1,721,039 research outputs found
Sodium doped hydrogen bonded clusters: Solvated electrons and size selection
No full textNeutral, sodium doped clusters feature special properties related to solvated electron formation which allow for a wide range of applications. In case of water, methanol and ethanol - but not for ammonia - the evolution of the ionization energy stops at small cluster sizes, reaching values similar to binding energies of solvated electrons in the liquid phase. Because of the appreciable lowering of the ionization energy, their photoionization can be performed free of fragmentation by standard lasers. This provides a very effective method for detection and, in combination with infrared excitation, for taking OH-stretch spectra of size selected, neutral clusters. (C) 2013 Elsevier B.V. All rights reserved.Deutsche Forschungsgemeinschaft [GRK 782, ZE 890 1-1
The Generation of a Compact n-Heptane Toluene Reaction Mechanism Using the Chemistry Guided Reduction (CGR) Technique
No full textThe present study describes the compilation and validation of a compact reaction mechanism for the oxidation of n-heptane, toluene and its mixtures using the Chemistry Guided Reduction (CGR) approach. By the module-wise composition of validated reaction schemes and the successive application of chemical lumping and redundant species retrieval for the n-heptane oxidation model, a compact mechanism is generated for reference fuel blends of n-heptane and toluene. The new mechanism is validated for recently published OH-concentrations histories and ignition times front shock tube studies, HCCl engine experiments and flame speed measurements. The good agreement between experiment and prediction demonstrates the general applicability of the CGR method
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
No full textOn 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-O-2) 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
Mechanisms and rates of the reactions C2H5+O and 1-C3H7+O
No full textThe mechanisms and rates of the reactions of the primary alkyl radicals ethyl and I-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 (C2H5COC2H5, C2H6 + CFCl3, C2H5I, C3H7COC3H7, SO2). 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: C2H5 + O --> C2H5O (reaction 1a)/C2H5O --> HCHO + CH3 (reaction 1a(1))/CH3CHO + H (reaction 1a(2))//C2H5 + O --> C2H4 + OH (reaction 1b). The absolute branching ratio was determined preferentially using diethyl ketone as the C2H5 radical source leading to (1a(1))/(1a(2))/(1b), 32/44/24. Relative branching ratios for the C2H5 radical sources C2H6 + Cl and C2H5I were derived as (1a(1))/(1a(2)) = 1/1.5 and 1/1.55, respectively. The overall rate coefficient of the reaction C2H5 + O was measured as k(1) = (1.04 +/- 0.1) X 10(14) cm(3) mol(-1) s(-1) and in addition k(C2H5 + OH) = (7.0 +/- 1) X 10(13) cm(3) mol(-1) s(-1). The mechanism and the rate of reaction 2 were found as 1-C3H7 + O --> 1-C3H7O (reaction 2a)/I-C3H7O --> HCHO + C2H5 (reaction 2a(1))/C2H5CHO + H (reaction 2a(2))//1-C3H7 + O --> C3H6 + OH (reaction 2b) (branching ratio (2a(1))/(2a(2))/(2b), 44/ 32/ 24) and k(2) = (8.2 +/- 1) X 10(13) cm(3) mol(-1) s(-1). The results are discussed in terms of statistical rate theory
Mechanism and rate of the reaction CH3+O- revisited
No full textThe 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. ash 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 + H-2 + 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) cm(3) mol(-1) s(-1)) leading to k(CH3 + O) = (7.6 +/- 1.4) x 10(13) cm(3) mol(-1) s(-1)
Experimental and modelling study of speciation and benzene formation pathways in premixed 1-hexene flames
