3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Photocatalytic degradation of methyl violet using modified radially aligned nano rutile TiO2-nanodiamonds composite(2020) Mugadza, Farirai; Farirai, MugadzaIn this work, a hydrothermal method was used to synthesize the radially aligned nano rutile (RANR) TiO2,usingTiCl4as a precursor. The synthesis temperatures, as well as the time involved in the refluxing step of the synthesis were varied to obtain the optimum morphology of the resulting TiO2. The optimum refluxing time for RANR TiO2 synthesis was determined to be 16 hours at 180°C. The synthesized RANR TiO2 with dandelion-like shapes had diameters ranging from 300 nm to 800 nm and an average diameter of 560 nm. The RANR TiO2 had BET surface area of 68 m2/g, which is higher than that of the commercially available Degussa P25 (45 m2/g).The RANR TiO2-nanodiamond composites were all synthesized in situ using the hydrothermal method with detonation nanodiamonds ranging from 0.1 to 1% mass loading. BET surface area analysis showed an increase in the surface area of the RANR TiO2 with an increase in the amount of nanodiamonds used in its modification. Raman spectra confirmed the presence of graphitic carbon and rutileTiO2in all the composite samples. The results obtained from XPS analysis showed that oxygen, carbon and titanium were all present in the sample but there was no evidence showing bond formation between titanium and carbon. RANR TiO2 was the most effective in dye degradation due to their nano rod structure, which increases light harvesting properties due to multiple reflections of light. All the other composites did generally well with respect to dandelions in the first hour, but then the rate of degradation decreased which could be attributed to the reduction in photocatalytic active sites due to blockage by reactants. A good dispersion of the nanodiamonds and RANR TiO2(0.1% loading) helped to create strong electronic interphase interactions. This helps to separate the photogenerated electrons and positive holes, thereby increasing photocatalytic efficiency. Calcination increased photocatalytic efficiency because of the increase in crystallinity of materials which reduces electron/hole recombination, the increase in crystallinity was shown by results from Raman spectroscopy. The photocatalyst recyclability studies showed that the recovery of the catalyst after each cycle and the re-use was not effective as the degradation efficiency decreased from 80% to 60% after 3 cyclesItem Fischer-Tropsch synthesis inside a nanoreactor(2017) Phaahlamohlaka, Tumelo NathanielCoal, biomass and natural gas are traditional energy carriers whose conversion via the Fischer-Tropsch process can be used to generate multiple hydrocarbon products such as fuels and fine chemicals. At the center of the Fischer-Tropsch process is the catalyst used for converting the syngas to hydrocarbons. Generally these catalysts using Co or Fe active sites are supported on high surface area inert materials such as silica and alumina. Our procedure in this thesis was to study some of the fundamental processes that affect Fischer-Tropsch catalysts (i.e. catalyst reduction, Ru as a reduction promoter and deactivation) using a so-called nanoreactor. This was done by loading metallic nanoparticles on either side of the nanoreactor surface. In this work hollow carbon sphere nanoreactors were mainly used as the catalyst support of choice to evaluate the processes mentioned above. First the effect of the hollow carbon sphere porosity on Fischer-Tropsch synthesis was evaluated using encapsulated Ru nanoparticles. Limited mass transfer limitations were observed on the mesoporous nanoreactor, thus suggesting the encapsulated nanoparticles were as accessible to reactants as the Ru nanoparticles loaded on the outside of the hollow spheres. Using mesoporous hollow carbon spheres the effect of hydrogen spillover on Co Fischer-Tropsch nanoparticles using a Ru promoter was evaluated by controlling the nanoparticle intimacy [of Ru and Co] by exploiting the hollow carbon sphere morphology. Primary hydrogen spillover was found to be more favorable in enhancing the Co nanoparticles extent of reduction and Fischer-Tropsch activity. However, secondary hydrogen spillover from the Ru nanoparticles to Co nanoparticles on the carbon shell was responsible for a complete reduction of the cobalt oxide when compared to an unpromoted Co Fischer-Tropsch catalyst on the hollow carbon spheres. It was also shown that the secondary hydrogen spillover led to the formation of highly hydrogenated products during Fischer-Tropsch synthesis. In terms of catalyst