Insights into the role of Ce and Sm in improving low-temperature NH3-SCR performance over Ce-Sm/Cu-SSZ-13 coupled catalysts

Abstract

The increasingly stringent requirements for controlling nitrogen oxide (NOx) emissions during the cold start conditions of diesel engines serve as a powerful driving force to enhance the low-temperature NH3-SCR performance of state-of-the-art commercial Cu-SSZ-13. In this study, the coupled catalysts were synthesized to create additional active sites for NO oxidation and NH3 adsorption/activation, and the synergistic effect between Cu species and CeO2/Sm2O3 leads to a substantial boost in the low-temperature NH3-SCR activity of CSZ. The results suggest that 6 % Ce-2 % Sm/CSZ, as the optimal coupled catalyst, achieves a NOx conversion of 93.1 % at 200 °C, significantly higher than that of CSZ. The coupled CeO2 and Sm2O3 enhance the number of both Brønsted and Lewis acid sites on CSZ, promoting the adsorption and activation of NH3. Therefore, 6 % Ce-2 % Sm/CSZ can form more NH+4 adsorbed on the Lewis acid sites, which reacts with free ionic nitrates to form NH4NO3. More importantly, the coupled Sm2O3 facilitates the conversion of NH4NO3 by NO to easily decomposable NH4NO2. In addition, additional oxygen vacancies provided by Ce3+ can adsorb O2 and promote the transport of oxygen ions, and electron donation from Sm3+ to [ZCu2+(OH)]+ enhances the low-temperature activity of the latter. Ultimately, the low-temperature NH3-SCR performance of CSZ is improved via a synergistic effect. The NH3-SCR reaction over 6 % Ce-2 % Sm/CSZ co-follows the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms