Kinetics of densification of porous silica gels: a structural and texturalstudy

The present work is devoted to the structural analysis of silica xerogel sa mples containing controlled pore sizes ranging from 25 to 270 Angstrom. It focuses on the kinetics of densification processes. Porous xerogels were tr eated in air at the fixed temperature of 1050 degreesC for periods varying from 10 min to 3 h and were allowed to coot down in open air. They were cha racterized by density and nitrogen adsorption-desorption measurements, as w ell as by Raman spectroscopy. The structural studies were based on the evol ution of the intensities and profiles of certain characteristic bands in th e Raman spectra. Among these are the two defects bands, D-1 and D-2 associa ted with the vibrations of four- and three-membered silica rings., respecti vely, and the v(Si-O-Si) band centered at 430 cm(-1). Analyses allowed a ch aracterization of the structural changes occurring with densification of th e amorphous matrix. In addition it was found that all modifications are bro ught to a halt after a characteristic time which depends on the initial por osity. It is important to note that the final microscopic structure is the same for all samples regardless of the initial pore size. (C) 2001 Elsevier Science B.V. All rights reserved.