Chemistry Department Seminar: Dr. Denisia Popolan-Vaida

Thursday, February 16, 2017 10 a.m. to noon

Denisia M. Popolan-Vaida
Department of Chemistry, University of California Berkeley Chemical Sciences Division, Lawrence Berkeley National Laboratory

Seminar Title: Investigating the Multifaceted Oxidation Chemistry of Hydrocarbons

Hydrocarbon oxidation reactions have been intensively investigated due to their importance in many areas of chemistry, such as combustion, polymer, lubricants, and atmospheric chemistry. In this talk, I will present results of how mass spectrometric methods and synchrotron based techniques can be used to selectively study the individual processes involved in hydrocarbon oxidation reactions important in combustion and atmospheric chemistry.

A high temperature (500-1100 K) jet-stirred reactor in conjunction with a high-resolution tunable synchrotron photoionization time-of-flight mass spectrometer technique offers a unique approach to monitor chemical transformations of key intermediates in well-defined conditions comparable to those in combustion engines. This experimental arrangement was used to reveal new insights into the mechanism of n-butane oxidation reaction. In addition to corroborating the observations of the formation of γ-ketohydroperoxide molecules, which are species with the C=O and C−OOH functional groups separated by one −CH2− group, it can be shown that these molecules can govern low-temperature chain-branching combustion. The use of partially deuterated n-butane provides evidence for a so-called Korcek mechanism of decomposition of the intermediate γ-ketohydroperoxide species into acid, ketone and aldehyde pairs. We found that the Korcek decomposition mechanism of γ-ketohydroperoxide is a substantial fraction of the organic acid production, but is unlikely to be a significant contributor to autoignition processes.

Synchrotron photoionization mass spectrometry, in conjunction with standard aerosol techniques, was used to examine the fundamental heterogeneous reactions between hydrocarbon droplets and gas phase radicals and molecules. Squalane molecules (no C=C bonds) and squalene molecules (6 C=C bonds) are two branched hydrocarbon molecules that represent ideal model systems to mimic the variety of reactive carbon sites typically found on hydrocarbon surfaces and may also occur in ambient organic aerosols. The reaction of I atoms with squalene and squalane sub-micron droplets was investigated to determine the effect of C=C bonds on the reaction probability. There is no evidence of product formation with the saturated hydrocarbon squalane droplets. In contrast, unsaturated squalene droplets are found to react with I atoms, despite the gas-phase reaction having very low probability. The increased reactivity is attributed to trapping, diffusion, and consequent reactivity in the unsaturated particles due to multiple interactions in the aerosol phase. Cl2 reactivity was also used to probe the heterogeneous reactivity of closed shell molecules with C=C double bonds in hydrocarbon droplets composed of unsaturated molecules, such as oleic acid, linoleic acid, linolenic acid, and squalene. Cl2 is found to react with these droplets and the reaction probability was observed to increase with the number of C=C double bonds in the particles. The results emphasize that reactions that have low probability in the gas phase can have an enhanced probability in the heterogeneous phase, broadening the scope of reactivity for aerosols in the atmosphere and other environments. 

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Physical Sciences Building: 160

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Graduate chemistry UCF Chemistry Undergraduate Seminar