Sites and Mechanism in Selective NOx Reduction · How Many of Which Cu(II) Site? W. F. Schneider CLEERS -4 Oct 2017 the 6-membered ring was counted as a surrogate for one dehy-drated

Sites and Mechanism in Selective NOx Reduction

William F. Schneider

Dept. of Chemical and Biomolecular EngineeringDept. of Chemistry and Biochemistry

[email protected]/~wschnei1

W. F. Schneider CLEERS - 4 Oct 2017

CLEERS WorkshopAnn Arbor, Michigan

October 4, 2017

The NSF/DoE NOx SCR Team


NDChris PaolucciAtun AnggaraHui LiSichi Li

PurdueFabio RibeiroRaj GounderJeff MillerNic Delgass

WSUJ-S McEwen

PNNLChuck PedenJanos SzanyiFeng Gao

CumminsAlex YezeretsNeil Currier

Objectives of NSF/DoE NOx SCR Team


“This project seeks to build a microscopically detailed model of catalyst performance under all operating conditions and throughout the life cycleaiming to optimize engine efficiency within emission constraints and to circumvent catalyst deactivation.”

Precise Synthesis

OperandoSpectro-scopies

Quantita-tive

Kinetics

Atomistic to Micro-

kinetic Modeling

NOx Selective Catalytic Reduction


Kwak, J. H., et al. J. Catal. 2012; 287, 203-209.

4 NH3 + 4 NO + O2 → 4 N2 + 6 H2O“Standard” SCR

Cu-SSZ-13 Catalyst

NH3 + O2 → products

Rates vs. Cu Loading @ Si:Al 5:1

• Standard SCR activity scales with number of isolated Cu2+

• Dry NO oxidation activity scales with Cu oxo species at higher Cu loadings


0.23 0.5

0.23 + 0.27

Bates et al. J. Catal. 2014, 312, 87–97.

Si

Cu(II)-SSZ-13 Composition Space


Si/Al

Cu/

Al

O

O

OO

–

AlO

O

OO

Cu Exchange in Zeolite Frameworks

• Cu(II) exhibits a strong preference for binding in 6-fold rings near 2 Al

• Agreement with DFT, UV-vis, EXAFS, and XRD, acid titrations


NH4-SSZ-13 (Si:Al 4.5 – 25)+

Cu(NO3)2 (aq)(Cu2+)

Cu2+

Z2Cu

Cu Exchange in Zeolite Frameworks

• Isolated Al bind Cu(II) as ZCu(II)OH• Agreement from DFT, UV-vis, EXAFS, and XRD, acid

titrations


NH4-SSZ-13 (Si:Al 4.5 – 25)+

Cu(NO3)2 (aq)(Cu2+)

ZCuOH

Relative Site Energies

• Does exchange favor ZCuOH or Z2Cu?

• Construct connecting reaction

• Evaluate in reference models


ZH

Z2CuH2O

0 eV

Paolucci et al. J. Am. Chem. Soc. 2016, 138, 6028

Relative Site Energies


ZCuOH

Z2H2

+0.6 eV

• Does exchange favor ZCuOH or Z2Cu?

• Construct connecting reaction

• Evaluate in reference models


In Situ Thermodynamic Screening

• Enumerate and rank all possible species by relative free energy


Relative Site Free Energies


DryHydratedPaolucci et al. J. Am. Chem. Soc. 2016, 138, 6028

How Many of Which Cu(II) Site?


the 6-membered ring was counted as a surrogate for one dehy-drated isolated Cu2+ ion. Fig. 9 shows the maximum amount ofdehydrated isolated Cu2+ ion in the 6-membered ring, per Alf, cal-culated for various silicon to framework aluminum (Si/Alf) atomicratios.

We previously showed that Cu+ charge compensated by a singleAl T-site (ZCu) preferred 6-membered ring sites in SSZ-13 [6]. Thefirst column of Table 3 reports the relative periodic GGA energies ofdehydrated, and isolated Cu2+ (Z2Cu) ions in the various ring loca-tions illustrated in Fig. 8. Energies are referenced to a dehydratedisolated Cu2+ in an 8-membered cage with 4NN Al and were deter-mined either in three 1 ! 1 ! 1 supercell models (Fig. 1a) that con-tained 2NN, 3NN, or 4NN Al sites in an 8-membered ring or in adoubled 2 ! 1 ! 1 supercell model that simultaneously presents4-, 6-, and 8-membered ring sites (Fig. S1c and d). Results fromthese two structural models differed by less than 0.1 eV, as shown

in Table S3. Geometry optimizations (Fig. S7) showed that a dehy-drated isolated Cu2+ ion maintained fourfold oxygen coordinationin the smaller 4- and 6-membered ring sites but attain onlythreefold coordination in the larger 2NN and 4NN 8-memberedring sites and twofold coordinated in 3NN 8-membered ring sites.Based on the relative energies in Table 3, a dehydrated isolatedCu2+ ion has a strong energetic preference for the 6-membered ringsites over 4 and 8-membered ring sites in SSZ-13.

