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M.Sc. (Chemistry) / In this dissertation the synthesis, characterization and the application of Ag, Au and Cu dendrimer encapsulated nanoparticles (DENs) in catalysis are described. Ag, Au and Cu-DENs were synthesized using G4-G6 PAMAM-OH and G4-G6 PAMAM-NH2 dendrimers as templates as well as stabilizers. NaBH4 was used as a reducing agent for the synthesis of DENs. Binding studies were carried out in order to determine the maximum capacity of the dendrimer to which the metal ions can be added. These binding studies were performed using UV-vis spectroscopy. The synthesis of these nanoparticles (NPs) was carried out at room temperature. For the synthesis of Ag and Au-DENs with PAMAM-NH2 dendrimers, the pH of the aqueous dendrimer solution was first adjusted to acidic condition (~pH 2) using HCl before the addition of the respective metal ion precursor to the dendrimer. This is done to avoid coordination of the metal ions to the primary amine groups on the periphery of the dendrimer, which might lead to particle agglomeration. These prepared DENs were characterized by UV-vis spectroscopy and high resolution transmission (HRTEM) microscopy. The synthesized DENs were evaluated as catalysts in the reduction of 4-nitrophenol to 4-aminophenol by NaBH4. This reaction was monitored by UV-vis spectroscopy by following the absorbance at 400 nm These DENs were all found to be active catalysts for the afore-mentioned process. The rate constant for the reduction process was observed to decrease as the concentration of 4-nitrophenol increased. As the concentration of NaBH4 is increased, the rate constant was also found to increase, however this increase was only observed to a maximum concentration of NaBH4. The Au-DENs prepared using G4 PAMAM-NH2 dendrimers were subsequently immobilized onto a titania support via the sol-gel (Ti-Au-s) and wetness impregnation (Ti-Au-w) methods. The titania supported Au NPs were characterized using HRTEM, powder X-ray diffraction (PXRD), thermal gravimetric analysis (TGA), inductive coupled plasma-optical emission spectroscopy (ICP-OES) and Brunauer Emmett Teller (BET) surface area analysis. The dendrimer template was removed by calcining at 500 oC. The catalytic activity of these supported Au NPs was investigated in the oxidation of styrene using tert-butyl hydroperoxide (TBHP) as an oxidant. Benzaldehyde and styrene oxide were observed as the major products. The catalyst prepared by wetness impregnation method was found to give the highest styrene conversion as compared to the one prepared via sol-gel method. At 60 oC, the catalyst prepared by sol-gel method was found to selectively produce benzaldehyde while on the other hand, the catalyst prepared by wetness impregnation selectively produce styrene oxide. The highest conversion of styrene was observed at 70 oC for both catalysts. Ti-Au-w catalyst was generally found to give the highest styrene conversion.

M.Sc. (Chemistry) / Please refer to full text to view abstract

A stable high temperature gold nano-catalyst: synthesis, characterization and application The ability of supported gold nanoparticles to catalyse many reactions even at very low temperatures has spurred a great deal of research into the eld. Reactions such as CO oxidation and NOx reduction have many industrial applications as well as uses in the motor industry for catalytic converters. The interest is both for scienti c as well as economic reasons as gold supplies far exceed all PGM supplies. Scienti cally gold catalysts are able to catalyze reactions from below 0°C, a feat that no PGM catalyst can achieve. The low temperature activity of gold catalysts will reduce the emission of pollutants during start up. Since the discovery and development of gold catalysts one of the most researched topics has been nding ways to stabilise the gold nanoparticles on the support surface. The importance of gold nanoparticle stability is crucial as the catalysts are only highly active if the gold nanoparticles are less than 5 nm in size. A number of companies have worked to develop gold catalysts that are stable for long durations at temperatures over 450°C with no signi cant progress made over the last two decades other than a catalyst produced by Toyota. In this thesis, literature reviews of current support materials as well as synthesis methods are investigated in order to determine reasons for the instability of current gold catalysts. Further, the Mintek Aurolite catalyst is tested and its deactivation mechanisms probed using in-situ VT-PXRD, Rietveld re nement, TEM, HR-TEM, as well as CO oxidation tests. Testing revealed aws in the support structure of the catalyst which resulted in dramatic deactivation. As titania is such a common support material for many reactions in industry as well as being known to be one of the best supports for gold it was chosen as a support material. However, as is revealed, in its current forms and morphologies it is unable to provide the thermodynamically stable and high surface areas that are required for a stable catalyst After the development of a robust and reproducible synthesis method for the deposition of gold and other PGM's a number of supports were tested. These include silica and zirconia as well as titania derivatives such as Degussa P25 and commercial anatase. Initially these supports o er high usable surface areas but after a relatively small amount of time complete deactivation occurs. Reasons for this deactivation are determined and the information gained is used to develop supports that can combat these deactivation processes. Phase pure nano anatase is synthesised which produced a support with an incredibly large surface area compared to the aforementioned supports. The catalyst was able to withstand temperatures over 450°C for longer durations compared to other catalysts exposed to the same conditions. However, the phase conversion of the anatase to its thermodynamically stable form rutile once again deactivated the catalyst with time. Finally a rutile nanosupport is developed with the desired morphology and thermodynamic stability needed for high temperature applications. The catalyst is able to withstand temperatures over 550°C for more than 200 hours as well as still being active after exposure to 810°C. The industrial Aurolite catalyst showed complete deactivation after just 12 hours at 500°C. The catalyst produced in this thesis has been shown to be one of the most stable and thermally resistant gold catalysts in the world.

