Architecture of Biomimetic Water Oxidation Catalyst with Mn4CaO5

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The Architecture of Biomimetic Water Oxidation Catalyst with Mn4CaO5 Cluster-like Structure Unit Zhibin Geng, Yu Sun, Yuan Zhang, YanXiang Wang, Liping Li, Keke Huang, Xiyang Wang, Jinghai Liu, Long Yuan, and Shouhua Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11041 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Applied Materials & Interfaces

The Architecture of Biomimetic Water Oxidation Catalyst with Mn4CaO5 Cluster-like Structure Unit Zhibin Geng a, Yu Sun a, Yuan Zhang a, Yanxiang Wang a, Liping Li a, Keke Huang a, Xiyang Wang a, Jinghai Liu a, b, Long Yuan a, and Shouhua Feng* a a

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, b

Inner Mongolia Key Lab of Chemistry of Natural Products and Synthesis of Functional

Molecules, College of Chemistry and Chemical Engineering, Inner Mongolia University for the Nationalities (IMUN), Tongliao 028000, People’s Republic of China KEYWORDS: biomimetic, water oxidation, Ca-birnessite, architecture

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ABSTRACT Mn4CaO5 cluster in green plant is considered as the ideal structure for water oxidation catalysis. However, this structure is difficult to be constructed in heterogeneous catalyst due to its distorted spatial structure and unique electronic state. Herein, we report the synthesis of 2-dimensional biomimetic Ca-Mn-O catalyst with Mn4CaO5 cluster-like structure through ultrasonic-assisted reduction treatment towards Ca-birnessite. The synergistic effect between ultrasonic and reduction successfully reduced the Mn oxidation state in Ca-birnessite without breaking the structure of MnO2 monolayers, forming a regular 2-dimensional structure with Mn4CaO5 cubane-like structure unit for the first time. The biomimetic catalyst shows a superior water oxidation activity (TOF = 3.43 s-1), which is the best in manganese-based heterogeneous catalyst to date. This work provides a new strategy for the precise synthesis of specific structure, and exhibits great prospect of bio-mimic in heterogeneous catalyst.

INTRODUCTION The absence of appropriate water oxidation catalyst is one of the great challenges that prevent the application of artificial photosynthesis. 1 - 5 In green plants, the highly active and earth abundant nature catalyst Mn4CaO5 cluster provides an excellent model for the designing of water oxidation catalyst.6,7 Mn4CaO5 cluster constitutes of a distorted oxo-bridged cubane structure with mixed valence of Mn (Mn3+3Mn4+), which is perfectly suitable for stepwise oxidation process in water oxidation.6-8 However, the distorted spatial structure and unique electronic state of Mn4CaO5 cluster make it difficult to be constructed in heterogeneous catalyst. Among all the heterogeneous catalysts, amorphous calcium-manganese oxides (ACMO)9-12 and Ca-birnessite13-16 have the closest structure to Mn4CaO5 cluster. Ca-birnessite consists of MnO2 monolayers with Ca2+ inserted to the interlayers.17-19 The edge-sharing MnO6 octahedra in


