3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Synthesis and characterisation of gold-rhodium nanocatalysts and their catalytic activity on carbon monoxide oxidation(2016-05-10) Rikhotso, Rirhandzu ShamaineGold nanoparticles are ideally suited as catalysts for selected low temperature reactions such as CO oxidation for catalytic convertors in the motor industry due to their high activity. But they are prone to sintering at high temperatures. Platinum-group-metal based catalysts are efficient at elevated temperatures and generally inactive at lower temperatures. This study explored the CO oxidation efficiency of gold nanoparticles and of a combination of gold and rhodium nanoparticles. Variables such as pH, loading concentration and type of support were varied to control the final properties of the Au based catalysts. Possible bimetallic systems of gold and rhodium were explored for wider temperature range activity than gold alone. All catalysts were characterised using Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-Ray Diffraction (XRD). Activity was measured using a temperature controlled, custom-built reactor linked to a gas chromatograph. The conditions yielding the smallest gold nanoparticles were established by adding 5, 8 or 10 wt.% loadings of chloroauric acid to aqueous suspensions of either TiO2 or SiO2 at pH 5, 7 or 9 and at 70-75 °C over 60 minutes. Each preparation was sealed in parafilm, aged in the dark at room temperature for 3 days, vacuum-filtered and subsequently calcined at 300 °C. Gold nanoparticles were smallest when deposited onto TiO2 instead of SiO2, at pH 7 and at a loading of 5 wt. %. A combination of gold and rhodium catalysts were subsequently prepared using these conditions, with the simultaneous addition of rhodium at 1, 3, 5 or 10 wt. % loading. Hydrolysis of gold is highly dependent upon pH, resulting in the synthesis of smaller particles under alkaline conditions. Catalytic activity of samples analysed at 70 and 150 °C was highest for gold nanoparticles below 5 nm, in agreement with previous studies. In the proposed bimetallic catalysts, it was difficult to distinguish gold and rhodium nanoparticles in TEM images, although EDS confirmed their combined presence on the TiO2 support. Particle sizes remained below 5 nm, appearing monodispersed on the TiO2 support except at 10 % rhodium loading where some nanoparticle aggregation was observed. CO oxidation activity showed an apparent temperaturedependent shift in the optimal rhodium loading. Au-TiO2 catalysts with a 5% loading showed the highest activity up to 350 °C for a period of 10 hours and the catalyst deactivated due to sintering. At 150 and 200 °C the Au/Rh-TiO2 catalyst remained active for more than 12 hours. It was concluded that the inclusion of rhodium is a potentially-favourable method for stabilising the activity of gold catalysts.Item Design and development of multifunctional Raman active noble metals nanoprobes for the detection of malaria and tuberculosis biomarkers(2016) Mlambo, MbusoSurface enhanced Raman spectroscopy (SERS) has emerged as a surface sensitive vibrational technique that leads to the enhancement of the Raman scattering molecules on or close to the surface of a plasmonic nanostructure. The enhancement is found to be in orders of 104 to 1015, which allows the technique to be sensitive enough to detect a single molecule. In this study, we report on the synthesis of different sizes of gold and silver nanoparticles (AuNPs and AgNPs) and gold nanorods (AuNRs). These are functionalized or co-stabilized with different stoichiometric ratios of HS-(CH2)11-PEG-COOH and alkanethiols (Raman reporters), i.e.; HS-(CH2)11-NHCO-coumarin(C), HS-(CH2)11-triphenylimidazole (TPI), HS- (CH2)11-indole (HSI), HS-(CH2)11-hydroquinone (HQ) to form mixed monolayer protected clusters (MMPCs). The alkanethiols were chosen as Raman reporters to facilitate the selfassembled formation of monolayers on the metal surface, thus resulting in stable MMPCs. The optical properties and stability of MMPCs were obtained using ultraviolet-visible (UVvis) spectrophometry and a zeta sizer. Size and shape of the as-synthesized nanoparticles were obtained using transmission electron microscopy (TEM). The tendency of thiolcapped nanoparticles to form self-assembled ordered superlattices was observed. Their Raman activities were evaluated using Raman spectroscopy, with the enhancement factor (EF) being calculated from the intensities of symmetric stretch vibrations of C-H observed in the region of about 2900 to 3000 cm-1 in all SERS spectra. In all four different alkanethiols (Raman reporters), smaller size metal nanoparticles (14 nm for AuNPs and 16 nm AgNPs) showed higher EF compared to 30 and 40 nm metal nanoparticles. The EF was observed to increase proportionally with stoichiometric ratios of alkanethiols from 1% iv | P a g e to 50%. The prepared MMPCs with small sizes were used as a SERS probe for the detection of malaria and tuberculosis biomarkers.Item Synthesis and characterization of bimetallic platinum nanoparticles for use in catalysis(2015) Mathe, Ntombizodwa RuthBimetallic platinum nanoparticles were synthesized for application as anode