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.)