Foi explorado o desenvolvimento de fotocatalisadores híbridos ZnO/Xerogel de carbono, visando aumentar a eficiência quântica do processo de degradação de poluentes orgânicos persistentes. Especificamente, foi estudada a preparação do híbrido ZnO-xerogel de carbono, sendo esta, a maior inovação tecnológica deste projeto, considerando que o efeito catalítico deste híbrido não foi estudado na literatura. O uso do xerogel de carbono na preparação do semicondutor-material carbonoso é justificado pela excelente condutividade elétrica, elevada área superficial e porosidade, além de sua estrutura porosa ser facilmente manipulada por modificações nos parâmetros de síntese. A escolha do tanino como precursor do xerogel visa à diminuição de custos e impactos ambientais, além de acrescentar valor à inovação tecnológica proposta. A influência do pH e rota de síntese foram avaliados, assim como a influência da temperatura de calcinação nas propriedades dos materiais. A espectroscopia por refletância difusa foi a técnica empregada para determinação da energia de gap das amostras. A morfologia, a análise elementar, a estrutura cristalina dos materiais, a estrutura química e a constituição dos materiais foram determinadas por microscopia eletrônica de varredura, espectrometria de energia dispersiva, difratometria de raios X, infravermelho e espectroscopia Raman, respectivamente. O balanço de cargas na superfície do material foi analisado pela metodologia do ponto de carga zero (PZC). A ação fotocatalítica do material foi avaliada através da decomposição de 4-clorofenol e bisfenol A, determinada através da espectroscopia no UV-Visível. Todos os materiais apresentam a estrutura cristalina hexagonal do óxido de zinco (wurtzita). Os materiais sem tanino em sua composição também apresentam a fase de hidroxicloreto de zinco mono-hidratado. Os difratogramas de raios-X e valores de bandgap obtidos confirmam a incorporação do carbono na estrutura cristalina do óxido de zinco. Os materiais produzidos via rota alcoólica apresentam menores valores de tamanho de cristalito e partícula, assim como um maior teor de grafite em sua composição e maior área superficial, enquanto que os materiais produzidos em solução aquosa apresentam menores valores de energia de gap. Todos os materiais apresentaram atividade fotocatalítica quando submetidos à radiação solar e visível, sendo que os materiais com proporções intermediárias entre xerogel de carbono e óxido de zinco se mostraram superiores para o processo de fotodegradação. Os materiais produzidos via alcoólica são superiores aos produzidos em solução aquosa, no que tange o processo de fotocatálise. Os valores máximos encontrados para a degradação de 4-clorofenol e bisfenol A foram de 88% e 78%, respectivamente, obtidos pelo material XZnC 3.0 (EtOH). A temperatura de calcinação e quantidade de hidróxido de potássio utilizados na síntese influenciam significativamente as propriedades dos materiais, de modo que as condições ótimas para a síntese dos materiais foram de calcinação em 300oC e 8 g de hidróxido de potássio utilizadas na síntese. O mecanismo de fotocatálise é fortemente influenciado pela geração de radicais hidroxilas e os materiais apresentam estabilidade para reciclo em processos industriais. / The development of ZnO/Carbon xerogel hybrid photocatalysts was studied, in order to increase the quantum efficiency of the degradation process of persistent organic pollutants. Specifically, the preparation of the ZnO-carbon xerogel hybrid was studied, being this the most significant technological innovation of this project, considering that the catalytic effect of this hybrid was not studied in the literature. The use of carbon xerogel in the preparation of the semiconductor-carbonaceous material is justified by its excellent electrical conductivity, high surface area and porosity, the latter being easily manipulated by modifications in the synthesis parameters. The choice of tannin as the precursor of the xerogel aims at reducing costs and environmental impacts, adding value to the proposed technological innovation. The influence of the pH and route of synthesis were evaluated, as well as the influence of the calcination temperature on the properties of the materials. Diffuse reflectance spectroscopy was the technique used to determine the gap energy of the samples. The morphology, elemental analysis, crystalline and chemical structure of the materials were determined by scanning electron microscopy, dispersive energy spectrometry, X-ray diffractometry, infrared and Raman spectroscopy, respectively. The charge balance on the material surface was analyzed by the point of zero charge methodology (PZC). The photocatalytic action of the material was evaluated by the decomposition of 4-chlorophenol and bisphenol A, determined by UV-Visible spectroscopy. All materials have the hexagonal crystalline structure of zinc oxide (wurtzite). The materials without tannin in their composition also present the zinc hydroxychloride monohydrate phase. The X-ray diffractograms and bandgap values obtained confirm the incorporation of the carbon in the crystalline structure of the zinc oxide. The materials produced via the alcohol route have lower values of crystallite and particle size, as well as a higher content of graphite in their composition and larger surface area, while the materials produced in aqueous solution have lower values of gap energy. All the materials presented photocatalytic activity when subjected to visible and solar radiation, and the materials with intermediate proportions between carbon xerogel and zinc oxide were superior for the photodegradation process. The materials produced via alcohol route are superior to those produced in aqueous solution, as far as the photocatalysis process is concerned. The maximum values found for the degradation of 4-chlorophenol and bisphenol A were 88% and 78%, respectively, obtained by the material XZnC 3.0 (EtOH). The calcination temperature and amount of potassium hydroxide used in the synthesis significantly influence the properties of the materials, so that the optimum conditions for the synthesis of the materials were calcination at 300 °C and 8 g of potassium hydroxide used in the synthesis. The mechanism of photocatalysis is strongly influenced by the generation of hydroxyl radicals and the materials present stability for recycling in industrial processes.

