Shock tube/time-of-flight mass spectrometer for high temperature kinetic studies

Initiation and Carbene Induced Radical Chain Reactions in CH2F2 Pyrolysis

High temperature dissociations of organic molecules typically involve a competition between radical and molecular processes. In this work, we use a modeling, experiment, theory (MET) framework to characterize the high temperature thermal dissociation of CH2F2, a flammable hydrofluorocarbon (HFC) that finds widespread use as a refrigerant. Initiation in CH2F2 proceeds via a molecular elimination channel; CH2F2→CHF+HF. Here we show that the subsequent self-reactions of the singlet carbene, CHF, are fast multichannel processes and a facile source of radicals that initiate rapid chain propagation reactions. These have a marked influence on the decomposition kinetics of CH2F2. The inclusion of these reactions brings the simulations into better agreement with the present and literature experiments. Additionally, flame simulations indicate that inclusion of the CHF+CHF multichannel reaction leads to a noticeable enhancement in predictions of laminar flame speeds, a key parameter that is used to determine the flammability of a refrigerant.

External standard calibration method for high-repetition-rate shock tube kinetic studies with synchrotron-based time-of-flight mass spectrometry

The signal levels observed from mass spectrometers coupled by molecular beam sampling to shock tubes are impacted by dynamic pressures in the spectrometer due to rapid pressure changes in the shock tube. Accounting for the impact of the pressure changes is essential if absolute concentrations of species are to be measured. Obtaining such a correction for spectrometers operated with vacuum ultra violet photoionization has been challenging. We present here a new external calibration method which uses VUV-photoionization of CO2 to develop time-dependent corrections to species concentration/time profiles from which kinetic data can be extracted. The experiments were performed with the ICARE-HRRST (high repetition rate shock tube) at the DESIRS beamline of synchrotron SOLEIL. The calibration experiments were performed at temperatures and pressures behind reflected shock waves of 1376 ± 12 K and 6.6 ± 0.1 bar, respectively. Pyrolytic experiments with two aromatic species, toluene (T5 = 1362 ± 22 K, P5 = 6.6 ± 0.2 bar) and ethylbenzene (T5 = 1327 ± 18 K, P5 = 6.7 ± 0.2 bar), are analyzed to test the method. Time dependent concentrations for molecular and radical species were corrected with the new method. The resulting signals were compared with chemical kinetic simulations using a recent mechanism for pyrolytic formation of polycyclic aromatic hydrocarbons. Excellent agreement was obtained between the experimental data and simulations, without adjustment of the model, demonstrating the validity of the external calibration method.

High-Temperature Dissociation of Neopentanol: Shock Tube/Photoionization Mass Spectrometry Studies

The pyrolysis mechanism of 2,2-dimethylpropan-1-ol (neopentanol) has been investigated at high temperatures (1128-1401K) and high pressures (5 and 15 bar). The experiments were performed in a miniature shock tube coupled to a time-of-flight mass spectrometer. Cations were generated by tunable vacuum ultraviolet photoionization resulting in multidimensional data sets containing mass and photoionization spectra and the time histories of species. At the elevated temperatures and pressures of this work, neopentanol was determined to dissociate primarily by the scission of a C-C bond yielding tert-butyl and hydroxymethyl radicals. These promptly form isobutene and formaldehyde by H-atom elimination. In the structurally similar molecule neopentane, roaming radical reactions have previously been found to be important under conditions close to the present work (1260-1459 K, 1.1 bar). There are two possible roaming radical reactions for neopentanol. However, no experimental evidence for these reactions was found at the elevated pressures in this study, and the dissociation of neopentanol is dominated by bond scission yielding radical products.

