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ORNL is managed by UT-Battelle, LLC for the US Department of Energy
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
2
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
3
• 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
4
• 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
5
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
6
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)
100 °C adsorption NOx (ppm)
NO
storage capacity unchanged
temperature (°C)
Increasing O2 has no effect on NO uptake
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TPD NOx (ppm)
temperature (°C)
100 °C adsorption NOx (ppm)
time (min)
50
100
200
400
25
NO ppm
800
1600N
O uptake rate increasing
NO
storage capacity unchanged
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%
100 °C adsorption NOx (ppm)
time (min)
H2O NO
uptake rate decreasing
150 °C adsorption NOx (ppm)
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)
100 °C adsorption NOx (ppm)
time (min)N
O uptake rate increasing
150 °C adsorption NOx (ppm)
time (min)
NO
uptake rate unchanged
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
uptake rate unchanged
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
storage capacity unchanged
100 °C adsorption NOx (ppm)
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