TiO2 photocatalysis is been widely studied for air and water purification applications; titanium dioxide is the most used semiconductor principally because its low cost, stability and chemical properties. However it only utilizes the UV portion of the solar spectrum as an energy source (less than 4% of the total sunlight energy). This behavior is due to its high band gap value of 3.2 eV. The modification of light harvesting properties of TiO2 by doping has become an important research topic to achieve an efficient operating range under UV and visible light. In addition, the structure and surface properties of photocatalysts play an important role. This thesis explores the effects of Zn doped TiO2, prepared by the sol-gel method, on its photocatalytic activity to decompose organics and the characterization of the doped samples. Since this study is part of a collaborative initiative, the samples were synthesized and provided by Dr. A. R. Phani from the Department of Physics, University of L'Aquila. Preliminary examination revealed a relatively low photocatalytic efficiency of the samples. The objective is to modify/improve its properties by high energy ball milling which is expected to generate accumulations of defects, particle size reduction and an increase in the active surface area. The characterization of doped and mechanochemically treated materials will be analyzed by optical diffuse reflectance measurements and optical absorption calculations using the Kubelka-Munk approach. The phase structure and particle size of the materials will be determined using X-ray diffraction (XRD). The BET surface area of the samples will be obtained using an Autosorb instrument. The photocatalytic properties will be studied by the analysis of decomposition of Methyl Orange in an aqueous solution. An aqueous photocatalytic tubular reactor with capability of operation using UV and/or fluorescent light will be designed and built.

京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第23976号 / 人博第1028号 / 新制||人||242(附属図書館) / 2022||人博||1028(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 吉田 寿雄, 教授 田部 勢津久, 教授 中村 敏浩 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

