H-atom Profiles Research Articles

Chemical kinetics investigation of toluene combustion in a shock tube using spectroscopic CO and H2O laser absorption

Toluene oxidation was investigated behind reflected shock waves using spectroscopic laser diagnostics to simultaneously measure carbon monoxide (CO) and water (H2O) time-history profiles. CO and H2O time histories are unique in the available database for toluene speciation as only a total of five H-atom profiles can be found in the literature and were carried out for pyrolysis instead. These new, high fidelity measurements can help in validating detailed chemical kinetics mechanisms over highly dilute conditions. The experiments cover three equivalence ratios (ɸ = 0.5, 1.0, and 2.0) with temperatures ranging from 1433 to 1921 K near atmospheric pressure. Comparisons with six models were conducted for a performance review: the Yuan et al. model shows the most accurate results and was selected to further understand toluene chemistry and identify the reaction pathways. Rate-of-production and sensitivity analyses support that toluene (A1CH3 in the model) decomposes via A1CH3 (+M) ⇄A1CH2 + H (+M) and mostly produces CO and H2O with the following sequences: A1CH3→ A1CH2→ A1CH2O → CH2O → HCO → CO and A1CH3→ A1CH2→ CH3→ H2O, respectively.

Direct measurements of channel specific rate constants in OH + C3H8 illuminates prompt dissociations of propyl radicals

OH + molecules are an important class of reactions in combustion and atmospheric chemistry. Consequently, numerous studies have measured rate constants for these processes over an extended temperature range. A large majority of these experimental studies have utilized the decay of [OH] profiles (monitored either by absorption or laser-induced fluorescence) to obtain total rate constants. However, there are limited direct measurements of channel specific rate constants in this important class of reactions, particularly at combustion relevant temperatures. In the present experiments, we have directly measured site-specific rate constants for abstraction of the secondary CH bond in OH + C3H8 at high temperatures. Atomic resonance absorption spectrometry (ARAS) was used to monitor the formation of H-atoms from shock-heated mixtures of tert-butylhydroperoxide and C3H8 at high temperatures. Simulations for the experimental H-atom profiles are sensitive only to abstraction of the secondary CH bond leading to unambiguous measurements of the rate constants for this reaction. Over the T-range, 921 K < T < 1146 K, rate constants from the present experiments for OH + C3H8 → H2O + i-C3H7 can be represented by the Arrhenius expression,k=(3.935±1.387)×10−11exp(−1681±362K/T)cm3molecule−1s−1Simulations of the lower temperature data (T < 1000 K) indicate that the H-atom profiles are also influenced to a minor extent by the thermal dissociation of iso-propyl, i-C3H7 → H + C3H6, at short time-scales. Direct dynamics calculations were performed to examine in greater detail the potential role of prompt dissociations of i-C3H7 and n-C3H7 (formed from the title reaction) in interpreting the lower temperature (< 1000 K) data from the present work. These simulations suggest that prompt dissociation of propyl radicals does not influence the present experimental observations but has a minor influence on higher temperature combustion simulations.

Quantitative atomic hydrogen measurements in premixed hydrogen tubular flames

Quantitative measurements of atomic hydrogen are reported in laminar premixed tubular flames using femtosecond two-photon laser-induced fluorescence. The H-atom fluorescence is corrected for collisional quenching by using local values of major-species concentrations and temperature measured by spontaneous Raman scattering. Lean hydrogen flames are sustained with two different diluents (N2, CO2) to investigate low–Lewis number flames under high curvature. When compared to planar stretched flames, the curved tubular flames enhance the H-atom concentration and temperature through increased preferential diffusion. Peak H-atom number densities on the order of 1015 per cm3 are measured, and absolute H-atom profiles show differences up to 40% in peak number density and flame radial position when compared to the pseudo one-dimensional flame model with detailed chemistry and transport. Although the overall agreement in absolute H-atom profiles is “good” considering the relative uncertainties in the model and experiment, the differences suggest the ability for this flame geometry to provide evidence for revision of molecular transport and chemical kinetic modeling in flames with substantial preferential diffusion. Peak absolute H-atom concentrations vary up to 30% depending on the assumed temperature dependency for the collisional quenching factors pointing to the need for high-temperature data for H-atom collisional quenching. Relative H-atom profiles in the flames are minimally affected by the collisional quenching.

