CATALYSIS

Dynamic multinuclear sites formedby mobilized copper ions in NOxselective catalytic reductionChristopher 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*

Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR)of nitrogen oxides (NOx) with ammonia (NH3), but the low-temperature rate dependenceon copper (Cu) volumetric density is inconsistent with reaction at single sites.We combinesteady-state and transient kinetic measurements, x-ray absorption spectroscopy, andfirst-principles calculations to demonstrate that under reaction conditions, mobilizedCu ions can travel through zeolite windows and form transient ion pairs that participate inan oxygen (O2)–mediated CuI→CuII redox step integral to SCR. Electrostatic tethering toframework aluminum centers limits the volume that each ion can explore and thus itscapacity to form an ion pair. The dynamic, reversible formation of multinuclear sites frommobilized single atoms represents a distinct phenomenon that falls outside theconventional boundaries of a heterogeneous or homogeneous catalyst.

Single-site heterogeneous catalysts prom-ise to combine the attractive features ofhomogeneous and heterogeneous catalysts:active sites of regular and tunable architec-ture that provide precise catalytic function,

integrated into a thermally stable, porous, solidhost that facilitates access of substrates to thosesites and separation of products from the catalyst(1). In the conventional definition, a single-sitecatalyst contains functionally isolated active sites,such that reaction rates per active site are inde-pendent of their spatial proximity (2). Singlemetalatoms incorporated into solid oxide supports arereported to follow this conventional single-sitebehavior in catalytic CO oxidation to CO2 (3–5),selective hydrogenation (6, 7), and water-gas shift(8, 9). Here we report that a nominally single-sitecatalyst (10) operates by dynamic, reversible, anddensity-dependent (non–mean field) interactionof multiple ionically tethered single sites, abehavior that lies outside the canonical defini-tion of a single-site heterogeneous catalyst (11).We discovered this phenomenon in the quest

for a molecularly detailed model to unify theseemingly disparate observations of the catalyticfunction of copper-exchanged chabazite (Cu-CHA)zeolites, materials used in emissions control forthe standard selective catalytic reduction (SCR) ofnitrogen oxides (NOx, x = 1, 2) with ammonia (12)

4NOþ O2 þ 4NH3 → 4N2 þ 6H2O ð1Þ

Chabazite is a small-pore zeolite composed ofcages (8 Å by 8 Å by 12 Å) interconnected by six-memberedring (6-MR)prismsandeight-memberedring (8-MR) windows (Fig. 1A). Substitution ofSi4+ by Al3+ within the framework introducesan anionic charge that is balanced by extra-lattice cations. After Cu ion exchange and high-temperature oxidation treatment, two isolatedCu site motifs are present: discrete CuII ions thatbalance two proximal Al centers and [CuIIOH]+

ions that balance single Al centers (10, 13, 14).Under low-temperature (<523 K) standard SCRconditions, ammonia coordinates to and liberatesCu ions from direct association with the zeolitesupport, and these solvated Cu ions act as theredox-active catalytic sites (15). At typical Cu ionvolumetric densities, standard SCR rates increaselinearly with Cu density, as expected for a single-site catalyst. As shown here, however, experimen-tal observations in the low–Cu density limit reveala portion of the catalytic cycle in which O2 acti-vation by transiently formed Cu pairs becomesrate limiting. These Cu pairs form from NH3-solvated Cu ions with mobilities restricted byelectrostatic attraction to charge-compensatingframework Al centers, leading to catalytic func-tion that is neither single site nor homogeneous.Recognizing the intermediacy of this distinct

catalytic state reconciles a number of contro-versies in Cu-zeolite SCR catalysis, including therole of the zeolite support in the catalytic mech-anism, the sensitivity of SCR rates to Cu densityunder different conditions of observation, theextent to which standard and the closely relatedfast SCR cycles (16) are connected through com-mon intermediates, the chemical processes thatlimit low-temperature NOx SCR reactivity, andthe origins of the apparent change in mech-anism at elevated temperatures. These observa-

tions provide insight into the design of improvedcatalysts for SCR. More broadly, they reveal adistinct class of catalytic materials characterizedby partially mobile ions that dynamically andreversibly form multinuclear active sites, a con-cept that may be exploited for a wide variety ofreactions.

