3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Photocatalytic degradation of methyl violet in water using TiO2/Cellulose-N-MWCNTs(2018) Mathebula, XiluvaABSTRACT TiO2-carbon based composites are of great significance in a wide range of applications including photocatalytic degradation. This is attributed to the high photodecomposition efficiency of the composites as compared to independent TiO2. Carbon materials such as cellulose polymer and multiwalled carbon nanotubes (MWCNTs) are considered as good supports for TiO2 owing to their unique properties such as lightweight, large surface area and high aspect ratio. Lately, the study of cellulose-MWCNTs composite has been an area of academic interest due to its large mass fraction, and prowess to facilitate toughening mechanisms in fiber bridging. However, a cost-effective method that can improve the dispersion and interfacial adhesion of the MWCNTs in the polymer is still required. Thus different modification methods of MWCNTs have been explored to increase the binding sites of the material. In this study, it was hypothesized that the cellulose’s potential as a TiO2 support can be improved by hybridizing it with MWCNTs resulting in high TiO2-C photocatalytic activity through synergistic effect. A catalytic decomposition of Fe-Co/CaCO3 was used over C2H2 to fabricate the MWCNTs. Thereafter, the MWCNTs were functionalized by (1) acid-treatment (referred to as fMWCNTs), (2) nitrogen doping by in situ and ex situ methods (referred to as in situ N-MWCNTs and ex situ N-MWCNTs, respectively) and (3) both acid treatment and nitrogen doping (referred to as in situ fN-MWCNTs and ex situ fN-MWCNTs). Moreover, cellulose-N-MWCNTs (C@fN-MWCNTs) hybrid was prepared by electrospinning a solution of cellulose acetate/in situ fN-MWCNTs (11/0.115) in DMAc at 25 kv and 1 mL/h. The prepared MWCNTs and cellulosic materials were further used as support materials of TiO2 in the photodegradation of methyl violet (MV 6B). The supported TiO2 catalysts were prepared by a sol-gel method and then analyzed using various techniques, such as transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectroscopy. iii TGA results revealed that in situ N-MWCNTs contained high impurities inclusive of as Fe, Co, Ca, and amorphous carbon which were identified by XRD analysis. Nevertheless, TGA, BET and TEM, showed that acid treatment of MWCNTs improves their purity, surface area and anchoring sites for the TiO2, respectively. Furthermore, SEM results showed that C@fNMWCNTs hybrid interacts with TiO2 better than cellulose fibers. This was in accord with the PL results which showed a reduction in the electron/hole recombination. However, the surface area of C@fN-MWCNTs was very low compared to cellulose fibers which resulted in low dye adsorption capacity by C@fN-MWCNTs. The photocatalytic degradation activity commercial TiO2 was enhanced by 3.7% and 5.6% after being supported on cellulose and C@fN-MWCNTs, respectively. Thus, incorporating in situ fNMWCNTs with cellulose did improve the cellulose’s potential as a TiO2 support. However, the overall photocatalytic degradation performance of TiO2/C@fN-MWCNTs was less than that of in situ TiO2/fN-MWCNTs. This may be due to the reduction in the surface area, which resulted in reduced adsorption and thus lowers degradation efficiency.Item The effect of the presence of species mimicking metal-support interactions adsorbed on a Co(0001) metal surface(2016) Mohotlhoane, Sifiso AlecThe adsorption of molecules on a metal surface is core in heterogeneous catalysis. Surface sensitive techniques such as low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) are key tools to study adsorption geometries and structures of molecules and atoms on a metal surface. As our first model system we investigated the dissociation of NO on Ir{100}. The LEED experimental results showed a p(2 X 2) diffraction pattern at 300 K using. In this study two options were explored: phase mixing where dissociated nitrogen and oxygen are on the same unit cell, as well as phase separation where both nitrogen and oxygen form their own separate unit cell which results in a p(2 x 2) unit cell. Calculations were done on atop, bridge and hollow sites, with only perpendicular parameters and vibrational amplitude being varied initially. Results for phase mixing calculations gave the lowest R-factor of 0.70 ± 0.11 for atop site. We further considered phase separation for hollow and bridge sites for nitrogen and oxygen respectively because these two sites were found to be the most stable sites using DFT from previous studies. The lowest R-factors were 0.37 ± 0.06 for nitrogen c(2x2) and 0.24 ± 0.13 for oxygen p(2 X 1) For oxygen significant row pairing of iridium atoms stabilized the structure as mentioned in previous studies. Therefore from our results it is evident that phase separation models the experimental data better than phase mixing. Nitrogen and oxygen form c(2 X 2) and p(2 X 1) overlayer structures respectively which in combination result in a p(2 X 2) pattern that is in agreement with experimental results. The second system involves enantio-selectivity and chiral resolution at the organic-inorganic interfaces. The d-serine molecule was adsorbed on the Cu{110} surface. Density functional theory (DFT) calculations were used as a benchmark for our CLEED calculations. LEED experiments showed a (- 1 + 2: 40) overlayer pattern for d-serine adsorbed on Cu{110} surface. Three