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Low-Temperature Passive NOx Adsorbers: Structure, Performance and Adsorption
Chemistry of Pd-Zeolites
Yang Zheng, Konstantin Khivantsev, Libor Kovarik, Mark Engelhard, Yilin Wang, Feng Gao, János Szanyi
Institute for Integrated Catalysis, Pacific Northwest National Laboratory Richland, WA99352 (USA)
Define the Problem
2
State of the art Cu zeolite SCR catalyst temperature
window Impr
oved
SC
R C
atal
yst
and
NH
3de
liver
y
Pass
ive
NO
x St
orag
e
Material Requirements
3
Key attributes of an effective passive NOxadsorber material:
qHigh NOx storage capacity up to ~150 °CqEfficient capture of NOx from a complex exhaust gas
mixtureqRelease of NOx in the temperature range of SCR catalyst
operationqLong-term hydrothermal stabilityqResistance to poisoning
PNA materials in this study
4
Catalyst Si/Al ICPPd (wt.%)
Pd/Al Na+
Exchangeable Pd(%)
Pd/Beta_0.50% 12.5 0.50 0.04 75.0
Pd/Beta_0.92% 12.5 0.92 0.07 11.2
Pd/ZSM-5_0.48% 15 0.48 0.04 73.8
Pd/ZSM-5_0.62% 15 0.62 0.06 65.3
Pd/SSZ-13_0.50% 12.5 0.50 0.04 1.4
Pd/SSZ-13_0.88% 12.5 0.88 0.06 9.1
Betazeolite(largepore)
ZSM-5zeolite(mediumpore)
SSZ-13zeolite(smallpore)
Experiments
5
Ø NOx adsorption/release:Ø gas composition (effects of H2O and CO)Ø catalyst preparation methodØ Si/Al ratio Ø hydrothermal agingØ active phase dispersion
Ø Microscopy: TEM (calcination/reduction/re-oxidation)
Ø SpectroscopyØ XPS (Pd) (oxidation/reduction with NO)Ø IR of adsorbed probe molecules (CO, NO)
NOx uptake/release: the effects of H2O and CO
6
High initial NOx storage efficiency over all three Pd-zeolites
Most of the NOx stored is released at T<200 °C
NOx/Pd ratio during NOx trapping at 100 °C for 10 min
Dry 200 ppm NOx feed
Pd/Beta_0.92% 1.26
Pd/ZSM-5_0.62% 0.88
Pd/SSZ-13_0.88% 1.08
0 10 20 30 40 50 600
50
100
150
200
250
300
350
100
200
300
400
500
Pd/Beta_0.92% Pd/ZSM-5_0.62% Pd/SSZ-13_0.88%
NO
x O
utle
t Con
cent
ratio
n (p
pm)
Time (min)Te
mpe
ratu
re (°
C)NOx
storage10 min
bypass10 min NOx
releaseat 10 C/min
no H2O; no CO
0 10 20 30 40 50 600
50
100
150
200
250
100
200
300
400
500
Pd/Beta_0.92% Pd/ZSM-5_0.62% Pd/SSZ-13_0.88%
NO
x O
utle
t Con
cent
ratio
n (p
pm)
Time (min)
Temperature
Tem
pera
ture
(°C
)
NOx/Pd ratio during NOx trapping at 100 °C for 10 min
200 ppm NOx +2.5% H2O
feed
Pd/Beta_0.92% 0.28
Pd/ZSM-5_0.62% 0.50
Pd/SSZ-13_0.88% 0.14
NOx storage efficiency significantly reduced by the presence of H2O in feed
Most of the NOx stored is released at T<200 °C
H2O; no CO
7
NOx uptake/release: both H2O and CO
0 10 20 30 40 50 600
50
100
150
200
250
100
200
300
400
500
Pd/Beta_0.92% Pd/ZSM-5_0.62% Pd/SSZ-13_0.88%
NO
x Out
let C
once
ntra
tion
(ppm
)
Time (min)
Temperature
Tem
pera
ture
(°C
)
NOx/Pd ratio during NOx trapping at 100 °C for 10
min
200 ppm NOx+200 ppm CO+2.5% H2O
Pd/Beta_0.92% 0.51
Pd/ZSM-5_0.62% 0.70
Pd/SSZ-13_0.88% 0.44
Ø In the presence of CO Pdions are reduced, and NOx uptake increases.
