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

Exposure of Pt/K/Al2O3 to 15 ppm SO2 reduces the NOx activity at 200, 300, and 400 °C at significantly different rates—1.5, 8.5, and 18.0 μmol NOx/(h gcat), respectively. During the initial sulfation, NOx conversion is directly linked to lean phase storage capacity, and sulfation does not impact the reduction kinetics since the amount of unconverted NOx 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 Al2O3; 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 NOx storage and release profiles. Additional analysis of these profiles suggests that SO2 initially adsorbs near Pt before interacting with other sites further away from Pt at 300 °C. At 400 °C, it appears that SO2 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.