Motivated by the evidence for Cu–Oy–Cu (y = 1, 2) clusters inZSM-5 [24,62–64], MOR [24], BEA [65], and Y [66] zeolites, we nextcalculated the structures and relative stabilities of these ZCuOyCuZaggregates within our SSZ-13 supercell models. Both y = 1 and y = 2clusters exhibited singlet and triplet spin states whose relative sta-bility depended on coordination environment [20,21]. In all cases,we optimized both states and report the lower energy results.Fig. 10a shows triplet ZCuOCuZ optimized in the 4NN 8-memberedring site. Cu ions adopted a twofold coordination to the zeolitelattice and were nearly symmetrically bridged by the central O atCu–O distance of about 1.75 Å. Figs. S8 and S9 compare optimizedCu–O–Cu structures computed at the other Alf pair sites. The Cuions in the larger 6- and 8-membered rings retained twofold coor-dination to the zeolite lattice by adjusting the Cu–O–Cu angle from107! to 132!. The ZCuOCuZ configuration was too large to beaccommodated entirely within a 4-membered ring; the optimizedcluster relaxed to retain twofold coordination involving O of con-nected rings. Table 3 compares relative energies of ZCuOCuZ refer-enced to the triplet 4NN 8-membered ring site. The Cu–O–Cudimers preferred coordination in an 8-membered ring over a 4-or 6-membered ring by 0.44 and 0.68 eV, respectively (Table 3).Further, the Cu dimer was more easily accommodated in 4NN Alf

sites than 3NN or 2NN in the 8-membered ring by 0.46 and0.76 eV, respectively.

CuO2Cu2+ is known to exhibit a coordination chemistry in whichthe O2 can end-on bridge the two Cu (l-1,2-O2), side-on chelate thetwo (l-g2:g2-O2), or dissociate into two oxo bridges (l-O,l-O)[21,22,25–27]. We found examples of all of these structures duringoptimizations of the various Al pair models (Figs. S10 and S11).Nearer Alf sites preferred the l-1,2-O2 structures while farthersites, like the 8-membered ring 4NN site shown in Fig. 10b, pre-ferred singlet l-g2:g2-O2. Cu again in general preferred to associ-ate with two lattice O near the Al sites in addition to the bridgingO2. Table 3 compares the relative energies of these Cu–O2–Cu2+ di-mers vs. Alf siting; as with the Cu–O–Cu2+ dimers, the Cu–O2–Cu2+

dimers preferred 8-membered ring sites over 4- and 6-memberedrings by 0.51 and 0.65 eV, respectively, and preferred the 4NN Alf

sites in 8-membered rings by 0.42–0.88 eV, respectively.We can use these results in combination with the redox reac-

tion steps of Eqs. (1) and (2) to compare the energies of O2 adsorp-tion and successive NO oxidation on ZCu, Z2Cu in a 6-membered

Fig. 8. Radial distribution of Alf–Alf pair sites vs. Alf–Alf separation in SSZ-13 atSi/Alf = 5/1. Al are distributed randomly obeying Löwenstein’s rule and densitiesaccumulated in 0.05 Å bins.

Fig. 9. The maximum isolated Cu2+ ion to Alf atomic ratio that can be accommo-dated in the 6-membered ring of SSZ-13, as a function of Si/Alf atomic ratio.

Table 3GGA-computed relative energies (eV) of Cu ions and dimers vs. Alf–Alf pair placement.

IsolatedCu2+

energy (eV)CuOCu2+

energy (eV)CuO2Cu2+

energy (eV)

4-Membered ring – 2NN –0.38 0.68 0.656-Membered ring – 3NN –1.51 0.44 0.518-Membered ring – 4NN 0 0 08-Membered ring – 3NN 0.23 0.46 0.428-Membered ring – 2NN –0.25 0.76 0.88

Fig. 10. The optimized periodic structure of atomic and diatomic oxygen adsorbedin the 8-membered ring of SSZ-13 of (a) ZCuOCuZ with fourth-nearest-neighbor(4NN) Alf pairs and (b) l-g2-g2-Cu2O2 with 4NN Alf pairs. Cu–O distances indicatedin Å. Yellow red, gray, and green spheres are Si, O, Cu, and Al, respectively. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

186 A.A. Verma et al. / Journal of Catalysis 312 (2014) 179–190

Verma et al. J. Catal., 2014, 312, 179–190

Large SSZ-13 supercell

Set Si:Al ratioSwap Al locationsObey Löwenstein’s rule

Pair site histogram

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.005 10 15 20 25 30 35 40 45

