Enhancing Understanding of State-of-the-Art Fe/zeolite Selective Catalytic Reduction Catalysts

Fe/zeolite selective catalytic reduction (SCR) catalysts are commercially used for NOx emissions reduction from diesel engines. In comparison to Cu/zeolite, these catalysts are widely reported to form less N2O as a byproduct of the SCR reactions between NOx and NH3, and Fe/zeolite SCR catalysts also achieve impressive deNOx at high temperatures. On the other hand, Fe/zeolite SCR is much less active than Cu/zeolite for low temperature NOx conversion under standard SCR conditions (NO+NH3). In this study, a state-of-the-art Fe/zeolite SCR catalyst is probed with a combination of BET surface area analysis, SEM/EDX physicochemical characterization, reactor-based performance and active site quantification, and in-situ DRIFTS. The results are split into 3 sections evaluating the impacts of: degreening, mild hydrothermal aging, and extreme hydrothermal aging. In a degreened condition, the impact of water vapor on standard and fast SCR is assessed, and the isothermal desorption of NH3 is reported. The Fe/zeolite catalyst’s hydrothermal durability is studied following both mild hydrothermal aging at 550°C as well as extreme hydrothermal aging at temperatures up to 950°C. In addition to standard and fast SCR performance, NH3 storage and temperature programmed desorption (TPD) and NO2 storage and TPD experiments are used to monitor the catalysts surface acidity and active Fe sites, respectively. A kinetic analysis of the standard SCR data is conducted to elucidate the mechanisms responsible for SCR activity loss. Finally, DRIFTS experiments are used to clarify the nature of NH3 storage and Fe site changes which occur upon degreening. The authors believe the results presented herein can support the industry wide efforts to continue to improve diesel emissions control.