The Study Of Surface/Interface Electronic State Regulation And Catalytic Performance For Perovskite Cobalt Oxides

Because of the increasing of harmful greenhouse gas emissions and energy demands,developing renewable resources is vital for energy and environmental sustainability.The design of high-efficiency and low-cost green catalysts remains a huge challenge.Perovskite oxides have been receiving more and more attentions in metal-air batteries,solid oxide fuel cells,automobile exhaust gas purification and electrocatalysis because of their good thermal stability,broad distribution for various elements,flexible structure,low price and excellent catalytic activity.In order to replace noble metals,the catalytic activity of perovskite catalysts needs to be further improved,however,the most critical factors for limiting their catalytic activity are:?1?low specific surface area;?2?native surface is dominated by inactive A-site cation;?3?the activity of surface sites is lower;?4?exposed crystal facet is usually a low-index plane.Conventional methods for modulating the catalytic activity of perovskite oxides are mainly doping different metal cations and increasing the specific surface area,but their activity has not yet achieved the desired goal.Therefore,exploring new regulatory strategies is very significant for the development of high-efficiency perovskite catalysts.In heterogeneous catalysis,chemical reaction process usually occurs on the surface and interface of the solid catalysts.The regulation of the surface and interface electronic states is crucial for improving the catalytic activity of solid materials.Therefore,in this thesis,based on the regulation of surface and interface electronic structure of perovskite cobalt oxides,we study the relationship between the surface/interface electronic state and catalytic performance at atomic scale,and deeply explore optimization mechanism for surface/interface electronic states.These studies provide important theoretical foundation for the development of a new generation of high-efficiency perovskite catalysts.Our specific work mainly focus on the following four aspects:oxygen defect regulation,high energy crystal plane regulation,activity regulation of surface site and interface heterostructure regulation:1.We developed urea pyrolysis and solution reduction methods to regulate surface oxygen defects of perovskite La_xSr_1-xCoO_3-?,and at atomic scale studied the relationship between oxygen defects,electronic structure and CO oxidation activity.?1?We designed a urea pyrolysis reaction to replace traditional H₂ anealing,namely that changing the mass ratio between urea and perovskite LSCO can control topological chemical reaction and regulate oxygen defect concentrations.The increase of urea content will improve oxygen defect concentrations and the distortion degree of CoO₆octahedron in perovskite LSCO,and the introduction of more oxygen vacancies narrowed the hybrid orbit between O 2p and Co 3d in perovskite LSCO and optimized the position of the O P-band center near the Fermi level,which significantly improved surface adsorption oxygen contents and surface oxygen ion mobility.?2?We prepared perovskite La_xSr_1-xCoO_3-??x=0.7,0.5,0.3?with different Sr contents.With the increase of Sr doping amount,surface Sr content significantl increased and its CO catalytic activity also accordingly increased.In order to create more oxygen vacancies on the surface of perovskite La_x Sr_1-xCoO_3-?,we developed a new strategy to remove certain lattice oxygen atoms via topological chemical reactions between highly active hydrogen atoms in KBH₄ aqueous solution and surface SrCoO_3-?layers,which futher induces surface reconstruction and create more surface oxygen vacancies.After treatment with KBH₄ solution,different Sr-doped perovskites have more surface Sr species and adsorbed oxygen species,which improves surface oxygen mobility.The two methods for defect design are very efficient and controllable,and their CO oxidation activity is greatly improved as surface oxygen defects increase.This study highlights the importance of anionic redox chemistry for catalytic activity in oxygen-deficient perovskite oxides.2.We developed a molten salt route to prepare perovskite cobalt oxides with high energy crystal facet.In NaCl-KCl molten salt system,K⁺ion,Na⁺ion,heating rate and reaction time have no obvious influence on exposed crystal facet.While Cl^-ion and Sr²⁺ion have strong electrostatic interaction with polar crystal plane,leading to the formation of different crystal facets.By controlling Sr ion concentration and reaction temperature,we successfully prepared LaCoO₃?100 and 110?,LaCoO₃?111?and La_0.7Sr_0.3CoO₃?111?microcrystals.The exposed?111?crystal facet has no effect on the valence state of Co atom,and the introduction of Sr ion leads to the formation of Co⁴⁺ions.The?100 and 110?crystal facets contain more A-site cations on the surface,while the high-energy?111?crystal facet contains more Co elements on the surface,which increases more surface active sites.In addition,we found that exposure of high energy crystal facet and the doping of Sr can enhance the hybridization between Co ions and O ions and make O P-band center closer to the Fermi level,which is critical for enhancing CO oxidation activity.Furthermore,La_0.7Sr_0.3CoO₃?111?shows the highest CO catalytic activity.This work provides a new synthetic route for the preparation of perovskite functional catalysts with high energy crystal facet.3.We selected high-activity and metastable La_0.4Sr_0.6CoO_3-?as perovskite model catalyst,and successfully activated surface oxygen site in perovskite LSCO through a two-step regulation strategy.First,we used urea pyrolysis reaction to optimize the interaction between Co⁴⁺ions and O^2-ions,which promotes the enrichment and phase separation of Sr.Second,we reduced surface segregations by chemical etching,which increases the number of surface active sites and retains original dominant structure.These regulations can effectively enhance charge transfer between the Co⁴⁺cation and O^2-anion and activate surface oxygen species in perovskite oxides,which makes more O₂^-species converted into active O₂^2-species.The increase of surface O₂^2-species effectively reduces the activation energy barrier of molecular oxygen and improves CO catalytic activity.And creating active O₂^2-species requires sufficient electron transfer at surface oxygen site,which is determined by surface segregation content and the interaction between Co cation and O anion.This work not only clearly illustrates the precise design route of activating lattice oxygen and relevant principle for the generation of surface O₂^2-species,but also is a step toward substituting noble metals for CO removal in the future.4.We proposed a wet chemical method to in situ construct Co₃O₄/La_0.3Sr_0.7CoO₃interface.Ethylene glycol as weak reducing agent and weak acid is used to react with Sr enrichment structure in the perovskite LSC,where Sr ion in SrCoO₃ layer is removed and the Co⁴⁺is also reduced,resulting in the formation of spinel Co₃O₄.While the bulk structure has no change,eventually creating the hybrid Co₃O₄/LSC with chemical interface.Compared with Co₃O₄,La_0.3Sr_0.7CoO₃ and mechanically mixed LSC+Co₃O₄,the hybrid Co₃O₄/LSC exhibits higher catalytic performance in CO oxidation,ORR and OER reactions and its reversible overpotential for oxygen electrode is lower than that of IrO₂ and Pt/C in 0.1 M/L KOH.The unusual catalytic activity stems from active surface lattice oxygen caused by synergistic chemical coupling effects between LSC and Co₃O₄,which efficiently reduces the activation energy barriers of surface anion oxygen reactions.We have also demonstrated that the generation of the chemical interface converts the original internal electron transfer mode between Co⁴⁺and O^2-in perovskite LSC to an external electron transfer mode between the O^2-in the perovskite LSC and Co₃O₄ lattices,which is also the root cause of activating surface lattice oxygen.