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Water Oxidation Reaction Catalyzed by Co3O4 Treated with Organic Compounds Paulo Vitor Cordeiro, and Nakédia Maysa Freitas Carvalho Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01962 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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Industrial & Engineering Chemistry Research

Water Oxidation Reaction Catalyzed by Co3O4 Treated with Organic Compounds Paulo Vitor O. Cordeiro and Nakédia M. F. Carvalho* Universidade do Estado do Rio de Janeiro, Instituto de Química, Rua São Francisco Xavier, 524, Edifício Haroldo Lisboa da Cunha, IQ, room 312a, Maracanã, 20550-013, Rio de Janeiro, RJ, Brazil. KEYWORDS. Water oxidation, cobalt oxides, organic molecules, Ce(IV), electrocatalysis.

ABSTRACT. This work describes the water oxidation reaction by the spinel Co3O4 nanopowder treated with coordinating organic compounds. The treatment of Co3O4 led to increase in the Co3+ species at the oxide surface, probably due to stabilization by the adsorbed organic compounds, as well as partial oxidation of Co2+ to Co3+ ions by oxygen from air. Oxygen evolution reaction (OER) tests were carried out against the sacrificial oxidant cerium(IV) ammonium nitrate (CAN) and higher TOF values were obtained for the treated samples in relation to the pure oxide, showing an influence of the Co3+/Co2+ ratio in the catalytic activity. The catalysts could be recycled up to five times without considerable loss of activity.

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INTRODUCTION Much effort has been devoted to the development of sustainable, carbon-neutral sources of energy.1 The conversion and storage of sunlight in electric and chemical energy by the water splitting is the most promising alternative to provide clean and sustainable hydrogen, the main candidate for fossil fuels replacement.2-4 The development of the so-called artificial photosynthetic system requires efficient catalysts to minimize the kinetic barriers of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which combined with the unfavorable thermodynamics of OER makes this half-reaction the key bottleneck for technological applications.1,5,6 The inspiration for the OER (eq. 1) catalyst design is the Mn4CaO5 cluster (OEC, oxygen evolving complex) of the Photosystem II (PSII) present in the photosynthesizing organisms, which oxidizes water at low overpotential and high turnover frequency.7 2H2O → O2 + 4H+ + 4e-

(eq. 1)

In the past decades, Earth-abundant transition metal oxides have been investigated in chemical and electrochemical water oxidation as alternative for the efficient noble metal compounds, for instance, cobalt oxides have shown high photo- and electrocatalytic activity.6,8,9 A variety of crystalline, amorphous, thin films, nano-structured Co3O4 spinel has been prepared and applied as catalyst in OER in alkaline medium.2,6,10-12 These materials have the advantage of high stability and abundance, low price, high redox capability, relatively good anticorrosion performance and favorable electrical conductivity. Recent works have demonstrated that for manganese oxides as MnO2, the presence of Mn3+ ion is fundamental for the catalytic activity.13,14 Disproportionation of Mn3+ in acid/neutral conditions into Mn2+ and Mn4+ is a drawback to produce efficient and stable catalysts.


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Stabilization of Mn3+ in MnO2 electrodes has been approached by the formation of N−Mn bonds via the coordination of poly(allylamine hydrochloride) to the manganese sites at the surface, resulting in decrease of the onset potential for the OER electroactivity at neutral pH.14 Modification of amorphous MnO2 powders by treatment with different oxidizable compounds resulted in partial reduction of Mn4+ ions to Mn3+, leading to improved turnover frequency up to 25 times in acidic pH range.15 By other side, it has been pointed out that cobalt OER catalysts are structurally unstable as high spin Co2+ state due to its lability.16 Besides, mechanistic proposals suggests that the oxidation of CoIII-OH to CoIV=O sites is an important step in the OER.17 This work describes the application of Co3O4 spinel in the water oxidation reaction in presence of the sacrificial oxidant Ce(IV) ammonium nitrate (CAN) after treatment with organic compounds that can interact and stabilize the Co3+ ions at the oxide surface. The Co3+/Co2+ ratio at the catalyst surface was determined by XPS and correlated with the improvement in the catalytic activity of the treated oxides.

EXPERIMENTAL SECTION Materials. Co3O4 nanopowder of particle size 98.5% of purity was purchased from Sigma-Aldrich.

