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 NH3-SCR catalyst for diesel vehicles, yet its performance is hindered at low-temperatures and further inhibited by NO2. These issues could be mitigated by MnOx-CeO2 modification, but the specific roles of MnOx and CeO2 remained unclear. Through a combination of characterization methods, TPD/TPSR and DRIFTS, a promotional mechanism was proposed. In the standard SCR reaction, MnOx and CeO2 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 NH3 species. MnOx 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 MnOx and CeO2 modification also reduced high-temperature activity of Cu-SSZ-13, the MnOx-CeO2 modification maintained high-temperature activity of Cu-SSZ-13 owing to the synergistic interaction between MnOx and CeO2 on Cu-SSZ-13. In the fast SCR reaction, MnOx modification promoted the reduction of NH4NO3 and inhibited N2O release by producing bidentate nitrate and monodentate nitrate, and CeO2 modification enhanced the reactivity and instability of NH4NO3, mitigating NH4NO3 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.