CLEERS WorkshopSeptember 19, 2018

Pd/Zeolite Passive HC/NOx Adsorbers

Trevor Lardinois1, Jason Bates1, Rajamani Gounder1, Robert B. Pace2,3, Yaying Ji2, Mark Crocker2,3, Jeroen Van der Mynsbrugge4, Alexis Bell4

1Charles D. Davidson School of Chemical Engineering, Purdue University2University of Kentucky Center for Applied Energy Research

3Department of Chemistry, University of Kentucky4Department of Chemical and Biomolecular Engineering, University of California, Berkeley

Objectives:• Obtain fundamental understanding of the chemistry of NOx adsorption and reduction in

Pd/zeolites• Tailor Pd/zeolites with respect to performance and durability for application as HC/NOx

adsorbers

Partners:University of California, Berkeley University of Kentucky Purdue UniversityOak Ridge National LaboratoryFord Motor Co.BASF

Pd2+

PdOH+

Pd/H-BEANO

Ethylene

Toluene

While this technology should be applicable to both gasoline and diesel vehicles, gasoline applications are emphasized in this project, given the importance of stoichiometric engines in the U.S. automotive market

Role of Zeolite Framework

• LT NO storage capacity: BEA > ZSM-5 > CHA• Strength of NO-Pd bond: CHA > ZSM-5 > BEA• HC adsorption capacity: BEA > ZSM-5 > CHA

BEA appears to be the most promising system for this application but is complex (9 discrete Al sites)

CHA represents a simple system, very amenable to study (only 1 framework Al site)

Y. Murata, T. Morita, K. Wada, H. Ohno, SAE Technical Paper 2015-01-1002.H. Chen, J.E. Collier, D. Liu, L. Mantarosie, D. Durán-Martin, V. Novák, R.R. Rajaram, D. Thompsett, Catal. Lett., 2016, 146, 1706-711.A. Vu, J. Luo, J. Li, W.S. Epling, Catal. Lett., 2017, 147, 745-750. Y. Zheng, L. Kovarik, M.H. Engelhard, Y. Wang, Y. Wang, F. Gao, J. Szanyi, J. Phys. Chem C, 2017, 121, 15793-15803Y. Ryou, J. Lee, S.J. Cho, H. Lee, C.H. Kim, D.H. Kim, Appl. Catal. B, 2017, 212, 140-149.J. Lee, Y. Ryou, S.J. cho, H. Lee, C.H. Kim, D.H. Kim, Appl. Catal. B, 2018, 226, 71-82. K. Khivantsev, F. Gao, L. Kovarik, Y. Wang, J. Szanyi, J. Phys. Chem. C, 2018, 122, 10820-10827. O. Mihai, L. Trandafilovic, T. Wentworth, F.F. Torres, L. Olsson, Top. Catal., 2018, https://doi.org/10.1007/s11244-018-1017-z A. Porta, T. Pellegrinelli, L. Castoldi, R. Matarese, S. Morandi, S. Dzwigaj, L. Lietti, Top. Catal., 2018, https://doi.org/10.1007/s11244-018-1045-8

Sample Preparation: NH4+ Back Exchange Parity Plot

4

Procedure for NH4+ back exchange

1. 50 gsoln / gzeo of 1 M NH4NO3

2. 20 hours at 65 oC3. Quantify Pd in supernatant

NH4+ selectively and quantitatively removes isolated Pd ions (measured by H2

TPR) in CHA and BEA zeolites Proportion of isolated Pd ions in BEA is insensitive to the Pd loading in range

studied

Pd-BEA-3 (0.1%)

Pd-CHA-1(1%)

Pd-BEA-1 (1%)

Pd-BEA-4(0.5%)

Sample preparation:1. Incipient wetness impregnation 2. Commercial BEA (Si/Al = 11)3. CHA synthesized in house (Si/Al = 14)

Ex Situ Microscopy on Pd-CHA

Data and analysis courtesy of Dr. K. Unocic at ORNL

DF-STEM

Small ~2 nm particles

Larger Pdparticles (<20nm)

Small ~2 nm particles

BF-STEM EDS

Summary

• Two sizes of Pd particles – small ~2 nm and large (< 20 nm)

• Small particles are uniformly distributed

• Findings consistent with relatively low content of ionic (isolated) palladium in as–prepared sample

