Titania-based photocatalysts have been extensively studied for the oxidative photodestruction of organic pollutants in wastewaters, releasing non-toxic substances such as CO2, HCl, and water. However, commercial exploitation of this process is limited by the fact that titania is only active under UV irradiation (wavelength below about 388 nm), which is only less than 5% of solar light energy. Sol-gel synthesised catalyst specimens were characterised to determine the correlation between preparation conditions on morphology (XRD, SEM), optical (bandgap energy level) and physicochemical properties (BET surface area, pore volume, acid site density, acid site strength and type) of the photocatalysts. These spesific properties would then be linked to their photoactivity using aqueous aliphatic and aromatic model pollutants. This study has demonstrated that sol-gel synthesised doped titania photocatalysts, especially Pt/TiO2, may be used to effectively degrade non-volatile acids (DL-malic acid, dichloroacetic acid, and p-hydroxybenzoic acid) under visible light and UV irradiation with significant photoactivity suitable for the solar light application of photocatalytic wastewater treatment. A significant drop in band-gap energy was found for all titania sol-gel catalysts doped with Pt, Co, and Ce with values between 1.41 to 1.78 eV. The BET areas of the photocatalysts were also higher (65-117 m2/g) than that of Degussa P25 (50 m2/g). The visible-light photomineralisation of the three pollutants with Pt-TiO2 specimen were further extended to evaluate the effects of major variables in a bubble-column photoreactor on the photodegradation activities. Those major variables were lamp intensity, oxygen concentration, initial pH, catalyst dosage, and inital pollutant concentration. All the three pollutants seemed to follow the Langmuir-Hinselwood model with dual adsorption sites which implicated a bimolecular surface rate-limiting step probably between the adsorbed organic substrate and a surface hydroxyl (or peroxy) radical. A study of the CeyCoxTi(1-x)O3+d perovskite was conducted to investigate the influence of metal composition and pH on the intrinsic optophysical attributes as well as p-hydroxybenzoic acid degradation under UV irradiation. The perovskite UV photoactivities were lower than that of pure TiO2 likely due to excessive loading (metal content) creating new oxide phases act as electron-hole recombination center, regardless better physicochemical attributes of some of the perovskite samples. The role of aging time and calcination temperature on the sol-gel synthesised TiO2 was also explored. Higher calcination temperature (from 250 to 700 0C) resulted in TiO2 photocatalysts with better crystallinity, which is important for OH group formation as active sites for photodegradation. Despite of some advantages from higher temperature preparation, some detrimental effects such as decreased acidity attributes, surface area, and pore volume were also observed. The significant red-shift of sol-gel synthesized TiO2 into visible light, especially for 250 0C specimen since 600 or 700 0C had extremely low activities, has promising implications that this specimen might be used for solar application to substitute Pt-doped TiO2 in order to produce a more cost effective photocatalyst. Aging period (1 to 14 days) did not have any discernible effect on the band-gap value and acid-site density. Even so, the highest acid site strength was obtained with an aging time of 10 days. From the overall perspective, aging time longer than 3 days did not bring noticeable benefits to both catalyst attributes and photoactivities.

Nanomaterials are currently attracting billions of dollars in research funding and are entering such diverse fields as the computing, communications, life science and energy sectors. The growing popularity of nanomaterials demands a comprehensive understanding of the means by which such materials can be produced including the effects of physical and chemical factors. One method of forming inorganic nanomaterials is the sol-gel process; a low temperature process combining the benefits of glass and plastics technology. Whilst the research community has ascertained that gravity is important and appears to affect the sol-gel process, no coherent picture of the role of gravity on the sol-gel process has been proposed. The flexibility of the sol-gel process, and the promise it holds for creating products as diverse as hydrogen fuel cell membranes through to protective coatings for space vehicles, make it an important area of study. This thesis addressed a fundamental gap in the scientific knowledge concerned with the sol-gel process: how and why does gravity affect the sol-gel process? The nanomaterial chosen for study was a xerogel, a dense compound with a high surface area which finds applications in high temperature ceramics, energy saving coatings, molecular filtration and thin film sensors. The xerogel was produced from an acid catalysed sol. 2ml samples of the sol were subjected to reduced, normal and high gravity levels, and the resultant xerogels were characterised through liquid and solid state NMR and nitrogen adsorption/desorption techniques. Viscosity and pH measurements were also recorded. Reduced gravity conditions were provided by NASA’s KC-135 aircraft which is capable of creating a 25 second window of 1x10−2 gravities. A centrifuge was utilised to simulate increased gravity environments and xerogels were formed between 2 and 70 gravities. Analysis of the results led to two major contributions to this field of scientific endeavour. It was concluded that (1) gravity affected the reaction pathways of the sol-gel process and (2) gravity directly altered the molecular structure of xerogels The second contribution was determined through the NMR studies, where it was shown that a reduction in gravity resulted in a molecular structure composed of extended branches of cyclic compounds. Due to a decrease in convection in reduced gravity the molecular structure of the sample was dominated by cyclisation. In terrestrial and high gravity the molecular structure grew through both bimolecularisation and cyclisation reactions. Thus the gravity level also determined the reaction pathway available within the sol by creating a more or less convective environment. This created a structure composed of cyclics (rings) and chains. As gelation and drying of the sol occurred there was a loss in Q4 group amount. Chains, having a higher energy configuration than rings, underwent repolymerisation. Short chains formed which reacted end-to-end to form small, stable rings. The rings packed together more closely within the liquid sol and delayed the formation of a spanning cluster. The greater the gravity level, the greater the extent of bimolecularisation reactions contributing to chain formation, in turn allowing a greater degree of repolymerisation of the molecular structure. Thus gel times increased as the gravity level increased. Again gravity directly affected the reaction pathway of the sol-gel process. In reduced gravity the sol gelled very quickly due to the formation of a cyclic structure which was not capable of repolymerisation. The final contribution of this thesis was the proposal of a mechanistic model. The model depicted the ffect of gravity on the formation of the molecular structure of a xerogel.

Würzburg, Universiẗat, Diss., 2008.

Thesis (M.S.)--Youngstown State University, 2008. / Includes bibliographical references (leaves 50-53). Also available via the World Wide Web in PDF format.

Zugl.: München, Techn. University, Diss., 2008.

Zugl.: München, Techn. Univ., Diss., 2008

Zugl.: Clausthal, Techn. Univ., Diss., 2008

Honors Project--Smith College, Northampton, Mass., 2009. / Includes bibliographical references.

Zugl.: Berlin, Humboldt-Univ., Diss., 2008

Thèse (M.Sc.)--Université Laval, 2008. / Bibliogr.: f. [60]-74. Publié aussi en version électronique dans la Collection Mémoires et thèses électroniques.