Experimental And Mechanistic Studies On Catalytic Reduction Of NO By Iron-based Spinel

Nitrogen oxides emitted from power plants cause environmental problems such as photochemical smog,acid rain,ozone holes and the greenhouse effect.To date,the selective catalytic reduction of NO_x with ammonia?NH₃-SCR?is considered as an efficient and highly selective denitrification technology for the removal of NO_x from the flue gas of coal-fired power plants.So far,V₂O₅-WO₃/TiO₂ and V₂O₅-MoO₃/TiO₂ are the commercially used SCR catalysts of coal-fired power plants.However,V₂O₅-based catalysts have some disadvantages,the toxicity of V₂O₅,the narrow temperature window and the formation of N₂O at high temperatures.In view of the shortcomings of current SCR catalysts,prepared a kind of low-temperature spinel catalysts,the related microscopic reaction mechanism was discussed by density functional theory.A series of Cu_xFe_3-xO₄ and CuMnFeO₄ spinel catalysts were prepared by a low-temperature sol-gel self-combustion synthesis method.The physicochemical properties of the catalysts were studied by SEM,XPS,XRD characterization analysis.The denitrification performance of Cu_xFe_3-xO₄ spinel and CuMnFeO₄ spinel catalysts was tested in a fixed bed reactor.The kinetic model of catalyst surface was established and the kinetic parameters of NO_x catalytic reaction were calculated.The limiting mechanism of catalyst denitrification was explored by changing the flue gas velocity on the catalyst surface.The results show that the particles of the Cu_xFe_3-xO₄ catalyst are relatively small,and the particle size of the Cu_xFe_3-xO₄ catalyst increases with the increase of Cu molar ratio.Cu,Mn and Fe atoms on the surface of CuMnFeO₄ catalyst exist in mixed valence state(Cu⁺,Cu²⁺,Fe²⁺and Fe³⁺).With the increase of Cu molar ratio,the agglomeration of catalyst nanoparticles appeared.The results show that the CuMnFeO₄ catalyst exhibits good denitrification performance in the temperature window of 180³00°C.The studies on the limiting mechanism show that the denitrification efficiency of the catalyst is controlled by mass transfer process when the surface velocity of the flue gas is less than21.1 cm/s.As the flue gas velocity is greater than 21.1 cm/s,the denitrification efficiency of the catalyst is affected by the surface reaction kinetics.CuMnFeO₄ catalyst has a certain ability to resist SO₂ and H₂O poisoning.Density functional theory calculation was used to study the chemical reaction mechanism on the surface of CuMnFeO₄ spinel catalyst.According to the adsorption energy and electronic structure analysis,the adsorption activity of NH₃ on the surface of CuMnFeO₄ catalyst was revealed.The interaction mechanism between NH₃ and surface active atoms of the catalyst was elucidated.The catalytic reaction mechanism of NO_x on the surface of CuMnFeO₄ catalyst was explained.The calculation results show that the maximum adsorption energy of NO on the surface of CuMnFeO₄ catalyst is 139.86 kJ/mol,and the maximum adsorption energy of NH₃ on the catalyst surface is 115.31 kJ/mol.The reactants such as NO and NH₃ are adsorbed on the Fe atoms.The reaction product such as N₂ is also adsorbed on Fe active site.During the formation of N₂,NH₂ generated from NH₃ deyhdrogenation reacts with NO molecule adsorbed on the catalyst surface to form HNO functional group.Subsequently,the HNO functional group further transforms into a NOH functional group,and reacts with NH radicals to form N₂ and H₂O.The adsorption and reduction mechanism of NO_x on the surface of MnFe₂O₄ spinel was studied by density functional theory.According to the adsorption energy,Milliken charge,molecular orbital and electronic structure analysis,the adsorption activity of NH₃on the surface of MnFe₂O₄ spinel was revealed.The interaction mechanism between NH₃and surface active atoms of the catalyst was explained.The reaction pathway and activation energy barrier of NO_x reduction on MnFe₂O₄ surface was investigated to reveal the catalytic reduction mechanism.The calculation results show that the excellent N₂selectivity of MnFe₂O₄ catalyst is closely related to the higher activation energy barrier of N₂O formation.The reaction pathway of N₂O formation includes:NO*+NO*?N₂O₂*+*?N₂O*+O*and N*+NO*?N₂O*+*.N₂O molecule formed in the first reaction path has an activation energy of 318.67 kJ/mol and a reaction heat of 70.25kJ/mol.The second reaction pathway presents an activation energy barrier of 56.78 kJ/mol,and a reaction heat of-57.30 kJ./mol.NO₂ is mainly produced from the oxidation reaction of NO?NO*+O*?NO₂*+*?.The activation energy barrier of this reaction is 111.51kJ/mol,and the reaction heat is 69.67 kJ/mol.The dominant reaction pathway of NO reduction with NH₃ is a three-step chemical reaction process,which is controlled by NH₃*+*?NH₂*+H*,NH₂*+NO*?NH₂NO*+*and NH₂NO*+*?N₂*+H₂O*.NH₃*+*?NH₂*+H*is the rate-limiting step of N₂ formation.