XRD patterns of MoO3/ ZrO2-TiO2 calcined at different temperature
Fig.2 shows the component of the catalyst is mainly amorphous when calcination temperature is below 400 ℃. As the temperature increasing, the morphology of MoO3/ ZrO2-TiO2 crystal appeared. The corresponding characteristic diffraction peaks of tetragonal phase (Zr(MoO4)2) occurred with the 2θ of 23.180°,30.526°and 50.033° at calcianation temperature of 500 ℃. The pattern shows crystal structure of anatase, it confirms the main constituents of the catalyst (MoO3/ ZrO2-TiO2) is tetragonal Zr (MoO4)2 phase mixed with TiO2 of anatase.
XRD patterns of MoO3/TiO2-ZrO2 at different calcination time
As shown in Fig.3, when the calcination time is 1hour and 2 hours, the XRD patterns (MoO3/TiO2-ZrO2) indicate that the components are mainly amorphous with calcination temperature of 500 ℃. When calcination time above 3 hours, the better crystal structures of MoO3/ZrO2-TiO2 are observed in the XRD patterns.
TEM patterns of MoO3/ZrO2-TiO2 calcined at different temperature
It can be seen from the TEM diagram of MoO3/TiO2-ZrO2.When the calcination temperature is 400 ℃, the catalyst mainly exists in amorphous state. With the increase of calcination temperature, the catalyst calcined at 500 ℃ mainly exists in crystalline phase. The selected area diffraction pattern in the lower right corner is concentric ring, which shows that the sample is polycrystalline, which is consistent with the XRD results. When the temperature continued to rise to 600 ℃, the specific surface area and catalytic activity of the catalyst decreased, the main reason is the sintering of catalyst.
BET N2 adsorption and desorption patterns of MoO3/ZrO2-TiO2 calcined at different temperature
As can be seen from table 1, with the increase of calcination temperature, the specific surface area of the catalyst decreases more sharply, which indicates that the catalyst has a certain degree of sintering. Combined with XRD, with the increase of calcination temperature, the intensity of diffraction peak increased to a certain extent, and the specific surface area of catalyst calcined at 600 ℃ decreased 224.795 compared with that at 400 ℃
Table 1. Specific surface area, average pore size and total pore volume of MoO3/ ZrO2-TiO2
specific surface area/m2·g-1
Average pore size/nm
Total pore volume/cm3·g-1
It can be seen from the figure that the isotherm characteristics of all samples are concave to the relative pressure axis. The adsorption curve is not consistent with the desorption curve, that is, there is a hysteresis loop. The P/P0 values of the catalysts calcined at 400 ℃ and 500 ℃ are 0.45- 0.99, which is a typical IV isotherm accompanied by H2 hysteresis loop. The characteristics of these isotherms also reflect the formation of intergranular mesopores. The catalyst calcined at 600 ℃ is characterized by type IV isotherm and H1 hysteresis loop, which is the result of homogeneous pore condensation. When the relative pressure is 0.75 and 0.9, the change slope of the isotherm is higher, which indicates that the mesoporous materials have better uniformity.
NH3-TPD of MoO3/TiO2-ZrO2calcined at different temperatures
Temperature programmed desorption method uses pyridine, ammonia, 2,6-dimethylpyridine and tert Butyl Ammonium as adsorbents. Because of NH3 have strong basicity (PKa = 9.2), high stability and small molecular volume, it’s can be adsorbed on acid sites with different acid strength and easily enter into the pore channels of porous materials ,it is an ideal acid probe molecule. The peak temperature is often related to the acid strength, peak area and acid amount .The desorption temperatures of NH3 were 100 ℃～150 ℃, 200～230 ℃ and 500～700 ℃, respectively, corresponding to the weak acid site desorption peak, medium strong acid site desorption peak and strong acid site desorption peak. It can be seen from Fig. 6 that NH3 has two strong acid sites (αand β) on MoO3/TiO2-ZrO2, the peak area is proportional to the amount of NH3 adsorbed on the sample. That is, it is directly proportional to the amount of acid in the sample, the weak desorption peak of β indicates that the content of strong acid site is less, the α - desorption peak of MoO3/TiO2-ZrO2 catalyst calcined at 500 ℃ was the strongest, indicating the highest content of weak acid. The results showed that the best hydrolysis effect of CFC-12 was obtained by MoO3/TiO2-ZrO2 catalyst calcined at 500 ℃,combined with the results of NH3-TPD characterization, the weak acid site of solid acid MoO3/TiO2-ZrO2 has strong catalytic activity for the hydrolysis of CFC-12.The calcination temperature has a great influence on the acidity of the catalyst, the β - desorption peak of MoO3/TiO2-ZrO2 catalyst calcined at 600 ℃ disappeared, the reason may be that higher calcination temperature is not conducive to the formation of medium strong acid sites.
FT-IR of MoO3/TiO2-ZrO2 calcined at different temperatures
Fig. 7 shows the FT-IR diagram at different calcination temperatures. It can be seen from the figure that the influence of calcination temperature on FT-IR diagram is great, the sample near 746 cm-1 can be attributed to the vibration peak of Zr-O bond, 500-700 cm-1 belongs to Ti-O vibration peak, near 990 cm-1 is the stretching vibration absorption of Mo = O in crystalline MoO3. In addition, there are strong absorption peaks near 1631 cm-1 and 3400 cm-1, 1631 cm-1 is the bending vibration peak of -OH adsorbed by metal atoms on the surface of composite oxides, 3400 cm-1 is the Strong absorption peak of stretching vibration of -OH. The absorption vibration peak of the catalyst calcined at 400 ℃ is weak, especially in the wavelet segment, the results show that the catalyst mainly exists in amorphous form and the results are consistent with those of XRD.
