(60c) Understanding of the Transition Metal Ion Properties and Their Impact on Various Catalytic Functions of Zeolite-Based SCR Catalysts

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. For example, it has been shown that at low temperatures, under standard SCR conditions, Cu-zeolites show higher steady state NOx conversion than Fe-zeolites and reverse is true at high temperatures. These differences have been reported in literature, but the underlying mechanisms were not very well understood mainly due to lack of comparable reference catalysts, which potentially introduces confounding effects for various reasons. 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. NOx conversion with NH₃ as a reductant and NO and NH₃ oxidation, and their storage were among the catalytic properties evaluated. The above catalytic properties were measured before and after sulfation of the SCR catalysts. Sulfation of the catalysts is achieved by exposing them to a mixture of SO₂ and SO₃ containing gas feed. The cumulative amount of sulfur stored on the catalyst and the rate of SOx release from sulfated SCR catalysts were determined using temperature programmed desorption under oxidizing conditions. Detailed experimental set up and conditions were described from our previous publications on SCR topic [1].

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.  

[1] Ashok Kumar, Michael A. Smith, Krishna Kamasamudram, Neal W. Currier, Hongmei An, Aleksey Yezerets, “Impact of different forms of feed sulfur on small-pore Cu-zeolite SCR catalyst”, Catalysis Today, 231 (2014) 75-82