NH3 formation over a lean NOX trap (LNT) system: Effects of lean/rich cycle timing and temperature

Christopher D. DiGiulio; Josh A. Pihl; Jae Soon Choi; James E. Parks; Michael J. Lance; Todd J. Toops; Michael D. Amiridis

doi:10.1016/j.apcatb.2013.09.012

NH₃ formation over a lean NO_X trap (LNT) system: Effects of lean/rich cycle timing and temperature

Christopher D. DiGiulio, Josh A. Pihl, Jae Soon Choi, James E. Parks, Michael J. Lance, Todd J. Toops, Michael D. Amiridis

Research output: Contribution to journal › Article › peer-review

54 Scopus citations

Abstract

A commercial lean NO_X trap (LNT) catalyst containing Pt, Pd, Rh, BaO, and CeO₂ was evaluated in this investigation. The effects of lean/rich cycle timing on the NO_X, CO and C₃H₆ conversions and on the NH₃ and N₂O selectivities were considered. Two distinct lean/rich cycling regimes were identified. At low temperatures, NO_X release and reduction were relatively slow processes. As a result, a longer, lower-concentration rich dose favored increased cycle averaged NO_X conversions. For example, extending the rich period from 5 to 15s at 250°C, while holding the overall reductant dose constant, resulted in an increase in cycle averaged NO_X conversion from 59 to 87%. At high temperatures, the opposite was found to be true. Above 450°C, NO_X release and reduction occurred very rapidly and shorter, higher concentration rich doses yielded significantly higher NO_X conversions. For example, extending the rich period from 5 to 15s at 500°C, while holding the overall rich dose constant, resulted in a decrease in the cycle averaged NO_X conversion from 76 to 54%. The selectivities to NH₃ and N₂O were found to be primarily a function of temperature, with both being higher at lower temperatures. The effect of cycle timing and reductant concentration were of secondary importance. In contrast, NH₃ and N₂O yields were significantly affected by the cycle timing since they depend on the NO_X conversion. Therefore, any combination of changes in the lean/rich timing protocol or reductant concentrations that resulted in increased NO_X conversion also resulted in increased NH₃ and N₂O yields for a given temperature. Thus, concerted control of NH₃ generation by varying the lean/rich cycle timing was demonstrated for this LNT catalyst. At both lower and higher temperatures, variations in the rich cycle duration resulted in NH₃/NO_X ratios that could extend the region of operation for a close-coupled LNT-SCR system, with the greatest effect observed at temperatures between 250 and 450°C. However, N₂O yield also increased with NH₃ yield under the same conditions.

Original language	English
Pages (from-to)	698-710
Number of pages	13
Journal	Applied Catalysis B: Environmental
Volume	147
DOIs	https://doi.org/10.1016/j.apcatb.2013.09.012
State	Published - Apr 5 2014

Funding

A portion of this work was sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program . The authors at ORNL thank program managers Ken Howden and Gurpreet Singh for their support. Additionally, the authors would like to acknowledge the contributions of Wei Li of General Motors and Davion Clark of Umicore for discussions and guidance in this portion of our research. This manuscript has been co-authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

Keywords

LNT-SCR
Lean NO trap (LNT)
NH formation
NO reduction
Selective catalytic reduction (SCR)

Access to Document

10.1016/j.apcatb.2013.09.012

Cite this

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title = "NH3 formation over a lean NOX trap (LNT) system: Effects of lean/rich cycle timing and temperature",

abstract = "A commercial lean NOX trap (LNT) catalyst containing Pt, Pd, Rh, BaO, and CeO2 was evaluated in this investigation. The effects of lean/rich cycle timing on the NOX, CO and C3H6 conversions and on the NH3 and N2O selectivities were considered. Two distinct lean/rich cycling regimes were identified. At low temperatures, NOX release and reduction were relatively slow processes. As a result, a longer, lower-concentration rich dose favored increased cycle averaged NOX conversions. For example, extending the rich period from 5 to 15s at 250°C, while holding the overall reductant dose constant, resulted in an increase in cycle averaged NOX conversion from 59 to 87%. At high temperatures, the opposite was found to be true. Above 450°C, NOX release and reduction occurred very rapidly and shorter, higher concentration rich doses yielded significantly higher NOX conversions. For example, extending the rich period from 5 to 15s at 500°C, while holding the overall rich dose constant, resulted in a decrease in the cycle averaged NOX conversion from 76 to 54%. The selectivities to NH3 and N2O were found to be primarily a function of temperature, with both being higher at lower temperatures. The effect of cycle timing and reductant concentration were of secondary importance. In contrast, NH3 and N2O yields were significantly affected by the cycle timing since they depend on the NOX conversion. Therefore, any combination of changes in the lean/rich timing protocol or reductant concentrations that resulted in increased NOX conversion also resulted in increased NH3 and N2O yields for a given temperature. Thus, concerted control of NH3 generation by varying the lean/rich cycle timing was demonstrated for this LNT catalyst. At both lower and higher temperatures, variations in the rich cycle duration resulted in NH3/NOX ratios that could extend the region of operation for a close-coupled LNT-SCR system, with the greatest effect observed at temperatures between 250 and 450°C. However, N2O yield also increased with NH3 yield under the same conditions.",

