Amorphous Cobalt Oxide Nanoparticles as Active Water-Oxidation CatalystsZheng Chen+,[a] Zhiyao Duan+,[b] Zhiliang Wang,[a] Xiaoyan Liu,[a] Lin Gu,[c] Fuxiang Zhang,*[a]

Michel Dupuis,*[d] and Can Li*[a]

Artificial photosynthesis requires the practical use of efficient,

robust, and economical water-oxidation catalysts (WOCs) forchemical-fuel production. The synthesis of amorphous cobalt

oxide nanoparticles (ca. 2 nm) is reported as a WOC with aturnover frequency up to 8.6 s@1 in the photocatalytic

Ru(bpy)32 +–Na2S2O8 system (bpy = 2,2’-bipyridyl). This activity is

unprecedented in heterogeneous cobalt-based WOCs and iseven comparable to that of a state-of-the-art homogeneous

cobalt-based polyoxometalate catalyst. With the help of experi-mental and computational X-ray absorption spectroscopy, the

atomic structure of the synthesized amorphous cobalt oxidenanoparticles was characterized, and it consists of a one-di-

mensional chain of dimeric edge-sharing CoO6 octahedra. The-

oretical calculations suggest that this structure was able topromote O@O bond coupling, unlike crystalline cobalt oxide

WOCs, which led to the enhanced water-oxidation activity.

Artificial photosynthesis, which converts solar energy into

chemical fuels, is a promising way to solve our energy and en-

vironmental challenges.[1] However, this whole process com-monly suffers from a significant loss in conversion efficiency;

this is partially due to the sluggish kinetics of water oxidation,which involves a four-proton-coupled electron-transfer trans-

formation.[2] To alleviate this problem, various water-oxidationcatalysts (WOCs) have been developed to accelerate the water-oxidation reaction thus to improve the overall efficiency of

solar-energy conversion.[3] Among the developed WOCs, iridi-um and ruthenium oxides fulfill the efficiency and stability re-

quirements, but the rareness of these elements prohibit their

large-scale use.[4] Intense efforts are currently being directedtowards the search for efficient, robust, and earth-abundant

WOCs with the ultimate aim to obtain practical artificial photo-synthetic systems.

A variety of cobalt-based homogeneous WOCs have beendemonstrated to be promising candidates.[5] For example, com-

plexes such as (TPA)Co-(m-OH)(m-O2)Co(TPA)(ClO4)3 (TPA = tris(2-

pyridylmethyl)amine) and Co4O4(py)4(OAc)4 (py = pyridine,OAc = acetate) were reported as active WOCs.[6] Homogeneous

WOCs often exhibit higher activity than their heterogeneouscounterparts and enable accurate structural determination and

a detailed kinetic study.[7] However, homogenous WOCs usuallyface challenges in terms of stability and synthetic costs.[8] A

breakthrough in the development of homogenous WOCs was

the discovery of a cobalt-based carbon-free polyoxometalatereported by Hill et al.[9] This catalyst was shown to possess

both high activity and good stability.Cobalt-based heterogeneous WOCs such as oxides and hy-

droxides are also interesting, because they are usually morerobust and cheaper than homogeneous WOCs and are thus

conducive to practical large-scale applications.[10] A series of

cobalt-based oxide nanostructures such as Co3O4 particles sup-ported by mesoporous silica, Co(OH)2 anchored on zeolite, and

LaCoO3 nanoparticles were reported to be promising WOCs.[11]

However, their activities still have to be significantly improvedso that they may compete with homogeneous WOCs.

Herein, we synthesized amorphous cobalt oxide (CoOx)

nanoparticles only 2 nm in size through a soft-chemicalmethod. The synthesized nanoparticles showed a turnover fre-quency (TOF) of approximately 8.6 s@1 and an apparent quan-

tum efficiency of 71 %, as tested in the typically used photoca-talytic Ru(bpy)3

2 +–Na2S2O8 (bpy = 2,2’-bipyridyl) system. The

synthetic route employed in this work is shown in Figure 1 a.Cobalt ions were mixed and complexed with tris(hydroxyme-

thyl)aminomethane (Tris) solution of various concentrations at

pH 8. The coordinated cobalt ions were subsequently oxidizedby H2O2 to form Tris-capped cobalt oxide (CoOx-Tris) at room

temperature. The formation of CoOx-Tris could be visualizedthrough a color change in the solution from transparent to

brown during the reaction (Figure S1 in the Supporting Infor-mation). The transmission electron microscopy (TEM) images

