| Description |
The research presented in this dissertation will focus on the electronic structure of size-selected Ptn (n ≤ 24) clusters supported on oxidized Si, as well as their catalytic activity, surface morphology, and electronic structure when Pt clusters are overcoated/undercoated by Al2O3 via 1-3 cycles of atomic layer deposition (ALD). The experiments found within this dissertation required the modification, design, and construction of a complex instrument capable of spanning pressures down to ultra high vacuum (UHV) levels while allowing for relatively high pressure ALD chambers and quick sample exchange. This was accomplished by designing and building a cluster deposition beamline capable of depositing size-selected metal clusters as well as the necessary sample holders and sample stages to make and analyze samples. Though the main goal was to determine how ALD overcoating/undercoating affects SiO2 supported Ptn clusters, during the course of this research, it was discovered that the inherent size effects on the electronic structure of clusters can also be reflected by white line and absorption edge in X-ray absorption near edge structure (XANES) spectra. This result will give implications for the interpretation of XANES on nano/subnanometer size metal clusters. In addition to the investigation of the size effects on the electronic structures of Ptn clusters via X-ray photoelectron spectroscopy (XPS) and XANES, the surface morphology, and catalytic activity of Ptn clusters with and without ALD modification were also probed by ion scattering spectroscopy (ISS), grazing-incidence small-angle scattering (GISAXS), and CO temperature programed desorption (TPD). Trimethylaluminum, the precursor of Al2O3 ALD, preferentially binds to “on-top†sites of Ptn clusters, and by depositing Al2O3 via 1 cycle of ALD, high-temperature CO binding sites are completely blocked and these sites are not recoverable. Chapter 1 gives a brief introduction into heterogeneous catalysis, and more specifically, the importance of size-selected clusters and the advantage of ALD-modified catalysts. Chapter 2 presents the design of the cluster deposition beamline, sample holder, and sample stages, and the description of the whole instrument. Chapter 3 focuses on the size effects of Ptn clusters on XANES spectra and its implication. Finally, Chapter 4 discusses the effects of deposited Al2O3 via ALD on the electronic structure, catalytic activity, and surface morphology of Ptn clusters. |
