Design Of Defect-rich High-entropy Oxide Catalysts And Their Application In Catalytic Benzyl Alcohol Oxidation

High-entropy oxides（HEOs）are special multi-oxide materials formed by five or more metallic elements in nearly equal substance ratios,with highly homogeneous compositions and lattice structures.This multivariate composition and the synergistic effect among the components make HEOs exhibit a series of unique properties（excellent structural stability and high configurational entropy）and application prospects（high-temperature materials,energy-storage materials,wide-area electrical materials,and catalysis,etc.）.In recent years,HEOs have received increasing attention in the field of catalysis.However,the current preparation of HEOs usually needs to be carried out under harsh conditions,such as high temperature and high pressure,which causes the catalysts to suffer from agglomeration,reduced specific surface area,decreased porosity,and reduced number of active sites,thus affecting their catalytic performance.Therefore,in this thesis,a series of defect-rich HEOs with large specific surface area,high porosity and a large number of active sites were constructed by using template methods（hard template method and self-sacrificial template method）from both synthesis and pyrolysis methods,and applied to the selective catalytic oxidation of benzyl alcohol for the preparation of benzaldehyde.In addition,we have conducted an in-depth and systematic study on the universality of the template method,the key factors affecting the phase formation,the microscopic phase transition mechanism,and the conformational relationship.This paper is divided into the following four parts to design and study the defect-rich HEOs catalysts.Part I:The green and economical Na Cl was utilized as an inorganic salt template to occupy a large number of positions in the precursors of HEOs by mechanical ball milling;in which the Na Cl eluted after the HEOs had formed a phase,thus releasing a large number of pores occupied by it and facilitating the enhancement of the porosity in the HEOs.A series of HEOs with good mesoporous structure,large specific surface area and single-crystal nature were prepared by this strategy,and the prepared HP-（Fe Cr Co Ni Cu）_xO_yshowed excellent catalytic activities(aromatic alcohols:99%BAL conversion,83.5%benzaldehyde selectivity and high stability;aromatic hydrocarbons:35.7%ethylbenzene conversion,95.4%acetophenone selectivity;propene(Combustion:T₁₀₀=583 K).Multiple material characterizations,catalytic performance data and DFT calculations indicate that the mesoporous structure of HP-（Fe Cr Co Ni Cu）_xO_yprovides abundant oxygen vacancies and adsorption sites,and the synergistic effect of the two together enhances the catalytic efficiency of the catalytic reaction.Part II:Based on the high-entropy MOF flower-like nanosheet structure and utilizing it as a self-sacrificial template for the oxygen-defect engineering strategy,the catalyst HEO-600 with abundant porosity,large specific surface area（46 m²/g）,good crystallinity,and a large number of oxygen vacancies was successfully prepared by treating the catalyst at 600°C.The catalyst showed a high benzyl alcohol conversion in catalytic oxidation of BAL up to 73.7%,which is 12.3 times higher than that of the comparison catalyst.The differences in the kinetic and thermodynamic behaviors of the two catalysts were explored in depth through structural characterization and theoretical calculations.This work shows that based on the advantages of the three-dimensional nanoflower-like structure of HEO-600,the molecular oxygen activation barrier can be significantly reduced through the comprehensive regulation of the active sites in HEO-600 and the reaction conditions,which can effectively enhance the efficiency of selective catalytic BAL.Part III:MW-HEO with abundant oxygen vacancies and high specific surface area（127 m²/g）was successfully prepared by taking advantage of microwave-endowed treatment of MOFs precursors with the advantages of more uniform and faster heating.The MW-HEO exhibited excellent catalytic activities at lower temperatures and atmospheric pressures(benzaldehyde yield:9132.58 mmol g^-1h^-1,furfural and aliphatic aldehydes selectivity higher than 90%)and far exceeded the currently reported feeder class catalyst.selectivity was higher than 90%)and far exceeded those of the currently reported catalysts.The catalyst was found to have good structural stability,and no obvious decrease in catalytic activity was observed after 11 consecutive cycles.The experimental results and DFT calculations showed that the rich mesoporous structure of the catalyst not only regulates the electronic states of the surface metal sites to promote the adsorption of reactants,but also regulates the concentration of oxygen vacancies in the catalyst,which in turn regulates the selectivity of benzaldehyde.Part IV:We developed a synthetic strategy to introduce mesoporous structures into HEOs via microwave nonthermal effect using various metal salts as precursors.Through this strategy,we successfully prepared a series of HEOs（m-Ni Mg Cu Zn Co:86 m²/g,m-Mn Cu Co Ni Fe:67 m²/g,and m-Fe Co Ni Cr Mn:5.4 m²/g）with excellent crystallinity and multilevel pore structures,demonstrating the universality of this strategy.We investigated the activity of the prepared HEOs catalysts in selective catalytic benzyl alcohol oxidation using m-Ni Mg Cu Zn Co as an example.It was found that the benzyl alcohol conversion of the high porosity catalyst（m-Ni-Ni Mg Cu Zn Co）was increased by about 2.75 times compared with that of the bulk catalyst（c-Ni Mg Cu Zn Co）.DFT calculations revealed that the difference in the binding energies to the reacting molecules between the different catalysts was the main factor for the significant difference in their catalytic activities.