stability, the nanoparticles, by virtue of being embedded inside the carbon nanoreactor shell, showed good stability against sintering and re-oxidation by added water during process conditions. Furthermore a simple design of a highly sinter resistant catalyst is presented by making a compact nanoreactor on a titania supported Co Fischer-Tropsch catalyst. This study illustrates the benefit of rationally designing and characterizing materials for a comprehensive understanding of important catalytic reaction processes; in this case our focus was on the reduction behavior and stability of Fischer-Tropsch catalysts.Item The effect of metal support interactions between a cobalt catalyst and carbon dots (Cdots) in cobalt fischer-tropsch catalysts(2018) Mokoloko, Lerato LydiaThe aim of this study was to introduce functionalized carbon dots (Cdots) as potential support and reducing agents for cobalt-based Fischer-Tropsch catalysts. Cdots with different physicochemical properties were synthesized from natural-based product such as ascorbic acid, sucrose and chitosan, using microwave-assisted reactions. Two types of Cdots, namely N-free Cdots (or Cdots) and N-doped Cdots, with particle sizes < 10 nm were produced. Both Cdots and N-doped Cdots have an amorphous structure, containing both sp2 and sp3 type carbon. The surface of Cdots contained mainly hydroxyl and carboxyl functional groups, while N-doped Cdots contained hydroxyl, carbonyl, and nitrogen-based (mainly pyridinic and pyrrolic) functional groups. Spinel Co3O4 nanoparticles with 10 nm crystallite sizes were synthesized from the “benzyl alcohol route”. Both the spinel Co3O4 nanoparticles and functionalized Cdots were used to prepare the inverse Co3O4/Cdots and Co3O4/N-doped Cdots catalysts. The in-situ XRD results showed that under H2 gas reduction conditions, the Co3O4/Cdots and Co3O4/N-doped Cdots catalysts increased the reduction temperature of Co3O4 to CoO and metallic Co, and Co (fcc) is the predominant form of metallic Co formed. However, when N-doped Cdots were used as both support and reducing agents, Co3O4 was reduced to both Co (fcc) and Co (hcp). The increase in reduction temperature of Co3O4 on Co3O4/Cdots and Co3O4/N-doped Cdots suggest the presence of strong metal to support interactions (SMSIs) between Co3O4 and functionalized Cdots. Further studies on the activity and selectivity of these prepared catalysts will be conducted under F-T reaction process.Item Osmium as a potential Fischer-Tropsch catalyst using hollow carbon spheres as support(2018) Tshepo, MolefeLittle attention has been given to the use of osmium as a catalyst or promoter for Fischer Tropsch synthesis (FTS). In this study the synthesis and use of osmium nanoparticles supported or encapsulated inside hollow carbon sphere (HCS) as a potential FT catalyst is reported. Os is not expected to be a good FT catalyst and this study provided for an evaluation of the role of Os on carbon as a catalyst in the FT reaction. The Os was encapsulated inside hollow carbon spheres (HCSs) as this will eventually allow for a study of spillover effects in FT reactions. The Os@HCS was prepared using sacrificial Stӧber silica spheres as a template. The SiO2 spheres were loaded with Os ((NH4)2OsCl6 metal precursor) via two methods: homogenous deposition precipitation (HDP) and wetness impregnation (WI). The Os/SiO2 was encapsulated by a carbon layer deposited by CVD using toluene. Removal of the sacrificial template gave Os@HCS (dOs = 11-17.5 nm; dHCS = 450-560 nm). Numerous physicochemical properties were used to reveal that the Os@HCS catalyst had been successfully synthesized. These included electron microscopy, PXRD, TGA, BET, and TPR studies. In the FTS evaluation, the Os@HCS and Os/SiO2 (as reference) showed low activity compared to conventional FT catalysts. The Os supported catalysts showed a product selectivity mainly to C1 and C2 hydrocarbons with high selectivity towards methane (> ~65%).Item Transesterification of animal fat to biodiesel over solid hydroxy sodalite catalyst in a batch reactor(2017) Makgaba, Chabisha PreciousOwing to the ongoing advancement in technology, escalating population sizes and urbanization rate, fossil fuels (coal, petroleum oil and natural gas) remain attractive as an energy source to run most of the daily operations. Consequent to heavy consumption of fossil fuels, the world faces detrimental challenges such as future energy security and environmental concerns. Combustion of fossil fuels results in emission of greenhouse gases such as CO2 and SO2 thereby contributing to global warming and acid rain problems. These alarming challenges drive the need