Passive-ammonia selective catalytic reduction (SCR): Understanding NH 3 formation over close-coupled three way catalysts (TWC)

Christopher D. Digiulio; Josh A. Pihl; James E.Parks Ii; Michael D. Amiridis; Todd J. Toops

doi:10.1016/j.cattod.2014.01.027

Passive-ammonia selective catalytic reduction (SCR): Understanding NH ₃ formation over close-coupled three way catalysts (TWC)

Christopher D. Digiulio, Josh A. Pihl, James E.Parks Ii, Michael D. Amiridis, Todd J. Toops

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

68 Scopus citations

Abstract

NH₃ formation was examined under steady-state and lean/rich cycling conditions over four commercial catalysts including: (1) a Pd-only, high precious metal loading (HPGM) three-way catalyst, (2) a Pd/Rh + CeO ₂, low precious metal loading (LPGM) three-way catalyst, (3) a combination of a HPGM and a LPGM (Dual-Zone) catalyst and (4) a lean NO _X trap (LNT) catalyst. The goal of this work was to evaluate these catalysts for their potential use as the upstream component in a passive-NH ₃ SCR configuration. NH₃ formation during steady-state operation was found to be dependent on the air-to-fuel ratio (AFR), temperature and catalytic formulation used. While all of the formulations produced significant amounts of NH₃ when operated under sufficiently rich conditions, in general the steady-state NH₃ yield decreased in the following order: HPGM Dual-Zone >> LPGM LNT. Under lean-rich cycling conditions that would be required for this mode of operation, lower air-to-fuel ratios were required to generate the same amount of NH₃ as under steady-state conditions. Results obtained with the LNT catalyst demonstrated that at moderate temperatures (i.e., 275-500 °C) NO_X storage capacity significantly increased the amount of NH₃ produced in relation to the amount of NO_X slipped. Consequently, the addition of an "optimum" amount of NO_X storage capacity in addition to well-controlled lean-rich timing, could significantly improve the performance of the three-way catalyst used as the upstream component in a passive-NH ₃ SCR configuration. When the CO, C₃H₆ and N₂O concentrations in the effluent were considered in addition to the NH₃ formation, an optimum temperature of 400-450 °C was determined for the operation of these catalysts.

Original language	English
Pages (from-to)	33-45
Number of pages	13
Journal	Catalysis Today
Volume	231
DOIs	https://doi.org/10.1016/j.cattod.2014.01.027
State	Published - Aug 1 2014

Funding

A portion of this research was sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program . The authors at ORNL wish to express their gratitude to program managers Ken Howden and Gurpreet Singh for their support. This manuscript has been co-authored by UT-Battelle, LLC, under Contract No. ( DE-AC05-00OR22725( with the U.S. Department of Energy. 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. Additionally, the authors would like to acknowledge the contributions of Wei Li, Chang Kim and Kushal Narayanaswamy of General Motors and Davion Clark and Christopher Owens of Umicore for valuable discussions and guidance in portions of this work.

Funders	Funder number
U.S. Department of Energy
Office of Energy Efficiency and Renewable Energy
UT-Battelle	DE-AC05-00OR22725

Keywords

Passive-NH3 SCR
Selective catalytic reduction (SCR)
Three way catalyst (TWC)

Access to Document

10.1016/j.cattod.2014.01.027

Cite this

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title = "Passive-ammonia selective catalytic reduction (SCR): Understanding NH 3 formation over close-coupled three way catalysts (TWC)",

abstract = "NH3 formation was examined under steady-state and lean/rich cycling conditions over four commercial catalysts including: (1) a Pd-only, high precious metal loading (HPGM) three-way catalyst, (2) a Pd/Rh + CeO 2, low precious metal loading (LPGM) three-way catalyst, (3) a combination of a HPGM and a LPGM (Dual-Zone) catalyst and (4) a lean NO X trap (LNT) catalyst. The goal of this work was to evaluate these catalysts for their potential use as the upstream component in a passive-NH 3 SCR configuration. NH3 formation during steady-state operation was found to be dependent on the air-to-fuel ratio (AFR), temperature and catalytic formulation used. While all of the formulations produced significant amounts of NH3 when operated under sufficiently rich conditions, in general the steady-state NH3 yield decreased in the following order: HPGM Dual-Zone >> LPGM LNT. Under lean-rich cycling conditions that would be required for this mode of operation, lower air-to-fuel ratios were required to generate the same amount of NH3 as under steady-state conditions. Results obtained with the LNT catalyst demonstrated that at moderate temperatures (i.e., 275-500 °C) NOX storage capacity significantly increased the amount of NH3 produced in relation to the amount of NOX slipped. Consequently, the addition of an {"}optimum{"} amount of NOX storage capacity in addition to well-controlled lean-rich timing, could significantly improve the performance of the three-way catalyst used as the upstream component in a passive-NH 3 SCR configuration. When the CO, C3H6 and N2O concentrations in the effluent were considered in addition to the NH3 formation, an optimum temperature of 400-450 °C was determined for the operation of these catalysts.",

