time-of-flight secondary ion mass spectrometry(ToF-SIMS) (fig. S11F) (19) and found higherCl content in CH3NH3PbI3(Cl) films than inCH3NH3PbI3 films without Cl. This techniqueprobes the top 2 nm of the film.Although perovskite solar cells have better ra-

diative efficiencies than many other types, suchas dye-sensitized, organic, or even cadmium tel-luride solar cells, they still suffer from greater non-radiative losses than inorganic materials such asgallium arsenide and are only at present approach-ing the radiative efficiencies of copper indiumgallium selenide (CIGS) (31). Our results identifya subpopulation of dark grains and grain bound-aries as specific targets for perovskite growth andpassivation studies, and show that local fluores-cence lifetime imaging provides a route by whichchanges in film processing can be evaluated toassess their influence on carrier recombinationin films. By removing these nonradiative path-ways to obtain uniform brightness with highemissivity across all grains, it is likely that wewillsee the performance of perovskite devices approachthe thermodynamic limits for solar cells andother light-emitting devices.

REFERENCES AND NOTES

1. T. C. Sum, N. Mathews, Energ. Environ. Sci. 7, 2518–2534 (2014).2. M. A. Green, A. Ho-Baillie, H. J. Snaith, Nat. Photonics 8,

506–514 (2014).3. J. Burschka et al., Nature 499, 316–319 (2013).4. G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely,

H. J. Snaith, Adv. Funct. Mater. 24, 151–157 (2014).5. G. Xing et al., Nat. Mater. 13, 476–480 (2014).6. G. E. Eperon et al., Energ. Environ. Sci. 7, 982–988 (2014).7. J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, Nano

Lett. 13, 1764–1769 (2013).8. National Center for Photovoltaics at the National Renewable

Energy Laboratory, Research cell efficiency records,www.nrel.gov/ncpv/ (accessed 11 April 2015).

9. S. D. Stranks et al., Science 342, 341–344 (2013).10. H. Zhou et al., Science 345, 542–546 (2014).11. O. D. Miller, E. Yablonovitch, S. R. Kurtz, IEEE J. Photovolt.

2, 303–311 (2012).12. N. K. Noel et al., ACS Nano 8, 9815–9821 (2014).13. J. You et al., ACS Nano 8, 1674–1680 (2014).14. P. W. Liang et al., Adv. Mater. 26, 3748–3754 (2014).15. S. D. Stranks et al., Phys. Rev. Appl. 2, 034007 (2014).16. F. Deschler et al., J. Phys. Chem. Lett. 5, 1421–1426 (2014).17. C. Wehrenfennig, M. Liu, H. J. Snaith, M. B. Johnston,

L. M. Herz, Energ. Environ. Sci. 7, 2269–2275 (2014).18. J. S. Manser, P. V. Kamat, Nat. Photonics 8, 737–743

(2014).19. See the supplementary materials on Science Online.20. G. Xing et al., Science 342, 344–347 (2013).21. Y. Yamada, T. Nakamura, M. Endo, A. Wakamiya, Y. Kanemitsu,

J. Am. Chem. Soc. 136, 11610–11613 (2014).22. M. Saba et al., Nat. Commun. 5, 5049 (2014).23. A. Abate et al., Nano Lett. 14, 3247–3254 (2014).24. S. Watanabe et al., Nat. Methods 8, 80–84 (2011).25. E. Edri et al., Nano Lett. 14, 1000–1004 (2014).26. W. J. Yin, T. Shi, Y. Yan, Adv. Mater. 26, 4653–4658

(2014).27. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami,

H. J. Snaith, Science 338, 643–647 (2012).28. P. Gao, M. Grätzel, M. K. Nazeeruddin, Energ. Environ. Sci.

7, 2448–2463 (2014).29. K. Munechika et al., Nano Lett. 11, 2725–2730 (2011).30. X. Wen et al., J. Phys. Chem. Lett. 5, 3849–3853 (2014).31. K. Tvingstedt et al., Sci. Rep. 4, 6071 (2014).32. S. De Wolf et al., J. Phys. Chem. Lett. 5, 1035–1039

(2014).33. C. H. Seager, Annu. Rev. Mater. Sci. 15, 271–302 (1985).34. J. S. Yun et al., J. Phys. Chem. Lett. 6, 875–880 (2015).35. Q. Dong et al., Science 347, 967–970 (2015).36. W. Nie et al., Science 347, 522–525 (2015).37. D. Shi et al., Science 347, 519–522 (2015).

38. M. Grätzel, Nat. Mater. 13, 838–842 (2014).39. G. Grancini et al., J. Phys. Chem. Lett. 5, 3836–3842

(2014).40. Y. Tidhar et al., J. Am. Chem. Soc. 136, 13249–13256

(2014).41. S. T. Williams et al., ACS Nano 8, 10640–10654 (2014).