The photocatalytic oxidation of trichloroethylene (TCE), ethylene. ethane and toluene on TiO2, Pt/TiO2 and Ag/TiO2 were investigated in a dedicated reactor set-up operated at room temperature and ambient pressure condition. The gas phase experiments were carried out for both single and binary mixtures of these chemicals to identify the role of Pt and Ag metallisation in the photocatalytic oxidation of different contaminants. In a single contaminant system, the presence of Pt enhanced the oxidation of ethylene, ethane and toluene but detrimental to the oxidation of TCE. In the oxidation of ethylene, Pt enhanced the oxidation by acting as catalyst and as electron sink. However, in ethane oxidation, the enhancement was solely associated to the ability of Pt to act as electron sink. The detrimental effect observed in TCE oxidation was attributed to Pt and Cl interaction, which formed a persistent inorganic chlorine species decreasing the overall Pt/TiO2 photocatalyst performance. Interestingly, Ag did not show any significant effect to the oxidation of any single system degradation. In binary system degradation, where TCE and another organic compound either ethylene, ethane or toluene were degraded simultaneously, Pt always caused a detrimental effect due to its strong interaction with Cl. However, the presence of Ag and Cl gives a more synergetic effect. Ag was found to provide sites to temporarily trap chlorine radicals as AgCl. Under illumination, electrons transferred from Cl to Ag forming chlorine radicals that could react with the surface contaminant enhancing its breakdown and mineralization.

The current work has developed a CFD model to characterise a pseudo-annular photocatalytic bubble column reactor. The model development was divided into three stages. Firstly, hydrodynamic assessment of the multiphase fluid flow in the vessel, which incorporated residence time distribution analysis both numerically and experimentally for validation purposes. Secondly, the radiation distribution of the UV source was completed. The final stage incorporated the kinetics for the degradation the model pollutant, sodium oxalate. The hydrodynamics were modelled using an Eulerian-Eulerian approach to the multiphase system with the standard k- turbulence model. This research established that there was significant deviation in the fluid behaviour in the pseudo-annular reactor when compared with traditional cylindrical columns due to the nature of the internal structure. The residence time distribution study showed almost completely mixed flow in the liquid phase, whereas the gas phase more closely represented plug flow behaviour. Whilst there was significant dependence on the superficial gas flow rate the mixing behaviour demonstrated negligible dependence on the liquid superficial velocity or the liquid flow direction, either co- or counter- current with respect to the gas phase. The light distribution was modelled using a conservative variant of the Discrete Ordinate method. The model examined the contribution to the incident radiation within the reactor of both the gas bubbles and titanium dioxide particles. This work has established the importance of the gas phase in evaluating the light distribution and showed that it should be included when examining the light distribution in a gas-liquid-solid three-phase system. An optimal catalyst loading for the vessel was established to be 1g/L. Integration of the kinetics of sodium oxalate degradation was the final step is developing the complete CFD model. Species transport equations were employed to describe the distribution of pollutant concentration within the vessel. Using a response surface methodology it was shown that the reaction rate exhibited a greater dependency on the lamp power that the lamp length, however, the converse was true with the quantum efficiency. This work highlights the complexity of completely modelling a photocatalytic system and has demonstrated the applicability of CFD for this purpose.

Magnetic photocatalysts were synthesised by coating a magnetic core with a layer of photoactive titanium dioxide. This magnetic photocatalyst is for use in slurry-type reactors in which the catalyst can be easily recovered by the application of an external magnetic field. The first attempt at producing this magnetic photocatalyst involved the direct deposition of titanium dioxide onto the surface of magnetic iron oxide particles. The photoactivity of these Fe3O4/TiO2 was lower than that of single-phase TiO2 and was found to decrease with an increase in the heat treatment. These observations were explained in terms of an unfavourable heterojunction between the titanium dioxide and the iron oxide core. Fe ion diffusion from the iron oxide core into the titanium dioxide matrix upon heat treatment, leading to a highly doped TiO2 lattice, was also contributing to the observed low activities of these samples. These Fe3O4/TiO2 particles were found to be unstable, with photodissolution of the iron oxide phase being encountered. This photodissolution was dependent on the heat treatment applied, the greater the extent of the heat treatment, the lower the incidence of photodissolution. This was explained in terms of the stability of the iron oxide phases present, as well as the lower photoactivity of the titanium dioxide matrix. In fact, the observed photodissolution was found to be induced-photodissolution. That is, the photogenerated electrons in the titanium dioxide phase were being injected into the lower lying conduction band of the iron oxide core, leading to its reduction and then dissolution. Thus, the approach of directly depositing TiO2 onto the surface of a magnetic iron oxide core proved ineffective in producing a stable magnetic photocatalyst. The introduction of an intermediate passive SiO2 layer between the titanium dioxide phase and the iron oxide phase inhibited the direct electrical contact and hence prevented the photodissolution of the iron oxide phase. Improvements in the photoactivity were seen to be due to the inhibition of both the electronic and chemical interactions between the iron oxide and titanium dioxide phases. Preliminary optimisation experiments revealed that a thin SiO2 layer is sufficient for inhibiting the photodissolution. The thickness of the TiO2 coating was found not to have a significant effect on the photocatalytic performance of the coated particles. Finally, heat treating for 20 minutes at 450??C was sufficient for converting the titanium dioxide into a photoactive phase, longer heating times had no beneficial effect on the photoactivity.