Turnover rates depend on the spatialdensity of single Cu sites

To assess the catalytic consequences of Cu iondensity, we prepared a series of Cu-CHA sam-ples with fixed framework Al composition(Si/Al = 15) and Cu/Al content ranging from 0.04to 0.44 [for synthesis methods and character-ization data, see supplementary materials (SM)section S1, figs. S1 to S4, and tables S1 and S2],corresponding to Cu volumetric densities (rCu)from (0.3 to 4.2) × 10–4 Cu Å–3. We denotesamples as Cu-CHA-X, where X indicates themean Cu-Cu distance (in Å), assuming a homo-geneous Cu distribution (table S2). Mean Cu-Cudistances vary from 40.7 to 16.6 Å from the leastto most heavily Cu-exchanged samples, and thehighest Cu loading corresponds to approximatelyone Cu ion per three chabazite cages (Fig. 1A).We observed that SCR rates increased lin-

early with Cu density at rCu > 1.9 × 10–4 Å–3

(Fig. 1B), as would be expected for a reactioncatalyzed by isolated Cu sites. Turnover rates,apparent reaction orders, and apparent activa-tion energies (table S3) are consistent with valuesreported for high–Cu density Cu-CHA samples(17, 18). By contrast, standard SCR rates varyquadratically with Cu density below rCu < 1.13 ×10–4 Å–3. All kinetic quantities are consistentwith those reported for Cu-dilute Cu-CHA sam-ples [Si/Al = 4.5, Cu/Al < 0.02 (17); Si/Al = 6,Cu/Al < 0.03 (18)]. We show that these two ki-netic regimes, characterized by distinct kineticparameters, reflect different rate-controlling ele-mentary steps and prevalent reactive intermedi-ates during steady-state NOx SCR.To probe themechanistic origins of this change

in kinetic behavior, we used x-ray absorptionnear-edge structure (XANES) spectroscopy toquantify Cu oxidation state during steady-statestandard SCR in operando. Figure 2 and tableS4 report the steady-state CuI fraction obtainedby XANES fitting (procedure detailed in SMsection S3) as a function of Cu density for threesamples in Fig. 1 (Si/Al = 15) and seven other Cu-CHA zeolites (Si/Al = 4.5, 16, and 25; table S4).Consistent with prior observation, the site-isolatedCuII ions observed ex situ evolve into a mixtureof CuI and CuII ions during standard SCRcatalysis, indicative of redox cycling betweenCuI and CuII oxidation states coupled with theSCR catalytic cycle (12). The CuII→CuI half-cycle(Fig. 2, inset) is accepted to occur on site-isolatedCuII, to consume one equivalent of NO and NH3

per CuII, and to produce N2 and H2O (15, 16).The inverse relationship between steady-state

CuI fraction and Cu volumetric density (Fig. 2)is inconsistent with the behavior expected ofa single-site catalyst, for which the active-site ox-idation state should depend only on the reaction

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Paolucci et al., Science 357, 898–903 (2017) 1 September 2017 1 of 6

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, MC50183, Columbus, IN 47201, USA.*Corresponding author. Email: wschneider@nd.edu (W.F.S.);rgounder@purdue.edu (R.G.)

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conditions and temperature. The XANES datashow that CuI is theminority oxidation state at thehighest Cu density and smallest mean Cu-Cuseparations (<~15 Å) during steady-state cata-lytic operation but that its proportion increaseswith decreasing Cu density to the point thatit becomes the majority oxidation state in themost dilute sample (meanCu-Cu distance ~29Å).In this dilute limit, the operando XANES andextended x-ray absorption fine structure (EXAFS)spectra become indistinguishable from those ofa CuI-CHA sample reduced in situ or of CuI(NH3)2in aqueous solution (fig. S5 and table S4) (10).This observation suggests that CuI→CuII oxi-dation rates increase with Cu ion density andimplies non–single site behavior in the oxidationhalf-cycle, a process that is not well understoodmechanistically beyond the observation that itconsumes O2 (12).The coordination states of site-isolated CuI

