Fundamental Studies of NOx Adsorber Materials

CLEERS Workshop #11May 13, 2008

Fundamental Studies of NOx Adsorber Materials

Do Heui Kim, Ja Hun Kwak,Chuck Peden, Janos SzanyiInstitute for Interfacial Catalysis

Pacific Northwest National Laboratory

TodayToday’’s Discussions Discussion• DOE/OFCVT-funded studies of BaO/Al2O3

Lean NOx Trap (LNT) materials• LNT material morphologies – new insights from FTIR,

computations, and ultra-high field NMR.• BaO on CeO2 – performance and sulfur poisoning.• Other support and alkaline earth oxide storage materials.

AcknowledgmentsAcknowledgmentsU. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy/FreedomCAR and Vehicle Technologies Program

Experiments performed in DOE/BER’s Environmental Molecular Sciences Laboratory located at PNNL, and in DOE/EE/VT’s High Temperature Materials Lab at ORNL

Summary of TP-XRD and TEM/EDX studies: Both ‘Monolayer’ and ‘Bulk’ Ba(NO3)2 morphologies present. These ‘phases’ can be distinguished spectroscopically.

Heat

NO2 adsorptionat 300K

Heatin NO2

Large Ba(NO3)2crystallites

Al2O3

BaO nanoparticles

Heat Ba(NO3)2nanoparticles

Ba(NO3)2particles

+thin Ba(NO3)2

layer

Al2O3

Al2O3Al2O3

Szanyi, Kwak, Hanson, Wang, Szailer, Peden,J. Phys. Chem. B 109 (2005) 7339-7344.

Observed practical implications of the Ba-phase morphology.

• From TPD experiments, the “monolayer” morphology is found to decompose at lower temperature in vacuum and in a reducing atmosphere than “bulk”nitrates.

• “Monolayer” Ba-phase is also easier to ‘de-sulfate’.• Formation of a high-temperature (deactivating?)

BaAl2O4 phase requires BaO coverages above 1 monolayer.

• Morphology model at least partially explains relatively small use of Ba species (often <20%) in storing NOx during typical lean-rich cycling.

FTIR after NOFTIR after NO22 adsorption on 2%,adsorption on 2%,8%8%--, and 20%, and 20%--BaO/AlBaO/Al22OO3 3 at 300Kat 300K

• Al2O3-bound nitrates (AN) decrease continuously with Bacoverage.

• Surface (“bidentate”– BN) and bulk (ionic – IN) nitrates are observed on BaO/Al2O3 catalysts. Their ratio (BN/IN) also decreases with BaO loading. 1800 1600 1400 1200

1254

12951315

1438

Abs

orba

nce

Wavenumbers (cm-1)

1582

20%

8%

2%

0.5

AN

BN

BNIN

AN

+NO

+NO

22

O OO O

NNBa(NOBa(NO33))22

Heat

Heat

Ba(NOBa(NO33))22NONO22

AlAl22OO33

AlAl22OO33AlAl22OO33

Heat

Heat

NO+NO+½½ OO22

Bridging Bridging NitratesNitrates(Surface)(Surface)

IonicIonicNitratesNitrates(bulk)(bulk)

==

OO

BaOBaO


-150-100-50050100150200

Chemcial shift(ppm)

Aloct

Altet

**

Aloct

Altet

Alpen??

-20020406080Chemical shift (ppm)


Altet

Aloct



Altet

Aloct

Alpen

SSBSSB

Use of a oneUse of a one--ofof--a kind Ultraa kind Ultra--High Field NMR in High Field NMR in the Environmental Molecular Science Lab at PNNLthe Environmental Molecular Science Lab at PNNL

JH Kwak, JZ Hu, DH Kim, J Szanyi, CHF Peden, Journal of Catalysis, 251 (2007) 189-194.

•• PentaPenta--coordinate Alcoordinate Al+3+3 ions ions readily observable in readily observable in γγ--AlAl22OO33;;

•• Are these species are located Are these species are located at the alumina surface?at the alumina surface?

