Preparation Of Keggin Type POMs-Based Composites And Their Photocatalytic Nitrogen Reduction

Ammonia is an important chemical product and a liquid fuel with great potential.Its industrial production not only consumes high energy,but also emits a huge amount of CO₂;Urea,as a downstream product of ammonia,consumes over 80%of ammonia production every year,and its industrial production also has the characteristics of high energy consumption and high carbon dioxide emissions.With the continuous intensification of energy consumption in human society and the continuous deterioration of the ecological environment,the photocatalytic nitrogen reduction technology with the characteristic of green,environmentally friendly,and sustainable,as an alternative to traditional nitrogen fixation processes,has attracted more and more researchers.Polyoxometalates（POMs）with their unique molecular structure and physical and chemical properties,have broad development prospects in the field of six-electron photocatalytic nitrogen fixation.However,the problems such as fast recombination of photo generated charge carriers,small specific surface area,easy self-aggregation,and difficulty in recovery caused by strong water solubility have severely limited the direct use of POMs in the field of photocatalysis.Based on these,in this dissertation,combining the POMs with suitable carriers,the role of POMs in catalytic reaction was discussed through the study of the structure-activity relationship of composites,which pointed out the direction for the design of high-performance POMs-based photocatalysts.The main research contents are as follows:（1）To solve the problem that POMs fall off easily in the catalytic process due to weak binding force between POMs and carrier,a series of LDHs with the POMs intercalated were prepared by using the electrostatic self-assembly strategy.After using suberic acid opened the anion exchange channel of LDHs materials blocked by carbonate ions,then PMo_12-xVx@ZnAl-LDHs（X=0,1,2,3,8）were prepared by anion exchange method.The performance test shows that the PMo9V3@ZnAl-LDH achieved a high ammonia yield of 89.16 μmol·h^-1·g^-1cat.By combining the characterization of the morphology and structure,in-situ DRIFTS,EPR and so on,the bidirectional effect of POMs intercalation on the composite was discussed,and the role of POMs in the catalytic reaction was revealed.The intercalation of POMs ions not only broadens the interlayer spacing of the composites and expands the light absorption range,but also increases the hydrophilicity of the material,leading to the generation of hydrogen by-products,promotes the improvement of the catalytic activity of the composites in general.POMs ions play the roles of electron repeater,photogenerated carrier excitation source and active site of water oxidation in the catalytic reaction.（2）To consider the problem of low specific surface area of 2D materials and explore whether POMs can be used as active site for nitrogen reduction,the Keggin-type SiWn was selected as guest molecule.Using the space limited strategy,the POMs were loaded in different cavities of MIL-101（Cr）by impregnation method and one-step solvothermal method,respectively.Two series of composites,X-MIL101（Cr）-I（I=indentation,X=0,100,250）and X-MIL-101（Cr）-S（S=solvothermal,X=50,100,200,300）,were finally obtained by adjusting the loading content of POMs.The performance test shows that the 100-MIL-101（Cr）-S achieved a high ammonia yield of 104.86 μmol·h^-1·g^-1cat.By combining the characterization of the morphology and structure,and mechanism testing,it was confirmed that the catalytic activity of the sample obtained by solvothermal method was higher,and the catalytic activity of the composites showed a typical "volcanic" trend due to the regulation of POMs loading.Keggin-type SiW₁₂ could effectively serve as the reaction site of nitrogen activation.The synergistic effect of SiW₁₂ and MIL-101（Cr）is the key to improve the photocatalytic performance.（3）Fine design and micro-regulation of the catalytic active site can change the electron cloud density and electron spin state of the active site,thus effectively promoting the improvement of catalytic reaction activity.The microchemical environment of the active site Keggin-type POMs was regulated by doping transition metal ions and changing the layout of metal ions,and the SiW9@MIL-101（Cr）,SiW9M3@MIL101（Cr）（M=Fe,Co,V,Mo）and D-SiW₉Mo₃@MIL-101（Cr）were obtained.The performance test shows that the SiW₉Mo₃@MIL-101（Cr）achieved a high ammonia yield of 185.67 μmol·h^-1·g^-1cat.Combined with the characterization of the morphology and structure,in-situ DRIFTS,and other mechanism tests,it was found that the increase in the electron cloud density of the active element W in POMs can effectively improve the efficiency of the composite material in photocatalytic ammonia synthesis.And it was also revealed that the catalyst SiW₉Mo₃@MIL-101（Cr）adopted a distal nitrogen hydrogenation reaction path.（4）To meet the requirement of active sites for the photocatalytic urea synthesis reaction from N₂ and CO₂,Keggin-type POMs（SiWMo）with dual active sites（tungsten as the nitrogen active site,molybdenum as the carbon dioxide active site）were designed and constructed.The POMs were loaded into the pores of the carrier MIL-101（Cr）by a one-step solvothermal method,and SiW₁₂-xMox@MIL101（Cr）（X=0,3,6,9,12）were obtained.The performance test shows that the SiW₆Mo₆@MIL-101（Cr）achieved a high urea yield of 1148 μg·h^-1·g^-1cat under optimal experimental conditions.Combined with the characterization of the morphology and structure,in-situ DRIFTS and density functional theory calculations,the successful construction of the dual-active site design strategy was proved.And it was clear that in the urea synthesis step,more energy was needed to cross the energy barrier for the formation of the tower-like intermediate*NCON*and form the intermediate*NHCONH2*,which was the speed determination of the photocatalytic urea synthesis reaction...