and CuII ions under standard SCR conditionsat 473 K have been explored previously in detail(10). X-ray absorption spectroscopy (10), x-rayemission spectroscopy (19, 20), and density func-tional theory (DFT)–based models (10) (includingab initio thermodynamic phase diagrams in figs.S17 and S18) show that NH3 outcompetes othergases present under standard SCR conditions,including H2O, for binding at both CuI andCuII ions, which are respectively two- and four-fold coordinated. Consistent with these find-ings, the standard SCR reaction rate is zeroorder with respect to water pressure (1 to 10%atm; fig. S19). Schematic illustrations of themost probable coordination states of CuI andCuII ions under these conditions are shown inthe inset of Fig. 2.In the high–Cu density samples (Fig. 2, sam-

ples c to g), the first-shell Cu coordination num-ber (CN) derived from operando EXAFS is three,consistent with the expectation for a nearly equi-

molar mixture of CuI and CuII [(10), table S4].The CN decreases to two in the fully reduced(Fig. 2, samples a to b), lowest–Cu density sam-ples, consistent with site-isolated CuI(NH3)2as the most abundant reactive intermediatepresent during steady-state standard SCR. Inthis limit, the SCR turnover rate is solely limitedby the CuI→CuII half-reaction. The increase inapparent O2 reaction order from 0.3 to 0.8 withdecreasing Cu density reinforces the increasingkinetic relevance of the CuI→CuII half-cycle asCu becomesmore dilute (Fig. 1). This change alsocorrespondswith the transition froma first-orderto a second-order dependence of SCR rate on Cu-ion density [Fig. 1, (18)]. From these observa-

tions, we conclude that the kinetically relevantO2-consuming step in the oxidation half-cycle issensitive to Cu density.

CuI site density requirements differ foroxidation with O2 and NO2

To probe the coupled roles of O2 and Cudensity in the oxidation half-cycle, Cu-CHA-29,Cu-CHA-20, and Cu-CHA-15 (Fig. 2, labels a, c,and h, respectively) were first reduced to theCuI state in flowing NO and NH3 (details in SMsection S3, figs. S6 to S8) (10). Then, sampleswere held under flowing O2, and the transientevolution of the Cu oxidation state was moni-tored by using XANES. The CuI fraction decayed

Paolucci et al., Science 357, 898–903 (2017) 1 September 2017 2 of 6

Fig. 1. Cu-density dependence of SCR rates. (A) The CHA cage (36) andschematic representation of theCu iondensities perCHAcage in samples a andg.(B)StandardNOxSCRrates (per volumecatalyst; 473K;measured in adifferentialreactor by using a gas mixture representative of practical low-temperature

application, including 2.5% H2O; details in SM section S2) and apparent O2

orders measured on Cu-CHA-X samples (Si/Al = 15, table S3) of increasingCu ion density. Colored line is a visual guide; regression fits to the quadratic (R2 =0.99) and linear (R2 = 0.99) kinetic regimes are detailed in SM section S2.

Fig. 2. Cu-density dependenceof operando Cu oxidationstate.The dependence of CuI

fraction on Cu ion volumetricdensity during steady-statestandard SCR at 473 K wasmeasured by XANES (details inSM section S3). Data pointsinclude samples a, f, and gshown in Fig. 1 (Si/Al = 15, filledsquares), samples at Si/Al =4.5 and Si/Al = 25 (opensquares), and comparableliterature data [open circleRef (19), Si/Al = 16; opentriangle Ref (17), Si/Al = 4.5].Inset shows NH3-solvated,isolated CuI and CuII speciespreviously observed and com-puted (10) to be present duringstandard SCR at 473 K. Gray,Cu; green, Al; yellow, Si; red,O; blue, N; and white, H. The colored arrow is a visual guide; error bars represent the absolute 5%uncertainty from linear combination XANES fitting (details in SM section S3).

