Improvement of CO2 uptake of activated carbons by treatment with mineral acids

Abstract

Three commercial activated carbons (ACs) supplied by Carbon, Norit and Desotec were modified by acid treatments. A significant CO2 capacity improvement of up to 36% was achieved. It was found that higher CO2 uptakes were related to removal of inorganic impurities i.e. mineral matter and increase of the submicropores volume i.e. smaller than 0.8nm. Sorbents were thoroughly characterized in terms of texture and chemical composition. CO2 uptake was determined by BET isotherm measurement carried out at 273K and pressure of up to 0.95 bar. The enhancement of CO2 uptake was observed especially for ACs with the highest initial mineral matter content. It was found that mineral matter in ACs plays the role of a ballast and additionally blocks adsorption places and submicropores volume thus lowers CO2 uptake. On the other hand it was proven that mineral matter may prevent the structure from collapsing and thus can stabilize high SSA. The AC surface oxidation effect on CO2 uptake was determined by comparison of ACs treated with HCl and HNO3. Both acids remove mineral matter content to the same degree, but detailed X-ray photoelectron spectroscopy (XPS) study revealed that in case of HNO3 treatment, AC surface was more oxidized which correlated with higher CO2 uptake. However observed uptake increase was attributed to increase of the submicropores volume.