Effect of Ceria on the Sulfation and Desulfation Characteristics of

Lean NOx Trap Catalysts

Y. Ji, V. Easterling, M. Crocker,Center for Applied Energy Research, University of Kentucky

T.J. ToopsFuels, Engines and Emissions Research Center, ORNL

J. Theis, J. Ura, R.W. McCabe,Ford Innovation and Research Center

May 13, 2007

• Role of ceria:- OSC for stoichiometric operation in lean-burn gasoline engines- water-gas shift activity for in situ H2 generation:

» facilitates LNT regeneration at low temperatures» facilitates LNT desulfation at moderate temperatures

- NOx storage at low temperatures (<350 °C)

• Use of ceria may prove critical for low temperature applications,e.g., LD diesel (typical FTP temperatures of ∼150-340 °C)

• Role of ceria largely ignored in literature LNT studies

J. Theis, J. Ura, C. Goralski Jr., H. Jen, E. Thanasiu, Y. Graves, A. Takami, H. Yamada, S. Miyoshi, SAE 2003-01-1160T. Morita, N. Suzuki, N. Satoh, K. Wada, H. Ohno, SAE 2007-01-0239.

Background: Ceria in LNT catalysis

Objectives

• Understand role of ceria in mitigating sulfur deactivation of LNT catalysts

• Quantify effect of ceria in fresh and aged catalysts

- This talk will focus on fresh/degreened catalysts

• Study divided into two parts:

- Powder model catalysts: Pt/Ba/Al2O3 with andwithout added Pt/CeO2

- DRIFTS and microreactor

- Monolithic model catalysts

DRIFTS STUDY

Part I: Powder model catalysts

DRIFTS studies performed in an environmental cell:- in situ observations during LNT sulfation and desulfation

Experiments also performed in a microreactor equipped with mass spectrometer:- measurement of NOx storage capacity after sulfation anddesulfation

- temperature-programmed desulfation

Two catalysts were prepared:(1) 1 wt% Pt / 20 wt% BaO / Al2O3 (“PBA”)(2) 1 wt% Pt / 20 wt% BaO / Al2O3 (74 wt%)

+ 1 wt% Pt / CeO2 (26 wt%), physical mixture (“PBAC”)

Sulfation of PBA without added H2O

2000 1800 1600 1400 1200 1000

0.1

Wavenumber (cm-1)

Abs

orba

nce

5 min4 min

20 min25 min30 min

15 min13 min6 min

1478

1560

1418

1323

1130

1039 • 300°C, 27 ppm SO2

• Strong nitrate bands

• Slow formation of surfaceBa sulfate

H. Abdulhamid, E. Fridell, J. Dawoody,M. Skoglundh, J. Catal. 241

1418, 1323 cm-1: mono-/bidentate Ba nitrate

1130 cm-1: surface/bulk Ba sulfate

1560 cm-1: Al-based nitrate

(2006) 200:

1168 cm-1 = bulk Ba sulfate1107 cm-1 = surface Ba sulfate

Sulfation of PBA with added H2O

1420/1320 cm-1: mono-/bidentate Ba nitrate

1122 cm-1: surface/bulk Ba sulfate

2000 1800 1600 1400 1200 1000 800

0.1

Wavenumber (cm-1)

Abs

orba

nce

1 min2 min

15 min

20 min

25 min

30 min

5 min10 min

1169 1038

1420

1320

1122

978

1108

• 300°C, 27ppm SO2

• Strong sulfate bands⇒ H2O facilitates sulfateformation

• Reduction in intensity ofnitrate bands towards end of experiment⇒ displacement of nitrateby sulfate

Sulfation of PBAC without added H2O

Wavenumber (cm-1)

Abs

orba

nce

0.1

2000 1800 1600 1400 1200 1000 800

1 min

20 min

25 min

30 min

5 min10 min

2 min

15 min

1542

10381140

1414

1323

1516

1480

1542, 1516 cm-1:Ce monodentate nitrate

1414, 1323 cm-1: Ba mono-/bidentate nitrate

1140 cm-1: surface/bulk BaSO4

• 300°C, 27ppm SO2

• Strong nitrate bands

• Slow formation of surfaceBa sulfate

• Shift in location of sulfates from 1130 to 1140 cm-1…ceria impact

Sulfation of PBAC with added H2O

Wavenumber (cm-1)