A rapid compression machine coupled with time-resolved molecular beam mass spectrometry for gas-phase kinetics studies

Rapid compression machines (RCMs) are used to simulate a single stroke of an internal combustion engine. After a high-speed compression process, a high-pressure and low-to-intermediate temperature condition can be obtained, under which ignition processes are usually studied. With the help of different diagnostic methods, the detailed speciation information of the ignition process can be quantified. In this study, the molecular beam mass spectrometry (MBMS) diagnostic method was applied on an RCM to realize time-resolved concentration profile measurements. To realize the combination between RCM and MBMS, particle dampers were adopted to suppress the vibrations of the RCM, and a novel flexible interface was designed to maintain a high vacuum, which ensured the safe and effective operation of a high-repetition-rate time-of-flight mass spectrometer (HRR-TOF-MS). The detailed configuration of this diagnostic method is presented, and the data acquisition system and data analysis method are described. The arrangement was validated through the investigation of the well-studied decomposition of 1,3,5-trioxane at temperatures between 697 and 777 K at 10 bars. The measured concentration profiles of 1,3,5-trioxane and formaldehyde were in good agreement with previous experimental and theoretical calculation results. The experimental results showed that the newly developed RCM coupled with the HRR-TOF-MS has advantages in time-resolved speciation measurements at low-to-intermediate temperatures and high pressures, and it can be applied in low-temperature combustion chemical kinetics studies.

Closed-loop recycling of rare liquid samples for gas-phase experiments

Many samples of current interest in molecular physics and physical chemistry exist in the liquid phase and are vaporized for use in gas cells, diffuse gas targets, or molecular gas jets. For some of these techniques, the large sample consumption is a limiting factor. When rare, expensive molecules such as custom-made chiral molecules or species with isotopic labels are used, wasting them in the exhaust line of the pumps is quite an expensive and inefficient approach. Therefore, we developed a closed-loop recycling system for molecules with vapor pressures below atmospheric pressure. Once filled, only a few valves have to be adjusted, and a cold trap must be moved after each phase of recycling. The recycling efficiency per turn exceeds 95%.

High-Temperature Unimolecular Decomposition of Diethyl Ether: Shock-Tube and Theory Studies

The unimolecular decomposition of diethyl ether (DEE; C₂H₅OC₂H₅) is considered to be initiated via a molecular elimination and a C-O and a C-C bond fission step: C₂H₅OC₂H₅ → C₂H₄ + C₂H₅OH (1), C₂H₅OC₂H₅ → C₂H₅ + C₂H₅O (2), and C₂H₅OC₂H₅ → CH₃ + C₂H₅OCH₂ (3). In this work, two shock-tube facilities were used to investigate these reactions via (a) time-resolved H-atom concentration measurements by H-ARAS (atomic resonance absorption spectrometry), (b) time-resolved DEE-concentration measurements by high repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS), and (c) product-composition measurements via gas chromatography/MS (GC/MS) after quenching the test gas. The experiments were conducted at temperatures ranging from 1054 to 1505 K and at pressures between 1.2 and 2.5 bar. Initial DEE mole fractions between 0.4 and 9300 ppm were used to perform the kinetics experiments by H-ARAS (0.4 ppm), GC/MS (200-500 ppm), and HRR-TOF-MS (7850-9300 ppm). The rate constants, k_total (k_total = k₁ + k₂ + k₃) derived from the GC/MS and HRR-TOF-MS experiments agree well with each other and can be described by the Arrhenius expression, k_total(1054-1467 K; 1.3-2.5 bar) = 10^12.81±0.22 exp(-240.27 ± 5.11 kJ mol^-1/RT) s^-1. From the H-ARAS experiments, overall rate constants for the bond fission channels, k₂₊₃ = k₂ + k₃ have been extracted. The k₂₊₃ data can be well described by the Arrhenius equation, k₂₊₃(1299-1505 K; 1.3-2.5 bar) = 10^14.43±0.33 exp(-283.27 ± 8.78 kJ mol^-1/RT) s^-1. A master-equation analysis was performed using CCSD(T)/aug-cc-pvtz//B3LYP/aug-cc-pvtz and CASPT2/aug-cc-pvtz//B3LYP/aug-cc-pvtz molecular properties and energies for the three primary thermal decomposition processes in DEE. The derived experimental data is very well reproduced by the simulations with the mechanism of this work. With regard to the branching ratios between bond fissions and elimination channels, uncertainties remain.

