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The effect of SiO 2 addition on the anatase-to-rutile phase transition was investigated by DTA, XRD, FTIR, and XPS. TiO 2 xerogels containing SiO 2 up to 20 mol% were prepared by mixing and hydrolyzing titanium tetraisopropoxide (TTIP) and tetraethylorthosilicate (TEOS) with HNO 3 as a catalyst. With increased amounts of SiO 2 in the xerogels, the following results were obtained: (1) the crystallization temperature of anatase increased from 415°C in pure TiO 2 to 609°C in 20-mol%-SiO 2 -containing xerogel in the DTA curves; (2) the formation temperature of rutile, according to quantitative XRD analysis, increased with increased SiO 2 content up to 5 mol% SiO 2 but became constant at higher SiO 2 contents; (3) the crystallinity of anatase became lower; and (4) the lattice parameter a of the anatase decreased slightly, but the parameter c decreased greatly up to 20 mol% SiO 2 . Although the added silicon atoms were considered from these results to be incorporated into the amorphous TiO 2 and anatase structures, the 29 Si MAS NMR spectra of the xerogels containing 10 mol% SiO 2 showed only tetrahedral silicon, with no indication of silicon in octahedral coordination. When calcined at higher temperatures, the xerogel showed polymerization of the SiO 4 tetrahedra in the NMR spectra and the Si-O-Si vibration in the FTIR spectra. The chemical composition of the xerogel surfaces, measured using XPS, showed increased SiO 2 content with increased calcining temperature, indicating the expulsion of silicon from inside the particles to form an amorphous SiO 2 surface layer. The formation of this amorphous SiO 2 surface layer was considered to be important in retarding the anataseto-rutile phase transition by suppressing diffusion between anatase particles in direct contact and limiting their ability to act as surface nucleation sites for rutile. These effects of silicon additions were similar to those observed in the ␥-Al 2 O 3 -to-␣-Al 2 O 3 transition.

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Structural models previously proposed for metakaolinite are examined in light of the most recent published experimental data and new information obtained by solid-state highresolution 29Si and 27AI NMR. A new model for metakaolinite is proposed, consisting of anhydrous regions of distorted AI-0 tetrahedra containing randomly distributed isolated residual hydroxyls associated with A1-0 configurations of regular octahedral and tetrahedpal symmetry. Such a structure, which can readily be formed from kaolinite by the removal of hydroxyls in certain sequences, accounts for the lack of a well-defined X-ray pattern and the persistence of =lo% residual hydroxyls in metakaolinite and is consistent with the most recent density data, bond lengths, and the new 29Si and z7AI NMR data.

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