Small‐angle X‐ray scattering and nitrogen adsorption study of the nanoporosity elimination in TEOS sonohydrolysis‐derived xerogels

Abstract

Small‐angle X‐ray scattering (SAXS) and nitrogen adsorption techniques were used to study the temperature and time structural evolution of the nanoporosity in silica xerogels prepared from acid‐ and ultrasound‐catalyzed hydrolysis of tetraethoxysilane (TEOS). Silica xerogels present a structure of nanopores of fully random shape, size, and distribution, which can be described by an exponential correlation function γ(r) = exp (–r/a), where a is the correlation distance, as predicted by the Debye, Anderson, and Brumberger (DAB) model. The mean pore size was evaluated as about 1.25 nm from SAXS and about 1.9 nm from nitrogen adsorption. The nanopore elimination in TEOS sonohydrolysis‐derived silica xerogels is readily accelerated at temperatures around 900 °C probably by the action of a viscous flow mechanism. The nanopore elimination process takes place in such a way that the pore volume fraction and the specific surface are reduced while the mean pore size remains constant. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Nonisothermal Study of the Kinetics of the Nanoporosity Elimination in Sonogels-derived Silica Xerogels

“…18 The pore size distribution from nitrogen adsorption isotherms has been found in the range of pore-width below about 5 nm, the incremental pore volume increasing exponentially in the micropore region. 17 However, the total pore volume as measured by the volume of nitrogen adsorbed at the nitrogen saturation pressure is in agreement with the value measured directly from mass and volume determinations. This means that the pores of the xerogel structure are interconnected.…”

“…We attribute the abrupt linear shrinkage at around 1070-1300 K (depending on the heating rate) to a quantitative measurement of the kinetics of the nanopore elimination process in the sonogel-derived silica xerogels. [15][16][17] We have estimated that ∆T ∼ (cFR 2 /4k)β should be the temperature difference between the surface and the center temperature of a cylinder sample under a "steady state" heat flow with a heating rate β, where R is the sample radius, c the specific heat, F the density, and k the thermal conductivity. Assuming c ∼ 8 × 10 2 J/kg‚K and k ∼ 0.5 W/m‚K (the last as a typical value for nanoporous xerogels 19 ), together with R ∼ 3 × 10 -3 m and F ∼ 1.6 × 10 3 kg/m 3 for our samples, we obtain ∆T ∼ 1.5 K for β ) 15 K/min, the most critical heating in our case, which is even a negligible value for the present purposes.…”

“…The pore mean size and the specific surface area were evaluated as 1.3 nm and 600 m 2 /g, as determined through the DAB model 17 by small-angle X-ray scattering, and 1.9 nm and 400 m 2 /g, as determined through the BET method by nitrogen adsorption. 17 The difference between the values obtained from these techniques is that the BET method is not be so well suited to microporous solids. 18 The pore size distribution from nitrogen adsorption isotherms has been found in the range of pore-width below about 5 nm, the incremental pore volume increasing exponentially in the micropore region.…”

“…Such a nanoporous structure is readily eliminated 16,17 at temperatures higher than about 1170 K (∼900 °C), probably above of the glass transition temperature of the xerogels. It has been found that the nanoporosity elimination in the sonogelderived xerogels under an isothermal hold gives place in such a mechanism that the pore volume fraction and the specific surface are diminished while the pore mean size is kept constant.…”

“…It has been found that the nanoporosity elimination in the sonogelderived xerogels under an isothermal hold gives place in such a mechanism that the pore volume fraction and the specific surface are diminished while the pore mean size is kept constant. 17 In this work, we present an in-situ study carried out by means of dilatometric thermal analysis of the nonisothermal kinetics of the nanoporosity elimination in silica xerogels prepared from acid and ultrasound catalyzed hydrolysis of TEOS.…”