Modification,Performance And Mechanisim Of Carbon Nitride Photocatalyst For Photocatalytic CO₂ Reduction

With the rapid development of the economy,a large amount of fossil energy is consumed,resulting in a sharp increase in the accumulation of CO₂gas,which brings about an energy and environmental crisis that seriously affects people’s lives.Photocatalytic CO₂ reduction technology is a new promising conversion technology to reduce CO₂ with the help of solar energy,which is the most promising technology to solve the energy crisis and environmental crisis in the world today by producing high value-added fuel and solving the greenhouse effect problem faced by the world today.However,photocatalysis technology relies heavily on the high efficiency and stability of photocatalysts,so the development of an efficient and stable photocatalytic material is a prerequisite and foundation for the industrialization of photocatalytic CO₂ reduction.Currently,polymeric carbon nitride（PCN）as a non-metallic photocatalytic material has received wide attention in photocatalytic CO₂ conversion.However,with the in-depth study of the material by researchers,PCN has gradually revealed some problems,including low photogenerated carrier separation efficiency and low light absorption efficiency.Therefore,in this thesis,three kinds of heterojunction composite photocatalysts,NiWO₄/PCN,PCN/Bi₁₂O₁₇Cl₂/PCN and Au-θ-Al₂O₃/PCN,were designed and prepared for the modification of PCN,and their physicochemical properties,microscopic morphological characteristics,photoelectrochemical properties,CO₂reduction performance,reaction mechanism and reaction product intermediates were systematically investigated.The results of the experimental studies are as follows:（1）NiWO₄/PCN heterojunction composite photocatalysts were prepared by a simple hydrothermal method combined with high-temperature calcination technique,and their photocatalytic performance was substantially improved compared with PCN and NiWO₄ alone.The photocatalytic experimental results showed that the NiWO₄/PCN heterojunction composite photocatalyst reached 100%selectivity for the photocatalytic reduction of CO₂to CO.After a 4 h performance test,the photocatalytic CO₂ reduction to CO yield of 10%-NiWO₄/PCN sample could reach21.2μmol g^-1,which was 2.58 times(8.2μmol g^-1)and 1.84 times(11.5μmol g^-1)than that of PCN and NiWO₄,respectively.Meanwhile,the NiWO₄/PCN heterojunction composite photocatalyst showed good stability and reusability after 16h of cycling stability test.The improved performance was mainly attributed to the formation of built-in electric field at the interface of p-type NiWO₄ and n-type PCN,which significantly improved the transfer and separation efficiency of photogenerated electrons and holes between them.（2）PCN/Bi₁₂O₁₇Cl₂/PCN sandwich heterojunction composite photocatalysts were prepared by a simple hydrothermal method.This unique sandwich structure enables the diffusion of electrons from PCN to Bi₁₂O₁₇Cl₂ under strong interfacial bonding to form a double interfacial built-in electric field,which can effectively drive the transport and separation of photogenerated carriers,thus improving the enhanced photocatalytic activity.After 4 h of photocatalytic experiments,the photocatalytic reduction of CO₂ to CO by PCN/Bi₁₂O₁₇Cl₂/PCN-3 can reach a yield of 1784.0μmol g^-1,which is 3.74 and 8.95 times that of PCN alone(477.6μmol g^-1)and Bi₁₂O₁₇Cl₂(199.4μmol g^-1),respectively;the reduction to CH₄ The yield of reduction to CH₄was up to 218.0μmol g^-1,which was 2.62 and 6.65 times higher than that of PCN alone(83.2μmol g^-1)and Bi₁₂O₁₇Cl₂(32.8μmol g^-1),respectively.Meanwhile,after16 h of cyclic reaction,the performance hardly decreased,indicating the good stability and reusability of the prepared PCN/Bi₁₂O₁₇Cl₂/PCN sandwich heterojunction composite photocatalyst.（3）The Au-θ-Al₂O₃/PCN heterojunction composite photocatalysts were prepared by a simple hydrothermal method combined with a high-temperature calcination technique,showing excellent photocatalytic CO₂ reduction performance.After 4 h of photocatalytic reaction,the yield of the best sample 10%-Au-θ-Al₂O₃/PCN photocatalytic CO₂ reduction to CO can reach 30.3μmol g^-1,which is about 4 times higher than that of PCN alone(7.8μmol g^-1),and it can still maintain stable photocatalytic activity after four cumulative 16 h runs,showing good stability and reusability.It is shown that Au nanoparticles can be used as an electron transport medium to enhance the charge transfer rate and generate surface local plasmon resonance effect to extend the visible light response range,while Au atoms doped in theθ-Al₂O₃ lattice can also induce the generation of oxygen vacancies to capture photogenerated electrons to improve the photogenerated carrier separation efficiency and act as the active site for the reaction,which in turn generates a synergistic catalytic effect and enhances the photocatalytic CO₂ reduction performance.