| Sewage treatment is one of the only way to solve water scarcity,protect the environment,and safeguard the life and health of human beings.Photocatalysis as a chemical technology is environmentally friendly,safe,sustainable and energy efficient.Due to their peculiar electronic structures’ distinctive physical and chemical characteristics,rare earth elements exhibit high performance and a wide range of applications and possibilities in the field of photocatalytic wastewater treatment.Pyrochlore-type A2B2O7-type compounds have recently gained a lot of study interest due to their structural flexibility,chemical stability,and other qualities.Based on this,this thesis focuses on pyrochlore rare earth titanates,it designs and prepares a series of RE2Ti2O7 compounds with different chemical compositions by sol-gel method;the crystal structure,magnetic properties and optical catalytic properties of RE2Ti2O7compounds are studied by selecting different rare earth elements;by exploring the dependence of structure and magnetism on temperature,it is found that there is magnetically induced anomalous thermal expansion behavior in Ce2Ti2O7,revealing the nature of magnetic lattice coupling;by measuring the efficiency of photocatalytic degradation of methylene blue,the possibility of RE2Ti2O7.series compounds for treating printing and dyeing wastewater was explored;the method of binary rare earth co-doping is used to optimize the efficiency of degrading methylene blue,and at the same time,it is extended to photocatalytic degradation of erythromycin,which provides a basis for understanding the physicochemical properties of rare earth titanate RE2Ti2O7.Firstly,the sol-gel method synthesized a cubic with defects(space group:Fd-3m)pyrochlore-type compound Ce2Ti2O7.X-ray diffraction combined with the Rietveld refinement method found that the Ce element was missing up to 18%in the lattice.Magnetic tests show that the absence of Ce will cause the change of Ce ion valence,which makes Ce2Ti2O7 exhibit rare high temperature(sub)ferromagnetism and apparent hysteresis behavior at room temperature.In addition,the compound has good thermal stability,no evident lattice symmetry transformation during the heating process,and the average axial thermal expansion coefficient is+11.2×10-6 K-1 in the temperature range of 25-700℃.Further research was carried out on anomalous magnetism and catalytic properties of the above defective cubic Ce2Ti2O7.By measuring the dependence of magnetization on temperature,it is found that Ce2Ti2O7 has two magnetic transitions from 5 K to 350 K:one is the magnetic order-disorder transition at 300 K,and the other is the magnetic composition change at 32 K;During the order-disorder transition,the thermal expansion of the Ce2Ti2O7 lattice is abnormal,and the average thermal expansion coefficient in the range of 123-298 K is+6.8×10-6 K-1,which is only half of the high temperature,indicating the strong coupling between the magnetism and the lattice of Ce2Ti2O7.The photocatalytic system analysis found that the Ce2Ti2O7 catalyst had a specific adsorption effect on methylene blue,which was in line with the pseudo-second-order reaction kinetic model,which is chemical adsorption.When the catalyst was at a temperature of 25℃,pH=7,and 50 mL of a 10 mg·L-1 methylene blue solution.,the degradation efficiency was 31.3%at 120 min.Through the co-doping of binary rare earth elements,pyrochlore-type La2-xCexTi2O7 series compounds with different La and Ce ratios were further synthesized.X-ray diffraction showed that La2-xCexTi2O7 had a monoclinic structure(space group:P21/m),and the highest Ce solid solubility is close to 40%,which is called La1.2Ce0.8Ti2O7.The photocatalytic degradation efficiency of La2Ti2O7 was 37.1%in 120 min at 25℃,pH=7,and 50 mL of 10 mg·L-1 methylene blue solution,which was slightly higher than that of cubic Ce2Ti2O7,which was 31.2%.Interestingly,the doping of Ce can significantly improve the degradation efficiency of La2-xCexTi2O7,La1.2Ce0.8Ti2O7 is close to the monoclinic phase solid solution limit,and the crystal structure instability helps the transfer of electrons.As a result,the degradation efficiency of La1.2Ce0.8Ti2O7 reaches the maximum value of 65.4%.In addition,La1.2Ce0.8Ti2O7 also has a specific photocatalytic degradation effect of erythromycin,and the maximum degradation rate can reach 49.2%.Based on La2-xCexTi2O7,several pyrochlore-type RE2Ti2O7(RE=Pr,Nd,Sm,Nd 1.4Ce0.6)with monoclinic structure(space group:P21/m)were further designed and synthesized.The photocatalytic degradation efficiency of methyl blue of this series of compounds was explored.The catalytic degradation efficiency of Pr2Ti2O7 in 120 min was 73.6%,and the degradation efficiencies of Nd2Ti2O7 and Sm2TiO7 were 87.10%and 90.87%,respectively.When a certain amount of Ce was incorporated into the Nd site,the catalytic degradation efficiency was further improved,and the catalytic degradation efficiency of Nd1.4Ce0.6Ti2O7 reached 92.46%after 120 min.The research shows that the optimal conditions for photocatalytic degradation of methylene blue by Nd1.4Ce0.6Ti2O7 are temperature 25℃,catalyst mass 0.2 g,pH=7,methylene blue solution dosage V=50 mL,methylene blue solution concentration c=5 mg·L-1,to provide a basis for the subsequent exploration of the photocatalytic degradation of printing and dyeing wastewater by RE2Ti2O7 compounds. |
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