ACKNOWLEDGMENTS

This material is based in part on work supported by the State ofWashington through the University of Washington Clean EnergyInstitute. D.W.D. acknowledges support from an NSF GraduateResearch Fellowship (DGE-1256082). S.M.V. acknowledges supportfrom a National Defense Science and Engineering GraduateFellowship. The research leading to these results has receivedfunding from the European Union Seventh Framework Program(FP7/2007-2013) under Grant Agreement No. 604032 of theMESO project. G.E.E. is supported by the Engineering and PhysicalSciences Research Council and Oxford Photovoltaics through aNanotechnology Knowledge Transfer Network Collaborative Award

in Science and Engineering. The authors gratefully acknowledgefunding from the National Institute for Biomedical Imaging andBioengineering (NIH grant EB-002027) supporting the National ESCAand Surface Analysis Center for Biomedical Problems and ToF-SIMSinstrumentation. D.W.D. thanks I. Braly, S. Braswell, D. Moerman, andB. Miller for valuable assistance. S.M.V. gratefully acknowledgesD. Graham for assistance with ToF-SIMS. Additional data, includingmaterials, methods, and key controls, are available online assupplementary materials (19).

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/348/6235/683/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S11

19 December 2014; accepted 14 April 201510.1126/science.aaa5333

CATALYSIS

Identification of molybdenum oxidenanostructures on zeolites for naturalgas conversionJie Gao,1 Yiteng Zheng,1 Jih-Mirn Jehng,2,3 Yadan Tang,2

Israel E. Wachs,2* Simon G. Podkolzin1*

Direct methane conversion into aromatic hydrocarbons over catalysts with molybdenum(Mo) nanostructures supported on shape-selective zeolites is a promising technology fornatural gas liquefaction. We determined the identity and anchoring sites of the initial Mostructures in such catalysts as isolated oxide species with a single Mo atom on aluminumsites in the zeolite framework and on silicon sites on the zeolite external surface. Duringthe reaction, the initial isolated Mo oxide species agglomerate and convert into carbidedMo nanoparticles. This process is reversible, and the initial isolated Mo oxide species canbe restored by a treatment with gas-phase oxygen. Furthermore, the distribution of the Monanostructures can be controlled and catalytic performance can be fully restored, evenenhanced, by adjusting the oxygen treatment.

Methane (CH4), the main component ofnatural gas, has the highest H-to-C ratioof all hydrocarbons; therefore, it is moreenvironmentally friendly in terms of CO2

emissions than oil or coal-derived fuels.However, 30 to 60% of natural gas reserves areclassified as “stranded” because shipping gas isnot economical, and the costs of liquefaction orbuilding a pipeline are usually prohibitively high(1–5). The problem of natural gas utilization is ex-acerbated by burning and venting of the associatedgas produced in the course of crude oil productionat remote locations. Conversion of methane intoshippable liquids can solve these problems butremains scientifically challenging (1–3, 6–8).

One of the technologies under development isdirect methane conversion into liquid aromatichydrocarbons in a single step (dehydroaromati-zation with the main reaction 6CH4 → C6H6 +9H2) using catalysts with Mo nanostructuressupported on shape-selective zeolites (2, 8–16).This technology offers two advantages over othermethane activation chemistries: Complete oxida-tion, as well as explosive combustion, is not possi-ble because of the absence of O2 or other oxidizingreagents, and processing can be performed atremote locations because no reagents are needed.The biggest issues in commercialization are rapidcatalyst deactivation and comparatively low single-pass conversion levels of ~10% (2, 8, 13–16). De-velopmentof improved catalysts hasbeenhinderedby a lack of molecular-level understanding of theidentity of the zeolite-supported Mo nanostruc-tures and their structural transformations.We studied Mo nanostructures supported on

ZSM-5 zeolites by combining quantum chemicalcalculations using density functional theory (DFT)with multiple spectroscopic techniques, includ-ing in situ ultraviolet-visible diffuse reflectance

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1Department of Chemical Engineering and Materials Science,Stevens Institute of Technology, Hoboken, NJ 07030, USA.2Operando Molecular Spectroscopy and Catalysis Laboratory,Department of Chemical Engineering, Lehigh University,Bethlehem, PA 18015, USA. 3Department of ChemicalEngineering, National Chung Hsing University, Taichung,Taiwan, Republic of China.*Corresponding author. E-mail: iew0@lehigh.edu (I.E.W.);simon.podkolzin@stevens.edu (S.G.P.)

