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Vibrational Spectroscopic Investigation of Dehydrogenation on Supported Platinum Nanoparticles, and Photoelectron Spectroscopy to Probe Chemistry at the Aqueous Interface

  • Author(s): Van Spyk, Marijke Helene Christy
  • Advisor(s): Hemminger, John C.
  • et al.
Abstract

This work focuses on determining the fundamental chemistry of molecules at interfaces important to catalysis and atmospheric chemistry using electron-detection based techniques. Instruments were either retrofitted, or designed and constructed to facilitate the study of interfaces in ultrahigh vacuum (UHV) or in near-ambient pressures. Chapters two and five overview theory and instrumental development for high resolution electron energy loss spectroscopy (HREELS) and liquid-microjet X-ray photoelectron spectroscopy (LJ-PES), respectively.

Chapters three and four cover vibrational spectroscopic studies completed at UC Irvine. The dehydrogenation of cyclohexane over alumina-supported platinum nanoparticles was found to differ from that over platinum surfaces in HREELS studies. HREELS was also used to test cleaning or activation protocols for nanoparticles supported on highly oriented pyrolytic graphite (HOPG). These are the first HREELS studies for cyclohexane dehydrogenation over supported nanoparticles, and results provide fundamental information about reactivity for this system.

Chapters six through eight cover LJ-PES studies on aqueous solutions completed at the ALS synchrotron facility in California, and at the BESSY synchrotron in Berlin, Germany. Here, the relative concentrations and chemistry of aqueous solutes at the air-water interface and in the bulk of solution were probed using LJ-PES. Chapter six investigates the interfacial deprotonation of aqueous ammonium, which varies with pH so that ammonium is depleted at the interface at low pH. Chapter seven describes the distribution of dications in magnesium chloride or sulfate solutions. It is found that sulfate ions "pull" magnesium cations away from the interface. These results are compared with ionic distributions for sodium chloride solutions that have well understood interfacial chemistry. Chapter eight focuses on interfacial solute distributions in ternary aqueous organic and ionic solutions. The addition of ethanol perturbs the ionic distribution for magnesium or sodium chloride solutions, in contrast to ethylene glycol addition, which does not. Additional studies on the interfacial behavior of acetonitrile and propionitrile (including the effects of potassium chloride on this behavior), and the distribution of lithium halide salts in aqueous solution are briefly mentioned. Results have ramifications for aqueous atmospheric aerosol chemistry, but are also interesting from a fundamental standpoint.

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