Reaction CH Research Articles

Kinetic study of reaction C2H5 + HO2 in a photolysis reactor with time-resolved Faraday rotation spectroscopy

The rate constant and branching ratios of ethyl reaction with hydroperoxyl radical, C2H5 + HO2 (1), a key radical-radical reaction for intermediate temperature combustion chemistry, were measured in situ for the first time in a photolysis Herriott cell by using mid-IR Faraday rotation spectroscopy (FRS) and UV-IR direct absorption spectroscopy (DAS). The microsecond time-resolved diagnostic technique in this work enabled the direct rate measurements of the target reaction at 40 and 80 mbar and reduced the experimental uncertainty considerably. C2H5 and HO2 radicals were generated by the photolysis of (COCl)2/C2H5I/CH3OH/O2/He mixture at 266 nm. By direct measurements of the transient profiles of C2H5, HO2 and OH concentrations, the overall rate constant for this reaction at 297 K was determined as k1(40 mbar) = (3.8 ± 0.8) × 10−11 cm3 molecule−1 s−1 and k1(80 mbar) = (4.1 ± 1.0) × 10−11 cm3 molecule−1 s−1. The direct observation of hydroxyl radical (OH) indicated that OH formation channel was the major channel with a branching ratio of 0.8 ± 0.1.

Mechanism and kinetics of the reaction CH3 + CH3CHO: Ab initio semiclassical transition state theory study

AbstractThe potential energy profile of the reaction between methyl radicals with acetaldehyde is theoretically investigated at different levels of theory prior to calculating the bimolecular rate constants of this reaction by semiclassical transition state theory (SCTST) and one‐dimensional master equation (1DME) modeling. The stationary points on the potential energy surface of the reaction CH3 + CH3CHO are characterized at the levels MP2/6‐311 + g(2d,2p) and CCD/6‐311 + g(2d,2p). To obtain more satisfying energies, single‐point calculations are performed at CCSD(T)/augh‐cc‐pVTZ+2df. It is shown that the title reaction proceeds via either hydrogen abstraction channels or the addition of methyl radical to acetaldehyde, forming an isopropoxy radical. Then, SCTST and 1DME modeling are used to compare the rate constants of distinct reaction channels. The aldehyde hydrogen atom abstraction by methyl radical producing CH4 + CH3CO is dominant both thermodynamically and kinetically. The calculated SCTST rate constants can be expressed as a function of temperature with cm3molecule−1s−1 (200‐3000 K). Hydrogen/deuterium (H/D) isotopic effects on the kinetics are computed for some specific abstraction channels, showing that tunneling indeed plays a vital role in the hydrogen abstraction process. Pressure dependencies of the addition reaction are followed using master equation calculations, illustrating that the stabilization of the isopropoxy radical is dominant at higher pressures and lower temperatures. In contrast, as pressure decreases and temperature increases, the energized radical prefers decomposing to new bimolecular sets.

Native and Introduced Trypanosome Parasites in Endemic and Introduced Murine Rodents of Sulawesi.

The Indonesian island of Sulawesi is a globally significant biodiversity hotspot with substantial undescribed biota, particularly blood-borne parasites of endemic wildlife. Documenting the blood parasites of Sulawesi's murine rodents is the first fundamental step towards the discovery of pathogens likely to be of concern for the health and conservation of Sulawesi's endemic murines. We screened liver samples from 441 specimens belonging to 20 different species of murine rodents from 2 mountain ranges on Sulawesi, using polymerase chin reaction (PCR) primers targeting the conserved 18S rDNA region across the protozoan class Kinetoplastea. We detected infections in 156 specimens (10 host species) with a mean prevalence of 35.4% (95% confidence interval [CI] = 30.9-39.8%). Sequences from these samples identified 4 infections to the genus Parabodo, 1 to Blechomonas, and the remaining 151 to the genus Trypanosoma. Within Trypanosoma, we recovered 17 haplotypes nested within the Trypanosoma theileri clade infecting 117 specimens (8 host species) and 4 haplotypes nested within the Trypanosoma lewisi clade infecting 34 specimens (6 host species). Haplotypes within the T. theileri clade were related to regional Indo-Australian endemic trypanosomes, displayed geographic structuring but with evidence of long-term connectivity between mountains, and had substantial phylogenetic diversity. These results suggest T. theileri clade parasites are native to Sulawesi. Conversely, T. lewisi clade haplotypes were recovered from both endemic and introduced rodents, demonstrated complete geographic separation between clades, and had low genetic diversity. These results suggest that the T. lewisi clade parasites invaded Sulawesi recently and likely in 2 separate invasion events. Our results provide the first records of metakinetoplastids in Sulawesi's rodents and highlight the need for more extensive sampling for pathogens in this biodiversity hotspot.

