TEXTURAL EVOLUTION AND PHASE-TRANSFORMATION IN TITANIA MEMBRANES .1. UNSUPPORTED MEMBRANES

Textural evolution in sol-gel derived nanostructured unsupported titania membranes has been studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermal gravimetry (TG), X-ray diffraction (XRD) and N2 adsorption. The anatase-to-rutile phase transformation kinetics were studied using the Avrami model. The precursor gel had a surface area of ca. 165 m2 g-1, which after heat treatment at 600-degrees-C for 8 h reduced to zero. Undoped titania-gel layers transformed to more than 95% rutile after calcination at 600-degrees-C for 8 h. The causes of surface-area reduction and pore growth were anatase crystallite growth and the enhanced sintering of rutile during transformation. Lanthanum oxide was identified as a suitable dopant for shifting the transformation temperature to ca. 850-degrees-C. Lanthanum oxide doped titania showed an improved stability of porous texture compared to that of the undoped titania membranes.