Evolution of Pore and Phase Structure of Sol-Gel Derived Lanthana Doped Titania at High Temperatures

Sol-gel derived porous titania was doped with lanthana at different dopant concentrations by a solution-sol mixing method to improve its phase and pore structure stability at high temperature. Very different effects of doping lanthana on the phase and pore structures of the sol-gel derived titania are found between the samples with low lanthana loadings (<0.27 g/100 m²) and those with high lanthana loadings (≥0.27 g/100 m²). The critical lanthana loading (0.27 g/100 m²) corresponds to the monolayer coverage of La₂O₃ on the grain surface of titania. At the low lanthana loadings (<0.27 g/100 m²), doped lanthana covers the surface of titania particles. This decreases the sintering rate and retards the anatase to rutile phase transformation. Doping lanthana also reduces the loss of surface area (or change of the pore structure) of heat-treated samples as a result of decreasing sintering and phase transformation rates. At the high lanthana loadings, lanthanum titanates are formed because of reactions between lanthana and titania which are facilitated at high temperature and lanthana concentration.