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Cu

: A

l

Si : Al

CuO

H : C

u total

Synthesized

Cu-SSZ-13 Composition Phase Diagram


Z2Cu

ZCuOH

Exchanges two protons4-fold coodinationNo autoreduction

Exchanges one proton3-fold coordinationAutoreduction


Ex situ Characterization of Cu Exchange


M:Al0 0.1 0.2 0.3 0.4 0.5

H:H

(par

ent)

0

0.2

0.4

0.6

0.8

1

1.2Monte CarloCuII ExchangeCoII Exchange

Si:Al 4.5 15 25

ν cm-1

3500355036003650370037503800

Inte

nsity

0.0

0.2

0.4

0.6

0.8

1.0

1.2Cu:Al 0.00Cu:Al 0.12Cu:Al 0.21Cu:Al 0.37Cu:Al 0.44

[CuOH]+

H+

Si:Al 15

Residual protons after exchange CuO—H vibrational stretch


Ex Situ Phase Diagram

[ZCuI]

[ZCuIIOH][ZCuIIO2]

[ZCuII(OH)(H2O)][ZCuIH2O]

Z[CuII(OH)(H2O)3]

2

1

3

Z[CuII(OH)(H2O)3] [ZCuIIOH] [ZCuI]1 2 3

2

1

3

[ZH]/ [ZCuI]

[Z2CuII]

[Z2CuII]O2

[Z2CuIIH2O]

Z2[CuII(H2O)4]

2 3 [Z2CuII]1 Z2[CuII(H2O)4]

1

2 O2

Atmosphere

He 3


ZCuOH Z2Cu 5% H2O


SCR Gas Binding Energies


Cu(II) Sites

ZCuOH

Z2Cu

Cu(I) SitesZCu

ZCu/ZNH4

HSE06+vdW

Z2Cu/NH3/H2O/O2 phase diagram

• NH3 covers all sites below 400˚C

• Oxidized and reduced Cu states close in free energy at 200˚C



Operando X-ray Spectroscopy vs Theory



SCR: Cu Redox on Solvated Cu Ions


CuIIOH(NH3)3 CuI (NH3)2

SCR?

CuII + NH3 + NO à CuI + N2 + H2O


Cu2+

Cu+/H+

Paolucci et al. Angew. Chemie 2014

SCR: Cu Redox on Solvated Cu Ions


ZCuIIOH(NH3)3 ZCuI (NH3)2

Reduction half-cycle

NO + NH3 N2 + x H2O

?O2 + NO + NH3

Ishant Khurana poster

Operando SCR Rate and Cu Oxidation State


Paolucci et al., Science 2017

rate~ k (Cu)2

TOF and Cu oxidation state sensitive to Cu density

Transient Cu(I) ⟶ Cu(II)


rate ⇠ k⇣[CuI]� [CuI]1

⌘2

• Approximately second order reoxidation

• k increases systematically with initial Cu density

• Recalcitrant CuI fraction decreases systematically with initial Cu density

• All at densities << 1 Cu/cage

CuI + O2 ��! CuII

Not single siteNot homogeneous Not mean-field kineticsPaolucci et al., Science 2017

Intercage Cu Transport



Intercage Cu Reactions


have the same shape as those for the bent structures in the single-T-site model.To examine the relationship between the ZCuO2CuZ isomers

identified here and previously,23 we constructed a potentialenergy surface within the single-T-site model (Figure 9) as afunction of Cu-Cu separation.23 All other parameters wererelaxed with the exception of the relative T-site orientation,which is fixed to a parallel arrangement (3), as suggested bythe larger models and by the preference for square-planarcoordination in isomers 6-8. For isomers 5 and 9 the relation-ship between the T-site orientations is found to be less important,and at Cu separations greater than 4.0 Å there is very littlebarrier to T-site rotation. C2 symmetry was preserved along theCu-Cu coordinate, and along this path a singlet (1A) and twotriplet (a3B and b3B) surfaces were followed. All five isomersdiscussed above (5-9) are captured on these potential energysurfaces.At Cu-Cu separations greater than 3.9 Å, the 3B surface is

lowest in energy. At approximately 4.2 Å the surface flattens,and if the Cu-O-O-Cu core is constrained to be planar, as inref 23, isomer 5 is obtained. Without this constraint, the Cu-O2-Cu angle (using the O2 center-of-mass) closes, and the 3Bsurface drops in energy as the Cu ions are brought together. Aminimum corresponding to isomer 8 is obtained at a Cu-Cudistance of 2.81 Å. Along this path the bridging O2 rotates withrespect to the Cu-Cu axis and the O-O separation increasesto 1.42 Å. The minimum is consistent with the bent isomer 8structures found with the multi-T-site models, although thestructure within the single-T-site model differs in two importantrespects: first, the Cu-O2-Cu angle is larger in the single-T-site model, and second, each Cu ion in the smaller model relaxes

