Textural Stability of Titania-Alumina Composite Membranes

Textural evolution (porosity reduction, pore and crystallite growth) in titania-alumina composite membranes has been studied using thermal analysis, X-ray diffraction, field emission scanning electron microscopy and N, physisorption techniques. The presence of alumina in the membranes improved the thermal stability of the porous texture by retarding the anatase-to-rutile phase transformation and grain growth of the anatase phase. Pure unsupported titania membranes lose their porosity completely after calcination at 600 "C for 8 h, whereas the titania-50 wt.% alumina composite membranes retained a porosity of ca. 40% even after calcination for 30 h at 800 "C. The anatase-to-rutile phase transformation temperatures for pure unsupported titania and the titania phase of the unsupported titania-alumina composite membranes (50 wt.% alumina) were found to be 580 and 960 "C, respectively, as observed from the DSC data. From XRD results it was found that pure unsupported titania and the unsupported titania-alumina (50 wt.% alumina) composite membranes transformed to more than 95% of rutile after heat treatment for 8 h at 600 and 900 "C, respectively. The anatase to rutile phase transformation kinetics were studied using DSC data and applying a non-isothermal form of the Avrami equation.