Ø Zeolite structure affects the NOx release temperature (governed by NOx diffusion in zeolite pores and channels?)
NOx adsorption at 100 °C followed by TPD (10 °C/min up to 500 °C). Feed gas mixture: 200 ppm NOx (185 ppm NO and 15 ppm NO2)+14% O2 + ~2.5% H2O + 200 ppm CO and balanced with N2 at a flow rate of 300 sccm
Both H2O and CO
Y, Zheng et al., J. Phys. Chem. C 2017, 121, 15793−15803
NO, NO2 and CO: both H2O and CO
8
100
200
300
400
500
0 10 20 30 40 50 600
50
100
150
200
250 Temperature
Tem
pera
ture
(°C
)
Pd/Beta_0.92% Pd/ZSM-5_0.62% Pd/SSZ-13_0.88%N
O O
utle
t Con
cent
ratio
n (p
pm)
Time (min)
100
200
300
400
500
0 10 20 30 40 50 600
50
100
150
200
250 Temperature
Tem
pera
ture
(°C
)
Pd/Beta_0.92% Pd/ZSM-5_0.62% Pd/SSZ-13_0.88%
CO
Out
let C
once
ntra
tion
(ppm
)
Time (min)
100
200
300
400
500
0 10 20 30 40 50 600
20
40
60
80 Temperature
Tem
pera
ture
(°C
)
Pd/Beta_0.92% Pd/ZSM-5_0.62% Pd/SSZ-13_0.88%
NO
2 Out
let C
once
ntra
tion
(ppm
)
Time (min)
NO
NO2
CO
Ø NO release profile is similar to that of NOx.
Ø NO2 is consumed by reaction with CO at T< 250 °C
Ø CO is consumed in PdOx and NO2 reduction during NOxuptake, and in CO oxidation at T>200 °C.
Theeffectofpreparationmethod
9
0 20 40 600
25
50
75
100
125
150
175
200
225
250
275
NO
x con
cent
ratio
n/ p
pm
Time/ min
IE; 550 C
NH4-form; AC; 650 C
NH4-form; IWI; 650 C
H-form; IWI; 550 C
~1wt% Pd/SSZ-13 (Si/Al=12)
Ø Lowest NOx storage was observed for the aqueous ion exchanged sample
Ø Autoclaved NH4/SSZ-13 in the presence of Pd showed better NOx storage than the IE sample.
Ø Best NOx storage performance was measured over the incipient wetness impregnated NH4/SSZ-13
Ø Very high Pd dispersion (i.e., Pd only in cationic position) may not be required for high NOx storage performance.
100 °C
500 °C
TheeffectofSi/AlratioinSSZ-13
10
0 20 40 60 800
50
100
150
200
250
NO
x con
cent
ratio
n/ p
pm
Time/ min
Si/Al 6 12 30
1wt% Pd/SSZ-13(from NH4-form by IWI)
Ø Both adsorption and desorption profiles are very similar for the same zeolite structure but different Si/Al ratio
Ø Pd/SSZ-13 with the lowest Si/Al ratio shows best NOxstorage performance (the most adsorption sites)
Si/Al6 12 30
NO/Pd: 1.25 1.14 0.4
100 °C
500 °C
TheeffectofPdOx phasedispersion
11
0 10 20 30 40 50 60 700
50
100
150
200
250
NO
x con
cent
ratio
n/ p
pm
Time/ min/
1 wt% Pd/(NH4)Y Si/Al 6 1 wt% Pd/(NH4)SSZ13 Si/Al 6
1% Pd/SSZ-13 (Si/Al=6) NO/Pd = 1.541% Pd/HY (Si/Al=6) NO/Pd 0.5
Ø Total NOx storage for 1% Pd/SSZ-13 (Si/Al=6) is ~3 times higher than that for 1% Pd/HY (Si/Al=6)
Ø Pd in HY (Si/Al=6) with large pores can accommodate most Pd in the micropore(Konigsberger)
Ø High Pd dispersion does not necessarily equate with high NOx storage capacity. It also strongly depends on the framework type.
