A NEW APPROACH TO QUANTIFY THE MICROPOROSITY OF ACTIVATED CARBONS BY ANALYZING THE N-2 77 K AND CO2/273 K ADSORPTION DATA BY THE SIMPLEX FLEXIBLE METHOD/

A mathematical model has been applied to N-2/77 K and CO2/273 K adsorp tion isotherms for a series of activated carbons prepared by carbonisi ng olive stones in N-2 and then activating them in CO, to six differen t levels of burn-off in the range 8-80%. Narrow and wide micropore vol umes of activated carbons were calculated from the Dubinin-Radushkevic h and Dubinin-Astakhov equations considering one, two and three microp ore size distributions in each sample, and allowing a variation of the micropore volume and characteristic energy of each distribution with the burn-off. The flexible simplex method was applied to obtain the pa rameters of each distribution in the mathematical model. Generally, it was found that increasing the number of micropore size distributions above two did not significantly improve fits. Each isotherm was fitted using six parameters at most. However, various constraints were impos ed, and the parameters were estimated from each isotherm using non-lin ear, least-squares regression analysis. The results obtained confirm t he valuable use of CO2/273 K adsorption to quantify the narrow micropo rosity of activated carbons. Differences between N-2/77 K and CO2/273 K adsorption in microporous activated carbons were due to the wide mic roporosity. An agreement between micropore volumes obtained from CO2/2 73 K adsorption and that corresponding to one of the two distributions of micropores obtained from N-2/77 K adsorption was obtained. The Dub inin-Radushkevich equation was more successful than the Dubinin-Astakh ov equation in the quantification of the microporosity with N-2/77 K a nd CO2/273 K. On the other hand, the exponent n of the Dubinin-Astakho v equation was better correlated with the burn-off of the carbons than with the parameter B. Copyright (C) 1996 Elsevier Science Ltd