Enhancing catalytic activity and sulfur resistance of Fe- or Ce-modified Cu-SSZ-13 catalysts for NH3-SCR in diesel exhaust system

Abstract

The Cu-SSZ-13 zeolite catalyst was synthesized via ion-exchange method, followed by Fe or Ce incorporation to improve catalytic activity and sulfur resistance for NH₃-SCR. The synthesized catalysts were evaluated by the tests of NH₃-SCR activity before and after sulfur poisoning process, along with various characterizations, including XRD, ICP-OES, BET, XPS, H₂-TPR and in-situ DRIFTS. The results demonstrate that the modified catalysts exhibit a broadened operation temperature window and superior sulfur resistance compared to Cu-SSZ-13, especially Fe-modified catalyst. The introduction of Fe or Ce can reduce the structural stability of Cu-SSZ-13 catalyst, which leads to the substitution of some weakly adsorbed Cu²⁺ ions with Fe or Ce ions in the CHA framework. The charge transfer between Fe/Ce and Cu ions effectively enhances the redox performance of the catalyst, but the partial substitution of Cu²⁺ ions in the 8 MR of Cu-SSZ-13 catalyst with Fe or Ce ions results in a slight decline in low-temperature activity. The incorporation of Fe mitigates sulfur poisoning on Cu active sites by modifying the composition of sulfate species, thereby improving the sulfur resistance of the Fe-modified catalyst. Ce modification promotes Cu⁺ reduction at lower temperatures, enhancing high-temperature activity. Compared with Fe/Cu-F, the content of Cu ions in Ce/Cu-F is 11.9 % higher, indicating Ce/Cu-F contains more active Cu. In addition, in-situ DRIFTS shows that the prepared Fe/Cu-SSZ-13 follows mechanisms of Eley-Rideal to undergo NH₃-SCR reaction. In the Fe-modified catalyst system, synergetic Cu-Fe dual active sites are identified to enhance surface Lewis acidity, while Cu sites exhibit higher activity.