catalysts for low temperature fuel cells such as direct methanol fuel cells (DMFCs). Two distinct synthesis procedures were used; namely conventional synthesis with post-synthesis heat treatment, and secondly polyol microwave-irradiation without further heat-treatment. The aim was to synthesize interesting and novel bimetallic nanostructures and relate their shape and morphologies to their methanol oxidation reaction (MOR) activities and their CO tolerance. Due to the high cost of the conventional synthesis processes as well as their use of harmful solvents, microwave-irradiation was explored as a possible synthesis procedure. It is a greener and more environmentally friendly approach with possibilities of mass production of the nanoparticles. For both the synthesis procedures, the reducing agent, the precursor salts, surfactants, pH of the solution and molar ratios were varied to determine the effect on the shape, size and ultimately the electrocatalytic activities of the Pt-Co and Pt-Ni nanoparticles. For the conventional synthesis procedure, the main parameter of comparison was the strength of the reducing agents, where NaBH4 and N2H4 were used under the same reaction conditions. In this study, the strength of the reducing agent affected the properties of the Pt-Co and Pt-Ni nanoparticles, such that, the stronger the reducing agent, the higher the degree of alloying and the more electrocatalytically active the materials. The drawback in the conventional synthesis was however low current outputs, in the microamps range, which necessitates a need to explore other synthesis procedures. Microwave-irradiation was thus used as an alternative synthesis procedure in an attempt to produce more active bimetallic platinum nanoparticles. Different reaction parameters were changed in this process to optimize the synthesis process, namely the pH of the solution, the amount of surfactant and the Pt-Ni molar ratio. In changing the reaction parameters, there was an observed change in the structure of the nanoparticles, with an average size in the order of 5 nm and different MOR activities. Furthermore, it was found that the activity was highest for the optimum amount of PVP and NaOH concentration of 500 mg and 1.0 M NaOH. In general, the MW synthesized nanoparticles achieved current values in the microamps to amps range, making it a more attractive synthesis procedure compared to the conventional method. The CO tolerance of the materials is an important aspect, as one of the main drawbacks of the commercial application of fuel cells is the propensity of Pt to get poisoned by CO during the methanol dissociation process. Therefore CO stripping measurements were performed on the MW-irradiated catalysts. The catalysts produced in this work showed good resistance towards CO. In general, the behaviours of the catalysts were dependent on the amount of surfactant and the molar ratio of the starting solution. The mechanism of CO tolerance in this case was determined as the bifunctional model, where the Ni-oxide and Ni-hydroxide species donate O to the electrooxidation of CO to CO2. In conclusion, the study of microwave-irradiated bimetallic nanoparticles performed here, resulted in highly active catalysts, which are even more active than commercial Pt/C nanoparticles.Item Silica and maghemite nanoparticles for the remediation of acid mine drainage-contaminated waters and Nanoparticle modification of metal uptake by a freshwater alga-Scenedesmus sp(2015-01-30) Etale, AnitaAims: The adsorptive removal of Cu, Mn, Hg and U by silica and maghemite nanoparticles (NPs) under acid mine drainage (AMD) conditions was investigated with the aim of assessing the applicability of NPs for remediation of AMD-contaminated water. The effect of NPs on metal uptake by algae, an increasingly popular remediation alternative, was also investigated. Methods: NP and algal metal removal were quantified by batch experiments using commercially prepared, bare and amine-functionalised silica-carbon hybrid NPs characterised for size, surface area, porosity, crystallinity, elemental composition and hydrodynamic size. Metal uptake by algae was quantified in the presence and absence of NPs. Results: Silica and maghemite NPs can be used for the adsorptive removal of Cu, Mn, Hg and U from AMD-contaminated surface and ground water. NP metal uptake was rapid and equilibria were attained within 5 minutes with silica and maghemite NPs, and within 45 minutes with amine-functionalised hybrid NPs. Adsorption efficiencies for Cu, Mn, Hg and U at pH 3 were 52, 56, 56 and 49%, respectively with silica and 56, 52, 75 and 50%, respectively, with maghemite NPs. Metal removal was enhanced by >10% in solutions containing ferric, manganese or sulphate ions, although Cu removal was inhibited in solutions with a >1 Mn:Cu ratio. Despite the presence of high affinity amine groups in hybrid NPs, Cu removal was only 52% due to the low surface area of the adsorbent. The comparative study with Hg, however, showed that surface area was not the only determinant of adsorption efficiency: maghemite NPs with a specific surface area ~15 times less than silica adsorbed 21% more Hg. Metal removal by Scenedesmus sp. was