Adsorption and Adsorbed Species of SO₂ during Its Oxidative Removal over Pitch-Based Activated Carbon Fibers

Adsorption and adsorbed species of SO 2 in humid air during its oxidative removal were studied over pitch-based activated carbon fibers (ACFs) to clarify the reaction scheme of continuous removal of SO 2, influence of reaction variables, and origin of activity enhancement by the calcination in inert atmosphere. The temperature-programmed desorption of SO 2 adsorbed species suggests that when both oxygen and water were present, the main species on ACF was hydrated SO 3 (H 2 SO 4 ), which desorbed in the form of SO 2 around 300 °C, and that the presence of both increased the amount of species by oxidizing and hydrating the adsorbed SO 2, which desorbed around 60 °C without oxidation and 200 °C with oxidation but without hydration. The amount of adsorbed H 2 SO 4 over ACF stayed at the same level after 25 h of reaction by balancing elution and adsorption of H 2 SO 4 to allow the steady removal of SO 2 . More water enhanced the adsorption of SO 2 at the initial stage but reduced the amount of adsorbed H 2 SO 4 at the steady state, eluting out H 2 SO 4 faster than the adsorption of SO 2 to keep more active sites open over ACF at the steady state. Hence, complete removal was achieved. A higher calcination temperature significantly increased the rate and amount of both oxidative and oxidative/hydrating adsorption, explaining the largest activity of ACF calcined at 1100 °C. The amounts of adsorbed SO 2 and evolved CO from OG-20As by calcination at 500−1000 °C are well correlated, indicating that the evolved CO creates an adsorption site on the ACF surface. A similar but different correlation was obtained among ACFs of different surface areas calcined at different temperatures.