Cooperative Spin Transition of Monodispersed FeN3 Sites withinGraphene Induced by CO AdsorptionQin-Kun Li,†,§ Xiao-Fei Li,‡ Guozhen Zhang,*,† and Jun Jiang*,†

†Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry forEnergy Materials), CAS Center for Excellence in Nanoscience, School of Chemistry and Materials Science, University of Scienceand Technology of China, Hefei, Anhui 230026, China‡School of Optoelectronic Science and Engineering, Institute of Fundamental and Frontier Sciences, University of Electronic Scienceand Technology of China, Chengdu, Sichuan 610054, China

*S Supporting Information

ABSTRACT: The significance of identifying the funda-mental mechanism of interactions between adjacentcatalytic active centers has long been underestimated.Utilizing density functional theory calculations, wedemonstrate controllable cooperative interaction betweentwo nearby Fe centers embedded on nitrogenatedgraphene aided by CO adsorption. The interconnectedadjacent Fe atoms respond cooperatively to CO moleculeswith communicative structural self-adaption and elec-tronic transformation. The adsorbed CO changes not onlythe spin of the active site it is attached to but also that ofits adjacent site. Consequently, the two adjacent Fe atomsfeature unique oscillatory long-range spin coupling. Ourtheoretical investigation suggests cooperative communi-cation between adjacent active sites on a single-atomcatalyst is nontrivial.

Atomic-level knowledge on the nature of active sites iscrucial for mechanistic understandings and performance

enhancement of heterogeneous catalysis.1−6 Especially, theinterplay between active sites and adsorbates, such as host−guest interactions, resulting in mutual impacts on theirrespective geometric and electronic structures, is well recog-nized and investigated.7−11 Meanwhile, the communicativeeffect between active sites has also been claimed as a commonand indispensablemechanism in nature.12 However, much less isrecognized about whether similar communication specificallyconnecting two individual active sites occurs in heterogeneouscatalysts.As emerging atomic-scale heterogeneous catalysts, single-

atom catalysts (SACs) comprising transition metal (TM) atomsembedded in a graphene matrix have drawn intensiveattention.11,13−16 Yet, the intersite communication in SACarchitecture is generally overlooked, although multiple adjacentsites exist under real synthesis condition.17 Recently, Reed et al.reported that CO adsorption in a metal−organic framework ofFe2Cl2(bbta) (H2bbta = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole)undergoes an abrupt lifting via a cooperative spin transitionmechanism.18 In contrast, such a phenomenon is absent in Fe-BTTri (H3BTTri =1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene)owing to the distant arrangement of neighboring iron sites.19

The authors ascribed this cooperative CO adsorption in

Fe2Cl2(bbta) to the magnetoelectronic interaction of adjacentiron sites. Additionally, Ren et al. found that monodispersed Mnatoms loaded in a graphene matrix exhibit distance-dependentspin coupling behavior via graphene-mediated inter-Mninteractions.20 These studies inspired us to examine whether asingle-atom site in a SAC communicates with its neighboringsites and behaves cooperatively.Herein, we predict cooperative spin transition of adjacent

single-atom sites upon stepwise CO adsorption in a Fe-basedSAC, where two adjacent Fe atoms were anchored ontonitrogenated graphene. The CO-induced local electronictransformation on one site triggers associated structuralrearrangement and electronic transition of adjacent Fe atoms,rendering possible such communicative long-range spintransition of two individual active sites.We have anchored two single Fe atoms, at a uniform distance(∼11 Å) to their nearest neighbors, separately onto a nitrogen-doped single vacancy (SV) of graphene as shown in Figure 1(b).For comparison, we have also depicted the completely isolatedsingle Fe site in Figure 1(a) and refer to these two structures asFeN3 and FeN3−FeN3, respectively. Experimentally, pyridinic-N, which exhibits a high stabilization effect on TM atoms, can beachieved in graphene with very high doping levels and faciletunability.21,22 A series of TM atoms have been preciselyembedded in SVs of nitrogenated graphene, namely, TMN3,