to a perfectly square-planar coordination. Both structural dif-ferences are attributable to the constraints imposed by the multi-T-site models, and both contribute to destabilization of thisisomer in these models.Rotating the O2 about the C2 axis introduces the second 3B

surface (b3B), which has a minimum at an even shorter Cu-Cu distance of 2.51 Å (9). Isomer 9 is 9.4 kcal mol-1 higher inenergy than isomer 8, is similar to the isomer 9 structures seenwith the multi-T-site models, and also has a short O-O distanceof 1.34 Å. The acute Cu-O2-Cu angle (73.0°) brings thecoordinating T sites into close proximity, matching the relativeligand orientation and distance observed in the multi-T-sitemodel clusters. In contrast to isomer 8, the structure of isomer9 within the single-T-site model closely mimics that found inthe multi-T-site models, and the constraints imposed by thelarger models have a less destabilizing effect.At Cu-Cu separations between 2.4 and 3.9 Å the 1A surface

is generally lower in energy than either 3B surface. Here wefind two minima and a transition state connecting them. Themore stable isomer, 7, has the shorter Cu-Cu (2.71 Å) andlonger O-O (2.28 Å) separations. The Cu2O22+ core is planar,and the Cu ions both have square-planar coordination. The otherisomer, 6, is also planar, but has longer Cu-Cu (3.42 Å) andshorter O-O (1.47 Å) distances. The low barrier betweenisomers 6 and 7 suggests a facile pathway for the formation orcleavage of an O-O bond, in good agreement with results foundin models of biological systems.32,35-37Despite the greater stability of the 1A isomers in the single-

T-site model, only 3B isomers are found in the larger, ZSM-5-specific models. These larger models incorporate steric effectsof the channel wall which prevent the formation of a planarCu2O22+ core and favor structures that place the O2 into thechannel void. While these particular coordination sites favorthe triplet ZCuO2CuZ isomers, other zeolites, or other coordina-tion sites within ZSM-5, may favor the singlet ones.As with ZCuOCuZ, we have investigated magnetic states for

O2-bridged oxocations, in part to understand the EPR activityof these species. We were unable to locate any antiferromagneticstates lower in energy than the triplet ground states discussedabove for ZCuO2CuZ. Again, alternative open- and closed-shellsinglet states are also higher in energy than the triplet. Otherstudies on related biological systems (with a [CuO2Cu]2+ core)utilizing DFT have found either no stable broken symmetrystate,36,37 or a stable broken symmetry state lower than anysymmetric state.25-27,35,38 The lack of general agreement on thisissue reflects both a sensitivity to the ligand environmentalaround the Cu ions and well-known difficulties in describingantiferromagnetic couplings in Cu-O-Cu systems.63 Thepresent DFT predictions of a singlet ground state for a flatCu2O22+ core and a triplet ground state for a puckered zeolite-supported geometry are consistent with previous biologicalstudies,32,35-38 and with DFT/CASPT233,34 and HF/CI29 calcula-tions on model NH3-ligated systems, respectively.

V. DiscussionAlthough the energetics of the three cluster models are

not directly comparable, within each we can calculate thebinding energy of the oxocation complexes relative to thecorresponding cluster with two Cu+ ions and an isolated O2molecule. Figure 10 compares the energies of reaction 1 in the

three multi-T-site cluster models. The single O-bridged oxoca-

Figure 8. Molecular orbital diagram for unligated [Cu(µ-1,2-O2)Cu]2+.

Figure 9. Energy vs Cu-Cu separation for ZCuO2CuZ within thesingle-T-sites model, and constrained to C2 symmetry. The zero ofenergy is the ZCuO2 + ZCu dissociation limit.

ZCu‚‚‚CuZ + x/2 O2f ZCuOxCuZ (1)

10458 J. Phys. Chem. B, Vol. 103, No. 47, 1999 Goodman et al.

Cluster Model Studies of Oxygen-Bridged Cu Pairs in Cu-ZSM-5 Catalysts

B. R. GoodmanDepartment of Chemistry, UniVersity of Illinois, Urbana, Illinois 61801

K. C. Hass* and W. F. SchneiderFord Research Laboratory, MD 3028/SRL, Dearborn, Michigan 48121-2053

J. B. AdamsDepartment of Chemical, Bio and Materials Engineering, Arizona State UniVersity, Tempe, Arizona 85287

ReceiVed: June 29, 1999; In Final Form: September 7, 1999

Effects of the support environment on the existence of Cu ion pairs in Cu-exchanged ZSM-5 catalysts areexamined using density functional theory. Results for the molecular and electronic structures of O- and O2-bridged Cu oxocations ([CuOCu]2+ and [CuO2Cu]2+) are presented, including two distinct isomers for thelatter. Both types of oxocations are predicted to be strongly bound for conditions likely to occur in the zeolite.The zeolite framework is represented by a variety of cluster models, including a previously established single-T-site model and larger multi-T-site models specific to particular binding sites in ZSM-5. With the largestmodels, bent Cu-Ox-Cu structures are found with Cu-Cu distances consistent with X-ray absorption datafor Cu-ZSM-5. Implications for catalytic chemistry, including a proposed pathway for O2 formation andsubsequent desorption from oxidized Cu sites in the zeolite, are discussed.

