Understanding Structure-Property Relationships for Palladium-Gold Nanoparticles as Colloidal Catalysts

Abstract

Bimetallic palladium-gold (PdAu) nanoparticle (NP) catalysts have been demonstrated for the better catalytic performance than monometallic Pd catalysts in various reactions; however, the enhancement mechanism is not completely clear for most reactions. This thesis addresses the investigation of PdAu NP catalysts with emphasis on the structureproperty relationships in water-phase reactions, using hydro dechlorination (HDC) of trichloroethene (TCE) as the model reaction. Catalyzed TCE HDC is a potential approach for water pollution control, in which colloidal Pd-decorated Au NPs (Pdf Au NPs) are known to be significantly better catalysts than monometallic Pd ones. X-ray absorption spectroscopy (XAS) of carbon-supported Pdf Au NPs with different surface Pd coverages verified their core-shell structure (Au-rich core and Pdrich shell). Structure evolution was observed upon heat treatment, in which Pd was in the form of surface Pd ensembles at room temperature. The metals formed a surface PdAu alloy or a bulk PdAu alloy above 200°C, as determined from the average coordination environment. Results suggested a new way to promote Pd catalysis, namely, by impregnating supported Pd catalysts with gold salt followed by thermal annealing; such post-impregnation with different heat treatments could lead to >15-fold increase in TCE HDC activity. Pd ensembles on the Au NP surface were demonstrated to be major active sites for TCE HDC as the reaction rates correlated strongly with the size of Pd ensembles determined from XAS. The geometric effect, in which atomic ensembles act as active sites, appeared to dominate over the mixed metal site effect and the electronic effect. Au NPs could stabilize surface Pd atoms in the metallic form, possibly leading to a set of highly active sites that is not present in monometallic Pd NPs. The TCE HDC reaction with Pdf Au NPs and Pd NPs was conducted as a closed batch system. Mass transfer effects in this three-phase reaction were assessed and quantified by analyzing observed reaction rates as functions of stirring rates and initial catalyst charges. The largest effect on observed reaction rates came from gas-liquid mass transfer. TCE HDC was modeled as a Langmuir-Hinshelwood mechanism involving competitive chemisorption of dihydrogen and TCE molecules.