# Elemental Mercury Removal over CeO\({}_{2}\)/TiO\({}_{2}\) Catalyst Prepared by Sol-Gel Method
## 3 Results and Discussion
### Characterizations
#### 3.5.3 Valence of Selected Elements  
X-ray photoelectron spectroscopy (XPS) tests were carried out for pure TiO\({}_{2}\) catalyst and CeO\({}_{2}\)/TiO\({}_{2}\) catalysts. The O 1s XPS spectra of the samples before and after the elemental mercury test are shown \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Catalyst** & **Ti/\%** & **O/\%** & **Ce/\%** & **V/\%** & **TiO\({}_{2}\)/\%** & **CeO\({}_{2}\)/\%** & **V\({}_{2}\)O\({}_{5}\)/\%** \\ \hline pure TiO\({}_{2}\) & 76.2 & 23.6 & - & - & 99.8 & - & - \\ CeO1.1 Ti & 64.6 & 25.9 & 8.53 & - & 90.2 & 8.52 & - \\ CeO2.7 Ti & 56.0 & 26.4 & 15.9 & - & 81.4 & 16.7 & - \\ CeO5.7 Ti & 33.9 & 38.4 & 22.3 & - & 63.1 & 31.4 & - \\ Ce1.0 Ti & 21.2 & 41.9 & 28.7 & - & 45.9 & 44.9 & - \\ CeO5.5 Ti-V & 44.0 & 25.9 & 26.2 & 0.161 & 65.0 & 31.1 & 0.483 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Main compositions of the catalysts. Figure 7: Crystalline structures of the catalysts. in Figure 8. Previous studies have shown that the oxygen on the catalyst can be divided into two main forms: adsorbed oxygen and lattice oxygen [33]. Two peaks could be observed in the XPS results of O 1s. The binding energy of adsorbed oxygen (O\(\alpha\)) was between 530.8 and 531.3 eV, while the binding energy of lattice oxygen (O\(\beta\)) ranged from 529.4 to 529.8 eV. The fraction of each type of oxygen could be obtained by calculating the proportion of the integral area of each peak. The results are summarized in Table 5. It was found that the fraction of adsorbed oxygen went up as the Ce content increased. Since the adsorbed oxygen was directly involved in the elemental mercury oxidation process, the Hg\({}^{0}\) removal efficiency of the catalyst increased after the addition of Ce. In addition, the proportion of adsorbed oxygen in the catalyst was lower than that in the fresh catalyst. Especially for pure TiO\({}_{2}\) catalyst, the fraction of O\(\alpha\) dropped from 45.6% to 27.8%. The main reason is that, unlike other catalysts, Ce\({}^{3+}\)/Ce\({}^{4+}\) pair was absent in pure TiO\({}_{2}\) catalyst, which could convert lattice oxygen into adsorbed oxygen. Figure 8: \(Cont.\)  
Ce0.5 Ti and pure TiO\({}_{2}\) catalysts were selected to test the Hg XPS spectra after the elemental mercury removal experiment. The results are shown in Figure 9. The Hg-related peaks could hardly be seen in the curve of pure TiO\({}_{2}\), since its mercury removal efficiency was low, as mentioned in Section 3.1. The elemental mercury removal performance improved after the addition of Ce. Mercury XPS spectra can be divided into two peaks, one of which is located near 99.8 eV, and the other is located near 104.4 eV. The peak with lower binding energy represents Hg\({}^{0}\), while the peak with higher binding energy represents HgO [20]. It can be found that main peak of the mercury in Ce0.5 Ti belonged to HgO, which indicates that chemical adsorption was the main mechanism of the Hg\({}^{0}\) removal over Ce0.5 Ti. In order to further study the Hg\({}^{0}\) removal mechanisms and to evaluate the effects of vanadium oxides on the valence of cerium in CeO\({}_{2}\)/TiO\({}_{2}\) catalyst, Ce0.5 Ti and Ce0.5 Ti-V were chosen to conduct the XPS tests before and after the elemental mercury removal experiment. The results of Ce XPS spectra \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Catalyst**} & \multicolumn{3}{c}{**Fraction/\%**} \\ \cline{3-4}  & & **O\(\alpha\)** & **O\(\beta\)** \\ \hline \multirow{2}{*}{Ce1.0 Ti} & fresh & 79.7 & 20.3 \\  & used & 69.4 & 30.6 \\ \multirow{2}{*}{Ce0.5 Ti} & fresh & 76.3 & 23.7 \\  & used & 64.3 & 35.7 \\ \multirow{2}{*}{Ce0.2 Ti} & fresh & 73.0 & 27.0 \\  & used & 60.0 & 40.0 \\ \multirow{2}{*}{Ce0.1 Ti} & fresh & 54.2 & 45.8 \\  & used & 48.0 & 52.0 \\ \multirow{2}{*}{pure TiO\({}_{2}\)} & fresh & 45.6 & 54.4 \\  & used & 27.8 & 72.2 \\ \hline \end{tabular}
\end{table} 
Table 5: The fraction of each type of oxygen.