Study On CuFe-SSZ-13 Catalysts For The Selective Catalytic Reduction Of NOx With NH₃

Nitrogen oxides?NOx?from diesel exhaust is one of the major air pollutants in urban.Selective catalytic reduction of NOx with NH₃?NH₃-SCR?is a well-proven technology,which was suitable for the catalytic and purification of NOx from diesel exhaust in our national.The core of this technology is the development of catalysts.Cu-SSZ-13 catalyst shows excellent catalytic performance and becomes a research hotspot in NH₃-SCR field.However,it has defects in the resistance to sulfur poisoning.Therefore,the transition metal Fe was loaded in Cu-SSZ-13 to improve its sulfur resistance.A series of CuFe-SSZ-13 catalysts were prepared to analyze the structure-activity relationship between the state of active species and NH₃-SCR activity.By further optimizing the preparation process,a bimetallic-based zeolite CuFe-SSZ-13 catalyst with excellent NH₃-SCR activity,hydrothermal stability and sulfur poisoning resistance was obtained.The mechanism of NH₃-SCR reaction and SO₂ poisoning were further investigated.It could provide theoretical basis for further optimization and practical application of CuFe-SSZ-13 catalysts.The main research results are as follows:?1?Fe was loaded into Cu-SSZ-13 catalysts through solid-state ion-exchange,homogeneous deposition precipitation and liquid ion-exchange to obtain three CuFe-SSZ-13catalysts(CuFe-SSZ-13_SSIE,CuFe-SSZ-13_HDP and CuFe-SSZ-13_IE).They showed significant difference in physicochemical properties.Both Cu²⁺and Fe³⁺are active species of three CuFe-SSZ-13 catalysts,which were beneficial to their NH₃-SCR activity.However,the difference in distribution of active species has a significant effect on hydrothermal stability.For CuFe-SSZ-13_SSIE,most active species accumulated inside the zeolite.During hydrothermal aging,the aggregation of these active species caused the collapse of catalyst structure,ultimately leading to the deactivation of CuFe-SSZ-13_SSIE.For CuFe-SSZ-13_IE,most Fe species was dispersed better on the surface.This distribution pattern inhibited the aggregation of active species effectively during aging,and thus improved the hydrothermal stability of CuFe-SSZ-13_IE.Fe loading improved the resistance to H₂O and SO₂ over Cu-SSZ-13.However,the effect of different preparation methods on the sulfur resistance of CuFe-SSZ-13 catalysts was not significant.Considering the catalytic activity,hydrothermal stability and sulfur poisoning resistance,liquid ion-exchange method was optimized to prepare CuFe-SSZ-13 catalyst.?2?On the basis of liquid ion-exchange method,the preparation conditions were further optimized,and Cu_2.3Fe_2.3-SSZ-13 catalyst with best catalytic activity,hydrothermal stability and sulfur resistance was obtained finally.Compared with Cu-SSZ-13,the hydrothermal stability and sulfur poisoning resistance of Cu_2.3Fe_2.3-SSZ-13 was improved obviously,but its NH₃-SCR activity was decreased at low temperatures.The low tempweature reaction mechanism of NH₃-SCR over Cu_2.3Fe_2.3-SSZ-13 was investigated by in situ diffuse reflectance infrared fourier transform spectroscopy?in situ DRIFTS?.When the reaction temperature was higher than 150°C,there were two reaction paths on the surface.First,NH₃ adsorbed species reacted with NO₂^-to form NH₄NO₂,which was then decomposed into N₂.Second,NO₃^-reacted directly with gaseous NH₃ to form N₂.When the reaction temperature dropped to 120°C,there was only the first reaction path.Fe loading weakened the reaction between NO₃^-and gaseous NH₃ over Cu-SSZ-13,ultimately leading to the decrease of NH₃-SCR activity over Cu_2.3Fe_2.3-SSZ-13 catalyst at low temperatures.?3?Based on the low temperature reaction mechanism of NH₃-SCR over Cu_2.3Fe_2.3-SSZ-13,its SO₂ resistance and poisoning mechanism were further studied.During sulfur treatment,Cu_2.3Fe_2.3-SSZ-13 was poisoned seriously in a short time,and the degree of poisoning increased with the temperature.However,the degree of poisoning was reduced under coexistence of H₂O and SO₂.After regeneration at high temperatures,the NH₃-SCR activity of Cu_2.3Fe_2.3-SSZ-13recovered at the medium and high temperature.Compared with the low temperature reaction path over Cu_2.3Fe_2.3-SSZ-13 before sulfation treatment,it changed greatly after sulfur poisoning.The reaction between NH₃ adsorbed species and NO₂^-was retained,but the reaction between gaseous NH₃ and NO₃^-was blocked after sulfur poisoning.Meanwhile,a new reaction path between gaseous NH₃ and NO⁺on SO₄^2-was added.However,the reaction rate of the new path was lower than that of original path,ultimately leading to the decrease in low temperature activity over Cu_2.3Fe_2.3-SSZ-13 catalyst after sulfur poisoning.