Synthesis of New Composites of Inorganic Polymers (Geopolymers) with Metal Oxide Nanoparticles and their Photodegradation of Organic Pollutants

<p>This thesis describes the development and performance of novel photocatalytic inorganic polymer (geopolymer) composites for photodegradation of environmentally harmful organic materials. Nanometer-sized cubic cuprous oxide nanoparticles and spherical Cu₂O/TiO₂ nano-heterostructures were synthesized via a precipitation method and then added to a metakaolinite-based geopolymer matrix prior to curing at ambient temperature. The morphology of the homogeneous oxide nanoparticle dispersion within the geopolymer matrix was demonstrated by SEM/EDS and HRTEM. FTIR spectroscopy confirmed the formation of a well-reacted geopolymer matrix that was unaffected by the insertion of the Cu₂O and Cu₂O/TiO₂ nanoparticles. The structures of these new composites were determined by ²⁷Al and ²⁹Si MAS NMR spectroscopy. ⁶³Cu NQR spectroscopy and XRD confirmed that the metal oxide nanoparticles are unchanged by their incorporation in the geopolymer composite and after the photodegradation reactions. The nitrogen adsorption-desorption isotherms were determined, providing information about the specific surface areas and total pore volumes of the composites. The action of the composites in the adsorption and photocatalytic destruction of the model organic compound MB was determined under dark and UV illumination conditions. Experiments in dark conditions and under UV irradiation showed that these materials efficiently remove a model organic pollutant (MB dye) from solution by a dual process of adsorption on the geopolymer matrix, and photodecomposition of the dye without destroying the geopolymer structure. The adsorption kinetics of the dye are best described by a pseudo first-order model and the adsorption process by Langmuir-Freundlich isotherms. In a novel extension of this research, the metakaolinite-based geopolymer matrix was modified with a surfactant (cetyltrimethylammonium bromide, CTAB), exploiting the cation exchange capacity of the geopolymers structure. The nano oxide composites were synthesised by adding different amounts of as-prepared metal oxide nanoparticles to the modified geoplymer to produce a hydrophobic photocatalyst composite with improved photocatalytic activity arising from the dispersion of the metal oxide nanoparticles in the external surfaces and interlayers of the geopolymer matrix. This method has the advantage of producing geopolymer composites with a stable pH which are more suitable for dye degradation studies. At concentrations >20 wt%, the photo-oxide component decreases the adsorption rate by blocking the active adsorption sites of the geopolymer. Under UV radiation, the composites remove the MB by a combination of adsorption and photodegradation, without deterioration of the geopolymer structure or the photoactive metal oxide component. In addition these studies show that the metal oxide-geopolymer nano composites have significantly improved photocatalytic activity compared with the oxide nanoparticles alone, because of the unique properties of these inorganic polymers. These results demonstrate that composites of nanosized Cu₂O particles and photoreactive TiO₂ in an aluminosilicate inorganic polymer matrix constitute new and novel materials with potential environmental protection applications to efficiently remove organic pollutants from water or the atmosphere.</p>

Synthesis of New Composites of Inorganic Polymers (Geopolymers) with Metal Oxide Nanoparticles and their Photodegradation of Organic Pollutants