Abs

orba

nce

2000 1800 1600 1400 1200 1000 800

0.1

1 min

20 min

25 min

30 min

5 min

10 min

2 min

15 min

1038

1508

1413

1325

1538

11141456

• 300°C, 27ppm SO2

• Spectra similar to those of PBA sulfated in absenceof water…but with muchgreater nitrate/sulfate ratio

• sulfate location shifts from 1122 to 1114

1538, 1508 cm-1:Ce monodentate nitrate

1413, 1325 cm-1: mono-/bidentate Ba nitrate

1114 cm-1: surface/bulk BaSO4

Sulfation of Pt/CeO2 with added H2O

Wavenumber (cm-1)

Abs

orba

nce

2000 1800 1600 1400 1200 1000 800

883

1315

1217

1096

995

1350

1540

1516

1246

1225

1540 and 1516 cm-1:Ce monodentate nitrate;

Other bands due to Cesulfate and sulfite species

• 300°C, 27 ppm SO2

• Strong sulfate bands

• Reduction in intensity ofnitrate bands towards end of experiment⇒ displacement of nitrateby sulfate

1 min

20 min

25 min

30 min

10 min

15 min

5 min

2 min

0.05

Comparison of sulfation behavior

2000 1800 1600 1400 1200 1000 800

PBA (w/o H2O)

PBAC (w/o H2O)

PBA (w/ H2O)

PBAC (w/ H2O)

Wavenumber (cm-1)

Abs

orba

nce

0.2

• Addition of H2O significantlyincreased the formation of sulfate relative to nitrate for PBA

→ H2O improved the Badispersion, resulting in greater SOx uptake by Ba phase?

→ Formation of surface HSO3-

facilitates sulfate storage on Ba phase?

Sulfate/Nitrate ratioPBA (w/o H2O): 0.38PBAC (w/o H2O): 0.40PBA (w/ H2O): 2.43PBAC (w/ H2O): 0.51

DRIFT spectra after desulfation at 450 °C

Wavenumber (cm-1)

Abs

orba

nce

2000 1800 1600 1400 1200 1000 800

0.1

PBA

PBAC

Pt/CeO2

1032

1107

1166

Desulfation conditions:1% H2, 5% H2O, balance Ar, 450 °C, 5 min

• Sulfur can be fully removedfrom Pt/CeO2 upon heating to450 °C

• Residual sulfur on both PBAand PBAC is associated withthe Ba phase

2000 1800 1600 1400 1200 1000 800

PBA

PBAC

Wavenumber (cm-1)

Abs

orba

nce

0.1

DRIFT spectra after desulfation at 550 °C

Desulfation conditions:1% H2, 5% H2O, balance Ar, 550 °C, 30 min

• Residual sulfur associatedwith the Ba phase notremoved at 550 °C

- 550°C limitation of the reactor

MICROREACTOR STUDY

Procedure for sulfation-desulfationexperiments

Powder reactorTesting procedure:- Pre-treat catalyst at 750 °C under 1% H2/Ar for 1 h- Measure NOx storage capacity (NSC) at 300 °C under

continuous lean conditions, 60 min (300 ppm NO, 10% O2, 5% CO2, 5% H2O, GHSV = 30,000 h-1)

- Sulfate catalyst at 300 °C, 120 min(27 ppm SO2)…~1 g S/L monolith equivalent…4 mg S/gcat

- Measure NSC as above- Desulfate catalyst at 700 °C, 60 min

(1% H2, 5% CO2, 5% H2O, balance Ar)- Measure NSC as above

PBA: Effect of sulfation on NSC

• Loss of NSC after sulfation (32% after 60 min)• Desulfation improves NSC at short times, no effect at longer storage times⇒ bulk storage sites not fully desulfated?

