Designed Synthesis And Property Studies Of Mesoporous Multicomponent Metal Oxides And The Composites

Mesoporous materials have received high attention from the fields of chemistry,materials,biology and other disciplines and developed into an important functional material due to their special structural characteristics e.g.,high specific surface area,large and uniformly adjustable pore size(2?50 nm),as well as open and ordered pore structure.Due to the diverse composition of pore wall,abundant defect sites on the surface,adjustable physical and chemical properties,low costs,excellent stability and synergistic effect of multiple metals,the mesoporous multicomponent metal oxide materials have gradually become an important functional mesoporous material.Recently,the development of traditional simple component mesoporous metal oxide materials has made remarkable achievements,such as mesoporous Ti O₂,and Zn O,etc.,but suitable porous strategy for multicomponent metal oxides is still in development primary stage.That stem from the complex synthesis system of mesoporous multicomponent metal oxides,and the main limitations are in the following.(1)The synthesis system is complex,and a suitable co-assembly synthesis system needs to be developed.The assembly process between multiple inorganic metal species and templates is sensitive to synthesis conditions.Therefore,it is necessary to strictly control reaction parameters and adjust the interaction between multiple components to form stable mesophase.(2)The multicomponent systems with different metal atomic radii and high mixing enthalpy usually require high temperature calcination for crystallization.Thermodynamically,how to slow down the collapse of mesoscopic structure in high temperature crystallization and maintain the well-defined porous structure of material is a difficult problem that needs to be overcome.(3)It is significant to in-depth understand the formation mechanism of mesoporous multicomponent metal oxide materials for achieving precise tuning of composition,mesoporous structure and crystallinity.(4)Develop the application performance of mesoporous multicomponent metal oxide materials,and reveal the structure-activity relationship.In order to promote the development of mesoporous multi-metal oxide materials,it is first necessary to continuously develop the new and efficient porous synthesis strategies.This dissertation takes mesoporous multicomponent metal oxides and their composite materials as the research object,and conducts research from four areas,including the design of synthesis method,the structural characterization,the performance application of mesoporous multicomponent metal oxide materials,and the structure-activity relationship exploration.The thesis mainly includes the following aspects:1.We designed an efficient and general co-assembly strategy to synthesize a series of mesoporous ABO₃ perovskite metal oxides with high crystallinity and open porous structure by cationic coordination and resol-assisted co-assembly method(La Mn O₃,La Fe O₃,La Co O₃,La Ni O₃,La_xSr_1-xMn O₃).In the synthesis process,multiple precursors interact together through hydrogen bonds and assemble into a stable inorganic-organic composite precursor during solvent evaporation.Crystalline mesoporous perovskite metal oxides were formed after calcination in air.The key reason of keeping porous structure intact after high temperature calcination is the resol acts as an important pore stabilizer.And the small molecule ligands of citric acid coordinate with various metal salts to avoid phase separation.After post-treatment with Na BH₄ solution,the prepared mesoporous La Mn O₃ has high pore volume,open nano-framework structure and abundant surface oxygen vacancies.As a heterogeneous catalyst,it presents significant catalytic activity and stability for the hydrogenation of furfural to furfuryl alcohol with conversion rate of 99%and selectivity of 96%.Combined with density functional theory calculations,the catalytic mechanism is elucidated,and it is found that the mesoporous La Mn O₃ nanocrystalline framework is beneficial to expose the oxygen-deficient sites,promote the interaction and interfacial electron transfer between the catalyst surface and the catalytic substrate molecules,and reduce the catalytic reaction energy barrier.This universal strategy provides a new idea for the synthesis of other kinds of mesoporous multicomponent metal-based materials.2.We have designed a modular co-assembly method for synthesis of ordered mesoporous perovskite oxides.The energy balance principle is proposed and reveals the interaction between inorganic and organic modules and the process of assembly balance,as well as the determinants of different mesoscopic structures.This strategy mainly consists of two key processes:multi-metal pre-coordination and resol-assisted co-assembly process.The pre-coordination process integrates various metal salts precursor into a stable multi-metal composite as inorganic module.Resol and templates as organic modules,various precursor modules were regularly assembled into inorganic-organic mesosstructured composites via hydrogen bond.During the calcination crystallization process,the resol plays a confinement-oriented role in the growth of the perovskite oxide,and acts as a pore structure stabilizer to slow down the collapse of the pore structure.In addition,we carefully explore the formation mechanism of ordered mesoporous perovskite metal oxides with different microstructures.Combinated experimental characterization and theoretical computational kinetic simulation,we explore the relationship between metal chelate precursor,resol,and templating agents.The synthesized mesoporous La₂Zr₂O₇exhibited excellent hydrogenation activity of 5-hydroxymethylfurfural(94%Conversion,99%Selectivity),and the hydrogenation process simulated by catalytic reaction kinetics and theoretical calculations revealed that the well-defined porous structure and activated Zr sites in catalyst facilitate the interaction between the catalyst and catalytic substrate and lower the energy barrier of catalytic reaction.3.Here,we design a gentle UV-induced Ostwald ripening method to synthesize mesoporous Ga₂O₃/Ga OOH heterojunction composites.By adjusting the crystallinity of mesoporous Ga₂O₃ after calcination and UV irradiation time,the products with different crystal phase can be achieved including mesoporous Ga₂O₃,mesoporous Ga OOH and mesoporous Ga₂O₃/Ga OOH heterojunction composite.They have good crystallinity and uniform pore structure.Moreover,the formation mechanism of mesoporous Ga₂O₃/Ga OOH heterojunction composites was described detailly by UV Raman spectroscopy,thermogravimetric curves and transmission electron microscopy.The closely-interacted heterojunction structure,well mesoporous channels and high specific surface area endow mesoporous Ga₂O₃/Ga OOH heterojunction composite with good light absorption,effective charge separation and transfer,which improves the photocatalytic performance of water splitting for hydrogen.Through spectroscopic and photoelectrochemical characterization,we summarize the energy band structure and possible electron transfer paths of mesoporous Ga₂O₃/Ga OOH heterojunction catalyst.This study provides a reference for constructing porous metal oxide heterojunction materials and subsequent photocatalysis research.In conclusion,this thesis proposes some effective solutions to the current challenges in developing mesoporous multicomponent metal oxide materials.In terms of synthesis methods,the additive-assisted soft template methods are proposed to solve the problems of complex and easily separable synthesis system and the porous channels collapse during high-temperature calcination.In terms of exploring synthesis mechanism,a variety of characterizations and the theoretical calculation are used to track the synthesis process.And the formation mechanism of different mesoporous multicomponent metal oxide materials in this thesis is summarized.In terms of exploring and revealing structure-activity relationships in performance applications,the as-made materials were applied to different performance fields in a targeted manner,according to their structure,composition and surface chemical properties.Then,we design effective performance comparison experiments and apply theoretical calculations to simulate catalytic process in order to reveal the structure-activity relationship from multiple perspectives.