[a] Z. Chen,+ Z. Wang, Prof. X. Liu, Prof. F. Zhang, Prof. C. LiState Key Laboratory of Catalysis, Dalian Institute of Chemical PhysicsChinese Academy of Sciences, Dalian National Laboratory for Clean Energy457 Zhongshan Road, Dalian 116023 (P.R. China)andUniversity of Chinese Academy of SciencesBeijing 100049 (P.R. China)E-mail : fxzhang@dicp.ac.cn

canli@dicp.ac.cn

[b] Dr. Z. Duan+

Department of Chemistry and Institute for Computational and EngineeringSciences, The University of Texas at AustinAustin, Texas 78712-1224 (USA)

[c] Prof. L. GuInstitute of Physics, Chinese Academy of SciencesBeijing 100190 (P.R. China)

[d] Prof. M. DupuisDepartment of Chemical and Biological Engineering and Computation andData-Enabled Science and Engineering ProgramUniversity at Buffalo, State University of New YorkBuffalo, New York 14260 (USA)E-mail : mdupuis2@buffalo.edu

[++] These authors contributed equally to this work.

Supporting Information and the ORCID identification number(s) for theauthor(s) of this article can be found under :https://doi.org/10.1002/cctc.201700882.

ChemCatChem 2017, 9, 3641 – 3645 T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3641

CommunicationsDOI: 10.1002/cctc.201700882

and histogram of the size distribution shown in Figure 1 b, c

confirm the existence of CoOx-Tris nanoparticles with an aver-age diameter of 2 nm with a narrow size distribution. A high-

angle annular dark-field scanning transmission electron micros-copy (HAADF-STEM) image of 2 nm CoOx-Tris is shown in Fig-

ure 1 d. The lack of diffraction spots or rings in the selected-area electron-diffraction (SAED) pattern indicates the amor-

phous nature of 2 nm CoOx-Tris. The average size of the ob-

tained CoOx-Tris nanoparticles could be tuned from 2 to6.5 nm by decreasing the Tris concentration from 0.1 to 0 m, as

shown in Figures S2 and S3 in the Supporting Information.The activity of CoOx-Tris toward water oxidation was evaluat-

ed by using the established Ru(bpy)32+–Na2S2O8 photocatalytic

system in borate buffer solution (pH 8) under visible-light irra-diation.[12] The reaction process is illustrated in the Supporting

Information Figure S4. No marked oxygen evolution could bedetected without CoOx-Tris (Figure S5), which indicated thatCoOx-Tris was actually driving the reaction. The amount of O2

produced increased as the size of the nanoparticles decreased

(Figure S6). The number of O2 molecules produced in the initial60 s after irradiation was used to estimate the TOF value. As

can be seen in Figure 2 a, the TOF normalized by the totalnumber of cobalt atoms in each sample linearly increases asthe size decreases. However, if we only consider surface cobaltatoms (estimated by surface/volume ratio assuming a sphericalshape and that every surface cobalt is active), the TOF per sur-

face cobalt atom stays relatively constant across the examined2–6.5 nm size range. Therefore, the higher activity of the small-

er CoOx-Tris nanoparticles comes from more exposed cobalt

atoms. Quantitatively, the 2 nm CoOx-Tris nanoparticles werefound to have a TOF of 8.6 s@1 by averaging the results of five

independent tests (Figure S7). The CoOx-Tris discussed here-after is the one prepared in 0.1 m Tris with an average diameter

of 2 nm. Notably, the maximal amount of O2 originating frompossible decomposition of residual H2O2 during the synthesis

of CoOx-Tris was also evaluated by the spectroscopic titrationmethod and was found to be less than 1.5 V 10@9 mol (Fig-

ure S8), which is by far lower than that of our water-oxidationtests (ca. 1.5 V 10@6 mol). Thus, the influence of residual H2O2 inthe CoOx-Tris sample during the water-oxidation tests could be

neglected. Comparing the activities of the different WOCs pub-lished is difficult, as the conditions under which the experi-ments were performed, including light intensity, buffer compo-nent, and concentration of the photosensitizer, vary. Neverthe-

less, the previously reported TOF value is the lower limit thatthe corresponding catalyst is capable of reaching in the photo-

catalytic Ru(bpy)32 +–Na2S2O8 system. As shown in the Support-

ing Information, Table S1, the TOF of our CoOx-Tris catalyst is atleast an order of magnitude higher than that of other reported

heterogeneous cobalt-based WOCs.To demonstrate the high activity further, we compared the

TOF of our amorphous CoOx-Tris nanoparticles with that of theNa10[Co4(H2O)2 (VW9O34)2]·35 H2O (Co-VWO) polyoxometalate,

which is the state-of-the-art homogeneous cobalt-based WOC.