100 °C
500 °C
Redispersion of PdOx phase upon HTA
12
EDS mapping analysis for Pd and Z-contrast STEM image of (a) fresh and (b) HTA Pd(1)/SSZ-13 IWI.
Schematic model to represent the change in the nature of Pd before (a) and after (b)
HTA treatment.
Y. Ryou et al., Applied Catalysis B: Environmental 212 (2017) 140–149
TEM
13
Calcined Reduced Re-oxidized
Pd/B
eta
Pd/Z
SM-5
Pd/S
SZ-1
3
Calcined:Pd in- and outside of the
zeolite framework
Reduced:Large Pd particles form
outside the zeolite, and small cluster inside the framework
Re-oxidized: Most of the Pd migrates back inside the zeolite
framework
XPS of Pd
14
After oxidation: After exposure to NO
After oxidation Pd is mostly in 4+ and 2+
oxidation states.
Much more Pd is in 4+ oxidation state in
Pd/SSZ-13 than in Pd/Beta (more Pd is outside the zeolite
framework)
After NO exposure most of the Pd is
present in 2+ oxidation state in both Pd/zeolites
Pd/Beta Pd/Beta
Pd/SSZ-13 Pd/SSZ-13
500 °C; 10% O2/He; 1 hr 150 °C; 2000 ppm NO/He; 1 hr
Y, Zheng et al., J. Phys. Chem. C 2017, 121, 15793−15803
DRIFTS of adsorbed CO on Pd/SSZ-13
15
2300 2200 2100 2000 1900 1800 1700
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
300 °C
250 °C
200 °C
150 °C
100 °C
50 °C
20 °C in He
2212
2190
2185
2163
2142
2126
2133
2112
2087
Abso
rban
ce
Wavenumber (cm-1)
189920 °C in CO
Pd/SSZ-13_0.88%*Pd2+-(CO)2
2215-2190 cm-1
Pd2+-CO2190-2135 cm-1
Pd+-CO2140-2120 cm-1
Pd0-CO (atop)2115-2000 cm-1
Pd0-CO (bridge)2000-1900 cm-1
Pd0-CO (hollow)<1900 cm-1
FTIR of adsorbed CO at 295 K:annealing vs. reduction in CO
16
2400 2300 2200 2100 2000 1900 1800 1700
0.000
0.005
0.010
0.015
0.020
0.025
0.030
1795
1922
1978
2115
21262130
21442191
199422
32-2
228
Abs
orba
nce/
a.u.
Wavenumbers/cm-1
2375
-237
0
2210 CO/1% Pd/SSZ-13 at 295 K
2400 2300 2200 2100 2000 1900 1800 17000.000
0.005
0.010
0.015
0.020
0.025
1796
1977
1980
-199
6
2095
21252130
2144
219022
10
Abs
orba
nce/
a.u.