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in the presence of O2 at different rates on the threesamples (Fig. 3A), and different fractions of CuI

(0.05 to 0.30) persisted at steady state (table S6).The dependence of the transient oxidation state

response on Cu density suggests some underlyingspatial requirements for CuI(NH3)2 ions to reactwith O2. After exploring various rate laws, wefound that an expression that is second order intotal CuI density [CuI] and offset by a recalcitrantfraction of CuI, [CuI]1, fits the data most sat-isfactorily [coefficient of determination (R2) =0.99 (a), 0.98 (c), and 0.99 (h), Fig. 2; details inSM section S4]. Normalizing to the initial CuI

density, [CuI]0, the integrated rate law becomes

CuI fraction¼ ½CuI�ðtÞ½CuI�0

¼ 1�½CuI�1=½CuI�01þ2k

�½CuI�0�½CuI�1�tþ ½CuI�1

½CuI�0(2)

where t is time and k is a pseudohomogeneoussecond-order rate constant for disappearance ofCuI. Solid lines in Fig. 3A denote best-fit regres-sions of the data to Eq. 2. Fitted k values of (1.0, 1.7,and 8.2) × 10−4 m3 mol Cu−1 s−1 for Cu-CHA-29,Cu-CHA-20 and Cu-CHA-15, respectively, increasedsystematically with Cu density. The variation in kdemonstrates that all the reaction sites are notkinetically equivalent. Further analysis belowsuggests that the oxidation kinetics cannot befaithfully captured by a mean-field model.

This second-order Cu dependence suggestsa pseudobimolecular reaction between twoCuI(NH3)2 ions and O2 during the transientexperiment

2�CuIðNH3Þ2

� þO2→

�ðNH3Þ2CuII–O2–CuIIðNH3Þ2

� ð3Þ

Oxygen-bridged Cu dimers are well known inCu-zeolite chemistry (21–23) and find analogiesin the biological chemistry of Cu that prevailsat lower temperatures (24–29). These dimerscan adopt various oxygen-bridging configura-tions (25, 30). To probe the plausibility of thisreaction between two caged CuI(NH3)2 centers,we turned to DFT (computational details inSM section S5).We first considered the energy landscape for

twoCuI(NH3)2 ions to occupy the sameCHAcage.Starting from two CuI(NH3)2 ions in adjacentcages that share an Al T-site vertex (Fig. 4, struc-ture A), the computed barrier for one CuI(NH3)2to diffuse through an 8-MR window into theadjacent CuI(NH3)2-containing cage (structure B)is 35 kJmol−1, consistent with prior estimates ofCuI(NH3)2 diffusion barriers into an empty cage(10, 31). The net pairing cost is 23 kJ mol−1. Thus,transport between adjacent Al-sharing cages isexpected to be facile at temperatures of catalyticinterest. Two CuI(NH3)2 ions bind O2 more ef-fectively (–59 kJ mol−1) than does an isolatedCuI(NH3)2 (–26 kJmol−1) (table S10 and fig. S14),and the O2 binding energymore than offsets the

energy cost for twoCuI(NH3)2 to cohabit the samecage. Initial reaction likely generates the end-onspin-triplet species shown in Fig. 4 (structure C),and further rearrangement and dissociation ofO2 across two CuI centers ultimately leads tothe di-oxo structure E, containing two four-fold-coordinated CuII centers. Conversion of structureC into D is spin forbidden; the effective barrierin similar ligand environments is estimated to be20 kJmol−1 (24). Subsequent conversion of struc-ture D into E occurs with a modest barrier. Therate-limiting process across this entire cascade(A to E, table S9) is themigration of CuI throughthe 8-MR; beyond that point, reaction energiesare computed to be independent of the zeolitecage. Thus, the primary role of the zeolite sup-port in this oxidation process is to regulate CuI

mobility.Structure E is consistent both with XANES-

observed CuII oxidation state and with the first-and second-shell features extracted from EXAFS(Fig. 3A, inset, and figs. S9 to S11) at the end ofthe transient O2 oxidation experiment. Thus,experiment and computation both reveal asecond CuII state of the catalyst, distinct fromframework-bound CuII observed after oxidativetreatments and from NH3-solvated CuII detectedduring low-temperature standard SCR (10, 20).Exposure of samples to NO and NH3 at 473 Kafter the transient O2 oxidation experiments re-duces all sites to the original mononuclear CuI

state, demonstrating that theSCRredox cycle canbeclosed through sequential stoichiometric reactions

