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Item Effect of fly ash composition on the synthesis of carbon nanomaterials(2016-05-10) Matshitse, Refilwe Manyama StephinaFly ash is a by-product generated during the combustion of coal for electricity gen- eration. Previous studies have shown that various waste fly-ashes (Japanese, Saudi Arabian, and Australian) contain trace quantities of transition metal elements which can be used in the synthesis of shaped carbon nanomaterials. A survey of the litera- ture has shown that no attempts to correlate the composition of a particular coal fly ash and the type or quantity of carbon nanomaterials (CNMs) that can be synthesized has been made. Neither has the effect of leached fly ash been tested for the synthesis of CNMs. Hence a study on the effect of the chemical composition of South African fly ash (collected from ESKOM’s Duvha power station in Mpumalanga) upon the chemical vapour deposition (CVD) synthesis of carbon nanostructures is justified. Untreated and chemically treated fly ash samples were used as catalysts in the CVD method to synthesize CNMs. In the latter case selective leaching experiments were conducted on the fly ash samples under acidic, basic and neutral conditions. Op- timal CNM synthetic conditions were achieved by initially flowing H2 gas to re- duce the metal oxides within the fly ash catalyst followed by the introduction of the carbon source (C2H2) at a temperature range of 600 - 800 ◦C. All samples were quantitatively and/or qualitatively characterized. Inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray fluorescence (XRF) techniques were used to quantify the metal ions which were removed from the fly ash samples. Fur- thermore, qualitative studies were conducted with (PXRD, and laser Raman spec- troscopy), morphological and surface area characterization techniques (SEM, TEM and BET) were used to investigate the synthesis of CNMs from the untreated and chemically treated fly ash samples. Results have shown that carbon nanofibers (CNFs) of different geometric morpholo- gies were synthesized at an optimal yield temperature of 700◦C. A combination of smooth, thin, wide, spiral platelet-like, stacked cup, and fishbone morphologies were reported when the untreated fly ash catalyst was used. Fly ash catalysts under acidic, basic and neutral treatments showed CNFs of varying sizes and specific morpholo- gies. Smooth graphitic platelet-like, stacked cup and platelet-like CNFs were re- ported when the fly ash catalyst was leached with neutral, basic and acidic solutions. Carbon nanofibre sizes with the IG ID ratios were reported as follows 115 nm (1.092), 52 nm (0.799), and 200 nm (0.960) under neutral, basic and acidic mediums respec- tively. Surface areas (41, 14 and 7) m2/g for the CNFs that were synthesised from the neutral, basic and acidic treated fly ash catalysts were related to the selective leaching of metals. The quality and quantity of CNFs obtained under acidic medium were associated with the leaching of iron (5.6%), cobalt (1.7%), calcium (20.4%), copper (12.5%), chromium (4.6%), magnesium (23.3%), manganese (15.2%) and nickel (2%) from the fly ash catalyst. Under a basic medium only chromium (0.2%), calcium (0.3%) and copper (7.4%) were removed. Significantly the best quality of CNFs was ob- tained when fly ash was treated under neutral conditions. Metal ions such as: cal- cium (3.7%), copper (3.8%), chromium (0.1%), and magnesium (1.3%) were mod- erately removed from the ash matrix. Therefore, composition and quantity of the fly ash catalyst had an effect on the synthesis of CNFs.Item The synthesis of nitrogen doped carbon spheres as supports for palladium catalysts in the hydrogenation of cinnamaldehyde(2016) Manikai, Sibongile Mary-AnneThe selective hydrogenation of cinnamaldehyde (CALD), an α,β-unsaturated aldehyde, at the carbonyl (C=O) and olefinic (C=C) groups is an important reaction since its products mainly cinnamyl alcohol (CA) and hydrocinnamaldehyde (HCALD) are important intermediates for the production of many chemicals in a wide range of industries (pharmaceuticals, flavouring, agrochemicals, perfume). In this study the synthesis of nitrogen doped carbon spheres (NCSs) as catalyst supports for the hydrogenation of CALD is reported. At the heart of the hydrogenation of CALD is the catalyst, since it provides the surface for the various reactions to take place. In this study, an in-depth study was conducted on the NCSs support, by varying pyrolysis time, pyrolysis temperatures and flow rates of gases to determine the physical and chemical properties. The effects of chemically modifying the surfaces of the NCS supports by functionalization with acid and doping with carbon were also investigated. NCSs which had undergone different pre-treatments procedures were then deposited with Pd nanoparticles using different metal deposition methods and the resultant catalysts