Well-defined Metal Nanostructures as Platform for Chemical Characterization and Catalytic Applications.

Abstract

Metal nanostructures with controlled sizes and shapes possess interesting optical, electronic and catalytic properties, making them suitable for a wide range of applications. The objectives of this dissertation were to 1) exploit the optical properties of the metal nanostructures to achieve selective molecular sensing using surface enhanced Raman spectroscopy (SERS) and high selectivity towards the desired product in heterogeneous catalysis, and 2) improve the stability of the metal nanostructures of controlled shape under catalytic reaction conditions. The optical properties of silver (Ag) nanostructures can be controlled by changing their shape. We have demonstrated that this effect can be employed to control the degree to which the Ag nanostructures enhance different vibrational bands of a molecule in SERS. We have shown that Ag nanocube and Ag nanosphere substrates with surface plasmon resonance peaks near 600 and 560 nm could be used to selectively detect species with 2100-2200 cm-1 and 900–1000 cm-1 Raman vibrational bands, respectively. This approach gives new insights in the design of plasmonic substrates for the selective molecular species detection. We have also demonstrated that the optical properties of copper (Cu) nanoparticles can be exploited under propylene oxidation conditions to favor the formation of the desired product, propylene oxide. We find that Cu nanocatalysts exhibit a sharp increase in selectivity to propylene oxide from ~20% to ~50% when illuminated with visible light. This increase in the selectivity is accompanied by light-induced change in the oxidation state of the surface Cu atoms from Cu-oxide (light off) to Cu metal (light on). This design principle gives new insights to control the reaction rate and product selectivity in heterogeneous catalysis. Finally, we have demonstrated a pretreatment method to improve the stability of metal nanostructures of controlled shape under catalytic reaction conditions. Ag nanocube catalysts pretreated at high temperature (400 °C) in oxygen atmosphere to remove the polymeric capping agent (polyvinylpyrrolidone) exhibit ~20% loss in activity under ethylene oxidation at 230 °C during the reaction time of 20 hours. However, we demonstrate that Ag nanocube catalysts pretreated with polysulfide solution at room temperature can exhibit stable activity for similar reaction conditions.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/100007/1/andmar_1.pd