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 degreesC, 700 degreesC and 900 degreesC. 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 N-2 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 degreesC 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 degreesC, resulting in particle and pore sizes growing from 2 nm to about 10 nm and the powders remained amorphous. At 900 degreesC both sintering and crystallization processes of TiO2 and Al2O3 were accelerated by the Ti4+ segregation; rutile and alpha -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.