5

6

H2 TPR on Pd-Beta and Reversibility

0.0

0.5

1.0

1.5

2.0

2.5

200 300 400 500 600H 2

Cons

umpt

ion

/ μm

ol g

-1s-1

Temperature / K

H-Beta

Pd-Beta-3 (0.1wt%)823 K oxidized

Pd-Beta-4 (0.5wt%)823 K oxidized

Pd-Beta-4 (0.5wt%)823 K re-oxidizedafter 1123 K reduction

x2

x10

x2

0.1% Pd-Beta (823 K oxidized)• H2/Pd (285–350 K) = 0.32 PdO• H2/Pd (350–550 K) = 0.67 Pd2+

0.5% Pd-Beta (823 K oxidized)• H2/Pd (280–360 K) = 0.25 PdO• H2/Pd (360–600 K) = 0.78 Pd2+

0.5% Pd-Beta (823 K oxidized after 1123 K reduction)• H2/Pd (262–360 K) = 1.0 PdO• H2/Pd (360–550 K) = 0.10 Pd2+

Pd cannot be re-dispersed after high T reduction

Pd/BEA Baseline: NOx Uptake Experiments

7

Experimental Details• 121.6 mg of Pd/BEA, 180-250 micron pellets,

pretreated at 500 oC in 10% O2.• 1007.5 sccm, 10 kPa O2, balance N2, bypass 10

minutes, switch to reactor at 100 oC for 10 minutes,40 minute temp ramp to 500 oC

NOx Uptake in first 10 minutesNOx / Pd = 1.15 & 0.92 (duplicate runs)NOx / Pd = 1.26*

*Zheng, et al., J. Phys. C., 2017, 121, 15793 - 15803

0.00

0.50

1.00

1.50

2.00

2.50

18001900200021002200230024002500

Unreduced Catalyst

0.000.100.200.300.400.500.600.700.800.901.00

1900200021002200230024002500

Catalyst reduced at 500 °C0 Minutes

10 minutes

20 minutes

30 minutes

40 minutes

50 minutes

60 minutes

Pd Speciation in Pd/BEA: Effect of Reduction Temperature (CO adsorption at 25 oC after Ar purge at 500 oC; H2 reduction at 100 oC, 300 oC, 500 oC)

2098

2098

2080

211821

402154

2193

2214

2193

1958

1958

• Very little ionic Pd remains after reduction at 500 oC

• Decreased CO adsorption after catalyst reduction at high temperature

Wavenumber (cm-1)

Wavenumber (cm-1)

Abso

rban

ce (a

.u.)

Abso

rban

ce (a

.u.)

8

0

40

80

120

160

0 20 40 60CO adsorption time (min)

Pd-C

O b

ands

pea

k ar

ea (a

.u.) Ar purge

Reduced@100 oC

Reduced@500 oC

Reduced@300 oC

Linear Pd-CO bands between 2230 and 2028 cm-1

NO Adsorption on Pd/BEANO adsorption at 25 oC after Ar purge at 500 oC for 1 h

Abso

rban

ce (a

.u.)

1500 2000 2500 3000 3500 Wavenumber (cm-1)

0.2

60 min

1 min

20 min

10 min

30 min

5 min

26441883,1844

16542466

3670/3645

3778

3740 3600

2166

9

IR band assignments:2400 – 2650 cm-1: Fermi resonance of

OH stretch*2166 cm-1: NO+

1883, 1844 cm-1: Cationic Pd-NO1654 cm-1: NO2 ads. on OH group

100 ppm NO, bal. N2

* M.F. Claydon, N. Sheppard, J. Chem. Soc., Chem. Comm., 1969, 1431-1433.

A.G. Pelmenschikov, J.H.M.C. van Wolput, J. Jaenchen, R.A. van Santen, J. Phys. Chem., 1995, 99, 3612-3617.

Effect of Reduction Temperature on NO AdsorptionNO adsorption at 25 oC after H2 reduction at 300 oC and 500 oC

Wavenumber (cm-1)

Wavenumber (cm-1)

Abso

rban

ce (a

.u.)