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spectroscopy (UV-vis DRS), in situ infrared (IR)spectroscopy, and operando Raman spectroscopyat elevated reaction temperatures with simulta-neous online mass spectrometry of reaction pro-ducts. We determined the identity and anchoringsites of the initial Mo oxide nanostructures andestablished structure-activity relationships. Thecatalytic activity can be fully restored by regener-ating initial Mo oxide nanostructures with a gas-phase O2 treatment. Furthermore, the activitycan even be enhanced by controlling the distri-bution of Mo oxide nanostructures by adjustingconditions of such anO2 regeneration treatment.Molybdenumnanostructures supported on ze-

olites were initially present in an oxide form afterMo deposition and an oxygen treatment at ele-vated temperatures (our samples were calcinedat 773 K) (17). The number of MoOx units in anaverage individual nanostructure was evaluatedusing the edge energy (Eg) of the in situ UV-visDRS spectra. TheEg values for the followingwell-defined Mo oxide reference compounds are pre-sented in Fig. 1A: (i) MoO6-coordinated Mo7-Mo12clusters, (ii) linear chains of alternatingMoO4 andMoO6 units, (iii) infinite layered sheets of MoO5

units, (iv) Mo2O7 dimer as MoO3-O-MoO3, (v) iso-lated MoO4 and MoO6 monomers, and (vi) aque-ousmolybdate anions as a function of the solutionpH (18). The Eg values in Fig. 1A exhibit a linearcorrelationwith the number of bridgingMo-O-Mocovalent bonds around the central Mo cation and,correspondingly, with the number of MoOx units

in a nanostructure. The Eg value for a representa-tive catalyst sample with 2 weight percent (wt %)Mo supported on a ZSM-5 (Si/Al = 15) zeolite,which is the most common zeolite evaluated formethanedehydroaromatization,was 4.8 eV,whichfalls in the range of isolated MoOx nanostructureswith a single Mo atom.The nature of theMo oxide nanostructureswas

further examined with in situ Raman spectros-copy by varying the concentration of Mo from 0.7to 3.3 wt % on a ZSM-5 zeolite support with aconstant Si/Al ratio of 15 (Fig. 1B) and by varyingthe Si/Al ratio from 15 to 140 at a constant Moconcentration of 1.3 wt % (Fig. 1C). The spectrumfor 1.3 wt % Mo on ZSM-5 with Si/Al = 15 isshown in both sets in Fig. 1, B andC, and a similarspectrum is shown in operando Ramanmeasure-ments with methane flow in fig. S1 (17). The ab-sence of sharpRamanbands fromcrystallineMoO3

nanoparticles (NPs) at 996, 815, and 666 cm−1 (19)or crystalline Al2(MoO4)3 at ~1004 and 1045 cm−1

(18, 20) indicates, in agreement with the UV-visresults in Fig. 1A, that Mo oxide was completelydispersed; any amorphousMo oxide species wouldcrystallize at the elevated pretreatment temper-ature of 773 K. Some spectra exhibited weakshoulder features at 950 cm−1 from Mo oxidespecies in zeolite framework vacancy defects andat 1026 cm−1 from Mo oxide species on extra-framework alumina NPs (17).For the ZSM-5 (Si/Al = 15) zeolite in Fig. 1B, a

single Raman band at 993 cm−1 was observed in

the Mo-O stretching region for all Mo concen-trations.However, at higher Si/Al ratios in Fig. 1C,a new band at 975 cm−1 was observed, and an ad-ditional band appeared at 984 cm−1 at the highestSi/Al = 140 (Fig. 1C). These three bands cannot beattributed to a single Mo oxide nanostructurebecause their relative intensities change with theSi/Al ratio. To determine the identity and anchor-ing sites of theseMooxide structures in the ZSM-5zeolite framework, variousmonomericMo oxidespecies were evaluated with DFT calculations,and the calculated normal vibrational modeswere compared with the experimental Ramanspectra.After calcination at 773 K, Mo was present in

its highest oxidation state of +6, as evidenced bythe absence of d-d transitions for reduced Mo inthe in situ UV-vis spectra. Our DFT calculationsshow that neutralMoO3 species on framework Sisites are unstable and that frameworkAl sites arerequired for anchoring (17). This result is in agree-ment with changes in the in situ IR spectra forsurface OH groups as a function of the Mo load-ing in fig. S2 (17) that showed preferential elim-ination of Brønsted acid sites (H+ on [AlO4]

–)after Mo deposition. On a site with two adjacentframework Al atoms, the stoichiometry of theMooxide species should beMo(=O)2

2+ as dioxo speciesto counterbalance the 2– charge of 2[AlO4]