23

and the spatial distance between two adjacent TM atoms canalso be controlled accurately by a high-energy electron beamtechnique.24 In light of such advanced synthesis techniques,FeN3−FeN3 will also be created under reasonable conditions.In FeN3−FeN3, both single Fe atoms were stabilized by

pyridinic-Nwith an overall binding energy of−8.58 eV, verifyingthe stabilization of anchoring Fe atoms onto SVs of nitrogenatedgraphene. Compared to FeN3, FeN3−FeN3 retains a nonplanarstructure with C2v symmetry, but the Fe−N bond length isslightly shortened by 0.02 or 0.03 Å. The ground state of FeN3−FeN3 is ferromagnetic (FM) with a total spin moment of 8.47 μBequally distributed on two Fe atoms, a little smaller than twicethat of FeN3 (4.35 μB). Such spin moment reduction on eachindividual Fe atom (3.41 μB) has been deemed highly relatedwith the shortening of the Fe−N bond length in an analogousFe−porphyrin molecule.25 We have also found that the

Received: July 23, 2018Published: November 1, 2018

Communication

pubs.acs.org/JACSCite This: J. Am. Chem. Soc. 2018, 140, 15149−15152

© 2018 American Chemical Society 15149 DOI: 10.1021/jacs.8b07816J. Am. Chem. Soc. 2018, 140, 15149−15152

Dow

nloa

ded

via

DA

LIA

N I

NST

OF

CH

EM

ICA

L P

HY

SIC

S on

Nov

embe

r 15

, 201

8 at

01:

14:0

9 (U

TC

).

See

http

s://p

ubs.

acs.

org/

shar

ingg

uide

lines

for

opt

ions

on

how

to le

gitim

atel

y sh

are

publ

ishe

d ar

ticle

s.

antiferromagnetic (AFM) state is less stable than the FM by10.05 meV, indicating noticeable ferromagnetic couplinginteractions between two adjacent Fe atoms.20,26 Further,Figure 1(c)−(f) show that most of the net spin-polarizedelectrons distribute in the vicinity of Fe atoms, indicating largespin polarization. With such uniform spin alignment on both Fesites, the FeN3−FeN3@graphene has illustrated 2D long-rangeferromagnetic ordering.Initially, we consider the first COmolecule adsorption on one

Fe site of FeN3−FeN3 shown in Figure 2. The CO moleculebonds to the Fe atom in an end-on fashion, and the C−O bondlength is elongated by 0.03 Å. The calculated adsorption energyis 1.86 eV, 0.11 eV higher than that in the FeN3 system (1.75 eV)

and indicating considerable adsorption enhancement comparedto a single Fe site. Upon CO adsorption, the geometricalstructure of FeN3−FeN3 adjusted accordingly. The Fe−N bondlength of the corresponding Fe site increases by 0.09 or 0.04 Å,whereas that in the neighboring Fe site shortens by 0.01 or 0.02Å. Such associated structural transformation upon moleculeadsorption cannot be observed in a single FeN3 framework.Moreover, the spin moment of the CO-attached Fe atom is

decreased to 2.81 μB. The reduction of net spin charge localizedon an Fe atom is accompanied by the spin polarization of theadsorbed CO, which is similar to spin-polarized N2 activation byFeN3@graphene.14 More interestingly, the structural trans-formation induced by CO adsorption leads to dramatic spintransition on the nearby Fe site, as illustrated in Figure 2(d);namely, the most energy-favorable spin ordering of FeN3−FeN3is shifted from ferromagnetic to ferrimagnetic with a total spinmoment of 1.12 μB. Such drastic total spin moment reduction ismainly due to the complete spin reversal to 3.39 μB of theneighboring Fe site. This spin transition process is furtherverified by the calculated charge density difference (see FigureS4 of the Supporting Information). These two evident structuraland electronic changes of the nearby Fe site signify theinterconnectedness of adjacent Fe centers and intersitecommunication. The long-range cooperative spin transitionupon CO adsorption indicates that adjacent Fe atoms interactindirectly via the graphene matrix,26 and this indirect exchangecoupling between adjacent Fe atoms can be effectively mediatedby the surface electron transfer of graphene20 (see Figure S4).Figure 3(c) depicts the optimized structure of a second CO