for exploration of alternative energy sources to reduce dependence on fossil fuels. Presented in this dissertation is a study of biodiesel, a biodegradable, non-toxic and environmentally benign energy source as an alternative to petroleum-based fuels. Chemically known as fatty acid alkyl ester (FAAE), biodiesel is commonly produced from vegetable oils or animal fats in addition to methanol by a catalysed transesterification reaction. Currently, biodiesel is more expensive than petroleum diesel due to high operation costs incurred during the production process. Despite the high prices, biodiesel production continues to grow on an industrial scale across the world as supported by policy measures and biofuel targets. Researchers have identified two main factors that contribute to high costs of biodiesel production; 1) type of feedstock and 2) type of catalyst used in the production process. Conventional methods of production use edible oils as feedstock. This becomes unjustified due to the potential price hikes in the food market owing to the prospective competition between fuel and food industries. As a result, numerous researchers reported on the use of cheap and non- edible feedstock oils such as waste cooking oil and animal fat. However, the challenge with the use of non-edible oils is their high content of free fatty acids (FFA) which is unattractive for a smooth transesterification process, more especially when homogeneous base catalysts are used. Homogeneous base catalysts are widely used in current industrial biodiesel production methods because they yield faster transesterification processes due to increased reaction rates. However, these types of catalysts are much sensitive to FFA, so when high FFA content feedstock is used, a saponification reaction occurs which consequently reduces the yield of biodiesel. An additional process unit is required to reduce the FFA content via esterification process prior to the main transesterification reaction. Furthermore, since the reaction mixture is homogeneously combined with the product, an additional process unit for product separation is required to recover the resulting biodiesel from the mixture, translating into additional production costs. Researchers are currently exploring the use of heterogeneous catalysts, which tend to avoid the saponification reaction and reduce the need for an esterification reaction used as oil pre-treatment step to reduce FFA content. This dissertation is therefore dedicated to attaining a economic and environmentally attractive process for biodiesel production using cheap non-edible beef tallow oil (BTO) and a heterogeneous hydroxy sodalite (H-SOD) catalyst. Some industrial operations such as zeolite manufacturing processes produce a low grade H-SOD as by products, which is in turn disposed as chemical waste and therefore induces ground water contamination concerns. Exploration on the use of H-SOD as catalyst can largely contribute to the environmental protective measures as a waste management process among other benefits. The use of H-SOD is extensively reported in current research development on membrane separation; limited research reports on the use of H-SOD material to catalyse chemical processes are present in literature. For the first time in open literature, H-SOD is reported as the solid catalyst for biodiesel production in this dissertation. The investigative study commenced with a preliminary study to gauge the feasibility of using H-SOD as a catalyst where a batch transesterification of waste cooking oil (WCO) was studied. The reaction was conducted at 60 ᵒC for 12 h at a methanol-to-WCO ratio of 7.5:1 using 3 wt. % H-SOD catalyst with a particle size of just below 300 Å, the stirring intensity was kept at 1000 rpm to ensure uniform mixing throughout the reaction. The product obtained after the reaction was analysed using a pre-calibrated Chromatography-Mass Spectrometer (GC-MS) described in Chapter 5, and the results demonstrated the possibility of catalysing a transesterification reaction using solid H-SOD. Under the same reaction conditions, the study was then extended to an investigation on the use of H-SOD to catalyze transesterification of BTO (4.53 % FFA) to FAME. The results showed that FAME was produced, at a yield of 39.6% and a conversion of 68.4%. Seeing that the yield and conversion obtained is relatively small compared to literature findings, the effect of some process conditions on the conversion and biodiesel yield were studied. The transesterification reaction was conducted with variations in the mixing intensity (700 – 1250 rpm), catalyst particle size (200 – 300 Å), reaction time (6 – 24 h) and reaction temperature (40-60 °C). The maximum performance of H-SOD catalyst for a transesterification of BTO was achieved with a conversion of 78.3% and biodiesel yield of 62.9% obtained at optimum conditions: a stirrer speed of 1000 rpm, with the smallest catalyst particle size of 200 Å at maximum temperature of 60 °C and 24 h reaction time. The values of activation energy, reaction constants and frequency factor obtained from the kinetic study were 0.0011 min-1, 5.52 x108 min-1 and 79.20 kJ/mol, respectively, and are within the range of the results reported in literature. As a result, solid H-SOD is recommended as a catalyst for the batch transesterification of BTO in a biodiesel production process.Item An experimental and thermodynamic study of iron catalyst activation and deactivation during Fischer Tropsch synthesis(2016) Gorimbo, JoshuaOne gram amounts of a commercial iron based catalyst were loaded into three reactors and reduced with syngas, hydrogen and carbon monoxide respectively. Fischer Tropsch experiments on the three reactors in parallel with the same operating conditions, namely 60 mL(NTP)/min, 1 bar gauge and 250 °C, were then conducted for extended periods and the gaseous products analysed. Initially (for about 150 hours) the three catalysts had quite different carbon monoxide conversions. After this until about 1000 hours the conversions were similar. However the distribution of products for the differently reduced catalyst was significantly different. This suggested that permanent changes had been done to the catalysts by the different reducing conditions. To try to understand what the differences during the reduction process might be, a thermodynamic analysis of the solid phases after reduction was done. Unfortunately because all the thermodynamic data for the possible carbides was not available this analysis was of limited value. However it did suggest that hydrogen reduced catalyst might contain more oxides and the carbon monoxide reduced catalyst might contain more carbides. Some electron microscope and XRD experiments supported these ideas and might account for the different selectivities of the differently reduced catalysts. Runs after about 5000 hours were done at different flowrates (60, 30 and 15 mL(NTP)/min) of syngas and again the big effects were on differences between the selectivities, the big effects being when going to the lowest flowrate. After about 12000 hours regeneration of the catalysts was then done by oxidation and then the same syngas reduction on all the catalysts. Runs were then done at different pressures (1, 10 and 20 bar gauge) and again selectivities were the biggest effects that remained, clearly showing the initial reduction had made permanent changes. In the final section some novel plots were used to try to make more sense of the results. It was shown that for all the catalysts the Olefin to Paraffin ratios were tied to each other under all conditions and that they were mainly a function of the conversions with much higher values at low conversions.Item Hydrogenation of carbon monoxide over modified cobalt-based catalysts(1991) Colley, Saul EricA disadvantage of the Fischer-Tropsch synthesis is that a broad product spectrum is obtained. Economic considerations however require an improvement in the optimization of the reaction to maximize the production of high value commercial products, in·particular, short chain olefins and high molecular weight hydrocarbons. [Abbreviated abstract. Open document to view full version]Item Direct methane transformation into higher hydrocarbons and oxy-products(1996) Eskendirov, IgorIn present thesis the results of a study of the combined action of a solid catalyst and a gas-phase inintiator, hydrogen peroxide, in the methane partial oxidation and oxidative coupling reactions are presented. [Abbreviated Abstract. Open document to view full version]Item Palladium (II) and iron (II) complexes derived from pyridyl-imine ligands as catalyst precursors for 1-hexene oligomerization and norbornene polymerization(2017) Khuzwayo, Pamela ZanelePyridyl-imine ligands L1-L4 were prepared by condensation of pyridine-2-carboxyaldehyde with an appropriate amine. Characterization by NMR spectroscopy, infrared spectroscopy, mass spectrometry and elemental analysis confirmed successful preparation in yields of 64-88%. These ligands were used to prepare Pd(II) complexes C1-C4, from PdCl2(CH3CN)2 and the corresponding pyridyl-imine ligand. 