Investigation of Aging Mechanisms in Lean NOx Traps · 2014. 3. 14. · Conditions: lean phase (6...

2008 DOE Merit Review 1

Investigation of Aging Mechanisms in Lean NOx Traps

PI: Mark Crocker

Center for Applied Energy Research,University of Kentucky

February 26, 2008

This presentation does not contain any proprietary or confidential information


• Understand main chemical and physical processes occurring during LNT aging

• Correlate washcoat composition with catalyst durability⇒ Effect of Pt, Rh, Ba, CeO2, CeO2-ZrO2

• Establish effect of LNT catalyst aging on NOxperformance and desulfation behavior

• Provide insights that will assist the design of more durable catalysts and/or allow for optimized catalyst operation as it ages

Purpose of Work


• Aftertreatment needed to meet NOx emission goals- Engine conditions that have the highest overall efficiency typically

give rise to NOx levels exceeding the 2010 regulations

• Limited durability of LNT catalysts impediment to widespread use

• Deeper understanding of “commercial” catalyst materials required:- role of individual components- effect on aging characteristics

• Providing insights that optimize catalyst operation over the lifetime of the vehicle– Improve catalyst efficiency and therefore minimize fuel penalty

Barriers


Approach

• Employ well characterized model catalysts which arerepresentative of 2nd generation LNT formulations⇒ use of ceria; also of relevance for lean-burn gasoline LNTs

• Examine effect of washcoat components/loadings on catalyst durability: ⇒ systematic variation of component concentrations:

Pt, Rh, Ba, CeO2(-ZrO2)

• Employ realistic aging protocol, with simultaneous measurement of catalyst NOx storage/reduction performance

• Perform detailed physico-chemical characterization of agedcatalysts

U. of Kentucky, ORNL, Ford and Umicore as partners


• Prepare model LNTs with sequential variation of component loadings: Pt, Rh, Ba, CeO2(-ZrO2)

• Evaluate de-greened catalysts to determine key component effects:- NOx performance (conversion, selectivity)- intra-catalyst chemistry- desulfation behavior

• Age catalysts in realistic and reproducible manner for subsequent studies (bench reactor + physico-chemical analysis)

Performance Measures


• Prepared model monolith and powder catalysts with well defined compositions

• Demonstrated that ceria improves low temperature NOxconversion and improves selectivity to N2

• Demonstrated that ceria significantly improves storage capacity and regeneration of LNTs, especially at low temperature

• Demonstrated that balanced ceria loading is required to maximize in situ H2 generation via WGS reaction

• Demonstrated that ceria improves catalyst desulfation

• Implemented LNT aging cycle on UK bench reactor

Accomplishments


Model Monolith Catalyst Compositions Prepared

LoadingComponent

Series 1 Series 2 Series 3

Pt, g/L (g/cuft) 3.53 (100) 3.53 (100) 3.53 (100), 2.65 (75), 1.77 (50)

Rh, g/L (g/cuft) 0.71 (20) 0.71 (20) 0.35 (10)

BaO, g/L 15, 30, 45 30 30

CeO2, g/L 0, 50, 100 - 50

CeO2-ZrO2, g/L - 50, 100 -

Al2O3, g/L Balance Balance Balance

• Target washcoat loading = 260 g/L• Actual average loading = 262 g/L, stand. dev. = 16.6 g/L (6.3%)• Monoliths coated at DCL Int. Inc.