Paolucci et al., Science 357, 898–903 (2017) 1 September 2017 3 of 6

Fig. 3. Kinetics of CuI oxidation by O2. (A) Temporal evolution of theXANES-measured CuI fraction is plotted for the Cu-CHA-29 (a, red),Cu-CHA-20 (c, blue), and Cu-CHA-15 (h, black) samples during transientoxidation in 10% O2 at 473 K. Least-squares fit to Eq. 2 is shown bysolid lines, and predicted recalcitrant CuI fractions are shown as horizontalbars. [CuI]1 was set by forcing the fit through the last (longest time)data point; [CuI](0)/[CuI]0 was set to 1 (full details in SM section S4).The CuI fractions reported contain an absolute 5% error from linear

combination XANES fits (details in SM section S3). Inset, the Fouriertransform of the k2-weighted EXAFS signal (FT[k2c(k)]) in R-space (R) ofCu-CHA-15 collected before O2 exposure and after the transient experiment.(B) Snapshots taken from simulated initial (time = 0) and final (time→1)CuI spatial distributions corresponding to the three samples (a, c, and h)in (A). CuI volumetric footprints are denoted by 9 Å–radius green spheres.Simulation results include decomposition of unoxidized CuI fraction intophysically isolated (Iso) and functionally isolated (MC) components.

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(SM section S6, fig. S12). Further, subsequent O2

treatment recovers the same transient responseand the same fraction of CuI sites that are un-responsive to O2 exposure (table S7), indicatingthat CuI ions do not irreversibly aggregate dur-ing O2 exposure, but return to their original site-isolated state after each reduction step.To verify that the residual CuI fraction did not

represent a physically inaccessible or chemicallydistinct site, transient O2 experiments were com-pared with an analogous transient NO2 exper-iment (details in SM section S3) on reduced formsof the same three samples (figs. S6 to S8). AfterNO2 exposure at 473 K, the Cu

I fraction decayedwith time as in the O2 experiment, but the CuI

absorption edges disappeared completely after~300 s. Further, the decrease in CuI fractionwith time is best described [R2 = 0.98 (a), 0.98(c), and 0.89 (h); details in SM section S4] by apseudo–first order rate expression with apparentrate constants (0.030 s−1) that are independent ofCu density (SM section S4, fig. S13). Thus, all CuI

sites are equivalently susceptible to oxidation byNO2. We compute the reaction energy for NO2

binding to CuI(NH3)2 to be –46 kJ mol–1 and togenerate a CuII center (SM section S5, fig. S14,and table S10).We conclude that the spatial proximity of

isolated CuI ions, and not the presence of minor-ity dimeric Cu species at low Cu densities, isresponsible for the transition in the standard SCRturnover rate from a quadratic to linear depen-dence on Cu ion density with increasing Cu den-sity (Fig. 1B). The steady-state and transientexperiments and DFT models are consistentwith a CuI→CuII half-cycle that combines twoCuI(NH3)2 complexes with one O2 molecule tocreate a previously unobserved binuclear CuII in-termediate (Eq. 3 and Fig. 4).

The available data exclude the possibility thatO2 activation occurs on a persistent minority frac-tion of Cu ion pairs within single zeolite cages(32). First, the fraction of isolated CuI(NH3)2complexes that could be reversibly oxidized withO2 (Fig. 3A) exceeds by 10-fold the fraction of Cupairs within a single cage if Cu were randomlydispersed on the zeolite support (SM section S7,fig. S15). Second, steady-state and transient ratesof CuI oxidation with O2 would exhibit a first-order dependence onCudensity, as observedwithNO2 as the oxidant. Rather, these results imply apseudohomogeneous reaction between equivalentsite-isolated CuI ions with mobilities constrainedin a manner that limits the total fraction of sitesreactive toward O2.

Solvation by ammonia confers mobilityto single Cu ions

To assess the mobility of CuI(NH3)2 complexesover time scales inaccessible to conventional abinitio molecular dynamics, we turned to ab initiometadynamics (SM section S8), taking Cu-Al coor-dination distance as the collective variable, usinga supercell with a minimum image distance of>10 Å (SM section S8), and sampling at 473 K.Free energywasminimized at a Cu-Al distance of4.7 Å and increased with Cu-Al separation untilthe Cu ion entered an 8-MR window separatingtwo cages, at 8 Å (Fig. 5). Free energy decreasedas the Cu ion moved into the adjacent cage,before increasing again as the Cu-Al distanceexceeded 9 Å. Comparison with a point-chargemodel indicates that electrostatics dominatethis distance-dependent free energy (SM sectionS9, Fig. 5). From the computed free-energylandscape, we estimate the hopping rate for aCuI(NH3)2 to leave its resting cage to be 6 × 106 s−1