<p>This thesis describes the development and performance of novel photocatalytic inorganic polymer (geopolymer) composites for photodegradation of environmentally harmful organic materials. Nanometer-sized cubic cuprous oxide nanoparticles and spherical Cu₂O/TiO₂ nano-heterostructures were synthesized via a precipitation method and then added to a metakaolinite-based geopolymer matrix prior to curing at ambient temperature. The morphology of the homogeneous oxide nanoparticle dispersion within the geopolymer matrix was demonstrated by SEM/EDS and HRTEM. FTIR spectroscopy confirmed the formation of a well-reacted geopolymer matrix that was unaffected by the insertion of the Cu₂O and Cu₂O/TiO₂ nanoparticles. The structures of these new composites were determined by ²⁷Al and ²⁹Si MAS NMR spectroscopy. ⁶³Cu NQR spectroscopy and XRD confirmed that the metal oxide nanoparticles are unchanged by their incorporation in the geopolymer composite and after the photodegradation reactions. The nitrogen adsorption-desorption isotherms were determined, providing information about the specific surface areas and total pore volumes of the composites. The action of the composites in the adsorption and photocatalytic destruction of the model organic compound MB was determined under dark and UV illumination conditions. Experiments in dark conditions and under UV irradiation showed that these materials efficiently remove a model organic pollutant (MB dye) from solution by a dual process of adsorption on the geopolymer matrix, and photodecomposition of the dye without destroying the geopolymer structure. The adsorption kinetics of the dye are best described by a pseudo first-order model and the adsorption process by Langmuir-Freundlich isotherms. In a novel extension of this research, the metakaolinite-based geopolymer matrix was modified with a surfactant (cetyltrimethylammonium bromide, CTAB), exploiting the cation exchange capacity of the geopolymers structure. The nano oxide composites were synthesised by adding different amounts of as-prepared metal oxide nanoparticles to the modified geoplymer to produce a hydrophobic photocatalyst composite with improved photocatalytic activity arising from the dispersion of the metal oxide nanoparticles in the external surfaces and interlayers of the geopolymer matrix. This method has the advantage of producing geopolymer composites with a stable pH which are more suitable for dye degradation studies. At concentrations >20 wt%, the photo-oxide component decreases the adsorption rate by blocking the active adsorption sites of the geopolymer. Under UV radiation, the composites remove the MB by a combination of adsorption and photodegradation, without deterioration of the geopolymer structure or the photoactive metal oxide component. In addition these studies show that the metal oxide-geopolymer nano composites have significantly improved photocatalytic activity compared with the oxide nanoparticles alone, because of the unique properties of these inorganic polymers. These results demonstrate that composites of nanosized Cu₂O particles and photoreactive TiO₂ in an aluminosilicate inorganic polymer matrix constitute new and novel materials with potential environmental protection applications to efficiently remove organic pollutants from water or the atmosphere.</p>

Synthetic Approaches to the Incorporation of Gallium and Germanium into Inorganic Polymer Structures

<p>New sol-gel and solid-state synthesis methods and combinations of these were developed for the preparation of several new inorganic polymers related to aluminosilicate inorganic polymers, attempting to substitute gallium and germanium for aluminium and silicon. Gallium could successfully substitute for aluminium, but germanium could not be substituted for silicon by these methods. Gallium silicate and gallium aluminosilicate inorganic polymers were synthesised from mixtures of KGaO2, KAlO2, KOH solutions with finely divided SiO2 (silica fume) using a combination of sol-gel and solid-state techniques. The products of these reactions were studied by X-ray powder diffraction (XRD), solid-state 27Al, 29Si, 71Ga and 39K nuclear magnetic resonance with magic-angle spinning (MAS NMR) and scanning electron microscopy (SEM). For the synthesis of these mixed gallium-aluminium silicate inorganic polymers, the optimal SiO2:(Ga2O3+Al2O3) ratio was found to be 7 and the Ga:Al ratio could range from 100% Ga to 100% Al, with all intermediate ratios yielding inorganic polymers. The products showed all the characteristics of a true inorganic polymer, being X-ray amorphous and containing gallium and/or aluminium in tetrahedral coordination states. 29Si MAS NMR showed the occurrence of Si(3Ga) and Si(2Ga) sites when gallium was present, and Si(3Al) and Si(2Al) sites when aluminium was present. Unreacted silica was also detected in these compounds by 29Si NMR and spherical silica particles were observed by SEM. Heat treatment of gallium silicate, gallium aluminosilicate and aluminosilicate inorganic polymers synthesised by variations of the sol-gel method was monitored by thermal analysis methods (DSC-TGA) which revealed a water loss at 75 [degrees]C and 160 [degrees]C followed by a phase transition at 950 [degrees]C. At this temperature the inorganic polymers crystallised to KGaSi2O6 and KAlSi2O6. The thermal behaviour of these samples was found to be different at 1200 [degrees]C; the high-temperature products derived from the gallium silicate inorganic polymers remained as crystalline KGaSi2O6 and retained their shape, while gallium aluminosilicate and aluminosilicate inorganic polymers melted and slumped, losing their shape and becoming X-ray amorphous. Attempts to substitute germanium for silicon in the inorganic polymer structure were unsuccessful. A sol-gel approach using GeO2 produced crystalline K6Ga6(GeO4)6(H2O)7. In an alternative solid-state approach, potassium germanate was synthesised and subsequently reacted with KGaO2 in a solidstate reaction to form partially amorphous hydraulic precursors; however, these did not set on the addition of water. A solid-state reaction of potassium germanate with KGa5O8 formed a partially amorphous precursor powder that set with the addition of water. However, the cured product was not amorphous, but proved to be crystalline K6Ga6(GeO4)6(H2O)7. In another approach, a sol-gel reaction of NaAlO2 solution and GeO2 with KOH solution set to an X-ray amorphous but brittle product. 27Al MAS NMR showed this to contain aluminium in both tetrahedral and octahedral coordination states. When KAlO2 was used instead of NaAlO2, the products were crystalline. The study of the structure of these germanium compounds is hindered by the inaccessibility of the germanium nuclide to MAS NMR. Nevertheless, the ability to synthesise a new category of materials by these new methods opens up the possibility of their potential applications as fluorescent materials and as components of optoelectronic devices.</p>

