Trimodal nanoporosity of porous glasses

V.A. Kreisberg, V.P. Rakcheev, T.V. Antropova

Proceedings of Sixth Seminar фPorous Glasses ы Special Glassesю, Szklarska Poreba (Poland), (2002) (in print)

ABSTRACT. The morphology problems of porous glass structure are extremely actual in studies of properties and applications of porous glasses. The quantitative methods for diagnostics of bi- and trimodal nanoporosity including mesopores with size from 2 to 50 nm and micropores with size less than 2 nm are absent at the present time. For study of polymodal nanoporosity of glasses the new method of diffusion diagnostics of micro- and mesoporosity of solids and films was developed. The method is based on the high-sensitive mass spectral registration of kinetics of gas desorption from pores of glass into high vacuum at temperature of liquid nitrogen. Three samples of porous glasses with various size of mesopores were investigated by the suggested method of diffusion diagnostics. The samples were produced by the leaching of alkali borosilicate glasses. The fraction 0.1ы0.135 mm of porous glasses were studied. Preliminary specific surface, mesopore volume, micropore volume and mesopore-size distribution for these samples were determined from the isotherms of nitrogen adsorption at 77.5 K in the ranges of monolayer and polymolecular condensation by фMicromeritics ASAP 2010ю. The kinetic curves of nitrogen, oxygen and argon desorption into vacuum at temperature 77.5 K from three samples of porous glasses were obtained. 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 row 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 not only detected the trimodal structure of pores but also characterized the width of peaks on distribution curve by pore size. 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. Gas diffusion in mesopores are mainly Knudsen diffusion although the effective diffusion coefficients are complicated function of coefficients of Knudsen diffusion, surface diffusion coefficient and HenryІs adsorption constant. Mesopore diffusion coefficient decreases with decreasing both the mean diameter of pore entrance and the pore diameter in maximum of mesopore distribution. Adsorption in micropores depends only slightly on adsorption pressure (in distinction to mesopores) and has volume character. Diffusion mechanism in micropores is not Knudsen one but 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. By method of diffusion diagnostics the volume of micropores of I and II types in investigated porous glasses. Micropore volume is equal to 3-6% from total pore volume of the glasses. Micropores surface is also estimated for different geometrical model of pores. The developed method of diffusion diagnostics of pores owing to the high sensitivity of mass spectral analysis is advisable to use in future for researching porous coatings and films for which the common adsorption methods are not acceptable due to small pore volume.

Laboratory of Catalysis and Gas Electrochemistry