-20020406080100

300 °C

400 °C

500 °C

600 °C

__

__

__

__

5-fold Al-atoms display ‘chemical’characteristics of being surface cations

5-fold Al cations increase at the expense of 6-fold cations after high

temperature annealing

Temperature

-20020406080100

500oC cal500oC H2O ads

5-fold cations disappear and octahedral Al increases after exposure to H2O

+ H2O

500 °C calcined500 °C + H2O

Lewis acidic 5Lewis acidic 5--fold Al sites on fold Al sites on γγ--AlAl22OO33 surfaces surfaces are nucleation sites for catalytic phases!are nucleation sites for catalytic phases!

JH Kwak, JZ Hu, DH Kim, J Szanyi, CHF Peden, Journal of Catalysis 251 (2007) 189-194.

55--fold sites are fully fold sites are fully titrated at ~4 weight titrated at ~4 weight % loading of % loading of BaOBaO on on 200 m200 m22/gm /gm γγ––AlAl22OO33..-20020406080

Chemical shift(ppm)

(i)

(ii)

(iii)

(iv)

-20020406080Chemical shift(ppm)

(i)

(ii)

(iii)

(iv)

0

1

2

3

4

0 0.5 1 1.5 2 2.5

BaO loading(wt%/Al)

Area

% o

f 23p

pm A

l

γ-Al2O3

Ba(.5%)/γ-Al2O3

Ba(1%)/γ-Al2O3

Ba(2%)/γ-Al2O3

Addition of a catalytic phase, Addition of a catalytic phase, BaOBaO, , quantitatively quantitatively ‘‘titratestitrates’’ 55--fold Al sites.fold Al sites.

The titration results consistent with The titration results consistent with expected distribution of expected distribution of γγ--AlAl22OO33 surfaces surfaces

• 4 weight % loading of BaO sufficient to titrate all 5-fold Al+3 sites.

• Assuming that BaO forms perfect 2D clusters or domains on the 200 m2/g γ–Al2O3 substrate, 1 ML of BaO will be reached at ~25% weight loading.

• Thus, ~16% of the alumina surface consists of 5-fold Al+3 sites.

Yates and coworkers, J. Phys. Chem. B 110 (2006) 4742, and Digne, et al., J. Catal. 226 (2004) 54, and references therein.

γ-Al2O3(100) γ-Al2O3(110)

γ-Al2O3(100) surfaces are estimated to be ~17% of the total surface area

γ-Al2O3(100) - ~17%

γ-Al2O3(110) - ~70-83%

γ-Al2O3(111) – stable?

Lewis acidic 5Lewis acidic 5--fold Al sites on fold Al sites on γγ--AlAl22OO33 surfaces surfaces are nucleation sites for catalytic phases!are nucleation sites for catalytic phases!

JH Kwak, JZ Hu, DH Kim, J Szanyi, CHF Peden, Journal of Catalysis 251 (2007) 189-194.

55--fold sites are fully fold sites are fully titrated at ~4 weight titrated at ~4 weight % loading of % loading of BaOBaO on on 200 m200 m22/gm /gm γγ––AlAl22OO33..-20020406080

Chemical shift(ppm)

(i)

(ii)

(iii)

(iv)

-20020406080Chemical shift(ppm)

(i)

(ii)

(iii)

(iv)

0

1

2

3

4

0 0.5 1 1.5 2 2.5

BaO loading(wt%/Al)

Area

% o

f 23p

pm A

l

γ-Al2O3

Ba(.5%)/γ-Al2O3

Ba(1%)/γ-Al2O3

Ba(2%)/γ-Al2O3

Addition of a catalytic phase, Addition of a catalytic phase, BaOBaO, , quantitatively quantitatively ‘‘titratestitrates’’ 55--fold Al sites.fold Al sites.

HRHR--TEM shows TEM shows BaOBaO monomers at monomers at low and low and dimersdimers a higher loadingsa higher loadings

2 weight %BaO/γ-Al2O3




JH Kwak, D Mei, C-W Yi DH Kim, CHF Peden, LF Allard, J Szanyi, Angew. Chemie, submitted.

