School of Chemistry - Theses
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ItemUltrasonic synthesis of advanced photocatalytic materials for use in continuous flow-through reactors( 2017)Water purification is a problem around the world. Every year, authorities introduce new standards on the amount of pollutants that could be released into the environment, and new compounds are continuously being added to the list of toxic waste. Some of these substances can be defined as persistent organic pollutants (POPs), and their degradation is generally difficult to achieve. In these cases, advanced oxidation processes are required for the decomposition of these hazardous molecules. In this thesis, the advanced oxidation process used is photocatalysis, which can be used to decompose almost any kind of organic pollutant. To enhance the photocatalytic activity of photocatalysts, researchers have focused their attention on reducing the size of the particles, reaching just a few nanometers. Unfortunately, no sufficient efforts were made in continuous flow studies, which are necessary if future industrial implementations are desired. In these systems, the photocatalyst can be used in two ways: dispersed or coated onto a surface. The former offers high activity, but its recovery at the end of the reaction can be difficult. Instead, the latter permits almost no effort in the catalyst retrieval, but its efficiency is significantly low. For these reasons, this thesis aims to investigate the conversion of nanosized catalysts into micron sized powders without a loss in activity. In this manner, the catalyst could be used as a dispersion, enhancing the degradation and reducing the costs involved in the filtration procedures. In order to accomplish this aim, microspheres were used as a template material. The studies on continuous flow systems and their comparison to batch systems, carried out in this thesis, could be useful for future industrial implementations. For the generation of microspheres, ultrasonic emulsification technique was utilized, and the fundamental principles of ultrasound, along with those of microspheres, photocatalysis, and continuous flow reactors are discussed in Chapter 1. In Chapter 2, a structured literature review examining ultrasonic emulsification, microencapsulation, photocatalysts, and studies on continuous flow reactors, is discussed. In this thesis, chitosan (a natural amino-polysaccharide used in a wide range of applications) was chosen as a shell material for the generation of the microspheres, while nano sized TiO2 and ZnO were used as model nanosized photocatalysts. In Chapter 3, materials, analytical methods, and e experimental details used in this thesis are discussed. Three continuous flow reactors are presented, along with a new type of ultra-bright LEDs used as a light source for the photocatalytic degradation of rhodamine B, metanil yellow and methylene blue. Chapter 4 is the first chapter of result and discussion section. The role of counter ions on controlling the properties of ultrasonically generated chitosan microspheres, produced via oilin- water emulsion technique, was investigated. Various acids were used to dissolve chitosan, and it was found that the conjugate bases of the acid used (which acted as counter ions to neutralize the positive charges of ammonium ions present in the chitosan backbone) played a significant role in controlling the size, size distribution, and stability of the chitosan microspheres. In Chapter 5, the development of micron sized photocatalysts was studied. Chitosan microspheres were used for the conversion of nano sized TiO2 and ZnO (25-50 nm) into micron sized particles, possessing a size of about 10 μm. The micron sized photocatalysts possessed a photocatalytic efficiency similar to that of the nano sized powders, which was investigated in both aqueous and gas phases. In addition, the mechanism on the formation of the micron sized structures was proposed. In Chapter 6, the comparison of the photocatalytic activity of batch and continuous flow systems was investigated, using the micron sized catalyst (TiO2) previously studied. It was found that the continuous flow system is able to increase the amount of decomposable dye of up to 110% compared to that reached by the batch system. In addition, the catalyst used was found to be suitable for such continuous flow studies, with no loss in activity over a period of 42 hours. In Chapter 7, the use of ultra-bright LEDs on continuous flow systems, and the ability to apply the theory of such systems on photocatalytic reactions, were studied. It was found that the consideration of the kinetics of the photocatalytic reaction being pseudo-first order is not entirely correct, and that the new type of light source is suitable for photocatalytic degradations. In Chapter 8, some concluding remarks have been provided.
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ItemEnergy efficiency and advantages of ultrasonic synthesis of nanomaterials( 2015)The physico-chemical effects of ultrasound (US) have been used widely for synthesising various materials. The focus of this project is to evaluate the energy efficiency and advantages of ultrasonic synthetic process. Poly(methyl methacrylate) and poly(methyl methacrylate)-CaCO3 nanocomposites were synthesised by conventional and US-assisted (USK) emulsion polymerization. Although the conversions obtained were similar for both processes, nanocomposites produced by USK were smaller with a narrower particle size distribution. In another study, the photocatalytic activity of CdS nanoparticles synthesized using US were compared with those synthesized using mechanical agitation on the basis of energy input. Samples synthesized using a US horn (USH) and a high shear homogeniser (HSH) showed a lower photocatalytic activity compared to those synthesized in an US bath (USB) and using mechanical stirring (NUS). However, when the power input per unit volume (W/L) is considered, the order of effectiveness of the catalysts is USB>NUS>HSH>USH, suggesting that the mild cavitation conditions generated in the USB process are sufficient to produce an efficient photocatalyst. Overall, US assistance provides improvement in conversions/yields and the dispersive effects help obtain smaller particle sizes and narrower size distributions. However, when the increased energy requirements are taken into account it is obvious that when combining US with conventional material synthesis techniques, it is imperative to choose not only the right amount of energy input but also, the right mode of US input in order to synthesize the most efficacious nanomaterials.