Shock-induced ignition and pyrolysis of high-pressure methane and natural gas mixtures

A high-pressure shock tube was used to study ignition delay times (IDT) of CH4/O2/Ar and natural gas/O2/Ar mixtures behind reflected shock waves. Reaction progress was monitored using sidewall pressure and direct laser absorption diagnostics of CH4 near 3.175 µm and ethylene near 10.532 µm. Stoichiometric, fuel-rich and fuel-lean mixtures of CH4/O2, highly dilute in argon, were studied over a temperature range from 1450 to 1850 K and pressures between 10 and 55 atm. Of note are the experiments conducted with fuel-rich mixtures, as there is a lack of literature data in this regime. In the current study, the methane absorption diagnostic provided a unique tool enabling both speciation of methane and a clear definition of ignition delay time. In addition to methane oxidation, we have measured ignition delay times of commercial natural gas blends over a temperature range of 1408–1541 K, at pressures near 12 atm, and at an equivalence ratio of 1. To understand the effects of minor constituents (such as ethane and propane) in commercial natural gas blends, ethylene concentration during pyrolysis experiments was monitored using a two-wavelength scheme (10.532 µm and 10.674 µm) using a CO2 gas laser. The deficiency of existing kinetic models towards predicting the high-temperature kinetics of natural gas blends was highlighted through our measurements. Therefore, this study also provides data critical for refining these models. Extensive sensitivity analysis emphasizes the importance of the reaction CH3+C2H6→CH4+C2H5 during natural gas pyrolysis, and the accuracy of the chemical kinetic models is significantly improved by using a revised reaction rate constant (Shao et al. 2019) for this reaction. These measurements extend the test conditions of earlier studies of methane and commercial natural gas.

Nonstationary processes in matrix-isolated methane probed by optical and current emission spectroscopy

Nonstationary processes in the Ar matrix doped with CH4 were studied with a focus on the behavior of radiolysis products – H atoms and CH radicals. Samples of 25 μm thickness were irradiated with 1.5 keV electron beam. It has been shown that because of the small penetration depth of electrons the bulk of the matrix is excited preferentially by photons with an energy of 9.8 eV, the most intense emission band of the matrix (excitons self-trapped into the configuration of Ar2∗), In other words “internal photolysis” occurs. The spectrally resolved emission of H atoms and CH radicals, along with the total yield of desorbing particles, were monitored in a correlated manner under the concurrent action of photons and external heating. These dependences, called nonstationary luminescence (NsL) and nonstationary desorption (NsD), were compared with the measured yield of thermally stimulated exoelectron emission (TSEE), that enabled us to reveal the appearance of the CH radical in the neutralization reaction CH3+ + e− → CH3∗ followed by dissociation of CH3∗. Thus, the CH radical was assumed to be a signature of the CH3∗ species. The antibate behavior of the CH NsL and the NsD in the temperature range 37–43 K, where diffusion in the Ar matrix occurs, was interpreted as a result of the recombination of CH3 radicals with the formation of C2H6 and the energy release spent for desorption. Self-oscillations (with τ of about 10 s) of increasing amplitude were detected, indicating self-oscillating processes in the bulk of the Ar matrix doped with 5% CH4.