I. IntroductionCu-exchanged ZSM-5 has high catalytic activity for NO

decomposition and selective catalytic reduction.1 An ongoingcontroversy surrounding Cu-ZSM-5 chemistry is the catalyti-cally active site: specifically, do isolated Cu ions or pairs ofCu ions dominate the catalytic chemistry? The first mechanismproposed for the NO decomposition reaction involved Cu ionpairs in the N-N bond forming step,2 and Cu pairs maysimilarly have a role in O2 dissociation or recombinationprocesses. Cu-Cu distances of 2.91 Å to 3.13 Å, attributed toCu pairs, have been observed in X-ray absorption fine structure(EXAFS) spectroscopy studies of Cu-ZSM-5 samples with highCu loadings,3-5 while separations of only 2.47 Å have beeninferred from more recent EXAFS studies.6 The sigmoidalrelation between NO turnover frequency and Cu exchange levelin NO decomposition can also be interpreted as evidence forthe participation of Cu pairs in catalysis,7-10 although analternative interpretation is that two or more mono-Cu sites exist,with the catalytically most active sites being less favorable forcation exchange.7 Temperature programmed reduction (TPR)data11,12 suggest that Cu pairs exist as oxygen-bridged “oxo-cations”, and perturbations of lattice vibrations observed withinfrared (IR) spectroscopy have been attributed to [CuOCu]2+species.11,13,14Electron paramagnetic resonance (EPR) data is also incon-

clusive on the issue of Cu pairs in Cu-ZSM-5. For example,an EPR signal attributed to [CuOCu]2+ oxocations is observedin Cu-Y15 but not in Cu-ZSM-5.16 The EPR silence in thelatter case has been attributed to antiferromagnetic coupling ofthe copper spins via superexchange,13 but a magnetic suscep-tibility study17 found no antiferromagnetism in dehydrated Cu-ZSM-5.The only relevant computational studies18-24 to date support

the existence of Cu pairs in Cu-ZSM-5. For instance, Sayle et

al. used empirical potentials to minimize the energy of systemswith 4 Cu(I), 4 Cu(II), and 4 OH- placed in a ZSM-518-20 ormordenite21 lattice. Of the generated sample systems in ZSM-5, 41% of the Cu ions formed pairs bridged by one or two OH.Because more than half of the Cu pairs had a mixed valent[Cu(II)-OH-Cu(I)]+ form, this species was suggested as apossible active site for catalysis. The bridged species were alsofound strongly anchored to the framework by Al-substitutedtetrahedral (T) sites. A similar study on mordenite found only10% of Cu in pairs, less than in the more catalytically activeZSM-5, lending further support to the importance of pairs.Another study22 used empirical potentials with moleculardynamics to generate sample [Cu(II)-O-Cu(II)]2+ systems inZSM-5. The oxocations and 6 T sites from one ring wereextracted and examined with minimal basis Hartree-Fock (HF)cluster calculations. Although this study found the [Cu(II)-O-Cu(II)]2+ cation to be bound in the zeolite model, it alsopredicted an unphysical charge distribution that calls the resultsinto question.In a recent density functional theory (DFT) study,23 we

examined a generic model of Cu-pairs in high-silica zeolites,in which two unlinked Al(OH)4- ligands were used to representcoordination to locally anionic portions of the zeolite framework.(Such portions will be generically labeled in this paper as Z.)We found both mono-oxo (ZCuOCuZ, 1) and dioxo (ZCuO2-

CuZ, 2) bridged species to be stable and likely to play significantroles in the local chemistry.23 The same model subsequently

10452 J. Phys. Chem. B 1999, 103, 10452-10460

10.1021/jp9922110 CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 11/05/1999


Intercage Cu Reactions


2 CuI CuII2O2

O2

Ene

rgy

(kJ

mol

-1)

-30

-20

-10

0

10

20

30

40

Cu O

xidation State

1.25

1.50

2.00

1.00

S3 to S1

(A)

(B)

-40

-50

-60

(C)(D)

(E)Reaction Coordinate


How far can a CuI travel?


CPMD metadynamics, 473 K

0

10

20

30

40

50

60

70Metadynamics free energy

Coulombic potential

3 4 5 6 7 8 9 10 11Cu-Al distance (Å)

1

2

3

∆F,∆

E (

kJ/m

ol)

1

2

3

~ 9 Å diffusion radius from ”home” Al


How Many Unique Pairs?


