The influence of sodium on the catalytic properties, low-temperature hydrothermal stability, and sulfur resistance of Cu-SAPO-34 for NO_x reduction by NH₃-SCR

In the actual diesel vehicle after-treatment systems, Cu-SAPO-34 selective catalytic reduction (SCR) of NO_x catalyst simultaneously experiences the failure of sodium (Na) poisoning, low-temperature hydrothermal (LTH) deactivation and SO₂ poisoning. This work investigated that the Na-loaded Cu-SAPO-34 catalysts were severally subjected to LTH treatment and sulfur poisoning for the first time. Our characterizations revealed that Na played entirely different roles in above two scenarios. In the LTH process, Na readily exchanges the protons of Si-OH-Al moieties on the zeolite framework, which inhibits the hydrolysis of the framework and further restrains the formation of SCR-inactive species. However, for SO₂ poisoning, the presence of Na accelerates the deactivation of Cu-SAPO-34 because more stable sulfate species are formed on the catalyst, which severely block the pores and reduce the active sites. It is expected that these results could assist the design of SCR catalysts to meet ultra-low NO_x emission.