1,721,014 research outputs found
The reaction of iso-propyl radicals with oxygen atoms: Rate coefficient, product branching, and relevance for combustion modeling
No full textReactions of hydrocarbon radicals with oxygen atoms are important in combustion, and both the rate coefficient and product branching have to be known for an accurate combustion modeling. In this work, the primary product formation in the reaction of the simplest open-chain secondary alkyl radical, iso-propyl (2-C3H7), with oxygen atoms in the gas phase was studied at room temperature and a pressure of 4 mbar. 2-C3H7 radicals were generated from diisopropylketone ((2-C3H7)(2)CO) and isopropyliodide (2-C3H7I), and O atoms were produced from SO2 by laser-flash photolysis at lambda = 193 nm, respectively. The reactants and products were detected by quantitative FTIR spectroscopy. The combined product analysis in the experiments with the different precursors leads to the following relative branching fractions: 2-C3H7 + O -> CH3CHO + CH3 (40%), CH3COCH3 + H (36%), C3H6 + OH (24%). The channel branching of the iso-propoxy (2-C3H7O) radical formed from the 2-C3H7 + O reaction was modeled using statistical rate theory with molecular and transition state data from G3MP2B3 calculations. The absolute rate of reaction was studied at room temperature and a pressure of 5.8 mbar. Laser-induced fluorescence (LIF) was used for the specific detection of the OH (v = 0,1) radicals, and the rate coefficient of the 2-C3H7 + O reaction was derived from the OH (v = 1) LIF-time profile leading to k(2-C3H7 + O) = (1.14 +/- 0.15) x 10(14) cm(3)/(mol.s) at 298 K. The OH-forming direct abstraction route and the channel to CH3CHO + CH3 may influence the flame speed of a propane flame. This is revealed, when the rate coefficient and channel branching of the 2-C3H7 + O reaction is incorporated in a suitable detailed reaction mechanism and target experiments are modeled in absence and presence of the title reaction. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved
The reaction of allyl radicals with oxygen atoms-rate coefficient and product branching
No full textThe primary product formation of the C(3)H(5) + O reaction in the gas phase has been studied at room temperature. Allyl radicals (C(3)H(5)) and O atoms were generated by laser flash photolysis at lambda = 193 nm of the precursors C(3)H(5)Cl, C(3)H(5)Br, C(6)H(10) (1,5-hexadiene), and SO(2), respectively. The educts and the products were detected by using quantitative FTIR spectroscopy. The combined product analysis of the experiments with the different precursors leads to the following relative branching fractions: C(3)H(5) + O -> C(3)H(4)O + H (47%), C(2)H(4) + H + CO (41'%), H(2)CO + C(2)H(2) + H (7%), CH(3)CCH + OH and CH(2)CCH(2) + OH (<5%). The rate of reaction has been studied relative to CH(3)OCH(2) + O and C(2)H(5) + O in the temperature range from 300 to 623 K. Here, the radicals were produced via the fast reactions of propene, dimethyl ether, and ethane, respectively, with atomic fluorine. Laser-induced multiphoton ionization combined with TOF mass spectrometry and molecular beam sampling from a flow reactor was used for the specific and sensitive detection of the C(3)H(5), C(2)H(5), and CH(3)COCH(2) radicals. The rate coefficient of the reaction C(3)H(5) + O was derived with reference to the reaction C(2)H(5) + O leading to k(C(3)H(5) + O) = (1.11 +/- 0.2) x 10(14) cm(3)/(mol s) in the temperature range 300-623 K. The C(3)H(5) + O rate and channel branching, when incorporated in a suitable detailed reaction mechanism, have a large influence on benzene and allyl concentration profiles in fuel-rich propene flames, on the propene flame speed, and on propene ignition delay times. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved
Exploring the chemical kinetics of partially oxidized intermediates by combining experiments, theory, and kinetic modeling
No full textCorrelation between aerosol yields and the decomposition of oxidized intermediates during gas phase ozonolysis at different pressures.</p
Shock-tube investigation of key reactions for chemiluminescence in various combustion systems
No full textExisting combustion systems, especially gas turbines in power generation applications must be optimized with regard to the reduction of pollutant emission and increase of efficiency. Combustion under fuel-lean conditions is beneficial for a significant reduction of NOx and soot formation. However, these operating conditions can lead to undesired combustion phenomena such as combustion-induced oscillations and flame flash back which must be avoided. For this purpose, fundamental knowledge of the underlying chemical processes is required. Non-intrusive optical methods such as the use of chemiluminescence are potential practical approaches to provide combustion relevant information for the development of combustion apparatus and process control. This requires knowledge of the formation reactions of chemiluminescence as well as adequate kinetics models that link the light intensity to relevant combustion parameters such as local heat release.