High temperature pyrolysis of 2-methyl furan

Experiments and theory reveal the complex dissociation of 2-methylfuran and the surprising importance of H-atom loss.

Shock-tube study of the decomposition of tetramethylsilane using gas chromatography and high-repetition-rate time-of-flight mass spectrometry

The decomposition of tetramethylsilane was studied in shock-tube experiments in a temperature range of 1270–1580 K and pressures ranging from 1.5 to 2.3 bar behind reflected shock waves combining GC/MS and HRR-TOF-MS.

A single-pulse shock tube coupled with high-repetition-rate time-of-flight mass spectrometry and gas chromatography for high-temperature gas-phase kinetics studies

Shock tubes are frequently used to investigate the kinetics of chemical reactions in the gas phase at high temperatures. Conventionally, two complementary arrangements are used where either time-resolved intermediate species measurements are conducted after the initiation of the reaction or where the product composition is determined after rapid initiation and quenching of the reaction through gas-dynamic processes. This paper presents a facility that combines both approaches to determine comprehensive information. A single-pulse shock tube is combined with high-sensitivity gas chromatography/mass spectrometry for product composition and concentration measurement as well as high-repetition-rate time-of-flight mass spectrometry for time-dependent intermediate concentration determination with 10 μs time resolution. Both methods can be applied simultaneously. The arrangement is validated with investigations of the well-documented thermal unimolecular decomposition of cyclohexene towards ethylene and 1,3-butadiene at temperatures between 1000 and 1500 K and pressures ranging from 0.8 to 2.4 bars. The comparison shows that the experimental results for both detections are in very good agreement with each other and with literature data.

High-Temperature Experimental and Theoretical Study of the Unimolecular Dissociation of 1,3,5-Trioxane

Unimolecular dissociation of 1,3,5-trioxane was investigated experimentally and theoretically over a wide range of conditions. Experiments were performed behind reflected shock waves over the temperature range of 775-1082 K and pressures near 900 Torr using a high-repetition rate time of flight mass spectrometer (TOF-MS) coupled to a shock tube (ST). Reaction products were identified directly, and it was found that formaldehyde is the sole product of 1,3,5-trioxane dissociation. Reaction rate coefficients were extracted by the best fit to the experimentally measured concentration-time histories. Additionally, high-level quantum chemical and RRKM calculations were employed to study the falloff behavior of 1,3,5-trioxane dissociation. Molecular geometries and frequencies of all species were obtained at the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and MP2/aug-cc-pVDZ levels of theory, whereas the single-point energies of the stationary points were calculated using coupled cluster with single and double excitations including the perturbative treatment of triple excitation (CCSD(T)) level of theory. It was found that the dissociation occurs via a concerted mechanism requiring an energy barrier of 48.3 kcal/mol to be overcome. The new experimental data and theoretical calculations serve as a validation and extension of kinetic data published earlier by other groups. Calculated values for the pressure limiting rate coefficient can be expressed as log10 k∞ (s(-1)) = [15.84 - (49.54 (kcal/mol)/2.3RT)] (500-1400 K).

Note: An improved driver section for a diaphragmless shock tube

Improvements to equipment lifetime and measurement reproducibility have been made by modifying the actuating mechanism of a diaphragmless shock tube that is used for high temperature gas kinetic studies. The modifications have two major benefits while retaining the simplicity of the original apparatus. First, the reproducibility of shock wave generation has been greatly improved and is demonstrated with 50 nearly identical experiments on the dissociation of cyclohexene at T2 = 1765 ± 13 K and P2 = 120 ± 1 Torr, demonstrating the capability for signal averaging over many experiments. Second, the lifetime of the bellows which forms the heart of the actuator is considerably improved, significantly increasing the time between replacements.