Enhanced CO2 Methanation Reaction in C1 Chemistry over a Highly Dispersed Nickel Nanocatalyst Prepared Using the One-Step Melt-Infiltration Method

The Paris Agreement requires the world to put the best efforts to reduce CO2 emissions, due to the global warming problems. As a promising technology corresponding to this greenhouse gas treatment, the CO2 methanation process a.k.a power to gas (PtoG), which catalytically converts CO2 into methane, has been in the limelight. To develop an efficient catalytic process, it is necessary to design a low-cost and high-efficiency catalyst for high CO2 conversion and CH4 selectivity. In this study, we have developed Ni/γ-Al2O3 catalysts by the one-step melt-infiltration method, where both aging and calcination are done in one pot. For enhancement of the catalytic activity and selectivity, sufficient Ni content (>25 wt %) and a high dispersion (<10 nm) are simultaneously required. Thus, the aging conditions of the melt-infiltration methods, e.g., time and temperature, were optimized for the high dispersion with sufficient Ni content (15–50 wt %). The catalytic performance tests were carried out under atmospheric pressure, 275 to 400 °C and gas hourly velocity (GHSV) = 25,000 h−1. And the various characteristic analyses (Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), H2-chemisorption, temperature programmed reduction (TPR), etc.) were performed to confirm the effects on the catalytic performance. As a result, based on the experiments and the characterization data, the 30 wt %-Ni catalyst (Ni particles size = 11 nm) showed the best CO2 conversion at 300 °C and the 20 wt % one having the highest Ni dispersion (Ni particles size = 8.8 nm), which showed the best intrinsic reaction rate and CH4 selectivity in the entire temperature range.

Autoignition studies of NH3/CH4 mixtures at high pressure

Autoignition delay times of NH3/CH4 mixtures with CH4 fractions of 0%, 5%, 10% and 50% were measured in a rapid compression machine at equivalence ratio φ = 0.5, pressures from 20 to 70 bar and temperatures from 930 to 1140 K. In addition, measurements were performed for NH3 mixtures with 10% CH4 at φ = 1.0 and 2.0. Methane shows a strong ignition-enhancing effect on NH3, which levels off at higher CH4 fractions, as the ignition delay time approaches that of pure methane. Autoignition delay times at 10% CH4 at φ = 0.5 and 1.0 are indistinguishable, while an increase of ignition delay times by factor of 1.5 was observed upon increasing φ to 2.0. The experimental data were used to evaluate six NH3 oxidation mechanisms capable of simulating NH3/CH4 mixtures. The mechanism previously used by the authors shows the best performance: generally, it predicts the measured ignition delay times to better than 30% for all conditions, except for 50% CH4 addition for which the differences increase up to 50% at the highest temperature. Sensitivity analysis based on the mechanism used indicates that under lean conditions the reaction CH4 + NH2 = CH3 + NH3 significantly promotes ignition for modest CH4 addition (5% and 10%), but becomes modestly ignition-inhibiting at 50% CH4. Sensitivity and rate-of-production analyses indicate that the ignition-enhancing effect of 50% CH4 addition is closely related to the formation and decomposition of H2O2. Flux analysis for NH3/CH4 mixtures indicates that CH4 + NH2 = CH3 + NH3 contributes substantially to the decomposition of methane early in the oxidation process, while CH3 + NO2 (+M) = CH3NO2 (+M) is a significant reservoir of NO2 at low temperature. Additionally, an anomalous pre-ignition pressure rise phenomenon, which is not reproduced by the simulations, was observed with high reproducibility for the NH3 mixture with 50% CH4 addition.

Direct Measurement of Radical-Catalyzed C6H6 Formation from Acetylene and Validation of Theoretical Rate Coefficients for C2H3 + C2H2 and C4H5 + C2H2 Reactions.

The addition of vinylic radicals to acetylene is an important step contributing to the formation of polycyclic aromatic hydrocarbons in combustion. The overall reaction 3C2H2 → C6H6 could result in large benzene yields, but without accurate rate parameters validated by experiment, the extent of aromatic ring formation from this pathway is uncertain. The addition of vinyl radicals to acetylene was investigated using time-resolved photoionization time-of-flight mass spectrometry at 500 and 700 K and 5–50 Torr. The formation of C6H6 was observed at all conditions, attributed to sequential addition to acetylene followed by cyclization. Vinylacetylene (C4H4) was observed with increasing yield from 500 to 700 K, attributed to the β-scission of the thermalized 1,3-butadien-1-yl radical and the chemically activated reaction C2H3 + C2H2 → C4H4 + H. The measured kinetics and product distributions are consistent with a kinetic model constructed using pressure- and temperature-dependent reaction rate coefficients computed from previously reported ab initio calculations. The experiments provide direct measurements of the hypothesized C4H5 intermediates and validate predictions of pressure-dependent addition reactions of vinylic radicals to C2H2, which are thought to play a key role in soot formation.