Ceria Addition Improves N2 Selectivity and Low Temperature Performance

NOx conversion

0102030405060708090

100

NO

xco

nver

sion

(%)

150 °C 250°C 350 °C 450 °CTemperature (°C)

0102030405060708090

100

Sele

ctiv

ity to

N2

(%)

2(%

)

150 °C 250°C 350 °C 450 °CTemperature (°C)

Selectivity to N2

• Ceria improves NOx conversion for T< 350°C– most significant at 150°C0 g CeO2/L

50 g CeO2/L100 g CeO2/L100 g CeO2-ZrO2/L

• Ceria-addition improves N2 selectivity – increases w/ OSC


Ceria Improves Effective NOxStorage Capacity at 150 °C

NOx storage capacity improves with increasing ceria loading− NO2 in effluent shows NO to NO2 oxidation not limiting for 30-0 and

30-50Rich phase NOx release also increases with ceria loading− More NOx released with ceria-based LNTs ⇒ reduction kinetics are

too slow30-0

0

400

800

1200

1600

20 50 80 110 140Time (sec)

Conc

entr

atio

n (p

pm)

NO NO2

30-50

0

400

800

1200

1600

0 30 60 90 120

Time (sec)

Conc

entr

atio

n (p

pm)

NO NO2

30-100

0

400

800

1200

1600

10 40 70 100 130Time (sec)

Con

cent

ratio

n (p

pm)

NO NO2

Key: 30-0 = 30 g BaO/Lcat. + 0 g CeO2/Lcat., etc.


Studies with Model Powder Catalysts Provide Basis for Explaining Monolithic Catalyst Results

In situ DRIFTS studies performed on two model powder catalysts: - PBA: 1 wt% Pt/BaO/Al2O3 (equivalent to monolith catalyst 30-0)- PBAC: PBA (74 wt%) + 1 wt% Pt/CeO2 (26 wt%), physical mixture(equivalent to monolith catalyst 30-50)

Abs

orba

nce

(a.u

.)

2000 1800 1600 1400 1200 1000

0.1

Wavenumber (cm -1)

PBAC

200 oC

400 oC

300 oC

Abs

orba

nce

(a.u

.)

2000 1800 1600 1400 1200 1000

0.1

Wavenumber (cm -1)

PBAC

200 oC

400 oC

300 oC

2000 1800 1600 1400 1200 1000

200 oC

300 oC

400 oC

0.1PBA

Wavenumber (cm -1)

Abs

orba

nce

(a.u

.)

2000 1800 1600 1400 1200 1000

200 oC

300 oC

400 oC

0.1PBA

Wavenumber (cm -1)

Abs

orba

nce

(a.u

.)

Conditions: lean phase (6 min): 300 ppm NO and 8% O2; rich phase (0.5 min): 5625 ppm CO, 3375 ppm H2, with 5% H2O and 5% CO2 added in both phases.

Rich

Lean

In situ DRIFTS spectra during lean-rich cycling: • In situ DRIFTS: − only a fraction of stored

NOx is purged during L/R cycling

− PBAC shows superior rich phase regeneration

• Microreactor results consistent

− PBAC shows superior dynamic NOx storage capacity while cycling between lean and rich


Intra-Catalyst Chemistry: H2 Concentration Profile from SpaciMS (T = 350 °C)

During OSC test During testing w/ NOx (rich phase)

0.0

1.0

2.0

3.0

4.0

5.0

12 15 18 21 24Time (s)

H 2 c

once

ntra

tion

(%) In Cat

0.25 Cat0.50 CatOut Cat

30-0

0.0

1.0

2.0

3.0

4.0

5.0

12 14 16 18 20 22 24Time (s)

H 2 c

once

ntra

tion

(%) In Cat


30-100 30-100

0.0

1.0

2.0

3.0

12 16 20 24Time (s)

H 2 c

once

ntra

tion

(%)

In Cat0.25 Cat0.50 CatOut Cat

30-0

0.0

0.5

1.0

1.5

2.0

2.5

12 16 20 24Time (s)

H 2 c

once

ntra

tion

(%) In Cat


High water-gas shift activity of 30-100 results in greatly increased H2 production compared to 30-0. However, H2 was largely consumed at the end of the bed for 30-100


Balanced Ceria/OSC Loading Required for Optimum Water-Gas Shift Activity

30-0

0.0

1.0

2.0

3.0

12 16 20 24Time (s)

