(233f) Peculiar Changes in Pt Accessibility and Morphology for Pt/Bao-Al2O3 Lean NoX Trap Catalysts with Different Sulfation Levels

Lean NOx trap (LNT) catalysts composed of Pt/BaO-Al₂O₃ represent a viable approach to meet the stringent NO_x emission standards being imposed on diesel engines. However, poisoning of LNTs by SO₂ represents a particularly challenging issue for this technology; thus, it is important to develop the more sulfur-durable LNT catalysts. As part of such an effort, we have been studying the effects of sulfation and desulfation on Pt accessibility and morphology changes in Pt/BaO-Al₂O₃ catalysts. In this work, a series of Pt/BaO-Al₂O₃ catalysts with three different sulfation levels (non-sulfated, partially-sulfated and fully-sulfated) were prepared. Pt accessibility after successive reductions in H₂ at elevated temperatures was measured by an irreversible volumetric H₂ chemisorption method, and the morphology of the sample after reduction at 800 °C was characterized by HRTEM and XRD diffraction techniques. Pt accessibility and morphology changes were found to have a complex dependence on the levels of sulfation. The H/Pt ratio decreased uniformly as a function of reduction temperature on the non-sulfated sample even though Pt particle sizes as estimated from XRD and TEM data showed negligible changes. Compared to the non-sulfated sample, the partially-sulfated one behaved quite differently, instead displaying an optimum H/Pt value at 500 °C and slight increase in Pt particle sizes after thermal aging at 800 °C. In further contrast to the above two samples, the fully sulfated sample gave very low H/Pt ratios for all reduction temperatures, indicating essentially complete poisoning by SO₂. Based on the observations, a mechanism is proposed to explain the peculiar changes of Pt accessibility and morphology with the different sulfation levels.