Effect of alkali metal poisoning on Cu-SSZ-13 in selective catalytic reduction with ammonia (NH3-SCR)

Abstract

Cu-SSZ-13 is widely recognized as an efficient catalyst for the selective catalytic reduction of NO_x with ammonia (NH₃-SCR). However, in practical stationary applications, it is frequently exposed to alkali metal contaminants, which can significantly influence its catalytic performance. In this study, the effects of different alkali metal salts (NaNO₃, KNO₃, NaCl and KCl) on Cu-SSZ-13 were systemically investigated (regarding the variation in the physicochemical properties and NH₃-SCR activity). A slight promotional effect (about 10% increase in NO_x conversion) was observed at low temperatures (<250 °C) for alkali metal poisoned samples, which is attributed to the enhanced presence of active ZCu^IIOH species, as evidenced by H₂-TPR analysis. Conversely, a marked deactivation (up to 50% decline in NO_x conversion) was observed at high temperatures (>400 °C), primarily due to the loss of Brønsted acid sites and isolated Cu²⁺ species, as indicated by NH₃-TPD, pyridine-IR, H₂-TPR and UV-vis spectroscopy. Notably, alkali metal chlorides (KCl and NaCl) induced more severe deactivation than their nitrate counterparts, likely due to a synergistic effect between framework dealumination and transformation of Cu²⁺ species, leading to the formation of CuAlO_x species. These findings provide deeper insight into the effect of alkali metal poisoning on the NH₃-SCR activity of Cu-SSZ-13, highlighting the necessity of preventing alkali metal chlorides from entering SCR systems to maintain catalyst efficiency.