Transition metal oxides and their doping for mercury removal from flue gas: the role of active oxygen and its regulation

Gaseous elemental mercury (Hg 0 ) from coal combustion poses severe environmental risks. Transition metal oxides are promising for Hg 0 removal, yet the role of active oxygen species remains unclear. This work systematically investigates the Hg 0 removal performance and mechanism of five transition metal oxides (Fe 2 O 3 , Fe 3 O 4 , MnO 2 , Mn 2 O 3 , Co 3 O 4 ) and their optimized binary transition metal oxides (Mn a Fe b O x ) prepared by mechanochemical method in N 2  + CO 2 and N 2  + O 2  + CO 2 atmospheres. The results indicate that the average Hg 0 removal efficiency under the two atmospheres follows the order of MnO 2  > Fe 2 O 3  > Co 3 O 4  > Mn 2 O 3  > Fe 3 O 4 , with oxidation escape being the primary Hg 0 removal way. The surface chemical active sites are the major factors influencing Hg 0 removal, where the initial content and consumption of oxygen vacancy are critical to their Hg 0 removal performance. MnO 2 and Fe 2 O 3 exhibit the highest Hg 0 adsorption energy and charge transfer quantity. Mechanochemically prepared Mn 3 Fe 2 O x exhibits the best Hg 0 removal performance, with 97.3% average Hg 0 removal efficiency under N 2  + O 2  + CO 2 atmosphere. Oxygen vacancy is the optimal Hg 0 adsorption active site on Fe 2 O 3 , Mn 2 O 3 , and Mn 3 Fe 2 O x , where Fe and Mn coordinate electron transfer through valence state conversion, forming Hg O and facilitating Hg 0 oxidation. The presence of O 2 replenishes the active oxygen species (lattice oxygen, adsorbed oxygen) on the surface, promoting Hg 0 adsorption by active oxygen, which thereby enhances the Hg 0 removal efficiency and Hg 0 adsorption rate. Identifying transition metal oxides with high oxygen vacancy content and ease consumption will help further improve their Hg 0 removal performance, which is vital for the development of metal oxide mercury removal adsorbents.