Engineering palladium-on-gold bimetallic nanoparticles as groundwater remediation catalysts
Nutt, Michael O'Neal
Wong, Michael S.
Doctor of Philosophy
Over the years, our groundwater sources have become contaminated with chlorinated organics, in particular trichloroethene (TCE) due to its exposure as a solvent to degrease metals and electronic parts in the automotive, metals, and electronic industries. Due to its prevalence and toxicity, TCE has been listed as one of the most hazardous organic compounds. Physical displacement methods (air stripping and carbon adsorption) are not as desirable as the catalytic breakdown of TCE into ethane using palladium-based materials. This thesis reports on the design, synthesis, characterization, and testing of Pd supported on gold nanoparticles (Au NPs) as a remediation catalyst for the hydrodechlorination (HDC) of TCE in water at ambient temperature and atmospheric pressure. The unique surface, structural, and electronic properties of 20 nm Pd/Au particles were found to contribute to the significantly enhanced HDC activity. Pd/Au NPs synthesized with smaller diameters (4 nm) reduced metals cost and increased effectiveness. Mass transfer effects during batch reactions were analyzed for the NP catalysts. Pd/Au NPs were immobilized on a support through electrostatic attractions, and demonstrated higher HDC activity compared to conventionally impregnated Pd and Pd-Au supported catalysts. Pd/A1 2O3, Pd NPs, and Pd/Au NP catalysts were found to be active for HDC of other chlorinated ethenes, with reaction mechanism studies supporting a sequential dechlorination pathway for TCE HDC. The reaction was found to be first-order in TCE and half-order in H2 for our Pd/Au catalysts, but was different from the half-order TCE and first-order H2 dependence of pure Pd catalysts. The Pd/Au nanostructure was more resistant to catalyst deactivation from chlorides and sulfides, relative to pure Pd catalysts. As a different example of using NPs as a catalyst support, a sol-gelation method was developed for the synthesis of molybdenum oxide (MoO3) supported on zirconia (ZrO2) NPs. It was hypothesized that ZrO 2 NPs can support amorphous MoO3 as a thermally stable, highly active oxidation catalyst. MoOX/ZrO2 containing up to 23 wt% amorphous MoOX was successfully synthesized and the nanostructure was analyzed in the context of colloid chemistry. They were more active for methanol oxidation on a gram-catalyst basis than conventionally prepared MoO X/ZrO2 catalysts, due to their higher MoOX content.
Chemical engineering; Environmental engineering