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

Paper #:
  • 2017-01-0939

  • 2017-03-28
Copper and iron-zeolite SCR catalysts are widely used in US and European diesel after-treatment system to achieve drastic reduction in NOx emissions. The type of transition metal ion in zeolite has a significant impact on their key performance and durability characteristics including the response to sulfur containing exhaust gas. In this article, we made an attempt to address the impact of only the transition metal ion on various SCR catalytic functions, by using a single source of zeolite BEA in the preparation of H-, Fe- and Cu-BEA samples. Similar molar amounts of copper and iron were ion-exchanged into H-BEA and washcoated on the cordierite support by Johnson Matthey Inc. Below 300C, Fe-BEA showed lower NOx conversion than Cu-BEA despite the former catalyst more active for NO oxidation to NO2, a potential reaction intermediate. Upon sulfation, the NOx reduction activity of Cu-BEA degraded much more than Fe-BEA SCR catalysts. Adsorption and desorption of various probe molecules such as NO2, SOx, and NH3 indicated that the Fe-BEA is relatively more acidic than Cu-BEA therefore, the former catalyst is prone to strong NH3 adsorption on Fe active species inhibiting the NOx conversion at lower temperatures. On the other hand, more acidic Fe-BEA shows lower affinity to acidic species such as sulfur oxides. This study reconciles a number of observed differences in catalytic properties of Fe- and Cu-zeolite SCR catalysts, mainly based on their acid-base characteristics. A systematic and comprehensive analysis, which yielded practically relevant insights into the catalyst performance and sulfation and desulfation behaviors, will be presented. -76=
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