle was coated by a Al2O3 layer to reduce catalytic reactions that could be caused by the metals in the stainless-steel nozzle.

The VP-JSR was coupled to two SVUV-PIMS systems at the Hefei Light Source. The first setup was installed at the combustion and flame beamline (BL03U). For this design, a three-stage sampling system was developed (Fig. 1b). This setup was used to study the low temperature oxidation of n-heptane at five and ten bar. The reaction products were first sampled using the stainless-steel nozzle, then the quartz nozzle, and finally to the nickel skimmer. The orifice diameter of the stainless-steel nozzle, quartz nozzle and nickel skimmer was 50 μm, 250 μm, and two mm, respectively. Moreover, the direct simulation Monte Carlo (DSMC) method [START_REF] Bird | Approach to Translational Equilibrium in a Rigid Sphere Gas[END_REF] was used to simulate the three-stage sampling system. The high mach number of argon expanding from the 50 μm stainless-steel nozzle (ca. 6 M) and the 250 μm nozzle (ca. 4 M) confirmed that the three-stage sampling system was molecular beam. Details of the simulation are provided in the Supplemental Material. For the one bar experiment, in the three-stage sampling the signal of the products was weak. The one bar experiment was conducted using two-stage sampling, as in the work of Herbinet et al. [START_REF] Herbinet | Experimental and modeling investigation of the low-temperature oxidation of n-heptane[END_REF]. The mass resolution of the time-of-flight mass spectrometer (TOF-MS) at the combustion and flame beamline was ∼2500 at m/z 40. For a better separation of the C/H/O composition of the low temperature oxidation intermediates, the five and ten bar experiment was also performed at the atomic and molecule physics beamline (BL09U). Mass spectra at selected temperatures were measured by TOF-MS with a mass resolution of ∼4000 at m/z 40 [START_REF] Xu | Comprehensive study of the low-temperature oxidation chemistry by synchrotron photoionization mass spectrometry and gas chromatography[END_REF]. In this experiment, the setup used two-stage sampling, as shown in Fig. 1c. Reaction products were first sampled using the stainless-steel nozzle, then directly to the nickel skimmer.

The orifice diameters of the stainless-steel nozzle and the nickel skimmer were 50 μm and two mm, respectively. The one bar experiment at the atomic and molecule physics beamline was also repeated. Mole fraction profiles measured by the two setups were consistent. Experimental conditions of the n-heptane low temperature oxidation investigated in this work are shown in Table 2. The first and fourth series were performed under the same conditions as those used in Refs. [START_REF] Herbinet | Experimental and modeling investigation of the low-temperature oxidation of n-heptane[END_REF] and [START_REF] Herbinet | An experimental and modeling study of the oxidation of n-heptane, ethylbenzene, and n-butylbenzene in a jet-stirred reactor at pressures up to 10 bar[END_REF], respectively, to validate the present setup. The data evaluation method of SVUV-PIMS was reported in previous work [START_REF] Herbinet | Experimental and modeling investigation of the low-temperature oxidation of n-heptane[END_REF][START_REF] Wang | Experimental Investigation of the Low Temperature Oxidation of the Five Isomers of Hexane[END_REF]. The mole fraction of the fuel was known at the unreacted temperature, its temperature-dependent mole fraction was calculated according to the corresponding signal and the gas expansion coefficient at each temperature. The mole fraction of the products was then obtained with the fuel as a reference.

Uncertainty for the quantified mole fraction was ±10% for the reactants, and ±20% for the species with a known photoionization cross section (PICS), and a factor of two for the species with unknown PICS. Mole fraction profiles discussed in this work are provided in the Supplementary Material. The n-heptane kinetic model by Zhang et al. [START_REF] Zhang | An updated experimental and kinetic modeling study of n-heptane oxidation[END_REF] was used to simulate the experiment; simulation was performed using Chemkin-Pro-software with the perfect-stirred reactor (PSR) module [START_REF]CHEMKIN-PRO 15112[END_REF]. Simulations were performed by fixing the reaction temperature, and the experimental conditions were used as input.

