Estimation of the dimensions of micropores in active carbons using the Dubinin-Radushkevich equation

Abstract

Decreases in the characteristic energy of the Dubinin-Radushkevich equation, E 0, are correlated with increases in micropore sizes determined by molecular probes, W m , using the empirical equation W m = 4.691 exp(−0.0666 E 0); a similar equation can also be used to correlate the more limited data relating E 0 to the Guinier radius of gyration, R g . A new, semiempirical method is proposed which relates E 0 to micropore sizes calculated from adsorption potentials in slit-shaped, model micropores. The two methods for estimating micropore size are applied to adsorption of Ar at 77 K on a steam-activated series of polymer carbons. Both methods show that micropore sizes increase progressively with activation, but for each carbon, the micropore size estimated from molecular probes is greater than that obtained using potentials in model micropores. The differences are attributed to different weightings of the pore size estimates in the micropore size distributions. Estimates based upon molecular probes are weighted towards the tail of the distribution, while those based on model pore potentials are weighted towards the smallest micropores. A method for estimating the size range of micropores is proposed where the lower limit is defined by the adsorption potential maximum in model pores and the upper limit is defined by the correlation between W m and E 0.