Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 263-285).

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

No description available.

The thermal decomposition of silver permanganate, pre-irradiated in BEPO and in a ⁶°C₀ Ϫ 'hot spot' has been investigated in the temperature range 100 - 125°C. The results are similar to those for irradiated KMn0₄ and the mechanism proposed for the latter is again suggested. The activation energy for the migration of point defects over the induction period is 1.03 ev. The decompositions of unirradiated and irradiated crystals differ in that the latter undergo physical disintegration over the acceleratory period. X-ray studies immediately prior to disintegration show strain and fragmentation in the irradiated crystal. An explanation involving the annealing of point defects at dislocation is advanced to explain the changes produced in the p/t plots with increased dosage, and fixed decomposition temperature. Summary, p. 94.

The first part of this work reports on the investigation of the formation constant of molecular silver chloride using ion exchange techniques. In order to determine the formation constant, it was necessary to develop a method of rinsing the sorbed molecular silver chloride from the resin. It was found that desorption with nitrobenzene would remove the sorbed molecular species. However, the formation constant was not calculated since the nitrobenzene used contained hydrogen ion and chloride ion which removed silver ion as well as the molecular species. In the study of the formation of silver sulfide by the reaction between the thioacetamide and silver ion, it was found that the order of the reaction with respect to silver ion was zero at pH values 0.38 and 1.24. The zero order rate constants at these pH values are 1.66 x 10^-6 moles liters^-1 sec^-1 and 9.16 x 10^-6 moles liters^-1 sec^-1 respectively. Qualitative experiments indicate that there is a change in mechanism at a pH value of approximately two.

M.S.

Solvent extraction techniques have been used to determine formation constants for molecular silver chloride and dichloro silver (I) ion in unsaturated aqueous solutions. Nitrobenzene was used to extract the molecular silver chloride from the aqueous silver chloride solution. The total concentration of silver in each phase was determined by radioassay techniques and related by an expression involving the equilibrium constants. The reciprocal of the distribution ratio times the activity of the chloride ion was plotted versus the activity of the chloride ion and portions of the curve were analyzed by the method of least squares to obtain values for the constants involved. The values obtained for K1, K2 and Kd were 1.96 X 103, 3.00 X 105, and 0.243 respectively. A second analysis of the experimental data was made by a least squares analysis of a plot of the logarithm of the reciprocal of the distribution ratio versus the logarithm of the activity of the chloride ion. The analyis was performed over the entire range of experimental points and the values obtained for K1, K2, and Kd were 1.62 X 103, 2.80 X 105, and 0.285 respectively.

M.S.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Neste trabalho é reportado o estudo de degradação do fenol via processo de fotocatálise heterogênea pelo uso de radiação eletromagnética visível em duas classes de fotocatalisadores: os baseados no TiO2 e os baseados na Ag. Os materiais baseados no TiO2 foram modificados com Ag2O e Ag3PO4 e foram preparados por meio do tratamento hidrotérmico de TiO2 comercial em meio altamente alcalino. Já os materiais baseados na Ag foram obtidos via reações de precipitação química, tendo sido preparados os seguintes compostos: Ag2O, Ag2CO3, Ag3PO4 e Ag2CrO4. A caracterização dos fotocatalisadores obtidos foi feita por meio das técnicas de microscopia eletrônica de varredura (MEV), difração de raios &#8722;X, DRX, fisissorção de N2, espectroscopias no ultravioleta &#8722; visível por refletância difusa, UV-Vis e no infravermelho, IV. Os resultados mostraram a obtenção de estruturas com morfologia associada a fios de TiO2 anatase modificados superficialmente com partículas esféricas de Ag2O e Ag3PO4, com elevado grau de cristalinidade e pureza de fase. O estudo fotocatalítico revelou que os materiais modificados com Ag2O e Ag3PO4 apresentaram melhor desempenho na fotodegradação do fenol em comparação com TiO2 puro, com eficiência de 60% de degradação em 5 horas de irradiação de fótons na região do visível. Em relação aos fotocatalisadores a base de Ag, foram obtidas partículas com boa distribuição de tamanho e morfologias associadas a cristais octaédricos de Ag2O, bastões de Ag2CO3, poliedros de Ag3PO4, bem como partículas multifacetas de Ag2CrO4. Os resultados indicaram que estes fotocatalisadores apresentaram elevada eficiência na fotodecomposição do fenol. Dentre eles, o Ag2O e o Ag3PO4 foram mais efetivos na remoção do poluente, com eficiência de 90% em apenas 2 horas de tratamento. Adicionalmente, avaliou-se a formação de radicais hidroxila durante a reação de oxidação, por meio da técnica da fotoluminescência utilizando a cumarina (COU) como uma molécula sonda. Como resultado foi observado que, com exceção do TiO2 puro,não houve formação da 7-hidroxicumarina (7HC) como produto fluorescente da reação da cumarina com os radicais &#8226;OH. Além disso, estes fotocatalisadores foram avaliados em relação à estabilidade e desempenho catalítico no reuso. A análise dos resultados obtidos por DRX e MEV dos fotocatalisadores após o uso em estudo fotocatalítico de degradação mostrou que houve a formação de prata metálica na estrutura dos mesmos. No entanto, os fotocatalisadores Ag2O e Ag2CO3 revelaram um aumento na atividade fotocatalítica após o reciclo catalítico, sugerindo que a modificação da estrutura contribuiu positivamente na eficiência da degradação do fenol. / In this, work it is reported the study of phenol degradation by heterogeneous photocatalysis process using TiO2-based and Ag-based photocatalysts materials, irradiated by visible light. TiO2-based catalysts were modified with Ag2O and Ag3PO4 and prepared by the alkaline hydrothermal treatment of commercial TiO2, while the Ag-based materials were obtained by chemical precipitation. It was prepared the following compounds: Ag2O, Ag2CO3</sub, Ag3PO4 and Ag2CrO4. Photocatalysts characterization was made by microscopy electron scanning (SEM), x ray diffraction (XRD), physisorption N2 method, UV-Vis diffuse reflectance spectroscopy and infrared spectroscopy (FT-IR). Anatase TiO2 materials showed wire-like morphology superficially modified with spherical particles of Ag2O or Ag3PO4, with high crystallinity and phase purity. The photocatalytic study found that the materials modified by Ag2O and Ag3PO4 showed better performance in phenol photodegradation than pure TiO2, with a degradation of 60% after 5 hours of irradiation by visible light. On the other hand, Ag-based catalysts exhibited particles with morphology associated to: Ag2O octahedrons crystals, Ag2CO3</sub rods, Ag3PO4 polyhedrons, and Ag2CrO4 multifaceted particles. Ag-based photocatalysts showed high efficiency in the phenol photocatalytic degradation by visible light. Among them, Ag2O and Ag3PO4 were the most effective, with 90% of pollutant degradation after only 2 hours of treatment. Moreover, the hydroxyl radicals production/role during the oxidative process was investigated by the photoluminescence technique (PL) using coumarin (COU) as a probe molecule. It was found that there was no formation of 7-hydroxycoumarin (7HC) as fluorescent product of the reaction of COU with radicals &#8226;OH, except using pure TiO2. Furthermore, the stability and catalytic performance during the reuse of these photocatalysts were evaluated. XRD and SEM analysis after catalyst use in the phenol photodegradation showed that metallic silver formation at their structures surface. Ag2O and Ag2CO3 photocatalysts showed an increase in photocatalytic activity after therecycle, suggesting that the structure modifications could contribute positively for phenol photodegradation efficiency.