Results and discussion

The experiment setup was first validated by comparison with n-heptane low-temperature oxidation data in the literature, as presented in Sections 3.1 and 3.2. Then, the n-heptane lowtemperature oxidation at 1, 5, and 10 bar was carried out in this setup, as reported in Section 3.3.

The reactivity of n-heptane oxidation, the species pool, and their distribution at different pressures are discussed. Furthermore, the prediction of the n-heptane kinetic model is compared to the newly obtained dataset and the observed deviations are discussed.

Validation of the setup at 1 bar

n-Heptane low temperature oxidation at one bar was studied by Herbinet et al. [START_REF] Herbinet | Experimental and modeling investigation of the low-temperature oxidation of n-heptane[END_REF], with a fuel initial mole fraction of 0.005 and a residence time of 2 s. The experiment was repeated by Zhang et al. [START_REF] Zhang | An updated experimental and kinetic modeling study of n-heptane oxidation[END_REF] in 2016, and Gao et al. [START_REF] Gao | Low to intermediate temperature oxidation studies of dimethoxymethane/n-heptane blends in a jet-stirred reactor[END_REF] in 2019, using GC. Good agreement was observed The C7H12O2, were likely to be the dione intermediate, with two keto groups, and were probably produced from the keto-hydroperoxides, or from the activated OOQOOH radical [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF].

Like C7H14O, the C7H14O2 was not quantified; its signal profile was presented for comparison. As shown in Fig. 2, the GC measured C7H12O2 profile was different from the measurement by SVUV-PIMS in this work; this difference was also observed by Herbinet et al. [START_REF] Herbinet | Experimental and modeling investigation of the low-temperature oxidation of n-heptane[END_REF]. These authors stated that GC measured C7H12O2 may arise from decomposition of keto-hydroperoxide in the transfer line between the JSR and the GC, because the GC measured C7H12O2 had a temperaturedependent profile similar to the keto-hydroperoxide. The signal profile of C7H12O2, measured by SVUV-PIMS in Herbinet et al., is also presented here; good agreement was observed for the two SVUV-PIMS data of C7H12O2.

Peroxide intermediates such as H2O2 and keto-hydroperoxide (C7H14O3), measured by SVUV-PIMS, are also presented in Fig. 2. H2O2 was quantified using the PICS reported by Dodson et al. [START_REF] Dodson | VUV Photoionization Cross Sections of HO2, H2O2, and H2CO[END_REF]. In the n-heptane low temperature oxidation work by Rodriguez et al. [START_REF] Rodriguez | Hydroperoxide Measurements During Low-Temperature Gas-Phase Oxidation of n-Heptane and n-Decane[END_REF], the mole fraction of H2O2 under the same experimental conditions was measured using continuous wave cavity ring-down spectroscopy (cw-CRDS). Although the temperature dependent profile of the two datasets was similar, there was a deviation factor of three for the mole fraction of H2O2. The source of the discrepancy was unclear, one possible reason was the uncertainty of the PICS of H2O2. The keto-hydroperoxide was not quantified; its signal profile is presented. The n-heptane low temperature model of Zhang et al. [START_REF] Zhang | An updated experimental and kinetic modeling study of n-heptane oxidation[END_REF] satisfactorily predicted the measured species (mole fraction profile and signal profile) in Fig. 2, except that the reactivity of n-heptane from 500 to 600 K was slightly over-predicted; the mole fraction of ethylene and 2-ethyl-5-methyl tetrahydrofuran was over-predicted, and the mole fraction of C7H12O2 dione was under-predicted. was similar. The present SVUV-PIMS experiment not only probed these species but also a much more detailed species pool for the n-heptane low temperature oxidation at ten bar. They could be categorized into groups with a molecular formula of C7H14Ox (x=0-3), C7H12Ox (x=0-4), C7H10Ox (x=0-4), CnH2n (n=2-6), CnH2n-2 (n=2-6), CnH2n+2O (n=1-3, 6), CnH2nO (n=1-6), CnH2n-2O (n=2-6), CnH2n-4O (n=4-6), CnH2n+2O2 (n=0-3, 7), CnH2nO2 (n=1-4, 6), CnH2n-2O2 (n=2-4), and CnH2n-4O2 (n=4-5). This species pool was similar to that measured during the n-heptane low temperature oxidation at one bar in Ref. [START_REF] Wang | n-Heptane cool flame chemistry: Unraveling intermediate species measured in a stirred reactor and motored engine[END_REF]. They were mainly alkenes, dienes, aldehyde/keto compounds, olefinic aldehyde/keto compounds, diones, cyclic ethers, peroxides, carboxylic acids, and alcohols/ethers. The free radicals were not studied in this work because they are quickly quenched at high pressure. However, the model prediction of the C7H12O2 diones was much lower than the experimental measurement, the simulated result was multiplied by a factor of 1000 to compare with the experimental measurement. The incomplete reaction pathways, such as the H-abstraction of keto-hydroperoxides [START_REF] Ranzi | New reaction classes in the kinetic modeling of low temperature oxidation of n-alkanes[END_REF][START_REF] Xing | Degradation of Carbonyl Hydroperoxides in the Atmosphere and in Combustion[END_REF], the reactions of the C7H14O cyclic ethers [START_REF] Wang | n-Heptane cool flame chemistry: Unraveling intermediate species measured in a stirred reactor and motored engine[END_REF], and the activated OOQOOH radical [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF], may have contributed to the dione formation.

