Abstract
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.