at 473 K, much faster than steady-state SCR

turnover rates (table S3), and the equilibriumfraction of Cu ions outside their resting cageto be 1.4 ×10−5 at 473 K.CuI(NH3)2 migration is thus rapid at 473 K

but constrained by electrostatic tethering tocharge-compensating framework Al sites. Wenext used this concept to predict the unoxidizedfraction of CuI(NH3)2 in the transient O2 ex-periments (Fig. 3A), assuming that Cu ions arerandomly associated with Al in the CHA lattice,that each Cu can access a limited diffusionvolume, and that only CuI ionswith overlappingdiffusion volumes can form an O2-bridged Cupair (Fig. 4). To exercise the model, we distrib-uted Al onto a periodic supercell of the CHAlattice following Löwenstein’s rule (33), occupiedsites with Cu following previously validated rules(10), counted the number of overlapping diffu-sion spheres at a given radius (Fig. 3B, t = 0)permitting each Cu to be counted only once, andrepeated until the average unoxidized CuI frac-tion converged (SM section S10 and fig. S16). Atthe end of a single simulation, the unoxidizedCuI fraction consisted of Cu ions that either wereinitially physically isolated from all other Cu (Iso)or were functionally isolated because they sharedoverlapping diffusion volumes with Cu ions thathad more than one potential partner, the losersin amolecular game ofmusical chairs (MC). Repre-sentative initial and final simulation snapshots areshown in Fig. 3B for Cu densities corresponding toCu-CHA-15, Cu-CHA-20, and Cu-CHA-29, and thepredicted fraction of unoxidized CuI assuming adiffusion radius of 9 Å are plotted as solid hor-izontal bars in Fig. 3A.Model results agree quan-titatively with both experimental observationand the metadynamics observation of a ~9 Åmaximum diffusion distance, implying that theCu sites are neither conventionally heterogeneous(immobile) nor homogeneous (mobility governedby molecular diffusion) and demonstrating thatthe fraction of spectator [CuI]∞ sites is a conse-quence of regulated and localized mobility due toelectrostatic tethering.These observations resolve the outstanding issues

regarding SCR catalysis raised in the introduction.NH3 solvates and mobilizes discrete Cu activesites under low-temperatureSCRconditions.Whenthe activation of O2 is not rate controlling, NH3-solvated Cu ions appear catalytically equivalent,such that rates increase linearlywith their numberdensity. In this regime, the zeolite framework itselfhas only a weak influence on the SCR turnoverrate (10) because it functions primarily as anionic host for homogeneous-like NH3-ligatedCuII complexes. Experiments performed on sam-ples with low Cu density revealed, however, thata homogeneous picture of the SCRmechanism isincomplete.Simulations reveal that NH3-solvated CuI ions

have sufficient mobility to travel through 8-MRCHAwindows, visit adjacent cages, and form Cusite pairs that activate O2 via Eq. 3. Because ofthis requirement for dynamic Cu pairing, SCRrates scale approximately second order with Cudensity under conditions in which the CuI→CuII

oxidation by O2 is rate determining, consistent

Paolucci et al., Science 357, 898–903 (2017) 1 September 2017 4 of 6

Fig. 4. Simulation of O2 adsorption and oxidation of two CuI(NH3)2 equivalents. DFT-computedenergy landscape is shown for the diffusion of CuI(NH3)2 through an 8-MR CHA window into anadjacent cage and subsequent bimolecular reaction with O2. All minima and transition stateswere computed here, except C to D, which is taken from (24). Gray, Cu; green, Al; red, O; blue,N; and white, H.

