Catalytic oxidation of SO2 in O2-aerated aqueous Solution: superior catalytic capability of KOH/urea-treated biochars and catalytic mechanism

The catalytic oxidation of SO2 (COS) by dissolved oxygen in aqueous solution allows to recycle sulfate salt/sulfuric acid from the SO2-containing flue gas, and high-performance catalysts are desired for an efficient COS process. In this study, the chestnut leaves-derived biochars treated by the co-pyrolysis process with KOH/urea exhibited a favorable capability to catalyze the oxidation of SO2 in aqueous solution. The BK-N-8 biochar catalyst was the optimal one due to its moderate nitrogen content and appropriate surface functional groups. It exhibited a fast initial conversion rate from SO2 to SO42− (6.50 μmol g−1 s−1), and an excellent cycling durability for the COS process. The material showed a catalytic activity that surpassed all the reported carbonaceous materials. The surface functionalities were revealed to be the decisive factor for the conversion rate and durability of catalytic capacity. The superiority of the BK-N-8 with pyridinic N-doped aromatic cluster (AC) was further investigated via density-functional-theory calculations. The epoxy groups from O2 dissociation at the edge sites of ACs played a pivotal role for COS process onto biochars. This study provides insightful information for an in-depth understanding of the mechanism of COS process catalyzed by carbonaceous materials.