PBA

0

50

100

150

200

250

300

0 10 20 30 40 50 60

Time (min)

Out

let N

Ox

(ppm

)Before sulfationAfter sulfationAfter desulfation

PBAC: Effect of sulfation on NSC

• Small loss of NSC after sulfation (8% after 60 min)• Desulfation improves NSC at short times, no effect at longer storage times⇒ bulk storage sites not fully desulfated?

PBAC

0

50

100

150

200

250

300

0 10 20 30 40 50 60

Time (min)

Out

let N

Ox

(ppm

)Before sulfationAfter sulfationAfter desulfation

Before sulfation

After sulfation

After desulfation

NOx stored (μmol/g)

400 271 322

Efficiency (%) 64 44 52

NOx stored (μmol/g)

373 343 339

Efficiency (%) 60 56 55

PBAC

PBA

Comparison of NSC between ceria-free and ceria-containing catalysts

Figures based on 60 min NOx storage time

LNT desulfation studies by TPR

Procedure:

- Pre-treat catalyst 450 o C (O2,15 min; then H2,15 min)

- Sulfate at 350 °C, lean conditions (100 ppm SO2, 8% O2,5% H2O, 5% CO2, balance N2)

- Desulfation by means of TPR: ramp to 800 °C (5 °C/min), continuous rich conditions (2% H2, 5% H2O, 5% CO2,balance N2)

- H2S, SO2 and COS monitored by CI-MS

TPR of 1 wt% Pt/Al2O3 sulfated to 1 g/L and 3 g/L monolith equivalent

0

200

400

600

800

1000

1200

1400

350 400 450 500 550 600 650 700 750 800

Temperature, oC

Con

cent

ratio

n, p

pm

SO2 (ppm)

H2S (ppm)

COS (ppm)

0

200

400

600

800

1000

1200

1400

350 400 450 500 550 600 650 700 750 800Temperature, oC

Con

cent

ratio

n, p

pm

SO2 (ppm)

H2S (ppm)

COS (ppm)

1 g S/L

3 g S/L

• 1 g S/L: H2S release peaks at370 °C

• 3 g S/L: SO2 release peaks at360 °C, H2S release at 380 °C

TPR of PBA sulfated to 1 g/L and 3 g/L monolith equivalent

0

100

200

300

400

350 400 450 500 550 600 650 700 750 800

Temperature, oC

Con

cent

ratio

n, p

pm

SO2 (ppm) H2S (ppm)COS (ppm)

0

100

200

300

400

350 400 450 500 550 600 650 700 750 800

Temperature, °C

Con

cent

ratio

n, p

pm

SO2 (ppm)

H2S (ppm)

COS (ppm)

1 g S/L

3 g S/L

• 380 °C = Al2O3 desulfation• 682 °C = surface BaSO4• 772 °C = bulk BaSO4

• 1 g S/L: H2S release peaks at695 °C

• 3 g S/L: H2S release at 380 °C,682 °C and 772 °C

TPR of PBAC sulfated to 1 g/L and 3 g/L monolith equivalent

0

100

200

300

400

500

600

350 400 450 500 550 600 650 700 750 800Temperature, oC

Con

cent

ratio

n, p

pm

SO2 (ppm)

H2S (ppm)

COS (ppm)

0

100

200

300

400

500

600

350 400 450 500 550 600 650 700 750 800Temperature, oC

Con

cent

ratio

n, p

pm

SO2 (ppm)

H2S (ppm)

COS (ppm)

1 g S/L

3 g S/L

• 463 °C = CeO2 desulfation• 684 °C = surface BaSO4• 775 °C = bulk BaSO4

• Ba loading in PBAC = 74% of that in PBA

• But, for PBAC at 1g S/L, sulfur release from Ba phase = ca. 50% of that from PBA

TPR of 1 wt% Pt/CeO2 sulfated to 3 g/L monolith equivalent

0

200

400

600

800

1000

1200

1400

350 400 450 500 550 600 650 700 750 800

Temperature, oC

Con

cent

ratio

n, p

pmSO2 (ppm)

H2S (ppm)

COS (ppm)