Furthermore, peroxide intermediates such as H2O2, methyl hydroperoxide, ethyl hydroperoxide, and the keto-hydroperoxides in n-heptane low temperature oxidation at ten bar were also measured by SVUV-PIMS (Fig. 4). Methyl hydroperoxide was quantified using photoionization cross calculated by Moshammer et al. [START_REF] Moshammer | Quantification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Elusive Intermediates during Low-Temperature Oxidation of Dimethyl Ether[END_REF]. Ethyl hydroperoxide and ketohydroperoxides were not quantified, only their signal profiles are presented. The kinetic model satisfactorily predicted the mole fraction of methyl hydroperoxide, the signal profiles of ethyl hydroperoxide and keto-hydroperoxides. However, the mole fraction of H2O2 was overestimated.

As discussed above, the H2O2 mole fraction measured at one bar by SVUV-PIMS was a factor of three lower than the cw-CRDS measurement. If the mole fraction is multiplied by a factor of three, it is close to the simulation at 650 K. However, the model predicted a rapid formation of H2O2 from 750 to 800 K, while the experiment did not behave this way.

n-heptane low-temperature oxidation from 1 to 10 bar

Discussions in Sections 3.1 and 3.2 establish that the VP-JSR, coupled to the SVUV-PIMS system, is a new tool suitable to study low temperature oxidation chemistry. Using this design, we studied the low temperature oxidation of n-heptane at one, five and ten bar, to evaluate the pressure effect on low temperature oxidation. The experimental conditions are shown in Table 2, i.e., n-heptane initial mole fraction of 0.005, residence time of 2 s, and equivalence ratio of 1.0.

The result showed that oxidations at one, five and ten bar had similar species categories/identification, but showed strong differences in reactivity and in the distribution of the mole fraction profiles, presented in the discussion to follow.

Figure 5 compares temperature-dependent profiles of reactants, final products, and initial oxidation intermediates of n-heptane from one to ten bar. The mole fraction of n-heptane, O2, CO, CO2, and H2O was quantified; the mole fraction of keto-hydroperoxides was not quantified because its PICS was unknown. Here, their relative ratio at one, five and ten bar is displayed by normalizing their signal with the initial signal of n-heptane (i.e., at 500 K) at 11.5 eV photon energy.