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with quantitative analysis of the transient oxida-tion of CuI with O2. In contrast with previousassertions (34), binuclear CuII intermediates canbe detected experimentally and are structurallydistinct from the mononuclear NH3-solvated CuII

observed in operando (10, 20), and the number ofsuch binuclear sites that can form on a givenCu-CHA sample can be quantitatively predicted.These results motivate a revision of previousmechanisms to incorporate this dynamic couplingof isolatedCu ions, as illustrated in Fig. 6. Isolated,NH3-solvatedCu

II ions charge-compensating eitherone (left cycle) or two (right cycle) framework Alsites are reduced by NO and NH3 to produceN2, H2O, and CuI(NH3)2. These two paths aredistinguished only by the fate of the protonthat is also generated (10). Mobile CuI(NH3)2species can diffuse and combine in a reactionthat consumes O2 and generates the CuII dimerintermediate observed here. We confirmed thatthis binuclear CuII complex reacts with two equiv-

alents of NO per Cu (SM section S6, fig. S12)to regenerate CuI(NH3)2 and close the SCR cycle.A key aspect of our model is that Cu ions

supported on the zeolite are neither mobile asprescribed by molecular diffusion processes,as in a homogeneous catalyst, nor segregatedinto separate ensembles of active and inactivesites, as in a heterogeneous catalyst. Rather,all Cu ions located within diffusion distancesof other ions can potentially form Cu pairsdynamically and reversibly during the SCRcycle, at rates influenced by the mobility andeffective diffusion distances of Cu ions. Thus,rates of O2-assisted oxidations are sensitive toCu proximity and could also be sensitive to thezeolite support composition and topology. Bycontrast, all NH3-solvated CuI sites are equiv-alently susceptible to first-order oxidation byNO2, independent of Cu spatial density, and aresubsequently reducible by NO and NH3. Theseobservations demonstrate that fast SCR, in which

oxidation capacity is provided by NO2 ratherthan O2

2NH3 þ NOþ NO2→ 2N2 þ 3H2O ð4Þ

is not linked through a common intermediateto standard SCR under conditions in which Cusites are solvated by NH3 (16). NO2 oxidants accel-erate SCR rates both by accelerating CuI oxida-tion kinetics and by engaging a larger fractionof Cu sites in the catalyst.At higher temperatures (>523K), standard SCR

rates are independent of Cu density (12, 18), the ap-parent activation energy increases to 140 kJmol−1

(18), and Cu ions lose their NH3 solvation shell(12), implicating the involvement of different O2

activation steps that do not occur at the Cu ionpairs formed dynamically at lower temperatures(<523 K). Of greater practical importance to NOx

emissions control is to increase SCR rates at evenlower temperatures (<473 K) (12). The resultshere imply that standard SCR onset (“light-off”)temperatures, which are observed to depend onzeolite composition, topology, andCu distribution(35), are sensitive to changes in rate-determiningO2 activation steps. Thus, optimization of Cuspatial distribution and promotion of Cumobilityare promising strategies for accelerating CuI

oxidation rates and improving low-temperatureSCR catalysts.

Outlook

Our results point to a previously unrecognizedcatalytic mechanism that embodies salient fea-tures of homogeneous and heterogeneous cat-alysts. This mechanism encompasses solventmobilization of discrete active site precursors(e.g., single metal ions) and ionic bonds to thesupport that limit their mobility. The active siteprecursor has an effective diffusion distance andoccupies a volumetric footprint that restricts itsinteractions only to other precursors withinoverlapping volumes; such catalytic behaviorcannot be described by mean-field, Langmuirkinetics. The ionic tethering motif provides op-portunities to confer catalytic benefits beyondimmobilization strategies based on covalent an-chors or tethers. This motif enables the in situdynamic generation of multinuclear complexesimplicated as active sites in O2 activation andtherefore could also apply to other reactions,such as the partial oxidation of methane tomethanol on Cu-zeolites. We expect that designparameters to regulate the mobility of activesites and their precursors would include thestructure, composition and electronic conduc-tivity of the support, and the molecules thatsolvate such sites to promote their mobility. Ma-nipulating these variables could open approachesto catalyst design for a wide variety of reactionsby combining knowledge from homogeneous andheterogeneous catalysis.

REFERENCES AND NOTES

1. J. M. Thomas, Proc. R. Soc. A 468, 1884–1903 (2012).2. M. Boudart, Chem. Rev. 95, 661–666 (1995).3. K. Ding et al., Science 350, 189–192 (2015).4. J. Jones et al., Science 353, 150–154 (2016).

