Boosting the low-temperature activity and SOx-tolerance of Al-rich Cu-CHA zeolite in NH3-SCR via a simple pretreatment strategy

Abstract

Selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) requires catalysts combining superior low-temperature performance and high SO_x tolerance in order to meet increasingly stringent regulations on heavy-duty diesel engine emissions. However, developing such catalysts is highly challenging. Herein, an Al-rich Cu-CHA zeolite was modified by a facile hydrogen pretreatment strategy, which resulted in boosted NO_x conversions to ca. 78 and 90 % at 175 and 200 °C, respectively. Moreover, SO_x (SO₂/SO₃=3/7) poisoning experiments revealed significantly higher SO_x tolerance over the modified Cu-CHA catalyst versus the pristine catalyst. H₂-TPR and kinetics analyses reveal that the pretreatment transformed aggregated Cu_xO_y species into isolated Cu²⁺ ions, thereby creating numerous active sites and promoting low-temperature SCR reactivity. Furthermore, the presence of more isolated Cu²⁺ ions and fewer 8MR-Z-[Cu(OH)]⁺ species after the pretreatment considerably inhibits the deposition of copper sulfate species, thereby enhancing the SO_x tolerance of the catalyst. In-situ DRIFTS demonstrates a higher number of key intermediate species (e.g., bridging/bidentate nitrates, NO⁺, and L-NH₃) on the modified Cu-CHA catalyst, and it also shows a “fast SCR” reaction response. This results in an enhanced deNO_x performance. Our study provides the feasibility of using a facile pretreatment strategy to develop effective SCR catalysts.