∂

# p

airs

Cu density

Cu density

Frac

tion

of p

aira

ble

Cu 18 Å collision diameter

Recalcitrant CuI fraction

• Distribute Cu onto framework at appropriate density

• Count “pairable” Cu– Within diffusion radius– No double counting


Electrostatically tethered, dynamic site pairsPaolucci et al., Science 2017

“Dual” Single Site SCR Mechanism


Single site reduction

Insensitive to framework

topologySi/Al, Cu/Al

Dual site oxidationSensitive to framework topology

Si/Al, Cu/Al

Single site reduction

Insensitive to framework

topologySi/Al, Cu/Al

NO

Cu(I)

N2 + 2 H 2O

O2

Cu(II)

N2 + H 2O

Cu(I)

Cu(II)

NO + 2 NH3

NO

N2 + 2 H 2O

NO + 2

NH 3

+ NH 4

+

+

N2 + H 2O+ NH 4

Mechanistic Implications

• At least at low temperature, not all atomically dispersed Cu participate in SCR cycle

• Apparent SCR rates connected to dynamic Cu ion mobility

• Microkinetics have a transport component that lies outside the domain of traditional mean field models


Transient Oxidation Rates


0

10

20

30

40

50

60

70Metadynamics free energy

Coulombic potential

3 4 5 6 7 8 9 10 11Cu-Al distance (Å)

1

2

3

∆F,∆

E (

kJ/m

ol)

+Kinetic Monte Carlo

10 % O2

nO2~ 0.2

nO2~ 0.3

nO2~ 1

rate(d) =

✓kT

h

◆e�(�G(d)+Ea)/kT (O2)

Mechanistic Implications

• At least at low temperature, not all atomically dispersed Cu participate in SCR cycle

• Apparent SCR rates connected to dynamic Cu ion mobility• Microkinetics have a transport component that lies outside the

domain of traditional mean field models

• How does SCR cycle close?• What is an appropriate, general rate model?• How do mechanism and rates change away from standard

conditions?• How does reaction get short-circuited? Eg, to make N2O?• How do catalytic sites change with sulfur and/or aging?


Needs/Opportunites

Fast SCR on Brønsted Sites


2 NH3 + 2 NO + 2 NO2 → 2 N2 + 3 H2O

“Fast” SCR

Li et al, ACS Catal 20172 NH3 + 4 NO2 → 2 N2O + 3 H2ON2O pathway?

Sulfur on Cu-SSZ-13

• Nature and reducibility of dominant sulfur species differs between Z2Cu to ZCuOH


Z2Cu

Tem

pera

ture

Cu(II)

Cu(II)

Cu(II)

[NH

3]=3

00 p

pm

ZCuOH

Cu(I)

Cu(II)

Cu(I)

Arthur Shih poster

Pathways to Dealumination


Nature and Consequences of Al - Al Interactions in SSZ-13 ZeoliteHui Li,1 Taebum Lee,1 Sichi Li,1 Anthony DeBellis,2 Subramanian Prasad,3 Imke Britta Mueller,4 Ahmad Moini,3 and William F. Schneider1*

1 Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556 (USA)2 BASF Corporation, 540 White Plains Road, Tarrytown, New York 10591 (USA)

3 BASF Corporation, 25 Middlesex-Essex Turnpike, Iselin, New Jersey 08830 (USA) 4 BASF SE, 67056 Ludwigshafen (Germany)

*[email protected]

SiO

AlO

AlO

Si

H H

0.0

0.5

1.0

1.5

2.0

PB

E re

lativ

e E

nerg

y (e

V)

3.0 4.0 5.0 6.0 7.0 8.0 9.0

-0.6-0.4

-0.2

0.0

0.2

0.4

0.60.8

PB

E re

lativ

e E

nerg

y (e

V)

Al-Al distance (Å)3.0 4.0 5.0 6.0 7.0 8.0 9.0 Brønsted sites (H+) exchange

Cation site (Cu2+) exchange

Introduction

Methods

• 36 T-site periodic supercell• DFT structural optimization by VASP 5.3.5

2 H+ exchange: 16 possible combinationsAll energies calculated

SiAl

OCuH

Color code

1 2 3

4

5 67

8

4NO + 4NH3 + O2 → 4N2 + 6H2O Standard Catalytic Reduction (SCR)

exhaust gas

urea inlet

SCR catalyst

ammonia slipcatalyst

• Cu-exchanged SSZ-13 widely used as on-board SCR catalyst• Synthetic SSZ-13 does not have Al-O-Al linkage• Fundamental understanding of Al locations and how Al environment influence Brønsted site (H+) and metal site (Mx+) speciation• How Al environment affect hydrothermal stability of zeolite

