Preparation Of Graphitic Carbon Nitride-Based Composite Photocatalysts For Carbon Dioxide Reduction

With the development of society,human demand for energy is increasing.At present,fossil fuels are the main source of energy,and their combustion emits a large number of CO₂,which is one of the main reasons for global warming.Using solar energy to convert CO₂ into valuable chemicals is one of the effective ways to alleviate the greenhouse effect and energy shortage.Photocatalytic reduction of CO₂ is a multi-electron reaction,and the effective transport and utilization of electrons is the key to achieve efficient conversion of CO₂.Especially,the design and development of photocatalyst with excellent performance is the key to improve the catalytic activity.In this thesis,In order to overcome the defects of small specific surface area,low light utilization rate and high recombination rate of photogenerated carriers of the pure g-C₃N₄,We have prepared high activity and high stability Type-II（chapter 2 and chapter 3）,direct Z-scheme（chapter 4 and chapter 5）and indirect Z-scheme（chapter 6）composite photocatalysts by combining g-C₃N₄ with different semiconductors.The phase structure,morphology characteristics,chemical composition,electronic environment,Photoelectrochemical properties,energy band structure and carrier separation and migration of the three composite heterojunction photocatalysts are systematically studied and clarified.First,Zn_0.2Cd_0.8S with photocatalytic CO₂ reduction ability is prepared by hydrothermal method,but this mateiral is easy to suffer photocorrosion and has poor stability.In order to solve this problem,g-C₃N₄ with stable physicochemical properties is coated on the surface of Zn_0.2Cd_0.8S.The method not only improves the stability of the catalyst,but also promotes the separation of photogenerated electrons and holes by the heterojunction interface formed between Zn_0.2Cd_0.8S and g-C₃N₄,thereby improving the photocatalytic CO₂ reduction activity.The performance test results show that the yield of CH₃OH is proportional to the initial concentration of CO₂.In the composite photocatalysts,CH₃OH yield for the optimum Zn_0.2Cd_0.8S/g-C₃N₄ is 11.54μmol·g^-1·h^-1,which is 2.60 times and 2.75 times of Zn_0.2Cd_0.8S and g-C₃N₄,respectively.Moreover,Zn_0.2Cd_0.8S/g-C₃N₄ shows good stability and the yield of CH₃OH is still 92.3%of the initial yield after 4 reaction cycles.Second,Carbon-doped TiO₂（C-TiO₂）is prepared by HF etching Ti₃Al C₂ and then calcined at high temperature.After that,Type-Ⅱg-C₃N₄/C-TiO₂ heterojunction composite photocatalyst with more specific surface area is synthesized by grinding and calcination.The synergistic effect between g-C₃N₄ and C-TiO₂ improves the separation efficiency of photogenerated electrons and holes,prolongs the lifetime of carriers,and makes them have more probability to participate in redox reactions.The experimental results show that the CO and CH₃OH yields of the optimum g-C₃N₄/C-TiO₂ heterojunction photocatalyst are 8.12μmol·g^-1·h^-1 and 12.65μmol·g^-1·h^-1,which are 3.85 times and 4.05 times of those of C-TiO₂,respectively.α-Fe₂O₃/g-C₃N₄ heterojunction photocatalyst with Z-scheme structure is prepared by hydrothermal method.Compared with the Type-II heterojunction composite photocatalyst,the carrier transport mechanism of the Z-scheme structure can allow electrons and holes to react in a conduction band with stronger reducing ability and a valence band with stronger oxidizing ability,respectively,thereby greatly improving the redox ability of the composite photocatalyst.In addition,in the Z-scheme structure,the recombination rate of photogenerated electron-hole pairs is greatly reduced,and the separation and migration rate of carriers are greatly improved,so that the high-efficiency photocatalytic CO₂ reduction activity is realized.The experimental results show that the CH₃OH yields of the optimumα-Fe₂O₃/g-C₃N₄ is 13.15μmol·g^-1·h^-1,which is 3.19 times higher than that of g-C₃N₄(4.12μmol·g^-1·h^-1).NiTiO₃ with perovskite structure and g-C₃N₄ are combined by ultrasonic calcination to form a direct Z-scheme heterojunction composite photocatalyst.The characterization results show that the Z-scheme heterojunction in NiTiO₃/g-C₃N₄ catalyst can distribute the electrons and holes in different phases,which realizes the spatial separation of carriers and contributes to the improvement of activity.The photocatalytic activity of NiTiO₃/g-C₃N₄ with 40%NiTiO₃ is the best,and the CH₃OH yield is 13.74μmol·g^-1·h^-1,which was 3.29 times of that of g-C₃N₄.Finally,Ag nanoparticles are deposited on the surface of Ui O-66 with high specific surface area by photo-deposition method,and then the Ag/Ui O-66@g-C₃N₄ photocatalyst with ternary Z-scheme structure is prepared by coating Ag/Ui O-66 with flocculent g-C₃N₄.The characterization tests show that the addition of Ui O-66 greatly improves the specific surface area of the photocatalyst and provides more active sites for the photocatalytic CO₂ reduction.Meanwhile,the ternary Ag/Ui O-66@g-C₃N₄ photocatalyst has stronger carrier separation and migration ability than the binary Ui O-66@g-C₃N₄ photocatalyst.The activity test results show that the CO and CH₃OH yields of Ag/Ui O-66@g-C₃N₄ can reach 11.45μmol·g^-1·h^-1 and 17.76μmol·g^-1·h^-1,respectively.