Controlled Synthesis And Facet-Dependent Catalytic Activities Of Palladium Nanocrystals

The fundmental understanding of structure-catalytic activity relationship is of great importance in the design of efficient catalysts. Nanoscale metal crystals (particularly noble metal nanocrystals) are a class of materials that can be used in catalytic organic reactions. Previously, the catalysts involved in the heterogeneous catalytic reaction systems usually possess multiple crystal facets on surface. This feature formed an obstacle to performing reliable investigations at the intersection of surface science and realistic catalytic systems. Along with the rapid development of materials synthesis,it becomes feasible tocontrollably synthesize metal nanocrystals with desired structures. The metal nanocrystals with uniform and well-defined surface facetswould be ideal candidates for catalysts to achieve high activity and selectivity.In this dissertation, the metal of palladium (Pd)has been chosen as the model system. As a result, we have been able to investigate the molecule-nanocrystal interactionsas well as the structure-activity relationship in various reaction systems.These findings provide fresh insights into the design and synthesis of metal nanocrystals for various catalytic reactions. The specific findings include:1. In general, surface facets of a nanocrystal play an important role in determining the species adsorption and reaction activation, and in turn, hold the key to tailoring its activity and selectivity in catalysis. In the work, we use Pd as a model system to perform the investigation where kinetics of atomic addition can be precisely controlled by simply using a syringe pump. In the Pd system, the majority of products is single-crystal, largely simplifying the influence from twinned structures. By manipulating the kinetics of atomic addition, we figure out that newly formed Pd atoms are preferentially added to the{100} facets of cuboctahedral seeds when the Pd atomic concentrations are intentionally controlled very low, leaving{111} facets on the resulted octahedral nanocrystals. The{111} facets formed at the surface of octahedrons enable us to investigating facet-dependent catalytic effects, with well-developed{100}-bound Pd nanocubes as a reference.2. Based on the controlled synthesis of Pd nanocrystals in the last chapter, we first employ single-facet Pd nanocrystals as a model system to investigate the facet-dependent behavior for molecular oxygen activation. In our investigation, two types of nanocrystals with different surface facets are used:nanocubes enclosed by{100} facets and octahedrons by{111} facets. The yield of singlet O2, characterized by probe molecules in the presence of various scavengers, exclusively demonstrates that singlet-02-analogous species is preferentially formed on{100} facets. Both the simulations and characterizations further elucidate that O2is more activated on the{100} facets via chemisorption. As facet control enables to tuning the capability of activating O2, we have been able to demonstrate that the surface facet of metal nanocrystals is a critical parameter to designing catalysts for organic oxidation and therapy agents for cancer treatment.3. Since the Pdâ†'O2electron transfer is responsible for O2activation, the charge state of metal surface may offer a knob for tuning its efficiency, which is fundamentally important to optimizing catalysts design for organic oxidations. Prior to our work, oxide semiconductors were generally considered as support materialsfor metal catalysts in organic reactions. In our work, we demonstrate using Pd-TiO2hybrid structures as a proof-of-concept model that plasmonic hot electrons can be injected into the conduction band of TiO2, in opposite to the function of Schottky junction, lowering the electron density of Pd surface. By varying the illumination intensity, it is feasible to modulate the charge state of Pd surface in such a metal-semiconductor hybrid configuration. This modulation enables enhancement of O2activation and in turn efficiency improvement of catalytic glucose oxidation by shedding appropriate light on the Pd-TiO2hybrid structures.4. Based on plasmon excitation, catalytic reactions can befacilitated through both plasmon-driven catalysis and photothermal conversion. In our work, we demonstrate the direct light harvesting in UV-to-visible range for hydrogenation reactions using the Pd nanostructures that are synthesized viaa one-pot Ru-assistedroute. Under the light illumination, hydrogenation reactions can be driven to different extentat room temperature depending on the light intensity. This unique characteristic of plasmonic nanostructures suggests that metal nanocrystals with high-index facets can be used in many catalytic heterogeneous reactions by harvesting solar energy instead of heat energy.