Synthesis Of Functional Carbon Nitride Based Photocatalytic Materials And Study On Oxidation Properties

Antibiotics misuse in medical treatment,livestock and poultry breeding and other industries leads to a large number of residues in municipal sewage,natural water and even drinking water,posing a serious threat to ecological balance and human health.Photocatalytic oxidation is considered as a green,clean and promising strategy for antibiotic pollution control.It is of great significance to develop and design efficient visible light catalysts for the catalytic degradation of antibiotics.Nonmetallic graphitic carbon nitride（g-C₃N₄）has been regarded as a promising visible light catalyst because of its suitable band gap,two-dimensional lamellar structure and superior chemical and thermal stability.However,the oxidation efficiency of g-C₃N₄ has strong limitations,mainly due to the fact that g-C₃N₄ is aπconjugated system composed of carbon and nitrogen atoms through sp² hybridization,and its photogenerated electrons can be randomly transferred throughπbonds in the plane composed of conjugated aromatic rings,resulting in the recombination of photogenerated electrons and holes.In this study,the main strategy is to overcome the rapid electron-hole recombination of g-C₃N₄ by realizing electron localization.Two effective ways of"copolymerization doping"and"defect modification"were proposed to construct a series of high-performance g-C₃N₄ photocatalytic materials.The influence mechanism of different modification strategies on photoelectron transport,band structure and photocatalytic oxidation performance was revealed by characterization analysis and theoretical simulation.In order to further break through the research bottleneck of limited oxidation efficiency and difficult recovery of g-C₃N₄-based materials,two feasible system optimization schemes were proposed:（1）the synergistic oxidation system of persulfate was constructed to greatly improve the oxidation efficiency and reveal the interface reaction mechanism;（2）The g-C₃N₄ based photocatalytic composite membrane with high flux,self-cleaning,antifouling and other superior functions was constructed to overcome the disadvantages of difficult recovery of powder materials;The microcosmic interaction between g-C₃N₄ and membrane matrix is also explained for the first time.The practical application of g-C₃N₄-based photocatalytic materials in water purification was promoted.The main research contents are as follows:（1）Regulation mechanism of photocatalytic properties of carbon nitride based onelectron localization strategyIn this study,three different modified g-C₃N₄-based photocatalytic materials including bridged triazole ring g-C₃N₄（NCN）,nitrogen-deficient g-C₃N₄（DCN）and carbon-deficient g-C₃N₄（CCN）were successfully prepared by one-step thermal polymerization with green,convenient and low cost.The electronic localization was realized and oxidation performance was improved.First,the analysis of electron spin resonance（ESR）and electron localization function（ELF）of NCN shows that the triazole ring acts as an electron-withdrawing group to delocalize and concentrate the isolated valence electrons in the conjugated aromatic ring,which inhibits the recombination of photogenerated electrons and holes.Second,the results of ESR and differential charge density（DCD）of DCN show that the introduction of nitrogen vacancy（N₂C）facilitates the rapid localization of unpaired electrons left by adjacent carbon atoms and other electrons in the conjugated system at the defect site through non-radiative transition,which is beneficial to space charge separation and photocatalytic activity.Third,photoluminescence spectra and DCD results show that the isolated valence electrons located in the conjugated aromatic ring tend to be confined in the central region of three adjacent N atoms to form an"electron trap"after the construction of a carbon vacancy（C-（N）₃）.The formation of localized electrons facilitates the separation of reduction and oxidation sites and drives electrons and holes to participate in surface reactions.In summary,three different"electronic localization"modification strategies have achieved the goal of improving the oxidation efficiency,and revealed the key technical points in the performance regulation mechanism of g-C₃N₄-based photocatalytic materials.（2）Oxidation mechanism of modified carbon nitride based photocatalytic materials coupled with persulfate systemIn order to improve the oxidation efficiency of g-C₃N₄-based photocatalytic materials,the above modified g-C₃N₄ were used to construct the photocatalytic synergistic persulfate’1+1>2’oxidation system,namely,system optimization Scheme 1.First,the difference of PS activation efficiency of different modified g-C₃N₄ was compared,and the results showed that PS/CCN system was particularly strong for the oxidative degradation of antibiotics.The mechanism of free radical transformation and electron transfer in the system was clearly elucidated by ESR detection and molecular simulation.The PS/CCN system had the highest adsorption energy（-18.340 e V）and the highest tensile strength of the peroxide bond（1.496?～1.519?）.Therefore,the effectiveness of the photocatalytic synergistic oxidation system is further confirmed,and the"electron trap"in CCN plays a key role in the activation of persulfate.（3）The mechanism of coupling functional membranes with modified carbonnitride photocatalytic materialsIn order to overcome the problem of difficult recycling of g-C₃N₄-based photocatalytic materials,a series of photocatalytic functional composite membranes were constructed by combining the above modified g-C₃N₄ with polyvinylidene fluoride（PVDF）by phase transformation method,namely,system optimization Scheme 2.Firstly,the composite membrane containing photocatalyst can degrade in situ the pollutants adsorbed on the membrane surface and membrane pores under light irradiation,thus improving the anti-pollution property of the membrane.Secondly,it was confirmed that the photocatalytic composite membrane retained good catalytic activity through the kinetic study of the oxidation degradation of antibiotics.Meanwhile,the pure water flux increased from 179.3L m^-2 h^-1 to 326.4 L m^-2 h^-1,and the irreversible pollution decreased from 36.9%to 9.2%.The influence mechanism of physical and chemical properties of g-C₃N₄photocatalyst on pore size distribution,permeability and retention of composite films was systematically analyzed.The non-covalent interaction between g-C₃N₄ and PVDF was revealed for the first time by reduced density gradient（RDG）analysis.This effect plays an important role in the uniform dispersion of catalyst in the casting solution,thus forming regular pore structure and improving the mechanical strength of the membrane.It is further confirmed that the structure design and performance control of catalysts are of great significance in the construction of photocatalytic functional composite membrane.This study has played a strong role in promoting the breakthrough of the practical application barrier of g-C₃N₄/PVDF photocatalytic composite membrane in wastewater treatment.Figures,72;Tables,20;References,208.