H 2 (

%)


30-100

0.0

1.0

2.0

3.0

12 16 20 24Time (s)

H 2 (%

)


30-100Z

0.0

1.0

2.0

3.0

41 45 49 53Time (s)

H 2 (

%)


0

1000

2000

3000

4000

5000

0 40 80 120

Time (s)

Out

let C

O(p

pm)

30-030-5030-10030-100Z Outlet CO concn.

during rich purge: low CO ⇒ consumption by WGS reaction and by stored oxygen

30-0

30-100Z:

30-100

T = 350 °C

100 g CeZrO2/Lcat.


Desulfation Studies: Effect of Catalyst Composition on Efficiency of Sulfur Removal

70

75

80

85

90

95

100

% S

ulfu

r rem

oved

650 °C 675 °C 700 °C

Desulfation temperature

30-030-5030-100Pt-50Pt-100

Key

• Beneficial effect of ceria confirmed• Reduction of precious metal content has adverse effect

:Pt-100 = same as 30-50 but with half the Rh loading (10 vs. 20 g Rh/cuft)Pt-50 = same as Pt-100 but with half the Pt loading (50 vs. 100 g Pt/cuft)

Conditions:Sulfation: 350 °C, 100 ppm SO2,10% O2, 5% CO2, 5% H2O, bal. N2; ca. 2 g S/L cat.;Desulfation: 1% H2, 10 min, bal. N2


• Goal is to subject model monolith catalysts to realistic accelerated aging for subsequent reactor studies and physico-chemical characterization

• Catalyst aging on automated bench reactor

• Initial aging to 50 cycles:ca. 50-75 k miles road equivalent based on fuel sulfur content

• Catalyst performance check every ~10 cycles

Catalyst Aging


LNT Aging Protocol

MODE #1Sulfur Exposure

30 minutesExposed to 0.50 – 1.0g S/L

L(s)/R(s) = 60/5

TInlet = 300ºC

MODE #2DeSOx10 minutes

L(s)/R(s) = 5/15Rich lambda=0.90

Tbed = DeSOx*

MODE #3DPF Regeneration

30 minutesLean only condition

Tbed = 650ºC

* Optimized DeSOx temperature that balance chemical and thermal deactivation

MODE #1Sulfur Exposure

30 minutesExposed to 0.50 – 1.0g S/L

L(s)/R(s) = 60/5

TInlet = 300ºC

MODE #2DeSOx10 minutes

L(s)/R(s) = 5/15Rich lambda=0.90

Tbed = DeSOx*

MODE #3DPF Regeneration

30 minutesLean only condition

Tbed = 650ºC

* Optimized DeSOx temperature that balance chemical and thermal deactivation

• Based on Ford protocol

• Optimized desulfation temperature balances desulfation with thermaldeactivation (→ 700 °C)

75

80

85

90

95

100

0 10 20 30 40 50 60

Cycle number

NO

x co

nver

sion

(%)

Aging profile for catalyst 30-100

(T = 300 °C)


• Technology transfer via Ford Motor Co. (project partner)

• Model monolith catalysts prepared in this project currently being utilized in Ford R&D (Bob McCabe): - fundamental studies pertaining to catalyst sulfation anddesulfation

- effect of catalyst composition on selectivity to differentN-species

• Joint U. of Kentucky/Ford project planned as follow up to current DOE-sponsored project (Ford University Research Program)

• Results of the project presented at conferences and in the literature:- 3 publications, 7 presentations to date

Technology Transfer


Publications, etc.

Publications:• Y. Ji, J.-S. Choi, T. J. Toops, M. Crocker, M. Naseri, Influence of Ceria on the NOx

Storage/Reduction Behavior of Lean NOx Trap Catalysts, Catal. Today, in press.• Y. Ji, T.J. Toops, M. Crocker, “Effect of Ceria on the Storage and Regeneration Behavior of a

Model Lean NOx Trap Catalyst”, Catal. Lett. 119 (2007) 257.• Y. Ji, T.J. Toops, U.M. Graham, G. Jacobs and M. Crocker, “A kinetic and DRIFTS study of

supported Pt catalysts for NO oxidation”, Catal. Lett. 110 (2006) 29.