enhanced by 12-27% in solutions containing NPs due to the greater sorption surface areas. NPs also modified metal partitioning in algal cells: intracellular concentrations were lower and extracellular concentrations higher in solutions containing NPs relative to controls (no NPs). Conclusion: Silica and maghemite NPs can be applied for the adsorptive removal of Cu, Mn, Hg and U from AMD-contaminated water and to improve the efficiency of phycoremediation by Scenedesmus sp. These findings also point to the possibility of retardation of metals by NPs during their transportation from tailings and contaminated sites. Their partitioning to NPs and the strength of the interactions thereof can determine the prevalence of the metals in solution or in the solid phase.Item Resin-gel synthesis and characterisation of copper and titanium mixed metal oxides nanoparticles(2014-08-21) Dziike, FaraiThe resin-gel method of synthesis successfully produced compounds of mixed metal oxides of copper titanium oxide powders of the form CuxTiyOZ with different compositions. These include Cu3TiO5, Cu3TiO4, Ti3Cu3O, Cu2Ti4O, Cu2Ti2O5 and Cu2TiO3. Heat-treatment of the powders at 300°C, 500°C, 700°C and 900°C for 1 hour was performed to determine the full composition/temperature phase diagram. The target particle size was in the 10- nanometer range, and for most of the samples, this size was achieved. Powder xray diffraction and transmission electron microscopy were the main techniques used to study the crystallization of these materials and their transformation to other polymorphic phases under different temperatures. Phase-match, particle size analysis and TEM imaging determined the properties and characteristics of the respective crystallographic phases of these materials. TEM analysis showed that some powders agglomerated while others exhibited both regular and irregular morphologies and polydisperse particle size distribution. Only a single unique phase was identified, but its structure could not be determined.Item Synthesis and application of novel functionalized nanostructured membranes incorporating N-doped CNT supported metal nanoparticles in water treatment(2013-08-06) Phao, NeoIn relation to conventional water treatment methods, membrane separation has acquired a great audience due to its wide applicability, reliability, low cost, low energy demands, and ease of use. However, membrane fouling has been identified as the main downsizing factor in the application of this technology. To address this issue, several studies have suggested the use of inorganic additives for enhancement of the membrane antifouling properties. In this study, silver (Ag) decorated nitrogen doped carbon nanotubes (N-CNTs) dispersed into polyethersulphone (PES) membranes for potential use in water treatment. Firstly, N-CNTs were synthesised using the chemical vapour deposition (CVD) method. The black soot was functionalised and characterised using transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and Brunner-Emmett-Teller (BET). The N-CNTs were found to have an average diameter of 15 nm. The functionalised N-CNTs were then decorated with Ag nanoparticles (AgNPs) using the Polyol method. The resultant product was also characterised using TEM, the AgNPs were found to have an average diameter of 6 nm. The N-CNTs and Ag/N-CNTs were then uniformly dispersed into (PES) membranes to form N-CNT/PES and Ag/N-CNT/PES blend membranes, respectively. The membranes were then characterised using several series of techniques including scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle analyser and a cross-flow filtration system. The blend membranes were investigated for any improved properties and tested for their efficiency in removing model pollutants (polyethylene glycol, humic acids, and bacteria) from water. The AFM results revealed a reduction in surface roughness from 23.9 nm for the pristine PES to 12.7 nm in the N-CNT/PES blend membranes. The mechanical stability increased from 3.7 MPa for the pristine PES to 4.4 MPa with a small addition of N-CNTs. Furthermore, the performance studies showed a 46% increase in pure water flux and a 13% increase in rejections for N-CNT blend PES membranes as compared to the pristine PES membrane. Antibacterial studies were also performed where Ag modified N-CNTs were found to inactivate Enterohaemorrhagic E. coli by almost twice the initial concentration in the bacterial suspensions. Finally, Ag/N-CNTs were immersed into PES membranes. The Ag/N-CNTs/PES membranes were then tested for their activity towards the bacteria.Item Microwave-assisted synthesis of β-CD polymers incorporating N-doped carbon nanotubes and silver nanoparticles for water purification(2013-07-29) Masinga, Sello PetrosThe pollution of water sources by chemical and biological species has created a serious water crisis all over the world. Such pollution has placed severe strains on the limited water sources resulting in the spread of waterborne diseases, which continue to be the leading causes of deaths in developing countries. Pollution by organic species still poses a serious health and environmental problem. Attempts to mitigate this problem are on-going and a number