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Studies of Heterogeneously Catalyzed Liquid-Phase Alcohol Oxidation on Platinum by Sum-frequency Generation Vibrational Spectroscopy and Reaction Rate Measurements

  • Author(s): Thompson, Christopher Mark
  • Advisor(s): Somorjai, Gabor A
  • et al.
Abstract

Sum-frequency generation (SFG) vibrational spectroscopy has been used to study the interfacial environment in heterogeneously catalyzed alcohol oxidation on platinum catalysts in the liquid phase. In this work, theoretical concepts relevant to studying the metal liquid interface with non-linear spectroscopy are discussed, and the design of a home-built flow reactor cell with in situ SFG capabilities is described. SFG data is correlated with reaction rate measurements in liquid-phase isopropanol and ethanol oxidation in order to deduce the effect of the surface environment on the catalyst activity and to elucidate the evolution of the surface throughout the course of the reaction. In conditions of aqueous oxidation of ethanol and isopropanol to acetaldehyde and acetone, respectively, SFG results show that water and alcohol competitively populate the first molecular layer. The reaction rate of isopropanol shows an enhancement in the concentration regime where both water and isopropanol are present in the interfacial region. Water is thus implicated in the reaction as a stabilizer of an intermediate or serving as a proton acceptor in a deprotonation step. Similar results are seen for ethanol oxidation, but the role of water appears to be different in ethanol oxidation. Non-resonant SFG is used as a probe for oxygen adsorption on the platinum surface, and it is seen that the surface is covered in oxygen during the oxidation reaction. A kinetic model is proposed where the build-up of oxygen on the catalyst surface is prevented by its reaction with surface hydrogen. A key part of the model is its inclusion of a mechanism for positive feedback through a proposed step involving hydrogen abstraction by an adsorbed oxygen species. A synthetic method for synthesizing nanoparticles of diameter less than 2 nm is reported using platinum ion reduction in the poly(amidoamine) dendrimer induced by ultra violet light irradiation. The new method is inspired by the need for a reduction method other than sodium borohydride, which we show is not effective in reducing platinum, because platinum ions are strongly stabilized by multidentate interaction with the dendrimer amide groups. The interfacial orientation of isopropanol on platinum nanoparticles under conditions of gas and liquid phase catalysis is investigated using SFG based orientational analysis. The SFG results show that the isopropanol adopts a much different configuration on platinum in the liquid phase compared to the gas phase. Finally, the adsorption of amino acids at the polystyrene/H2O and SiO2/H2O interfaces is shown to have significant effect on the orientation and/or concentration of water at both interfaces. The effect of these molecules on water spans the range from highly disordering to highly ordering and depends on the amino acid identity.

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