molecule adsorption on the neighboring Fe site. The second CO

binding energy (1.70 eV) is 0.16 eV weaker than the first one,which suggests that FeN3−FeN3 may not possess similarcooperative CO adsorption to Fe2Cl2(bbta). We attribute thisto the robust configuration of the graphene matrix, unlike theflexible structure and facile occurrence of notable geometricrearrangements triggered by gas uptake in a MOF or biologicalenzymes.18,27 Since adsorption energy is deemed to be a crucialand fundamental descriptor in catalysis,28 we expect that thedensity of the active site in SACs can affect its catalyticperformance via intersite communication.Upon the second CO adsorption, the Fe−N bond lengths

readjust slightly and C2v symmetry is restored. The Fe−N bondlength of the first CO-attached Fe site is shortened by 0.01 or0.02 Å, whereas it is elongated by 0.03 or 0.09 Å in the secondCO-attached Fe site. In the ground state after two COadsorptions, the spin moment of corresponding Fe sites

Figure 1.Top view of the optimized supercell structure of FeN3 (a) andFeN3−FeN3 (b). (c, d) Top and (e, f) side view of net spin density ofFeN3 and FeN3−FeN3 at an isosurface value of 1 × 10−3 e Å−3.

Figure 2. Top view of the optimized structure of FeN3 (a) and FeN3−FeN3 (c) after the first CO molecule is adsorbed. Side view of the netspin density of (b) FeN3 and (d) FeN3−FeN3 at an isosurface value of 1× 10−3 e Å−3.

Figure 3. (a) Top and (b) side view of CO-induced charge densitydifference at an isosurface value of 5 × 10−4 e Å−3. Yellow and bluebubbles represent charge accumulation and depletion, respectively. (c)Top view of the optimized structure of FeN3−FeN3 after two COmolecules are adsorbed on the two individual Fe sites. (d) Side view ofnet spin density at an isosurface value of 1 × 10−3 e Å−3.

Journal of the American Chemical Society Communication

DOI: 10.1021/jacs.8b07816J. Am. Chem. Soc. 2018, 140, 15149−15152

15150

decreases from 3.39 μB to 2.89 μB, whereas that of theneighboring site with preadsorbed CO rises a little by 0.08 μB.The equal (2.89 μB) but antiparallel spin moment on eachindividual Fe site unambiguously indicates the 2D long-rangeAFM orders on Fe sites. This AFM spin ordering is completelydifferent from the FM spin ordering when Fe active sites are freefrom CO adsorption, although both structures possess the samesymmetry. More specifically, this subsequent CO adsorption inturn leads to electron transfer (Figure 3(a,b)), changes the spinof its neighboring site (Figure 3(d)), and tailors their local spinto be equal and antiparallel, further demonstrating the existenceof intersite communication.Considering the coordinatively unsaturated Fe centers of

FeN3−FeN3, we further probe a third CO molecule adsorptionon one of the Fe centers, and the optimized structure is shown inFigure 4(a). A third CO adsorption energy is 1.23 eV, ∼0.60 eV

weaker than the first CO adsorption, resulting from the lowerresidual coordination number of the Fe center and the repulsionfrom the preadsorbed CO. The third CO adsorption shortensthe Fe−Nbond length by 0.01 or 0.06 Å in the corresponding Fesite and 0.01 or 0.02 Å in the nearby Fe site. This transformationis ensued by the notable spin transition depicted in Figure 4(b).FeN3−FeN3 is in FM spin ordering with a total spin moment of4.36 μB, in which 2 CO- and 1 CO-attached Fe centers bearparallel-aligned spin moments of 1.03 and 2.96 μB, respectively.Noteworthily, the third CO molecule again reverses the spinmoment of the adjacent Fe center, which differs from thescenario of one CO adsorption. The contrast between 1 CO and3 CO adsorption on FeN3−FeN3 suggests that cooperativeelectronic transformation via intersite communication canmodulate long-range magnetic orderings on magnetic TMN3frameworks.As a fourth CO molecule binds to the neighboring Fe site