– andmaintain Mo in the +6 oxidation state. The size ofisolated Mo dioxo species serves as a geometricrestriction, which determines the acceptable rangeof separation distances between the two anchor-ing framework Al-atom sites. Because ZSM-5 is aSi-rich zeolite, Lowenstein’s rule prohibits one Alatom to be the first neighbor of another Al atomin the framework as Al-O-Al. An arrangement ofAl-O-Si-O-Al with two Al atoms as second neigh-bors was not found experimentally, based on 27Alnuclear magnetic resonance (NMR) and addi-tional characterization for ZSM-5 samples withSi/Al > 8 (21, 22). Finally, an arrangement of Al-O-(Si-O)2-Al with two Al atoms as third neigh-bors must be the only possible double Al-atomanchoring sites for Mo dioxo species. Our DFTresults confirm that twoAl atoms as fourth neigh-bors in Al-O-(Si-O)3-Al can serve only as two in-dividual single anchoring sites (17).Although the exact distribution of Al atoms

among different framework sites in ZSM-5 zeo-lites is currently not well understood, it can bevaried by adjusting the zeolite synthesis proce-dure. For example, the number of Al atoms asdouble anchoring sites in the arrangement Al-O-(Si-O)2-Al can be varied from 4 to 46% for ZSM-5samples with Si/Al = ~20, based on characteri-zation with hydrated Co cations (22). The frac-tion of Al atoms as double anchoring sitestypically decreases, but not proportionally, withthe increasing Si/Al ratio for the same synthesisprocedure (22). Our evaluation of Al-O-(Si-O)2-Alarrangements in ZSM-5 shows that these sitescan serve as double Al-atom anchoring sites ifthey are located in the same channel, but not inthe same plane. Additional classification of doubleAl-atom anchoring sites is provided in fig. S4 (17).A representative Mo(=O)2

2+ dioxo structure on

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Fig. 1. Spectroscopic measurements.(A) Electronic edge values based on insitu UV-vis spectra of reference Mo oxidecompounds exhibit a linear correlationwith the number of bridging Mo-O-Mocovalent bonds around the central Mocation. The value of 4.8 eV for 2 wt %Mo/ZSM-5 (Si/Al = 15) corresponds toMo oxide species with a single Mo atom.(B and C) In situ Raman spectra ofMo/ZSM-5 catalysts under oxygen flowat 773 K as a function of (B) Mo loading forconstant Si/Al = 15 and (C) Si/Al ratio forconstant 1.3 wt % Mo loading with bandassignments to Mo oxide species based onDFTcalculations. a. u., arbitrary units.

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an anchoring site with a pair of Al atoms in T8and T12 framework positions is shown in Fig. 2.In this nanostructure, the Mo atom is bridge-bonded to two framework Al atoms through twoneighboring framework oxygen atoms and ter-minated with two additional oxygen atoms. Thenormal vibrational modes obtained with DFTcalculations for these terminal oxygen atoms inMo(=O)2

2+ are summarized in Table 1. The sym-metric stretch (ns) is calculated to be at 992 cm−1.The calculated geometries and normal vibration-al modes for the Mo(=O)2

2+ nanostructure onother double Al-atom anchoring sites with twobridging frameworkO atoms are similar (table S2and fig. S6) (17). On a site with a single frameworkAl atom, the stoichiometry of Mo oxide speciesshould be Mo(=O)2(OH)+ to counterbalance the1– charge of [AlO4]

– and maintain Mo in the +6oxidation state. The vibrationalmode for the sym-metric stretch of the terminal oxygen atoms inthese Mo species is predicted to be at 975 cm−1,based on evaluation of geometries and vibra-tionalmodes of theMo(=O)2(OH)+ nanostructureanchored on single Al-atom sites in T8 (Table 1)and other ZSM-5 framework positions (table S1and fig. S5) (17).Raman spectroscopy gives rise to strong bands

of symmetric stretches (ns) and weaker bands ofasymmetric stretches (nas), with the latter some-times being undetectable. In our previous studiesof MoO3/SiO2 (19, 20), nas for Mo(=O)2 was notobserved forMo loadings below 4wt%. Therefore,only ns is expected to be observed for lower Moloadings. A comparison of the dominant Ramanbands at 975 and 993 cm−1 in Fig. 1 with thecalculated symmetric stretch values (ns) in Table1 (975 and 992 cm−1) allowed us to assign thesebands to two distinct isolated Mo dioxo speciesanchored on, respectively, single and double Al-atom framework sites.The identification of the isolated Mo oxide

structures provided insight as to how they wereaffected by themain catalyst formulation param-eters: the Mo loading and Si/Al ratio. At a lowSi/Al= 15,Mooxide species preferentially anchoredon sites with two Al atoms (band at 993 cm−1 inFig. 1B). Even at the highest Mo loading of 3.3 wt%, the Al/Mo atomic ratio is 2.8, which allowedall Mo atoms to be anchored on double Al-atomsites. However, when the Si/Al ratio increased,the number of Al atoms per unit volume of thezeolite decreased, and the number of sites withtwoAl atoms should have decreasedmore rapidlythan the overall number of Al atoms. As a result,at higher Si/Al ratios of 25 and 40 in Fig. 1C, thedominant band was at 975 cm−1, arising fromMo(=O)2OH species anchored on sites with oneAl atom. The identification of single and doubleAl-atom anchoring sites is in agreement withprevious findings that each Mo atom displacesone H+ from framework [AlO4]

