New Insights into the Mechanisms and Active Site Requirements of Low Temperature NOx SCR with

Ammonia on Cu-SSZ-13 Zeolites

NO + ½ O2 473 K

NO + NH3 473 K

Cu(II)

Cu(I)

oxidized

reduced

Atish Parekh, John Di Iorio, Ishant Khurana, Arthur Shih, Jonatan Albarracin,

W. Nicholas Delgass, Fabio H. Ribeiro, Jeffrey T. Miller, Rajamani Gounder

School of Chemical Engineering Purdue University West Lafayette, IN

Christopher Paolucci, Hui Li, Sichi Li,

William F. Schneider

Chemical and Biomolecular Engineering University of Notre Dame

South Bend, IN

Aleksey Yezerets (and team)

Cummins, Inc. Columbus, IN

Acknowledgements

Funding: NSF/DOE Advanced Combustion Engines GOALI: 1258715-CBET Collaborations: Washington St., PNNL, ANL

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

Understanding the mechanism and active sites for SCR on Cu-SSZ-13

NO + ½ O2 473 K

NO + NH3 473 K

Cu(II)

Cu(I)

oxidized

reduced

§  Active site(s) §  What are they? §  How many are there? §  Do they change during reaction? §  How to control their formation?

§  Mechanism(s) and Kinetics §  What are the elementary steps? §  What is the role of the oxidant? §  What do we measure when we

measure a rate?

A Catalysis Science Approach

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

Understanding the mechanism and active sites for SCR on Cu-SSZ-13

WHY? §  Active site(s) §  What are they? §  How many are there? §  Do they change during reaction? §  How to control their formation?

§  Mechanism(s) and Kinetics §  What are the elementary steps? §  What is the role of the oxidant? §  What do we measure when we

measure a rate?

A Catalysis Science Approach

… copper and ammonia? … a zeolite?

Quantifying and controlling the Cu active sites in Cu-SSZ-13

J. R. Di Iorio, R. Gounder, Chemistry of Materials, (2016) DOI: 10.1021/acs.chemmater.6b00181

Cu-CHA (Cu-SSZ-13) zeolite: Catalyst for NOx selective catalytic

reduction (SCR) with NH3

•  Design variables: •  Zeolite framework •  Si/Al ratio •  Cu/Al ratio

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

O

Si Si

O

Si Si

O O

Al

Cu2+

OH

PAIRED AL SITES

Cu2+ sites (replace 2 H+)

Si

O

Al

O

Si Al

O

Si

O

Cu2+

ISOLATED AL SITES

[CuOH]+ sites (replace 1 H+)

Copper cation density and structure depend on the framework Al distribution

… how to control this?

Quantifying and controlling the Cu active sites in Cu-SSZ-13

J. R. Di Iorio, R. Gounder, Chemistry of Materials, (2016) DOI: 10.1021/acs.chemmater.6b00181

(TMAda+) N

N +

organic SDA

Na+

inorganic SDA

+

low charge density

high charge density

forms isolated Al (1 Al per CHA cage)

forms paired Al in CHA

N+

Zeolites are made using structure-directing

agents (SDAs)

•  Organic or inorganic compounds

•  Space-fillers or templates that form a certain framework

•  Cationic charges can site framework Al atoms (anionic charges)

O

Si Si

O

Si Si

O O

Al

Cu2+

OH

PAIRED AL SITES

Cu2+ sites (replace 2 H+)

Si

O

Al

O

Si Al

O

Si

O

Cu2+

ISOLATED AL SITES

[CuOH]+ sites (replace 1 H+)

Copper cation density and structure depend on the framework Al distribution

… how to control this?

