Sulfate Storage and Stability on Lean NOx Trap Components

Sulfate Storage and Stability on Lean NOx Trap Components

Todd J. Toops, Nathan Ottinger,Justin Roop, Josh A. Pihl,

Jae-Soon Choi and William P. Partridge

Oak Ridge National LaboratoryDOE Management Team:

Ken Howden, Gurpreet Singh

2 Managed by UT-Battellefor the U.S. Department of Energy

CLEERS reference catalyst is a complex multicomponent system

• Umicore catalyst contributed as the CLEERS reference LNT

• Ba-based sample with Pt, Pd and Rh

• Relies on Al2 O3 , CeOx -ZrO2 , and MgAl-oxide as supports

• Several studies exist to understand the performance and kinetics of this LNT

Void: monolith channel

Al-rich

Ce & Zr-rich

Ba-rich

CordieriteMg & Al-rich

Void: monolith channel

Al-rich

Ce & Zr-rich

Ba-rich

CordieriteMg & Al-rich


Detailed materials analysis has led to current understanding of associations• Ba is primarily associated with

ceria-zirconia phase• Mg-Al phase makes up a large

part of the washcoat• PGM primarily associated with

Ba/Ce-Zr and Al2 O3 phases-1.2

-1-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2

Al Ba Ce Mg Pd Pt Rh S Zr

a

SXN1

Mg

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


b

SXN1

Ce

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


a

SXN1

Mg

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


a

SXN1

Mg

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


b

SXN1

Ce

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


b

SXN1

Ce

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


c

SXN1

Ba

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


d

SXN1

Pt

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


c

SXN1

Ba

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


c

SXN1

Ba

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


d

SXN1

Pt

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2


d

SXN1

Pt

30 40 50 60 700

200

400

600

800

♦ CeO2◘ Cordierite○ MgAl2O4

♦

♦

♦

◘

◘

◘○

○ ◘

2θ

Cou

nts

◘◘

◘ ◘

30 40 50 60 700

200

400

600

800

♦ CeO2◘ Cordierite○ MgAl2O4

♦

♦

♦

◘

◘

◘○

○ ◘

2θ

Cou

nts

◘◘

◘ ◘J.-S. Choi

et al., Catalysis Today, in press.


Key questions that motivate study

• Why is Ba storage phase supported on Ceria-Zirconia rather than Alumina?

• What phase does sulfur adsorb to during lean rich cycling?

• What is role of Mg-Al phase?


Catalysts and mixtures chosen to represent washcoat components

• Catalysts studied not only represent the commercial LNT components, but also enable comparative studies

• PBCZ+MA is most representative of the Umicore sample

Pt Ba Al2O3 MgAl2O4 CeO2-ZrO2

MgAl2O4 - - - 100 -Pt/MgAl2O4 1 - - 99 -Pt/Al2O3 1.5 - 98.5 - -Pt/Ba/Al2O3 1.5 20 79.2 - -Pt/CeO2-ZrO2 1 - - - 99Pt/Ba/CeO2-ZrO2 0.8 20 - - 79.2

LNT componentsMgAl2 O4 (MA)

Pt/MgAl2 O4 (PMA)Pt/Al2 O3 (PA)

Pt/Ba/Al2 O3 (PBA)Pt/CeO2 -ZrO2 (PCZ)

Pt/Ba/CeO2 -ZrO2 (PBCZ)

Pt/Al2 O3 + MgAl2 O4 (PA+MA)Pt/Ba/CeO2 -ZrO2 + MgAl2 O4 (PBCZ+MA)


Results focus on sulfation and desulfation with microreactor• Microreactor system relies on MS for primary effluent analysis• To increase sensitivity to sulfur, second oxidation reactor employed in conjunction with SO2

analyzer– Additional Oxygen stream introduced– UV fluorescence SO2 analyzer

• requires dilution, but sensitive to ppb levels SO2

thermocouple

LNTcomponent

Oxidation catalyst

Fast Switching ValveFor lean rich cycling

1000°C200-1000°C

mass spectrometer

+SO2 analyzer

AddedO2

thermocouple

LNTcomponent

Oxidation catalyst

Fast Switching ValveFor lean rich cycling

1000°C200-1000°C

mass spectrometer

+SO2 analyzer

AddedO2


Support effect: Ceria-Zirconia versus Alumina


Several studies have shown contribution of Ceria in mixtures with Pt/Ba/Alumina• Low temperature improvement with ceria

• Multi-valent properties of ceria results in oxygen storage– Impacts nitrogen-based products and selectivity– Decreased NH3 slip