An accurate description of chemiluminescence fundamentally depends on the corresponding ground-state chemistry. For small hydrocarbons such as CH4 and C2H2 detailed reaction mechanisms already exist which were used as a base for the development of OH* and CH* sub-mechanisms in the present work. The present work was devoted to study the formation reactions of OH* and CH* chemiluminescence in shock tubes time-resolved detection of the emission with a photomultiplier with narrowband interference filters. The signals were compared to the corresponding excited-state species concentrations from simulations where based on established ground-state mechanisms, OH* and CH* kinetics models were compiled and validated with the experimental data from the present work. Based on the present work, the reactions H + O + M = OH* + M and CH + O2 = OH* + CO are identified as the main OH* formation channels in hydrogen and hydrocarbon oxidation and their corresponding rate coef-ficients are determined as (1.5±0.45)×1013 exp(−25.0 kJ mol−1/RT) cm6mol−2s−1 and (8.0±2.56)×1010 cm3mol−1s−1, respectively. For CH* chemiluminescence the reactions C2 + OH = CH* + CO and C2H + O = CH* + CO are the most important formation reactions and their underlying rate coefficients are (5.7±3.02)×1013 cm3mol−1s−1 and (1.0±0.53)×1012 exp(−10.9 kJ mol−1/RT) cm3mol−1s−1, respectively.
While for small hydrocarbons well-known ground-state mechanisms are available, reliable kinetics models for ethanol oxidation, especially for high temperatures, are sparse. Therefore, the formation of important intermediates and products (e.g., OH, C2H2, and CO2) was studied for ethanol oxidation by time-of-flight mass spectrometry and ring-dye laser absorption spectroscopy under shock-tube conditions. The experimental data were compared to simulations using different reaction mechanisms from the literature and recommendations for the improvement of the corresponding mechanisms were suggested.Stoßwellenuntersuchung von Schlüsselreaktionen der Chemilumineszenz in verschiedenen Verbrennungssystemen
Bestehende Verbrennungssysteme, insbesondere Gasturbinen für die Erzeugung von Strom, müssen in Hinblick auf die Reduzierung des Rohstoffeinsatzes und des Ausstoßes von Emissionen optimiert werden. Hierbei kann die Verbrennung unter mageren Mischungsbedingungen zu einer signifikanten Reduzierung der Stickoxid- und Rußbildung führen. Diese Betriebszustände führen jedoch teilweise zu unerwünschten Schwingungen und Flammen-rückschlag innerhalb der Brennkammer, die vermieden werden müssen. Hierfür ist ein grundlegendes Wissen über den zugrundeliegenden Verbrennungsprozess erforderlich. Nicht-invasive optische Methoden wie das Flammenleuchten sind potentielle Ansätze zur Bereitstellung von verbrennungsrelevanten Informationen für die Entwicklung von Verbrennungskonzepten und deren Regelung. Dies erfordert jedoch zum einen die Kenntnis über die Bildungsreaktionen der Chemilumineszenz und zum anderen sind geeignete Kinetikmodelle zur Beschreibung erforderlich.
