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Understanding NO Adsorption and Desorption on Pd-Exchanged Zeolite Passive NOx Adsorbers

Sreshtha S. Majumdar, Josh A. PihlOak Ridge National Laboratory

2019 CLEERS WorkshopAnn Arbor, MI17th September 2019

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Acknowledgements

• Funding & guidance from DOE VTO Program Managers:– Ken Howden, Gurpreet Singh, Mike Weismiller

• Catalyst samples & guidance from Johnson Matthey:– Haiying Chen

• Collaboration with University of Virginia:– Kevin Gu, Bill Epling, Chris Paolucci

• Discussions with the ORNL Team: – Todd Toops, Melanie DeBusk, Jim Parks

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• ORNL conducts a survey of industrial participants in CLEERS every two years to identify high priority research topics

• 2017 survey showed that traps for low temperature emissions control are among the highest priority topics

• Passive NOx adsorbers (PNA) was the highest rated topic for diesel applications

• Hydrocarbon Traps (HCT) was the second highest rated topic for gasoline applications

HD

PF

PNA

HCT

SCR

DOC

TWC

LNT

other

LDMD GasolineDiesel

Particulate Filter

Passive NOx Adsorber

Hydrocarbon Trap

Selective Catalytic Reduction

Diesel Oxidation Catalyst

Three-Way Catalyst

Lean NOx Trap

6.5-105.5-6.54.5-5.53.5-4.50-3.5Avg.Score

*Cross-cut Lean Exhaust Emissions Reduction Simulationshttps://cleers.org/

2017 CLEERS* Industry Priorities Survey showed continuing interest in trap materials for low temperature emissions control

https://cleers.org/

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• ORNL’s R&D activities under CLEERS are currently focused on understanding and modeling the operation and aging of PNAs and HCTs

• Our efforts are directed towards gaining a better understanding of the underlying chemistry in order to propose reaction mechanisms which will then lay the foundation for a model

Experiments Mechanism

Model

Objective: propose a mechanism consistent with experiments to aid modeling efforts for PNAs

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Synthetic exhaust flow reactor and DRIFTS experiments used to reveal the chemistry underlying NO adsorption on a PNA• Obtained catalyst core sample from Johnson Matthey

– model dCSCTM component– Pd-exchanged ZSM-5

• Pd loading: 50 g/ft3 (1.8 g/l)– washcoated on a 400 cells/in2 cordierite monolith

• Degreened at 600 °C for 4 h under 10% O2/7% H2O/N2

• Conducted dozens of NO storage-release cycles to stabilize the cycle-to-cycle PNA performance

• Measured NO uptake and release on a synthetic exhaust flow reactor:– isothermal NO adsorption/TPD– varied concentrations, storage T

• Investigated surface intermediates with DRIFTS

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NO exposure conditionsVariable Baseline

ConditionsEvaluation

RangeNO 200 ppm (25-1600)CO 200 ppm (50-800)O2 10% (1-13)H2O 7% (5-13)CO2 0% (0-13)T 100°C (75-225)SV 30000 h-1

pretreat, cool conditionsO2 10%H2O 7%T 600-100°CSV 30000 h-1

PNA isothermal storage/TPD experiments enable reproducible measurements of capacity and stability

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5%

7%

9%

11%

13%

0%

CO2

time (min)

100 °C adsorption NOx (ppm)

NO

uptake rateunchanged

temperature (°C)

TPD NOx (ppm)

NO

storage capacity unchanged

NO: Pd ~0.18

CO2 has no effect on PNA NO uptake/release

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4%

7%

10%

13%

1%

O2 TPD NOx (ppm)

NO

uptake rate unchangedtime (min)


NO


temperature (°C)

Increasing O2 has no effect on NO uptake

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TPD NOx (ppm)

temperature (°C)


time (min)

50

100

200

400

25

NO ppm

800

1600N

O uptake rate increasing

NO


Increasing NO increases rate of uptake (but not capacity)

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TPD NOx (ppm)Isothermal adsorption NOx (ppm)

75 °C

100 °C

125 °C

150 °C

175 °C

200 °C

225 °C

time (min) temperature (°C)

IncreasingNO storage:Decreased

H2O competition?