Synthetic Approaches to the Incorporation of Gallium and Germanium into Inorganic Polymer Structures

<p>New sol-gel and solid-state synthesis methods and combinations of these were developed for the preparation of several new inorganic polymers related to aluminosilicate inorganic polymers, attempting to substitute gallium and germanium for aluminium and silicon. Gallium could successfully substitute for aluminium, but germanium could not be substituted for silicon by these methods. Gallium silicate and gallium aluminosilicate inorganic polymers were synthesised from mixtures of KGaO2, KAlO2, KOH solutions with finely divided SiO2 (silica fume) using a combination of sol-gel and solid-state techniques. The products of these reactions were studied by X-ray powder diffraction (XRD), solid-state 27Al, 29Si, 71Ga and 39K nuclear magnetic resonance with magic-angle spinning (MAS NMR) and scanning electron microscopy (SEM). For the synthesis of these mixed gallium-aluminium silicate inorganic polymers, the optimal SiO2:(Ga2O3+Al2O3) ratio was found to be 7 and the Ga:Al ratio could range from 100% Ga to 100% Al, with all intermediate ratios yielding inorganic polymers. The products showed all the characteristics of a true inorganic polymer, being X-ray amorphous and containing gallium and/or aluminium in tetrahedral coordination states. 29Si MAS NMR showed the occurrence of Si(3Ga) and Si(2Ga) sites when gallium was present, and Si(3Al) and Si(2Al) sites when aluminium was present. Unreacted silica was also detected in these compounds by 29Si NMR and spherical silica particles were observed by SEM. Heat treatment of gallium silicate, gallium aluminosilicate and aluminosilicate inorganic polymers synthesised by variations of the sol-gel method was monitored by thermal analysis methods (DSC-TGA) which revealed a water loss at 75 [degrees]C and 160 [degrees]C followed by a phase transition at 950 [degrees]C. At this temperature the inorganic polymers crystallised to KGaSi2O6 and KAlSi2O6. The thermal behaviour of these samples was found to be different at 1200 [degrees]C; the high-temperature products derived from the gallium silicate inorganic polymers remained as crystalline KGaSi2O6 and retained their shape, while gallium aluminosilicate and aluminosilicate inorganic polymers melted and slumped, losing their shape and becoming X-ray amorphous. Attempts to substitute germanium for silicon in the inorganic polymer structure were unsuccessful. A sol-gel approach using GeO2 produced crystalline K6Ga6(GeO4)6(H2O)7. In an alternative solid-state approach, potassium germanate was synthesised and subsequently reacted with KGaO2 in a solidstate reaction to form partially amorphous hydraulic precursors; however, these did not set on the addition of water. A solid-state reaction of potassium germanate with KGa5O8 formed a partially amorphous precursor powder that set with the addition of water. However, the cured product was not amorphous, but proved to be crystalline K6Ga6(GeO4)6(H2O)7. In another approach, a sol-gel reaction of NaAlO2 solution and GeO2 with KOH solution set to an X-ray amorphous but brittle product. 27Al MAS NMR showed this to contain aluminium in both tetrahedral and octahedral coordination states. When KAlO2 was used instead of NaAlO2, the products were crystalline. The study of the structure of these germanium compounds is hindered by the inaccessibility of the germanium nuclide to MAS NMR. Nevertheless, the ability to synthesise a new category of materials by these new methods opens up the possibility of their potential applications as fluorescent materials and as components of optoelectronic devices.</p>