3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Influence of oxidation on leaf decomposition in acid mine water(2017) Mohasoa, Bongani PeterAcidification of freshwater systems by Acid Mine Drainage (AMD) is a persistent risk to aquatic ecosystems in South Africa, particularly in Gauteng and Mpumalanga. From several studies that have been conducted, it is clear that AMD has profound effects on aquatic life and functionality of the ecosystem. One of the ecosystem processes affected by AMD is the decomposition process. It has been established that AMD-affected streams inhibit the decompositon process. [Abbreviated Abstract. Open document to view full version]Item High temperature oxidation and corrosion behaviour of titanium aluminide alloy Ti-52.5AI-10.0Ni-0.2Ru (at.%)(2016) Mantyi, Hadio CapriceThe alloys Ti-52.5Al-10.0Ni (at.%) and Ti-52.5Al-10.0Ni-0.2Ru (at.%) were made by mixing, and melting their powders in a button arc furnace under an argon atmosphere. The high temperature oxidation and room temperature corrosion of behaviour of the alloys was investigated. Isothermal oxidation in air at 950°C for 120 hours and 720 hours was done. Cyclic oxidation behaviour of the alloys was also investigated in air and in a hot salt (Na2SO4) environment. The corrosion tests were conducted in 5 wt% and 25 wt% HCl. All the samples were characterised using scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffraction and hardness measurements. On solidification, the Ti-52.5Al-10.0Ni (at.%) alloy formed dendrites of γ-TiAl (~55 at.% Al) surrounded by a eutectic of γ-TiAl + Ti2NiAl3 (τ3) phases. Most of the nickel was found in the Ti2NiAl3 (τ3) phase (~12 at.%) with trace amounts in the dendrites (~0.5 at.%). The Ti-52.5Al-10.0Ni-0.2Ru (at.%) alloy formed dendrites of γ-TiAl (~53 at.% Al) surrounded by a eutectic of γ-TiAl + Ti2NiAl3 (τ3). Most of the nickel (~15 at.%) and ruthenium (~0.3 at.%) were in solid solution in the Ti2NiAl3 (τ3) phase, although small amounts of both metals were present in the dendrites (~1 at.% Ni and 0.1 at.% Ru). Under isothermal oxidation conditions, both alloys showed good oxidation resistance with a low mass gain (< 2%). The alloys formed a continuous scale of TiO2 and Al2O3 with good adherence to the substrate, but as exposure time increased, the scale was severely degraded and exfoliated from the surface. Cyclic oxidation conditions were more aggressive for both alloys. The Ti-52.5Al-10.0Ni-0.2Ru (at.%) alloy was more resistant and formed a nickel-rich sub-surface zone between the substrate and intermixed oxide layer. Both alloys had a fairly good corrosion resistance in HCl due to the presence of nickel. They formed a thin and non-continuous Al2O3 oxide scale on the surface of the γ-TiAl dendrites, with Ti3NiAl2O on the γ-TiAl + Ti2NiAl3 (τ3) eutectic regions. The acid mainly corroded the τ3 phase, thus attacking the eutectic and leaving the γ-TiAl dendrites exposed.Item Catalytic oxidation of carbon monoxide and dimethyl ether synthesis over gold-containing catalysts(2008-06-27T07:18:34Z) Ntho, Thabang AbrahamIn recent years, the catalytic properties of finely dispersed gold particles on oxide support materials have attracted much attention. Such catalysts are active for several types of oxidation reactions, in particular low-temperature carbon monoxide oxidation. The watergas- shift (WGS) reaction and the selective oxidation of CO in the presence of hydrogen are possible applications of gold-based catalysts. In this thesis we attempted to detail the key issues relevant to the deactivation of supported gold catalysts. A new aspect of the CO oxidation deactivation mechanism was comprehensively discussed. It was found that titanate nanotube supported gold catalyst (Au/TN), prepared by deposition precipitation, deactivated due to the formation of bicarbonate species on gold-sites. Moisture prevented the formation and accumulation of these species and also promoted the reaction. The Au/TN catalyst was characterised by HRTEM, in-situ DRIFTS-Mass spectrometry, BET, etc. Titanium dioxide (TiO2) is widely used as support material for various important industrial catalysts and its modification may suite specific catalytic requirements. In this work we have confirmed that the incorporation of nitrogen (N) into TiO2 increases the concentration of oxygen ion vacancies. When tested for CO oxidation, the nitrogendoped titania supported gold catalyst, Au/TiO2-xNx, was found to be a poor and unstable catalyst compared to the pure titania supported gold catalyst, Au/TiO2. Both catalysts were characterised by XRD, Raman spectroscopy, DRS-UV visible spectroscopy, TPO, BET, HRTEM etc. New Cu-based methanol synthesis catalysts, prepared by co-precipitation and deposition precipitation, were physically mixed with _-Al2O3 and tested for the direct single-step DME synthesis from syngas. The catalysts exhibited good CO conversion and DME selectivity. The loading of gold on the methanol synthesis components of the bifunctional catalysts promoted CO conversion and the WGS reaction. In addition, Au suppressed iv methanation on the bifunctional catalysts. The catalysts were characterised by TPR, XRD, BET and XRF spectroscopy.