structures from DFT calculations with the lowest energy were used for CLEED calculations. These structures differed by the way they bond to the surface and molecular interactions. Calculations were carried out on these three structures and the structure with intra-dimer bonding was the best structure. The searches for this structure were further optimized by introducing pairing of the atoms in the row reconstruction on the copper surface and angle search. The lowest value obtained was 0.37 ± 0.09, which suggests that further understanding of this system is needed. The ultra-high vacuum (UHV) chamber was fully commissioned and is now ready for TPD and XPS studies.Item Investigation of the desulfurization of petroleum distillates using novel ionic liquids(2016) Sefoka, Ramogohlo EuniceThe use of fuels (from crude oil) in vehicles is responsible for one of the biggest environmental challenges; SO2 emission. As a result most countries regulate their sulfur emissions, with the goal of getting to the use of 10 ppm sulfur fuels. These stringent fuel sulfur content requirements have resulted in intensive research being directed at alternative desulfurization technologies which will ensure the treatment of fuels to acceptable sulfur levels. Extractive desulfurization using ionic liquids (IL) may be considered as one of the most promising of these technologies and is the subject of the study presented in this work. This study served two major purposes: (1) to investigate the capacity as well as key parameters which affect the extraction efficiency of the IL; 1-butyl-3-methylimidazolium octylsulfate as a solvent for deep extractive desulfurization of real Fluid Catalytic Cracking Unit (FCCU) diesel fuel samples collected from a typical South African Refinery, (2) to study/find suitable solvents for the regeneration of sulfur-loaded 1-butyl-3-methylimidazolium octylsulfate and the efficiency and effectiveness of the regenerated IL in the desulfurization of diesel fuel. 1-butyl-3-methylimidazolium octylsulfate was selected due to its properties i.e. good extractive ability for S-compounds and insolubility in fuel oils. A 22.1% sulfur removal was achieved in the desulfurization of FCCU feed stream diesel fuel, while 96% sulfur removal was achieved for FCCU product stream diesel fuel. These results show that the IL is more effective in the selective removal of sulfur (S) from FCCU diesel product than from FCCU feed stream, suggesting that fuel sulfur content and stream composition affects the extraction efficiency and effectiveness of the IL. Based on thermodynamic considerations, hexane was selected as the most suitable solvent for the re-extraction of sulfur from spent IL. Regenerated IL was used for desulfurization of diesel and achieved highest sulfur removal of 95% and the IL was regenerated up to four times without appreciable decrease in efficiency. The results obtained herein show that ILs are effective in the desulfurization of real diesel oil samples when the sulfur concentration is not very high. Further studies on the recoverability of ILs as well as their environmental impact need to be done to support findings in this study.Item Removal of toxic metals and recovery of acid from acid mine drainage using acid retardation and adsorption processes(2016) Nleya, YvonneThe remediation of acid mine drainage (AMD) has received much attention over the years due to the environmental challenges associated with its toxic constituents. Although, the current methods are able to remediate AMD, they also result in the loss of valuable products which could be recovered and the financial benefits used to offset the treatment costs. Therefore, this research focused on the removal of toxic heavy metals as well as the recovery of acid using a low cost adsorbent and acid retardation process, respectively. In the first aspect of the study, three low cost adsorbents namely zeolite, bentonite clay and cassava peel biomass were evaluated for metal uptake. The adsorption efficiencies of zeolite and bentonite, was found to be less than 50% for most metal ions, which was lower compared to the 90% efficiency obtained with cassava peel biomass. Subsequently, cassava peel biomass was chosen for further tests. The metal removal efficiency using the cassava biomass was in the order Co2+> Ni2+> Ca2+> Mn2+> Fe3+> Mg2+. The highest metal removal was attained at 2% adsorbent loading and 30 ˚C solution temperature. Amongst the equilibrium models tested, the experimental data was found to fit well with the Langmuir isotherm model. Column studies using the immobilized cassava waste biomass suggested that the breakthrough curves of most metal ions did not resemble the ideal breakthrough curve, due to the competitive nature of the ions present in the AMD used in this study. However, the experimental data from the column tests was found to correlate well with the Adam-Bohart model. Sulphuric acid recovery from the metal barren solution was evaluated using Dowex MSA-1 ion exchange resins. The results showed that sulphuric acid can be recovered by the resins via the acid retardation process, and could subsequently be upgraded to near market values of up to 70% sulphuric acid using an evaporator. Water of re-usable quality could also be obtained in the acid upgrade process. An economic evaluation of the proposed process also showed that it is possible to obtain revenue from sulphuric acid which could be used to offset some of the operational costs.