Synthesis, characterization and photocalytic activity of TiO2 and TiO2 based quaternary mixed metal oxides supported on carbon and fly ash based materials

Hlekelele, Lerato
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The fast industrialization that has been the key to the increased quality of the lives of humans and other animals has not been without its disadvantages. The epitome to this is the continued generation of coal fly ash (CFA) which is the by-product of burning coal for the production of electricity. Millions of tons of this material are produced all over the world and it is both dangerous and expensive to dispose of. Furthermore, industrialization has also come with the production of various useful chemicals, some of which end up contaminating water systems. An example of these problematic chemicals is classed as endocrine disrupting chemicals (EDCs) because they affect the hormonal system of animals. Bisphenol-A (BPA) is an example of an EDC and it interferes with male reproductive organs and has been detected in the environment, including water systems. This, therefore, necessitates that methods of cleaning water be improved. Semiconductor photocatalysis, especially TiO2, has been shown to be a potentially effective method of removing organic contaminants from water because of its high mineralization efficiency and simplicity. However, efforts to use TiO2 for photocatalysis in the visible region has remained inefficient because of its large band gap and high recombination rate of the photoinduced electrons (e-) and holes (h+). It is for these reasons that the focus of this research was to synthesize highly active photocatalysts for the photodegradation of BPA. This was achieved by immobilizing TiO2 on nitrogen-doped carbon nanotubes (NCNTs), carbon nanofibers (CNFs) and zeolites. Furthermore, this research involved modifying TiO2 with Ag nanoparticles and also by incorporating W6+ and Zn2+ into the crystal lattice of TiO2 to form quaternary mixed metal oxides (QMMOs). NCNTs were synthesized using a 2-stage chemical vapor deposition technique by decomposing melamine on the surface of CFA in a nitrogen atmosphere at various temperatures (800, 850 and 900 °C). The morphology, nitrogen content, crystallinity and thermal stability of the NCNTs were found to vary with synthesis temperature. The NCNTs synthesized at 850 °C and the CNFs were purified and functionalized by stirring them in 5 % HF for 24 h followed by heating to 60 °C in HNO3 and H2SO4 (1:3) for 12 h. The confirmation of the removal of CFA from the NCNTs as well as CNFs and their functionalization was confirmed by various analytical techniques. TiO2 nanoparticles were loaded onto these materials at different loadings (1, 5 and 20 % m/m of shaped carbon nanomaterials (SCNMS)) by a surfactant wrapping sol-gel/hydrothermal method. The formation of these composites was confirmed by various analytical techniques. It was observed from photoluminescence (PL) measurements that the emission intensity of these composites was lower than that of TiO2 indicating reduced recombination of e- and h+. The efficiency at which TiO2 and these composites photodegraded BPA was measured, where it was found that the composites showed higher efficiencies relative to that of TiO2 except for those that consisted of 20 % CNFs/NCNTs (m/m). In a separate study, zeolites (CFA_Zeo) were synthesized by fusing CFA with NaOH followed by hydrothermally treating the mixture. Zeolitization of CFA yielded rod-like materials which were found to be zeolite-X. Thereafter TiO2 was loaded onto various masses of CFA_Zeo (15 and 30 % m/m of CFA_Zeo) by the resin gel technique and then these composites were decorated with 1 % Ag nanoparticles by the NaOH-urea deposition precipitation technique. Here it was observed by TEM that the CFA_Zeo rods were completely coated with TiO2 nanoparticles and that the Ag nanoparticles were on top of these. The photocatalytic efficiencies of TiO2 and these composites were tested under UV and visible light. The composites were shown to have superior photocatalytic activities to that of TiO2 by itself under both UV and visible light (λ > 400 nm). The incorporation of W6+ and Zn2+ at various concentrations (up to 30 % m/m) into the crystal lattice of TiO2 (anatase phase) was achieved by the resin gel technique to form QMMOs. This was confirmed by PXRD where it was shown that the diffraction patterns of the QMMOs were similar to that of TiO2 except for the observation that the (101) reflection of the QMMOs was slightly shifted relative to that of TiO2. The optical absorption signature of the QMMOs and TiO2 were studied using UV-diffuse reflectance spectroscopy (DRS). It was found that they were similar except for the observation that QMMOs had an absorption peak in the visible range whereas the absorption spectrum of TiO2 was flat in this region. TiO2 and the QMMOs were composited with 10 % (m/m) NCNTs using a hydrothermally modified resin gel technique. The formation of the composites was confirmed by TGA, TEM, and PL. The photocatalytic efficiencies of TiO2, QMMOs and TiO2/QMMOs loaded on NCNTs were evaluated under UV-light (λ > 400 nm). The QMMOs loaded on NCNTs showed the highest efficiencies whereas TiO2 showed the least activity
Dissertation in fulfillment of the requirement for the degree Philosophiae Doctor in school of chemistry in the Faculty of Science of the University of the Witwatersrand