Reinvestigation of Water Oxidation Catalyzed by a Dinuclear Cobalt

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Re-Investigation of Water Oxidation Catalyzed by a Dinuclear Cobalt-Polypyridine Complex: Identification of CoOx as a Real Heterogeneous Catalyst Jia-Wei Wang, Pathik Sahoo, and Tong-Bu Lu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00798 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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Re-Investigation of Water Oxidation Catalyzed by a Dinuclear Cobalt-Polypyridine Complex: Identification of CoOx as a Real Heterogeneous Catalyst Jia-Wei Wang, Pathik Sahoo, and Tong-Bu Lu* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China.

* To whom correspondence should be addressed. Fax: +86-20-84112921. E-mail: [email protected].

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ABSTRACT: Recently, a dinuclear cobalt complex, [(TPA)CoIII(µ-OH)(µ-O2)CoIII(TPA)] (ClO4)3 (1, TPA = tris(2-pyridylmethyl)amine), has been reported as a homogeneous catalyst for electrochemical and photochemical water oxidation (Wang et al. Angew. Chem. Int. Ed. 2014, 53, 14499.). During the re-investigation of the reported water oxidation catalyst (WOC) of 1, several characterizations such as EDTA and bipyridine titrations, electrochemistry, SEM, EDX, ICP-AES, TEM, XPS and UV-vis spectroscopy have revealed that the water oxidation may happen due to the formation of CoOx as a real heterogeneous WOC, and 1 itself lacks the ability to catalyze water oxidation. This manuscript presents a practical and simple procedure to clarify whether the water oxidation is catalyzed really by a molecular catalyst or not.

KEYWORDS: Heterogeneous versus homogeneous water oxidation catalysis, dinuclear cobalt complex, electrocatalysis, photocatalysis, identification, cobalt oxide

INTRODUCTION Catalytic water splitting is an appealing way for using solar energy or electricity to produce hydrogen, which has been regarded as the most promising alternative energy source for its cleanness, effectiveness and renewability.1,

2

The water splitting process includes two half

reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).3 Since OER is associated with the transfer of multiple electrons/protons and O-O bond formation in a high energy barrier process, it becomes kinetically slow and leads to significant consumption of overpotential in the water-splitting process.4 Thus the design and synthesis of efficient water oxidation catalysts (WOCs) based on earth-abundant elements5-18 remain great challenge.


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Recently, cobalt-based WOCs have been extensively investigated for their remarkable performance,19-24 in which soluble cobalt complexes as homogeneous WOCs11, 25-32 are appealing for their controllable redox properties and facile characterizations on active intermediates, which are favorable to the related mechanistic studies.33 However, determining whether these homogeneous WOCs retain their molecular integrities during catalysis or merely act as precursors of genuinely active heterogeneous WOCs such as films or nanoparticles is important and complicated.34-36 The clarification is extremely challenging for the studies on cobalt-based homogeneous WOCs. This is mainly because a hardly detectable amount of cobalt oxide (CoOx) can be generated from a homogeneous WOC, which behaved as an efficient heterogeneous catalyst during water oxidation reaction.10,

11

A typical example of this challenge is the

cobalt-based polyoxometelate, [Co4(H2O)2(PW9O34)2]10- (Co4POM),25, presumed to be a stable homogeneous WOC,25,

37

37

which had been

was exploited latter to discover that the

Co4POM was decomposed under electrochemical water oxidation at lower overpotential (< 500 mV), and deposited as an amorphous CoOx film at a glassy carbon (GC) electrode to behave as a heterogeneous WOC.34 The catalytic activity of CoOx film was also observed in the absence of molecular Co4POM. However, subsequent studies demonstrated that both Co4POM and CoOx would contribute to the electrocatalytic activity at higher overpotential (> 600 mV),38 and Co4POM is indeed the active WOC in homogeneous light driven reaction conditions.35, 39 In some other cases, the cubane Co4O4 clusters in Co4O4(Ac)4(py)440 and its derivatives were primarily regarded as homogeneous WOCs,41 and it was proven lately that the observed water oxidation activity originates from Co(II) impurities introduced from their synthesis rather than the Co4O4 clusters.36 Therefore, it should be very careful to identify the real active species for water oxidation reactions.


