Critical effect of sol ageing conditions on thermal stability and organization of titania mesoporous powder

Abstract

An attempt has been made to study the effect of sol ageing time and temperature on the mesostructural order, thermal stability, and adsorption–desorption behavior of mesoporous titania prepared by the templating method. Two different sets of sols were prepared: the first group were aged for 0.0, 1, 2, 3, 4, 5, 6, or 19 h at room temperature and the second set were aged at 50 °C for 0.0, 1, 4, 8, or 14 days. The gels obtained were subsequently subjected to humidity treatment and drying followed by calcination at 350, 400, 450, or 500 °C for 2 h. Dynamic light scattering (DLS), small-angle X-ray diffraction, wide-angle X-ray diffraction, Raman spectroscopy, and high-resolution transmission microscopy, among other techniques, were used to characterize the samples. DLS studies revealed the critical effect of ageing conditions (time and temperature) on micellar aggregation and arrangement of the prepared sols. Whereas room temperature sol ageing for different times (0.0–19 h) resulted in the formation of large aggregates/clusters, smaller micellar aggregates were formed by ageing at 50 °C. A fully ordered hexagonal mesostructure (p6 mm) was obtained when calcined samples were prepared from sols aged for 14 days. However, for calcined samples prepared from unaged sol a wormlike structure was observed. Moreover, for sols aged at 50 °C, increasing the ageing period improved the mesostructural order, which led to higher specific surface areas, pore volumes, mesoporosity, and thermal stability. Finally, it was shown that the size and size distribution of the oxo-clusters/aggregates formed in the prepared sols and their subsequent complementary condensation during drying had a substantial effect on the final mesostructural order of the calcined samples.