DecreasingNO storage:stability of

adsorbed NO

1Consistent with Chen et al., Catal Lett 2016 146, 1706 (JM)

NO uptake initially increases with adsorption temperature, then decreases1

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5%

7%

9%

11%

13%


time (min)

H2O NO

uptake rate decreasing


time (min)

NO

uptake rate unchanged

2Consistent with Zheng et al., J. Phys. Chem. C 2017, 121, 15793 (PNNL)

Increasing H2O decreases NO uptake2 at 100 °C, but not at 150 °C

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50

100

200

400

800

CO(ppm)


time (min)N

O uptake rate increasing


time (min)

NO


3Consistent with Vu et al., Catal Lett 2017 147, 745 (UVA)

Increasing CO increases NO uptake3 at 100 °C, but not at 150 °C

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CO (ppm) H2O (%) 100 °C adsorption NOx (ppm)

0 0

200 0

200 7

NO


time (min)

Under dry conditions, NO uptake is lower and CO does not impact the rate of NO uptake

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175 °CNO+O2+H2O

100 °CNO+O2+H2O+CO

100 °CNO+O2+H2O

100 °CNO+O2

dryPre-Ox

Kubelka-Munk (a.u.)

Wavenum

bers (cm-1)

187318381818

1640

1583

Pd2+NO

Al AlSi

OO O

O- -

Pd2+NO

Al AlSi

OO O

O- -

- H+O H

NO species not associated with

Pdn+

DRIFTS reveals three distinct NO/Pd adsorption configurations

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TPD NOx (ppm)

temperature (°C)

simult.CO+NO

CObefore

NO

NObefore

CO

NO



time (min)

NO

uptake rate decreasing

CO NO(ppm) (ppm)

200 200

200 200

200 200

CO increases the rate of NO uptake, but not the total equilibrium storage capacity

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Pd2+NO

Al AlSi

OO O

O- -Pd2+

Al AlSi

OO O

O- -

Pd2+NO

Al AlSi

OO O

O- -

- H+O H

Al AlSi

OO O

O- -

H+OH

H

Pd2+-O

H

O H

H

Al AlSi

OO O

O- -

- H+Pd2+ O HCO

n H2O

SLOW

NO n-1 H2O

CO

n-1 H2O

NO

NO

CO

H2O inhibition at low T

T < 150 °C

T ≥ 150 °C

CO promotionat low T

No H2O or CO effects at high T

1873 cm-1

1818 cm-1

Heat

H2O

T ≥ 150 °C

Heat, O2

NO

T ≥ 250 °C

NO displaces CO

Proposed mechanism captures key gas composition & temperature effects, is consistent with DRIFTS observations, and provides a foundation for modeling efforts

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NO Storage (Wet)Notes

100 °C 150°C

H2O rate ↓3 --• Some amount of H2O is helpful for higher NO adsorption on the PNA• Under wet conditions, H2O competes for NO storage sites < 150 °C

CO rate ↑2 --• In absence of H2O, CO has no effect• Under wet conditions, CO mitigates H2O inhibition effect < 150 °C• No NO-CO co-adsorbed species observed

T capacity ↑ till 150 °C, then ↓1

• H2O inhibition < 150 °C, diminished at higher Ts• NO stability/capacity decreases at > 150 °C

Literature with similar observations: 1Chen et al., Catal Lett 2016 146, 1706 (JM)2Vu et al., Catal Lett 2017 147, 745 (UVA)3Zheng et al., J. Phys. Chem. C 2017, 121, 15793 (PNNL)

Proposed mechanism based on reactor experiments and DRIFTS observations provides a foundation for modelling

Conclusions

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• Continue flow reactor experiments on adsorption/desorption phenomena

• Use DRIFTS to identify surface adsorbates

• Develop consistent adsorption/desorption mechanism and modeling strategies

• Investigate other Pd-zeolites to see if behavior is similar (or not)

Future Work

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Thank You!

Questions? Comments?

[email protected]@ornl.gov

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