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Inspired

by

a

recent

report

on

a

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cobalt

(III)

dinuclear

complex

[(TPA)CoIII(µ-OH)(µ-O2)CoIII(TPA)](ClO4)3 (1, TPA = tris(2-pyridylmethyl)amine, Scheme 1),42 which can act as a homogeneous catalyst for electrochemical and photochemical water oxidation

in

borate

buffer

at

pH

8.0,

a

[(BPMEN)CoIII(µ-OH)(µ-O2)CoIII(BPMEN)](ClO4)3

new (2,

dinuclear BPMEN

cobalt(III) =

complex,

N,N’-dimethyl-N,N’-

bis(pyridin-2-ylmethyl)ethane-1,2-diamine, Scheme 1) was synthesized to explore its water oxidation catalytic activity. However, no catalytic activity was observed in the cyclic voltammetry (CV) on complex 2 (Figure 2), in spite of its structural similarity to 1. The unexpected inactivity of complex 2 thrusts us to re-investigate the electrocatalytic efficiency of 1 exclusively. During the re-investigation, we found strong experimental evidence to confirm that the in-situ generated CoOx is the predominantly heterogeneous WOC instead of the complex 1 in sodium borate buffer at pH 8.0, under both electrochemical and photochemical water oxidation processes as determined by bipyridine (bpy) and EDTA titrations, followed by several characterization techniques like scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), inductively coupled plasma-atomic emission spectrometer (ICP-AES), X-ray photoelectron spectroscopy (XPS), UV-vis techniques and high-resolution transmission electron microscope ( HRTEM) with elemental mapping. It is difficult to trace CoOx particle in DLS experiment under photocatalytic condition due to the scarcity and the adherence force of CoOx to magnetic stirring bars or reaction vessels, which may often result in indecisive conclusion. Our finding, exemplified on a cobalt complex, may be extended to other molecular systems and demonstrates that the confirmation of CoOx can be achieved by the titration of chelating agent like bpy and TEM measurements.


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Scheme 1. The structures for 1 and 2.

EXPERIMENTAL SECTION

Materials.

The

ligands

of

TPA

(tris(2-pyridylmethyl)amine)43

and

BPMEN

(N,N’-dimethyl-N,N’-bis(pyridin-2-ylmethyl)ethane-1,2-diamine)44 were synthesized according to literature methods. [Ru(bpy)3](ClO4)2 was prepared following the reported method.45 All of the other chemicals are commercially available and used without further purification. The purity of Argon is 99.999%. All solutions were prepared with Milli-Q ultrapure water (> 18 MΩ) unless otherwise stated. ITO glass (Rs = 6-7 Ohms) was purchased from Zhuhai Kaivo Electronic Components Co. Ltd.

Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive and should be handled in small quantities with care.

Characterization. Electron paramagnetic resonance (EPR) spectra were collected on an EPR spectrometer (BRUKER A300). UV–vis spectra were determined on a Shimadzu UV-2600 spectrophotometer. SEM images and EDX spectra were collected by a field emission scanning electron microscope (FEI, Quanta 400). The XPS data were collected on an ESCA Lab250


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instrument. The content of cobalt ions in solution (5% HNO3 digested) was determined by a TJA IRIS

(HR)

inductively

coupled

plasma-atomic

emission

spectrometer

(ICP-AES).

Electrochemical measurements were carried out using an electrochemical workstation (CHI 620E). Unless otherwise stated, all potentials were footnoted as vs. NHE. TEM measurements were operated on a FEI Tecnai G2 F30 (300 kV with elemental mapping) or a JEM-2010HR (200 kV).