No full textAn existing detailed and broadly validated kinetic scheme is augmented to capture the flame chemistry of 1-hexene under stoichiometric and fuel rich conditions including benzene formation pathways. In addition, the speciation in a premixed stoichiometric 1-hexene flame (flat-flame McKenna-type burner) has been studied under a reduced pressure of 20-30 mbar applying flame-sampling molecular-beam time-of-flight mass spectrometry and photoionization by tunable vacuum-ultraviolet synchrotron radiation. Mole fraction profiles of 40 different species have been measured and validated against the new detailed chemical reaction model consisting of 275 species and 3047 reversible elementary reactions. A good agreement of modelling results with the experimentally observed mole fraction profiles has been found under both stoichiometric and fuel rich conditions providing a sound basis for analyzing benzene formation pathways during 1-hexene combustion. The analysis clearly shows that benzene formation via the fulvene intermediate is a very important pathway for 1-hexene. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved
Kinetic Modeling of NO x Formation and Consumption during Methanol and Ethanol Oxidation
No full textCH2Cl and CHCl2 Radical Chemistry: The Formation by the Reactions CH3Cl + F and CH2Cl2 + F and the Destruction by the Reactions CH2Cl + O and CHCl2 + O
No full textThe primary product formation of the reactions CH2Cl + O and CHCl2 + O in the gas phase has been Studied around room temperature. The coupling of a conventional discharge flow reactor via a molecular sampling system to a mass spectrometer with electron impact ionization allowed the determination of labile and stable species (set-up A). The radicals are formed by H atom abstraction in the reactions CH3Cl + F and CH2Cl2 + F. The product analysis leads to the following branching fractions relative to precursor consumption: For CH2Cl + O, the channel HCHO + Cl yields 19 % and CO + HCl + H yields 43 %, the contributions of the labile species HCO is found but not quantified. For CHCl2 + O the channel CO + HCl + Cl yields 70 %, CICHO and the labile CICO are detected but not quantified. The comparison to CH3 + O shows the stepwise increase of channel fractions for the CO forming Mines by chlorination of the methyl radical. The rates of the reactions have been studied relative to CH3 + O and CH3OCH2 + O. Laser-induced multiphoton ionization combined with TOF mass spectrometry and molecular beam sampling front a flow reactor (set-up B) was used for the specific and sensitive detection of the CH2Cl, CHCl2, CH3, and CH3OCH2 radicals. The rate coefficient of the reactions CH2Cl + O was derived with reference to the reaction CH3OCH2 leading to k = (8.1 +/- 1.8) x 10(13) cm(3)/(mol.s) and for CHCl2 + O with reference to CH3 + O leading to k = (3.8 +/- 1.9) X 10(13) cm(3)/(mol.s). For CH3Cl + F and CH2Cl2 + F the rate coefficients have been determined with set-Lip A leading to k = (14.3 +/- 0.9)X 10(13) cm(3)/(mol.s) for CH3Cl + F and k = (8.4 +/- 3.8) X 10(13) cm(3)/(mol.s) for CH2Cl2 + F. Only a negligible temperature dependence in the temperature range from 250-360 K was observed for all reactions studied
Laserinduzierte Fluoreszenz von Iod in der Gasphase
No full textDie Fluoreszenz von Festkörpern und Flüssigkeiten ist ein weitläufig bekanntes Phänomen, das unter anderem auch im Alltag beobachtet werden kann. In diesem Versuch wird gezeigt, dass auch Stoffe in der Gasphase zu einer Fluoreszenz angeregt werden können – am Beispiel von gasförmigen Iod. Diese, durch grünes Laserlicht induzierte faszinierende Fluoreszenzentstehung sowie deren ‐löschung, können dabei anhand eines didaktisch angepassten Energiestufenmodell im fortgeschrittenen Chemieunterricht anschaulich erarbeitet werden. Darüber hinaus ermöglicht das Experiment viele Anknüpfungspunkte zu den grundlegenden Themen des Chemie‐ und Physikunterrichts wie Absorption, Emission, Strahlungs‐ sowie Energieübergänge Translation abstract en The fluorescence of solids and liquids is a well‐known phenomenon, also in everyday life. Within this experiment, the fluorescence of a substance in the gaseous phase is demonstrated by the example of gaseous iodine. In order to explain this fascinating observation, induced by green laser light, as well as its quenching in the presence of nitrogen, a didactically adjusted energy stage model is presented. This model is suitable for advanced chemistry class. In addition, the experiment offers several links to fundamental topics of chemistry and physics education, such as absorption, emission, and radiation‐ and energy transitions
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