# Elemental Mercury Removal over CeO\({}_{2}\)/TiO\({}_{2}\) Catalyst Prepared by Sol-Gel Method
## 3 Results and Discussion
### The Effect of Vanadium on Catalytic Performance  
In this study, unlike the CeO\({}_{2}\)/TiO\({}_{2}\) catalysts, V\({}_{2}\)O\({}_{5}\)-CeO\({}_{2}\)/TiO\({}_{2}\) catalyst was also prepared by sol-gel method. Vanadium was added into Ce0.5 Ti to obtain Ce0.5 Ti-V, which contained 0.5 wt% V\({}_{2}\)O\({}_{5}\). The effect of vanadium on the elemental mercury removal process over the catalyst was studied under various working conditions. The results can be found in Figure 5. In the temperature zone from 150 to 450 \({}^{\circ}\)C, the Hg\({}^{0}\) removal efficiencies of Ce0.5 Ti and Ce0.5 Ti-V respectively ranged from 36.1% to 79.3% and 69.1% to 96.3%, as shown in Figure 5a. The Hg\({}^{0}\) removal performance of catalyst increased obviously after the addition of vanadium. Specifically, the value of \(\eta\) was over 95% when the temperature was between 250 and 400 \({}^{\circ}\)C. Figure 5b shows the Hg\({}^{0}\) removal efficiencies of the catalysts under various O\({}_{2}\) concentrations. It can be noted that the trends of the two curves were similar as the O\({}_{2}\) concentration increased. The Hg\({}^{0}\) removal efficiency was higher in the presence of O\({}_{2}\), but \(\eta\) went up slowly with the further increase of O\({}_{2}\) concentration. In comparison, the elemental mercury removal performance of the catalyst improved after the addition of vanadium. According to the literature, vanadium oxides could perform similar functions as the cerium oxides in the process of elemental mercury removal [14]. Since vanadium is similar to cerium, which has many valence states (V\({}^{5+}\) and V\({}^{4+}\)), it can also promote the oxidation and removal of elemental mercury through the Mars-Maessen mechanism. V\({}^{5+}\) could take part in the redox reaction as the oxidant because of its strong oxidation. Then, the production, V\({}^{4+}\), could react with the oxygen in the flue gas and turn back to V\({}^{5+}\) again. The process can be described as follows:  
\[\text{Hg}^{0}_{(\text{g})}\to\text{Hg}^{0}_{(\text{ads})} \tag{6}\]  
\[\text{Hg}^{0}_{(\text{ads})}+\text{V}_{2}\text{O}_{5}\to\text{HgO}+\text{V}_ {2}\text{O}_{4} \tag{7}\]  
\[\text{V}_{2}\text{O}_{4}+\frac{1}{2}\text{O}_{2}\to\text{V}_{2}\text{O}_{5} \tag{8}\]The effect of HCl concentration on the Hg\({}^{0}\) removal efficiency is shown in Figure 5c. Higher concentration of HCl promoted the elemental mercury removal performances of both catalysts. Ce0.5 Ti-V showed higher efficiency than Ce0.5 Ti because vanadium oxides, just like cerium oxides, could provide abundant lattice oxygen, which is quite important for the oxidation of Hg\({}^{0}\). The process could also be explained by Equations (6)-(8). When the ratio of NH\({}_{3}\) to NO was 1:1, the efficiency of Ce0.5 Ti-V remained higher than 91% under various NO and NH\({}_{3}\) concentrations, as shown in Figure 5d. Similar results could be found in Figure 5e, which shows the effect of NH\({}_{3}\) to NO ratio on \(\eta\) of these two catalysts. The Hg\({}^{0}\) removal efficiency of Ce0.5 Ti-V still went down as the ratio of  
Figure 5: Catalytic performance after the addition of vanadium; **(a)** the effect of temperature; **(b)** the effect of O\({}_{2}\); **(c)** the effect of HCl; **(d)** the effect of NO and NH\({}_{3}\); **(e)** the effect of the ratio of NH\({}_{3}\)/NO; **(f)** the effect of SO\({}_{2}\). NH\({}_{3}\) to NO increased. However, compared with Ce0.5 Ti, the catalyst with the addition of vanadium showed much better resistance to higher NH\({}_{3}\) to NO ratio. The elemental mercury removal efficiency of Ce0.5 Ti-V was still higher than 95% when the NH\({}_{3}\) to NO ratio was 1.2. Figure 5f shows the influence of SO\({}_{2}\) concentration on the Hg\({}^{0}\) removal efficiency. As mentioned in Section 3.2.5, \(\eta\) would decline with the introduction of SO\({}_{2}\), since the functional groups on the catalyst's surface would be affected.