- 學位論文
以微波水熱法合成氧化鈦奈米管之吸附與光催化潛勢研究
Titanate Nanotubes Synthesized by a Microwave Hydrothermal Method: Study on Their Adsorption and Photocatalytic Potential
摘要
氧化鈦奈米管為近年新興之奈米材料，其具有高比表面積、優良吸附與光催化能力，而被廣泛應用於各領域研究中。以微波水熱法可於三小時內製備完成氧化鈦奈米管，其較傳統水熱法以更短的時間、更省的能源以及更簡潔的程序製成，且保有其吸附與光催化效能。 以微波水熱法製備之氧化鈦奈米管主要是由鈉、鈦、氧以及氫原子所構成，其化學結構式主要可歸為NaxH2-xTi3O7類型。氧化鈦奈米管表面之鈉原子含量受製備時所注入之微波功率影響甚大，隨微波功率之增加而使表面更多鈉原子嵌入管壁，且鈉原子可藉由離子交換機制與污染物反應，進而使污染物穩固吸附於管壁。氧化鈦奈米管管壁之鈦原子則於其吸附或光催化反應中皆未有明顯作用或變化。 本研究以微波水熱法製備氧化鈦奈米管，探討其吸附與光催化潛勢，包括對重金屬鉛離子之吸附潛勢鑑定、對全氟辛酸之光催化潛勢鑑定，以及同時添加半導體硫化鎘與氧化鈦奈米管光催化水中氨氮之研究，最後則探討氧化鈦奈米管之表面化學反應機制。 研究結果發現，氧化鈦奈米管之吸附能力比光催化能力優良。以比表面積50 m2/g之二氧化鈦於強鹼環境中以微波能輔助加熱可製備出比表面積為原材料三倍之高的氧化鈦奈米管。由於氧化鈦奈米管之高比表面積提供大量活性吸附位置，其可於短時間內完成吸附反應並達平衡，且近乎完全去除水中鉛離子，以等溫吸附模式計算可得知每克氧化鈦奈米管之最大吸附潛勢為2,000 mg Pb(II)/g。氧化鈦奈米管之等電位點約在pH值為3，提高反應環境之pH值使得氧化鈦奈米管之表面愈呈負電性，而有利於吸附重金屬陽離子；反之則有利於吸附陰離子污染物。氧化鈦奈米管雖可作為觸媒輔助光催化反應，但由於異相光催化效應促使固態氧化鈦奈米管無法於水溶液環境中提升其光催化效率，且有遮蔽效應之產生。水中氨氮與全氟辛酸兩污染物於氧化鈦奈米管之光催化反應系統下可達50%之去除效率，且觸媒之存在可提升反應途徑之完整性。
並列摘要
Titanate nanotubes are nano-materials with special properties, such as high specific area, adsorption capacity and strong photocatalytic capability; and they have been widely studied for applications in various fields recently. Titanate nanotubes can be synthesized within three hours by a microwave hydrothermal method, which is simpler and more time and energy efficient than conventional hydrothermal methods. This study aimed at evaluating the adsorption capacity and photocatalytic capability of titanate nanotubes, made by the microwave hydrothermal method, in removing some common pollutants from aqueous media, and exploring the reaction mechanisms. Titanate nanotubes synthesized by a microwave hydrothermal method are mainly consist of sodium, titanium, oxygen, and hydrogen atoms, and their chemical structure can be represented as NaxH2-xTi3O7. The sodium content on the surfaces of titanate nanotubes was increased with irradiation power; this phenomenon could be attributed to the intercalation of sodium atoms in the TNT nanostructure which enlarges the space between inter-layers. The sodium ions on the surfaces can exchange with target ions, as well as be adsorbed onto the inner and outer surfaces of titanate nanotubes. However, the titanium ions on the surfaces of titanate nanotubes had minor effects on their adsorption and photocatalytic reactions. Titanate tubes used in this study were synthesized from commercial titanium dioxide (Degussa P25). Lead, ammonia, and perfluorooctanoic acid (PFOA) were chosen as the target polluants. The research firstly evaluated the removal of lead ions from water by titanate nanotubes through adsorption. It then evaluated their photocatalyic capability in reducing concentrations of ammonia nitrogen and perfluorooctanoic acid in aqueous solutions. For the case of ammonia removal, the nanotubes were combined with cadmium sulfide to enhance their photocatalytic capacity. Effects were also made to discuss plausible mechanisms for the adsorption and photocatalyitc reactions. The experimental results indicate that these titanate nanotubes are good adsorbents. Titanate nanotubes synthesized by the microwave hydrothermal method have specific surface area three times larger than that of Degussa P25 (50 m2/g). Due to their high specific surface areas, they could remove almost 100% of lead ions from water through adsorption and the reaction was fast and reached equilibrium within 30 minutes. The maximum adsorption capacity was found to be 2,000 mg Pb(II)/g of titanate nanotubes. The pHZPC of titanate nanotubes is around 3, thus they are more favorable in adsorbing cations under higher pH values and in adsorbing anions under acidic conditions. Although titanate nanotubes could be catalysts in photocatalytic removal of pollutants, they were not effective as expected. It might be caused by the fact that these reactions were heterogeneous in nature and the shielding effects caused by the porous structure of the nanotubes. About half of ammonia nitrogen and perfluorooctanoic acids in aqueous solutions could be removed by titanate nanotubes under UV irradiation, and the presence of the nanotubes as photocatalysts could enhance the completeness of the reactions.