Quantifying and controlling the Cu active sites in Cu-SSZ-13

J. R. Di Iorio, R. Gounder, Chemistry of Materials, (2016) DOI: 10.1021/acs.chemmater.6b00181

Zeolites of fixed composition of matter … but different arrangements of matter

•  Framework Al pairs counted by Co2+ titration •  Both samples have >95% framework Al and

can exchange monovalent cations (Na+, NH4+)

•  Residual H+ sites counted using methods for selective titration with NH3

•  Sample with only isolated Al sites ( ) exchanges only [CuOH]+ species

•  Sample with 16% paired Al sites ( ) exchanges Cu2+ first, then [CuOH]+ species

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.1 0.2 0.3 0.4 0.5

Res

idua

l H+

(per

tota

l Al)

Exchanged Cu (per total Al)

m = -1

m = -2100% Isolated Al16% Paired Al

Copper Cation Exchange(SSZ-13, Si/Al = 15)0 200 400 600 800 1000

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0 2000 4000 6000 8000 10000

Exch

ange

d C

o2+

(per

tota

l Al)

[Co2+]/[H+]2 in exchange solution at equilibrium

200 400 600

Co2+ Exchange Isotherms(SSZ-13, Si/Al = 15)

(100% isolated Al)

(16% paired Al)

Quantifying and controlling the Cu active sites in Cu-SSZ-13

J. R. Di Iorio, R. Gounder, Chemistry of Materials, (2016) DOI: 10.1021/acs.chemmater.6b00181

Zeolites of fixed composition of matter … but different arrangements of matter

•  CHA synthesized only with low charge density organic cations (TMAda+) contain only isolated Al (Si/Al = 15, 20, 25, 30)

•  CHA synthesized with mixtures of organic and inorganic SDAs can be used to control Al pairing (at fixed Si/Al ratio)

•  High charge density cations (Na+) induce Al pairing

•  Synthetic strategy can be extended to control Al distribution in other zeolite frameworks

Synthesis methods to control Al distribution open new opportunities to structural and catalytic diversity to zeolites of fixed composition

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.00 0.03 0.06 0.09

Satu

ratio

n C

o2+

Exch

ange

(per

tota

l Al)

Na+ Incorporated withinSolid Product (per total Al)

100% Isolated Al6 independent samples

Quantification of Paired Al Sites (SSZ-13, Si/Al = 15)

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

“Cu”

Understanding the mechanism and active sites for SCR on Cu-SSZ-13

Need to combine insights from experiment and theory…

in operando

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

8.96 8.97 8.98 8.99 9.00 9.01 Photon Energy (keV)

Nor

mal

ized

Abs

orpt

ion

(A. U

.)

Cu-SSZ-13 (Cu/Al = 0.16, Si/Al = 4.5), 473 K

NO+NH3+O2

Standard SCR reaction conditions: 473 K, 320 ppm NO, 320 ppm NH3, 10% O2, 5% H2O, balance He

0

20

40

60

80

100

NO+NH3+O2 (Standard SCR)

NO+O2 NH3+O2 NO+NH3 Is

olat

ed C

u(I)

(%)

Gas Composition

0.16

Cu/Altot Cu(I)

NO+O2 NH3+O2

NO+NH3

0.11

Standard SCR on Cu-SSZ-13 involves a Cu2+ / Cu+ redox cycle

F. H. Ribeiro, J. T. Miller (Paolucci et al., Angew. Chem. Int. Ed. 53 (2014) 11828-11833)

NO + ½ O2 473 K

NO + NH3 473 K

Cu(II)

Cu(I)

oxidized

reduced

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

Standard SCR on Cu-SSZ-13 involves a Cu2+ / Cu+ redox cycle

What are the elementary steps and mechanisms for standard SCR?

