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Title: Effect of Barium Loading on the Desulfation of Pt-BaO/Al2O3 Studied by H2 TPRX, TEM, Sulfur K-edge XANES, and In Situ TR-XRD

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
914209
Report Number(s):
BNL-78777-2007-JA
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: J. Phys. Chem. B; Journal Volume: 110
Country of Publication:
United States
Language:
English
Subject:
national synchrotron light source

Citation Formats

Kim,D., Szanyi, J., Kwak, J., Szailer, T., Hanson, J., Wang, C., and Peden, C. Effect of Barium Loading on the Desulfation of Pt-BaO/Al2O3 Studied by H2 TPRX, TEM, Sulfur K-edge XANES, and In Situ TR-XRD. United States: N. p., 2006. Web. doi:10.1021/jp060119f.
Kim,D., Szanyi, J., Kwak, J., Szailer, T., Hanson, J., Wang, C., & Peden, C. Effect of Barium Loading on the Desulfation of Pt-BaO/Al2O3 Studied by H2 TPRX, TEM, Sulfur K-edge XANES, and In Situ TR-XRD. United States. doi:10.1021/jp060119f.
Kim,D., Szanyi, J., Kwak, J., Szailer, T., Hanson, J., Wang, C., and Peden, C. Sun . "Effect of Barium Loading on the Desulfation of Pt-BaO/Al2O3 Studied by H2 TPRX, TEM, Sulfur K-edge XANES, and In Situ TR-XRD". United States. doi:10.1021/jp060119f.
@article{osti_914209,
title = {Effect of Barium Loading on the Desulfation of Pt-BaO/Al2O3 Studied by H2 TPRX, TEM, Sulfur K-edge XANES, and In Situ TR-XRD},
author = {Kim,D. and Szanyi, J. and Kwak, J. and Szailer, T. and Hanson, J. and Wang, C. and Peden, C.},
abstractNote = {},
doi = {10.1021/jp060119f},
journal = {J. Phys. Chem. B},
number = ,
volume = 110,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • Desulfation processes were investigated over sulfated Pt BaO/Al2O3 with different barium loading (8 wt% and 20 wt%) by using H2 temperature programmed reaction (TPRX), transmission electron microscope (TEM) with energy dispersive spectroscopy (EDS), sulfur K-edge X-ray absorption near-edge spectroscopy (XANES), and in situ time-resolved X-ray diffraction (TR-XRD) techniques. Both sulfated samples (8 wt% and 20 wt%) form sulfate species (primarily BaSO4) as evidenced by S K-edge XANES and in situ TR-XRD. However, the desulfation behavior is strongly dependant on the barium loading. Sulfated Pt BaO(8)/Al2O3, consisting predominantly of surface BaO/BaCO3 species, displays more facile desulfation by H2 at lower temperaturesmore » than sulfated Pt BaO(20)/Al2O3, a material containing primarily bulk BaO/BaCO3 species. Therefore, after desulfation with H2 up to 1073 K, the amount of the remaining sulfur species on the former, mostly as BaS, is much less than on the latter. 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. It is concluded that the removal of sulfur is significantly easier at lower barium loading. This finding can potentially be important in developing more sulfur resistant LNT catalyst systems.« less
  • Desulfation by hydrogen of pre-sulfated Pt(2wt%) BaO(20wt%)/Al2O3 with various sulfur loading (S/Ba = 0.12, 0.31 and 0.62) were investigated by combining H2 temperature programmed reaction (TPRX), x-ray photoelectron spectroscopy (XPS), in-situ sulfur K-edge x-ray absorption near-edge spectroscopy (XANES), and synchrotron time-resolved x-ray diffraction (TR-XRD) techniques. We find that the amount of H2S desorbed during the desulfation in the H2 TPRX experiments is not proportional to the amount of initial sulfur loading. The results of both in-situ sulfur K-edge XANES and TR-XRD show that at low sulfur loadings, sulfates were transformed to a BaS phase and remained in the catalyst, rathermore » than being removed as H2S. On the other hand, when the deposited sulfur level exceeded a certain threshold (at least S/Ba = 0.31) sulfates were reduced to form H2S, and the relative amount of the residual sulfide species in the catalyst was much less than at low sulfur loading. Unlike samples with high sulfur loading (e.g., S/Ba = 0.62), H2O did not promote the desulfation for the sample with S/Ba of 0.12, implying that the formed BaS species originating from the reduction of sulfates at low sulfur loading are more stable to hydrolysis. The results of this combined spectroscopy investigation provide clear evidence to show that sulfates at low sulfur loadings are less likely to be removed as H2S and have a greater tendency to be transformed to BaS on the material, leading to the conclusion that desulfation behavior of Pt BaO/Al2O3 lean NOx trap catalysts is markedly dependent on the sulfation levels.« less
  • Desulfation by hydrogen of presulfated Pt (2 wt %)-BaO(20 wt %)/Al{sub 2}O{sub 3} with various sulfur loading (S/Ba = 0.12, 0.31, and 0.62) were investigated by combining H{sub 2} temperature programmed reaction (TPRX), X-ray photoelectron spectroscopy (XPS), in situ sulfur K-edge X-ray absorption near-edge spectroscopy (XANES), and synchrotron time-resolved X-ray diffraction (TR-XRD) techniques. We find that the amount of H{sub 2}S desorbed during the desulfation in the H{sub 2} TPRX experiments is not proportional to the amount of initial sulfur loading. The results of both in situ sulfur K-edge XANES and TR-XRD show that at low sulfur loadings, sulfates weremore » transformed to a BaS phase and remained in the catalyst rather than being removed as H{sub 2}S. On the other hand, when the deposited sulfur level exceeded a certain threshold (at least S/Ba = 0.31) sulfates were reduced to form H{sub 2}S, and the relative amount of the residual sulfide species in the catalyst was much less than at low sulfur loading. Unlike samples with high sulfur loading (e.g., S/Ba = 0.62), H{sub 2}O did not promote the desulfation for the sample with S/Ba of 0.12, implying that the formed BaS species originating from the reduction of sulfates at low sulfur loading are more stable to hydrolysis. The results of this combined spectroscopy investigation provide clear evidence to show that sulfates at low sulfur loadings are less likely to be removed as H{sub 2}S and have a greater tendency to be transformed to BaS on the material, leading to the conclusion that desulfation behavior of Pt-BaO/Al{sub 2}O{sub 3} lean NO{sub x} trap catalysts is markedly dependent on the sulfation levels.« less
  • 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 LIII XAFS (X-ray absorption fine structure). All of the samples studied (Al2O3, BaO/Al2O3, Pt/Al2O3 and Pt-BaO/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. Even if bariummore » and aluminum sites are available for SO2 to form sulfate, for the case of the BaO(8)/Al2O3 sample, where the barium coverage is about 0.26 ML, S XANES spectroscopy results show that barium sulfates are preferentially produced 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 LIII XAFS results show that the first coordination sphere around the Pt atoms in the Pt particles is dependent upon the redox nature of the gas mixture used during the sulfation process. Sulfation under reducing environments (e.g. SO2+H2) leads to formation of Pt-S bonds, while oxidizing conditions (e.g. SO2+O2) continue to show the presence of Pt-O bonds. In addition, the former condition was found to give rise to a higher degree of Pt sintering than the latter 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 sulfur.« less
  • 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 LIII 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, themore » 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 LIII 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/H2) leads to formation of Pt-S bonds, while oxidizing conditions (e.g., SO2/O2) 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 sulfur.« less