Designed Synthesis And Property Studies Of Crystalline Mesoporous Transition Metal Oxides And Their Composites

Since the twentieth century,transition metal oxides have drawn great attention because of their low cost and wide application fields.With the continuous research on the micro-nano structure and morphology of materials,researchers have reported that mesoporous materials possess the characteristics of abundant oxygen vacancy sites,high specific surface areas,developed pore structure,tunable pore size and structure,making them have application potential in energy conversion and storage,catalysis,adsorption and separation and other fields.Therefore,more and more researchers have exhibited tremendous interests in the subject of introducing mesoporous structures into transition metal oxides,and a series of representative mesoporous transition metal oxides have been successfully prepared.However,the existing synthesis strategies still have some limitations:First,the synthesis method lacks universality,and the proposed synthesis strategy is only applicable to one or several specific transition metal elements.Second,the reaction yield is low and it is hard to get more than grams of yields in a single reaction.Third,it is difficult to precisely control the pore size of the mesopores.The existing methods of adjusting the pore size by controlling the chain length of the surfactants not only have a complicated synthesis process of the surfactants,but also have limited in the tunable pore size range.In addition,the mesoporous structure of the transition metal oxide material with high crystallization temperature is easy to collapse and difficult to maintain during the calcination process.Therefore,the exploration of a universal synthetic method for massive preparation of transition metal oxides with adjustable pore sizes,and the development of synthetic methods suitable for mesoporous transition metal oxides possess high crystallization temperatures is very important to promote the advances of metal oxide materials.This thesis is mainly focused on the designed synthesis and performance research of mesoporous transition metal oxides and corresponding composites.A series of mesoporous transition metal oxides and metal oxides composite with high crystallization temperature have been synthesized using two different synthesis strategies,and the properties of materials have been thoroughly studied.The major achievements are described as following:A general ligand-assisted self-assembly method is proposed for the synthesis of mesoporous transition metal oxides with high crystallinity and high specific surface area.In this method,a carboxyl-containing ligand is used as a complexing agent to slow down the hydrolysis and condensation rate of the precursors.At the same time,the ligand interacts with the PEO block in P123 through hydrogen bonding.This two kinds of effect synergistically ensure the controllable co-assembly of template micelles and metal ions.During the solvent evaporation process,X-ray diffraction,transmission electron microscopy,and N₂ sorption results exhibited that the prepared mesoporous metal oxide was composed of numerous highly crystalline nanoparticles and possess closed-packed mesostructures with uniform pore-size distributions.This method has successfully synthesized a series of mesoporous transition metal oxides?Co₃O₄,Mn₂O₃,Fe₃O₄,NiO,CuO,ZnO,Cr₂O₃?and composite metal oxide materials?Co₃O₄/Fe₃O₄,Co₃O₄/NiO,Fe₃O₄/NiO?.The crystalline Co₃O₄/Fe₃O₄ composite as an electrocatalyst exhibits high catalytic activity during the electrocatalytic oxygen generation reaction.At the current density of 10 mA cm-2,the overpotential of the oxygen-producing reaction is 322 mV,which indicates that this method is not only of great significance in synthesis,but also provides more possibilities for the development of electrocatalytic reactions.Highly crystalline manganese trioxide material with interconnected mesoporous structure and controllable pore size was prepared by a ligand-coordinated self-assembly method,and used as high-performance cathode material for rechargeable aqueous zinc ion battery.The degree of coordination between Mn²⁺and citric acid ligand plays an important role in the formation of the mesoporous structure.The pore diameter of the material is adjustable from 3.2 nm to 7.3 nm,and the specific surface area is adjustable from 55 m²·g^-1 to 260m²·g^-1.The unique mesoporous structure and good crystallinity make it exhibit high capacity as a zinc ion battery cathode material(capacity is233mAh·g^-1 at a current density of 1C),superior rate performance?current density is The capacity can still be maintained above 70%at 10C?and excellent cycle stability?the capacity is still maintained at about 90%after 3000 charge and discharge cycles?.In addition,for the first time,we deeply studied the battery reaction mechanism of manganese trioxide as a zinc ion anode material,and a zinc ion/hydrogen ion insertion/extraction mechanism was proposed.These results suggest that rational design of the nanoporous architecture for electrode materials can effectively improve the battery performance.A novel holey lamellar high-entropy oxide material(Co_0.2Ni_0.2Cu_0.2Mg_0.2Zn_0.2O)was prepared by a simple anchoring and merging process.The material exhibited ultra-high catalytic activity as a heterogeneous catalyst for the aerobic oxidation of benzyl alcohol.Due to the abundant oxygen defects and various catalytic active sites in the material,the conversion of the catalysis can reach up to 98%in only two hours,which is much higher than that previous reported noble metal catalysts and non-precious metal catalysts in the literature for this reaction.By rationally adjusting the parameters of the catalytic reaction?the content of each component in the catalyst and the duration of the catalytic reaction?,we can selectively optimize benzoic acid or benzaldehyde as the main product of the benzyl alcohol oxidation reaction.The experimental results and theoretical calculations provide a deeper understanding of the catalytic mechanism and reveal that the existence of abundant oxygen defects in the material is the main reason for its display of ultra-high catalytic activity.Our method of preparing holey lamellar high-entropy oxides with multiple catalytically active centres and rich oxygen defects provides new ideas for the development of heterogeneous catalysts for the oxidation of aromatic alcohols.