Regulation Of Exsolution And Properties Of Fe/Mn-based Perovskite Oxides

Perovskite type oxides have the advantages of high stability,strong adjustability and low price.Their spin,orbit,charge and lattice coupling produce abundant physical and chemical properties,which are widely used in the fields of harmful gas purification,energy storage and conversion.The surface modification of perovskite oxides by chemical synthesis often endows them with better properties or even new functions.The exsolution of perovskite is an effective method to control the surface of perovskite oxides.It refers to the process that some or all of the active metal elements with catalytic activity in perovskite oxides are separated from the lattice by chemical reaction,and finally a new phase is formed on the surface of the perovskite.Compared with the traditional time-consuming deposition or impregnation technology,the method is more cost-effective and time-efficient in the preparation of more fine and evenly distributed active metal nanoparticles.The key to the preparation of high performance catalysts is to obtain the second phase with controllable composition,density,distribution and size.The composition,density and size of nanoparticles can be basically controlled by adjusting A-site defects,element doping,temperature and environment by exsolution.In addition,the composite prepared by exsolution has strong metal-oxide interface interaction and excellent resistance to high temperature thermal agglomeration.The obtained metal-oxide has strong electronic,lattice,orbital and other interaction,which greatly improves the catalytic performance and shows good stability.Therefore,the exsolution of perovskite has been widely used in the field of energy catalysis.In this paper,starting from the regulation of the structure and electronic state of materials,the change of perovskite crystal structure and electronic state before and after exsolution and the influence mechanism on catalytic performance are explored.Meanwhile,new methods to control exsolution are sought to expand the traditional synthesis methods.The basic rules of exsolution regulation are systematically summarized,the essence of exsolution is revealed,and the performance-oriented strategy to optimize the preparation of materials for directional exsolution is clarified:1.We controlled the exsolution of A-site deficient perovskite La_0.9Fe_0.92Ru_0.08-xO₃(LFRO)with low Ru content,and constructed RuO₂/LFRO composites.The changes of crystal structure and electronic state of perovskite before and after exsolution and the relationship between structure and OER catalytic performance were investigated.After heat treatment at 550 ^oC in 5%H₂/Ar,Ru nanoparticles were exsolved from the lattice of LFRO and formed embedded structure with the perovskite.Compared with LFRO,RuO₂/LFRO composite has higher OER performance,which is mainly due to the formation of electrochemically active RuO₂ nanoparticles and the improvement of conductivity.In addition,it was found that the Ru nanoparticles can return to the perovskite lattice at 550 ^oC in air,which verifies the reversibility of the exsolution.This study proposed a new strategy for the preparation of composite catalysts,which provides an effective way to reduce the dosage of precious metals in different catalytic fields.2.We use Co-doped lanthanum ferrite as a model to investigate the effect of doping on the B-site exsolution of Fe-based perovskite.Co Fe alloy can be exsolved from La_0.9Fe_0.92Ru_0.08-xO₃(LFRO) after heat treatment at 500°C in a reduced atmosphere,whereas Fe will not be exsolved from La_0.9FeO₃(LFO).Density functional theory calculation(DFT)reveals that the stability of LFCO decreased after Co is doped into the lanthanum ferrite perovskite lattice and the formation energy of the Co-Fe bond on the surface of LFCO is lower than that of Fe-Fe in LFO,which promises an easier exsolution of CoFe alloy than the pristine Fe cluster.In addition,due to the strong interaction and charge transfer between the exsolved CoFe alloy and the perovskite,as well as the longer Fe-O bond after exsolution,the exsolved composite can act as an excellent bifunctional electrocatalyst for oxygen evolution and oxygen reduction reactions.Our work not only reveals the mechanism of the alloy exsolution in Fe-based perovskites but also provides a potential route to prepare the highly efficient electrocatalysts.3.We combine the exsolution and phosphating process to construct the composite materials with various active components and realize the further functionalization of the materials.Firstly,CoFe@La_0.9Fe_0.9-x Co(0.9-y) O₃-delta composite was prepared via the exsolution of La_0.9Fe_0.9Co_0.1O₃.Then,P-Co Fe@La_0.9Fe_0.9Co_0.1O₃ composite was constructed by phosphating process.The composite of phosphate,phosphide,alloy and perovskite was realized by simple temperature control,and the electrocatalyst with various active components was obtained.Due to the strong interaction between the Co Fe and La_0.9Fe_0.9-x Co(0.9-y) O₃-delta,the appropriate adsorption capacity of the catalyst after phosphating,the coating of phosphate on the surface and the introduction of Fe⁴⁺in the parent perovskite,the OER performance of the composite was gradually optimized.Compared with CoFe@La_0.9Fe_0.9-x Co(0.9-y) O₃-delta and P-La_0.9Fe_0.9Co_0.1O₃,it has the lowest overpotential and the highest current density.The strategy described in this paper provides a new way for the design and synthesis of highly efficient electrocatalysts,which has a wide application prospect in electrochemical storage and conversion.4.We developed a new solvent induced exsolution method and applied it to Mn-based perovskite.Combined with the Jahn-Teller disproportionation effect of Mn³⁺and bond strength difference in Mn-based perovskite,a preparation strategy was proposed to exsolved vacancy ordered two-dimensional(2D)non layered?-MnO₂ nanosheets from charge ordered perovskite La_0.5Sr_0.5MnO)3(LSMO).X-ray absorption spectroscopy(XAS),quantitative analysis of dissolved ions and the stochastic surface walking(SSW)were used to explore the mechanism of solvent induced exsolution of?-MnO₂nanosheets at atomic scale.When protons are introduced,La³⁺and Sr²⁺in LSMO crystals are selectively dissolved,and then Mn³⁺disproportionates into Mn²⁺and Mn⁴⁺.Mn²⁺dissolved in solution,and the corner-shared Mn⁴⁺O₆ octahedron is transformed into edge-shared octahedron,forming a special thin 2D structure and Mn-vacancy ordered?-MnO₂.Compared with other manganese oxides,the obtained?-MnO₂ has good thermal stability and water oxidation activity.The new strategy is expected to be extended to the synthesis of other two-dimensional nanosheets with non-ayered structure,which provides a new means for the design and preparation of efficient catalysts.