3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Synthesis and functionalization of gallium nitride nanostructures for gas sensing and catalyst support(2014-01-10) Kente, ThobekaWe report the role of a double step heat treatment process in the synthesis of novel GaN nanostructures (NSs) using a two stage furnace following a catalyst free vapour-solid growth mechanism. Morphological analysis revealed that GaN NSs were composed of rod-like structures with average diameter of 250 nm and accumulated particulates of GaN with diameter of ~ 12 – 16 nm providing enhanced surface area. The wurtzite phase of GaN nanorods of agglomerated nanoclusters was synthesized at temperatures as low as 750 °C. An X-ray photoelectron spectroscopic study confirmed formation of GaN. The surface areas of the GaN NSs were high at ~20 m2/g with respect to that expected for solid nanorod structures. The GaN NSs were of high crystallinity and purity as revealed by structural studies. Raman spectral analysis showed stronger intensity of the A1(LO) mode with respect to that for E2(high) mode indicating the high electronic quality of the sample. A photoluminescence study revealed the dominant presence of a defect band around 1.7-2.1 eV corresponding to nitrogen di-vacancies. Subsequent annealing in NH3 has demonstrated a compensation of the defect state and evolution of a band edge peak with possible hydrogen compensation of surface states. We also report the role of activated carbon on Ga2O3 to make GaN/C nanostructure composites using a single stage furnace. TEM analysis showed that GaN/C nanostructures gave different morphologies with different ratios of GaN/C. The surface areas of these materials showed an increase as the ratio of activated carbon was increased. PXRD showed that a ratio of Ga2O3: C of 1:0.5 (w/w) was sufficient to form GaN. TGA revealed that the ratios of Ga2O3: C of 1:0.5 – 1:2 gave materials that were thermally stable. Raman spectra showed that the material had excellent electronic properties. The material with a Ga2O3/C 1:2 ratios showed a poor gas response due to the change in reference value of resistance with the variation of hydrogen concentration. iv This study also provides the first investigation of GaN as a catalyst support in hydrogenation reactions. The GaN NSs were synthesized via chemical vapour deposition (CVD) in a double stage furnace (750 ºC) while nitrogen doped carbon spheres (NCSs) were made by CVD in a single stage furnace (950 ºC). TEM analysis revealed that the GaN NSs were rod-like with average diameters of 200 nm, while the NCSs were solid with smoother surfaces, and with diameters of 450 nm. Pd nanoparticles (1 and 3% loadings) were uniformly dispersed on acid functionalized GaN NSs and NCSs. The Pd nanoparticles had average diameters that were influenced by the type of support material used. The GaN NSs and NCSs were tested for the selective hydrogenation of cinnamaldehyde in isopropanol at 40 and 60 °C under atmospheric pressure. A comparative study of the activity of the nanostructured materials revealed that the order of catalyst activity was 3% Pd/GaN >3% Pd/NCSs > 1% Pd/NCSs > 1% Pd/GaN. However, 100% selectivity to hydrocinnamaldehyde (HCALD) was obtained with 1% Pd/GaN at reasonable conversion rates.Item Promotion effects of iron in carbon nanotubes synthesis to make supports for Fischer-Tropsch synthesis catalysts(2011-03-11) Kente, ThobekaIron (Fe), catalysts were prepared by incipient wetness impregnation method using CaCO3 as a supporter. 0.6% Cu, 0.4% K, 0.6% Cu/0.4% K, 0.1 ml of Tetraethyl orthosilane (TEOS) in 2ml water were used as promoters for the Fe catalysts. The promoted catalysts were then employed in the synthesis of carbon nanotubes (CNTs) using a chemical vapor deposition (CVD) method. Transmission electron microscopy (TEM) revealed that Cu increased the diameter of the CNTs and caused them to be coiled, thereby altering the shape of the CNTs. The addition of K resulted in no recognizable change in the microstructure of these tubes. The CNTs obtained looked similar to those obtained when Fe was used without a promoter. However, the diameter of the tubes also increased due to K addition. Thermol Gravimetric Analysis (TGA) results showed that the addition of K to the Fe catalyst (Figure 20, p56, 57) resulted in the enhancement in thermal stability of the CNTs. BET analysis revealed that Cu increased the surface area while K decreased the surface area of this solid material. The Cu/K promoted catalyst produced CNTs with diameters of 60 nm which was the same as unpromoted catalyst. However, the CNT diameter distribution was quiet different for the two catalysts. The surface area was less than that of the unpromoted catalyst. The Fe/Cu/K synthesized CNTs were coiled and they looked more like the product produced from Fe/Cu CNTs. Thus the Cu catalyst has the dominant effect in determining the CNT morphology. When SiO2 was used as a promoter no change in the diameter distribution of the CNTs could be detected. Thus while changes to the carbon surface may have occurred at the atomic level, the changes were not detected at the TEM resolution used. The surface area was also less than that of CNTs produced over the unpromoted catalyst. Both promoted and unpromoted Fe/CNT were tested for FT synthesis. The activity, selectivity and CO conversions were recorded. The K was found to strongly influence the production of the hydrocarbon yield and increased the CO conversion. The K promoted catalyst increased the CO reaction rate, and increased the olefinity and the alpha value. The effect of K on the olefinity of the C2 hydrocarbon ranged from 0.04 to 0.26. An increase in FTS activity is also observed for the K promoted catalyst. Cu decreased the reduction temperature of Fe oxides as noted by TPR studies. The Cu promoted catalyst showed a high selectivity to methane and a decrease of C5+ hydrocarbons, the C2 olefins also decreased.