Catalytic activity of 2-D bimetallic surfaces and 3-D metal organic frameworks




Han, Sungmin (Ph. D. in chemistry)

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The first part of this dissertation involves understanding the fundamentals of 2-D palladium (Pd) - gold (Au) bimetallic surfaces. The special catalytic capabilities of Pd-Au bimetallic surfaces are mainly attributed to ensemble effects related to the compositions of inert Au atoms and active Pd atoms. In particular, we focus on the activation of O₂ and various oxidative reactions on the Pd–Au surfaces. Using molecular beam techniques under ultrahigh vacuum (UHV), we prove that H₂O and O₂ admolecules form hydrogen bonded clusters, which enhance O₂ activation. The direct dissociative adsorption of O₂ is also possible on the Pd–Au surfaces. We experimentally estimate the activation barriers for O₂ dissociation and the reactivity of oxygen adatoms as a function of the Pd coverage on the Au(111) surface, where a relatively low O₂ dissociation barrier (high Pd coverages) corresponds to a higher reaction barrier for oxygen adatoms (due to the higher Pd coverages). Based on these results, we also discover that acetaldehyde molecules can be selectively oxidized to acetic acid on less Pd deposited surfaces, since the small Pd ensembles on this surface can prevent the decomposition of acetate, which is an intermediate state in the formation of acetic acid. The second part of this dissertation involves growing and analyzing HKUST-1 metal-organic framework (MOF) thin films. MOFs are a new class of ultra-porous material based on inorganic-organic hybrid structures. We explain a new growth method for HKUST-1 thin films by sequential deposition of Cu and H₃BTC under vacuum. This procedure resulted in first MOF thin film to be controllably grown under vacuum, and the strategy can be applied in various applications. Since this growth method allows delicate quantity control of Cu, we successfully grow 4-6 nm or 8-12 nm Cu nanoparticles (NP) incorporated in HKUST-1 thin films. Applying temperature programmed desorption under vacuum, Cu NP incorporated HKUST-1 thin films show different catalytic activity for the methanol oxidation depending on the Cu NP sizes. The film with smaller Cu NP’s has improved selectivity for formaldehyde, and the film with larger Cu NP’s generates formaldehyde along with other products, CO₂ and H₂.



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