Single-atom Co-N-C catalyst for efficient Hg-0 oxidation at low temperature

Oxidation of Hg-0 to Hg2+ is considered to be a promising method to remove Hg-0 from flue gas. Chemisorbed oxygen on catalyst surface is the main active site during Hg-0 oxidation process. Thus, how to efficiently activate gaseous oxygen forming more chemisorbed oxygen is a big challenge in this field. Inspired by this, metal Co based catalysts with an aggregation state of single Co atoms were prepared by pyrolysis of the redesigned bimetallic Zn/Co metal-organic frameworks precursor. SEM, TEM, HR-TEM and XRD results show that the catalysts have a high Co dispersion degree with lower Co doping content. HAADF-STEM has confirmed that Co-2.5-SAs@NC (NC stands for nitrogen-doped carbon) has the aggregation state of single Co atoms. EXAFS results show that single Co atoms mainly coordinate with N and stay in Co-N-2. Furthermore, DFT results determine that the adsorption energy of O-2 on Co-N-2 is much higher than that on the surface of traditional metal oxide catalyst Co3O4. It means single Co atoms have a better ability of transforming gaseous oxygen into chemisorbed oxygen. Thanks to the highly coordinated unsaturated state of single Co atoms and the better ability of transforming gaseous oxygen into chemisorbed oxygen, Co-2.5-SAs@NC gains an excellent Hg-0 oxidation efficiency of near 100% at high GHSV of 180, 000 h(-1) even at a low temperature of 60 degrees C. With these advantages, Co-2.5-SAs@NC appears to be a promising catalyst for Hg-0 oxidation from flue gas.