INVESTIGATION OF THE STRUCTURE OF MICROPOROUS TI-SI MIXED OXIDES BY X-RAY, UV REFLECTANCE, FT-RAMAN, AND FT-IR SPECTROSCOPIES

A series of high surface area, microporous, Ti-Si mixed oxides having a wide range of elemental compositions was prepared by a sol-gel method. Since the materials were noncrystalline, X-ray absorption spectroscopy at the Ti K edge was used to probe the first-shell coordination environment around Ti. Incorporation of Si into the mixed oxide shortened the average Ti-0 bond distance derived from EXAFS and disrupted the normal octahedral coordination of pure titania. The shortest Ti-0 bond distance was 1.82 Angstrom and was attributed to Ti primarily residing in tetrahedral sites, which was also indicated by a significant pre-edge peak present in Ti K edge XANES. Shifts in the UV absorption edges for the mixed oxides were attributed to titania domain sizes decreasing with increasing Si content in accord with the well-known quantum size effect. The smallest domain size was estimated as less than 1 nm for a sample with a Ti:Si atomic ratio of 1:8. Lattice vibrations in the materials were probed with FT-Raman and FT-IR absorption spectroscopies, and the results were consistent with a shortening of the Ti-0 bond and the formation of Ti-0-Si linkages in the mixed oxides. In situ experiments showed that adsorbed water profoundly affects the local environment around surface Ti atoms.