Doctor of Philosophy (PhD)
Physics and Astronomy
We have investigated the morphology and electronic structure of two basic classes of systems: metal oxide surfaces that catalyze the formation of environmentally persistent free radicals (EPFRs) from aromatic precursors, and Au and Cu nanoparticles that may be suitable catalysts for the catalytic oxidation of CO or hydrogenation of CO2. First, we examine the adsorption behavior of phenol on rutile TiO2(110) and ultrathin films of alumina prepared on a NiAl(110) substrate. Electron paramagnetic resonance studies show that exposure of both γ-alumina and titania powder to phenol at 250°C results in the formation of persistent phenoxyl radicals. EELS studies of phenol dosed on single-crystal titania and alumina show that phenol adsorbed at elevated temperature demonstrates a significantly narrower HOMO-LUMO gap than molecular phenol in the gas phase or physisorbed molecular phenol. Ultraviolet photoelectron spectroscopy shows direct evidence of charge transfer from high-temperature adsorbed phenol to electronic states of TiO2(110) usually associated with the accumulation of charge at surface oxygen vacancies, providing direct evidence of a frequently-hypothesized radical formation mechanism. Second, we deposit and characterize Au nanoparticles on the self-assembled hexagonal boron nitride “nanomesh” prepared on a Rh(111) substrate. STM studies show that at all levels of coverage, Au clusters almost always remain confined to the nanomesh “pores” and are restricted in size to < 3 nm diameter. XPS studies suggest that the resulting Au clusters are negatively charged, and for < 1 ML Au coverage, the electronic properties of most of the clusters formed are dominated by final-state effects that arise from the reduced dimensionality of the smallest clusters (one or two Au layers). A similar morphology for Au deposited on ZnO(10-10) has been previously observed; however, we find that the Au-ZnO interaction instead results in positively charged clusters. Cu on ZnO(10-10) grows as three-dimensional clusters even at very small coverage and shows positive charging similar to Au. It is clear that catalytically relevant properties of supported metal nanoclusters are strongly influenced by interactions with the support, even if the cluster morphology is identical for particles on various different substrates.
Document Availability at the Time of Submission
Release the entire work immediately for access worldwide.
Patterson, Matthew C., "Single-crystal metal oxides and supported metal nanoclusters as model catalyst sytems" (2013). LSU Doctoral Dissertations. 1312.