Preparation of lanthanum-doped TiO2 photocatalysts by coprecipitation

The lanthanum-doped TiO2 (La3+-TiO2) photocatalysts were prepared by coprecipitation and sol-gel methods. Rhodamine B was used as a model chemical in this work to evaluate the photocatalytic activity of the catalyst samples. The optimum catalyst samples were characterized by XRD, N-2 adsorption-desorption measurement, SEM and electron probe microanalyses to find their differences in physical and chemical properties. The experimental results showed that the La3+-TiO2 catalysts prepared by coprecipitation exhibited obviously higher photocatalytic activities as compared with that prepared by the conventional sol-gel process. The optimum photocatalysts prepared by the coprecipitation and sol-gel process have similar adsorption equilibrium constants in Rhodamine B solution and particle size distribution in water medium although there are larger differences in their surface area, morphology and pore size distribution. The pores in the sol-gel prepared catalysts are in the range of mesopores (2-50 nm), whereas the pores in the coprecipitation prepared catalysts consist of bigger mesopores and macropores (> 50 nm). The morphology of the primary particles and agglomerates of the La3+-TiO2 catalyst powders was affected by doping processes. The inhibition effect of lanthanum doping on the phase transformation is greater in the coprecipitation process than in the sol-gel process, which could be related with the different amount of Ti-O-La bonds in the precursors. This finding could be used for preparing the anatase La3+-TiO2 catalysts with more regular crystal structure through a higher heat treatment temperature. The optimum amount of lanthanum doping is ca. 1.0 wt.% and the surface atomic ratio of [O]/[Ti] is ca. 2.49 for 1.0 wt.% La3+-TiO2 catalysts prepared by the two processes. The obviously higher photocatalytic activity of the La3+-TiO2 samples prepared by the coprecipitation could be mainly attributed to their more regular anatase structure and more proper surface chemical state of Ti3+ species. The optimum preparation conditions are 1.0 wt.% doping amount of lanthanum ions, calcination temperature 800 degrees C and calcination time 2 h using the coprecipitation process. As compared with the sol-gel process, the coprecipitation process used relatively cheap inorganic raw materials and a simple process without organic solvents. Therefore, the coprecipitation method provides a potential alternative in realizing large scale production.