Controllable Synthesis Of Manganese Dioxide Nanomaterials By Precursor Transformation And Their Catalytic Performances

The adjustment of synthesis,shape and structure for nanomaterials and their further practical applications have become the main research priorities in the field of materials.As a typical transition metal oxide,manganese dioxide(MnO₂)is characterized by a good deal of content,low cost,environmentally friendly,variable crystal shape and different morphologies.It is favored by researchers and widely used in catalysis,electrochemistry and other fields.In this paper,different crystal forms and preparation methods of MnO₂ were used as the starting point,and MnO₂-based precursors were obtained by different preparation methods,and then further processed to obtain MnO₂ products with specific morphology or crystal form.In addition,different catalytic reactions were used as probes to explore the relationship between different crystal forms,morphologies as well as doping elements of MnO₂ and catalytic reaction.The main work of the thesis are as follows:(1)The amorphous manganese dioxide was obtained by microwave-assisted hydrothermal method in the absence of template and surfactant,and then Ce-doped?-MnO₂nanoparticles were obtained by high temperature solid phase method by using the amorphous manganese dioxide as the precursor.The obtained?-MnO₂ nanoparticles had larger specific surface area and more structural defects.XPS results showed that the doping of Ce element further distorts the lattice of MnO₂,giving rise to more oxygen vacancies,thus a range of surface reactive oxygen species were generated on the surface of Ce/?-MnO₂.In addition,the NH₃-TPD results showed that Ce also brought abundant acidic sites for?-MnO₂,which was conducive to the adsorption and activation of NH₃.Therefore,at a volume space velocity of280,000 h^-1,Ce-doped?-MnO₂ achieved a 90%conversion rate at 137 ^oC,and the temperature window above 90%reached 267 ^oC(137-404 ^oC),which was better than the?-MnO₂synthesized by the hydrothermal method and commercial?-MnO₂ catalytic performance.This method provides a new idea for the preparation of transition metal-doped?-MnO₂nanoparticles.(2)Rod-like(HS),prismatic(HN)and half-tube-like(HC)of?-MnO₂ nanomaterials were prepared by one-step hydrothermal method using?-MnO₂ as precursor,and three different acids(H₂SO₄,HNO₃ and HCl)with the same concentration were used as solvent.The?-MnO₂showed different morphologies when only the solution anions were different,which indicated that the anions contained in the solution had a great influence on the growth of?-MnO₂nanomaterials.At the same time,the three different morphologies of?-MnO₂ were analyzed by various characterization techniques,and it was found that only the specific surface area of the three different morphologies of?-MnO₂ was different obviously,while other physical and chemical properties were not significantly different.?-MnO₂-HS and?-MnO₂-HC have larger specific surface area than?-MnO₂-HN,so they had more surface active sites,and exhibited higher methylene blue degradation conversion rate.Furthermore,the crystal transformation mechanism from?-MnO₂ precursor to?-MnO₂ was investigated by XRD characterization,which followed the growth mechanism of dissolution and crystallization.(3)MnCO₃ was selected as precursor,and a centain amount of CH₃COOK was added to Grinding evenly,then the?-MnO₂-K nanospheres(?-MnO₂-K)with K⁺enrichment on the surface were prepared by high temperature solid phase method.The products were washed several times and dried off as the control group(?-MnO₂-KL).With toluene catalytic combustion as the probe reaction,we found that?-MnO₂-K had superior toluene catalytic activity(at 202 ^oC realized the conversion rate of 90%)and excellent stability.Meanwhile,XRD,FESEM,HRTEM,XPS and other characterization techniques had been used.As a result,the crystal form and morphology of?-MnO₂ enriched with K⁺are close to that of?-MnO₂ KL,but?-MnO₂-K has larger specific surface area,better reducibility and more oxygen species adsorbed on the surface.The kinetic analysis results showed that the activation energy of?-MnO₂-K was nearly twice as much as that of?-MnO₂-KL,and the reaction rate per unit specific surface area was twice as much as that of?-MnO₂-KL while the specific surface area of?-MnO₂-K was similar to that of?-MnO₂-KL.Therefore,?-MnO₂-K exhibited excellent catalytic activity,which also confirmed the co-catalytic effect of surface K⁺.