From one to ten bar, reactivity was promoted in both the initial reaction temperature and the conversion of the fuel. The higher reactivity, between 550 and 600 K, increased reaction reactivity further in the negative temperature coefficient (NTC) zone. Compared to the one bar experiment, the five and ten bar low temperature oxidation showed weak NTC behavior. Higher reactivity was also indicated by the formation of CO, H2O, CO2, C2H4, and C3H6, especially in the 650-770 K range. The higher reactivity between 550 and 600 K at higher pressure was also confirmed by the higher formation of keto-hydroperoxides. At five and ten bar, the initial formation temperature of the keto-hydroperoxides was lower than that at one bar; higher pressure also promoted the formation of the keto-hydroperoxides, as shown in Fig. 5h. The kinetic model satisfactorily predicted the pressure effect on the conversion of n-heptane and O2, and the formation of CO, H2O, C2H4, and keto-hydroperoxides. It was noted that the temperature corresponding to the maximum mole fraction of keto-hydroperoxides was on the order of one bar > five bar > ten bar, but the model prediction at ten bar was slightly higher than the measurement. Moreover, the model under predicted the formation of CO2 (Fig. 5d) and over predicted the formation of formaldehyde (Fig. 5g) at five and ten bar. These two intermediates involved in the reactions of the base model-and a thorough analysis of the mechanism-is needed in future work. Signal profile (*) of keto-hydroperoxides is presented.

The pressure effect on the formation of H2O2 and formic acid is shown in Fig. 6. The experiment proved that formation of H2O2 was promoted when the pressure was increased from one to ten bar. The promoting effect of increased pressure on H2O2 formation was predicted well by the simulation, however, the kinetic model significantly over predicted the mole fraction of H2O2 (Fig. 6a). A large difference was observed in the NTC zone at five and ten bar (ca. a factor of 20 at 750 K). The ROP analysis revealed that H2O2 was predominantly formed from the selfreaction of HO2 radicals at 750 K, which was related to the reactions in the base model, such as HCO, CH2O, CH3O, and CH3O2. Evaluating these reactions in the base model is outside the scope of this work.

The experiments showed no apparent pressure effect on the formation of formic acid from one to ten bar, while such an effect was not predicted by the simulation, where a significant promotion of formic acid formation at five and ten bar was observed. Also, the mole fraction of formic acid was over predicted by a factor of five, nine, and 11 at one, five and ten bar, respectively.

ROP analysis showed that formic acid was predominantly formed from the following reaction steps:

CH2O + OH = HOCH2O (R1) HOCH2O = HOCHO + H (R2)

As mentioned by Wang et al. [START_REF] Wang | n-Heptane cool flame chemistry: Unraveling intermediate species measured in a stirred reactor and motored engine[END_REF], the rate constant of R1 in the model was overestimated by adopting the rate constant from Taylor et al. [START_REF] Taylor | Kinetic and mechanistic studies of the reaction of hydroxyl radicals with acetaldehyde over an extended temperature range[END_REF]. This caused high carbon flux to formic acid and explained the overestimation of formic acid at one, five and ten bar. Furthermore, our experiment showed a slight effect of pressure on the formation of formaldehyde (Fig. 5g), but the model predicted that the formation of formaldehyde was promoted at five and ten bar, which further promoted formic acid formation at five and ten bar (Fig. 6b).

The pressure effect on the formation of C7H12O2 intermediate is shown in Fig. 7. The GC measurement by Herbinet et al. [START_REF] Herbinet | An experimental and modeling study of the oxidation of n-heptane, ethylbenzene, and n-butylbenzene in a jet-stirred reactor at pressures up to 10 bar[END_REF] measured two dione intermediates (i.e., 2,4-heptadione and 3,5-heptadione) during n-heptane low temperature at ten bar. The mole fraction of 2,4-heptadione was much higher than that of 3,5-heptadione. In this work, it was not possible to separate the isomers or to quantify them. The relative ratio of the total C7H12O2 in Fig. 7 revealed that the effect of pressure on its formation was negligible. This observation was contrary to the conclusion of Herbinet et al. [START_REF] Herbinet | An experimental and modeling study of the oxidation of n-heptane, ethylbenzene, and n-butylbenzene in a jet-stirred reactor at pressures up to 10 bar[END_REF], who stated that high pressure promoted the formation of diones. The simulation for the C7H12O2 diones in Fig. 7 also indicated that high pressure was favorable for dione formation. One possible explanation is that the SVUV-PIMS signal of C7H12O2 included other intermediates, such as isomers with a carbonyl and a cyclic ether functional group, proposed by Wang et al. [START_REF] Wang | n-Heptane cool flame chemistry: Unraveling intermediate species measured in a stirred reactor and motored engine[END_REF]. Error bar for the experimental data in this work is given.

