## Mercury Removal Methods
### Catalytic Oxidation  
Some transition metals or transition metal oxides can serve as catalysts to promote the oxidation of elemental mercury. Studies indicate that iron (Fe)-based materials can achieve a good Hg\({}^{0}\) oxidation efficiency [100; 101; 102]. However, some scholars have pointed out that when bituminous coal is used, Fe-based materials exhibit a lower Hg\({}^{0}\) oxidation efficiency, and the concentrations of the flue gas components can also have an impact [103; 104; 105]. [PERSON] et al. [106] prepared a novel Fe-based nanomaterial and tested its mercury removal performance in simulated flue gas. The results showed that the nano-Fe-Si material prepared by the hydrothermal method had an excellent mercury removal efficiency and adsorption capacity. The reaction occurring on the material surface was the rate-limiting step. NO and HCl were beneficial for the Hg\({}^{0}\) removal, while SO\({}_{2}\) inhibited the mercury removal  
Figure 8: SEM images of Ag nanoparticles (**a**) before Hg adsorption; (**b**) after Hg adsorption [99]. efficiency. [PERSON] et al. [107] used hematite to prepare a series of magnetic Fe-based spinel materials, which could be easily recycled and have the potential to be used in industrial applications. Manganese (Mn) exhibits various oxidation states, with a valence ranging from +2 to +6, meaning that manganese oxides possess active lattice oxygen [108]. Compared to other catalysts, Mn-based catalysts can achieve high Hg\({}^{0}\) oxidation efficiency at lower temperatures [109; 110; 111]. Studies have shown that Mn-based catalysts exhibit good catalytic effects at temperatures below 170 \({}^{\circ}\)C [112]. Mixing MnO\({}_{2}\) with Ca(OH)\({}_{2}\) or Al\({}_{2}\)O\({}_{3}\) can catalyze the Hg0 oxidation, but high concentrations of SO\({}_{2}\) could inhibit the mercury oxidation reactions [113; 114]. To address this, some scholars have proposed the addition of Mo to prevent the sulfur poisoning of Mn-based catalysts [115]. [PERSON] et al. [108] achieved a good adsorption performance by adding manganese oxides to alumina and vermiculite. [PERSON] et al. [116] used a wet impregnation method to add Mn to commercial magnesite, creating a novel adsorbent with high Hg removal efficiency at low temperatures. [PERSON] et al. [117] incorporated manganese oxides into TiO\({}_{2}\), achieving high mercury removal efficiency. The characterization results indicated that the Mn\({}^{4+}\) and lattice oxygen in oxides play a crucial role in the oxidation of elemental mercury. Copper (Cu) is a relatively low-cost transition metal element that can switch between the +1 and +2 oxidation states. Copper oxides possess oxygen storage capacity and exhibit good catalytic oxidation abilities [118]. [PERSON] et al. [119] coated porous carbon materials with Cu using a chemical deposition method. The results showed that loading Cu onto the materials led to a significant improvement in the mercury removal efficiency, despite the decrease in the specific surface area and total pore volume (Figure 9). [PERSON] et al. [120] prepared CuO\({}_{x}\)-Al\({}_{2}\)O\({}_{3}\) materials, achieving good mercury removal effects. When mixed with activated carbon in a certain proportion, it can help reduce the operational costs of ACI. [PERSON] et al. [121] modified activated coke with copper oxides, demonstrating that the modification altered the surface functional groups of the activated coke and enhanced its mercury removal capacity. Cobalt (Co) is a common catalyst component with strong catalytic oxidation abilities. Scholars have combined cobalt oxides with TiO\({}_{2}\) or Al\({}_{2}\)O\({}_{3}\), obtaining a favorable mercury removal performance [122; 123]. To further enhance the mercury removal and sulfur resistance capabilities of catalysts, researchers have mixed various metal oxides, including Mn, Cu, Co and Al, and studied the synergistic effects on their catalytic oxidation performance [124].