Developing gold-based nanostructures to study catalytic reactions in water
Heck, Kimberly N.
Wong, Michael S.
Doctor of Philosophy
Gold-based catalysts are effective for reactions that occur in water. They are not well understood though, with regard to the nature of active sites and surface reaction mechanisms. Water presents interesting challenges in performing catalytic studies, as it interferes with infrared spectroscopy commonly used to detect surface intermediates under reaction conditions. Insights leading to improved catalysts can be gained if gold-based nanostructures could be designed, engineered, and tested for a given chemical reaction. Two gold-based water-phase catalytic reactions were considered for this thesis: glycerol oxidation and hydrodechlorination of chlorinated ethenes. Glycerol is a by-product of biofuel production, and is considered a possible non-petroleum feedstock for chemicals if efficient conversion processes exist. It can be oxidized using Au catalysts in alkaline solution, but the surface reaction mechanism is not known and the role of basic pH is not fully understood. Gold nanoshells (Au NSs) were used for the first time to study glycerol oxidation through surface-enhanced Raman spectroscopy (SERS). Raman bands for surface-adsorbed glyceric acid, the major reaction product, were detected. High oxygen content and high pH values led to carbon monoxide surface species, indicating carbon-carbon cleavage. When the glycerolate:O2 ratio was constant, higher pH led to advance decomposition, due to the activation of O2 by adsorbed hydroxide ions. The catalytic HDC of chlorinated ethenes can potentially remove these contaminants from groundwater. This reaction occurs at room temperature and uses palladium-coated gold nanoparticles (Pd/Au NPs), which are more efficient then pure Pd. The Au NSs were coated with Pd and used to study the surface intermediates of 1,1-dichloroethene HDC through SERS. Pathways were ascertained through careful Raman band assignments to probable chemical species and analysis of bulk reaction products, leading to the formulation of a reaction mechanism. Gold enhances Pd catalysis for HDC, presumably through the generation of palladium-based active sites, though the true active site is unknown. The effects of chloride and sulfide on the activity of Pd/Au NPs for trichloroethene HDC were studied to provide information about active sites and deactivation properties. The activity of Pd/Au NPs was unaffected by NaCl, while that of the pure Pd catalysts decreased. Pd/Au NPs were resistant to sulfide poisoning compared to pure Pd catalysts. This increased resistance is attributed to the formation of small Pd islands on the Au NPs.
Physical chemistry; Chemical engineering