K. Hoyermann

A detailed chemical reaction mechanism for the oxidation of hydrocarbons and its application to the analysis of benzene formation in fuel-rich premixed laminar acetylene and propene flames

On the basis of existing detailed kinetic schemes a general and consistent mechanism of the oxidation of hydrocarbons and the formation of higher hydrocarbons was compiled for computational studies covering the characteristic properties of a wide range of combustion processes. Computed ignition delay times of hydrocarbon–oxygen mixtures (CH4-, C2H6-, C3H8-, n-C4H10-, CH4 + C2H6-, C2H4, C3H6-O2) match the experimental values. The calculated absolute flame velocities of laminar premixed flames (CH4-, C2H6-, C3H8-, n-C4H10-, C2H4-, C3H6-, and C2H2-air) and the dependence on mixture strength agree with the latest experimental investigations reported in the literature. With the same model concentration profiles for major and intermediate species in fuel-rich, non-sooting, premixed C2H2-, C3H6- air flames and a mixed C2H2/C3H6 (1:1)-air flame at 50 mbar are predicted in good agreement with experimental data. An analysis of reaction pathways shows for all three flames that benzene formation can be described by propargyl combination.

Low-pressure hydrogen/oxygen flame studies

Abstract Composition and temperature profiles are reported for lean, near stoichiometric and rich, low-pressure hydrogen/oxygen flames. A discussion of the consistency checks used to assure the soundness of the analysis and various methods to estimate radical concentrations is presented. The mechanism of water formation in the various flame systems is discussed.

Kinetics of the reaction of hydroxyl radicals with ethylene and with C3 hydrocarbons

The rates of reaction of hydroxyl radicals with ethylene, propane, propylene, methylacetylene, and allene have been measured at room temperature in a discharge-flow system using electron spin resonance detection. The stoichiometries (n=Δ[OH]/Δ[R]) were obtained by mass spectrometric analysis of the reacted gas under similar, although not completely identical conditions. The primary rate constants for the C3-hydrocarbons obtained by combining the two are given as: OH + propane k6=(5.0 ± 1.0)× 1011 cm3 mol–1 s–1+ propylene k7=(3.0 ± 1.0)× 1012 cm3 mol–1 s–1+ methylacetylene k8=(5.7 ± 1.0)× 1011 cm3 mol–1 s–1+ allene k9=(2.7 ± 1.5)× 1012 cm3 mol–1 s–1. The values of n as well as the nature of the products provide some information on the mechanisms involved.A value of k5=(1 ± 0.3)× 1012 cm3 mol–1 s–1 was obtained for the reaction of OH + ethylene.

Mechanism and rate of the reaction CH3+ O—revisited

The primary products and the rate of the reaction of methyl radicals with oxygen atoms in the gas phase at room temperature have been studied using three different experimental arrangements: (A) laser flash photolysis to produce CH3 and O from the precursors CH3I and SO2 (the educts and the products were detected by quantitative FTIR spectroscopy); (B) the coupling of a conventional discharge flow reactor via a molecular sampling system to a mass spectrometer with electron impact ionization, which allowed the determination of labile and stable species; (C) laser induced multiphoton ionization combined with a TOF mass spectrometer-molecular beam sampling-flow reactor, which was used for the specific and sensitive detection of the CH3, CD3, C2H5 and C2D5 radicals and the determination of rate coefficients. The branching ratio of the reaction channels was determined by the experimental arrangements (A) and (B) leading to CH3 + O --> HCHO + H (55 +/- 5)% --> CO + H2 + H (45 +/- 5)%. The rate coefficients of the normal and deuterated methyl and ethyl radicals with atomic oxygen showed no isotope effect: k(CD3 + O)/k(CH3 + O) = 0.99 +/- 0.12, k(C2D5 + O)/k(C2H5 + O) = 1.01 +/- 0.07 (statistical error, 95% confidence level). The absolute rate coefficient of the reaction CH3 + O was derived with reference to the reaction C2H5 + O (k = 1.04 x 10(14) cm3 mol(-1) s(-1)) leading to k(CH3 + O) = (7.6 +/- 1.4) x 10(13) cm3 mol(-1) s(-1).

