The effect of temperature on the structural and textural evolution of sol-gel Al₂O₃-TiO₂ mixed oxides

A series of Al₂O₃-TiO₂ mixed oxides with different TiO₂ concentrations (6%, 14% and 44% by weight) and the single oxides were prepared by the sol-gel method. The hydroxide precursors were transformed into their respective oxides and calcined at 500°C, 700°C and 900°C. The evolution of the morphology, microstructure, texture and phase transformations were followed by X-ray diffraction, DTA-TGA, Scanning and Transmission Electron Microscopy with selected area electron diffraction and N₂ physisorption. The dried gels were formed by nanometric particles closely related to AlOOH (boehmite). The Ti⁴⁺ was highly dispersed and presumably incorporated into the AlOOH structure, affecting the morphology, the aggregation state and the atomic short range order. The mixed metallic oxides calcined at 500°C were amorphous and formed by nanometric particles and pore sizes of about 2 nm. For these samples the surface area increased as the TiO₂ concentration increased especially at high Ti concentration. The sintering process started at 700°C, resulting in particle and pore sizes growing from 2 nm to about 10 nm and the powders remained amorphous. At 900°C both sintering and crystallization processes of TiO₂ and Al₂O₃ were accelerated by the Ti⁴⁺ segregation; rutile and α-alumina were present as separate phases. The textural properties were also affected, the surface area decreased and the average pore diameters increased for the same thermal treatment.