(Figure 4(c) and (d)), it recovers the geometric symmetry ofFeN3−FeN3, and the calculated adsorption energy is 1.20 eV.Along with the increasing binding with CO molecules, theresidual spin moment of each individual Fe center is partiallyscreened to 1.02 μB. The spin on two individual Fe atoms is inantiparallel alignment with a total spin moment of 0 μB. It throwsthe overall spin ordering back to the AFM state, similar to thecase of two CO adsorptions.This oscillatory behavior of spin transition between FM and

AFM states might be accounted for by the Ruderman−Kittel−Kasuya−Yosida (RKKY) interaction between Fe centers via thegraphene surface.20,26,29 The CO adsorption leads to electrondepletion and intersite charge redistribution on the graphenematrix, which is expected to modify the intersite exchangecoupling strength and spin ordering of adjacent Fe atoms (see

Supplemental Note 3 and Figure S4). A similar long-rangeferrimagnetic order has been observed between iron phthalo-cyanine (FeFPc) and manganese phthalocyanine (MnPc)molecules coassembled onto Au(111) substrates. It has beenascribed to the RKKY interaction mediated by surface-stateelectrons of the Au(111) substrate.26 Furthermore, FM−AFMoscillation of magnetic adatoms caused by the RKKY interactioncan be tuned by charge-carrier concentration on the substrate.20

We have also investigated O2 adsorption on FeN3−FeN3 andfound similar cooperative spin transitions to CO adsorption.Intriguingly, the O−O bond is stretched to a larger extent in thepresence of an adjacent Fe site (more details in the SupportingInformation), suggesting a potentially alternative strategy ofoxygen activation by intersite communication of active sites.In summary, we have demonstrated, from theoreticalperspectives, cooperative spin transition between adjacent Fesites in SACs induced by small-molecule adsorption. A CO-induced local electronic transition on one Fe active site cantrigger adjoint structural adaption of adjacent Fe sites throughintersite communication. Consequently, communicative Fe sitesanchored onto a graphene matrix establish tunable long-rangemagnetic ordering in the presence of an external stimulus. This isdifferent from previous works26,30 where the spin-bearingmolecules are assembled in specific patterns onto conductivesubstrates.Most importantly, our results indicate the impact of proximity

of active sites of SACs on their catalytic activities is non-negligible. Since high-density single-atom sites are regarded as akey factor for higher activity of SACs, it will naturally lead to theproximity of active sites.31 Hence it is necessary to carefullyestimate the effect of intersite cooperativity when constructinghigh-density SACs. Noticeably, when preparing this paper, threedifferent groups reported their observations of cooperativecommunications in SACs17 and metal nanocatalysts,32,33

respectively. These exciting discoveries strongly support ourvision on cooperative effects of neighboring active sites incatalysis and prompt us to continue explorations.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/jacs.8b07816.

Computational details, structural adjustment of activesites in Table S1 and S2, electronic structure andsupplemental note about CO and O2 adsorption details(PDF)

■ AUTHOR INFORMATIONCorresponding Authors*guozhen@ustc.edu.cn*jiangj1@ustc.edu.cn

ORCIDXiao-Fei Li: 0000-0002-0851-3885Guozhen Zhang: 0000-0003-0125-9666Jun Jiang: 0000-0002-6116-5605Present Address§Department of Materials Science and NanoEngineering, RiceUniversity, Houston, Texas 77005, United States.

