Soltuion-based Controllable Synthesis of Manganese Dioxides and Their Activities
In recent years,with the rise of the concept of green synthesis,the exploration of simple and low-eaergy preparation methods to control the synthesis of nanomaterials has attracted widespread attention.Manganese dioxide(MnO2)has many characteristics such as abundance,low price,environmental friendliness,good catalytic performance and chemical stability.It is a novel material that exhibits many properties in physics and chemistry(such as catalysis,adsorption and electrochemistry and so on).They are often applied as catalysts,ion sieves,cathode materials for lithium-ion batteries,etc.,so it is of far-reaching significance to achieve controlled synthesis of manganese dioxide and its properties.In this thesis,based on the structural characteristics of manganese dioxide,the controlled synthesis of manganese dioxide nanomaterials with special morphology based on liquid phase method was proposed.The structure-activity relationship of manganese dioxide catalytic performance was studied.The experimental part of this thesis mainly includes the following four parts:(1)In the absence of surfactant and templating agent,γ-MnO2 with a dendritic morphology of sea urchin was synthesized by one-step hydrothermal method using sodium chloride as an etchant.The sea urchin is composed of "nano trees",each of which has a length of about 10 μm and a width of about 2.5 μm,and the branches of the "nanotree"exhibit a quadruple symmetrical structure with respect to the trunk.The growth mechanism of γ-MnO2 in the dendritic morphology of sea urchin was studied by changing the reaction time.The γ-MnO2 was then used for electrochemical measurement of the oxygen reduction reaction.The test results showed that the half-wave potential of LSV curve was 0.72 V,which exhibited good ORR performance.The favorable electrochemical ORR result is attributed to that the dislocation defects is easier to form due to the unique dendritic morphology,and the quadruple symmetrical structure can also provide more active sites.(2)Using a redox reaction of manganese sulfate and potassium persulfate,a series of α-MnO2 with the same morphology was synthesized by adjusting the reaction temperature.The ORR performance showed that the a-MnO2-80 synthesized at 80℃ had a potential of 0.79 V at a current density of-3 mA/cm 2 and an active mass of 59.6 A gox-1,showing the most excellent ORR activity.Through systematic characterization methods(XRD,Raman,FESEM,HRTEM,XPS,BET,CV,Ⅰ-Ⅴ),it was found that although the a-MnO2 synthesized at different temperatures had the same morphology,the a-MnO2-60 synthesized at 60℃ had too many defects and poor crystallinity,resulting in reduced conductivity;whileα-MnO2-140 synthesized at 140℃ had good crystallinity and conductivity,but the content of Mn3+ was pretty low and the active sites were less.The sample a-MnO2-80 synthesized at 80℃ achieved an equilibrium of conductivity and active sites,and thus having an optimum oxygen reduction performance.(3)In the absence of surfactants and templating agents,we synthesized a-MnO2 with a hollow urchin-like structure by one-step hydrothermal method.The hollow urchin was self-assembled by nanorods which has a diameter of 20～50 nm and a length of about 1 μm.The hollow urchin-like α-MnO2 was used in the ORR/OER reaction.The ORR/OER performance showed that the potential difference between OER and ORR was only 0.99 V,which had superior ORR/OER performance.Characterized by XRD,Raman,FESEM,HRTEM,SAED,XPS,BET and O2-TPD,it was found that proton not only corroded the core of the sample to form a hollow structure in high temperature environment when using hydrothermal method,but also corroded the surface of the nanorods,forming more Mn3+which improved the ORR/OER performance.(4)Ag-OMS-2 nanowires were synthesized by low temperature liquid phase method using Ag under acidic conditions with amorphous manganese dioxide as precursor By adjusting the reaction ratio of the precursor,it was found that the synthesis of Ag-OMS-2 nanowires can be successfully induced only when the molar ratio of NaMnO4 and MnSO4 was more than 0.6.Moreover,the growth mechanism of Ag-OMS-2 was systematically studied by reaction time experiments.Characterization techniques(XRD,SEM,HRTEM,XPS)show that silver ions are distributed in the monodisperse form in the Ag-OMS-2 tunnel,and the valence state of Ag was also between 0 and+1,with certain metal properties,which can effectively improve the conductivity of the sample.Electrochemical performance tests showed that the OER and ORR potential difference of the sample was only 0.87 V,which had excellent ORR/OER performance(5)Using manganese carbonate microspheres as precursors,β-MnO2 mesoporous microspheres with high specific surface area and mesoporous structure were successfully prepared by high temperature solid phase reaction.The structure of β-MnO2 mesoporous microspheres were characterized by XRD,Raman,FESEM,HRTEM,BET,XPS,H2-TPR and NH3-TPD.It was found that the crystal orientation of the as-prepared β-MnO2 was different when compared to that of conventional β-MnO2 nanorods.In addition,we applied the two β-MnO2 samples to the NH3-SCR reaction.The results showed that when the NO conversion of β-MnO2 mesoporous microspheres reached 50%,the reaction temperature was only 60℃,and the window of reaction temperature was larger.The high activity was closely related to the high content of Mn4+,low reduction temperature and large specific surface area of β-MnO2 mesoporous microspheres.