Abso

rban

ce (a

.u.)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

15001600170018001900200021002200

Catalyst Reduced at 300 °C

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

15001600170018001900200021002200

Catalyst Reduced at 500 °C

10 Minutes

20 Minutes

30 Minutes

40 Minutes

50 Minutes

60 Minutes

Limited NO adsorption after catalyst reduction at 300 oCbut no adsorption after reduction at 500 oC

NO adsorption sites correspond to Pd ions

10

11

Can Reduced Pd be Re-Oxidized/Re-Dispersed? CO Adsorption on Pd-BEA: Repeated Reduction/Oxidation Cycles

• Sample was reduced at 500 °C under hydrogen for 1 h, oxidized at 500 °C under air for 1 h; CO adsorption was conducted at 25 °C for 1 h in both cases

• Main bands observed at 2098 cm-1 and 1950 cm-1 (Pd0) on reduced catalyst • Intensities of main bands relatively unchanged from one cycle to the next, although

2211 and 2190 cm-1 bands (Pd3+/Pd4+) largely disappear11

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1900200021002200230024002500

Abso

rban

ce (a

.u.)

Wavenumbers (cm-1)

CO Adsorption after Reduction

1st Cycle

2nd Cycle

3rd Cycle

4th Cycle

5th Cycle

2098

2189

0

0.2

0.4

0.6

0.8

1

1.2

1900210023002500

Abso

rban

ce (a

.u.)

Wavenumbers (cm-1)

CO Adsorption after Oxidation

1st Cycle

2nd Cycle

3rd Cycle

4th Cycle

5th Cycle

2146

2115

2098

2212

12

1900 2000 2100 2200 2300 2400 2500

Abso

rban

ce (a

.u.)

Wavenumbers (cm-1)

0.5

2 min

5 min10 min

20 min30 min45 min60 min

2214

2193

2154

2140 21

1820

9820

80

1958

Pd-CHA: Pd speciationComparison of Pd-CO bands for Pd-CHA and Pd-BEA

(pre-treatment in Ar at 500 oC for 1 h)

Pd-CHA-1 Pd-BEA

Compared to Pd-BEA, Pd-CHA shows fewer CO bands. CO distribution over the two samples is also different. Relative to other bands,

the bands at 2211 and 2190 cm-1 (Pd3+/Pd4+) are more significant for Pd-CHA than Pd-BEA.

Wavenumber (cm-1)

1 min2 min

5 min

10 min

20 min

30 min

1600 1700 1800 1900 2000 2100 2200 2300

1.0

Abso

rban

ce (a

.u.)

2211 21

9021

6421

4221

2621

13

1900

13

Pd-CHA: Comparison of Pd-CO Bands after H2 Reduction (CO adsorption at 25 oC for 2 min)

1900 2000 2100 2200 2300 2400 2500

Ar@500 C

H2@100 C

H2@300 C

Abso

rban

ce (a

.u.)

Wavenumber (cm-1)

0.5

2210

2189

2142

2163

2125

2365

2350

Comparison of the Pd-CO bands clearly shows that CO adsorption declines after reduction, with a very significant drop after reduction at 300 oC.

14

Pd-CHA: Reduced at 100 °C

2.0

1000 1500 2000 2500 3000 3500

Abso

rban

ce (a

.u.)

Wavenumber (cm-1)

30 min 30 min

NO + CO co-adsorption @100 ° CNO adsorption @100 °C

In the NO-only case, evolution of OH bands and background are significant;Pd-NO band is almost unobservable

With CO present, Pd-CO and Pd-NO bands are observed simultaneously; Pd-CO region consistent with formation of Pd(CO)(NO)2+ complex

CO promotes NO adsorption

15

1600 1800 2000 2200 2400

0.5

Abso

rban

ce (a

.u.)

Wavenumber (cm-1)

2 min

5 min

10 min

20 min

1 min

1600 1800 2000 2200 2400

0.5

Abso

rban

ce (a

.u.)

Wavenumber (cm-1)

2 min

5 min

10 min

20 min

1 min

Abso

rban

ce (a

.u.)

1600 1800 2000 2200 2400

1.0

Wavenumber (cm-1)

2 min

5 min

10 min

20 min

1 min

Pd-CHA: Can Reduced Pd be Re-Oxidized/Re-Dispersed?

Ar purged at 500 °C H2 reduced at 300 °C Re-oxidized at 500 °C

Compared to Ar purge, Pd-CO bands become weaker after reduction After being re-oxidized at 500 °C, the original Pd species are re-formed to some

extent

16

1600 1800 2000 2200 2400

0.5Ab

sorb

ance

(a.u

.)

Wavenumber (cm-1)

2 min

5 min

10 min

20 min

30 min

1 min

Pd-CHA: Effect of calcination at 750 °C(CO adsorption at 25 °C)

1600 1800 2000 2200 2400

0.5

2 min

5 min

10 min

20 min

30 min

1 min

15 min

Abso

rban

ce (a

.u.)