• Water gas-shift activity: CO + H2 O H2 + CO2

• Improved durability

• Sulfur detected on ceria in physical mixtures– Lower desulfation temperature


Catalyzed supports have very different sulfur uptake behavior • SO2 breakthrough measured while lean-rich cycling

at 400°C– 30 ppm SO2 included in both lean and rich feeds

• 120s lean, 10s rich– Expose catalysts to 5.5 mg S/gcat (~3 g/L)

• Each evaluation involves three sulfation steps and three desulfation steps– First two desulfations to

700°C with 1h hold time• Catalyzed supports

evaluated without Ba support

• Ceria-Zirconia supported catalysts trap more SO2 during cycling

Gas Lean RichNO 300 ppm -O2 10% -H2 - 0.71%CO - 1.19%CO2 5% 5%H2O 5% 5%SO2

* 30 ppm 30 ppmAr balance balance

= Pt/Alumina= Pt/Ceria-Zirconia

012345678

0 1000 2000 3000 4000 5000Storage time (s)

Sulfu

r (pp

m)

PAPCZ


Pt/Alumina has broad desulfation profile compared to Pt/Ceria-Zirconia• Temperature programmed reduction using rich phase (H2 /CO/H2 O/CO2 )

– 3rd desulfation to 1000°C; Designed for total sulfur removal• Onset of sulfur release lower

for Pt/Al2 O3 , but release profile is broad– T20% = 464°C– T90% = 704°C– ∆T = 240°C– Demonstrates

heterogeneity of sites• Pt/Ceria-Zirconia has sharp

release profile– T20% = 494°C; – T90% = 570°C– ∆T = 74°C

0

2

4

6

8

10

12

14

400 500 600 700 800 900 1000Temperature (oC)

Sulfu

r (

mol

/gca

t-min

) PAPCZ= Pt/Ceria-Zirconia

= Pt/Alumina


Ceria-Zirconia support decreases desulfation temperature significantly• PBA samples have sharp SO2

release profile during sulfation– small pulses on PBCZ

• Introducing Ba transforms release profiles– Minor Alumina-based S – More S on ceria-zirconia

• PBCZ sulfur releases at much lower temperature– PBA:

• T20% = 570°C • T90% = 855°C

– PBCZ: • T20% = 535°C • T90% = 797°C 0

1

2

3

4

5

400 500 600 700 800 900 1000Temperature (oC)

Sulfu

r (m

ol/g

cat-m

in)

PBA

PBCZ

0123456789

10

0 1000 2000 3000 4000 5000Storage time (s)

Sulfu

r (pp

m)

PBAPBCZ = Pt/Ba/Ceria-Zirconia

= Pt/Ba/Alumina


Pt/Ba/Alumina + Pt/Ceria desulfation profiles results in more distinct S release• With a physical mixture there is more

separation between desulfation peaks– Ceria release is distinct from Ba

• Results suggest Ba supported directly on Pt/Ce-Zr impacts Ba sulfate stability – Also allows more sulfur on support

Pt/Ba/Alumina

Pt/Ceria + Pt/Ba/Alumina

Y. Ji

et al., Catalysis Letters 127:1-2 (2009) 55.

0

1

2

3

4

5

400 500 600 700 800 900 1000Temperature (oC)

Sulfu

r (m

ol/g

cat-m

in)

PBA

PBCZ


Ceria-Zirconia also appears to modify nitrates stored; Peak shift identified • DRIFT spectra after 30 min of

NOx storage at 200°C – 300 ppm NO, 10% O2 , 5% H2 O– Spectra taken before

sulfating and referenced to catalyst at t=0

• At 200°C, Nitrates observed on Ce-Zr support – not significant on alumina

• Ba-based nitrate peak locations appear to shift due to support– PBA: 1350 and 1410 cm-1

– PBCZ: 1300 and 1410 cm-1 0

0.1

0.2

9001100130015001700

0

0.1

0.20

0.05

0.10

0.05

0.1

Abs

orba

nce PA

PBA

PBCZ

Wavenumber (cm-1)

PCZ

1540

1350

1030 100012451600

1580

1410

12401000

1540 1525

13001410

1010

1300


Why does sulfur pulse during cycling on Pt/Ba/Alumina but not on Pt/Ba/Ceria-Zirconia?• Always flowing 30 ppm SO2

