Designed Synthesis And Catalytic Property Studies Of Hierarchical Porous Metal And Metal Oxide Materials

Hierarchical porous material is defined as a material with two or more kinds of pore structures with different pore sizes in the same main material.The pore size is distributed step by step,which has the advantages of all levels of pore structures.Each level of pore structures is composed of the next level of pore structures with hierarchical advantages.Because the coupling effect of hierarchical pore structure produces functional synergistic effect,hierarchical porous materials show more excellent and multifunctional synergistic effect than single pore structure in function and property.At present,the synthesis methods for preparing hierarchical porous materials are limited,and most of them are only suitable for the synthesis of hierarchical porous non-metallic base materials.There are few reports on the preparation methods for hierarchical porous metal and metal oxide materials.At the same time,metal and metal oxide precursors（usually alkoxide）in traditional sol-gel synthesis system for nanomaterials,difficult to control the hydrolysis polymerization process is prone to macro phase separation.For other metal salts（such as nitrate,sulfate,and chloride）,weak interfacial interactions between metal ions and surfactant molecules are difficult to drive the assembly of surfactant micelles and entry into the framework.In addition,the structure of hierarchical porous metal and metal oxide materials is prone to collapse during crystallization process,which greatly affects the structural stability and porosity of materials.How to expand the synthesis method to prepare hierarchical porous metal and metal oxide materials with more diverse element composition and more complex structure has become a frontier problem.Therefore,it is of great practical significance to develop effective methods to solve the above problems in order to promote the further development of the field of hierarchical porous metal and metal oxide material synthesis.This paper by macroporous/mesoporous hierarchical porous metal and metal oxide materials as the research object,from the design of the synthetic method,structure characterization,application performance and relationships between structure and activity to carry out the research.The thesis mainly includes the following aspects:1.We have designed a double-base plate cooperative assembly strategy to construct three-dimensional ordered macroporous/mesoporous（3DOM/m）noble metal materials using PS-b-PAA micelles and polymer colloidal crystals as templates.Using double layer FTO glasses as substrates can effectively stabilize the ordered structure of polymer colloidal crystals during the synthesis process.The composite micelles composed of block copolymer PS-b-PAA and noble metal ions infiltrated into the macropores of the opal template by capillary force.After aging,reduction and elution,highly stable 3DOM/m porous structure can be obtained.The prepared 3DOM/m noble metal materials have abundant mesoporous,well-ordered and tunable macropores（160～320 nm）and stable hierarchical pore structure.Benefit from the characteristics of the hierarchical pore structure,the prepared 3DOM/m Pd film can effectively improve the molecular mass transfer effect in the process of catalytic reaction,increase the active site of the reaction,enhance the roughness of the material,and show excellent performance in the electrocatalytic oxidation of formic acid(the mass activity up to 95.59 m A·mg^-1),much higher than Pd materials with single porous structure as well as commercial Pd.This study provides an effective method for the preparation and application of 3D ordered macroporous/mesoporous noble metal materials in electrochemical catalysis,and provides a universal method for the design of other hierarchical porous materials as candidates for energy storage and transformation.2.A confined self-assembly strategy was developed to successfully prepare three-dimensional ordered macroporous/mesoporous（3DOM/m）hierarchical porous high-entropy metal oxides（HEOs）materials,which extended the hierarchical pore structure to the field of metal oxides with more diverse elemental composition.This method avoids the problems of few active sites,low mass transfer efficiency and structure collapse caused by traditional high-temperature synthesis methods.By adjusting the type and amount of metal nitrate,this method has successfully synthesized a series of HEOs materials,and expanded the metal elements to 9 for the first time and avoided the occurrence of heterogeneous separation,which fully demonstrates the reliability and universality of this method.The synthesized3DOM/m HEOs nanomaterial has three-dimensional ordered open macroporous structure,abundant mesoporous,high crystallinity and ultra-high specific surface area(up to 123 m²·g^-1).The 3DOM/m structure significantly improves the surface area and mass transfer efficiency,and provides abundant catalytic active sites and oxygen vacancies.Combined with the excellent intrinsic properties of HEOs,the prepared3DOM/m HEOs nanoskeleton material has broad application prospects in the selective catalytic oxidation of alcohols.The conversion of P-benzyl alcohol reached72%in the atmospheric pressure catalytic oxidation process,and the selectivity was close to 95%,which illustrate it is a kind of efficient and stable alcohol oxidation catalyst.This work provides a novel and practical method for the preparation of hierarchical porous HEOs materials with great application potential.3.We have proposed a ligand-assisted hard template method for preparation of multi-shelled mesoporous hollow metal hydroxide and oxide nanospheres,which further expands the diversity of hierarchical porous metal and metal oxide nanostructures.Using Si O₂ spheres as template,ascorbic acid（AA）as ligand and HMT as chelating agent,multi-shell mesoporous hollow metal hydroxide and oxide nanospheres with hierarchical porous structure were successfully prepared through the collaborative self-assembly strategy of layer and layer coating.The synthesized nanospheres have monodispersion and uniform particle size,and the particle size is precisely adjustable from 280 nm to 690 nm.They have high specific surface area and macroporous/mesoporous hierarchical porous structure of stable multi-shelled layers.The obtained multi-shelled mesoporous hollow nanocomposites loaded with gold nanoparticles show excellent performance in the field of inert hydrocarbon catalysis.The conversion of ethylbenzene in the solvent-free catalytic oxidation process is28.2%,and the selectivity is close to 90%,which is a kind of efficient and stable ethylbenzene oxidation catalyst.Moreover,it has excellent performance in solvent-free catalytic oxidation of a series of inert hydrocarbons.This work broadens the preparation methods of multi-shell mesoporous hollow metal oxide materials and promotes the application of hierarchical porous metal and metal oxide materials in organic catalysis.In conclusion,this thesis proposes some effective solutions to the challenges in the development of macroporous/mesoporous hierarchical metals and metal oxide materials.In terms of synthesis methods,this thesis proposes synthesis strategies such as double-base plate cooperative assembly strategy,confined self-assembly strategy and ligand-assisted hard template method to solve the problems of particle aggregation,phase separation and pore structure collapse caused by high temperature calcination in the synthesis system of macroporous/mesoporous hierarchical porous metal and metal oxides.In terms of the study of synthesis mechanism,this paper tracks the synthesis process by applying a variety of characterization methods,and summarizes the formation mechanism of macroporous/mesoporous hierarchical porous metal and metal oxides with different porous structure.In performance of application and reveal the structure-activity relationship,according to the sample of the components,structure and intrinsic chemical properties,the obtained materials have been applied to the targeted application.And the relationship between the performance of the catalyst and the mesoporous/mesoporous hierarchical porous structure has been revealed from many angles through the design of effective comparison experiments.