Effect of Transition Metal Ion Properties on the Catalytic Functions and Sulfation Behavior of Zeolite-Based SCR Catalysts

Copper- and Iron- based metal-zeolite SCR catalysts are widely used in US and European diesel aftertreatment systems to achieve drastic reduction in NOx emission. These catalysts are highly selective to N₂ under wide range of operating conditions. Nevertheless, the type of transition metal has a significant impact on the key performance and durability parameters such as NOx conversion, selectivity towards N₂O, hydrothermal stability, and sensitivity to fuel sulfur content. In this study, we explained the differences in the performance characteristics of these catalysts based on their relative acidic-basic nature of transition metal present in these catalysts using practically relevant gas species present in diesel exhaust such as NO₂, SOx, and NH₃. These experiments show that Fe-zeolite has relatively acidic nature as compared to Cu-zeolite that causes NH₃ inhibition and hence explains low NOx conversion on Fe-zeolite at low temperature under standard SCR conditions. Similarly, relatively basic nature of Cu-zeolite results in more prominent SOx storage and performance deactivation in the presence of acidic contaminant like SOx and required higher temperatures for desulfation as compared to Fe-zeolite.