Factors controling the SO2 removal by porous carbons:: relevance of the SO2 oxidation step

Activated carbons (AC) and activated carbon fibres (ACF) with different surface chemistry and porosity have been studied to analyse the SO2 retention in presence of O-2 at room temperature. Samples surface chemistry was studied using temperature programmed desorption (TPD) and H-2 temperature programmed reaction experiments. The porous texture was determined by CO2 and N-2 adsorption isotherms at 273 K and 77 K, respectively. SO2 adsorption experiments were performed at 313 K on fresh and heat treated (N-2 1173 K) samples using gas mixtures of 2000 ppm SO2 in N-2 or 2000 ppm SO2/5% O-2 in N-2 Adsorption experiments on heat treated samples show that the presence of surface oxygen complexes impedes the SO2 adsorption and its oxidation to SO3. Additionally, no correlation has been found between the: amount of SO2 adsorbed and the number of active sites created by the evolution of oxygen complexes during heat treatment. The results obtained have been explained using the fundamentals of gas adsorption for microporous solids. The SO2 uptake is analyzed considering the SO2 oxidation to SO3 as a new variable that is strongly affected by the pore size distribution. An optimum pore size exists (i.e. pore size of about 7 Angstrom) in which the oxidation of SO2 to SO3 is favoured. A pore width enlargement decreases the conversion of SO2 to SO3 and, thus, the total amount of SO2 retained by the carbon sample. (C) 2000 Elsevier Science Ltd. All rights reserved.