Die Beschreibung der Chemilumineszenz erfordert genaue Kenntnis über die zugrundeliegende Grundzustandschemie. Für einfache Kohlenwasserstoffverbindungen wie z.B. CH4 oder C2H2 existieren bereits gut validierte Modelle, die in der vorliegenden Arbeit als Basis für die Entwicklung von OH*- und CH*-Mechanismen verwendet wurden. Im Rahmen dieser Arbeit wurden die Bildungsreaktionen der OH*- und CH*-Chemilumineszenz in Stoßwellenreaktoren mit Hilfe von Emissionsmessungen untersucht. Hierbei wurde das Flammleuchten mit einer Kombination aus Photomultiplier und schmalbandigem Interferenzfilter zeitaufgelöst gemessen. Basierend auf etablierten Mechanismen zur Beschreibung der Grundzustandschemie wurden Kinetikmodelle für OH*- und CH*-Chemilumineszenz aufgestellt und mithilfe der experimentellen Daten validiert. Die Reaktionen H + O + M = OH* + M und CH + O2 = OH* + CO wurden als Hauptreaktionen für die Bildung von OH* bei der Oxidation von Wasserstoff oder Kohlenwasserstoffen identifiziert und ihre zugrundeliegenden Geschwindigkeitskoeffizienten wurden ermittelt mit (1.5±0.45)×1013 exp(−25.0 kJ mol−1/RT) cm6mol−2s−1 bzw. (8.0±2.56)×1010 cm3mol−1s−1. Für CH*-Chemilumineszenz wurden die Reaktionen C2 + OH = CH* + CO und C2H + O = CH* + CO als wichtigste Bildungsreaktionen identifiziert und mit den Geschwindigkeitskoeffizient (5.7±3.02)×1013 cm3mol−1s−1 bzw. (1.0±0.53)×1012 exp(−10.9 kJ mol−1/RT) cm3mol−1s−1.
Während für kleine Kohlenwasserstoffe etablierte Mechanismen vorliegen, ist der Reaktionsmechanismus der Verbrennung von Ethanol, insbesondere bei hohen Temperaturen, nur unzureichend bekannt. Daher wurde im Rahmen dieser Arbeit die Bildung von wichtigen In-termediaten und Produkten (u.a. OH, C2H2, CO2) bei der Oxidation von Ethanol im Stoßwellenrohr mittels Flugzeit-Massenspektrometrie und Farbstoff-Ringlaser-Absorptionsspektroskopie untersucht und mit verschiedenen Reaktionsmechanismen verglichen, die zusätzliche Daten zur Verbesserung und weiteren Validierung der bestehenden Modelle liefern
Infrared Detection of Criegee Intermediates Formed during the Ozonolysis of beta-Pinene and Their Reactivity towards Sulfur Dioxide
No full textRecently, direct kinetic experiments have shown that the oxidation of sulfur dioxide to sulfur trioxide by reaction with stabilized Criegee intermediates (CIs) is an important source of sulfuric acid in the atmosphere. So far, only small CIs, generated in photolysis experiments, have been directly detected. Herein, it is shown that large, stabilized CIs can be detected in the gas phase by FTIR spectroscopy during the ozonolysis of beta-pinene. Their transient absorption bands between 930 and 830 cm(-1) appear only in the initial phase of the ozonolysis reaction when the scavenging of stabilized CIs by the reaction products is slow. The large CIs react with sulfur dioxide to give sulfur trioxide and nopinone with a yield exceeding 80%. Reactant consumption and product formation in time-resolved beta-pinene ozonolysis experiments in the presence of sulfur dioxide have been kinetically modeled. The results suggest a fast reaction of sulfur dioxide with CIs arising from b-pinene ozonolysis.DFG [GRK 782
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Mechanische Aspekte der Stickoxidbildung unter motorischen Bedingungen: Kinetische Modellierungen und Stoßwellenexperimente
No full textAufbereitung von Harnstoff-Wasser-Lösung zur Emissionskontrolle von Stickoxiden: homogene Gasphasenreaktionen und Partikelbildung
Full text linkDer Stickstoffoxidgehalt des Abgases von Dieselfahrzeugen wird heute großteils mithilfe von Ammoniak in selektiver katalytischer Reduktion (SCR) reduziert. In mobilen Anwendungen wird ein Vorläufer des Reduktionsmittels in Form von Harnstoff-Wasser-Lösung (HWL) als feines Spray vor dem SCR-Katalysator getaktet eingespritzt.