2NN, 4MRLöwenstein’s rule violation, 1NN

2NN, meta-Z2Cu 3NN, para-Z2Cu 3NN in 8MR 4NN in 8MR

2NN, meta-Z2Cu 3NN, para-Z2Cu2NN, 4MRLöwenstein’s rule violation, 1NN 3NN in 8MR 4NN in 8MR

• GGA/PBE energies• Normalized to E=0 for infinite separation

• Energy minimized for Al-O-Al configuration• Energy approach 0 at larger Al-Al distances

• GGA/PBE energies• Normalized to E=0 for para-Z2Cu structure• Energy minimized for para-Z2Cu, 3NN in 6MR meta-Z2Cu 0.15 eV higher• All other structures at least 0.8 eV higher in energy• Cu prefers to located in 6MR at larger Al-Al distances

ZE = E - EZ2H2 Z-H

4NN

4NN

• Löwenstein’s rule (no Al-O-Al) for synthesized zeolites is not a consequence of the energy landscape• Low energy for Al-O-Al structures may be the cause of Al aggregation during hydrothermal aging• Exchanged Cu2+ can help stabilizing 6MR and the zeolite structure

Conclusions

1 Cu2+ exchange: Local minima exploredLowest energy structure picked

Nomenclature:6 membered-ring (6MR), 2 nearest-neighbor (2NN)




*[email protected]

SiO

AlO

AlO

Si

H H

0.0

0.5

1.0

1.5

2.0

PB

E re

lativ

e E

nerg

y (e

V)

3.0 4.0 5.0 6.0 7.0 8.0 9.0

-0.6-0.4

-0.2

0.0

0.2

0.4

0.60.8

PB

E re

lativ

e E

nerg

y (e

V)



Introduction

Methods



SiAl

OCuH

Color code

1 2 3

4

5 67

8


exhaust gas

urea inlet

SCR catalyst









ZE = E - EZ2H2 Z-H

4NN

4NN


Conclusions



Pathways to Dealumination





*[email protected]

SiO

AlO

AlO

Si

H H

0.0

0.5

1.0

1.5

2.0

PB

E re

lativ

e E

nerg

y (e

V)

3.0 4.0 5.0 6.0 7.0 8.0 9.0

-0.6-0.4

-0.2

0.0

0.2

0.4

0.60.8

PB

E re

lativ

e E

nerg

y (e

V)



Introduction

Methods



SiAl

OCuH

Color code

1 2 3

4

5 67

8


exhaust gas

urea inlet

SCR catalyst









ZE = E - EZ2H2 Z-H

4NN

4NN


Conclusions






*[email protected]

SiO

AlO

AlO

Si

H H

0.0

0.5

1.0

1.5

2.0

PB

E re

lativ

e E

nerg

y (e

V)

3.0 4.0 5.0 6.0 7.0 8.0 9.0

-0.6-0.4

-0.2

0.0

0.2

0.4

0.60.8

PB

E re

lativ

e E

nerg

y (e

V)



Introduction

Methods



SiAl

OCuH

Color code

1 2 3

4

5 67

8


exhaust gas

urea inlet

SCR catalyst









ZE = E - EZ2H2 Z-H

4NN

4NN


Conclusions



Group Publications


CHAPTER ONE

Catalysis Science of NOx SelectiveCatalytic Reduction WithAmmonia Over Cu-SSZ-13 andCu-SAPO-34C. Paolucci*, J.R. Di Iorio†, F.H. Ribeiro†, R. Gounder†,W.F. Schneider*,1*University of Notre Dame, Notre Dame, IN, United States†School of Chemical Engineering, Purdue University, West Lafayette, IN, United States1Corresponding author: e-mail address: [email protected]

Contents

1. Introduction 41.1 Selective Catalytic Reduction 61.2 Metal-Exchanged Zeolite Catalysts 71.3 Scope and Structure of This Review 11

2. Synthesis of Zeolites 132.1 Synthetic Control of Al Distribution in Zeolites 132.2 Synthesis of SSZ-13 Zeolites 152.3 Synthesis of SAPO-34 Molecular Sieves 192.4 Copper Exchange in Zeolite and SAPO Frameworks 20

3. Ex Situ Characterization of Cu-SSZ-13 and Cu-SAPO-34 233.1 DFT-Based Analysis of Cu Speciation 243.2 Ambient Conditions 293.3 High-Temperature Oxidative Conditions 363.4 Vacuum and Inert Pretreatments 473.5 Hydrogen Temperature-Programmed Reduction 503.6 Characterization Following NO Dosing 53

4. In Situ and Operando Characterization 594.1 DFT Models of NH3 Adsorption in Cu-SSZ-13 594.2 Selective NH3 Titration of H+ Sites in H-Form and Cu-Exchanged Zeolites 624.3 Vibrational Spectroscopy 664.4 Magnetic Spectroscopy With NH3 704.5 X-Ray Spectroscopy With NH3 704.6 Operando X-Ray Spectroscopy 74

5. Catalytic Activity and Mechanism 765.1 Differential Standard SCR Kinetics 785.2 Standard SCR Mechanism Near 200°C 83

Advances in Catalysis, Volume 59 # 2016 Elsevier Inc.ISSN 0360-0564 All rights reserved.http://dx.doi.org/10.1016/bs.acat.2016.10.002

1

Author's personal copy

Cite as: C. Paolucci et al., Science 10.1126/science.aan5630 (2017).