– Both lean and rich cycles• No SO2 breakthrough on PCZ

– Impact of stored oxygen• PBA has minimal oxygen storage

– Sharp releases possibly due to SO2 on Pt unable to spillover

• PBCZ has muted release– Mixed effects

0E+00

1E-12

2E-12

3E-12

4E-12

0 1000 2000 3000 4000

Time (s)

Inte

nsity

(a.u

.)PBA MS data for SO2

Ba sites heavily sulfated; SO2

spillover becomes inhibited

0123456789

10

0 1000 2000 3000 4000 5000Storage time (s)

Sulfu

r (pp

m)


= Pt/Ba/Alumina

0123456789

10

0 1000 2000 3000 4000 5000Storage time (s)

Sulfu

r (pp

m)


= Pt/Ba/Alumina

On order of 40ppm SO2


Proposed scheme: oxygen storage in ceria enables sulfation during rich phase

• SO2 in feed, and on Pt, reacts w/ stored oxygen, forms sulfate

Sulfated Ce-Zr

SO2 SO2

Lean sulfation CeO2

Rich sulfation

CeO2

Sulfated Ce-Zr CeSO4CeSO4

Sulfated Ba

SO2 SO2

BaO BaO

•Ce-Zr and Ba sites near heavily sulfated;

•SO2

spillover becomes inhibited and SO2 population on Pt increases

Oxidized CeO2

capable of forming sulfates with incoming SO2

and Pt-SO2

BaO

(or BaCO3

) unable to form sulfates in rich phase

Sulfated BaBaO BaO

SO2 SO2

SO2 SO2

BaO BaO

• For PBA, SO2 on Pt and in rich feed leaves reactor

• For PBCZ, some of the desorbed and incoming SO2 readsorbs on ceria


What is role of Mg-Al phase?


Umicore catalyst has high MgAl2 O4 content but mechanism of benefit is unknown• Up to 40%wt MgAl2 O4 (MA) in the washcoat• Not strongly-coordinated with platinum group metals (PGM) via EPMA

• Sulfation profile on MA-only results in fast SO2 breakthrough– With PGM, it’s an effective SO2 trap (PMA)

• Physical mixture of PA+MA mimics PMASO2

breakthrough measured while lean-rich cycling at 400°C

0

5

10

15

20

25

0 1000 2000 3000 4000 5000Storage time (s)

Sulfu

r (pp

m)

PAMAPMAPA+MA MgAl2

O4

Pt/Al2

O3

+ MgAl2

O4Pt/Al2

O3

Pt/MgAl2

O4


Pt proximity to sulfate not critical for sulfur trap or release• Sulfates on MgAl2 O4 are very

highly stable

• Physical mixture of PA+MA mimics PMA

• Although some highly stable sulfates still exist…– significantly less than

predicted by physical mixture

– also indicates sulfur is being stored on MA phase

0123456789

400 500 600 700 800 900 1000Temperature (oC)

Sulfu

r (

mol

/gca

t-min

) PAMA

0123456789

400 500 600 700 800 900 1000Temperature (oC)

Sulfu

r (

mol

/gca

t-min

) PMAPA+MA

MgAl2

O4

Pt/Al2

O3

+ MgAl2

O4

Pt/Al2

O3

Pt/MgAl2

O4


Desulfation of Umicore-like sample suggests Mg-Al phase adsorbs SO2• PBCZ+MA most closely mimics Umicore formulation• Desulfation TPR shows that there is a small amount of extra SO2 released at ~800°C

– Demonstrates ability of Mg-Al phase to participate in sulfur trapping and protection of PBCZ phase

0123456789

400 500 600 700 800 900 1000Temperature (oC)

Sulfu

r (m

ol/g

cat-m

in)

PBCZPBCZ+MAMA

Indicates MgAl2

O4

adsorption of SO2

MgAl2

O4

Pt/Ba/Ceria-Zirconia

+ MgAl2

O4

Pt/Ba/

Ceria-Zirconia


Summary• Consistent with earlier study on materials characterization of intact

Umicore LNT Ceria-Zirconia, Alumina, and Mg/Al-phase act as sulfur traps– Direct contact with PGM not required– Relative S-trapping ability: Ceria-zirconia > Alumina > Mg-Al

• Oxygen storage plays key role in ability of ceria to store and shield Ba- phase from sulfur storage

• Using Ceria-Zirconia as a Ba support modifies both sulfate stability and nitrates formed– Implies strong support interaction with Ba-phase


Acknowledgments

This research was sponsored by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy,

Vehicle Technologies Program. Ken Howden and Gupreet Singh

Umicore Automotive CatalystsOwen Bailey

Sulfate Storage and Stability on Lean NOx Trap Components

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