tested for the hydrogenation of CALD.Item Synthesis of carbon nanofibers and their subsequent use as catalyst supports for Fischer-Tropsch synthesis(2014-07-07) Phaahlamohlaka, Tumelo NathanielIn this study the synthesis and use of carbon nanofibers (CNFs) as catalysts supports for Fischer Tropsch synthesis is reported. The synthesis of carbon nanofibers with two distinct morphologies was optimized based on the reports in the literature that the straight (SCNF) and helical (CCNF) carbon nanofibers grow on Cu catalysts with different particle sizes. To selectively grow CNFs with a single morphology Cu catalysts were designed using different synthesis procedures (by using unsupported, coated and silica supported catalysts). The prepared copper oxide (CuO) nanoparticles were characterized by techniques such as TEM, XRD and nitrous oxide chemisorption. These techniques showed that the unsupported and coated CuO catalyst precursors has large particle sizes (range 100-300 nm) and thus had low Cu atomic surface area, while the supported CuO catalysts displayed low particles sizes in the nanoscale regime (<20 nm) and hence had high atomic surface area. Preparation of CNFs was carried out 300 using acetylene (C2H2) gas as the carbon source. Cu catalysts with large particle sizes resulted in straight CNFs and the small supported Cu nanoparticles grew helical CNFs because of the change in the nanoparticle surface energy during adsorption of the acetylene gas and the silica (SiO2) support effects that limited Cu nanoparticles from sintering (i.e. final particles size 60 nm). Soxhlet extraction proved to be an invaluable step in removing adsorbed polycyclic aromatic hydrocarbons. Because of the low thermal stability of these CNFs the materials were then annealed at higher temperatures ranging from 500-1400 in an inert environment (passing N2 gas). The helical CNFs snapped under high temperature annealing ( 900 ) resulting in shorter lengths in comparison to the straight CNFs. BET analysis of the annealed CNFs indicated that the CNFs annealed at 500 and 900 have increased surface area and have a mesoporous pore structure with the surface area ranging from 200-350 m2/g. Raman and Fourier transform IR spectroscopy indicated that the CNFs annealed at 500 and 900 , (which were the main material of interest because of their high surface area and thermal stability) had different hybridized carbon content. CNFs annealed at 500 contained both sp2 and sp3 hybridized carbon while annealing the CNFs at 900 resulted in a complete rehybridization of the carbon content to sp2. The carbon sp3 content in the CNFs annealed at 500 therefore implied that CNFs annealed at this temperature are more defective in comparison to the CNFs annealed at 900 . Since it is well known that material functionalities are affected by the amount of defects present inside the different CNFs were then applied as catalyst supports for Fischer Tropsch synthesis (FTS) to compare the support effects on cobalt active sites. The CNF surfaces were first modified by functionalization using concentrated HNO3 solution. The preparation of the catalyst systems was performed by a simple HDP method using urea. The CNFs and the FT catalysts were characterized using different techniques such as XRD, TEM, BET, TPR and Raman spectroscopy. Reactor studies performed at 220 (P = 8 bar, GHSV= 1200 mL.h-1. ) showed the catalysts had activities with CO conversion ranging from 25-45%. It was observed that catalysts supported on CNFs annealed at 500 displayed higher average activities of about 15% (based on the CO conversions) in relation to the catalysts supported on CNFs annealed at 900 . Catalysts showed minimal water gas shift reaction and high methane selectivity (i.e. 20-30%) which can be attributed to the small Co crystallite sizes and low pressure reaction conditions.Item Nickel and copper catalysed synthesis of carbon fibers(2014-01-10) Maubane, Manoko StephinaStructured carbon nanomaterials have attracted considerable interest because of their unique structures and outstanding properties. Among other structured carbon nanomaterials, carbon nanofibers (CNFs) have been the subject of study for several decades with particular interest having been paid towards their synthesis and application. However, control over the size and shape of these materials still remains a challenge. Three main components necessary for the synthesis of CNFs are the catalyst or template, the carbon source and the source of energy/power. It has been noted that catalyst morphology and the carbon source plays an important role in controlling CNF growth and morphology. As such one of the main challenges is to produce the catalyst particles that would yield the desired CNF morphology. In this study, we investigated methods for controlling the size and morphology of CNFs by synthesizing Ni and Cu catalysts of particular morphology, while using C2H2 and trichloroethylene (TCE) as a carbon source for the synthesis of CNFs. A mixture of TCE/C2H2 was also employed as a carbon source for comparison. The catalysts and synthesized CNFs were characterized by different techniques such as TEM, XRD, TPR, TGA, Raman spectroscopy, IR spectroscopy, etc. The synthesis of Ni nanoparticles (NPs) was achieved by reduction of Ni(acetate)2 with hydrazine (35%). CNFs were synthesized by deposition of TCE, C2H2 and their mixtures using a chemical vapor deposition technique (CVD) in the temperature range 400-800 oC. N2 and CO2 were used as carrier gases. TEM analysis of the Ni particles as a function of time revealed that the Ni underwent a morphological change with time. Further, as the temperature of the reaction changed, so did the shape of the carbon materials. The shapes changed from structures showing bilateral growth at T = 400 oC to tripod-like structures and multipod-like structures at T = 450 oC and T = 500 oC respectively. Irregular shaped materials were observed at T > 500 oC. It was also found that when acetylene or an acetylene/trichloroethylene mixture was used at T = 450 oC, helical (> 80%) and linear fibers were produced respectively. It was also demonstrated that the flow rate of H2, N2 and CO2 had a dramatic influence on the morphology of CNFs. CO2/TCEwas found to produce linear fibers with controlled sizes at 800 oC. The results demonstrated that the formation of tripod CNFs only occurs in a very narrow parameter regime. Manoko S. Maubane The preheating of the TCE prior to its deposition over a Ni particle catalyst was achieved using a double stage CVD reactor. TCE was subjected to high temperatures prior to its deposition at low temperatures. Results showed that the decomposition temperature was the key parameter in the synthesis of CNFs. It was found that during the decomposition, TCE breaks down into different species/radicals which then adsorb onto the catalyst particle to give CNFs of different morphology. Raman studies revealed that the synthesized CNFs showed an increase in graphitic nature when the temperature in the first reactor of a two stage reactor was increased. Decomposition of C2H2 was also performed over Cu NPs, and Cu modified catalysts (Cu@SiO2 and Cu/SiO2) with different silica coatings at 300 oC. These catalysts were prepared by reduction of Cu(acac)2 with hydrazine (35%). TEM images revealed that coiled CNFs were only produced from Cu/SiO2 grown in the presence of H2 (> 90 %; d = 60-70 nm). IR spectra of all the CNFs indicated the presence of surface C=C, C=O, CH3 and CH2 moieties, and that the ratios of peak intensities of C=O/CHx and C=C/CHx species indicated the variable CNF surface that was produced by the gases and the Cu particles used. It was thus revealed that the CNFs produced by different Cu catalysts have different chemical and physical properties and that these properties correlate with catalyst particle size and the gas mixtures used. CuO and SrO modified Cu catalysts (with different Cu/Sr ratios) were also employed using the CVD method for the synthesis of CNFs at 300 oC. These catalysts were prepared by a coprecipitation method. The TEM images of the CNFs revealed a mixture of straight and coiled CNFs with a broad diameter distribution (50-400 nm) dependent on the Cu/Sr ratio of the catalyst used. IR and TGA analysis revealed that the chemical composition of the CNFs changed as the SrO content changed. The SrO content also affected the Cu particle size and influenced the morphology of the Cu particles from which the CNFs grew.Item The synthesis of the quinones dehydroherbarin, anhydrofusarubin and the acetal core of marticin(2013-08-06) Pillay, AdushanThe syntheses of the naturally occurring naphthoquinone fungal metabolites dehydroherbarin and anhydrofusarubin as well as the acetal core of marticin are described in this thesis. Starting from 2,4-dimethoxybenzaldehyde, dehydroherbarin was prepared in 11 steps in an overall yield of 4.5 %. The naphthalene segment of dehydroherbarin was constructed and functionalized utilizing reactions including a Stobbe condensation, O-allylation, and a Claisen rearrangement with the key step being a regioselective phenyliodine bis(trifluoroacetate) mediated methanol addition to the naphthalene. The pyran ring was assembled by a lithium aluminium reduction followed by Wackertype oxidation reaction. Two unnatural synthetic naphthoquinones, (3R,4R) 3-hydroxy-7,9-dimethoxy-3- methly-5,10-dioxo-3,4,5,10-tetrahydro-1H-benzo[g]isochromen-4-yl nitrate and 3,4- dihydroxy-7,9-dimethoxy-3-methyl-3,4-dihydro-1H-benzo[g]isochromene-5,10-dione, were also produced on route to accessing dehydroherbarin. Anhydrofusarubin was synthesized from 2,4,5-trimethoxybenzaldehyde in 12 steps in an overall yield of 5.3 % by employing the same synthetic methodology developed towards the assembly of dehydroherbarin. To our knowledge, this represents the first formal synthesis of anhydrofusarubin. The assembly of model system of the 6,6-bicyclic pyran ring arrangement found in the naturally occurring naphthoquinone marticin is also described. 