Mechanism Of Photocatalytic Degradation Of Organic Pollutants By 4-hydroxycoumarin And CeO₂/g-C₃N₄

Photodegradation is a process in which the radicals generated by radiation and photocatalyst are extremely active,and the pollutants are completely degraded by the processes of addition,substitution and electron transfer between free radicals and organic pollutants.It has become an important area for research on water pollution control technology.New green and efficient photocatalysts have been the focus of research in this field.As an important chemical raw material,nitrobenzene（NB）brings serious environmental problems while facilitating human production and life.Its degradation in the natural environment has rarely been reported.Coumarin compounds are widely used as natural organic compounds（NOMs）in plants,and are mainly used for assembling enzyme substrates,synthesizing fluorescent dyes,and drug synthesis.However,as an organic substance widely present in the natural environment,its role in environmental photodegradation has not been reported.In addition,the dye wastewater which is difficult to be naturally degraded has complex composition,high toxicity and stable structure,and poses a serious threat to the ecological environment.Photocatalytic degradation,a green degradation method for dyes,has become a research hotspot.Among them,photocatalyst is a key factor affecting the efficiency of photocatalytic degradation.Graphite phase carbonitride（g-C₃N₄）as a photocatalyst has many excellent properties（such as high chemical and thermal stability,economical source of raw materials,simple preparation method and visible light responsiveness）,which has attracted wide attention of researchers in various countries.However,the catalyst has problems of complicated fabrication process,low specific surface area,narrow absorption range of visible light,and easy recombination of photogenerated electrons and holes,and greatly limits the further improvement of catalytic performance.In this regard,this study explored the reduction of nitrobenzene by 4-hydroxycoumarin（4HC）in sunlight,revealing a degradation pathway of organic pollutants under natural conditions;and a new g-C₃N₄ based catalyst CeO₂/g-C₃N₄ was synthesized,the degradation of organic dyes was carried out,and the photocatalytic mechanism of CeO₂/g-C₃N₄catalyst was analyzed.Finally,the similarities and differences of photocatalytic mechanism between natural organic and synthetic catalysts were compared.1.4HC is used as a representative of coumarins to study the typical refractory degradation of 4HC under different light sources（sunlight,500 W mercury lamp and 500 W xenon lamp）.Photocatalytic reduction of organic pollutants,nitrobenzene（NB）.We first used NB（80μM）solution and 500 W mercury lamp for light/dark contrast test,and added 4HC（160μM）/no 4HC comparison experiment.The results showed that 4HC has the ability of photocatalytic degradation of NB.After adding 4HC,NB can degrade 60.1%after 12 hours of exposure to mercury lamps.Then we studied the relationship between the optimal concentration ratio of NB and 4HC and different light sources.Under mercury lamp,NB:4HC=1:8,NB degradation is best（96.6%degradation at 4h）;under xenon lamp,NB:4HC The degradation effect was best at 1:1（9.6%degradation at 4h）;under sunlight,NB:4HC=1:2,the degradation was best（degradation of 17.3%at 4h）.This indicates that the intensity of ultraviolet light plays a key role in the photochemical reaction of 4HC and NB.To further analyze the photoreaction process,we determined the reaction product by LC-MS/MS and 1H NMR.The results showed that the main product of NB photocatalytic reduction by 4HC was 4-aminophenol.Subsequently,we combined the transient absorption spectra of 4HC under experimental conditions and related literatures to determine that 4HC becomes 4HC excited state（S₁）after illumination,and its decay time is about 250 ps.It was confirmed by electron paramagnetic resonance（EPR）experiments that hydroxyl radicals（·OH）were formed during the reaction.Combined with the above experimental results,we proposed the reaction mechanism between 4HC and NB.We also studied various influencing factors（gas environment,solvent,pH）and the results showed.The addition of H₂ and alcohol promoted the degradation of NB.The presence of O₂ inhibited the degradation of NB.The pH value had no significant effect on the degradation of NB,which further confirmed the reaction mechanism proposed by us.2.In this study,CeO₂/g-C₃N₄ light with different CeO₂ doping ratios（0.5%,1.5%,2.5%,3.5%）was prepared by a single heating method using simple raw materials（melamine,Ce（NO₃）₃·6H₂O and NH₄Cl）.Catalytic material,followed by photodegradation（500 W xenon lamp）of Rhodamine B（RhB）and photolysis of hydrogen produced by water（300 W xenon lamp,λ>420 nm）The results show that the photocatalytic performance of CeO₂/g-C₃N₄ is significantly improved compared with pure g-C₃N₄,and the catalytic performance of 2.5%CeO₂/g-C₃N₄ is the best.Under the illumination of xenon lamp for 40 min,the degradation rate of RhB（20 mg/L）catalyzed by 2.5%CeO₂/g-C₃N₄ was 96.4%,while that of pure g-C₃N₄ was only 14.8%.The photocatalytic hydrogen production efficiency of 2.5%CeO₂/g-C₃N₄(1.787mmol·g^-1·h^-1)was 21.5 times that of pure g-C₃N₄ hydrogen production efficiency(0.083mmol·g^-1·h^-1).Subsequently,we characterized the synthesized materials by means of SEM,TEM,XRD,XPS,UV-DRS,FTIR,BET and other material characterization methods.The electrochemical conversion performance of different materials was compared by electrochemical workstations,and finally the influencing factors（pH,Investigation of solvents）.Based on the experimental results,we proposed the photocatalytic mechanism of CeO₂/g-C₃N₄.Comparing the photocatalytic processes and mechanisms of 4HC and CeO₂/g-C₃N₄,we found that natural organic matter is excited by ultraviolet light and becomes an excited state,thereby degrading organic pollutants.After the photochemical reaction is completed,the natural organic matter becomes another product and cannot be reused.The synthetic catalyst undergoes a redox reaction with photogenerated holes and photogenerated electrons,and the catalyst itself has high stability and can be recovered and reused after the reaction is completed.