Mechanisms and rates of the reactions C2H5+O and 1-C3H7+O

The mechanisms and rates of the reactions of the primary alkyl radicals ethyl and l-propyl with oxygen atoms at room temperature and low pressure (around 5 mbar) have been studied using two independent experimental arrangements. The reactants were generated by UV-laser flash photolysis with different precursors (C 2 H 5 COC 2 H 5 , C 2 H 6 +CFCl 3 , C 2 H 5 I, C 3 H 7 COC 3 H 7 , SO 2 ). Stable species concentrations were measured quantitatively by Fourier transform IR and OH radical concentrations of the ground ( v =0) and first vibrational ( v =1) state by time-resolved laser-induced fluorescence. For both reaction 1 and reaction 2, the mechanism is explained in terms of the formation and subsequent decomposition of a chemically activated alkoxy radical and a competing abstraction channel leading directly to OH and the alkene: C 2 H 5 +O→C 2 H 5 (reaction Ia)/C 2 H 5 O→HCHO+CH 3 (reaction la 1 )/CH 3 CHO+H (reaction 1a 2 )//C 2 H 5 +O→C 2 H 4 +OH (reaction 1b). The absolute branching ratio was determined preferentially using diethyl ketone as the C 2 H 5 radical source leading to (1a 2 )/(1a 2 )/(1b), 32/44/24. Relative branching ratios for the C 2 H 5 radical sources C 2 H 6 +Cl and C 2 H 5 I were derived as (1a 1 /(1a 2 )=1/1.5 and 1/1.55, respectively. The overall rate coefficient of the reaction C 2 H 5 +O was measured as k 1 =(1.04±0.1)×10 14 cm 3 mol −1 s −1 and in addition k (C 2 H 5 +OH)=(7.0±1)×10 13 cm 3 mol −1 s −1 . The mechanism and the rate of reaction 2 were found as 1-C 3 H 7 +O→1-C 3 H 7 O (reaction 2a)/1-C 3 H 7 O→HCHO+C 2 H 5 (reaction 2a 1 )/C 2 H 5 CHO+H (reaction 2a 2 )//1.C 3 H 7 +O→C 3 H 6 +OH (reaction 2b) (branching ratio (2a 1 )/(2b), 44/32/24 and k 2 =(8.2±1)×10 3 mo −1 s −1 The results are discussed in terms of statistical rate theory.

Experimental and mathematical study of a hydrogen-oxygen flame

Fuel-rich premixed hydrogen-oxygen flames were stabilized on cooled porous plate burners at pressures around 10 mm Hg. The concentrations of H 2 , O 2 , H 2 O, H, and OH were measured by means of mass spectroscopy, gas chromatography, uv-absorption spectroscopy, and ESR spectroscopy. For one flame, the system of equations describing the propagation of the flame was solved numerically with respect to the interactions between the flame and the flameholder. These profiles, which were computed according to the boundary conditions and the kinetic data, are in agreement with the measured profiles. A discussion on the accuracy of concentration and temperature measurements is given, taking into account diffusion, thermal diffusion, heat transfer, and “two-body collision” equilibrium. From the experimental and mathematical work, it was found that, in these fuel-rich hydrogen-oxygen flames, water is produced at low temperatures by the chain mechanism H+O 2 +M→HO 2 +M H+HO 2 →OH+OH OH+H 2 →H 2 O+H, with the chain length of the order of 10. At high temperatures, water is produced by the branching mechanism H+O 2 →OH+O (2) O+H 2 →OH+H OH+H 2 →H 2 O+H (1) The derived rate coefficients k 1 =1×10 13 exp (−4800/ RT ); 500°K T k 2 =2.3×10 14 exp (−16800/ RT ); 650°K T 3 , °K) are in good agreement with rate constants reported in the literature.

