# Elemental Mercury Removal over CeO\({}_{2}\)/TiO\({}_{2}\) Catalyst Prepared by Sol-Gel Method
## 4 Conclusions  
In this study, a series of powdery CeO\({}_{2}\)/TiO\({}_{2}\) catalysts with various Ce contents, together with a pure TiO\({}_{2}\) catalyst, were prepared by sol-gel method. The influences of reaction temperature and flue gas components on the elemental mercury removal efficiency were investigated. The simultaneous removal performance of NO and Hg\({}^{0}\) and the effect of vanadium on the catalyst performance were also considered. Advanced characterization methods were employed to explore the mechanisms of NO and Hg\({}^{0}\) removal of the novel catalysts. The following conclusions can be drawn:  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Catalyst**} & \multicolumn{2}{c}{**Fraction/\%**} \\ \cline{2-3}  & **Ce\({}^{3+}\)** & **Ce\({}^{4+}\)** \\ \hline fresh Ce0.5 Ti & 26.10 & 73.90 \\ used Ce0.5 Ti & 34.35 & 65.65 \\ fresh Ce0.5 Ti-V & 25.35 & 74.65 \\ used Ce0.5 Ti-V & 34.09 & 65.91 \\ \hline \hline \end{tabular}
\end{table}
Table 6: The fraction of each type of cerium.  
Figure 11: Mechanisms of Hg\({}^{0}\) and NO removal over the catalysts.  
1. The Hg\({}^{0}\) removal efficiency of CeO\({}_{2}\)/TiO\({}_{2}\) catalyst was much higher than that of pure TiO\({}_{2}\) catalyst. The optimal reaction temperature was 350 \({}^{\circ}\)C. Ce0.2 Ti catalyst showed the best performance among all samples, since an excessive Ce loading value would cause the blockage of the catalyst.
2. Both O\({}_{2}\) and HCl would benefit the removal of elemental mercury. However, higher NH\({}_{3}\)/NO ratio and SO\({}_{2}\) concentration could bring negative effects on the Hg\({}^{0}\) removal process. When the NH\({}_{3}\)/NO ratio was set as 1:1, the Hg\({}^{0}\) removal efficiency increased firstly and then decreased as the NO and NH\({}_{3}\) concentration rose from 0 to 600 \(\mu\)L-L\({}^{-1}\).
3. The Hg\({}^{0}\) removal efficiency increased remarkably after the addition of vanadium. Elemental mercury could be oxidized by V\({}^{5+}\), which is a strong oxidant. Besides, V\({}_{2}\)O\({}_{5}\)-CeO\({}_{2}\)/TiO\({}_{2}\) catalyst showed good resistance to NH\({}_{3}\) and SO\({}_{2}\).
4. Excellent simultaneous removal performance of NO and Hg\({}^{0}\) could be obtained within the temperature window from 300 to 350 \({}^{\circ}\)C. The SCR process was reinforced, since NO could be oxidized into NO\({}_{2}\) by Ce\({}^{4+}\). NO\({}_{2}\) also has a synergistic effect on the Hg\({}^{0}\) removal over the catalysts.
5. The content of each component in the catalyst can be controlled accurately by the sol-gel method. Cerium oxides distributed evenly on the catalyst's surface unless the Ce loading value was high enough. The proportion of adsorbed oxygen in the catalyst increased significantly after Ce was added. Chemical reaction, instead of physical adsorption, would be the dominant mechanism of Hg\({}^{0}\) removal for the catalyst with high Ce content. Ce\({}^{4+}\)/Ce\({}^{3+}\) pair and V\({}^{5+}\)/V\({}^{4+}\) pair were of great importance in the removal mechanisms of NO and Hg\({}^{0}\).  
Conceptualization, Q.L.; methodology, Q.L. and M.C.; investigation, Q.L., M.C., and Y.H.; writing--original draft preparation, Q.L.; writing--review and editing, C.W. and D.C.; supervision, D.C.; project administration, D.C.; funding acquisition, C.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Natural Science Basic Research Plan in Shaanxi Province of China (2019 JM-067, 2020 JQ-061) and the Chinese Postdoctoral Science Foundation (2019T120286).  
The authors acknowledge financial support from the Natural Science Basic Research Plan in Shaanxi Province of China (2019 JM-067, 2020 JQ-061) and the China Postdoctoral Science Foundation (2019T120286).  
The authors declare no conflict of interest.