of methods are employed currently. Activated carbon and reverse osmosis are some of the current techniques that are used for the removal of organics in water. However, these techniques are limited in the removal of pollutants at lower concentrations (ng/L). Recent studies demonstrated the efficient removal of organics by nanoporous cyclodextrin (CD) polymers, a class of nanomaterials with great potential in absorbing organic pollutants from water. This project reports on the synthesis of β-cyclodextrin (β-CD) based polymer nanocomposite materials (nanocomposites) that have been blended with nitrogen-doped carbon nanotubes (N-CNTs) and silver (Ag) nanoparticles for water treatment. Prior to this study, the synthesis of these nanocomposites has been based on a conventional method that involves heating the reactants in a round bottom flask for 16 – 24 h. In this study a new method that is efficient, greener and time saving is reported. This facile method involved synthesizing the polymer nanocomposites under microwave irradiation wherein complete synthesis of the polymer nanocomposites was achieved in 10 min. N-CNTs were first synthesized via modified chemical vapour deposition method (CVD) using a 10wt% Fe-Co/CaCO3 catalyst. The N-CNTs were found to contain ~ 2% nitrogen by CN and XPS analysis. The N-CNTs were of high purity and were oxidized with acid functional groups (-COOH, -C=O, -OH) using nitric acid under reflux. Zeta potential studies indicated that the quantity of acid functional groups increased with increase in acid treatment time. The functionalised N-CNTs (fN-CNTs) were then decorated with Ag nanoparticles using microwave irradiation and further polymerized with β-CD using hexamethylene diisocyanate (HMDI) as the linker in an industrial microwave under an inert gas atmosphere of N2. Two types of polymer nanocomposites were synthesized namely, N-CNTs/β-CD and Ag/N-CNTs/β-CD. Different synthesis parameters such as microwave power and time were varied during the synthesis of these composites to study their effect on the result materials. Different level of power, 400 W, 600 W and 800 W were tested and surface area and morphology data indicated that all these powers can be used in synthesising the polymer composites. The optimum power used was 600 W, which gave highly porous, less densely packed morphology and a higher surface area of the polymers. The synthesis time was varied for 10 min, 15 min and 30 min. An irradiation time of 10 min was found to be sufficient for the synthesis of the nanocomposites. The polymers showed an efficient removal of p-nitrophenol, bisphenol A and trichloroethylene (TCE) from spiked water as confirmed by UV-Vis spectroscopy and GC/MS analysis.Item Investigating the effect of gold-palladium bimetallic nanoparticles on TiO2 and the catalytic activity in CO oxidation(2013-04-29) Ntholeng, NthabisengIn recent years, studies have shown that supported Au catalysts have high activity for CO oxidation at low or ambient temperatures. However, the activity of these catalysts is dependent on a lot of synthesis conditions as reproducibility of small sized gold particles is hard. In this study supported Au catalysts were prepared via deposition precipitation-method (DP). The small sized Au particles were supported on TiO2 (P25). The suitable synthesis conditions such as pH, aging, metal loading and catalyst pre-treatment were investigated in order to obtain optimum synthesis conditions. The catalysts were characterized with TEM, XRD, and HRTEM. It was found that 3.7 nm Au particles were best synthesized when Au metal loading is 3% at pH 8 and aged for 72 h. The suitable calcination temperature was 200 °C. It was found that the Au particle size was 4.5 nm when Au was supported on SiO2 thus making TiO2 a suitable support. Bimetallic catalyst was synthesized via DP where Pd metal was incorporated as the second metal. It was found that the type of bimetallic formed was heterostructed where both metals where separately attached on the support. The interatomic distance measured from HRTEM results confirmed that both metal were individually attached on the support. XRD results showed that there was no Au-Pd alloy phase or PdO confirming that the Pd metal on the support was indeed in metallic form. Carbon monoxide (CO) oxidation reactions were undertaken in a tubular glass flow reactor. The monometallic Au catalyst showed superior activity at 200 °C with almost 100% CO conversion. It was also observed that the activity of these catalyst decreased as temperature increased. The CO-TPD studies showed that as temperature increased there was a low CO adsorption due to a decrease in adsorption sites. Varying Pd composition in the bimetallic catalyst did not enhance catalytic activity. However, 25Au75Pd catalyst showed a better conversion as compared to other Au-Pd catalysts. Temperatures studies on bimetallic catalysts showed that as temperature increased there was a decrease in activity. The observed decrease could be attributed to catalyst formation of large particle aggregates. It was also assumed that the low activity was due to how these catalysts were prepared as there was no surfactant utilized during preparation.