POROUS NANOCOMPOSITES AS CATALYST SUPPORTS .1. 2ND-PHASE STABILIZATION, THERMAL-STABILITY AND ANATASE-TO-RUTILE TRANSFORMATION IN TITANIA-ALUMINA NANOCOMPOSITES

Porous titania-alumina nanocomposites, with alumina contents in the range of 1 to 50 wt.-%, were prepared by mixing the respective sols to study the stabilization of the titania (matrix) phase by the alumina (second) phase. It was noted that the total stabilization effect could not be explained based on the chemical modification of the titania phase by alumina. An additional physical stabilization effect was proposed based on some circumstantial evidences. Pore structure and phase stability of these nanocomposites were studied using thermal analysis, X-ray diffraction, Raman spectroscopy, and nitrogen physisorption techniques. The presence of alumina in the composite improved the thermal stability of the titania phase by retarding the anatase-to-rutile phase transformation and the grain growth of the anatase phase. The anatase-to-rutile phase transformation temperatures for pure titania and the titania phase of the titania-alumina nanocomposite were found to be around 560 and 1000 degrees C, respectively, from the DSC data. The surface area contributed by the titania phase in the composite was calculated to be in the range of 50 to 100 m(2)/g at 600 degrees C for 8 h, whereas pure titania lost its whole surface area at this temperature and time.