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TRANSCRIPT
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