Catalyst preparation. 1.00 g of Co3O4 was added to a 500 mL aqueous organic compound solution at 1.67×10-4 mol L-1 and stirred for 30 minutes. The treated oxide, Co3O4-L, was filtered


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in a membrane of 0.45 µm (Millipore®) under reduced pressure and dried at 70 oC for 24 hours under air.

Catalyst characterization. The X-ray powder diffraction (XRD) patterns were measured in a Rigaku Ultima IV diffractometer with Ni-filtered Cukβ radiation source (α = 0.15406 nm, graphite monochromator, 40 kV and 20 mA) in the 2θ angle range between 5o and 80o, at a step width of 0.1o, counting 5 s between each step. Thermogravimetric (TG) analyses were carried out in a TGA Q50, TA Instruments, equipped with a high precision balance to monitor the mass variation as a function of time. Around 4 mg of the samples was heated from 10 oC to 600 oC, at 10 oC min-1 (error of ±5 oC), under N2 flow (100 mL min-1). Scanning electron microscopy (SEM) images were obtained using a JEOL JSM - 6510LV (Jeol technics Ltd, Tokyo, Japan) microscope, where the catalysts samples were previously covered with Au during 820 s (Denton Vacuum model DESKV), images of the samples were recorded at an operating current of 20 mA. Textural analyses of the catalysts were determined by N2 adsorption at −196 ◦C in an ASAP 2020 V3.01 E equipment, Micromeritics. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Scientific Escalab 250Xi equipment, having a monochromatized Al K Alpha (E = 14.866 eV) as the excitation source, a base pressure in the vacuum chamber of 9.0×10−10 mbar and a flood gun source and Ar+ ions in order to prevent surface charging. The spectra were adjusted by a combination of Gaussian and Lorentzian functions. Cobalt binding energy in eV was corrected in respect to the C1s line, considering a reference value of 284.8 eV. The error in the XPS spectra is ±0.2 eV.


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Oxygen

evolution

measurements.

Oxygen

concentration

was

determined

by

SevenExcellenceTM dissolved oxygen meter from Mettler Toledo, using the optical sensor InLab OptiOxTM. The oxidation reactions were carried out at 25 oC, in a three-necked round bottomed flask, which one of the necks is connected to an addition funnel, the second neck is connected to the oxygen fluorescent probe, and the third neck is connected to an argon purge valve. The oxygen probe was calibrated in air and in a free-oxygen aqueous sodium sulfite solution. In a typical reaction, 30 mg of catalyst was dissolved in 40 mL of deionized and degasified water and purged with argon until the oxygen detected in the probe is zero and stable. 10 mL of 0.500 mol L-1 of CAN in degasified water in argon was added to the catalyst suspension through the addition funnel, giving a final concentration of 0.100 mol L-1 of CAN at 50 mL. The oxygen concentration was acquired in every 5 seconds. The reaction initial pH was 0.65 (pH meter 827 pH lab Metrohm). Recycle tests were carried out in a larger reaction batch (100 mg of catalyst, 80 mL of 0.100 mol L-1 of CAN). After each cycle the catalyst was filtered in a Millipore membrane of 0.45 µm, washed with water and dried under air at room temperature. After that, a new batch of oxidant solution was added proportionally to the recovered mass of catalyst.

RESULTS AND DISCUSSION

Catalyst Characterization. Commercial spinel Co3O4 nanopowder was treated by different coordinating organic compounds, such as: 2-picolinic acid (pic), 2,6-dipicolinic acid (dipic), 1,10-phenantroline (ophen), sodium acetate (ac), pyridine (py), 1,2-ethylenediamine (en). The molecules were selected


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based on structural similarities with the amino acid residues in the PSII Mn4CaO5 cluster, which contains nitrogenated and carboxylate groups. Besides, these molecules are known to interact with metal transition ions such as cobalt. XRD profiles of the oxides (Figure S1) show the characteristic peaks of the crystalline spinel Co3O4 phase (JCPDS no. 42-1467),2 with no considerable change after the treatment. The morphology of the Co3O4 samples was observed by SEM images, presented in Figure 1, which showed agglomerated dense spheres. Treated oxides showed smaller, more porous particles. The oxide Co3O4 presented moderate BET surface area (Table 1), 39 m2 g-1, and mesoporisity according to the N2 adsorption-desorption isotherm (Figure S2), with pores in the range of 18.6 nm (Table S1). Slightly smaller BET areas (36 – 38 m2 g-1) as well as smaller pore volume (Table S2) were found for the treated oxides, probably by the adsorption of organic molecules at the surface, differently of the reported nanolayered Mn oxides treated with similar molecules that showed higher BET areas.15 Less than 0.4 wt.% of organic content was incorporated in the oxides, as estimated by thermogravimetric analyses (Figure S3, Table S3), except for o-phen that presented 1.38 wt.%.

