Preparation, Characterization And Photocatalytic Properties Of Modified Graphitic Carbon Nitride

With the continuous exploitation and consumption of fossil energy,energy shortage and environment pollution have become two major problems hindering the sustainable development of today's society.Photocatalysis is considered as one of the most promising methods to solve the above problems.Graphite carbon nitride(g-C₃N₄)is a new type of non-metallic inorganic semiconductor material.Due to its advantages of excellent stability,stable visible light response and moderate band structure,it is regarded as a highly efficient catalyst driving photocatalytic technology to decompose water to produce hydrogen and degrade pollutions.However,the inherent defects of g-C₃N₄,such as high producing hydrogen reaction energy barrier,low specific surface area and easy recombination of photogenerated charge-hole,seriously limit its application in the field of photocatalysis.In this work,three strategies were proposed to modify g-C₃N₄,including:surface-loading Pt single atom,doping alkai metals(Na?K?Li)and constructing homojunctions.The main research contents and results of this work are as follows:(1)Preparation of carbon nitride loading platinum single atom catalysts by microwave-assisted and its photocatalytic performance study:We used g-C₃N₄ as the carrier,Pt Cl₂ as the platinum source and acetonitrile and trimethylamine as the solvent,single atom Pt catalysts(SA-Pt/g-C₃N₄)with uniform dispersion and high stability was successfully prepared by microwave hydrothermal synthesis.Pt was observed on the surface of g-C₃N₄ as an isolated and uniform single atom in the high-angle annular dark field image of scanning transmission electron microscope.X-ray photoelectron spectroscopy shows that the coordination structure between Pt and N/C in g-C₃N₄ triazine ring is formed.Steady-state fluorescence spectra shows that loading Pt can greatly inhibit the electron-hole pair recombination,making more electrons participate in the catalytic reaction.As a hydrogen-producing co-catalyst,Pt with uniform load on the surface not only significantly improves the photocatalytic hydrogen production performance of g-C₃N₄,but also maximizes the atomic utilization rate of precious metals.The hydrogen production rate of SA-Pt1.5/g-C₃N₄ reached 4.02 mmol g^-1 h^-1,which is 27 times that of g-C₃N₄,and the monatomic Pt showed extremely high stability in the photocatalytic process.(2)Preparation of alkali metal doped carbon nitride and its photocatalytic performance study:Alkali metal-doped carbon nitride(Na-K-Li-DCN-NF)was obtained by calcining the molten salt system after hydrolyzing and stripping bulk carbon nitride(DCN)by Na OH.The product were tested by XRD,TEM,BET and other characterization methods,and the properties of producing hydrogen and degrading Rh B degradation under visible light were investigated.The hydrogen production rate of Na-K-Li-DCN-NF was228?mol/h,30 times that of DCN,and showed excellent catalytic stability.When Na-K-Li-DCN-NF was irradiated for 120 min,the degradation rate of Rh B was 97%,and the kinetic rate was 5 times that of DCN.The reason lies in that nano-rod structure with high specific surface area can provide a large number of photocatalytic reaction active sites.Doping with Na,K and Li improves separation effect of electron-hole pair,and also change the band structure of carbon nitride,making it more favorable for photocatalytic reaction.(3)Construction of alkali metal doped carbon nitride homojunctions and its photocatalytic performance study:The alkali metal doped modified carbon nitride and graphite carbon nitride(UCN)were combined to form homojunctions(M-DCN-NF-UCNx).The photocatalytic hydrogen production performance of M-DCN-NF-UCN2 was the best,and its hydrogen production rate was 280?mol/h,1.3times that of M-DCN-NF and 37 times that of DCN.The interfacial region generated by constructing homojunctions is favorable for the separation and migration of photogenerated carriers.Photoexcited electrons are more easily transferred to the surface of the catalyst to participate in the reduction reaction under the action of internal electric field.