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Porous silica gels and TiO2/SiO2 mixed oxides prepared via the sol-gel process: characterization by spectroscopic techniques Article uri icon

abstract

  • Titania/silica mixed oxides prepared by sol-gel techniques are characterized by diffuse reflectance FTIR, Raman and 29Si solid state CP/MAS-NMR spectroscopy. The degree of cross-linking of the silica network is inferred from the frequency of the v(Si{single bond}O{single bond}Si) skeletal vibration. Gels prepared by complete hydrolysis of the alkoxide precursors (tetra-ethoxy-orthosilicate, TEOS, and tetra-isopropoxy-orthotitanate, TIOT, respectively) prior to mixing exhibit TiO2 crystallites embedded in an amorphous silica matrix. Raman spectroscopy is used to study the dependence of TiO2 crystallite size on hydrolysis pH. In an alternative two-stage hydrolysis procedure, 2 mol of water are first added per mole of TEOS, followed by mixing with the appropriate amount of TIOT, and completion of the hydrolysis. No crystalline domains are detected in the resulting oxides. The formation of Si{single bond}O{single bond}Ti bridges, as monitored by the intensity of a characteristic 945-960 cm-1 vibration, is particularly prominent if both steps of the two-stage hydrolysis procedure are performed in an acidic medium. A redispersion treatment in base, designed to induce a pore ripening process, if found to be only effective when applied to the wet gel, i.e., prior to drying. 29Si NMR experiments are used to investigate the processes occurring upon drying at 873 K. The extent of condensation reactions, by which Si{single bond}O{single bond}Si bridges are formed with concomitant desorption of water, is inferred from the fraction of completely cross-linked Si({single bond}O{single bond}Si)4 sites. 1H-29Si cross-polarization dynamics are found to depend strongly on the degree of cross-linking, as well as on the number of available surface silanol groups. © 1992 Elsevier Science Publishers B.V. All rights reserved.

publication date

  • 1992-01-01