– sites in ZSM-5with Si/Al = 40 and twoH+ in ZSM-5 with Si/Al =15 (23). At the highest Si/Al = 140 shown in Fig. 1C,when the corresponding Al/Mo ratio fell belowunity to 0.8, therewere not even enough single Al-atom sites for stabilizing all Mo atoms. For thiscatalyst formulation,Mooxide specieswere forced

to be stabilized, not in the zeolite pores but on theleast preferable Si sites on the external surface ofthe zeolite. A newband at 984 cm−1 for Si/Al = 140in Fig. 1C is consistent with our previous Ramanspectra for Mo oxide species supported on amor-phous SiO2 (19, 20). Our DFT calculations con-firmed that Mo dioxo species did not stabilize inzeolite pores in the absence of Al sites and thatthe structure of isolated Mo(=O)2 dioxo speciesas (Si-O-)2Mo(=O)2 on the external surface of thezeolite (Fig. 1; full details in fig. S8 and table S4)(17) is similar to that on SiO2. These findings arealso supported by the in situ IR spectra of thesurface OH region for ZSM-5 (Si/Al = 15) as afunction of the Mo loading in fig. S2 (17). Theintensity of the peak at 3608 cm−1 for OH groupson framework Al sites (24) decreased through re-placement by Mo oxide species at low Mo load-ings, followed by a decrease in the intensity of thepeak at 3745 cm−1 for OH groups on the externalsurface Si sites (24) at higher Mo loadings. TheisolatedMooxide structurespreferentially anchoredon double Al-atom framework sites, then singleAl-atom framework sites, and finally Si sites onthe external surface of the zeolite. The isolatedMo oxide nanostructures anchored on these threetypes of zeolite sites are shown schematically inFig. 1 and with 3D animation in movie S1.Dynamic changes of Mo nanostructures under

reaction and regeneration conditions were eval-uated by simultaneously collecting operandoRaman spectroscopy and online mass spectrom-etry measurements, first with CH4 flow at 953 to1053 K (fig. S1) (17) and then under regenerationconditions with gas-phase O2 flow at 773 K (figs.

S10 and S11) (17). Upon CH4 introduction, CO2

was the only initial carbon-containing product,and the Raman band at 993 cm−1 for the isolatedMo oxide structures gradually disappeared. Be-cause CH4 was the only reactant, Mo oxide nano-structures reduced tooxycarbide or carbide species.Several studies with different techniques, such asx-ray absorption fine structure, Mo LIII edge x-rayabsorption near-edge structure, and 95Mo NMR,provide direct evidence that the reduced Mophase is a carbide with the stoichiometry ofMoCxor MoCxOy and that the initial oxide species ag-glomerate into particles with a size of ~0.6 nm(25–28). After the induction period, CO2 forma-tion stopped, the Raman band at 993 cm−1 for theinitial Mo oxide species was no longer observed(Fig. 2B and fig. S1) (17), and the catalyst per-formed CH4 dehydroaromatization with C6H6 asthe main hydrocarbon product.Our results demonstrate that an O2 treatment

can reverse both the carbide formation and theagglomeration ofMo nanostructures. TheRamanspectra at 753 K for the initial catalyst with iso-latedMo oxide structures and for the regeneratedcatalyst after reaction in Fig. 2 are similar, with asingle band at 993 cm−1 and a shoulder feature at950 cm−1. The similarity in the Raman band po-sitions and intensities before reaction and afterregeneration indicates that the regeneration con-verts carbided Mo NPs into an oxide phase,redisperses this phase into isolated oxide nano-structures with a single Mo atom, and allowstheseMo oxide species to diffuse and then stabilizeon substantially the same zeolite anchoring sitesas in the initial catalyst before the reaction.

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Fig. 2. Operando Ramanspectra of 2 wt%Mo/ZSM-5(Si/Al = 15). Spectra (A) afterinitial pretreatment withgas-phase oxygen, (B) duringreaction with methane, and(C) after regeneration withgas-phase oxygen are shown.The spectra demonstratethat the initial Mo(=O)2

2+

nanostructures anchored ondouble Al-atom framework sites(shown schematically on theright and in a zeolite pore below)with a vibrational mode at993 cm−1 are recovered afterregeneration.