A miniature high repetition rate shock tube

A miniature high repetition rate shock tube with excellent reproducibility has been constructed to facilitate high temperature, high pressure, gas phase experiments at facilities such as synchrotron light sources where space is limited and many experiments need to be averaged to obtain adequate signal levels. The shock tube is designed to generate reaction conditions of T > 600 K, P < 100 bars at a cycle rate of up to 4 Hz. The design of the apparatus is discussed in detail, and data are presented to demonstrate that well-formed shock waves with predictable characteristics are created, repeatably. Two synchrotron-based experiments using this apparatus are also briefly described here, demonstrating the potential of the shock tube for research at synchrotron light sources.

Thermal dissociation of ethylene glycol vinyl ether

The pyrolysis of ethylene glycol vinyl ether (EGVE), an initial product of 1,4-dioxane dissociation, was examined in a diaphragmless shock tube (DFST) using laser schlieren densitometry (LS) at 57 ± 2 and 122 ± 3 Torr over 1200-1800 K. DFST/time-of-flight mass spectrometry experiments were also performed to identify reaction products. EGVE was found to dissociate via two channels: (1) a molecular H atom transfer/C-O scission to produce C(2)H(3)OH and CH(3)CHO, and (2) a radical channel involving C-O bond fission generating ˙CH(2)CH(2)OH and ˙CH(2)CHO radicals, with the second channel being strongly dominant over the entire experimental range. A reaction mechanism was constructed for the pyrolysis of EGVE which simulates the LS profiles very well over the full experimental range. The decomposition of EGVE is clearly well into the falloff region for these conditions, and a Gorin model RRKM fit was obtained for the dominant radical channel. The results are in good agreement with the experimental data and suggest the following rate coefficient expressions: k(2,∞) = (6.71 ± 2.6) × 10(27) × T(-3.21)exp(-35512/T) s(-1); k(2)(120 Torr) = (1.23 ± 0.5) × 10(92) × T(-22.87)exp(-48 248/T) s(-1); k(2)(60 Torr) = (2.59 ± 1.0) × 10(88) × T(-21.96)exp(-46283/T) s(-1).

A shock tube with a high-repetition-rate time-of-flight mass spectrometer for investigations of complex reaction systems

A conventional membrane-type stainless steel shock tube has been coupled to a high-repetition-rate time-of-flight mass spectrometer (HRR-TOF-MS) to be used to study complex reaction systems such as the formation of pollutants in combustion processes or formation of nanoparticles from metal containing organic compounds. Opposed to other TOF-MS shock tubes, our instrument is equipped with a modular sampling unit that allows to sample with or without a skimmer. The skimmer unit can be mounted or removed in less than 10 min. Thus, it is possible to adjust the sampling procedure, namely, the mass flux into the ionization chamber of the HRR-TOF-MS, to the experimental situation imposed by species-specific ionization cross sections and vapor pressures. The whole sampling section was optimized with respect to a minimal distance between the nozzle tip inside the shock tube and the ion source inside the TOF-MS. The design of the apparatus is presented and the influence of the skimmer on the measured spectra is demonstrated by comparing data from both operation modes for conditions typical for chemical kinetics experiments. The well-studied thermal decomposition of acetylene has been used as a test system to validate the new setup against kinetics mechanisms reported in literature.

Towards cleaner combustion engines through groundbreaking detailed chemical kinetic models

In the context of limiting the environmental impact of transportation, this critical review discusses new directions which are being followed in the development of more predictive and more accurate detailed chemical kinetic models for the combustion of fuels. In the first part, the performance of current models, especially in terms of the prediction of pollutant formation, is evaluated. In the next parts, recent methods and ways to improve these models are described. An emphasis is given on the development of detailed models based on elementary reactions, on the production of the related thermochemical and kinetic parameters, and on the experimental techniques available to produce the data necessary to evaluate model predictions under well defined conditions (212 references).

