Preparation Of High-Efficiency Manganese Oxides And Application Of Catalytic Oxidation And Adsorption For Hg~0 Removal

Mercury was a global pollutant has aroused many environmental problems around the world.After decades of hard work,the Minamata Convention on Mercury was finally approved and signed,aims at limiting the use of mercury product and decreasing the emission of mercury to the environment.As the largest mercury emission country,China faced serious problems about mercury control.Mercury emission from coal-fired power plant often regarded as the biggest source among the anthropogenic emission sources.In addition,mercury existed as elemental mercury?Hg⁰?was hard to be removed.Generally,two methods were applied to remove Hg⁰ from coal-fired flue gas:1)Catalytic oxidation of Hg⁰ to the oxidized mercury(Hg²⁺)and then use wet desulphurization devices to removal Hg²⁺;2)Adsorption of Hg⁰ to particle-bond mercury?Hg^p?and then use dust collection devices to remove Hg^p.No matter catalytic oxidation method or adsorption method,the key was the development of novel catalyst and adsorbent.Manganese oxides?MnOx?were cheap transition metal oxides and they have high catalytic oxidation performance for Hg⁰.The oxidized mercury combined with the surface oxygen on the surface of MnOx.MnOx also can be regarded as high efficiency adsorbent for Hg⁰.However,MnOx often suffers from the problems of low activity at high temperature,SO₂ poison effect,particles aggregation and low electron conductivity.To solve these problems,a series of manganese oxides were designed and synthesized.The mechanism for Hg⁰ removal over these oxides were also discussed.In addition,some novel technologies such as simultaneous remove NOx and mercury were investigated in this study.The major conclusions are summarized as follows:1.Different crystal MnO₂??-,?-and?-MnO₂?with one dimensional morphologies were synthesized using hydrothermal method.?-MnO₂ has a belt-like morphology,?-MnO₂ has a rod-like morphology as well as?-MnO₂ has a roller-like morphology.These materials were tested for Hg⁰removal and the mechanism was also discussed.The results indicated that crystal structure of MnO₂ has obvious effect on Hg⁰ removal.?-MnO₂ has a better Hg⁰ removal performance than?-MnO₂,but?-MnO₂ nearly has no activity for Hg⁰.Low temperature?100-200 ^oC?was beneficial for Hg⁰removal.When the temperature was higher than 200 ^oC,MnO₂ could lost part of activity.O₂ takes part in the reaction.The surface adsorbed oxygen of MnO₂ related to the mercury capture.The abundant surface oxygen could result in high mercury capacity.The reducibility of MnO₂ was also an important parameter,the higher oxidation performance results in higher Hg⁰ removal performance.Mercury combined with surface oxygen on the surface of MnO₂.Therefore,duing Hg⁰ removal process,catalytic oxidation of Hg⁰ to Hg²⁺and the adsorption of Hg²⁺with O were the keys for mercury chemical adsorption.2.Graphene supported MnOx were synthesized for Hg⁰ catalytic oxidation and adsorption.It was found that MnOx particles grow on the surface of graphene sheets and that the particles become smaller and more uniform.MnOx/graphene has a better Hg⁰ removal performance than pure MnOx.During the reaction,graphene offers the electrons which was beneficial for the catalytic oxidation.High temperature was also not favorable for Hg⁰ removal,but the poison effect was not as serious as pure MnOx.HCl nearly has no effect for Hg⁰ removal.MnOx/graphene has no dependence on HCl like other carbon based sorbents.But O₂ has a significant effect on Hg⁰ removal.The adsorbed mercury can release from the surface of MnOx/graphene using thermal desorption method,resulted in the regeneration of the sorbent.3.To simultaneously remove Hg⁰ and NOx at low temperature from flue gas in coal-fired power plants,an efficient NH₃-SCR LaMnO₃perovskite oxide was chosen as the catalyst for Hg⁰ removal.The results indicated that LaMnO₃ exhibits high mercury capacity.The special perovskite oxide structure benefits catalytic oxidation and promoted Mn⁴⁺to Mn³⁺.La modified MnOx enlarged the BET surface area of MnOx as well as surface oxygen which offer more room for mercury adsorption.Higher temperature was not favorable for Hg⁰ removal.NO and Hg⁰ have competitive adsorption relationship on the surface of LaMnO₃.O₂enhanced the Hg⁰ removal performance by re-oxidation of reduced Mn³⁺to Mn⁴⁺and providing additional adsorbed oxygen.NO enhanced the Hg⁰removal performance.However,NH₃ exhibited negative effects on Hg⁰removal.The results indicated that the ad-NH₃ species prevent Hg⁰adsorption,but the ad-NO₂ species were beneficial for Hg⁰ oxidation.The effects of SO₂ and H₂O were also investigated,and the results indicated that they inhibited Hg⁰ removal.4.To solve the problems of the inactivation at high temperature and SO₂ poison effect,a series of Fe-Sn modified MnOx complex metal oxides were prepared for Hg⁰ removal.The results indicated that Fe-Sn-MnOx metal oxide has the highest Hg⁰ removal performance among the prepared metal oxides.MnOx can homo-disperse in the crystal structure of SnO₂which resulted in higher surface area.A small amount of Fe could further enlarge the surface area.In addition,with the addition of Sn element,the reaction temperature window was enlarged.SnO₂ could adsorb O₂ from simulated flue gas at high temperature.The adsorbed O₂ can be changed to O^2-which was beneficial for mercury capture.With the addition of Fe element,it protected the Mn active site from the poison of SO₂.The numbers sulfate species on the surface of complex metal oxides decreased.5.Mercury-temperature programmed desorption?Hg-TPD?method was employed to clarify mercury species over Mn-based oxides.The Hg⁰removal mechanism over MnOx was ascribed to chemical-adsorption.HgO was the primary mercury species adsorbed on the surface of MnOx.Rare earth?Ce?element,main group element?Sn?and transition metal elements?Zr and Fe?were chosen for modification of MnOx.Hg-TPD results indicated that the binding strength of mercury between these binary oxides followed the order of Sn-MnOx<Ce-MnOx^MnOx<Fe-MnOx<Zr-MnOx.Sn-MnOx had a weak bond of mercury?Hg-O?,while Zr-MnOx had a strong bond?Hg?O?.Ce-MnOx and Fe-MnOx had similar bonds compared with pure MnOx.Moreover,the effects of SO₂ and NO were investigated based on Hg-TPD analysis.SO₂ had a poison effect on Hg⁰ removal,and the weak bond of mercury can be easily destroyed by SO₂.NO was favorable for Hg⁰ removal,and the bond strength of mercury was enhanced.In addition,we design a series of novel manganese oxide, such as 3D porous materials using carbon sphere and 1D?-MnO₂,spinel LiMn₂O₄ metal oxide and Ce-Sn binary oxide supported LaMnO₃ complex oxides.Their Hg⁰ removal performances were also evaluated.In this study,pure manganese oxides?MnO₂?,supported manganese oxides?MnOx/graphene?,special structure manganese oxides?LaMnO₃?and complex manganese oxides?Fe-Sn-MnOx?were prepared for Hg⁰removal.They exhibited various properties and they can be used in many places.It offers new idea for material design and preparation,and it also offers novel technology for simultaneously remove Hg⁰ and NOx.