Magister Scientiae - MSc / TiO2 has been broadly used as a standard photocatalyst due to its high stability, low cost, relatively low toxicity, and excellent photocatalytic performance in comparison to other semiconductor materials. However, the large band gap of TiO2 limits its use as a photocalyst due to the high energy required for excitation of the electrons in the UV region. Research on the reduction of TiO2 band gap to the visible region of the spectrum has been explored with little success. Therefore, this study focusses on shifting the band gap of TiO2 catalyst from the UV region to the visible region by doping with graphene and nitrogen-doped graphene to form TiO2-G and TiO2-NG nanocomposites, respectively. The N-doped graphene support was prepared by doping the graphene oxide with nitrogen through Hydrothermal process, followed by the reduction of the materials. Fourier Tranform Infrared (FTIR) spectroscopy confirmed the successful doping of graphene to N-graphene by the appearance of C-N and N-H vibrational modes on the spectra. The XRD results show the fingerprint patterns of TiO2 and N-graphene, which confirms the successful preparation of the nanocomposites. Morphological studies of the nanocomposites using transmission electron microscopy (TEM) show the TiO2 nanowires dispersed on graphene related supports. The optical band gap of TiO2 from UV-Vis spectroscopy was found to be 3.2eV, which decreased to 2.7eV and 2.5eV upon incorporation of grapheme and N-graphene, respectively. These results prove the success in the achievement of the aim in this study. When electrochemical studies were further conducted on the materials, TiO2-NG was found to possess better electrochemical properties with fast electron kinetics observed on the impendance spectroscopy results. The results obtained justified use of TiO2-NG photocatalyst the optimal material for organic mineralisation in Advanced Oxidation Processes (AOPs).

ABSTRACT The purpose of this research was to investigate the modified photocatalyst(TiO2) from the heterogeneous photocatalysis of perchloroethylene(PCE) products distribution by different operating conditions using near UV/TiO2 hope that could enhance the PCE¡¦s conversion and mineralization rate and explore the reaction pathways.The modified photocatalyst of this research was completed with Ag/TiO2¡BAu/TiO2¡BPt/TiO2¡BWO3/TiO2 and AC/TiO2. The added species except activied carbon was 10 % weight of photocatalyst(TiO2) others the noble metal (sillver¡Bgold and platinum) and tungsten oxide(WO3) were 0.5 % weight of TiO2. Then the experiments were conducted by varying relative humidity(R.H.=0~60 %), oxygen concentration(0~21 %), and retention time(0.38~0.89 sec). Glass beds coated with modified Degussa P-25 anatase TiO2 were filled in a Pyrex glass reactor. TiO2 was illuminated by four 10 watts ultraviolet(UV) lamps. Results from QA/QC experiments indicated that PCE could not be photodegradated by near UV of wavelength 365 nm. However, It can be decomposed quickly through heterogeneous photocatalysis. And it also find that no modified photocatalyst would be envenomed in photoactivied continuance test. The best PCE conversion and mineralization rate of modified photocatalyst were WO3/TiO2 and AC/TiO2. The modified photocatalyst Ag/TiO2 were the same as TiO2 but Au/TiO2 and Pt/TiO2 were worse to TiO2. The highest converstion rate of PCE could top to 99.5 %¡F The experiment showed that PCE was decomposed as oxygen concentration and retention time increased. But a higher concentration of oxygen was not efficient on the increase of PCE conversion. The conversion ratio of PCE could be inhibited at higher relative humidities. The end products observed from UV/TiO2 heterogeneous photocatalytic reactions included CHCl3, CCl4, C2HCl5, C2Cl6, COCl2, CCl3CClO, Cl2, HCl, CO, and CO2. The major chlorinated compound was Cl2 when the photocatalytic reactions proceed at higher oxygen concentration and less humid conditions. As water vapor existed, the major chlorinated compound became HCl. The major product was CO2 during the entire experimental process of heterogeneous photocatalytic reactions.

Thesis (Ph.D.)--North Carolina State University, 2008. / Adviser: Ollis, David F. Includes bibliographical references.

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Photocatalysis has been attracting much research interest because of its wide applications in renewable energy and environmental remediation. Among the photocatalysts, metal oxide semiconductors are the best choice due to distinctive properties, durability and cost effectiveness. However, the mechanisms of photocatalysis are still not entirely clear and the photocatalytic activity of the metal oxide materials needs to be improved. Therefore, this thesis concentrates on the study of the photocatalytic mechanism and the factors affecting the photocatalytic activity. The photocatalysis of different commercial metal oxide nanoparticles has been studied. Different photocatalytic experiments have been designed to find out the mechanisms of photocatalysis. It is found that the dominant mechanism of photocatalysis is direct charge transfer between metal oxide and organic compounds. Reactive oxygen species, such as hydroxyl radicals also play a minor role in the photocatalysis. ZnO tetrapods were synthesized by evaporating Zn powder in the flow of Ar gas at high temperature in this study. They exhibit excellent photocatalytic activity. By the comparison between ZnO tetrapods and different ZnO nanoparticles, it is found that the photocatalytic activity of the metal oxide is strongly affected by the native defects, especially by the nonradiative defects. 1D TiO2 nanotubes and ZnO nanorods were synthesized by anodization and hydrothermal growth respectively. ZnO nanorods with alumina and titania shell were prepared by solution-based methods. These structures are interesting as supported catalysts, which is important for practical applications, since it enables simple removal of photocatalyst from treated water. The core shell structures are expected to improve the stability of ZnO. The results also imply that the photocatalytic avtivity of materials is affected by the native defects. Two different solution based methods hydrothermal growth and electrodeposition to grow CuxO layer were used. The hydrothermal grown CuxO exhibit excellent photocatalytic activity and good photocorrosion resistance. It was also found that the photocatalytic activity of the CuxO prepared by hydrothermal methods can be recovered by simple immersion of the sample into the precursor solution. / published_or_final_version / Physics / Master / Master of Philosophy

