Molecular Modification Of Carbon Nitride And Its Photocatalytic Properties

The conversion of solar energy into hydrogen and reduction of carbon dioxide（CO₂）into highly value-added chemicals using semiconductor photocatalysis is an effective way to address energy shortages and the greenhouse effect.Graphitic phase carbon nitride（g-C₃N₄,CN）is widely used for photocatalytic water cracking for H₂and CO₂reduction because of its suitable energy band structure and wide source of raw materials.However,the conventionally prepared CN has insufficient light absorption capacity and photogenerated charges are extremely easy to compound,resulting in its low photocatalytic activity.To improve the photocatalytic activity of CN,a series of modified CN-based complex catalysts were produced in this thesis by altering the CN structure and charge distribution through molecular grafting,elemental doping and construction defect methods.The structural and compositional aspects,photogenerated electron-hole distribution and separation efficiency of the composite photocatalysts were determined,and their photocatalytic performance and its mechanism were investigated.The research is described in the following summary:（1）The supramolecular precursor assembled by melamine,cyanuric acid and benzene melamine was calcined,and eosin Y（EY）was grafted by reflux to prepare ey-Ph CN（EY-Ph CN）,which was rich in carbon vacancy,benzene ring and EY.The prepared EY-Ph CN nanosheets can realize the directional transfer of electrons and the separation of electron-hole space.On EY-Ph CN catalyst supported by 1%Pt,the optimal H₂yield is 4.44 mmol g^-1h^-1,and the apparent quantum yield at 420 nm is27.38%.In addition,EY-Ph CN showed high CO₂reduction efficiency when H₂O was used as reducing agent,and the yields of CO and CH₄within 5 h of light were 61.91and 13.75μmol g^-1,respectively,which were higher than those of CN(CO:24.60μmol g^-1and CH₄:8.51μmol g^-1).（2）The Br-doped and C-and N-deficient CN（CN_V-mCN-Br）was synthesized by the mechanical and chemical method of ball-milling and calcination of the N-containing precursor with 6-bromopyridine-3-formaldehyde（Br Py CHO）.In the CN_V-m CN-Br frame,the mechanical force generated by ball milling and the HBr gas induced by Br Py CHO decomposition result in the simultaneous formation of C and N vacancies and C-Br bonds.The prepared CN_V-m CN-Br has nano-flake morphology,and the electron-hole separation is realized.CN_V-m CN-Br loaded with 1%Pt showed excellent photocatalytic H₂evolution rates in distilled water and simulated seawater,which were 22.21 and 34.41 mmol g^-1h^-1,respectively,7.7 and 4.4 times that of the original CN.（3）Bi and Sn bimetallic modified benzene doped CN(BS_0.5-Ph CN)was prepared by solvothermal method.Under visible light,BS_0.5-Ph CN showed better CO₂reduction efficiency when H₂O was used as reducing agent.The yields of CO and CH₄were 55.34 and 12.43μmol g^-1,respectively,after 5 h illumination,which were higher than those of CN(CO:8.54μmol g^-1,CH₄:3.79μmol g^-1).The mechanism of CO₂reduction was analyzed.