Structure-activity Relationship And Reaction Mechanism Of NH₃-SCR Catalysts

With the development of modern industry and the increasing number of motor vehicles,more and more air pollutants have been released into the atmosphere.Nitrogen oxides?NO_x?are major air pollutants,which contribute to environmental issues such as acid rain,photochemical smog,atmospheric oxidant ozone increase and haze.The elimination of NO_x pollution has become an important issue in environmental protection.Recently,the selective catalytic reduction?SCR?of NO_x with NH₃ is the most widely used technology and has been industrialized for reducing NO_x emission.However,the traditional V₂O₅-WO₃/TiO₂ catalyst suffers from some disadvantages including the toxicity of vanadium,the narrow operational temperature window,and the deterioration of the structure and constituents after high-temperature aging,which restrict its wider application.Hence,the development of environmental-friendly catalysts with the wider operated temperature window and improved NH₃-SCR activity has become a hot research topic.In recent years,lots of research work focus on the modification of the existent catalyst,while the structure-activity relationship and reaction mechanism of the oxide and molecular sieve catalysts should be further studied.Therefore,it is of great theoretical and practical significance to investigate the structure-activity relationship and reaction mechanism of NH₃-SCR catalysts,which could guide the development and application of these catalysts.In this paper,a variety of characterization and research methods were applied to carry out a series of systematical and in-depth investigation to detect the accurate structural and physical-chemical properties for Fe-based oxide catalyst,Mn-based bifunctional catalyst,and Cu-based molecular sieve catalyst.The structure-activity relationship and reaction mechanism of these catalysts were studied by the contrast experiments and the measurement of intrinsic activity.Density functional theory?DFT?calculations were also used to clarify the active sites and uncover the reaction mechanism at molecular/atomic-level.Fe-based oxide catalysts possess excellent NH₃-SCR activity,N₂ selectivity,as well as SO₂resistance at medium-high temperature,while the low-temperature activity is poor.W_aFeO_x composite oxides were synthesized and characterized in detail using HRTEM,HAADF-STEM and XAFS.The results indicate that the short-range ordered W-O-Fe structures with the atomic-level interaction between W and Fe act as the active sites,which broaden the active temperature window and provide the high activity at low-medium temperature.Meanwhile,the short-range ordered W-O-Fe structures possess a superior thermal stability,which retain the amorphous state and cover on the surface of the crystallines after aging at 800 ^oC for 5 h.These reserved short-range ordered W-O-Fe structures maintain the excellent activity at high temperature?300-450 ^oC?.Tungsten,which exhibits character a similar to that of Mo,was doped into the Fe₂O₃ to obtain Mo_x-Fe₂O₃ composite oxides with good low-medium temperature NH₃-SCR activity.The structural properties were deeply characterized using HRTEM,HAADF STEM,XPS and XAFS.The results confirmed that only a small part of Mo atoms participated into the formation of Fe₂?MoO₄?₃ microcrystals,and the majorities were doped into Fe₂O₃ matrix to occupy the Fe site with octahedral coordination.The doped Mo atoms led to the enhanced Lewis acidity of the adjacent Fe site resulting in the improved SCR activity.The structural models of Mo atom doped Fe₂O₃ matrix and the corresponding NH₃ adsorption were simulated by DFT calculation.The DFT results are consistent with the experimental data,confirming the catalytic active sites are not the dopant atoms themselves but rather the adjacent Fe atoms whose electronic structure was modified by the dopant atoms resulting in enhanced Lewis acidic properties of the catalyst.In consideration of the low N₂ selectivity of Mn₂O₃ catalyst,the bifunctional V_a-MnO_x catalysts were fabricated,in which the synergistic effect between Mn₂O₃ and Mn₂V₂O₇provided the improved SCR activity and N₂ selectivity at low temperature.The structural models of oxides and surfaces,the adsorption and activation of NH₃ as well as the formation and decomposition of active intermediates were studied in virtue of DFT calculations to clarify the reaction intermediates,detailed reaction pathway,and reaction mechanism.Therein,NH₃ is initially activated by Mn₂O₃ to form an NH₂ intermediate.Transfer of NH₂ to Mn₂V₂O₇ then takes place which facilitates the capture of gaseous NO leading to the formation of NH₂NO over Mn₂V₂O₇,whereafter NH₂NO is efficiently converted to the preferred N₂ rather than the undesired by-product,N₂O.Cu-based molecular sieves with CHA structure have become a research hotspot of SCR catalyst for diesel vehicle due to their good high temperature activity and thermal stability.Cu_x-SAPO-44 zeolites with CHA structure were synthesized by ion-exchange method,and the Cu contents were tunable by changing the concentration of CuSO₄ solution.XAFS and EPR characterization of Cu species and the measurement of activity confirmed that the isolated Cu²⁺ions in zeolites were active sites for SCR reaction,and stable even after the hydrothermal aging.The reaction kinetics revealed that the activation energy of Cu_x-SAPO-44catalyst was 38.2±1.9 kJ/mol,which could be a promising candidate as a SCR catalyst for deNO_x in after-treatment systems for diesel vehicles.