Reaction mechanisms and rate constants of PAH growth in astrophysical environments
Alexander M. Mebel
Florida International University
The presentation will overview results of quantum chemical calculations of potential energy surfaces combined with RRKM-Master Equation calculations of reaction rate constants, carried out in order to unravel reaction mechanisms of the growth of polycyclic aromatic hydrocarbons (PAHs) at temperatures and pressures relevant to the interstellar medium or to carbon-rich circumstellar environments. We will describe our recent efforts directed toward the development of a comprehensive mechanism of PAH growth and consider possible formation routes to two-ring PAHs, naphthalene and indene, as well as the Hydrogen Abstraction aCetylene Addition (HACA) and Hydrogen Abstraction Vinylacetylene Addition (HAVA) growth mechanisms of larger PAHs. The computational results will be compared with the experimental findings by R. Kaiser’s (University of Hawaii at Manoa) and M. Ahmed’s (LBL) groups utilizing a pyrolytic chemical reactor and product identification by means of photoionization spectroscopy using the quasi-continuous tunable vacuum ultraviolet light from the Advanced Light Source. In particular, we will describe the C6H5 + C4H4 and C10H7 (1-/2-naphthyl) + C4H4 reactions and show that they can form naphthalene, phenathrene, and anthracene even at very low temperatures in the interstellar medium. Alternatively, the HACA mechanism is feasible only in high-temperature circumstellar environments. We will consider prototype C2H2 addition steps as well as formation of phenanthrene from a biphenylyl radical and formation of pyrene from a phenanthryl radical. The complementary nature of the HACA and HAVA mechanisms and their role in the build-up of two-dimensional graphene-type nanostructures and three-dimensional carbonaceous nanostructures holding corannulene units through the incorporation of five-membered rings will be discussed.
Organic Acid and Carbonyl Formation from γ-Ketohydroperoxide Decomposition in n-Butane Oxidation
Denisia M. Popolan-Vaida,[a],[b] Arkke J. Eskola,[c] Brandon Rotavera,[c] Jessica F. Lockyear,[b] Zhandong Wang,[d] S. Mani Sarathy,[d] Arnas Lucassen,[c] Kai Moshammer,[c] Philippe Dagaut,[e] Nils Hansen,[c] Stephen R. Leone,[b] and Craig A. Taatjes[c]
[a] Department of Chemistry, University of Central Florida, Orlando, Florida 32816, USA
[b] Department of Chemistry, University of California, Berkeley, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
[c] Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551 USA
[d] King Abdullah University of Science and Technology, Clean Combustion Research Center, Thuwal 23955-6900, Saudi Arabia
[e] Centre National de la Recherche Scientifique, INSIS, 45071 Orléans Cedex 2, France
Autoignition of hydrocarbon-air mixtures plays a crucial role in the development of new low-emission, high efficiency engine technologies and relies on chain-branching reactions. A key chain-branching step in autoignition is the decomposition of ketohydroperoxides to form an oxy radical and OH. Other pathways compete with chain-branching, such as “Korcek” dissociation of γ-ketohydroperoxide to a carbonyl and an acid. A high temperature (500-1100 K) jet-stirred reactor in conjunction with a high-resolution tunable synchrotron photoionization time-of-flight mass spectrometer offers a unique experimental 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 low temperature oxidation reaction. In addition to corroborating the observations of the ketohydroperoxide species in the oxidation of butane, the use of partially deuterated butane provides evidence for the Korcek mechanism of decomposition of the intermediate ketohydroperoxide species into acid, ketone and aldehyde pairs, through the observation of the partially deuterated acetone and formic acid Korcek pair. It was found that the Korcek decomposition mechanism of γ-ketohydroperoxide is a substantial fraction of the organic acid production, but it is unlikely to be a significant perturbation on the autoignition process. The results provide experimental bounds that enable the construction of more realistic and accurate kinetic mechanisms for autoignition chemistry.
Electrospinning of natural and synthetic polymers
Nelly Mateeva, Jamie Hamilton, Brittney Jackson, Christopher Weider
Florida A&M University
Nanostructured materials with high surface area are of tremendous importance for many industrial applications, such as production of catalysts, sensors, and thermoelectric materials. Electrospinning, a method which applies high voltage to a solution, or melt of a polymeric material, allows for the synthesis of fibers from nano- to micro- size with versatile properties. High surface-to-volume ratio and the availability of functional groups enable post-modification and further processing of the material. Most synthetic polymers are easy to electrospin, however proteins are notoriously difficult to convert to individual, bead-less fibers. The reasons for this are not well understood and we explored the effect of many parameters, such as viscosity, delivery rate, solvent, etc., on the production of nanofibers from egg while lysozyme (HEWL). Our group also created several metal-polymer composite materials that involving transition metals, with possible application in catalysis.