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Masters of Science in Engineering. / The partial oxidation of methane by air to synthesis gas over supported cobalt catalysts was studied. The investigation included analysis of the products of this reaction at various temperatures, and of the structure of the catalysts using powder X-ray diffraction techniques. (Abbreviation abstract) / Andrew Chakane 2018

A thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfillment of the requirements for the degree, Doctor of philosophy Degree (PhD) in chemistry Johannesburg, 2013. / Nitrogen 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.

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Masters of Science in Engineering. / The partial oxidation of methane by air to synthesis gas over supported cobalt catalysts was studied. The investigation included analysis of the products of this reaction at various temperatures, and of the structure of the catalysts using powder X-ray diffraction techniques. The most effective catalyst for this reaction was found to be metallic cobalt supported on rhombohedral alumina (prepared as lO%Co/C/'r-A103)' In the presence of this catalyst 96% of tile feed was completely converted to synthesis gas (CO: 2H2) at lOOO°C. This catalyst showed no evidence of coking or loss of activity at lOfO°C over a period of 180 hours. The reaction mechanism is thought to occur in two stages over two distinct zones of the catalyst, Complete reaction of O2 with CH4 to form CO2 and H20 is followed, in the second stage, by reforming and the water gas shift reaction to produce synthesis gas. / Andrew Chakane 2018

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment for the degree of Master of Science in Chemistry. Johannesburg 2016. / In the current study, different platinum-hollow carbon sphere catalysts were synthesized for use as electrocatalysts in low temperature fuel cells such as proton exchange membrane fuel cells (PEMFCs). The support material was synthesized via a hard templating method using mesoporous silica (synthesized using a modified Stöber method) as a sacrificial template. In fuel cells, one aim is to ensure that as much platinum as possible is present on a given electrode while keeping the entirety of the catalytic layer as thin as possible (i.e. with the minimum amount of carbon). One approach to achieving this was to make the hollow carbon spheres as small as possible, starting of course with the templating material. It was found that tailoring the molar ratios between the two co-solvents (that is water and ethanol) during Stöber synthesis was the key to achieving particles as small as approximately 150 nm with a uniform shape, size, and significant yields of up to 5.00 g. Another focal point in terms of the template material was achieving a silica structure that consisted of a solid core, and a distinctly mesoporous shell. Two different surfactants were explored in order to fabricate these structures; namely octadecyltrimethoxysilane (C18TMS) and cetyltrimethylammonium bromide (CTAB). It was found that of the two, the C18TMS resulted in more distinctly formed mesoporous silica layers with higher measured specific surface areas. Because the type of support material greatly influences the catalytic behaviour of the loaded catalysts, two different carbonization techniques were explored; namely the bubbling method using toluene as a carbon source, and a nanocasting method where resorcinol formaldehyde (RF) was the carbon source. The toluene-synthesized hollow carbon spheres had advantages over their RF-synthesized counterparts in that they were more thermally stable and had a more graphitic crystalline carbon framework. The RF-synthesized carbon, however, possessed a pseudo-capacitance due to surface carbon-oxygen groups, as well as a higher specific surface area, which resulted in the RF-carbon cyclic voltammetry profile spanning a larger current range in microampere per square centimetre. The effect of the size of the support materials was also explored; comparing 350 nm and 150 nm hollow carbon spheres. Besides the type of carbon, the metal precursor used to synthesize the catalyst nanoparticles was also explored, with either platinum(II)chloride (PtCl2) or platinum(II)acetylacetonate [Pt(acac)2] being used as the platinum source. It is also known that achieving high metal yields using conventional methods is quite difficult, and so an easier, quicker and less wasteful method was also explored; comparing the traditional wet-impregnation (WI) method with a chemical vapour deposition (CVD) method. Ultimately, it was found that platinum loaded on top of small-sized toluene-synthesized hollow carbon spheres using the CVD method and Pt(acac)2 as the metal precursor was the better catalyst in terms of oxygen reduction (determined using linear sweep voltammetry measurements); outperforming even commercial Pt/C catalysts as a result of improved mass transfer afforded by the voided cores of the hollow carbon spheres. The ability of a catalyst to withstand the reaction conditions present in a PEM fuel cell (i.e. oxygen-rich environments) was also considered. The stability of the catalysts was tested using chronoamperometry measurements in an oxygen-saturated perchloric acid solution. It was evident that the platinum loaded on the inner shells of the hollow carbon spheres showed far superior stability to those loaded on the outside surface. This was attributed to the qualities bestowed by the carbon shell around the platinum nanoparticles, protecting said platinum against the consequences of support corrosion due to the oxygenated environment; consequences such as metal sintering and interaction with surrounding carbon supports for example. These encapsulated catalysts, however, displayed a decrease in electrocatalytic activity compared to the catalysts with top-loaded platinum. In conclusion, the study of different platinum-carbon catalysts studied in the current work revealed that (a) loading platinum on top of small sized toluene-synthesized hollow carbon spheres using a CVD method and Pt(acac)2 as a metal precursor resulted in a highly active oxygen reduction catalyst, while (b) loading platinum on the inside surface of the hollow carbon spheres under the dame conditions resulted in a more electrocatalytically stable catalyst. / LG2017