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MnO2 monolayers and the upper Ca2+ ion form a cubane-like motif which is analogous to the Mn4CaO5 cluster (Figure 1).8 However, compared with Mn4CaO5 cluster, great imperfections still exist in the biomimetic structure of Ca-birnessite. The most important distinction lies in the oxidation state of Mn, the average oxidation state of Mn in Ca-birnessite is 3.6-3.9,13-16 which is much higher than that of 3.25 in Mn4CaO5 cluster.6 Mn3+ has been proved to be the active Mn species in water oxidation catalysis,20-23 the low percentage of Mn3+ in Ca-birnessite leads to a lower catalytic activity for Ca-birnessite compared to Mn4CaO5 cluster.13-16 Furthermore, the layered structure of Cabirnessite locks most biomimetic Ca-Mn-O cubane-like structure unit in the interlayer, which limits its water oxidation activity.8 In order to construct an efficient biomimetic water oxidation catalyst, it is crucial to reduce Mn oxidation state and delaminate layered structure of Cabirnessite. However, the structure of Ca-birnessite is instable which tends to convert into other structures in reduction environment, 24 reducing Mn oxidation state without breaking its advantageous structures remains a challenge. Thus, a specific method is needed for the modification of Ca-birnessite. Herein, we describe the successful fabrication of biomimetic Ca-Mn-O catalyst by an ultrasonic assisted reduction treatment of Ca-birnessite. During this procedure, ultrasonic treatment synergizes with reduction treatment, smoothly reduced most Mn4+ into Mn3+ without breaking the structure of MnO2 monolayer, and delaminated the layered structure of Cabirnessite to expose the interlayer active sites, forming a catalyst with biomimetic Ca-Mn-O cubane-like structure unit (Figure 1). The biomimetic catalyst exhibits excellent heterogeneous water oxidation activity, which indicates the great prospect of bio-mimic in artificial photosynthesis.


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Figure. 1 Schematic diagram of ultrasonic assisted reduction treatment of Ca-birnessite. Individually reduction or ultrasonic treatment cannot fabricate biomimetic catalyst.

RESULT AND DISCUSSION Biomimetic Ca-Mn-O catalysts were synthesized by reducing nanosize Ca-birnessite precursor with NaBH4 solution in ultrasonic environment. The structural change of Ca-birnessite under the modification treatment is identified by powder X-ray diffraction (XRD) patterns (Figure 2a). Cabirnessite pattern shows typical layered birnessite characters,16,25,26 the peak positions of (001) and (002) crystal facets are at 2θ of ~11o and ~23o, respectively, indicating a interlayer distance of ~7.8 Å. The peaks at 2θ of ~37o (100) and ~66o (110) reflect the structure of MnO2 monolayers. After the modification treatment, the peaks of (001) and (002) planes faded in CM-1, CM-2, and CM-3, which means the layered structure of Ca-birnessite was broken. Meanwhile,


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the structure of MnO2 monolayers in biomimetic catalysts remained unchanged according to the strong signals of (100) and (110) planes. In biomimetic catalysts, CM-3 shows a new signal at 2θ of ~18o, which implied that phase change may occur at high reductant dosage. We are able to determine the amount of Mn vacancy sites in MnO2 monolayers by the lineshape characteristics at 2θ of ~45-55o.27,28 There are few Mn vacancy sites in Ca-birnessite judging from the smooth lineshape at 2θ of ~45o, while the scattering dip at 2θ of ~45o and the hump at ~50-55o 2θ indicating more vacant layer sites in the biomimetic catalysts.27 The vacant layer sites are capped by metal cations, Ca2+ or Mn3+ precipitated from monolayer (Figure 1).27 Figure 2b shows the Raman spectra of four samples. Two major features can be recognized at 575 and 638 cm-1 for all samples. In birnessite, the Raman band at 638 cm−1 can be regarded as the symmetric stretching vibration ν2(Mn–O) of MnO6 groups, and the band located at 575 cm−1 usually attributed to the ν3(Mn–O) stretching vibration in the basal plane of MnO2 monolayers.29 The similarity in Raman spectra between Ca-birnessite and biomimetic catalysts indicates that the structure of MnO2 monolayers remains unchanged in the modification treatment. The band at 575 cm−1 is slightly weaker from Ca-birnessite to CM-1, CM-2 and CM-3, which can be related to the decreases of Mn4+ percentage.29


