## 3 Result and Discussion  
### Characterization of Catalysts  
Figure 1 shows the FT-IR spectra of ammonium molybdate (a), MoO\({}_{3}\) (b), and Ce/MoO\({}_{3}\)/SiO\({}_{2}\) (c). Curve (a) shows an ammonium ion peak at 1404 cm\({}^{-1}\), the peaks at 889 cm\({}^{-1}\), 839 cm\({}^{-1}\), and 642 cm\({}^{-1}\) are the molybdate ions peaks. Curve (b) shows that the peaks below 1000 cm\({}^{-1}\) are the peaks of the molybdenum oxygen species, and the peak at 988 cm\({}^{-1}\) is the peak of the Mo=O. The peaks at 878 cm\({}^{-1}\), 821 cm\({}^{-1}\), and 673 cm\({}^{-1}\) are the peaks of the Mo-O-Mo [22]. Curve (c) shows that the peaks at 805 cm\({}^{-1}\) and 1097 cm\({}^{-1}\) are the infrared absorption of the carrier silica [23], which caused the molybdenum oxygen species to be covered. It was indicated that in the preparation process of catalyst, ammonium molybdate had been decomposed into MoO\({}_{3}\) and highly dispersed on the catalyst. Catalytic performance for the catalyst was provided by the MoO\({}_{3}\) active center.  
Figure 2 shows the powder X-ray diffractions of ammonium molybdate (a), MoO\({}_{3}\) (b), and Ce/MoO\({}_{3}\)/SiO\({}_{2}\) (c). Curve (b) shows the diffraction peaks at 2\(\theta\) = 12.6\({}^{\circ}\), 18.8\({}^{\circ}\), 22.3\({}^{\circ}\), 25.1\({}^{\circ}\), 26.2\({}^{\circ}\), 27.2\({}^{\circ}\), 29.6\({}^{\circ}\), 30.5, and 38.0 that are attributed to MoO\({}_{3}\)[24]. Curve (c) shows no  
Figure 1: FT-IR spectra of ammonium molybdate (a), MoO\({}_{3}\) (b), and Ce/MoO\({}_{3}\)/SiO\({}_{2}\) (c).  
obvious characteristic diffraction peak, which illustrated that MoO\({}_{3}\) was evenly dispersed on the catalyst, and that the catalyst Ce/MoO\({}_{3}\)/SiO\({}_{2}\) was an amorphous structure.  
Figure 3 shows the N\({}_{2}\) adsorption-desorption isotherm (a) and pore size distribution (b) of Ce/MoO\({}_{3}\)/SiO\({}_{2}\). In Figure 3a an obvious hysteresis loop between the relative pressure of 0.60-0.85 is shown. According to the IUPAC classification, this should belong to the type IV adsorption isotherm. This feature was also very consistent with the characteristics of mesoporous materials [25]. In Figure 3b it is shown that the average pore sizes are mainly distributed around 3.35 nm, the specific surface area is 673.133 m\({}^{2}\)/g, and the pore volume is 0.565 cm\({}^{3}\)/g. More reaction sites were provided by abundant pore structure and larger specific surface area to accelerate the oxidation reaction rate.  
Figure 4 shows the XPS spectra of Ce/MoO\({}_{3}\)/SiO\({}_{2}\). In Figure 4a it is shown that the electron binding energy corresponding to Mo 3\(d\) in the catalyst is 233.00 eV, indicating the presence of Mo. Since the atomic percentage of Ce was only 0.19%, the characteristic peak of Ce was not detected in the full spectrum. In Figure 4b it is shown that the electron binding energies of Mo 3\(d_{5/2}\) and Mo 3\(d_{3/2}\) are 233.00 eV and 236.18 eV. According to the comparison of NIST data, it could be concluded that the Mo in the catalyst was +6 formal existence [26]. In Figure 4c it is shown that the electron binding energies corresponding to Ce 3\(d_{5/2}\) and Ce 3\(d_{3/2}\) are 882.40 eV and 886.39 eV. According to the comparison of NIST data, it could be concluded that the Ce in the catalyst was +4 formal existence [27].  
Figure 3: _Cont._  
Figure 2: X-ray diffractions of ammonium molybdate (a), MoO\({}_{3}\) (b), and Ce/MoO\({}_{3}\)/SiO\({}_{2}\) (c).  
Figure 4: XPS spectra of Ce/MoO\({}_{3}\)/SiO\({}_{2}\): (**a**) survey, (**b**) Mo 3\(d\), and (**c**) Ce 3\(d\).  
Figure 3: Pore structure parameters of Ce/MoO\({}_{3}\)/SiO\({}_{2}\). (**a**) Adsorption desorption isotherm; (**b**) pore size distribution.