Table 1. Catalyst characterization data. Catalyst

Co3+/Co2+b Oads/Olattb

Co/Ob

BET Surface

Organic content

Area (m²/g)

(wt%)a

Co3O4

38.83

-

1.91

0.42

0.54

Co3O4-ac

36.54

0.37

2.13

0.51

0.48

Co3O4-en

36.15

0.26

2.10

0.58

0.40

Co3O4-pic

37.78

0.12

2.39

0.55

0.45

Co3O4-dipic

36.55

0.39

2.04

0.48

0.46

Co3O4-py

36.04

0.29

2.05

0.58

0.44

Co3O4-o-phen

37.06

1.38

1.94

0.83

0.41

a

determined by thermogravimetric analyses; b determined by XPS analyses.


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Figure 1. SEM images of the catalysts.

To investigate the surface composition and metal valence states, we performed X-ray photoelectron spectroscopy (XPS) on the catalysts (Figure 2). From the survey spectra (Figure S4) it is possible to observe the presence of cobalt and oxygen, with atomic percentages and binding energies (Table S4 and S5) compatible with Co3O4 for all catalysts. Besides Co and O, carbon could also be detected and its atomic percentage increased considerably in the treated oxides. For the oxide with higher organic content Co3O4-o-phen, nitrogen was also found. The comparison of the organic content determined by XPS with the bulk content determined by thermogravimetric analysis, indicates that the organic molecules are preferentially adsorbed at the oxide surface. From the Co2p spectra of the catalysts (Figure S5) it is possible to observe a subtle difference in the 2p1/2 and 2p3/2 lines. According to Biesinger et al.,17 Co2+ and Co3+ species present very close 2p3/2 binding energies, 780.9 and 779.6 eV, respectively, which requires careful fitting of the data for reliable determination of the cobalt oxidation states by XPS. An asymmetric peak shape is usually observed, and is accompanied by the satellite peaks at 789.6 and 785.2 eV for


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Co2+ and Co3+, respectively.9,17 Figure 2(a) and S6 show the Co2p3/2 spectra for selected catalysts, where is possible to observe that the peak of Co3O4-pic and Co3O4-o-phen are at lower BE, both compared to Co3O4, indicative of more Co3+ ions at the surface. Fitting of the Co2p3/2 lines (Figure S7, Table S6) were carried out,17 and the Co3+/Co2+ atomic ratio (Table 1) was calculated from the areas of the peaks, for Co3O4 a ratio of 1.91 was obtained, close to the theoretical value. For the treated oxides the Co3+/Co2+ ratio increased, for instance Co3O4-pic shows the highest Co3+/Co2+ ratio: 2.35. The O1s XPS line is presented in Figure 2(b) and a different peak shape and area were observed for Co3O4, Co3O4-pic and Co3O4-o-phen. The oxygen species such as peroxide O- (BE = 530.8 eV), superoxide O2- (BE = 531.8 eV) and hydroxide ions/carbonate/water adsorbed (BE = 532.8 eV) over the catalyst surface, as well as the lattice oxygen in the form of oxide O2- (BE = 529.6 eV),18 could be determined (Figure S8, Table S7). The ratio Oads/Olatt increased after the treatment compared to the pure oxide, and also indicates a more oxidized catalyst surface, in agreement with the increase in the O/Co atomic ratio (Table 1). The amount of oxygen and cobalt in Co3O4-o-phen is considerably smaller than for Co3O4, indicating a higher coverage of the oxide surface by o-phen, what is consistent with the larger organic content found by TG. This can be explained by the high coordination ability of o-phen as ligand in regard to the transition metal ions. The observed increase in Co3+ species at the oxide surface observed by XPS can be caused by the partial oxidation of Co2+ ions by oxygen from air as well as the stabilization of Co3+ after treatment with the organic molecules, that could act as ligands and form N-Co interaction through adsorption, as proposed previously for MnO2 electrodes.13,14 It is well known that Co3+ is more stabilized than Co2+ by common ligands in the octahedral crystal field, as a consequence,