We synthesized Co-VWO according to the method previouslyreported.[13] The structure of the synthesized Co-VWO catalyst

was confirmed by various characterization techniques (Sup-porting Information, Figure S9 and Table S2). As indicated in

Figure 2 b and the Supporting Information, Figure S10, the TOFof Co-VWO toward water oxidation was measured to be 5.2 s@1

Figure 1. a) Synthesis route to CoOx nanoparticles. b) Size-distribution histo-gram c) TEM image and d) HAADF-STEM image of CoOx-Tris nanoparticles.Inset : selected electron-diffraction patterns. The CoOx nanoparticle sampleused in (a)–(c) were prepared in 0.1 m Tris solution.

Figure 2. Photocatalytic water-oxidation activity of CoOx-Tris examined inthe Ru(bpy)3

2 +–Na2S2O8 system. a) Water-oxidation activity of CoOx-Tris as afunction of nanoparticle size. b) Water-oxidation activities of CoOx-Tris, Co-VWO, and CoOOH. All activity tests were performed in 50 mm borate buffer(pH 8.0) containing 1 mm Ru(bpy)3

2 + and 10 mm Na2S2O8.

ChemCatChem 2017, 9, 3641 – 3645 www.chemcatchem.org T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3642

Communications

under conditions identical to those used for the CoOx-Tris cata-lyst. Hence, the activity of amorphous CoOx-Tris nanoparticles

developed in this work is even higher than that of the most-active homogeneous cobalt-based molecular WOC.

The quantum efficiency of the CoOx nanoparticles was calcu-lated to be 71 % on the basis of the number of O2 moleculesevolved and the irradiation spectrum of the halogen lamp(Supporting Information, Figure S11). Notably, the quantum ef-ficiency is underestimated, because we assumed every incident

photon was absorbed by the Ru(bpy)32+ solution. Besides, with

fast water oxidation on CoOx-Tris, O2 evolution will quicklyreach a plateau as a result of the accumulation of protons anda rapid decrease in the pH value of the reaction solution (Fig-

ure S12). Oxygen evolution can only be partially resumed afteradding NaOH to adjust the pH value of the reaction solution

to the original pH 8, as degradation of the Ru(bpy)32 + sensitiz-

er is inevitable.[14] Encouraged by the extraordinary water-oxi-dation activity of CoOx-Tris, we incorporated it on the surface

of a Ti-modified Fe2O3 photoanode[15] (Ti-Fe2O3) to evaluate itsstability during water oxidation, as the CoOx-Tris nanoparticles

with a size of only 2 nm are difficult to isolate from the reac-tion solution in the Ru(bpy)3

2 +–Na2S2O8 system. As shown in

Figure S13, the water-oxidation current of the CoOx/Ti-Fe2O3

photoanode could be well kept for at least 4 h, which is indica-tive of the robustness of the CoOx-Tris WOC.

Previous studies illustrated that the amorphous morphologyof the cobalt-based oxides played an important role in enhanc-

ing their water-oxidation activities.[16] We also found that ouramorphous CoOx-Tris nanoparticles exhibited much higher ac-

tivity than crystalline CoOOH synthesized according to

Ref. [17] . The structure and crystallinity of CoOOH synthesizedby us were confirmed by X-ray diffraction (XRD) (Figure S14)

and SAED results (Figure S15). According to our test underidentical conditions, the TOF for water oxidation on 2 nm

CoOx-Tris nanoparticles was 28 times higher than that on crys-talline CoOOH (Figure 2 b). This result signals the intrinsic dif-ference between the cobalt active sites in the amorphous

nanoparticles and the crystalline cobalt oxides.To resolve the physical origin of the high activity of CoOx-

Tris, we combined experimental and computational methodsto establish the structure–function relationship of the CoOx

nanoparticles. Regarding the oxidation state of the Co cationsin CoOx-Tris, satellite peaks at binding energies of 786.2 and

802.6 eV assigned to CoII in the Co 2p X-ray photoelectronspectrum are weak,[18] which suggests that CoOx-Tris consistsmostly of CoIII (Figure S16). Similarly, the Co K-edge position in

the X-ray absorption near-edge structure (XANES) spectrum(Figure S17) of CoOx-Tris (7722.1 eV) is closer to that of

CoIIIOOH (7723.2 eV) than to that of CoIICl2 (7717.8 eV). On thebasis of this evidence and also considering the fact that CoOx-