Synthesis. The dinuclear cobalt(III) complexes 1 and 2 were synthesized following the literature method.42 Anal. Calcd for [(TPA)Co(µ-OH)(µ-O2)Co(TPA)](ClO4)3·H2O (1⋅H2O): C 40.64, H 3.69, N 10.53%; found: C 40.38, H 3.28, N 10.65%. Anal. Calcd for [(BPMEN)Co(µ-OH)(µ-O2)Co(BPMEN)](ClO4)3·2H2O (2⋅2H2O): C 36.89, H 4.74, N 10.75%; found: C 36.87, H 4.71, N 10.74%.

X-ray Crystallography. Single-crystal X-ray diffraction data for 2 were collected at 150 K on an Agilent Technologies Gemini A Ultra system, with Mo/Kα (λ = 0.71073Å) radiation. The empirical absorption corrections were applied using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. The structure was solved using direct method and refined by the full-matrix least-squares method on F2, which yielded the positions of all non-hydrogen atoms. These were refined first isotropically and then anisotropically. All of the hydrogen atoms of the ligand were placed in calculated positions with fixed isotropic thermal parameters and included in the structure factor calculations in the final stage of full-matrix least-squares refinement. All the calculations were performed using the SHELX-97 program.46


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The crystallographic data are summarized in Table S1 (CCDC number 1456616), and selected bond distances and angles are given in Table S2. Electrode Pretreatment. Prior to experiments, the glassy carbon (GC) electrode (0.07 cm2) was polished with 0.3 and 0.05 µm Al2O3 slurry for 3 min each to obtain a mirror surface, followed by sonication in distilled water for ~60 seconds to remove debris, and was thoroughly rinsed with Milli-Q ultrapure water. The indium-tin-oxide (ITO) glass slides (2.0 cm2, 1.0 cm2 immersed in electrolyte) were cleaned by sonication in acetone, ethanol and Milli-Q ultrapure water for ~10 min, respectively.

CV Experiments. Unless otherwise stated, CV measurements were generally conducted under an argon atmosphere in a three-electrode system. A Pt foil was used as a counter electrode. The reference electrode was an Ag/AgCl electrode (0.200 V vs. NHE) and the working electrode was a 3 mm GC electrode. All experiments remained at room temperature (24~25 °C).

Controlled Potential Electrolysis (CPE) and Oxygen Detection. CPE experiments were performed in a gas-tight, one-compartment (35 mL solution in 66 mL volume), three-electrode cell without iR compensation. A Pt foil was used as a counter electrode. The reference electrode was an Ag/AgCl electrode and the working electrode was an ITO electrode. When it comes to the oxygen detection, the same cell would be firmly sealed up and purged with argon at least 1 h. The O2 product analysis was determined using a calibrated Ocean Optics FOXY probe. The phase shift of the O2 sensor on the FOXY probe, recorded at 20 s intervals, was converted into the dissolved concentration of O2 in the electrolyte using a two-point calibration curve (air, 20.9% O2; and high purity argon, 0% O2). After recording the dissolved concentration of O2 for


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0.5 h in the absence of an applied potential, electrolysis was initiated at 1.31 V without iR compensation. Bulk electrolysis with O2 sensing was continued for 1 h. However, the oxygen detection for dissolved concentration cannot be utilized for Faradaic efficiency calculation but only for confirming the presence of evolved oxygen with a higher sensitivity. Because in such a stagnant system, the evolved oxygen would first saturate the electrolyte near the working electrode with slow diffusion to surroundings, confusing the exact volume related to the detected [O2]. All experiments remained at room temperature (24~25 °C). Photocatalytic Oxygen Evolution Reaction. After mixing the borate buffer solution (50 mM, pH = 8.0) of 1 (5.0 µM) or Co(ClO4)2·6H2O with [Ru(bpy)3](ClO4)2 (0.4 mM) and Na2S2O8 (3 mM) in the absence or presence of bpy, the reaction mixture was purged with argon 5 min and then initiated with light irradiation. The total volume of the solution was 1.0 mL. The generation of dissolved oxygen was analyzed by a Hanstech Clark-type electrode irradiated under an LED light (λ = 470 ± 5 nm, with a light intensity of 150 mW·cm−2, and irradiated area of approximate 0.5 cm2). Each photocatalytic reaction was repeated three times to confirm the reliability of the data.