In this study nanocrystalline TiO2 was synthesized by a sol-gel process. The effect of praseodymium (Pr) doping and calcination conditions on the textural properties of nano structured particles and photocatalytic activity were examined. Samples were synthesized by hydrolyzation of titanium tetra iso-propoxide (TTIP) and calcination at different temperatures. Characterizations of the samples were carried out using XRD, BET, XPS, TEM, and EDAX analyses. It was observed that anatase to rutile transformation is favored by higher calcination temperatures. The XRD analysis indicated that the anatase structure is stabilized by Pr doping and rutile phase formation at higher calcination temperatures is inhibited by the addition of Pr. It was also observed that Pr doping enhances the surface area and inhibit crystal growth. Phase stabilization effect of Pr doping was also confirmed by XPS results. EDAX analysis revealed that Pr is dispersed atomically in the crystal structure. The Photoluminescence analysis by 325 nm excitation indicated the emissions at 608 and 621 nm which can be attributed by photon up-conversion. The photocatalytic activities of the samples were measured by methylene blue degradation and phenol mineralization reactions. The photocatalytic activities of the Pr doped samples were also found higher than undoped TiO2 samples.

A titanium dioxide powder co-doped with boron and zirconium was prepared by mechanical ball milling. Photocatalytic performance of the powder was evaluated by degradation of methylene blue (MB) solution under UV illumination. XRD patterns were refined by Rietveld analysis method to obtain accurate lattice parameters and position of the atoms in the crystal structure of TiO2. XRD analysis indicated that the B and/or Zr doped TiO2 powders composed of anatase and did not exhibit any additional phase. Rietveld analysis suggested that dopant B and Zr elements were successfully weaved into crystal structure and distorted the lattice of TiO2. The highest distortion was obtained by co-doping. SEM investigations confirmed that mechanical ball milling technique led to a decrease in particle size of TiO2 powder. XPS analysis revealed that dopant B and Zr atoms did not appear in any form of compound including Ti and O elements. Results of photocatalytic activity test suggested that boron and zirconium co-doped TiO2 particles exhibited a better visible light response and photocatalytic activity than that of mono element doped TiO2 (i.e. B-TiO2 and Zr-TiO2) and undoped TiO2 particles. A 20% improvement in photocatalytic activity of reference TiO2 powder (powder ball milled without dopant addition) was achieved by B and Zr co-doping. The enhanced photocatalytic activity is attributed to synergistic effects of B-Zr co-doping the lattice of TiO2 as well as particle size reduction.

