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Title: Relationship of Pt Particle Size with the NOx Storage Performance of Thermally Aged Pt/BaO/Al2O3 Lean NOx Trap Catalysts

Abstract

Relationship between Pt particle size and NOx storage performance was investigated over a model Pt/BaO/Al2O3 and an enhanced lean NOx trap catalyst (LNT). These catalysts were treated at elevated temperature to mimic the effect of thermal aging encountered during the desulfation step in NOx trap catalyst regeneration. Combination of in situ time-resolved X-ray Diffraction (TR-XRD), DRIFT measurement after CO adsorption and TEM observation over the thermally aged samples clearly shows the sintering behavior of Pt crystalline as a function of time and temperature. Under elevated temperatures, the crystalline growth of Pt occurs within a short time (e.g. 1.5 hr) followed by the gradual increase with time. NOx storage reaction performed after successive thermal treatments of catalyst under oxygen and in situ XRD experiment have allowed us to correlate the NOx storage performance with phase change. Comparing the simple Pt-BaO/Al2O3 sample with the enhanced one which shows much less Pt sintering, it was confirmed that the Pt crystalline size plays a critical role in determining the NOx storage activity, in other words, the retention of the small Pt particles after thermal aging is crucial to maintain a higher activity. In conclusion, the prevention of Pt sintering when operated at elevated temperaturemore » must be a key factor to design the more durable LNT catalyst, thus potentially implying the practical importance for the improvement of the LNT technology.« less

Authors:
; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
898102
Report Number(s):
PNNL-SA-49255
VT0401000; TRN: US200705%%412
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Industrial and Engineering Chemistry Research, 45(26):8815-8821
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; PLATINUM; PARTICLE SIZE; SORPTIVE PROPERTIES; NITROGEN OXIDES; ADSORPTION; AGING; CATALYSTS; PERFORMANCE; REGENERATION; BARIUM OXIDES; ALUMINIUM OXIDES; AIR POLLUTION CONTROL; Lean NOx trap; Pt-BaO/Al2O3; thermal aging; Pt sintering; in situ XRD; NOx storage; TEM

Citation Formats

Kim, Do Heui, Chin, Ya-Huei, Muntean, George G., Yezerets, Aleksey, Currier, Neal, Epling, William S., Chen, H.-Y., Hess, Howard .., and Peden, Charles HF. Relationship of Pt Particle Size with the NOx Storage Performance of Thermally Aged Pt/BaO/Al2O3 Lean NOx Trap Catalysts. United States: N. p., 2006. Web. doi:10.1021/ie060736q.
Kim, Do Heui, Chin, Ya-Huei, Muntean, George G., Yezerets, Aleksey, Currier, Neal, Epling, William S., Chen, H.-Y., Hess, Howard .., & Peden, Charles HF. Relationship of Pt Particle Size with the NOx Storage Performance of Thermally Aged Pt/BaO/Al2O3 Lean NOx Trap Catalysts. United States. doi:10.1021/ie060736q.
Kim, Do Heui, Chin, Ya-Huei, Muntean, George G., Yezerets, Aleksey, Currier, Neal, Epling, William S., Chen, H.-Y., Hess, Howard .., and Peden, Charles HF. Wed . "Relationship of Pt Particle Size with the NOx Storage Performance of Thermally Aged Pt/BaO/Al2O3 Lean NOx Trap Catalysts". United States. doi:10.1021/ie060736q.
@article{osti_898102,
title = {Relationship of Pt Particle Size with the NOx Storage Performance of Thermally Aged 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, H.-Y. and Hess, Howard .. and Peden, Charles HF},
abstractNote = {Relationship between Pt particle size and NOx storage performance was investigated over a model Pt/BaO/Al2O3 and an enhanced lean NOx trap catalyst (LNT). These catalysts were treated at elevated temperature to mimic the effect of thermal aging encountered during the desulfation step in NOx trap catalyst regeneration. Combination of in situ time-resolved X-ray Diffraction (TR-XRD), DRIFT measurement after CO adsorption and TEM observation over the thermally aged samples clearly shows the sintering behavior of Pt crystalline as a function of time and temperature. Under elevated temperatures, the crystalline growth of Pt occurs within a short time (e.g. 1.5 hr) followed by the gradual increase with time. NOx storage reaction performed after successive thermal treatments of catalyst under oxygen and in situ XRD experiment have allowed us to correlate the NOx storage performance with phase change. Comparing the simple Pt-BaO/Al2O3 sample with the enhanced one which shows much less Pt sintering, it was confirmed that the Pt crystalline size plays a critical role in determining the NOx storage activity, in other words, the retention of the small Pt particles after thermal aging is crucial to maintain a higher activity. In conclusion, the prevention of Pt sintering when operated at elevated temperature must be a key factor to design the more durable LNT catalyst, thus potentially implying the practical importance for the improvement of the LNT technology.},
doi = {10.1021/ie060736q},
journal = {Industrial and Engineering Chemistry Research, 45(26):8815-8821},
number = ,
volume = ,
place = {United States},
year = {Wed Dec 20 00:00:00 EST 2006},
month = {Wed Dec 20 00:00:00 EST 2006}
}
  • A simple liquid water treatment applied to fresh and thermally aged Pt(2wt%)-BaO(20wt%)/Al2O3 lean NOx 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 BaAl2O4), exposing the sample to this liquid water treatment promotes the formation of BaCO3 crystallites (about 15 – 25 nm of its size) without changing the Pt particle size.more » Such transformations of the barium species are found to significantly promote NOx uptake from 250 °C to 450 °C. The increase in the NOx 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 NOx trap catalysts since water is plentiful in the exhaust of diesel or lean-burn engines.« less
  • 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,more » 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.« 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
  • 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