# Elemental Mercury Removal over CeO\({}_{2}\)/TiO\({}_{2}\) Catalyst Prepared by Sol-Gel Method
## 3 Results and Discussion
### Simultaneous Removal of Hg\({}^{0}\) and NO  
The novel CeO\({}_{2}\)/TiO\({}_{2}\) catalysts in this study are not only used to remove the gaseous elemental mercury from flue gas, its ability to reduce the NO emission also needs to be taken into account. In this section, the simultaneous removal performances of Hg\({}^{0}\) and NO from simulated flue gas over pure TiO\({}_{2}\) catalyst and CeO\({}_{2}\)/TiO\({}_{2}\) catalysts were investigated at various temperatures. The ratio of NH\({}_{3}\) to NO was 1:1, and the detailed working conditions are listed in Table 2. The results are presented in Figure 6. Similar to Hg\({}^{0}\) removal performance, pure TiO\({}_{2}\) catalyst showed much lower efficiency than other catalysts in terms of NO removal. The highest NO removal efficiency of pure TiO\({}_{2}\) was only about 26.4%. By observing the \(\eta^{\prime}\) of each CeO\({}_{2}\)/TiO\({}_{2}\) catalyst, it can be found that NO removal efficiency showed a similar trend to Hg\({}^{0}\) removal efficiency. The value of \(\eta^{\prime}\) reached its highest peak at 300 or 350 \({}^{\circ}\)C. The slight difference was that the NO removal efficiency increased more significantly than Hg\({}^{0}\) removal efficiency as the temperature rose from 150 to 200 \({}^{\circ}\)C. In general, the value of \(\eta^{\prime}\) was slightly higher than that of \(\eta\). When the reaction temperature was between 250 and 350 \({}^{\circ}\)C, the NO removal efficiencies of Ce0.1 Ti, Ce0.2 Ti, and Ce0.5 Ti were above 85%. That indicates that the novel CeO\({}_{2}\)/TiO\({}_{2}\) catalysts could provide excellent simultaneous removal performance of Hg\({}^{0}\) and NO, especially from 300 to 350 \({}^{\circ}\)C. It is worth noting that this temperature range matched well with the working zone of commercial SCR catalyst, which makes it possible to replace some of the traditional catalyst with the novel CeO\({}_{2}\)/TiO\({}_{2}\) catalysts. After being oxidized over the novel catalyst, the mercury pollutants would eventually be eliminated by other APCDs, such as WFGD and PMCDs.  
Actually, some studies have been focused on the NO removal performance of Ce catalyst due to the outstanding oxygen storage capacity and the existence of the Ce\({}^{3+}\)/Ce\({}^{4+}\) pair [29; 30]. Since Ce\({}^{4+}\) is a strong oxidant, it could react with NO, which is the dominant constituent of NO\({}_{x}\), to produce NO\({}_{2}\) and Ce\({}^{3+}\). The consumed Ce\({}^{4+}\) could be replenished by the reaction between Ce\({}^{3+}\) and O\({}_{2}\) in the flue gas. Then, the redox reaction between NO\({}_{2}\) and NH\({}_{3}\) occurs. This process could promote the NO removal in the SCR system [31]. The reaction equations are as follows:  
\[\text{NO}+2\text{CeO}_{2}\rightarrow\text{NO}_{2}+\text{Ce}_{2}\text{O}_{3} \tag{9}\]  
\[\text{Ce}_{2}\text{O}_{3}+\frac{1}{2}\text{O}_{2}\to 2\text{CeO}_{2} \tag{10}\]  
\[6\text{NO}_{2}+8\text{NH}_{3}\rightarrow 7\text{N}_{2}+12\text{H}_{2}\text{O} \tag{11}\]  
In order to confirm the oxidation of NO, the concentrations of NO and NO\({}_{2}\) were measured at the outlet after cutting off NH\({}_{3}\) from the simulated flue gas. The initial concentration of NO was 200 \(\mu\)L-L\({}^{-1}\) in each case. Part of NO was oxidized to NO\({}_{2}\) over the catalysts, as shown in Table 3. It is worth noting that NO\({}_{2}\) could enhance the elemental mercury oxidation and removal as the oxidant based on the analyses in Section 3.2.3. Considering the concentration of nitrogen oxides in flue gas is much higher than that of gaseous elemental mercury, Equation (9) has a synergistic effect on the Hg\({}^{0}\) removal over the CeO\({}_{2}\)/TiO\({}_{2}\) catalysts.