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

Exposure of Pt/K/Al₂O₃ to 15 ppm SO₂ 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 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 NO_x 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. De-sulfurization up to 800 °C using a temperature programmed reduction (TPR) procedure and rich conditions with both CO₂ and H₂O 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. H₂S and SO₂ were the primary de-sulfurization products with H₂S having a maximum release between 690 and 755 °C, while SO₂ 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.