INCORPORATION OF TITANIA INTO PRESHAPED SILICA MONOLITH STRUCTURES

A series of titania-silica honeycombs with various titanium profiles across the walls of the cells have been prepared by impregnation and homogeneous precipitation-deposition methods. Although the latter method seems to be very useful to deposit an atomic layer of TiO2 on high surface area SiO2 microspheres, it does not work if the silica is preshaped in monolith structures. However, quite varied titanium profiles can already be prepared by impregnating the silica monoliths with acid solutions of TiCl3. As the transport of Ti3+ from the solution to the pores is controlled by diffusion, convex profiles are usually obtained at short impregnation times, but these tend to flatten off at very long times. Moreover, the amount of titanium incorporated can be reasonably increased by consecutive impregnation steps. Drying of impregnates is also very critical. If drying takes place at 398 K, very sharp titanium profiles are obtained owing to the fast evaporation of water, and the simultaneous Ti(OH)4 precipitation, on the outer walls surface. Scanning electron microscopy-energy dispersive X-ray and X-ray photoelectron spectroscopy techniques appear to be very useful to monitor the macrodistribution of TiO2 across the monolith walls. Further information on TiO2 distribution, including crystal size and location, is derived from X-ray diffraction and mercury porosimetry.