1H-NMR, 13C-NMR, FT-IR, mass spectrometry and elemental analysis confirmed coordination. Attempts to prepare target Fe(II) complexes C5-C8 by reacting the ligands with anhydrous FeCl2 were unsuccessful. Infrared data suggested coordination of ligands to the Fe centre, however mass spectrometry and elemental analysis data revealed that target complexes were not obtained. Pd(II) complexes C1-C4 were evaluated as catalyst precursors for 1-hexene oligomerization and norbornene polymerization using methylaluminoxane (MAO) as co-catalyst. The oligomerization of 1-hexene was investigated in a neat reaction media at various Al:Pd ratios. All investigated complexes were found to be inactive for the oligomerization of 1-hexene. From 1H-NMR spectroscopy and GC-MS analysis it was observed that the product distribution was mainly a mixture of 2-hexene and 3-hexene isomers. Parameters such as temperature and time did not have any significant influence towards the productivity of 1-hexene oligomers. Norbornene polymerization studies were carried out with Pd(II) complex C4 in toluene at room temperature. This complex was found to exhibit good activity for norbornene polymerization, producing a vinyl bicyclic polymer, confirmed with infrared and solid state 13C-NMR spectroscopy. Increasing the amount of co-catalyst (MAO) and temperature did not have any significant influence on the activity and monomer conversion. However, increasing reaction time was observed to have a significant influence on the activity.Item Synthesis and performance evaluation of Co/H-ZSM-5 bi-functional catalyst for Fischer-Tropsch Synthesis(2016) Matamela, KhuthadzoThe motivation behind this study is the need to manage and reduce wastes, in particular waste tyre and biomass, while in turn recovering energy from these carbonaceous materials. These wastes were gasified to produce synthetic gas which served as a feed to the Fischer-Tropsch Synthesis process to produce hydrocarbons. The formed hydrocarbons can be used as fuels for different purpose like transportation, domestic and industrial heating systems. Cobalt supported on zeolite catalysts are used because of their high acidic sites present in the zeolite that can break the Anderson-Schultz-Flory polymerization kinetics and also because cobalt-based catalysts are preferred for low temperature Fischer-Tropsch (LTFT) synthesis process due to their negligible water and carbon dioxide formation as well as stability and life span. In this research, a bi-functional Co/H-ZSM-5 catalyst was synthesized, characterized and evaluated for direct production of hydrocarbons at different process conditions. The bi-functional catalyst was prepared by incipient wetness impregnation method of an aqueous cobalt solution as the source of cobalt metal onto an H-ZSM-5 zeolite support, thereafter dried at 120 °C and calcined at 400 °C to obtain the finished Co/H-ZSM-5 catalyst. Physicochemical analyses performed included, Nitrogen Physisorption at 77 K to determine the surface area, pore volume and size of the synthesized catalyst. Also the N2 adsorption was used to determine the adsorptive properties of the catalyst. X-ray diffraction at 2θ region between 10 to 90 ° by using Co-Kα radiation (λ=1.79026 Å) was used to determine the material crystallinity, structure and composition. For the morphology and elemental composition of the catalyst, a Scanning Electron Microscopy coupled with an Energy Dispersive X-ray Spectroscopy was used. Thermal stability of the catalyst was checked using a Thermal Gravimetric Analyzer to determine how the catalyst degraded with time when temperature was increased uniformly. Reducibility of the catalyst was determined by using Temperature Programmed Reduction equipment in a hydrogen environment from room temperature to 900 °C. Transmission Electron Microscopy was used to check the catalyst morphology, and the dispersion of the metal-oxide particles within the catalyst support. The bi-functional zeolite supported catalyst was found to possess a surface area of 292 m2/g, pore volume of 0.18 cm3/g and pore size of 2.83 nm. The catalyst morphology was found to be irregular and aggregated-circular shape with a particle size of about 2.5 ± 0.5 μm. The embedded cobalt-oxide particles were obtained to be about 8 ± 3 nm located closer to the surface of the support and were reduced to metallic cobalt of 25% composition, at 330 °C in a hydrogen rich environment with an expected hydrogen consumption of 133 %. The process conditions under study involved flow rate, pressure and temperature and synthetic gas of different H2/CO ratio. The Synthetic gas mixture was purchased from Afrox and prepared in a way to mimic or simulate the syngas mixture expected from gasification of the waste tyre and biomass. However the study mainly focused on Hydrogen, Carbon Monoxide and