Textural properties of Al2O3-TiO2 mixed oxides synthesized by the aqueous sol method

Al2O3-TiO2(AT(x)) mixed oxides, with x = 0, 5, 10, 15 and 20 wt.% TiO2 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 degreesC. The resulting mixed oxides were thermally stable up to 900 degreesC. The crystallite size of the hydroxides and the mixed oxides was affected by the incorporation of TiO2 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 Al-27 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. (C) 2004 Elsevier Inc. All rights reserved.