Roles of Pt and BaO in the sulfation of Pt/BaO/Al2O3 lean NOx trap materials: Sulfur K-edge XANES and Pt L-III XAFS studies

The roles of barium oxide and platinum during the sulfation of Pt-BaO/Al2O3 lean NOx trap catalysts were investigated by S K edge XANES (X-ray absorption near-edge spectroscopy) and Pt L-III XAFS (X-ray absorption fine structure). All of the samples studied [Al2O3, BaO(x; x = 8 or 20 wt %)/Al2O3, Pt(2.5 wt %)/Al2O3, and Pt(2 wt %)-BaO(x; x = 8 or 20 wt %)/Al2O3] were pre-sulfated prior to the X-ray absorption measurements. It was found that barium oxide itself has the ability to directly form barium sulfate even in the absence of Pt and gas-phase oxygen. In the platinum-containing samples, the presence of Pt-O species plays an important role in the formation of sulfate species. For the case of the BaO(8)/Al2O3 sample, where the barium coverage is about 0.26 ML, both baria and alumina phases are available for sulfation. S XANES results show that barium sulfates are formed preferentially over aluminum sulfates. When oxygen is absent from the gas phase, the sulfation route that involves Pt-O is eliminated after the initially present Pt-O species are completely consumed. In this case, formation of sulfates is suppressed unless barium oxide is also present. Pt L-III XAFS results show that the first coordination sphere around the Pt atoms in the Pt particles is dependent upon the gas mixture used during the sulfation process. Sulfation under reducing environments (e.g., SO2/H-2) leads to formation of Pt-S bonds, while oxidizing conditions (e.g., SO2/O-2) continue to show the presence of Pt-O bonds. In addition, a reducing environment was found to cause Pt sintering in greater extent than an oxidizing one. This result explains why samples sulfated under reducing conditions had lower NOx uptakes than those sulfated under oxidizing conditions. Therefore, our results provide needed information for the development of optimum practical operation conditions (e.g., sulfation or desulfation) for lean NOx trap catalysts that minimize deactivation by sulfurclose161

Promotional effects of H(2)O treatment on NO(x) storage over fresh and thermally aged Pt-BaO/Al(2)O(3) lean NO(x) trap catalysts

A simple liquid water treatment applied to fresh and thermally aged Pt(2 wt%)-BaO(20 wt%)/Al(2)O(3) lean NO(x) trap catalysts at room temperature induces morphological and structural changes in the barium species as followed by XRD and TEM analysis. During the water treatment, liquid water sufficient to fill the catalyst pore volume is brought into contact with the samples. It was found that irrespective of the original barium chemical state (highly dispersed BaO or crystalline BaAl(2)O(4)), exposing the sample to this liquid water treatment promotes the formation of BaCO(3) crystallites (about 15-25 nm of its size) without changing the Pt particle size. Such transformations of the barium species are found to significantly promote NO(x) uptake from 250 to 450 C. The increase in the NO(x) uptake for the water-treated samples can be attributed to an enhanced Pt-Ba interaction through the redistribution of barium species. These results provide useful information for the regeneration of aged lean NO(x) trap catalysts since water is plentiful in the exhaust of diesel or lean-burn enginesclose121

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

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 hereclose111