Co3O4 Nanorods with a Great Amount of Oxygen Vacancies for Highly Efficient Hg0 Oxidation from Coal Combustion Flue Gas

Oxidizing elemental mercury (Hg-0) to Hg2+ is an effective way to remove Hg-0 from flue gas. Surface-active oxygen species are considered to be important active sites in Hg-0 oxidation process. The concentration enhancement of surface-active oxygen species is a primary challenge for this technology. Oxygen vacancies can easily capture and activate gaseous oxygen, forming more surface-active oxygen species, which may lead to a better Hg-0 oxidation efficiency. Co3+ in Co3O4 can generate oxygen vacancies through the reduction of Co3+ to Co2+, and the oxygen vacancies formation process is controlled by Co2+/Co3+ ratio. Inspired by this, Co3O4 nanorods exposing (220) facet with a high Co3+/Co2+ ratio were successfully synthesized. Raman and X-ray photoelectron spectroscopy (XPS) results show that the high concentration of Co3+ leads to more oxygen vacancies. It results in a better catalytic performance for Co3O4, nanorods whose Hg-0 oxidation efficiency remains above 90% at 180 000 h(-) in the temperature range of 100-300 degrees C. After 2880 min reaction, the Hg-0 oxidation efficiency of Co3O4 nanorods reduces to about 72%, and it recovers to the original level after in situ thermal treatment at 550 degrees C, suggesting a great renewable property. Furthermore, XPS results of Co3O4 nanorods before and after the reaction show that the concentrations of Co3+ and surface-active oxygen decrease after the reaction. The reaction mechanism was revealed based on these results. Hg-0 reacts with surface-active oxygen forming HgO, and the consumed oxygen is replenished by gaseous O-2. Co3+/Co2+ redox couple can improve the electron-transfer activity to enhance the Hg-0 oxidation efficiency in the presence of O-2. The effects of flue gas components on the Hg-0 oxidation efficiency are also investigated. O-2 and NO have positive effects, while H2O and SO2 have negative effects on the Hg-0 removal process. However, Co3O4 nanorods still have an efficiency of 75% even in the presence of 8% H2O and 200 ppm SO2.