Explosive desorption from solid methane and methane-containing argon matrices

The total yield of emitted particles from solid CH4 and Ar matrices doped with CH4 was studied under irradiation with electrons. The explosive desorption was detected for these systems. Strong bursts of particles were delayed with respect to the start of irradiation indicating the need for accumulation of radiolysis products. The study was performed using optical and current spectroscopy – cathodoluminescence (CL) together with thermally stimulated exoelectron emission (TSEE) and thermally stimulated post-desorption (TSpD). In the CL spectra, the products of radiation-induced transformation of CH4 molecules were recorded: H, CH, C. Dose dependencies of the optical emissions were measured in a correlated manner with the total yield of particles. It was found that the particles burst correlates with an increase in the concentration of H atoms. Scenario suggested for the delayed particle emission incorporates the recombination of CH3 radicals appearing simultaneously with H atoms in the reaction CH4 → CH3 + H.

Radiation absorption on MHD free convective laminar flow over a moving vertical porous plate, viscous dissipation and chemical reaction with suction under the influence of transverses magnetic field

Abstract The aim of the present chapter is to investigate the problem of Study on MHD boundary layer flow over a moving vertical porous plate with heat generation and chemical reaction in the presence of thermal absorption. The governing equations of the flow are transformed into nonlinear ordinary differential equations and then solved by using Runge-Kutta-fourth order along with shooting method. Comparison between the existing literature and the present study was carried out and found to be in excellent agreement. The influence of the various parameters like Magnetic parameter (M), thermal Grashof number (Gr), mass Grashof number (Gc), permeability parameter (K), Prandtl number (Pr), Dufour number (Df), Soret number (Sr), Schmidt number (Sc), Heat generation (B), Radiation Parameter (Ra) and Chemical reaction (Ch) on the flow are analyzed and discussed through graphs in detail. The values of the skin friction coefficient, Nusselt number and the Sherwood number for different physical parameters are also tabulated.

Effects of non-equilibrium excitation on methane oxidation in a low-temperature RF discharge

The kinetic effects of non-equilibrium excitation by direct electron impact on low-temperature oxidation of CH4 were investigated by experiment and simulation. We focused on the vibrational-electronic-chemistry coupling of methane and oxygen molecules under conditions of immediate reduced electric field strengths of 30–100 Td in an RF dielectric barrier discharge. A detailed plasma chemistry mechanism governing the oxidation processes in an He/CH4/O2 combustible mixture was proposed and studied by including a set of electron impact reactions, dissociative recombination reactions, reactions involving vibrationally- and electronically- excited species, and important three-body recombination reactions. A linear increase in reactant consumption with an increase in plasma power was observed experimentally. This suggested the presence of decoupling between the molecular excitation by plasma and the low-temperature chemistry. However, CO formation showed a non-linear trend, with its formation increasing with lower energy inputs and decreasing at higher energy inputs. By modelling the chemical kinetic sensitivity and reaction pathways, we found that the formation of radicals via the chain propagation reactions CH4 + O(1D) → CH3 + OH, and O2(a1Δg) + H → O + OH was mainly accelerated by the electronically excited species O(1D) and O2(a1Δg). The numerical simulation also revealed that under conditions of incomplete relaxation, the vibrational species CH4(v) and O2(v) enhanced chain propagating reactions, such as CH4(v) + O → CH3 + OH, CH4(v) + OH → CH3 + H2O, O2(v) + H → O + OH, thus stimulating the production of active radicals and final products. Specifically, for an E/N value of 68.2 Td in a stoichiometric mixture (0.05 CH4/0.1 O2/0.85 He), O(1D), CH4(v13), and O2(v) were estimated to contribute to 12.7%, 3.6%, and 3.8% of the production of OH radicals respectively. The reaction channel CH4(v13) + OH → H2O + CH3 was estimated to be responsible for 1.6% of the H2O formation. These results highlight the strong roles of vibrational states in a complex plasma chemistry system and provide new insights into the roles of excited species in the low-temperature oxidation kinetics of methane.