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Effects of regeneration timewith O2 on the iden-tity ofMo nanostructures and on catalytic perform-ance with CH4 after regeneration were evaluatedby combining additional Raman spectroscopicmeasurements with reaction testing. Ramanspectra were collected in O2 flow at 773 K fortwo 1.3 wt % Mo/ZSM-5 (Si/Al = 15 and 25)catalysts after their deactivation in reactionwith CH4. The evolution of Raman spectra as afunction of regeneration time in figs. S10A andS11A (17) shows that isolated Mo oxide nano-structures were regenerated sequentially. IsolatedMo(=O)2 species anchored on double Al-atomframework sites were regenerated first, as evi-denced by a single initial Raman band at 993 cm−1.With increased regeneration time, a secondRamanband at 975 cm−1 caused by Mo(=O)2OH speciesanchored on single Al-atom sites appeared andgrew in intensity. Finally, a third Raman band at984 cm−1 due to Mo(=O)2 species anchored on Sisites on the external surface of the zeolite ap-peared and grew in intensity for the catalystwith alower Al concentration in the zeolite (Si/Al = 25instead of 15). These direct spectroscopic observa-tions demonstrate that exposure to gas-phase O2

first regenerates isolated Mo oxide nanostruc-tures anchored on sites with two Al atoms, thenforces these species tomigrate to siteswith one Alatom and, eventually, to Si sites on the externalsurface of the zeolite.A comparison of C6H6 formation rates in CH4

conversion as a function of time on stream for afresh 1.3 wt % Mo/ZSM-5 catalyst (Si/Al = 25)versus the same catalyst after deactivation in thereaction with CH4 and subsequent regeneration

for 120 min (Fig. 3A) demonstrates that the cat-alytic performance can be fully restored. TheC6H6 formation rates after regenerationmatchedthose for the fresh catalyst. Additional reactionresults for C6H6 and H2 formation rates for two1.3 wt % Mo/ZSM-5 (Si/Al = 15 and 25) catalystsas a function of regeneration time (figs. S10 andS11) (17) show that both the overall activity andselectivity to C6H6 fully recovered after regener-ation. Thus, rapid catalyst deactivation can besuccessfully addressed by regeneration with gas-phase O2, and the catalyst lifetime can be ex-tended by repeated regeneration cycles.Correlations between the structure of the ini-

tial Mo oxide species and catalytic performancecan be established by comparing the evolution ofthe Raman spectra with changes in reaction ratesas a function of regeneration time in figs. S10 andS11 (17). The catalytic activity was restored onceMo oxide nanostructures on double Al-atomframework sites were regenerated (after ~20min).With increased regeneration time, these isolatedMo oxide species migrated from double to singleAl-atom zeolite framework sites, and the catalyticperformancewith CH4 remainedunchanged. Fur-thermore, the catalytic performance of a regen-erated catalyst can be optimized and may exceedthat of a fresh catalyst if the regeneration treat-ment is stopped before Mo oxide nanostructuresare forced to migrate to Si anchoring sites on theexternal surface of the zeolite. Specifically for the1.3 wt %Mo/ZSM-5 (Si/Al = 25) catalyst, Mo oxidenanostructureswere regenerated andmoved fromdouble to single Al-atom zeolite framework sitesfor regeneration times between 20 and 100 min

(fig. S11A) (17). Notably, Mo nanostructures re-mained anchored on zeolite framework Al siteswhen the regeneration was limited to this dura-tion, and the rates of C6H6 formation for suchregenerated catalyst samples actually exceededthose for the fresh catalyst. The C6H6 formationrates for a catalyst regenerated for 100 min inFig. 3A exceeded those for the same catalyst be-fore deactivation during the initial time on streamperiod. In contrast, when the regeneration timewas extended beyond 100 min, Mo oxide nano-structures were forced to migrate from Al frame-work sites to Si anchoring sites on the externalsurface of the zeolite. This change in the anchor-ing sites caused the catalytic activity to decreaseto the level of the fresh catalyst, and the C6H6

formation rates for the catalyst regenerated for120 min (Fig. 3A) matched those for the fresh cat-alyst. With time on streamwith CH4, the catalyticactivity declined likely becauseofmigration, growth,and coking of Mo NPs, and the performance forall regenerated catalysts eventually became in-distinguishable. However, in the first 60 min oftime on stream, the benzene formation rates inFig. 3A and fig. S11C (17) were dependent on theidentity of the initial Mo oxide nanostructures.For understanding these initial activity differ-

ences, transition-stateDFT calculationswere usedfor comparing CH4 activation over catalytic Mocarbide nanostructures anchored on the identi-fied three types of anchoring sites: double andsingle Al-atom zeolite framework sites and Sisites on the external surface of the zeolite. Thecalculations compared the first step (breakingof the CH3-H bond in methane), which is likelythe rate-determining step, over the Mo carbidenanostructure with a stoichiometry ofMo4C2 onthe three zeolite anchoring sites (29). The CH4

activationmechanism is similar for all anchoringsites, as illustrated in Fig. 3, which compares re-action pathways for a double Al-atom anchoringsite inside a zeolite framework pore (Fig. 3, B, E,and F) and a Si site on the external surface of the