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Figure. 2 (a) Powder XRD patterns and (b) Raman spectra of Ca-birnessite, CM-1, CM-2, and CM-3; (c) TEM and (d) HRTEM images of Ca-birnessite; (e)TEM and (f) HRTEM images of CM-2. The composition of as-prepared catalysts was identified by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The Ca/Mn ratios of Ca-birn, CM-1, CM-2, and CM-3 are 1/3.6, 1/5.5, 1/10, and 1/11, respectively (Table 1). The Ca concentration decreases with the increasing of reductant. Elemental mapping of CM-2 proved that Ca2+ was uniformly distributed in biomimetic catalyst (Figure S1). TEM images of Ca-birnessite and CM-2 are shown in Figure 2. The morphology of Cabirnessite is a disordered accumulation of layered crystals (Figure 2c), the HRTEM image shows the surface of Ca-birnessite is smooth (Figure 2d). After the modification treatment, the accumulation morphology of Ca-birnessite generally remains unchanged, the micro-scale morphology of CM-2 is still a disordered accumulation (Figure 2e). However, great change is observed at the nano-scale morphology, the surface of CM-2 is rough and the thickness is uneven (Figure 2f), which can be assigned to the accumulation of 2D nanosheets. 30 Combined the


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morphology changes with XRD results, we consider that the layered structure of Ca-birnessite was delaminated in modification treatment, and the delaminated monolayers were in-situ accumulated to form a rough and more disordered structure. The delamination of layered structure is also proved by AFM images (Figure S2), Cabirnessite is larger sheet with a thickness of around 3.2 nm, CM-2 is smaller sheet with a thickness of around 0.8 nm. As the thickness of one layer in Ca-birnessite is 0.78 nm according to XRD pattern in Figure 2, Ca-birnessite sheet has 4 layers while CM-2 is monolayer. Thus, the layered structure of Ca-birnessite is delaminated in the ultrasonic assisted reduction treatment. The variation of surface area from Ca-birnessite to biomimetic catalyst also reflects the delamination of layered structure, BET surface areas of CM-1, CM-2, and CM-3 are much larger than that of Ca-birnessite (Table 1). The delamination of layered structure exposes interlayer Ca2+ ions and increases surface area, which are favorable for water oxidation catalysis. The electronic state of manganese is the key factor of water oxidation catalysis. XPS of asprepared samples were taken to identify the oxidation state of Mn in samples (Figure 3). For the Mn 2p XPS signal, the peaks shift to lower binding energy from Ca-birnessite, CM-1, CM-2, to CM-3, indicating the oxidation state of manganese decreases with the increasing of reductant dosage (Figure 3a). The binding energy of Mn 2p3/2 of Ca-birnessite is 642.7 eV, which means most Mn ions are Mn4+, while the signal at 641.0 ± 0.2 eV indicates the presence of Mn3+.31 CM1 also exhibits the characters of both Mn3+ and Mn4+,32 but the content of Mn3+ ions is higher, while CM-2 and CM-3 are mainly consist of Mn3+. The accurate oxidation state of Mn ions is calculated by the energy separation of Mn 3s XPS.33,34 In Figure 3b, we fitted and measured Mn 3s of catalysts, the energy separations of Ca-birnessite, CM-1, CM-2, and CM-3 are 4.73 eV, 4.82 eV, 5.18 eV, and 5.26 eV. According to the linear relationship between energy separation


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and Mn oxidation,35 their Mn oxidation states are 3.6, 3.5, 3.1, and 3.0, respectively (Table 1). The higher Mn oxidation state of CM-1 indicates that Mn4+ ions in Ca-birnessite is less sensitive in low reductant dosage, while most Mn4+ ions can be reduced to Mn3+ ions in higher reductant dosage.

