Preparation And Photocatalytic Performance Of C/g-C₃N₄ Composites By BPSM Hard Template Method

With the development of society,the water pollution caused by environmental problems becomes more and more serious.Water pollution caused by organic pollutants,because of the complex composition and other problems,the degradation process is particularly difficult.Therefore,it is very important to find an effective method to solve water pollution.Among them,the photocatalytic technology stands out because of the advantages of mild conditions,thorough degradation and green environmental protection.Among a wide variety of semiconductor photocatalysts,graphitic carbon nitride?g-C₃N₄?,as a non-metallic catalyst material,has attracted great attention in the field of photocatalysis with the characteristics of stable physicochemical properties,responsiveness to visible light and simple preparation process.However,through the research of scholars,it is found that g-C₃N₄ itself has some shortcomings,such as small specific surface area,low catalytic site;low conductivity,only 10^-9Sm^-1,its inherent insulation greatly reduces the transport rate of photogenerated carriers,resulting in its high probability of photogenerated electron hole recombination,seriously affecting the photocatalytic performance.In order to improve the existing problems of g-C₃N₄,the current methods to improve its photocatalytic efficiency mainly include:composite with other materials?semiconductor materials,carbon materials?,structural control and precious metal deposition.Among them,the preparation of carbon/graphite carbon nitride composite with large specific surface area and high conductivity by hard template method and carbon material composite is one of the effective strategies to improve the photocatalytic performance of g-C₃N₄.In this paper,on the basis of the previous research of our group,bridged polysilsesquioxane?BPSM?is proposed to be used as the silicon source for the first time;dicyandiamide?DCDA?and partial formaldehyde-methylolated dicyanodiamid?PHDC?are used as the nitrogen source?carbon source?precursor,and by virtue of the hydrogen bond of Si-O,-NH₂,C-O,O-H in BPSM group and DCDA,PHDC,the precursor is embedded in silicon oxide three-dimensional network A series of carbon/graphite carbon nitride composites?C/g-C₃N₄?were prepared by sol-gel process of BPSM-precursor,then calcined and etched by high temperature.The effects of bridged monomers,kinds of organic bridging groups and preparation conditions on the structure and properties of C/g-C₃N₄ were systematically studied.Finally,the photocatalytic properties and mechanism of C/g-C₃N₄ were studied by using methylene blue?MB?as a simulated organic pollutant.The specific research is as follows:?1?Using 3-Glycidoxypropyltrimethoxysilane?GPTS?and?3-Chloropropyl?trimethoxysilane?CPTS?silane coupling agents as raw materials,four kinds of BPSM with polyamines as bridged organic groups?DETA-GPTS,DETA-CPTS,EDA-GPTS,EDA-CPTS?were prepared by reacting with ethylenediamine?EDA?and diethylenetriamine?DETA?respectively.?2?PHDC is nitrogen source?carbon source?,and four kinds of BPSM,DETA-CPTS,DETA-GPTS,EDA-CPTS and EDA-GPTS,are used as silicon source.Hydrothermal synthesis method is adopted to prepare BPSM-PHDC sol-gel by BPSM in situ hydrolysis,then high temperature roasting and template removal.FT-IR,XRD,SEM,UV-Vis,TG,BET and other characterization methods were used to systematically characterize the crystal form and microstructure of the material.The effects of preparation conditions such as the kinds of silane coupling agents,bridged organic groups and BPSM/PHDC ratio on the structural parameters,carbon content and photocatalytic properties of the materials were discussed.The photocatalytic properties of the composite were determined by photocatalytic degradation of methylene blue?MB?.The results show that the composites prepared by four BPSM do not change the structure of the materials.Compared with PHDC,most of the composites have fold like lamellar structure,and the carbon elements are distributed on the surface of g-C₃N₄,which enhances the absorption and absorption of visible light.The specific surface areas of DEA-CPTS-PHDC-x?DEA-GPTS-PHDC-x?EA-CPTS-PHDC-x and EA-GPTS-PHDC-x are higher than those of CN obtained directly from the pyrolysis of dicyandiamide,DEA-GPTS-PHDC-x and EA-GPTS-PHDC-x are higher than DEA-CPTS-PHDC-x and EA-CPTS-PHDC-x.With the increase of BPSM dosage,the content of inorganic carbon also increased,among which DEA-GPTS-PHDC-x and EA-GPTS-PHDC-x introduced more inorganic carbon than DEA-CPTS-PHDC-x and EA-CPTS-PHDC-x.The pore size of the composite decreases with the increase of inorganic carbon content,and the pore structure is mainly microporous.The introduction of appropriate amount of inorganic carbon is beneficial to the enrichment of organic matter and the improvement of photocatalytic ability.The specific surface area of DEA-CPTS-PHDC-2 prepared by DETA-CPTS is 190.1 m²/g,which is 17.3 times of that of CN.The photocatalytic properties of the composite were determined by photocatalytic degradation of methylene blue?MB?.The results show that the photocatalytic rate constant k of DEA-CPTS-PHDC-2 is 0.023min^-1,and the photocatalytic performance is 5.93 times that of CN.?3?Using dicyanodiamide?DCDA?as nitrogen source and four BPSM?DETA-GPTS,DETA-CPTS,EDA-CPTS,EDA-GPTS?as carbon source and silicon source,the BPSM-DCDA-sol-gel was prepared by in-situ hydrolysis of BPSM by hydrothermal synthesis method,and then the carbon composite graphite carbon nitride composite material was obtained by HF etching template.The effects of preparation conditions such as silane coupling agent,bridged organic group and BPSM/DCDA ratio on the structural parameters,carbon content and photocatalytic properties of the materials were discussed.The results show that the structure of s-triazine units or tri-s-triazine subunits of g-C₃N₄ has not changed,only the degree of polymerization has decreased.The carbon produced by pyrolysis of organic bridging group in BPSM is compounded on the surface of g-C₃N₄,which improves the absorption of visible light.Among them,DEA-GPTS-DCDA and EA-GPTS-DCDA composites prepared by two kinds of BPSM,DETA-GPTS and DEA-GPTS,contain more inorganic carbon.By etching SiO₂,the composite shows more voids,and the particle size decreases with the increase of carbon content.The specific surfaces of DEA-CPTS-DCDA-x and DEA-GPTS-DCDA-x composites prepared by DETA-CPTS and DETA-GPTS increase with the increase of BPSM addition,and the specific surfaces of EA-CPTS-DCDA-x and EA-GPTS-DCDA-x composites prepared by EDA-CPTS and EDA-GPTS increase with the increase of BPSM addition.The photocatalytic experiments show that the photocatalytic efficiency of DEA-CPTS-DCDA-3,DEA-GPTS-DCDA 2,EA-CPTS-DCDA-3 and EA-GPTS-DCDA-2 is the best among the same type of composite materials,and the rate constants are 0.023min^-1,0.012min^-1,0.015min^-1 and0.010min^-1,respectively.The maximum specific surface area of DEA-CPTS-DCDA-3is 152.2 m²/g,which is 13.8 times of CN.The photocatalytic experiment shows that the degradation rate of DEA-CPTS-DCDA-3 to MB reaches 95.5%,which greatly improves the catalytic ability,and the photocatalytic performance is 5.93 times that of CN.The mechanism of photocatalysis was verified by adding free radical catcher.The results show that h⁺ is the main active group.