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

Abstract

Oxidizing elemental mercury (Hg0) to Hg2+ is an effective way to remove Hg0 from flue gas. Surface-active oxygen species are considered to be important active sites in Hg0 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 Hg0 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 Hg0 oxidation efficiency remains above 90% at 180 000 h– in the temperature range of 100–300 °C. After 2880 min reaction, the Hg0 oxidation efficiency of Co3O4 nanorods reduces to about 72%, and it recovers to the original level after in situ thermal treatment at 550 °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. Hg0 reacts with surface-active oxygen forming HgO, and the consumed oxygen is replenished by gaseous O2. Co3+/Co2+ redox couple can improve the electron-transfer activity to enhance the Hg0 oxidation efficiency in the presence of O2. The effects of flue gas components on the Hg0 oxidation efficiency are also investigated. O2 and NO have positive effects, while H2O and SO2 have negative effects on the Hg0 removal process. However, Co3O4 nanorods still have an efficiency of 75% even in the presence of 8% H2O and 200 ppm SO2.