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Study on the catalytic activity of surface-modified anatase TiO2 nanocrystal
- Alternative Title
- Study on the catalytic activity of surface-modified anatase TiO2 nanocrystal
- Author(s)
- Yu, Xiaomei
- Alternative Author(s)
- Yu Xiaomei
- Advisor
- Yu Kwon Kim
- Department
- 일반대학원 에너지시스템학과
- Publisher
- The Graduate School, Ajou University
- Publication Year
- 2017-08
- Language
- eng
- Keyword
- Applied Chemistry
- Alternative Abstract
- Anatase TiO2 nanosheets with large {001} facets are synthesized in a hydro thermal process by using HF as a shape-controlling agent. A successive post treatment with either H2 or NH3 gases is carried out under a gas flow at elevated temperatures of 500 – 800 °C. The photocatalytic performance of these prepared TiO2 catalysts is evaluated from a photodegradation of methylene blue in an aqueous solution under light irradiation. A shape effect is observed from the as-synthesized aTiO2 for higher photoactivities with the optimized ratio of crystal facet {001} close to 60%. Further enhancement on photoactivity is also achieved after the chemical etching treatment of the as-synthesized aTiO2 in a dilute NaOH (or HF) solution to remove the surface layers with significant changes in the surface defects as well as the surface morphology. The changes, especially in the surface defects which favor the formation of surface O- species under ambient condition are considered to play an important role in improving the photoactivity. Furthermore, post treatments of aTiO2 with reducing gases such as H2 may change the concentration and distribution of the intrinsic Ti3+ defects, which including the oxygen vacancies and Ti3+ interstitials in between the surface and bulk of nanocrystals. We find that the enhanced photoactivity after hydrogenation is closely related to such changes in defects. Compared with aTiO2, in our case, about 10 times higher photoactivity is obtained with hydrogenated TiO2 (H-TiO2) at 600 °C for 16 h treatment which is suggested due to a high surface-to-bulk defect ratio and a no uniform distribution of defects between the bulk and surface. Here we can see that the surface/bulk defects can be controlled by kinetically controlling of hydrogenation conditions, which in turn, will improve the photocatalytic performance of aTiO2. Other dopant element such as N is also introduced by annealing of aTiO2 under NH3 flow, which is investigated to have a promoting effect on photoactivity especially under visible light (Halogen lamp) irradiation. The optimized doping condition (temperature, time, gas flow rate) with generally enhanced photoactivity observed in our research is considered attributed to the detailed distribution of doped N species within the TiO2 matrix. A N-rich shell is obtained near the surface of N-TiO2 and proved by dramatic decrease in photoactivity after removing of this shell layer through post-chemical-etching in HF aqueous solution. While, the enhancement of photoactivity is favorable with the presence of no uniform distribution of N dopants which may improve the charge separation in TiO2 nanocrystals. The thermal catalytic performance of our as-synthesized aTiO2 nanosheets in CO oxidation reaction is also studied with the TiO2-supported Pt catalysts over a wide range of reaction conditions such as pressures of 1 – 100Torr and temperatures of 300 – 500 K. Well prepared TiO2 nanocrystals with 1 – 3wt% Pt loading are oxidized and reduced at 300 °C before being exposed to the reactants of CO and O2 gases in a well-defined ratio. The reaction rate changes based on the systematic measurements and the temperature-dependent reaction rates are observed to follow Arrhenius behavior. We observe that the initial stage of reaction is strongly influenced by the initial surface condition. Also, two distinct temperature ranges with different kinetics are investigated to show different slopes in Arrhenius plot. The partially oxidized Pt particles are characterized in a form of mixed phase of metallic Pt, PtO and PtO2 which provide various active sites for catalytic CO oxidation. The sudden change in the slope of Arrhenius plots is considered due to the formation of Pt oxides at high temperatures under oxidizing condition which is proposed to switch the reaction channels over new reaction sites at the boundary of Pt oxides. For the conversion of ubiquitous solar energy into clean chemical fuels such as H2, SnS has also been studied as a strong potential candidate as a photocatalyst in applications such as water splitting for its appropriate band gap and high optical coefficient. In our study, a two-dimensional orthorhombic SnS thin films in shape of nanodisk are prepared by dissociation of SnS2 particles in Ar plasma followed by sulfur reduction. The thickness of the SnS films varies from 10 – 200 nm in a controllable way and a thickness-dependent effect is observed on its physical properties such as the optical band gap, the absorption coefficient and the flat band potential. A photoelectrocataltytic water splitting is carried out in a three-electrode system with SnS as the working electrode, Pt as the counter and Ag/AgCl as the reference electrode. The hydrogen evolution rate is determined from the gas chromatography (GC) measurements. We find that with positive bias voltage of ESnS/(Ag/AgCl)=0.4 V, enhanced H2 evolution occurs over Pt while O2 production is suppressed over SnS surface under UV irradiation. However, the opposite bias voltage of -0.56 V leads to about 4 times higher of H2 evolution rate which obtained from the SnS surface, the current flow is suppressed. We propose band alignment models for our water splitting system for the explanation of the underlying mechanism.
- Fulltext
- Appears in Collections:
- Graduate School of Ajou University > Department of Energy Systems > 4. Theses(Ph.D)
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