Characterisation of Activated Carbons with High Surface Area and Variable Porosity Produced from Agricultural Cotton Waste by Chemical Activation and Co-activation

Abstract

Activated carbons were prepared by phosphoric acid activation of cotton stalks in nitrogen (chemical activation) and steam/nitrogen atmospheres (co-activation) at various temperatures in the 500–800°C range and at different H3PO4 acid to cotton stalk impregnation ratios (0.3–3). The BET surface area and the pore structure of activated carbons were evaluated from nitrogen adsorption data at 77 K in relation to process conditions. The results showed that the surface area and the porosity of the activated carbons were strongly dependent on the impregnation ratio, temperature and the atmosphere. Increasing impregnation ratio favours the development of mesopores, whereas the temperature has a negative effect on the surface area and porosity up to 700°C and thereafter it promotes the creation of micropores at 800°C. Steam promotes formation of narrower micropores in the activated carbons prepared at higher temperatures and the formation of mesopores at lower temperatures. Transmission electron microscopy analysis also identifies mesopore structures in the mesoporous activated carbon and microporous structures in the microporous activated carbons. The effect of activation temperature and atmosphere on surface chemistry of activated carbon, in terms of carbon, oxygen and phosphorus functionalities, was extensively studied using X-ray photoelectron spectroscopy. The results showed that carbon content increased as the activation temperature was increased from 500 to 800°C in both atmospheres and as the atmosphere was changed from nitrogen to steam/nitrogen mixture in both temperatures, while the oxygen and phosphorus contents decreased due to the removal of phosphorus compounds.