RESEARCH ARTICLES

First release: 17 August 2017 www.sciencemag.org (Page numbers not final at time of first release) 1

Single-site heterogeneous catalysts promise to combine the attractive features of homogeneous and heterogeneous cata-lysts: active sites of regular and tunable architecture that provide precise catalytic function, integrated into a thermal-ly-stable, porous solid host that facilitates access of sub-strates to those sites and separation of products from the catalyst (1). In the conventional definition, a single-site cata-lyst contains functionally isolated active sites, such that re-action rates per active site are independent of their spatial proximity (2). Single metal atoms incorporated into solid oxide supports are reported to follow this conventional sin-gle-site behavior in catalytic CO oxidation to CO2 (3–5), se-lective hydrogenation (6, 7), and water-gas shift (8, 9). Here, we report that a nominally single-site catalyst (10) operates by dynamic, reversible, and density-dependent (non-mean-field) interaction of multiple ionically tethered single sites, a behavior that lies outside the canonical definition of a sin-gle-site heterogeneous catalyst (11).

We discovered this phenomenon in the quest for a mo-lecularly detailed model to unify the seemingly disparate observations of the catalytic function of copper-exchanged chabazite (Cu-CHA) zeolites, materials used in emissions control for the standard selective catalytic reduction (SCR) of nitrogen oxides (NOx, x = 1, 2) with ammonia (12):

2 3 2 24NO O 4NH 4N 6H O+ + → + (1)

Chabazite is a small-pore zeolite composed of cages (8 × 8 × 12 Å) interconnected by 6-membered-ring (6-MR) prisms and 8-membered ring (8-MR) windows (Fig. 1A). Substitu-tion of Si4+ by Al3+ within the framework introduces an ani-

onic charge that is balanced by extra-lattice cations. After Cu ion exchange and high temperature oxidation treatment, two isolated Cu site motifs are present: discrete CuII ions that balance two proximal Al centers and [CuIIOH]+ ions that balance single Al centers (10, 13, 14). Under low tem-perature (<523 K) standard SCR conditions, ammonia coor-dinates to and liberates Cu ions from direct association with the zeolite support, and these solvated Cu ions act as the redox-active catalytic sites (15). At typical Cu ion volumetric densities, standard SCR rates increase linearly with Cu den-sity, as expected for a single-site catalyst. As shown below, however, experimental observations in the low-Cu-density limit reveal a portion of the catalytic cycle in which O2 acti-vation by transiently formed Cu pairs becomes rate-limiting. These Cu pairs form from NH3-solvated Cu ions with mobili-ties restricted by electrostatic attraction to charge-compensating framework Al centers, leading to catalytic function that is neither single-site nor homogeneous.

Recognizing the intermediacy of this distinct catalytic state reconciles a number of controversies in Cu-zeolite SCR catalysis, including the role of the zeolite support in the cat-alytic mechanism, the sensitivity of SCR rates to Cu density under different conditions of observation, the extent to which standard and the closely-related fast SCR cycles (16) are connected via common intermediates, the chemical pro-cesses that limit low-temperature NOx SCR reactivity, and the origins of the apparent change in mechanism at elevated temperatures. These observations provide insight into the design of improved catalysts for SCR. More broadly, they

Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction Christopher Paolucci,1 Ishant Khurana,2 Atish A. Parekh,2 Sichi Li,1 Arthur J. Shih,2 Hui Li,1 John R. Di Iorio,2 Jonatan D. Albarracin-Caballero,2 Aleksey Yezerets,3 Jeffrey T. Miller,2 W. Nicholas Delgass,2 Fabio H. Ribeiro,2 William F. Schneider,1* Rajamani Gounder2* 1Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA. 2Charles D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA. 3Cummins Inc., 1900 McKinley Avenue, MC 50183, Columbus, IN 47201, USA. *Corresponding author. Email: [email protected] (W.F.S.); [email protected] (R.G.)

Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of NOx with NH3, but the low-temperature rate dependence on Cu volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an O2-mediated CuI → CuII redox step integral to SCR. Electrostatic tethering to framework Al centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.

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Sites and Mechanism in Selective NOx Reduction · How Many of Which Cu(II) Site? W. F. Schneider CLEERS -4 Oct 2017 the 6-membered ring was counted as a surrogate for one dehy-drated