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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:7349 |
| Date | 05 November 2012 |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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