Catalytic Reactor/UHV Surface ScienceCatalytic Reactor/UHV Surface ScienceApparatus for Model Catalyst StudiesApparatus for Model Catalyst Studies

LEED Optics

Side View

Residual Gas Analyzer

FTIR AbsorptionSpectroscopy

High Pressure Catalytic Cell

Gas Chromatograph

Hemispherical Analyzer

Reactor for Testing Mechanisms Reactor for Testing Mechanisms on Model Catalystson Model Catalysts

Model Catalyst Synthesis Strategy

NiAl(110)

NiAl(110)AlOx

γ-Al2O3

O2; T

NiAl(110)AlOx

γ-Al2O3

NiAl(110)AlOx

γ-Al2O3

Ba

O2; T

Ozensoy, E.; Szanyi, J.; Peden, C.H.F. J. Phys. Chem. B 109 (2005) 3431-3436; 15977-15984.Ozensoy, E.; Peden, C.H.F.; Szanyi, J. J. Phys. Chem. B 110 (2006) 17001-17008; 17009-17014.

Identical FTIR features observed for Identical FTIR features observed for 300K NO300K NO22 adsorption on model BaO/Aladsorption on model BaO/Al22OO33

1800 1600 1400 1200

1254

12951315

1438

Abs

orba

nce

Wavenumbers (cm-1)

1582

20%

8%

2%

0.5

AN

BN

BNIN

AN

1900 1800 1700 1600 1500 1400 1300 1200 1100

1315

1332

1463

15721592

1623

1645

IRA

S In

tens

ity (A

rbitr

ary

Uni

ts)

Wavenumber (cm-1)

Alumina Film 0.15 ML Ba/Alumina 0.30 ML Ba/Alumina 0.75 ML Ba/Alumina

0.025 %

Szailer, T.; Kwak, J.H.; Kim, D.H.; Szanyi, J.; Wang, C.M.; Peden, C.H.F., Catal. Today 114 (2005) 86.Yi, C.W.; Kwak, J.H.; Peden, C.H.F.; Wang, C.M.; Szanyi, J., J. Phys. Chem. C 111 (2007) 14942.

UHV IRAS Studies: NOUHV IRAS Studies: NO22 adsorption adsorption on Model on Model BaOBaO Surface at 90 KSurface at 90 K

At the lowest NO2 exposure:* both NO2

- & NO3- are

present* no adsorbed NO2/N2O4

At high NO2exposures:* NOx

- intensities saturate

• N2O4 ice grows1800 1700 1600 1500 1400 1300 1200 1100

0.000

0.002

0.004

0.006

17651772

1737

14291342

1272

IRA

S In

tens

ity (A

rbitr

ary

Uni

ts)

Wavenumber (cm-1)

1259

NO3-

NO2-

11001200130014001500160017001800

Wavenumber/cm-1

Abs

orba

nce/

cm-1

0.2

Surface (expt.)

Bulk (expt.)

Bulk (calc.)

Surface (calculated)

11001200130014001500160017001800

Wavenumber/cm-1

Abs

orba

nce/

cm-1

0.2

Surface (expt.)

Bulk (expt.)

Bulk (calc.)

Surface (calculated)

NO2 Adsorption On:(BaO)1/Al2O3(100) BaO(100)

JH Kwak, D Mei, C-W Yi DH Kim, CHF Peden, LF Allard, J Szanyi, Angew. Chemie, submitted.