NotesThe authors declare no competing financial interest.

Figure 4. (a, c) Top view of the optimized structure of three and fourCO molecules, respectively, adsorbed on the two individual Fe sitesembedded at graphene. (b, d) Corresponding side views of net spindensity at an isosurface of 1 × 10−3 e Å−3.

Journal of the American Chemical Society Communication

DOI: 10.1021/jacs.8b07816J. Am. Chem. Soc. 2018, 140, 15149−15152

15151

■ ACKNOWLEDGMENTS

This work was financially supported by MOST (Nos.2014CB848900, 2018YFA0208702), NSFC (Nos. 21790351,21703221, 21633006), and the Fundamental Research Fundsfor the Central Universities (WK2060030027). The Super-computing Center of University of Science and Technology ofChina is acknowledged for the computing resource. Z.G.Z. isgrateful to Prof. George Schatz and Dr. Jia Chen for helpfuldiscussions and comments.

■ REFERENCES(1) Zambelli, T.; Wintterlin, J.; Trost, J.; Ertl, G. Identification of the″active sites″ of a surface-catalyzed reaction. Science 1996, 273, 1688.(2) Nørskov, J. K.; Bligaard, T.; Hvolbæk, B.; Abild-Pedersen, F.;Chorkendorff, I.; Christensen, C. H. The nature of the active site inheterogeneous metal catalysis. Chem. Soc. Rev. 2008, 37, 2163.(3) Ding, K.; Gulec, A.; Johnson, A. M.; Schweitzer, N. M.; Stucky, G.D.; Marks, L. D.; Stair, P. C. Identification of active sites in COoxidation and water-gas shift over supported Pt catalysts. Science 2015,350, 189.(4) Bruix, A.; Rodriguez, J. A.; Ramírez, P. J.; Senanayake, S. D.; Evans,J.; Park, J. B.; Stacchiola, D.; Liu, P.; Hrbek, J.; Illas, F. A New Type ofStrong Metal-Support Interaction and the Production of H2 throughthe Transformation of Water on Pt/CeO2 (111) and Pt/CeOx/TiO2(110) Catalysts. J. Am. Chem. Soc. 2012, 134, 8968.(5) Pfisterer, J. H.; Liang, Y.; Schneider, O.; Bandarenka, A. S. Directinstrumental identification of catalytically active surface sites. Nature2017, 549, 74.(6) Wu, Y.; Jiang, J.; Weng, Z.; Wang, M.; Broere, D. L.; Zhong, Y.;Brudvig, G. W.; Feng, Z.; Wang, H. Electroreduction of CO2 Catalyzedby a Heterogenized Zn-Porphyrin Complex with a Redox-InnocentMetal Center. ACS Cent. Sci. 2017, 3, 847.