Figure. 3 XPS of Ca-birnessite, CM-1, CM-2, and CM-3: (a) Mn 2p; (b) Mn 3s. (c) O 1s XPS spectra of CM-2. (d) Mn L-edge X-ray absorption spectroscopy of Ca-birn and CM-2. Figure 3c shows O 1s XPS spectra of CM-2, the peaks at 530, 531.3, and 532.4 eV can be assigned to metal-O-metal bond, hydroxyl oxygen, and oxygen in absorbed water. The high concentration of hydroxyl could attribute to the charge balance while most Mn4+ ions were reduced to Mn3+. The Ca 2p XPS spectrum of CM-2 is shown in Figure S23, the Ca/Mn ratio calculated from peak areas of XPS spectra according with the result from ICP-AES, which indicates that Ca2+ concentration at surface is the same with that in the whole material. Detailed structural and electronic state changes are studied by X-ray absorption spectroscopy (XAS) at Mn L-edge and K-edge. Figure 3d shows the Mn L-edge XAS spectra of Ca-birnessite


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and CM-2. The Ca-birnessite spectrum exhibits obviously Mn4+ characteristic with a low energy t2g peak and a high energy eg peak at L3-edge, while CM-2 spectrum corresponds to Mn3+ characteristic spectrum which has a broad eg peak caused by strong Jahn-Teller distortion.36 This result indicated the reduction of Mn oxidation in the fabrication.

Table 1 Characteristics of synthesized catalyst

Sample

Mn Oxidation state

SBET (m2 g−1)

Ca:Mn ratio

TOF c (s-1)

TOF d (s-1)

Ca-birna

3.6

103

1 : 3.6

0.17

0.13

CM-1

3.5

193

1 : 5.5

0.67

1.44

CM-2

3.1

208

1 : 10

3.43

3.11

CM-3

3.0

205

1 : 11

3.03

2.84

C-Mn2O3b

3.0

11

-

0.13

0.02

a

Ca-birn is annealed sample. bCommercial Mn2O3. cWater oxidation activity measured by visible light/Ru(bpy)32+/S2O82-. Catalytic activities are expressed by Turnover Frequency (TOF) which indicates oxygen generation numbers per catalytic site (mmolO2 molMn-1 s-1). dWater oxidation activity measured by Ce4+.

K-edge spectra of as-prepared catalysts are shown in Figure 4a, the edge-rise energies is indicative of the average oxidation state of manganese.10,37 The edge position of catalyst shifts to lower energy from Ca-birnessite, CM-1, CM-2, to CM-3, indicating the reduction of average Mn oxidation state. The average oxidation state of manganese is obtained via its linear correlation to edge position,10,38 the average Mn oxidation states of Ca-birnessite, CM-1, CM-2, and CM-3 are 3.6, 3.4, 3.1, and 3.0, respectively (Figure 4a inset). This result is consistent with the XPS result. The bonding structure of the as-prepared catalysts can be confirmed by fourier-transformed extended X-ray absorption fine structure spectrum (EXAFS).12,39 The peak positions in EXAFS


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spectra correspond to the distances between Mn atoms to the backscattering atoms (Figure 4b),13 curve fitting of the spectrum is shown in Figure S7. The first peak at R’ = ~1.5 Å corresponds to the Mn-O distances in MnO6 octahedra, and the second peak at R’ = ~2.4 Å corresponds to the di-µ-oxido connected Mn-Mn distances of neighboring MnO6 octahedra. 13,17 The characteristic of these two peaks changed little after the modification treatment of Ca-birnessite, showing that the basic structural unit of MnO2 monolayers remain unchanged in the modification treatment. Meanwhile, the peak intensities of these two peaks weakened with the increasing of reductant dosage, which could be caused by the structural disorder or lattice distortion originated from Jahn-Teller effect of Mn3+ and the increasing of Mn vacancy in MnO2 monolayers.40 The presence of a peak at R’ = ~3 Å is predicted for Ca-Mn distance of cubane-like metaloxido motif.101713 This peak could be found in all catalysts, Ca-birnessite spectra shows the highest peak intensity due to its highest Mn coordination number to Ca2+. Owing to calcium losses and Mn vacancy, the peak intensities of modified catalysts gradually reduce (Figure 4b inset). The increasing of Mn vacancy is also proved by the decreases of long-range order of MnO2 monolayers. The peak at R’ = ~5 Å usually attributed to three collinearly arranged Mn ions interconnected by di-µ-oxido bridges,13,41 the intensity of the corresponding peak reflects the extent of long-range order of the catalyst.42 We found that the long-range order of biomimetic catalysts decreases compared with that of Ca-birnessite, CM-3 which has the highest reduction degree shows the lowest peak intensity.


