The Study Of Manganese Oxide Catalysts For Heterogeneous Water Oxidation

It is an ideal way to solve the energy problem by photocatalytically splitting water into hydrogen,which utlizizes solar energy to produce clean fuel.Water oxidation reaction plays an important role in the process of photocatalytic water splitting,which oxidates water molecules into oxygen and releases protons.As a four electron process,the water oxidation reaction is much more difficult than hydrogen evlution reaction,which has become an important factor restricting the improvement of overall photocatalytic water splitting activity.Therefore,it is significant to develop a cheap,efficient and stable water oxidation catalyst.Manganese is a cheap,environmentally friendly,and earth abundant element.Manganese oxide is one of the most promising catalysts for water oxidation which is more suitable for mass application than noble metal oxide and cobalt oxide.In green plant,the water oxidation complex?OEC?in photosynthesis is just composed of manganese oxide clusters,which provides an excellent example for artificial manganese oxide water oxidation catalyst.In the longterm study of manganese oxide catalyst and the OEC,people summarized the structure-activity relationship of manganese oxide water oxidation catalyst.Mn3+ is generally considered the active manganese ion in water oxidation;flexible structure and long Mn-O bond is conducive to water oxidation catalysis;Ca2+ doping can significantly improve water oxidation activity of manganese oxide.However,the existed manganese oxides can only partly meet these conditions,resulting in lower water oxidation activity.Therefore,it is necessary to regulate the structure and electronic state of manganese oxide in order to improve the water oxidation activity of the catalyst.In this thesis,we fabricate several kinds of manganese oxide catalyst for the water oxidation catalysis based on regulation of structure and electronic state:1,?-MnO₂–Mn₃O₄ nanocomposites were synthesized via a mild reduction condition.This material is composed of ?-MnO₂ nanosheets and Mn₃O₄ nanoparticles.Owing to the different structure parameters of MnO6 octahedral in ?-MnO₂ and Mn₃O₄,the interface between two phases formed disordered structure,weak Mn-O bond,and Mn mixed valence of Mn3+/Mn4+.This configuration is favorable for water oxidation catalysis.Benefit from the active interface,the water oxidation activity of ?-MnO₂–Mn₃O₄ nanocomposites is much higher than that of the single component ?-MnO₂ nanosheets or Mn₃O₄ nanoparticles.By controlling the reduction dose,?-MnO₂–Mn₃O₄ nanocomposites of different compositions can be obtained,the catalyst containing 38% Mn₃O₄ and 62% ?-MnO₂ hold the best water oxidation activity of TOF = 0.93 s-1.We chatacterized the structure,electronic states and water oxidation activity of ?-MnO₂–Mn₃O₄ nanocomposites,explored the growth mechanism and the structure-activity relationship of the catalyst,and analyzed the effect of annealing treatment on the structure and properties of catalyst.2,We utilize an ultrasonic assisted reduction process to optimise the structure and electronic state of typical Ca-birnessite,obtaining a modified Ca-birnessite catalyst which is more suitable for water oxidation catalysis.In the modification treatment,the ultrasonic treatment synergizes with the reduction process,reducing most Mn4+ in Ca-birnessite into active Mn3+,delaminating the layer structure to expose active assisted ion Ca2+,and forming a similar structure to the natural water oxidation center Mn4CaO5.According to the XRD spectra,the layered structure of the modified Ca-birnessite catalyst completely disappeares,while the structure of MnO₂ thin film remains unchanged.The TEM characterization showed that the modified Ca-birnessite is a disordered stacking structure of monolayer MnO₂ with a high specific surface area.The XPS spectra showed that the average valence of manganese is close to +3.The water oxidation activity of the modified Ca-birnessite catalyst is up to TOF = 3.43 s-1,which is much higher than typical Ca-birnessite.In addition,we studied the synthesis mechanism,the best synthesis condition,the best water oxidation condition and the effect of annealing treatment on the material.3,Perovskite type calcium manganese oxide water oxidation catalysts are prepared by sol-gel method.The perovskite structure has excellent stability,when the composition,structure and electronic states of perovskite type manganese oxides meet with the requirements of water oxidation catalysis,an excellent water oxidation catalyst with both high activity and high stability will be formed.A series of La1-xCaxMnO3 catalysts with different structures and electronic states have been synthesized by adjusting the ratio of La/Ca of A site in perovskite manganese oxide.The morphology of La1-xCaxMnO3 catalyst is nearly spherical.The XRD spectra show that it is a pure perovskite structure,and the lattice structure changes with the ratio changes of lanthanum and calcium.Many factors influence the water oxidation activity of La1-xCaxMnO3 catalysts,including Mn3+ content,lattice ordered degree and surface Ca2+ number,conductivity,etc.Thus,water oxidation activity of the catalyst is determined by the comprehensive effect of a number of factors.The best catalyst for the oxidation water is La0.25Ca0.75MnO3 with an average manganese valence of +3.6,large numbers of surface Ca2+ ions and the larger lattice disordered are the main factors of high activity.We studied the relationship between the composition,structure,electronic state and the properties of perovskitealcium manganese oxides.In this thesis,the composition,structure and electronic state of manganese oxides were regulated by the methods of moderate reduction,ultrasonic treatment and control of feed ratio.Several kinds of fine manganese oxide water oxidation catalysts are fabricated.The composition and structure of these catalysts are closer to the natural water oxidation center of Mn4CaO5,and show high water oxidation activities and stability.Compared with the previous research,we pay more attention to the regulation of chemical bond and atomic scale,in order to obtain the active sites for the water oxidation catalysis directly,which provides a new perspective for the further improvement of manganese oxide water oxidation catalysts.