Homogene chemische Prozesse, die zwischen der HWL-Dosierstelle und dem Eingang des Katalysators ablaufen, werden in der aktuellen Arbeit unter-sucht. Der Fokus liegt hauptsächlich auf Gasphasenreaktionen, die nach der thermischen Zersetzung von Harnstoff vor dem Katalysator ablaufen. Es wird gezeigt, wie das für die schnellen SCR-Reaktionen im Oxidationskatalysator oxidiertes Stickstoffdioxid einen Reaktionsweg von Ammoniak zu Stickstoff-monoxid ermöglicht und dadurch die Reduktion in der Gasphase kontrapro-duktiv macht. Auch der fördernde Effekt und der Wirkungsmechanismus von erhöhtem Druck sowie Temperatur wurden erforscht, die bei „pre-turbine“ und „closed-coupled“ Anordnungen die Zusammensetzung der Gasmischung beeinflussen können. Neben Ammoniak wird auch Isocyansäure in der Harnstoffzersetzung freigesetzt. Für Reaktionen von Isocyansäure mit Stickstoffoxiden und mit Ammoniak unter abgasrelevanten Bedingungen wurde ein Mechanismus als Teil der Arbeit entwickelt und für Simulationen verwendet.
Neben Gasphasenreaktionen wurde die Verdampfung und die thermo-chemische Zersetzung von Einzeltröpfchen des HWL-Sprays simuliert, um eine
mögliche Partikelbildung vorherzusagen. Zudem wurden Möglichkeiten und Grenzen zur Eliminierung dieser Partikel mit Temperaturerhöhung geschildert.
Die durch diese Arbeit entstandenen neuen Kenntnisse ermöglichen es, Harnstoff-SCR Systeme noch realitätsnaher zu betrachten, diese genauer zu modellieren und dadurch eine sicherere und effizientere Stickstoffoxidreduktion zu realisieren
Rate coefficients for cycloalkyl + O reactions and product branching in the decomposition of chemically activated cycloalkoxy radicals: an experimental and theoretical study
No full textThe kinetics of cycloalkyl + O reactions were studied with respect to their rate coefficients and the product branching ratios from the decomposition of the chemically activated cycloalkoxy radicals. Rate coefficients for the reactions of cyclohexyl (c-C6H11), cycloheptyl (c-C7H13) and cyclooctyl (c-C8H15) radicals with oxygen atoms were determined with an experimental setup consisting of a discharge flow reactor with molecular beam sampling and REMPI/TOF-MS detection. The following rate coefficients were obtained (units: cm(3)/mol(-1) s(-1)): k(c-C6H11 + O) = (1.33 +/- 0.24) x 10(14)(T/298 K)(0.11) (T = 250-600 K), k(c-C7H13 + O) = (1.85 +/- 0.25) x 10(14) (T = 298 K), k(c-C8H15 + O) = (1.56 +/- 0.20) x 10(14)(T/298 K)(0.66+/-0.15) (T = 268-363 K). Stable products were determined by quantitative FTIR spectroscopy. The decomposition of the cycloalkoxy radicals leads besides beta-C-H bond fission (yields: 24% for c-C6H11O, 20-25% for c-C8H15O) mainly to alkyl radicals by ring-opening via beta-C-C bond cleavage. These open-chain alkyl radicals further decompose mainly by beta-C-C bond scission. An increase of the total pressure from 4 mbar to 1 bar had no effect on the product distribution for the reaction c-C6H11 + O, whereas for the reaction c-C8H15 + O further decomposition of the ring-opening product is significantly suppressed at 1 bar. The experimental results on the channel branching and its pressure dependence were rationalized with the statistical rate theory. A comparison of the experimental and modeling results indicates a significant influence of hindered internal rotations (HIRs) on the reactions of the ring-opening products. The harmonic approximation to describe these modes was shown to be inadequate, while a treatment as one-dimensional HIRs led to a significantly improved agreement between experimental and modeling results. Implications of our findings for the formation of secondary organic aerosol and high-temperature combustion are discussed
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