RESULTS AND DISCUSSION

Synthesis and Structure. Both 1 and 2 were prepared according to the reported synthetic procedure of 1 using TPA and BPMEN as ligands.42 Elemental analysis and EPR (Figure S1) were conducted over the prepared bulk samples, which confirmed their high purity, and ruled out the co-existence of paramagnetic Co(II) impurities in diamagnetic Co(III) complexes 1 and 2. The result of X-ray structural analysis reveals that the structure of 2 is similar to that of 1 (Figure


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1), in which each Co(III) is six-coordinated to four nitrogen atoms from the BPMEN ligand, and two oxygen atoms from one µ-OH- and one µ-O22- groups, respectively, and two [CoIII(BPMEN)]3+ units are connected by a µ-OH- and a µ-O22- bridge to form a dinuclear Co(III) complex of 2 (Figure 1). In 2, the Co1-O2 (1.872(2) Å) and Co2-O3 (1.862(2) Å) distances are shorter than the Co1-O1 (1.935(2) Å) and Co2-O1 (1.941(2) Å) distances, indicating the stronger coordination interactions between two Co(III) ions and the µ-O22- bridge. In 1, the Co1-O2 (1.844 Å) and Co2-O3 (1.852 Å) distances42 are even shorter than the corresponding distances in 2, further demonstrating the coordination interactions between two Co(III) ions and µ-O22- bridge in 1 are very strong. The O2-O3 distance of 1.405(3) Å is in the typical peroxo range of 1.4-1.5 Å.42

Figure 1. The crystal structure of 2, with ellipsoids drawn at the 30% probability level.


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Electrochemical Studies. Initially, the catalytic property of 2 was investigated by CV in 0.1 M borate buffer at pH 8.0 (Figure 2). Its CV showed one reversible wave at 1.05 V, which can be assigned to CoIIICoIII/CoIVCoIII. In addition, as shown in Figure 2, the CV of 2 exhibited an insignificant current enhancement compared to the background, indicating its negligible catalytic ability for water oxidation. In contrast, in the CV of 1 under the same conditions (Figure 2), an obviously enhanced current above the background was observed, consistent with the reported results.42 Therefore 1 is far more active than 2 in electrocatalytic water oxidation as determined by the CV results. To investigate such inefficient catalytic ability of 2 in spite of their similar structures, the electrochemical properties of 1 were exclusively investigated. Unexpectedly, an increase in the catalytic currents was observed in the repeated CV scanning of 1 (Figure 3a), which is a typical feature of electrodeposition of an active species as a heterogeneous WOC on the electrode surface.47, 48 In addition, after the above scans for 30 cycles, the GC electrode was rinsed with water several times but not polished. Then the GC electrode was cycled in fresh, catalyst-free borate buffer (0.1 M, pH 8.0), and a significant catalytic current was observed (Figure 3b), further demonstrating the electrodeposition of active species on the electrode surface.10, 34


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Figure 2. The CVs of 0.5 mM 1 (red line) and 2 (blue line) in 0.1 M sodium borate buffer (pH 8.0) at a 3 mm GC electrode at the 30th scan. Scan rate = 100 mV/s. Background (black line) is shown for comparison.


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Figure 3. (a) Repeated CV scanning (30 cycles) of 1 (0.5 mM) in 0.1 M sodium borate buffer (pH 8.0) at a 3 mm GC electrode with a scan rate of 100 mV/s. (b) The CV of the GC electrode cycled 30 times with 0.5 mM 1 in 0.1 M sodium borate buffer (pH 8.0) (red solid line). The electrode was rinsed off, but not polished, then cycled in catalyst-free electrolyte at pH 8.0 (red dash line). Background (black) is shown for comparison.


