Inherent thermal regeneration performance of different MnO₂ crystallographic structures for mercury removal

Manganese oxides with different crystallographic structures were investigated for gas-phase elemental mercury removal. The inherent thermal regeneration performance and mechanism of α- and γ-MnO₂ were studied. The manganese dioxides were found to possess a mercury removal efficiency of higher than 96% even after 120 min mercury exposure except for β-MnO₂ which removed much less mercury than Mn₂O₃. The α-MnO₂ was found to have a higher recyclability of mercury capture and better durability for regeneration than γ-MnO₂. During the first 1 h of exposure, α-MnO₂ showed an excellent mercury capacity of 128 μg/g over 5 regeneration cycles. While for γ-MnO₂, the mercury capacity of the fifth cycle was reduced to 68.74 μg/g, which is much lower than 131.42 μg/g for the first cycle. The microstructure of α-MnO₂ was maintained throughout regeneration cycles due to its capability to retain lattice oxygen. In comparison, γ-MnO₂ experienced reconstruction and phase transformation induced by oxygen vacancies due to lattice oxygen loss during regeneration process, leading to a degradation in mercury capture. The α-MnO₂ oriented composite was found to be better developed into a regenerable catalytic sorbent for mercury removal from flue gases of coal-fired power plants.