Covalent Organic Porous Polymers And Their Derived Carbon-immobilized Metal Nanoparticles For The Catalytic Methanolysis Of Ammonia Borane

Hydrogen energy is considered as an ideal alternative to fossil fuels because of its high energy density and water as the only by-product.Vigorously developing hydrogen economy is not only conducive to economic growth,but also in line with the goal of sustainable development.At present,the biggest problem to restrict the development of hydrogen economy is the storage and transportation of hydrogen.Ammonia borane（AB）,with a mass hydrogen storage density of 19.6 wt%,is safe and stable at room temperature,which makes it easy to store and transport.Ammonia borane has been considered as a great potential chemical hydrogen storage material.Due to high hydrogen purity,efficient regeneration of byproduct,as well as its possible application at low temperatures,the methanolysis of AB has been regarded as an ideal way to release hydrogen.In order to solve the problems of low activity and poor cycling stability of the catalysts for AB methanolysis,a series of supported and confined metal nanoparticles（NPs）catalysts were developed with high nitrogen-containing covalent organic porous polymers（COPs）and their derived carbon materials as supports in this thesis.The effects of the structure and chemical composition of the supports,and preparation conditions of the catalysts on the particle size of metal NPs and the hydrogen production rate were studied.The specific research contents are as follows:（1）Rh NPs catalyst confined by covalent organic porous polymers（PC-COP）was prepared by M-N coordination reduction method.FT-IR and XPS characterizations confirm the coordination state between Rh ions and N species of PC-COP.This coordination structure helps to improve the dispersion,reduce the particle size,and inhibit the aggregation of metal NPs.Then,the catalyst was used for the methanolysis of AB.The effects of the preparation conditions of the catalyst and methanolysis reaction conditions on the hydrogen production rate were investigated.The results show that the solvent polarity and metal loading affect the size of metal NPs.When the volume ratio of methanol to dichloromethane is 2:1,the particle size of Rh NPs is the smallest,only 2.0 nm.With the increase of metal loading,the particle size of Rh NPs increases.Therefore,with the same support,the activity of the catalyst is only related to the particle size of metal NPs.Rh/PC-COP with Rh loading of 1.91 wt%shows the highest catalytic activity for the methanolysis of AB,and the TOF value can reach 505min^-1.（2）Two-dimensional（2D）porous aromatic framework（PAFs）materials can self-assemble to form multilayer stacked structures due to theπ-πinteractions between layers,which will reduce the surface utilization of PAFs nanosheets and seriously affect the practical application of PAFs materials.The vertical 2D-2D porous aromatic frameworks-graphene based composites maximize the utilization of PAFs nanosheets and provide a large specific surface area and abundant slit pores for the loading of metal nanoparticles and the transport of reaction substrates.Firstly,monolayer benzidine functionalized graphene oxide molecular pillars（BZ-GO）were prepared by N-Boc protection method,and then vertical porous aromatic frameworks-graphene composites PAF-GO-V1 were prepared by a step-growth strategy.The specific surface area of PAF-GO-V1 is measured to be larger than that of the composites PAF-GO-P with parallel structure,which is four times that of its sub-components PAFs and GO.Then,Co/PAF-GO-V1 was prepared by ultrasonic impregnation reduction method with PAF-GO-V1 as the support.TEM characterization shows that the average particle size of Co NPs in Co/PAF-GO-V1 is only 1.4 nm,which is less than that of Co/GO,Co/BZ-PAF,as well as Co NPs without support.The catalyst was used to catalyze the methanolysis of AB.The effects of specific surface area of the supports and particle size of Co NPs on the methanolysis performance of AB were investigated.The results show that large specific surface area and pore volume played an important role in the confinement and dispersion of metal NPs.In addition,the abundant nitrogen in the support provides sufficient anchor sites for metal NPs.The electron transfer and interaction between nitrogen and metal can actively promote the decomposition of AB.Co/PAF-GO-V1 shows the highest catalytic activity with a TOF value of 47.6 min^-1.In addition,a series of isotopic experiments were performed,and it is found for the first time that the cleavage of the O-H bond in methanol is involved in the rate-determining step（RDS）of AB methanolysis.（3）A new spherical porphyrin framework material（BPDA-POF）was prepared by solvothermal method.Subsequently,a series of nitrogen doped carbon spheres（POF-NCT,T is the pyrolysis temperature）were prepared by the pyrolysis of POFs.Ultrafine Ru NPs were encapsulated in the pores of nitrogen doped carbon spheres by ultrasonic impregnation reduction method.Reconstructed slices tomographic characterization confirms that the highly dispersed Ru NPs with a mean size of 3.2 nm are successfully encapsulated in the pores of POF-NC600.The nanoscale confining effect of the pores can effectively prevent the aggregation and loss of Ru NPs.The effects of nitrogen species in Ru@POF-NCT on the particle size of metal NPs and the rate of hydrogen production were investigated.The results show that the anchoring of pyridine nitrogen in POF-NCT can lead to the high dispersion,small particle size,as well as high catalytic activity of Ru NPs.（4）Hollow structure nitrogen doped carbon materials with ultra-low density,high specific surface area and pore volume,good substrate accessibility,as well as rich nitrogen sites,are especially suitable for stabilizing metal NPs as the support.Using Si O₂ nanospheres as hard template,a hollow nitrogen doped carbon nanosphere（HNCS）was prepared by coating porphyrin frameworks（POFs）on Si O₂,pyrolyzing in an inert atmosphere,and etching off the templates.The effect of pyrolysis temperature on the physical properties of HNCS was studied.The results show that HNCS800 exhibits stable hollow structure,the largest specific surface area and pore volume,as well as the most defect sites.Then,the hollow carbon nanospheres were employed as the support to immobilize Co NPs by ultrasonic impregnation reduction method.The average particle size of Co NPs in Co/HNCS800 is measured to be only 2.5 nm,which is much smaller than that of Co NPs without support.The catalyst was used for the methanolysis of AB to produce hydrogen,and the TOF value can reach up to 57.9 min^-1.The high catalytic activity of the Co/HNCS800 catalyst can be attributed to the high specific surface area of HNCS800,ultrafine metal NPs,as well as electron transfer between nitrogen and metal.