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

Energies: DFT (HSE-06); Entropies: Potential of Mean Force (Isothermal AIMD + PBE), Mobile Adsorbates

-100 -80 -60 -40 -20

0 20 40 60 80

100

NH3 H2O O2 NO N2

ΔG

ads (

473

K) (

kJ/m

ol)

Adsorbate

Cu2+

Reduction via NO-assisted N-H dissociation of NH3 at Cu2+ sites

+ NO (ΔE = -9)

+ O2 (ΔE = 87) (ΔE = 119)

ΔE in kJ/mol

NO2

(OH)2

Facile oxidation

by NO2

Selective nature of reduction half-cycle: NO-assisted NH3 dissociation

W. F. Schneider (Paolucci et al., Angew. Chem. Int. Ed. 53

(2014) 11828-11833)

Cu cations are solvated by ammonia during low temperature SCR

W. F. Schneider

Thermodynamically most stable configurations of Cu cations at low temperature SCR conditions

(theory “in operando”)

Isolated Al

Paired Al

Cu cations are solvated by ammonia during low temperature SCR

EXAFS does not show Cu-zeolite coordination during low temperature SCR conditions

(experiment in operando)

Isolated Al

Paired Al

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

NO + ½ O2 473 K

NO + NH3 473 K

Cu(II)

Cu(I)

oxidized

reduced

Does Cu2+ or [CuOH]+ matter? Does the zeolite matter?

Cu cations are solvated by ammonia during low temperature SCR

1

2

3

4

5

Stan

dard

SC

R R

ate

(473

K)

(10-

6 mol

NO

g-1

s-1

)

Standard SCR reaction conditions: 473 K, 320 ppm NO, 320 ppm NH3, 8% CO2, 10% O2, 6% H2O, balance He

CHA (5)

0

Total Cu content (10-3 mol Cu g-1) 0 0.5 1 1.5

What do we measure when we measure a rate?

CHA (15)

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

MFI

BEA

(Cu)1 (O2)0.3 (NO)0.7

(NH3)0

Cu reduction rates do not depend on zeolite topology or composition

Reduction rate-limiting

1

2

3

4

5

Stan

dard

SC

R R

ate

(473

K)

(10-

6 mol

NO

g-1

s-1

)

Standard SCR reaction conditions: 473 K, 320 ppm NO, 320 ppm NH3, 8% CO2, 10% O2, 6% H2O, balance He

CHA (5)

0

Total Cu content (10-3 mol Cu g-1) 0 0.5 1 1.5

What do we measure when we measure a rate?

CHA (15)

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

MFI

BEA

Cu re-oxidation rates depend on zeolite topology or composition

Oxidation rate-limiting

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

NO + ½ O2 473 K

NO + NH3 473 K

Cu(II)

Cu(I)

oxidized

reduced

§  Synthesis of model samples §  DFT/ Molecular dynamics §  Operando XAS §  Kinetics §  Spectroscopy

Different mechanisms and active site requirements for NO oxidation

How do Cu(I) diammine complexes in reduced Cu-SSZ-13 re-oxidize?

Different mechanisms and active site requirements for NO oxidation

4 NH3 + 4 NO + O2 4 N2 + 6 H2O 473 K

NO2 473 K

NO + NH3 473 K

Cu(II)

Cu(I)

1 Cu 2 NO + O2

473 K

1 Cu 2 Cu

Fast SCR is “faster” than standard SCR for two reasons…

… and the site requirements (for using NO2 vs. O2 as the oxidant) are fundamentally different …

A) Cu(O2)(NH

3)2 B) Cu(NO

2)(NH

3)2

+1.51 / +1.47 +1.80 / +2.27

Conclusions and Acknowledgements

§  Financial Support

§  NSF/DOE Advanced Combustion Engines GOALI: 1258715-CBET

NSF/DOE GOALI TEAM Cummins – June 2013

§  Purdue §  Shane Bates §  Vince Kispersky §  Anuj Verma §  John Harwood

§  PNNL §  Chuck Peden §  Feng Gao §  Janos Szanyi

§  Washington St. §  Jean-Sabin McEwen §  Kathy Helling §  Renqin Zhang

§  Cummins §  Aleskey Yezerets §  … and the team

WHY?

… copper and ammonia? … a zeolite?

Impact of automotive emissions catalysis (SCR)

research on catalysis science

§  Homogeneous catalysts in operando supported ionically

§  Synthetic methods to control the Al distribution in zeolites