Understanding Structure-Property Relationships for Palladium-Gold Nanoparticles as Colloidal Catalysts
Author
Fang, Yu-Lun
Date
2011Advisor
Wong, Michael S.
Degree
Doctor of Philosophy
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.
Keyword
Chemical engineering