Structure of chemical vapor deposition titania/silica gel

The structure of porous silica gel/titania synthesized using chemical vapor deposition (CVD) of titania via repeated reactions of TiCl4 with the surface and subsequent hydrolysis of residual Ti-CI bonds at different temperatures was investigated by means of low-temperature nitrogen adsorption-desorption, X-ray diffraction (XRD), IR spectroscopy, and theoretical methods. A globular model of porous solids with corpuscular structure was applied to estimate the porosity parameters of titania/silica gel adsorbents. The utilization of this model is useful, for example, to predict conditions for synthesis of titania/silica with a specified structure. Analysis of pore parameters and fractal dimension suggests that the porosity and fractality of samples decrease with increasing amount of TiO2 covering the silica gel surface in a nonuniform layer, which represents small particles embedded in pores and larger particles formed at the outer surface of silica globules. Theoretical simulation shows that the Si-O-Ti linkages between the cover and the substrate can be easily hydrolyzed, which is in agreement with the IR data corresponding to the absence of a band at 950 cm(-1) (characteristic of Si-O-Ti bridges) independent of the concentration of CVD-titania. (C) 1999 Academic Press.