published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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Fischer-Tropsch Synthesis (FTS) is known as the most practical way to convert natural gas to hydrocarbon products including synthetic fuel depending on the catalysts and operating conditions. Australia has 25% of world's natural gas resources hence Australia's crude oil dependency can be reduced extensively by developing catalysts that will facilitate the technique of converting natural gas to synthetic fuel. Molybdenum nitride has been employed in this study for FTS because of its superior mechanical strength, stability, exceptional resistance to carbon deposition & suifur poisoning. In particular, molybdenum nitride is endowed with similar electronic properties to those of noble metals. Other transition metal nitrides such as Co nitride and Co-Mo nitride were also investigated in this study. The physicochemical attributes of nitride catalysts were examined by BET surface area, particle dispersion, acid site strength & concentration, and surface elemental composition. Gas to solid nitridation kinetic was thermogravimetrically monitored. CO hydrogenation activity was measured in a fixed bed reactor using various syngas compositions and temperatures at atmospheric pressure. The effect of nitridation conditions on catalytic properties of nitrides was investigated via 23 factorial design. It has revealed that nitridation parameters; temperature, nitriding gas composition (H2:NH3) and nitridation reaction time were all significantly influencing catalyst properties. The optimal nitridation condition was 973 K, H2:NH3=1: 1, and 4 hours of nitriding time which gave higher alkene selectivity. 20 wt% M02N/Ah03 was found to be the better FT catalyst compare to catalysts with lower Mo loading and other inorganic oxide supports. Nitridation kinetic studied by thermogravimetric analysis showed that successful nitridation of transition metal oxide precursor was dependent of nitridation temperature and hydrogen concentration. Co-Mo nitride has several forms of nitride species, COS.47N, C03M03N, MoN, and Mo2N. It was shown that COS.47N was the most active component favouring the CO hydrogenation rate and alkene selectivity. Mechanistically-based kinetic models suggested that methanation over Co nitride occurs mainly via surface carbon while surface oxygenated intermediates were accountable for methanation over Co-Mo nitride and Mo nitride.

Thesis (M.S.)--University of Delaware, 2008. / Principal faculty advisors: Daniel Chester, Dept. of Computer & Information Sciences; and Christiana Honsberg, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references (leaves 124-133). Also available in an electronic version.

Thesis (Ph. D.)--University of Hong Kong, 1995. / Includes bibliographical references.

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2006. / Title from title screen (site viewed May 23, 2007). PDF text: 205 p. : ill. ; 2.40Mb UMI publication number: AAT 3237051. Includes bibliographical references. Also available in microfilm and microfiche formats.

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abstract: III-nitride alloys are wide band gap semiconductors with a broad range of applications in optoelectronic devices such as light emitting diodes and laser diodes. Indium gallium nitride light emitting diodes have been successfully produced over the past decade. But the progress of green emission light emitting devices has been limited by the incorporation of indium in the alloy, mainly due to phase separation. This difficulty could be addressed by studying the growth and thermodynamics of these alloys. Knowledge of thermodynamic phase stabilities and of pressure - temperature - composition phase diagrams is important for an understanding of the boundary conditions of a variety of growth techniques. In this dissertation a study of the phase separation of indium gallium nitride is conducted using a regular solution model of the ternary alloy system. Graphs of Gibbs free energy of mixing were produced for a range of temperatures. Binodal and spinodal decomposition curves show the stable and unstable regions of the alloy in equilibrium. The growth of gallium nitride and indium gallium nitride was attempted by the reaction of molten gallium - indium alloy with ammonia at atmospheric pressure. Characterization by X-ray diffraction, photoluminescence, and secondary electron microscopy show that the samples produced by this method contain only gallium nitride in the hexagonal phase. The instability of indium nitride at the temperatures required for activation of ammonia accounts for these results. The photoluminescence spectra show a correlation between the intensity of a broad green emission, related to native defects, and indium composition used in the molten alloy. A different growth method was used to grow two columnar-structured gallium nitride films using ammonium chloride and gallium as reactants and nitrogen and ammonia as carrier gasses. Investigation by X-ray diffraction and spatially-resolved cathodoluminescence shows the film grown at higher temperature to be primarily hexagonal with small quantities of cubic crystallites, while the one grown at lower temperature to be pure hexagonal. This was also confirmed by low temperature photoluminescence measurements. The results presented here show that cubic and hexagonal crystallites can coexist, with the cubic phase having a much sharper and stronger luminescence. Controlled growth of the cubic phase GaN crystallites can be of use for high efficiency light detecting and emitting devices. The ammonolysis of a precursor was used to grow InGaN powders with different indium composition. High purity hexagonal GaN and InN were obtained. XRD spectra showed complete phase separation for samples with x < 30%, with ~ 9% indium incorporation in the 30% sample. The presence of InGaN in this sample was confirmed by PL measurements, where luminescence from both GaN and InGaN band edge are observed. The growth of higher indium compositions samples proved to be difficult, with only the presence of InN in the sample. Nonetheless, by controlling parameters like temperature and time may lead to successful growth of this III-nitride alloy by this method. / Dissertation/Thesis / Ph.D. Physics 2011

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