THE STRUCTURE OF THE SILICA SURFACE AND ITS MODIFICATION BY THERMAL-TREATMENT

The studies of the vibrational spectra and quantum-chemical calculatio ns based on cluster models of surface structures of silica are reviewe d. The process of thermal dehydroxylation of silica includes two stage s. At the first step (500 - 600-degrees-C), condensation of hydrogen-b onded surface silanol groups occurs with the formation of slightly str ained siloxane bonds. Relaxation of the framework of silica in the vic inity of the siloxane bonds decreases the strains. At the second stage (600 - 1200-degrees-C), a dramatic reconstruction of separate surface sites occurs as a result of removal of free silanol groups, and the p roperties of the remaining surface hydroxyl groups change. Electron-ac ceptor properties of the surface of modified silica are influenced by the extent of distortion of the fragments involving a modifying compon ent incorporated in the silica framework. The spectral data and the ra te of removal of surface silanol groups reflect the sate of the modify ing additive in the structure of silica.