Construction Of Polymer Photocatalysts And Regulation Of Surface/Interface Reactive Activity

The conversion of solar energy to chemical energy can be realized by absorbing sunlight through semiconductors to drive catalytic reactions.Polymer photocatalytic materials such as g-C₃N₄and resorcinol-formaldehyde resins have the advantages of easy structural tunability,wide visible light absorption range,and easy large-scale synthesis,which have potential applications in the direction of photochemical water splitting for H₂production,photocatalytic synthesis of H₂O₂,and photocatalytic degradation of organic pollutants.However,polymer photocatalytic materials are mainly synthesized by artificial preparation,and the raw materials,synthesis approach and process have important effects on the reaction performance of polymer photocatalysts.The pristine polymer photocatalytic materials also suffer from poor separation and transfer properties of photo-generated carriers,low surface/interface reactivity.In recent years,various modification studies have been conducted on polymer photocatalysts in the field of photocatalysis.However,the efficiency of photocatalytic synthesis of H₂O₂and degradation of organic pollutants by polymer photocatalysts still needs to be improved,and the precise construction of polymer photocatalysts and the regulation of surface/interface activity are of great scientific and practical significance.In this paper,g-C₃N₄and resorcinol-formaldehyde resins were selected as typical representatives of polymer photocatalysts.The formation mechanism of K-doped g-C₃N₄and the effects of organic byproducts on the photocatalytic properties of K-doped g-C₃N₄were systematically investigated;The activity and mechanism of surface N-Hydroxymethylation of g-C₃N₄photocatalysts for the efficient production of H₂O₂were studied.RFS/Fe OOH composites and the regularity of photo-Fenton degradation of pollutants were explored.The main study contents and conclusions are as follows:（1）In order to evaluate the performance and application value of K-doped g-C₃N₄photocatalyst objectively and comprehensively,K-doped g-C₃N₄was prepared by the classical KOH/melamine,KOH/dicyandiamide or KOH/urea thermal polymerization.SEM,XRD,XPS and FTIR characterization tests confirmed the successful production of K-doped g-C₃N₄by thermal polymerization method.However,it was found that organic byproducts were universally present in the K-doped g-C₃N₄samples synthesized by thermal polymerization through washing treatment of the produced samples and the analysis of the dissolved material composition.The dissolved amount of byproducts was positively correlated with the amount of KOH added.The organic byproducts are very stable and cannot be degraded by the photocatalytic action of K-doped g-C₃N₄,and they are not easily removed by washing with ethylene glycol,acetic acid,DMSO,deionized water at room temperature.The secondary calcination treatment also cannot effectively remove these organic byproducts.By comprehensive comparison,hot-water（75°C）washing was an acceptable approach for the removal of the organic byproducts from K-doped g-C₃N₄.Taking K-doped g-C₃N₄photocatalytic degradation of dimethyl phthalate（DMP）and photocatalytic synthesis of H₂O₂as examples,it was confirmed that the organic byproducts have negative influence on the application of K-doped g-C₃N₄photocatalysis.Specifically,these byproducts could be dissolved into the photocatalytic system of K-doped g-C₃N₄as new and stable pollutants or impurity.The formation of organic byproducts in K-doped g-C₃N₄could be attributed to the fact that the presence of the KOH results in insufficient thermal polymerization of melamine,dicyandiamide or urea into expected g-C₃N₄.（2）Through the reaction of formaldehyde with the amino groups（-NH₂）on the g-C₃N₄surface,N-hydroxymethyls groups（-NH-CH₂-OH）were introduced on typical g-C₃N₄photocatalysts.With the introduction of-NH-CH₂-OH groups on the surface,the photocatalytic activity of g-C₃N₄to produce H₂O₂in pure water system was greatly improved.Compared with the pristine g-C₃N₄photocatalyst,the modified g-C₃N₄photocatalysts had over 12.8 times higher activity for H₂O₂production,and about 84.3%activity was retained after four cycles of repeated testing（the total reaction time was 28 h）.In addition,the K-g-C₃N₄and Cv-g-C₃N₄samples showed significantly enhanced activity in the H₂O₂production after the introduction of-NH-CH₂-OH groups.The experimental investigations and density functional theory（DFT）calculations indicated that the introduction of-NH-CH₂-OH on the g-C₃N₄surface did not change its morphology,light absorption intensity and edges,band positions,charge separation and transfer properties,but markedly improved the adsorption,activation of O₂molecules and H₂O dehydrogenation properties,which was more energetically favorable in the reduction kinetics of O₂to H₂O₂.By adding Fe²⁺to the g-C₃N₄/500-85°C-3h photocatalytic system,a homogeneous photo-Fenton reaction was successfully constructed by directly using H₂O₂produced from the modified samples for efficient degradation of rhodamine B（Rh B）.（3）Fe OOH/RFS composites were constructed by coupling Fe OOH thin layer on the surface of resorcinol-formaldehyde resin（RFS）nanospheres,and the resultant composites were investigated as photocatalysts to achieve a heterogeneous photo-Fenton degradation of organic pollutants.Based on the contact coupling interaction between RFS nanospheres（300～400 nm in diameter）and Fe OOH thin layers,the Fe OOH/RFS heterojunction could effectively promote the separation and transfer properties of photogenerated carriers between the interfaces.Meanwhile,the heterogeneous photo-Fenton system was developed to degrade organic pollutants by using the property that H₂O₂can synthesized by RFS under visible light illumination,and Fe³⁺in Fe OOH can be partially reduced to Fe²⁺by the photogenerated electrons of the composites.Using Rh B and DMP as model organic pollutants,the effect and reaction mechanism of Fe OOH/RFS photo-Fenton system in degrading organic pollutants were investigated.The 6Fe OOH/RFS composite showed the highest degradation activity,with a rate constant about seven times higher than that of the RFS system.And the repeated experiments demonstrated that the constructed photo-Fenton system for the degradation of pollutants can remain stable.Combined with the capture agent addition experiment,EPR and photoelectrochemical test,the analysis shows that·OH is the main active species for degrading pollutants in Fe OOH/RFS heterogeneous photo-Fenton system,which is mainly generated by the reaction of H₂O₂produced by photocatalysis and Fe²⁺.The findings of the design and construction of g-C₃N₄and resorcinol-formaldehyde resins photocatalysts and their surface/interface activity modulation can provide valuable experimental methods and technical references for the design and preparation of the same type of photocatalytic materials,as well as useful references for the construction of efficient photocatalytic reaction systems.