3 g S/L

• CeO2 desulfation occurs at 451 °C

MONOLITH STUDY

Monolithic catalyst compositions prepared:

Total washcoat loading = 260 g/L

Part II: Monolithic catalysts

Catalyst code / LoadingComponent

30-0 30-50

100 100

20

30

50

Balance

20

30

0

Balance

30-100 Pt-100 Pt-50

Pt, g/cuft 100 100 50

Rh, g/cuft 20 10 10

BaO, g/L 30 30 30

CeO2, g/L 100 50 50

Al2O3, g/L Balance Balance Balance

Ford Protocol for sulfation/desulfation

• Sample sulfation:1/1 L/R cycles, 350 °C, 9 ppm SO2, 500 pm NO, 5% O2 (L), 10% CO2, 10% H2O, 15 h; equivalent to 6.2 g S/L cat.

• Desulfation #1:Rich conditions: 1.2% CO, 0.4% H2, 10% CO2, 10% H2O, 5 min.; T = 675-750 ° C

• Measurement of lean NOx storage efficiency (#1):5/3 minute L/R cycles, 350 °C

• Desulfation #2 • Measurement of lean NOx storage efficiency (#2)• Desulfation #3• Measurement of lean NOx storage efficiency (#3)• Sulfur “burnout”

Rich conditions, 10 min, 750 ° C• Measurement of lean NOx storage efficiency (#4)

Effect of catalyst composition on required desulfation temperature: catalyst 30-0 (no ceria)

Catalyst 30-0

0

5

10

15

20

25

30

35

40

45

50

1 2 3 4

Desulfation temperature

Lean

NO

x St

orag

e Ef

ficie

ncy

(%)

DeSOx #1 DeSOx #2 DeSOx #3 Burnout

675 °C 725 °C700 °C 750 °C

• Required desulfation temperature for optimal NSC ≥ 750 °C

5 min lean storage data at 350 °C

Effect of catalyst composition on required desulfationtemperature: catalyst Pt-100 (30 g/L ceria)

• Required desulfation temperature for optimal NSC ≈ 725 °C,vs. ~750 °C for catalyst w/o ceria (30-0)

0

5

10

15

20

25

30

35

40

45

50

675C 8/1 700C 8/2 725C 8/3 750C 8/4

Desulfation temperature

Lean

NO

x St

orag

e Ef

ficie

ncy

(%)

1st 5-3 2nd 5-3 3rd 5-3 Burnout, 5-3 @ 750Catalyst Pt-100

675 °C 725 °C700 °C 750 °C

5 min lean storage data at 350 °C

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

• Beneficial effect of ceria confirmed

• Reduction of precious metal content has adverse effect

Sulfation at 350 °C, continuous lean phase (1 h), sulfation to 6 g S/L

Desulfation by ramping to temp. indicated under 2% H2, 5% CO2, 5% H2O (10 min hold)

Conclusions (1)

DRIFTS studies indicate that H2O exerts a promotingeffect on Ba sulfation

BaSO4 formation is partially suppressed in the presenceof CeO2

Consequently, CeO2-containing catalysts are better ableto store NOx during/after sulfation than Ba-onlyformulations

Pt/CeO2 can be completely desulfated at 450 °C in H2/Ar

For catalyst PBA, BaSO4 shows two decompositionmaxima in H2:~685 °C (surface BaSO4) and ~775 °C (bulk BaSO4)

Conclusions (2)

Desulfation of surface BaSO4 occurs at a highertemperature for a physical mixture of Pt/Al2O3 + BaO/Al2O3 than for Pt/BaO/Al2O3 (PBA)⇒ spillover of Hads facilitates decomposition of surfacesulfate (as for nitrate*)

The beneficial effect of ceria with respect to LNTdesulfation has been demonstrated for fully formulatedmonolithic catalysts

* I. Nova, L. Lietti, L. Castoldi, E. Tronconi, P. Forzatti, J. Catal. 239 (2006) 244.

Acknowledgements

Monolith catalyst preparation:

DCL Int.: Mojghan NaseriShazam Williams

Funding:

Department of Energy(Vehicle Technologies Program)