Investigation of the Structure of Microporous Ti-Si Mixed Oxides by X-ray, UV Reflectance, FT-Raman, and FT-IR Spectroscopies

Abstract

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-O bond distance derived from EXAFS and disrupted the normal octahedral coordination of pure titania. The shortest Ti-O 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-O bond and the formation of Ti-O-Si linkages in the mixed oxides. In situ experiments showed that adsorbed water profoundly affects the localmore » environment around surface Ti atoms. 44 refs., 11 figs., 3 tabs.« less