Mechanism of O2-promoted NO adsorption on activated carbon: An experimental and computational study

Activated carbon can be used to control NOx emissions in low-temperature flue gas, while improving its performance is challenging. NO adsorption on activated carbon was found significantly promoted with the presence of O2. Based on the analysis of NO adsorption characteristics under different atmospheres and corresponding desorption characteristics, NO oxidation, NO2 migration and further conversion on activated carbon are proposed to cause the significant increase in NO adsorption capacity. Quantum chemical calculations conducted on the adsorption and reaction pathways of NO, O2, and NO2 on different carbonaceous models clarified that the pathway of O2-promoted NO adsorption on activated carbon can be divided into two stages. In the first stage, O2 adsorbs on active carbon atoms near the hydroxyl functional group and undergoes an oxidation reaction with gaseous NO to generate NO2. In the second stage, the desorbed NO2 is re-absorbed on the saturated carbon atoms; the N-down adsorption configuration of NO2 will not convert, while the O-down adsorption configuration of NO2 will oxidize the active site to form C-O and release NO; the C-O further reacts with the gaseous NO2 to form NO3- which stably remains. There are two types of active sites involved in the pathway. The active carbon atoms near the hydroxyl functional groups mainly play a catalytic role in the oxidation of NO, while the saturated carbon atoms mainly store nitrogen-containing components. The results can help improve the performance of activated carbon in adsorbing NO in low-temperature flue gas.