Tris is synthesized in the presence of oxidizing H2O2, we be-

lieve that the CoOx-Tris sample is mainly composed of CoIII.The local structural information of CoOx-Tris was probed by

the extended X-ray absorption fine structure (EXAFS) (Fig-ure S18). The k3-weighted EXAFS spectra and Fourier trans-

forms (FT) of CoOx-Tris, Co3O4, and CoOOH are shown in Fig-ure 3 a, b, and the fitted structural parameters are summarized

in Table S3. Details of the fitting process can be found in the

Supporting Information. The fitted NCo-O for CoOx-Tris is approx-imately 5.6, which evidences the existence of CoO6 octahedra

in the sample. The R value for the Co–Co vector is approxi-mately 2.89 a, which is characteristic of edge-sharing CoO6 oc-

tahedra. The NCo-Co value was fitted to be 0.9. This unusually

low value suggests that CoOx-Tris is likely to have a structureof low dimensionality and is probably 1 D in character. The ra-

tionale behind this statement is that crystalline CoOOH ismade of 2 D CoO2 planes consisting of closed-packed edge-

sharing CoO6 octahedra with a NCo-Co value of 6. Furthermore,the EXAFS study of the Co-Pi (Pi = phosphate) catalyst reportedby Nocera et al. resolved that the active site of the catalyst was

composed of molecular-dimensional Co oxo/hydroxo clus-ters.[19] The NCo-Co in the Co-Pi catalyst was found to vary from

3.4 to 4.5 depending on the size of the cobalt-oxide clusters.Therefore, the atomic structure of CoOx-Tris is distinct fromthat of CoOOH and Co-Pi, which consists of extended or mo-lecular 2 D planes of edge-sharing CoO6 octahedra. To decrease

the NCo-Co value further, a 1 D-CoOx model having edge-sharingCoO6 octahedra chains is a reasonable proposal for the struc-ture of CoOx-Tris.

We further approached structural determination of CoOx-Trisby simulating the EXAFS spectrum through first-principles

methods. In the method, we combined molecular dynamics onthe DFT level (DFT-MD) to sample the equilibrium structures

and used the FEFF method to calculate the electron-scattering

properties[20] to simulate the EXAFS spectrum without any pa-rameterization or fitting. This method was successfully applied

to the structural determination of metal nanoparticles and Co-Pi.[21] For Co3O4 and CoOOH, the method reproduced the ex-

perimental spectra well, as shown in Figure 3 d, and the fittedstructural parameters from the simulated EXAFS spectra are in

Figure 3. a) The k3-weighted EXAFS oscillations for CoOx-Tris (red), CoOOH(blue), and Co3O4 (black). b) Fourier-transformed k3-weighted EXAFS spectraof CoOx-Tris (red), CoOOH (blue), and Co3O4 (black) at the cobalt K-edge.c) Structure of a 1 D mix-sharing model. Co dark blue, O red, H cyan; CoO6

octahedra gray. d) Fourier-transformed k3-weighted EXAFS spectrum of themixed sharing model (green). For comparison, the experimental spectrum ofCoOx-Tris (dashed red) and simulated spectra of Co3O4 (black), and CoOOH(blue) are also shown.

ChemCatChem 2017, 9, 3641 – 3645 www.chemcatchem.org T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3643

Communications

good agreement with the experimental data (Supporting Infor-mation, Table S4). For CoOx-Tris, we constructed a 1 D model of

CoOx-Tris as shown in Figure 3 c consisting of mixed corner-and edge-sharing CoO6 octahedra as guided by the structural

inferences obtained from the experimental EXAFS analysis. Thismodel yielded a clear NCo-Co value of 1, which is consistent with

experimental EXAFS analysis. As can be seen in Figure 3 d, thesimulated EXAFS spectrum for this 1 D CoOx model is also ingood agreement with the experimental data for CoOx-Tris. A

CoOx structure similar to the present 1 D CoOx model was seenin a cobalt-based metal–organic framework (MOF).[22] Notably,we also tested a structural model with a NCo-Co value of 2, andit turned out that the Co–Co peak became significantly higher

as a result of an increase in the coordination number and stiff-ening of the Co–Co vector (Figure S19). The unusual structure

of CoOx-Tris consisting of linked dimeric edge-sharing CoO6 oc-

tahedra could be explained by the early stage of nucleationeither due to the presence of Tris molecules hindering further

nucleation or the mild synthesis conditions such as low tem-perature. Hence, by combining experimental and theoretical

EXAFS, we propose that the 1 D CoOx structure is a reasonableproposal for the structure of CoOx-Tris.