Insights into nitromethane combustion from detailed kinetic modeling – Pyrolysis experiments in jet-stirred and flow reactors

The pyrolysis of nitromethane highly diluted in helium was studied in a plug flow reactor and in a jet-stirred reactor at 1.07 bar and over the temperature range from 500 to 1100 K. Mole fraction profiles of major products and of intermediates were identified with gas chromatography and Fourier transform infrared spectroscopy. Using these experimental data, as well as published ones, we have developed a newly compiled model for the prediction of the pyrolysis and of the oxidation of nitromethane in jet-stirred and flow reactors, freely propagating, and burner-stabilized premixed flames, as well as in shock-tubes. The experimental results from the present work and from the literature are interpreted with the help of the kinetic model derived here. This study mainly focuses on the analysis of speciation in different reactors. Among the nitrogenous species, NO is found to be a major product for pyrolysis and oxidation. The model suggests that for nitromethane pyrolysis and oxidation the thermal dissociation channel to CH3 and NO2 is the main reaction path for the nitromethane degradation followed by the H-atom abstraction channel. The most sensitive reactions for nitromethane pyrolysis in a flow reactor and during pyrolysis and oxidation in a jet-stirred reactor are found to be CH3NO2(+M) ⇋ CH3 + NO2(+M) and CH3 + NO2 ⇋ CH3O + NO. The reaction CH3 + NO2 ⇋ CH3O + NO is found to be the most important reaction for all conditions studied. In a burner-stabilized premixed flame, as the mixture gets richer, the thermal dissociation channel CH3NO2(+M) ⇋ CH3 + NO2(+M) becomes more important as the contribution of the H-atom abstraction channel is decreased. Furthermore, in the burner-stabilized premixed flames, it was found that NO is mainly formed via NO2: NO2 + H ⇋ NO + OH, NO2 + CH3 ⇋ CH3O + NO. The model provided an overall reasonable agreement with the experimental data. However, for pyrolysis conditions, future work is desirable to improve predictions of intermediate species. This work extends the kinetic database and helps to improve the understanding of nitromethane chemistry. The kinetic model presented in this work can serve as a base model for hydrocarbons and oxygenated fuels higher than C2 and nitrogen-containing compounds higher than C1 as well as for pure nitrogen compounds.

Experimental and kinetic study on di-tertiary butyl peroxide auto-ignition and its effect on the ignition of n-heptane

Di-tertiary butyl peroxide is commonly used as ignition improver. In this work, auto-ignition properties of di-tertiary butyl peroxide and its promoting effect on n-heptane ignition were investigated systematically using a shock-tube facility. Ignition delay times were determined by measuring the time interval between electronically excited CH* emission and reflected shock pressure signals monitored at shock-tube sidewall. An empirical correlation for ignition delay times of di-tertiary butyl peroxide as a function of temperature, pressure and equivalence ratio was deduced by linear regression analysis. The quantitative effect of di-tertiary butyl peroxide on n-heptane ignition was obtained. The addition of di-tertiary butyl peroxide can dramatically decrease the ignition delay time of n-heptane, especially at high pressures and low temperatures. A two-step dissociation mechanism of di-tertiary butyl peroxide was combined with an n-heptane combustion mechanism to model the ignition characteristics. Sensitivity and reaction pathway analyses were carried out to identify the dominant reactions and fuel consumption paths during the ignition process, and to analysis the effect of di-tertiary butyl peroxide addition. The chain branching reaction CH3 + HO2 = CH3O + OH plays a dominant role in ignition promotion.

Significance of reaction CH3 + NO = H2CN + OH in two-stage ignition of nitromethane

Two-stage ignition of nitromethane diluted in Ar is investigated at various equivalence ratios (0.2–0.5), dilution levels (93%–97%), and pressures (4–16 atm) with a heated shock tube apparatus. OH* emission profiles are used to determine ignition delay times. The second-stage ignition delay time increases with increasing equivalence ratio or dilution level, but is insensitive to pressure especially at high dilution levels. The first-stage ignition delay time is unaffected by the present experimental parameters above 1150 K, below which it decreases with increasing equivalence ratio or pressure. An updated model reconsidering reaction CH3 + NO = H2CN + OH is examined against the experimental measurements from present work and literature, including ignition delay times, laminar burning velocities, and species mole fraction profiles. At fuel-rich condition, this reaction has a significant contribution to the formation of OH radical during the second-stage ignition. The two-stage ignition and intensity evolution of OH peaks are well predicted and explained kinetically by the updated model.

