Preparation Of Porous Perovskite Oxides By Etching Method And Their Performance For Catalytic Oxidation Of Ethyl Acetate

Volatile organic compounds（VOCs）,which affect human health due to their toxicity,carcinogenicity and mutagenicity,and destroy the ozone layer,are precursors of photochemical smog,posing a serious threat to ecological safety.Among them,ethyl acetate（EA）is a very important industrial solvent,and the harm to the environment and human body caused by the large amount of emission should not be underestimated.Therefore,it is urgent to take effective ways to control EA pollution.Catalytic oxidation method is considered as one of the effective means to eliminate EA because of its high efficiency,no secondary pollution and good economy.The development of efficient catalysts is the key to the research of catalytic oxidation treatment technology.Perovskite（ABO₃）composite metal oxides are suitable for catalytic oxidation due to their unique physicochemical properties.However,its high synthesis temperature and small specific surface area lead to low exposure of the active site,which seriously hinders its application in practice.How to design and synthesize perovskite oxides with high specific surface area and high active site exposure,and improve their catalytic oxidation performance of EA is the focus and difficulty of current research.In this thesis,we designed a selective etching method to synthesize perovskite type metal oxides with high specific surface area and porous morphology for the catalytic oxidation of EA reaction to investigate the constitutive relationship between the physical and chemical properties of perovskite oxides and the catalytic oxidation performance.In order to enhance the specific surface area of perovskite,the porous LaCoO₃ was obtained through the acetic acid selective etching process of in-situ generated La₂O₃ particles using acetic acid,derived from a pre-synthesized composite of LaCoO₃-La₂O₃ for the catalytic combustion of EA.Characterization by XRD,ICP-OES,TEM,XPS,H₂-TPR and O₂-TPO showed that the etching of La₂O₃ with acetic acid not only generated surface Ladefects,but also improved the porosity,the exposure of active sites and the redox properties of the material.In addition,LaCoO₃ synthesized by three methods,i.e.,sol-gel method,co-precipitation method,and acetic acid selective etching method,were compared to study the effects of different preparation methods on the grain size,specific surface area,redox capacity,oxygen mobility,Co³⁺ content,and catalytic activity for removal EA of LaCoO₃.The catalytic oxidation of EA over perovskite LaCoO₃ was also examined when NO was introduced into the reaction,as well as the effect of the introduction of a small amount or excess of water vapor on the catalytic oxidation activity,respectively.In order to explore the versatility of the acetate selective etching method,LaBO_3E with different B-site（B = Ni,Mn,and Fe）elements was prepared by this method for the catalytic oxidation of EA.The amount of oxygen vacancies in LaBO_3E was found to have a linear relationship with its catalytic oxidation performance of EA（LaMnO_3E >LaNiO_3E > LaFeO_3E）by series of characterizations.Comparing LaBO_3S（prepared by sol-gel method）and LaBO_3E（prepared by cetic acid selective etching method）,it was found that the acetic acid selective etching method was significantly better than the sol-gel method.The T90 of the series catalysts were arranged as the order: LaMnO_3E（200 ℃）> LaNiO_3E（207 ℃）> LaMnO_3S（244 ℃）> LaNiO_3S（250 ℃）> LaFeO_3E（269 ℃）> LaFeO_3S（290 ℃）.Additionally,the analysis of intermediate products resulting from the catalytic oxidation of EA revealed that the reaction temperature played a crucial role in determining the types of intermediate products.Specifically,when the temperature below 160 ℃,acetic acid and ethanol emerged as the main intermediate products of EA oxidation,while acetaldehyde generated when the temperatures higher than 160 ℃,and EA is completely oxidized when the temperature raised to 210 ℃.In order to further explore the etching method to enhance the catalytic performance of perovskite LaMnO₃,the nitric acid etching method was employed to successfully prepare perovskite LaMnO₃（HD-LMO）with dense grain boundaries,high specific surface area,and a hierarchical dimensionality structure,by controlling the microscopic dimension of the perovskite.Compared with LaMnO₃（LMO）prepared by the sol-gel method,HD-LMO demonstrates remarkable catalytic oxidation capabilities towards EA,as evidenced by its T90 value of 184 ℃.This enhanced performance can be attributed to its hierarchical dimensionality,substantial specific surface area（212.3 m²/g）,abundant oxygen vacancies,and excellent redox properties.The presence of high-density grain boundaries in HD-LMO effectively facilitates the migration of surface oxygen,thereby significantly reducing the energy barrier associated with the catalytic oxidation reaction.This study also examines the alterations in the catalytic oxidation efficiency of HD-LMO in response to long-term exposure,cycling,water resistance and high space velocity,demonstrating its remarkable stability.The nitric acid etching method provides an innovative strategy for designing efficient chalcogenide catalysts for catalyzing reactions such as VOC oxidation.