Effect of MoO3/ZrO2-TiO2 on the hydrolysis rate of CFC-12
Effect of calcination temperature on the hydrolysis rate of CFC-12
It can be seen in Fig.8 that the hydrolysis rate increases with the increase of temperature from 300 ℃ to 500 ℃, and decreases after 500 ℃. The highly hydrolysis rate reaches 98.65 % at 500 0C. It is studied and analyzed that the occurrence of the catalyst is due to the temperature being too high, the catalyst is sintered, and the activity is lowered.
Effect of calcination time on hydrolysis rate of CFC-12
Fig.9 is based on a previous study of the optimal calcination temperature. The effect of calcination time on the hydrolysis rate of CFC-12 at the optimum calcination temperature was investigated. The experimental results show that the catalytic hydrolysis rate of CFC-12 is up to 98.65% when the calcination time is 3 hours, When the calcination time is less than 3 hours, the calcination is incomplete and the structure is incomplete, so the hydrolysis rate is not too high. After calcination for 3 hours, the main structure of the solid acid catalyst MoO3/ZrO2-TiO2 is tetragonal Zr (MoO4)2 doped anatase TiO2 structure. The hydrolysis rate decreased with the increase of calcination time, which is mainly due to the corrosion of the catalyst by HCl and HF produced by the hydrolysis of CFC-12 and calcination time is too long, the catalyst will be sintered, the dispersion of TiO2 in ZrO2 will be reduced, resulting in the specific surface area and activity of the catalyst will be reduced.
Effect of catalytic hydrolysis temperature on the hydrolysis rate of CFC-12
Fig.10 shows that the hydrolysis rate of CFC-12 increases with the increase of catalytic hydrolysis temperature. The hydrolysis rate of CFC-12 reaches 98.65% at 400 ℃,it remains unchanged after 400 ℃.The rate of hydrolysis gradually increases due to the endothermic process of the reaction of CFC-12: CF2Cl2＋H2O→CO＋HF＋HCl. According to the mass spectrum analysis, there is a certain amount of CO2 in the hydrolysis products of CFC-12, which may be produced by the reaction of CF2Cl2＋O2＋H2O→CO2＋HF＋HCl, and the oxygen contained may come from dissolved oxygen in the air. The formation rate of COx (CO and CO2) are 72.44%, which may indicates that the catalytic hydrolysis of CFC-12 over MoO3/ZrO2-TiO2 is complete. All the above investigations indicate that the solid acid of MoO3/ ZrO2-TiO2 composite with the calcination temperature of 500。C and calcination time of 3 hours in the catalytic hydrolysis of CFC-12 acts as a effective catalyst with high catalytic activity and selectivity. The solid acid catalystMoO3/ZrO2-TiO2 prepared by calcining at 500 ℃ for 3 hours is one of the ideal catalysts for catalytic hydrolysis of CFC-12.
The effect of water vapor concentration on the hydrolysis rate of CFC-12
Fig.11 shows that water vapor concentration has significant effect on the hydrolysis rate of CFC-12. In the absence of water vapor, the hydrolysis of CFC-12 was 14.31%.The hydrolysis rate of CFC-12 increased with the increase of steam concentration, and decreases after 83.18%.When the steam concentration was 83.18%, the hydrolysis rate reached the maximum value of 98.65%.The hydrolysis rate decreased with the further increase of steam concentration. The main reason is that the reaction rate is accelerated and the gas-solid contact time is shortened with the increase of water vapor concentration, as well as the lower reaction temperature as more water vapor involved in the system. It is concluded that the optimum volume fraction of water vapor is 83.18%.
The effect of reaction time on the hydrolysis rate of CFC-12
As shown in Figure 12, when the reaction time is less than 10 hours, the hydrolysis rate of CFC-12 is more than 98.00%, indicating that the solid acid of MoO3/ZrO2-TiO2 catalyst has good thermal stability. The hydrolysis rate of CFC-12 drops from 98.00% to 65.34% as the reaction time reaches 20 hours. When the reaction time exceeds 20 hours, the hydrolysis rate decreased slowly. All of the above studies show that the solid acid of MoO3/ZrO2-TiO2 composite with the calcination temperature of 500oC and calcination time of 3 hours in the catalytic hydrolysis of CFC-12 has high thermal stability.
MoO3/TiO2-ZrO2 SEM diagram of the reaction before and after
The SEM images of MoO3/ZrO2-TiO2 composite before and after the catalytic hydrolysis reaction of CFC-12 were given in Fig.13. The MoO3/ZrO2-TiO2 composite is the shape of lumpy, and has a good crystal structure with calcination temperature of 500 ℃ and calcination time of 3 hours., which is consistent with the XRD results. In addition to the complete crystal form, amorphous particles appear after the reaction, mainly due to the introduction of SiO2, which indicates that MoO3/ZrO2-TiO2 catalyst has high stability and long service life.
Characterization of MoO3/TiO2-ZrO2 EDS after reaction
The EDS diagram showed that no fluorine(F) was detected after the reaction, indicating that fluorine products did not participate in the reaction, and elemental carbon(C) appeared mainly due to the introduction of conductive adhesive in the test. The results show that MoO3/ZrO2-TiO2catalysthas high selectivity and stability.