摘要
以TiO2半导体为主的多相光催化氧化技术因与传统污染处理技术相比具有许多优点而倍受青睐;但是,目前以TiO2为基础的光催化技术还存在量子效率低、太阳能利用率低等技术难题,如何解决这些难题是当前光催化科学发展所面临的重大挑战。
金属离子掺杂是改善催化剂光催化性能的有效途径,而稀土离子所具有的特殊电子结构能有效地对TiO2进行表面改性。目前,稀土离子掺杂的TiO2光催化剂主要用于液相光催化,将其用于气相光催化反应的研究较少。采用原位红外光谱考察其对多种典型有机污染物的气相光催化降解过程也鲜见报道。
本文采用改进的溶胶-凝胶法和浸渍法制备了TiO2掺杂稀土离子La3+、Y3+、Gd3+、Er3+、Nd3+、Pr3+的RE/TiO2光催化剂。以气相光催化降解典型的有机污染物乙烯、溴代甲烷为模型反应,考察稀土离子掺杂对TiO2气相光催化性能的影响。运用ICP、FTIR、XRD、TEM、BET、TG/DTA、Raman、DRS、SPS等多种表征手段,深入探讨稀土离子对TiO2的表面改性机理,阐明稀土离子掺杂TiO2催化剂光催化性能得到改善的原因。另外,利用原位红外技术考察了RE/TiO2样品光催化降解乙烯、丙酮、苯的反应,对其光催化降解有机污染物的过程进行较深入的研究。
结果表明,与纯TiO2相比,掺杂La3+、Y3+、Gd3+、Er3+、Nd3+的RE/TiO2样品上乙烯、溴代甲烷的转化率均有不同程度的提高,CO2的生成量也有相应增加,说明掺杂这些稀土离子可以使TiO2的光催化活性增强,而且表现出较强的矿化能力。但是,掺杂Pr3+的TiO2的光催化性能没有提高甚至降低。催化剂的表征结果显示,稀土离子La3+、Y3+、Gd3+、Er3+、Nd3+、Pr3+掺杂后,TiO2样品的锐钛矿含量增加,比表面积增大,粒径变小,表现出量子尺寸效应;稀土离子掺杂后TiO2样品的吸收边发生蓝移,表面光电压的响应阈值增大,使得光致电子-空穴对的氧化还原能力增强;此外,Pr3+除外的其它稀土离子掺杂的TiO2样品的表面光电压信号增强,导致光生电子-空穴对的分离效率提高,这些都可能是TiO2掺杂稀土离子La3+、Y3+、Gd3+、Er3+、Nd3+后光催化活性提高的原因。虽然Pr/TiO2催化剂的锐钛矿相含量增加、比表面积增大、粒径变小、吸收边发生蓝移、SPS的响应阈值增大,但是由于具有可变价态的Pr3+离子容易成为空穴捕获的不可逆陷阱,使得Pr/TiO2的表面光电压信号减弱,光生电子-空穴对没有得到有效分离,故显示出较差的光催化活性。
原位红外光谱研究乙烯、丙酮、苯的光催化降解结果表明,乙烯可以被光催化氧化生成CO2和水,而丙酮、苯被光催化氧化除了生成CO2和水外,还可能有其它较稳定的产物生成。
TiO2-based heterogeneous photocatalysis as a method of environmental remediation receives more attentions as compared to the traditional methods because it has many advantages. However, there are still a lot of problems in TiO2-based photocatalysis such as low quantum efficiency and little utilization of solar energy and so on. As a result, it is a great challenge confronted to photocatalysis that how to solve these problems and make the photocatalytic technique be widely used in practice.
Metal ions doping is considered to be an efficient way in respect of enhancing the photocatalytic activity. Rare earth elements have special electronic structure, so they can serve as electron trap and suppress the recombination of electron-hole pairs. A lot of investigations have been carried out on photocatalytic degradation of organic contaminants over rare-earth-doped TiO2 catalysts. By far, rare-earth-doped TiO2 photocatalysts have mainly been used in liquid photocatalysis rather than gas-phase photocatalysis. The in situ FTIR study of gas-phase photocatalytic degradation of some typical organic pollutants using rare-earth-doped TiO2 catalysts have seldom been reported, too.
In this paper, TiO2 doped with La3+, Y3+, Gd3+, Er3+, Nd3+, Pr3+ (labeled as RE/TiO2, RE=La, Y, Gd, Er, Nd, Pr) photocatalysts were prepared by the sol-gel and impregnation methods. The photocatalytic degradations of ethylene and bromomethane were used as model reactions to evaluate the photocatalytic activity under UV light irradiation. The influences of rare earth doping on the structure of catalysts and their photocatalytic performance were systematically studied by using ICP, FTIR, XRD, TEM, BET, TG/DTA, Raman, DRS and SPS. The photocatalytic processes of ethylene, acetone, benzene were investigated by in situ FTIR and the degradation mechanisms were discussed.
The results revealed that through La3+, Y3+, Gd3+, Er3+, Nd3+ doping, the conversion of ethylene and bromomethane over TiO2 remarkably enhanced ; At the same time, the production of CO2 increased. That is, TiO2 exhibited higher photocatalytic activity than pure TiO2 through La3+, Y3+, Gd3+, Er3+, Nd3+ doping. However, Pr3+ doping decreased the photocatalytic activity of TiO2. The characteristic
results showed that La3+, Y3+, Gd3+, Er3+, Nd3+, Pr3+ doping could inhibit the anatase to rutile phase transformation , decrease particle sizes and increase the specific surface area. The DRS and SPS spectras of rare-earth-doped TiO2 catalysts showed blue shifts in the band gap transition, which could yield larger redox potential. Furthermore, the surface photovoltage intensity of rare-earth-doped TiO2 catalysts increased except by Pr3+ doping, which could lead to the effective separation of photogenerated electron-hole pairs. All the above might result in the improvement of the photocatalytic activity over TiO2 through La3+, Y3+, Gd3+, Er3+, Nd3+ doping, while the decrease of the photocatalytic activity over Pr/TiO2 catalyst might be due to its changeable electrovalence which made Pr3+ easily act as irreversible trap of hole.
The photocatalytic degradation of ethylene, acetone, benzene studied by in situ FTIR showed that ethylene could be oxidized to CO2 and H2O under UV light, but acetone and benzene were converted to some other steady products besides CO2 and H2O.
引文
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