【作者】 陈亦琳；

【导师】 付贤智； 李旦振；

【作者基本信息】 福州大学 ， 物理化学， 2006， 博士

【摘要】 虽然以TiO₂为代表的半导体光催化氧化技术能在室温下实现有机污染物的氧化分解,但仍存在光催化过程量子效率低（～4 %）而难以大规模应用的重大问题。本文以Pt/TiO₂和Pt/TiO₂-xNx为光催化剂,首次在富氧的光催化气氛中加入微量氢气,发现了光催化过程的H₂-O₂耦合效应,考察了H₂-O₂耦合效应对挥发性有机污染物光催化氧化降解效率的影响。初步揭示了H₂-O₂耦合效应的规律及其本质原因,为提高光催化量子效率提供一条崭新的途径。采用溶胶-凝胶技术制备了TiO₂溶胶,利用浸渍法在催化剂表面修饰贵金属Pt和掺杂N元素,得Pt/TiO₂和Pt/TiO₂-xNx光催化剂。运用X射线粉末衍射、低温N2吸附、紫外-可见漫反射光谱、透射电镜和X射线光电子能谱等分析手段对催化剂体相及表面结构进行详细表征,结合光催化反应性能的评价结果,讨论了催化剂的组成和结构对其光催化性能的影响。运用程序升温脱附仪、傅立叶变换红外光谱、表面光电压谱、电场诱导表面光电压谱、瞬态光电导谱、荧光光谱、电子顺磁共振谱和荧光分子探针等分析技术表征氢、氧在催化剂表面的吸附行为,考察了H₂-O₂耦合对光生载流子的分离及界面迁移、催化剂表面活性氧物种的生成、光催化氧化降解污染物的产物分布的影响等。结果表明,当反应体系处于30℃、O₂气氛和紫外光照下,催化剂Pt/TiO₂（Pt含量1wt.%）对苯的转化率和矿化率分别为3%和0%,引入微量H₂后（H₂/O₂比例0.02）,转化率和矿化率分别提高至71%和78%,且催化剂不失活,当H₂/O₂比例升至0.31,催化剂具有最高的光催化活性,苯完全被转化和矿化; 在H₂/O₂比例为0.02的条件下,热处理温度300℃时,催化剂具有最佳光催化性能,Pt含量5～12 wt.%时,催化剂对苯完全矿化; 对所考察的模型污染物,催化剂的活性顺序为:环己烷<丙酮<苯<甲苯<乙苯; H₂和O₂在催化剂表面的解离吸附是产生H₂-O₂耦合效应的必要条件; Pt（0）是产生H₂-O₂耦合效应的活性中心; H₂-O₂耦合效应提高了Pt/TiO₂光生载流子的分离效率,促进了羟基自由基的生成,抑制有机难降解产物在催化剂表面的生成和沉积,使催化剂保持较高、稳定的活性。H₂-O₂耦合效应同样适用于Pt/TiO₂-xNx利用可见光降解有机污染物。30℃时,催化剂对苯的转化率和矿化率分别为18 %和80 %,且催化剂不失活; 对模型污染物的光催化降解活性顺序为:环己烷<苯<甲苯<乙苯<丙酮<乙烯。H₂-O₂耦合效应显著提高了Pt/TiO₂-xNx光生载流子的分离效率和羟基自由基的生成率。更多还原