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the oxidation of Co2+ in Co3+ is more favored and increases the Co3+/Co2+ ratio after treatment. Furthermore, Co3+ possess a more oxidizing power, what could be advantageous for OER catalysis. From the comparison of the nitrogen XPS spectra of Co3O4-o-phen with pure o-phen, it was possible to observe that the N1s binding energy in the oxide was 0.1 eV higher (Figure S9 and Table S8). This slight difference indicates a weak interaction, probably an adsorption of ophen at the Co3O4 surface. More pronounced shift would be observed if a classic coordination occurred, as described for polymer-bound o-phen coordinated to Cu1+ and Fe3+ ions, which also showed shift to higher binding energies.19,20

24000 20000

16000

(b)

3+

2+

Co2p3/2

Co

Co

Co3O4

Intensity (a.u.)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co3O4-pic Co3O4-o-phen

12000

18000

O1s

-

OH

2-

O

-

O

2-

Olatt

15000

Co3O4

12000

Co3O4-o-phen

Co3O4-pic

9000 6000 3000

8000

0

786

784

782

780

778

776

534

Binding Energy (eV)

532

530

528

526

Binding Energy (eV)

Figure 2. XPS spectra of the Co2p3/2 (a) and O1s (b) lines of Co3O4, Co3O4-pic and Co3O4-ophen.

OER activity. Co3O4 has been described as efficient catalyst in the photo and electrochemical oxidation of water, however, it has not been applied in the chemical oxidation against sacrificial oxidants, a simple and rapid method for screening and tuning of new potential catalysts.3 The pure and treated Co3O4 oxides prepared were tested towards CAN (cerium(IV) ammonium nitrate) as a


9


non-oxo transfer one-electron oxidant. Figure 3(a) shows the oxygen evolution, where it is possible to observe that the treated oxides are more active than Co3O4, except for o-phen where a negative effect was observed. TOF value calculated per cobalt atom (Figure 3(b), Table S9) for Co3O4 is 0.76 mmolO2 molCo-1 s-1, very high compared to the extensively explored manganese oxides, for instance, 40-folds more active than nanolayered Mn oxides (around 0.02 mmolO2 molMn-1 s-1),15 and 10-folds more active than manganese oxide supported on carbon black (0.07 mmolO2 molCo-1 s-1).21 The highest TOF values were obtained for Co3O4-dipic, Co3O4-en and Co3O4-py, with 1.07 mmolO2 molCo-1 s-1 for Co3O4-dipic. However, the positive increment was not as high as that observed for nanolayered Mn oxides, where 25 times more active catalyst was obtained after treatment with Mn(ClO4)2.15 The same Mn oxide treated with pyridine (TOF around 0.045 mmolO2 molMn-1 s-1)15 presented an increment of 2.3, while Co3O4-py presented an increment of 1.3, nevertheless, 23-fold higher TOF was observed for Co3O4-py. Five recycle tests were carried out with Co3O4 and Co3O4-pic and the oxides remained active against the oxidant CAN (Figure 3(c)). XRD spectra of the recovered catalysts (Figure S10) confirmed the integrity of the crystalline phase after the fifth test.

(b) Co3O4 -1

0.04

Co3O4-o-phen Co3O4-py

-1

0.05

TOF (mmolO2 molCo s )

(a)

nO2 (mmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Co3O4-pic Co3O4-en

0.03

Co3O4-ac Co3O4-dipic

0.02 0.01 0.00 40

60

80

100

Time (s)

120

140

1.0 0.8 0.6 0.4 0.2 0.0 c c 4 en -py -en pic -pi 4-a 3O -ph -di O4 O4 o3O4 3O Co o3 O4 4-o o3 C 3 Co C C O 3 Co Co

Catalyst


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(c)

Co3O4

1.6

Co3O4-pic

-1

-1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2

0.8

0.4

0.0 1st

2nd

3rd

4th

5th

Cycle

Figure 3. Oxygen evolution (a), TOF values (b) and recycle (c), from reaction of the Co3O4 oxides (30 mg) with aqueous 0.1 mol L-1 CAN (50 mL) at 25 oC.

