Inhibition of SO₂ for Low-Temperature NO Adsorption on Activated Carbon

Activated carbon can be used for NOx removal from low to medium temperature flue gas, but SO2 inhibits NO adsorption. Isothermal adsorption experiments and temperature-programmed desorption experiments were conducted to investigate the effect of SO2 on NO adsorption at low temperatures (−20 to 30 °C). Quantum chemical calculations were performed to reveal the active sites and specific reaction pathways involved in the effect of SO2. As the temperature dropped from 30 to −20 °C, NO adsorption enhanced 4.18-fold. However, the presence of SO2 reduced the NOx adsorption amount by 96.5 to 99.1% and inhibited NO2 formation in the product gas. SO2 significantly suppressed both physical and chemical adsorption of NOx and altered the chemical adsorption forms of NOx on activated carbon. In the presence of SO2, the desorption peaks corresponding to adsorbed NO3– completely disappeared, and the peaks corresponding to adsorbed C-ONO and C-NO2 decreased dramatically. The adsorption and desorption experiments demonstrated that SO2 inhibits the oxidation of NO during the adsorption process and affects the subsequent conversion of NO2. Quantum chemical calculations indicated that gaseous SO2 and NO competitively oxidize at the same active sites near hydroxyl functional groups, with comparable reaction barriers. However, the oxidation products of SO2 are difficult to desorb, leading to site deactivation and hindering the NO oxidation reaction. Gaseous SO2 also reacts with adsorbed C-ONO on the activated carbon surface, further reducing NOx adsorption. Enhancing activated carbon’s catalytic oxidation activity for NO via surface modification could mitigate SO2’s inhibitory effect on NO adsorption.