Kinetics of densification of porous silica gels: a structural and textural study

The present work is devoted to the structural analysis of silica xerogel samples containing controlled pore sizes ranging from 25 to 270 A. It focuses on the kinetics of densification processes. Porous xerogels were treated in air at the fixed temperature of 1050 °C for periods varying from 10 min to 3 h and were allowed to cool down in open air. They were characterized by density and nitrogen adsorption–desorption measurements, as well as by Raman spectroscopy. The structural studies were based on the evolution of the intensities and profiles of certain characteristic bands in the Raman spectra. Among these are the two defects bands, D1 and D2 associated with the vibrations of four- and three-membered silica rings, respectively, and the ν(Si–O–Si) band centered at 430 cm−1. Analyses allowed a characterization of the structural changes occurring with densification of the amorphous matrix. In addition it was found that all modifications are brought to a halt after a characteristic time which depends on the initial porosity. It is important to note that the final microscopic structure is the same for all samples regardless of the initial pore size.