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July 26, 2006 Sulfur K edge XANES and TR XRD Studies of Pt-BaO/Al2O3 lean NOx Trap Catalysts: Effects of Barium Loading on DesulfationD.H. Kim1, J. Szanyi1, J.H. Kwak1, T. Szailer1, J.C. Hanson2, C. Wang1, and C.H.F. Peden1 Sulfur K edge X ray absorption near-edge spectroscopy (XANES) and in situ time-resolved X ray diffraction (TR XRD) are used to show that the removal of sulfur (in the form of BaSO4) from Pt-BaO(x)/Al2O3 (x = wt% BaO) catalysts is strongly dependant on barium loading. Sulfated Pt-BaO(8)/Al2O3, consisting predominantly of monolayer BaO/BaCO3 species, displays more facile desulfation by H2 at lower temperatures than sulfated Pt-BaO(20)/Al2O3, a material containing primarily particulate BaO/BaCO3 species. This suggests that the initial morphology differences between the two samples play a crucial role in determining the extent of desulfation and the temperature at which it occurs, a result that may be important in developing more sulfur-resistant LNT catalyst systems.
Internal combustion engines operating under lean-burn conditions, such as diesel engines, exhibit high fuel efficiency. Removal of harmful NOx emissions from the exhaust in the presence of excess oxygen, however, presents a great challenge to the catalysis community because traditional three-way catalysts are ineffective under these conditions. Among the approaches being considered, urea and hydrocarbon selective catalytic reduction (SCR), and lean-NOx traps (LNTs, aka NOx storage/reduction (NSR) catalysts or NOx adsorbers) are promising technologies. In LNT technology, an active oxide (alkali and/or alkaline earth) material takes up NOx under lean engine operation conditions and stores them as nitrates. In a brief rich cycle, these nitrates are released from the active oxide catalyst component, and reduced to N2 on the precious metal component of the catalyst. Because even low concentrations of SO2 in the emission gradually reduces the ability of the active phase to store NOx, the resistance of the material to SO2 poisoning remains a critical issue. Meanwhile, since we have shown that NOx adsorption/desorption chemistry is strongly dependent on the loading of barium, an important question concerns the variation of the desulfation chemistry as a function of barium content in the LNT formulation. As such, we performed a multi-spectroscopy study to understand desulfation processes on Pt-BaO/Al2O3 LNT materials with varying barium loadings. In particular, we investigated the desulfation behavior of pre-sulfated Pt-BaO(8 or 20 wt%)/Al2O3 catalysts using H2 temperature programmed reaction (TPRx). These two BaO loadings were chosen because we have previously shown that the Ba-phase morphologies are significantly different; notably, BaO consists of a monolayer "coating" on the alumina surface in Pt-BaO(8)/Al2O3, while this monolayer phase coexists with a "particulate" or bulk-like BaO phase in the Pt-BaO(20)/Al2O3 sample. Thus, we also followed the changes in catalyst morphology and sulfur oxidation states during desulfation processes using synchrotron time-resolved x ray diffraction (TR XRD) and sulfur K edge x-ray absorption near-edge spectroscopy (XANES), which were performed on the NSLS beamlines X7B and X19A, respectively.
Figure 1 shows the H2 TPRx spectra of sulfated Pt-BaO(8)/Al2O3 and Pt-BaO(20)/Al2O3 samples, obtained by ramping the temperature of these samples in a H2/He flow while continually monitoring the product gases with a mass spectrometer. H2S is the primary product of the reaction between H2 and sulfur species on the sample. H2S is formed at higher temperature for the sample with higher barium loading, implying that the type of barium sulfate species formed upon uptake of SO2 is different depending on the loading of barium species - surface or 'monolayer' sulfates for Pt-BaO(8)/Al2O3, and 'bulk' BaSO4 for the Pt-BaO(20)/Al2O3 sample. In addition, the amount of H2S produced over Pt-BaO(8)/Al2O3 is two times larger than that of the sample with higher barium loadings, which suggests a more facile desulfation of 'monolayer' BaSO4. Sulfur K edge XANES experiments were carried out to investigate changes in the oxidation states of sulfur as a function of H2 reduction temperature. We collected samples after H2 TPRx up to 553 K, 743 K and 1073 K (see arrows in Figure 1). After H2 TPRx up to 553 K for the sulfated Pt-BaO(8)/Al2O3 sample, the spectrum in Figure 2(a) contains a small peak at 2472 eV, which can be assigned to a sulfide-like (S2-) species, while the main sulfate (SO42-) peak is unchanged. After H2 TPRx up to 1073 K, the sulfate peaks nearly disappears, while there is an increase in features from lower oxidation state sulfur species (sulfide-like and sulfite-like (SO32-)). The sulfated Pt-BaO(20)/Al2O3 sample shows qualitatively similar behavior as shown in Figure 2(b). However, compared with the sample with lower barium loading, Pt-BaO(20)/Al2O3 contains a significantly larger amount of residual sulfur species of all types after H2 TPRx up to 1073 K, which is consistent with the H2 TPRx results.
Figure 3 shows a series of XRD patterns obtained during H2 TPRx for the sulfated Pt-BaO(20)/Al2O3 sample. The room temperature XRD contains peaks assigned to BaSO4. Up to about 773 K, the BaSO4 phase is unchanged. However, above 773 K, diffraction peaks associated with BaS appear and continue to grow with increasing temperature, along with a corresponding drop in the intensities of the BaSO4 peaks. Compared with the Pt-BaO(20)/Al2O3 sample, Pt-BaO(8)/Al2O3 contains much smaller amounts of BaS, confirming that residual sulfur species were present at much lower concentrations for the lower barium loading sample.
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The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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