SURFACE CHARACTERIZATION OF MICROPOROUS SOLIDS WITH HE ADSORPTION AND SMALL-ANGLE X-RAY-SCATTERING

The adsorption isotherms of He at 4.2 K and N2 at 77 K on pitch-based activated carbon fiber (ACF) and ACF treated at 1473 K in Ar (ACF-1473) were determined. Also their small angle X-ray scattering (SAXS) spectra were measured in air. The amount of He adsorption expressed in terms of the adsorbed volume using Steele's theoretical density of the adsorbed He layer was compared with that of N2 adsorption. The He adsorption isotherm had much steeper uptake below P/P0 = 0.02; the amount of He adsorption below P/P0 = 0.02 was greater than that of N2 adsorption by more than 50%; heat-treatment of ACF increased the difference between He and N2 adsorption at low pressure. Both He and N2 adsorption isotherms were analyzed by the Gaussian distribution. The N2 adsorption provided completely different peak positions of the micropore size distribution of ACF and ACF-1473, while He adsorption led to slight difference in the mean micropore width. The presence of necked micropores in ACF-1473 was evidenced by the distinct adsorption behaviors of He and N2. The distribution of the Guinier gyration radius was determined from SAXS data and the mean Guinier gyration radius was obtained from the Maxwellian distribution. The mean gyration radius and the width of the slit-shaped scattering entity by the thickness plot of the SAXS data gave an average geometry of micropores. The slit width from the thickness plot was very close to that from molecular adsorption. The heat treatment did not change the micropore geometry of ACF from the SAXS analysis. The difference between molecular adsorption and SAXS results on the heat treatment was attributed to the necked pore structure.