Design and understanding of selective catalysts using colloidal nanoparticle synthesis
Keywords: Chemical Engineering.

Catalysts are a critical component of the chemical industry that produces products that we rely on every day. A common geometry for industrial catalysts is comprised of metal nanoparticles dispersed on oxide supports. Numerous works have shown how control over the geometry of this catalyst motif can be leveraged to create catalysts with improved catalytic activity, selectivity, and stability. However, traditional catalyst synthesis techniques often do not provide the required control over important catalyst parameters such as nanoparticle size and alloy nanoparticle composition. In the works outlined in this thesis, colloidal synthesis is used to produce highly uniform nanoparticles with precise control over their geometry. First, a seed-mediated colloidal synthetic technique was developed to produce dilute (or single atom) alloy nanoparticles. Electron microscopy, UV-Vis spectroscopy, and X-ray absorption measurements were used to verify the formation of Pd/Au and Pd/Ag alloy nanoparticles with control over the Pd ensemble size down to single atoms. The Pd/Au nanoparticles were then used to carry out selective oxidation of alcohols in the presence of hydrogen and oxygen with attention paid to the impact of Pd ensemble size within Au on activity. It was found that Pd/Au alloy nanoparticles outperformed both pure metals, with the most dilute levels of Pd offering the largest improvement in catalytic activity while maintaining nearly complete selectivity to the desired product. Beyond improving catalytic activity, this work shows how the ability to tune several catalyst parameters independently enables better understanding of catalytic mechanisms. In particular, the mechanism of selective oxidation reactions in which a selective oxidant, likely hydrogen peroxide, is created from a mixture of hydrogen and oxygen over Au and Pd/Au catalysts are described. Through systematic testing, it was demonstrated that while Au nanoparticles need an interface with a metal oxide to produce the selective oxidant, Pd/Au nanoparticles carried out selective oxidation without contact with metal oxides. Furthermore, this work shows that once mobile oxidants are produced, they are able to diffuse through the catalytic bed do different catalytic sites. Beyond selective oxidation, this thesis describes how colloidal Ru nanoparticles can be used to determine the number of sites on an oxide support capable of hosting single metal atoms. This finding has implications for the creation of highly dispersed catalysts that exhibit reaction selectivity unique from nanoparticle catalysts. Despite its ability to create highly uniform catalysts, colloidal synthesis is currently mostly confined to the laboratory scale due to the relatively high cost and small scale at which nanoparticles are produced. While other groups are working to translate batch processes into continuous flow systems, little has been done to address the waste of expensive synthesis solvents that drive up the cost of colloidal nanoparticles. To address solvent waste, this work examines processes for recycling nanoparticle synthesis solvents and demonstrates the potential to produce uniform mono and bimetallic nanoparticles over many reuses. This work opens the door to closed loop processes where the colloidal nanoparticles are produced continuously requiring only metal precursors and a small amount of surfactants. Such a process would allow superior colloidal nanoparticle catalysts developed in the laboratory to be translated to more industrially relevant scales. Together, the projects outlined in this thesis demonstrate the potential of colloidally synthesized nanoparticles to improve the activity of selective catalytic reactions and to uncover mechanistic insights in complex, multi-site transformations. It also provides a scheme for how these and other laboratory scale discoveries can be transferred to industrial application