Preparation Of Cerium Oxide/carbon Nitride-based Heterojunction By Molten Salt Method And Its Photocatalytic Performanc

Cerium dioxide and carbon nitride of graphite phase are both photocatalytic materials with excellent redox performance.By combining the two,the construction of cerium dioxide/carbon nitride based heterojunction can promote the effective separation of electrons and holes,and further modify the heterojunction,which is expected to achieve a substantial improvement in the photocatalytic performance and adsorption performance of the heterojunction.In this paper,monocatalic（CeO₂ and g-C₃N₄）and binary（CeO₂-g-C₃N₄）photocatalytic materials were prepared successfully by a simple one-step molten salt method.BiOCl and La OCl were introduced into g-C₃N₄ and CeO₂-g-C₃N₄,respectively,to explore the synergistic mechanism of layered chlorine oxides on photocatalytic performance of heterojunction.The effects of crystalline carbon on photocatalytic and adsorption properties of heterojunction were investigated by introducing crystalline carbon into CeO₂-g-C₃N₄ under different roasting conditions.The microstructure,energy band structure,oxygen vacancy content and surface C/N ratio of the photocatalytic materials were studied by SEM,TEM,XPS,UV and PL characterization methods.The photocatalytic properties of CeO₂ prepared by molten salt method with K₂CO₃/Li₂CO₃,Li Cl/KCl,Ca Cl₂/Na Cl,Ca Cl₂/KCl and Na Cl/KCl were studied.The results showed that CeO₂ prepared by K₂CO₃/Li₂CO₃molten salt system had the best photocatalytic performance,and the degradation rate of methylene blue（MB）was 95.91%after 120 min illumination.The effects of p H=4-10 and reaction temperature of 25-65^oC on the photocatalytic performance of CeO₂ were investigated.It was found that the photocatalytic performance of CeO₂ first increased and then decreased with the increase of reaction temperature.When the reaction temperature was 45 ^oC,the degradation rate of MB reached 100%after 90 min of reaction.With the increase of p H,the degradation rate of MB showed an increasing trend.When p H=10,the degradation rate of MB was99.36%after 120 min reaction.By adjusting the ratio of CeO₂ and g-C₃N₄ precursor,the CeO₂-g-C₃N₄heterojunction was successfully prepared.With the content of CeO₂ in the heterojunction from 0.5-0.7,the exposed crystal plane of g-C₃N₄ in the heterojunction gradually changed from（100）and（002）to（100）,（110）,（200）and（120）crystal plane transition,where CeO₂（0.6）-g-C₃N₄ is the intermediate state.The transformation of exposed crystal surface of g-C₃N₄ in CeO₂-g-C₃N₄ heterojunction contributes to the significant increase of its specific surface area.With the gradual increase of doping ratio,Ce³⁺percentage and surface oxygen O_sur content showed an increasing trend,among which CeO₂（0.7）-g-C₃N₄ Ce³⁺percentage（26.85%）and surface oxygen O_sur content（0.795）were the highest,which was beneficial to the improvement of its photocatalytic performance.CeO₂（0.7）-g-C₃N₄ has the best photocatalytic performance,and the degradation rates of methylene blue and tetracycline are 90.2%and 80.1%,respectively,under light conditions for 120 min.·OH and h⁺were the main active free radicals in the photocatalytic degradation of tetracycline by CeO₂（0.7）-g-C₃N₄.BiOCl-g-C₃N₄ heterojunction was successfully prepared by adjusting the ratio of BiOCl and g-C₃N₄ precursor,in which BiOCl（0.2）-g-C₃N₄ was the optimal doping amount of BiOCl and had the best photocatalytic performance.The results showed that the degradation rate of tetracycline was 91.4%and k=0.01766 min^-1 under 120 min illumination.BiOCl has 2D structure,and appropriate doping amount of BiOCl can effectively improve the specific surface area of BiOCl-g-C₃N₄ heterojunction.According to the photoelectric performance results,appropriate doping amount of BiOCl is helpful to improve the separation efficiency of photogenerated electrons and holes in BiOCl-g-C₃N₄ heterojunction.A ternary photocatalyst of La OCl/CeO₂-g-C₃N₄ with excellent photocatalytic performance was designed and firstly synthesized by a facile KCl-Li Cl molten salt method.The 60%La OCl/40%CeO₂-g-C₃N₄ with a 6:4 weight ratio of La OCl to CeO₂has an optimal degradation ratio（nearly 100%）for methylene blue（MB）with an impressive cyclic stability.Both of adsorption and photodegradation contribute to the high MB removal ratio,and kinetic study results show that the rate constant of photocatalytic degradation route is 0.0174 min^-1,which is less than that of adsorption process(2.4406 min^-1).The excellent adsorption performance of60%La OCl/40%CeO₂-g-C₃N₄ is attributed to its nagetive zeta potential（-35.40 m V）and larger average pore size.The photochemical charaterizations suggest that the60%La OCl/40%CeO₂-g-C₃N₄ form a dual Z-scheme heterojunction,which enhances its separation efficency of photogenerated e^-/h⁺with an outstanding redox ability,and further promotes the photocatalytic acivity.A novel CeO₂-C-g-C₃N₄ Z-scheme heterojunctions with significantly enhanced photocatalytic performance were prepared for the first time by a facile one-step molten KCl-Li Cl method.The formation mechanism of CeO₂-C-g-C₃N₄ was discussed,where g-C₃N₄ turned to C by carbonizing and the C content was adjusted by controlling the synthesis conditions.The photocatalytic property of the ternary composites first increased and then decreased with synthesis temperature and time.The 580^oC 4h CeO₂-C-g-C₃N₄ exhibited the best performance towards MB（Methylene blue）and TC（Tetracyclines）degradation with a removal degree of 99.9%and 92.5%,respectively.The surface C/N ratio was a key index that influenced the photocatalytic performance,and was calculated to be 0.918-1.352 for various CeO₂-C-g-C₃N₄ catalysts.The 580^oC4h CeO₂-C-g-C₃N₄ had an appropriate C/N ratio of 0.930 and a narrow band gap.The formed Z-scheme 2D heterojunctions and generated C clearly promoted the separation of photo-excited e^-/h⁺.Besides,it was indicated that e^-and·O₂^-were the main active species for TC photodegradation.The 580^oC 4h CeO₂-C-g-C₃N₄ exhibited a large BET surface area,high negative zeta potential,strongπ-πelectron donor-acceptor interaction,and an excellent MB adsorption ability.It was evident that chemisorption on catalyst surface played an important role for adsorbing MB.The formation mechanism of CeO₂-C-g-C₃N₄ and the effects of the surface C/N ratio on catalytic and adsorption property,determined in this work,offer new insights for preparing and structuring highly effective photocatalysts,which is expected to promote further photocatalytic and adsorption applications.The typical organic pollutant sodium oxalate in alumina produced by Bayer process was used as the target degradation agent for photochemical degradation experiments.The optimal reaction conditions were as follows:reaction temperature 40-45 ^oC;Sodium hydroxide 100 g/L;Sodium aluminate 20 g/L.When oxidants H₂O₂,sodium persulfate and potassium persulfate were added into the reaction system,the photochemical degradation rates of sodium oxalate were increased to 47.32%,74.47%and 74.20%,respectively.