Study On Sulfur Poisoning Resistance Of CuFe-SSZ-13 Catalysts For Selective Catalytic Reduction Of NO_x With NH₃

As one of the main urban air pollutants,nitrogen oxides（NO_x）can cause serious environmental pollution.Diesel vehicle exhaust is one of the main sources of NO_x emission.NH₃ selective catalytic reduction of NOx（NH₃-SCR）is a very effective and mature technology,which has been widely used for diesel exhausts purification.The key of this technology is the development of catalysts.Much attention has been paid to Cu-SSZ-13 catalyst in the field of NH₃-SCR recently due to its excellent catalytic activity.In this study,Fe was loaed on the Cu-SSZ-13 with different metheods to prepare Cu Fe-SSZ-13 catalysts,and the prepareration conditions of each method were optimized,aiming to improve the sulfur poisoning resistance of Cu-SSZ-13 catalyst.The optimized Cu Fe-SSZ-13 catalysts were characterized with several methods to analyze the physicochemical properties and the existence state of active species of the fresh and sulfur-poisoning Cu Fe-SSZ-13 catalysts.And the structure-activity relationship was also investigated.Finally,the reasons for the improvement of the sulfur-poisoning resistance ability were explained from mechanism,aiming to provide a theoretical basis for further study and improvement of sulfur-poisoning resistance of NH₃-SCR catalysts.Specific research results are as follows:（1）Fe-Cu-SSZ-13_HDP was prepared by homogeneous deposition-precipitation method;two kind of core-shell structure Cu-SSZ-13@Fe_xO_y catalysts were obtained by soft template method and self-assembly method,respecitively.The homogeneous deposition-precipitation method was optimized,Fe-Cu-SSZ-13_HDP with 2.28 wt.%Cu loading and 2.2 wt.%loading was obtained.Two kind of core-shell catalysts preparation methods were optimized and Cu-SSZ-13@Fe_xO_y catalyst with 0.75 wt.%Fe loading prepared by self-assembly method was obtained.Fe-Cu-SSZ-13_HDP and Cu-SSZ-13@Fe_xO_y prepared by self-assembly method both have better sulfur poisoning resistance than Cu-SSZ-13.The Cu-SSZ-13@Fe_xO_y catalyst with 4.36 wt.%Fe loading prepared by soft template method was also obtained,but its sulfur poisoning resistance is lower than Cu-SSZ-13.（2）The physicochemical properties and sulfate species of Fe-Cu-SSZ-13_HDP and Cu-SSZ-13@Fe_xO_y catalysts were characterized and analyzed.The relationship between physicochemical properties and the sulfur poisoning resistance of the catalysts was investigated.Fe loading didn’t affect the crystal structure of Fe-Cu-SSZ-13_HDPand Cu-SSZ-13@Fe_xO_y.After Fe loading,Fe-Cu-SSZ-13_HDPcatalyst has more L-acid sites on suface.And the redox ability of catalyst decreased,which was beneficial to the improvement of sulfur poisoning resistance,due to the inhibition of the oxidation of SO₂ to SO₃.After the sulfur poisoning,some Fe species on the catalyst were not vulcanized and could maintain a part of NH₃-SCR activity,which was conducive to the improvement of sulfur poisoning resistance.The number of B-acid sites and L-acid sites on the surface of Cu-SSZ-13@Fe_xO_y catalyst has decreased,and the redox ability of the catalyst decreased,too.After sulfur poisoning,fewer ammonium sulfate and copper sulfate were observed on the surface of Cu-SSZ-13@Fe_xO_y catalyst,which is important for the improvement of the sulfur resistance ability of the catalyst.（3）The differences of reaction path between Fe-Cu-SSZ-13_HDP,Cu-SSZ-13@Fe_xO_y and Cu-SSZ-13 after poisoning were compared to explain the reason of the improvement of sulfur poisoning resistance ability of catalysts.After sulfur poisoning,the low temperature reaction path of Cu-SSZ-13 catalyst has changed,the reaction path between gaseous NH₃ and NO₃^-was completely blocked.Only the reaction path between adsorbed NH₃ species and NO+O₂ existed,but the reaction activity was very slow,which was the reason for the significant decrease of Cu-SSZ-13 catalytic activity after sulfur poisoning.The poisoned Fe-Cu-SSZ-13_HDP catalyst retained two reaction paths.Firstly,the reaction path between gaseous NH3 and NO₃^-was not completely blocked,and secondly,the adsorbed NH₃ species on the passivated active site surface could react slowly with NO+O₂,which was the reason for the improved resistance to poisoning of the catalyst.The reaction path of gaseous NH₃ and NO₃^-also exists in the poisoned core-shell Cu-SSZ-13@Fe_xO_y catalyst,and the reaction paths of adsorbed NH₃ species and NO+O₂ can occur on the Cu active sites in the catalyst.Both reaction path exists in Cu-SSZ-13@Fe_xO_y catalyst are the reasons for the improvement of sulfur poisoning resistance ability.