Combustion Synthesis And Dealloying Of Porous CoAl₃ Intermetallics

Porous Co-Al intermetallics have the advantages of low density,large specific surface area,and excellent high-temperature oxidation resistance of traditional Al-based intermetallics and promising applications in the field of catalysis due to their quasicrystalline structure.The nanostructured materials have a high specific surface area and size effect,which can provide sufficient surface reactive sites and facilitate the adsorption of reactants and the desorption of intermediates and final products,therefore are also widely used in the research of catalysis.Because of the problems of high cost,low catalytic efficiency,and poor environmental tolerance of existing catalysts for hydrogen production from electrolytic water,this thesis proposes to combine the advantages of high tolerance,high catalytic properties,and high specific surface area of porous Co-Al catalysts and nanostructured materials in catalysis,and prepare porous CoAl₃ precursors with high micron pores by thermal explosion,and then remove Al from porous CoAl₃ by dealloying.Elements in the porous Co-Al by dealloying to prepare porous Co-based catalytic materials with nanostructures.The pore formation mechanism and near-net forming of porous CoAl₃ intermetallics are systematically investigated,and the adjustment of the dealloying prepared porous Co-based catalysts with various nanostructures.The electrochemical properties of the catalysts are tested.The specific findings are as follows:The porous CoAl₃ intermetallics are prepared by mixing and pressing Co powder and Al powder in the atomic ratio of 1:3,then by thermal explosion,and the pore formation mechanism and high-temperature oxidation resistance are investigated.The results show two exothermic reactions during the sintering of porous CoAl₃:the solid-phase diffusion at 616℃and the thermal explosion reaction at 646℃.The heat released from the two reactions causes the sample to undergo expansion,reaching a maximum of 104%at 800°C,corresponding to an open porosity of 53%.The pore sources of the porous samples include interstitial pores left during the raw press,Kirkendall pores generated during solid-phase diffusion,and in-situ pores generated by Al melting after the thermal explosion reaction.The porous CoAl₃ exhibited excellent high-temperature oxidation resistance after cyclic oxidation at 700°C for 130 h.The mass increased by only 1.52%because a continuous layer of protective oxide film,Al₂O₃,could be formed on the skeleton surface.To solve the spalling and cracking phenomena of porous CoAl₃ intermetallics prepared by thermal explosion,the effect of isothermal treatment on the near-net formation of porous CoAl₃ is investigated.The phase transition process and diffusion kinetics of Co/Al diffusion couples in the solid-phase diffusion stage are also analyzed.The results show that the isothermal treatment in the range of 510-550°C contributes to reducing the exothermic intensity of the thermal explosion reaction,but only keeping it at 530°C for 30 min can promote the near-net formation of porous CoAl₃.The addition of the isothermal treatment does not affect the phase composition of the final product but leads to changes in the type and depth of the diffusion layer and affects the microscopic morphology of the skeleton.Based on the thermodynamic and kinetic analysis,find that the formation of the CoAl phase can consume as much raw material as possible before the occurrence of the thermal explosion reaction and reduce the exothermic intensity of the subsequent thermal explosion reaction.And the low diffusion activation energy of the CoAl phase(Q₁=1.66 k J mol^-1)is the key to the preferential formation in the Co/Al diffusion couple.The chemical dealloying is used to remove Al elements from porous CoAl₃.The effects of dealloying parameters（concentration and type of etching solution,atmosphere and temperature of dealloying,and addition of reducing agent and third component）on nanostructure formation are investigated.The electrochemical properties of the prepared Co-based catalysts are also characterized.The results show that various nanostructures,such as nanorods,nanosheets,and nanoparticles,can be obtained by adjusting the dealloying process parameters.The nanosheet structures exhibit more uniformity.XPS and Raman characterize the obtained nanosheet structured catalysts,and the nanosheets’main composition is Co₃O₄ or Co₃O₄/Cu O.The electrochemical performance tests of the nanosheets revealed that the synthesized Dealloying-CoAl₃&10 wt%Cu exhibited better OER performance than Dealloying-CoAl₃ because Co₃O₄ and Cu O could form heterogeneous structures.This thesis includes 57 Figures,11 Tables,and 111 References.