High Temperature Shock Tube and Theoretical Studies on the Thermal Decomposition of Dimethyl Carbonate and Its Bimolecular Reactions with H and D-Atoms

The shock tube technique was used to study the high temperature thermal decomposition of dimethyl carbonate, CH3OC(O)OCH3 (DMC). The formation of H-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range of 1053-1157 K at pressures ∼0.5 atm. The H-atom profiles were simulated using a detailed chemical kinetic mechanism for DMC thermal decomposition. Simulations indicate that the formation of H-atoms is sensitive to the rate constants for the energetically lowest-lying bond fission channel, CH3OC(O)OCH3 → CH3 + CH3OC(O)O [A], where H-atoms form instantaneously at high temperatures from the sequence of radical β-scissions, CH3OC(O)O → CH3O + CO2 → H + CH2O + CO2. A master equation analysis was performed using CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for all thermal decomposition processes in DMC. The theoretical predictions were found to be in good agreement with the present experimentally derived rate constants for the bond fission channel (A). The theoretically derived rate constants for this important bond-fission process in DMC can be represented by a modified Arrhenius expression at 0.5 atm over the T-range 1000-2000 K as, kA(T) = 6.85 × 10(98)T (-24.239) exp(-65250 K/T) s(-1). The H-atom temporal profiles at long times show only minor sensitivity to the abstraction reaction, H + CH3OC(O)OCH3 → H2 + CH3OC(O)OCH2 [B]. However, H + DMC is an important fuel destruction reaction at high temperatures. Consequently, measurements of D-atom profiles using D-ARAS allowed unambiguous rate constant measurements for the deuterated analog of reaction B, D + CH3OC(O)OCH3 → HD + CH3OC(O)OCH2 [C]. Reaction C is a surrogate for H + DMC since the theoretically predicted kinetic isotope effect at high temperatures (1000 - 2000K) is close to unity, kC ≈ 1.2 kB. TST calculations employing CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for reactions B and C are in good agreement with the experimental rate constants. The theoretical rate constants for these bimolecular processes can be represented by modified Arrhenius expressions over the T-range 500-2000 K as, kB(T) = 1.45 × 10(-19)T(2.827) exp(-3398 K/T) cm(3) molecule(-1) s(-1) and kC(T) = 2.94 × 10(-19)T(2.729) exp(-3215 K/T) cm(3) molecule(-1) s(-1).

Experiment and theory on methylformate and methylacetate kinetics at high temperatures: Rate constants for H-atom abstraction and thermal decomposition

The shock tube technique was used to study the high temperature thermal decomposition of methylformate (MF) and methylacetate (MA). The formation of H-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range of 1194–1371K at pressures ∼0.5atm. The H-atom profiles were simulated using a detailed chemical kinetic mechanism for MF and MA thermal decomposition. The simulations were used to derive rate constants for sensitive decomposition and H-abstraction reactions in MF and MA. In methylformate, the most sensitive reactions that determine H-atom profiles are: (A)CH3OC(O)H→HCO2+CH3(B)CH3OC(O)H+H→CH3OCO+H2 where H is formed from HCO2→H+CO2. In methylacetate the most sensitive reactions affecting H-atom formation are: (C)CH3OC(O)CH3→CH3+OC(O)CH3(D)CH3OC(O)CH3+H→CH2OC(O)CH3+H2 Minor sensitivity was observed for the energetically higher lying bond fission, (E)CH3OC(O)CH3→CH3+CH3OCO and H-atom abstraction from MA by CH3 through, (F)CH3OC(O)CH3+CH3→CH2OC(O)CH3+CH4(G)CH3OC(O)CH3+CH3→CH3OC(O)CH2+CH4 Unlike MF, where H-atoms are formed instantaneously at high-temperatures from (A), in MA, H-atoms form from the CH3 radicals (through CH3+CH3→C2H4+2H) generated primarily through the C–O bond fission channel (C) with minor contributions from (E). A master equation analysis was performed using CCSD(T)/cc-pv∞z//B3LYP/6-311++G(d,p) energetics and molecular properties for all thermal decomposition processes in MF and MA. The theoretical predictions were found to be in good agreement with the present experimentally derived rate constants for the bond fissions. TST calculations employing CCSD(T)/cc-pv∞z//MP2/aug-cc-pvtz energies and molecular properties for reactions (B) and (D) (the only sensitive abstraction processes in MF and MA) are in good agreement with the experimental rate constants. The theoretically derived rate constants for these processes can be represented by modified Arrhenius expressions for the bond fissions at 0.5atm over the T-range 1000–2000K and for the bimolecular abstractions over the 500–2000K regime. kA(T)=9.79×1068T-15.95exp(-57,434K/T)s-1kB(T)=5.67×10-19T2.50exp(-3188K/T)cm3molecule-1s-1kC(T)=1.42×1084T-19.60exp(-63,608K/T)s-1kD(T)=1.18×10-18T2.58exp(-3714K/T)cm3molecule-1s-1kE(T)=1.90×1082T-19.30exp(-64,724K/T)s-1 Our theoretical predictions for MA+CH3 give over the T-range 500–2000K, kF(T)=2.12×10-25T3.93exp(-4440K/T)cm3molecule-1s-1kG(T)=3.40×10-25T3.88exp(-4149K/T)cm3molecule-1s-1 To our knowledge this is the first study providing experimentally derived rate constant values for the primary bond fission and abstraction reactions in MF and MA.