DFT Calculations of Stable DFT Calculations of Stable NOxNOx Species on Species on Dispersed Dispersed BaOBaO and Bulk and Bulk BaOBaO SurfacesSurfaces

1540-1600 cm-1

1200-1230 cm-11430-1475 cm-1

1280-1310 cm-1

+NO

+NO

22

O OO O

NNBa(NOBa(NO33))22

Heat

Heat

Ba(NOBa(NO33))22NONO22

AlAl22OO33

AlAl22OO33AlAl22OO33

Heat

Heat

NO+NO+½½ OO22

Bridging Bridging NitratesNitrates(Surface)(Surface)

IonicIonicNitratesNitrates(bulk)(bulk)

==

OO

BaOBaO


The γ- to θ-Al2O3 phase transition, between 900-1000 °C, can be followed by XRD and 27Al NMR

JH Kwak, JZ Hu, AC Lukaski, DH Kim, J Szanyi, CHF Peden, J. Phys. Chem. C (2008) in press.

10 20 30 40 50 60 70

2 theta( o)

i.

ii

iii.

(a)

▼▼

▼▼

▼: θ-Al2O3

-300306090120

Chemical shift(ppm)

i

ii

iii

(b)

72.8

11.8

77.2

13.8

: θ-Al2O3

γ-Al2O3

γ-Al2O3

500 °C

800 °C

1000 °C

XRD 27Al NMR

γ-Al2O3

θ-Al2O3

10 20 30 40 50 60 70

2 theta( o)

i

ii

iii

(a)

Addition of barium oxide stabilizes Addition of barium oxide stabilizes γγ--AlAl22OO33 to to a high temperature phase transition at 1000 a high temperature phase transition at 1000 °°CC

γ-Al2O3

Ba(2%)/γ-Al2O3

Ba(8%)/γ-Al2O3

Could this stabilization Could this stabilization be related to be related to occupation of surface occupation of surface 55--coordinate Al sites?coordinate Al sites?


γγ--AlAl22OO33 Thermal Stability As A Function Of Thermal Stability As A Function Of Crystallite Structure And Lanthanum Oxide ContentCrystallite Structure And Lanthanum Oxide Content

0

20

40

60

80

100

120

140

160

Surf

ace

Are

a, m

2/g

14N4 14HP 14HPL1 14HPL3

550 C, 3 Hr

1000 C, 24 Hr

1100 C, 24 Hr

1200 C, 24 Hr

Sasol Promotional Literature, S.L. Baxter, private communication.

Addition of Addition of lanthanalanthana stabilizes stabilizes γγ--AlAl22OO33 to a to a high temperature phase transition at 1000 high temperature phase transition at 1000 °°CC

-300306090120

Chemical shift(ppm)

i

ii

(b)

27Al NMR

undopedγ-Al2O3

lanthana-dopedγ-Al2O3

Calcined at 500 °C

γ-Al2O3 has a much larger amount of 5-fold Al+3 ions

10 20 30 40 50 60 70

2 theta( o)

(a)

ii

i▼

▼▼ ▼

▼: α-Al2O3Calcined at 1000 °C

XRD

: α-Al2O3


TodayToday’’s Discussions Discussion

• DOE/OFCVT-funded studies of BaO/Al2O3 Lean NOx Trap (LNT) materials• LNT material morphologies – new results

from FTIR, computations, and ultra-high field NMR.

• BaO on CeO2 – performance and sulfur poisoning.

• Other support and alkaline earth oxide storage materials.

We have initiated studies of We have initiated studies of LNTsLNTs that that operate at higher temperatures.operate at higher temperatures.

0

10

20

30

40

50

60

70

150 250 350 450Temperature(oC)

Abs

orpt

ion

Cap

acity

(%)

Al2O3MgAl2O4

0

20

40

60

80

100

0 5 10 15 20 25 30

Time(min)

NO

x le

vel(%

of b

y-pa

ss)

Al2O3MgAl2O4

350oC

We discovered that supporting BaO on MgAl2O4 produced much more active materials at higher temperatures.

Recently published work from Toyota Recently published work from Toyota demonstrate that MgAldemonstrate that MgAl22OO44 is also an is also an

improved support material for Kimproved support material for K--based based LNTsLNTs..

600 600 °°CC

Takahashi, et al., Toyota,Appl. Cat. B 77 (2007) 73-78.