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Figure. 4 X-ray absorption spectroscopy at Mn K-edge of Ca-birnessite, CM-1, CM-2, and CM3: (a) XANES spectra, the linear correlation of edge position and average Mn oxidation state is shown in the inset, edge position is obtained at half-height of the absorption edge. (b) EXAFS spectra, each peak relates to a specific binding motif, shown as black bidirectional arrow. Red, blue, and green balls represent O, Mn, and Ca ions, respectively. The peak at R’ = ~3 Å is MnCa distance, shown enlarged in the inset.


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According to the characterization results, we concluded the mechanism and reaction process of modification treatment. In the ultrasonic assisted reduction treatment, most Mn4+ ions in Cabirnessite are reduced into Mn3+ ions while the structure of MnO2 monolayers remain besides the generation of some Mn vacancies, and the layered structure of Ca-birnessite was delaminated. But the controlled experiments show that individually ultrasonic treatment or reduction treatment cannot play the same role as in ultrasonic assisted reduction treatment. Phase change will take place when Ca-birnessite precursor is simply treated by reductant, and individual ultrasonic treatment cannot delaminate layered structure of Ca-birnessite, seen in Figure S3-S5. Thus, there should be an synergistic effect between ultrasonic treatment and reduction treatment, which facilely reduces the oxidation state of Mn without changing the structure of MnO2 monolayers, forming a MnO2 monolayer mainly constitute of Mn3+O6 octahedra which is rare so far.28 Meanwhile, after the treatment, MnO2 monolayers exhibit high degree of lattice disorder, which could be caused by Jahn-Teller effect of Mn3+ and Mn vacancy. The three neighboring distorted MnO6 octahedra and upper Ca2+ and Mn3+ ions form a cubane-like structure analogous to Mn4CaO5 cluster (Figure 1). In order to further explore the mechanism of ultrasonic assisted reduction reaction, we studied the impact of ultrasonic power and time in the fabrication of biomimetic catalyst. Ca-birnessite precursor is treated with different ultrasonic power (120 W, 300 W, 500 W, and 950 W) for 1 min, and 300 W ultrasonic bath for different time (10 s, 30 s, 1 min, 5min, 20min), other conditions are the same to CM-2 (Figure S6). Obviously, lower ultrasonic power (120 W) cannot delaminate layered structure of Ca-birnessite, and higher ultrasonic power (950 W) will damage the structure of MnO2 monolayer, the appropriate ultrasonic power is 300-500 W. Meanwhile, the layered structure is delaminated merely after 10 s’ treatment, indicating that ultrasonic


