Date of Award

2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Materials Science & Engineering

First Advisor

Eric A. Stach

Abstract

Major industrial processes rely on heterogeneous catalysis to transform and process chemicals. In fact, many chemical reactions are accelerated with metallic nanocatalysts. The reduction of the environmental impact of the chemical industry is necessary to address today's challenges. This can be only done by creating innovative nanocatalysts, requiring lower temperatures and pressures to accelerate a chemical reaction with a high yield. However, new nanostructures for heterogeneous catalysis are not always well understood, especially at the atomic scale. Additionally, catalysts are usually precious transition metals (Pt, Pd, Ru, Rh) which may benefit from being diluted into an alloyed phase with a less expensive metal to adjust the activity and selectivity of the catalyst. This leads to complex bimetallic structures, which can substantially change under reaction conditions. Understanding the structure of nanocrystals and dynamical changes in reaction conditions is crucial to guide the rational design of active, selective, and stable catalysts. To this end, transmission electron microscopy (TEM) with ex situ and in situ techniques is a valuable tool to perform diagnostics at the nanoscale. To understand changes in activity and selectivity in Au-Pd nanoparticles used for the hydrogenation of alkynes, ex situ and in situ TEM was performed to track segregation phenomena and changes of facets. The analysis of Au-Pd particles revealed segregation of Pd to the surface for systems with high Pd concentrations. Changes in facets were also observed in reductive and oxidative environments. The analysis was expanded to other bimetallic systems, namely Co-Pt and Ni-Cu systems. Ex situ and in situ TEM and X-ray absorption spectroscopy (XAS) showed dynamical changes in O2-rich and H2-rich environments at elevated temperatures, which could be linked to catalytic properties. To further investigate bimetallic particles, Cu-Ru, Cu-Rh, and Cu-Ir nanoparticles with a core-shell configuration were synthesized and studied with TEM and catalytic testing. The remarkable catalytic properties of these systems are promising for catalytic reactions involving precious metals. Finally, electron pair distribution (ePDF) function analysis was performed on the Cu-precious metals particles to gain further insight into the crystallographic properties of these alloys. The combination ex situ and in situ characterization techniques with the synthesis of novel bimetallic nanocatalysts expands the knowledge on dynamical changes in bimetallic nanocrystals and widens the tools to achieve sustainable catalysis.

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