Study of Alkoxide Silica Gels by Infrared Spectroscopy

The infrared spectra of silica gels made from alkoxides are different from those made from fumed silica, and from the infrared spectra of other amorphous silicas in which the SiO 2 network is relatively complete. One important feature of these spectra is the occurrence of an absorption band at 960 cm −1 due to the vibration of dangling -Si-OH bonds in the alkoxide gels, which is not present in many other silicas. There also are less well-defined absorption bands due to structural defects for the alkoxide gels, but their interpretation is speculative. Dried gels made with more concentrated reactants contain more defects, and heat treatment reduces the concentration of dangling bonds, especially above 900°C. For gels containing fluorine, two absorption bands appear at 932 and 980 cm −1 , and these are interpreted as arising from terminal -SiF and -SiF 2 groups, respectively.