Diffusion diagnostics of microporosity in nanoporous amorphous silicas and porous glasses

V.A. Kreisberg, V.P. Rakcheev

Proceedings of the 6th ESG Conference ф2002 Glass Odysseyю, Montpellier (France), (2002) (in print)

ABSTRACT. At the present time the quantitative methods for diagnosing micropores (less than 2 nm) in porous systems with mesopores (2 ы 50 nm) are absent. However the availability of micropores affects many adsorptive, adhesive, mechanical and capillary properties of these systems because of high adsorptive potential of micropores. Producing micropores is possible in glass structure under the action of high energies. Polymodal nanoporosity of silica materials was studied by the new method of diffusion diagnostics based on the high-sensitive mass spectral registration of kinetics of gas desorption from pores into high vacuum at temperature of liquid nitrogen. Subjects of investigation were polydispersional samples of amorthous silica and porous glasses with different mean size of mesopores. Distinguishing in the diffusion drag of the gas transport in the pores of various morphology results in the essential difference in the type of isothermal kinetic curves of desorption of which the corresponding diffusion coefficients can be calculated. The gas diffusion drag decreases in the row from nitrogen to argon, i.e. in the direction of decreasing the value of quadrupole moment of the molecules. The computer fitting of kinetic curves of nitrogen, oxygen and argon desorption at 77.5 K by diffusion equations for spherical symmetry revealed the availability microporous substructure in mesopores silicas and glasses. For silicas with globular structure bimodal nanoporosity was detected. Trimodal nanoporous structure of the porous glasses consists essentially of transporting mesopores with mean diameter from 3.8 to 10.3 nm and two kinds of adsorbing micropores with diameter 0.3-0.4 and 0.6-0.7 nm what corresponds 1-2 diameters of gas molecules. The developed method of diffusion diagnostics enabled not only to determine mean sizes of pores but also to characterize the width of all peaks on distribution curve by pore size which depended on method of producing porous glasses by the leaching of alkali borosilicate glasses. Gas adsorption in micropores does not depend (or depends slightly) on adsorption pressure in distinction to mesopores and has a character of volume filling or condensation. By the method of diffusion diagnostics the volume of micropores of I and II types was determined. Micropore volume in porous glasses is equal to 3-6% from total pore volume what is correlated with estimations from the isotherms of nitrogen adsorption at 77.5 K by as - method. For amorthous silicas the volume of micropores depends on method of synthesis and on the size of globules. Micropores surface was also estimated for different geometrical model of pores. Gas diffusion in mesopore are mainly Knudsen diffusion although the effective diffusion coefficients are the complicated function of coefficients of Knudsen diffusion, surface diffusion coefficient and HenryІs adsorption constant. Diffusion mechanism from micropores is surface-diffusion one. Micropore diffusion coefficients depend also on pore diameter and can be related to coefficients of gas diffusion in zeolite channels of similar size. The perspective direction for the use of the method of diffusion diagnostics of pore morphology is its application for researching the systems with small pore volume - nanoporous coatings, films and surface defective layers on optical materials what is possible owing to the high sensitivity of mass spectral analysis and is not possible by common adsorption methods. In this case the algorithm of calculation will be include diffusion equations for one-sided plate symmetry.

Laboratory of Catalysis and Gas Electrochemistry