Wavenumbers/cm-1
2235
-222
8
CO/1% Pd/SSZ-13 at 295 K(CO reduced; annealed)
Ø CO2 forms upon CO exposure at 296 K: CO is reducing PdOx speciesØ Large decrease in peaks associated with Pd2+(CO)2
FTIR of adsorbed NO on Pd,H/ZSM-5
17
Standard (S)—773 K in 10% O2, 2 h; 773 K in He, 1 h; cool to RT in HeReductive (R)—773 K in 4.2% CO, 2 h; 773 K in He for 1 h; cool to RT in He
NO+
Pd2+….NO Pd0….NO
JOURNAL OF CATALYSIS 172, 453–462 (1997)
After oxidation (S) After reduction (R)
FTIR: NO on 1 and 3 wt% Pd/SSZ-13 at 353 K
2200 2000 1800 1600
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
1750
1862
1880
1920
1811-1804
Abs
orba
nce/
a.u.
Wavenumbers/cm-1
2168
-217
8NO/1% Pd/SSZ-13 at 295 K
2400 2200 2000 1800 16000.00
0.01
0.02
0.03
0.04
0.05
0.06
Abs
orba
nce/
a.u.
Wavenumbers/cm-1
NO+O2/1% Pd/SSZ-13 at 295 K
1% Pd/SSZ-13
+O2
18
2200 2000 1800 1600 14000.00
0.01
0.02
0.03
0.04
0.05
1539
1660
-166
3
1859
-186
419
19
1811-1804
Abs
orba
nce/
a.u.
Wavenumbers/cm-1
2167
-217
9
NO/3% Pd/SSZ-13 at 295 K
2300 2200 2100 2000 1900 1800 1700 1600 15000.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Abs
orba
nce/
a.u.
Wavenumbers/cm-1
NO+O2/3% Pd/SSZ-13 at 295 K
3% Pd/SSZ-13
+O2
FTIR: NO+O2 at 295 K
19
2400 2200 2000 1800 1600 1400
0.00
0.05
0.10
0.15
156516
25164018
1518
05
1864
1980
~2195
Abs
orba
nce/
a.u.
Wavenumbers/cm-1
NO+O2/5%-Pd/SSZ-13 (PH=3) T = 295 K
2160-2167
2400 2200 2000 1800 1600 14000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
1575
1640
1650
1674
1964
Abs
orba
nce/
a.u.
Wavenumbers/cm-1
NO+O2/5%-Pd/Beta (PH=3)
T = 295 K2134
2400 2200 2000 1800 1600 14000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
157516
2016
4716
70
1838
1880
1958
NO+O2/5%-Pd/ZSM-5 (PH=3)
T = 295 K
Abs
orba
nce/
a.u
Wavenumbers/cm-1
21355%Pd/SSZ-13 5%Pd/Beta 5%Pd/ZSM-5
Ø Similar IR bands after NO adsorption (NO on Pd2+ sites)Ø After O2 addition NO2 forms rapidly and disproportionates to NO+ and NO3
-
Ø On Pd/SSZ-13 and Pd/ZSM-5 N2O3 formed initially converts to N2O4 fastØ On Pd/Beta large amount of stable N2O3 forms
Conclusions
20
Ø Pd-loaded zeolites are efficient low temperature NOx adsorber materials.
Ø NOx adsorption/release strongly depends on theØ Zeolite structure and compositionØ Metal loadingØ Gas compositionØ Adsorber treatment (fresh vs. HTA)Ø Adsorber preparation method (IE, AC, IWI)
Ø Several Pd-containing species are present in/on Pd/zeolites
Ø In oxidized Pd-zeolites large fraction of Pd is present in high (+4 and +3)oxidation states
Ø NO and CO are able to reduce most of the Pd(IV) to Pd(II) (PdOx, PdxOy and Pd(OH)x)
Ø Pd is highly mobile in/on zeolite structuresØ Hydrothermal aging reduces NOx adsorption efficiencyØ The presence of CO seems to stabilize NO adsorption on Pd2+ ions
Acknowledgements
21
ØDOEOfficeofEnergyEfficiencyandRenewableEnergy/VehicleTechnologiesOfficeforfinancialsupportofthisproject.
ØEMSL,aDOEuserfacility,locatedatPNNL.
ØCRADApartners(Cummins,Inc.andJohnsonMatthey,Inc.)