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theses

Elucidating structural and compositional polymorph changes of multi-phase semiconductors in a quick and quantitative manner is important for their manufacturing and applications in catalysis alike. Derivative peak fitting of diffuse-reflectance UV-visible spectra (DPR) is presented as an inexpensive, fast and quantitative method to estimate both the composition of a multi-phase semiconductor sample as well as the band gap energies of each component semiconductor in the mixture. Compositional measurements from DPR were in good quantitative agreement with XRD and Raman analysis. The application of in situ UV-Visible Spectroscopy DPR allowed for the on-stream determination of the onset temperature for rutilization during calcination. The synthesis and modification of anatase phase titanium dioxide was investigated. High pressure high temperature annealing (HPHT) under various gases lacking a partial pressure of oxygen induces the formation of paramagnetic defects as measured by Electron Paramagnetic Resonance (EPR) and X-ray Photoelectron Spectroscopy (XPS). The presence of these defects increased the metal-free photocatalytic activity of the samples towards hydrogen evolution from photocatalytic methane steam reforming (MSR) under UV illumination. The high concentration of unique Ti3+ defect sites generated during annealing catalyze hydrogen evolution, avoiding the need for precious metal cocatalysts, while anatase lacking these defects is inactive. Metal-free hydrogen evolution from MSR over the annealed anatase and is compared to hydrogen evolution over the annealed sample with a nickel cocatalyst. The addition of nickel cocatalyst increased the photocatalytic methane activation as well as the hydrogen production rate between 773 K and 973 K. Under UV illumination catalyst activity was stable for hydrogen generation from MSR at a steam to carbon ratio of 0.9 for over 8 hours, owing to the regeneration of Ti3+ defects by UV photoexcitation, preventing coking on the surface, while activity decayed in the dark due to coking on the nickel cocatalyst. This work shows that the implementation of nickel supported on defect-rich anatase TiO2 for highly stable photocatalytic hydrogen evolution from MSR with a major reduction in coking at low steam to carbon ratios. Low pressure flame synthesis using a burner-stabilized premixed stagnation flat flame was employed to produce high surface area, carbon doped, anatase TiO2 with a particle size below the quantum confinement limit. These nanoparticles had a 3.5-fold decrease in particle size (7 nm) and a 12-fold increase in surface area (187 m2 g-1) compared to commercial anatase. When the as synthesized particles are heated above 473 K, a majority of the carbon in the sample migrates to the surface and burns off, while this process decreases the overall surface area of the sample, it slightly increases the particle size therefore decreases the band gap energy while increasing the average pore diameter, and the photocatalytic activity as measured by photocatalytic water reduction.

ETD_8847

Ph.D.

Includes bibliographical references

by Ashley M. Pennington

doi:10.7282/T3KS6W0B

ETD doctoral

The author owns the copyright to this work.