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

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 NH<inf>3</inf>-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 NH<inf>3</inf> adsorption/activation, and the synergistic effect between Cu species and CeO<inf>2</inf>/Sm<inf>2</inf>O<inf>3</inf> leads to a substantial boost in the low-temperature NH<inf>3</inf>-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 CeO<inf>2</inf> and Sm<inf>2</inf>O<inf>3</inf> enhance the number of both Brønsted and Lewis acid sites on CSZ, promoting the adsorption and activation of NH<inf>3</inf>. Therefore, 6 % Ce-2 % Sm/CSZ can form more NH⁺<inf>4</inf> adsorbed on the Lewis acid sites, which reacts with free ionic nitrates to form NH<inf>4</inf>NO<inf>3</inf>. More importantly, the coupled Sm<inf>2</inf>O<inf>3</inf> facilitates the conversion of NH<inf>4</inf>NO<inf>3</inf> by NO to easily decomposable NH<inf>4</inf>NO<inf>2</inf>. In addition, additional oxygen vacancies provided by Ce³⁺ can adsorb O<inf>2</inf> 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 NH<inf>3</inf>-SCR performance of CSZ is improved via a synergistic effect. The NH<inf>3</inf>-SCR reaction over 6 % Ce-2 % Sm/CSZ co-follows the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms.