Mechanistic insights into the enhanced low-temperature activity of Cu-SSZ-13 by MnOx-CeO2 modification for standard SCR reaction and fast SCR reactions

Cu-SSZ-13 has been commercially utilized as NH 3 -SCR catalyst for diesel vehicles, yet its performance is hindered at low-temperatures and further inhibited by NO 2 . These issues could be mitigated by MnO x -CeO 2 modification, but the specific roles of MnO x and CeO 2 remained unclear. Through a combination of characterization methods, TPD/TPSR and DRIFTS, a promotional mechanism was proposed. In the standard SCR reaction, MnO x and CeO 2 modification facilitated NO oxidative activation by reducible Mn species and active oxygen species, promoting the L -H pathway by nitrite/nitrate species reacting with adsorbed NH 3 species. MnO x modification proved particularly effective for NO oxidative activation, generating monodentate nitrate and nitrite as active intermediates, thereby contributing to the superior low-temperature activity. Although the MnO x and CeO 2 modification also reduced high-temperature activity of Cu-SSZ-13, the MnO x -CeO 2 modification maintained high-temperature activity of Cu-SSZ-13 owing to the synergistic interaction between MnO x and CeO 2 on Cu-SSZ13. In the fast SCR reaction, MnO x modification promoted the reduction of NH 4 NO 3 and inhibited N 2 O release by producing bidentate nitrate and monodentate nitrate, and CeO 2 modification enhanced the reactivity and instability of NH 4 NO 3 , mitigating NH 4 NO 3 inhibition on NO reduction. This work provided an improved understanding of the standard SCR and fast SCR reactions of Cu-SSZ-13-based catalysts at low temperatures.