(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 NOx 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.