Paolucci et al., Science 357, 898–903 (2017) 1 September 2017 5 of 6

Fig. 6. Proposed low-temperatureSCR catalytic cycle. Reductionsteps proceed on site-isolated CuII

ions residing near one (left-handcycle) or two (right-hand cycle)framework Al centers with con-strained diffusion of CuI ions intosingle cages and oxidation byO2 (inner step). NH4

+ is formed andconsumed in the right-hand cycleto maintain stoichiometry andcharge balance. Gray, Cu; yellow,Si; red, O; blue, N; and white, H.

NO

Cu(I)

N2 + 2 H2O

O2

Cu(II)

N2 + H2O

Cu(I)

Cu(II)

NO + 2 NH3

NO

N2 + 2 H2O

NO + 2

NH 3

+ NH 4

+

+

N2 + H2O+ NH4

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

ΔF

/ΔE

(kJ

mol

-1)

1

2

3

Fig. 5. Simulated CuI(NH3)2 diffusion up to 11 Å from charge-compensating Al. On left, themetadynamics-computed free energy at 473 K of CuI(NH3)2 in the 72–T site CHA supercell versusCu-Al distance. The red line is the energy profile predicted from a point-charge electrostatic model,described in SM section S9. Labeled are reactant state (1) [CuI(NH3)2 in the same cage as Al],transition state (2) [CuI(NH3)2 diffusion through 8-MR], and product state (3) [CuI(NH3)2 inthe neighboring cage without Al]. Corresponding representative CuI(NH3)2 configurations fromthe trajectories are shown on the right. Gray, Cu; green, Al; blue, N; and white, H.

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ACKNOWLEDGMENTS

Full details of experiments and computational models are provided inthe supplementary materials. Financial support was provided by the

National Science Foundation GOALI program under award numbers1258715-CBET (Purdue) and 1258690-CBET (Notre Dame), theNational Science Foundation Faculty Early Career DevelopmentProgram under award number 1552517-CBET (R.G.), Cummins, Inc.(I.K., A.J.S., A.A.P., J.D.A.-C.). and The Patrick and Jane Eilers GraduateStudent Fellowship for Energy Related Research (C.P.). Use of theAdvanced Photon Source is supported by the U.S. Department ofEnergy, Office of Science, and Office of Basic Energy Sciences, underContract No. DE-AC02-06CH11357. We thank the Center forResearch Computing at Notre Dame, and EMSL, a DOE Office ofScience User Facility sponsored by the Office of Biological andEnvironmental Research and located at Pacific Northwest NationalLaboratory, for computational resources.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/357/6354/898/suppl/DC1Materials and MethodsFigs. S1 to S20Tables S1 to S11References (37–68)Database S1

2 May 2017; accepted 7 July 2017Published online 17 August 201710.1126/science.aan5630

Paolucci et al., Science 357, 898–903 (2017) 1 September 2017 6 of 6

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reduction selective catalyticxDynamic multinuclear sites formed by mobilized copper ions in NO

Rajamani GounderAlbarracin-Caballero, Aleksey Yezerets, Jeffrey T. Miller, W. Nicholas Delgass, Fabio H. Ribeiro, William F. Schneider and Christopher Paolucci, Ishant Khurana, Atish A. Parekh, Sichi Li, Arthur J. Shih, Hui Li, John R. Di Iorio, Jonatan D.

originally published online August 17, 2017DOI: 10.1126/science.aan5630 (6354), 898-903.357Science 

, this issue p. 898; see also p. 866Sciencewas mobilizing the copper ions to pair up as they activated oxygen during the catalytic cycle.in and out of their cagelike structures. In this case, though, x-ray absorption spectroscopy suggested that the ammonia

flowPerspective by Janssens and Vennestrom). Zeolite catalysts generally fix metals in place while the reacting partners found that these copper ions may move about during the reaction (see theet al.ammonia and oxygen. Paolucci

Copper ions in zeolites help remove noxious nitrogen oxides from diesel exhaust by catalyzing their reaction withX-ray vision spies copper on the move

ARTICLE TOOLS http://science.sciencemag.org/content/357/6354/898

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2017/08/16/science.aan5630.DC1

CONTENTRELATED http://science.sciencemag.org/content/sci/357/6354/866.full

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