Determination of rate parameters of cyclohexane and 1-hexene decomposition reactions

Peukert et al. recently published (Int. J. Chem. Kinet. 2010; 43: 107–119) the results of a series of shock tube measurements on the thermal decomposition of cyclohexane (c-C6H12) and 1-hexene (1-C6H12). The experimental data included 16 and 23 series, respectively, of H-atom profiles measured behind reflected shock waves by applying the ARAS technique (temperature range 1250–1550 K, pressure range 1.48–2.13 bar). Sensitivity analysis carried out at the experimental conditions revealed that the rate coefficients of the following six reactions have a high influence on the simulated H-atom profiles: R1: c-C6H12 = 1-C6H12, R2: 1-C6H12 = C3H5 + C3H7, R4: C3H5 = aC3H4 + H; R5: C3H7 = C2H4 + CH3; R6: C3H7 = C3H6 + H; R8: C3H5 + H = C3H6. The measured data of Peukert et al. were re-analysed together with the measurement results of Fernandes et al. (J. Phys. Chem. A 2005; 109: 1063–1070) for the rate coefficient of reaction R4, the decomposition of allyl radicals. The optimization resulted in the following Arrhenius parameters: R1: A = 2.441 × 1019, E/R = 52,820; R2: A = 3.539 × 1018, E/R = 42,499; R4: A = 8.563 × 1019, n = −3.665, E/R = 13,825 (high pressure limit); R4: A = 7.676 × 1031n = −3.120, E/R = 40,323 (low pressure limit); R5: A = 3.600 × 1012, E/R = 10699; R6: A = 1.248 × 1017, E/R = 28,538; R8: A = 6212 × 1013, E/R = −970. The rate parameters above are in cm3, mol, s, and K units. Data analysis resulted in the covariance matrix of all these parameters. The standard deviations of the rate coefficients were converted to temperature dependent uncertainty parameter f(T). These uncertainty parameters were typically f = 0.1 for reaction R1, f = 0.1–0.3 for reaction R2, below 0.5 for reaction R8 in the temperature range of 1250–1380 K, and above 1 for reactions R4–R6.

Formation of H‐atoms in the pyrolysis of cyclohexane and 1‐hexene: A shock tube and modeling study

AbstractCyclohexane (cC6H12) plays an important role in the combustion of practical liquid fuels, as a major component of naphthenic compounds. Therefore, the pyrolysis of cyclohexane was investigated by measuring the formation of H‐atoms. The thermal decomposition of 1‐hexene (1‐C6H12) was also studied, because of the assumption that 1‐hexene is the sole initial product of cyclohexane decomposition. For cyclohexane, the measurements were performed over a temperature range of 1320–1550 K, at pressures ranging from 1.8 to 2.2 bar; 1‐hexene experiments were done at temperatures between 1250 and 1380 K and pressures between 1.5 and 2.5 bar. For each experiment, the time‐dependent formation of H‐atoms was measured behind reflected shock waves by using the method of atomic resonance absorption spectrometry. For the dissociation of 1‐hexene to n‐propyl (C3H7) and allyl (C3H5) radicals, the following Arrhenius expression was derived: kR2(T) = 2.3 × 1016 exp(−36,672 K/T) s−1. For cyclohexane, overall rate coefficients (kov) were deduced for the global reaction cC6H12 → products + H from the H‐atom time profiles; the following temperature dependency was obtained: kov(T) = 4.7 × 1016 exp(−44,481 K/T) s−1. For both sets of rate coefficient values, an uncertainty of ±30% is estimated. Especially concerning the isomerization cC6H12 → 1‐C6H12, our experimental results are in excellent agreement with the rate coefficient values given by Tsang (Tsang, W. Int J Chem Kinet 1978, 10, 1119–1138). A reaction model was assembled that is able to reproduce the H‐atom profiles measured for both sets of experiments. According to this model, H‐atoms are mostly stemming from the thermal decomposition of allyl radicals (C3H5), which arise from the decomposition of 1‐hexene. Furthermore, it will be shown that the recombination of allyl radicals with H‐atoms to propene (C3H6) also represents a very important subsequent reaction. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 107–119, 2011