Summary and conclusion

In this work a variable pressure JSR (up to ten bar) system was developed, which adopted the molecular beam sampling and synchrotron radiation photoionization mass spectrometry for species identification and quantification.

Figure 1 .

 1 Figure 1. The stainless-steel nozzle with Al2O3 coating and the mach number distribution map of the Ar after the sampling by the 50 μm stainless steel nozzle and 250 μm quartz nozzle (a), the schematic of the VP-JSR coupled to the SVUV-PIMS system at Combustion and Flame beamline (b), and at Atomic and Molecule Physics beamline (c) of Hefei Light Source.

  among these three datasets, making these experimental conditions a good validation target for the present VP-JSR. In the Herbinet et al. data, both the GC and SVUV-PIMS were used to probe the low temperature oxidation intermediates. When GC data were not available in the following comparison, only the SVUV-PIMS data in Herbinet et al. are presented.A comparison of mole fraction profiles of the fuel and typical low temperature oxidation intermediates is shown in Fig.2. The measured n-heptane profile agreed well with the measurement of Herbinet et al.[START_REF] Herbinet | Experimental and modeling investigation of the low-temperature oxidation of n-heptane[END_REF]. C2H4, CH2O, and CH3OH were quantified using the PICS measured by Cool et al.[START_REF] Cool | Selective detection of isomers with photoionization mass spectrometry for studies of hydrocarbon flame chemistry[END_REF][START_REF] Cool | Photoionization cross sections for reaction intermediates in hydrocarbon combustion[END_REF] and Dodson et al.[START_REF] Dodson | VUV Photoionization Cross Sections of HO2, H2O2, and H2CO[END_REF]. They were in good agreement with Herbinet et al., considering the experimental uncertainty. The mole fraction for CH2O, measured by CRDS in Rodriguez et al.[START_REF] Rodriguez | Hydroperoxide Measurements During Low-Temperature Gas-Phase Oxidation of n-Heptane and n-Decane[END_REF], is also presented for comparison. The mole fraction measured by CRDS was slightly higher than the simulation and the PIMS measurement in this work. For the cyclic ether isomers (C7H14O), we presented the mole fraction profile of 2-ethyl-5-methyl tetrahydrofuran, measured by GC, the major component of the cyclic ether in n-heptane low temperature oxidation. The SVUV-PIMS data could not be quantified because their PICSs were unknown. The signal profile of C7H14O measured by SVUV-PIMS was compared to the GC measured mole fraction.

Figure 2 .

 2 Figure 2. Data comparison of n-heptane low-temperature oxidation at 1 bar, in this work, with the measurement by Herbinet et al. [41] and Rodriguez et al. [27] (P = 1 bar, τ = 2 s, ф = 1.0 and 𝑥 𝑓𝑢𝑒𝑙 𝑖𝑛𝑙𝑒𝑡 = 0.005). Error bar for the experimental data in this work is given. SVUV-PIMS data of C7H14O, C7H12O2, and C7H14O3 are signal profiles (*).

3. 2

 2 Validation of the setup at 10 bar n-Heptane low temperature oxidation at ten bar was studied by Dagaut et al.[START_REF] Dagaut | Experimental study of the oxidation of n-heptane in a jet stirred reactor from low to high temperature and pressures up to 40 atm[END_REF], with the initial fuel mole fraction of 0.001, equivalence ratio of 1.0, and residence time of 1 s. Similar work was done by Herbinet et al.[START_REF] Herbinet | An experimental and modeling study of the oxidation of n-heptane, ethylbenzene, and n-butylbenzene in a jet-stirred reactor at pressures up to 10 bar[END_REF] at ten bar, with initial fuel mole fraction of 0.001, equivalence ratio of 1.0, and residence time of 2 s. The experiment by Herbinet et al. was repeated in this work. The experiment with 1 s residence time was not repeated because it required a total flow rate higher than 20 L/min., which exceeds the range of the mass flow controllers used in this work.

Figure 3

 3 Figure3shows the mole fraction profiles of n-heptane, O2, CO, CO2, H2O, formaldehyde

Figure 3 .

