In this study, we prepared four nano catalysts including CeO2、CeO2/Apatite、TiO2 and CeO2/TiO2 for the catalytic wet air oxidation of methylene blue(MB). Via rate of color removal and COD removal, performance of the catalysts was evaluated to screen for the optimal one and reaction condition of the most active catalyst was also investigated.
The results indicated that the efficacy of WAO without catalyst of 100 ppm methylene blue improved with temperature at O2 partial pressure of 1.5Mpa, which implied heat decomposition occurred in a certain temperature.
The characterization of catalysts of SEM shows the diameter of CeO2、TiO2 and CeO2/TiO2 catalysts are around 15 to 21nm, and the BET analysis are around 39 to 47m2/g. With the smallest diameter and largest surface area, TiO2 catalyst performed the most active among the four. At the condition of 100 ppm methylene blue concentration, 180℃, 1.0 g/L the catalyst loading and 1.5 MPa of O2 pressure, the color removal and COD removal after 4 hrs reaction are 95% and 38.5%, respectively. The rate of COD removal at temperatures of 180~200°C increased as reaction temperature increased, while at temperatures of 200~220°C, the COD removal decreased with increase in reaction temperature, suggesting an optimum temperature for the operation of this catalyst of 200°C. An optimal loading was found at 1.0 g/L, when free radical formation is higher at a lower catalyst loading (0.5~1.5g/L), while at a higher loading (1.5~3.0g/L), free radical destruction dominates. The suitable O2 partial pressure occurred at 1.0Mpa. The intensity of the TiO2 crystals peaks increased calcination temperature increased, while the 515℃ calcinated-temperature catalyst is superior to 300°C and 700℃calcinated-temperature catalyst on CWAO performance with his more structure defects.
As for the assessment of catalyst regeneration and durability, it was found that the order of color removal for 50ppm MB with fresh catalyst and regenerated catalyst was: calcined again > reused > fresh catalyst > alcohol > rinsed with acetone, and also the order of COD removal was: fresh catalyst > calcined again > reused > alcohol > rinsed with acetone, which showed calcined again was the best catalyst regeneration method in the study.
In addition, we selected the second most active catalyst, CeO2/HA, for the CWAO of MB by varying Cerium content. The result indicated that the most active catalyst was CeO2/HA(1:1/8), which performed better activity than TiO2 catalyst. The efficacy of this catalyst increased as the MB concentration increased. At the condition of 2600ppm MB solution, it enabled 99.75% of color removal and 91.89% of COD removal after 4 hrs reaction, which confirmed this catalyst was feasible for high concentration of reactants.