Kinetic Measurements of CO+ and CO2+ Reactions with N and O Atoms for Models of the Martian Atmosphere
Jake Tenewitz1, Tri Le1, Shaun G. Ard2, Nicholas S. Shuman2, Albert A. Viggiano2, and Joshua J. Melko1
1. University of North Florida, Department of Chemistry, Jacksonville, FL
2. Air Force Research Laboratory, Kirtland Air Force Base, Albuquerque, NM
We have measured rate constants for CO+ and CO2+ reacting with N and O atoms using a flow tube apparatus equipped with a microwave discharge atom source. We report new room-temperature rate constants for these reactions that are much less efficient than previously thought, and the reaction of CO2+ + O is observed to yield O2+ exclusively, in contrast to an existing measurement in the literature. Experimental work was supplemented by molecular structure calculations. Calculated pathways show the sensitivity of kinetic barriers to theoretical method, and indicate high level ab initio methods are required for accurate energetics. Our findings suggest that models of planetary atmospheres and the interstellar medium need to be updated accordingly. We will highlight a recently published model of the Martian atmosphere utilizing our new values.
Tracking the ultrafast charge carrier dynamics at the surface of photocatalytic materials
Mihai E. Vaida1, Brett M. Marsh2, Bethany Lamoureux,2 and Stephen R. Leone2
1 Department of Physics, University of Central Florida, Florida 32816, USA
2 Departments of Physics and Chemistry, University of California, Berkeley, California 94720, USA and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
The electronic structure of semiconductor substrates decorated with co-catalytic centers, i.e. metal and metal oxides small clusters and particles are important because of their many potential applications in photochemistry and catalysis. Understanding electronic structure and ultrafast photoinduced charge carrier dynamics at the co-catalytic particles is a defining principle that will guide advances in the next generation of photocatalysts to produce storable fuels from sustainable inputs.
Time resolution, surface sensitivity and element specificity are technical ingredients required to investigate ultrafast photoinduced processes of charge migration, localization and recombination at the surface of photocatalytic materials. All these requirements are fulfilled by a new experimental technique based on pump-probe photoelectron spectroscopy in conjunction with femtosecond extreme ultraviolet (XUV) laser pulses that will be presented in this contribution. The ultrafast electron and hole charge state dynamics at photocatalytic surfaces is investigated by monitoring the ultrafast photoinduced transient charging of the overlayers, particles, or clusters at surface.
Gold clusters grown on 10 ML MgO(100)/Mo(100) are investigated as a model system for using static XUV photoemission as a probe of electronic character versus cluster size. As the size of the Au clusters is increased, a gradual shift in the photoemission onset up to the Fermi energy indicates a change in the character of the gold clusters from non-metallic to metallic. The results are compared with theoretical work and previous investigations to validate the PES method. Static photoemission is then further utilized to monitor the electronic structure of Zn clusters on p-Si(100) as a function of Zn deposition. The transition from non-metallic to metallic Zn character is observed at 0.16 ML of Zn coverage. Furthermore, femtosecond pump-probe XUV photoemission spectroscopy technique is employed to induce a charge transfer from the p-Si(100) substrate to the Zn clusters and to measure in real time the charge trapping at the Zn cluster as well as the subsequent charge relaxation. The ultrafast charge carrier dynamics is investigated as the Zn dimensionality is increased from small clusters composed of a very few atoms to large particles to extended Zn films.
Origins of Life: Prebiotic Chemistry in Simulated Hydrothermal Vent Environments via Calcium Carbonate, Barium Carbonate and Iron Sulfide Chemical Garden Catalysis.
Arthur P. Omran and Oliver Steinbock
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306-4390, USA
Hydrothermal environments could be the setting for where life originated. Hydrothermal vent chimneys such as black and white smokers can be simulated in the lab using pump injected chemical garden tubes. The hydrothermal vent chimneys are examples of chemical gardens themselves. To simulate white smoker hydrothermal vents, we have synthesized calcium carbonate and barium carbonate chemical garden tubes, by injecting calcium chloride and barium chloride respectively, into sodium silicate solution. To simulate black smoker hydrothermal vents, we have synthesized iron (II) sulfide chemical garden tubes, by injecting iron (II) chloride into sodium silicate solution containing sodium sulfide. We have characterized these precipitation products spectroscopically and with x-ray diffraction. We then expose these tubes to hydrothermal conditions and added formaldehyde. We found that these tubes act as a heterogeneous catalyst for the formose reaction and produce various sugars associated with the reaction. Furthermore, we found that at lower starting pH values for our system the calcite tubes act as a catalyst for the Cannizzaro reaction producing formic acid and methanol. We verified the products of these reactions using 1H NMR. Moreover, the presence of organic species in the system does not inhibit the precipitation formation of the chemical garden tubes. Finally, we demonstrate that the carbonate tubes evolve a bicarbonate buffer. We believe that synthesis and transport in a hydrothermal environment could form, and subsequently protect via buffering, biomonomers setting the stage for further chemical evolution.