## 3 Results and Discussion
### Mercury Capture Mechanism  
Finally, XPS analysis was conducted to explore the adsorption mechanisms of Co\({}_{4}\)Fe\({}_{1}\)-ZSM-5. Figure 12a,b shows the Co 2p spectra of the fresh and spent samples. It can be discovered from the figures that both ratios of Co\({}^{3+}\)/Co\({}^{2+}\) (from 1.37 to 0.84) and Fe\({}^{3+}\)/Fe\({}^{2+}\) (from 1.29 to 0.93) declined after Hg\({}^{0}\) adsorption, implying that Co\({}^{3+}\) and Fe\({}^{3+}\) play a crucial role in the removal of Hg\({}^{0}\) and undergo a redox-reduction reaction to a lower valance state [2]. As can be seen from Figure 12c, the asymmetry of O 1s spectra suggested the existence of various oxygen species. After curve-fitting, those peaks in the range of 530.2-530.5 eV could be indexed to the lattice oxygen of metal oxides (denoted as O\({}_{\alpha}\)), while the peaks at 532.9-533.2 eV were associated with the lattice oxygen of ZSM-5 (denoted as O\({}_{\beta}\)), as shown in Figure 12c [17]. According to Table 2, the concentration of O\({}_{\alpha}\) dropped to 5.9%, suggesting that the lattice oxygen of metal oxides was primarily responsible for the oxidation of Hg\({}^{0}\). As for Hg 4f spectra, there were two photoelectron peaks detected at 103.2 and 104.0 eV after deconvolution (Figure 12d). The former can be assigned to Si 2p for SiO\({}_{2}\), while the latter was attributable to Hg 4f\({}_{5/2}\) for HgO, being consistent with the Hg-TPD result shown in Figure 11a that the captured mercury species existed mainly in the form of HgO on the adsorbent surfaces. Moreover, it can be concluded that the Mars-Maessen reaction routine is dominant in the Hg\({}^{0}\) adsorption process over Co\({}_{4}\)Fe\({}_{1}\)-ZSM-5 (Equations (5)-(12)) [45; 54; 55]. More specifically, Hg\({}^{0}\) is first physically adsorbed on the adsorbent surface, which is followed by the combination with lattice oxygen of metal oxides to form HgO. Meanwhile, Co\({}^{3+}\) and Fe\({}^{3+}\) are reduced. Finally, gaseous O\({}_{2}\) acts as an oxidant to re-oxidize the reduced cobalt and iron species into Co\({}^{3+}\) and Fe\({}^{3+}\). Figure 11: (**a**) Hg-TPD curves on sorbent at 3 \({}^{\circ}\)C/min. (**b**) Effect of regeneration temperature on Hg\({}^{0}\) removal performance. (**c**) Effect of regeneration atmosphere on Hg\({}^{0}\) removal performance. (**d**) Hg\({}^{0}\) removal efficiency of the sample over five repeated adsorption-regeneration; condition: N\({}_{2}\) + 5% O\({}_{2}\) + 200 ug/m\({}^{3}\) Hg\({}^{0}\). Reaction condition: N\({}_{2}\) + 5% O\({}_{2}\) + 200 \(\mu\)g/m\({}^{3}\) Hg\({}^{0}\). \[\rm{Hg^{0}(g)\to Hg^{0}(ads)} \tag{5}\]  
\[\rm{Hg^{0}(ads)+Co_{x}O_{y}\to Hg\text{-}O\equiv Co_{x}O_{y-1}} \tag{6}\]  
\[\rm{Hg^{0}(ads)+Fe_{x}O_{y}\to Hg\text{-}O\equiv Fe_{x}O_{y-1}} \tag{7}\]  
\[\rm{Hg^{0}(ads)+Co_{x}O_{y}\to Hg\text{Co}_{x}O_{y}} \tag{8}\]  
\[\rm{Hg^{0}(ads)+Fe_{x}O_{y}\to Hg\text{Fe}_{x}O_{y}} \tag{9}\]  
\[\rm{HgO\;(ads)\to HgO\;(g)} \tag{10}\]  
\[\rm{Co_{x}O_{y-1}+1/2O_{2}\to Co_{x}O_{y}} \tag{11}\]  
\[\rm{Fe_{x}O_{y-1}+1/2O_{2}\to Fe_{x}O_{y}} \tag{12}\]  
On the basis of this result, it can be concluded that adsorbent oxidative ability contributed to the capture of Hg\({}^{0}\) for the existence of the adsorbed mercury species in the form of Hg-O-M compounds. On the basis of the H2-TPR profiles shown in Figure 7, Co4 Fe1-ZSM-5 adsorbent exhibited a relative high oxidative ability, which, to some extent, had a positive effect on the oxidation of Hg\({}^{0}\) and thus stimulated the capture of Hg0. Meanwhile, the optimized textural structures could be beneficial for the mass transfer of the reactant, enabling Co4 Fe1-ZSM-5 with a satisfactory Hg\({}^{0}\) removal performance. In addition, Co4 Fe1-ZSM-5 was found to effectively capture Hg\({}^{0}\) in SO2- or NO-containing flue gas. It possesses a superior recycle attribute, hindering that this material can be a potential adsorbent for the practical application. Figure 12: XPS spectra of fresh and spent Co\({}_{4}\)Fe\({}_{1}\)-ZSM-5: (a) Co 2p, (b) Fe 2p, (c) O1s, (d) Hg 4f.