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To further investigate the active species deposited on the working electrode surface, CPE experiment was conducted over a piece of ITO electrode (1.0 cm2) at 1.31 V (560 mV overpotential). The reason we used ITO electrode instead of a 3 mm GC electrode as used in the reported paper42 is due to its larger surface, better surface-deposit attractive force34 and high electrochemical stability at high anodic potential (GC electrode may get oxidized to carbon dioxide36). As shown in Figure 4, during 3 h CPE with 0.5 mM 1 in 0.1 M borate buffer at pH 8.0, the electrolytic current exhibited an induction period at the beginning, and reached a current density of approximate 0.19 mA/cm2, with the formation of a yellow thin film at the surface of ITO electrode (Figure S2). Then the ITO electrode was rinsed with water several times for subsequent surface analysis. As shown in Figure S3, its SEM image shows the topography of the deposited active species on the electrode surface (Figure S3a), and the EDX spectrum indicates the existence of trace amount of cobalt species on the surface of ITO electrode (Figure S3b). In addition, the result of ICP-AES measurement suggests ~1.5 µg/cm2 cobalt ion was deposited on the ITO electrode after 3 h CPE. All the above results demonstrate a heterogeneous, active cobalt species in situ formed on the surface of ITO electrode.18, 48 Moreover, the above ITO electrode after 3 h CPE was rinsed with water, followed by the CPE test in a catalyst-free borate buffer solution (0.1 M, pH 8.0), a significant current enhancement of approximate 0.19 mA/cm2 was observed compared to the background (Figure 4). These observations clearly demonstrate that 1 merely acts as a precursor for the generation of a Co-based active species deposited on the ITO electrode as a real heterogeneous WOC.


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Figure 4. Current density traces obtained in controlled potential electrolysis at 1.31 V vs NHE with an ITO electrode (1.0 cm2) in 0.1 M sodium borate buffer (pH 8.0), with 0.5 mM 1 (red solid line), and in catalyst-free electrolyte (red dash line). Background (black line) is shown for comparison.

Figure 5. XPS patterns for samples A, B and C, respectively.


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A comparative XPS experiment was conducted to investigate the chemical composition of the observed cobalt-based active species electrodeposited on the ITO electrode. Three samples, an ITO after the above 3 h CPE (sample A); an ITO after 1 h CPE in 0.1 M borate buffer with 0.05 mM Co(ClO4)2·6H2O (sample B), which has been proven to generate CoOx on the surface of the ITO electrode;49 and an ITO coated with 1 on its surface (sample C), were prepared. From Figure 5 it can be found that the XPS spectra of A and B are very similar, which both exhibit similar peak intensities and positions (Co 2p3/2 of 781.0 eV for sample A and 780.8 eV for sample B, Co 2p1/2 of 796.2 eV for sample A and 796.1 eV for sample B). In contrast, the XPS spectrum of C is significantly different from those of samples A and B, with the higher binding energies of Co 2p3/2 and Co 2p1/2 peaks located at 781.6 and 797.0 eV, respectively, demonstrating the electrodeposition of active species on the ITO surface is CoOx rather than complex 1.

Chelation Experiments. The CoOx heterogeneous WOC could originate from the release of cobalt ions from 1. To identify it, bpy was added to the buffer during the CV measurements to chelate the released cobalt ions. As shown in Figure 6, the catalytic current decreased along with increasing concentrations of bpy, confirming that cobalt ions are involved in the observed catalytic activity. In addition, CPE with 0.5 mM 1 and 0.3 mM bpy exhibited a substantially suppressed current density of ~0.02 mA/cm2 (Figure S4a), and the subsequent CV scans with the used ITO electrode after CPE showed identical current density as the background (Figure S4b), indicating no electrodeposition of CoOx on the ITO surface during CPE in the presence of bpy, which was further confirmed by the SEM and EDX results (Figure S5). Moreover, CPE experiments with 0.5 mM 1 in the absence and presence of 0.3 mM bpy were carried out in a


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gas-tight one-compartment cell, and the evolved oxygen was analyzed by a FOXY fluorescent sensor, which was immersed in an unstirred electrolyte to detect the concentration of dissolved oxygen. As shown in Figure 7, a significant amount of evolved oxygen was detected in the absence of bpy, whereas no oxygen was detected in the presence of bpy. Moreover, when EDTA was used as a chelating agent, similar results were also observed (Figure S6 and S7). The above results rule out the possibility of 1 as a homogeneous electrocatalyst, and the observed catalytic ability of 1 originates the in situ formed cobalt oxides from the release of cobalt ions from 1 as the real heterogeneous WOC. Once the released cobalt ions are chelated by bpy or EDTA, the catalytic activity of 1 was not observed.

