Ultrasound initiated radical formation and electron transfer reactions in aqueous Solutions

© 2013 Dr. Leena Chandhi DharmarathneThis thesis investigates the ultrasound induced radical formation and electron transfer reactions in the presence of various electron acceptors in order to develop a deeper understanding of sonochemical reactions in general. The reduction reactions of organic free radicals which are generated from the sonochemical decomposition of aliphatic alcohols in argon saturated aqueous solutions, with electron acceptors, namely, ferricyanide, methyl viologen and zwitter viologen were studied at an ultrasound frequency of 355 kHz and the results are discussed in Chapter 3. The number of organic radicals generated was found to be related to the amount of adsorbed alcohol molecules at bubble/solution interface for any bulk concentration of alcohol. Most of the radicals generated stem from the thermal decomposition of alcohol molecules that have evaporated into the cavitation bubble core. The maximum rate for the reduction of ferricyanide was significantly higher than that for the reduction of methyl viologen. The differences observed were attributed to the reaction of methyl viologen cation with pyrolytically generated free radicals. It is noteworthy to mention that most of the reactions occur at the bubble/solution interface. The formation of hydrated electrons, during the sonication of argon saturated aqueous alkali solutions was examined and presented in Chapter 4. The detection of hydrated electrons was achieved by quantifying its reaction with the same one-electron acceptors, ferricyanide and methyl viologen. The results revealed that hydrated electrons were generated predominately at the bubble/solution interface and the local concentration was estimated to be > (1.5 - 5.5) x 10-3 M. Under the experimental conditions used, the half-life of the hydrated electron was estimated to be less than about 60 ns. Sonoluminescence (SL) and sonochemiluminescence (SCL), generated by acoustic cavitation, were used to generate molecular hydrogen in a photo catalytic process and the results are presented in Chapter 5. The same photocatalytic process has been widely studied using solar energy. Ru(bipy)32+ as a photosensitizer, methyl viologen as an electron relay agent, EDTA as a sacrificial electron donor and colloidal platinum as a catalyst were used in the present study. Apart from radicals produced via SL and SCL excitation of the sensitizer, sonochemically generated radicals and accompanied reactions were also involved in the photocatalytic process. So the other radical reaction pathways limited the efficacy of the photocatalytic hydrogen production to some extent. Despite the sonochemical side reactions, the in situ photon generation was sufficiently intense to generate appreciable amounts of molecular hydrogen. Finally, the reduction of adsorbed solutes on the surface of metal nanoparticles and accompanying electron transfer reactions were examined and the results are discussed in Chapter 6. Specifically, the reduction of adsorbed cadmium ions on the surface of silver nanoparticles in the presence of iso-propyl alcohol was explored. Evidence for the reduction of cadmium ions was sought by monitoring changes in the shape of the surface plasmon absorption band of the colloidal silver. However, spectral evidence for the reduction of cadmium ions was not found in the current study under the experimental conditions used