Mechanisms and rates of the reactions of CH3O and CH2OH radicals with H atoms

The reactions of hydrogen atoms with methoxy (CH 3 O) and hydroxy methyl (CH 2 OH) radicals were studied directly at low pressures (0.1–2 torr) using fast flow reactors and a Laval nozzle reactor. Samples from the reaction center were withdrawn continuously by a molecular beam system and transferred into the ion source, of a mass spectrometer. Chemical modulation combined with digital synchronous ion counting was used for high-sensitivity concentration measurements. High resolution mass spectra and ionization curves allowed the specific detection of labile and stable species. Various isotopically labelled CH 3 O and CH 2 OH radicals were produced via H/D atom abstraction from CH 3 OH, CH 3 OD, CD 3 OH, CD 3 OD by F atoms. The measurements of the primary products of the reactions of isotopically labelled radicals with H/D atoms establish the following mechanisms: C H 3 O + H → H C H O + H 2 ( a ) → C H 3 O H ° → M C H 3 O H ( M ) → C H 3 + O H ( b ) C H 2 O H + H → H C H O + H 2 ( a 1 ) → C H O H + H 2 ( a 2 ) → C H 3 O H ° → M C H 3 O H ( M ) → C H 3 + O H ( b ) The abstraction reactions (a) were found to be the major routes (~75%). The ration of the rate constants was determined as k ( C H 3 O + H ) / k ( C H 2 O H + H ) = 2 / 3 By measuring the constants of the reaction CH 2 OH+H and of the reference reaction C 2 H 5 +H simultaneously, the absolute rate coefficients were obtained k ( C H 3 O + H ) = 2 ⋅ 1 0 1 3 c m 3 / m o l s k ( C H 2 O H + H ) = 3 ⋅ 1 0 1 3 c m 3 / m o l s

The detection of CH3CO, C2H5, and CH3CHO by rempi/mass spectrometry and the application to the study of the reactions H+CH3CO and O+CH3CO

The discharge flow reactor technique combined with molecular beam sampling and either resonance enhanced multi-photon ionization (REMPI)/mass spectrometry or low energy electron impact ionization (EI)/mass spectrometry has been used to study the specific and sensitive detection of radicals and molecules and the reaction of radicals with atoms. The acetyl radical CH 3 CO shows a REMPI mass spectrum at m/e =43 in the wavelength region 395–415 nm and at 430 nm. The ethyl radical C 2 H 5 is detected by multi-photon ionization at the wavelength 400–460 nm by the parent ion m/e =29. Acetyldehyde is monitored sensitively by a (2+1) Rydberg transition at 440.6 nm with ion fragmentation at m/e =15, 29, 43. The reaction mechanism of CH 3 CO radicals with H atoms at low pressure (0.01–0.1; 0.5–10 mbar) and temperature (293–300 K) is given by CH 3 CO+H→CH 3 +HCO (7a) →CH 3 CHO (7b) →CH 2 CO+H 2 (7c) Reaction channel (a) is the main route at low pressure (1 mbar) and the ratio of (a)/(b) is discussed in a model of chemical activation/collision stabilization. The rate constant k 1 was determined relative to the reaction C 2 H 5 +H leading to k 7 =(3.3+2)·10 13 cm 3 /mol· s The ratio of the rate constants of acetyl CD 3 CO and vinoxy CD 2 CHO radicals with D atoms was determined as k 7 dd (CD 3 CO+D)/ k 11− d (CD 2 CHO+D)=1.6+0.6 For the reaction CH 3 CO+O→CH 3 +CO 2 (12a) →OH+CH 2 CO (12b) the route (12a) dominates. The rate of this reaction (12) was measured using the reaction C 2 H 5 +O as a standard: k 12 =(1.2±0.4)·10 14 cm 3 /mol· s

The reactions of benzyl radicals with hydrogen atoms, oxygen atoms, and molecular oxygen using EI/REMPI mass spectrometry