Figure 6. CVs of 0.5 mM 1 in the absence (red line) and in the presence of 0.1 (green line), 0.2 (blue line) and 0.3 mM (orange line) bpy in 0.1 M sodium borate buffer (pH 8.0), 3 mm GC electrode, scan rate = 100 mV/s. Background (black line) is shown for comparison.


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Figure 7. Dissolved oxygen production measured by a fluorescent sensor with 0.5 mM 1 in the absence (dot line) and in the presence of 0.3 mM bpy (red solid line) at ITO electrodes (1.0 cm2) in sodium borate buffer (pH 8.0) during CPE at 1.31 V vs NHE. Background (dash line) is shown for comparison.

An important issue is the source of cobalt ions: is 1 unstable in alkaline borate buffer, or does 1 get dissociated at oxidation conditions? On one side, UV-vis measurements for 1 in 0.1 M borate buffer at pH 8.0 within 3 h illustrated that negligible amount of 1 was dissociated (Figure S8), demonstrating 1 is stable in borate buffer. Moreover, a 0.5 mM solution of 1 was treated with 0.5 mM bpy in 0.1 M borate buffer at pH 8.0 for 3 h, and negligible changes in absorbance were observed (Figure S9), indicating 1 is even stable in the presence of 1 equivalent of bpy in borate buffer. On the other side, a 15% decrement in the absorbance at 380 nm of UV-vis spectra of 1 was observed after 3 h CPE (Figure S10), indicating its instability under oxidative condition. An alternative explanation behind such instability might be the occurrence of irreversibly


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dissociative structural changes on the dinuclear complex during the oxidative CPE experiment.18, 42

Hence, the cobalt ions were released from the dissociation of 1 under the oxidative condition,

and subsequently transformed to CoOx as a real heterogeneous WOC. Photochemical Studies. Lastly, photocatalytic water oxidation was also carried out to testify the catalytic activity of 1 as a homogeneous WOC following the reported experimental conditions42 (see the Experimental Section). As shown in Figure 8, the photocatalytic water oxidation reaction with 5 µM 1 generated ca. 250 µM oxygen within a 1 min duration, consistent with the reported data.42 To nullify the probable contribution of cobalt ions (source of CoOx), 0.2 equivalent of bpy (1 µM) was added and the oxygen yield was decreased drastically to ca. 80 µM. Moreover, with increasing concentrations of bpy, the oxygen yield was further inhibited and eliminated to zero in the presence of 1 equivalent of bpy. In addition, the high stability of 1 in the presence of 1 equivalent of bpy in borate buffer (Figure S10) has ruled out the probability of ligand exchange reaction under ambient condition. Otherwise the presence of 20% bpy in the reaction system would not cause ~70% decrease of oxygen generation (decreasing from 250 µM to 80 µM). The above results indicate that the water oxidation may happen due to the formation of CoOx as a real heterogeneous WOC, and 1 itself may lack the ability to catalyze water oxidation. Interestingly, under identical conditions, we found that approximately 0.5 µM Co2+ (5% cobalt source) can account for the observed activity of 5 µM 1, and it needs 4 µM of bpy to totally quench the activity of Co(ClO4)2·6H2O (0.5 µM) (Figure S11). These observations strongly confirm that about 5% of free Co2+ (released from 1 during photochemical reaction) was involved in the photocatalytic system with 1.