 3 Figure 3. Comparison of data of n-heptane low-temperature oxidation at 10 bar measured in this work, by Herbinet et al. [28], and by the simulation of Zhang et al. [39] (P = 10 bar, τ = 2 s, ф = 1.0 and 𝑥 𝑓𝑢𝑒𝑙 𝑖𝑛𝑙𝑒𝑡 = 0.001). Simulated mole fraction of C7H12O2 in (l) is multiplied by a factor of 1000. Error bar for the experimental data in this work is given. We note that the SVUV-PIMS data for C7H14O and C7H12O2 is signal profile (*) scaled to match the mole fraction profile measured by GC.

Figure 4 .

 4 Figure 4. Experimental (symbols) and simulated (lines) profiles of H2O2 (a), methyl hydroperoxide (b), ethyl hydroperoxide (c), and keto-hydroperoxides (d) in n-heptane lowtemperature oxidation (P = 10 bar, τ = 2 s, ф = 1.0 and 𝑥 𝑓𝑢𝑒𝑙 𝑖𝑛𝑙𝑒𝑡 = 0.001). Error bar for the experimental data in this work is given. The signal profiles (*) of ethyl hydroperoxide and ketohydroperoxides are presented.

Figure 5 .

 5 Figure 5. Experimental (symbols) and simulated (lines) profile of n-heptane, O2, CO, CO2, H2O, C2H4, CH2O, and keto-hydroperoxides in n-heptane low-temperature oxidation (P = 1, 5, and 10 bar, τ = 2 s, ф = 1.0 and 𝑥 𝑓𝑢𝑒𝑙 𝑖𝑛𝑙𝑒𝑡 = 0.005). Error bar for the experimental data in this work is given.

Figure 6 .

 6 Figure 6. Experimental (symbols) and simulated (lines) profile of H2O2 and formic acid in n-heptane low-temperature oxidation (P = 1, 5, and 10 bar, τ = 2 s, ф = 1.0 and 𝑥 𝑓𝑢𝑒𝑙 𝑖𝑛𝑙𝑒𝑡 = 0.005).

Figure 7 .

 7 Figure 7. Experimental (symbols) and simulated (lines) profile of C7H12O2 in n-heptane lowtemperature oxidation (P = 1, 5, and 10 bar, τ = 2 s, ф = 1.0 and 𝑥 𝑓𝑢𝑒𝑙 𝑖𝑛𝑙𝑒𝑡 = 0.005). Signal profile (*) of C7H12O2 is presented.

  The setup was validated by repeating experiments on the low temperature oxidation of n-heptane at one and ten bar. The good agreement between the data from this work and in the literature showed that the VP-JSR system is suitable for study of low temperature oxidation chemistry. Compared to customary JSR systems that use GC, GC-MS, and FTIR to measure low temperature oxidation products, the molecular beam sampling, coupled with synchrotron vacuum ultraviolet photoionization mass spectrometry, produced a more comprehensive measurement of the intermediates-especially the peroxides, carboxylic acids, and oxygenated intermediates with multiple functional groups. The development of the VP-JSR-SVUV-PIMS setup in this work is useful to the study of low temperature oxidation mechanisms, providing a comprehensive dataset for examining chemical kinetic models. The kinetic model of Zhang et al. could predict n-heptane low temperature oxidation at one, five and ten bar, but needs to improve the reaction mechanism for some specific species, especially hydrogen peroxide, formic acid, carbon dioxide, the cyclic ether, and the dione intermediates. A detailed analysis of the pressure effect on the low temperature oxidation could be the goal of future work.

Table 1 .

 1 The typical JSR systems used in the combustion community

	Research Team	JSR volume (cm 3 )	Working pressure (bar)	Analytic method	Reference
		35	1-40	GC/FTIR	[23,24]
	ICARE-CNRS	37	1	CRDS	[13]
		35	10	APCI-Orbitrap MS	[19,20]
		85	1	GC/FTIR	[7,25]
		85	1	CRDS	[12,26]
	LRGP-CNRS	85	1	SPI-MS	[14,27]
		65.4	1-10	GC	[28,29]
	KAUST	76 76	1 1	GC CI-Orbitrap MS	[30] [17,18]
	ALS	33.5	1	SVUV-PIMS	[16,31,32]
	NSRL	102	1	SVUV-PIMS	[15,33]
	SOLEIL	60	1	PEPICO	[21,22]
	IET-CAS	65	1	GC	[34]
	SJTU	103	1	GC	[35]
	XJTU	87	1	GC	[36,37]
	Princeton	0.5	100	GC	[11]

Table 2 .

