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Title: Design of a reaction protocol for decoupling sulfur removal and thermal aging effects during desulfation of Pt-BaO/Al2O3 lean NOx trap catalysts

Abstract

A novel reaction protocol was designed to decouple the effects of thermal deactivation from those due to, for example, incomplete de-sulfation during regeneration steps of Ba-based lean NOx trap catalysts. The protocol was applied to two samples: a Pt-BaO/Al2O3 model catalyst, and an enhanced model sample doped with promoter species. The results obtained from the reaction protocol demonstrate that regeneration (desulfation) temperatures need to be maintained below those that lead to significant Pt sintering in order to prevent permanent deactivation. In addition, the modified reaction protocol allows us to compare the regeneration behavior of samples with varying degrees of sulfation, while other approaches have difficulty differentiating the effects of thermal aging from those of sulfation. We believe that this approach provides a convenient way both to assess the relative sensitivities of various catalysts to regeneration conditions, and to develop regeneration strategies that minimize the separate but often linked deactivation effects of sulfation and high temperatures.

Authors:
; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
909674
Report Number(s):
PNNL-SA-53243
Journal ID: ISSN 0888-5885; IECRED; 19800; VT0401000; TRN: US200723%%5
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Industrial and Engineering Chemistry Research, 46(9):2735-2740; Journal Volume: 46; Journal Issue: 9
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; AGING; CATALYSTS; REGENERATION; DESULFURIZATION; PLATINUM; BARIUM OXIDES; ALUMINIUM OXIDES; NITROGEN OXIDES; AIR POLLUTION CONTROL; TEMPERATURE CONTROL; Environmental Molecular Sciences Laboratory

Citation Formats

Kim, Do Heui, Chin, Ya-Huei, Muntean, George G., Yezerets, Aleksey, Currier, Neal, Epling, William S., Chen, Haiying, Hess, Howard .., and Peden, Charles HF. Design of a reaction protocol for decoupling sulfur removal and thermal aging effects during desulfation of Pt-BaO/Al2O3 lean NOx trap catalysts. United States: N. p., 2007. Web. doi:10.1021/ie061542d.
Kim, Do Heui, Chin, Ya-Huei, Muntean, George G., Yezerets, Aleksey, Currier, Neal, Epling, William S., Chen, Haiying, Hess, Howard .., & Peden, Charles HF. Design of a reaction protocol for decoupling sulfur removal and thermal aging effects during desulfation of Pt-BaO/Al2O3 lean NOx trap catalysts. United States. doi:10.1021/ie061542d.
Kim, Do Heui, Chin, Ya-Huei, Muntean, George G., Yezerets, Aleksey, Currier, Neal, Epling, William S., Chen, Haiying, Hess, Howard .., and Peden, Charles HF. Wed . "Design of a reaction protocol for decoupling sulfur removal and thermal aging effects during desulfation of Pt-BaO/Al2O3 lean NOx trap catalysts". United States. doi:10.1021/ie061542d.
@article{osti_909674,
title = {Design of a reaction protocol for decoupling sulfur removal and thermal aging effects during desulfation of Pt-BaO/Al2O3 lean NOx trap catalysts},
author = {Kim, Do Heui and Chin, Ya-Huei and Muntean, George G. and Yezerets, Aleksey and Currier, Neal and Epling, William S. and Chen, Haiying and Hess, Howard .. and Peden, Charles HF},
abstractNote = {A novel reaction protocol was designed to decouple the effects of thermal deactivation from those due to, for example, incomplete de-sulfation during regeneration steps of Ba-based lean NOx trap catalysts. The protocol was applied to two samples: a Pt-BaO/Al2O3 model catalyst, and an enhanced model sample doped with promoter species. The results obtained from the reaction protocol demonstrate that regeneration (desulfation) temperatures need to be maintained below those that lead to significant Pt sintering in order to prevent permanent deactivation. In addition, the modified reaction protocol allows us to compare the regeneration behavior of samples with varying degrees of sulfation, while other approaches have difficulty differentiating the effects of thermal aging from those of sulfation. We believe that this approach provides a convenient way both to assess the relative sensitivities of various catalysts to regeneration conditions, and to develop regeneration strategies that minimize the separate but often linked deactivation effects of sulfation and high temperatures.},
doi = {10.1021/ie061542d},
journal = {Industrial and Engineering Chemistry Research, 46(9):2735-2740},
number = 9,
volume = 46,
place = {United States},
year = {Wed Apr 25 00:00:00 EDT 2007},
month = {Wed Apr 25 00:00:00 EDT 2007}
}
  • 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
  • A combination of H2 TPRX, TR-XRD and XPS analysis has been used to investigate the effects of CO2 on the desulfation of pre-sulfated Pt BaO/Al2O3 samples. The results demonstrate that the presence of CO2 promotes the removal of sulfur species, especially at temperatures below 500 °C, with a corresponding suppression of BaS formation, thus resulting in a lower amount of residual sulfur on the sample after desulfation.
  • The desulfation mechanisms of pre-sulfated Pt-BaO/{gamma}-Al{sub 2}O{sub 3} lean NOx trap catalysts were investigated under isothermal conditions (600 C) using H{sub 2} as the reductant. Sulfates were found to be reduced first with H{sub 2} to produce SO{sub 2}, followed by a reaction between SO{sub 2} and H{sub 2} to produce H{sub 2}S. Gas analysis during the rich pulse reveals that the sulfur removal efficiency is initially proportional to the H{sub 2} concentration. At constant H{sub 2} concentration the overall desulfation efficiency decreases in the order of H{sub 2}/CO{sub 2}/H{sub 2}O > H{sub 2}/CO{sub 2} > H{sub 2}/H{sub 2}O >more » H{sub 2}, as confirmed by XPS analysis of residual sulfur in the desulfated samples. H{sub 2}O limits the evolution of SO{sub 2} at an early stage of the rich pulse and enhances the production of H{sub 2}S in later stages of reduction. CO{sub 2} is involved in both the formation of COS and the production of H{sub 2}O (via the reverse water-gas shift reaction), therefore, resulting in an increased overall efficiency.« less
  • The effects of K loading and thermal aging on the structural properties and high temperature performance of Pt/K/Al2O3 lean NOx trap (LNT) catalysts were investigated using in situ X-ray diffraction (XRD), temperature-programmed decomposition/desorption of NOx (NOx-TPD), transmission electron microscopy (TEM), NO oxidation and NOx storage tests. In situ XRD results demonstrate that KNO3 becomes extremely mobile on the Al2O3 surface, and experiences complex transformations between orthorhombic and rhombohedral structures, accompanied by sintering, melting and thermal decomposition upon heating. NOx storage results show an optimum K loading around 10% for the best performance at high temperatures. At lower K loadings wheremore » the majority of KNO3 stays as a surface layer, the strong interaction between KNO3 and Al2O3 promotes KNO3 decomposition and deteriorates high-temperature performance. At K loadings higher than 10%, the performance drop is not caused by NOx diffusion limitations as for the case of barium-based LNTs, but rather from the blocking of Pt sites by K species, which adversely affects NO oxidation. Thermal aging at 800 ºC severely deactivates the Pt/K/Al2O3 catalysts due to Pt sintering. However, in the presence of potassium, some Pt remains in a dispersed and oxidized form. These Pt species interact strongly with K and, therefore, do not sinter. After a reduction treatment, these Pt species remain finely dispersed, contributing to a partial recovery of NOx storage performance.« less