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However, the results of the previous DLS measurements indicate that no CoOx was formed during the illumination in the reaction system using 1 as a WOC, and a control experiment with Co(ClO4)2 showed the formation of large particles, suggesting that the active catalyst in the homogeneous system was not CoOx.42 An alternative explanation, consistent with both our results of the above bpy chelation experiment and those of Wang et al,42 is that the chelating agent may compete with substrate for the active site when the catalyst is in its oxidized state. Nonetheless, Fujita et al have suggested that DLS detection might leave out very small particles generated in water oxidation system,49 and thus the above DLS measurement cannot exclusively rule out the presence of CoOx nanoparticles in the reported photocatalytic system.

Figure 8. Time profiles of photocatalytic water oxidation in a 1.0 mL reaction solution containing 1 (5.0 µM), [Ru(bpy)3](ClO4)2 (0.4 mM) and Na2S2O8 (3 mM) with bpy (0, 1, 2, 5 µM) in 50 mM sodium borate buffer (pH 8.0) with background subtraction. The Clark cell was kept constant at 25 °C, and the system was irradiated using LEDs (λ = 470 ± 5 nm, light intensity = 150 mW/cm2). The arrow indicates the start of the irradiation.


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To get the direct evidence for the existence of heterogeneous CoOx catalyst formed during the photochemical reaction, after the photo-catalysis reaction, the solution was taken on the Cu grid to identify trace of CoOx under TEM (Figure S12). The results clearly demonstrate the existence of CoOx in the solution, which is consistent with the conclusion based on the results of bpy chelation. It should point out that if a magnetic stirring bar was used during the photochemical experiment, no heterogeneous CoOx can be observed by TEM, as the formed CoOx was adhered on the surface of magnetic stirring bar.

CONCLUSION

In conclusion, while exploring the reasons behind catalytic inefficiency of 2 in water oxidation, a previously reported complex 1 with analogous structure was reinvestigated exclusively. During the investigation, we have found that it is the slow degradation of water soluble complex 1 into CoOx as a real heterogeneous WOC in electrocatalytic process. Herein, our detailed investigations on 1 have elucidated the following conclusions: (1) The dinuclear cobalt(III) complex 1 lacks the ability to catalyze water oxidation, and 1 is not a homogeneous electrochemical/photochemical WOC. Recently, Liu et al found that Co2+Td site in Co3O4 acts as the active site for water oxidation via the formation of CoOOH species, while Co3+Oh tends to form stable bond with OH groups, thus limiting its catalytic activity.50 From the structures of 1 and 2 it can be found that Co3+ indeed forms stable coordination bond with µ-O22- group, and this may be the reason that both 1 and 2 lack the ability to catalyze water oxidation. From this point we may conclude that one should avoid using Co(III)-oxygen complexes as molecular WOCs, as the stable Co3+-O bond will limit their catalytic activity; (2) The formed CoOx originated from the degradation of 1 at oxidative condition contributes to the catalytic activity observed in


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literature. (3) The presence of CoOx generated in photocatalytic systems would be difficult to detect by DLS measurement, and we found that the generated nanoparticles in the reaction system can be directly observed by TEM analysis. Such finding, exemplified on a cobalt complex, may be extended to other molecular systems for the clarification of homogeneous or heterogeneous WOCs. All the above investigations elucidate that the true identity of the active catalyst must be clarified prior to a detailed interrogation of the reaction mechanism,36 which can avoid false orientations for the design of new molecular WOCs based on an incorrect mechanism. ASSOCIATED CONTENT Supporting Information. Crystallographic data and additional figures as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by the 973 program of China (2012CB821706, 2014CB845602), NSFC (21331007), and NSF of Guangdong Province (S2012030006240).


ACS Catalysis

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Table of Contents Graphic

Re-Investigation of Water Oxidation Catalyzed by a Dinuclear Cobalt-Polypyridine Complex: Identification of CoOx as a Real Heterogeneous Catalyst Jia-Wei Wang, Pathik Sahoo, and Tong-Bu Lu*

A reported dinuclear cobalt(III) complex as a homogeneous water oxidation catalyst has been found to be ineffective in electrocatalytic and photocatalytic water oxidation, and CoOx has been identified as the genuine heterogeneous catalyst for water oxidation.


Reinvestigation of Water Oxidation Catalyzed by a Dinuclear Cobalt

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