NONO22 TPD indicates enhanced performance TPD indicates enhanced performance may be related to better dispersion of may be related to better dispersion of BaOBaO

on the MgAlon the MgAl22OO44 surface.surface.

100 200 300 400 500 600 7000

500

1000

1500

2000

2500

NO from decomposition NO2 from decomposition

open symbol: NO, NO2 from NO2 TPD

NO

x co

ncen

tratio

n (p

pm)

Temperature (oC) 100 200 300 400 500 600 7000

500

1000

1500

2000

NO from decomposition NO2 from decomposition

open symbol: NO, NO2 from NO2 TPD

NO

x co

ncen

tratio

n (p

pm)

Temperature (oC)

BaO/Al2O3 BaO/MgAl2O4

Transmission electron microscopy (TEM) micrographs also indicate better Pt dispersion on MgAl2O4-supported LNT.

A ceriaA ceria--supported catalyst is much more active per supported catalyst is much more active per amount of amount of BaBa, and much more readily , and much more readily desulfateddesulfated

than an aluminathan an alumina--supported Ptsupported Pt--BaOBaO LNT.LNT.

J.H. Kwak, D.H. Kim, J. Szanyi, and C.H.F. Peden, Appl. Catal. B (2008) in press.

0

50

100

150

200

250

0 500 1000 1500

Time(sec)

NO

x con

cent

ratio

n(pp

m)

—— 250oC

—— 300oC—— 350oC—— 400oC

2%Pt/10%BaO/CeO2

0

10

20

30

40

50

60

70

200 300 400

Reaction temperature(oC)

NO

x con

vers

ion(

%) f

or 3

0 m

in

—○— Pt/BaO/Al2O3

—○— Pt/BaO/CeO2

2%Pt/10%BaO/Al2O3

2%Pt/10%BaO/CeO2

0

10

20

30

40

50

60

70

Fresh

NO

x con

vers

ion(

%) f

or 3

0 m

in

□ Pt/BaO/CeO2

■ Pt/BaO/Al2O3

After sulfation 600oCdesulfation

250 °C

A ceriaA ceria--supported catalyst is much more active per supported catalyst is much more active per amount of amount of BaBa, and much more readily , and much more readily desulfateddesulfated

than an aluminathan an alumina--supported Ptsupported Pt--BaOBaO LNT.LNT.

J.H. Kwak, D.H. Kim, J. Szanyi and C.H.F. Peden, Appl. Catal. B (2008) in press.

155160165170175

0

1000

2000

3000

4000

5000

6000

Binding Energy (eV)

c/s

Sulfate 169.3 eV

(a)

(b)(c)

(d)

155160165170175

0

1000

2000

3000

4000

5000

6000

Binding Energy (eV)

c/s

Sulfate 169.3 eV

155160165170175

0

1000

2000

3000

4000

5000

6000

Binding Energy (eV)

c/s

Sulfate 169.3 eV

(a)

(b)(c)

(d)2%Pt/10%BaO/CeO2

2%Pt/20%BaO/Al2O3

(d) desulfated(b) fresh

(a) fresh(c) desulfated

Summary and Conclusions• The morphology of BaO/Al2O3 LNT materials is remarkably dynamic

during NOx storage and reduction. A “monolayer’” of Ba(NO3)2 forms on the alumina surface in addition to large “bulk” Ba(NO3)2 particles. Recent results provide clear evidence that “monolayer” BaO is chemically distinct from “bulk” BaO; i.e., the surface chemistry of BaO/Al2O3 is quite different than “bulk” BaO.

• These different morphologies display dramatically different behavior with respect to NOx removal temperature, formation of a deactivating high-temperature BaAl2O4 phase, and temperature requirements of desulfation.

• On the basis of a recent CLEERS priorities poll, we have initiated studies of LNT materials that operate at higher temperatures than the baseline Pt/BaO/alumina. Both novel supports (MgAl2O4, CeO2, etc.) and alternative storage materials are included in this new work.

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Fundamental Studies of NOx Adsorber Materials

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