Computational study on C1- versus C3-regioselectivity in Pd(II)-catalyzed olefination/dearomatization of 2-naphthyl ureas

The regioselectivity in Pd(II)-catalyzed CH direct functionalization of C2-substituted naphthalenes has been a long-term issue in synthetic organic chemistry due to its existence of competitive reaction sites, including C1-, C3- and even C4-sites. Our recent achievement in Pd(II)-catalyzed C1-dearomatization of 2-naphthyl ureas encouraged us to find out the key factors for determining the C1- and C3- regioselectivity of 2-naphthyl ureas based on density functional theory (DFT). In this work, we identified a plausible reaction pathway, which led to the experimental C1-functionalized product with a reasonable energy profile. A homologous pathway giving C3-spirocycle was also calculated to investigate the nature of C1-regioselectivity in this reaction. According to the calculated results, we found that in our reaction CH cleavage was the regioselectivity-determining step, and that C1 was the kinetically favorable site in this step due to electronic effect which makes C1 a more vulnerable site to electrophiles compared to C3, as reflected by the condensed Fukui function and its larger proportion of the highest occupied molecular orbital. Energy decomposition analyses and electronic structure analyses were further performed, which were in good accordance with our conclusion yet indicated C1 was a more sterically hindered site. This result provided us with the inspiration that steric hindrance may play an important role in frequently reported transition metal-catalyzed C3-arylation in the literature. We then selected a recently reported reaction Pd-catalyzed C3-arylation of 2-naphthyl aniline to study the regioselectivity. The result was in line with our assumption that in C3-arylation of 2-naphthyl aniline the preferential functionalization of C3-site is due to its less steric hindrance than C1-site. This work is thus instructive for understanding the regioselectivity in Pd(II)-catalyzed CH direct functionalization of C2-substituted naphthalenes.

Effect of structure and composition on the CO2 hydrogenation properties over bimodal mesoporous CeCo composite catalyst

The CeO2-δ and CeCo composite catalysts with different Ce/Co mole ratio were prepared by hard-template method for CO2 catalytic hydrogenation. The physical and chemical properties of these catalysts were characterized by H2-TPR, XRD, BET, TEM, ICP, and in-situ XPS. The characterization results indicated that the Co ions can be dissolved into the CeO2 lattice to form Ce-O-Co solid solution, which promotes the reduction of corresponding precursor to form Co active species, surface oxygen vacancies, and the Ce3+-□-Co0 structure. The pore structure of the CeCo composite catalysts can be affected by the Ce/Co mole ratio, and the bimodal mesoporous structure can be obtained at relative high Ce/Co mole ratio. The synergistic effect from ordered bimodal mesoporous structure, surface oxygen vacancies, and metallic Co active species can effectively promote CO2 hydrogenation reaction. The CeCo05 composite catalyst with Ce/Co mole ratio of 0.5 exhibited superior CO2 hydrogenation catalytic activity and high stability, the CO2 catalytic hydrogenation reaction rate and CH4 selectivity reached 4.10 × 10−5 mol/gcat/s and 93.3% at 400 °C, respectively. This work indicated that the CeCo composite catalyst is a promising candidate for CO2 hydrogenation reaction.

Theoretical Study on Reactions of Alkylperoxy Radicals.

We carried out a theoretical study on geometries, relative energies of stationary points, and reaction rate constants for ethyl + O2, propyl + O2, and butyl + O2 reactions, which are important reactions in the low-temperature oxidation of corresponding alkanes. Geometries with CCSD(T)/aug-cc-pVTZ for the ethyl + O2 system are adopted as the benchmark to choose a proper exchange-correlation functional for geometry optimization. Our results show that B3LYP with 6-311+G(d,p) can provide reliable structures for this system, and structures of the other two systems are determined with this functional. The performances of the explicitly correlated CCSD(T)-F12a and the locally correlated DLPNO-CCSD(T) methods on barrier heights and reaction energies are evaluated by comparing their results with those of CCSD(T)/aug-cc-pVQZ for the ethyl + O2 system. Our results indicate that reliable energy differences for this system are achieved with CCSD(T)-F12a using the cc-pVDZ-F12 basis set, and this method is employed in calculating single-point energies for the other two systems. The single-reference equation-of-motion spin-flip coupled-cluster method is adopted to obtain the potential energy surface of the barrierless reaction C2H5· + O2 → CH3CH2OO·, and the results are compared with those using broken-symmetry density functional theory and the Morse potential. Differences between energies with these methods are <1.6 kcal/mol, but the difference in the rate constants could be sizable at temperatures <500 K, and rate constants obtained in this work are reliable only for temperatures >500 K. Pressure-dependent rate constants for these reactions are determined using the Rice-Ramsperger-Kassel-Marcus/Master equation method. The obtained reaction energies, barrier heights, and rate constants could be valuable for reactions between the large alkane radical and O2, which are important in the low-temperature combustion of fuels such as kerosene and gasoline.

