Characteristics of Desulfation Behavior for Presulfated Pt-BaO/CeO2 Lean NOx Trap Catalyst: The Role of the CeO2 Support

Abstract

The desulfation of presulfated Pt-BaO/CeO2) lean NOx trap catalyst was investigated by H-2 TPRX (temperature programmed reaction), in situ TR-XRD (time-resolved X-ray diffraction), and in situ S K-edge XANES (Xray absorption near edge spectroscopy) techniques Compared with Pt-BaO/Al2O3 materials, I reductive treatment in H-2 for the CeO2-supported sample up to 1073 K removes, at most, only a very small amount of sulfur species. However, the results Of in Situ TR-XRD measurements demonstrate that the quantity of a BaS phase formed oil Pt-BaO/CeO2 IS Much smaller than that on Pt-BaO/Al2O3, implying that the formation of BaS crystallites. which Occurs during the reduction from Sulfate (SO42-) to sulfide (S2-), is significantly Suppressed in the CeO2-supported catalyst. As the desulfation temperature increases under reducing Conditions (m H-2), in Situ S XANES spectra show that. compared with alumina-supported samples, the reduction temperature for Sulfates (S6+) decreases by about 150 K. Concomitantly, the formation Of Sulfur species with lower oxidation states (SI-S') is enhanced The absolute intensities of S XANES spectra before and after desulfation are very similar, implying that the amount of sulfur-contaming species removed during the reductive treatment is negligible, in agreement with the results of H-2 TPRX These results Suggest that H2S produced by the reduction of BaSO4 is readily readsorbed oil the ceria support to form ceria-sulfur complexes (e.g., Ce2O2S) The high affinity of ceria for H2S, combined with the ease of reducibility of the ceria support material gives rise to Various oxidation states of sulfur after high-temperature H-2 treatments. Thus, the results of this study clearly show that the ceria Support strongly affects the overall desulfaction mechanism. The intrinsic role of the ceria support during desulfation and its effect oil the overall NOx storage processes are discussed oil the basis of the characterization results obtained here