keywords = "LNT-SCR, Lean NO trap (LNT), NH formation, NO reduction, Selective catalytic reduction (SCR)",

author = "DiGiulio, {Christopher D.} and Pihl, {Josh A.} and Choi, {Jae Soon} and Parks, {James E.} and Lance, {Michael J.} and Toops, {Todd J.} and Amiridis, {Michael D.}",

year = "2014",

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doi = "10.1016/j.apcatb.2013.09.012",

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T2 - Effects of lean/rich cycle timing and temperature

AU - DiGiulio, Christopher D.

AU - Pihl, Josh A.

AU - Choi, Jae Soon

AU - Parks, James E.

AU - Lance, Michael J.

AU - Toops, Todd J.

AU - Amiridis, Michael D.

PY - 2014/4/5

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N2 - A commercial lean NOX trap (LNT) catalyst containing Pt, Pd, Rh, BaO, and CeO2 was evaluated in this investigation. The effects of lean/rich cycle timing on the NOX, CO and C3H6 conversions and on the NH3 and N2O selectivities were considered. Two distinct lean/rich cycling regimes were identified. At low temperatures, NOX release and reduction were relatively slow processes. As a result, a longer, lower-concentration rich dose favored increased cycle averaged NOX conversions. For example, extending the rich period from 5 to 15s at 250°C, while holding the overall reductant dose constant, resulted in an increase in cycle averaged NOX conversion from 59 to 87%. At high temperatures, the opposite was found to be true. Above 450°C, NOX release and reduction occurred very rapidly and shorter, higher concentration rich doses yielded significantly higher NOX conversions. For example, extending the rich period from 5 to 15s at 500°C, while holding the overall rich dose constant, resulted in a decrease in the cycle averaged NOX conversion from 76 to 54%. The selectivities to NH3 and N2O were found to be primarily a function of temperature, with both being higher at lower temperatures. The effect of cycle timing and reductant concentration were of secondary importance. In contrast, NH3 and N2O yields were significantly affected by the cycle timing since they depend on the NOX conversion. Therefore, any combination of changes in the lean/rich timing protocol or reductant concentrations that resulted in increased NOX conversion also resulted in increased NH3 and N2O yields for a given temperature. Thus, concerted control of NH3 generation by varying the lean/rich cycle timing was demonstrated for this LNT catalyst. At both lower and higher temperatures, variations in the rich cycle duration resulted in NH3/NOX ratios that could extend the region of operation for a close-coupled LNT-SCR system, with the greatest effect observed at temperatures between 250 and 450°C. However, N2O yield also increased with NH3 yield under the same conditions.

AB - A commercial lean NOX trap (LNT) catalyst containing Pt, Pd, Rh, BaO, and CeO2 was evaluated in this investigation. The effects of lean/rich cycle timing on the NOX, CO and C3H6 conversions and on the NH3 and N2O selectivities were considered. Two distinct lean/rich cycling regimes were identified. At low temperatures, NOX release and reduction were relatively slow processes. As a result, a longer, lower-concentration rich dose favored increased cycle averaged NOX conversions. For example, extending the rich period from 5 to 15s at 250°C, while holding the overall reductant dose constant, resulted in an increase in cycle averaged NOX conversion from 59 to 87%. At high temperatures, the opposite was found to be true. Above 450°C, NOX release and reduction occurred very rapidly and shorter, higher concentration rich doses yielded significantly higher NOX conversions. For example, extending the rich period from 5 to 15s at 500°C, while holding the overall rich dose constant, resulted in a decrease in the cycle averaged NOX conversion from 76 to 54%. The selectivities to NH3 and N2O were found to be primarily a function of temperature, with both being higher at lower temperatures. The effect of cycle timing and reductant concentration were of secondary importance. In contrast, NH3 and N2O yields were significantly affected by the cycle timing since they depend on the NOX conversion. Therefore, any combination of changes in the lean/rich timing protocol or reductant concentrations that resulted in increased NOX conversion also resulted in increased NH3 and N2O yields for a given temperature. Thus, concerted control of NH3 generation by varying the lean/rich cycle timing was demonstrated for this LNT catalyst. At both lower and higher temperatures, variations in the rich cycle duration resulted in NH3/NOX ratios that could extend the region of operation for a close-coupled LNT-SCR system, with the greatest effect observed at temperatures between 250 and 450°C. However, N2O yield also increased with NH3 yield under the same conditions.

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KW - NH formation

KW - NO reduction

KW - Selective catalytic reduction (SCR)

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