Graduate Thesis Or Dissertation
 

Platinum Nanoparticles in Transition Alumina: An Atomic Resolution Study of Interfaces

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/q524jw530

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  • The phases of transition alumina are scientifically interesting due to their high specific surface area, favorable acid/ base characteristics, and chemical and hydrothermal stability which make this metallic oxide a prime candidate for use as a support for metallic nanoparticles (NPs) in heterocatalysis. Interactions at alumina/Pt interfaces are important for catalysis, but poorly understood due to the microstructural complexity of transition alumina and challenges associated with characterization of interfaces. The goal of this work is to compare experimentally determined atomic-level structural information from aberration-corrected scanning transmission electron microscopy (STEM) with density functional theory (DFT) based structural models and quantum mechanical STEM image simulations to characterize the chemistry and atomic-level structure of Pt/γ and Pt/θ-interfaces. In this work, dense transition-alumina layers are formed using high-energy ion implantation to amorphize near surface layers of α-alumina wafers which are then recrystallized into the desire alumina form via thermal annealing. Self-implanted alumina wafers are used to determine viability of producing oriented γ-alumina layers. Pt-implanted alumina is processed to produce Pt/γ-alumina and Pt/θ-alumina interfaces in the form of faceted Pt NPs embedded in alumina. Conventional transmission electron microscopy (TEM), image-corrected TEM, and x-ray diffraction (XRD) are used to characterize the microstructure of γ-alumina and Pt/transition-alumina systems. The recrystallization of amorphized alumina occurs epitaxially to the α-alumina with parallel alignment close-packed O planes of (0006)α and (111)γ or (􀀁201)𝜃𝜃. Faceted Pt NPs possess a (111)Pt||(111)γ;[1􀀁10]𝛾𝛾||[1􀀁10]𝑃𝑃𝑃𝑃 orientation in γ-alumina and a (􀀁201)𝜃𝜃 ||(111)Pt; [132]θ|| [1􀀁10]𝑃𝑃𝑃𝑃 orientation in θ-alumina. In both systems the NPs are primarily bound by {111}Pt facets. Probe-corrected STEM was used to collect atomic resolution images of the faceted interfaces to determine the structure and chemical terminations of γ- and θ-alumina at the interfaces. This is facilitated by comparison of the experimental images to quantum mechanical scanning (S)TEM simulations of density functional theory (DFT) based model structures. Oxygen termination of the alumina is found to be the dominant chemical termination for both (111)γ||(111)Pt and ( 01)𝜃𝜃||(111)Pt interfaces.
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