Ammoniation and nitridation of highly disperse silica

Infrared spectroscopy, X-ray scattering intensity measurements and chemical analysis of the N content have been used to study the reaction of flowing NH3 with highly disperse silica (aerosil) at elevated temperatures. During interaction with flowing NH3 (ammoniation) at temperatures ≤ 723 K, only SiNH2 groups were formed by substitution of SiOH groups on aerosil. At temperatures higher than 773 K under flowing NH3, both SiNH2 groups and Si-NH-Si groups (silazane bridges) were formed, which could be identified clearly in the IR spectra. Silazane bridges appear at temperatures ≥ 1073 K, replacing not only surface siloxane bridges but also siloxane bridges in the bulk. With increasing reaction time, nitrogen contents up to 25 mass% were introduced into the SiO2 lattice by a reaction of aerosil with flowing NH3 at 1373 K. X-ray scattering intensity measurements have shown that the incorporation of nitrogen in the aerosil structure gives rise to a drastic rearrangement of the amorphous SiO2 structure. Both the primary structure order and the mid-range order of the aerosil were strongly changed after high temperature ammoniation. The treatment of aerosil in flowing N2 at 1373 K results in formation of cristobalite.