Presentations:• Y. Ji, T.J. Toops and M. Crocker, “Effect of Ceria Addition on the Regeneration of a Model LNT

Catalyst”, Tri-State Catalysis Society Fall Symposium, Lexington, KY, November 19, 2007.• Y. Ji, T.J. Toops, J.-S. Choi, M. Crocker, “Composition-Activity Relationships in Lean NOx Trap

Catalysts”, Europacat VIII, Turku, Finland, August 26-30, 2007, paper P14-9.• Y. Ji, T.J. Toops, J.-S. Choi, M. Crocker, “Investigation of Aging Mechanisms in Lean NOx Trap

Catalysts”, 1st Annual DOE Semi-Mega Merit Review Meeting, Crystal City, VA, June 18-19, 2007.• Y. Ji, T.J. Toops, J.-S. Choi, M. Crocker, “Effect of CeO2 on the Storage and Regeneration

Behavior of Lean NOx Traps”, 20th North American Catalysis Society Meeting, Houston, TX, June 17-22, 2007, paper O-S4-41.

• Y. Ji, T.J. Toops, J.-S. Choi, M. Crocker, “The Effect of CeO2 on the Performance of Lean NOxTrap Catalysts”, 10th Cross-Cut Lean Exhaust Emissions Reduction Simulation (CLEERS) Workshop, Dearborn, MI, May 1-3, 2007.

• Y. Ji, T.J. Toops and M. Crocker, “Effect of Ceria Addition on the Regeneration of a Model LNT Catalyst”, Southeastern Catalysis Society Fall Symposium, Asheville, NC, September 18, 2006.

• Y. Ji, T.J. Toops, U.M. Graham, G. Jacobs and M. Crocker, “A kinetic and DRIFTS study of supported Pt catalysts for NO oxidation”, 9th Cross-Cut Lean Exhaust Emissions Reduction Simulation (CLEERS) Workshop, Dearborn, MI, May 3-4, 2006.


• Complete accelerated aging of monolith catalysts according to Ford protocol

• Characterize NOx storage and reduction properties of aged model monolith catalysts:→ ORNL bench reactor, with use of spaci-MS

• Performed detailed physico-chemical characterization of aged catalysts:→SEM, TEM, N2 physisorption, H2 chemisorption, XRD

• Derivation of LNT deactivation model:→ spatial resolution possible?

Plans for Next Fiscal Year


Summary

• Model LNT catalysts have been prepared & characterized with systematic variation of Pt, Rh, Ba and CeO2(-ZrO2) loadings

• Correlations have been identified between catalyst performance and CeO2, Ba and Pt loading

• SpaciMS studies have revealed a strong dependency of intra-catalyst H2 concentration profiles on the ceria/oxygen storage content in model LNT catalysts⇒ need for balanced OSC

• The model catalysts are currently being aged (accelerated LNT aging cycle)

• In the next phase, the aged catalysts will be characterized (NOx storage and reduction, physico-chemical analysis)

Purpose of WorkBarriersApproachPerformance MeasuresAccomplishmentsCeria Improves Effective NOx �Storage Capacity at 150 °CIntra-Catalyst Chemistry: H2 Concentration Profile from SpaciMS (T = 350 °C)Balanced Ceria/OSC Loading Required for Optimum Water-Gas Shift Activity Desulfation Studies: Effect of Catalyst Composition on Efficiency of Sulfur Removal LNT Aging ProtocolTechnology TransferPublications, etc.Plans for Next Fiscal YearSummary

Investigation of Aging Mechanisms in Lean NOx Traps · 2014. 3. 14. · Conditions: lean phase (6...

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