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assisted reduction is a very fast reaction. But the obtained catalyst is not very stable in ultrasonic reduction environment, after 5 min’ treatment, other phases appeared seen from the impurity peak at around 20o, and the catalyst is completely reduced into Mn3O4 after 20 min’ treatment. According to the experiment result, we consider that the synergistic effect is mainly caused by the cavitation of the ultrasonic wave,43 which facilitates the splitting of BH4- and accelerate the reaction rate. In the cavitation bubble, BH4- is more likely to split into B2H6 and H-, which could be predicted by the generation of large number of bubbles from the suspension. Then, massive Hcrash on Ca-birnessite promoted by ultrasonic wave, most of the Mn4+ on the surface of Cabirnessite are reduced into Mn3+ in a very short time, having no time to carry out phase transition. This assumption could be proved by the rapid reaction rate that the Ca-birnessite is completely transferred into biomimetic catalyst after merely 10 s’ treatment. According to XPS and ICPAES result, Ca2+ ions in Ca-birnessite gradually lost as the increases of reductant, which could be caused by delamination of layered structure and the increase of vacant sites in monolayer. The remaining Ca2+ is evenly distributed in the whole material, as proved by ICP-AES, XPS, and Elemental mapping by STEM-XEDS (Figure S1). There are two main methods of measuring heterogeneous water oxidation activity of catalyst: photochemical water oxidation and chemical water oxidation test, both methods were used to investigate the water oxidation activities of catalysts. Photochemical water oxidation test is conducted by typical Ru(bpy)32+/Na2S2O8 system under visible light illumination (Figure 5a). The activity is calculated by oxygen generation rate per manganese ion (mmolO2 molMn-1 s-1). Reaction system without catalyst was also tested to ensure no oxygen is generated in the absence of catalyst (Figure S8). All the as-prepared catalysts show catalytic activity in water oxidation experiments. As expected, biomimetic catalysts perform much better than Ca-birnessite, the


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activities of Ca-birnessite, CM-1, CM-2, and CM-3 are 0.17, 0.67, 3.43, and 3.03 s-1, respectively (Table 1). The activity of CM-2 is 20 times higher than typical Ca-birnessite.

Figure. 5 (a)(b) Photochemical water oxidation tests of Ca-birnessite and biomimetic Ca-Mn-O catalysts. Commercial Mn2O3 (C-Mn2O3) is also tested for comparation. Condition: phosphate buffer (pH=7) with 1 mM Ru(bpy)32+ and 5 mM Na2S2O8, illuminated with 300 W Xenon lamp. (c)(d) Chemical water oxidation tests of samples. Condition: 0.25 M Ce4+. Chemical water oxidation activities of catalysts were tested by single-electron oxidant ceric ammonium nitrate (CAN). The measurement result is similar to photochemical water oxidation tests (Figure 5c), the activities of Ca-birnessite, CM-1, CM-2, and CM-3 are 0.13, 1.44, 3.11, and


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2.84 s-1 (Table 1), respectively. Biomimetic Ca-Mn-O catalyst is one of the best heterogeneous water oxidation catalysts so far (Table S1 and Table S2). We investigated the major parameters which influence the catalytic activity of biomimetic CaMn-O catalysts. The electronic state of manganese is a key factor for water oxidation catalysis.2022

Biomimetic catalysts with Mn oxidation state of approximately 3 (CM-2 and CM-3) show

much better activities over the catalysts with higher Mn oxidation state (Ca-birnessite and CM-1), indicating the vital function of Mn3+ ions or eg1 configuration. Surface Ca2+ ions serve as coordination centers of water molecule and reduce overpotential of intermedium conversion. Higher Ca2+ content should be the reason for the better performance of CM-2 than CM-3. For Ca-birnessite, although it contains high concentration of Ca2+ ions, the interlayer location of Ca ions lead to a poor water oxidation activity compared to the modified catalyst CM-1. The delamination of layered structure exposes interlayer Ca2+ ions and increases the surface area of Ca-birnessite, obtaining a better structure for water oxidation. Thus, the best catalyst CM-2 with proper structure and composition exhibits the highest activity. In CM-2, an cubane-like structure similar to Mn4CaO5 cluster could be formed by the combination of three di-µ-oxido connected MnO6 octahedral in MnO2 monolayer, a Ca2+ ion above empty layer tetrahedra, and a Mn3+ ion capped on layer vacancy (Figure 1). Due to this advantageous structure and electronic state, biomimetic Ca-Mn-O catalyst performs enhanced water oxidation activity over other manganesebased heterogeneous catalysts.