Gas-phase reactivity of CH3C(O) CH3 with OH radicals at interstellar Temperatures (T = 11.7 – 64.4 K) using the CRESU technique

AbstractThe rate coefficients, k(T= 11.7 – 64.4 K), for the gas-phase reaction between OH radicals and acetone, CH3C(O) CH3, have been measured using the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique, the most suitable one to cool down gases below the freezing point without gas condensation. The experimental k(T) was found to increase as temperature was lowered and is several orders of magnitude higher for low temperature than k(300 K). No pressure dependence of k(20 K) and k(64 K) was observed, while k(50 K) at the largest gas density is twice higher than the average values found at lower gas densities. The obtained values of k(11.7 K) and k(21.1 K) were 2.45’10-10 and 1.39’10-10 cm3 molecule-1 s-1, respectively.

Investigation of OH* chemiluminescence and heat release in laminar methane–oxygen co-flow diffusion flames

OH* chemiluminescence is one of the major spontaneous emission in flames, and often applied in combustion diagnostics to indicate flame structure, strain rate, equivalence ratio, heat release rate, etc. In this work, OH* chemiluminescence in the laminar methane–oxygen co-flow diffusion flames was investigated. A high resolution ultra-violet imaging system was used to capture the OH* chemiluminescence images. Numerical simulations of the experimental cases were performed based on OH* chemiluminescence reaction mechanism. The numerical results show good agreement with the experimental measurements. It's found that there are two OH* distribution zones in laminar methane–oxygen co-flow diffusion flames. Analysis on the production pathway of OH* chemiluminescence shows that the reaction H + O + M = OH* + M (R1) is the major formation reaction of OH* chemiluminescence in laminar methane–oxygen diffusion flames. The increase of diluent addition in oxidizer will lead to the dominant OH* production pathway changing from the reaction R1 to the reaction CH + O2 = OH* + CO (R2). The OH* distribution characteristics under different global oxygen-fuel equivalence ratios indicate that OH* chemiluminescence can be employed as an appropriate indicator to characterize the combustion condition. Moreover, the correlation between integrated heat release rate and integrated OH* concentration is derived for the oxygen-deficient flames. The integrated heat release rate can be predicted in terms of integrated OH* concentration, methane flow rate and global oxygen-fuel equivalence ratio.

Shock Tube Measurement of the C2H4 + H ⇔ C2H3 + H2 Rate Constant.

The rate constant for the reaction C2H4 + H ⇔ C2H3 + H2 was studied behind reflected shock waves at temperatures between 1619 and 1948 K and pressures near 10 atm in a mixture of C2H4, CH4, H2, and argon. C2H4 time histories were measured using laser absorption of a CO2 gas laser near 10.53 μm. Experimental mixtures were designed to optimize sensitivity to the title reaction with only weak sensitivity to secondary reactions. Two mechanisms, FFCM1 and ARAMCO v2, are used for data analysis. The well-selected operating conditions and Monte Carlo sampling data analysis procedure resulted in mechanism-independent reaction rate constant measurements with a 2σ uncertainty of ±35%. The current data disagree with a broadly used theoretical calculation (Knyazev et al. (1996)), but they are in good consensus with one of the review studies (Baulch et al. (2005)), k = (3.9 × 1022) T3.62 exp(-5670/ T) cm3 molecule-1 s-1. To the best of our knowledge, this work provides the first high-temperature study of the C2H4 + H ⇔ C2H3 + H2 reaction rate constant with well-defined uncertainty.