The reactions of benzyl radicals (C 7 H 7 ) with H atoms, O atoms and O 2 molecules were studied at low pressure (around 1 mbar) and room temperature in multiple discharge flow reactor arrangements by molecular beam sampling and mass spectrometry. The ionization of labile and stable species was performed using electron impact ionization (EI) and resonance enhanced multiphoton ionization (REMPI) by an excimer pumped dye laser arrangement. The C 7 H 7 radicals were produced by the reactions of toluene (C 7 H 8 ) with halogen atoms C 7 H 8 +Cl→C 7 H 7 +HCl (1) C 7 H 8 +F→C 7 H 7 +HF (2a) →C 6 H 5 F+CH 3 (2b) →C 7 H 7 F+H (2c) The rate of the Rxn. (1) was determined with respect to the reference reaction C 2 H 6 +Cl leading to k 1 =3.5 10 13 cm 3 /mol s. The mechanism of the Rxn. (3) is described by C 7 H 7 +H→C 7 H 8 (3) The use of D atoms establishes this mechanism and the absence of an isotope exchange reaction C 7 H 7 +D→C 7 H 7 D (3D) C 7 H 7 +D→CHin7H 6 D+H Relative rate constants were measured with reference to the reaction CH 3 +D→CH 2 D+H leading to k 3 =3.3. 10 14 cm 3 /mol s and k 3D =1.8 10 14 cm 3 /mol s. The mechanism of reaction (4) C 7 H 7 +O→C 6 H 5 CHO+H (4a) →C 6 H 6 +HCO (4b) shows comparable probability for (4a) and (4b). With respect to the rate of the reaction CH 3 +O the rate constant k 4 was found as k 4 =3.3 10 14 cm 3 /mol s. The rate of the reaction C 7 H 7 +O 2 →products (5) under the low pressure and temperature conditions was estimated to be k ≥3 10 11 cm 3 /mol s.

Reactions of phenoxy radicals in the gas phase

The mechanisms and rates of the formation and the reactions of the phenoxy radical with atoms andradicals in the gas phase have been studied at room temperature (around 295 K) and low pressure (1–5 mbar) using the discharge flow technique. Samples were withdrawn continuously by a molecular-beam-sampling system coupled to a mass spectrometer with electron impact ionization for a specific and sensitive detection by applying low ionization energy and single-ion counting. Phenoxy radicals were produced by the reaction of phenol with chlorine atoms C6H5OH+Cl→C6H5O+HCl (1) and the rate coefficient k1 was determined to be k1=(1.43±0.25)·1014cm3/mol s (relative to the reference reaction C2H6+Cl, k0=3.4·1013 cm3/mol s). Phenoxy radicals react with atoms according to C6H5O+O→C6H4O2 (quinone)+H (2a) →C5H5+CO2 (2b) with reaction (2a) being the main pathway, and a rate coefficient of k2=(1.68±0.35)·1014 cm3/mol s (measured relative to the standard reaction (01) C2H5+O, k01=1.31·1014 cm3/mol s) and C6H5O+H→C6H6O/C6H5OH (3) with C6H6O being cyclohexadienone, and a rate coefficient k3=4·1013 cm3/mol s. (The reference reaction was (02) C2H5+H, k02=3.6·1013/mol s, and the ratio of the rate constants was determined as k3/k02=1.12±0.03.) Phenoxy radicals combine with the alkyl radicals C2H5 and CH3 in two different pathways with comparable efficiency:C6H5O+C2H5→C6H5OC2H5 (phenetol) (4a) →C2H5C6H4OH (ethylphenol) (4b) C6H5O+CH3→C6H5OCH3 (anisol) (5a) →CH3C6H4OH (cresol) (5b) ((4a):(4b)=63:37; (5a):(5b)=59:41.) The combination reaction of phenoxy radicals is fast:C6H5O+C6H5O→(C6H5O)2 k6=(2+2/−1)·1013 cm3/mol s (6)