Experimental and theoretical investigation of the self-reaction of phenyl radicals

A combination of experiment and theory is applied to the self-reaction kinetics of phenyl radicals. The dissociation of phenyl iodide is observed with both time-of-flight mass spectrometry, TOF-MS, and laser schlieren, LS, diagnostics coupled to a diaphragmless shock tube for temperatures ranging from 1276 to 1853 K. The LS experiments were performed at pressures of 22 +/- 2, 54 +/- 7, and 122 +/- 6 Torr, and the TOF-MS experiments were performed at pressures in the range 500-700 Torr. These observations are sensitive to both the dissociation of phenyl iodide and to the subsequent self-reaction of the phenyl radicals. The experimental observations indicate that both these reactions are more complicated than previously assumed. The phenyl iodide dissociation yields approximately 6% C(6)H(4) + HI in addition to the major and commonly assumed C(6)H(5) + I channel. The self-reaction of phenyl radicals does not proceed solely by recombination, but also through disproportionation to benzene + o-/m-/p-benzynes, with comparable rate coefficients for both. The various channels in the self-reaction of phenyl radicals are studied with ab initio transition state theory based master equation calculations. These calculations elucidate the complex nature of the C(6)H(5) self-reaction and are consistent with the experimental observations. The theoretical predictions are used as a guide in the development of a model for the phenyl iodide pyrolysis that accurately reproduces the observed laser schlieren profiles over the full range of the observations.

A diaphragmless shock tube for high temperature kinetic studies

A novel, diaphragmless shock tube (DFST) has been developed for use in high temperature chemical kinetic studies. The design of the apparatus is presented along with performance data that demonstrate the range and reproducibility of reaction conditions that can be generated. The ability to obtain data in the fall off region, confined to much narrower pressure ranges than can be obtained with a conventional shock tube is shown, and results from laser schlieren densitometry experiments on the unimolecular dissociation of phenyl iodide (P(2)=57+/-9 and 122+/-7 torr, T(2)=1250-1804 K) are presented. These are compared with results similar to those that would be obtained from a classical shock tube and the implications for extrapolation by theoretical methods are discussed. Finally, the use of the DFST with an online mass spectrometer to create reproducible experiments that can be signal averaged to improve signal/noise and the quality of mass peaks is demonstrated; something that is not possible with a conventional shock tube where each experiment has to be considered unique.

Shock tube study of dissociation and relaxation in 1,1-difluoroethane and vinyl fluoride

This paper reports measurements of the thermal dissociation of 1,1-difluoroethane in the shock tube. The experiments employ laser-schlieren measurements of rate for the dominant HF elimination using 10% 1,1-difluoroethane in Kr over 1500-2000 K and 43 < P < 424 torr. The vinyl fluoride product of this process then dissociates affecting the late observations. We thus include a laser schlieren study (1717-2332 K, 75 < P < 482 torr in 10 and 4% vinyl fluoride in Kr) of this dissociation. This latter work also includes a set of experiments using shock-tube time-of-flight mass spectrometry (4% vinyl fluoride in neon, 1500-1980 K, 500 < P < 1300 torr). These time-of-flight experiments confirm the theoretical expectation that the only reaction in vinyl fluoride is HF elimination. The dissociation experiments are augmented by laser schlieren measurements of vibrational relaxation (1-20% C(2)H(3)F in Kr, 415-1975 K, 5 < P < 50 torr, and 2 and 5% C(2)H(4)F(2) in Kr, 700-1350 K, 6 < P < 22 torr). These experiments exhibit very rapid relaxation, and incubation delays should be negligible in dissociation. An RRKM model of dissociation in 1,1-difluoroethane based on a G3B3 calculation of barrier and other properties fits the experiments but requires a very large DeltaE(down) of 1600 cm(-1), similar to that found in a previous examination of 1,1,1-trifluoroethane. Dissociation of vinyl fluoride is complicated by the presence of two parallel HF eliminations, both three-center and four-center. Structure calculations find nearly equal barriers for these, and TST calculations show almost identical k(infinity). An RRKM fit to the observed falloff again requires an unusually large DeltaE(down) and the experiments actually support a slightly reduced barrier. These large energy-transfer parameters now seem routine in these large fluorinated species. It is perhaps a surprising result for which there is as yet no explanation.