Formation of H-atoms in the Pyrolysis of 1,3-butadiene and 2-butyne: A Shock Tube and Modelling Study

Abstract For investigating the pyrolysis of 1,3-butadiene (1,3-C4H6) and 2-butyne (2-C4H6), reactive gas mixtures highly diluted with argon as bath gas were prepared. The experiments were carried out in a high purity shock tube device over a temperature range of about 1500–1800 K at total pressures between 1.2 and 1.9 bar. The time-dependent formation of H-atoms was measured behind reflected shock waves by using the very sensitive method of atomic resonance absorption spectrometry (ARAS). A detailed chemical kinetic reaction mechanism consisting of 33 elementary reactions and 26 species was used to model the experimentally obtained H-atom profiles. From kinetic modelling, with help of sensitivity and reaction flux analysis, it was concluded that reaction R 1 2-C4H6 ↔ 2-C4H5 + H is crucial for the observed formation of H-atoms during the thermal decomposition of both investigated species and within the investigated range of temperatures and pressures. Moreover, at temperatures above about 1650 K, the decay of propargyl radicals (C3H3) turns out to contribute significantly to the amount of produced H-atoms. The following rate expressions were obtained for three reactions (R 1–R 3) – among them the isomerisation from 2-butyne to 1,3-butadiene – important with respect to the formation of H-atoms within the investigated parameter range. The uncertainties are estimated to be ±30%: 2-C4H6 ↔ 2-C4H5 + H (R 1) 2-C4H6 ↔ 1,2-C4H6 (R 2) 1,3-C4H6 ↔ C2H2 + C2H4 (R 3) k 1=3.8⋅1015exp(-44871K/T)s-1 k 2=6.9⋅1013exp(-32496K/T)s-1 k 3=7.0⋅1012exp(-33768K/T)s-1

The branching ratio in the thermal decomposition of H 2CO

The thermal decomposition of H2CO has been investigated in reflected shock-wave experiments at temperatures between 2004 and 2367 K. The quantitative temporal formation of H atoms in the reactions, (1a) H2CO+Kr→HCO+H+Kr and HCO+Kr→CO+H+Kr, were measured by the atomic resonance absorption spectrometric (ARAS) technique. The product HCO radicals instantaneously decompose giving a second H atom. The experiments were carried out under conditions where secondary reaction perturbations were negligible. The observed H-atom profiles could be reproduced using a two-step mechanism, reactions (1a) and (1b), H2CO+Kr→H2+CO+Kr. The resulting values for the branching ratio, k1a/(k1a+k1b) range between 6.7 and 12.2%. The data yield second-order rate constants, k1a=1.019×10−8 exp(−38,706 K/T) and k1b=4.658×10−9 exp(−32,110 K/T) cm3, molecule−1 s−1, respectively. The rate data and branching ratio results are compared to earlier determinations. Lastly, the data are theoretically rationalized using three theoretical formalisms. Single-channel theoretical calculations are carried out with the semiempirical Troe and with the RRKM-Gorin methods, and these are compared to multichannel RRKM calculations using the Unimol code.

Numerical modeling of the filament-assisted diamond growth environment

A numerical model of the filament-assisted diamond growth environment has been developed and used to calculate temperature, velocity, and species concentration profiles, accounting both for transport and detailed chemical kinetics. The computed hydrocarbon concentrations agree well with previously measured values, when allowance is made for 3D effects not included in our model. Upper-bound, diffusion-limited film growth rates for various assumed growth species have been computed, and it has been found no hydrocarbon species other than CH3, C2H2, or CH4 can account for measured diamond growth rates. The effect of thermal diffusion on H-atom profiles has been examined, and found to be only 10%. Although the environment is far from thermodynamic equilibrium, several reactions are close to partial equilibrium throughout the region from the filament to the substrate. It is also shown that homogeneous H-atom recombination is too slow to explain the experimentally observed decrease in the concentration of H with increasing initial methane concentration.