Carbon dioxide as the dominant constituents of a waste tyre produced syngas. The bi-functional, Co/H-ZSM-5 performance evaluation was compared to commercial Co/SiO2 catalyst under similar conditions. The performance evaluation and comparison was made based on conversion and selectivity at different conditions. The process conditions considered were a flow rate of 1200, 2400 and 3600 GHSV (ml/gcat.hr), a pressure of 2, 8 and 15 bar, Low Temperature Fischer-Tropsch (LTFT) process at 220 and 250 °C was used, with a syngas composition that included H2/CO ratio of 1.5, 2.5 and 2.5 with 5 % of CO2 present in the reactant feed. The combination of 2 bar, 1200 GHSV and temperature of 220 °C and 1.5 of ratio was considered as low process conditions. While the combination of 15 bar, 1200 GHSV, 250 °C and ratio of 2.5 was considered as high process condition. Three pre-calibrated GCs (two online and one offline) were used to analyze the reaction products and the feed and the integrated peak-data analyses was captured by the use of a Data Apex Chromatograph software package known as Clarity ® (v. 2.5). The captured and analyzed data was used to calculate conversion and selectivity according to the methods reported in literature. With regard to the effect of process conditions, at low process conditions, the bi-functional catalyst, Co/H-ZSM-5, resulted in a 3 % CO conversion, while the commercial Co/SiO2 catalyst, resulted in 15 % of CO conversion. However the bi-functional catalyst was more selective to gasoline range products and 16 % selectivity to C5 hydrocarbons was obtained and 79 % to C6+, as compared to selectivities of 4 and 75 % for C5 and C6+ respectively, for Co/SiO2 catalyst. Also Co/SiO2 was found to be more selective to Olefins, the undesired products, with a selectivity of about 91 % to C6+ hydrocarbons as compared to a selectivity of 87 % for C6+ hydrocarbon obtained by using the bi-functional Co/H-ZSM-5 catalyst. Methane production was high for the Co/SiO2 catalyzed reaction, (about 13 % selectivity) with some quantity of water produced, as compared to 3 % methane selectivity for Co/H-ZSM-5 catalyst with no water produced during the reaction. At low process condition, both catalysts were less prone to middle distillates hydrocarbon production. At high process conditions, a CO conversion of about 54 and 68 % was obtained by Co/H-ZSM-5 and Co/SiO2 catalyst respectively. At these conditions the H-ZSM-5 supported catalyst was observed to produce more methane, about 53 % selectivity while for Co/SiO2 catalyst it was obtained to be 35 % selective to methane, with 66 and 7 % of C6+ olefin and paraffin selectivity respectively. Co/H-ZSM-5 offered 9 % selectivity to C6+ per olefin and paraffin hydrocarbons. The commercial catalyst showed an orderly manner of distributing products at these conditions while the bi-functional catalyst randomly distributed the formed products with a high selectivity to middle olefin distillates. In terms of CO2 co-feeding in the reactant feed, both CO and CO2 were hydrogenated to hydrocarbons. A CO conversion of about 73 % was obtained by Co/H-ZSM-5 catalyzed reaction while for Co/SiO2 catalyzed reaction a conversion of 70 % was obtained. About 63 and 75 % of CO2 conversion was obtained by H-ZSM-5 and SiO2 supported catalyst. These results were obtained at high process conditions. No change in paraffin selectivity was observed when comparing a state in which CO2 was present and absent, however olefin selectivity is significantly affected by the presence of CO2. Thus, an increase in olefin selectivity is observed with Co/SiO2, achieving 76 % of C6+ Olefin from 66 % and Co/H-ZSM-5 increasing middle olefin distillated from 25 to about 30 % of selectivity. Based on the performance evaluation the bi-functional catalyst was proven to yield higher hydrocarbons from a simulated waste-tyre synthetic gas with no requirement of downstream hydrocracking, since the bi-functional catalyst cut-off higher hydrocarbons due to its acidic sites. While the metallic sites of the catalyst, catalyzes the reaction of synthetic gas to hydrocarbons. This type of catalyst with both metallic sites and acidic sites is a hybrid-catalyst commonly known as bi-functional catalyst (Kang et al., 2014). At low process conditions the bi-functional Co/H-ZSM-5 catalyst is found to be more preferred while at higher process conditions the commercial catalyst was found to be more preferred, however in the presence of CO2 co-feeding, either catalyst can be used, but if water elimination is required the bi-functional catalyst is more suitable for the process.
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