Textural properties of Al 2O 3–TiO 2 mixed oxides synthesized by the aqueous sol method

Abstract

Al 2O 3–TiO 2(AT x ) mixed oxides, with x=0, 5, 10, 15 and 20 wt.% TiO 2 content, were prepared by the aqueous sol method. The resulting hydroxides were dried either in a conventional air static oven or by the freeze drying (F) method, the samples were labeled as AT x and ATF x , respectively. The hydroxides were transformed into their corresponding oxides by calcination at 600 °C. The resulting mixed oxides were thermally stable up to 900 °C. The crystallite size of the hydroxides and the mixed oxides was affected by the incorporation of TiO 2 into the alumina structure. The mixed oxides obtained by both drying methods presented a greater surface area than the corresponding pure alumina phase. Furthermore, the mixed oxides dried by the freeze drying method developed a higher surface area than the corresponding mixed oxides dried by the conventional method. Noticeable effects related with the pore size of the materials obtained after the drying processes were observed: The oxides dried by freezing developed a smaller pore size with a narrower, unimodal pore size distribution (PSD) if compared to the corresponding oxides dried by the conventional method, which showed a bimodal and broad PSD. All the materials presented a type IV N 2 adsorption–desorption isotherm, characteristic of mesoporous materials. Electron microscopy studies also showed some differences in the textural and structural properties of the samples obtained under different treatments. The titanium dioxide phases were determined by electron diffraction and high resolution electron microscopy methods. The 27Al MAS-NMR spectra of the starting aluminas, AT x=0 and ATF x=0 , consisted of two broad asymmetric peaks at approximately 63.5–64 and 6.29 ppm which are associated with a tetrahedral and octahedral aluminum coordination, respectively.