Figure 4(a) shows the relation between TOF and the catalysts surface area, where a direct linear dependence is expected for heterogeneous catalysts. However, in our case a higher area did not lead to higher activity, indicating that the improvement in the oxygen evolution by the treated oxides are not due to textural differences.15 For Co3O4-o-phen, a much lower activity was observed than what would be expected from its area, what is probably related to the high organic content observed for this catalyst and possible partial occlusion of the active sites. For treated manganese oxides with oxizable compounds, the surface area was not the only determinant of the catalytic rate, but the percentage of Mn3+ ions had a strong effect to form catalytically active centers that promoted the OER by CAN.15 A correlation between TOF and Co3+/Co2+ ratio of the prepared cobalt oxides (Figure 4(b)), shows an increase in the activity with the increase of Co3+ species at the surface of the catalyst. Co3O4-dipic presented the highest TOF and a Co3+/Co2+ ratio of 2.03, after this value a decrease in the activity was observed, indicating an optimum amount of Co3+ at the catalyst surface. In the case of Co3O4-o-phen, the low TOF can be related


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to the strong and high adsorption of this molecule with Co3+ and coverage of the catalytic sites, which disfavored the OER reaction.

(b) Co3O4-en

-1

1.0 0.9

Co3O4-py

Co3O4-ac

Co3O4-pic Co3O4

0.8 0.7 0.6 0.5 2

Co3O4-o-phen

0.4 36.0

Co3O4-dipic

1.1

Co3O4-dipic

-1

-1 -1

1.1


(a) TOF (mmol O2 mol Co s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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36.5

37.0

37.5

R = 0.2644

38.0

38.5

39.0

Co3O4-en

Co3O4-py

1.0 0.9

Co3O4-pic

Co3O4-ac Co3O4

0.8 0.7 0.6 0.5

Co3O4-o-phen

0.4 1.8

1.9

2.0

2.1 3+

-1

2.2

2.3

2.4

2+

Co /Co

Surface area (m² g )

Figure 4. TOF versus area (a) and Co3+/Co2+ ratio (b) plots, from reaction of the Co3O4 oxides (30 mg) with aqueous 0.1 mol L-1 CAN (50 mL) at 25 oC.

The OER electroactivity of the catalysts was also investigated (Figure S11, Tables S10) in FTO modified electrodes in pH 13.0 and 1.0 and the onset potential for the catalysts are around 0.25 V smaller than that of FTO substrate. However, the treated oxides did not led to significant improvement of the overpotential and TOF values. In conclusion, Co3O4 oxide was treated with coordinating organic molecules, what resulted in increase of Co3+ species in the oxide surface probably due to stabilization by adsorption and interaction with the organic molecules, combined with partial oxidation of Co2+ in Co3+ favored under the octahedral crystal field environment. The OER activity against the sacrificial oxidant CAN was tested and high activity and stability was obtained. Furthermore, the treatment in the oxide improved the catalytic activity and it was possible to relate this effect to the Co3+/Co2+


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ratio at the catalyst surface. For the electrocatalytic activity, high values of overpotential were obtained and the improvement of the treated oxides was not so pronounced as that for the chemical oxidation tests. The oxides could be recycled up to five times, showing high stability against CAN. From the present study it was possible to show that Co3+ species play an important role in the OER catalytic activity and stabilization of the Co3+ ions could lead to more efficient cobalt catalysts.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: …... XRD data, nitrogen adsorption-desorption isotherms, thermogravimetric curves and weight loss data, XPS data, electrocatalytic tests and calculated TOF values (PDF). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (N.M.F.C.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources


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We acknowledge National Council for Scientific and Technological Development – CNPq (Universal 2014: 459256/2014-9 and PQ-2/2015) and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro – FAPERJ (APQ1 2014/01: E-26/111.377/2014 and Jovem Cientista do Nosso Estado 2015: E-26/202.859/2015) for supporting this project.

ACKNOWLEDGMENT We acknowledge Laboratório de Caracterização Estrutural/DEMa at Universidade de São Paulo for the SEM analyses and Laboratório Multiusuário de Espectroscopia de Fotoelétrons na Região de Raios X (XPS) at Universidade Federal do Rio de Janeiro for the analyses. REFERENCES (1) Pham, H. H.; Cheng, M.-J.; Frei, H.; Wang, L.-W. Surface Proton Hopping and FastKinetics Pathway of Water Oxidation on Co3O4 (001) Surface. ACS Catal. 2016, 6, 5610. (2) Zhang, P.; Han, X.; Hu, H.; Gui, J.; Li, M.; Qiu, J. In-situ growth of highly uniform and single crystalline Co3O4 nanocubes on graphene for efficient oxygen evolution. Catal. Comm. 2017, 88, 81. (3) Kärkäs, M.D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev. 2014, 114, 11863. (4) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474.


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SYNOPSIS.


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Graphical abstract 338x190mm (96 x 96 DPI)


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Water Oxidation Reaction Catalyzed by Co3O4 ... - ACS Publications

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