Zinc oxide (ZnO) nanostructures, especially nanowires, have been one of the most important semiconductive materials used for photocatalysis due to their unique material properties and remarkable performance. In this project, vertically-aligned ZnO nanowires on glass substrate have been synthesized by using the facile hydrothermal methods with the help of pre-coated ZnO seeding layer. The crystalline structure, morphology and UV-Vis transmission spectra of the as-synthesized sample were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and Ultra-violet Visible (UV-Vis) Spectrophotometer. The photocatalytic activity of the sample was examined for the photocatalytic degradation of methyl orange (MO) as the test dye in aqueous solution under UV-A irradiation. The extent of direct hydrolysis of the MO dye under UV light without the photocatalysts was first measured to eliminate the possible contribution from the undesired variables to the overall efficiency. The effects of pH and initial concentration of the MO solution, as well as the nanowire growth time, on the photocatalytic efficiency have been investigated, in order to determine the optimal conditions for photocatalytic applications of ZnO nanowires in the industry. Furthermore, the reproducibility of the experimental methods used in this project was tested to ensure the reliability of the experimental results obtained; and the reusability of the prepared ZnO nanowire arrays were also evaluated to investigate the stability of the products for photocatalytic applications in a large scale. In addition, a micro-chamber based microfluidic device with integrated ZnO nanowire arrays has been fabricated and used for photodegradation studies of MO solution under continuous-flow conditions. As expected, the micro-chamber based approach exhibited much improved photodegradation efficiency as compared to the conventional method using bulk dye solution. The effects of the flow rate and chamber height of the microfluidic device have also been investigated in order to determine the optimal experimental conditions for photodegradation reactions in microfluidic devices.

The photo-oxygenation of 2-methylpropane and cyclohexane using visible light in aqueous acidic uranyl ion solutions at ambient temperature and pressure has been undertaken. For 2-methylpropane in the absence of oxygen, the main product (≈90%) is 2-methyl-2-propanol with a quantum yield of 0.021 ± 0.001. In the presence of molecular oxygen, both 2-methyl-2-propanol and 2-propanone (acetone) are found. Based on this, and results of gamma radiolysis of aqueous 2-methylpropane to give 'tert'-butyl radical by electronically excited uranyl ion is proposed. In the absence of oxygen, the quantum yield of 2-methyl-2-propanol shows a sigmoidal dependence on the concentration of perchloric acid. A two-species kinetic model involving an acid-base dissociation of the uranyl ion in the excited state accounts for the observed features. The addition of sodium perchlorate to the 2-methylpropane system has an inhibitory effect on the quantum yield. Excited-state ion pairing between the uranyl ion and perchlorate anion is proposed. Consequently, the two-species acid-base model is expanded upon to yield a three-species acid-base-perchlorate model that seems to account for the results from 0.01-0.4 M perchloric acid concentration. Potassium peroxydisulfate is proven effective to increase the quantum yield of 2-methyl-2-propanol from 0.021 ± 0.001 to greater than unity (1.5 ± 0.1) indicating the existence and importance of thermal chain reactions involving sulfate radical anion. The quantum yield of 2-methyl-2-propanol is found to be dependent on the concentrations of 2-methylpropane, perchloric acid and potassium peroxydisulfate, and inversely dependent on the light intensity. The net consumption of uranyl ion is zero in the presence of potassium peroxydisulfate meaning that the uranyl ion is regenerated, making this a catalytic cycle in terms of uranyl ion. The oxygenation of cyclohexane using photo-excited aqueous uranyl ion gives cyclohexanol and cyclohexanone as the two main isolated products (54%). The overall mechanism is expected to be similar to that for the 2-methylpropane system. Refluxing of this substrate with a reducing agent (CaH2 or LiAlH4) is required prior to photolysis to achieve consistent quantum yields of both products due to thermal autoxidation reactions. The presence of molecular oxygen is found to be important in determining the ratio of alcohol to ketone in the product distribution. Potassium peroxydisulfate significantly enhances the quantum yield of cyclohexanone, leaving the quantum yield of cyclohexanol relatively unchanged (as compared to without added potassium peroxydisulfate), while uranyl ion is not consumed. Possible reactions involving cyclohexyl and cyclohexyl peroxyl radicals are given to account for the experimental results.