Synergistic interaction between the γ-Al2O3 washcoat and Cu-SSZ-13 for enhancing NH3-SCR performance

The development of coated monolithic catalysts that combine high catalytic activity with high N<inf>2</inf> selectivity remains a significant challenge for NOx abatement in diesel exhaust gas. The slurry with Cu-SSZ-13 (CS), silica sol and γ-Al<inf>2</inf>O<inf>3</inf> derived from alumina sol calcination was deposited on honeycomb cordierite monolith (CC) to obtain coated monolithic catalysts. This study aims to unravel the synergistic interaction between the γ-Al<inf>2</inf>O<inf>3</inf> washcoat and CS as an active component to enhance the catalytic performance of CS. The CS-SiAl-4/CC displays excellent NOx purification performance in the absence of water vapor as well as outstanding hydrothermal aging resistance. The results indicate that γ-Al<inf>2</inf>O<inf>3</inf> exhibits excellent adhesion and uniform dispersion on the CS surface without disrupting CS's crystalline structure. The coordination environment, redox properties and amount of Cu species are not affected by γ-Al<inf>2</inf>O<inf>3</inf>. Furthermore, γ-Al<inf>2</inf>O<inf>3</inf> features a substantial number of both Brønsted and Lewis acid sites, which play a crucial role in storing NH<inf>3</inf> and providing NH<inf>4</inf>⁺. In situ DRIFTS results demonstrate that CS-SiAl-4/CC can adsorb more NOx species (free ionic nitrates and monodentate nitrates), which react with NH<inf>3</inf> adsorbed on the Lewis acid sites through the Langmuir-Hinshelwood (L-H) mechanism. NH<inf>4</inf>⁺ plays a more important role as the main active intermediates, and its reaction with gaseous NO suggests the Eley-Rideal (E-R) mechanism. More importantly, bidentate nitrates can convert to monodentate nitrites, which serve as the active species for the SCR reaction. This phenomenon is likely the main reason for the superior SCR performance of CS-SiAl-4/CC compared to CS-Si/CC.