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Bimetallic Palladium Catalysis and the Role of Secondary Metals in Reaction Activity and Selectivity
Pd bimetallic nanoparticles are versatile catalytic materials for electrochemical, thermal, and organic catalysis. In this dissertation, we explore how the composition and morphology of the Pd bimetallic structure influence both the electronic properties and local geometry of the Pd active site. Through careful design of core-shell and alloy nanostructures, we are able to enhance the efficiency of the electrocatalytic oxygen reduction reaction (ORR) and the diastereoselectivity in a hydroxyl-directed olefin hydrogenation reaction.
Metallic Pd is among the most active catalysts for electrocatalytic ORR in acidic and alkaline media, but significant overpotentials are still required to achieve device-relevant catalytic current densities. One strategy to tune the redox properties of surface Pd active sites is to utilize a core-shell structure, in which the core metal modulates the electronic properties of a thin Pd shell. Because the electronic effect of the core dissipates rapidly with increasing shell thickness, precise and uniform monolayer structures are required. In this work, we develop a colloidal ligand-exchange deposition strategy to synthesize core-shell structures of Au@Pd with precise submonolayer, monolayer, and multilayer structure. Using these materials, we show that Pd shell thickness correlates directly to the redox potential of surface Pd atoms and subsequently to the required overpotential for electrochemical ORR.
In addition to tuning the electronic properties of Pd active sites, bimetallic morphology can also be used to control active site ensemble geometry. In this effort, we synthesize Pd-Cu alloy nanoparticles with well-defined Pd and Cu surface distribution in order to catalyze diastereoselective hydroxyl-directed olefin hydrogenation. Directed hydrogenation, typically catalyzed by cationic Rh and Ir complexes, is utilized in organic synthesis to generate highly-functionalized and diastereomerically-pure alcohol products. No nanoparticle catalyst has been shown to achieve equivalently high diastereoselectivity through substrate direction. In our Pd-Cu alloy design, we anticipate that the hydroxyl directing group preferentially binds the more oxophilic Cu atom, bringing the olefin to the Pd active site in a well-defined orientation and delivering hydrogen exclusively from the same face as the hydroxyl group. Using terpinen-4-ol as a model substrate, we show that a Pd3Cu/SiO2 catalyst, thermally annealed under both N2 and H2 to generate an intermetallic surface, is capable of achieving high conversion and excellent diastereoselectivity toward the directed hydrogenation.