THE REACTION OF HEXAMETHYLDISILAZANE AND SUBSEQUENT OXIDATION OF TRIMETHYLSILYL GROUPS ON SILICA STUDIED BY SOLID-STATE NMR AND FTIR

The reactions of hexamethyldisilazane (HMDS) on silica preheated at 200-820 degrees C were carried out using atomic layer epitaxy (ALE), a technique based on saturating gas-solid reactions. This led to highly reproducible numbers of trimethylsilyl groups bonded to the silica surface. The trimethylsilyl groups produced were subsequently oxidized in air with and without bound chromium acetylacetonate complex (SiOCr(acac)(2)) present. The silica surface at different reaction stages was characterized by carbon determination, H-1 MAS NMR, Si-29 CPMAS NMR, C-13 CPMAS NMR, and FTIR. The reaction with the terminal H-bonded OH groups, in addition to isolated OH groups, on silica preheated at lower temperatures (<450 degrees C) was confirmed, and the number of these species on 200 degrees C preheated silica was found to be around 0.4 OH/nm(2) The secondary reaction by dissociative chemisorption of ammonia was observed to occur on highly dehydroxylated silica. On pure oxidized silylated silica samples, in addition to a significant amount of geminols, silanetriol groups (OSi(OH)(3)) were also produced. Their number could be increased even further by adsorbing Cr(acac)3 prior to oxidation to create ''hot spots''. This made it possible to use a lower decomposition temperature, thus preventing the formation of siloxane bridges. The results suggest that the use of large samples (5-8 g) of silylated preheated silica, with and without an additional compound for total or partial oxidation, can lead to a stabilized modified silica surface.