Exploring The Performance Of Manganese Oxides Catalyst For Gaseous Ozone Decomposition

Ozone in the stratosphere prevents short-wavelength ultraviolet rays from reaching the earth surface,which is beneficial for the livings on the earth.But tropospheric ozone is considered as a hazardous pollutant,and prolonged exposure into an ozone atmosphere with high concentrations are harmful to humans,including neurological disorders,increased frequency of respiratory symptoms and reduced immune function.Ozone can also degrade building materials,reduce crop yields and cause slow growth of trees.In addition,ozone is also regarded as greenhouse gas.Therefore,the elimination of ozone is of great importance for environmental protection and human health.Among all ozone elimination technologies,catalytic decomposition has been widely studied due to its high efficiency,economy and safety.Up to date,the main problem for catalytic ozone decomposition is the serious deactivation of catalysts,especially for ozone decomposition under high relative humidity condition,which largely limits the industrial application of catalysts.Thus,there is an urgent need to develop a stable and active catalyst.In this thesis,carbon-coated nitrogen-doped manganese oxide catalysts and x Mn/y C-T catalysts with different calcination temperatures and manganese loadings were prepared,providing two novel strategies to solve catalyst deactivation.The performances of the catalysts for ozone decomposition were evaluated.The catalysts were analyzed by XRD,BET,SEM,TEM and XPS,etc.In addition,the catalyst stability and the deactivation mechanism induced by water vapor were explored.The main studies and results are shown as following:1.A series of nitrogen-doped Mn O-Mn₂N_0.86@C catalysts were prepared by pyrolysis of manganese acetate and melamine mixtures.Nitrogen-doping into the manganese oxides was observed at a high pyrolysis temperature,and this resulted in the increase of the oxygen vacancy density on the surface of the catalyst.Furthermore,the Mn species of the tubular Mn O-Mn₂N_0.86@C-850 catalyst was coated by a carbon layer,and this could significantly improve the water vapor resistance of the catalyst under high humidity.The ozone decomposition conversion reached nearly 100%using the Mn O-Mn₂N_0.86@C-850 catalyst,and this is clearly better than the widely studied OMS-2 catalyst and active carbon.Combined with the results of the catalytic performance and the Mn species in the catalysts,the ozone decomposition activities were revealed in the order:Mn₂N_0.86>Mn O₂>Mn O>Mn₃O₄>Mn₂O₃.The Mn O-Mn₂N_0.86@C-850 catalyst showed the highest activity and stability,and this can be ascribed to the existence of a high surface area and increased oxygen vacancy density on the surface of the catalyst through nitrogen doping,as well as the presence of a hydrophobic carbon layer.2.A series of Mn O_x catalyst supported by carbon sphere were prepared by calcining mixtures of manganese acetate and carbon spheres under nitrogen atmosphere,and its performances for ozone decomposition under high humidity condition（RH=90%）were evaluated.The calcination temperature and the ratio（of manganese acetate to carbon sphere）have significant influences on the catalytic stability for ozone decomposition over Mn/C catalysts.Among all the Mn/C catalysts,1Mn/3C-900 catalyst showed the most robust catalytic performance（ozone conversion remaining 100%after 6 h）under high humidity condition（RH=90%）.The size of Mn O_x particles on the surface of Mn/C-900 catalyst increased gradually from21 nm to 108 nm with the increase of Mn loading,which might result in the change of the oxygen vacancy density and Mn²⁺content of Mn O_x species.1Mn/3C-900 catalyst with Mn O_x particle size of 36 nm possessed highest surface oxygen vacancies and Mn²⁺content of Mn O_x species on surface,thus,the most stable activity was observed on the 1Mn/3C-900 catalyst.The high catalytic activity of 1Mn/3C-900 catalyst remained for even 70 h under dry condition.However,the significant deactivation of the 1Mn/3C-900 catalyst was observed with the presence of both water vapor（RH=90%）and ozone.Besides the competitive adsorption of water vapor on catalyst surface,the obvious decrease of oxygen vacancy density and Mn²⁺content on the catalyst surface induced by water vapor during the reaction should also be responsible to remarkable deactivation.