Characterization of sulfated BaO-based NOx trap

Two types of BaSO4 were found on sulfated BaO/Pt, Rh/Al2O3 catalysts. DTG and Raman spectroscopic characterization indicate that type I BaSO4 has smaller crystallite size (∼3 nm) and is reducible at low temperatures, whereas type II BaSO4 has larger crystallite size (>10 nm) and is reducible at relatively higher temperatures. The Ba-Pt interaction affects both the nature of BaSO4 formed and the desulfation mechanism. The closer the Ba to Pt, the stronger the electronic interaction between the Pt and the SO bond on Ba, as indicated by a systematic Raman band shift to lower energies. Pt also promotes hydrogen spillover, which enhances reduction of small BaSO4 crystallites. Reduction of large/bulk BaSO4 crystallites mostly relies on gas-solid phase reaction between sulfate and H2 and therefore is not significantly affected by Pt. The nature of the precious metal also affects the sulfation process. Pt promotes sulfation on both barium and alumina, whereas Pd or Rh only facilitates BaSO4 formation. The extent of electronic interaction between precious metal and (Ba) SO bond is: Pt > Pd > Rh.