(7) Parent, L. R.; Pham, C. H.; Patterson, J. P.; Denny, M. S., Jr;Cohen, S. M.; Gianneschi, N. C.; Paesani, F. Pore Breathing of Metal-Organic Frameworks by Environmental Transmission ElectronMicroscopy. J. Am. Chem. Soc. 2017, 139, 13973.(8) Schneemann, A.; Bon, V.; Schwedler, I.; Senkovska, I.; Kaskel, S.;Fischer, R. A. Flexible metal-organic frameworks. Chem. Soc. Rev. 2014,43, 6062.(9) Wackerlin, C.; Chylarecka, D.; Kleibert, A.; Muller, K.; Iacovita,C.; Nolting, F.; Jung, T. A.; Ballav, N. Controlling spins in adsorbedmolecules by a chemical switch. Nat. Commun. 2010, 1, 61.(10) Shekhah, O.; Belmabkhout, Y.; Chen, Z.; Guillerm, V.; Cairns,A.; Adil, K.; Eddaoudi, M.Made-to-order metal-organic frameworks fortrace carbon dioxide removal and air capture. Nat. Commun. 2014, 5,4228.(11) Jiang, K.; Siahrostami, S.; Akey, A.-J.; Li, Y.; Lu, Z.; Lattimer, J.;Hu, Y.; Stokes, C.; Gangishetty, M.; Chen, G.; Zhou, Y.; Winfield, H.;Cai, W.-B.; David, B.; Karen, C.; Nørskov, J. K.; Cui, Y.; Wang, H.Transition-metal single atoms in a graphene shell as active centers forhighly efficient artificial photosynthesis. Chem. 2017, 3, 950.(12) Changeux, J.-P. Allostery and the Monod-Wyman-Changeuxmodel after 50 years. Annu. Rev. Biophys. 2012, 41, 103.(13) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu,J.; Li, J.; Zhang, T. Single-atom catalysis of CO oxidation using Pt1/FeOx. Nat. Chem. 2011, 3, 634.(14) Li, X.-F.; Li, Q.-K.; Cheng, J.; Liu, L.; Yan, Q.; Wu, Y.; Zhang, X.-H.; Wang, Z.-Y.; Qiu, Q.; Luo, Y. Conversion of dinitrogen to ammoniaby FeN3-embedded graphene. J. Am. Chem. Soc. 2016, 138, 8706.(15) Deng, D.; Chen, X.; Yu, L.; Wu, X.; Liu, Q.; Liu, Y.; Yang, H.;Tian, H.; Hu, Y.; Du, P.; Si, R.; Wang, J.; Cui, X.; Li, H.; Xiao, J.; Xu, T.;Deng, J.; Yang, F.; Duchesne, P. N.; Zhang, P.; Zhou, J.; Sun, L.; Li, J.;Pan, X.; Bao, X. A single iron site confined in a graphene matrix for thecatalytic oxidation of benzene at room temperature. Sci. Adv. 2015, 1,No. e1500462.(16) Fei, H.; Dong, J.; Feng, Y.; Allen, C. S.; Wan, C.; Volosskiy, B.; Li,M.; Zhao, Z.; Wang, Y.; Sun, H.; An, P.; Chen, W.; Guo, Z.; Lee, C.;