11- (hydroxymethyl)-9-methyl-10,13-dioxatricyclo[7.3.1.02,7]trideca-2(7),4-diene-3,6- dione was produced racemically in 9 steps, starting from 2,5-dihydroxyacetophenone in a 3.3 % overall yield, with the key reaction again being a Wacker-type oxidation reaction The related 6,7-bicyclic acetal compound, 3,6-dimethoxy-9-methyl-10,13- dioxatricyclo[7.3.1.02,7]trideca-2,4,6-triene, was also made inadvertently in 7 steps, from 2,5-dihydroxyacetophenone in a 6.5 % overall yield.Item Synthesis, characterisation and activity of ruthenium/N-doped multi-walled carbon nanotubes catalysts(2013-07-29) Mabena, Letlhogonolo FortunateNitrogen doped carbon nanotubes (N-CNTs) were synthesised using thermal-Chemical Vapour Deposition (CVD). The obtained material was purified, characterised and used as a support for ruthenium nanoparticles. The catalytic performance of the Ru/N-CNTs was investigated in different chemical reactions. Thus, this thesis is divided into two sections. The synthesis of the nanomaterials, the catalyst performance of nanomaterials in the oxygen reaction reduction (ORR) and activity in the oxidation of styrene and benzyl alcohol. In the first section N-CNTs were synthesised using a thermal-CVD method in a horizontal split-tube furnace. The reactions were carried out in a tubular quartz reactor. Cyclohexanol was used as carbon source, aniline as a nitrogen source and ferrocene as catalyst. A mixture of cyclohexanol-aniline-ferrocene was placed in a quartz boat that was directly introduced in the centre of the first furnace and vaporised at 280 °C. The resultant vapours were transferred to the second furnace where the N-CNTs were grown at a temperature of 900°C under the carrier gas flow (nitrogen or 5% H2 balanced in argon gas). The N-CNTs formed had a fairly crystalline structure, constituted by a periodical bamboo like structure with tubes diameters of 35 - 100 nm and nitrogen content up to 1.3 at. %.The N-CNTs with 0.8 at.% were selected to be used becaused of the quality and the amount of CNTs produced. N-CNTs were then used to support ruthenium (Ru) nanoparticles using a microwave assisted reduction technique. The synthesised nanostructured materials were characterised by TEM, SEM, TGA, and XRD. The TEM images of the Ru catalysts supported on N-CNTs revealed homogenous dispersion of Ru nanoparticles with a narrow sizes distribution and small particle size with an average diameter of 2.5 nm when 500 W power was used. In the second section, part A; four catalysts with different Ru wt. % supported on N-CNTs were prepared: the amount of Ru deposited on the N-CNTs was varied between 0 –10 wt. %. The activity of the prepared nanocatalysts towards the oxygen reduction reaction (ORR) was characterised using the rotating disk electrode and voltammetry techniques. The ORR activity was higher at lower concentrations of Ru on N-CNTs. The 4e- pathway of ORR was more favourable on 2 and 5 % Ru loaded N-CNTs than as 10 % Ru loaded N-CNTs. In Part B; prepared Ru/CNT and Ru/N-CNT catalysts were calcined and used for the liquid-phase oxidation reaction of styrene and benzyl alcohol. The influence of various reaction parameters such as reaction time, catalyst mass, solvent nature and reaction temperature were evaluated. It is interesting to note that the RuO2 on carbon material catalyst was more active for styrene oxidation than for benzyl alcohol oxidation reaction. The conversion of styrene was 41 % and the selectivity to benzaldehyde was 85 % when 5 % RuO2/CNTs catalyst was used with 1,4-dioxane as a solvent at 80 °C in 4 h. The highest conversion of benzyl alcohol was 11 % also with 85 % benzaldehyde selectivity. The benzyl alcohol oxidation was performed at 110 °C for 5 h. Ru/N-CNTs were shown to exhibit better activity for a styrene oxidation reaction. Therefore further investigations on the activity of nitrogen doped carbon nanotubes (N-CNTs*) prepared by reaction of acetylene (C2H2) and acetonitrile (CH3CN) at 700 °C over a 10 % Fe-Co supported on calcium carbonate (CaCO3) catalyst was investigated for styrene oxidation. In this case the nitrogen doped carbon nanotubes (N-CNTs*) with 2.2 at. % nitrogen content was used. A 5 % Ru/N-CNT* catalyst was highly selective as compared to the previous N-CNT supports used in the styrene oxidation reaction. Comparing the support it was deduced that the nitrogen present in the support is playing a major role. With the increase in the nitrogen content in the matrix of the CNTs the conversion of styrene decreased but with an increase in the selectivity. The selectivity towards benzaldehyde was 96 % after 4 h when N-CNTs* were used as support for the styrene conversion reaction. In comparison for the RuO2 on CNTs and N-CNTs the styrene conversions were 85 and 87 % respectively.