Sulfation of potassium-based lean NOx trap while cycling between lean and rich conditions: I. Microreactor study

Abstract Exposure of Pt/K/Al 2 O 3 to 15 ppm SO 2 reduces the NO x activity at 200, 300, and 400 °C at significantly different rates—1.5, 8.5, and 18.0 μmol NO x /(h g cat ), respectively. During the initial sulfation, NO x conversion is directly linked to lean phase storage capacity, and sulfation does not impact the reduction kinetics since the amount of unconverted NO x was constant or decreased with increasing sulfation time. A portion of sulfur stored at 200 °C desorbs upon mild heating to 400 °C while cycling between lean and rich conditions. This apparently is a result of sulfur being released from Al 2 O 3 ; however, performance is not significantly recovered as much of the sulfur is re-adsorbed on the K-phase. This is apparent from analysis of the NO x storage and release profiles. Additional analysis of these profiles suggests that SO 2 initially adsorbs near Pt before interacting with other sites further away from Pt at 300 °C. At 400 °C, it appears that SO 2 either preferentially adsorbs near Pt and then quickly diffuses along the surface to other less proximal sites, or it directly adsorbs on sites further away from Pt. De-sulfurization up to 800 °C using a temperature programmed reduction (TPR) procedure and rich conditions with both CO 2 and H 2 O restored 73–94% of the LNT performance at 300 and 400 °C. However, the recovered performance measured at 200 °C was only 34–49% of the original NO x reduction activity. H 2 S and SO 2 were the primary de-sulfurization products with H 2 S having a maximum release between 690 and 755 °C, while SO 2 had a peak release between 770 and 785 °C. The sulfation temperature does not have a significant impact on the recovered performance, the de-sulfurization products or the sulfur release temperature.