Sulfation and Desulfation Mechanisms on Pt–BaO/Al2O3 NOx Storage-Reduction (NSR) Catalysts

NOx storage-reduction (NSR) catalyst is regarded as the promising solution to effectively remove NOx emitted from lean-burn engines. However, sulfur poisoning remains the big obstacle for NSR technology to overcome. Here fundamental studies on NSR catalyst, especially focusing on the various characteristics of sulfur poisoning and regeneration mechanism, were reviewed. Various aspects of sulfation mechanism such as the roles of Pt and Ba, and the types of sulfate species were investigated. It was found that the desulfation of barium sulfate with H2 resulted in the transformation into barium oxide (BaO) or barium sulfide (BaS) depending on the amount of barium loading and the sulfur loading, based on the combined temperature programmed reaction with hydrogen, Sulfur K-edge X-ray absorption near-edge spectroscopy and time-resolved X-ray diffraction results. H2O played a critical role in converting BaS into BaO, thus promoting the desulfation by producing the additional H2S. The overall sulfation and desulfation scheme was proposed by integrating the results obtained in this study.