【Abstract】 TiO₂-based photocatalytic oxidation has received wide interest as a promising technique for environmental remediation,because the complete mineralization of many organic pollutants can be conducted at ambient conditions. However, the low quantum efficiency （ 4 %） of photocatalytic process has awfully hindered the industrialization. In this paper, the H₂-O₂ synergistic effect on photooxidation of volatile organic compounds （VOCs） on Pt/TiO₂ and Pt/TiO₂-xNx has been investigated for the first time. Mechanisms are proposed to elucidate this promoting effect of the H₂ addition. Hopefully, this work may open a new door towards improving the quantum efficiency of pollutant photodegradation. Titanium dioxide sol was prepared by a sol-gel technique. Wet impregnation method was applied to prepare Pt/TiO₂ and Pt/TiO₂-xNx photocatalysts. Powder X-ray diffraction （XRD）, nitrogen sorption （BET） at 77 K, UV-Vis diffuse reflectance spectra （DRS）, transmission electron microscopy （TEM） and X-ray photoelectron spectroscopy （XPS） were used to characterize the structure and composition of samples including crystal phase composition, crystalline sizes, typical specific surface area, optical absorption, and surface chemical state, respectively. Taking account of the characterization and photocatalytic analyses, relationship among structure, composition and photocatalytic performance of catalysts was studied. Temperature programmed desorption （O₂-TPD and H₂-TPD） was used to investigate the physisorption and chemisorption of hydrogen and oxygen on samples. Surface photovoltage spectroscopy （SPS）, electric field induced surface photovoltage spectroscopy （EFISPS）, time-resolved photoconductivity （TRPC）, photoluminescence （PL）, spin-trapped electron paramagnetic resonance （EPR）, terephthalic acid photoluminescence probing technique （TA-PL） and Fourier transform infrared spectroscopy （FT-IR） were used to study H₂-O₂ synergistic effect on the separation efficiency of photogenerated electron-hole pairs, formation of surface oxygenous radicals and product distribution. Results show that unprecedented photocatalytic activity and durability of Pt/TiO₂ for decomposing benzene have been obtained by adding trace H₂ into an O₂-rich photooxidation system. In pure O₂ atmosphere, the conversion of benzene is very low （3%）, and no detectable CO2 is observed at 30oC. Surprisingly, when the feeding gas contains both H2 and O2 （H2/O2 ratio: 0.02）, the photocatalytic conversion and mineralization ratio of benzene rapidly increase up to 71% and 78%, respectively. Exceptional stability of Pt/TiO2 in H2-O2 atmosphere is demonstrated by repeated use of the catalyst four times or successional operation for 50 h. The complete decomposition of benzene is achieved when the H2/O2 ratio is 0.31. The calcination temperature and the content of Pt for Pt/TiO2 have remarkable effects on the photocatalytic performance of the catalyst. When calcined at 300 oC, Pt/TiO2 exhibits the highest photocatalytic performance in H2-O2 atmosphere （H2/O2 ratio: 0.02）. The conversion and mineralization of benzene are 76 % and 82 %, respectively. While the content of Pt ranges from 5¹2 wt.%, complete mineralization of benzene is observed on Pt/TiO2. The order of photodegradation efficiencies for VOCs is cyclohexane < acetone < benzene < toluene < ethylbenzene. Among Ni, Pd, Pt, Cu, Ag, Au, Fe, Co, Ru and Rh modification on TiO2, the H2-O2 synergistic effect is only observed for Pd/TiO2 and Pt/TiO2 samples. The reason may be ascribed to the dissociative adsorption of H2 and O2 on the surface of Pt particles. Moreover, the more H2 and O2 are adsorbed on catalyst, simultaneously, the higher photocatalytic activity is exhibited. Pt（0） seems to be the active site on Pt/TiO2. With an increase in the content of Pt（0）, more and more benzene can be photodegraded. The H2-O2 synergistic effect in the photocatalysis is studied by FT-IR, spin-trapping EPR, SPS and PL. Results demonstrate that the introduced H2 has several beneficial effects on heterogeneous photocatalysis, namely, a comparative clean surface of Pt/TiO2 with no persistent aromatic intermediates, an increased surface hydroxyl radical in photocatalytic process, and an enhanced separation efficiency of photogenerated electron-hole pairs. When it turns to Pt/TiO2-xNx catalyst irradiated by visible light, results also indicate that the high photocatalytic activity and durability of the photocatalyst for decomposing benzene have been obtained in H2-O2 photooxidation system at 30 oC. The photocatalytic conversion and mineralization of benzene rapidly increase up to 18 % and 80 %, respectively, compared with no photodegradation in O2 atmosphere. The order of photodegradation efficiencies for volatile organic compounds is cyclohexane < benzene < toluene < ethylbenzene < acetone < ethylene.The H2-O2 synergistic effect on the Pt/TiO2-xNx photocatalytic system is studied by EFISPS and TA-PL. Results demonstrate that the introduced H2 has severalbeneficial effects on heterogeneous photocatalysis, including an increased surface hydroxyl radical in photocatalytic process, and an enhanced separation efficiency of photogenerated electron-hole pairs.更多还原