keywords = "Passive-NH3 SCR, Selective catalytic reduction (SCR), Three way catalyst (TWC)",

author = "Digiulio, {Christopher D.} and Pihl, {Josh A.} and Ii, {James E.Parks} and Amiridis, {Michael D.} and Toops, {Todd J.}",

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T2 - Understanding NH 3 formation over close-coupled three way catalysts (TWC)

AU - Digiulio, Christopher D.

AU - Pihl, Josh A.

AU - Ii, James E.Parks

AU - Amiridis, Michael D.

AU - Toops, Todd J.

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N2 - NH3 formation was examined under steady-state and lean/rich cycling conditions over four commercial catalysts including: (1) a Pd-only, high precious metal loading (HPGM) three-way catalyst, (2) a Pd/Rh + CeO 2, low precious metal loading (LPGM) three-way catalyst, (3) a combination of a HPGM and a LPGM (Dual-Zone) catalyst and (4) a lean NO X trap (LNT) catalyst. The goal of this work was to evaluate these catalysts for their potential use as the upstream component in a passive-NH 3 SCR configuration. NH3 formation during steady-state operation was found to be dependent on the air-to-fuel ratio (AFR), temperature and catalytic formulation used. While all of the formulations produced significant amounts of NH3 when operated under sufficiently rich conditions, in general the steady-state NH3 yield decreased in the following order: HPGM Dual-Zone >> LPGM LNT. Under lean-rich cycling conditions that would be required for this mode of operation, lower air-to-fuel ratios were required to generate the same amount of NH3 as under steady-state conditions. Results obtained with the LNT catalyst demonstrated that at moderate temperatures (i.e., 275-500 °C) NOX storage capacity significantly increased the amount of NH3 produced in relation to the amount of NOX slipped. Consequently, the addition of an "optimum" amount of NOX storage capacity in addition to well-controlled lean-rich timing, could significantly improve the performance of the three-way catalyst used as the upstream component in a passive-NH 3 SCR configuration. When the CO, C3H6 and N2O concentrations in the effluent were considered in addition to the NH3 formation, an optimum temperature of 400-450 °C was determined for the operation of these catalysts.

AB - NH3 formation was examined under steady-state and lean/rich cycling conditions over four commercial catalysts including: (1) a Pd-only, high precious metal loading (HPGM) three-way catalyst, (2) a Pd/Rh + CeO 2, low precious metal loading (LPGM) three-way catalyst, (3) a combination of a HPGM and a LPGM (Dual-Zone) catalyst and (4) a lean NO X trap (LNT) catalyst. The goal of this work was to evaluate these catalysts for their potential use as the upstream component in a passive-NH 3 SCR configuration. NH3 formation during steady-state operation was found to be dependent on the air-to-fuel ratio (AFR), temperature and catalytic formulation used. While all of the formulations produced significant amounts of NH3 when operated under sufficiently rich conditions, in general the steady-state NH3 yield decreased in the following order: HPGM Dual-Zone >> LPGM LNT. Under lean-rich cycling conditions that would be required for this mode of operation, lower air-to-fuel ratios were required to generate the same amount of NH3 as under steady-state conditions. Results obtained with the LNT catalyst demonstrated that at moderate temperatures (i.e., 275-500 °C) NOX storage capacity significantly increased the amount of NH3 produced in relation to the amount of NOX slipped. Consequently, the addition of an "optimum" amount of NOX storage capacity in addition to well-controlled lean-rich timing, could significantly improve the performance of the three-way catalyst used as the upstream component in a passive-NH 3 SCR configuration. When the CO, C3H6 and N2O concentrations in the effluent were considered in addition to the NH3 formation, an optimum temperature of 400-450 °C was determined for the operation of these catalysts.

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