We performed DFT calculations to estimate the theoretical

water-oxidation activity of CoOx-Tris by using a computationalhydrogen electrode model.[23] This method was previously suc-

cessful in predicting trends in electrochemical activity on oxidesurfaces.[24] Figure 4 illustrates the free-energy diagrams of the

water-oxidation reaction for Co3O4, CoOOH, and the present1 D CoOx model by using the traditional four-step, proton-cou-

pled electron-transfer mechanism. The binding energies of the

OH*, O*, and OOH* intermediates used to construct the free-energy diagrams are summarized in the Supporting Informa-

tion, Table S5. As can be seen in Figure 4 a, b, the overpoten-tials (OPs) of the Co3O4 and CoOOH surfaces, based on the

structural models in Figure S20, were predicted to be 0.83 and0.78 V, respectively, in good agreement with a previousstudy.[25] In contrast, the 1 D CoOx model was found to have a

much lower OP of 0.48 V. The decrease in the OP arises fromstronger stabilization of the O* intermediate in the 1 D CoOx

model relative to that on the Co3O4 and CoOOH surfaces. Theincreased stabilization is attributed to the formation of an un-

usual O* adsorption structure, whereby O* interacts with anearby OH ligand to form an O@OH species instead of a dan-

gling O*.In this O@OH species, the O–O coupling has occurred. The

distance between the O and OH moieties is 1.508 a, and the

Co@O bond length is 1.935 a. The Co cation remains in an oxi-dation state of 3 + , as evidenced by its zero magnetic

moment (low-spin state of 3d6). In contrast, Co cations with O*adsorbed on the Co3O4 and CoOOH surfaces are assigned to

an oxidation state of 4 + , as they have magnetic moments of

1 mB (low-spin state of 3d5). The dangling O* species on thesesurfaces also have magnetic moments of 1 mB. These findings

support the formation of a Co4 +@superoxo (O@) bond on thesolid surfaces. It appears that it is more difficult to stabilize ad-

sorbed O* through charge transfer from a Co3 + cation to forma Co4 +@O@ bond in 1 D CoOx. Rather, the electrophilic O* spe-

cies interacts with a nearby nucleophilic OH ligand and forms

an O@OH bond. We believe that the high activity of CoOx-Tris

could be due to facile O@O bond coupling as a result of theformation of the O@OH species.

In summary, we synthesized ultrasmall amorphous Co-basedoxide nanoparticles. The turnover frequency (TOF) of the 2 nm

CoOx-Tris nanoparticles towards water oxidation was approxi-mately 8.6 s@1, and the apparent quantum efficiency was 71 %.

To our best knowledge, this activity is higher than that report-ed to date for other Co-based heterogeneous water-oxidationcatalysts (WOCs). The activity is even comparable to that of the

homogeneous Na10[Co4(H2O)2 (VW9O34)2]·35 H2O catalyst. Onthe basis of extended X-ray absorption fine structure (EXAFS)

analysis and first-principles simulations, the atomic structure ofthe amorphous CoOx nanoparticles was resolved to be a 1 D

chain of dimeric edge-sharing CoO6 octahedra. On the basis of

the structure, theoretical calculations suggested that a facileO@O coupling step was the key to an overpotential that was

lower than those of solid cobalt-oxide surfaces. Given the ex-cellent activity of the amorphous CoOx-Tris nanoparticles, we

believe it to be a promising candidate for use in artificial pho-tosynthetic devices.

Figure 4. Free-energy diagram for water oxidation on Co3O4, CoOOH, and1 D CoOx surfaces. The dashed lines indicate the step free energy for anideal electrocatalyst. The potential-determining step is marked with a redline. The surface structures with various reaction intermediates are shownalongside the free-energy diagram. DG = Gibbs free-energy change,h= overpotential.

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Acknowledgements

This work was supported by the Basic Research Program of

China (973 Program : 2014CB239403), National Natural ScienceFoundation of China (21373210, 21522306, 21633009). F.Z.

thanks the priority support from the “Hundred Talents Program”

of the Chinese Academy of Sciences. This work also benefitedfrom the BL14W1 beamline at the Shanghai Synchrotron Radia-

tion Facility (SSRF).We thank Prof. Licheng Sun from Dalian Uni-versity of Technology for helpful discussions.

Conflict of interest

The authors declare no conflict of interest.

Keywords: amorphous materials · cobalt · oxidation · waterchemistry · X-ray absorption spectroscopy

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Manuscript received: May 31, 2017

Accepted manuscript online: June 8, 2017Version of record online: August 2, 2017

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