CONCLUSIONS In summary, the Mn4CaO5 cluster-like structure unit is first constructed in heterogeneous catalyst for efficient water oxidation catalysis. In detail, a 2-dimensional biomimetic Ca-Mn-O


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catalyst is fabricated via an ultrasonic-assisted reduction treatment of Ca-birnessite. The synergetic effect between ultrasonic and reduction treatment facilely reduced most Mn4+ in Cabirnesstie into Mn3+ without breaking its native advantageous structure of MnO2 monolayers, forming a biomimetic Mn4CaO5 cubane-like structure. Furthermore, the layered structure of Cabirnessite is delaminated in the treatment, resulting in higher surface area and more active sites. This is the first time of obtaining a heterogeneous catalyst with analogous structure to Mn4CaO5 cluster in both spatial structure and electronic state. The biomimetic catalyst exhibits an excellent water oxidation activity of TOF = 3.43 s-1, which is the highest in manganese-based heterogeneous catalyst to date. This work not only provides a powerful strategy for the design and fabrication of efficient catalyst, but also proved that heterogeneous catalyst can be essentially improved by mimicking nature active structure.

EXPERIMENTAL SECTION Synthesis of Ca-birnessite Precursor 3.16 g KMnO4 and 44.1 g CaCl2 •2H2O were dissolved in 750 mL deionized water, and the solution was moved into 1 L round-bottom flask. Then, 200 mL of ethyl acetate was added into the round-bottom flask to form a biphasic system, ethyl acetate was the upper layer. The flask was kept on water bath with refluxing devices at 80 oC for 10 h, brown Ca-birnessite precipitates were formed at the bottom of flask. The products were washed with deionized water for several times, and dried in lyophilizer. Fabrication of Biomimetic Ca-Mn-O Catalyst 0.2 g of the as-prepared Ca-birnessite was ultrasonic dispersed in 50 ml deionized water. Then, 20 mL NaBH4 aqueous solutions contains 10 mmol, 30 mmol, and 50 mmol NaBH4 were added


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to the suspensions, respectively. The suspensions were kept in ultrasonic environment for 1 min at room temperature, and washed with large amount of deionized water for several times. The obtained samples were dried in lyophilizer, and annealed at 200 oC for 5 h. The three products were named CM-1, CM-2, and CM-3 (Table 1). A portion of Ca-birnessite was also heated at 200 oC for 5 h for comparison. Water Oxidation Tests A typical Ru(bpy)32+-Na2S2O8 system was used to characterize photochemcial water oxidation properties. 44 , 45 2.5 mg as-prepared sample was added to a quartz vessel containing 20 mL phosphate buffer (pH=7) with 1 mM Ru(bpy)32+ and 5 mM Na2S2O8, the concentration of sample was 125 ppm. The suspension was purged by bubbling nitrogen for 10 min, then, the vessel was sealed and illuminated with a 300 W Xenon lamp with filters for UV (420 nm cutoff). Dissolved oxygen concentration was measured by a Clark-type oxygen electrode at 20 oC controlled by water recirculator. Chemical water oxidation test is carried out with CeIV as a single-electron oxidant.9,13 1 mg asprepared sample was dispersed to 20 mL deionized water in a vessel, the suspension purged by bubbling nitrogen for 10 min. 5 mmol ceric ammonium nitrate (CAN) was added to the suspension under stirring and quickly sealed the vessel. Dissolved oxygen concentration was measured by a Clark-type oxygen electrode at 20 oC controlled by water recirculator.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional experimental results, detailed evidence of reaction mechanism, optimize of water oxidation condition, the influence of anneal treatment for the samples. (PDF)


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Funding Sources This work was supported by National Natural Science Foundation of China (grants 21427802, 21671076, and 21621001). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Beamline BL14W1 of Shanghai Synchrotron Radiation Facility for providing the beam time. The authors thank beamline BL11U of National Synchrotron Radiation Laboratory Facility (NSRL) for providing the beam time.

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Table of Contents


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Architecture of Biomimetic Water Oxidation Catalyst with Mn4CaO5

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