School of Chemistry - Theses
Permanent URI for this collection
Search Results
1 - 3 of 3
-
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.
-
ItemMesoporous Ti-based nanomaterials for photocatalysis and energy storage( 2017)Inspired by the discovery of the photocatalytic phenomenon in splitting water, enormous efforts have been devoted to the research of TiO2 materials. This has led to various applications ranging from photovoltaics and photocatalysis to batteries and sensors, which can be roughly divided into ‘energy’ and ‘environmental’ categories. In general, the effectiveness in the practical applications depend not only on the intrinsic properties of the TiO2 material, but also on modification to the material, including composition, morphology, and the compositional modification. As a photocatalyst, TiO2 is a wide band gap semiconductor (3.0-3.2 eV) that can be used to decompose organic compounds under ultraviolet light irradiation. An efficient strategy to extend the light response to the visible range and thus improve photocatalytic activity is by designing a heterojunction semiconductor. In this thesis pristine anatase TiO2 microspheres were used to prepare mesoporous TiO2/g-C3N4 microspheres via a nano-coating procedure followed by calcination, where the porous TiO2 acts as the active supporting scaffold and g-C3N4 as the visible light sensitizer. The composite microspheres were 8.5 folds more active in degrading phenol under visible light irradiation than mesoporous g-C3N4. Furthermore, starting with mesoporous TiO2 hollow microspheres, mesoporous brookite/anatase TiO2/g-C3N4 hollow microspheres were prepared via a facile nanocoating procedure that showed mixed phases of brookite (48 %), anatase (44 %), and rutile (8 %), incorporated with a g-C3N4 coating layer. The mesoporous hollow microspheres exhibited a unique hollow shell morphology of packed TiO2/g-C3N4 nanosheets, and a remarkable 5-fold increase in degrading phenol under visible light irradiation compared to mesoporous g-C3N4. Besides visible light photocatalysis, TiO2 can be used as an anode material for lithium-ion batteries, as it shows good gravimetric performance (336 mAh g-1) and excellent cyclability. To overcome the poor rate behaviour, slow lithium-ion diffusion, and high irreversible capacity decay, TiO2 nanomaterials with tuned compositions and morphologies are being investigated. Here, a promising TiO2 material has been prepared that comprises a mesoporous ‘yolk-shell’ spherical morphology in which the core is anatase TiO2 and the shell is TiO2(B). The electrochemical results indicate high specific reversible capacity at moderate current (330.0 mAh g-1) and cyclability (98 % capacity retention after 500 cycles). Lithium-sulphur batteries have attracted considerable attention as they have high specific capacity (1675 mAh g-1) and the abundance of sulphur, makes them one of the more promising next-generation battery technologies. However, commercialization of LSBs has generally been hampered by low sulphur utilization and poor long-term cyclability. These issues can be addressed, in part, by producing cathodic additives to encapsulate sulphur and polysulphides during the charge/discharge process. Mesoporous Magnéli Ti4O7 microspheres were prepared via an in-situ carbothermal reduction that exhibited large pore volume (0.39 cm3 g-1) and high surface area (197.2 m2 g-1). Strong chemical bonding of the polysulphides to Ti4O7, along with effective physical trapping in the mesopores and voids of the matrix, give superior reversible capacity (1317.6 mAh g-1) and cyclability (88 % capacity retention after 400 cycles). Ti-based materials with carefully tuned compositions, porosity, and morphologies have been constructed and tested in photocatalytic and energy storage applications revealing promising potential.
-
ItemHierarchically porous titania nanostructures with high crystallinity: synthesis and photocatalytic application( 2017)Water pollution is one of the most pressing issues affecting society, consequently using titanium dioxide (TiO2) as a photocatalyst for the treatment of polluted water has attracted immense attention over past decades. However, low photocatalytic performance as a result of the fast recombination of photogenerated electron-hole pairs, few active sites and poor light utilization has restrained its real application. This thesis reports the synthesis of various novel TiO2 photocatalysts with high crystallinity and tailored nanostructures obtained by sol-gel chemistry, templating, self-assembly, solvothermal treatment and calcination. Mixed-phased hierarchically porous TiO2 networks (PTN) were prepared through sol-gel chemistry and a templating technique, followed by calcination. The PTN materials possessed reduced contact areas between TiO2 nanocrystals, significantly retarding the anatase to rutile transformation and rutile crystal growth. Compared to control samples prepared without the template, hierarchical PTN materials showed enhanced photocatalytic activity towards the degradation of methylene blue (MB) under UV light illumination. The material calcined at 600 °C for 6 h contained 15.4 % rutile and had a specific surface area of 32.2 m2 g-1, giving the highest photocatalytic activity. This enhancement was attributed to optimal rutile content and increased active sites resulting from the high surface area. Micrometer-size, monodisperse amorphous TiO2 spheres with controllable sizes were fabricated through a sol-gel process. The monodispersity, spherical shape and size were tuned by varying experimental parameters including the amount of structure-directing hexadecylamine, salt species and concentration, water amount and reaction temperature. The diameter of the spheres was determined by a competitive process between the solubility of Ti oligomers and the hydrolysis rate of titanium isopropoxide, the TiO2 precursor. Spheres with diameters up to 5.39 ± 0.68 um were achieved. The amorphous TiO2 spheres were readily converted by a solvothermal treatment and calcination process to anatase TiO2 spheres with three fascinating morphologies: ‘fluffy’ core/shell, yolk/shell and hollow nanostructures. Direct evidence was found that a surface seeding and subsequent inwards hollowing through an Ostwald ripening process lead to the formation of diverse nanostructures. The hollow microsphere calcined at 650 °C displayed a higher degradation MB rate than the benchmark, commercial Degussa (Evonik) P25. The superior photocatalytic activity of the anatase hollow structures resulted from the unique hollow structure, hierarchically porous shell and high crystallinity. The amorphous TiO2 spheres were also readily converted by a solvothermal process to pure anatase TiO2 with high thermal stability. The resultant microspheres were composed of well-crystallized anatase nanocrystals with a uniform size of 24 nm and a 77 nm pore after calcination at 900 °C. The superior thermal stability was primarily attributed to increased Ti-O-Ti bond strength and narrow crystal size distribution. Microspheres calcined at 800 or 900 °C displayed higher photocatalytic performance than P25 treated at the same temperatures. The excellent performance of the microspheres was attributed to the retention of anatase phase, presence of large pores, high crystallinity and high surface area. Overall, TiO2 photocatalyst nanostructures were fabricated by sol-gel chemistry, templating, self-assembly, solvothermal and calcination processes, and exhibited UV light photocatalytic activity that surpassed P25.