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Time-resolved broadband cavity-enhanced absorption spectroscopy for chemical kinetics

Sheps, Leonid S.; Chandler, D.W.

Experimental measurements of elementary reaction rate coefficients and product branching ratios are essential to our understanding of many fundamentally important processes in Combustion Chemistry. However, such measurements are often impossible because of a lack of adequate detection techniques. Some of the largest gaps in our knowledge concern some of the most important radical species, because their short lifetimes and low steady-state concentrations make them particularly difficult to detect. To address this challenge, we propose a novel general detection method for gas-phase chemical kinetics: time-resolved broadband cavity-enhanced absorption spectroscopy (TR-BB-CEAS). This all-optical, non-intrusive, multiplexed method enables sensitive direct probing of transient reaction intermediates in a simple, inexpensive, and robust experimental package.

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TYPE SAND Report YEAR 2013
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Low-temperature combustion chemistry of biofuels: Pathways in the low-temperature (550-700 K) oxidation chemistry of isobutanol and tert-butanol

Proceedings of the Combustion Institute

Welz, Oliver W.; Savee, John D.; Eskola, Arkke J.; Sheps, Leonid S.; Osborn, David L.; Taatjes, Craig A.

Butanol isomers are promising next-generation biofuels. Their use in internal combustion applications, especially those relying on low-temperature autoignition, requires an understanding of their low-temperature combustion chemistry. Whereas the high-temperature oxidation chemistry of all four butanol isomers has been the subject of substantial experimental and theoretical efforts, their low-temperature oxidation chemistry remains underexplored. In this work we report an experimental study on the fundamental low-temperature oxidation chemistry of two butanol isomers, tert-butanol and isobutanol, in low-pressure (4-5.1 Torr) experiments at 550 and 700 K. We use pulsed-photolytic chlorine atom initiation to generate hydroxyalkyl radicals derived from tert-butanol and isobutanol, and probe the chemistry of these radicals in the presence of an excess of O2 by multiplexed time-resolved tunable synchrotron photoionization mass spectrometry. Isomer-resolved yields of stable products are determined, providing insight into the chemistry of the different hydroxyalkyl radicals. In isobutanol oxidation, we find that the reaction of the a-hydroxyalkyl radical with O2 is predominantly linked to chain-terminating formation of HO2. The Waddington mechanism, associated with chain-propagating formation of OH, is the main product channel in the reactions of O2 with b-hydroxyalkyl radicals derived from both tert-butanol and isobutanol. In the tert-butanol case, direct HO2 elimination is not possible in the b-hydroxyalkyl + O2 reaction because of the absence of a beta C-H bond; this channel is available in the b-hydroxyalkyl + O2 reaction for isobutanol, but we find that it is strongly suppressed. Observed evolution of the main products from 550 to 700 K can be qualitatively explained by an increasing role of hydroxyalkyl radical decomposition at 700 K. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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TYPE Journal Article YEAR 2013
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Low-temperature combustion chemistry of biofuels: Pathways in the initial low-temperature (550 K-750 K) oxidation chemistry of isopentanol

Physical Chemistry Chemical Physics

Welz, Oliver W.; Zador, Judit Z.; Savee, John D.; Ng, Martin Y.; Meloni, Giovanni; Fernandes, Ravi X.; Sheps, Leonid S.; Simmons, Blake S.; Lee, Taek S.; Osborn, David L.; Taatjes, Craig A.

The branched C 5 alcohol isopentanol (3-methylbutan-1-ol) has shown promise as a potential biofuel both because of new advanced biochemical routes for its production and because of its combustion characteristics, in particular as a fuel for homogeneous-charge compression ignition (HCCI) or related strategies. In the present work, the fundamental autoignition chemistry of isopentanol is investigated by using the technique of pulsed-photolytic Cl-initiated oxidation and by analyzing the reacting mixture by time-resolved tunable synchrotron photoionization mass spectrometry in low-pressure (8 Torr) experiments in the 550-750 K temperature range. The mass-spectrometric experiments reveal a rich chemistry for the initial steps of isopentanol oxidation and give new insight into the low-temperature oxidation mechanism of medium-chain alcohols. Formation of isopentanal (3-methylbutanal) and unsaturated alcohols (including enols) associated with HO 2 production was observed. Cyclic ether channels are not observed, although such channels dominate OH formation in alkane oxidation. Rather, products are observed that correspond to formation of OH via β-C-C bond fission pathways of QOOH species derived from β- and γ-hydroxyisopentylperoxy (RO 2) radicals. In these pathways, internal hydrogen abstraction in the RO 2 QOOH isomerization reaction takes place from either the -OH group or the C-H bond in α-position to the -OH group. These pathways should be broadly characteristic for longer-chain alcohol oxidation. Isomer-resolved branching ratios are deduced, showing evolution of the main products from 550 to 750 K, which can be qualitatively explained by the dominance of RO 2 chemistry at lower temperature and hydroxyisopentyl decomposition at higher temperature. © 2012 The Owner Societies.

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TYPE Journal Article YEAR 2012
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Results 51–72 of 72
Results 51–72 of 72