Preparation Of G-C₃N₄-based Photocatalyst And Its Photocatalytic Performance

The most urgent solution for the energy crisis and environmental problems is to maximize the utilization of solar radiation through the development and exploration of sustainable technologies.Heterogeneous semiconductor photocatalysis has become an important technology to solve these above two problems,one of which,g-C₃N₄ with advantages of low cost,easy preparation,non-toxic properties,high oxidation ability,chemical blunt,photochemical stability and environment friendly,is widely used in the field of the photocatalytic degradation of pollutants.In addition,from view of the thermodynamics,photochemical and sustainability,g-C₃N₄ is regarded as an effective photocatalyst.However,the photocatalytic performance of bulk g-C₃N₄ is limited due to its low surface area,insufficient light absorption and rapid recombination of photo-generated carriers.In order to overcome the inherent defects of the bulk g-C₃N₄,the photocatalytic performance of g-C₃N₄ was improved by changing morphology structure,doping with metal or non-metal elements and coupling with permonosulfate（PMS）,etc.Meanwhile,structural composition,morphological characteristics,optical properties and photochemical properties of catalysts were analyzed by characterization technology.Combined with the photocatalytic degradation data,the possible catalytic degradation mechanism of pollutants was proposed,which provided a new idea and method for the practical development and application of g-C₃N₄.Specific research contents are as follows:（1）Porous graphite phase carbon nitride（PCN）was prepared through calcining monocycyanamide（NH₂CN）and ammonium chloride（NH₄Cl）as raw materials.After that,the porous g-C₃N₄（Co-PCN）with different mass of doped Co（1%,3%,5%,10%）was obtained by a simple two-step calcination method.Under visible light irradiation,the degradation efficiency of Rhodamine B（RB）by Co-PCN（5%）was up to 98.1%,which was 1.28 and 2.18 times that of PCN and g-C₃N₄,respectively.The results of X-ray Diffraction（XRD）and Fourier Transform infrared spectroscopy（FTIR）confirmed that cobalt has been successfully doped in g-C₃N₄ without destroying crystal structure of g-C₃N₄.Transmission Electron Microscopy（TEM）showed that Co-PCN possesses abundant pore structure.In addition,the optical and electrochemical test showed that the photocurrent intensity of Co-PCN was 1.5 times that of g-C₃N₄,and meanwhile its photoluminescence intensity was also lower than that of g-C₃N₄.According to the K-M curves,Co-PCN band gap was adjusted from 2.44 e V（g-C₃N₄）to 2.28 e V.The active species including·O₂^-and h⁺are dominated in photocatalytic degradation.Combined with the analysis of semiconductor band theory,the excellent degradation performance of Co-PCN can be attributed to the synergistic result of stronger visible light absorption capacity obtained by adjusting the size of band gap and the hoster-guest interaction between Co and g-C₃N₄.（2）Porous nitrogen defect g-C₃N₄（DPCN）was prepared by one-step calcination with different sizes of silica as the template using monocyandiamide as raw material.Choosing Rhodamine B（RB）as target pollutant,the effect of three factors,including the size of silicon dioxide,the precursor proportion,calcination method（temperature programmed,secondary calcination）on the photocatalytic performance of catalysts was studied.The results indicated that under the condition of silicon dioxide with size of400 nm,the precursor proportion of 1:1 and the programmed heating,DPCN exhibited RB degradation as high as 88.83%,which is 2.91 times that of g-C₃N₄.The morphology characterization results show that the pore size of DPCN is about 400nm,and its porous two-dimensional plane structure is beneficial to increase the contact area of pollutants and improve photocatalytic reaction efficiency.Compared with bulk g-C₃N₄,DPCN shows higher photogenerated charge separation efficiency,and the photocurrent density is twice that of bulk g-C₃N₄.The unique nitrogen defect structure plays an important role in the expansion of visible light absorption range and the effective separation of the photogenerated carriers,which is beneficial to the improvement of photocatalytic performance.·O₂^-and·OH are the main active species in photocatalytic degradation.（3）In this work,carbon and oxygen codoping g-C₃N₄（CO-C₃N₄）is prepared through one-step calcination of the melamine-oxalic acid aggregates.The visible light-assisted photocatalytic degradation efficiency of the tetracycline hydrochloride（TCH）for CO-C₃N₄ is significantly enhanced by introducing the peroxymonosulfate（PMS）,and the apparent rate constant is greatly increased from 0.01966 min^-1 in CO-C₃N₄/vis system to 0.07688 min^-1 in CO-C₃N₄/PMS/vis system.It is found that carbonyl for CO-C₃N₄ might offer the possible reactive sites for PMS activation and collection sites of photo-generated electrons,greatly accelerating carrier’s separation for PMS activation.The favorable conditions,such as the higher catalyst dosage,higher PMS amount and alkaline p H,contribute to TCH degradation.The deleterious effects of co-existing anions on the TCH degradation efficiency are ranked in a decline:H₂PO₄^->SO₄^2->HCO₃^->NO₃^->Cl^-,and it may be affected by the type and amounts of anions and active radicals generated.The radical trapping tests and electron spin resonance（ESR）detection display that·O₂^-,h⁺,¹O₂,·OH and·SO₄^-all contribute to TCH degradation.Meanwhile,the degradation mechanism,intermediates and degradation pathway of TCH are revealed in CO-C₃N₄/PMS/vis system.This study will offer a new insight for constructing the coupled advanced oxidation technology with multi-reaction synergy to achieve effective treatment of organic wastewater.