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Due to industrialization and population growth, environmental contamination caused by organic pollutants is becoming an increasing problem worldwide. Environmental pollution on a global scale, particularly water pollution, has drawn scientists’ attention to the vital need for environmentally clean and friendly chemical processes. The demand for higher quality water has increased due to population growth, more stringent health regulations and economic development. Untreated wastewater contains a variety of organic compounds with variable toxicities as well as carcinogenic and mutagenic properties. Most contaminants in wastewater contain aromatic rings, which are generally resistant to chemicals, photochemicals and biological degradation.These compounds are very persistent in the environment and have a high potential to negatively affect human health and the ecosystem. Therefore, the removal or degradation of hazardous material and contaminants from wastewater is a significant global challenge. This thesis reported on the synthesis of titanium dioxide by using a peroxo method. This synthesis was done in the presence of a number of fluoride-containing surfacemodifying agents to determine the effects of these agents on particle growth, shape and crystallinity. Further, studies were carried out to investigate the modification of F-modified TiO2 with the deposition of Au colloids and an Au9 cluster. A different deposition method is employed in the synthesis of the TiO2-Au materials to gain a catalyst with the highest photocatalytic activity. The performance of the catalyst was further investigated through pre-treatment and post-treatment of the materials. Finally, several of the synthesised materials were trialled as photocatalysts using industrial dye Reactive Blue 19 (RB19) as an organic pollutant.

The development of sustainable, environmentally benign oxidation processes of organic compounds is an important task for chemical industry. This challenge can be addressed by designing catalysts that enable the utilisation of molecular oxygen as an oxidant. The work in this thesis is focused on the development of heterogeneous catalysts for the selective aerobic oxidation of various organic compounds. The first part of the thesis (Chapters 3 and 4) covers the study of bifunctional gold catalysts for the solvent-free aerobic oxidation of cyclohexene, with a particular focus on tuning the selectivity of the catalyst. Various characterisation techniques (such as TEM, diffuse-reflectance UV-Vis spectroscopy, XPS), catalytic experiments and kinetic studies were used to investigate the nature of catalyst functionality and establish the optimal structure of a gold catalyst. The second part of the thesis (Chapter 5) covers the study of the photocatalytic activity of hydrous ruthenium oxide deposited on TiO₂ in the aerobic oxidation of amines to nitriles under irradiation with visible light. The effect of the wavelength of the utilised light, applicability of the Sun as light source and water as a solvent were investigated. High catalytic activity of ruthenium-based catalyst was demonstrated for various benzylic and aliphatic amines. Various mechanistic studies were performed, based on which the mechanism of photocatalysis was suggested.

Photocatalysis is a promising technology that can effectively convert the solar energyinto sustainable green energy. However, theoretical studies on the molecular mechanisms of photocatalytic reactions are rare. This thesis is devoted to investigate several typical photocatalytic reactions on the surfaces of the most popular photocatalysis TiO2 with density functional theory. We start our study with the characterization of both the free and trapped hole on the surface generated by the light. The oxidation of physisorbed H2O molecule by the hole trapped at bridge oxygen on rutile TiO2(110) surface has been studied. The hole is found to transferto the molecule via the anti-bonding orbital as a result of the hybridization between the hole orbital and the HOMO of the molecule. The energy and symmetry mismatching between the trapped hole orbital and the HOMO of the molecule explains why the trapped hole cannot directly transfer to the chemisorbed H2O molecule. On the other hand, we have found that the chemisorbed H2O moleculecan be more efficiently oxidized by the free hole with a lower barrier and higher reaction energy compared to the oxidation by the trapped hole. In this reaction, the free hole is transferred to the chemisorbed H2O after the dissociation. This is different from the oxidation of chemisorbed H2O on anatase TiO2(101) surface by free hole, in which the hole is transferred concertedly with the dissociation of themolecule.     In order to understand the hole scavenger ability of organic molecules, the oxidation of three small organic molecules (CH3OH, HCOOH and HCOH) onanatase TiO2(101) surface has been systematically investigated. The concerted hole and proton transfer is found for all these molecules. The calculations suggestthat both kinetic and thermodynamic effects need to be considered to correctly describe the hole transfer process. The order of hole scavenging power is found tofollow: HCOH &gt; HCOOH &gt; CH3OH &gt; H2O, which agrees well with experiments.     Photo-selective catalytic reduction of the NO by NH3 and the photooxidationof CO by O2 are closely related to the environment application. Both reactionsinvolve the formation and/or breaking of non R–H bonds. The mechanism for the photoreduction of NO proposed by experiment has been verified by our calculations.The role of the hole is to oxidize the adsorbed NH3 into ·NH2 radical, which canform a NH2NO complex with a gaseous NO molecule easily. The photooxidation of CO by O2 is the first multi-step photoreaction we ever studied. By combining thepotential energy surfaces at the ground and excited state we have found that thehole and electron both take part in the reaction. A molecular mechanism which is in consistent with various experiments is proposed.     These studies show that density functional theory is a powerful tool for studying the photocatalytic reaction. Apparently, more work needs to be done in orderto improve the performance of the existing materials and to design new ones thatcan take advantage of the solar light more efficiently / <p>QC 20140522</p>

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