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Fig. 3. Catalytic activity of Mo nanostructures. (A) Benzene formationrates in methane conversion over a 1.3 wt % Mo/ZSM-5 (Si/Al = 25) catalyst.The catalytic activity declines with time on stream but is fully restored after120 min of gas-phase oxygen regeneration. Initial activity is enhanced bycontrolling the distribution of Mo nanostructures: Activity is higher after100 min of regeneration when Mo oxide nanostructures are anchored mostly

on framework Al sites and not forced to migrate to Si sites on the externalsurface of the zeolite. (B to F) Reaction mechanism calculations show thatthe energy barrier for methane activation with the formation of CH3 and Hspecies on the surface of the catalyst is lower when a Mo nanostructure isanchored on a framework Al site [(B), (E), and (F)] versus on a Si site on theexternal surface [(B) to (D)].

Table 1. DFT-calculated vibrational modes for isolated Mo oxide species supported on ZSM-5 (cm–1):symmetric stretch (ns), asymmetric stretch (nas), and bend (d).

Mo species ns O=Mo=O nas O=Mo=O d O=Mo=O

Mo(=O)2 on a double Al-atom site 992 972 388Mo(=O)2OH on a single Al-atom site 975 962 356

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zeolite (Fig. 3, B toD). TheCH4 initially approachesan exposed Mo atom, an atom that is not directlybonded to the zeolite. In the transition state (Fig. 3,C and E), CH4 forms aMo-CH3-H-C cycle in whichthe C atom of CH4 binds to the exposed Mo atomand, simultaneously, one of the H atoms of CH4

binds to a C atom in the carbide. Thus, aMo-C pairof atoms in the Mo carbide nanostructure servesas a single catalytic active site. This dual Mo-C siteactivates CH4 in a scissoringmotion that producesaCH3 groupbonded toMoand anHatombondedto C of the carbide (Fig. 3, D and F). Although themechanismofCH4 activation is similar, differencesin geometries and electronic properties of Mocarbide nanostructures anchored on Al and Sisites lead to differences in their catalytic properties.The CH4 activation energy over the Mo carbideanchored on the double Al-atom site of 112 kJ/molin Fig. 3E is lower than 140 kJ/mol for the Si sitein Fig. 3C. The transition state for the single Al-atom anchoring site is analogous to that for thedouble Al-atom site in Fig. 3E, with a comparableactivation energy of 117 kJ/mol (table S6) (17).The CH4 reaction is therefore predicted to bedominated by the activity of Mo nanostructuresanchored on framework Al sites. This computa-tional result is consistent with known experimen-tal observations that the catalytic activity of Monanostructures depends strongly on the Si/Alratio of the supporting zeolite and declines sub-stantiallywhenAl framework sites are lost throughdealumination (2, 8, 13–15, 30).The obtained information on the identity of

Mo structures, their regeneration, and their in-fluence on catalytic activity opens new oppor-tunities for rational design of improved catalystformulations and for optimizing reaction condi-tions for direct conversion of natural gas intoliquid transportation fuels and valuable feed-stocks for the chemical industry. It is importantto control the distribution of Mo oxide speciesand limit their anchoring to framework Al sitesbecause initialMo oxide nanostructures anchoredonAl sites of the zeolite framework are convertinginto carbided Mo NPs with higher catalytic ac-tivity than those produced by initial Mo oxidespecies anchored on Si sites. The number anddistribution of single and double Al-atom anchor-ing sites can be optimized by adjusting a zeolitesynthesis procedure. The number of Si anchoringsites on the external surface of the zeolite can bereduced, or these Si sites can be eliminated com-pletely by adjusting the Mo deposition pro-cedure. Furthermore, the catalytic performanceof Mo species and their periodic regenerationcan be optimized by adjusting catalyst formula-tions (for example, with promoter metals) andchanging the temperatures of the reaction andregeneration, flow rates, and other reactionconditions.

