Pore characteristics of activated carbons from the phosphoric acid chemical activation of cotton stalks

Activated carbons were prepared by phosphoric acid activation of cotton stalks in a nitrogen atmosphere at various temperatures in the 500–800 °C range and at different H3PO4 acid to cotton stalk impregnation ratios (0.3–3). In addition pyrolysis was undertaken in a thermogravimetric analyser in the presence of different ratios of phosphoric acid in order to establish the nature of the biomass to carbon transformations involved. It was established that the total activation process of H3PO4-impregnated cotton stalks occurred in four stages with the main degradation at 740 °C, compared with 330 °C for raw cotton stalks. The effects of impregnation ratio and activation temperature on the yield and adsorption capacities of activated carbon were evaluated. The chemical composition of the carbons was investigated by elemental analysis and infrared spectroscopy. The impregnation ratio and activation temperature show a strong influence on the yields and the porous texture of the resultant activated carbons. It was demonstrated that increasing impregnation ratio favours the development of mesopores especially at high activation temperature. The activated carbons showed BET surface areas ranging from 330 to 1720 m2 g−1, total pore volumes of 0.15–1.23 cm3 g−1 with mesopore volumes between 0 and 0.61 cm3 g−1. Results suggest the practical feasibility of phosphoric acid activation of cotton stalks, which produces high quality activated carbons with high fractions of micropores and mesopores.

► High surface area (1720 m2 g−1) activated carbons have been produced.
► Process conditions can be altered to produce microporous or mesoporous carbons.
► Four stages of thermal degradation of the biomass have been identified.
► Phosphates interact with the biomass influencing pore formation.