CLEERS Workshop #11 May 13, 2008 Fundamental Studies of NOx Adsorber Materials Do Heui Kim, Ja Hun Kwak, Chuck Peden , Janos Szanyi Institute for Interfacial Catalysis Pacific Northwest National Laboratory
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
Szanyi, Kwak, Hanson, Wang, Szailer, Peden,J. Phys. Chem. B 109 (2005) 7339-7344.
-150-100-50050100150200
Chemcial shift(ppm)
Aloct
Altet
**
Aloct
Altet
Alpen??
-20020406080Chemical shift (ppm)
-20020406080Chemical shift (ppm)
Altet
Aloct
-20020406080Chemical shift (ppm)
-20020406080Chemical shift (ppm)
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
2 weight %BaO/γ-Al2O3
8 weight %BaO/γ-Al2O3
8 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
Szanyi, Kwak, Hanson, Wang, Szailer, Peden,J. Phys. Chem. B 109 (2005) 7339-7344.
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?
JH Kwak, JZ Hu, AC Lukaski, DH Kim, J Szanyi, CHF Peden, J. Phys. Chem. C (2008) in press.
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
JH Kwak, JZ Hu, AC Lukaski, DH Kim, J Szanyi, CHF Peden, J. Phys. Chem. C (2008) in press.
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.