Wavenumber (cm-1)

Ar purged at 500 °C H2 reduced at 500 °C

• Compared to calcination at 550 °C (as prepared), little change is observed in the distribution of Pd-CO bands after sample re-calcination at 750 °C

• Notably, Pd-CO bands are still significant even after reduction at 500 °C for the sample calcined at 750 °C

Results consistent with calcination at high temperature improving Pd dispersion, such that after reduction Pd sintering is less severe

Y. Ryou, J. Lee, S.J. Cho, H. Lee, C.H. Kim, D.H. Kim, Appl. Catal. B, 2017, 212, 140-149.

Computational Studies: CO and NO Adsorption

∆Gads [kJ/mol] T = 298.15 K, P = 1 atm; quasi-RRHO

LOT: T8 (g): ωB97X-D/Def2-SV(P); T8 (QM/MM): ωB97X-D/Def2-TZVPD // ωB97X-D/Def2-SV(P)

CO interacts more strongly with Pd sites than NO; CO and NO adsorb most strongly on Pd0 + H-Z

H-Z Pd0 + H-Z Pd+Z- PdOH+Z-

COT8 (g) +7 -157 -88T8 (QM/MM) +11 -152 -97NOT8 (g) +11 -89 -79 -54T8 (QM/MM) +13 -87 -63

T696 Cluster Model for CHA Used in QM/MM Calculations

(Avoids boundary effects)

18

19

isolated Al proximate Al pairHZ ZH—HZ

Pd0+HZ ↔ PdH+ Z- ZH—Pd0+HZPd+ Z- ZH—Pd+Z-

Z- Pd2+ Z-

PdO+HZ ↔ PdOH+ Z- ZH—PdO+HZ ↔ ZH—PdOH+ Z-

Possible Pd Exchange Sites at Isolated Al Atoms and Proximate Al Pairs in Zeolites

(Pd oxidation state = 0, 1, 2)

Configurational Search To assess structural and energetic variations between chemically equivalent configurations

NNNN Al pair in 6R

21

• Configurational search revealed significant variations (up to ~50 kJ/mol) in free energy between different configurations of Pdn+

sites. • Characteristics of Pd2+,

Pd+ and Pd0 are relatively independent of their specific environment (isolated Al, NNN or NNNN Al pairs).

Configurational Search: Summary

Lowest free energy configurations for Pdn+

(n=0,1,2) sites at isolated Al and proximate Al pairs in CHA.

Interaction of PdO with CHA

22

For isolated Al and NNN Al pairs, the interaction of PdO with the Brønsted acid protons was also investigated. Regardless of the starting configuration, PdOwas found to protonate, forming PdOH+ Z-.

Lowest free energy configurations for PdO interacting with isolated Al (PdOH+Z-, left) and NNN Al pairs (PdOH+Z- H+Z-, center). Configuration of water molecule adsorbed at Pd2+Z- Z-

site (right).

Summary (1)

• Pd/BEA and Pd/CHA:- NO adsorption sites correspond to Pd ions- reduction with H2 at ≥ 300 oC results in decreased CO and NO adsorption- re-oxidation at 500 oC (after reduction at 500 oC) unable to bring catalyst completely back to original state

• Pd/CHA:- presence of CO promotes NO adsorption via formation of Pd(CO)(NO)2+

complex- calcination at 750 oC appears beneficial w.r.t. preventing Pd sintering and retaining adsorption capacity after high temperature reduction

• For practical applications, protection against rich transients will be necessary

23

Summary (2)

• CO interacts more strongly with Pd sites than NO; CO and NO adsorb most strongly on Pd0 + H-Z

• To assess structural and energetic variations between chemically equivalent configurations, a full configurational search was performed (> 100 different geometries)

• This configurational search revealed significant variations (up to ~50 kJ/mol) in free energy between different configurations of Pdn+ sites.

• Characteristics of Pd2+, Pd+ and Pd0 are relatively independent of their specific environment (isolated Al, NNN or NNNN Al pairs).

• For isolated Al and NNN Al pairs, PdO was found to protonate, forming PdOH+ Z-

24

Acknowledgements

• Ford Motor Co.:Christine LambertAndrew (Bean) GetsoianJoe TheisJason LupescuJustin Ura

• U. California, Berkeley:Martin Head-Gordon

• ORNL: Vitaly PrikhodkoKinga UnocicJae-Soon Choi

• BASF:Saeed Alerasool

• DOE (Vehicle Technologies Office):Aaron YocumKen Howden