The Study On The Construction Of High Performance CuAl-LDO/CNTs Catalysts And The Denitration Behavior Of NH₃-SCR

The selective catalytic reduction of NO_x by NH₃?NH₃-SCR?has become the most widely used one for flue gas denitration at domestic and foreign currently due to its advantages of high removal efficiency,large gas treatment capacity and easy control of reaction conditions.In view of the operating temperature window of current commercial catalysts can not well match the situation of low-temperature denitration condition.Based on the practical significance of low temperature flue gas treatment and research,it is of great value to develop low temperature NH₃-SCR catalysts with high efficiency,stability and friendly environment.Because of its friendly environment,price moderate and good redox ability,Cu-based oxides have become the most potential catalysts for low-temperature NH₃-SCR.However,its poor dispersion,stability and SO₂ resistance are the key factors what restricting its application.Although the Cu_y AlO_x composite metal oxides derived from layered double hydroxides?LDHs?precursor possess better dispersion on active centers and higher deNO_x activity than these of CuO/?-Al₂O₃ which are prepared by traditional impregnation method,the exposure of active centers remains limited,which owing to the nano-scale LDHs particles are easily influenced by the high surface energy,the strong hydrogen bonding between the layers and the Jahn-Teller distortion effect of Cu²⁺resulting in the serious layers stacking during the drying and roasting process.Therefore,the deNO_x performance and stability of Cu_yAlO_x preparing from CuAl-LDHs are still not satisfactory,and the H₂O/SO₂ resistance has not been markedly improved.This paper based on the excellent structure and physicochemical property of layered double hydroxides?LDHs?and carbon nanotubes?CNTs?,the CuAl-LDH/CNTs were assembly prepared in virtue of the induction effect of LDH nucleation by the electron-rich defect sites on the surface of CNTs.And then the CNT-doped Cu-based oxide denitration catalysts were obtained by structural topological transition of CuAl-LDH/CNTs at high temperature.The high dispersion of Cu-based active center is realized by the effective barrier of CNTs to the hydrotalcite-like layers,meanwhile,the coordinated construction of Cu₂O/CuO in the surface of the catalysts and the strengthening of the sulfur resistant property are accomplished with the help of thermal reduction and hydrophobic properties of CNTs,thus promoting deNO_x performance of Cu-based oxide catalysts.Combined with the calculation of density functional theory?DFT?,the synergic catalytic mechanism between Cu₂O and CuO was explored in depth,providing a theoretical basis for the structural design,optimization and application of the high-performance Cu-based oxide deNO_x catalyst for NH₃-SCR.The main research contents are as follows:?1?The effects of assembly methods of CNTs and CuAl-LDHs?in-situ assembly,stir assembly,mechanical assembly?on the structure morphology of the precursor and the denitration performance of the calcined products were investigated.The results showed that the CuAl-LDH/CNTs prepared by three kinds of assembly methods can disperse CuAl-LDH in varying degrees.Owing to the electron-rich defect sites of CNTs can induce in situ nucleation of CuAl-LDH on its surface during the in-situ assembly process,which making the CNTs can better play its steric hindrance effect,and the advantage dispersion of CuAl-LDH is realized.As a result,the CuAl-LDH/CNTs?I?prepared by this method shows a looser stacking form and a thinner lamellar structure.Thus,the CNTs doped Cu-based oxides denitration catalysts?CuAl-LDO/CNTs?I??derived from the high temperature structural topological transformation of CuAl-LDH/CNTs?I?has better dispersion of Cu-based active components.The surface acidity as well as redox ability of the catalysts were significantly enhanced,and leading to excellent NH₃-SCR denitration performance in low temperature?the NO_x conversion above 80%,and N₂ selectivity above 90%within 180-300??.?2?A series of CuAl-LDO/CNTs-x denitration catalysts were prepared via in-situ assembly method,the effects of CNTs content on the surface structure,chemical properties and deNO_x performance of the catalyst were investigated,and the mechanism of the enhancement of sulfur tolerance by the introduction of CNTs was revealed.Research results showed that changing the amount of CNTs can advantageously control valence distribution of CuO_x,and optimize the surface acidity as well as redox ability.When CuAl-LDH/CNTs-1.0?the mass percentage of CNTs was 21.48%?as the catalyst,the conversion of NO_x can exceed 90%within 180-300?.In addition,the synergistic effect between CNTs and CuAl-LDO promoted the activation and decomposition of ammonium sulfate at lower temperatures and achieve a dynamic balance between the formation and decomposition of ammonium sulfate.Thus,the excessive accumulation of ammonia sulfate on the catalyst surface was avoided to some extent,and the SO₂ resistance was improved obviously.?3?The synergic catalytic mechanism of Cu₂O/CuO for NH₃-SCR was revealed deeply by density functional theory?DFT?calculation.The calculation results showed that the Cu₂O and CuO active sites have clear division of labor and cooperation in the reaction process.As the main active center of the catalysts,CuO is responsible for the adsorption and activation of NH₃ and NO reactants,while Cu₂O is responsible for the rapid generation of active oxygen species on the surface.Under the synergistic effect of Cu₂O and CuO,the formation of reaction intermediate species was accelerated through the transmission of the surface active oxygen species,and the catalytic reaction cycle is completed quickly.