The Study Of Gas-phase Photocatalytic Degradation Of Organic Pollutants Over TiO₂ Doped With Rare Earth Lons

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.