 2 The experimental conditions of n-heptane low-temperature oxidation. The highlighted conditions were used to validate the experimental setup.

	Fuel	Pressure	Residence time	Equivalence	Fuel mole fraction	O2 mole fraction
		(bar)	(s)	ratio
		1	2	1.0	0.005	0.0525
		5	2	1.0	0.005	0.0525
	n-heptane
		10	2	1.0	0.005	0.0525
		10	2	1.0	0.001	0.0105

Table 3

 3

presents the species pool measured by Dagaut et al., Herbinet et al., and this group. Here, only the molecular formula measured by GC and SVUV-PIMS was compared. It can be seen that the species pool measured by Dagaut et al. and Herbinet et.al. (mainly stable intermediates)

Table 3 .

 3 Intermediate species measured during n-heptane oxidation in a JSR at 10 bar. The initial n-heptane mole fraction is 0.001, and equivalence ratio is 1.0. The measurements by Dagaut et al.[START_REF] Dagaut | Experimental study of the oxidation of n-heptane in a jet stirred reactor from low to high temperature and pressures up to 40 atm[END_REF] and Herbinet et al.[START_REF] Herbinet | An experimental and modeling study of the oxidation of n-heptane, ethylbenzene, and n-butylbenzene in a jet-stirred reactor at pressures up to 10 bar[END_REF] were performed using GC/GC-MS, while the measurement in this work was realized with SVUV-PIMS.

	m/z	Dagaut	Herbinet	This work	m/z Dagaut Herbinet	This work
	26	--	--	C2H2	86 C5H10O	--	C4H6O2/C5H10O
	28	C2H4	C2H4	C2H4	88	--	--	C4H8O2
	30	CH2O	CH2O	CH2O	90	--	--	C4H10O2
	32	--	CH4O	CH4O	94	--	--	C7H10
	34	--	--	H2O2	96	--	--	C7H12/C6H8O/C5H4O2
	40	--	--	C3H4	98	C7H14	C7H14	C7H14/C6H10O/C5H6O2
	42	C3H6	C3H6	C3H6/C2H2O	100	--	--	C6H12O
	44	C2H4O	C2H4O	C2H4O	102	--	--	C6H14O
	46	--	--	CH2O2/C2H6O	110	--	--	C7H10O
	48	--	--	CH4O2	112	--	--	C7H12O
	54	--	--	C4H6	114 C7H14O	C7H14O	C7H14O
	56	C4H8/C3H4O	C4H8	C4H8/C3H4O	116	--	--	C6H12O2
	58	C3H6O	C3H6O	C3H6O/C2H2O2	118	--	--	C5H10O3
	60	C2H4O2	--	C2H4O2/C3H8O	126	--	--	C7H10O2
	62	--	--	C2H6O2	128	--	C7H12O2	C7H12O2
	68	C5H8	--	C5H8/C4H4O	130	--	--	C7H14O2
	70 C5H10/C4H6O	C5H10	C5H10/C4H6O	132	--	--	C7H16O2
	72	C4H8O	C4H8O	C4H8O/C3H4O2	140	--	--	C7H8O3
	74	--	--	C3H6O2	142	--	--	C7H10O3
	76	--	--	C3H8O2	144	--	--	C7H12O3
	80	--	--	C6H8	146	--	--	C7H14O3
	82	--	--	C6H10/C5H6O	158	--	--	C7H10O4
	84	C6H12	--	C6H12/C5H8O/C4H4O2 160	--	--	C7H12O4

Acknowledgements

This work was supported by National Natural Science Foundation of China (51976208, 61871172), by Hefei Science Center,CAS (2020HSC-KPRD001), and by the DNL Cooperation Fund, CAS (DNL202005).