Preparation of lanthanum-doped TiO₂ photocatalysts by coprecipitation

Abstract

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, N₂ 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 °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.