A new approach to quantify the microporosity of activated carbons by analysing the N2/77 K and CO2/273 K adsorption data by the simplex flexible method

A mathematical model has been applied to N₂/77 K and CO₂/273 K adsorption isotherms for a series of activated carbons prepared by carbonising olive stones in N₂ and then activating them in CO₂ to six different levels of burn-off in the range 8–80%. Narrow and wide micropore volumes of activated carbons were calculated from the Dubinin-Radushkevich and Dubinin-Astakhov equations considering one, two and three micropore 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 parameters 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 imposed, and the parameters were estimated from each isotherm using non-linear, least-squares regression analysis. The results obtained confirm the valuable use of CO₂/273 K adsorption to quantify the narrow microporosity of activated carbons. Differences between N₂/77 K and CO₂/273 K adsorption in microporous activated carbons were due to the wide microporosity. An agreement between micropore volumes obtained from CO₂/273 K adsorption and that corresponding to one of the two distributions of micropores obtained from N₂/77 K adsorption was obtained. The Dubinin-Radushkevich equation was more successful than the Dubinin-Astakhov equation in the quantification of the microporosity with N₂/77 K and CO₂/273 K. On the other hand, the exponent n of the Dubinin-Astakhov equation was better correlated with the burn-off of the carbons than with the parameter B.