Unveiling Secondary-Ion-Promoted Catalytic Properties of Cu-SSZ-13 Zeolites for Selective Catalytic Reduction of NO x .

The incorporation of secondary metal ions into Cu-exchanged SSZ-13 zeolites could improve their catalytic properties in selective catalytic reduction of NO x with ammonia (NH 3 -SCR), but their essential roles remain unclear at the molecular level. Herein, a series of Cu-Sm-SSZ-13 zeolites have been prepared by ion-exchanging Sm ions followed by Cu ions, which exhibit superior NH 3 -SCR performance. The NO conversion of Cu-Sm-SSZ-13 is nearly 10% higher than that of conventional Cu-SSZ-13 (175-250 °C) after hydrothermal ageing, showing an enhanced low-temperature activity. The Sm ions are found to occupy the six-membered rings (6MRs) of SSZ-13 by X-ray diffraction Rietveld refinement and aberration-corrected scanning transmission electron microscopy. The Sm ions at 6MRs can facilitate the formation of more active [ZCu 2+ (OH)] + ions at 8MRs, as revealed by temperature-programmed reduction of hydrogen. X-ray photoelectron spectroscopy and density functional theory (DFT) calculations indicate that there exists electron transfer from Sm 3+ to [ZCu 2+ (OH)] + ions, which promotes the activity of [ZCu 2+ (OH)] + ions by decreasing the activation energy of the formation of intermediates (NH 4 NO 2 and H 2 NNO). Meanwhile, the electrostatic interaction between Sm 3+ and [ZCu 2+ (OH)] + results in a high-reaction energy barrier for transforming [ZCu 2+ (OH)] + ions into inactive CuO x species, thus enhancing the stability of [ZCu 2+ (OH)] + ions. The influence of the ion-exchanging sequence of Sm and Cu ions into SSZ-13 is further investigated by combining both experiments and theoretical calculations. This work provides a mechanistic insight of secondary ions in regulating the distribution, activity, and stability of Cu active sites, which is helpful for the design of high-performance Cu-SSZ-13 catalysts for the NH 3 -SCR reaction.