Chen, D.; Shakir, I.; Liu, M.; Hu, T.; Li, Y.; Kirkland, A. I.; Duan, X.;Huang, Y. General synthesis and definitive structural identification ofMN4C4 single-atom catalysts with tunable electrocatalytic activities.Nat. Catal. 2018, 1, 63.(17) Li, H.; Wang, L.; Dai, Y.; Pu, Z.; Lao, Z.; Chen, Y.; Wang, M.;Zheng, X.; Zhu, J.; Zhang, W.; Si, R.; Ma, C.; Zeng, J. Synergeticinteraction between neighbouring platinum monomers in CO2hydrogenation. Nat. Nanotechnol. 2018, 13, 411.(18) Reed, D. A.; Keitz, B. K.; Oktawiec, J.; Mason, J. A.; Runcevski,T.; Xiao, D. J.; Darago, L. E.; Crocella, V.; Bordiga, S.; Long, J. R. A spintransition mechanism for cooperative adsorption in metal−organicframeworks. Nature 2017, 550, 96.(19) Reed, D. A.; Xiao, D. J.; Gonzalez, M. I.; Darago, L. E.; Herm, Z.R.; Grandjean, F.; Long, J. R. Reversible CO scavenging via adsorbate-dependent spin state transitions in an iron(II)-triazolate metal-organicframework. J. Am. Chem. Soc. 2016, 138, 5594.(20) Ren, J.; Guo, H.; Pan, J.; Zhang, Y.-F.; Yang, Y.; Wu, X.; Du, S.;Ouyang, M.; Gao, H.-J. Interatomic Spin Coupling in ManganeseClusters Registered on Graphene. Phys. Rev. Lett. 2017, 119, 176806.(21) Susi, T.; Kotakoski, J.; Arenal, R.; Kurasch, S.; Jiang, H.;Skakalova, V.; Stephan, O.; Krasheninnikov, A. V.; Kauppinen, E. I.;Kaiser, U.; Meyer, J. C. Atomistic description of electron beam damagein nitrogen-doped graphene and single-walled carbon nanotubes. ACSNano 2012, 6, 8837.(22) Liu, Y.; Shen, Y.; Sun, L.; Li, J.; Liu, C.; Ren, W.; Li, F.; Gao, L.;Chen, J.; Liu, F.; Sun, Y.; Tang, N.; Cheng, H.-M.; Du, Y. Elementalsuperdoping of graphene and carbon nanotubes. Nat. Commun. 2016,7, 10921.(23) Lin, Y.-C.; Teng, P.-Y.; Yeh, C.-H.; Koshino, M.; Chiu, P.-W.;Suenaga, K. Structural and chemical dynamics of pyridinic-nitrogendefects in graphene. Nano Lett. 2015, 15, 7408.(24) He, Z.; He, K.; Robertson, A.W.; Kirkland, A. I.; Kim, D.; Ihm, J.;Yoon, E.; Lee, G.-D.; Warner, J. H. Atomic structure and dynamics ofmetal dopant pairs in graphene. Nano Lett. 2014, 14, 3766.(25) Bhandary, S.; Ghosh, S.; Herper, H.; Wende, H.; Eriksson, O.;Sanyal, B. Graphene as a reversible spin manipulator of molecularmagnets. Phys. Rev. Lett. 2011, 107, 257202.(26) Girovsky, J.; Nowakowski, J.; Ali, M. E.; Baljozovic, M.;Rossmann, H. R.; Nijs, T.; Aeby, E. A.; Nowakowska, S.; Siewert, D.;Srivastava, G.; Wackerlin, C.; Dreiser, J.; Decurtins, S.; Liu, S.-X.;Oppeneer, P. M.; Jung, T. A.; Ballav, N. Long-range ferrimagnetic orderin a two-dimensional supramolecular Kondo lattice. Nat. Commun.2017, 8, 15388.(27) Eaton, W. A.; Henry, E. R.; Hofrichter, J.; Mozzarelli, A. Iscooperative oxygen binding by hemoglobin really understood? Nat.Struct. Biol. 1999, 6, 351.(28) Norskov, J. K.; Studt, F.; Abild-Pedersen, F.; Bligaard, T.Fundamental Concepts in Heterogeneous Catalysis; John Wiley & Sons,2014.(29) Khajetoorians, A. A.; Wiebe, J.; Chilian, B.; Lounis, S.; Blugel, S.;Wiesendanger, R. Atom-by-atom engineering and magnetometry oftailored nanomagnets. Nat. Phys. 2012, 8, 497.(30) Garnica, M.; Stradi, D.; Barja, S.; Calleja, F.; Díaz, C.; Alcamí,M.;Martín, N.; De Parga, A. L. V.; Martín, F.; Miranda, R. Long-rangemagnetic order in a purely organic 2D layer adsorbed on epitaxialgraphene. Nat. Phys. 2013, 9, 368.(31) Wang, A.; Li, J.; Zhang, T. Heterogeneous single-atom catalysis.Nat. Rev. Chem. 2018, 2, 65.(32) Zou, N.; Zhou, X.; Chen, G.; Andoy, N. M.; Jung, W.; Liu, G.;Chen, P. Cooperative communication within and between singlenanocatalysts. Nat. Chem. 2018, 10, 607.(33) Suchorski, Y.; Kozlov, S. M.; Bespalov, I.; Datler, M.; Vogel, D.;Budinska, Z.; Neyman, K. M.; Rupprechter, G. The role of metal/oxideinterfaces for long-range metal particle activation during CO oxidation.Nat. Mater. 2018, 17, 519.

Journal of the American Chemical Society Communication

DOI: 10.1021/jacs.8b07816J. Am. Chem. Soc. 2018, 140, 15149−15152

15152