REFERENCES AND NOTES

1. E. McFarland, Science 338, 340–342 (2012).2. J. H. Lunsford, Catal. Today 63, 165–174 (2000).3. S. G. Podkolzin, E. E. Stangland, M. E. Jones,

E. Peringer, J. A. Lercher, J. Am. Chem. Soc. 129, 2569–2576(2007).

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157, 190–200 (1995).11. F. Solymosi, A. Erdöhelyi, A. Szöke, Catal. Lett. 32, 43–53

(1995).12. J. Z. Zhang, M. A. Long, R. F. Howe, Catal. Today 44, 293–300

(1998).13. Y. Xu, X. Bao, L. Lin, J. Catal. 216, 386–395 (2003).14. Z. R. Ismagilov, E. V. Matus, L. T. Tsikoza, Energy Environ. Sci 1,

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ACKNOWLEDGMENTS

The work in S.G.P.’s group at Stevens Institute of Technology wassupported by the NSF under grant CBET-1133987. The work inI.E.W.’s group at Lehigh University was supported by the NSF undergrant CBET-1134012. The Materials Studio software was used undera collaborative research license from BIOVIA Corp. in San Diego,California. Author contributions: J.G. and Y.Z. obtained thecomputational and reaction-testing results and discussed the overallresults; J.-M.J. and Y.T. obtained the experimental spectroscopicdata and discussed the overall results; I.E.W. conceived andsupervised the spectroscopic experiments and interpreted theresults; and S.G.P. conceived and supervised the calculationsand reaction testing, interpreted the results, and prepared theinitial manuscript.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/348/6235/686/suppl/DC1Materials and MethodsFigs. S1 to S12Tables S1 to S6ReferencesMovie S1

16 January 2015; accepted 26 March 2015Published online 9 April 2015;10.1126/science.aaa7048

INORGANIC CHEMISTRY

A synthetic Mn4Ca-cluster mimickingthe oxygen-evolving centerof photosynthesisChunxi Zhang,1* Changhui Chen,2 Hongxing Dong,2* Jian-Ren Shen,3

Holger Dau,4* Jingquan Zhao1

Photosynthetic splitting of water into oxygen by plants, algae, and cyanobacteria iscatalyzed by the oxygen-evolving center (OEC). Synthetic mimics of the OEC, which iscomposed of an asymmetric manganese-calcium-oxygen cluster bound to protein groups,may promote insight into the structural and chemical determinants of biological wateroxidation and lead to development of superior catalysts for artificial photosynthesis. Wesynthesized a Mn4Ca-cluster similar to the native OEC in both the metal-oxygen core andthe binding protein groups. Like the native OEC, the synthetic cluster can undergo fourredox transitions and shows two magnetic resonance signals assignable to redox andstructural isomerism. Comparison with previously synthesized Mn3CaO4-cubane clusterssuggests that the fourth Mn ion determines redox potentials and magnetic properties ofthe native OEC.

The oxygen-evolving center (OEC) in photo-system II (PSII) of plants, algae, and cyano-bacteria facilitates splitting of water intoO2, protons, and electrons (1–4). Crystallo-graphic structures (5–8) reveal that the core

of the OEC consists of a Mn3CaO4 cubane motifand a “dangler” Mn linked via two bridging ox-ides, forming a distinct asymmetricMn4Ca-cluster(Fig. 1A). This cluster is coordinated to four water

molecules, one imidazole, and six carboxylategroups of the amino acid residues of the PSIIpolypeptides (Fig. 1C). The structure of the OECas well as the oxidation states of the four manga-nese ions undergo changes during the water-oxidation reaction cycle, or S-state cycle (4, 9, 10).Spectroscopic results and computational chem-istry have provided insight in reaction interme-diates and mechanisms (4, 9–16). The lability of

690 8 MAY 2015 • VOL 348 ISSUE 6235 sciencemag.org SCIENCE

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Identification of molybdenum oxide nanostructures on zeolites for natural gas conversionJie Gao, Yiteng Zheng, Jih-Mirn Jehng, Yadan Tang, Israel E. Wachs and Simon G. Podkolzin

originally published online April 9, 2015DOI: 10.1126/science.aaa7048 (6235), 686-690.348Science 

, this issue p. 686Sciencetreatments.that form during this reaction. They were also able to recover and even enhance the zeolite catalytic activity after oxygen

identified the active Mo nanostructures as well as deactivated carbide specieset al.from low conversion efficiency. Gao transported liquid is conversion to benzene over zeolites containing molybdenum (Mo). However, this method suffersway to transport it to markets. One route proposed for converting its main component, methane, into a more readily

Natural gas often escapes or is deliberately burned at remote exploration sites because there is no economicalWaste not, want not

ARTICLE TOOLS http://science.sciencemag.org/content/348/6235/686

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2015/04/08/science.aaa7048.DC1

REFERENCES

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